Co-reporter:John D. Watkins;Steven D. Bull
The Journal of Physical Chemistry C September 3, 2009 Volume 113(Issue 35) pp:15629-15633
Publication Date(Web):2017-2-22
DOI:10.1021/jp905068r
Power ultrasound is used to “mobilize” droplets of 1,2-dichloroethane (DCE) on a glassy carbon electrode surface in an aqueous electrolyte environment. Voltammetric methods are employed to investigate the effect of ultrasound on (i) the mass transport in the aqueous phase, (ii) the mass transport in the DCE−aqueous two-phase system, and (iii) the triple-phase boundary anion extraction reaction coupled to oxidation of n-butylferrocene (nBuFc) in the organic phase. Optimized conditions comprise a 13 mm diameter ultrasonic horn (24 kHz) with 15 W/cm2 power output at a distance of 15 mm from a 2.83 cm2 glassy carbon working electrode in 32 cm3 of aqueous solution. Mass transport in the aqueous phase is probed for the reduction of hexaammineruthenium(III) chloride in aqueous 0.1 M KCl supporting electrolyte, and an increase in mass transport induced by the DCE droplets is observed. Triple-phase-boundary ion transfer reactions are studied for the oxidation of nBuFc in DCE in the presence of aqueous 0.1 M NaBPh4, KPF6, NaClO4, and phosphate buffer pH 1. The hydrophobicity of the transferring electrolyte anion is observed to shift the electrochemical response according to the standard transfer potential. For phosphate electrolyte media, rather than phosphate transfer, n-butylferricenium cation transfer into the aqueous phase and iron(III) phosphate formation occur. The beneficial effect of adding tetrabutylammonium hexafluorophosphate electrolyte into the organic DCE phase is demonstrated; however, triple-phase-boundary processes in the absence of intentionally added electrolyte in the organic phase are feasible.
Co-reporter:Katherine Lawrence, Fengjie Xia, Rory L. Arrowsmith, Haobo Ge, Geoffrey W. Nelson, John S. Foord, Mónica Felipe-Sotelo, Nick D. M. Evans, John M. Mitchels, Stephen E. Flower, Stanley W. Botchway, Daniel Wolverson, Gazi N. Aliev, Tony D. James, Sofia I. Pascu, and Frank Marken
Langmuir October 7, 2014 Volume 30(Issue 39) pp:11746-11752
Publication Date(Web):October 7, 2014
DOI:10.1021/la404866s
A novel two-photon-fluorescent N,O-heteroatom-rich carbon nanomaterial has been synthesized and characterized. The new carbon nanoparticles were produced by hydrothermal conversion from a one-photon-fluorescent poly(4-vinylpyridine) precursor (P4VP). The carbonized particles (cP4VP dots) with nonuniform particle diameter (ranging from sub-6 to 20 nm with some aggregates up to 200 nm) exhibit strong fluorescence properties in different solvents and have also been investigated for applications in cell culture media. The cP4VP dots retain their intrinsic fluorescence in a cellular environment and exhibit an average excited-state lifetime of 2.0 ± 0.9 ns in the cell. The cP4VP dots enter HeLa cells and do not cause significant damage to outer cell membranes. They provide one-photon or two-photon fluorescent synthetic scaffolds for imaging applications and/or drug delivery.
Co-reporter:Yuanyang Rong;Karen J. Edler;Craig Evans;Andrew J. Wain;Richard Malpass-Evans;Stuart H. Taylor;Daping He;Mariolino Carta;Neil B. McKeown;Adrian Sanchez-Fernandez;Tomos J. Clarke;John M. Mitchels
Langmuir November 10, 2015 Volume 31(Issue 44) pp:12300-12306
Publication Date(Web):2017-2-22
DOI:10.1021/acs.langmuir.5b02654
Vacuum carbonization of organic precursors usually causes considerable structural damage and collapse of morphological features. However, for a polymer with intrinsic microporosity (PIM-EA-TB with a Brunauer–Emmet–Teller (BET) surface area of 1027 m2g–1), it is shown here that the rigidity of the molecular backbone is retained even during 500 °C vacuum carbonization, yielding a novel type of microporous heterocarbon (either as powder or as thin film membrane) with properties between those of a conducting polymer and those of a carbon. After carbonization, the scanning electron microscopy (SEM) morphology and the small-angle X-ray scattering (SAXS) Guinier radius remain largely unchanged as does the cumulative pore volume. However, the BET surface area is decreased to 242 m2g–1, but microporosity is considerably increased. The new material is shown to exhibit noticeable electrochemical features including two pH-dependent capacitance domains switching from ca. 33 Fg–1 (when oxidized) to ca. 147 Fg–1 (when reduced), a low electron transfer reactivity toward oxygen and hydrogen peroxide, and a four-point-probe resistivity (dry) of approximately 40 MΩ/square for a 1–2 μm thick film.
Co-reporter:Daping He, Elena Madrid, Barak D. B. Aaronson, Lian Fan, James Doughty, Klaus Mathwig, Alan M. Bond, Neil B. McKeown, and Frank Marken
ACS Applied Materials & Interfaces March 29, 2017 Volume 9(Issue 12) pp:11272-11272
Publication Date(Web):March 13, 2017
DOI:10.1021/acsami.7b01774
A thin film of Nafion, of approximately 5 μm thickness, asymmetrically deposited onto a 6 μm thick film of poly(ethylene terephthalate) (PET) fabricated with a 5, 10, 20, or 40 μm microhole, is shown to exhibit prominent ionic diode behavior involving cation charge carrier (“cationic diode”). The phenomenon is characterized via voltammetric, chronoamperometric, and impedance methods. Phenomenologically, current rectification effects are comparable to those observed in nanocone devices where space-charge layer effects dominate. However, for microhole diodes a resistive, a limiting, and an overlimiting potential domain can be identified and concentration polarization in solution is shown to dominate in the closed state.Keywords: desalination; ion channels; ion pump; nanofluidics; sensor; water;
Co-reporter:Daping He;Erwan Rauwel;Richard Malpass-Evans
Journal of Solid State Electrochemistry 2017 Volume 21( Issue 7) pp:2141-2146
Publication Date(Web):16 February 2017
DOI:10.1007/s10008-017-3534-2
Silver microparticles (ca. 1 μm average size clustered into cage-like aggregates of 10–20 μm diameter) are shown to adhere to a glassy carbon electrode surface to give voltammetric current responses, which are considerably enhanced/stabilised when applying a coating with a molecularly rigid polymer of intrinsic microporosity (PIM-EA-TB). In preliminary voltammetric experiments characteristic Ag(0/I) surface oxidation and back-reduction processes are observed in aqueous phosphate buffer (associated with silver phosphate layer formation on the silver surface). In contrast to the oxidation, which is dominated by a nucleation process causing a sharp well-defined current signal, for the back-reduction stochastic current responses are observed possibly associated with density fluctuations in the surrounding liquid phase (“Brownian activation”) as an essential part of the mechanism of conversion of surface-oxidised silver back to silver metal.
Co-reporter:Yuanyang Rong;Daping He;Richard Malpass-Evans;Mariolino Carta
Electrocatalysis 2017 Volume 8( Issue 2) pp:132-143
Publication Date(Web):2017 March
DOI:10.1007/s12678-016-0347-5
Polymers of intrinsic microporosity (PIM or here PIM-EA-TB) offer a highly rigid host environment into which hexachloroplatinate(IV) anions are readily adsorbed and vacuum carbonised (at 500 °C) to form active embedded platinum nanoparticles. This process is characterised by electron and optical microscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical methods, which reveal that the PIM microporosity facilitates the assembly of nanoparticles of typically 1.0 to 2.5-nm diameter. It is demonstrated that the resulting carbonised “Pt@cPIM” from drop-cast films of ca. 550-nm average thickness, when prepared on tin-doped indium oxide (ITO), contain not only fully encapsulated but also fully active platinum nanoparticles in an electrically conducting hetero-carbon host. Alternatively, for thinner films (50–250 nm) prepared by spin coating, the particles become more exposed due to additional loss of the carbon host. In contrast to catalyst materials prepared by vacuum-thermolysed hexachloroplatinate(IV) precursor, the platinum nanoparticles within Pt@cPIM retain high surface area, electrochemical activity and high catalyst efficiency due to the molecular rigidity of the host. Data are presented for oxygen reduction, methanol oxidation and glucose oxidation, and in all cases, the high catalyst surface area is linked to excellent catalyst utilisation. Robust transparent platinum-coated electrodes are obtained with reactivity equivalent to bare platinum but with only 1 μg Pt cm−2 (i.e. ~100% active Pt nanoparticle surface is maintained in the carbonised microporous host).
Co-reporter:Murilo F. Gromboni, Dyovani Coelho, Lucia H. Mascaro, Adam Pockett, Frank Marken
Applied Catalysis B: Environmental 2017 Volume 200(Volume 200) pp:
Publication Date(Web):1 January 2017
DOI:10.1016/j.apcatb.2016.06.059
•Thin alumina barrier coating can enhance photo-activity of bismuth vanadate photo-anode materials.•There are two distinct potential domains with different enhancement characteristics and mechanisms.•The thickness of alumina films can be controlled with a sol-gel coating to provide optimized performance.Nanostructured semiconductor photoanodes play an important role in solar fuel generation, and the design of the semiconductor − aqueous electrolyte interface can be crucial in enhancing the energy conversion efficiency. We have investigated the effects on photoelectrochemical oxygen evolution for monoclinic nanostructured BiVO4 films uncoated and coated with microporous sol-gel Al2O3 “over-layers”. Variation of the thickness of the Al2O3 coating (formed by surface sol-gel deposition and annealing at 435 °C) led to a reduction of pseudo-capacitance and allowed optimization of the quantum efficiency. Exploration of the photocurrent enhancement as a function of applied potential reveals two distinct potential domains/mechanisms: (i) a low bias region enhancement effect (assigned to a lowering of the rate of external recombination of electrons with oxygen) and (ii) a high bias region of enhancement (assigned to higher charge carrier mobility due to less trapping in surface states).Download high-res image (159KB)Download full-size image
Co-reporter:Alexander N. Bondarchuk, Laurence M. Peter, Gabriela P. Kissling, Elena Madrid, Josué A. Aguilar-Martínez, Zuhayr Rymansaib, Pejman Iravani, Murilo Gromboni, Lucia H. Mascaro, Aron Walsh, Frank Marken
Applied Catalysis B: Environmental 2017 Volume 211(Volume 211) pp:
Publication Date(Web):15 August 2017
DOI:10.1016/j.apcatb.2017.04.047
•Nanostructured hematite has been produced from a surfactant ink on ITO substrates with an ink-jet printing method.•Photo-activity is observed in aqueous NaOH but substantial improvements in photo-currents are observed after vacuum-annealing.•The formation of sub-surface states is suggested and discussed in terms of the photo-transient responses.Transparent nano-structured hematite (α-Fe2O3) films of approximately 550 nm thickness on tin-doped indium oxide (ITO) have been obtained conveniently by ink-jet printing of a Fe(NO3)3/Brij® O10 precursor ink and subsequent annealing at 500 °C in air. When illuminated with a blue LED (λ = 455 nm, ca. 100 mW cm−2), the hematite films exhibited photocurrents of up to 70 μA cm−2 at 0.4 V vs. SCE in 0.1 M NaOH electrolyte. Thermal annealing in vacuum at 500 °C for 2 h increased photocurrents more than three times to 230 μA cm−2 in agreement with previous literature reports for pure hematite materials. These results suggest that a simple ink-jetting process with surfactants is viable. The effects of vacuum-annealing on the photoelectrical properties of α-Fe2O3 films are discussed in terms of a sub-surface state templating hypothesis based on data gathered from photo-transients, field emission scanning electron microscopy, X-ray photoelectron spectroscopy analysis, X-ray diffraction, photocurrent spectra, and cyclic voltammetry.Download high-res image (108KB)Download full-size image
Co-reporter:Naiara Hernandez, Jesus Iniesta, Vicente Montiel Leguey, Robert Armstrong, Stuart H. Taylor, Elena Madrid, Yuanyang Rong, Rémi Castaing, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown, Frank Marken
Applied Materials Today 2017 Volume 9(Volume 9) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.apmt.2017.06.003
•Direct “one-step” conversion of the polymer of intrinsic microporosity PIM-EA-TB-H2 to a microporous heterocarbon for applications in electrochemistry.•Slow hydration of microporous aided by protons and perchlorate to give “pH-switchable” capacitance response.•Carbonization temperature during PIM carbonization appears to be linked to pore size and microporosity effects in the resulting heterocarbon.A nitrogen-containing polymer of intrinsic microporosity (PIM-EA-TB-H2; nitrogen adsorption surface area 846 m2 g−1) is vacuum carbonized at 700 °C and thereby directly without post-treatment converted into a microporous heterocarbon (cPIM; N2 adsorption surface area 425 m2 g−1). Nitrogen functionalities in the polymer backbone are retained in the heterocarbon and appear responsible for unusual time-, electrolyte-, and pH-dependent properties. Electrochemical characterization suggests a high specific capacitance (typically 50 F g−1) but only after prolonged immersion in aqueous HClO4. The time-dependent increase in capacitance during immersion is assigned to slow hydration and ingress of HClO4 into hydrophobic micropores (H2SO4 or H3PO4 are more hydrophilic and much less effective). Once hydrated, the microporous heterocarbon exhibits pH-dependent capacitance “switching” over a wide pH range and analytical applications as “capacitive” pH sensor are proposed.Download high-res image (155KB)Download full-size image
Co-reporter:Miguel A. Montiel, Jesus Iniesta, Andrew J. Gross, Frank Marken
Electrochimica Acta 2017 Volume 224(Volume 224) pp:
Publication Date(Web):10 January 2017
DOI:10.1016/j.electacta.2016.11.103
•Generator-collector voltammetry in low electrolyte conditions.•Analytical current responses insensitive to electrolyte levels.•Diffusion effects within the dual-plate microtrench electrode and pre-electrolysis effects.A gold-gold dual-plate microtrench electrode system based on two oppositely placed gold surfaces with 5 mm length, 17 μm average depth, and 6 μm inter-electrode gap is employed in generator-collector configuration in a four-electrode cell (counter electrode, reference electrode, and two independent working electrodes denoted “generator” – with scanning potential – and “collector” – with fixed potential). The dual-plate microtrench electrodes were investigated for (i) the reduction of Ru(NH3)63+, (ii) the oxidation of ferrocenemethanol, and (iii) the oxidation of iodide in aqueous media, all as a function of supporting electrolyte concentration. It is shown that due to the inter-electrode feedback character of the generator-collector currents, well-defined steady state sensor responses are obtained for the collector electrode even in the absence of added electrolyte. The variation in the mass transport limited steady state current (measured at the collector electrode) with addition/removal of supporting electrolyte remains low (compared to unexpectedly stronger effects caused by the switch between reduction and oxidation conditions at the collector electrode). Microtrench electrode systems are suggested for sensing applications without/with varying levels of supporting electrolyte.Download high-res image (125KB)Download full-size image
Co-reporter:Budi Riza Putra, Mariolino Carta, Richard Malpass-Evans, Neil B. McKeown, Frank Marken
Electrochimica Acta 2017 Volume 258(Volume 258) pp:
Publication Date(Web):20 December 2017
DOI:10.1016/j.electacta.2017.11.130
•Ionic diode inversion upon precipitaion of KClO4 leads to sensor function.•Potassium accumulation at the PIM .| Nafion interface can be used for sensing.•PIM | Nafion interface are introduced as novel reaction spaces.“Heterojunction” ionic diodes based on a Nafion cation conductor and a polymer of intrinsic microporosity (PIM) interfaced at a 6 μm thickness polyethylene-terephthalate (PET) film with 20 μm diameter microhole exhibit rectification effects in cation (K+) flux. When combined with the precipitation reaction of potassium cations with perchlorate anions to give insoluble KClO4 (solubility product ca. 1.05 × 10−2 M2 at 25 °C) at the PIM | Nafion interface, inversion of the rectification/diode effect occurs and the formerly “open” state changes into a “closed” state due to blocking of ion flow. The localised interfacial precipitation reaction is due to up to three orders of magnitude accumulation of K+ at the PIM | Nafion interface and shown to be fast and reversible. The effects of K+/ClO4− concentration and of Na+ interference are considered. The blocking process within the heterojunction ionic diode is suggested to be dynamic/fast and potentially useful as a diode sensor mechanism based on solubility product dependent precipitation.The interfaec between Nafion and a polymer of intrinsic microporosity can be reversibly blocked with KClO4 to give a potassium sensor response.Download high-res image (189KB)Download full-size image
Co-reporter:Barak D. B. Aaronson;David Wigmore;Marcus A. Johns;Janet L. Scott;Igor Polikarpov
Analyst (1876-Present) 2017 vol. 142(Issue 19) pp:3707-3714
Publication Date(Web):2017/09/25
DOI:10.1039/C7AN00918F
Cellulose films as well as chitosan-modified cellulose films of approximately 5 μm thickness, reconstituted from ionic liquid media onto a poly(ethylene-terephthalate) (PET, 6 μm thickness) film with a 5, 10, 20, or 40 μm diameter laser-drilled microhole, show significant current rectification in aqueous NaCl. Reconstituted α-cellulose films provide “cationic diodes” (due to predominant cation conductivity) whereas chitosan-doped cellulose shows “anionic diode” effects (due to predominant anion conductivity). The current rectification, or “ionic diode” behaviour, is investigated as a function of NaCl concentration, pH, microhole diameter, and molecular weight of the chitosan dopant. Future applications are envisaged exploiting the surface charge induced switching of diode currents for signal amplification in sensing.
Co-reporter:Daping He, Dong Sheng He, Jinlong Yang, Ze-Xian Low, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown, and Frank Marken
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 34) pp:22425
Publication Date(Web):August 11, 2016
DOI:10.1021/acsami.6b04144
A molecularly rigid polyamine based on a polymer of intrinsic microporosity (PIM-EA-TB) is shown to capture and stabilize platinum nanoparticles during colloid synthesis in the rigid framework. Stabilization here refers to avoiding aggregation without loss of surface reactivity. In the resulting rigid framework with embedded platinum nanoparticles, the volume ratio of platinum to PIM-EA-TB in starting materials is varied systematically from approximately 1.0 to 0.1 with the resulting platinum nanoparticle diameter varying from approximately 4.2 to 3.1 nm, respectively. Elemental analysis suggests that only a fraction of the polymer is “captured” to give nanocomposites rich in platinum. A transition occurs from electrically conducting and electrochemically active (with shorter average interparticle distance) to nonconducting and only partially electrochemically active (with longer average interparticle distance) polymer–platinum composites. The conducting nanoparticle network in the porous rigid macromolecular framework could be beneficial in electrocatalysis and in sensing applications.Keywords: electrocatalysis; fuel cells; membrane; percolation; stabilization; tunneling
Co-reporter:Elena Madrid, Mark A. Buckingham, James M. Stone, Adrian T. Rogers, William J. Gee, Andrew D. Burrows, Paul R. Raithby, Veronica Celorrio, David J. Fermin and Frank Marken
Chemical Communications 2016 vol. 52(Issue 13) pp:2792-2794
Publication Date(Web):11 Jan 2016
DOI:10.1039/C5CC09780K
Ionic transport (for applications in nanofluidics or membranes) and “ionic diode” phenomena in a zeolitic imidazolate framework (ZIF-8) are investigated by directly growing the framework from aqueous Zn2+ and 2-methylimidazole as an “asymmetric plug” into a 20 μm diameter pore in a ca. 6 μm thin poly-ethylene-terephthalate (PET) film.
Co-reporter:Dharmendra Kumar Yadav, Vellaichamy Ganesan, Frank Marken, Rupali Gupta, Piyush Kumar Sonkar
Electrochimica Acta 2016 Volume 219() pp:482-491
Publication Date(Web):20 November 2016
DOI:10.1016/j.electacta.2016.10.009
•Ag@MOF-5(Zn) nanocomposite synthesisied in a hydrothermal process and investigated by electrochemical methods.•Mechanism of nitrophenol adsorption and electrochemically driven release discovered.•Application in electroanalytical detection of nitrophenol derivatives suggested.Classical Metal-Organic Frameworks (MOFs), although able to accumulate chemicals from solution, are usually electrochemically “inactive”. Here, it is demonstrated for the zinc-containing MOF-5(Zn) (and MOF-5W(Zn)) system, that silver incorporation (Ag@MOF-5(Zn), prepared via a solvothermal process) can be used to assist/promote release and electrochemical oxidation of accumulated nitrophenols (2-methyl-4-nitrophenol, 4-nitrophenol, and 2-nitrophenol). Nitrophenols belong to a group of compounds that are present in diesel exhaust and considered harmful pollutants. The enhanced electrochemical detection of nitrophenols at a glassy carbon electrode modified with Ag@MOF-5(Zn) is suggested to be due to analyte accumulation with estimated Langmuirian binding constants of 40 × 103 M−1 (for 2-methyl-4-nitrophenol and 4-nitrophenol) and 15 × 103 M−1 (for 2-nitrophenol) and electrochemical detection/conversion with a current enhancement of more than one order of magnitude due to potential driven release from Ag@MOF-5(Zn). Surface characterization and electrochemical techniques suggest that silver is present in Ag@MOF-5(Zn) in metallic form and probably also embedded into the framework. This silver incorporation changes the electrochemical oxidation behavior towards nitrophenols in MOF-5(Zn) from “inactive” to “active”. The new class of metal@MOF materials is highlighted as practical nano-composites.Figure optionsDownload full-size imageDownload as PowerPoint slide
Co-reporter:Sunyhik D. Ahn, Karthik Somasundaram, H.Viet Nguyen, Erik Birgersson, Jim Yang Lee, Xiangming Gao, Adrian C. Fisher, Paul E. Frith, Frank Marken
Electrochimica Acta 2016 Volume 188() pp:837-844
Publication Date(Web):10 January 2016
DOI:10.1016/j.electacta.2015.11.143
•Experiments confirm well behaved limiting current with Levich-type equation quantitatively agreeing with uniformly planar diffusion.•Theory developed to link the case of rotating disc voltammetry with the more complex rocking disc voltammetry.•Close agreement of rocking disc voltammetry and rotating disc voltammetry when comparing the average angular movement for both methods.Rocking disc electrode voltammetry (RoDE) is introduced as an experimentally convenient and versatile alternative to rotating disc voltammetry. A 1.6 mm diameter disc electrode is employed with an overall rocking angle of θ = 90 degree applied over a frequency range of 0.83 Hz to 25 Hz. For a set of known aqueous redox systems (the oxidation of Fe(CN)64− in 1 M KCl, the reduction of Ru(NH3)63+ in 0.1 M KCl, the oxidation of hydroquinone in 0.1 M pH 7 phosphate buffer, the oxidation of I− in 0.125 M H2SO4, and the reduction of H+ in 1 M KCl) the mass transport controlled limiting current Ilim is demonstrated to follow in good approximation the Levich-type expression Ilim=0.111 nFAcD2/3v−1/6Θf with n, the number of electrons transferred per molecule diffusing to the electrode surface, F, the Faraday constant, A, the geometric area, c, the concentration of the active redox species, D, the diffusion coefficient, v, the kinematic viscosity, θ is the overall rocking angle in degree, and f, the rocking rate in Hz. Quantitative theory is developed based on a two-dimensional (2D) axisymmetric laminar flow model accounting for the conservation of mass, momentum and species along with the kinematic analysis of a “four-bar mechanism” to obtain the rocking motion.
Co-reporter:Uday Pratap Azad, Dharmendra Kumar Yadav, Vellaichamy Ganesan and Frank Marken
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 33) pp:23365-23373
Publication Date(Web):29 Jul 2016
DOI:10.1039/C6CP04758K
Four polypyridyl redox catalysts Fe(bp)32+, Fe(ph)32+, Fe(dm)32+, and Fe(tm)32+ (with bp, ph, dm, and tm representing 2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, and 3,4,7,8-tetramethyl-1,10-phenanthroline, respectively) are investigated for the electrocatalytic oxidation of three analytes (nitrite, arsenite, and isoniazid). The poly-pyridyl iron complex is exchanged into a Nafion film immobilized on a glassy carbon electrode, which is then immersed in 0.1 M Na2SO4. Cyclic voltammetry is employed for the evaluation of the mechanism and estimation of kinetic parameters. The electrocatalytic behaviour going from low to high substrate concentration is consistent with the Albery–Hillman cases of “LEty” switching to “LEk” (changing from the first order in the substrate to half order in the substrate), denoting a process that occurs in a reaction zone close to the electrode surface with diffusion of charge (from the electrode surface into the film) and of anionic or neutral analyte (from the Nafion–solution interface into the film). The relative hydrophobicity of the iron polypyridyl catalyst within the film is shown to affect both the diffusion of charge/electrons and analyte within the film with Fe(tm)32+ providing the mildest catalyst. All three analytes, nitrite, isoniazid, and arsenite, exhibit linear calibration ranges beneficial for analytical applications in the micro-molar to the milli-molar range.
Co-reporter:Daping He, Yuanyang Rong, Mariolino Carta, Richard Malpass-Evans, Neil B. McKeown and Frank Marken
RSC Advances 2016 vol. 6(Issue 11) pp:9315-9319
Publication Date(Web):18 Jan 2016
DOI:10.1039/C5RA25320A
There remains a major materials challenge in maintaining the performance of platinum (Pt) anode catalysts in fuel cells due to corrosion and blocking of active sites. Herein, we report a new materials strategy for improving anode catalyst stability based on a protective microporous coating with an inert and highly rigid (non-blocking) polymer of intrinsic microporosity (PIM-EA-TB). The “anti-corrosion” effect of the PIM-EA-TB coating is demonstrated with a commercial Pt catalyst (3–5 nm diameter, 40 wt% Pt on Vulcan-72) and for three important fuel cell anode reactions: (i) methanol oxidation, (ii) ethanol oxidation, and (iii) formic acid oxidation.
Co-reporter:Yuanyang Rong, Adam Kolodziej, Elena Madrid, Mariolino Carta, Richard Malpass-Evans, Neil B. McKeown, Frank Marken
Journal of Electroanalytical Chemistry 2016 Volume 779() pp:241-249
Publication Date(Web):15 October 2016
DOI:10.1016/j.jelechem.2015.11.038
Anion uptake and charge transport in a polymer of intrinsic microporosity (here PIM-EA-TB) is investigated for three cases: (i) the oxidation of ferrocene embedded into a thin film of PIM-EA-TB on a glassy carbon electrode, (ii) the reduction of protons absorbed into a thin film of PIM-EA-TB on a platinum electrode, and (iii) the potential-driven transport of anions and protons in an asymmetrically deposited free-standing PIM-EA-TB membrane working as a current rectifier or “ionic diode”. In all three cases the competing effects of the diameter and hydrophobicity (size and hydration energy) of the anion are important. For free-standing membranes very high ionic diode rectification ratios (> 103 at ± 1 V) are observed in particular for thicker deposits of PIM-EA-TB and for chloride or perchlorate containing electrolyte.
Co-reporter:James Weber;Dr. Andrew J. Wain;Dr. Heidi Piili;Ville-Pekka Matilainen;Dr. Anne Vuorema; Gary A. Attard; Frank Marken
ChemElectroChem 2016 Volume 3( Issue 6) pp:1020-1025
Publication Date(Web):
DOI:10.1002/celc.201600098
Abstract
Stainless-steel rods were manufactured by laser additive manufacturing (LAM or “3D-printing”) from a stainless-steel (316 L) powder precursor, and then investigated and compared to conventional stainless steel in electrochemical experiments. The LAM method used in this study was based on “powder bed fusion”, in which particles with an average diameter of 20–40 μm are fused to give stainless-steel rods of 3 mm diameter. In contrast to conventional bulk stainless-steel (316 L) electrodes, for 3D-printed electrodes, small crevices in the surface provide residual porosity. Voltammetric features observed for the 3D-printed electrodes immersed in aqueous phosphate buffer are consistent with those for conventional bulk stainless steel (316 L). Two chemically reversible surface processes were observed and tentatively attributed to Fe(II/III) phosphate and Cr(II/III) phosphate. Galvanic exchange is shown to allow improved platinum growth/adhesion onto the slightly porous 3D-printed stainless-steel surface, resulting in a mechanically robust and highly active porous platinum deposit with good catalytic activity toward methanol oxidation.
Co-reporter:Sunyhik D. Ahn;Adam Kolodziej;Richard Malpass-Evans;Mariolino Carta
Electrocatalysis 2016 Volume 7( Issue 1) pp:70-78
Publication Date(Web):2016 January
DOI:10.1007/s12678-015-0284-8
The free radical 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4B-TEMPO) is active as an electrocatalyst for primary alcohol oxidations when immobilised at an electrode surface and immersed into an aqueous carbonate buffer solution. In order to improve the catalytic process, a composite film electrode is developed based on (i) carbon microparticles of 2–12 μm diameter to enhance charge transport and (ii) a polymer of intrinsic microporosity (here PIM-EA-TB with a BET surface area of 1027 m2 g−1). The latter acts as a highly rigid molecular framework for the embedded free radical catalyst with simultaneous access to aqueous phase and substrate. The resulting mechanism for the oxidation of primary alcohols is shown to switch in reaction order from first to zeroth with increasing substrate concentration consistent with a kinetically limited process with competing diffusion of charge at the polymer layer-electrode interface (here the “LEk” case in Albery-Hillman notation). Reactivity optimisation and screening for a wider range of primary alcohols in conjunction with DFT-based relative reactivity correlation reveals substrate hydrophobicity as an important factor for enhancing catalytic currents. The PIM-EA-TB host matrix is proposed to control substrate partitioning and thereby catalyst reactivity and selectivity.
Co-reporter:Elena Madrid, Philip Cottis, Yuanyang Rong, Adrian T. Rogers, James M. Stone, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown and Frank Marken
Journal of Materials Chemistry A 2015 vol. 3(Issue 31) pp:15849-15853
Publication Date(Web):15 Jul 2015
DOI:10.1039/C5TA04092B
We describe ion current rectification using a Polymer of Intrinsic Microporosity (PIM) based on Tröger's base (PIM-EA-TB). When deposited asymmetrically over one (or more) 20 μm diameter hole(s) in 6 μm thick poly(ethylene terephthalate) and investigated in a two-compartment electrochemical cell with acidified aqueous NaCl on both sides, novel ionic diode effects are observed.
Co-reporter:Meng Li, Zhiqian Guo, Weihong Zhu, Frank Marken and Tony D. James
Chemical Communications 2015 vol. 51(Issue 7) pp:1293-1296
Publication Date(Web):25 Nov 2014
DOI:10.1039/C4CC07891H
A novel electrochemically and fluorescence active boronic ester sensor molecule has been developed containing ferrocene and naphthalimide as the redox and fluorophore units. The combinations of iron (Fe3+) ions, sodium L-ascorbate, and fluoride (F−) ions can be used to produce a molecular system displaying INHIBIT logic, due to indirect fluorescence quenching.
Co-reporter:Meng Li, Weihong Zhu, Frank Marken and Tony D. James
Chemical Communications 2015 vol. 51(Issue 78) pp:14562-14573
Publication Date(Web):28 Aug 2015
DOI:10.1039/C5CC04976H
Boronic acids can bind with 1,2- or 1,3-diols to form five or six-membered cyclic complexes and also can interact with Lewis bases to generate boronate anions. Therefore, boronic acid functionalised compounds and materials are highly topical and now employed in (i) functional materials, (ii) for attaching/sensing bio-molecules and proteins, and (iii) for microbial electrochemistry as well as being widely developed as chemical sensors and tools in health diagnostics. In this review, we address the recent progress of boronic acid-based electrochemical sensors both in solution processes and surface processes for the detection of biological analytes. This feature article will be of interest to chemists, chemical engineers, biochemists, the sensor community, but also researchers working with protein and microbial systems.
Co-reporter:Khadijeh Nekoueian, Christopher E. Hotchen, Mandana Amiri, Mika Sillanpää, Geoffrey W. Nelson, John S. Foord, Philip Holdway, Antoine Buchard, Stephen C. Parker, and Frank Marken
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 28) pp:15458
Publication Date(Web):June 24, 2015
DOI:10.1021/acsami.5b03654
Covalently grafted KolliphorEL (a poly(ethylene glycol)-based transporter molecule for hydrophobic water-insoluble drugs; MW, ca. 2486; diameter, ca. 3 nm) at the surface of a glassy-carbon electrode strongly affects the rate of electron transfer for aqueous redox systems such as Fe(CN)63–/4–. XPS data confirm monolayer grafting after electrochemical anodization in pure KolliphorEL. On the basis of voltammetry and impedance measurements, the charge transfer process for the Fe(CN)63–/4– probe molecule is completely blocked after KolliphorEL grafting and in the absence of a “guest”. However, in the presence of low concentrations of suitable ferrocene derivatives as guests, mediated electron transfer across the monolayer via a shuttle mechanism is observed. The resulting amplification of the ferrocene electroanalytical signal is investigated systematically and compared for five ferrocene derivatives. The low-concentration electron shuttle efficiency decreases in the following sequence: (dimethylaminomethyl)ferrocene > n-butyl ferrocene > ferrocene dimethanol > ferroceneacetonitrile > ferroceneacetic acid.Keywords: amplification; Cremophor; PEGylation; sensor; tunneling; voltammetry;
Co-reporter:Daping He, Yuanyang Rong, Zongkui Kou, Shichun Mu, Tao Peng, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown, Frank Marken
Electrochemistry Communications 2015 Volume 59() pp:72-76
Publication Date(Web):October 2015
DOI:10.1016/j.elecom.2015.07.008
•Accelerated corrosion testing with commercial Pt fuel cell catalyst shows that significantly slower corrosion occurs with PIM-EA-TB applied over the catalyst layer.•Not only migration, Ostwald ripening, and deactivation of the Pt catalyst, but also slower carbon support degradation is observed and confirmed with microscopy.•Microporosity of PIM-EA-TB allows unimpeded transport of reagents and products while protecting the catalyst.The limited stability of fuel cell cathode catalysts causes a significant loss of operational cell voltage with commercial Pt-based catalysts, which hinders the wider commercialization of fuel cell technologies. We demonstrate beneficial effects of a highly rigid and porous polymer of intrinsic microporosity (PIM-EA-TB with BET surface area 1027 m2 g− 1) in accelerated catalyst corrosion experiments. Porous films of PIM-EA-TB offer an effective protective matrix for the prevention of Pt/C catalyst corrosion without impeding flux of reagents. The results of electrochemical cycling tests show that the PIM-EA-TB protected Pt/C (denoted here as PIM@Pt/C) exhibit a significantly enhanced durability as compared to a conventional Pt/C catalyst.
Co-reporter:Christopher E. Hotchen, Ian J. Maybury, Geoffrey W. Nelson, John S. Foord, Philip Holdway and Frank Marken
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 17) pp:11260-11268
Publication Date(Web):02 Apr 2015
DOI:10.1039/C5CP01244A
“Amplified” electron transfer is observed purely based on electron transfer kinetic effects at modified carbon surfaces. An anodic attachment methodology is employed to modify the surface of glassy carbon or boron doped diamond electrodes with poly-ethylene glycols (PEGs) for polymerisation degrees of n = 4.5 to 9.1 (PEG200 to PEG400). Voltammetry and impedance data for aqueous Fe(CN)63−/4− suggest systematic PEG structure-dependent effects on the standard rate constant for heterogeneous electron transfer as a function of PEG deposition conditions and average polymer chain length. Tunnel distance coefficients are polymerisation degree dependent and estimated for shorter PEG chains, β = 0.17 Å−1 for aqueous Fe(CN)63−/4−, consistent with a diffuse water–PEG interface. In contrast, electron transfer to 1,1′-ferrocene-dimethanol (at 1 mM concentration) appears un-impeded by PEG grafts. Mediated or “amplified” electron transfer to Fe(CN)63−/4− based on the 1,1′-ferrocene-dimethanol redox shuttle is observed for both oxidation and reduction with estimated bimolecular rate constants for homogeneous electron transfer of kforward = 4 × 105 mol dm3 s−1 and kbackward = 1 × 105 mol dm3 s−1. Digital simulation analysis suggests an additional resistive component within the PEG graft double layer.
Co-reporter:H. Al-Kutubi, L. Rassaei, W. Olthuis, G. W. Nelson, J. S. Foord, P. Holdway, M. Carta, R. Malpass-Evans, N. B. McKeown, S. C. Tsang, R. Castaing, T. R. Forder, M. D. Jones, D. He and F. Marken
RSC Advances 2015 vol. 5(Issue 96) pp:78746-78746
Publication Date(Web):16 Sep 2015
DOI:10.1039/C5RA90086G
Correction for ‘Polymers of intrinsic microporosity as high temperature templates for the formation of nanofibrous oxides’ by H. Al Kutubi et al., RSC Adv., 2015, 5, 73323–73326.
Co-reporter:H. Al Kutubi, L. Rassaei, W. Olthuis, G. W. Nelson, J. S. Foord, P. Holdway, M. Carta, R. Malpass-Evans, N. B. McKeown, S. C. Tsang, R. Castaing, T. R. Forder, M. D. Jones, D. He and F. Marken
RSC Advances 2015 vol. 5(Issue 89) pp:73323-73326
Publication Date(Web):24 Aug 2015
DOI:10.1039/C5RA15131G
The highly rigid molecular structure of Polymers of Intrinsic Microporosity (PIM) – associated with a high thermolysis threshold – combined with the possibility to fill intrinsic micropores allows the direct “one-step” templated conversion of metal nitrates into nano-structured metal oxides. This is demonstrated here with PIM-EA-TB and with PIM-1 for the conversion of Pr(NO3)3 to Pr6O11.
Co-reporter:Meng Li;Dr. Su-Ying Xu;Dr. Andrew J. Gross;Jules L. Hammond;Dr. Pedro Estrela;James Weber;Dr. Karel Lacina; Tony D. James; Frank Marken
ChemElectroChem 2015 Volume 2( Issue 6) pp:867-871
Publication Date(Web):
DOI:10.1002/celc.201500016
Abstract
The interaction of ferrocene-boronic acid with fructose is investigated in aqueous 0.1 M phosphate buffer at pH 7, 8 and 9. Two voltammetric methods, based on 1) a dual-plate generator–collector micro-trench electrode (steady state) and 2) a square-wave voltammetry (transient) method, are applied and compared in terms of mechanistic resolution. A combination of experimental data is employed to obtain new insights into the binding rates and the cumulative binding constants for both the reduced ferrocene-boronic acid (pH dependent and weakly binding) and for the oxidised ferrocene-boronic acid (pH independent and strongly binding).
Co-reporter:Adam Kolodziej, Sunyhik D. Ahn, Mariolino Carta, Richard Malpass-Evans, Neil B. McKeown, Robert S.L. Chapman, Steven D. Bull, Frank Marken
Electrochimica Acta 2015 160() pp: 195-201
Publication Date(Web):
DOI:10.1016/j.electacta.2015.01.106
Co-reporter:Lucia H. Mascaro;Adam Pockett;John M. Mitchels
Journal of Solid State Electrochemistry 2015 Volume 19( Issue 1) pp:31-35
Publication Date(Web):2015 January
DOI:10.1007/s10008-014-2495-y
A one-step method of preparing photoelectrochemically active nanostructured BiVO4 films is reported based on thermolysis (500 °C in air) of a polyethylene glycol (PEG300) “paint-on” precursor solution containing Bi3+ (as nitrate) and VO43− (as the metavanadate ammonium salt). Films are formed directly on tin-doped indium oxide (ITO) substrates and characterised by electron microscopy (scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS)), X-ray diffraction, Raman spectroscopy, and photoelectrochemistry. The nanocrystalline film exhibited typically up to 52 % incident photon to current efficiency (IPCE) at 1.0 V vs. saturated calomel electrode (SCE) in aqueous 0.5 M Na2SO4 with oxalate, strongly enhancing photocurrents.
Co-reporter:Dawid Kaluza;Martin Jönsson-Niedziólka
Journal of Solid State Electrochemistry 2015 Volume 19( Issue 5) pp:1277-1283
Publication Date(Web):2015 May
DOI:10.1007/s10008-014-2722-6
Voltammetry and chronoamperometry-based reactivity screening of water-insoluble microparticles is demonstrated for a mediated reaction. The solid-solid 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated electrolytic conversion of diphenylcarbinol to benzophenone is investigated by employing TEMPO and its derivatives (4-carboxy-TEMPO, 4-methoxy-TEMPO, 4-amino-TEMPO, 4-acetamido-TEMPO) in alkaline carbonate buffer at pH 11. Chronoamperometry experiments reveal (at least) two reaction stages with (i) a high onset current (ca. 30 s) with nucleation and growth of product and (ii) a first-order transport-controlled reaction (ca. 5 min) leading to bulk conversion. It is shown that the type of TEMPO derivative and the pH (but not the TEMPO concentration) affect the reaction rate. The 4-acetamido-TEMPO derivative is the most effective mediator.
Co-reporter:Jules L. Hammond, Andrew J. Gross, Pedro Estrela, Jesus Iniesta, Stephen J. Green, C. Peter Winlove, Paul G. Winyard, Nigel Benjamin, and Frank Marken
Analytical Chemistry 2014 Volume 86(Issue 14) pp:6748
Publication Date(Web):June 23, 2014
DOI:10.1021/ac501321e
Thiols and disulfides are ubiquitous and important analytical targets. However, their redox properties, in particular on gold sensor electrodes, are complex and obscured by strong adsorption. Here, a gold–gold dual-plate microtrench dual-electrode sensor with feedback signal amplification is demonstrated to give well-defined (but kinetically limited) steady-state voltammetric current responses for the cysteine-cystine redox cycle in nondegassed aqueous buffer media at pH 7 down to micromolar concentration levels.
Co-reporter:Yuanyang Rong, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown, Gary A. Attard, Frank Marken
Electrochemistry Communications 2014 Volume 46() pp:26-29
Publication Date(Web):September 2014
DOI:10.1016/j.elecom.2014.06.005
A water-insoluble Polymer with Intrinsic Microporosity (or PIM, here for the particular case of the Tröger Base system PIM-EA-TB, BET area ca. 103 m2 g− 1) is demonstrated to act as a rigid host environment for highly water-insoluble molecular catalysts, here tetraphenylporphyrinato-iron (FeTPP), surrounded by aqueous solution-filled micropores. A PIM-EA-TB film containing catalyst is deposited onto the electrode and immersed for voltammetry (i) with 4-(3-phenyl-propyl)-pyridine to give an organogel, or (ii) bare directly into aqueous solution. The porous host allows processes to be optimised as a function of solution phase, composition, and catalyst loading. Effective electron transfer as well as effective electrocatalysis is reported for aqueous oxygen and peroxide reduction. Given the use of completely water-insoluble catalyst systems, the methodology offers potential for application with a wide range of hitherto unexplored molecular electrocatalysts and catalyst combinations in aqueous media.Rigid microporous PIM-EA-TB poly-amine holds a molecular FeTPP catalyst in an aqueous electrolyte environment.
Co-reporter:Christopher E. Hotchen, Gary A. Attard, Steven D. Bull, Frank Marken
Electrochimica Acta 2014 Volume 137() pp:484-488
Publication Date(Web):10 August 2014
DOI:10.1016/j.electacta.2014.06.001
•A convenient one-step synthesis of nano-platinum is suggested based on PEG solution thermolysis.•Stable nano-fibrous platinum on ITO is formed and characterised in 0.5 M H2SO4.•Effective methanol oxidation electrocatalysis is demonstrated with catalyst activity comparable to other nano-platinum materials.The electrochemically reducing nature of the poly-(ethylene-glycol) solvent (PEG200) is exploited in a one-step electroless deposition of nano-fibrous platinum electrocatalyst. K2PtCl6 is dissolved in poly-(ethylene-glycol) and applied to a substrate (here tin-doped indium oxide, ITO). The deposition process is conducted by rapid heating in a furnace in air. Upon ramping the temperature up (with 10 °C per minute to 500 °C) a complex sequence of nucleation and growth reactions leads via (i) formation of nano-fibrous platinum and (ii) complete removal of all organic components directly to an electrochemically highly active platinum deposit. When characterised in aqueous 0.5 M H2SO4, typical poly-crystal Pt surface oxidation and hydrogen adsorption features were observed. Electrocatalysis is demonstrated for methanol oxidation, for which the rate of catalysis per electrochemically active area is increased compared to that for a conventional polycrystalline platinum macro-disc
Co-reporter:Fengjie Xia, Mu Pan, Shichun Mu, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown, Gary A. Attard, Ashley Brew, David J. Morgan, Frank Marken
Electrochimica Acta 2014 Volume 128() pp:3-9
Publication Date(Web):10 May 2014
DOI:10.1016/j.electacta.2013.08.169
•Intrinsically microporous polymers are introduced as electrode modifier.•Binding of indigo carmine and PdCl4− are demonstrated.•Formation of Pd metal and catalysis are demonstrated.Two polymers (i) the polymer of intrinsic microporosity (or PIM) ethanoanthracene TB-PIM (P1, PIM-EA-TB, MW 70 kDa, BET surface area 1027 m2 g−1) and (ii) the structurally less rigid polymer based on dimethyldiphenylmethane units (P2, BDMPM-TB, MW 100 kDa, BET surface area 47 m2g−1) are compared to highlight the benefits of the newly emerging PIM membrane materials in electrocatalysis and nanostructure formation. Binding sites and binding ability/capacity in aqueous environments are compared in films deposited onto glassy carbon electrodes for (i) indigo carmine dianion immobilisation (weakly binding from water–ethanol) and (ii) PdCl42− immobilisation (strongly binding from acidic media). Nano-lamella growth for Pd metal during electro-reduction of PdCl42− is observed. Electrocatalytic oxidation of formic acid (at pH 6) is investigated for P1 and P2 as a function of film thickness. The more rigid high BET surface area PIM material P1 exhibits “open-pore” characteristics with much more promising electrocatalytic activity at Pd lamella within polymer pores.
Co-reporter:Mohammad A. Hasnat, Andrew J. Gross, Sara E. C. Dale, Edward O. Barnes, Richard G. Compton and Frank Marken
Analyst 2014 vol. 139(Issue 3) pp:569-575
Publication Date(Web):09 Dec 2013
DOI:10.1039/C3AN01826A
Generator–collector electrode systems are based on two independent working electrodes with overlapping diffusion fields where chemically reversible redox processes (oxidation and reduction) are coupled to give amplified current signals. A generator–collector trench electrode system prepared from two tin-doped indium oxide (ITO) electrodes placed vis-à-vis with a 22 μm inter-electrode gap is employed here as a sensor in aqueous media. The reversible 2-electron anthraquinone-2-sulfonate redox system is demonstrated to give well-defined collector responses even in the presence of oxygen due to the irreversible nature of the oxygen reduction. For the oxidation of dopamine on ITO, novel “Piranha-activation” effects are observed and chemically reversible generator–collector feedback conditions are achieved at pH 7, by selecting a more negative collector potential, again eliminating possible oxygen interference. Finally, dopamine oxidation in the presence of ascorbate is demonstrated with the irreversible oxidation of ascorbate at the “mouth” of the trench electrode and chemically reversible oxidation of dopamine in the trench “interior”. This spatial separation of chemically reversible and irreversible processes within and outside the trench is discussed as a potential in situ microscale sensing and separation tool.
Co-reporter:Grace E. M. Lewis, Sara E. C. Dale, Barbara Kasprzyk-Hordern, Anneke T. Lubben, Edward O. Barnes, Richard G. Compton and Frank Marken
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 35) pp:18966-18973
Publication Date(Web):05 Aug 2014
DOI:10.1039/C4CP02943G
Two types of generator–collector electrode systems, (i) a gold–gold interdigitated microband array and (ii) a gold–gold dual-plate microtrench, are compared for nitrobenzene electroanalysis in aerated aqueous 0.1 M NaOH. The complexity of the nitrobenzene reduction in conjunction with the presence of ambient levels of oxygen in the analysis solution provide a challenging problem in which feedback-amplified generator–collector steady state currents provide the analytical signal. In contrast to the more openly accessible geometry of the interdigitated array electrode, where the voltammetric response for nitrobenzene is less well-defined and signals drift, the voltammetric response for the cavity-like microtrench electrode is stable and readily detectable at 1 μM level. Both types of electrode show oxygen-enhanced low concentration collector current responses due to additional feedback via reaction intermediates. The observations are rationalised in terms of a “cavity transport coefficient” which is beneficial in the dual-plate microtrench, where oxygen interference effects are suppressed and the analytical signal is amplified and stabilised.
Co-reporter:Hakikulla H. Shah, Rayya A. Al-Balushi, Mohammed K. Al-Suti, Muhammad S. Khan, Frank Marken, Anna L. Sudlow, Gabriele Kociok-Köhn, Christopher H. Woodall, Paul R. Raithby and Kieran C. Molloy
Dalton Transactions 2014 vol. 43(Issue 25) pp:9497-9507
Publication Date(Web):30 Apr 2014
DOI:10.1039/C3DT52914B
Three new neutral di-ferrocenyl-ethynylpyridinyl copper complexes, [L2(CuCl)2(PPh3)2] (2), [L2(CuBr)2(PPh3)2] (3), and [L2(CuI)2(PPh3)2] (4) were synthesized from the ferrocenyl-ethynylpyridine ligand (L) (1), the appropriate copper halide CuX (with X = Cl−, Br−, I−) and triphenylphosphine. These neutral complexes were fully characterized by spectroscopic methods and by single crystal X-ray crystallography. Cyclic voltammetry in dichloroethane revealed chemically reversible ferrocenyl oxidation signals followed by characteristic “stripping reduction peaks” showing evidence for oxidation-product electro-crystallization. Scanning electron microscopy confirmed spontaneous formation of crystalline oxidation products with three distinct morphologies for X = Cl−, Br−, I−. Energy dispersive X-ray elemental analysis data show Fe:P ratios of 1:2.0, 1:2.1 and 1:2.1 for electro-crystallization products of complexes 2, 3, and 4, respectively, indicating the presence of two [PF6]− anions in the vicinity of the dioxidized complexes, and suggesting product formulae [2]2+[PF6]−2, [3]2+[PF6]−2 and [4]2+[PF6]−2.
Co-reporter:Sunyhik D. Ahn, Paul E. Frith, Adrian C. Fisher, Alan M. Bond, Frank Marken
Journal of Electroanalytical Chemistry 2014 Volumes 722–723() pp:78-82
Publication Date(Web):1 May 2014
DOI:10.1016/j.jelechem.2014.02.018
•Voltammetric methodology based on rocking disc is introduced.•With single-disc experiments uniform mass transport and hydrodynamic current enhancements are observed.•A predictive equation for the mass transport controlled limiting current is proposed.•For dual-semi-disc electrode modulation effects are demonstrated in generator–collector mode.A hydrodynamic electrochemical method based on a rocking disc system with 90° amplitude and a frequency range of 0.83–16.7 Hz is reported. The system is versatile and similar in characteristics to the familiar rotating disc voltammetry method (e.g. providing a uniform diffusion layer but without sliding contacts), except for the more complex flow pattern imposed by the oscillatory movement. A Levich-type mass transport limited current response linear with the square root of rocking frequency is obtained. Additionally, a dual-semi-disc generator–collector system is demonstrated and characterised for the one-electron quasi-reversible Fe(CN)63−/4− redox couple. Generator–collector current efficiencies vary from 3% to 6% and characteristic transient concentration modulation effects are observed suitable for Fourier transform analysis. Rotational asymmetry in the flow pattern is detected.
Co-reporter:Muhammad Saleem Abid, Safeer Ahmed, Ejaz Ahmed, Sabir Hussain, Josef Kiermaier, Frank-Michael Matysik, Gabriele Kociok-Köhn, John M. Mitchels, Frank Marken
Journal of Photochemistry and Photobiology A: Chemistry 2014 Volume 276() pp:65-70
Publication Date(Web):15 February 2014
DOI:10.1016/j.jphotochem.2013.11.012
•“Roll-on” nano-CIGSe films on ITO substrates show photo-transients consistent with photo-hydrogenation processes.•Photo-generator-collector experiments with a Pt microelectrode provide information about product formation during photo-transient measurements.•Fumaric acid is an ideal irreversible quencher giving succinic acid without solution phase recombination pathways.Commercial nano-CIGSe powder (nominal CuIn0.7Ga0.3Se2, average particle size < 100 nm) is applied to tin-doped indium oxide (ITO) electrodes in a simple “roll-on” process from n-hexanol to give thin photo-electrochemically active films. Films “as deposited” and “annealed” in selenium atmosphere at 350 °C are compared with the latter performing substantially better. Perhaps surprisingly, photo-transient voltammetry suggests only insignificant effects when changing aqueous electrolyte from Eu(NO3)3 to KNO3 or when changing electrolyte concentration, most likely due to a dominating electron-quenching process caused by dissolved dioxygen. However, when under inert argon atmosphere, generator-collector photo-transient voltammetry with a 50 μm Pt collector microelectrode placed close to the CIGSe film surface proves photo-induced H2 formation in 0.1 M KNO3 or in 0.1 M HNO3 (and it confirms electron-quenching by O2 in the presence of air). In aqueous 0.1 M HNO3 effective electron-quenching (photo-hydrogenation) with fumaric acid is observed with current yields of ca. 60% for fumarate to succinate conversion. Detrimental loss of photoelectrochemical activity (probably via photo-corrosion) with time occurs.
Co-reporter:Meng Li;Grace E. M. Lewis; Tony D. James; Yi-Tao Long;Dr. Barbara Kasprzyk-Hordern;Dr. John M. Mitchels; Frank Marken
ChemElectroChem 2014 Volume 1( Issue 10) pp:
Publication Date(Web):
DOI:10.1002/celc.201402304
Co-reporter:Meng Li;Grace E. M. Lewis; Tony D. James; Yi-Tao Long;Dr. Barbara Kasprzyk-Hordern;Dr. John M. Mitchels; Frank Marken
ChemElectroChem 2014 Volume 1( Issue 10) pp:
Publication Date(Web):
DOI:10.1002/celc.201402303
Abstract
The front cover artwork is provided by the groups of Prof. Tony D. James, Prof. Frank Marken, Dr. Barbara Kasprzyk-Hordern and Dr. John M. Mitchels (University of Bath, UK) as well as Prof. Yi-Tao Long (East China University of Science and Technology, P.R. China). The image shows a coupled electron transfer–phosphate transfer, which is driven between two electrodes to provide a phosphate-sensitive current response. Read the full text of the article at 10.1002/celc.201402181.
Co-reporter:Meng Li;Grace E. M. Lewis; Tony D. James; Yi-Tao Long;Dr. Barbara Kasprzyk-Hordern;Dr. John M. Mitchels; Frank Marken
ChemElectroChem 2014 Volume 1( Issue 10) pp:1640-1646
Publication Date(Web):
DOI:10.1002/celc.201402181
Abstract
Anion transfer for highly hydrophilic phosphate and hydroxide anions into a water-immiscible organic phase, 3-(4-phenylpropyl)-pyridine (or PPP), is driven with the tetraphenylporphyrinato manganese(II/III) (or TPPMn) redox system and facilitated with a hydrophobic oil-based boronic acid ((3-(1,3-dioxo-6-propylamino-1H-benzo[de]isoquinolin-2(3 H)-yl)phenyl)boronic acid). Both 1) experiments with random arrays of microdroplets (transient) and 2) experiments in a gold–gold dual-plate oil-filled microtrench generator-collector configuration (approaching steady state), show that phosphate transfer is facilitated by boronic acid. At pH 7.5 a switch in mechanism occurs from phosphate transfer to hydroxide transfer. Accelerated charge transport is observed lateral to the oil|water interface. Improved boronic acid facilitators and nanotrench electrodes are discussed in terms of future feasibility for phosphate sensing applications.
Co-reporter:Thomas R. Bartlett;Dr. Safeer Ahmed;Dr. Floriana Tuna; David Collison; Gary J. Blanchard; Frank Marken
ChemElectroChem 2014 Volume 1( Issue 2) pp:400-406
Publication Date(Web):
DOI:10.1002/celc.201300090
Abstract
Photoelectrochemical processes are investigated for chromoionophore I (ETH 5294) dissolved in 4-(3-phenylpropyl)pyridine (PPP) and deposited in the form of a microdroplet array (through evaporation deposition from a solution in acetonitrile) onto a 5 mm diameter basal plane pyrolytic graphite (BPPG) electrode. Stable biphasic dark voltammetric responses (two electron/two proton) are observed in phosphate buffer solution (from pH 2 to 12) with a switch in reactivity at pH 5 due to a biphasic protonation step. The photoelectrochemical activity at pH 2 is investigated further by phototransient amperometry. The protonated chromoionophore I is shown to be the photoactive component (supported by EPR data) and “hole transfer” at the liquid|liquid interface to aqueous oxalate is demonstrated. This interfacial hole-transfer process can be “switched off” by hydrophobic anions (PF6−), which compete for cationic liquid|liquid PPP surface binding sites. Implications for light harvesting and liquid semiconductor properties are discussed.
Co-reporter:Dr. Elena Madrid;Yuanyang Rong;Dr. Mariolino Carta; Neil B. McKeown;Richard Malpass-Evans; Gary A. Attard;Tomos J. Clarke; Stuart H. Taylor; Yi-Tao Long; Frank Marken
Angewandte Chemie International Edition 2014 Volume 53( Issue 40) pp:10751-10754
Publication Date(Web):
DOI:10.1002/anie.201405755
Abstract
A highly rigid amine-based polymer of intrinsic microporosity (PIM), prepared by a polymerization reaction involving the formation of Tröger’s base, is demonstrated to act as an ionic diode with electrolyte-dependent bistable switchable states.
Co-reporter:Katherine Lawrence, Fengjie Xia, Rory L. Arrowsmith, Haobo Ge, Geoffrey W. Nelson, John S. Foord, Mónica Felipe-Sotelo, Nick D. M. Evans, John M. Mitchels, Stephen E. Flower, Stanley W. Botchway, Daniel Wolverson, Gazi N. Aliev, Tony D. James, Sofia I. Pascu, and Frank Marken
Langmuir 2014 Volume 30(Issue 39) pp:11746-11752
Publication Date(Web):2017-2-22
DOI:10.1021/la404866s
A novel two-photon-fluorescent N,O-heteroatom-rich carbon nanomaterial has been synthesized and characterized. The new carbon nanoparticles were produced by hydrothermal conversion from a one-photon-fluorescent poly(4-vinylpyridine) precursor (P4VP). The carbonized particles (cP4VP dots) with nonuniform particle diameter (ranging from sub-6 to 20 nm with some aggregates up to 200 nm) exhibit strong fluorescence properties in different solvents and have also been investigated for applications in cell culture media. The cP4VP dots retain their intrinsic fluorescence in a cellular environment and exhibit an average excited-state lifetime of 2.0 ± 0.9 ns in the cell. The cP4VP dots enter HeLa cells and do not cause significant damage to outer cell membranes. They provide one-photon or two-photon fluorescent synthetic scaffolds for imaging applications and/or drug delivery.
Co-reporter:Dr. Katherine Lawrence;Charlotte L. Baker; Tony D. James;Dr. Steven D. Bull;Dr. Ruth Lawrence;Dr. John M. Mitchels; Marcin Opallo; Omotayo A. Arotiba; Kenneth I. Ozoemena; Frank Marken
Chemistry – An Asian Journal 2014 Volume 9( Issue 5) pp:1226-1241
Publication Date(Web):
DOI:10.1002/asia.201301657
Abstract
Functionalized carbon nanoparticles (or blacks) have promise as novel active high-surface-area electrode materials, as conduits for electrons to enzymes or connections through lipid films, or as nano-building blocks in electroanalysis. With previous applications of bare nanoblacks and composites mainly in electrochemical charge storage and as substrates in fuel cell devices, the full range of benefits of bare and functionalized carbon nanoparticles in assemblies and composite (bio)electrodes is still emerging. Carbon nanoparticles are readily surface-modified, functionalized, embedded, or assembled into nanostructures, employed in bioelectrochemical systems, and incorporated into novel electrochemical sensing devices. This focus review summarizes aspects of a rapidly growing field and some of the recent developments in carbon nanoparticle functionalization with potential applications in (bio)electrochemical, photoelectrochemical, and electroanalytical processes.
Co-reporter:Frank Marken
Journal of Solid State Electrochemistry 2014 Volume 18( Issue 12) pp:3215
Publication Date(Web):2014 December
DOI:10.1007/s10008-014-2672-z
Co-reporter:Katherine Lawrence, Geoffrey W. Nelson, John S. Foord, Mónica Felipe-Sotelo, Nick D. M. Evans, John M. Mitchels, Tony D. James, Fengjie Xia and Frank Marken
Journal of Materials Chemistry A 2013 vol. 1(Issue 14) pp:4559-4564
Publication Date(Web):12 Feb 2013
DOI:10.1039/C3TA10198C
Negatively charged carbon nanoparticles (surface-phenylsulfonated) are “wrapped” in a poly(4-vinylpyridine) cationomer and hydrothermally converted into a pH-responsive core–shell nano-composite. With a “thin shell” this nano-material (ca. 20–40 nm diameter) is water-insoluble but readily dispersed into ethanol and deposited onto electrodes. Zeta-potential measurements suggest a point of zero charge (PZC) at ca. pH 4.5 with negative functional groups dominating in the more alkaline range and positive functional groups dominating in the acidic range. XPS data suggest carboxylate and pyridinium-like functional groups. This is further confirmed in voltammetric measurements for adsorbed cations (methylene blue) and adsorbed anions (indigo carmine). The specific capacitance reaches a maximum of 13 F g−1 at the PZC explained here tentatively by a “shell charging” effect within the nanoparticle shell.
Co-reporter:Hakikulla H. Shah ; Rayya A. Al-Balushi ; Mohammed K. Al-Suti ; Muhammad S. Khan ; Christopher H. Woodall ; Anna L. Sudlow ; Paul R. Raithby ; Gabriele Kociok-Köhn ; Kieran C. Molloy
Inorganic Chemistry 2013 Volume 52(Issue 20) pp:12012-12022
Publication Date(Web):October 9, 2013
DOI:10.1021/ic401803p
Three new tetra-ferrocenylethynylpyridinyl copper complexes, L4(CuI)4 (3), L4(CuBr)2 (4), and L4(CuCl)2 (5) have been prepared from the reaction of ferrocenylethynylpyridine (L)(2) with copper halides CuX (with X = I–, Br–, Cl–).The ligand 2 and the complexes 3–5 have been fully characterized by spectroscopic methods. The structures of 2–4 have been confirmed by single-crystal X-ray crystallography. 2 forms a dimer in the crystalline-state through C–H··N hydrogen bonds. 4 and 5 are dimers and 3 a tetramer, in all cases linked through Cu–X··Cu bridging interactions. Cyclic voltammetry in dichloroethane showed chemically reversible multiferrocenyl oxidation signals with evidence for product electro-crystallization. The oxidation products were isolated by electrodeposition onto a Pt disc electrode and investigated by scanning electron microscopy which confirmed the spontaneous formation of crystalline oxidation products with distinctive morphologies. Energy dispersive X-ray elemental analysis shows the presence of hexafluorophosphate (counterion) with the P:Fe ratio of 1:1, 0.5:1, and 1:1 for the electrocrystallized products 3, 4, and 5, respectively, suggesting the formulas [3]4+(PF6–)4, [4]2+(PF6–)2, and [5]4+(PF6–)4 for the electro-crystallized products.
Co-reporter:Hakikulla H. Shah, Rayya A. Al-Balushi, Mohammed K. Al-Suti, Muhammad S. Khan, Christopher H. Woodall, Kieran C. Molloy, Paul R. Raithby, Thomas P. Robinson, Sara E. C. Dale, and Frank Marken
Inorganic Chemistry 2013 Volume 52(Issue 9) pp:4898-4908
Publication Date(Web):April 17, 2013
DOI:10.1021/ic3024887
A new series of bis(ferrocenylethynyl) complexes, 3–7, and a mono(ferrocenylethynyl) complex, 8, have been synthesized incorporating conjugated heterocyclic spacer groups, with the ethynyl group facilitating an effective long-range intramolecular interaction. The complexes were characterized by NMR, IR, and UV–vis spectroscopy as well as X-ray crystallography. The redox properties of these complexes were investigated using cyclic voltammetry and spectroelectrochemistry. Although there is a large separation of ∼14 Å between the two redox centers, ΔE1/2 values in this series of complexes ranged from 50 to 110 mV. The appearance of intervalance charge-transfer bands in the UV–vis–near-IR region for the monocationic complexes further confirmed effective intramolecular electronic communication. Computational studies are presented that show the degree of delocalization across the Fc–C≡C–C≡C–Fc (Fc = C5H5FeC5H4) highest occupied molecular orbital.
Co-reporter:David O. Miles, Dongmei Jiang, Andrew D. Burrows, Jonathan E. Halls, Frank Marken
Electrochemistry Communications 2013 Volume 27() pp:9-13
Publication Date(Web):February 2013
DOI:10.1016/j.elecom.2012.10.039
[Co(bdc)(DMF)] (Co-MOF-71 bdc = 1,4-benzenedicarboxylate DMF = N,N-dimethylformamide) has been previously reported to be a suitable supercapacitor material in spite of being electrically insulating and redox silent. The phenomenon can be explained due to a facile surface hydrolysis of Co-MOF-71 microcrystals in alkaline aqueous media via “conformal” transformation into a porous cobalt hydroxide. The resulting volume decrease during transformation of the MOF precursor (without a significant change in shape) is beneficial and ensures formation of a more active porous cobalt hydroxide product with high pseudo capacitance and electrocatalytic activity. The high “molecular scale porosity” of the MOF precursor offers benefits due to conformal transformation into products with high “nano-scale porosity”.Co-MOF-71 is converted to a porous electrochemically active cobalt hydroxide without changing the crystal appearance.Highlights► Co-MOF-71 is not a supercapacitor material due to its being insulating and redox silent. ► Conformal transformation of Co-MOF-71 to cobalt hydroxide is demonstrated. ► Electrochemical activity and electrocatalysis with the resulting cobalt hydroxide are observed.
Co-reporter:Jonathan E. Halls, Sunyhik D. Ahn, Dongmei Jiang, Luke L. Keenan, Andrew D. Burrows, Frank Marken
Journal of Electroanalytical Chemistry 2013 Volume 689() pp:168-175
Publication Date(Web):15 January 2013
DOI:10.1016/j.jelechem.2012.11.016
Small redox active molecules such as alizarin red S are readily adsorbed and bound into redox-inactive metal–organic framework hosts such as UMCM-1. Redox activity of the bound guest molecule is of interest for electrochemical conversions and electrocatalysis within pores. For the reduction of alizarin red S charge compensating proton uptake into pores is expected. However, it is shown here that alizarin red S redox processes in UMCM-1 immersed in aqueous electrolyte, ethanolic electrolyte, and in acetonitrile electrolyte media are dominated instead by surface processes and the potential driven reductive release of leuco-alizarin red S into the surrounding solution. Self-mediation via released alizarin red S occurs, whereas in acetonitrile a cobaltocene redox mediator is employed for the potential driven redox release of alizarin red S from the UMCM-1 host lattice.Graphical abstractHighlights► Redox activity of alizarin red S in metal-organic framework pores. ► Host degradation with associated expulsion of alizarin red S. ► Buffer effects on “pore redox processes”.
Co-reporter:Katherine Lawrence, Tomoki Nishimura, Peter Haffenden, John M. Mitchels, Kazuo Sakurai, John S. Fossey, Steven D. Bull, Tony D. James and Frank Marken
New Journal of Chemistry 2013 vol. 37(Issue 7) pp:1883-1888
Publication Date(Web):13 Mar 2013
DOI:10.1039/C3NJ00017F
Pyrene-appended boronic acids self-assemble onto graphitic surfaces and are shown here to self-assemble also around negatively charged carbon nanoparticles of ca. 8–18 nm diameter (Emperor 2000, Cabot Corp.). The resulting boronic acid modified nanoparticles are water-insoluble and readily deposited onto graphite electrodes, for example for electroanalytical applications. Boronic acid binding to the ortho-quinol caffeic acid is demonstrated and the mechanism based on ligand fluxionality is further investigated. Electrodes based on nanoparticle aggregates are sensitive to caffeic acid with Langmuirian binding constants estimated as 106 mol−1 dm3. The molecular structure of the pyrene-appended boronic acid affects the ability to bind caffeic acid as well as, more importantly, the fluxional transformation of the strongly bound (high oxidation potential) into the weakly bound (low oxidation potential) form. The resulting design principles for improved boronic acid receptors are discussed.
Co-reporter:Jonathan E. Halls, Sunyhik D. Ahn, Dongmei Jiang, Luke L. Keenan, Andrew D. Burrows, Frank Marken
Journal of Electroanalytical Chemistry 2013 710() pp: 2-9
Publication Date(Web):
DOI:10.1016/j.jelechem.2013.08.019
Co-reporter:Fengjie Xia, Mu Pan, Shichun Mu, Matthew D. Jones, Gabriele Kociok-Köhn, Shik Chi Tsang, Frank Marken
Electrochimica Acta 2013 110() pp: 663-669
Publication Date(Web):
DOI:10.1016/j.electacta.2013.01.014
Co-reporter:Safeer Ahmed, Ibrahim A.I. Hassan, Hayley Roy, and Frank Marken
The Journal of Physical Chemistry C 2013 Volume 117(Issue 14) pp:7005-7012
Publication Date(Web):March 13, 2013
DOI:10.1021/jp400962t
Commercial nano-WO3 (10–100 nm average particle size) is dispersed in n-hexanol and applied to tin-doped indium oxide (ITO) substrates in a simple “roll-on” process. As-deposited and annealed (500 °C) films are compared and shown to be photoactive for the formation of hypochlorite in neutral aqueous NaCl (e.g., seawater). Annealing at 500 °C in air improves photocurrents most likely due to improved interparticle charge transport (e.g., removal of hydration or n-hexanol surface layers). In phototransients (interpreted here in the limiting case of a weakly associated nanoparticle aggregate as opposed to the limiting case of a single-crystal semiconductor), evidence for the presence of both holes (as O(-I), fast moving) and electrons (as W(V), slow moving) is obtained in particular in as-deposited films. Bipotentiostat experiments reveal the presence of chlorine as a reaction intermediate close to the photoanode when immersed in 3 M NaCl. A “molecular conduit” effect with adsorbed Co(II/III) sepulchrate is observed to significantly enhance the photocurrents at as-deposited electrodes (but not at annealed electrodes).
Co-reporter:Sara Shariki, Owen T. L. Cox, David A. Tickell, Marta P. Pereira Morais, Jean M. H. van den Elsen, Tony D. James, Sara E. C. Dale, Simon Bending and Frank Marken
Journal of Materials Chemistry A 2012 vol. 22(Issue 36) pp:18999-19006
Publication Date(Web):09 Aug 2012
DOI:10.1039/C2JM31089A
An electrolyte–stable polymer composite sensor film based on 2.4% boronic acid substituted polyacrylamide (poly[acrylamide-co-3-(methacrylamido)-phenylboronic acid] or P(A-MPBA)) and polydiallyl-dimethyl-ammonium chloride (or PDDACl) is obtained in a coil-by-coil deposition procedure. Based on AFM evidence, the resulting coil aggregate deposits show ca. 5–10 nm diameter and the average film thickness increases with ca. 1 nm per deposition cycle consistent approximately with the solution coil diameter. In electrochemical experiments alizarin red S is employed as a boronic acid indicating redox probe with a Langmuirian binding constant of ca. K = 4 × 106 M−1 and two distinct redox responses tentatively assigned here to two isomeric forms of the complex in oxidized and reduced state. Inter- and intra-coil electron transfer and electron hopping occurs during alizarin red S reduction and back-oxidation. Both smooth and mesoporous ITO substrates are employed.
Co-reporter:Richard A. Webster, Fengjie Xia, Mu Pan, Shichun Mu, Sara E.C. Dale, Shik Chi Tsang, Frank W. Hammett, Chris R. Bowen, Frank Marken
Electrochimica Acta 2012 Volume 62() pp:97-102
Publication Date(Web):15 February 2012
DOI:10.1016/j.electacta.2011.11.095
Methods are developed for using a carbon nanofiber membrane electrode assembly (cnMEA) for three-electrode electrochemical measurements in unsupported liquid or gaseous media. The carbon nanofiber membrane (ca. 20 μm thickness) is hot-pressed from both sides onto a Nafion™ ion conductor film (ca. 50 μm thickness). The resulting cnMEA is employed as both a counter electrode and working electrode (with an external SCE reference electrode) for the liquid phase oxidation of hydroquinone in (i) 0.5 M, (ii) 0.05 M, and (iii) 0.005 M aqueous NaHCO3, and (iv) in pure demineralised water. Well-defined voltammetric responses are obtained and pH effects are investigated. A gas–salt (di-ammonium sulphate) contact between the cnMEA working electrode and SCE reference electrode is then employed in a gas flow system to obtain voltammetric responses in contact with a humidified gas (N2, 80% relative humidity). Reversible redox cycling of entrapped hydroquinone is demonstrated and humidity and pH effects are observed. Ammonia gas effects on Faradaic currents are explored. Voltammetric measurements provide direct insight into the MEA processes and conditions.
Co-reporter:Charles Y. Cummings, Guillaume Zoppi, Ian Forbes, Diego Colombara, Laurence M. Peter, Frank Marken
Electrochimica Acta 2012 Volume 79() pp:141-147
Publication Date(Web):30 September 2012
DOI:10.1016/j.electacta.2012.06.095
The co-electrodeposition of copper and indium from a pH 3 tartrate bath onto 4.8 cm × 2.5 cm Mo and MoSe2 substrates is studied and conditions are optimised for CuIn alloy films. Selenisation at ca. 500 °C for 30 min in selenium vapour gives CuInSe2 (or CISe). Mapping using the photo-electrochemical reduction of Eu(NO3)3 is used to asses the relative photoactivity as a function of position and surface treatment.Etching of detrimental CuxSe phases is investigated with 5% and 0.5% (w/w) aqueous KCN. The slower 0.5% (w/w) KCN etch allows better process control, and re-annealing at 500 °C for 30 min followed by further etching significantly improved the photo-activity. However, over the large area local pinhole recombination effects are substantial. An alternative low temperature film optimisation method is proposed based on (i) KCN over-etch, (ii) hypochlorite (5%, w/w) pinhole removal (Mo etch), and (iii) a final KCN etch to give good and more uniform activity.
Co-reporter:Jonathan E. Halls, Alberto Hernán-Gómez, Andrew D. Burrows and Frank Marken
Dalton Transactions 2012 vol. 41(Issue 5) pp:1475-1480
Publication Date(Web):26 Jul 2011
DOI:10.1039/C1DT10734H
Metal–organic framework (MOF) materials based on zinc(II) and aluminium(III) dicarboxylate frameworks with covalently attached ferrocene functional redox groups were synthesised by post-synthetic modification and investigated by voltammetry in aqueous and non-aqueous media. In the voltammetry experiments, ferrocene oxidation occurs in all cases, but chemically reversible and stable ferrocene oxidation without decay of the voltammetric response requires a “mild” dichloroethane solvent environment. The voltammetric response in this case is identified as “surface-confined” with fast surface-hopping of electrons and without affecting the bulk of MOF microcrystals. In aqueous media a more complex pH-dependent multi-stage redox process is observed associated with chemically irreversible bulk oxidation and disintegration of the MOF framework. A characteristic 30 mV per pH unit dependence of redox potentials is observed attributed to a “framework effect”: the hydroxide-driven MOF framework dissolution.
Co-reporter:Fengjie Xia, Mu Pan, Shichun Mu, Yuli Xiong, Karen J. Edler, Ilaria Idini, Matthew D. Jones, Shik Chi Tsang and Frank Marken
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 45) pp:15860-15865
Publication Date(Web):11 Oct 2012
DOI:10.1039/C2CP42659E
Carbon nanoparticles with phenylsulfonate negative surface functionality (Emperor 2000, Cabot Corp.) are coated with positive chitosan followed by hydrothermal carbonization to give highly pH-responsive core–shell nanocarbon composite materials. With optimised core–shell ratio (resulting in an average shell thickness of ca. 4 nm, estimated from SANS data) modified electrodes exhibit highly pH-sensitive resistance, capacitance, and Faradaic electron transfer responses (solution based, covalently bound, or hydrothermally embedded). A shell “double layer exclusion” mechanism is discussed to explain the observed pH switching effects. Based on this mechanism, a broader range of future applications of responsive core–shell nanoparticles are envisaged.
Co-reporter:Richard A. Webster, John D. Watkins, Robert J. Potter and Frank Marken
RSC Advances 2012 vol. 2(Issue 11) pp:4886-4890
Publication Date(Web):22 Mar 2012
DOI:10.1039/C2RA20514A
Electrochemical ammonia oxidation is of interest in waste treatment as well as in electrochemical sensing applications and demonstrated here at a carbon nanofibre (“bucky-paper”) electrode. The electrode is placed at the aqueous electrolyte | gas interface, and current (cyclic voltammetry) as well as ambient differential electrochemical mass spectrometry (DEMS, cyclic voltbarometry) data are recorded as a function of solution composition and pH. The oxidation of oxalate to CO2 is employed as a test and calibration system. Anodic polarization of the carbon nanofibre membrane in inert aqueous electrolyte is shown to result in direct sustained anodic CO2 evolution. In alkaline aqueous media (starting at pH 9) significant levels of nitrogen from ammonia are produced in competition to CO2 formation from carbon nanofibres without the need for additional catalysts. However, for applications with low level ammonia, catalysts will be required to minimize current losses, carbon nanofibre corrosion, and side product formation.
Co-reporter:Liza Rassaei, Markus Herrmann, Sergey N. Gordeev, Frank Marken
Journal of Electroanalytical Chemistry 2012 Volume 686() pp:32-37
Publication Date(Web):15 October 2012
DOI:10.1016/j.jelechem.2012.09.022
Self-assembled films of nanoparticulate TiO2-phytate are prepared on a gold disc–gold disc junction electrode with approximately 6 μm inter-electrode gap. Charge diffusion dominated by inter-particle hopping of electrons is investigated (in contrast to bulk charge transport) for films immersed in 0.1 M phosphate buffer solution at pH 7 using gold–gold generator–collector voltammetry and chronoamperometry techniques. Numerical simulation based on a basic “randomly propagating” finite difference approach is employed to reveal first approximation potential-dependent apparent diffusion coefficients and apparent concentrations for electrons (Dapp increases from 9 × 10−11 to 18 × 10−11 m2 s−1 and capp increases from 0.2 to 37 M when going from −0.8 to −1.2 V vs. SCE). A mechanistic model for inter-particle versus intra-particle electron hopping is developed. Applications of the inter-particle charge hopping process in electrochemical sensing are suggested.Graphical abstractHighlights► Electron hopping in TiO2 nanoparticle film assembly is treated quantitatively. ► Transient chronoamperometry measurements at gold–gold junctions are performed. ► A numerical simulation tool is employed and inter-particle electron hopping dissected. ► Sensor applications are suggested based on inter-particle space effects.
Co-reporter:Norahim Bin Ibrahim, Katherine Lawrence, Tony D. James, Fengjie Xia, Mu Pan, Shichun Mu, John M. Mitchels, Frank Marken
Sensors and Actuators B: Chemical 2012 Volume 161(Issue 1) pp:184-190
Publication Date(Web):3 January 2012
DOI:10.1016/j.snb.2011.10.009
Carbon nanoparticles of ca. 9–18 nm diameter (Emperor 2000™) are surface-modified by covalently linking l-dopa-boc (boc-protected l-3,4-dihydroxyphenylalanine) with a surface coverage of approximately 100 per particle (or 3 × 1013 cm−2). In solution environments these redox-active nanoparticles provide chemically stable and pH-sensitive voltammetirc responses (reversible 2-electron 2-proton oxidation) over a pH range from 2 to 12.When mixed into Dowex 50 Wx4 cation exchanger or Dowex 50 1x2 anion exchanger and placed in contact with a glassy carbon electrode in a flow of humidified gas, the l-dopa-boc-modified carbon nanoparticles provide pH-sensitive surface probes to monitor the surface conditions. In a two-terminal cell it is demonstrated that gas flow measurements are possible with both modified cation and anion exchanger particles in contact to glassy carbon electrodes. The anion exchanger particles allow pH control after pre-conditioning in phosphate buffer. Loading-dependent sensitivity to ammonia gas is investigated and high sensitivity to ammonia is observed for Dowex 50 1x2 anion exchanger pre-equilibrated in phosphate buffer pH 3 and decorated with l-dopa-boc-modified carbon nanoparticles. Responses are observed with sequential injections of 1 cm3 ammmonia into a gas flow-through device.
Co-reporter:John D. Watkins, Christopher E. Hotchen, John M. Mitchels, and Frank Marken
Organometallics 2012 Volume 31(Issue 7) pp:2616-2620
Publication Date(Web):January 10, 2012
DOI:10.1021/om2006112
This report describes exploratory experimental findings for electrochemical processes in nonpolar solvents (hexane, toluene, and dichloroethane). Conventional 3 mm diameter glassy-carbon-disk electrodes are used in contact with a crystalline salt electrolyte (ammonium nitrate) immersed in nonpolar solvents. The insoluble salt is employed as a “surface thin film electrolyte”, with humidity causing electrical connection from the working electrode to the SCE counter-reference electrode. The organic solvents are employed without intentionally added electrolyte. Humidity in the nonpolar solvents is shown to be essential for the processes to work. The oxidation of decamethylferrocene is demonstrated as a test organometallic redox system. The electrochemical reduction of Au(III) in toluene (solubilized with tetraoctylammonium bromide, TOABr) is employed to demonstrate and visualize the reaction zone around salt crystal|working electrode contact points. Gold nanowire bundle formation is observed, presumably due to an ordered interfacial surfactant microphase at salt|electrode contact points. The triple phase boundary nature of these processes is discussed, and future applications are suggested.
Co-reporter:Sara E. C. Dale;Anne Vuorema;Ellen M. Y. Ashmore;Barbara Kasprzyk-Horden;Mika Sillanpää;Guy Denuault
The Chemical Record 2012 Volume 12( Issue 1) pp:143-148
Publication Date(Web):
DOI:10.1002/tcr.201100008
Abstract
The formation of gold-gold junction electrodes for application in electroanalysis is described here based on electro-deposition from a non-cyanide gold plating bath. Converging growth of two hemispherical gold deposits on two adjacent platinum microelectrodes (both 100 µm diameter in glass, ca. 45 µm gap) followed by careful etching in aqueous chloride solution was employed. During growth both gold hemispheres “connect” and during etching “disconnection” is evident in a drop in current. Gold-gold junctions with sub-micron gaps are formed and applied for the electroanalytical detection of sub-micromolar concentrations of hydroquinone in 0.1 M phosphate buffer pH 7 (Erev = 0.04 V vs. SCE) and sub-micromolar concentration of dopamine in 0.1 M phosphate buffer pH 7 (Erev = 0.14 V vs. SCE). The potential future uses in analysis and limitations of gold-gold junction electrodes are discussed. DOI 10.1002/tcr.201100008
Co-reporter:Samuel Eyley, Sara Shariki, Sara E.C. Dale, Simon Bending, Frank Marken, and Wim Thielemans
Langmuir 2012 Volume 28(Issue 16) pp:6514-6519
Publication Date(Web):April 10, 2012
DOI:10.1021/la3001224
Ferrocene-decorated cellulose nanowhiskers were prepared by the grafting of ethynylferrocene onto azide functionalized cotton-derived cellulose nanowhiskers using azide–alkyne cycloaddition. Successful surface modification and retention of the crystalline morphology of the nanocrystals was confirmed by elemental analysis, inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The coverage with ferrocenyl is high (approximately 1.14 × 10–3 mol g–1 or 4.6 × 1013 mol cm–2 corresponding to a specific area of 61 Å2 per ferrocene). Cyclic voltammetry measurements of films formed by deposition of ferrocene-decorated nanowhiskers showed that this small spacing of redox centers along the nanowhisker surface allowed conduction hopping of electrons. The apparent diffusion coefficient for electron (or hole) hopping via Fe(III/II) surface sites is estimated as Dapp = 10–19 m2s–1 via impedance methods, a value significantly less than nonsolvated ferrocene polymers, which would be expected as the 1,2,3-triazole ring forms a rigid linker tethering the ferrocene to the nanowhisker surface. In part, this is believed to be also due to “bottleneck” diffusion of charges across contact points where individual cellulose nanowhiskers contact each other. However, the charge-communication across the nanocrystal surface opens up the potential for use of cellulose nanocrystals as a charge percolation template for the preparation of conducting films via covalent surface modification (with applications similar to those using adsorbed conducting polymers), for use in bioelectrochemical devices to gently transfer and remove electrons without the need for a solution-soluble redox mediator, or for the fabrication of three-dimensional self-assembled conducting networks.
Co-reporter:John D. Watkins, Sunyhik D. Ahn, James E. Taylor, Steven D. Bull, Philip C. Bulman-Page, Frank Marken
Electrochimica Acta 2011 Volume 56(Issue 19) pp:6764-6770
Publication Date(Web):30 July 2011
DOI:10.1016/j.electacta.2011.05.075
An amphiphilic carbon nanofibre membrane electrode (ca. 50 nm fibre diameter, 50–100 μm membrane thickness) is employed as an active working electrode and separator between an aqueous electrolyte phase (with reference and counter electrode) and an immiscible organic acetonitrile phase (containing only the redox active material). Potential control is achieved with a reference and counter electrode located in the aqueous electrolyte phase, but the electrolysis is conducted in the organic acetonitrile phase in the absence of intentionally added supporting electrolyte. For the one-electron oxidation of n-butylferrocene coupled to perchlorate anion transfer from aqueous to organic phase effective electrolysis is demonstrated with an apparent mass transfer coefficient of m = 4 × 10−5 m s−1 and electrolysis of typically 1 mg n-butylferrocene in a 100 μL volume. For the two-electron reduction of tetraethyl-ethylenetetracarboxylate the apparent mass transfer coefficient m = 4 × 10−6 m s−1 is lower due to a less extended triple phase boundary reaction zone in the carbon nanofibre membrane. Nevertheless, effective electrolysis of up to 6 mg tetraethyl-ethylenetetracarboxylate in a 100 μL volume is demonstrated. Deuterated products are formed in the presence of D2O electrolyte media. The triple phase boundary dominated mechanism and future microreactor design improvements are discussed.Graphical abstractHighlights► Amphiphilic carbon nanofiber membrane employed in electro-synthesis. ► Triple phase boundary process within a carbon membrane. ► Electrochemical deuteration in a liquid|liquid micro-reactor system. ► Triple phase boundary reaction zone effects in electro-synthesis.
Co-reporter:Sara E.C. Dale, Charles Y. Cummings, Frank Marken
Electrochemistry Communications 2011 Volume 13(Issue 2) pp:154-157
Publication Date(Web):February 2011
DOI:10.1016/j.elecom.2010.11.038
Electrochemical processes at the electrode | ammonium chloride salt crystal | gas triple phase boundary are detected under the condition that (i) microscopic contact points of salt crystals to a suitable working electrode are formed and (ii) the relative humidity level is adjusted to allow sufficient surface ion conduction in a thin water film within the salt matrix. In this preliminary report, redox systems such as Fe(CN)63−/4−, hydroquinone/benzoquinone, decamethylferrocene+/0, ferrocenedimethanol+/0, and Cu2+/+/0 are investigated embedded into an ammonium chloride salt matrix (by mechanical grinding) and then brought into contact with a macroscopic gold disc electrode. Voltammetric current responses are demonstrated consistent with the processes in the microphase formed at the salt | electrode | gas triple phase boundary. Copper metal electro-deposition is observed. Direct electrochemistry of dioxygen from the gas phase at the salt crystal | gold electrode contact is observed and gas sensing applications are proposed.
Co-reporter:Frank Marken, John D. Watkins and Andrew M. Collins
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 21) pp:10036-10047
Publication Date(Web):13 Apr 2011
DOI:10.1039/C1CP20375D
Ion transfer at liquid|liquid junctions is one of the most fundamental processes in nature. It occurs coupled to simultaneous electron transfer at the line junction (or triple phase boundary) formed by the two liquids in contact to an electrode surface. The triple phase boundary can be assembled from a redox active microdroplet deposit of a water-immiscible liquid on a suitable electrode surface immersed into aqueous electrolyte. Ion transfer voltammetry measurements at this type of electrode allow both thermodynamic and kinetic parameters for coupled ion and electron transfer processes to be obtained. This overview summarises some recent advances in understanding and application of triple phase boundary redox processes at organic liquid|aqueous electrolyte|working electrode junctions. The design of novel types of electrodes is considered based on (i) extended triple phase boundaries, (ii) porous membrane processes, (iii) hydrodynamic effects, and (iv) generator–collector triple phase boundary systems. Novel facilitated ion transfer processes and photo-electrochemical processes at triple phase boundary electrodes are proposed. Potential future applications of triple phase boundary redox systems in electrosynthesis, sensing, and light energy harvesting are indicated.
Co-reporter:Anne Vuorema, Sara Shariki, Mika Sillanpää, Wim Thielemans, Geoffrey W. Nelson, John S. Foord, Sara E. C. Dale, Simon Bending and Frank Marken
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 20) pp:9857-9862
Publication Date(Web):18 Apr 2011
DOI:10.1039/C1CP20532C
In this report we demonstrate a versatile (and potentially low-cost) cellulose nano-whisker-based surface carbonisation method that allows well-defined films of TiO2 nanoparticles surface-modified with carbon to be obtained. In a layer-by-layer electrostatic deposition process based on TiO2 nanoparticles, cellulose nano-whiskers, and poly(diallyl-dimethylammonium) or PDDA are employed to control the ratio of surface carbon to TiO2. Characterisation based on optical, AFM, XRD, and XPS methods is reported. Electrochemical measurements suggest improved access to surface states, dopamine binding at the anatase surface, and surface redox cycling aided by the thin amorphous carbon film in mesoporous TiO2. In future, the amorphous carbon layer method could be applied for surface processes for a wider range of semiconductor or insulator surfaces.
Co-reporter:Fengjie Xia, Sara E. C. Dale, Richard A. Webster, Mu Pan, Shichun Mu, Shik Chi Tsang, John M. Mitchels and Frank Marken
New Journal of Chemistry 2011 vol. 35(Issue 9) pp:1855-1860
Publication Date(Web):04 Jul 2011
DOI:10.1039/C1NJ20421A
The electrochemical oxidation of formic acid to CO2 is facile at nano-palladium catalysts. In conventional electrochemical systems this process is conducted in aqueous phase and the resulting formation of poorly soluble CO2 gas can limit the kinetics. Here, an alternative electrochemical system with the gas phase in closer contact to the palladium nanoparticle catalyst is investigated based on a glassy carbon electrode and a solid salt electrolyte. It is demonstrated that the reaction zone of salt (here (NH4)2SO4 is most effective), palladium nanoparticle catalyst, and gas phase, is where the electrochemical oxidation process occurs. The effects of the type of salt, the partial pressure of formic acid, and the gas flow rate are investigated. Preliminary data for the oxidation of hydrogen gas at the salt|palladium|electrode contact are reported. A significant salt effect on the palladium catalysed reactions is observed and potential future applications of “salt cells” in sensing are discussed.
Co-reporter:Charles Y. Cummings, Jay D. Wadhawan, Takuya Nakabayashi, Masa-aki Haga, Liza Rassaei, Sara E.C. Dale, Simon Bending, Martin Pumera, Stephen C. Parker, Frank Marken
Journal of Electroanalytical Chemistry 2011 Volume 657(1–2) pp:196-201
Publication Date(Web):1 July 2011
DOI:10.1016/j.jelechem.2011.04.010
Focused ion beam (FIB) machining allowed a sub-micron trench to be cut through tin-doped indium oxide (ITO) film on glass to give a generator – collector junction electrode with narrow gap (ca. 600 nm). A layer-by-layer deposited film composed of a dinuclear ruthenium(II)-bis(benzimidazolyl)pyridine-phosphonate (as the negative component) and nanoparticulate TiO2 (ca. 6 nm diameter, as the positive component) was formed and investigated first on simple ITO electrodes and then on ITO junction electrodes. The charge transport within this film due to Ru(II/III) redox switching (electron hopping) was investigated and an apparent diffusion coefficient of ca. Dapp = 2 (± 1) × 10−15 m2 s−1 was observed with minimal contributions from intra-molecular Ru–Ru interactions. The benefits of FIB-cut ITO junctions as a tool in determining charge hopping rates are highlighted.Graphical abstractProcesses in a layer-by-layer deposited film of TiO2 nanoparticles and a di-ruthenium(II) metal complex are investigated in a focused ion beam cut trench in an ITO electrode.Highlights► Focused ion beam cutting of ITO electrode. ► Electron hopping in nanostructured films. ► Apparent diffusion parameters are obtained.
Co-reporter:Andrew M. Collins, Gary J. Blanchard, Jonathan Hawkett, David Collison, and Frank Marken
Langmuir 2011 Volume 27(Issue 10) pp:6471-6477
Publication Date(Web):April 13, 2011
DOI:10.1021/la2010584
Voltammetric responses of pentoxyresorufin in 4-(3-phenylpropyl)-pyridine (PPP) microdroplets immersed in aqueous electrolyte are investigated in the absence and in the presence of light. The reduction of pentoxyresorufin to leuco-pentoxyresorufin in the dark is shown to occur in a two-electron, two-proton process sensitive to the aqueous pH and the PPP|aqueous electrolyte interfacial tension. No significant net photoelectrochemical current responses are observed, although transient responses indicative of distinct electron and hole charge carriers are seen in the presence of pentoxyresorufin. EPR evidence confirms the formation of radical intermediates upon illumination. As a coreactant, duroquinone in the PPP microdroplet phase is investigated and also shown to undergo two-electron, two-proton reduction (to duroquinol) without significant photoelectrochemical activity. When investigated in combination, pentoxyresorufin acts as a photocatalyst for the oxidation of duroquinol to duroquinone. Wavelength-resolved photovoltammetry experiments clearly implicate pentoxyresorufin as the primary photoexcited intermediate. The photoelectrochemical mechanism is explained on the basis of the presence of a long-lived (possibly charge-separated) photoexcited intermediate in the PPP microphase. Implications for light-energy harvesting are discussed.
Co-reporter:Charles Y. Cummings, Paul E. Frith, Guillaume Zoppi, Ian Forbes, Keith D. Rogers, David W. Lane, Frank Marken
Thin Solid Films 2011 Volume 519(Issue 21) pp:7458-7463
Publication Date(Web):31 August 2011
DOI:10.1016/j.tsf.2010.12.107
A novel electro-deposition method based on a rocking disc system with π/3 amplitude and variable frequency is introduced. Uniform copper films were deposited from a 0.1 M CuSO4/3.0 M NaOH/0.2 M sorbitol bath directly onto 12.1 cm2 Mo/MoSe2 substrates with X-ray diffraction showing a thickness variation of ±5% over this area. Investigation of the mass transport conditions suggests (i) uniform diffusion over the sample, (ii) a rate of mass transport proportional to the square root of the rocking rate, and (iii) turbulent conditions, which are able to dislodge gas bubbles during electro-deposition.
Co-reporter:Yan-Jun Huang, Yun-Bao Jiang, John S. Fossey, Tony D. James and Frank Marken
Journal of Materials Chemistry A 2010 vol. 20(Issue 38) pp:8305-8310
Publication Date(Web):20 Aug 2010
DOI:10.1039/C0JM01510E
The highly water insoluble N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate is synthesised and investigated for sensor applications. This amphiphilic molecule is immobilised by evaporation of an acetonitrile solution at a basal plane pyrolytic graphite (HOPG) electrode surface and is shown to provide a monolayer film. By varying the amount of deposit partial or full coverage can be achieved. The N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer acts as an active receptor for 1,2-dihydroxy-benzene (catechol) derivatives in aqueous media. The ability to bind alizarin red S is investigated and the Langmuirian binding constant determined as a function of pH. It is shown that the immobilised boronic acid monolayer acts as sensor film for a wider range of catechols. A comparison of Langmuirian binding constants for alizarin red S (1.4 × 105 mol−1 dm3), catechol (8.4 × 104 mol−1 dm3), caffeic acid (7.5 × 104 mol−1 dm3), dopamine (1.0 × 104 mol−1 dm3), and L-dopa (8 × 103 mol−1 dm3) reveals that a combination of hydrophobicity and electrostatic interaction causes considerable selectivity effects.
Co-reporter:Michael J. Bonné, Ewan Galbraith, Tony D. James, Matthew J. Wasbrough, Karen J. Edler, A. Toby A. Jenkins, Matthew Helton, Anthony McKee, Wim Thielemans, Elefteria Psillakis and Frank Marken
Journal of Materials Chemistry A 2010 vol. 20(Issue 3) pp:588-594
Publication Date(Web):13 Nov 2009
DOI:10.1039/B918308F
Cellulose nanofibrils from sisal of typically 4–5 nm diameter and ca. 250 ± 100 nm length are reconstituted into thin films of ca. 6 µm thickness (or thicker freestanding films). Pure cellulose and cellulose composite films are obtained in a solvent evaporation process. A boronic acid appended dendrimer is embedded as a receptor in the nanofibrillar cellulose membrane. The number of boronic acid binding sites is controlled by varying the dendrimer content. The electrochemical and spectrophotometric properties of the nanocomposite membrane are investigated using the probe molecule alizarin red S. Pure cellulose membranes inhibit access to the electrode. However, the presence of boronic acid receptor sites allows accumulation of alizarin red S with a Langmuirian binding constant of ca. 6000 ± 1000 M−1. The 2-electron 2-proton reduction of immobilized alizarin red S is shown to occur in a ca. 60 nm zone close to the electrode surface. With a boronic acid dendrimer modified nanofibrillar cellulose composition of 96 wt% cellulose and 4 wt% boronic acid dendrimer, the analytical range for alizarin red S in aqueous acetate buffer pH 3 is approximately 10 µM to 1 mM.
Co-reporter:Liza Rassaei, Michaela Nebel, Neil V. Rees, Richard G. Compton, Wolfgang Schuhmann and Frank Marken
Chemical Communications 2010 vol. 46(Issue 5) pp:812-814
Publication Date(Web):25 Nov 2009
DOI:10.1039/B920154H
Microwave induced activation of electrochemical processes at microelectrodes (ca. 0.8 µm diameter) immersed in aqueous electrolyte media is shown to be driven by (i) continuous stable cavitation (giving rise to Faradaic current enhancements by up to three orders of magnitude) and (ii) transient discharge cavitation on the µs timescale (giving rise to cathodic plasma current spikes and more violent surface erosion effects).
Co-reporter:Liza Rassaei and Frank Marken
Analytical Chemistry 2010 Volume 82(Issue 17) pp:7063
Publication Date(Web):August 5, 2010
DOI:10.1021/ac101303s
A novel glucose sensing concept based on the localized change or “modulation” in pH within a symmetric gold−gold junction electrode is proposed. A paired gold−gold junction electrode (average gap size ca. 500 nm) is prepared by simultaneous bipotentiostatic electrodeposition of gold onto two closely spaced platinum disk electrodes. For glucose detection in neutral aqueous solution, the potential of the “pH-modulator” electrode is set to −1.5 V vs saturated calomel reference electrode (SCE) to locally increase the pH, and simultaneously, either cyclic voltammetry or square wave voltammetry experiments are conducted at the sensor electrode. A considerable improvement in the sensor electrode response is observed when a normal pulse voltammetry sequence is applied to the modulator electrode (to generate “hydroxide pulses”) and the glucose sensor electrode is operated with fixed bias at +0.5 V vs SCE (to eliminate capacitive charging currents). Preliminary data suggest good linearity for the glucose response in the medically relevant 1−10 mM concentration range (corresponding to 0.18−1.8 g L−1). Future electroanalytical applications of multidimensional pulse voltammetry in junction electrodes are discussed.
Co-reporter:Niancai Cheng, Haifeng Lv, Wei Wang, Shichun Mu, Mu Pan, Frank Marken
Journal of Power Sources 2010 Volume 195(Issue 21) pp:7246-7249
Publication Date(Web):1 November 2010
DOI:10.1016/j.jpowsour.2010.05.039
An experimentally simple process is reported in aqueous solution and under ambient conditions to prepare highly dispersed and active Pd/C catalyst without the use of a stabilizing agent. The [Pd(NH3)4]2+ ion is synthesized with gentle heating in aqueous ammonia solution without formation of Pd(OH)x complex intermediates. The adsorbed [Pd(NH3)4]2+ on the surface of carbon (Vulcan XC-72) is reduced in situ to Pd nanoparticles by NaBH4. The Pd/C catalyst obtained is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that highly dispersed Pd/C catalyst with 20 wt.% Pd content and with an average Pd nanoparticle diameter of 4.3–4.7 nm could be obtained. The electrochemical measurements show that the Pd/C catalyst without stabilizer has a higher electro-oxidation activity for formic acid compared to that of a Pd/C catalyst prepared in a traditional high temperature polyol process in ethylene glycol.
Co-reporter:Charles Y. Cummings, Alistair H. Roweth, Andrew K.Z. Ching, A. Toby A. Jenkins, John M. Mitchels, Sara Shariki, Soon Yee Liew, Wim Thielemans, Darren A. Walsh, Frank Marken
Electrochemistry Communications 2010 Volume 12(Issue 12) pp:1722-1726
Publication Date(Web):December 2010
DOI:10.1016/j.elecom.2010.10.006
Molecular layer-by-layer assembly from pre-saturated aqueous solutions of Fe3+ and phytate is employed to build up iron phytate deposits on tin-doped indium oxide (ITO) electrodes. Globular films with approximately 1 nm growth per layer are observed by AFM imaging and sectioning. In electrochemical experiments the iron phytate films show well-defined voltammetric responses consistent with an immobilised Fe(III/II) redox system in aqueous (LiClO4, NaClO4, KClO4, phosphate buffer) and in ethanolic (LiClO4, NaClO4, NBu4ClO4) electrolyte solutions. The Fe(III/II) redox system is reversible and cation insertion/expulsion occurs fast on the timescale of voltammetric experiments even for more bulky NBu4+ cations and in ethanolic solution. Peak shape analysis and scan rate dependent midpoint potentials suggest structural changes accompanying the redox process and limiting propagation. Iron phytate is proposed as a versatile and essentially colourless cation electro-insertion material and as a potential energy storage material.
Co-reporter:K. Firoz Babu, M. Anbu Kulandainathan, Ioannis Katsounaros, Liza Rassaei, Andrew D. Burrows, Paul R. Raithby, Frank Marken
Electrochemistry Communications 2010 Volume 12(Issue 5) pp:632-635
Publication Date(Web):May 2010
DOI:10.1016/j.elecom.2010.02.017
For the case of the commercially available metal-organic framework (MOF) structure BasoliteTM F300 or Fe(BTC) with BTC = benzene-1,3,5-tricarboxylate, it is shown that the Fe(III/II) electrochemistry is dominated by reductive dissolution rather than ion insertion (which in marked contrast is dominating the behaviour of Fe(III/II) open framework processes in Prussian blues). Solid Fe(BTC) immobilised onto graphite or platinum working electrodes is investigated and it is shown that well-defined and reversible Fe(III/II) reduction responses occur only on platinum and in the presence of aqueous acid. The process is shown to follow a CE-type mechanism involving liberation of Fe(III) in acidic media, in particular for high concentrations of acid. Effective electrocatalysis for the oxidation of hydroxide to O2 (anodic water splitting) is observed in alkaline aqueous media after initial cycling of the potential into the reduction potential zone. A mechanism based on a MOF-surface confined hydrous iron oxide film is proposed.
Co-reporter:Mikhail Yu Vagin, Arkady A. Karyakin, Anne Vuorema, Mika Sillanpää, Helen Meadows, F. Javier Del Campo, Montserrat Cortina-Puig, Philip C. Bulman Page, Yohan Chan, Frank Marken
Electrochemistry Communications 2010 Volume 12(Issue 3) pp:455-458
Publication Date(Web):March 2010
DOI:10.1016/j.elecom.2010.01.018
Electrochemical processes at liquid–liquid–electrode interfaces involve simultaneous ion transfer and electron transfer. When driven at triple phase boundary electrode systems, electron and ion transfer occur in the same interfacial reaction zone. In this report, preliminary work with a novel electrode system based on two coupled triple phase boundary reaction zones is described. An interdigitated gold band array with 7 μm gold bands separated by 13 μm gaps is employed immersed in aqueous electrolyte with a water-immiscible solution of the redox system N,N-diethyl-N′N′-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP) immobilized on the surface. Well-defined generator–collector feedback currents are observed which depend on the volume of deposit, the concentration of the redox system, and the nature of the aqueous electrolyte.
Co-reporter:John D. Watkins, Fumihiro Amemiya, Mahito Atobe, Philip C. Bulman-Page, Frank Marken
Electrochimica Acta 2010 Volume 55(Issue 28) pp:8808-8814
Publication Date(Web):1 December 2010
DOI:10.1016/j.electacta.2010.07.104
Unstable acetonitrile | aqueous emulsions generated in situ with ultra-turrax agitation are investigated for applications in dual-phase electrochemistry. Three modes of operation for liquid | liquid aqueous–organic electrochemical processes are demonstrated with no intentionally added electrolyte in the organic phase based on (i) the formation of a water-soluble product in the aqueous phase in the presence of the organic phase, (ii) the formation of a product and ion transfer at the liquid | liquid–electrode triple phase boundary, and (iii) the formation of a water-insoluble product in the aqueous phase which then transfers into the organic phase.A three-electrode electrolysis cell with ultra-turrax agitator is employed and characterised for acetonitrile | aqueous 2 M NaCl two phase electrolyte. Three redox systems are employed in order to quantify the electrolysis cell performance. The one-electron reduction of Ru(NH3)63+ in the aqueous phase is employed to determine the rate of mass transport towards the electrode surface and the effect of the presence of the acetonitrile phase. The one-electron oxidation of n-butylferrocene in acetonitrile is employed to study triple phase boundary processes. Finally, the one-electron reduction of cobalticenium cations in the aqueous phase is employed to demonstrate the product transfer from the electrode surface into the organic phase. Potential applications in biphasic electrosynthesis are discussed.
Co-reporter:Niancai Cheng, Richard A. Webster, Mu Pan, Shichun Mu, Liza Rassaei, Shik Chi Tsang, Frank Marken
Electrochimica Acta 2010 Volume 55(Issue 22) pp:6601-6610
Publication Date(Web):1 September 2010
DOI:10.1016/j.electacta.2010.06.014
The electrochemical formation of a palladium nanoparticle catalyst composite material has been investigated. A carbon nanoparticle–chitosan host film deposited onto a carbon substrate electrode has been employed to immobilize PdCl2 as catalyst precursor. A one-step electrochemical reduction process gave Pd nanoparticles within the chitosan matrix with different levels of loading, on different carbon substrates, and with a reproducible catalyst particle diameter of ca. 3–5 nm. High activity for formic acid oxidation has been observed in aqueous phosphate buffer medium. The oxidation of formic acid has been investigated as a function of pH and maximum catalyst activity was observed at pH 6. When varying the formic acid concentration, limiting behaviour consistent with a “resistance effect” has been observed. A flow cell system based on a screen-printed carbon electrode has been employed to establish the effect of hydrodynamic conditions on the formic acid oxidation. Both increasing the convective-diffusion mass transport rate and increasing the concentration of formic acid caused the oxidation peak current to converge towards the same “resistance limit”. A mechanistic model to explain the resistance effect based on CO2 flux and localized CO2 gas bubble formation at the Pd nanoparticle modified carbon nanoparticle–chitosan host film has been proposed.
Co-reporter:John D. Watkins, Ruth Lawrence, James E. Taylor, Steven D. Bull, Geoffrey W. Nelson, John S. Foord, Daniel Wolverson, Liza Rassaei, Nick D. M. Evans, Silvia Antón Gascon and Frank Marken
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 18) pp:4872-4878
Publication Date(Web):17 Mar 2010
DOI:10.1039/B927434K
The surface functionalities of commercial sulfonate-modified carbon nanoparticles (ca. 9–18 nm diameter, Emperor 2000) have been converted from negatively charged to positively charged via sulfonylchloride formation followed by reaction with amines to give suphonamides. With ethylenediamine, the resulting positively charged carbon nanoparticles exhibit water solubility (in the absence of added electrolyte), a positive zeta-potential, and the ability to assemble into insoluble porous carbon films via layer-by-layer deposition employing alternating positive and negative carbon nanoparticles. Sulfonamide-functionalised carbon nanoparticles are characterised by Raman, AFM, XPS, and voltammetric methods. Stable thin film deposits are formed on 3 mm diameter glassy carbon electrodes and cyclic voltammetry is used to characterise capacitive background currents and the adsorption of the negatively charged redox probe indigo carmine. The Langmuirian binding constant K = 4000 mol−1dm3 is estimated and the number of positively charged binding sites per particle determined as a function of pH.
Co-reporter:David J. Fermín, Frank Marken
Journal of Electroanalytical Chemistry 2010 Volume 646(1–2) pp:1-2
Publication Date(Web):15 July 2010
DOI:10.1016/j.jelechem.2010.06.017
Co-reporter:Charles Y. Cummings, Guillaume Zoppi, Ian Forbes, Phillip J. Dale, Jonathan J. Scragg, Laurie M. Peter, Gabriele Kociok-Köhn, Frank Marken
Journal of Electroanalytical Chemistry 2010 Volume 645(Issue 1) pp:16-21
Publication Date(Web):15 June 2010
DOI:10.1016/j.jelechem.2010.03.031
Mo/MoSe2 thin film electrodes formed by selenisation of molybdenum are investigated as chemically robust substrates for the electro-deposition of InCu precursor layers for CuInSe2 (CISe) solar cell absorber films. Exposure of molybdenum films to selenium vapour at 550 °C produces thin and chemically robust heterostructures of Mo/MoSe2. These films exhibit the characteristics of a degenerate semi-conductor and provide close to metallic electrical conductivity for electro-deposition processes in both acidic and alkaline aqueous media. The Mo/MoSe2 films are characterised by cyclic voltammetry for the reduction of Ru(NH3)63+ in aqueous 0.1 M KCl, for the reduction of 0.1 M In3+ in aqueous 0.5 M LiCl pH 3, and for the reduction of 0.1 M Cu2+ in aqueous 3 M NaOH with 0.2 M d-sorbitol. In all three cases well-defined and reversible voltammetric responses are observed. For the formation of CISe films initially In3+ is deposited potentiostatically followed by electro-deposition of Cu2+ and selenisation at 550 °C in selenium vapour. Mechanically stable CISe films are produced and preliminary photo-electrochemical data demonstrate the effects of changing the stoichiometry.
Co-reporter:Andrew M. Kelly, Najoua Katif, Tony D. James and Frank Marken
New Journal of Chemistry 2010 vol. 34(Issue 7) pp:1261-1265
Publication Date(Web):26 Apr 2010
DOI:10.1039/C0NJ00166J
We have developed a permethylated ferrocene redox system with a butylamine substituent for application in liquid | liquid ion sensors. The steric hindrance associated with the methyl groups results in an electrochemical system where the ferricenium derivative is chemically inert and the redox system remains chemically reversible, even for applications in aqueous or biphasic media. N,N-Butyl-decamethylferrocenyl-amine is soluble in hydrophobic organic solvents, such as 4-(3-phenylpropyl)pyridine (PPP) and N-octyl-pyrrolidone (NOP), and is employed here under “microphase” conditions, deposited in the form of microdroplets onto an electrode and immersed in aqueous buffer solutions. It is shown that under these conditions, electron transfer and proton transfer are only weakly coupled, and that anion transfer dominates the microphase redox process over the entire pH range. The corresponding biphasic Pourbaix diagram is discussed.
Co-reporter:Alberto Fattori, Laurence M. Peter, Stephen R. Belding, Richard G. Compton, Frank Marken
Journal of Electroanalytical Chemistry 2010 640(1–2) pp: 61-67
Publication Date(Web):
DOI:10.1016/j.jelechem.2010.01.004
Co-reporter:Andrew M. Collins;Xiaohang Zhang;Jonathan J. Scragg; Gary J. Blanchard;Dr. Frank Marken
ChemPhysChem 2010 Volume 11( Issue 13) pp:2862-2870
Publication Date(Web):
DOI:10.1002/cphc.200000094
Abstract
Rhodamine B base is employed as a photoactive redox system in a liquid | liquid | electrode triple phase boundary electrochemical experiment. Microdroplets of the water-immiscible 3-(4-phenylpropyl)-pyridine (PPP) containing rhodamine B are deposited onto a basal plane pyrolytic graphite electrode surface which is then immersed into an aqueous solution containing 0.5 M phosphate buffer solution (pH 12). In cyclic voltammetry experiments, it is shown that dark reduction of rhodamine B to leuco-rhodamine B in PPP occurs in two steps—both one electron processes—and the re-oxidation occurs in a single two-electron process. Voltammetric signals are consistent with Na+ transfer (for the first reduction step) and proton transfer (for the second reduction step) coupled to the electron transfer. A proton-driven disproportionation reaction (with kdis=3 mol−1 dm3 s−1 at pH 12) is observed for the one-electron reduced intermediate. In the presence of light, a new photocatalysed oxidation process for the two-electron conversion of leuco-rhodamine B to rhodamine B is observed. Photoexcitation of rhodamine B is shown to trigger an effective photo-comproportionation mechanism. Quantitative insights into the dark and photomechanisms are obtained by comparison of rhodamine B and rhodamine B octadecylester and by applying an approximate numerical simulation procedure based on DigiSim.
Co-reporter:Alberto Fattori, Laurence M. Peter, Hongxia Wang, Hidetoshi Miura and Frank Marken
The Journal of Physical Chemistry C 2010 Volume 114(Issue 27) pp:11822-11828
Publication Date(Web):June 23, 2010
DOI:10.1021/jp103808u
The indoline dyes D102, D131, D149, and D205 have been characterized when adsorbed on fluorine-doped tin oxide (FTO) and TiO2 electrode surfaces. Adsorption from 50:50 acetonitrile−tert-butanol onto fluorine-doped tin oxide (FTO) allows approximate Langmuirian binding constants of 6.5 × 104, 2.0 × 103, 2.0 × 104, and 1.5 × 104 mol−1 dm3, respectively, to be determined. Voltammetric data obtained in acetonitrile/0.1 M NBu4PF6 indicate reversible one-electron oxidation at Emid = 0.94, 0.91, 0.88, and 0.88 V vs Ag/AgCl(3 M KCl), respectively, with dye aggregation (at high coverage) causing additional peak features at more positive potentials. Slow chemical degradation processes and electron transfer catalysis for iodide oxidation were observed for all four oxidized indolinium cations. When adsorbed onto TiO2 nanoparticle films (ca. 9 nm particle diameter and ca. 3 μm thickness on FTO), reversible voltammetric responses with Emid = 1.08, 1.16, 0.92, and 0.95 V vs Ag/AgCl(3 M KCl), respectively, suggest exceptionally fast hole hopping diffusion (with Dapp > 5 × 10−9 m2 s−1) for adsorbed layers of all four indoline dyes, presumably due to π−π stacking in surface aggregates. Slow dye degradation is shown to affect charge transport via electron hopping. Spectroelectrochemical data for the adsorbed indoline dyes on FTO-TiO2 revealed a red-shift of absorption peaks after oxidation and the presence of a strong charge transfer band in the near-IR region. The implications of the indoline dye reactivity and fast hole mobility for solar cell devices are discussed.
Co-reporter:John D. Watkins, Stuart M. MacDonald, Paul S. Fordred, Steven D. Bull, Yunfeng Gu, Kamran Yunus, Adrian C. Fisher, Philip C. Bulman-Page, Frank Marken
Electrochimica Acta 2009 Volume 54(Issue 27) pp:6908-6912
Publication Date(Web):30 November 2009
DOI:10.1016/j.electacta.2009.06.083
A dynamic acetonitrile | aqueous electrolyte phase boundary in contact with platinised Teflon working electrodes is investigated. High concentrations of salt in the aqueous phase (2 M NaCl and 0.1 M NaClO4) ensure immiscibility and the polar nature of acetonitrile aids the formation of a well-behaved triple phase boundary reaction zone. The one-electron oxidation of tert-butylferrocene in the organic phase without intentionally added electrolyte is studied. The limiting current for the flowing triple phase boundary process is shown to be essentially volume flow rate independent. The process is accompanied by the transfer of perchlorate from the aqueous into the organic phase and the flux of anions is shown to be approximately constant along the dynamic acetonitrile | aqueous electrolyte | platinum line interface. A high rate of conversion (close to 100%) is achieved at slow volume flow rates and at longer platinum electrodes.
Co-reporter:Liza Rassaei, Elena Vigil, Robert W. French, Mary F. Mahon, Richard G. Compton, Frank Marken
Electrochimica Acta 2009 Volume 54(Issue 26) pp:6680-6685
Publication Date(Web):1 November 2009
DOI:10.1016/j.electacta.2009.06.062
In situ microwave activation is investigated for the electrodeposition of a metal (gold) and for a metal oxide (hydrous Ti(IV) oxide) onto tin-doped indium oxide (ITO) film electrodes. It is demonstrated that localized microwave heating of the ITO film can be exploited to affect electrodeposition processes.The electrochemically reversible and temperature sensitive one-electron redox system Fe(CN)63−/4− was employed in aqueous solution in order to calibrate the average surface temperature at the ITO film electrode. In the presence of microwave radiation the average electrode surface temperature reached ca. 363 K whereas under the same conditions the bulk solution temperature reached ca. 313 K. Therefore localized heating of the ITO film appears to be important.The rate of electrodeposition of gold from an aqueous 1 mM tetrachloroaurate(III) solution in 0.1 M KCl (adjusted to pH 2) is enhanced by microwave activation. However, the morphology of deposits remains un-effected. Hydrous titanium (IV) oxide films were electrodeposited from an aqueous solution of 1 mM TiCl3 in 0.1 M acetate buffer pH 4.7. Dense films with blocking character were obtained with conventional heating but a fibrous more open deposit forms in the presence of microwaves.
Co-reporter:Anne Vuorema, Philip John, Marjo Keskitalo, Mary F. Mahon, M. Anbu Kulandainathan and Frank Marken
Physical Chemistry Chemical Physics 2009 vol. 11(Issue 11) pp:1816-1824
Publication Date(Web):14 Jan 2009
DOI:10.1039/B814149E
Anthraquinone immobilised onto the surface of indigo microcrystals enhances the reductive dissolution of indigo to leuco-indigo. Indigo reduction is driven by glucose in aqueous NaOH and a vibrating gold disc electrode is employed to monitor the increasing leuco-indigo concentration with time. Anthraquinone introduces a strong catalytic effect which is explained by invoking a molecular “wedge effect” during co-intercalation of Na+ and anthraquinone into the layered indigo crystal structure. The glucose-driven indigo reduction, which is ineffective in 0.1 M NaOH at 65 °C, becomes facile and goes to completion in the presence of anthraquinone catalyst. Electron microscopy of indigo crystals before and after reductive dissolution confirms a delamination mechanism initiated at the edges of the plate-like indigo crystals. Catalysis occurs when the anthraquinone–indigo mixture reaches a molar ratio of 1:400 (at 65 °C; corresponding to 3 μM anthraquinone) with excess of anthraquinone having virtually no effect. A strong temperature effect (with a composite EA≈ 120 kJ mol−1) is observed for the reductive dissolution in the presence of anthraquinone. The molar ratio and temperature effects are both consistent with the heterogeneous nature of the anthraquinone catalysis in the aqueous reaction mixture.
Co-reporter:Robert W. French, Sergey N. Gordeev, Paul R. Raithby, Frank Marken
Journal of Electroanalytical Chemistry 2009 Volume 632(1–2) pp:206-210
Publication Date(Web):1 July 2009
DOI:10.1016/j.jelechem.2009.04.018
Simultaneous bipotentiostatic electro-deposition of gold onto two closely spaced platinum disc electrodes produces paired gold electrode junctions. The potential applied during gold deposition onto the two electrodes is chosen to be slightly offset (by 0.5 mV) to allow for an automatic cut-off mechanism to control the resulting gap size for the paired diffusion junction. The key parameter controlling the gap size is identified as the deposition potential (in relation to the reversible potential for gold deposition) and paired gold electrode junctions are prepared reproducibly from ca. 5 μm estimated gap size (−250 mV overpotential during deposition) down to approximately 300 nm estimated gap size (−20 mV overpotential during deposition). The resulting paired junction electrodes are characterized for the reduction of alizarin red S and it is shown that for junctions with ca. 300 nm gap size: (i) the low concentration sensitivity is improved to allow sub-micromolar concentration measurements and (ii) due to faster mass transport within the smaller gap a transition from reversible to irreversible electron transfer can be observed.
Co-reporter:Stuart M. MacDonald;John D. Watkins;Stephen D. Bull;Iwan R. Davies;Yunfeng Gu;Kamran Yunus;Adrian C. Fisher;Philip C. Bulman Page;Yohan Chan;Claire Elliott
Journal of Physical Organic Chemistry 2009 Volume 22( Issue 1) pp:52-58
Publication Date(Web):
DOI:10.1002/poc.1424
Abstract
A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junctions in contact with a boron-doped diamond (BDD) working electrode. The rectangular flow cell is calibrated for both single-phase liquid flow and biphasic liquid–liquid flow for the case of (i) the immiscible N-octyl-2-pyrrolidone (NOP)–aqueous electrolyte system and (ii) the immiscible acetonitrile–aqueous electrolyte system. The influence of flow speed and liquid viscosity on the position of the phase boundary and mass transport-controlled limiting currents are examined. In contrast to the NOP–aqueous electrolyte case, the acetonitrile–aqueous electrolyte system is shown to behave close to ideal without ‘undercutting’ of the organic phase under the aqueous phase. The limiting current for three-phase boundary reactions is only weakly dependent on flow rate but directly proportional to the concentration and the diffusion coefficient in the organic phase. Acetonitrile as a commonly employed synthetic solvent is shown here to allow effective three-phase boundary processes to occur due to a lower viscosity enabling faster diffusion. N-butylferrocene is shown to be oxidised at the acetonitrile–aqueous electrolyte interface about 12 times faster when compared with the same process at the NOP–aqueous electrolyte interface. Conditions suitable for clean two-phase electrosynthetic processes without intentionally added supporting electrolyte in the organic phase are proposed. Copyright © 2008 John Wiley & Sons, Ltd.
Co-reporter:Robert W. French
Journal of Solid State Electrochemistry 2009 Volume 13( Issue 4) pp:609-617
Publication Date(Web):2009 April
DOI:10.1007/s10008-008-0698-9
A diffusion junction between two paired gold electrodes is created in a bipotentiostatic electro-deposition process. Gold metal is deposited simultaneously on two adjacent disc electrodes (100 μm diameter, approximately 125 μm separation) until short-circuit conditions trigger the end point of the electro-deposition. Symmetric gold junctions with typically 5 μm average inter-electrode gap size, 140 μm gap length, and approximately 18 μm junction depth are obtained. These paired gold electrodes are employed in generator–collector mode to give well-defined steady-state feedback currents even for extremely low concentrations of analyte (sub-μM) and without any contributions from capacitive charging. Four redox systems are investigated spanning a wide range of diffusion coefficients: (1) the one-electron oxidation of iodide to iodine, (2) the two-electron oxidation of hydroquinone to benzoquinone, (3) the two-electron reduction of alizarin red S, and (4) the one-electron oxidation of the redox protein cytochrome c. Consistent results for these redox systems suggest that (1) the junction zone between the two electrodes is dominating the behaviour of the electrode in particular for the slower diffusing systems and (2) the paired gold electrode junction can be calibrated and employed for electroanalysis at very low concentrations and for a wider range of analytically relevant redox systems.
Co-reporter:Najoua Katif;Rachel A. Harries;Andrew M. Kelly
Journal of Solid State Electrochemistry 2009 Volume 13( Issue 10) pp:1475-1482
Publication Date(Web):2009 October
DOI:10.1007/s10008-008-0709-x
The transfer of the α-hydroxy-carboxylates of glycolic, lactic, mandelic and gluconic acid from the aqueous electrolyte phase into an organic 4-(3-phenylpropyl)-pyridine (PPP) phase is studied at a triple-phase boundary electrode system. The tetraphenylporphyrinato complex MnTPP dissolved in PPP is employed to drive the anion transfer reaction and naphthalene-2-boronic acid (NBA) is employed as a facilitator. In the absence of a facilitator, the ability of α-hydroxy-carboxylates to transfer into the organic phase improves, consistent with hydrophobicity considerations giving relative transfer potentials (for aqueous 0.1 M solution) of gluconate>glycolate>lactate>mandelate. In the presence of NBA, a shift of the reversible transfer potential to more negative values is indicating fast reversible binding (the mechanism for the electrode process is EICrev) and the binding constants are determined as Kglycolate = 2 M−1, Kmandelate = 60 M−1, Klactate = 130 M−1 and Kgluconate = 2,000 M−1. The surprisingly strong interaction for gluconate is rationalised based on secondary interactions between the gluconate anion and NBA.
Co-reporter:Liza Rassaei, Mika Sillanpää, Frank Marken
Electrochimica Acta 2008 Volume 53(Issue 19) pp:5732-5738
Publication Date(Web):1 August 2008
DOI:10.1016/j.electacta.2008.03.051
Surface functionalised carbon nanoparticles of ca. 8 nm diameter co-assemble with chitosan into stable thin film electrodes at glassy carbon surfaces. Robust electrodes for application in sensing or electrocatalysis are obtained in a simple solvent evaporation process. The ratio of chitosan binder backbone to carbon nanoparticle conductor determines the properties of the resulting films. Chitosan (a poly-d-glucosamine) has a dual effect (i) as the binder for the mesoporous carbon composite structure and (ii) as binding site for redox active probes. Physisorption due to the positively charged ammonium group (pKA ≈ 6.5) occurs, for example, with anionic indigo carmine (a reversible 2e−–2H+ reduction system in aqueous media). Chemisorption at the amine functionalities is demonstrated with 2-bromo-methyl-anthraquinone in acetonitrile (resulting in a reversible 2e−–2H+ anthraquinone reduction system in aqueous media). Redox processes within the carbon nanoparticle-chitosan films are studied and at sufficiently high scan rates diffusion of protons (buffer concentration depended) is shown to be rate limiting. The chemisorption process provides a much more stable interfacial redox system with a characteristic and stable pH response over a pH 2–12 range. Chemisorption and physisorption can be employed simultaneously in a complementary binding process.
Co-reporter:Charles Y. Cummings, Michael J. Bonné, Karen J. Edler, Matthew Helton, Anthony McKee, Frank Marken
Electrochemistry Communications 2008 Volume 10(Issue 11) pp:1773-1776
Publication Date(Web):November 2008
DOI:10.1016/j.elecom.2008.09.018
Nanoparticle film voltammetry is employed to explore the presence and reactivity of surface-stabilised iron redox centers at the interface of immobilised Fe2O3 nanoparticles of ca. 4 nm diameter and aqueous buffer media. Mesoporous films of Fe2O3 nanoparticles on tin-doped indium oxide (ITO) substrates are formed in a layer-by-layer deposition process from aqueous colloidal Fe2O3 and aqueous cyclohexyl-hexacarboxylate followed by thermal (500 °C) removal of the organic binder content. Both reversible oxidation and reversible reduction responses for Fe(III) are observed in phosphate and carbonate buffer media in the “underpotential” zone. Higher oxidation states of iron formed anodically (here tentatively assigned to Fe(IV)) are shown to be inert in phosphate buffer media but reactive towards the oxidation of glucose in carbonate buffer media.
Co-reporter:Lorena Vidal, Alberto Chisvert, Antonio Canals, Elefteria Psillakis, Alexei Lapkin, Fernando Acosta, Karen J. Edler, James A. Holdaway, Frank Marken
Analytica Chimica Acta 2008 Volume 616(Issue 1) pp:28-35
Publication Date(Web):26 May 2008
DOI:10.1016/j.aca.2008.04.011
Chemically surface-modified (tosyl-functionalized) carbon nanoparticles (Emperor 2000 from Cabot Corp.) are employed for the extraction and electrochemical determination of phenolic impurities such as benzophenone-3 (2-hydroxy-4-methoxybenzophenone) or triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol). The hydrophilic carbon nanoparticles are readily suspended and separated by centrifugation prior to deposition onto suitable electrode surfaces and voltammetric analysis. Voltammetric peaks provide concentration information over a 10–100 μM range and an estimated limit of detection of ca. 10 μM (or 2.3 ppm) for benzophenone-3 and ca. 20 μM (or 5.8 ppm) for triclosan.Alternatively, analyte-free carbon nanoparticles immobilized at a graphite or glassy carbon electrode surface and directly immersed in analyte solution bind benzophenone-3 and triclosan (both with an estimated Langmuirian binding constants of K ≈ 6000 mol−1 dm3 at pH 9.5) and they also give characteristic voltammetric responses (anodic for triclosan and cathodic for benzophenone-3) with a linear range of ca. 1–120 μM. The estimated limit of detection is improved to ca.5 μM (or 1.2 ppm) for benzophenone-3 and ca. 10 μM (or 2.3 ppm) for triclosan. Surface functionalization is discussed as the key to further improvements in extraction and detection efficiency.
Co-reporter:Stuart M. MacDonald, Marcin Opallo, Andreas Klamt, Frank Eckert and Frank Marken
Physical Chemistry Chemical Physics 2008 vol. 10(Issue 26) pp:3925-3933
Publication Date(Web):22 May 2008
DOI:10.1039/B803582B
Understanding liquid|liquid ion transfer processes is important in particular for naturally occurring species such as carboxylates. In this study electrochemically driven mono-, di-, and tri-carboxylate anion transfer at the 4-(3-phenylpropyl)pyridine|aqueous electrolyte interface is investigated experimentally for a triple phase boundary system at graphite electrodes. The tetraphenylporphyrinato-Mn(III/II) redox system (Mn(III/II)TPP) dissolved in the water-immiscible organic phase (4-(3-phenylpropyl)pyridine) is employed for the quantitative study of the structure–Gibbs transfer energy correlation and the effects of the solution pH on the carboxylate transfer process. For di- and tri-carboxylates the partially protonated anions are always transferred preferentially even at a pH higher than the corresponding pKa. COSMO-RS computer simulations are shown to provide a quantitative rationalisation as well as a powerful tool for predicting Gibbs free energy of transfer data for more complex functionalised carboxylate anions. It is shown that the presence of water in the organic phase has a major effect on the calculated Gibbs free energies.
Co-reporter:Anne Vuorema, Philip John, Marjo Keskitalo, M. Anbu Kulandainathan, Frank Marken
Dyes and Pigments 2008 Volume 76(Issue 2) pp:542-549
Publication Date(Web):2008
DOI:10.1016/j.dyepig.2006.06.044
The reduction of indigo (dispersed in water) to leuco-indigo (dissolved in water) is an important industrial process and investigated here for the case of glucose as an environmentally benign reducing agent. In order to quantitatively follow the formation of leuco-indigo two approaches based on (i) rotating disk voltammetry and (ii) sonovoltammetry are developed. Leuco-indigo, once formed in alkaline solution, is readily monitored at a glassy carbon electrode in the mass transport limit employing hydrodynamic voltammetry. The presence of power ultrasound further improves the leuco-indigo determination due to additional agitation and homogenization effects.While inactive at room temperature, glucose readily reduces indigo in alkaline media at 65 °C. In the presence of excess glucose, a surface dissolution kinetics limited process is proposed following the rate law ⅆnleuco-indigo/ⅆt=k×cOH−×Sindigoⅆnleuco-indigo/ⅆt=k×cOH−×Sindigo where nleuco-indigo is the amount of leuco-indigo formed, k = 4.1 × 10−9 m s−1 (at 65 °C, assuming spherical particles of 1 μm diameter) is the heterogeneous dissolution rate constant, cOH−cOH− is the concentration of hydroxide, and Sindigo is the reactive surface area. The activation energy for this process in aqueous 0.2 M NaOH is EA = 64 kJ mol−1 consistent with a considerable temperature effects. The redox mediator 1,8-dihydroxyanthraquinone is shown to significantly enhance the reaction rate by catalysing the electron transfer between glucose and solid indigo particles.
Co-reporter:Liza Rassaei, Michael J. Bonné, Mika Sillanpää and Frank Marken
New Journal of Chemistry 2008 vol. 32(Issue 7) pp:1253-1258
Publication Date(Web):26 Mar 2008
DOI:10.1039/B800331A
Thin chitosan–carbon nanoparticle films (ca. 2 nm average thickness increase per layer) are assembled onto tin-doped indium oxide (ITO) electrode substrates in a layer-by-layer deposition process employing carbon nanoparticles of ca. 8 nm average diameter and an aqueous solution of chitosan (poly-D-glucosamine, low molecular weight, 75–85% deacetylated). Chitosan introduces amine/ammonium functionalities which are employed for the immobilization of redox systems (i) via physisorption of indigo carmine and (ii) via chemisorption of 2-methyleneanthraquinone. The number of binding sites within the chitosan–carbon nanoparticle film is controlled by changing the thickness of the film deposit or by changing the chitosan content, which is varied by changing the chitosan concentration during layer-by-layer deposition. Voltammetric characteristics and stability of the chemisorbed and physisorbed redox systems are reported as a function of pH. The physisorbed redox system is expelled from the film at a pH consistent with the pKA of chitosan (approximately 6.5). However, the 2-methyleneanthraquinone redox system remains stable even in alkaline media and only a minor inflection in the plot of midpoint potentials vs. pH indicates the film deprotonation process at the pKA of chitosan.
Co-reporter:Elizabeth V. Milsom, Hayley A. Dash, A. Toby A. Jenkins, Marcin Opallo, Frank Marken
Bioelectrochemistry 2008 Volume 72(Issue 1) pp:1-2
Publication Date(Web):February 2008
DOI:10.1016/j.bioelechem.2007.07.006
Facile demetallation occurs upon contact of the methemoglobin with a mesoporous TiO2 host in phosphate buffer media at pH 5.5 but not in acetate buffer media. As a result, voltammetric signals previously attributed to hemoglobin-based redox processes have to be re-interpreted.
Co-reporter:Liza Rassaei;Mika Sillanpää
Journal of Solid State Electrochemistry 2008 Volume 12( Issue 6) pp:747-755
Publication Date(Web):2008 June
DOI:10.1007/s10008-007-0424-z
A porous silicate is obtained from octa-anionic \({\text{Si}}_{8} {\text{O}}^{{8 - }}_{{20}} \) cage-like poly-silicate (PS) and Ru3+ cations in an ethanol-based layer-by-layer assembly process. Electrochemical experiments (voltammetry and impedance spectroscopy) confirm the formation of redox-active ruthenium centers in the form of hydrous ruthenium oxide throughout the film deposit. Oxidation of Ru(III) to Ru(IV) at a potential below 0.5 V vs saturated Calomel electrode (SCE) is reversible, but a potential positive of 0.5 V vs SCE is associated with an irreversible change in reactivity, which is characteristic for very small hydrous ruthenium oxide nanoparticles. Further voltammetric experiments are performed in aqueous phosphate buffer solutions, and the effects of number of layers, scan rate, and pH are investigated. Three aqueous redox systems are studied in contact with the PS–Ru3+ films. The reduction of cationic methylene blue adsorbed onto the negative surface of the nanocomposite silicate is shown to occur, although most of the bound methylene blue appears to be electrochemically inactive either bound to silicate or buried into small pores. The PS–Ru3+-catalyzed oxidations of hydroquinone and arsenite(III) are investigated. Scanning electron microscopy images show that a macroscopically uniform porous surface is formed after deposition of 50 layers of the PS–Ru3+ nanocomposite. However, atomic force microscopy images demonstrate that in the initial deposition stages, irregular island growth occurs. The average rate of thickness increase for PS–Ru3+ nanocomposite films is 6 nm per deposition cycle.
Co-reporter:Charles Y. Cummings;Susan J. Stott
Journal of Solid State Electrochemistry 2008 Volume 12( Issue 12) pp:1541-1548
Publication Date(Web):2008 December
DOI:10.1007/s10008-008-0508-4
The formation of variable-thickness CeO2 nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO2) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO2 films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO4, is formed during electrochemical reduction of CeO2 in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO2 is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects.
Co-reporter:Frank Marken
Journal of Solid State Electrochemistry 2008 Volume 12( Issue 10) pp:1183
Publication Date(Web):2008 October
DOI:10.1007/s10008-008-0555-x
Co-reporter:Anne Vuorema;Philip John;Marjo Keskitalo
Journal of Applied Electrochemistry 2008 Volume 38( Issue 12) pp:
Publication Date(Web):2008 December
DOI:10.1007/s10800-008-9617-0
Electrochemical determination of redox active dye species is demonstrated in indigo samples contaminated with high levels of organic and inorganic impurities. The use of a hydrodynamic electrode system based on a vibrating probe (250 Hz, 200 μm lateral amplitude) allows time-independent diffusion controlled signals to be enhanced and reliable concentration data to be obtained under steady state conditions at relatively fast scan rates up to 4 V s−1. In this work the indigo content of a complex plant-derived indigo sample (dye content typically 30%) is determined after indigo is reduced by addition of glucose in aqueous 0.2 M NaOH. The soluble leuco-indigo is measured by its oxidation response at a vibrating electrode. The vibrating electrode, which consisted of a laterally vibrating 500 μm diameter gold disc, is calibrated with Fe(CN)63−/4− in 0.1 M KCl and employed for indigo determination at 55, 65, and 75 °C in 0.2 M NaOH. Determinations of the indigo content of 25 different samples of plant-derived indigo are compared with those obtained by conventional spectrophotometry. This comparison suggests a significant improvement by the electrochemical method, which appears to be less sensitive to impurities.
Co-reporter:Mohamed A. Ghanem, Richard G. Compton, Barry A. Coles, Elefteria Psillakis, M. Anbu Kulandainathan, Frank Marken
Electrochimica Acta 2007 Volume 53(Issue 3) pp:1092-1099
Publication Date(Web):20 December 2007
DOI:10.1016/j.electacta.2007.01.065
The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN)63−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.
Co-reporter:Stuart M. MacDonald, Paul D.I. Fletcher, Zheng-Gang Cui, Marcin Opallo, Jingyuan Chen, Frank Marken
Electrochimica Acta 2007 Volume 53(Issue 3) pp:1175-1181
Publication Date(Web):20 December 2007
DOI:10.1016/j.electacta.2007.01.072
Stabilised liquid|liquid interfaces between an organic 4-(3-phenylpropyl)-pyridine (PPP) phase and an aqueous electrolyte phase are obtained in the presence of suitable nanoparticles. The use of nanoparticulate stabilisers (ca. 30 nm diameter laponite or 9–18 nm diameter carbon) in “Pickering” emulsion systems allows stable organic microdroplets to be formed and these are readily deposited onto conventional tin-doped indium oxide (ITO) electrodes. In contrast to the electrically insulating laponite nanoparticles, conducting carbon nanoparticles are shown to effectively catalyse the simultaneous electron transfer and ion transfer process at triple phase boundary junctions.Anion transfer processes between the aqueous and organic phase are driven electrochemically at the extensive triple phase junction carbon nanoparticle|4-(3-phenylpropyl)-pyridine|aqueous electrolyte. The organic phase consists of a redox active reagent 5,10,15,20-tetraphenyl-21H,23H-porphinato manganese(III) (MnTPP+), 5,10,15,20-tetraphenyl-21H,23H-porphinato iron(III) (FeTPP+), or proto-porphyrinato-IX iron(III) (hemin) dissolved in 4-(3-phenylpropyl)-pyridine (PPP). The composition of the aqueous electrolyte phase determines the reversible potential for the Nernstian anion transfer process. The methodology is shown to be versatile and, in future, could be applied more generally in liquid|liquid electroanalysis.
Co-reporter:Michael J. Bonné, Elizabeth V. Milsom, Matthew Helton, Wim Thielemans, Shelley Wilkins, Frank Marken
Electrochemistry Communications 2007 Volume 9(Issue 8) pp:1985-1990
Publication Date(Web):August 2007
DOI:10.1016/j.elecom.2007.05.010
Porous composite films containing cellulose nanofibrils (from sisal) and TiO2 nanoparticles (ca. 6 nm diameter) are obtained in a layer-by-layer assembly process. Each layer consists of ca. 0.18 μg cellulose nanofibrils and ca. 0.72 μg TiO2 (determined by QCMB) and adds a thickness of ca. 16 nm (by AFM) to the uniform deposit. The TiO2 nanophase is creating conducting pathways for electrons in a relatively open cellulose structure (ca. 82% open pores) potentially suitable for the immobilization of large redox proteins such as methemoglobin.Methemoglobin is shown to readily adsorb into the cellulose–TiO2 film. However, electrochemical responses for the immobilized methemoglobin in aqueous 0.1 M phosphate buffer at pH 5.5 suggest that facile demetallation occurs. Experiments with Fe3+ in the absence of protein result in voltammetric responses indistinguishable from those observed for immobilized methemoglobin. In the presence of ethylenediamine tetraacetic acid (EDTA) the voltammetric signals for the Fe3+ immediately disappear. Complementary experiments conducted in 0.1 M acetate buffer at pH 5.5 demonstrate that methemoglobin can indeed be immobilized in electrochemically active form and without demetallation loss of the voltammetric signal in the presence of EDTA. Demetallation appears to occur (i) in the presence of phosphate, (ii) at pH 5.5, (iii) and in the presence of a charged surface.
Co-reporter:Stuart M. MacDonald, John D. Watkins, Yunfeng Gu, Kamran Yunus, Adrian C. Fisher, Galyna Shul, Marcin Opallo, Frank Marken
Electrochemistry Communications 2007 Volume 9(Issue 8) pp:2105-2110
Publication Date(Web):August 2007
DOI:10.1016/j.elecom.2007.05.031
A microfluidic double channel device was employed to study reactions at a flowing liquid∣liquid interface in contact with a gold electrode. The rectangular flow cell was calibrated for both single phase liquid flow and biphasic liquid∣liquid flow for the case of the immiscible N-octyl-2-pyrrolidone (NOP)∣aqueous electrolyte system. The influence of flow direction and speed and liquid viscosity on the position of the phase boundary was examined.The Ru(NH3)63+/2+ redox system was employed in aqueous solution to calibrate the flow cell in the absence and in the presence of the organic NOP phase. A significant “undercutting” of the organic phase into the aqueous phase was observed in particular for shorter gold band electrodes. The triple phase boundary reaction zone was visualized with a colour reaction based on the oxidation of N-benzylaniline. An approximate expression can be given to predict the mass transport controlled limiting currents even under two-phase flow conditions. Next, n-butylferrocene in NOP (without intentionally added electrolyte) was employed as the organic redox system with 0.1 M NaClO4 as the adjacent aqueous electrolyte phase. Under these conditions the electrochemical reaction only proceeded at the organic liquid∣aqueous liquid∣solid electrode triple phase boundary reaction zone and significant currents were observed. In contrast to the processes at conventional liquid∣electrode interfaces, these currents decreased with an increasing flow rate. The level of conversion at the triple phase boundary reaction zone can be further enhanced (i) at sufficiently slow flow rates and (ii) at larger electrodes. Bulk electrosynthetic processes are feasible, but the reactor design has to be further improved.
Co-reporter:Elizabeth V. Milsom, Jan Novak, Munetaka Oyama, Frank Marken
Electrochemistry Communications 2007 Volume 9(Issue 3) pp:436-442
Publication Date(Web):March 2007
DOI:10.1016/j.elecom.2006.10.018
Structured films of TiO2 (anatase) nanoparticles (ca. 6 nm diameter) and gold nanoparticles (nominal 20 nm diameter) are formed via a layer-by-layer deposition procedure. TiO2 nanoparticles are deposited with a Nafion polyelectrolyte binder followed by calcination to give a mesoporous thin film electrode. Gold nanoparticles are incorporated into this film employing a poly(diallyldimethylammonium chloride) polyelectrolyte binder followed by calcination to give a stable mesoporous TiO2–gold nanocomposite. This methodology allows well-defined and structured films to be formed which are re-usable after a 500 °C heat treatment in air.Electrochemical experiments are performed in aqueous KCl and buffer solutions and for the oxidation of nitric oxide, NO, and nitrite in phosphate buffer solution. It is shown that the NO oxidation occurs as a highly effective electrocatalytically amplified process at the surface of the gold nanocomposite probably with co-evolution of oxygen, O2. In contrast, the oxidation of nitrite to nitrate occurs at the same potential but without oxygen evolution. A mechanistic scheme for the amplified NO detection process is proposed.
Co-reporter:Mandana Amiri, Saeed Shahrokhian, Elefteria Psillakis, Frank Marken
Analytica Chimica Acta 2007 Volume 593(Issue 1) pp:117-122
Publication Date(Web):12 June 2007
DOI:10.1016/j.aca.2007.04.042
A film composed of carbon nanoparticles and poly(diallyldimethylammonium chloride) or CNP-PDDAC is formed in a layer-by-layer deposition process at tin-doped indium oxide (ITO) substrates. Excess positive binding sites within this film in aqueous phosphate buffer at pH 9.5 are quantified by adsorption of iron(III)phthalocyanine tetrasulfonate and indigo carmine. Both anionic redox systems bind with Langmuirian characteristics (K ≈ 105 mol−1 dm3) and show electrochemical reactivity throughout the film at different thicknesses. Therefore, the electrical conductivity in CNP-PDDAC films is good and the positive binding sites are approximately 140 pmol cm−2 per layer. Structural instability of the CNP-PDDAC film in the presence of high concentrations of iron(III)phthalocyanine tetrasulfonate or indigo carmine is observed.Triclosan, a widely used anti-bacterial and anti-fungal agent, exists in aqueous media at pH 9.5 as a negatively charged chlorinated poly-aromatic phenol. Due to the negative charge, triclosan is readily accumulated into CNP-PDDAC films with an efficiency consistent with that expected for simple electrostatic interaction with the cationic binding sites. Oxidation of bound triclosan occurs at 0.6 V versus SCE in a chemically irreversible process. The CNP-PDDAC film electrode is renewed by rinsing in organic solvent and the triclosan oxidation response is shown to correlate with the triclosan concentration in solution from 0.5 to 50 μM. Applications of the CNP-PDDAC film electrode (or improved versions of it) in analysis or in anodic extraction are proposed.
Co-reporter:Michael J. Bonné, Matthew Helton, Karen Edler, Frank Marken
Electrochemistry Communications 2007 Volume 9(Issue 1) pp:42-48
Publication Date(Web):January 2007
DOI:10.1016/j.elecom.2006.08.034
Cellulose films of variable thickness are electro-deposited from aqueous alkaline thiourea solution onto polished boron-doped diamond substrates in an anodic process. Films with “net-like” topography are formed and shown to consist of both cellulose-I and cellulose-II components. Properties of these films are investigated.When immersed in aqueous electrolyte solution, ion partitioning into the electro-deposited cellulose films occurs. The accumulation and release of two aqueous redox systems, Ru(NH3)63+/2+ and methylviologen2+/+, is reported. Relatively slow diffusion of these cations is observed within cellulose (approximately 5 orders of magnitude slower when compared to diffusion in aqueous media). For the methylviologen2+/+ redox system partitioning leads to irreversibility in the voltammetric response and to the preferred formation of aggregates immobilized within the cellulose film.
Co-reporter:Veronica Saez, Jose Gonzalez-Garcia, M. Anbu Kulandainathan, Frank Marken
Electrochemistry Communications 2007 Volume 9(Issue 5) pp:1127-1133
Publication Date(Web):May 2007
DOI:10.1016/j.elecom.2007.01.018
A new methodology for the electro-deposition and stripping of highly reactive iron nanoparticles at boron-doped diamond electrodes is proposed. In aqueous 1 M NH4F iron metal readily and reversibly electro-deposits onto boron-doped diamond electrodes. The effects of deposition potential, FeF63- concentration, deposition time, and mass transport are investigated. Power ultrasound (24 kHz, 8 W cm−2) is employed to achieve enhanced mass transport conditions. Scanning electron microscopy images of iron nanoparticles grown to typically 20–30 nm diameter are obtained. It is shown that a strongly and permanently adhering film of iron at boron-doped diamond can be formed and transferred into other solution environments.The catalytic reactivity of iron nanoparticle deposits at boron-doped diamond is investigated for the reductive dehalogenation of trichloroacetate. The kinetically limited multi-electron reduction of trichloroacetate is dependent on the FeF63- deposition conditions and the solution composition. It is demonstrated that a stepwise iron catalysed dechlorination via dichloroacetate and monochloroacetate to acetate is feasible. This methodology in conjunction with power ultrasound offers a novel, clean, and very versatile electro-dehalogenation methodology. The role of fluoride in the surface electrochemistry of iron deserves further attention.
Co-reporter:Elizabeth V. Milsom, Hayley A. Dash, Toby A. Jenkins, Catherine M. Halliwell, Adam Thetford, Natalie Bligh, Wojciech Nogala, Marcin Opallo, Frank Marken
Journal of Electroanalytical Chemistry 2007 Volume 610(Issue 1) pp:28-36
Publication Date(Web):15 November 2007
DOI:10.1016/j.jelechem.2007.06.021
Thin mesoporous nanocomposite films of SnO2–poly(diallyldimethylammonium chloride) are formed in a layer-by-layer deposition process from SnO2 nanoparticles (nominal 15 nm diameter) and poly(diallyldimethylammonium chloride) binder. Atomic force imaging and electron microscopy suggest that each deposition cycle is adding ca. 10 nm of SnO2–poly(diallyldimethammonium chloride) to the film thickness. Data from cyclic voltammetry suggest that SnO2–poly(diallyldimethylammonium chloride) films are stable and n-type semiconducting but actually electrically sufficiently conducting (similar to mesoporous SnO2) over a wide range of applied potentials.The immobilization of redox proteins into the mesoporous structure is attempted for the heme proteins methemoglobin (bovine) and cytochrome P450cam (CYP101). Very similar voltammetric responses for these systems indicate denaturation in the presence of polyelectrolyte or the charged SnO2 surface. In addition, it is shown that methemoglobin and cytochrome P450cam are even demetallated to give Fe(III/II) immobilized in electrochemically active form within the SnO2–poly(diallyldimethylammonium chloride) film. Most of this Fe(III/II) can be sequestered and identified in a test reaction with ethylenediaminetetraacetate (EDTA).
Co-reporter:M. Anbu Kulandainathan, Clive Hall, Daniel Wolverson, John S. Foord, Stuart M. MacDonald, Frank Marken
Journal of Electroanalytical Chemistry 2007 Volume 606(Issue 2) pp:150-158
Publication Date(Web):15 August 2007
DOI:10.1016/j.jelechem.2007.05.018
This study highlights the effects of the electrochemical surface pre-treatment of polished polycrystalline boron-doped diamond electrodes in ethanol on electron transfer to organic redox systems. A novel “activation” procedure based on cathodic polarisation in ethanol (0.01 M NBu4PF6) is proposed and shown to be highly effective in promoting electron transfer to the aqueous Fe(CN)63-/4- redox system. For redox systems in acetonitrile, effects on electron transfer processes are strongly dependent on the type of electron transfer process. For decamethylferrocene+/0, decamethylcobaltocene+/0, methyviologen2+/+, and for methylviologen+/0 redox systems essentially reversible electron transfer is observed irrespective of the pre-treatment of the boron-doped diamond electrode. For the benzoquinone0/− redox system insignificant changes occur but for the benzoquinone−/2− process a more dramatic change in electron transfer kinetics is observed (after cathodic polarisation in ethanol) consistent with an improved interfacial electron transfer (a 4.6 × increase in peak current occurs). Finally, the tetraethyl-ethylenetetracarboxylate0/− system is investigated as a model olefin redox system. Again changes in reactivity occur (a 4.0 × increase in peak current) at the cathodically pre-treated boron-doped diamond electrode. XPS surface analysis data reveal only insignificant changes in the chemical composition of the boron-doped diamond surface before and after activation and therefore a predominantly electronic mechanism for the “activation” process is proposed. In future selectivity effects for electron transfer at boron-doped diamond electrode surfaces could be introduced intentionally and used beneficially for chemo-selective electro-organic processes.
Co-reporter:Michael J. Bonné, James E. Taylor, Alan K. Carver, Matthew Helton, Frank Marken
Journal of Electroanalytical Chemistry 2007 Volume 601(1–2) pp:211-219
Publication Date(Web):15 March 2007
DOI:10.1016/j.jelechem.2006.11.017
Cotton is an excellent cellulose-based absorbent mainly due to amorphous cellulose regions in the nanocrystalline (cellulose-I) natural fiber structure. It is shown here for the case of a woven cotton textile with ca. 20 μm fiber diameter and ca. 270 μm thickness that cation and anion absorption and diffusion processes occur within cotton fibers. An electrochemical approach based on voltammetry is developed to investigate and quantify absorption and leaching processes as well as the chemical reactivity within cotton fibers.The metal complexes Fe(CN)63-, Ru(NH3)63+, [Fe(II)(N,N-bis(pyridin-2-yl-methyl)-1,1-bis(pyridin-2-yl)-1-aminoethane)Cl]+ or Fe(BBA)Cl+, and [Fe(III)(protoporphyrinato-IX)(H2O)(OH)]2− or hemin are readily absorbed into cotton in the electrochemically active form. The ability of Fe(CN)63- and Ru(NH3)63+ to absorb appears to be predominantly due to “entrapment” without specific interaction with the amorphous cellulose. The approximate rate of diffusion of these ions in cellulose is estimated to be a factor 4 slower when compared to the rate of diffusion in solution. In contrast, Fe(BBA)Cl+ and hemin diffuse much slower (by a factor 24 and 70, respectively) within the cotton matrix. The slower diffusion is correlated with “specific” binding of the metal complexes to sites within the amorphous cellulose. In addition, Fe(BBA)Cl+ exhibits a new redox process within the cotton matrix indicating a change in chemical reactivity when compared to that observed in aqueous solution environments.
Co-reporter:Mohamed A. Ghanem, Harriet Hanson, Richard G. Compton, Barry A. Coles, Frank Marken
Talanta 2007 Volume 72(Issue 1) pp:66-71
Publication Date(Web):15 April 2007
DOI:10.1016/j.talanta.2006.09.025
In situ microwave activation has been applied to the electro-deposition and stripping of palladium metal (which is widely used as a catalyst) at cavitation resistant boron-doped diamond electrodes. Focused microwave radiation leading to heating, boiling, and cavitation is explored as an option to improve the speed and sensitivity of the analytical detection procedure. The deposition and anodic stripping of palladium by linear sweep voltammetry in 0.1 M KCl (pH 2) solution and at boron-doped diamond electrodes is shown to be strongly enhanced by microwave activation due to both (i) the increase in mass transport and (ii) the increase in the kinetic rate of deposition and stripping.The temperature at the electrode surface is calibrated with the reversible redox couple Fe(CN)64−/Fe(CN)63− and found to be reach 380 K. In the presence of microwave radiation, the potential of onset of the deposition of palladium is strongly shifted positive from −0.4 to +0.1 V versus SCE. The optimum potential for deposition in the presence of microwaves is −0.4 V versus SCE and the anodic stripping peak current is shown to increase linearly with deposition time. Under these conditions, the stripping peak current varies linearly with the palladium concentration down to ca. 2 μM. At concentration lower than this a logarithmic variation of the stripping peak current with concentration is observed down to ca. 0.1 μM (for 5 min pre-concentration in presence of microwave radiation).
Co-reporter:Elizabeth V. Milsom;Jan Novak;Stephen J. Green
Journal of Solid State Electrochemistry 2007 Volume 11( Issue 8) pp:
Publication Date(Web):2007 August
DOI:10.1007/s10008-006-0247-3
The formation of variable thickness TiO2 nanoparticle-Nafion® composite films with open pores is demonstrated via a layer-by-layer deposition process. Films of about 6 nm diameter TiO2 nanoparticles grow in the presence of Nafion® by “clustering” of nanoparticles into bigger aggregates, and the resulting hierarchical structure thickens with about 25 nm per deposition cycle. Film growth is characterized by electron microscopy, atomic force microscopy, and quartz crystal microbalance techniques. Simultaneous small-angle X-ray scattering and wide-angle X-ray scattering measurements for films before and after calcination demonstrate the effect of Nafion® binder causing aggregation. Electrochemical methods are employed to characterize the electrical conductivity and diffusivity of charge through the TiO2-Nafion® composite films. Characteristic electrochemical responses are observed for cationic redox systems (diheptylviologen2+/+, \({\text{Ru}}{\left( {{\text{NH}}_{3} } \right)}^{{3 + /2 + }}_{6} \), and ferrocenylmethyl-trimethylammonium2+/+) immobilized into the TiO2-Nafion® nanocomposite material. Charge conduction is dependent on the type of redox system and is proposed to occur either via direct conduction through the TiO2 backbone (at sufficiently negative potentials) or via redox-center-based diffusion/electron hopping (at more positive potentials).
Co-reporter:Frank Marken, Ujjal Kumar Sur, Barry A. Coles, Richard G. Compton
Electrochimica Acta 2006 Volume 51(Issue 11) pp:2195-2203
Publication Date(Web):15 February 2006
DOI:10.1016/j.electacta.2005.04.081
The effects of high intensity microwave radiation in electrochemistry are summarized and discussed. In situ microwave activation of electrochemical processes has been introduced recently and is possible by placing a carefully designed electrochemical cell directly into a microwave cavity. Self-focusing of intense microwave radiation occurs into a region close to the electrode | solution (electrolyte) interface of a microelectrode placed into the electrochemical cell. The electrode diameter and the electrode material strongly affect the observed mass transport enhancement and temperature effects. Experiments have been conducted to determine the temperature at the electrode surface electrochemically and to quantify the rate of chemical processes which occur in the vicinity of the electrode under high intensity microwave conditions. The effects of microwaves in a wide range of solvent systems from aqueous solutions to organic solvents (DMSO, acetonitrile, DMF, formamide) and in an ionic liquid (BMIM+PF6−) have been investigated. Considerable current and temperature enhancements are observed in all solvents and are explained based on the interaction of microwaves with the liquid (electrolyte) and the physical properties of the liquids or solutions.
Co-reporter:Susan J. Stott, Roger J. Mortimer, Sandie E. Dann, Munetaka Oyama and Frank Marken
Physical Chemistry Chemical Physics 2006 vol. 8(Issue 46) pp:5437-5443
Publication Date(Web):26 Oct 2006
DOI:10.1039/B610391J
Titanium carbide (TiC) nanoparticles are readily deposited onto tin-doped indium oxide (ITO) electrodes in the form of thin porous films. The nanoparticle deposits are electrically highly conducting and electrochemically active. In aqueous media (at pH 7) and at applied potentials positive of 0.3 V vs. SCE partial anodic surface oxidation and formation (at least in part) of novel core-shell TiC–TiO2 nanoparticles is observed. Significant thermal oxidation of TiC nanoparticles by heating in air occurs at a temperature of 250 °C and leads first to core-shell TiC–TiO2 nanoparticles, next at ca. 350 °C to TiO2 (anatase), and finally at temperatures higher than 750 °C to TiO2 (rutile). Electrochemically and thermally partially oxidized TiC nanoparticles still remain very active and for some redox systems electrocatalytically active. Scanning and transmission electron microscopy (SEM and TEM), temperature dependent XRD, quartz crystal microbalance, and voltammetric measurements are reported. The electrocatalytic properties of the core-shell TiC–TiO2 nanoparticulate films are surveyed for the oxidation of hydroquinone, ascorbic acid, and dopamine in aqueous buffer media. In TiC–TiO2 core-shell nanoparticle films TiO2 surface reactivity can be combined with TiC conductivity.
Co-reporter:Sascha Förster, Frank-Michael Matysik, Mohamed A. Ghanem and Frank Marken
Analyst 2006 vol. 131(Issue 11) pp:1210-1212
Publication Date(Web):08 Sep 2006
DOI:10.1039/B608974G
A new experimental development concerning microwave-assisted electrochemical detection in conjunction with capillary electrophoresis is presented. Focused microwaves are readily incorporated into end-column detection systems for capillary electrophoresis, they induce strong localised thermal activation at microelectrodes, and they affect and modulate, in particular, signals for chemically irreversible redox processes.
Co-reporter:Michael J. Bonné, Christopher Reynolds, Stuart Yates, Galyna Shul, Joanna Niedziolka, Marcin Opallo and Frank Marken
New Journal of Chemistry 2006 vol. 30(Issue 3) pp:327-334
Publication Date(Web):06 Feb 2006
DOI:10.1039/B514348A
The transfer of ions between an aqueous and an organic phase is driven electrochemically at a triple phase junction graphite | 4-(3-phenylpropyl)pyridine | aqueous electrolyte. Tetraphenylporphyrinato (TPP) metal complexes (MnTPP+, FeTPP+, CoTPP) and hemin readily dissolve in the organic 4-(3-phenylpropyl)pyridine phase and undergo oxidation/reduction processes which are coupled to liquid | liquid ion transfer. In order to maintain charge neutrality, each one-electron oxidation (reduction) process is coupled to the transfer of one anion (here PF6−, ClO4−, SCN−, NO3−, OCN−, or CN−) from the aqueous (organic) into the organic (aqueous) phase. The range of anions studied allows effects of hydrophobicity and complex formation (association of the anion to the metal center) to be explored. A preliminary kinetic scheme is developed to quantify complex formation, facilitated anion transfer, and competing cation transfer processes. The effects of the organic solvent on the ion transfer processes are explored. Very strong binding and specific effects are observed for the reversible cyanide transfer process.
Co-reporter:D.V. Bavykin, E.V. Milsom, F. Marken, D.H. Kim, D.H. Marsh, D.J. Riley, F.C. Walsh, K.H. El-Abiary, A.A. Lapkin
Electrochemistry Communications 2005 Volume 7(Issue 10) pp:1050-1058
Publication Date(Web):October 2005
DOI:10.1016/j.elecom.2005.07.010
TiO2 nanotubes (8–20 nm outer diameter and 3–5 nm inner diameter) grown via alkaline hydrothermal synthesis are characterised and compared to 6 nm diameter TiO2 (anatase) nanoparticles. Zeta potential, voltammetric, and titration experiments reveal that, in contrast to anatase nanoparticles (p.z.c. ca. 6), TiO2 nanotubes carry a stronger negative surface charge (p.z.c. ca. 3, acidic protons ca. 2 × 10−3 mol g−1, electrostatic cation adsorption sites in neutral solution ca. 7 × 10−5 mol g−1) and, under neutral conditions, offer electrostatic binding sites for cations.When immobilised onto an inert boron-doped diamond substrate, TiO2 nanotubes show electrochemical reactivity due to reversible Ti(IV) reduction, which is very similar to that observed for anatase nanoparticles. Three cationic redox systems, Meldola’s blue, Ni2+, and cytochrome c, are immobilised on the TiO2 nanotube surface; the binding ability and the number of binding sites are quantified voltammetrically. Redox proteins, such as cytochrome c, adsorb readily and irreversibly. Well-defined voltammetric signals for the immobilised protein are observed in an aqueous buffer. TiO2 nanotubes are shown to be novel, inert substrates for both inorganic and biological electrocatalysts.
Co-reporter:Mohamed A. Ghanem, Richard G. Compton, Barry A. Coles, Antonio Canals and Frank Marken
Analyst 2005 vol. 130(Issue 10) pp:1425-1431
Publication Date(Web):26 Aug 2005
DOI:10.1039/B507167D
The direct electroanalysis of complex formulations containing α-tocopherol (vitamin E) is possible in micellar solution and employing microwave-enhanced voltammetry. In the presence of microwave radiation substantial heating and current enhancement effects have been observed at 330 µm diameter glassy carbon electrodes placed into a micellar aqueous solution and both hydrophilic and highly hydrophobic redox systems are detected. For the water soluble Fe(CN)63−/4− redox system in micellar aqueous solutions of 0.1 M NaCl and 0.1 M sodium dodecylsulfate (SDS) at low to intermediate microwave power, thermal effects and convection effects are observed. At higher microwave power, thermal cavitation is induced and dominates the mass transport at the electrode surface. For the micelle-soluble redox systems tert-butylferrocene and 2,5-di-tert-butyl-1,4-benzoquinone, strong and concentration dependent current responses are observed only in the presence of microwave radiation. For the oxidation of micelle-soluble α-tocopherol current responses at glassy carbon electrodes are affected by adsorption and desorption processes whereas at platinum electrodes, analytical limiting currents are obtained over a wide range of α-tocopherol concentrations. However, for the determination of α-tocopherol in a commercial formulation interference from proteins is observed at platinum electrodes and direct measurements are possible only over a limited concentration range and at glassy carbon electrodes.
Co-reporter:Mohamed A. Ghanem, Richard G. Compton, Barry A. Coles, Antonio Canals, Anne Vuorema, Philip John and Frank Marken
Physical Chemistry Chemical Physics 2005 vol. 7(Issue 20) pp:3552-3559
Publication Date(Web):30 Aug 2005
DOI:10.1039/B509784C
The oxidation of glucose is a complex process usually requiring catalytically active electrode surfaces or enzyme-modified electrodes. In this study the effect of high intensity microwave radiation on the oxidation of glucose in alkaline solution at Au, Cu, and Ni electrodes is reported. Calibration experiments with the Fe(CN)63−/4− redox system in aqueous 0.1 M NaOH indicate that strong thermal effects occur at both 50 and 500 μm diameter electrodes with temperatures reaching 380 K. Extreme mass transport effects with mass transport coefficients of kmt > 0.01 m s−1
(or kmt > 1.0 cm s−1) are observed at 50 μm diameter electrodes in the presence of microwaves. The electrocatalytic oxidation of glucose at 500 μm diameter Au, Cu, or Ni electrodes immersed in 0.1 M NaOH and in the presence of microwave radiation is shown to be dominated by kinetic control. The magnitude of glucose oxidation currents at Cu electrodes is shown to depend on the thickness of a pre-formed oxide layer. At 50 μm diameter Au, Cu, or Ni electrodes microwave enhanced current densities are generally higher, but only at Au electrodes is a significantly increased rate for the electrocatalytic oxidation of glucose to gluconolactone observed. This rate enhancement appears to be independent of temperature but microwave intensity dependent, and therefore non-thermal in nature. Voltammetric currents observed at Ni electrodes in the presence of microwaves show the best correlation with glucose concentration and are therefore analytically most useful.
Co-reporter:Norahim Bin Ibrahim, Katherine Lawrence, Tony D. James, Fengjie Xia, Mu Pan, Shichun Mu, John M. Mitchels, Frank Marken
Sensors and Actuators B: Chemical (3 January 2012) Volume 161(Issue 1) pp:184-190
Publication Date(Web):3 January 2012
DOI:10.1016/j.snb.2011.10.009
Carbon nanoparticles of ca. 9–18 nm diameter (Emperor 2000™) are surface-modified by covalently linking l-dopa-boc (boc-protected l-3,4-dihydroxyphenylalanine) with a surface coverage of approximately 100 per particle (or 3 × 1013 cm−2). In solution environments these redox-active nanoparticles provide chemically stable and pH-sensitive voltammetirc responses (reversible 2-electron 2-proton oxidation) over a pH range from 2 to 12.When mixed into Dowex 50 Wx4 cation exchanger or Dowex 50 1x2 anion exchanger and placed in contact with a glassy carbon electrode in a flow of humidified gas, the l-dopa-boc-modified carbon nanoparticles provide pH-sensitive surface probes to monitor the surface conditions. In a two-terminal cell it is demonstrated that gas flow measurements are possible with both modified cation and anion exchanger particles in contact to glassy carbon electrodes. The anion exchanger particles allow pH control after pre-conditioning in phosphate buffer. Loading-dependent sensitivity to ammonia gas is investigated and high sensitivity to ammonia is observed for Dowex 50 1x2 anion exchanger pre-equilibrated in phosphate buffer pH 3 and decorated with l-dopa-boc-modified carbon nanoparticles. Responses are observed with sequential injections of 1 cm3 ammmonia into a gas flow-through device.
Co-reporter:Elena Madrid, Mark A. Buckingham, James M. Stone, Adrian T. Rogers, William J. Gee, Andrew D. Burrows, Paul R. Raithby, Veronica Celorrio, David J. Fermin and Frank Marken
Chemical Communications 2016 - vol. 52(Issue 13) pp:NaN2794-2794
Publication Date(Web):2016/01/11
DOI:10.1039/C5CC09780K
Ionic transport (for applications in nanofluidics or membranes) and “ionic diode” phenomena in a zeolitic imidazolate framework (ZIF-8) are investigated by directly growing the framework from aqueous Zn2+ and 2-methylimidazole as an “asymmetric plug” into a 20 μm diameter pore in a ca. 6 μm thin poly-ethylene-terephthalate (PET) film.
Co-reporter:Liza Rassaei, Michaela Nebel, Neil V. Rees, Richard G. Compton, Wolfgang Schuhmann and Frank Marken
Chemical Communications 2010 - vol. 46(Issue 5) pp:NaN814-814
Publication Date(Web):2009/11/25
DOI:10.1039/B920154H
Microwave induced activation of electrochemical processes at microelectrodes (ca. 0.8 µm diameter) immersed in aqueous electrolyte media is shown to be driven by (i) continuous stable cavitation (giving rise to Faradaic current enhancements by up to three orders of magnitude) and (ii) transient discharge cavitation on the µs timescale (giving rise to cathodic plasma current spikes and more violent surface erosion effects).
Co-reporter:Meng Li, Weihong Zhu, Frank Marken and Tony D. James
Chemical Communications 2015 - vol. 51(Issue 78) pp:NaN14573-14573
Publication Date(Web):2015/08/28
DOI:10.1039/C5CC04976H
Boronic acids can bind with 1,2- or 1,3-diols to form five or six-membered cyclic complexes and also can interact with Lewis bases to generate boronate anions. Therefore, boronic acid functionalised compounds and materials are highly topical and now employed in (i) functional materials, (ii) for attaching/sensing bio-molecules and proteins, and (iii) for microbial electrochemistry as well as being widely developed as chemical sensors and tools in health diagnostics. In this review, we address the recent progress of boronic acid-based electrochemical sensors both in solution processes and surface processes for the detection of biological analytes. This feature article will be of interest to chemists, chemical engineers, biochemists, the sensor community, but also researchers working with protein and microbial systems.
Co-reporter:Michael J. Bonné, Ewan Galbraith, Tony D. James, Matthew J. Wasbrough, Karen J. Edler, A. Toby A. Jenkins, Matthew Helton, Anthony McKee, Wim Thielemans, Elefteria Psillakis and Frank Marken
Journal of Materials Chemistry A 2010 - vol. 20(Issue 3) pp:NaN594-594
Publication Date(Web):2009/11/13
DOI:10.1039/B918308F
Cellulose nanofibrils from sisal of typically 4–5 nm diameter and ca. 250 ± 100 nm length are reconstituted into thin films of ca. 6 µm thickness (or thicker freestanding films). Pure cellulose and cellulose composite films are obtained in a solvent evaporation process. A boronic acid appended dendrimer is embedded as a receptor in the nanofibrillar cellulose membrane. The number of boronic acid binding sites is controlled by varying the dendrimer content. The electrochemical and spectrophotometric properties of the nanocomposite membrane are investigated using the probe molecule alizarin red S. Pure cellulose membranes inhibit access to the electrode. However, the presence of boronic acid receptor sites allows accumulation of alizarin red S with a Langmuirian binding constant of ca. 6000 ± 1000 M−1. The 2-electron 2-proton reduction of immobilized alizarin red S is shown to occur in a ca. 60 nm zone close to the electrode surface. With a boronic acid dendrimer modified nanofibrillar cellulose composition of 96 wt% cellulose and 4 wt% boronic acid dendrimer, the analytical range for alizarin red S in aqueous acetate buffer pH 3 is approximately 10 µM to 1 mM.
Co-reporter:Uday Pratap Azad, Dharmendra Kumar Yadav, Vellaichamy Ganesan and Frank Marken
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 33) pp:NaN23373-23373
Publication Date(Web):2016/07/29
DOI:10.1039/C6CP04758K
Four polypyridyl redox catalysts Fe(bp)32+, Fe(ph)32+, Fe(dm)32+, and Fe(tm)32+ (with bp, ph, dm, and tm representing 2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, and 3,4,7,8-tetramethyl-1,10-phenanthroline, respectively) are investigated for the electrocatalytic oxidation of three analytes (nitrite, arsenite, and isoniazid). The poly-pyridyl iron complex is exchanged into a Nafion film immobilized on a glassy carbon electrode, which is then immersed in 0.1 M Na2SO4. Cyclic voltammetry is employed for the evaluation of the mechanism and estimation of kinetic parameters. The electrocatalytic behaviour going from low to high substrate concentration is consistent with the Albery–Hillman cases of “LEty” switching to “LEk” (changing from the first order in the substrate to half order in the substrate), denoting a process that occurs in a reaction zone close to the electrode surface with diffusion of charge (from the electrode surface into the film) and of anionic or neutral analyte (from the Nafion–solution interface into the film). The relative hydrophobicity of the iron polypyridyl catalyst within the film is shown to affect both the diffusion of charge/electrons and analyte within the film with Fe(tm)32+ providing the mildest catalyst. All three analytes, nitrite, isoniazid, and arsenite, exhibit linear calibration ranges beneficial for analytical applications in the micro-molar to the milli-molar range.
Co-reporter:Hakikulla H. Shah, Rayya A. Al-Balushi, Mohammed K. Al-Suti, Muhammad S. Khan, Frank Marken, Anna L. Sudlow, Gabriele Kociok-Köhn, Christopher H. Woodall, Paul R. Raithby and Kieran C. Molloy
Dalton Transactions 2014 - vol. 43(Issue 25) pp:NaN9507-9507
Publication Date(Web):2014/04/30
DOI:10.1039/C3DT52914B
Three new neutral di-ferrocenyl-ethynylpyridinyl copper complexes, [L2(CuCl)2(PPh3)2] (2), [L2(CuBr)2(PPh3)2] (3), and [L2(CuI)2(PPh3)2] (4) were synthesized from the ferrocenyl-ethynylpyridine ligand (L) (1), the appropriate copper halide CuX (with X = Cl−, Br−, I−) and triphenylphosphine. These neutral complexes were fully characterized by spectroscopic methods and by single crystal X-ray crystallography. Cyclic voltammetry in dichloroethane revealed chemically reversible ferrocenyl oxidation signals followed by characteristic “stripping reduction peaks” showing evidence for oxidation-product electro-crystallization. Scanning electron microscopy confirmed spontaneous formation of crystalline oxidation products with three distinct morphologies for X = Cl−, Br−, I−. Energy dispersive X-ray elemental analysis data show Fe:P ratios of 1:2.0, 1:2.1 and 1:2.1 for electro-crystallization products of complexes 2, 3, and 4, respectively, indicating the presence of two [PF6]− anions in the vicinity of the dioxidized complexes, and suggesting product formulae [2]2+[PF6]−2, [3]2+[PF6]−2 and [4]2+[PF6]−2.
Co-reporter:Meng Li, Zhiqian Guo, Weihong Zhu, Frank Marken and Tony D. James
Chemical Communications 2015 - vol. 51(Issue 7) pp:NaN1296-1296
Publication Date(Web):2014/11/25
DOI:10.1039/C4CC07891H
A novel electrochemically and fluorescence active boronic ester sensor molecule has been developed containing ferrocene and naphthalimide as the redox and fluorophore units. The combinations of iron (Fe3+) ions, sodium L-ascorbate, and fluoride (F−) ions can be used to produce a molecular system displaying INHIBIT logic, due to indirect fluorescence quenching.
Co-reporter:Yan-Jun Huang, Yun-Bao Jiang, John S. Fossey, Tony D. James and Frank Marken
Journal of Materials Chemistry A 2010 - vol. 20(Issue 38) pp:NaN8310-8310
Publication Date(Web):2010/08/20
DOI:10.1039/C0JM01510E
The highly water insoluble N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate is synthesised and investigated for sensor applications. This amphiphilic molecule is immobilised by evaporation of an acetonitrile solution at a basal plane pyrolytic graphite (HOPG) electrode surface and is shown to provide a monolayer film. By varying the amount of deposit partial or full coverage can be achieved. The N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer acts as an active receptor for 1,2-dihydroxy-benzene (catechol) derivatives in aqueous media. The ability to bind alizarin red S is investigated and the Langmuirian binding constant determined as a function of pH. It is shown that the immobilised boronic acid monolayer acts as sensor film for a wider range of catechols. A comparison of Langmuirian binding constants for alizarin red S (1.4 × 105 mol−1 dm3), catechol (8.4 × 104 mol−1 dm3), caffeic acid (7.5 × 104 mol−1 dm3), dopamine (1.0 × 104 mol−1 dm3), and L-dopa (8 × 103 mol−1 dm3) reveals that a combination of hydrophobicity and electrostatic interaction causes considerable selectivity effects.
Co-reporter:Katherine Lawrence, Geoffrey W. Nelson, John S. Foord, Mónica Felipe-Sotelo, Nick D. M. Evans, John M. Mitchels, Tony D. James, Fengjie Xia and Frank Marken
Journal of Materials Chemistry A 2013 - vol. 1(Issue 14) pp:NaN4564-4564
Publication Date(Web):2013/02/12
DOI:10.1039/C3TA10198C
Negatively charged carbon nanoparticles (surface-phenylsulfonated) are “wrapped” in a poly(4-vinylpyridine) cationomer and hydrothermally converted into a pH-responsive core–shell nano-composite. With a “thin shell” this nano-material (ca. 20–40 nm diameter) is water-insoluble but readily dispersed into ethanol and deposited onto electrodes. Zeta-potential measurements suggest a point of zero charge (PZC) at ca. pH 4.5 with negative functional groups dominating in the more alkaline range and positive functional groups dominating in the acidic range. XPS data suggest carboxylate and pyridinium-like functional groups. This is further confirmed in voltammetric measurements for adsorbed cations (methylene blue) and adsorbed anions (indigo carmine). The specific capacitance reaches a maximum of 13 F g−1 at the PZC explained here tentatively by a “shell charging” effect within the nanoparticle shell.
Co-reporter:Jonathan E. Halls, Alberto Hernán-Gómez, Andrew D. Burrows and Frank Marken
Dalton Transactions 2012 - vol. 41(Issue 5) pp:NaN1480-1480
Publication Date(Web):2011/07/26
DOI:10.1039/C1DT10734H
Metal–organic framework (MOF) materials based on zinc(II) and aluminium(III) dicarboxylate frameworks with covalently attached ferrocene functional redox groups were synthesised by post-synthetic modification and investigated by voltammetry in aqueous and non-aqueous media. In the voltammetry experiments, ferrocene oxidation occurs in all cases, but chemically reversible and stable ferrocene oxidation without decay of the voltammetric response requires a “mild” dichloroethane solvent environment. The voltammetric response in this case is identified as “surface-confined” with fast surface-hopping of electrons and without affecting the bulk of MOF microcrystals. In aqueous media a more complex pH-dependent multi-stage redox process is observed associated with chemically irreversible bulk oxidation and disintegration of the MOF framework. A characteristic 30 mV per pH unit dependence of redox potentials is observed attributed to a “framework effect”: the hydroxide-driven MOF framework dissolution.
Co-reporter:Frank Marken, John D. Watkins and Andrew M. Collins
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 21) pp:NaN10047-10047
Publication Date(Web):2011/04/13
DOI:10.1039/C1CP20375D
Ion transfer at liquid|liquid junctions is one of the most fundamental processes in nature. It occurs coupled to simultaneous electron transfer at the line junction (or triple phase boundary) formed by the two liquids in contact to an electrode surface. The triple phase boundary can be assembled from a redox active microdroplet deposit of a water-immiscible liquid on a suitable electrode surface immersed into aqueous electrolyte. Ion transfer voltammetry measurements at this type of electrode allow both thermodynamic and kinetic parameters for coupled ion and electron transfer processes to be obtained. This overview summarises some recent advances in understanding and application of triple phase boundary redox processes at organic liquid|aqueous electrolyte|working electrode junctions. The design of novel types of electrodes is considered based on (i) extended triple phase boundaries, (ii) porous membrane processes, (iii) hydrodynamic effects, and (iv) generator–collector triple phase boundary systems. Novel facilitated ion transfer processes and photo-electrochemical processes at triple phase boundary electrodes are proposed. Potential future applications of triple phase boundary redox systems in electrosynthesis, sensing, and light energy harvesting are indicated.
Co-reporter:Grace E. M. Lewis, Sara E. C. Dale, Barbara Kasprzyk-Hordern, Anneke T. Lubben, Edward O. Barnes, Richard G. Compton and Frank Marken
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 35) pp:NaN18973-18973
Publication Date(Web):2014/08/05
DOI:10.1039/C4CP02943G
Two types of generator–collector electrode systems, (i) a gold–gold interdigitated microband array and (ii) a gold–gold dual-plate microtrench, are compared for nitrobenzene electroanalysis in aerated aqueous 0.1 M NaOH. The complexity of the nitrobenzene reduction in conjunction with the presence of ambient levels of oxygen in the analysis solution provide a challenging problem in which feedback-amplified generator–collector steady state currents provide the analytical signal. In contrast to the more openly accessible geometry of the interdigitated array electrode, where the voltammetric response for nitrobenzene is less well-defined and signals drift, the voltammetric response for the cavity-like microtrench electrode is stable and readily detectable at 1 μM level. Both types of electrode show oxygen-enhanced low concentration collector current responses due to additional feedback via reaction intermediates. The observations are rationalised in terms of a “cavity transport coefficient” which is beneficial in the dual-plate microtrench, where oxygen interference effects are suppressed and the analytical signal is amplified and stabilised.
Co-reporter:Christopher E. Hotchen, Ian J. Maybury, Geoffrey W. Nelson, John S. Foord, Philip Holdway and Frank Marken
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 17) pp:NaN11268-11268
Publication Date(Web):2015/04/02
DOI:10.1039/C5CP01244A
“Amplified” electron transfer is observed purely based on electron transfer kinetic effects at modified carbon surfaces. An anodic attachment methodology is employed to modify the surface of glassy carbon or boron doped diamond electrodes with poly-ethylene glycols (PEGs) for polymerisation degrees of n = 4.5 to 9.1 (PEG200 to PEG400). Voltammetry and impedance data for aqueous Fe(CN)63−/4− suggest systematic PEG structure-dependent effects on the standard rate constant for heterogeneous electron transfer as a function of PEG deposition conditions and average polymer chain length. Tunnel distance coefficients are polymerisation degree dependent and estimated for shorter PEG chains, β = 0.17 Å−1 for aqueous Fe(CN)63−/4−, consistent with a diffuse water–PEG interface. In contrast, electron transfer to 1,1′-ferrocene-dimethanol (at 1 mM concentration) appears un-impeded by PEG grafts. Mediated or “amplified” electron transfer to Fe(CN)63−/4− based on the 1,1′-ferrocene-dimethanol redox shuttle is observed for both oxidation and reduction with estimated bimolecular rate constants for homogeneous electron transfer of kforward = 4 × 105 mol dm3 s−1 and kbackward = 1 × 105 mol dm3 s−1. Digital simulation analysis suggests an additional resistive component within the PEG graft double layer.
Co-reporter:Sara Shariki, Owen T. L. Cox, David A. Tickell, Marta P. Pereira Morais, Jean M. H. van den Elsen, Tony D. James, Sara E. C. Dale, Simon Bending and Frank Marken
Journal of Materials Chemistry A 2012 - vol. 22(Issue 36) pp:NaN19006-19006
Publication Date(Web):2012/08/09
DOI:10.1039/C2JM31089A
An electrolyte–stable polymer composite sensor film based on 2.4% boronic acid substituted polyacrylamide (poly[acrylamide-co-3-(methacrylamido)-phenylboronic acid] or P(A-MPBA)) and polydiallyl-dimethyl-ammonium chloride (or PDDACl) is obtained in a coil-by-coil deposition procedure. Based on AFM evidence, the resulting coil aggregate deposits show ca. 5–10 nm diameter and the average film thickness increases with ca. 1 nm per deposition cycle consistent approximately with the solution coil diameter. In electrochemical experiments alizarin red S is employed as a boronic acid indicating redox probe with a Langmuirian binding constant of ca. K = 4 × 106 M−1 and two distinct redox responses tentatively assigned here to two isomeric forms of the complex in oxidized and reduced state. Inter- and intra-coil electron transfer and electron hopping occurs during alizarin red S reduction and back-oxidation. Both smooth and mesoporous ITO substrates are employed.
Co-reporter:Anne Vuorema, Philip John, Marjo Keskitalo, Mary F. Mahon, M. Anbu Kulandainathan and Frank Marken
Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 11) pp:NaN1824-1824
Publication Date(Web):2009/01/14
DOI:10.1039/B814149E
Anthraquinone immobilised onto the surface of indigo microcrystals enhances the reductive dissolution of indigo to leuco-indigo. Indigo reduction is driven by glucose in aqueous NaOH and a vibrating gold disc electrode is employed to monitor the increasing leuco-indigo concentration with time. Anthraquinone introduces a strong catalytic effect which is explained by invoking a molecular “wedge effect” during co-intercalation of Na+ and anthraquinone into the layered indigo crystal structure. The glucose-driven indigo reduction, which is ineffective in 0.1 M NaOH at 65 °C, becomes facile and goes to completion in the presence of anthraquinone catalyst. Electron microscopy of indigo crystals before and after reductive dissolution confirms a delamination mechanism initiated at the edges of the plate-like indigo crystals. Catalysis occurs when the anthraquinone–indigo mixture reaches a molar ratio of 1:400 (at 65 °C; corresponding to 3 μM anthraquinone) with excess of anthraquinone having virtually no effect. A strong temperature effect (with a composite EA≈ 120 kJ mol−1) is observed for the reductive dissolution in the presence of anthraquinone. The molar ratio and temperature effects are both consistent with the heterogeneous nature of the anthraquinone catalysis in the aqueous reaction mixture.
Co-reporter:John D. Watkins, Ruth Lawrence, James E. Taylor, Steven D. Bull, Geoffrey W. Nelson, John S. Foord, Daniel Wolverson, Liza Rassaei, Nick D. M. Evans, Silvia Antón Gascon and Frank Marken
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 18) pp:NaN4878-4878
Publication Date(Web):2010/03/17
DOI:10.1039/B927434K
The surface functionalities of commercial sulfonate-modified carbon nanoparticles (ca. 9–18 nm diameter, Emperor 2000) have been converted from negatively charged to positively charged via sulfonylchloride formation followed by reaction with amines to give suphonamides. With ethylenediamine, the resulting positively charged carbon nanoparticles exhibit water solubility (in the absence of added electrolyte), a positive zeta-potential, and the ability to assemble into insoluble porous carbon films via layer-by-layer deposition employing alternating positive and negative carbon nanoparticles. Sulfonamide-functionalised carbon nanoparticles are characterised by Raman, AFM, XPS, and voltammetric methods. Stable thin film deposits are formed on 3 mm diameter glassy carbon electrodes and cyclic voltammetry is used to characterise capacitive background currents and the adsorption of the negatively charged redox probe indigo carmine. The Langmuirian binding constant K = 4000 mol−1dm3 is estimated and the number of positively charged binding sites per particle determined as a function of pH.
Co-reporter:Elena Madrid, Philip Cottis, Yuanyang Rong, Adrian T. Rogers, James M. Stone, Richard Malpass-Evans, Mariolino Carta, Neil B. McKeown and Frank Marken
Journal of Materials Chemistry A 2015 - vol. 3(Issue 31) pp:NaN15853-15853
Publication Date(Web):2015/07/15
DOI:10.1039/C5TA04092B
We describe ion current rectification using a Polymer of Intrinsic Microporosity (PIM) based on Tröger's base (PIM-EA-TB). When deposited asymmetrically over one (or more) 20 μm diameter hole(s) in 6 μm thick poly(ethylene terephthalate) and investigated in a two-compartment electrochemical cell with acidified aqueous NaCl on both sides, novel ionic diode effects are observed.
Co-reporter:Anne Vuorema, Sara Shariki, Mika Sillanpää, Wim Thielemans, Geoffrey W. Nelson, John S. Foord, Sara E. C. Dale, Simon Bending and Frank Marken
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 20) pp:NaN9862-9862
Publication Date(Web):2011/04/18
DOI:10.1039/C1CP20532C
In this report we demonstrate a versatile (and potentially low-cost) cellulose nano-whisker-based surface carbonisation method that allows well-defined films of TiO2 nanoparticles surface-modified with carbon to be obtained. In a layer-by-layer electrostatic deposition process based on TiO2 nanoparticles, cellulose nano-whiskers, and poly(diallyl-dimethylammonium) or PDDA are employed to control the ratio of surface carbon to TiO2. Characterisation based on optical, AFM, XRD, and XPS methods is reported. Electrochemical measurements suggest improved access to surface states, dopamine binding at the anatase surface, and surface redox cycling aided by the thin amorphous carbon film in mesoporous TiO2. In future, the amorphous carbon layer method could be applied for surface processes for a wider range of semiconductor or insulator surfaces.
Co-reporter:Fengjie Xia, Mu Pan, Shichun Mu, Yuli Xiong, Karen J. Edler, Ilaria Idini, Matthew D. Jones, Shik Chi Tsang and Frank Marken
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 45) pp:NaN15865-15865
Publication Date(Web):2012/10/11
DOI:10.1039/C2CP42659E
Carbon nanoparticles with phenylsulfonate negative surface functionality (Emperor 2000, Cabot Corp.) are coated with positive chitosan followed by hydrothermal carbonization to give highly pH-responsive core–shell nanocarbon composite materials. With optimised core–shell ratio (resulting in an average shell thickness of ca. 4 nm, estimated from SANS data) modified electrodes exhibit highly pH-sensitive resistance, capacitance, and Faradaic electron transfer responses (solution based, covalently bound, or hydrothermally embedded). A shell “double layer exclusion” mechanism is discussed to explain the observed pH switching effects. Based on this mechanism, a broader range of future applications of responsive core–shell nanoparticles are envisaged.
Co-reporter:Stuart M. MacDonald, Marcin Opallo, Andreas Klamt, Frank Eckert and Frank Marken
Physical Chemistry Chemical Physics 2008 - vol. 10(Issue 26) pp:NaN3933-3933
Publication Date(Web):2008/05/22
DOI:10.1039/B803582B
Understanding liquid|liquid ion transfer processes is important in particular for naturally occurring species such as carboxylates. In this study electrochemically driven mono-, di-, and tri-carboxylate anion transfer at the 4-(3-phenylpropyl)pyridine|aqueous electrolyte interface is investigated experimentally for a triple phase boundary system at graphite electrodes. The tetraphenylporphyrinato-Mn(III/II) redox system (Mn(III/II)TPP) dissolved in the water-immiscible organic phase (4-(3-phenylpropyl)pyridine) is employed for the quantitative study of the structure–Gibbs transfer energy correlation and the effects of the solution pH on the carboxylate transfer process. For di- and tri-carboxylates the partially protonated anions are always transferred preferentially even at a pH higher than the corresponding pKa. COSMO-RS computer simulations are shown to provide a quantitative rationalisation as well as a powerful tool for predicting Gibbs free energy of transfer data for more complex functionalised carboxylate anions. It is shown that the presence of water in the organic phase has a major effect on the calculated Gibbs free energies.
![[1,1'-Biphenyl]-4,4'-dicarboxylic acid, 2-amino-](/data/chemimg/3723700/1240557-01-0.png)
![[1,1'-Biphenyl]-4,4'-dicarboxylic acid, 2-amino-](/data/chemimg/3723700/1240557-01-0_b.png)
![1,2-Ethanediol, 1,2-bis[4-(trifluoromethyl)phenyl]-, (1R,2S)-rel-](http://img.cochemist.com/ccimg/173100/173035-63-7.png)
![1,2-Ethanediol, 1,2-bis[4-(trifluoromethyl)phenyl]-, (1R,2S)-rel-](http://img.cochemist.com/ccimg/173100/173035-63-7_b.png)
![Benzenemethanamine, N-[[4-(trifluoromethyl)phenyl]methylene]-](http://img.cochemist.com/ccimg/79200/79128-82-8.png)
![Benzenemethanamine, N-[[4-(trifluoromethyl)phenyl]methylene]-](http://img.cochemist.com/ccimg/79200/79128-82-8_b.png)
![Xanthylium,3,6-bis(diethylamino)-9-[2-[(octadecyloxy)carbonyl]phenyl]-, chloride (1:1)](/data/chemimg/939500/65603-19-2.png)
![Xanthylium,3,6-bis(diethylamino)-9-[2-[(octadecyloxy)carbonyl]phenyl]-, chloride (1:1)](/data/chemimg/939500/65603-19-2_b.png)
![Octadecanamide,N-[9-(diethylamino)-5H-benzo[a]phenoxazin-5-ylidene]-](http://img.cochemist.com/ccimg/125900/125829-24-5.png)
![Octadecanamide,N-[9-(diethylamino)-5H-benzo[a]phenoxazin-5-ylidene]-](http://img.cochemist.com/ccimg/125900/125829-24-5_b.png)
![Iron, chloro[5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-5-12)-](/data/chemimg/1024600/16456-81-8.png)
![Iron, chloro[5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-5-12)-](/data/chemimg/1024600/16456-81-8_b.png)
![Thioperoxydicarbonimidicdiamide ([(H2N)C(NH)]2S2)](http://img.cochemist.com/ccimg/3300/3256-06-2.png)
![Thioperoxydicarbonimidicdiamide ([(H2N)C(NH)]2S2)](http://img.cochemist.com/ccimg/3300/3256-06-2_b.png)
![Ferrate(2-), [7,12-diethenyl-3,8,13,17-tetramethyl-21H,23H-porphine-2,18-dipropanoato(4-)-κN21,κN22,κN23,κN24]-, hydrogen (1:2), (SP-4-2)-](/data/chemimg/522800/14875-96-8.png)
![Ferrate(2-), [7,12-diethenyl-3,8,13,17-tetramethyl-21H,23H-porphine-2,18-dipropanoato(4-)-κN21,κN22,κN23,κN24]-, hydrogen (1:2), (SP-4-2)-](/data/chemimg/522800/14875-96-8_b.png)
![1,1'-Diheptyl-[4,4'-bipyridine]-1,1'-diium bromide](http://img.cochemist.com/ccimg/6200/6159-05-3.png)
![1,1'-Diheptyl-[4,4'-bipyridine]-1,1'-diium bromide](http://img.cochemist.com/ccimg/6200/6159-05-3_b.png)
![Cobalt(2 ), (1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane-κN3,κN6,κN10,κN13,κN16,κN19)-, (OC-6-11)-](/data/chemimg/639000/63218-22-4.png)
![Cobalt(2 ), (1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane-κN3,κN6,κN10,κN13,κN16,κN19)-, (OC-6-11)-](/data/chemimg/639000/63218-22-4_b.png)
![Manganese, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)-](/data/chemimg/546000/31004-82-7.png)
![Manganese, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)-](/data/chemimg/546000/31004-82-7_b.png)
