Co-reporter:Honglan Qi, Justin J. Teesdale, Rachel C. Pupillo, Joel Rosenthal, and Allen J. Bard
Journal of the American Chemical Society September 11, 2013 Volume 135(Issue 36) pp:13558-13566
Publication Date(Web):August 27, 2013
DOI:10.1021/ja406731f
Two new 2,2′-bipyridine (bpy) derivatives containing ancillary BODIPY chromophores attached at the 5- and 5′-positions (BB3) or 6- and 6′-positions (BB4) were prepared and characterized. In this work, the basic photophysics, electrochemistry, and electrogenerated chemiluminescence (ECL) of BB3 and BB4 are compared with those previously reported for a related bpy-BODIPY derivative (BB2) (J. Phys. Chem. C 2011, 115, 17993–18001). Cyclic voltammetry revealed that BB3 and BB4 display reversible 2e– oxidation and reduction waves, which consist of two closely spaced (50–70 mV) 1e– events. This redox behavior is consistent with the frontier molecular orbitals calculated for BB3 and BB4 and indicates that the 2,2′-bipyridine spacer of each bpy-BODIPY homologue does not facilitate efficient electronic communication between the tethered indacene units. In the presence of a coreactant such as tri-n-propylamine (TPA) or benzoyl peroxide (BPO), BB3 and BB4 exhibit strong ECL and produce spectra that are very similar to their corresponding photoluminescence profiles. The ECL signal obtained under annihilation conditions, however, is significantly different and is characterized by two distinct bands. One of these bands is centered at ∼570 nm and is attributed to emission via an S- or T-route. The second band occurs at longer wavelengths and is centered around ∼740 nm. The shape and concentration dependence of this long-wavelength ECL signal is not indicative of emission from an excimer or aggregate, but rather it suggests that a new emissive species is formed from the bpy-BODIPY luminophores during the annihilation process.
Co-reporter:Caleb M. Hill, Jiyeon Kim, Nataraju Bodappa, and Allen J. Bard
Journal of the American Chemical Society May 3, 2017 Volume 139(Issue 17) pp:6114-6114
Publication Date(Web):April 9, 2017
DOI:10.1021/jacs.6b12104
Described here is a semiquantitative theoretical treatment of the kinetics of outer sphere electrochemical reactions. The framework presented here, which is based on simple physical arguments, predicts heterogeneous rate constants consistent with previous experimental observations (k0 > 10 cm/s). This theory is applied to the analysis of voltammetry experiments involving ultramicroelectrodes modified with thin, insulating oxide films where electronic tunneling between the electrode and redox species in solution (metal–insulator–solution tunneling) is expected to play a prominent role. It is shown that analysis of the voltammetric response of an outer sphere redox couple can be used to track changes in the structure of the adsorbed insulating layer.
Co-reporter:Hyun S. Ahn and Allen J. Bard
Analytical Chemistry August 15, 2017 Volume 89(Issue 16) pp:8574-8574
Publication Date(Web):July 20, 2017
DOI:10.1021/acs.analchem.7b02799
Transition metal phosphides have been investigated heavily as hydrogen evolution reaction (HER) catalysts. One of the most active transition metal phosphides, CoP, has been tested for its stability and operability under mild conditions that it may be exposed to in its applications (photoelectrochemistry and artificial photosynthesis). Surface-interrogation scanning electrochemical microscopy (SI-SECM) revealed that CoP HER catalyst is vulnerable to oxidation (by oxygen and chemical oxidants). The degradation mechanism was shown to be surface oxidation by dioxygen, followed by acid etching of the oxidized layer. The compositional integrity (unity ratio of cobalt and phosphorus) was maintained throughout the film decomposition progress.
Co-reporter:Zhenxing Liang, Hyun S. Ahn, and Allen J. Bard
Journal of the American Chemical Society April 5, 2017 Volume 139(Issue 13) pp:4854-4854
Publication Date(Web):March 9, 2017
DOI:10.1021/jacs.7b00279
The hydrogen evolution reaction (HER) on Ni in alkaline media was investigated by scanning electrochemical microscopy under two operating modes. First, the substrate generation/tip collection mode was employed to extract the “true” cathodic current associated with the HER from the total current in the polarization curve. Compared to metallic Ni, the electrocatalytic activity of the HER is improved in the presence of the low-valence-state oxide of Ni. This result is in agreement with a previous claim that the dissociative adsorption of water can be enhanced at the Ni/Ni oxide interface. Second, the surface-interrogation scanning electrochemical microscopy (SI-SECM) mode was used to directly measure the coverage of the adsorbed hydrogen on Ni at given potentials. Simulation indicates that the hydrogen coverage follows a Frumkin isotherm with respect to the applied potential. On the basis of the combined analysis of the Tafel slope and surface hydrogen coverage, the rate-determining step is suggested to be the adsorption of hydrogen (Volmer step) in the investigated potential window.
Co-reporter:Dr. Xiao Yang;Dr. Li Ji;Dr. Xingli Zou;Dr. Taeho Lim;Dr. Ji Zhao; Edward T. Yu; Allen J. Bard
Angewandte Chemie 2017 Volume 129(Issue 47) pp:15274-15278
Publication Date(Web):2017/11/20
DOI:10.1002/ange.201707635
AbstractElectrodeposition of Si films from a Si-containing electrolyte is a cost-effective approach for the manufacturing of solar cells. Proposals relying on fluoride-based molten salts have suffered from low product quality due to difficulties in impurity control. Here we demonstrate the successful electrodeposition of high-quality Si films from a CaCl2-based molten salt. Soluble SiIV−O anions generated from solid SiO2 are electrodeposited onto a graphite substrate to form a dense film of crystalline Si. Impurities in the deposited Si film are controlled at low concentrations (both B and P are less than 1 ppm). In the photoelectrochemical measurements, the film shows p-type semiconductor character and large photocurrent. A p–n junction fabricated from the deposited Si film exhibits clear photovoltaic effects. This study represents the first step to the ultimate goal of developing a cost-effective manufacturing process for Si solar cells based on electrodeposition.
Co-reporter:Dr. Xiao Yang;Dr. Li Ji;Dr. Xingli Zou;Dr. Taeho Lim;Dr. Ji Zhao; Edward T. Yu; Allen J. Bard
Angewandte Chemie International Edition 2017 Volume 56(Issue 47) pp:15078-15082
Publication Date(Web):2017/11/20
DOI:10.1002/anie.201707635
AbstractElectrodeposition of Si films from a Si-containing electrolyte is a cost-effective approach for the manufacturing of solar cells. Proposals relying on fluoride-based molten salts have suffered from low product quality due to difficulties in impurity control. Here we demonstrate the successful electrodeposition of high-quality Si films from a CaCl2-based molten salt. Soluble SiIV−O anions generated from solid SiO2 are electrodeposited onto a graphite substrate to form a dense film of crystalline Si. Impurities in the deposited Si film are controlled at low concentrations (both B and P are less than 1 ppm). In the photoelectrochemical measurements, the film shows p-type semiconductor character and large photocurrent. A p–n junction fabricated from the deposited Si film exhibits clear photovoltaic effects. This study represents the first step to the ultimate goal of developing a cost-effective manufacturing process for Si solar cells based on electrodeposition.
Co-reporter:Stephen J. Percival, Jeffrey E. Dick, and Allen J. Bard
Analytical Chemistry 2017 Volume 89(Issue 5) pp:
Publication Date(Web):February 8, 2017
DOI:10.1021/acs.analchem.6b04832
The cathodic dissolution of platinum, resulting from the oxygen reduction reaction (ORR) or hydrogen peroxide reduction on platinum, has been investigated. Highly oxidizing hydroxyl radicals (OH•) are believed to be the species responsible for the platinum dissolution phenomenon. These radicals are produced from the ORR byproduct, hydrogen peroxide, through a 1 electron reduction pathway (H2O2 + e– → OH• + OH–). Platinum ultramicroelectrodes (UMEs) were polarized sufficiently negative to drive the ORR or H2O2 reduction on the platinum surface, mainly using square wave potential pulses but constant applied potential and cyclic voltammetry (CV) were also investigated. The dissolved platinum was detected using a femtomolar level detection technique which involves reducing platinum ions to platinum metal species followed by an electrocatalytic amplification of proton reduction on an inert carbon fiber electrode. This method has allowed the quantification of the amount of platinum metal dissolved into the solution, from which the rate of platinum dissolution could be determined. Additionally, the detection method demonstrates the platinum is dissolved into the solution as an ionic species and does not form metallic nanoparticles.
Co-reporter:Jiyeon Kim, Jeffrey E. Dick, and Allen J. Bard
Accounts of Chemical Research 2016 Volume 49(Issue 11) pp:2587
Publication Date(Web):October 27, 2016
DOI:10.1021/acs.accounts.6b00340
Metal clusters are very important as building blocks for nanoparticles (NPs) for electrocatalysis and electroanalysis in both fundamental and applied electrochemistry. Attention has been given to understanding of traditional nucleation and growth of metal clusters and to their catalytic activities for various electrochemical applications in energy harvesting as well as analytical sensing. Importantly, understanding the properties of these clusters, primarily the relationship between catalysis and morphology, is required to optimize catalytic function. This has been difficult due to the heterogeneities in the size, shape, and surface properties. Thus, methods that address these issues are necessary to begin understanding the reactivity of individual catalytic centers as opposed to ensemble measurements, where the effect of size and morphology on the catalysis is averaged out in the measurement.This Account introduces our advanced electrochemical approaches to focus on each isolated metal cluster, where we electrochemically fabricated clusters or NPs atom by atom to nanometer by nanometer and explored their electrochemistry for their kinetic and catalytic behavior. Such approaches expand the dimensions of analysis, to include the electrochemistry of (1) a discrete atomic cluster, (2) solely a single NP, or (3) individual NPs in the ensemble sample. Specifically, we studied the electrocatalysis of atomic metal clusters as a nascent electrocatalyst via direct electrodeposition on carbon ultramicroelectrode (C UME) in a femtomolar metal ion precursor. In addition, we developed tunneling ultramicroelectrodes (TUMEs) to study electron transfer (ET) kinetics of a redox probe at a single metal NP electrodeposited on this TUME. Owing to the small dimension of a NP as an active area of a TUME, extremely high mass transfer conditions yielded a remarkably high standard ET rate constant, k0, of 36 cm/s for outer-sphere ET reaction. Most recently, we advanced nanoscale scanning electrochemical microscopy (SECM) imaging to resolve the electrocatalytic activity of individual electrodeposited NPs within an ensemble sample yielding consistent high k0 values of ≥2 cm/s for the hydrogen oxidation reaction (HOR) at different NPs. We envision that our advanced electrochemical approaches will enable us to systematically address structure effects on the catalytic activity, thus providing a quantitative guideline for electrocatalysts in energy-related applications.
Co-reporter:Jiyeon Kim; Christophe Renault; Nikoloz Nioradze; Netzahualcóyotl Arroyo-Currás; Kevin C. Leonard
Journal of the American Chemical Society 2016 Volume 138(Issue 27) pp:8560-8568
Publication Date(Web):June 17, 2016
DOI:10.1021/jacs.6b03980
Understanding the relationship between the structure and the reactivity of catalytic metal nanoparticles (NPs) is important to achieve higher efficiencies in electrocatalytic devices. A big challenge remains, however, in studying these relations at the individual NP level. To address this challenge, we developed an approach using nanometer-scale scanning electrochemical microscopy (SECM) for the study of the geometric property and catalytic activity of individual Pt NPs in the hydrogen oxidation reaction (HOR). Herein, Pt NPs with a few tens to a hundred nm radius were directly electrodeposited on a highly oriented pyrolitic graphite (HOPG) surface via nucleation and growth without the necessity of capping agents or anchoring molecules. A well-defined nanometer-sized tip comparable to the dimensions of the NPs and a stable nanogap between the tip and NPs enabled us to achieve lateral and vertical spatial resolutions at a nanometer-scale and study fast electron-transfer kinetics. Specifically, the use of two different types of redox mediators: (1) outer-sphere mediator and (2) inner-sphere mediators could differentiate between the topography and the catalytic activity of individual Pt NPs and measure a large effective rate constant of HOR, keff0 of ≥2 cm/s as a lower limit at each Pt NP. Consequently, the size, shape, spatial orientation and the catalytic activity of Pt NPs could be determined at an individual level in nanoscale SECM where imaging accompanied by theoretical modeling and analysis. This approach can be easily extended to quantitatively probe the effects of the surface property, such as capping agent effects on the catalytic activity of a variety of metal NPs for the design and assessment of NP catalysts.
Co-reporter:Jeffrey E. Dick
Journal of the American Chemical Society 2016 Volume 138(Issue 27) pp:8446-8452
Publication Date(Web):June 13, 2016
DOI:10.1021/jacs.6b03202
We report the electrochemical detection of femtomolar amounts of cobalt, iridium, nickel, and iron ions in solution by electrocatalyst formation and amplification. The metal oxides of these ions can be formed electrochemically and can catalyze the oxidation of water. Alternatively, the reduction of metal ions to metals, such as the reduction of IrCl63– to iridium, is capable of electrocatalytically reducing protons to molecular hydrogen, as shown previously with Pt. These events, which manifest themselves in amperometry, correspond to the formation of electrocatalytic nuclei on the electrode surface, capable of electrocatalytically oxidizing water or reducing protons. An analysis of the frequency of anodic blips compared to theory implies that the requirement for water oxidation is 10 ± 1 ions of cobalt, 13 ± 4 ions of iridium, and 11 ± 3 ions of nickel. A similar analysis for iridium reduction and the corresponding catalytic reduction of protons implies that 6 ± 2 ions of iridium are required for proton reduction. These numbers are confirmed in an analysis of the time of first nucleation event, i.e. the time at which the first blip on the amperometric i–t experiment occurs. We further show that the anodic blips in detecting nickel increase in intensity upon increasing amounts of iron ions in solution to a ratio of Ni/Fe of ∼5, surprisingly close to that for bulk electrocatalysts of Ni–Fe.
Co-reporter:Haiqiang Deng, Jeffrey E. Dick, Sina Kummer, Udo Kragl, Steven H. Strauss, and Allen J. Bard
Analytical Chemistry 2016 Volume 88(Issue 15) pp:7754
Publication Date(Web):July 7, 2016
DOI:10.1021/acs.analchem.6b01747
Co-reporter:Jiyeon Kim and Allen J. Bard
Analytical Chemistry 2016 Volume 88(Issue 3) pp:1742
Publication Date(Web):December 23, 2015
DOI:10.1021/acs.analchem.5b03965
We demonstrate a new experimental approach to measure heterogeneous electron transfer rates. We adapted the classical Koutecký-Levich model for a rotating disk electrode (RDE) to a general heterogeneous electrochemical kinetic study with ultramicroelectrodes (UMEs) even for fast redox systems, where different sizes of UMEs are used to modulate the mass transfer rate (m). Subsequently, a linear plot of (1/current density) vs 1/m at different potentials can be created from the obtained steady state voltammograms, which is analogous to the traditional Koutecký-Levich plot. A simple numerical treatment with a slope and y-intercept from a linear plot allows for extracting kinetic parameters. A unifying treatment is presented for the steady state quasi-reversible, irreversible, and reversible voltammograms for a simple electron transfer reaction at UMEs. This new experimental approach with submicrometer to ∼micrometer sized UMEs exceeds the mass transfer rates achieved by conventional electrochemical methods using rotating electrodes or solely tens of micrometer sized electrodes, thus enables us to study much faster heterogeneous electron transfer kinetics with simple instrumentation. The method should be particularly useful in studying particle size and structure effects.
Co-reporter:Jiyeon Kim, Christophe Renault, Nikoloz Nioradze, Netzahualcóyotl Arroyo-Currás, Kevin C. Leonard, and Allen J. Bard
Analytical Chemistry 2016 Volume 88(Issue 20) pp:10284
Publication Date(Web):September 23, 2016
DOI:10.1021/acs.analchem.6b03024
We report the crucial components required to perform scanning electrochemical microscopy (SECM) with nanometer-scale resolution. The construction and modification of the software and hardware instrumentation for nanoscale SECM are explicitly explained including (1) the LabVIEW code that synchronizes the SECM tip movement with the electrochemical response, (2) the construction of an isothermal chamber to stabilize the nanometer scale gap between the tip and substrate, (3) the modification of a commercial bipotentiostat to avoid electrochemical tip damage during SECM experiments, and (4) the construction of an SECM stage to avoid artifacts in SECM images. These findings enabled us to successfully build a nanoscale SECM, which can be utilized to map the electrocatalytic activity of individual nanoparticles in a typical ensemble sample and study the structure/reactivity relationship of single nanostructures.
Co-reporter:Hsien-Yi Hsu, Li Ji, Minshu Du, Ji Zhao, Edward T. Yu, and Allen J. Bard
The Journal of Physical Chemistry C 2016 Volume 120(Issue 35) pp:19890-19895
Publication Date(Web):August 9, 2016
DOI:10.1021/acs.jpcc.6b07850
A variety of PbI2/MAPbI3 perovskites were prepared and investigated by a rapid screening technique utilizing a modified scanning electrochemical microscope (SECM) in order to determine how excess PbI2 affects its photoelectrochemical (PEC) properties. An optimum ratio of 2.5% PbI2/MAPbI3 was found to enhance photocurrent over pristine MAPbI3 on a spot array electrode under irradiation. With bulk films of various PbI2/MAPbI3 composites prepared by a spin-coating technique of mixed precursors and a one-step annealing process, the 2.5% PbI2/MAPbI3 produced an increased photocurrent density compared to pristine MAPbI3 for 2 mM benzoquinone (BQ) reduction at −0.4 V vs Fc/Fc+. As a result of the relatively high quantum yield of MAPbI3, a time-resolved photoluminescence quenching experiment could be applied to determine electron–hole diffusion coefficients and diffusion lengths of PbI2/MAPbI3 composites, respectively. The diffusion coefficients combined with the exciton lifetime of the pristine 2.5% PbI2/MAPbI3 (τPL = 103.3 ns) give the electron and hole exciton diffusion lengths, ∼300 nm. Thus, the 2.5% PbI2/MAPbI3 led to an approximately 3.0-fold increase in the diffusion length compared to a previous report of ∼100 nm for the pristine MAPbI3 perovskite. We then demonstrated that the efficiency of liquid-junction solar cells for 2.5% excess PbI2 of p-MAPbI3 was improved from 6.0% to 7.3%.
Co-reporter:Hyun S. Ahn
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 14) pp:2748-2752
Publication Date(Web):July 6, 2016
DOI:10.1021/acs.jpclett.6b01276
The hydrogen evolution reaction (HER) on an electrodeposited a-MoS2 electrode was investigated by a surface-selective electrochemical titration technique by application of surface interrogation scanning electrochemical microscopy. In a mildly acidic (pH 4.6) environment, the saturated surface hydride coverage of MoS2 was determined to be 31%, much higher than that expected for a crystalline nanoparticle. The HER rate constant of a surface molybdenum atom was measured for the first time in situ to be 3.8 s–1 at a 600 mV overpotential. At high Mo–H coverages, a change in the nature of the active sites was observed upon consumption of Mo–H by HER.
Co-reporter:Hsien-Yi Hsu; Li Ji; Hyun S. Ahn; Ji Zhao; Edward T. Yu
Journal of the American Chemical Society 2015 Volume 137(Issue 46) pp:14758-14764
Publication Date(Web):November 2, 2015
DOI:10.1021/jacs.5b09758
A liquid junction photoelectrochemical (PEC) solar cell based on p-type methylammonium lead iodide (p-MeNH3PbI3) perovskite with a large open-circuit voltage is developed. MeNH3PbI3 perovskite is readily soluble or decomposed in many common solvents. However, the solvent dichloromethane (CH2Cl2) can be employed to form stable liquid junctions. These were characterized with photoelectrochemical cells with several redox couples, including I3–/I–, Fc/Fc+, DMFc/DMFc+, and BQ/BQ•– (where Fc is ferrocene, DMFc is decamethylferrocene, BQ is benzoquinone) in CH2Cl2. The solution-processed MeNH3PbI3 shows cathodic photocurrents and hence p-type behavior. The difference between the photocurrent onset potential and the standard potential for BQ/BQ•– is 1.25 V, which is especially large for a semiconductor with a band gap of 1.55 eV. A PEC photovoltaic cell, with a configuration of p-MeNH3PbI3/CH2Cl2, BQ (2 mM), BQ•– (2 mM)/carbon, shows an open-circuit photovoltage of 1.05 V and a short-circuit current density of 7.8 mA/cm2 under 100 mW/cm2 irradiation. The overall optical-to-electrical energy conversion efficiency is 6.1%. The PEC solar cell shows good stability for 5 h under irradiation.
Co-reporter:Hyun S. Ahn
Journal of the American Chemical Society 2015 Volume 137(Issue 2) pp:612-615
Publication Date(Web):January 5, 2015
DOI:10.1021/ja511740h
Despite exhaustive spectroscopic investigations on the CoPi oxygen-evolving catalyst over the past several years, little is known about the surface cobalt sites and intermediates in direct contact with water that are responsible for the actual catalysis. Many studies thus far have been limited to ex situ characterizations or bulk film measurements, often in the absence of solvent. Here we describe an investigation of the CoPi catalyst by surface interrogation scanning electrochemical microscopy (SI-SECM). This method should allow us to selectively study surface atoms separately from the bulk in a solvent-filled environment. By SI-SECM, independent titrations of surface CoIII and CoIV were performed, yielding a direct measurement of the surface active-site density of a CoPi electrode (11 Co/nm2). The pseudo-first-order reaction rate constants of CoIII and CoIV with water were determined to be 0.19 and >2 s–1, respectively, through time-dependent titration measurements.
Co-reporter:Hyun S. Ahn
Journal of the American Chemical Society 2015 Volume 138(Issue 1) pp:313-318
Publication Date(Web):December 8, 2015
DOI:10.1021/jacs.5b10977
Nickel-iron mixed metal oxyhydroxides have attracted significant attention as an oxygen evolution reaction (OER) catalyst for solar fuel renewable energy applications. Here, we performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of NiIV and FeIV in NiOOH, FeOOH, and Ni1–xFexOOH (0 < x < 0.27) electrodes. Most importantly, two types of surface sites exhibiting “fast” and “slow” kinetics were found, where the fraction of “fast” sites in Ni1–xFexOOH matched the iron atom content in the film. This finding provides experimental support to the theory-proposed model of active sites in Ni1–xFexOOH. The OER rate constant of the “fast” site was 1.70 s–1 per atom.
Co-reporter:Minshu Du; Lishan Cui; Yi Cao
Journal of the American Chemical Society 2015 Volume 137(Issue 23) pp:7397-7403
Publication Date(Web):May 18, 2015
DOI:10.1021/jacs.5b03034
Both the ligand effect and surface strain can affect the electrocatalytic reactivity. In that matter exists a need to be fundamentally understood; however, there is no effective strategy to isolate the strain effect in electrocatalytic systems. In this research we show how the elastic strain in a platinum nanofilm varies the catalytic activity for the oxygen reduction reaction, a key barrier to the wide applications of fuel cells. NiTi shape memory alloy was selected as the substrate to strain engineer the deposited Pt nanofilm in both compressively and tensilely strained states by taking advantage of the two-way shape memory effect for the first time. We demonstrate that compressive strain weakens the Pt surface adsorption and hence improves the ORR activity, which reflects in a 52% enhancement of the kinetic rate constant and a 27 mV positive shift of the half-wave potential for the compressively strained 5 nm Pt compared to the pristine Pt. Tensile strain has the opposite effect, which is in general agreement with the proposed d-band theory.
Co-reporter:Yan Li, Haiqiang Deng, Jeffrey E. Dick, and Allen J. Bard
Analytical Chemistry 2015 Volume 87(Issue 21) pp:11013
Publication Date(Web):October 2, 2015
DOI:10.1021/acs.analchem.5b02968
We report the collisions of single emulsion oil droplets with extremely low dielectric constants (e.g., benzene, ε of 2.27, or cyclohexane, ε of 2.02) as studied via emulsion droplet reactor (EDR) on an ultramicroelectrode (UME). By applying appropriate potentials to the UME, we observed the electrochemical effects of single-collision signals from the bulk electrolysis of single emulsion droplets. Different hydrophobic redox species (ferrocene, decamethyl-ferrocene, or metalloporphyrin) were trapped in a mixed benzene (or cyclohexane) oil-in-water emulsion using an ionic liquid as the supporting electrolyte and emulsifier. The emulsions were prepared using ultrasonic processing. Spike-like responses were observed in each i–t response due to the complete electrolysis of all of the above-mentioned redox species within the droplet. On the basis of these single-particle collision results, the collision frequency, size distribution, i–t decay behavior of the emulsion droplets, and possible mechanisms are analyzed and discussed. This work demonstrated that bulk electrolysis can be achieved in a few seconds in these attoliter reactors, suggesting many applications, such as analysis and electrosynthesis in low dielectric constant solvents, which have a much broader potential window.
Co-reporter:Yao Meng and Allen J. Bard
Analytical Chemistry 2015 Volume 87(Issue 6) pp:3498
Publication Date(Web):February 24, 2015
DOI:10.1021/acs.analchem.5b00052
Cu(II)Cl complexes can undergo stepwise electrochemical reduction to Cu(I)Cl and then to Cu. In this paper, the Nernst equation is used to simultaneously calculate the temperature-dependent stability constants of both mononuclear Cu(II)Cl and Cu(I)Cl complexes for the first time. First, voltammograms were recorded at a Pt ultramicroelectrode (UME) with a variable of free chloride concentration at a given temperature. Using the equilibrium of Ag/AgCl redox couple, the free chloride activities were measured on an Ag electrode in different NaCl solutions, adjusting for the influence of ionic strength. This electrochemical technique was proved to be feasible at 25 and 90 °C. With increasing temperature, the Cu(II)Cl complex favored a lower coordination number and the CuCl2 complex prevailed, whereas the CuCl42– was predicted to be unstable. The CuCl2– and CuCl32– complexes were still the primary species for the Cu(I)Cl complex.
Co-reporter:Jeffrey E. Dick;Adam T. Hilterbrand;Aliaksei Boika;Jason W. Upton
PNAS 2015 Volume 112 (Issue 17 ) pp:5303-5308
Publication Date(Web):2015-04-28
DOI:10.1073/pnas.1504294112
We report observations of stochastic collisions of murine cytomegalovirus (MCMV) on ultramicroelectrodes (UMEs), extending
the observation of discrete collision events on UMEs to biologically relevant analytes. Adsorption of an antibody specific
for a virion surface glycoprotein allowed differentiation of MCMV from MCMV bound by antibody from the collision frequency
decrease and current magnitudes in the electrochemical collision experiments, which shows the efficacy of the method to size
viral samples. To add selectivity to the technique, interactions between MCMV, a glycoprotein-specific primary antibody to
MCMV, and polystyrene bead “anchors,” which were functionalized with a secondary antibody specific to the Fc region of the
primary antibody, were used to affect virus mobility. Bead aggregation was observed, and the extent of aggregation was measured
using the electrochemical collision technique. Scanning electron microscopy and optical microscopy further supported aggregate
shape and extent of aggregation with and without MCMV. This work extends the field of collisions to biologically relevant
antigens and provides a novel foundation upon which qualitative sensor technology might be built for selective detection of
viruses and other biologically relevant analytes.
Co-reporter:Dr. Hyun S. Ahn ; Allen J. Bard
Angewandte Chemie 2015 Volume 127( Issue 46) pp:13957-13961
Publication Date(Web):
DOI:10.1002/ange.201506963
Abstract
Single-nanoparticle collisions were observed on an n-type silicon electrode (600 μm diameter) passivated by a thin layer of amorphous TiO2, where the current steps occurred by tunneling electron transfer. The observed collision frequency was in reasonable agreement with that predicted from theory. The isolated electrode, after a collision experiment, with a Pt/TiO2/n-Si architecture was shown to retain the photoelectrochemical properties of n-Si without photocorrosion or current decay. The Pt/TiO2/n-Si electrode produced 19 mA cm−2 of photocurrent density under 100 mW cm−2 irradiation from a xenon lamp during oxygen evolution without current fading for over 12 h.
Co-reporter:Dr. Hyun S. Ahn ; Allen J. Bard
Angewandte Chemie International Edition 2015 Volume 54( Issue 46) pp:13753-13757
Publication Date(Web):
DOI:10.1002/anie.201506963
Abstract
Single-nanoparticle collisions were observed on an n-type silicon electrode (600 μm diameter) passivated by a thin layer of amorphous TiO2, where the current steps occurred by tunneling electron transfer. The observed collision frequency was in reasonable agreement with that predicted from theory. The isolated electrode, after a collision experiment, with a Pt/TiO2/n-Si architecture was shown to retain the photoelectrochemical properties of n-Si without photocorrosion or current decay. The Pt/TiO2/n-Si electrode produced 19 mA cm−2 of photocurrent density under 100 mW cm−2 irradiation from a xenon lamp during oxygen evolution without current fading for over 12 h.
Co-reporter:Ki Min Nam, Eun Ah Cheon, Won Jung Shin, and Allen J. Bard
Langmuir 2015 Volume 31(Issue 39) pp:10897-10903
Publication Date(Web):September 15, 2015
DOI:10.1021/acs.langmuir.5b01780
We describe a composite of the n-type semiconductors for the photoelectrochemical oxygen evolution reaction (OER). A simple drop-casting technique of mixed precursors and a one-step annealing process were used in the synthesis of the WO3/CuWO4 composite. The composite showed improved photocurrent for water oxidation compared to either of the two compounds individually. We discuss possible electron–hole separation mechanisms in two semiconductors comprising a primary photon-absorbing semiconductor of CuWO4 with a secondary semiconductor of WO3. When the WO3/CuWO4 composite is simultaneously irradiated, the photogenerated hole from the WO3 valence band transfers to CuWO4, which results in an enhanced charge separation of CuWO4. Furthermore, the OER catalytic activity of manganese phosphate (MnPO) was compared to manganese oxide nanoparticles (Mn2O3) by electrochemical measurements, showing that the manganese phosphate was more efficient for the OER reaction. To investigate the effect of catalysts on semiconductors, manganese phosphate was deposited on the WO3/CuWO4 composite. The result demonstrates the promise of manganese phosphate for improving the photocurrent as well as the stability of the WO3/CuWO4 composite.
Co-reporter:Fahe Cao ; Jiyeon Kim
Journal of the American Chemical Society 2014 Volume 136(Issue 52) pp:18163-18169
Publication Date(Web):December 5, 2014
DOI:10.1021/ja511602v
The short-lived intermediate N,N-dimethylaniline (DMA) cation radical, DMA•+, was detected during the oxidation of DMA in MeCN with 0.1 M tetra-n-butylammonium hexafluorophosphate. The detection was accomplished at steady state by scanning electrochemical microscopy (SECM) with ultramicroelectrodes using the tip generation/substrate collection mode. Cyclic voltammetry (CV) with a 2 mm Pt electrode indicates that DMA oxidation in acetonitrile is followed by a dimerization and two electrochemical reactions, which is consistent with previous results. The DMA•+ intermediate is detected by SECM, where the DMA•+ generated at the ca. 500 nm radius Pt tip is collected on a 5 μm radius Pt substrate when the gap between the tip and the substrate is a few hundred nanometers. Almost all of the DMA•+ is reduced at the substrate when the gap is 200 nm or less, yielding a dimerization rate constant of 2.5 × 108 M–1·s–1 based on a simulation. This is roughly 3 orders of magnitude larger than the value estimated by fast-scan CV. We attribute this discrepancy to the effects of double-layer capacitance charging and adsorbed species in the high scan rate CV.
Co-reporter:David Eisenberg ; Hyun S. Ahn
Journal of the American Chemical Society 2014 Volume 136(Issue 40) pp:14011-14014
Publication Date(Web):September 22, 2014
DOI:10.1021/ja5082475
n-BiVO4 is a promising semiconductor material for photoelectrochemical water oxidation. Although most thin-film syntheses yield discontinuous BiVO4 layers, back reduction of photo-oxidized products on the conductive substrate has never been considered as a possible energy loss mechanism in the material. We report that a 15 s electrodeposition of amorphous TiO2 (a-TiO2) on W:BiVO4/F:SnO2 blocks this undesired back reduction and dramatically improves the photoelectrochemical performance of the electrode. Water oxidation photocurrent increases by up to 5.5 times, and its onset potential shifts negatively by ∼500 mV. In addition to blocking solution-mediated recombination at the substrate, the a-TiO2 film—which is found to lack any photocatalytic activity in itself—is hypothesized to react with surface defects and deactivate them toward surface recombination. The proposed treatment is simple and effective, and it may easily be extended to a wide variety of thin-film photoelectrodes.
Co-reporter:Jiyeon Kim ; Byung-Kwon Kim ; Sung Ki Cho
Journal of the American Chemical Society 2014 Volume 136(Issue 23) pp:8173-8176
Publication Date(Web):May 23, 2014
DOI:10.1021/ja503314u
We describe the fabrication of a nanometer-size electrode based on an insulating TiO2 film and a metal nanoparticle (NP). The TiO2 film is deposited on the conducting Pt surface of an ultramicroelectrode (UME) to block electron transfer (ET) to solution species. The film thickness is, however, thin enough to enable tunneling to Pt NPs; thus, the subsequent contact of metal NP to the TiO2 film restores the ET to solution species solely on the NP surface via facile electron tunneling. Consequently, the composite of UME/metal oxide film/NP offers nm-scale active area. The TiO2 film is electrochemically deposited on the Pt UME (Pt UME/TiO2), monitoring the cyclic voltammetry (CV) of ferrocenemethanol until the oxidation wave just disappears. A single Pt NP is captured in a collision experiment by observing the current increase upon contact of the Pt NP with the Pt UME/TiO2 by means of Pt NP-mediated electrochemical reduction of Fe(CN)63–. The resultant Pt UME/TiO2/Pt NP (or tunneling UME, T-UME) showed long-term stability and robustness with well-defined electrochemical response, suggesting applicability as a novel nm-size electrode for CV and steady-state measurements such as those with scanning electrochemical microscopy (SECM). Here, we employed the T-UME to measure SECM approach curves and showed remarkable approach capability for a nm-size SECM probe.
Co-reporter:Byung-Kwon Kim ; Aliaksei Boika ; Jiyeon Kim ; Jeffrey E. Dick
Journal of the American Chemical Society 2014 Volume 136(Issue 13) pp:4849-4852
Publication Date(Web):March 18, 2014
DOI:10.1021/ja500713w
We report an electrochemical study of the collisions of single droplets in an emulsion by two methods. In the first method, an electroactive redox species, for example, ferrocene, inside a toluene-in-water emulsion droplet (but not in the continuous phase) is measured by chronoamperometry during a collision with an ultramicroelectrode (UME). Here, a blip or spike type of collision signal is observed, representing electrolysis of the droplet contents. In the second method, electrochemical oxidation of an electroactive redox species in the continuous aqueous phase is hindered by a droplet blocking collision. In this case, a staircase current decrease is observed. From an analysis of single soft particle collision data, one can find the emulsion droplet size distribution and the droplet contents.
Co-reporter:Aliaksei Boika and Allen J. Bard
Analytical Chemistry 2014 Volume 86(Issue 23) pp:11666
Publication Date(Web):November 4, 2014
DOI:10.1021/ac502944n
We report for the first time how electrophoretic migration of ions and charged nanoparticles (NPs) in low electrolyte concentration solutions affects positive feedback in scanning electrochemical microscopy (SECM). The strength of the electric field in the gap between either the tip and the substrate, or the tip and counter electrodes, is shown to increase proportionally to the decrease in gap size. This field affects the flux of the charged redox species as expected for dilute electrolyte solutions. However, the shape of the normalized approach curve is unaffected by the electrophoretic migration. We also report that the rate of collisions of charged insulating NPs with the tip electrode decreases as the tip is brought closer to the substrate electrode. This rather unexpected result (negative feedback) can be explained by the blocking of the particle flux with the glass insulating layer around the metal microwires. Observation of simultaneous changes in the faradaic current at the tip and substrate electrodes due to particle collisions with the tip confirms a high rate of mass transport between the two electrodes under the conditions of positive feedback SECM.
Co-reporter:Jiyeon Kim, Jodi L. Connell, Marvin Whiteley, and Allen J. Bard
Analytical Chemistry 2014 Volume 86(Issue 24) pp:12327
Publication Date(Web):November 17, 2014
DOI:10.1021/ac5036204
We report a novel strategy for studying a broad range of cellular behaviors in real time by combining two powerful analytical techniques, micro-3D printing and scanning electrochemical microscopy (SECM). This allows one, in microbiological studies, to isolate a known number of cells in a micrometer-sized chamber with a roof and walls that are permeable to small molecules and observe metabolic products. In such studies, the size and spatial organization of a population play a crucial role in cellular group behaviors, such as intercellular interactions and communication. Micro-3D printing, a photolithographic method for constructing cross-linked protein microstructures, permits one to compartmentalize a small population of microbes by forming a porous roof and walls around cells in situ. Since the roof and walls defining the microchamber are porous, any small molecules can freely diffuse from the chamber to be detected and quantified using SECM. The size of the chamber and the roof permeability can be obtained by SECM using a small probe molecule, ferrocenemethanol (FcMeOH). The chamber permeability to FcMeOH can be tuned by varying printing parameters that influence the cross-linking density of the proteinaceous material. These analyses establish a versatile strategy as a sensitive platform to quantitatively monitor small molecules produced by microbes.
Co-reporter:Jodi L. Connell;Jiyeon Kim;Jason B. Shear;Marvin Whiteley
PNAS 2014 Volume 111 (Issue 51 ) pp:18255-18260
Publication Date(Web):2014-12-23
DOI:10.1073/pnas.1421211111
Microbes frequently live in nature as small, densely packed aggregates containing ∼101–105 cells. These aggregates not only display distinct phenotypes, including resistance to antibiotics, but also, serve as building
blocks for larger biofilm communities. Aggregates within these larger communities display nonrandom spatial organization,
and recent evidence indicates that this spatial organization is critical for fitness. Studying single aggregates as well as
spatially organized aggregates remains challenging because of the technical difficulties associated with manipulating small
populations. Micro-3D printing is a lithographic technique capable of creating aggregates in situ by printing protein-based
walls around individual cells or small populations. This 3D-printing strategy can organize bacteria in complex arrangements
to investigate how spatial and environmental parameters influence social behaviors. Here, we combined micro-3D printing and
scanning electrochemical microscopy (SECM) to probe quorum sensing (QS)-mediated communication in the bacterium Pseudomonas aeruginosa. Our results reveal that QS-dependent behaviors are observed within aggregates as small as 500 cells; however, aggregates
larger than 2,000 bacteria are required to stimulate QS in neighboring aggregates positioned 8 μm away. These studies provide
a powerful system to analyze the impact of spatial organization and aggregate size on microbial behaviors.
Co-reporter:Hyun S. Park, Hyung-Wook Ha, Rodney S. Ruoff, Allen J. Bard
Journal of Electroanalytical Chemistry 2014 716() pp: 8-15
Publication Date(Web):
DOI:10.1016/j.jelechem.2013.08.036
Co-reporter:Xinsheng Zhang ; Huicheng Li ; Shijun Wang ; Fu-Ren F. Fan
The Journal of Physical Chemistry C 2014 Volume 118(Issue 30) pp:16842-16850
Publication Date(Web):March 27, 2014
DOI:10.1021/jp500395a
The use of tantalum as a highly effective dopant for hematite photoelectrochemistry (PEC) has shown contradictory results in previous reports. We show here through screening of different compositions by scanning electrochemical microscopy that Ta doping significantly improves the PEC performance of dropcast films on fluorine-doped tin oxide (FTO). In studies with larger electrodes, a 2% Ta-doped hematite photoanode fabricated at 500 °C shows the highest improvement of photoactivity, which is ∼32 times higher than pure hematite even under visible light. At fabrication temperature higher than 500 °C (e.g., 600, 680 °C), the substrate FTO becomes more resistive and the dopant Ta prefers to segregate from the bulk phase (α-Fe2O3) and forms tantalum fluoride oxide (TaO2F), which may act as charge-carrier recombination centers, and the corresponding Ta-doped samples show much lower photoactivities. Ta-doped hematite samples show stronger (110) diffraction as compared with the pure α-Fe2O3. We show that the doping of Ta induced a preferential growth along the {001} basal plane, which has been reported to have good conductivity. We found the conductivity of the Ta-doped hematite was improved up to at least about one order of magnitude after the incorporation of Ta, with the improved carrier mobility decreasing recombination of the photogenerated holes and electrons.
Co-reporter:Jinho Chang
Journal of the American Chemical Society 2013 Volume 136(Issue 1) pp:311-320
Publication Date(Web):December 13, 2013
DOI:10.1021/ja409958a
Fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) were used to investigate the reduction of Sn(IV) as the hexabromo complex ion in a 2 M HBr–4 M NaBr medium. CV at scan rates to 100 V/s and SECM indicated the reaction pathway involves ligand-coupled electron transfer via an ECEC-DISP process: (1) one-electron reduction of SnIVBr62– to SnIIIBr63–; (2) bromide dissociation of the reduced SnIIIBr63– to SnIIIBr52–; (3) disproportionation of the reduced 2SnIIIBr52– to SnIVBr5– and SnIIBr53–; (4) one-electron reduction of SnIIIBr52– to SnIIBr53–; (5) bromide dissociation from SnIIBr5 to SnIIBr42–. The intermediate Sn(III) species was confirmed by SECM3–, where the Sn(III) generated at the Au tip was collected on a Au substrate in the tip generation/substrate collection mode when the distance between the tip and substrate was a few hundred nanometers.
Co-reporter:Jun Hui Park ; Scott N. Thorgaard ; Bo Zhang
Journal of the American Chemical Society 2013 Volume 135(Issue 14) pp:5258-5261
Publication Date(Web):March 26, 2013
DOI:10.1021/ja4000917
We describe the electrochemical detection of single nanoparticle (NP) attachment on a nanoelectrode by the increase in the active electrode area. The attachment of gold NP-decorated single wall carbon nanotubes (Au-SWCNTs) was observed by their current–time transients for ferrocenemethanol (FcMeOH) oxidation. Since the attached Au-SWCNT increases the electroactive area available for FcMeOH oxidation, the current increases after attachment of the particle. The “staircase” shape of the current response establishes that the particles do not become deactivated for the outer-sphere electron transfer reaction after attachment. Au-SWCNTs migrate to and are held at the nanoelectrode by an electric field. However, SWCNTs that are not decorated with a gold NP produce only a sharp transient (“blip”) response.
Co-reporter:Honglan Qi ; Yu-Han Chen ; Chien-Hong Cheng
Journal of the American Chemical Society 2013 Volume 135(Issue 24) pp:9041-9049
Publication Date(Web):May 24, 2013
DOI:10.1021/ja403166s
The electrochemistry and electrogenerated chemiluminescence (ECL) of three phenanthrene derivatives, 3,6-diphenyl-9,10-bis-(4-tert-butylphenyl)phenanthrene (TphP, T1), 3,6-di(naphthalen-2-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TnaP, T2), and 3,6-di(pyrene-1-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TpyP, T3), are investigated in an acetonitrile:benzene (v:v = 1:1) solvent. Cyclic voltammetry (CV) of the three derivatives shows reversible reduction waves and less chemically reversible oxidation waves at low scan rates. The CV character becomes more reversible, and the stability of the radical cations increases as the conjugation of the substituent groups appended to the phenanthrene increases. This finding indicates that the radical ion stabilities in phenanthrene derivatives are drastically improved by increasing the conjugation of the substituent groups; thus, electrochemically stable radical ions can be obtained by introducing more conjugated groups to the phenanthrene center. Additionally, ECL is generated for all compounds by radical ion annihilation, and the ECL spectrum shows good agreement with the fluorescence emission, assigned as emission by the S-route. ECL efficiencies for radical ion annihilation are 0.004 for TphP, 0.16 for TnaP, and 0.25 for TpyP, respectively, and the ECL efficiency increases with the conjugation of the substituent groups appended to the phenanthrene increases. Radical ion annihilation produced by potential steps exhibits asymmetric ECL transients in which the cathodic ECL pulse is smaller than the anodic pulse due to the instability of the radical cation. These molecules can produce a stronger ECL, which can be observed by the naked eye in a lighted room, on reduction in the presence of a coreactant (benzoyl peroxide).
Co-reporter:Mei Shen ; Xu-Hui Zhu
Journal of the American Chemical Society 2013 Volume 135(Issue 24) pp:8868-8873
Publication Date(Web):May 30, 2013
DOI:10.1021/ja312189k
We report here the electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) from a solution, film, and nanoparticles (NPs) of a red-emitting dithienylbenzothiadiazole molecular fluorophore [4,7-bis(4-(n-hexyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole, 1a], which has a donor–acceptor–donor configuration. In addition, the quenching of the fluorescence of the organic NPs by KI was investigated. The 1a film and NPs exhibit two absorbance peaks at 350 and ∼504 nm that are red-shifted compared to those of 1a dissolved in solution (340 and 486 nm). Fluorescence quenching of 1a NPs does not follow a linear Stern–Volmer relationship; i.e., the fluorescence emission with excitation wavelength at either 350 or 504 nm decreased with increasing concentration of KI. Static quenching and heterogeneity related to the size distribution of the 1a NPs are proposed to explain the nonlinearity. A lifetime of 4.49 ± 0.04 ns was found for 1a organic NPs in water saturated with N2. After addition of KI, the fluorescence lifetime decreased to 3.1 ns. The fluorescence emission of 1a film/NPs is red-shifted (∼17 nm) compared with that of 1a solution in dichloromethane (DCM). Solution ECL was generated in DCM through an annihilation reaction, while film and NP ECL could be generated in water through oxidation with a coreactant, tri-n-propylamine (TPrA). A film of 1a with thickness of 100–900 nm was prepared by drop-casting 1a in DCM on fluorine-doped tin oxide, and the ECL of the 1a film was found in phosphate-buffered saline solution with TPrA. Both 1a in solution and the 1a film produce strong ECL (Ifilm = 0.14Isolution). The ECL spectrum of 1a in solution, produced by electron-transfer annihilation of the reduced and oxidized forms, consists of a single peak with maximum emission at about 637 ± 4 nm, ∼20 nm red-shifted from its fluorescence, while the ECL spectrum of 1a film produced by reaction with TPrA consists of a single peak with maximum emission at 642 ± 3 nm, a 10 nm red shift compared with the fluorescence of 1a film. Organic fluorescent 1a NPs were prepared by a reprecipitation method in water saturated with N2, and they were characterized by transmission electron microscopy, absorbance, fluorescence, and ECL. Strong ECL was also generated from the organic NPs in water by reduction with K2S2O8 coreactant.
Co-reporter:Kevin C. Leonard
Journal of the American Chemical Society 2013 Volume 135(Issue 42) pp:15890-15896
Publication Date(Web):September 24, 2013
DOI:10.1021/ja407395m
We report a new method of scanning electrochemical microscopy (SECM) that can be used to separate multireactional electrochemical interfaces, i.e., electrodes at which two or more reactions occur (and hence two partial currents flow) at the same time. This was done with a modified tip generation/substrate collection mode where the two reactions occur on the tip electrode, and the substrate electrode is held at a potential to collect only one of the products, allowing the determination of the individual partial currents. Thus, by using the substrate electrode current and the difference between the tip and substrate electrode currents, the two reactions occurring on the tip electrode can be separated. As a test case for this new method, we investigated proton reduction on Mn, a reaction that, because of the highly corrosive nature of Mn, to our knowledge has never before been directly measured. This test was carried out using a Mn tip electrode and a Pt substrate electrode. Using a three-dimensional COMSOL Multiphysics simulation, we were able to accurately determine the tip/substrate distance with this electrode, and by fitting simulations to experimental data, we were able to determine an exchange current density, log(j0) = −4.7 ± 0.7 A cm–2, for proton reduction on Mn in strong acid. This result corrects a literature value and was used in a pattern recognition algorithm reported in a companion manuscript.
Co-reporter:Kevin C. Leonard
Journal of the American Chemical Society 2013 Volume 135(Issue 42) pp:15885-15889
Publication Date(Web):September 24, 2013
DOI:10.1021/ja407394q
Here we demonstrate the use of a previously reported pattern recognition algorithm to evaluate correlations between 50 different materials properties of the elements and their kinetics for the hydrogen evolution reaction in acid. We determined that the melting point and bulk modulus of the elements quantitatively gave the highest correlations of all materials properties investigated. We also showed that the melting point and bulk modulus correlations held true for a popular hydrogen evolution catalysts alloy, NiMo, and a previously untested material, MoSi2. In addition, we quantified the previously known relationship between the d-band center of an element and its kinetics for hydrogen evolution, and found that the melting point and bulk modulus correlations have correlations that are similar to but slightly stronger than those of the d-band center.
Co-reporter:Ashis K. Satpati, Netzahualcóyotl Arroyo-Currás, Li Ji, Edward T. Yu, and Allen J. Bard
Chemistry of Materials 2013 Volume 25(Issue 21) pp:4165
Publication Date(Web):October 9, 2013
DOI:10.1021/cm401635v
The scanning electrochemical microscope (SECM) was used to characterize the atomic layer deposition (ALD) of TiO2 on indium-doped tin oxide (ITO) substrates by studying electron transfer through pores in the thin films (1–5 nm thickness). The extent of electron transfer, and thus the porosity of the films, was evaluated by transient electrochemistry. These studies show that ALD deposition of TiO2 on ITO does not produce pinhole-free films but rather porous deposits with electrochemical behavior similar to that of microelectrode arrays up to about 30 ALD cycles. All the experimental results are explained in the context of a numerical model developed by finite element analysis and corroborated by complementary conductive atomic force microscopy (cAFM) results that directly reveal localized, nanoscale current conduction paths in thinner TiO2 layers with a transition to more spatially uniform conduction in the thickest layers. SECM images demonstrate the existence of pinholes even on films that have been subjected to more than 100 ALD cycles (thicknesses larger than 4 nm).Keywords: atomic layer deposition (ALD); finite element modeling; nanoporous films; scanning electrochemical microscopy (SECM);
Co-reporter:Shanmugam Senthil Kumar and Allen J. Bard
Analytical Chemistry 2013 Volume 85(Issue 1) pp:292
Publication Date(Web):November 27, 2012
DOI:10.1021/ac302732x
The background electrogenerated chemiluminescence (ECL) emission observed only upon electrochemical oxidation of tri-n-propylamine (TPrAH) on a platinum electrode is a limiting factor in ECL analytical techniques and is poorly understood. We studied this reaction in aerated acetonitrile (MeCN) solution with TPrAH oxidized at a constant potential at the Pt surface and observed ECL spectra with an emission band at 630 nm, which is characteristic of the emission of the dimeric 1Δg state of O2. No ECL emission was observed when the same solution was deaerated. This background ECL emission is attributed to the reaction between dissolved oxygen and two different products of TPrAH oxidation: the TPrAH• radical that reduces O2 to the superoxide ion and the TPrAH•+ radical cation that oxidizes this species to singlet O2.
Co-reporter:Jun Hui Park, Hongjun Zhou, Stephen J. Percival, Bo Zhang, Fu-Ren F. Fan, and Allen J. Bard
Analytical Chemistry 2013 Volume 85(Issue 2) pp:964
Publication Date(Web):December 14, 2012
DOI:10.1021/ac3025976
We investigate the principle of the open circuit potential (OCP) change upon a particle collision event based on mixed potential theory and confirmed by a mimic experiment in which we studied the changes in the OCP when two different electrodes (Pt and Au) are brought into contact in a solution that contains some irreversible redox couples. A micrometer-sized Au ultramicroelectrode, when connected in parallel to a Pt micro- or nanoelectrode, showed clearly measurable OCP changes whose magnitude matches well with that predicted by a simplified mixed potential theory for a pair of different electrode materials. On the basis of the study, each electrode establishes a different mixed potential involving two or more half reactions that have different heterogeneous electron transfer kinetics at different electrodes and the OCP changes are very sensitive to the relative ratio of the rate constant of the individual half reaction at different materials.
Co-reporter:Hong Zhao, Jinho Chang, Aliaksei Boika, and Allen J. Bard
Analytical Chemistry 2013 Volume 85(Issue 16) pp:7696
Publication Date(Web):July 12, 2013
DOI:10.1021/ac4016769
High concentrations of copper chloride solutions (in the molar range) are used in several industrial applications. In this work, we investigated the species distribution of copper chloride complexes and how to measure the copper concentration precisely at high concentrations using electrochemical methods, by including migrational effects. The latter, in fact, can be useful in determining the nature of the species in solution undergoing electron transfer at the electrode. The study indicates that the main species of Cu(II) complexes in high chloride concentration is CuCl42– and the main species of Cu(I) complexes are CuCl2– and CuCl32–. However insoluble CuCl is an intermediate in the process and can deactivate the electrode surface. This can be ameliorated by increasing the temperature or Cl– concentration. Under these conditions, voltammetry with an ultramicroelectrode (UME) can measure copper concentration with good precision even at 1 M Cu(II) concentrations in a few molar chloride. The main charge of the species can be determined by fitting to a migration model.
Co-reporter:Mohammad Mahdi Najafpour, Kevin C. Leonard, Fu-Ren F. Fan, Mahmoud Amouzadeh Tabrizi, Allen J. Bard, Cecil K. King'ondu, Steven L. Suib, Behzad Haghighi and Suleyman I. Allakhverdiev
Dalton Transactions 2013 vol. 42(Issue 14) pp:5085-5091
Publication Date(Web):24 Jan 2013
DOI:10.1039/C3DT32864C
Inspired by Nature's catalyst, a nano-size layered manganese–calcium oxide showed a low overvoltage for water oxidation in acidic solutions, which is comparable to platinum.
Co-reporter:Dongping Zhan, Xiao Li, Alexander B. Nepomnyashchii, Mario A. Alpuche-Aviles, Fu-Ren F. Fan, Allen J. Bard
Journal of Electroanalytical Chemistry 2013 Volume 688() pp:61-68
Publication Date(Web):1 January 2013
DOI:10.1016/j.jelechem.2012.07.008
The viability of living fibroblast cells cultured on a Petri dish was determined by scanning electrochemical microscopy (SECM). Three kinds of feedback modes for single living cells were derived with different redox couples: positive, negative and consumptive. The consumptive feedback was unique and distinguished uptake from the response caused by cell deformation. The toxicity of Ag+ ion on the living fibroblast cells was investigated by using both ferrocenemethanol (FcMeOH) and oxygen (O2) as electrochemical mediators. The results showed that Ag+ ions could be taken up and then reduced to form sub-micrometer and micrometer sized metal silver particles. The metal silver particles stained fibroblast cells and give a positive feedback with FcMeOH. These were also identified by the results of both SECM and energy dispersive X-ray spectroscopy (EDX). The amount of consumption of dissolved O2 decreased with time, showing the cell status as the cells died. Since O2 is directly involved in cell metabolism and the products of O2 reduction are not re-oxidized by metal silver particles trapped on fibroblast cells, it is a good SECM mediator to characterize drug effects on cell viability. The results suggest the importance of choice of redox mediator in characterizing cell viability.Highlights► Three SECM modes were used to study viability of single living fibroblast cells. ► FcMeOH+ positive feedback from the cells decreased as the cells died. ► Fibroblast cell viability was monitored by observing dissolved O2 concentration. ► Silver ion was toxic to fibroblast cells and Ag(0)-containing clusters were formed. ► The silver toxic dose found with SECM agreed with the results of other methods.
Co-reporter:Ki Min Nam, Hyun S. Park, Heung Chan Lee, Benjamin H. Meekins, Kevin C. Leonard, and Allen J. Bard
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 16) pp:2707-2710
Publication Date(Web):July 26, 2013
DOI:10.1021/jz401334k
Scanning electrochemical microscopy (SECM) was used to identify more efficient p-type photocatalysts for H+ reduction in the Pb–Bi–Mo trimetal oxide system. An atomic ratio of (1:1:1) between Pb, Bi, and Mo showed higher photocurrent than other spots in screening experiments. The crystal structure of Pb–Bi–Mo oxide was studied by X-ray diffraction, indicating that a composite of p-PbMoO4 and n-Bi2O3 coexisted as a heterostructure. Photoelectrochemical performance of the p-PbMoO4/n-Bi2O3 composite electrode showed enhanced stability for the H+ and O2 reduction reactions. We propose a reaction mechanism to explain this stabilization of a p-type semiconductor.Keywords: p-PbMoO4/n-Bi2O3; p-type photocatalyst; p-type semiconductor; Pb−Bi−Mo trimetal oxide system; scanning eletrochemical microscopy;
Co-reporter:Jun Hui Park ; Aliaksei Boika ; Hyun S. Park ; Heung Chan Lee
The Journal of Physical Chemistry C 2013 Volume 117(Issue 13) pp:6651-6657
Publication Date(Web):March 8, 2013
DOI:10.1021/jp3126494
We report that conductive single nanoparticle (NP) collisions can involve a significant component of the mass transport to the electrode of the charged NPs by migration. Previously, collision events of catalytic NPs were described as purely diffusional using random walk theory. However, the charged NP can also be attracted to the electrode by the electric field in solution (i.e., migration) thereby causing an enhancement in the collision frequency. The migration of charged NPs is affected by the supporting electrolyte concentration and the faradaic current flow. A simplified model based on the NP transference number is introduced to explain the migrational flux of the NPs. Experimental collision frequencies and the transference number model also agreed with more rigorous simulation results based on the Poisson and Nernst–Planck equations.
Co-reporter:Chinmoy Bhattacharya, Heung Chan Lee, and Allen J. Bard
The Journal of Physical Chemistry C 2013 Volume 117(Issue 19) pp:9633-9640
Publication Date(Web):April 10, 2013
DOI:10.1021/jp308629q
Bi2WO6 microelectrode arrays on FTO glass substrates were fabricated by a picoliter solution dispensing technique using Bi(NO3)3 as the Bi source and (NH4)6H2W12O40 as the W source in ethylene glycol. The scanning electrochemical microscope modified by using an optical fiber in place of an ultramicroelectrode was employed for rapid screening of the Bi2WO6 arrays and to investigate the effect of 12 different dopants on the photocatalytic oxidation of SO32–. Among the different dopant compositions, addition of 12% Zn showed a photocurrent enhancement of up to 80% compared with that of the pure Bi2WO6. This result was further confirmed with bulk electrode studies for SO32– and water oxidation. UV–vis absorption, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray diffraction studies were carried out with the photocatalysts to elucidate the role of Zn in the bulk semiconductors. Absorbed photon-to-current efficiency and incident photon-to-current efficiency determinations further confirm the enhancement of photoelectrochemical behavior upon addition of Zn to Bi2WO6 photocatalysts.
Co-reporter:Alexander B. Nepomnyashchii, Allen J. Pistner, Allen J. Bard, and Joel Rosenthal
The Journal of Physical Chemistry C 2013 Volume 117(Issue 11) pp:5599-5609
Publication Date(Web):March 7, 2013
DOI:10.1021/jp312166w
A set polyethylene glycol (PEG)-appended BODIPY architectures (BOPEG1–BOPEG3) have been prepared and studied in CH2Cl2, H2O:CH3CN (1:1) and aqueous solutions. BOPEG1 and BOPEG2 both contain a short PEG chain and differ in substitution about the BODIPY framework. BOPEG3 is comprised of a fully substituted BODIPY moiety linked to a PEG polymer that is roughly 13 units in length. The photophysics and electrochemical properties of these compounds have been thoroughly characterized in CH2Cl2 and aqueous CH3CN solutions. The behavior of BOPEG1–BOPEG3 correlates with established rules of BODIPY stability based on substitution about the BODIPY moiety. Electrogenerated chemiluminescence (ECL) for each of these compounds was also monitored. BOPEG1, which is unsubstituted at the 2- and 6-positions dimerized upon electrochemical oxidation while BOPEG2, which contains ethyl groups at the 2- and 6-positions, was much more robust and served as an excellent ECL luminophore. BOPEG3 is highly soluble in water due to the long PEG tether and demonstrated modest ECL activity in aqueous solutions using tri-n-propylamine (TPrA) as a coreactant. As such, BOPEG3 represents the first BODIPY derivative that has been shown to display ECL in water without the need for an organic cosolvent, and marks an important step in the development of BODIPY based ECL probes for various biosensing applications.
Co-reporter:Hyun S. Park, Kevin C. Leonard, and Allen J. Bard
The Journal of Physical Chemistry C 2013 Volume 117(Issue 23) pp:12093-12102
Publication Date(Web):May 15, 2013
DOI:10.1021/jp400478z
Reaction kinetics and surface coverage of water oxidation intermediates at a W/Mo-BiVO4 photoanode were studied using surface interrogation scanning electrochemical microscopy (SI-SECM). Adsorbed hydroxyl radicals (OH•) were produced during water oxidation at the semiconductor surface under UV–visible irradiation and were subsequently electrochemically titrated by tip-generated reductant without irradiation. The IrCl62–/3– redox couple was used to determine the surface concentration of OH• in acidic solution. On W/Mo-BiVO4, ∼6% of the absorbed photons generate surface OH• with a coverage of 5.8 mC cm–2. Less than 1% of the irradiated photons were eventually used for water oxidation under high intensity irradiation (∼1 W cm–2) at the photoanode. Assuming that the primary decay mechanism of the adsorbed OH• on W/Mo-BiVO4 is dimerization to produce hydrogen peroxide (H2O2), the rate constant was determined to be 4 × 103 mol–1 m2 s–1. A faster decay rate of OH• was observed in the presence of excess methanol (a radical scavenger) in aqueous solution. In addition, quantitative analysis of the water oxidation processes at W/Mo-BiVO4 along with the quantum efficiency for the oxygen evolution reaction was determined using SECM.
Co-reporter:Dr. Hyun S. Park;Dr. Heung Chan Lee;Dr. Kevin C. Leonard;Dr. Guanjie Liu ; Dr. Allen J. Bard
ChemPhysChem 2013 Volume 14( Issue 10) pp:2277-2287
Publication Date(Web):
DOI:10.1002/cphc.201201044
Abstract
Photoelectrochemical water splitting to generate H2 and O2 using only photon energy (with no added electrical energy) has been demonstrated with dual n-type-semiconductor (or Z-scheme) systems. Here we investigated two different Z-scheme systems; one is comprised of two cells with the same metal-oxide semiconductor (W- and Mo-doped bismuth vanadate), that is, Pt-W/Mo-BiVO4, and the other is comprised of the metal oxide and a chalcogenide semiconductor, that is, Pt-W/Mo-BiVO4 and Zn0.2Cd0.8Se. The redox couples utilized in these Z-scheme configurations were I−/IO3− or S2−/Sn2−, respectively. An electrochemical analysis of the system in terms of cell components is shown to illustrate the behavior of the complete photoelectrochemical Z-scheme water-splitting system. H2 gas from the unbiased photolysis of water was detected using gas chromatography–mass spectroscopy and using a membrane-electrode assembly. The electrode configuration to achieve the maximum conversion efficiency from solar energy to chemical energy with the given materials and the Z-scheme is discussed. Here, the possibilities and challenges of Z-scheme unbiased photoelectrochemical water-splitting devices and the materials to achieve practical solar-fuel generation are discussed.
Co-reporter:Sung Ki Cho ; Hyun S. Park ; Heung Chan Lee ; Ki Min Nam
The Journal of Physical Chemistry C 2013 Volume 117(Issue 44) pp:23048-23056
Publication Date(Web):October 11, 2013
DOI:10.1021/jp408619u
We report that oxide composite electrodeposition can be used for the facile preparation of metal-doped BiVO4 photoelectrodes for photoelectrochemical water oxidation. The photoactivity of electrodeposition film was improved by the addition of a small amount of tungstic acid particles during the electrodeposition. These particles are incorporated in the deposit and finally generate tungsten-doped bismuth vanadate. The suspended particles in the plating solution were electrostatically attracted to the cathode and accordingly incorporated into the deposit (electrostatic deposition). WO3 nanoparticles (NPs) can be used instead of tungstic acid, to yield a BiVO4 with different properties. Enhanced photoelectrochemical (PEC) water oxidation was confirmed via scanning electrochemical microscopy (SECM) by detecting increased oxygen evolution with using optical fiber incorporating a ring electrode.
Co-reporter:Kevin C. Leonard, Ki Min Nam, Heung Chan Lee, Soon Hyung Kang, Hyun S. Park, and Allen J. Bard
The Journal of Physical Chemistry C 2013 Volume 117(Issue 31) pp:15901-15910
Publication Date(Web):July 9, 2013
DOI:10.1021/jp403506q
A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO4/WO3 composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO3 with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO4/WO3 resulted in a 2.5-fold increase in the photocurrent compared to pristine WO3 for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO3 and ZnWO4/WO3 electrodes. X-ray diffraction showed the composite nature of the WO3 and ZnWO4 mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO3 has a smaller band gap than ZnWO4, while Mott–Schottky analysis determined that ZnWO4 has a more negative flat-band potential than WO3. A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO4/WO3 composite was formed in a single annealing step.
Co-reporter:Aliaksei Boika, Scott N. Thorgaard, and Allen J. Bard
The Journal of Physical Chemistry B 2013 Volume 117(Issue 16) pp:4371-4380
Publication Date(Web):October 23, 2012
DOI:10.1021/jp306934g
The individual adsorption events of sub-μm silica and polystyrene spheres (310–530 nm in diam.) were detected by monitoring the blocking of redox mediator diffusion to Pt ultramicroelectrode (UME) substrates by the adsorbing spheres. Under the diffusion limited oxidation of FcMeOH and at low supporting electrolyte concentrations, the negatively charged spheres arrive at the electrode by electrophoretic migration. Sphere adsorption monitoring experiments consisted of long-time (1000–5000 s) chronoamperograms recorded in solutions with fM concentrations of spheres and different concentrations of supporting electrolyte. Trends in the heights of the step features with time reflect changing surface coverage of spheres, and coupled step features in the chronoamperograms suggest dynamic rearrangement of spheres on the surface. Numerical simulations of diffusion blocking at electrodes by adsorbing particles as well as mass transport of particles under migration were also performed, and show good agreement with the experimental data collected.
Co-reporter:Hongjun Zhou ; Jun Hui Park ; Fu-Ren F. Fan
Journal of the American Chemical Society 2012 Volume 134(Issue 32) pp:13212-13215
Publication Date(Web):July 27, 2012
DOI:10.1021/ja305573g
Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.
Co-reporter:Joaquín Rodríguez-López ; Mei Shen ; Alexander B. Nepomnyashchii
Journal of the American Chemical Society 2012 Volume 134(Issue 22) pp:9240-9250
Publication Date(Web):May 15, 2012
DOI:10.1021/ja301016n
Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)3]2+ was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (kann > 107 M–1 s–1). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)3]2+ to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.
Co-reporter:Seong Jung Kwon
Journal of the American Chemical Society 2012 Volume 134(Issue 26) pp:10777-10779
Publication Date(Web):June 14, 2012
DOI:10.1021/ja304074f
This study demonstrates a highly sensitive sensing scheme for the detection of low concentrations of DNA, in principle down to the single biomolecule level. The previously developed technique of electrochemical current amplification for detection of single nanoparticle (NP) collisions at an ultramicroelectrode (UME) has been employed to determine DNA. The Pt NP/Au UME/hydrazine oxidation reaction was employed, and individual NP collision events were monitored. The Pt NP was modified with a 20-base oligonucleotide with a C6 spacer thiol (detection probe), and the Au UME was modified with a 16-base oligonucleotide with a C6 spacer thiol (capture probe). The presence of a target oligonucleotide (31 base) that hybridized with both capture and detection probes brought a Pt NP on the electrode surface, where the resulting electrochemical oxidation of hydrazine resulted in a current response.
Co-reporter:Shijun Wang ; Sarah F. Swingle ; Heechang Ye ; Fu-Ren F. Fan ; Alan H. Cowley
Journal of the American Chemical Society 2012 Volume 134(Issue 27) pp:11056-11059
Publication Date(Web):June 21, 2012
DOI:10.1021/ja301765v
A p-type boron arsenide photoelectrode was prepared from a material consisting of a thin layer of boron arsenide on a boron substrate. The structure of the material was identified using X-ray diffraction and scanning electron microscopy, and the surface composition was determined by means of X-ray photoelectron spectroscopy. The electrode was found to be photoactive under both visible light and UV–vis irradiation and displayed a photocurrent of ∼0.1 mA/cm2 under UV–vis irradiation at an applied potential of −0.25 V vs Ag/AgCl. Mott–Schottky plots for this boron arsenide electrode displayed an estimated flat-band potential near the onset photopotential. The estimated indirect band gap, as determined from incident photon-to-electron conversion efficiency plots, is 1.46 ± 0.02 eV.
Co-reporter:Honglan Qi ; Jinho Chang ; Sameh H. Abdelwahed ; Khushabu Thakur ; Rajendra Rathore
Journal of the American Chemical Society 2012 Volume 134(Issue 39) pp:16265-16274
Publication Date(Web):September 4, 2012
DOI:10.1021/ja3057997
The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a series of π-stacked poly(fluorenemethylene) oligomers (Fn, n = 1–6) were investigated. The pendant cofacially oriented fluorene moieties are essentially in contact with each other by Van der Waals interaction promoting electronic delocalization in these species. All six compounds give successive cyclic voltammetric one-electron (1e) oxidations in 1:1 acetonitrile/benzene (MeCN/Bz), and the multiple 1e transfer properties of all these compounds were confirmed by chronoamperometric experiments with an ultramicroelectrode and digital simulations. The potentials for oxidation of the successive 1e transfers can be explained in terms of electrostatic interactions among the fluorenes. The monomer (F1) shows one irreversible wave, while F2 shows two reversible 1e waves. F3 shows only two reversible 1e oxidation waves, which is consistent with the large energy to remove a third electron because of the greater electrostatic repulsion, so the third wave is shifted toward more positive potentials. Both F4 and F5 show three reversible 1e oxidation waves, while F6 shows four reversible 1e waves. The removal of the first electron from an oligomer becomes easier as n increases. The stability of the radical cations also increases with n. The removal of consecutive electrons from Fn can be correlated with the distance between fluorene moieties. No reduction peaks were observed except for some broad ones at ∼−3.2 V vs SCE in THF, which is consitent with the wide highest occupied molecular orbital–lowest unoccupied molecular orbital gap in these compounds (absorbance at about 300 nm). No characteristic annihilation ECL signal was observed for these compounds in 1:1 MeCN/Bz mixed solvent. However, the ECL of F6 in the presence of the coreactant C2O42– showed a long-wavelength ECL emission that was proposed to be electrolyzed byproduct from the radical cation.
Co-reporter:Seong Jung Kwon
Journal of the American Chemical Society 2012 Volume 134(Issue 16) pp:7102-7108
Publication Date(Web):March 27, 2012
DOI:10.1021/ja300894f
We investigated the electrochemical detection of single iridium oxide nanoparticle (IrOx NP) collisions at the NaBH4-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response (“blip”) for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrOx NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrOx NP collision, adsorption, and subsequent deactivation.
Co-reporter:Jungdon Suk ; Palani Natarajan ; Jarugu Narasimha Moorthy
Journal of the American Chemical Society 2012 Volume 134(Issue 7) pp:3451-3460
Publication Date(Web):January 17, 2012
DOI:10.1021/ja209894q
The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a variety of 2- and 4-fold anthracene-functionalized tetraarylbimesityls, AB1–4, were investigated. AB1–4 compounds contain a bimesityl core with 2- and 4-fold anthracene functionalities, which generate a rigid D2d-symmetric structure. Cyclic voltammetry of AB1 and AB2 showed two reversible, closely spaced one-electron transfers for both oxidation and reduction, and that of AB3 and AB4 showed four reversible, closely spaced one-electron transfers for oxidation and reduction in a benzene/acetonitrile solution. The multielectron transfer properties of all four compounds were confirmed by chronoamperometric experiments with an ultramicroelectrode and digital simulations. These serve as models to probe how interacting groups on a molecule affect the energies of successive electron transfers. AB1–4 compounds are highly fluorescent in nonaqueous solvents and display blue-green emission. They produce very strong ECL with emission at 480 nm, near that of the photoluminescence spectra that can be assigned to emission by direct formation of the singlet via the S-route.
Co-reporter:Yanqing Cong, Hyun S. Park, Hoang X. Dang, Fu-Ren F. Fan, Allen J. Bard, and C. Buddie Mullins
Chemistry of Materials 2012 Volume 24(Issue 3) pp:579
Publication Date(Web):January 9, 2012
DOI:10.1021/cm203269n
Tantalum cobalt nitride photocatalysts were prepared using a simple drop coating method on a Ta foil substrate followed by thermal ammonia treatment, and their photoelectrochemical (PEC) properties for water oxidation under visible light were studied. The resulting Ta0.9Co0.1Nx films showed a photocurrent of ca. 1.5 mA/cm2 (12 times higher than that of Ta3N5) under 100 mW/cm2 visible light irradiation at 0.7 V vs Ag/AgCl in a 0.1 M Na2SO4 aqueous solution (pH 11). The good PEC performance was attributed to the introduction of cobalt and the formation of cobalt nitride, which efficiently facilitates electron transfer and suppresses the recombination of photogenerated electron–hole pairs. Some cobalt nitride could further be oxidized to generate cobalt oxide, which serves as an efficient electrocatalyst for water oxidation. The enhanced visible light activity and film stability under light irradiation make tantalum cobalt nitride a promising semiconductor for PEC water oxidation.Keywords: photo-oxidation of water; photoelectrochemistry; tantalum cobalt nitride photocatalysts;
Co-reporter:Chaokang Gu, Brent C. Norris, Fu-Ren F. Fan, Christopher W. Bielawski, and Allen J. Bard
ACS Catalysis 2012 Volume 2(Issue 5) pp:746
Publication Date(Web):March 16, 2012
DOI:10.1021/cs3000107
Studies of the hydrogen evolution reaction (HER) were carried out on base-inhibited vapor phase polymerized (BI-VPP) poly(3,4-ethylenedioxythiophene) (PEDOT)-poly(ethylene glycol) (PEG) thin films synthesized on several substrates, including gold, glassy carbon, and titanium, in an acidic electrolyte. The proton reduction overpotential became smaller and the current–potential (i-E) scans moved toward the thermodynamic potential with time. However, they did not move below the proton reduction potential of the substrate. Collectively, the results suggested to us that the polymer film was porous and that the porosity increased with time when the electrode was immersed in solution, with the hydrogen evolution catalysis taking place on the conducting substrate beneath the polymer rather than on our BI-VPP PEDOT-PEG thin films. Comparison of the HER on gold substrates with different impurities was also studied and traces of Pt and Pd at the ppm level significantly improved the HER activity of Au.Keywords: base-inhibited vapor-phase polymerization (BI-VPP); conducting polymer; hydrogen evolution reaction; PEDOT; PEDOT-PEG;
Co-reporter:Alexander B. Nepomnyashchii, Robert J. Ono, Dani M. Lyons, Christopher W. Bielawski, Jonathan L. Sessler and Allen J. Bard
Chemical Science 2012 vol. 3(Issue 8) pp:2628-2638
Publication Date(Web):02 May 2012
DOI:10.1039/C2SC20263H
The electrochemical properties of oligomers of thiophene (with number of monomer units, n, from 2 to 12) and fluorene (n = 3 to 7) were investigated. Both sets of oligomers were characterized by the presence of two oxidation and two reduction waves as determined by cyclic voltammetry (CV), with the reversibility of the waves depending on the structural properties of the compounds. The addition or removal of a third electron was found to be difficult relative to the second, a finding shown for conjugated oligomers with chain lengths up to 7 in the case of the fluorenes and up to 12 for the thiophenes. The oligothiophenes showed a larger separation between the electrochemical waves for the same chain length, and also substantial electrogenerated chemiluminescence (ECL) signals, whose intensity increased with oligomer size. In contrast, the ECL intensity of the fluorene oligomers was essentially independent of chain length. The ECL spectra for the thiophene dodecamer were obtained with concentrations as low as 20 pM, a result that reflects a high ECL efficiency, close to that of the well-known ECL standard Ru(bpy)32+. Oligomers were also formed on electrochemical reduction of an appropriately functionalized dimer in the presence of benzoyl peroxide producing a longer wavelength emission (maximum at ∼540 nm) as opposed to the spectrum of the dimer (λem = 390 nm).
Co-reporter:Jinho Chang, Kevin C. Leonard, Sung Ki Cho, and Allen J. Bard
Analytical Chemistry 2012 Volume 84(Issue 11) pp:5159
Publication Date(Web):May 12, 2012
DOI:10.1021/ac300863r
In this paper, we present a technique to rapidly and directly examine ultramicroelectrodes (UMEs) by white light vertical scanning interferometry (VSI). This technique is especially useful in obtaining topographic information with nanometer resolution without destruction or modification of the UME and in recognizing tips where the metal is recessed below the insulating sheath. Two gold UMEs, one with a metal radius a = 25 μm and relative insulating sheath radius RG = 2 and the other with a = 5 μm and RG = ∼1.5, were examined, and the average depth of the gold recessions was determined to be 1.15 μm and 910 nm, respectively. Electrodeposition of gold was performed to fill the recessed hole, and the depth was reduced to ∼200 nm. With the electrodeposited gold electrode and a conventional microelectrode (a = 25 μm) as a tip and substrate, respectively, a tip/substrate distance, d, of 600 nm was achieved allowing scanning electrochemical microscopy (SECM) in positive feedback mode at a close distance, which is useful for measuring fast kinetics.
Co-reporter:Ashis K. Satpati and Allen J. Bard
Analytical Chemistry 2012 Volume 84(Issue 21) pp:9498
Publication Date(Web):October 2, 2012
DOI:10.1021/ac302349m
We report a simple method of preparation of carbon paste ultramicroelectrodes (UMEs) for use as probe tips in scanning electrochemical microscopy (SECM). Carbon paste UMEs were prepared by packing the carbon paste into a chemically etched tip of a Pt-UME or a pulled glass capillary. Carbon-based UMEs are attractive in micrometer to nanometer gap experiments and in electrodeposition of single metal nanoparticles for electrocatalytic studies because of their high overpotential in proton and oxygen reduction. We have demonstrated the preparation of conically shaped carbon paste UMEs, appropriate for SECM measurements and micrometer to nanometer gap experiments.
Co-reporter:Dodzi Zigah, Joaquín Rodríguez-López and Allen J. Bard
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 37) pp:12764-12772
Publication Date(Web):07 Aug 2012
DOI:10.1039/C2CP40907K
The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH(ads) generated photoelectrochemically at the surface of a nanostructured TiO2 substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl62−/3− offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH(ads) saturation coverage of 338 μC cm−2 was found in our nanostructured samples by its reduction with the electrogenerated IrCl63−. The decay kinetics of ˙OH(ads) by dimerization to produce H2O2 were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ∼kOH = 2.2 × 103 mol−1 m2 s−1. A radical scavenger, such as methanol, competitively consumes ˙OH(ads) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k′MeOH ∼ 1 s−1.
Co-reporter:Alexander B. Nepomnyashchii, Robert J. Ono, Dani M. Lyons, Jonathan L. Sessler, Christopher W. Bielawski, and Allen J. Bard
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 15) pp:2035-2038
Publication Date(Web):July 11, 2012
DOI:10.1021/jz300828r
Thiophene oligomer nanoparticles (NPs) were studied by fluorescence spectroscopy and electrogenerated chemiluminescence (ECL). Distinct spectroscopic differences between aggregates or NPs of thiophene hexamers having differing substitution patterns of solubilizing alkyl groups were observed. The α,ω-unsubstituted thiophene hexamer, Hexamer-2, exhibited fluorescence properties that were similar in solution and as colloidal NPs; there was only a small red shift compared with what was observed for the discrete system dissolved in tetrahydrofuran (THF). In contrast, the oligomer substituted in the α,ω-positions with branched alkyl substituents (Hexamer-1) displayed a gradual bathochromic shift of the fluorescence maximum in proportion to the amount of a poor solvent (water) added to the THF solution. Moreover, the fluorescence characteristics for the oligomer(s) dissolved in a mixture of THF and water were similar to those seen by annihilation ECL in a mixture of benzene/acetonitrile. On this basis, we conclude that annihilation ECL may be a useful technique for monitoring the formation of organic nanoparticles.Keywords: aggregation; branched thiophenes; ECL; electrochemiluminescence; oligothiophenes; sexithiophene;
Co-reporter:Dr. Sung Ki Cho;Dr. Fu-Ren F. Fan ; Allen J. Bard
Angewandte Chemie International Edition 2012 Volume 51( Issue 51) pp:12740-12744
Publication Date(Web):
DOI:10.1002/anie.201206789
Co-reporter:Yanqing Cong, Hyun S. Park, Shijun Wang, Hoang X. Dang, Fu-Ren F. Fan, C. Buddie Mullins, and Allen J. Bard
The Journal of Physical Chemistry C 2012 Volume 116(Issue 27) pp:14541-14550
Publication Date(Web):June 1, 2012
DOI:10.1021/jp304340a
Tantalum nitride (Ta3N5) is a promising material for photoelectrochemical (PEC) water oxidation with a narrow band gap (2.1 eV) that can effectively utilize visible light in the solar spectrum. Ta3N5 nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta3N5 was over 3 times higher than that of a dense regular Ta3N5 film in 0.1 M Na2SO4 aqueous solution at pH 11. Several electrocatalysts (IrO2 nanoparticles (NPs), Co3O4 NPs, cobalt phosphate, and Pt NPs) were used to modify Ta3N5 NTs for PEC water oxidation. The photocurrent of Ta3N5 NTs modified with IrO2 and Co3O4 was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta3N5 NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta3N5 photoanodes for water oxidation, which can reach as high as 88% for a Co3O4–Ta3N5 NTs photoanode, but is less than 15% at best, for Ta3N5 without the electrocatalyst. The results indicate that cobalt oxide and cobalt phosphate are promising candidates as electrocatalysts on Ta3N5 for water oxidation because Co is an earth-abundant material.
Co-reporter:Dr. Sung Ki Cho;Dr. Fu-Ren F. Fan ; Allen J. Bard
Angewandte Chemie 2012 Volume 124( Issue 51) pp:12912-12916
Publication Date(Web):
DOI:10.1002/ange.201206789
Co-reporter:Dipankar Koley
PNAS 2012 Volume 109 (Issue 29 ) pp:
Publication Date(Web):2012-07-17
DOI:10.1073/pnas.1201555109
Oxidative stress induced in live HeLa cells by menadione (2-methyl-1,4-napthaquinone) was studied in real time by scanning
electrochemical microscopy (SECM). The hydrophobic molecule menadione diffuses through a living cell membrane where it is
toxic to the cell. However, in the cell it is conjugated with glutathione to form thiodione. Thiodione is then recognized
and transported across the cell membrane via the ATP-driven MRP1 pump. In the extracellular environment, thiodione was detected
by the SECM tip at levels of 140, 70, and 35 µM upon exposure of the cells to menadione concentrations of 500, 250, and 125 µM,
respectively. With the aid of finite element modeling, the kinetics of thiodione transport was determined to be 1.6 × 10-7 m/s, about 10 times faster than menadione uptake. Selective inhibition of these MRP1 pumps inside live HeLa cells by MK571
produced a lower thiodione concentration of 50 µM in presence of 500 µM menadione and 50 µM MK571. A similar reduced (50%
drop) thiodione efflux was observed in the presence of monoclonal antibody QCRL-4, a selective blocking agent of the MRP1
pumps. The reduced thiodione flux confirmed that thiodione was transported by MRP1, and that glutathione is an essential substrate
for MRP1-mediated transport. This finding demonstrates the usefulness of SECM in quantitative studies of MRP1 inhibitors and
suggests that monoclonal antibodies can be a useful tool in inhibiting the transport of these MDR pumps, and thereby aiding
in overcoming multidrug resistance.
Co-reporter:Alexander B. Nepomnyashchii ; Martin Bröring ; Johannes Ahrens
Journal of the American Chemical Society 2011 Volume 133(Issue 48) pp:19498-19504
Publication Date(Web):October 24, 2011
DOI:10.1021/ja207545t
The electrochemistry of several difluoroboradiaza-s-indacene (BODIPY) compounds lacking substituent groups in the meso (8)- and/or 3 (α)-positions was investigated. Chemical and electrochemical dimerization was demonstrated, and the dimerization depended on the character of substitution. The chemical dimerization was achieved by oxidative coupling using FeCl3 in CH2Cl2 at 0 °C. The electrochemical dimerization proceeded via anodic oxidation to the radical cation and monitored by both cyclic voltammetry (CV) and electrogenerated chemiluminescence (ECL). An available open 3-position was important for the formation of the dimer. The resulting 3,3′-dimer produced a second peak in the CV oxidation and also the appearance of a longer wavelength ECL peak at 656 nm, which is considerably shifted from the parent peak at 532 nm. No dimerization was seen for BODIPY molecules in which only the meso 8-position was unsubstituted, either by chemical or electrochemical means, demonstrating that dimerization occurs at position 3.
Co-reporter:Khalid M. Omer ; Sung-Yu Ku ; Jian-Zhang Cheng ; Shu-Hua Chou ; Ken-Tsung Wong
Journal of the American Chemical Society 2011 Volume 133(Issue 14) pp:5492-5499
Publication Date(Web):March 17, 2011
DOI:10.1021/ja2000825
A new D−A−π−A−D molecule (Spiro-BTA) containing two 2,1,3-benzothiadiazole (BTA) as the acceptor (A) and triphenylamine as the donor (D) bridged by a spirobifluorene moiety has been synthesized. The novel D−A molecule shows intense red emission (612 nm) with a high PL quantum yield (ΦPL = 0.51) in a solid film. A cyclic voltammogram of Spiro-BTA in 1:2 MeCN:benzene/0.1 M Bu4NPF6 shows two reversible oxidation waves and one reversible reduction wave. The first oxidation wave and reduction wave were assigned as two successive electron transfer peaks separated by ∼50 mV related to the oxidation of the two noninteracting donors and the reduction of the two noninteracting acceptors, respectively. Electrogenerated chemiluminescence (ECL) of Spiro-BTA upon cyclic oxidation and reduction in MeCN:benzene 1:2 shows a very bright and stable red emission that could be seen in a well-lit room. Using a reprecipitation method, well-dispersed organic nanoparticles (NPs) of the Spiro-BTA were prepared in aqueous solution. The nanoparticles were analyzed by dynamic light scattering (DLS) and scanning electron microscopy (SEM), yielding a NP size (without surfactant) of 130 ± 20 nm, while with surfactant, 100 ± 20 nm. Bathochromic shifts of absorption spectra (∼16 ± 2 nm), as compared to that of the dissolved Spiro-BTA in THF, were observed for both NPs in water and as a thin film. While blue shifts (14 ± 2 nm) were observed for the photoluminescence (PL). The PL intensity of the Spiro-BTA nanoparticles was slightly enhanced (ΦPL of nanoparticles in water = 48%) over that of the dissolved Spiro-BTA in THF. The ECL of the organic Spiro-BTA nanoparticles in aqueous solution could be observed upon oxidation with tri-n-propylamine as a coreactant.
Co-reporter:Jungdon Suk ; Zhiyong Wu ; Lei Wang
Journal of the American Chemical Society 2011 Volume 133(Issue 37) pp:14675-14685
Publication Date(Web):August 1, 2011
DOI:10.1021/ja203731n
We report the electrochemical characterization and the observation of excimer emission from a series of 9-naphthylanthracene-based dimer- and trimer-bridged high steric hindrance aromatic groups during photoluminescence (PL) measurements in the solid state and in solution electrogenerated chemiluminescence (ECL) measurements. Cyclic voltammetry of 4,4′-bis(9-(1-naphthyl)anthracen-10-yl)biphenyl (4A) and 1,3,5-tris(9-(1-naphthyl)anthracen-10-yl)benzene (4C) showed two or three reversible, closely spaced one-electron transfers on oxidation in dichloromethane. The ECL emission spectra of 4A and 4C resulting from the annihilation reaction in benzonitrile showed two bands: one at the same wavelength as the PL peak in the solution state, and a broad band at longer wavelength. With a coreactant, such as peroxydisulfate, ECL spectra showed a single peak that was less broad in shape. PL measurement in the solid state and measurement of representative time traces of PL intensity, lifetimes, and picosecond time-correlated single-photon counting confirmed excimer emission at long wavelength. A reprecipitation method was used to prepare well-dispersed organic nanoparticles (NPs) of 4A in both aqueous and acetonitrile solutions. The smallest stable size of NPs produced was ∼15 ± 6 nm, as analyzed by transmission electron microscopy. These organic NPs produced stable and weak ECL emission from the annihilation reaction in both aqueous and MeCN solutions. With a coreactant, such as peroxydisulfate, the ECL signal on reduction was sufficiently strong to obtain an ECL spectrum.
Co-reporter:Alexander B. Nepomnyashchii, Martin Bröring, Johannes Ahrens, and Allen J. Bard
Journal of the American Chemical Society 2011 Volume 133(Issue 22) pp:8633-8645
Publication Date(Web):May 12, 2011
DOI:10.1021/ja2010219
Synthesis of the C8 BODIPY monomers, dimers, and trimers, a C8 polymer, and N8 aza-BODIPY monomer and dimer was carried out. Methyl and mesityl C8-substituted monomers, dimers, and trimers were used. Dimers, trimers, and polymer were formed chemically through the β–β (2/6) positions by oxidative coupling using FeCl3. A red shift of the absorbance and fluorescence is observed with addition of monomer units from monomer to polymer for C8 dyes. The aza-BODIPY dye shows red-shifted absorbance and fluorescence compared with the C8 analogue. Cyclic voltammetry shows one, two, and three one-electron waves on both reduction and oxidation for the monomer, dimer, and trimer, respectively, for the C8 BODIPYs. The separation for the reduction peaks for the C8 dimers is 0.12 V compared with 0.22 V for the oxidation, while the trimers show separations of 0.09 V between reduction peaks and 0.13 V for oxidation peaks. The larger separations between the second and third peaks, 0.25 V for the oxidation and 0.2 V for the reduction, are consistent with a larger energy to remove or add a third electron compared with the second one. The BODIPY polymer shows the presence of many sequential one-electron waves with a small separation. These results provide evidence for significant electronic interactions between different monomer units. The aza-BODIPY dye shows a reduction peak 0.8 V more positive compared to the C8 compound. Aza-BODIPY dimer shows the appearance of four waves in dichloromethane. The separation between two consecutive waves is around 0.12 V for reduction compared with 0.2 V for oxidation, which is comparable with the results for the C8 dyes. Electrogenerated chemiluminescence (ECL) of the different species was obtained, including weak ECL of the polymer.
Co-reporter:Mei Shen, Netzahualcóyotl Arroyo-Currás, and Allen J. Bard
Analytical Chemistry 2011 Volume 83(Issue 23) pp:9082
Publication Date(Web):October 22, 2011
DOI:10.1021/ac2021294
Scanning electrochemical microscopy (SECM) tips with rounded glass insulation around the metal wire (radius a = 5 μm) were fabricated (apparent RG < 1.1, where RG is the ratio of the radius of the insulation sheath divided by the electrode radius), and their SECM feedback approach curves were studied in solutions of tris(2,2′-bipyridine)ruthenium(2+) (Rubpy) in acetonitrile and ferrocenemethanol in water with a platinum disk as the substrate electrode (radius as = 1 mm). Considerable enhancement of the normalized feedback current, IT(L) = iT/ iT,∞, where L = d/a and d is the distance traveled by the SECM tip, was observed in both systems (e.g., IT(L) = 15 in organic solutions and IT(L) = 30 in aqueous solutions) with good electrode alignment. This shows that tip-to-substrate gaps of ca. d = 110 nm can be achieved. To account for any deviations from the usual disk UME behavior and currents caused by possible changes in the tip electrode geometry, simulations of the feedback response were performed for a 2D axisymmetric environment. All simulated results match in a point-to-point comparison with experimental values (average relative standard deviation (RSD) = 0.01 ± 0.005).
Co-reporter:Seong Jung Kwon, Hongjun Zhou, Fu-Ren F. Fan, Vasily Vorobyev, Bo Zhang and Allen J. Bard
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 12) pp:5394-5402
Publication Date(Web):28 Feb 2011
DOI:10.1039/C0CP02543G
Collisions of several kinds of metal or metal oxide single nanoparticles (NPs) with a less catalytic electrode surface have been observed through amplification of the current by electrocatalysis. Two general types of current response, a current staircase or a current blip (or spike) are seen with particle collisions. The current responses were caused by random individual events as a function of time rather than the usual continuous current caused by an ensemble of a large number of events. The treatment of stochastic electrochemistry like single NP collisions is different from the usual model for ensemble-based electrochemical behaviour. Models for the observed responses are discussed, including simulations, and the frequency of the steps or blips investigated for several systems experimentally.
Co-reporter:Joel Rosenthal, Alexander B. Nepomnyashchii, Julia Kozhukh, Allen J. Bard, and Stephen J. Lippard
The Journal of Physical Chemistry C 2011 Volume 115(Issue 36) pp:17993-18001
Publication Date(Web):August 10, 2011
DOI:10.1021/jp204487r
Two new 2,2′-bipyridine (bpy)-based ligands with ancillary BODIPY chromophores attached at the 4- and 4′-positions were prepared and characterized, which vary in the substitution pattern about the BODIPY periphery by either excluding (BB1) or including (BB2) a β-alkyl substituent. Both absorb strongly throughout the visible region and are strongly emissive. The basic photophysics and electrochemical properties of BB1 and BB2 are comparable to those of the BODIPY monomers on which they are based. The solid-state structures and electronic structure calculations both indicate that there is negligible electronic communication between the BODIPY moieties and the intervening bpy spacers. Electrogenerated chemiluminescence spectra of the two bpy-BODIPY derivatives are similar to their recorded fluorescence profiles and are strongly influenced by substituents on the BODIPY chromophores. These 2,2′-bipyridine derivatives represent a new set of ligands that should find utility in applications, including light-harvesting, photocatalysis, and molecular electronics.
Co-reporter:Jungdon Suk, Jian-Zhang Cheng, Ken-Tsung Wong, and Allen J. Bard
The Journal of Physical Chemistry C 2011 Volume 115(Issue 30) pp:14960-14968
Publication Date(Web):June 22, 2011
DOI:10.1021/jp205621e
We report the synthesis, electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a donor (D)–acceptor (A) molecule of form D–A−π–A–D, 4,4′-(7,7′-(9,9-bis(6-azidohexyl)-9H-fluorene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole7,4-diyl))bis(N,N-diphenylaniline) (Azide-BTA), and its organic nanoparticles (NPs). Azide-BTA consists of two 2,1,3-benzothiadiazole (A) and triphenylamine (D) groups at the ends bridged by a fluorene (π) moiety. Cyclic voltammetry (CV) of Azide-BTA showed a single reversible reduction wave (Eored = −1.48 V vs SCE) and two reversible oxidation waves (Eo1,ox = 0.92 V, Eo2,ox = 1.34 V vs SCE). The first oxidation and reduction waves were assigned two reversible, closely spaced one-electron transfers. Azide-BTA exhibited a large solvatochromic effect in the emission ranging from yellow (580 nm) to red (633 nm). The ECL spectrum resulting from the annihilation reaction showed a bright and single peak with a maximum at 652 nm in mixed solvent of acetonitrile and benzene (MeCN/Bz). Using a reprecipitation method, we prepared well-dispersed and spherical organic NPs of Azide-BTA in an aqueous solution. The size of the NPs was controlled by the preparation conditions, i.e., concentration of Azide-BTA in THF, water temperature, stirring rate, and method of dropping into water. The smallest and most stable size of the NPs was 16 ± 6 nm as analyzed by TEM. Those organic NPs showed stable and moderate intensity in annihilation ECL emission in an aqueous solution. With a coreactant, such as peroxydisulfate, an ECL spectrum was obtained that showed a single broader peak relative to that produced by annihilation from dissolved Azide-BTA molecules.
Co-reporter:Jungdon Suk, Khalid M. Omer, Thomas Bura, Raymond Ziessel, and Allen J. Bard
The Journal of Physical Chemistry C 2011 Volume 115(Issue 31) pp:15361-15368
Publication Date(Web):June 30, 2011
DOI:10.1021/jp201844s
The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of three BODIPY derivatives, PB, MCPB, and DCPB, are investigated. Cyclic voltammetry of all three derivatives shows at low scan rates reversible reduction waves and less reversible oxidation waves (consistent with lack of substitution in the 2 and 6 positions). The three compounds are highly fluorescent in MeCN emitting in the green to the red regions. All of the three compounds produce intense ECL, strong enough to be seen with the naked eye in a lighted room, but are not stable under continued pulsing of potential because of instability of the radical cation, ultimately producing an insulating polymer layer on the electrode surface. The ECL spectra are at essentially the same wavelengths as the photoluminescence (PL) spectra for MCPB and DCPB, assigned as emission by the T- or ST-route. The ECL spectra of PB show four peaks at 516, 581, 663, and 752 nm. The peaks at 581, 663, and 752 nm are not present in the PL spectrum. The first ECL peak is attributed to the singlet excited state at essentially the same wavelength as PL, while oligomers or other byproducts are probably responsible for the additional two ECL peaks (581 and 663 nm) based on the results from bulk electrolysis and mass spectroscopy.
Co-reporter:Heechang Ye ; Hyun S. Park
The Journal of Physical Chemistry C 2011 Volume 115(Issue 25) pp:12464-12470
Publication Date(Web):May 25, 2011
DOI:10.1021/jp200852c
Oxygen evolution reaction (OER) electrocatalyst arrays for photoelectrochemical (PEC) water oxidation were fabricated on a metal oxide semiconductor photoelectrode, W-doped BiVO4 (BiVW-O). The electrocatalysts (IrOx, Pt, Co3O4) were prepared on a drop cast film of BiVW-O on fluorine-doped tin oxide (FTO) with a picoliter solution dispenser or photodeposition with light irradiation through an optical fiber. The prepared arrays were tested for PEC water oxidation in 0.2 M sodium phosphate buffer (pH 6.8) using scanning electrochemical microscopy modified with an optical fiber. Pt and Co oxide electrocatalysts showed an enhanced photocurrent for PEC water oxidation, while the other metal oxide catalysts including IrOx, which is known as an excellent water oxidation electrocatalyst on a metal substrate, were not effective. These results were confirmed with bulk film studies. A cobalt phosphate (Co-Pi) electrocatalyst was also tested as a bulk film on BiVW-O and showed improvement for PEC water oxidation. Preliminary characterization (X-ray diffraction and X-ray photoelectron spectroscopy) was also performed for these catalysts. The results indicate that considerations of the semiconductor photocatalyst/electrocatalyst interface are important in determining the effectiveness of materials for the photodriven OER.
Co-reporter:Hyun S. Park ; Kyoung Eun Kweon ; Heechang Ye ; Eunsu Paek ; Gyeong S. Hwang
The Journal of Physical Chemistry C 2011 Volume 115(Issue 36) pp:17870-17879
Publication Date(Web):July 26, 2011
DOI:10.1021/jp204492r
Metal doping of the metal oxide photoelectrocatalyst, BiVO4, dramatically increases its activity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for their photoelectrochemical performance and to optimize the used dopant material concentrations with this photocatalyst. For example, adding Mo to W-doped BiVO4 enhanced the performance. The photocatalytic activity was examined on larger electrodes by means of photoelectrochemical and electrochemical measurements. The developed photoelectrocatalyst, W- and Mo-doped BiVO4, shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO4. Factors that affect performance are discussed, and enhanced separation of excited electron–hole pairs by doping onto the semiconductor is suggested by first-principles density-functional theory (DFT) calculations. Distortion of the crystal structure of monoclinic scheelite-like BiVO4 by addition of W and Mo doping predicted by DFT is also revealed by X-ray diffraction and Rietveld refinement analysis. The results indicate that the consecutive doping of W and Mo into the metal oxide photocatalyst introduces improved electron–hole separation without a significant change of the band gap or the material's optical properties.
Co-reporter:Heechang Ye ; Hyun S. Park ; Vahid A. Akhavan ; Brian W. Goodfellow ; Matthew G. Panthani ; Brian A. Korgel
The Journal of Physical Chemistry C 2011 Volume 115(Issue 1) pp:234-240
Publication Date(Web):December 13, 2010
DOI:10.1021/jp108170g
Photoelectrochemical (PEC) studies of CuInSe2 (CIS) and Cu(In1−xGax)Se2 (CIGS) thin films prepared by electrodeposition and spray coating of nanocrystals were carried out. PEC reductions of methyl viologen (MV) in aqueous solution and ethyl viologen (EV) in acetonitrile were used to evaluate the thin-film performance by measuring the photocurrent. Deposition of a CdS layer on CIS and CIGS films strongly enhanced the photocurrent; for example, a nanocrystal-CIS film showed about 100 times higher photocurrent with a thin CdS layer. Capacitance measurements and Mott−Schottky plots were obtained to find the flat band potential. Incident photon to current conversion efficiencies (IPCE) and absorbed photon to current conversion efficiencies (APCE) obtained from PEC measurements were about 20% and 40−70%, respectively.
Co-reporter:Fen Zhang ; Vladimir Roznyatovskiy ; Fu-Ren F. Fan ; Vincent Lynch ; Jonathan L. Sessler
The Journal of Physical Chemistry C 2011 Volume 115(Issue 5) pp:2592-2599
Publication Date(Web):January 12, 2011
DOI:10.1021/jp110482v
A picoliter solution dispenser was used to fabric-ate photosensitizer arrays on mesoporous TiO2 electrodes. The scanning electrochemical microscopy (SECM) technique, modified by replacing the usual ultramicroelectrode (UME) with an optical fiber and using the photooxidation of iodide in acetonitrile in a photoelectrochemical (PEC) cell, was shown to be useful for the initial screening of potential PEC photosensitizers. This SECM technique allows for the rapid identification of new dyes and also can be used to investigate the synergetic effect of multiple dyes for application in dye-sensitized solar cells (DSSCs). This technique was specifically demonstrated via the synthesis and analysis of a new, bis-bithiophene functionalized porphyrin derivative, wherein the modified SECM technique was used to carry out an initial test of its PEC efficiency relative to other dyes. The PEC properties of bulk films based on this new porphyrin derivative were then investigated and the results shown to be in good agreement with those obtained using the SECM method.
Co-reporter:Xiaole Chen;Xiuhui Liu;Dipankar Koley;Marvin Whiteley;Matthew M. Ramsey
PNAS 2011 Volume 108 (Issue 7 ) pp:2668-2673
Publication Date(Web):2011-02-15
DOI:10.1073/pnas.1018391108
Quantitative detection of hydrogen peroxide in solution above a Streptococcus gordonii (Sg) bacterial biofilm was studied in real time by scanning electrochemical microscopy (SECM). The concentration of hydrogen
peroxide was determined to be 0.7 mM to 1.6 mM in the presence of 10 mM glucose over a period of 2 to 8 h. The hydrogen peroxide
production measured was higher near the biofilm surface in comparison to Sg grown planktonically. Differential hydrogen peroxide production was observed both by fluorometric as well as by SECM measurements.
The interaction between two different species in a bacterial biofilm of Sg and Aggregatibacter actinomycetemcomitans (Aa) in terms of hydrogen peroxide production was also studied by SECM. One-directional y-scan SECM measurements showed the unique
spatial mapping of hydrogen peroxide concentration across a mixed species biofilm and revealed that hydrogen peroxide concentration
varies greatly dependent upon local species composition.
Co-reporter:Jungdon Suk
Journal of Solid State Electrochemistry 2011 Volume 15( Issue 11-12) pp:2279-2291
Publication Date(Web):2011 December
DOI:10.1007/s10008-011-1449-x
This review discusses briefly the preparation, electrochemistry, and electrogenerated chemiluminescence (ECL) as well as spectroscopic properties of organic nanoparticles. Organic nanoparticles, ranging from several tens of nanometers to hundreds of nanometers in diameter, were successfully prepared by various methods. Using a simple reprecipitation method, organic nanoparticles of a very small size can be prepared and show unique electrochemical and ECL characteristics. As with inorganic nanoparticles, organic nanoparticles suggest possible applications, like labels for the analysis of biological materials with ECL.
Co-reporter:Dipankar Koley;Marvin Whiteley;Matthew M. Ramsey
PNAS 2011 Volume 108 (Issue 50 ) pp:19996-20001
Publication Date(Web):2011-12-13
DOI:10.1073/pnas.1117298108
Bacteria are social organisms that possess multiple pathways for sensing and responding to small molecules produced by other
microbes. Most bacteria in nature exist in sessile communities called biofilms, and the ability of biofilm bacteria to sense
and respond to small molecule signals and cues produced by neighboring biofilm bacteria is particularly important. To understand
microbial interactions between biofilms, it is necessary to perform rapid, real-time spatial quantification of small molecules
in microenvironments immediately surrounding biofilms; however, such measurements have been elusive. In this study, scanning
electrochemical microscopy was used to quantify small molecules surrounding a biofilm in 3D space. Measuring concentrations
of the redox-active signaling molecule pyocyanin (PYO) produced by biofilms of the bacterium Pseudomonas aeruginosa revealed a high concentration of PYO that is actively maintained in the reduced state proximal to the biofilm. This gradient
results in a reduced layer of PYO that we have termed the PYO “electrocline,” a gradient of redox potential, which extends
several hundred microns from the biofilm surface. We also demonstrate that the PYO electrocline is formed under electron acceptor-limiting
conditions, and that growth conditions favoring formation of the PYO electrocline correlate to an increase in soluble iron.
Additionally, we have taken a “reactive image” of a biofilm surface, demonstrating the rate of bacterial redox activity across
a 2D surface. These studies establish methodology for spatially coordinated concentration and redox status measurements of
microbe-produced small molecules and provide exciting insights into the roles these molecules play in microbial competition
and nutrient acquisition.
Co-reporter:Joaquín Rodríguez-López
Journal of the American Chemical Society 2010 Volume 132(Issue 14) pp:5121-5129
Publication Date(Web):March 12, 2010
DOI:10.1021/ja9090319
The surface interrogation mode of scanning electrochemical microscopy (SECM) is extended to the in situ quantification of adsorbed hydrogen, Hads, at polycrystalline platinum. The methodology consists of the production, at an interrogator electrode, of an oxidized species that is able to react with Hads on the Pt surface and report the amounts of this adsorbate through the SECM feedback response. The technique is validated by comparison to the electrochemical underpotential deposition (UPD) of hydrogen on Pt. We include an evaluation of electrochemical mediators for their use as oxidizing reporters for adsorbed species at platinum; a notable finding is the ability of tetramethyl-p-phenylenediamine (TMPD) to oxidize (interrogate) Hads on Pt at low pH (0.5 M H2SO4 or 1 M HClO4) and with minimal background effects. As a case study, the decomposition of formic acid (HCOOH) in acidic media at open circuit on Pt was investigated. Our results suggest that formic acid decomposes at the surface of unbiased Pt through a dehydrogenation route to yield Hads at the Pt surface. The amount of Hads depended on the open circuit potential (OCP) of the Pt electrode at the time of interrogation; at a fixed concentration of HCOOH, a more negative OCP yielded larger amounts of Hads until reaching a coulomb limiting coverage close to 1 UPD monolayer of Hads. The introduction of oxygen into the cell shifted the OCP to more positive potentials and reduced the quantified Hads; furthermore, the system was shown to be chemically reversible, as several interrogations could be run consecutively and reproducibly regardless of the path taken to reach a given OCP.
Co-reporter:Mei Shen ; Joaquín Rodríguez-López ; Ju Huang ; Quan Liu ; Xu-Hui Zhu
Journal of the American Chemical Society 2010 Volume 132(Issue 38) pp:13453-13461
Publication Date(Web):September 2, 2010
DOI:10.1021/ja105282u
We report here the electrochemistry and electrogenerated chemiluminescence (ECL) of a red-emitting dithienylbenzothiadiazole-based molecular fluorophore (4,7-bis(4-(4-sec-butoxyphenyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole, 1b). 1b contains two substituted thiophene groups as strong electron donors at the ends connected directly to a strong electron acceptor, 2,1,3-benzothiadiazole, in the center. Each thiophene moiety is substituted in position 2 by 3,5-di(1-naphthyl)phenyl and in position 3 by 4-sec-butoxyphenyl. Cyclic voltammetry of 1b, with scan rate ranging from 0.05 to 0.75 V/s, shows a single one-electron reduction wave (E°red = −1.18 V vs SCE) and two nernstian one-electron oxidation waves (E°1,ox = 1.01 V, E°2,ox = 1.24 V vs SCE). Reduction of the unsubstituted 2,1,3-benzothiadiazole center shows nernstian behavior with E°red = −1.56 V vs SCE. By comparison to a digital simulation, the heterogeneous electron-transfer rate constant for reduction, kr° = 1.5 × 10−3 cm/s, is significantly smaller than those for the oxidations, ko° > 0.1 cm/s, possibly indicating that the two substituted end groups have a blocking effect on the reduction of the benzothiadiazole center. The ECL spectrum, produced by electron-transfer annihilation of the reduced and oxidized forms, consists of a single peak with maximum emission at about 635 nm, consistent with the fluorescence of the parent molecule. Relative ECL intensities with respect to 9,10-diphenylanthracene are 330% and 470% for the radical anion−cation and radical anion−dication annihilation, respectively. Radical anion (A−•)−cation (A+•) annihilation produced by potential steps shows symmetric ECL transients during anodic and cathodic pulses, while for anion (A−•)−dication (A2+•) annihilation, transient ECL shows asymmetry in which the anodic pulse is narrower than the cathodic pulse. Digital simulation of the transient ECL experiments showed that the origin of the observed asymmetry is asymmetry in the amount of generated charges rather than instability of the electrogenerated species.
Co-reporter:Seong Jung Kwon ; Fu-Ren F. Fan
Journal of the American Chemical Society 2010 Volume 132(Issue 38) pp:13165-13167
Publication Date(Web):September 1, 2010
DOI:10.1021/ja106054c
We describe the electrochemical detection of single iridium oxide nanoparticle (IrOx NP) collisions on a NaBH4-treated Pt ultramicroelectrode (UME). We observe single NP events through the enhanced current by electrocatalytic water oxidation, when IrOx contacts the electrode and transiently sticks to it. The overall current transient consists of repeated current spikes that return to the background level, superimposed on a current decay, rather than the staircase response seen where an NP sticks on the UME. Here each event produces a unique current spike (or “blip”). The frequency of the spikes was directly proportional to the particle concentration, and the peak current increased with the applied potential. The observed current is very sensitive to the material and surface state of the measuring electrode; a NaBH4-treated Pt UME was important in obtaining reproducible results.
Co-reporter:Alexander B. Nepomnyashchii ; Sangik Cho ; Peter J. Rossky
Journal of the American Chemical Society 2010 Volume 132(Issue 49) pp:17550-17559
Publication Date(Web):November 22, 2010
DOI:10.1021/ja108108d
Electrochemistry and electrogenerated chemiluminescence (ECL) of selected substituted BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been studied. The location and nature of substituents on positions 1−8 are important in predicting the behavior, and especially the stability, of the radical ions formed on electron transfer. Dyes with unsubstituted positions 2, 6, and 8 show a kinetic contribution to both oxidation and reduction. Dyes with only unsubstituted positions 2 and 6 and a substituted 8 position show chemically reversible reduction but irreversible oxidation. Unsubstituted positions 2 and 6 tend to show dimer formation on oxidation. Completely substituted dyes show nernstian oxidation and reduction. Oxidation and reduction studies of simple BODIPY dyes show an unusually large separation between the first and second reduction peaks and also the first and second oxidation peaks, of about 1.1 V, which is very different from that observed for polycyclic hydrocarbons and other heteroaromatic compounds, where the spacing is usually about 0.5 V. Electronic structure calculations confirmed this behavior, and this effect is attributed to a greater electronic energy required to withdraw or add a second electron and a lower relative solvation energy for the dianion or dication compared with those of the polycyclic hydrocarbons. ECL was generated for all compounds either by annihilation or by using a co-reactant.
Co-reporter:Thiruvengadam Munisamy, Allen J. Bard
Electrochimica Acta 2010 Volume 55(Issue 11) pp:3797-3803
Publication Date(Web):15 April 2010
DOI:10.1016/j.electacta.2010.01.097
Electrodeposition of Si on substrates such as Ni, Ag and GC from acetonitrile and tetrahydrofuran containing SiCl4 and SiHCl3 as precursors is reported. The deposits readily oxidize on exposure to air but XPS analysis of a deposit with minimum contact with air confirms that elemental Si (2p, 99.5 eV) is electrodeposited. The deposits contained C, O, N and Cl impurities; annealing under argon improved the quality of the deposits. Studies related to initial stages of Si growth using cyclic voltammetry and chronoamperometry suggest that growth occurs via four stages (1) nucleation and localized growth, (2) growth in between islands, (3) growth inhibition and (4) slow growth stage and the growth is inhibited after a layer (∼2 nm) of Si is electrodeposited on the substrate.
Co-reporter:Allen J. Bard;Hongjun Zhou ;Seong Jung Kwon
Israel Journal of Chemistry 2010 Volume 50( Issue 3) pp:267-276
Publication Date(Web):
DOI:10.1002/ijch.201000014
Abstract
Recent experiments on the observation of collisions of single nanoparticles (NPs) with an electrode through amplification of the current by electrocatalysis are described. Systems in which the particles adhere to the electrode upon collision produce a step and staircase response, while those in which particles only interact for a short time with the electrode produce a spike or blip, with little change in the steady state current. Examples of both behaviors, e. g., Pt NPs on a Au electrode for hydrazine oxidation (staircase response) and IrOx NPs on a Pt electrode for water oxidation (blip response) are shown. Controlling the nature of the electrode surface is important in generating useful responses, for example, in the case of gold NPs on an oxidized Pt electrode for borohydride oxidation.
Co-reporter:Allen J. Bard;Hongjun Zhou ;Seong Jung Kwon
Israel Journal of Chemistry 2010 Volume 50( Issue 3) pp:
Publication Date(Web):
DOI:10.1002/ijch.201090007
Co-reporter:Qian Wang;Joaquín Rodríguez-López; Dr. Allen J. Bard
ChemPhysChem 2010 Volume 11( Issue 13) pp:2969-2978
Publication Date(Web):
DOI:10.1002/cphc.201000183
Abstract
The volume created by the positioning of two scanning electrochemical microscope (SECM) probes (tip and substrate) at a micrometric distance defines a “picoliter beaker” where homogeneous electron-transfer reactions are studied. The SECM is used to concurrently electrogenerate in situ two reactive species and to evaluate the possibility of detecting their reactivity. Two reaction cases are studied: the first, called the “reversible case”, occurs when the electrochemically generated species at the substrate electrode can also react at the tip to yield the same product as the reaction in the gap. The second case, named the “irreversible case”, occurs when the electrochemically generated species at the substrate are not able to react at the tip. Digital simulations are performed and compared to experimental studies. These show that an unusual compensation between collection and feedback effects render the analysis inapplicable in the “reversible case”. The “irreversible case” is shown experimentally.
Co-reporter:Joaquín Rodríguez-López ; Alessandro Minguzzi
The Journal of Physical Chemistry C 2010 Volume 114(Issue 43) pp:18645-18655
Publication Date(Web):October 14, 2010
DOI:10.1021/jp107259h
The surface interrogation mode of scanning electrochemical microscopy (SI-SECM), an in situ method for the quantification of adsorbed intermediates at electrodes at open circuit, was used to evaluate the rate of reaction of four redox mediators of different reducing power (methyl viologen+ > Ru(NH3)62+ > Fe(II)[EDTA]2− > Fe(CN)64−) with electrogenerated platinum oxides (PtOx) at different pHs. The rate constant determined by SI-SECM was sensitive to the nature of the Pt oxide formed at different potentials. A simplified model with finite element method simulation was used to fit the SI-SECM experimental response. The extraction of kinetic constants and their comparison to the potential (reducing power) of the mediator yielded a Marcus-theory type relationship. Although these are clearly not outer sphere reactions, if subjected to the Marcus theory treatment, one can extract an estimated value of λ = 1.3 ± 0.3 eV for the reduction of “place exchanged” oxide, which forms with oxide coverage values larger than 0.5 (as PtO). Steady state feedback experiments were also conducted in which the reduced form of the mediators was oxidized at Pt with different oxide coverage, i.e., over a wide range of potentials. A comparison of SI-SECM kinetic analysis with the Pt substrate at open circuit to the case where an electrode bias is applied indicates that for Fe(II)[EDTA]2−, the feedback current follows the oxide coverage at the Pt substrate. We suggest that thin layers of oxide provide a chemical route in which the oxide mediates oxidation of the reduced electrogenerated species; this route is governed by the reactivity of the different species of oxide formed at the electrode and not necessarily by other effects, such as direct electron tunneling through the oxide layer.
Co-reporter:Hongjun Zhou, Fu-Ren F. Fan, and Allen J. Bard
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 18) pp:2671-2674
Publication Date(Web):August 30, 2010
DOI:10.1021/jz100963y
After growing a thin layer of oxide (PtOx) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH4 oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH4 oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification from NaBH4 oxidation. The current response is affected by NP concentration and the applied potential.Keywords (keywords): collision; electrocatalytic amplification; nanoparticles; surface modification; ultramicroelectrode;
Co-reporter:Dipankar Koley
PNAS 2010 Volume 107 (Issue 39 ) pp:16783-16787
Publication Date(Web):2010-09-28
DOI:10.1073/pnas.1011614107
Changes in HeLa cell morphology, membrane permeability, and viability caused by the presence of Triton X-100 (TX100), a nonionic
surfactant, were studied by scanning electrochemical microscopy (SECM). No change in membrane permeability was found at concentrations
of 0.15 mM or lower during an experimental period of 30 to 60 min. Permeability of the cell membrane to the otherwise impermeable,
highly charged hydrophilic molecule ferrocyanide was seen starting at concentrations of TX100 of about 0.17 mM. This concentration
level of TX100 did not affect cell viability. Based on a simulation model, the membrane permeability for ferrocyanide molecules
passing though the live cell membrane was 6.5 ± 2.0 × 10-6 m/s. Cells underwent irreversible permeabilization of the membrane and structural collapse when the TX100 concentration reached
the critical micelle concentration (CMC), in the range of 0.19 to 0.20 mM. The impermeability of ferrocyanide molecules in
the absence of surfactant was also used to determine the height and diameter of a single living cell with the aid of the approach
curve and probe scan methods in SECM.
Co-reporter:Heechang Ye, Joowook Lee, Jum Suk Jang and Allen J. Bard
The Journal of Physical Chemistry C 2010 Volume 114(Issue 31) pp:13322-13328
Publication Date(Web):July 15, 2010
DOI:10.1021/jp104343b
A picoliter solution dispenser was used to fabricate various n-type BiVO4 based photocatalyst arrays having different compositions and a scanning electrochemical microscopy (SECM) technique modified by replacing a normal ultramicroelectrode with an optical fiber was used for fast screening of the effective photocatalysts for Na2SO3 (as a sacrificial reductant) and water oxidation. Bi/V/W oxide with a ratio of 4.5:5:0.5 showed ∼4.6× higher photocurrent than Bi/V oxide without W and this result was confirmed with bulk film studies. Preliminary characterization (XRD, XPS, and EIS) was also performed for these catalysts. This material was tested for photoelectrochemical water oxidation with a Pt ring optical fiber to detect the product (O2) of photoelectrochemical oxidation reaction.
Co-reporter:Mei Shen, Joaquín Rodríguez-López, Yi-Ting Lee, Chin-Ti Chen, Fu-Ren F. Fan and Allen J. Bard
The Journal of Physical Chemistry C 2010 Volume 114(Issue 21) pp:9772-9780
Publication Date(Web):May 6, 2010
DOI:10.1021/jp911451v
We report here the electrogenerated chemiluminescence (ECL) of a red fluorophore diphenylaminospirobifluorenylfumaronitrile (FPhSPFN). FPhSPFN contains two diphenylamino groups as strong electron donors at the ends linked through a nonplanar bulky spirobifluorene linker to a common fumaronitrile group as a strong electron acceptor. The cyclic voltammetry of this compound shows two one-electron transfer reduction waves and one single overall two-electron oxidation wave consisting of closely spaced waves with a peak separation of 63 mV. Chronoamperometry with an ultramicroelectrode (UME) confirmed that each reduction wave involves a 1e transfer and that the oxidation wave contains an overall 2e transfer. The first reduction wave is Nernstian, Eo1,red = −1.09 V versus SCE, and the second reduction wave, Eo2,red = −1.50 V versus SCE, produces a film on the electrode. By comparison to a digital simulation, the oxidation is assigned to two reversible, closely spaced, one-electron processes for oxidation with Eo1,ox = 1.05 V and Eo2,ox = 1.11 V versus SCE. A large wavelength shift (∼35 nm) was observed in ECL as compared to photoluminescence (PL). The ECL spectrum showed a maximum emission at 708 nm with a red-shifted shoulder at around 750 nm. ECL was studied in mixtures of benzene/acetonitrile at different composition ratios; an increase in the ratio of the ECL intensity of the shoulder with respect to the main peak emission was observed with increasing solvent polarity. ECL generated with BPO as a coreactant also showed the red-shifted shoulder at the same ratio as that with annihilation.
Co-reporter:Guanjie Liu
The Journal of Physical Chemistry C 2010 Volume 114(Issue 41) pp:17509-17513
Publication Date(Web):September 29, 2010
DOI:10.1021/jp1058879
CuInSe2 and CuInMSe2 (M = Ga, Zn, Cd) arrays were prepared from precursor aqueous:glycerol (3:1) solutions with a robotic piezoelectric dispenser on fluorine-doped tin oxide (FTO) conductive glass. Studies of the photoresponse of spots on the array by scanning electrochemical microscopy (SECM) with an optical fiber tip showed that different annealing temperatures affected the position on the array of the optimal spot (the spot that showed the highest photocurrent). Depositing a thin layer of CdS on the CIS-250 array improved the photoactivity of the spots and also affected the optimal spot position. Adding 20−40% Ga into the CIS array increased the photocurrent, but addition of Zn or Cd did not.
Co-reporter:Guanjie Liu ; Chongyang Liu
The Journal of Physical Chemistry C 2010 Volume 114(Issue 49) pp:20997-21002
Publication Date(Web):November 15, 2010
DOI:10.1021/jp1058116
Quaternary-alloyed ZnxCd1−xSySe1−y arrays were prepared from four precursor aqueous/glycerol (3:1) solutions with a robotic piezoelectric dispenser on fluorine-doped tin oxide (FTO) conductive glass. Hydrazine (N2H4) was used for protecting the selenium source, dimethylselenourea, from oxidation so that fabrication of the arrays could be carried out under ambient air conditions. The absorption spectra of the photocatalyst spots (230-μm size) on the array were measured with a fiber-optic/CCD detector system. The band gaps were a function of the elemental composition and ranged from 1.8 to 3.6 eV. The photoelectrochemical properties were evaluated by scanning electrochemical microscopy in the photoelectrochemical mode using an optical fiber tip attached to a Xe lamp as the excitation source. The spot with a precursor composition Zn0.3Cd0.7S0.8Se0.2 (elemental ratio, 1:2.12:1.75:0.81) showed the highest photocurrent under 150 W Xe lamp irradiation. The difference of the photocurrent onset indicated that addition of Zn raises the conduction band position of CdSySe1−y.
Co-reporter:Qian Wang ; Joaquín Rodríguez-López
Journal of the American Chemical Society 2009 Volume 131(Issue 47) pp:17046-17047
Publication Date(Web):November 11, 2009
DOI:10.1021/ja907626t
Scanning electrochemical microscopy surface interrogation (SI-SECM) in the cyclic voltammetry mode was successfully used to detect and quantify adsorbed CO on a Pt electrode by reaction with electrogenerated Br2. The two-electrode setup used in this new technique allowed the production of Br2 on an interrogator tip, which reported a transient positive feedback above a Pt substrate at open circuit as an indication of the reactivity of this halogen with CO(ads). Br− and CO2 are shown to be the main products of the reaction (in the absence of O2), which may involve the formation of bromophosgene as a hydrolyzable intermediate. Under saturation conditions, CO(ads) was reproducibly quantified at the polycrystalline Pt surface with θCO ≈ 0.5. The reaction is shown to be blocked by the action of pre-adsorbed cyanide, which demonstrates the surface character of the process. The formation of CO2 as an end product was further tested in a bulk experiment: addition of Pt black to a mixture of Br2 in 0.5 M H2SO4 through which CO was bubbled gave a precipitate of BaCO3 in a saturated solution of Ba(OH)2. The use of SI-SECM allowed access to a reaction that would otherwise be difficult to prove through conventional electrochemistry on a single electrode.
Co-reporter:Chong-yang Liu
Journal of the American Chemical Society 2009 Volume 131(Issue 18) pp:6397-6401
Publication Date(Web):April 21, 2009
DOI:10.1021/ja806785x
We show that pristine PMMA can spontaneously transfer electrons to species in a liquid, thereby inducing a variety of electron transfer reactions. The electrons that are transferred we call cryptoelectrons; these have a surface density of the order of 5 × 1013 cm−2 and are at a considerably more negative reduction potential than the PMMA bonding electrons. For example, metal ions including Ag+, Cu2+, and Pd2+ were reduced and plated on a PMMA surface and Fe(CN)63− was reduced to Fe(CN)64−. Moreover, protons were reduced when PMMA powder was dropped into a slightly acidic solution, resulting in a pH increase and hydrogen generation. Chemiluminescence was produced in a solution containing Ru(bpy)32+ and S2O82− with the addition of PMMA powder. These results clearly demonstrate that there are available electrons in PMMA that can participate in redox reactions at a rather negative potential. We also show that contacting PMMA with Teflon depletes this electronic surface charge. However, the PMMA used for a redox reaction or contacted with Teflon that was depleted of the electronic surface charge could be recharged by contacting with a suitable reductant.
Co-reporter:Jum Suk Jang, Ki Youl Yoon, Xiaoyin Xiao, Fu-Ren F. Fan and Allen J. Bard
Chemistry of Materials 2009 Volume 21(Issue 20) pp:4803
Publication Date(Web):September 30, 2009
DOI:10.1021/cm901056c
Ag−Fe2O3 and Sn-doped Ag−Fe2O3 nanocomposite photocatalysts were optimized by scanning photoelectrochemical microscopy (SPECM) as a potential photoelectrochemical material for visible light irradiation (λ ≥ 420 nm). The photocurrent vs potential response and the action spectra were investigated in a three-electrode cell. Among different mole ratios, the composition of nanocomposite with 50% Ag−50% Fe (atomic percentage) showed a dramatic improvement in photocurrent on a thin film electrode under visible light (λ ≥ 420 nm). The addition of 2 atomic % Sn (relative to Fe) produced an Ag−Fe2O3 nanocomposite with the largest photocurrent in alkaline solution under visible light irradiation (λ ≥ 420 nm).
Co-reporter:Changhoon Jung, Carlos M. Sánchez-Sánchez, Cheng-Lan Lin, Joaquín Rodríguez-López and Allen J. Bard
Analytical Chemistry 2009 Volume 81(Issue 16) pp:7003
Publication Date(Web):July 23, 2009
DOI:10.1021/ac901096h
The electrochemical oxidation of formic acid was studied by the tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM), extending the number of applications of SECM in electrocatalysis. Formic acid was generated at a Hg on Au ultramicroelectrode (UME) tip by reduction of CO2 in a 0.1 M KHCO3 solution saturated with this gas. The electrocatalytic activity of different Pd−Co bimetallic compositions was evaluated using a Pd−Co electrocatalyst array formed by spots deposited onto glassy carbon (GC) as a SECM substrate. The SECM tip, which generated a constant formic acid flux, was scanned above the array and the oxidation current generated when formic acid was collected by active electrocatalytic spots was displayed as a function of tip position. This generated a SECM image that showed the electrocatalytic activity of each spot. SECM screening identified Pd50Co50 (Pd/Co = 50:50, atomic ratio) as a better electrocatalyst toward the formic acid oxidation than pure Pd or Pt in 0.1 M KHCO3 solution and this result was confirmed by cyclic voltammetry. Positive feedback was observed for the most active compositions of Pd−Co which suggests fast reaction kinetics and chemical reversibility during the oxidation of formic acid to CO2. Moreover this feedback increases the contrast between active and non-active spots in this imaging mode.
Co-reporter:Carlos M. Sánchez-Sánchez and Allen J. Bard
Analytical Chemistry 2009 Volume 81(Issue 19) pp:8094
Publication Date(Web):September 2, 2009
DOI:10.1021/ac901291v
We present data for H2O2 production at eight different materials tested as electrocatalysts for the oxygen reduction reaction (ORR) in 0.5 M H2SO4 (Hg, Au, Ag, Cu, Pt, Pd, Pd80Co20, and Au60Cu40) using scanning electrochemical microscopy (SECM) as an alternative to the widely used rotating ring-disk electrode (RRDE) method. The amount of H2O2 is related to the total number of electrons, n, found in the O2 reduction reaction, with n = 2 showing only H2O2 production and n = 4 showing no H2O2 formation. From the SECM study Hg shows n close to 2, whereas Pt and Pd80Co20 show n-values near 4. The other materials show intermediate n-values as a function of potential.
Co-reporter:Cheng-Lan Lin, Joaquín Rodríguez-López and Allen J. Bard
Analytical Chemistry 2009 Volume 81(Issue 21) pp:8868
Publication Date(Web):October 9, 2009
DOI:10.1021/ac901434a
The micropipet delivery−substrate collection (MD−SC) mode of scanning electrochemical microscopy (SECM) is demonstrated. This new mode is intended for the study and imaging of electrochemical as well as electrocatalytic reactions of neutral species that cannot be generated electrochemically. The spontaneous transfer of the analyte from an organic solvent across an interface between two immiscible electrolyte solutions (ITIES) and its diffusion into the aqueous solution served as the mechanism to deliver it to the substrate, where the corresponding electrochemical or electrocatalytic reaction is carried out. High-resolution SECM images of ferrocenemethanol (FcMeOH) oxidation, benzoquinone (BQ) reduction, and the formic acid oxidation reaction (FAOR) at a Pt microelectrode substrate were successfully acquired. Furthermore, this new mode was used for the screening of electrocatalyst arrays for the methanol oxidation reaction (MOR), with the optimization of an efficient candidate, Pt80Ce20. Digital simulations produced quantitative information about the expected current response at the substrate in the proposed MD−SC mode of SECM.
Co-reporter:Anahita Izadyar, Shiuh-Tzung Liu, Pi-Tai Chou, Allen J. Bard
Journal of Electroanalytical Chemistry 2009 Volume 635(Issue 1) pp:7-12
Publication Date(Web):1 October 2009
DOI:10.1016/j.jelechem.2009.07.020
We report the electrogenerated chemiluminescence (ECL) of 2-oxa-bicyclo[3.3.0]octa-4,8-diene-3,6-dione (OBDD). Cyclic voltammetry at 100 mV/s shows two chemically reversible reduction waves but an irreversible oxidation wave. As opposed to the usual radical ion annihilation ECL, for this compound, ECL was observed in an acetonitrile solution just upon oxidation to the radical cation (D+) in the absence of any additional coreactant. This suggests that OBDD can produce the needed reducing species upon its oxidation, so ECL is probably generated by the reaction of D+ with a strongly-reducing free radical produced during OBDD electrooxidation. Production of CO2 upon oxidation was found by bulk electrolysis and mass spectrometry.
Co-reporter:Xiao Li, Allen J. Bard
Journal of Electroanalytical Chemistry 2009 Volume 628(1–2) pp:35-42
Publication Date(Web):1 April 2009
DOI:10.1016/j.jelechem.2009.01.002
The viability and activity of HeLa cells were probed using scanning electrochemical microscopy (SECM). The feedback generated by HeLa cells during scanning depends on the electrochemical mediator. Living HeLa cells generated positive feedback when ferrocene methanol (FcMeOH) was oxidized at the tip, showing that the cells reduced FcMeOH+. The positive feedback with FcMeOH changed to negative feedback when the HeLa cells were exposed to toxic treatments, i.e. CN− or UVC radiation, suggesting that FcMeOH+ reduction can be used to monitor cell activity. Living HeLa cells also accumulate FcMeOH after exposure times of a few h, but the presence of mM concentrations of FcMeOH has no apparent effect on the cell viability. The effect of Ag+ (known to be toxic to bacteria at the 10 μM level) on HeLa cells was probed using the FcMeOH as an indicator. The activity of the HeLa cells was not affected in a culture medium containing Ag+ up to 10 mM. The uptake of Ag+ by living and dead HeLa cells was small and nearly the same, indicating that even at high Ag+ concentrations in the culture medium, only a small amount of Ag+ is accumulated within the cells.
Co-reporter:Jum Suk Jang, Joowook Lee, Heechang Ye, Fu-Ren F. Fan and Allen J. Bard
The Journal of Physical Chemistry C 2009 Volume 113(Issue 16) pp:6719-6724
Publication Date(Web):2017-2-22
DOI:10.1021/jp8109429
Scanning electrochemical microscopy in the photoelectrochemical (PEC) mode was used to search for more efficient doped iron oxide photocatalysts under visible light irradiation (λ ≥ 420 nm). Iron oxide doped with one or two different metal cations was investigated to improve its PEC performance. Among various dopants, Sn or Ti as single dopants and Be or Al as codopants showed an improved photocurrent response of Fe2O3 under visible light irradiation (λ ≥ 420 nm). Fe2O3 doped with 4% Sn(IV) and 6% Be(II) showed the highest photocurrent as well as a good photosensitivity and stability in alkali solution (0.2 M NaOH) under UV and visible light irradiation.
Co-reporter:Khalid M. Omer and Allen J Bard
The Journal of Physical Chemistry C 2009 Volume 113(Issue 27) pp:11575-11578
Publication Date(Web):April 7, 2009
DOI:10.1021/jp901038h
We report the preparation, characterization, and electrogenerated chemiluminescence (ECL) of rubrene nanoparticles (NPs) and 9,10-diphenylanthracene (DPA) nanorods. The organic NPs were prepared in an aqueous phase using a simple reprecipitation method, i.e., injecting a solution of the hydrocarbon in an organic solvent into water. The resulting NPs can be collected and redispersed to form stable colloidal solutions in water. Rubrene forms spherical nanoparticles (NPs) (diameter ∼50 nm), while DPA initially forms nanorods with an average size of ∼500 nm in length and ∼50 nm in diameter. DPA nanorods grew gradually with time into wires with diameters of ∼1 μm and lengths of ∼10 μm. ECL emission from these NPs was observed upon electrochemical oxidation in aqueous solutions containing different co-reactants, such as tri-n-propylamine for rubrene and an oxalate ion for DPA NPs. The ECL intensity from rubrene NPs was significantly higher than that from DPA NPs because of the smaller size, and thus there is a higher diffusion coefficient for rubrene as compared to that of DPA NPs.
Co-reporter:Xiaoyin Xiao, Shanlin Pan, Jum Suk Jang, Fu-Ren F. Fan and Allen J. Bard
The Journal of Physical Chemistry C 2009 Volume 113(Issue 33) pp:14978-14982
Publication Date(Web):July 28, 2009
DOI:10.1021/jp904811f
The electrocatalytic properties of individual single Pt nanoparticles (NPs) can be studied electrochemically by measuring the current−time (i−t) responses during single NP collisions with a noncatalytic ultramicroelectrode (UME). The Pt NPs are capped with citrate ions or a self-assembled monolayer (SAM) of alkane thiols terminated with carboxylic acid that affect the observed i−t responses. By varying the length of the SAMs or the composition of a mixed monolayer, we have studied the effect of adsorbed molecules on the catalytic activity of Pt NPs at the single particle level through electrocatalytic amplification of single NP collisions. Collisions of single NPs were triggered and recorded as individual current steps whose amplitude represents the reactivity of single Pt NPs for the reaction of interest, here hydrazine oxidation, at a given electrode potential. The catalytic properties of Pt NPs are dependent not only on the nature of the interaction between the adsorbed monolayer and the catalytic NP surface, but also on the rate of electron transfer through the SAMs, governed by their length.
Co-reporter:Chong-yang Liu, Allen J. Bard
Chemical Physics Letters 2009 480(4–6) pp: 145-156
Publication Date(Web):
DOI:10.1016/j.cplett.2009.08.045
Co-reporter:KhalidM. Omer;Sung-Yu Ku;Ken-Tsung Wong;AllenJ. Bard
Angewandte Chemie International Edition 2009 Volume 48( Issue 49) pp:9300-9303
Publication Date(Web):
DOI:10.1002/anie.200904156
Co-reporter:Alessandro Minguzzi, Mario A. Alpuche-Aviles, Joaquin Rodríguez López, Sandra Rondinini and Allen J. Bard
Analytical Chemistry 2008 Volume 80(Issue 11) pp:4055
Publication Date(Web):April 30, 2008
DOI:10.1021/ac8001287
Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation−tip collection (SG−TC) imaging mode with a 100 µm diam tip. Pure IrO2 and Sn1−xIrxO2 combinatorial mixtures were prepared by a sol−gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG−TC setup in determining the properties of the composite materials and imaging arrays.
Co-reporter:Joowook Lee, Heechang Ye, Shanlin Pan and Allen J. Bard
Analytical Chemistry 2008 Volume 80(Issue 19) pp:7445
Publication Date(Web):August 22, 2008
DOI:10.1021/ac801142g
A method for rapid screening of photocatalysts employing a form of scanning electrochemical microscopy (SECM) is described. A piezoelectric dispenser was used to deposit arrays composed of ∼300-μm-size photocatalyst spots with different compositions onto conducting glass, fluorine-doped tin oxide substrate. The scanning tip of the SECM was replaced by a fiber optic connected to a xenon lamp and was rapidly scanned over the array. In this arrangement, the photocatalytic performance of the spots was evaluated by measuring the photocurrent at the substrate of the array. A fiber optic with a ring electrode can also be used to electrochemically detect products of the photoreaction. Several iron oxide-based bimetallic oxide combinations were found to exhibit enhanced photocatalytic activity, when compared to pure α-Fe2O3. These combinations included iron−palladium, iron−europium, and iron−rubidium in specific ratios. A trimetallic bismuth−vanadium−zinc oxide combination was also found to show a higher photocurrent, by ∼40%, compared to BiVO3.
Co-reporter:Alexander B. Nepomnyashchii, Mario A. Alpuche-Aviles, Shanlin Pan, Dongping Zhan, Fu-Ren F. Fan, Allen J. Bard
Journal of Electroanalytical Chemistry 2008 Volume 621(Issue 2) pp:286-296
Publication Date(Web):15 September 2008
DOI:10.1016/j.jelechem.2008.01.025
Cyclic voltammetry was used to determine the stability constants for the complexation of cadmium and zinc ions with hydroxyl-terminated polyamidoamine generation 2 dendrimers with an ethylenediamine core (PAMAM G2-OH) at a mercury ultramicroelectrode. Cadmium and zinc ions were found to form poly-metal complexes through the interactions with interior amino groups rather than hydroxyl groups and the maximum number of metal ions per dendrimer or stoichiometry was 4. Complexation is accompanied by a significant decrease in the limiting current and by measuring the changes in the magnitude of cathodic current with the analytical dendrimer:metal ion ratio and fitting the resulting curves, the four cumulative consecutive complexation constants were determined: 1.3 × 105 M−1, 1.6 × 109 M−2, 3.3 × 1012 M−3 and 4.6 × 1014 M−4 for Cd2+ and 1.0 × 106 M−1, 1.6 × 109 M−2, 4.6 × 1014 M−3 and 2.6 × 1017 M−4 for Zn2+. The stronger interaction of zinc ions with PAMAM G2-OH compared to cadmium was accompanied with a sharper decrease of the cathodic current with the addition of dendrimer and a larger half-wave potential shift than cadmium. This was rationalized by evaluating the changes of the diffusion coefficients for these two different systems by electrochemical means.
Co-reporter:Allen J. Bard
Journal of Solid State Electrochemistry 2008 Volume 12( Issue 1) pp:103-104
Publication Date(Web):2008 January
DOI:10.1007/s10008-007-0397-y
Co-reporter:Allen. J. Bard;Fu-Ren F. Fan;Dongping Zhan
PNAS 2008 Volume 105 (Issue 34 ) pp:12118-12122
Publication Date(Web):2008-08-26
DOI:10.1073/pnas.0805286105
We report that silver ion (Ag+) uptake is enhanced by 4-aminopyridine (4-AP), a well known voltage-sensitive potassium ion channel (Kv) blocker. Both bacterial (Escherichia coli) and mammalian (3T3 fibroblast) cells were used as model systems. Ag+ uptake was monitored with a scanning electrochemical microscope with an amperometric Ag+ ion-selective electrode (Ag+-ISE) and the respiration rates of E. coli cells were measured by oxygen reduction at an ultramicroelectrode. The results showed that not only the amount but also the
rate of silver uptake by the cells increased significantly when 4-AP was added to the solution. For fibroblasts, the Ag+ uptake rate was 4.8 × 107 ions per cell per sec without 4-AP compared with 1.0 × 108 ions per cell per sec with 0.2 mM 4-AP. For E. coli cells, the uptake rate was 1.5 × 104 ions per cell per sec without 4-AP vs. 3.5 × 104 ions per cell per sec with 0.5 mM 4-AP and 5.9 × 104 ions per cell per sec with 1 mM 4-AP. Thus, 4-AP might be useful where silver is used as antimicrobial agent to speed its
uptake.
Co-reporter:Matthew M. Sartin ; Franck Camerel ; Raymond Ziessel
The Journal of Physical Chemistry C 2008 Volume 112(Issue 29) pp:10833-10841
Publication Date(Web):July 1, 2008
DOI:10.1021/jp8011848
B8amide is a large molecule based on a derivative of the BODIPY laser dye PM567. Both molecules exhibit highly stable radical ions and similar absorbance and photoluminescence spectra. For B8amide, the redox potentials are E° = −1.40 and 0.96 V vs SCE for reduction and oxidation, respectively, and for PM567, they are E° = −1.37 and 0.94 V vs SCE. The absorbance and fluorescence λmax are 526 and 537 nm, respectively, for B8amide, and 516 and 533 nm for PM567. Whereas the ECL spectrum of PM567 consists of a single peak with λmax = 555 nm, the ECL of B8amide possesses peaks at 551 and 741 nm. The 741 nm peak includes a shoulder that extends to even longer wavelengths. The intensity ratio I741/I551 increases as a function of pulsing time, implying that film formation contributes to the 741 nm peak. Preliminary results also suggest a lower intensity ratio for lower bulk concentrations, which usually indicates dimer, excimer, or aggregate formation; however, excimer formation for BODIPY compounds has been observed only at 650−670 nm. Phosphorescence has been observed at 770 nm, although such emission is rarely observed in solution at room temperature. The proposed film on the electrode may stabilize the triplets long enough for them to relax radiatively. The ECL transients of B8amide are highly stable, despite drastic alternations of intensity with each pulse. The light pulses observed on reduction are initially about 30 times smaller than those that appear on oxidation. Bulk electrolysis results indicate polymerization of the radical cation, but this is too slow to account for the dramatic intensity difference between pulses.
Co-reporter:Anahita Izadyar, Khalid M. Omer, Yunqi Liu, Shiyan Chen, Xinjun Xu and Allen J. Bard
The Journal of Physical Chemistry C 2008 Volume 112(Issue 50) pp:20027-20032
Publication Date(Web):2017-2-22
DOI:10.1021/jp807202d
We report the electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of donor−acceptor compounds containing a quinoxaline derivative as the central core coupled with fluorene (CFPQ, CFPP, and MFPQ) and triphenylamine moieties (MAPQ). Cyclic voltammetry of all four derivatives show reversible reduction waves (assigned to the formation of the radical anion) localized in the quinoxaline moiety (the acceptor group), whereas oxidation waves show behavior that depends upon the nature of the donor group. With a pair of strong donor groups, triphenylamine, (MAPQ), exhibits a two electron oxidation wave due to the two noninteracting donor moieties. The fluorene derivatives (CFPQ, CFPP, and MFPQ) show less reversible oxidation waves at scan rates of 100 mV/s, but CFPP and CFPQ become reversible at higher scan rates. In their absorbance and emission spectra, all of the compounds, except MFPQ, exhibit large Stokes shifts, which are related to the rearrangement of the excited-state and also to solvent effects. ECL was observed for MAPQ, CFPQ, and CFPP in MeCN/benzene solutions that generally agree with the photoluminescence spectra, suggesting that the energy of annihilation is sufficient to form singlet excited states (the S-route). The ECL of MAPQ was strong enough to be seen with the naked eye. MFPQ did not produce ECL, probably because of the instability of the radical cation.
Co-reporter:Allen J. Bard
ACS Nano 2008 Volume 2(Issue 12) pp:2437
Publication Date(Web):December 23, 2008
DOI:10.1021/nn800801z
Single-molecule studies, including those of single enzyme molecules, have led to important new insights about the effects of environment and configuration on the behavior of these molecules. Such information is not available from ensemble studies. Most of these have been based on spectroscopic approaches, but there have been relatively few attempts at single-molecule electrochemistry. In a paper in this issue, progress toward studying a single enzyme molecule by protein film voltammetry is described. This Perspective reviews briefly past work on nanoelectrodes and electrochemical and spectroelectroelectrochemical single-molecule studies, as well as examples of the type of information obtained in past studies of enzymes.
Co-reporter:Janine Mauzeroll;Omeed Owhadian;Terrence J. Monks
PNAS 2004 Volume 101 (Issue 51 ) pp:17582-17587
Publication Date(Web):2004-12-21
DOI:10.1073/pnas.0407613101
The cytotoxicity of menadione on hepatocytes was studied by using the substrate generation/tip collection mode of scanning
electrochemical microscopy by exposing the cells to menadione and detecting the menadione-S-glutathione conjugate (thiodione) that is formed during the cellular detoxication process and is exported from the cell by
an ATP-dependent pump. This efflux was electrochemically detected and allowed scanning electrochemical microscopy monitoring
and imaging of single cells and groups of highly confluent live cells. Based on a constant flux model, ≈6 × 106 molecules of thiodione per cell per second are exported from monolayer cultures of Hep G2 cells.
Co-reporter:Janine Mauzeroll
PNAS 2004 Volume 101 (Issue 21 ) pp:7862-7867
Publication Date(Web):2004-05-25
DOI:10.1073/pnas.0402556101
The uptake of menadione (2-methyl-1,4-naphthoquinone), which is toxic to yeast cells, and its expulsion as a glutathione complex
were studied by scanning electrochemical microscopy. The progression of the in vitro reaction between menadione and glutathione was monitored electrochemically by cyclic voltammetry and correlated with the
spectroscopic (UV–visible) behavior. By observing the scanning electrochemical microscope tip current of yeast cells suspended
in a menadione-containing solution, the export of the conjugate from the cells with time could be measured. Similar experiments
were performed on immobilized yeast cell aggregates stressed by a menadione solution. From the export of the menadione-glutathione
conjugate detected at a 1-μm-diameter electrode situated 10 μm from the cells, a flux of about 30,000 thiodione molecules
per second per cell was extracted. Numerical simulations based on an explicit finite difference method further revealed that
the observation of a constant efflux of thiodione from the cells suggested the rate was limited by the uptake of menadione
and that the efflux through the glutathione-conjugate pump was at least an order of magnitude faster.
Co-reporter:José L. Fernández Dr.;Nicolas Mano Dr.;Adam Heller Dr. Dr.
Angewandte Chemie 2004 Volume 116(Issue 46) pp:
Publication Date(Web):23 NOV 2004
DOI:10.1002/ange.200461528
Schnelles Screening: Bei der Optimierung des Enzym-Gewichtsanteils in Enzymelektroden liefert die hier beschriebene elektrochemische Rastermikroskopie (SECM) Ergebnisse (siehe Bild), die gut mit denen übereinstimmen, die mit einer klassischen rotierenden Scheibenelektrode erhalten werden. Dabei benötigt sie aber nur ein Zehntel der Zeit und ein Hunderttausendstel der Polymer- und Enzymmenge.
Co-reporter:José L. Fernández Dr.;Nicolas Mano Dr.;Adam Heller Dr. Dr.
Angewandte Chemie International Edition 2004 Volume 43(Issue 46) pp:
Publication Date(Web):23 NOV 2004
DOI:10.1002/anie.200461528
Rapid screening: For optimizing the weight fraction of enzyme in enzyme electrodes, the scanning electrochemical microscopy (SECM) screening method described gives results (see picture) in good agreement with those obtained by the traditional rotating-disk-electrode method but in one tenth of the time and with a reduction in the amount of polymer and enzyme required by a factor of 100 000.
Co-reporter:Chong-yang Liu
and
Allen J. Bard
Nature 2002 418(6894) pp:162
Publication Date(Web):
DOI:10.1038/nature00875
Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance1. Metal–insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles2, 3. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell4. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (~100 μm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10-13 A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.
Co-reporter:Fardad Forouzan;Michael V. Mirkin;Fardad Forouzan;Michael V. Mirkin
Israel Journal of Chemistry 1997 Volume 37(Issue 2‐3) pp:155-163
Publication Date(Web):21 NOV 2013
DOI:10.1002/ijch.199700019
The adsorption kinetics and self-assembly of hexadecyl mercaptan on gold have been investigated by scanning electrochemical microscopy (SECM), chronoamperometry, and cyclic voltammetry. The developed methodology allows one to evaluate the surface coverage and the average size of the defects in the monolayer film from the effective rate constant of electron transfer. Two kinetic regimes of self-assembly were identified: a rapid initial adsorption of hexadecyl mercaptan onto a clean gold surface from 5 mM solution (more than 90% coverage obtained in 1 to 5 min), and a slower subsequent annealing of a thiol monolayer resulting in a more compact film. Typically, a long-chain-length thiol-treated gold surface acts as an electronically insulating surface after about 1 h. The SECM images of partially covered gold surfaces were always featureless, suggesting that the defects in the film were smaller than 0.5 μm for any exposure time ≥ 1 min.
Co-reporter:Fu-Ren F. Fan
Nano Letter () pp:
Publication Date(Web):May 21, 2008
DOI:10.1021/nl8009236
We demonstrate a novel method of observing single particle collision events with electrogenerated chemiluminescence (ECL). A single event is characterized by the enhancement of ECL intensity during the collision of an individual platinum nanoparticle (Pt NP) on an indium tin oxide electrode, which catalyzes the oxidation of Ru(bpy)32+ and a coreactant, for example, tri-n-propylamine (TPrA), present in the solution. Every collision produces a unique photon spike whose amplitude and frequency can be correlated with the size and concentration of the Pt NPs. A large amplification of ECL intensity can occur by choosing an appropriate measuring electrode and using high concentrations of Ru(bpy)32+ and the coreactant.
Co-reporter:Byung-Kwon Kim; Jiyeon Kim
Journal of the American Chemical Society () pp:
Publication Date(Web):January 23, 2015
DOI:10.1021/ja512065n
We report here the electrochemistry of emulsion droplets by observing single emulsion droplet collisions with selective electrochemical reduction on an ultramicroelectrode (UME). With appropriately applied potentials at an UME, we can observe the electrochemical effects of single collision signals from the complete electrolysis of single emulsion droplets, or selective electrolysis of redox species in single emulsion droplets. This was observed with nitrobenzene (NB), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ionic liquid. The NB, TCNQ, and ionic liquid act as emulsion material, redox specie, and emulsifier (and electrolyte), respectively. NB emulsions and NB (TCNQ) emulsions were made by ultrasonic processing. During the amperometric current–time (i–t) curve measurement with NB/water emulsion at −0.65 V, reduction of NB emulsion droplets was measured. In the case of less negative potentials, e.g., at −0.45 V with a NB (TCNQ) emulsion, selective reduction of TCNQ in NB droplet was measured. Spike-like responses from electrolysis of NB or TCNQ in each experiment were observed. From these single-particle collision results of NB and NB (TCNQ) emulsions, the collision frequency, size distribution, i–t decay behavior of emulsion droplets, and possible mechanisms are discussed.
Co-reporter:Honglan Qi ; Justin J. Teesdale ; Rachel C. Pupillo ; Joel Rosenthal
Journal of the American Chemical Society () pp:
Publication Date(Web):August 27, 2013
DOI:10.1021/ja406731f
Two new 2,2′-bipyridine (bpy) derivatives containing ancillary BODIPY chromophores attached at the 5- and 5′-positions (BB3) or 6- and 6′-positions (BB4) were prepared and characterized. In this work, the basic photophysics, electrochemistry, and electrogenerated chemiluminescence (ECL) of BB3 and BB4 are compared with those previously reported for a related bpy-BODIPY derivative (BB2) (J. Phys. Chem. C 2011, 115, 17993–18001). Cyclic voltammetry revealed that BB3 and BB4 display reversible 2e– oxidation and reduction waves, which consist of two closely spaced (50–70 mV) 1e– events. This redox behavior is consistent with the frontier molecular orbitals calculated for BB3 and BB4 and indicates that the 2,2′-bipyridine spacer of each bpy-BODIPY homologue does not facilitate efficient electronic communication between the tethered indacene units. In the presence of a coreactant such as tri-n-propylamine (TPA) or benzoyl peroxide (BPO), BB3 and BB4 exhibit strong ECL and produce spectra that are very similar to their corresponding photoluminescence profiles. The ECL signal obtained under annihilation conditions, however, is significantly different and is characterized by two distinct bands. One of these bands is centered at ∼570 nm and is attributed to emission via an S- or T-route. The second band occurs at longer wavelengths and is centered around ∼740 nm. The shape and concentration dependence of this long-wavelength ECL signal is not indicative of emission from an excimer or aggregate, but rather it suggests that a new emissive species is formed from the bpy-BODIPY luminophores during the annihilation process.
Co-reporter:Caleb M. Hill; Jiyeon Kim
Journal of the American Chemical Society () pp:
Publication Date(Web):August 17, 2015
DOI:10.1021/jacs.5b04519
Here, a new methodology is proposed for treating electrochemical current densities in metal-insulator-metal nanoparticle (M-I-MNP) systems. The described model provides broad, practical insights about MNP-mediated electron transfer to redox species in solution, where electron transfer from the underlying electrode to a MNP via tunneling and heterogeneous electron transfer from the MNP to redox species in solution are treated as sequential steps. Tunneling is treated through an adaptation of the Simmons model of tunneling in metal–insulator–metal structures, and explicit equations are provided for tunneling currents, which demonstrate the effect of various experimental parameters, such as insulator thickness and MNP size. Overall, a general approach is demonstrated for determining experimental conditions where tunneling will have a measurable impact on the electrochemistry of M-I-MNP systems.
Co-reporter:Khalid M. Omer ; Sung-Yu Ku ; Yu-Chen Chen ; Ken-Tsung Wong
Journal of the American Chemical Society () pp:
Publication Date(Web):July 15, 2010
DOI:10.1021/ja104160f
We report the synthesis, electrochemistry, and electrogenerated chemiluminescence of a series of star-shaped donor−acceptor (D−A) molecules. The star-shaped molecules consist of an electron-deficient 1,3,5-triazine core with three fluorene arms substituted with diarylamino (TAM1−TAM3) or carbazolyl (TAM4) electron donors. Cyclic voltammetry of TAM1−TAM3 shows that the reduction consists of one wave of single electron transfer to the core, while the oxidation exhibits a single peak of three sequential electron-transfer processes, with the formation of a trication. The carbazole-containing molecule TAM4 after oxidation undergoes a subsequent rapid chemical reaction to produce a dimer (via the overall coupling of two radical cations with the loss of two protons). The dimer electrooxidizes more easily than the monomer of TAM4. With continuous cycling on the oxidation side, a conductive polymer film is formed on the surface of the working electrode. Because of the presence of the acceptor (triazine) center and strong donors in the arms (diarylamine or carbazole), TAM1−TAM3 exhibit large solvatochromic effects with emissions ranging from deep blue (428 nm) to orange-red (575 nm) depending on the solvent polarity. These star-shaped molecules show high PL quantum yields of 0.70−0.81. The electrogenerated chemiluminescence (ECL) of TAM1−TAM3 in nonaqueous solutions showed strong ECL that could be seen with the naked eye in a well-lit room. Because the enthalpy of annihilation is higher than the energy required for formation of the singlet excited state, the ECL emission is believed to be generated via S-route annihilation. However, TAM4 shows weak annihilation ECL because of the production of polymer film on the electrode surface during oxidation cycles. However, by limiting the potential region only to the reduction side and using benzoyl peroxide (BPO) as a coreactant, strong ECL of TAM4 can be obtained.
Co-reporter:Mei Shen
Journal of the American Chemical Society () pp:
Publication Date(Web):August 15, 2011
DOI:10.1021/ja206136h
The kinetics of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)) oxidation and reduction in acetonitrile were investigated by steady-state voltammetry using scanning electrochemical microscopy (SECM). The SECM setup was placed inside a drybox for carrying out experiments in an anhydrous atmosphere and in the absence of oxygen. The standard rate constant, k°, for Ru(bpy) oxidation at a Pt electrode (radius, a = 5 μm) was 0.7 ± 0.1 cm/s, which is smaller than k° for Ru(bpy) reduction measured under the same conditions (≥3 cm/s). This is attributed to the 2,2′-bipyridine ligands having an electron-transfer (ET) blocking effect on the oxidation of the ruthenium(II) center, as opposed to the reduction, which involves ET to the exposed ligands. Thus, tunneling effects may be important in considering the ET in this molecule.
Co-reporter:Seong Jung Kwon, Hongjun Zhou, Fu-Ren F. Fan, Vasily Vorobyev, Bo Zhang and Allen J. Bard
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 12) pp:NaN5402-5402
Publication Date(Web):2011/02/28
DOI:10.1039/C0CP02543G
Collisions of several kinds of metal or metal oxide single nanoparticles (NPs) with a less catalytic electrode surface have been observed through amplification of the current by electrocatalysis. Two general types of current response, a current staircase or a current blip (or spike) are seen with particle collisions. The current responses were caused by random individual events as a function of time rather than the usual continuous current caused by an ensemble of a large number of events. The treatment of stochastic electrochemistry like single NP collisions is different from the usual model for ensemble-based electrochemical behaviour. Models for the observed responses are discussed, including simulations, and the frequency of the steps or blips investigated for several systems experimentally.
Co-reporter:Dodzi Zigah, Joaquín Rodríguez-López and Allen J. Bard
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 37) pp:NaN12772-12772
Publication Date(Web):2012/08/07
DOI:10.1039/C2CP40907K
The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH(ads) generated photoelectrochemically at the surface of a nanostructured TiO2 substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl62−/3− offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH(ads) saturation coverage of 338 μC cm−2 was found in our nanostructured samples by its reduction with the electrogenerated IrCl63−. The decay kinetics of ˙OH(ads) by dimerization to produce H2O2 were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ∼kOH = 2.2 × 103 mol−1 m2 s−1. A radical scavenger, such as methanol, competitively consumes ˙OH(ads) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k′MeOH ∼ 1 s−1.
Co-reporter:Mohammad Mahdi Najafpour, Kevin C. Leonard, Fu-Ren F. Fan, Mahmoud Amouzadeh Tabrizi, Allen J. Bard, Cecil K. King'ondu, Steven L. Suib, Behzad Haghighi and Suleyman I. Allakhverdiev
Dalton Transactions 2013 - vol. 42(Issue 14) pp:NaN5091-5091
Publication Date(Web):2013/01/24
DOI:10.1039/C3DT32864C
Inspired by Nature's catalyst, a nano-size layered manganese–calcium oxide showed a low overvoltage for water oxidation in acidic solutions, which is comparable to platinum.
Co-reporter:Alexander B. Nepomnyashchii, Robert J. Ono, Dani M. Lyons, Christopher W. Bielawski, Jonathan L. Sessler and Allen J. Bard
Chemical Science (2010-Present) 2012 - vol. 3(Issue 8) pp:NaN2638-2638
Publication Date(Web):2012/05/02
DOI:10.1039/C2SC20263H
The electrochemical properties of oligomers of thiophene (with number of monomer units, n, from 2 to 12) and fluorene (n = 3 to 7) were investigated. Both sets of oligomers were characterized by the presence of two oxidation and two reduction waves as determined by cyclic voltammetry (CV), with the reversibility of the waves depending on the structural properties of the compounds. The addition or removal of a third electron was found to be difficult relative to the second, a finding shown for conjugated oligomers with chain lengths up to 7 in the case of the fluorenes and up to 12 for the thiophenes. The oligothiophenes showed a larger separation between the electrochemical waves for the same chain length, and also substantial electrogenerated chemiluminescence (ECL) signals, whose intensity increased with oligomer size. In contrast, the ECL intensity of the fluorene oligomers was essentially independent of chain length. The ECL spectra for the thiophene dodecamer were obtained with concentrations as low as 20 pM, a result that reflects a high ECL efficiency, close to that of the well-known ECL standard Ru(bpy)32+. Oligomers were also formed on electrochemical reduction of an appropriately functionalized dimer in the presence of benzoyl peroxide producing a longer wavelength emission (maximum at ∼540 nm) as opposed to the spectrum of the dimer (λem = 390 nm).
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