The cyclic voltammetric responses of individual palladium-coated carbon nanotubes are reported. Upon impact—from the solution phase—with the electrified interface, the nanoparticles act as individual nanoelectrodes catalyzing the hydrogen-oxidation reaction. At high overpotentials the current is shown to reach a quasi-steady-state diffusion limit, allowing determination of the tube length. The electrochemical response of the individual nanotubes also reveals the system to be modulated by the electrical contact between the electrode and carbon nanotube. This modulation presents itself as fluctuations in the recorded Faradaic current.

We report the electrocatalytic dehalogenation of trichloroethylene (TCE) by single soft nanoparticles in the form of Vitamin B₁₂-containing droplets. We quantify the turnover number of the catalytic reaction at the single soft nanoparticle level. The kinetic data shows that the binding of TCE with the electro-reduced vitamin in the Co^I oxidation state is chemically reversible.

The in situ electrochemical sizing of individual gold nanorods is reported. Through the combination of electrochemical dissolution and the use of a surface-bound redox tag, the volume and surface area of the nanorods are measured, and provide the aspect ratio and the size of the nanorods. Excellent independent agreement is found with electron microscopy analysis of the nanorods, establishing the application of nano-impact experiments for the sizing of anisotropic nanomaterials.

We demonstrate that the concentration of a red blood cell solution under physiological conditions can be determined by electrochemical voltammetry. The magnitude of the oxygen reduction currents produced at an edge-plane pyrolytic graphite electrode was diagnosed analytically at concentrations suitable for a point-of-care test device. The currents could be further enhanced when the solution of red blood cells was exposed to hydrogen peroxide. We show that the enhanced signal can be used to detect red blood cells at a single entity level. The method presented relies on the catalytic activity of red blood cells towards hydrogen peroxide and on surface-induced haemolysis. Each single cell activity is expressed as current spikes decaying within a few seconds back to the background current. The frequency of such current spikes is proportional to the concentration of cells in solution.

The cyclic voltammetric responses of individual palladium-coated carbon nanotubes are reported. Upon impact—from the solution phase—with the electrified interface, the nanoparticles act as individual nanoelectrodes catalyzing the hydrogen-oxidation reaction. At high overpotentials the current is shown to reach a quasi-steady-state diffusion limit, allowing determination of the tube length. The electrochemical response of the individual nanotubes also reveals the system to be modulated by the electrical contact between the electrode and carbon nanotube. This modulation presents itself as fluctuations in the recorded Faradaic current.

We report the electrocatalytic dehalogenation of trichloroethylene (TCE) by single soft nanoparticles in the form of Vitamin B₁₂-containing droplets. We quantify the turnover number of the catalytic reaction at the single soft nanoparticle level. The kinetic data shows that the binding of TCE with the electro-reduced vitamin in the Co^I oxidation state is chemically reversible.

The in situ electrochemical sizing of individual gold nanorods is reported. Through the combination of electrochemical dissolution and the use of a surface-bound redox tag, the volume and surface area of the nanorods are measured, and provide the aspect ratio and the size of the nanorods. Excellent independent agreement is found with electron microscopy analysis of the nanorods, establishing the application of nano-impact experiments for the sizing of anisotropic nanomaterials.

We demonstrate that the concentration of a red blood cell solution under physiological conditions can be determined by electrochemical voltammetry. The magnitude of the oxygen reduction currents produced at an edge-plane pyrolytic graphite electrode was diagnosed analytically at concentrations suitable for a point-of-care test device. The currents could be further enhanced when the solution of red blood cells was exposed to hydrogen peroxide. We show that the enhanced signal can be used to detect red blood cells at a single entity level. The method presented relies on the catalytic activity of red blood cells towards hydrogen peroxide and on surface-induced haemolysis. Each single cell activity is expressed as current spikes decaying within a few seconds back to the background current. The frequency of such current spikes is proportional to the concentration of cells in solution.

A novel electrochemical method has been developed for the determination of the oxygen solubility in cetyltrimethylammonium bromide (CTAB) micelles. The electroreduction of oxygen is studied in aqueous phosphate buffer solutions, using a microelectrode. The addition of micelle-forming surfactants to solutions pre-saturated with oxygen leads to a reduction of the oxygen signal, allowing the oxygen uptake by the micelles to be measured. For CTAB micelles, an oxygen concentration of 6.7±0.72 mm was observed, which was shown to remain constant with increasing CTAB concentration in the bulk solution. The method has general applicability.

We report the voltammetry of alloy nanoparticles by using cyclic voltammetry to study gold–silver alloy nanoparticles. Through careful comparison with the response of pure gold nanoparticles, pure silver nanoparticles, and their mixture, the contrasting electrochemical behavior of the alloy nanoparticles is established, providing insights into the electrochemical stability of gold–silver alloy nanoparticles in chloride-containing media.

The charging and controlled oxidative doping of single organometallic ferrocene nanoparticles is reported in aqueous sodium tetrafluoroborate using the nano-impacts method. It is shown that ferrocene nanoparticles of approximately 105 nm diameter are essentially quantitatively oxidatively doped with the uptake of one tetrafluoroborate anion per ferrocene molecule at suitably high overpotentials. By using lower potentials, it is possible to achieve low doping levels of single nanoparticles in a controlled manner.

We successfully exploited the natural highly efficient activity of an enzyme (catalase) together with carbon electrodes to produce a hybrid electrode for oxygen reduction, very appropriate for energy transformation. Carbon electrodes, in principle, are cheap but poor oxygen reduction materials, because only two-electron reduction of oxygen occurs at low potentials, whereas four-electron reduction is key for energy-transformation technology. With the immobilization of catalase on the surface, the hydrogen peroxide produced electrochemically is decomposed back to oxygen by the enzyme; the enzyme natural activity on the surface regenerates oxygen, which is further reduced by the carbon electrode with no direct electron transfer between the enzyme and the electrode. Near full four-electron reduction of oxygen is realised on a carbon electrode, which is modified with ease by a commercially available enzyme. The value of such enzyme-modified electrode for energy-transformation devices is evident.

Colloidal suspensions of Bi₂O₃ nanoparticles were studied in aqueous solution using imaging and electrochemical techniques. Nanoparticle tracking analysis revealed the particles to be agglomerated. In contrast, electrochemical detection via the nano-impacts technique showed almost exclusive detection of monomeric nanoparticles. Comparison of the two techniques allows the conclusion to be drawn that the agglomeration/deagglomeration of the nanoparticles is reversible. A minimum rate constant for the deagglomeration process was estimated.

Core–shell nanoparticles (NPs) are amongst the most promising candidates in the development of new functional materials. Their fabrication and characterization are challenging, in particular when thin and intact shells are needed. To date no technique has been available that differentiates between intact and broken or cracked shells. Here a method is presented to distinguish and quantify these types of shells in a single cyclic voltammetry experiment by using the different electrochemical reactivities of the core and the shell material. A simple comparison of the charge measured during the stripping of the core material before and after the removal of the shell makes it possible to determine the quality of the shells and to estimate their thickness. As a proof-of-concept two multifunctional examples of core–shell NPs, Fe₃O₄@Au and Au@SnO₂, are used. This general and original method can be applied whenever core and shell materials show different redox properties. Because billions of NPs are probed simultaneously and at a low cost, this method is a convenient new screening tool for the development of new multifunctional core–shell materials and is hence a powerful complementary technique or even an alternative to the state-of-the-art characterization of core–shell NPs by TEM.

We report the use of single Vitamin B₁₂ nanodroplets to mediate the reduction of oxygen in neutral buffer. Electron transfer to single Vitamin B₁₂ nanodroplets is observed using the nano-impacts method and shown to be quantitative. The mechanism of mediated oxygen reduction by single VB₁₂ droplets is revealed as via both Co^II and Co^I reduced from Co^III in VB₁₂ through one or two electron transfer followed by the four-electron reduction of oxygen.

The influence of capping agents on the oxidation of silver nanoparticles was studied by using the electrochemical techniques of anodic stripping voltammetry and anodic particle coulometry (“nano-impacts”). Five spherical silver nanoparticles each with a different capping agent (branched polyethylenimine (BPEI), citrate, lipoic acid, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP)) were used to perform comparative experiments. In all cases, regardless of the capping agent, complete oxidation of the single nanoparticles was seen in anodic particle coulometry. The successful quantitative detection of the silver nanoparticle size displays the potential application of anodic particle coulometry for nanoparticle characterisation. In contrast, for anodic stripping voltammetry using nanoparticles drop casting, it was observed that the capping agent has a very significant effect on the extent of silver oxidation. All five samples gave a low oxidative charge corresponding to partial oxidation. It is concluded that the use of anodic stripping voltammetry to quantify nanoparticles is unreliable, and this is attributed to nanoparticle aggregation.

The geometry of quasi-spherical nanoparticles is investigated. The combination of SEM imaging and electrochemical nano-impact experiments is demonstrated to allow sizing and characterization of the geometry of single silver nanoparticles.

We report the use of single Vitamin B₁₂ nanodroplets to mediate the reduction of oxygen in neutral buffer. Electron transfer to single Vitamin B₁₂ nanodroplets is observed using the nano-impacts method and shown to be quantitative. The mechanism of mediated oxygen reduction by single VB₁₂ droplets is revealed as via both Co^II and Co^I reduced from Co^III in VB₁₂ through one or two electron transfer followed by the four-electron reduction of oxygen.

We report mediated charge transfer across a nanoparticle that Faradaically interacts with its surrounding solution while impacting on an electrochemically inactive electrode. To this end, two different aspects of the process are elucidated and interconnected: The Faradaic reaction at the particle surface and electron tunnelling between the electrode and the particle. Results demonstrate that the charge transfer can be described through a binary model, in which the current switches between the limiting Faradaic current and no current at all as a function of the electrode–particle distance, while the response is largely unrelated to the analyte’s formal potential. This finding allows a significantly simplified modelling approach in future studies.

Electrostatic interactions between surface-charged nanoparticles (NPs) and electrodes studied using existing techniques unavoidably and significantly alter the system being analyzed. Here we present a methodology that allows the probing of unperturbed electrostatic interactions between individual NPs and charged surfaces. The uniqueness of this approach is that stochastic NP impact events are used as the probe. During a single impact, only an attomole of the redox species reacts and is released at the interface during each sensing event. As an example, the effect of electrostatic screening on the reduction of negatively charged indigo NPs at a mercury microelectrode is explored at potentials positive and negative of the potential of zero charge. At suitable overpotentials fully driven electron transfer is seen for all but very low (<0.005 M) ionic strengths. The loss of charge transfer in such dilute electrolytes is unambiguously shown to arise from a reduced driving force for the reaction rather than a reduced population of NPs near the electrode, contradicting popular perceptions. Electrostatics were found not to significantly affect the reactivity of the studied NPs. Importantly, the presented technique is general and can be applied to a wide variety of NPs, including metals, metal oxides and organic compounds.

Mercury(I) chloride (Hg₂Cl₂) nanoparticles (NPs) are synthesised for the first time by using two different techniques. First, particles are formed by implosion of a calomel nanolayer, induced by partial electrolysis at a mercury hemisphere microelectrode. The resulting NPs are then characterised by the nanoimpact method, demonstrating the first time metal chloride NPs have been sized by this technique and showing the ability to form and study NPs in situ. Second, Hg₂Cl₂ NPs are synthesised by using the precipitation reaction of Hg₂(NO₃)₂ with KCl. The NPs are characterised on both mercury and carbon microelectrodes and their size is found to agree with TEM results.

The oxidative and reductive electrochemistry of DPPH nanoparticles is studied via the nanoimpacts method and with drop-casted layers used to modify an electrode surface. In the former case the currents associated with single NP impacts are used to measure the electron transfer kinetics, whereas in contrast it is shown that the reduced diffusion caused by the overlap of adjacent diffusion layers in the latter case leads to more electrochemically reversible behaviour.

Partially blocked electrodes (PBEs) are important; many applications use non-conductive nanoparticles (NPs) to introduce new electrode functionalities. As aggregation is a problem in NP immobilization, developing an in situ method to detect aggregation is vital to characterise such modified electrodes. We present chronoamperometry as a method for detection of NP surface aggregation and semi-quantitative sizing of the formed aggregates, based on the diffusion limited current measured at PBEs as compared with the values calculated numerically for different blocking feature sizes. In contrast to voltammetry, no approximations on electrode kinetics are needed, making chronoamperometry a more general and reliable method. Sizing is shown for two modification methods. Upon drop casting, significant aggregation is observed, while it is minimized in electrophoretic NP deposition. The aggregate sizes determined are in semi-quantitative agreement with ex situ microscopic analysis of the PBEs.

Carbon nanotubes decorated with ultra-small metal nanoparticles are of great value in catalysis. We report that individual multiwalled carbon nanotubes decorated with ultra-small palladium nanoparticles can be detected by using the nano-impacts method. The high conductivity and reactivity of each decorated carbon nanotube is directly evidenced; this is achieved through studying the proton-reduction reaction for the underpotential deposition of hydrogen onto the nanoparticles decorated on the carbon nanotube walls. The reductive spikes from current amplification are analyzed to estimate the approximate length of the decorated carbon nanotubes, revealing that the decorated carbon nanotubes are electroactive along its entire length of several micrometers.

The influence of nanoparticle aggregation on anodic stripping voltammetry is reported. Dopamine-capped silver nanoparticles were chosen as a model system, and melamine was used to induce aggregation in the nanoparticles. Through the anodic stripping of the silver nanoparticles that were aggregated to different extents, it was found that the peak area of the oxidative signal corresponding to the stripping of silver to silver(I) ions decreases with increasing aggregation. Aggregation causes incomplete stripping of the silver nanoparticles. Two possible mechanisms of ‘partial oxidation’ and ‘inactivation’ of the nanoparticles are proposed to account for this finding. Aggregation effects must be considered when anodic stripping voltammetry is used for nanoparticle detection and quantification. Hence, drop casting, which is known to lead to aggregation, is not encouraged for preparing electrodes for analytical purposes.

Typical laser-dependent methods such as nanoparticle tracking analysis (NTA) and dynamic light scattering (DLS) are not able to detect nanoparticles in an optically opaque medium due to scattering or absorption of light. Here, the electrochemical technique of ‘nano-impacts’ was used to detect nanoparticles in solution in the presence of high levels of alumina particulates causing a milky white suspension. Using the ‘nano-impacts’ method, silver nanoparticles were successfully detected and sized in the model opaque medium. The results obtained compared well with those using transmission electron microscopy (TEM), an ex situ method for nanoparticle size determination. The ability to use the ‘nano-impacts’ method in media unmeasurable to competitor techniques confers a significant advantage on the electrochemical approach.

Recent progress in the theory and practice of voltammetry is surveyed and evaluated. The transformation over the last decade of the level of modelling and simulation of experiments has realised major advances such that electrochemical techniques can be fully developed and applied to real chemical problems of distinct complexity. This review focuses on the topic areas of: multistep electrochemical processes, voltammetry in ionic liquids, the development and interpretation of theories of electron transfer (Butler–Volmer and Marcus–Hush), advances in voltammetric pulse techniques, stochastic random walk models of diffusion, the influence of migration under conditions of low support, voltammetry at rough and porous electrodes, and nanoparticle electrochemistry. The review of the latter field encompasses both the study of nanoparticle-modified electrodes, including stripping voltammetry and the new technique of ‘nano-impacts’.

The field of nanoparticle (NP) sizing encompasses a wide array of techniques, with electron microscopy and dynamic light scattering (DLS) having become the established methods for NP quantification; however, these techniques are not always applicable. A new and rapidly developing method that addresses the limitations of these techniques is the electrochemical detection of NPs in solution. The ‘nano-impacts’ technique is an excellent and qualitative in situ method for nanoparticle characterization. Two complementary studies on silver and silver bromide nanoparticles (NPs) were used to assess the large radius limit of the nano-impact method for NP sizing. Noting that by definition a NP cannot be larger than 100 nm in diameter, we have shown that the method quantitatively sizes at the largest limit, the lower limit having been previously reported as ∼6 nm.[1]

Surface modification of boron-doped diamond (BDD) with copper phthalocyanine was achieved using a simple and convenient dropcast deposition, giving rise to a microcrystalline structure. Both unmodified and modified BDD electrodes of different surface terminations (namely hydrogen and oxygen) were compared via the electrochemical reduction of oxygen in aqueous solution. A significant lowering of the cathodic overpotential by about 500 mV was observed after modification of hydrogen-terminated (hydrophobic) diamond, while no voltammetric peak was seen on modified oxidised (hydrophilic) diamond, signifying greater interaction between copper phthalocyanine and the hydrogen-terminated BDD. Oxygen reduction was found to undergo a two-electron process on the modified hydrogen-terminated diamond, which was shown to be also active for the reduction of hydrogen peroxide. The lack of a further conversion of the peroxide was attributed to its rapid diffusion away from the triple phase boundary at which the reaction is expected to exclusively occur.

The antibacterial properties of silver are strongly controlled by the redox couple of silver/silver(I). This work reports the influence of phosphate anions on silver nanoparticle oxidation, which is important given the abundance of phosphate species in biological systems. The three different species of anions were found to have a varying degree of influence on silver oxidation with the order PO₄³⁻>HPO₄²⁻>H₂PO₄⁻. It was found that in the presence of phosphate anions, the silver oxidation potential shifts to a less positive value, which indicated the increasing ease of the oxidation reaction of silver. Given that the interplay between silver and its cation is crucial to its antibacterial properties and significant concentrations of the HPO₄²⁻ anion are present at biological pH (near neutral), it is essential that the influence of the dibasic anion (HPO₄²⁻) on silver oxidation dynamics be considered for biological systems.

The oxidative doping of single poly(N-vinylcarbazole) (PVK) nanoparticles is reported in aqueous sodium perchlorate using the nanoimpact method. Complete oxidative doping of single PVK nanoparticles with a size of approximately 120 nm is demonstrated, showing for the first time a simple strategy to synthesize and characterize doped polymeric nanoparticles at the single nanoparticle level.

Encapsulating liposomes are widely used for controlled drug delivery. We report the use of nano-impact experiments for the electrochemical attomolar quantification of the liposome load, uniquely at the single liposome level, using vitamin C encapsulated liposomes as a model. The size of the liposomes and their picomolar concentration are also determined in biological buffer in real time.

The electrochemistry of nitroblue tetrazolium chloride (NBTC) was studied in aqueous solutions of pH 6.97 on a glassy carbon macroelectrode and at a carbon fibre microelectrode; values of near unity for the reduction transfer coefficient and 7.2×10⁻⁶ cm² s⁻¹ for the diffusion coefficient were generated. The electrochemically produced diformazan was shown to adsorb on the glassy carbon surface if the potential was held at −0.35 V [vs. saturated calomel electrode (SCE)]. A carbon paste electrode, fabricated by using dioctyl phthalate and graphite powder, was used as a nonenzymatic sensor. The sensitivity of the diformazan oxidation signal to the presence of superoxide was exploited to detect superoxide, which is voltammetrically visible at about +0.69 V (vs. SCE). The paste electrode was first immersed in aqueous superoxide solutions. It was subsequently equilibrated with NBTC by immersing it into aqueous NBTC solutions. The reduction of NBTC (by superoxide) thus took place in the paste, which allowed quantification of the superoxide in the aqueous phase by means of the diformazan oxidation signal. Values for the practical limit of detection and the sensor sensitivity, 0.059 nM and 1.79 μA nM⁻¹ respectively, were obtained.

Organic nanoparticles are attracting many significant applications in bio-imaging and nanomedicine. However, in comparison with intensive studies of metal and inorganic nanoparticles, the research of organic nanoparticles is still at a very early stage due to their complex physical and chemical properties. Most recently, there are increasingly significant interests in developing nano-capacitors and nano-scale energy storage and sensor devices based on organic materials. Herein, we report for the first time the kinetics and mechanism of electron transfer to individual organic nanoparticles, using indigo nanoparticles as a model system, via analysis of the charge transferred in the reduction of individual organic nanoparticles. We conclude the indigo nanoparticles display irreversible (slow) electron transfer kinetics and that the charge transfer is the rate-determining step in the reductive dissolution of the nanoparticles; protonation and detachment of molecules from the nanoparticles occurs after this rate-determining step. The transfer coefficient, α, of the electron transfer is found to be 0.25±0.05, whilst the rate constant (k⁰) is determined as the composite parameter , where is the formal potential, which is found to have the value of 1.7×10⁻⁸ m s⁻¹. Given the high importance of the electron transfer mechanism for organic nanomaterials, this report may have great significance on future research of organic nanomaterials for diverse applications.

In some important electrochemical systems, the degree of electro-reduction (or electro-oxidation) of the reactant at the electrode surface depends on the extent of a surface-catalysed reaction that involves intermediates formed by electron transfer. The catalytic properties of the electrode surface towards this heterogeneous reaction, therefore, can control the final product of important processes, such as the electro-reduction of oxygen and some organic compounds. The modelling of the EC_hetE_fd mechanism (where C_het is a heterogeneous chemical reaction and E_fd indicates that the second electron transfer is fully driven) is considered in this paper at nanoparticle-modified electrodes and homogeneous surface macroelectrodes. The influences of the chemical and electrochemical kinetics as well as the characteristics of the mass transport are investigated. The results enable us to propose procedures for the identification and characterisation of the surface-catalysed process and for the optimisation of electrode modifications.

Cu underpotential deposition (UPD) is compared between a Au macroelectrode and Au nanoparticles ranging between 1.8 nm and 123 nm in size in order to identify whether the capping agent or nanoparticle size affects this reaction. It was found that the surface coverage of Cu was markedly decreased in the presence of the citrate capping agent, while a strong size dependence was also observed, with nanoparticles smaller than 60 nm displaying significantly less defined Cu UPD responses. These results highlight the influence of both the capping agent and nanoparticle size for UPD applications, such as the electrochemical determination of surface areas and the formation of bimetallic surfaces.

We theoretically and experimentally investigate the influence of partial surface blocking on the electrochemistry of nanoparticles impacting at an electrode. To this end, we introduce an analytical model for the adsorption of single blocking molecules on the electrode and calculate the resulting fractional electrode coverage. We find that even small amounts of adsorbed molecules can fully suppress detection of impacts of nanoparticles while the electrode characteristics in the detection of electroactive molecules hardly change. Our findings are supported by experimental data on the indigo nanoparticle electroreduction at a carbon microelectrode (radius 5.5 μm) in aqueous solution. We find that nanoimpacts are fully suppressed in the presence of acetone at concentrations of 250 nm, which have a negligible effect on the electrode kinetics of the Fe(CN) couple.

The oxidative doping of single poly(N-vinylcarbazole) (PVK) nanoparticles is reported in aqueous sodium perchlorate using the nanoimpact method. Complete oxidative doping of single PVK nanoparticles with a size of approximately 120 nm is demonstrated, showing for the first time a simple strategy to synthesize and characterize doped polymeric nanoparticles at the single nanoparticle level.

Encapsulating liposomes are widely used for controlled drug delivery. We report the use of nano-impact experiments for the electrochemical attomolar quantification of the liposome load, uniquely at the single liposome level, using vitamin C encapsulated liposomes as a model. The size of the liposomes and their picomolar concentration are also determined in biological buffer in real time.

The use of micro- and nanoelectrodes and their arrays has become commonplace in modern electrochemistry. Numerical simulation is often required for detailed analysis of voltammetric data and this relies upon an understanding of the prevailing mass transport operating under the experimental conditions. The theoretical basis of our understanding of mass transport, particularly diffusion and migration, has developed greatly in recent years. We review both theoretical and experimental studies which have probed the mass transport at micro- and nanoelectrodes and their arrays. DOI 10.1002/tcr.201100032

The electrochemistry of formic acid, carbon monoxide and methanol have been investigated and evaluated in combination with hydrazine. Hydrazine was observed to display the anticipated steady-state oxidation waves at platinum (Pt) microelectrodes by cyclic voltammetry, and upon introduction of carbon monoxide (CO) gas, the Pt surface was fully passivated (prior to CO oxidation). However, the two individual responses of hydrazine and formic acid (HCOOH) are to be additive when combined in solution. No detrimental effects were observed upon the hydrazine voltammetry, even in the presence of excess formic acid, despite formic acid clearly displaying characteristic self-poisoning tendencies (primarily due to the formation of CO) in its own voltammetry. Effects intermediate to those of CO and formic acid were observed when methanol was present. Currents were essentially additive at low methanol content, but hydrazine oxidation current decreased by about 40 % when an 100-fold excess of methanol was present, corresponding to poisoning by methanol dehydrogenation intermediates. These results are discussed with relevance to mixed fuels for more flexible or powerful fuel cells, and the possible formation of a random microelectrode array (templated by strongly adsorbed poison) on the microelectrode surface.

We report the catalytic anthraquinone-mediated reduction of oxygen at a boron-doped diamond electrode. Scheme of squares modelling confirms the existence of and reveals the role of the semiquinone intermediates, which are shown to have an exceptional reactivity towards oxygen (as compared to the di-reduced anthraquinone).

The volatilisation of ferrocene (Fc), dissolved in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C₄mpyrr][NTf₂], to the gas phase has been indirectly monitored by cyclic voltammetry and chronoamperometry. Simulation of the observed trends in concentration with time using a simple model allowed quantification of the process. Volatilisation of dissolved Fc under flowing wet and dry dinitrogen gas (N₂) was found to be kinetically limited with a rate constant in the region of 2×10⁻⁷ cm s⁻¹. The activation energy of diffusion for Fc was found to be 28.2±0.7 kJ mol⁻¹, while the activation energy of volatilisation of Fc from [C₄mpyrr][NTf₂] to dry N₂ was found to be 85±2 kJ mol⁻¹.

An edge plane pyrolitic graphite (EPPG) electrode was modified by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N₂⁺BF₄⁻), giving an EPPG–AQ2-modified electrode of a surface coverage below a monolayer. Cyclic voltammograms simulated using Marcus–Hush theory for 2e⁻ process assuming a uniform surface gave unrealistically low values of reorganisation energies, λ, for both electron transfer steps. Subsequently, two models of surface inhomogeneity based on Marcus–Hush theory were investigated: a distribution of formal potentials, E′, and a distribution of electron tunneling distances, r₀. The simulation of cyclic voltammograms involving the distribution of formal potentials showed a better fit than the simulation with the distribution of tunneling distances. Importantly the reorganization energies used for the simulation of E′ distribution were similar to the literature values for adsorbed species.

Graphitic electrodes find widespread use throughout electrochemistry; understanding their fundamental electrochemical properties is imperative. It is widely thought that graphite edge plane sites exhibit faster rates of electron transfer as compared to basal plane sites. Hitherto the different rates of electron transfer at the edge and basal sites have been inferred indirectly using diffusional systems. To avoid possible complications we alternatively study a surface-bound system to simplify the interpretation. The voltammetric response of graphitic-surface-bound anthraquinone monosulfonate (AQMS) with varying pH, reveals two distinct voltammetric responses, ascribed as being due to the basal and edge plane sites; where the pK_a s associated with the reduced anthraquinone are found to differ for the two sites. Through modelling of the system based upon a “scheme of squares” mechanism it is possible to conclude that both the thermodynamics and kinetics of the species differ in the two environments in which the rate of electron transfer at the basal plane site is shown to be 2–3 orders of magnitude slower than that of the edge plane site. This work provides the first example of a voltammetric response which is purely due to electron transfer at a basal plane site. Further, we believe this is the first time a full “scheme of squares” model has been used for the quantitative analysis of a diffusionless 2H⁺2e⁻ system.

Electrode kinetic data for the electro-reduction of 4-nitrophenol in aqueous solution are compared for bulk silver macro-electrodes and arrays of silver nanoparticles of size 15–50 nm. The electrode kinetics and mechanism change qualitatively and quantitatively.

The electrochemical reduction of benzoic acid (BZA) has been studied at platinum micro-electrodes (10 and 2 µm diameters) in acetonitrile (MeCN) and six room temperature ionic liquids (RTILs): [C₂mim][NTf₂], [C₄mim][NTf₂], [C₄mpyrr][NTf₂], [C₄mim][BF₄], [C₄mim][NO₃] and [C₄mim][PF₆] (where [C_nmim]⁺ = 1-alkyl-3-methylimidazolium, [NTf₂]⁻ = bis(trifluoromethylsulphonyl)imide, [C₄mpyrr]⁺ = N-butyl-N-methylpyrrolidinium, [BF₄]⁻ = tetrafluoroborate, [NO₃]⁻ = nitrate and [PF₆]⁻ = hexafluorophosphate). Based on the theoretical fitting to experimental chronoamperometric transients in [C₄mpyrr][NTf₂] and MeCN at several concentrations and on different size electrodes, it is suggested that a fast chemical step preceeds the electron transfer step in a CE mechanism (given below) in both RTILs and MeCN, leading to the appearance of a simple one-electron transfer mechanism.

The six RTIL solvents and MeCN were saturated with BZA, and potential-step chronoamperometry revealed diffusion coefficients of 170, 4.6, 3.2, 2.7, 1.8, 0.26 and 0.96 × 10⁻¹¹ m² s⁻¹ and solubilities of 850, 75, 78, 74, 220, 2850 and 48 mM in MeCN and the six ionic liquids, respectively, at 298 K. The high solubility of BZA in [C₄mim][NO₃] may suggest a strong interaction of the dissolved proton with the nitrate anion. Although there are relatively few literature reports of solubilities of organic solutes in RTILs at present, these results suggest the need for further studies on the solubilities of organic species (particularly acids) in RTILs, because of the contrasting interaction of dissolved species with the RTIL ions. Chronoamperometry is suggested as a convenient methodology for this purpose. Copyright © 2008 John Wiley & Sons, Ltd.

The reduction of methyl benzoate was studied in tetrahydrofuran (THF), 0.1 M TBAP, both at platinum and glassy carbon macroelectrodes and at platinum microelectrodes. While cyclic voltammograms obtained at macroelectrodes show extensive distortion due to ohmic drop, this distortion is negligible for cyclic voltammograms obtained at microelectrodes. The system methyl benzoate/methyl benzoate radical anion was found to be chemically reversible. The fitting of chronoamperometric measurements using the Shoup and Szabo's expression obtained at microdisc electrodes allowed us to estimate the diffusion coefficient of methyl benzoate. Cyclic voltammetry was modelled allowing the determination of the formal potential, , of the transfer coefficient, α, and of the standard heterogeneous kinetic constant for the electron transfer, k₀. Furthermore, it was found that the diffusion coefficient for the radical anion produced at the electrode surface has an apparent value of 2.1 × 10⁻⁶ ± 0.5 cm² s⁻¹ (25 °C) which is ca. 10 times smaller than the value obtained for the diffusion coefficient of methyl benzoate (2.1 × 10⁻⁵ ± 0.1 cm² s⁻¹). Taking into account the species present in solution, this result could only be explained by a strong ion pairing between the methyl benzoate anion radical and the tetra-n-butylammonium cation used as an electrolyte ion in the solution. The ion pair is found to be stable on the time scale of cyclic voltammetry and no evidence of any following homogeneous chemical reaction was found. Further studies involved the performance of cyclic voltammetry and chronoamperometry at low temperatures (down up to −81 °C) and the analysis of this data allowed the determination of the activation energy, E_A, through an Arrhenius plot, both for the diffusion coefficient and for the electron transfer reaction. Copyright © 2008 John Wiley & Sons, Ltd.

This review describes the protocol, procedures and methods for electrochemical studies in THF and at low temperature that have been developed in the course of the last 5 years in our laboratory. Electrochemical studies in THF benefit from a large accessible potential window. In practice, it is however necessary to avoid the presence of humidity in the electrochemical cell. A specific reference electrode had to be designed for those measurements. Microelectrodes, the voltammetric response of which is negligibly affected by the resistivity of the medium, were preferred to macroelectrodes. Moreover, a methodology has been developed for the quantitative analysis of both voltammetric and chronoamperometric curves obtained for the microelectrodes. The fitting of chronoamperometric measurements using the Shoup and Szabo's expression allows us to estimate the diffusion coefficient of the substrate. The modelling of the cyclic voltammograms measured over a large range of scan rates allows the confirmation of the diffusion coefficient of the substrate D_A, the determination of the diffusion coefficient of the electrogenerated molecule D_B, of the formal potential E, of the transfer coefficient α and of the standard heterogeneous rate constant for the electron transfer k₀. Typically, systems are investigated at temperatures ranging from room temperature to 192 K. Parameters obtained at various temperatures are used to extract, through Arrhenius plots, the activation energies both for the diffusion coefficient E_A(D) and for the electron transfer reaction E_A(k₀). Copyright © 2009 John Wiley & Sons, Ltd.

We present the novel use of photoelectrochemistry to detect and monitor the trajectory of moving spheres. Using an array of individually addressable electrodes under illumination and potentiostatted so that a photocurrent is generated, the motion of a sphere is detected by means of measuring “dark” transients as the shadow cast by the moving sphere passes over each electrode. The method can used to determine the size and velocity of a single ball, or simultaneously track two spheres in collision.

Modification of carbon materials such as graphite and glassy carbon in bulk quantities using diazonium salts is developed. We used both 4-nitrobenzenediazonium tetrafluoroborate and 1-antharaquinonediazonium chloride to modify graphite and glassy carbon surfaces. Experiments were carried out in the presence and absence of hypophosphorous acid and the mechanism involved in both cases were studied using cyclic voltammetry. The observed peak potentials for both the 4-nitrophenyl and 1-anthraquinonyl modified materials were found to differ depending on whether or not the hypophosphorous acid reducing agent was used. In the absence of hypophosphorous acid the derivatisation reaction was inferred to go through a cationic intermediate, whilst in the presence of the hypophosphorous acid the mechanism likely involves either a purely radical intermediate or a mixture of radical and cationic species. Derivatisation experiments from 5 to 70°C allowed us to determine the optimum derivatisation temperature for both cases, in the presence and absence of hypophosphorous acid. Optimum temperature was 20°C for the former and 35°C for the later. Copyright © 2008 John Wiley & Sons, Ltd.