Heterogeneous Electron Transfer Reactions Research Articles

Efficacy of films of uncapped gold nanoparticle as electrodes for direct electron transfer to redox proteins

Electrochemical manipulation of enzymes is expected to enable the rapid detection of marker molecules and the efficient production of valuable materials. We have fabricated an Au nanoparticle (AuNP) thin film electrode in our previous study, which enables more enhanced heterogeneous electron transfer reactions than a conventional planner Au electrode. In this study, electrochemical evaluation of cytochrome c was performed using the AuNP thin film electrodes with and without self-assembled monolayer (SAM) modification to assess its practicality for protein electrochemistry. The 4-pyridinethiol- and 7-carboxy-1-heptanethiol-modified AuNP thin film electrodes showed much larger faradaic current values attributed to redox reactions compared to those of the Au electrodes. The results indicate that the nanostructural effects of the AuNP thin film electrode are beneficial. Importantly, even when the non-modified (bare) AuNP thin film electrode was applied, it exhibited the clear electrochemical response of cyt c, while the bare Au electrode showed no such a response. This study provides evidence that the AuNP thin film electrode functions as a potent bioelectrocatalysts due to the nanostructure-specific properties.

Electrocatalytic Oxidation of Nitrite Using a Composite of Reduced Graphene Oxide and Cobalt‐4‐Pyridylporphyrin Coordination Polymer

AbstractThe present work describes the electrocatalytic oxidation of nitrite in the presence of a composite of coordination polymer of cobalt‐4‐pyridylporphyrin and electrochemically reduced graphene oxide. The electrode was modified by drop‐casting a suspension of graphene oxide and polymeric cobalt‐4‐pyridylporphyrin followed by reduction through cyclic voltammetry in the potential range 0.0 V to −1.5 V. The electrocatalytic investigations were carried out by voltammetric (cyclic, linear sweep, differential pulse and hydrodynamic) and by amperometric methods. The LOD and the working range obtained through different techniques are compared. The rate constant for this heterogeneous electron transfer reaction is 2.45×10−2 cms−1.

Expanding the Scanning Electrochemical Microscopy Toolset for Evaluating Electron Transfer Kinetics of Concentrated and Specialized Redox Electrolytes on Carbon Surfaces

Performing quantitative measurements of heterogeneous electron transfer reactions at electrodes (e.g., determining k0 and a) under conditions of high redox active molecule concentration in solution, such as those encountered in electrolytes for redox flow batteries (RFBs), presents significant technical challenges. For example, attempting to use transient voltammetry methodology quickly leads to limitations imposed by iR drop, as currents can be large and resistance, especially in non-aqueous systems, can be high. Additionally, using steady-state techniques based on convective principles, such as the rotating disk electrode (RDE), involve the use of relatively large equipment, with moving parts, and usually with requirements of tens to hundreds of mL of solution, which may be limiting for exploring new compounds at high concentration.To address these issues, here we introduce the use of scanning electrochemical microscopy (SECM) for the determining the rates of electron transfer of redox active species at high concentration in non-aqueous [1] and other unconventional systems, such as deep eutectic solvents. SECM enables the resolution of higher rates of electron transfer compared to transient voltammetry and RDE, and it also enables experiments with spatial resolution, enabling the exploration of reactivity on various sites of a heterogeneous electrode. In this presentation, we will describe theoretical and practical aspects of these measurements, and we will demonstrate their utility using two model systems. The first one consists on the observation of inherent rate asymmetries observed for ferrocene derivatives, ubiquitous redox probes, at the graphite/carbonate solvent interface. These asymmetries are not present on aqueous systems or metallic electrodes, demonstrating the ability of the SECM technique to capture unique behaviors. In the second case, we will discuss how SECM measurements can be used on deep eutectic solvents such as ethaline to address how interactions between redox actives and the electrode or the solvent systems modulate the rate of electron transfer. By systematically modifying the strength of interactions by means of molecule choice, solvent composition, and electrode type and structure, we can gain insight into the processes that determine reactivity at redox flow battery electrodes. This insight is critical for exploring the interfacial aspects such as adsorption, passivation, and double-layer effects on kinetics of electron transfer.[1] Gaddam, R.; Sarbapalli, D.; Howard, J.; Curtiss, L.A.; Assary, R.S.; Rodríguez-López, J. An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces Chem: Asian J., 2023, 18, 2, e202201120. Figure 1

Use of phase angle in alternating voltammetry on redox self-assembled monolayers with intermolecular interactions

We present a methodology based on the study of the frequency dependence of the phase angle in Alternative Voltammetry to estimate the standard electrochemical rate constant for heterogeneous electron transfer reactions in the case of strong intermolecular interactions occur between the redox forms. This work is a follow-up to our previous study which was dedicated only to the study of peak currents.

Electrodeless Synthesis of Low Dispersity Au Nanoparticles and Nanoclusters at an Immiscible Micro Water/Ionic Liquid Interface.

Owing to their biocompatibility, optical, and catalytic properties, Au nanoparticles (NPs) have been the subject of much research. Since smaller NPs have enhanced catalytic properties and NP morphology greatly impacts their effectiveness, controlled and reproducible methods of generating Au NPs are still being sought. Herein, Au NPs were electrochemically generated at a water|ionic liquid (w|IL) immiscible micro-interface, 25 µm in diameter, using a redox active IL and compared to results at a water|oil (w|o) one. The liquid|liquid interface is advantageous as it is pristine and highly reproducible, as well as an excellent means of species and charge separation. In this system, KAuCl4 dissolved in the aqueous phase reacts under external potential control at the water|P8888TB (tetraoctylphosphonium tetrakis(pentafluorophenyl)borate) with trioctyl(ferrocenylhexanoyl)phosphonium tetrakis(pentafluorophenyl)borate (FcIL), an electron donor and redox active IL. FcIL was prepared with a common anion to P8888TB, which greatly enhances its solubility in the bulk IL. Simple ion transfer of AuCl4− and AuCl(4−γ)(OH)γ− at the w|P8888TB micro-interface were characterized voltammetrically as well as their heterogeneous electron transfer reaction with FcIL. This interfacial reaction generates Au NPs whose size can be thermodynamically controlled by modifying the pH of the aqueous phase. Critically, at low pH, nanoclusters, <1.7 nm in diameter, were generated owing to inhibited thermodynamics in combination with the supramolecular fluidic nature of the IL microenvironment that was observed surrounding the as-prepared NPs.

Efficient removal of estrogenic compounds in water by MnIII-activated peroxymonosulfate: Mechanisms and application in sewage treatment plant water

In this paper, the degradation of three endocrine-disrupting chemicals (EDCs): bisphenol A (BPA), 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) by manganite (γ-MnOOH) activated peroxymonosulfate (PMS) was investigated. Preliminary optimisation experiments showed that complete degradation of the three EDCs was achieved after 30 min of reaction using 0.1 g L−1 of γ-MnOOH and 2 mM of PMS. The degradation rate constants were determined to be 0.20, 0.22 and 0.15 min−1 for BPA, E2 and EE2, respectively. Combining radical scavenging approaches, Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) analyses, we revealed for the first time that about 40% of EDCs degradation can be attributed to heterogeneous electron transfer reaction involving freshly generated Mn(IV), and 60% to sulfate radical degradation pathway. The influence of various inorganic ions on the γ-MnOOH/PMS system indicated that removal efficiency was slightly affected by chloride and carbonate ions, while nitrate and nitrite ions had negligible impacts. The application of γ-MnOOH/PMS system in real sewage treatment plant water (STPW) showed that degradation rate constants of EDCs decreased to 0.035–0.048 min−1 and complete degradation of the three EDCs after 45 min. This study provides new insights into the reactivity of combined γ-MnOOH and PMS, and opens new ways for the application of Mn-bearing species in wastewater treatment technologies.

Gold nanoparticle-based eco-friendly ink for electrode patterning on flexible substrates

Au nanoparticles functionalized by short poly(ethylene glycol) chain ligands were suspended in a hydroalcoholic medium. This suspension was then printed by inkjet onto a Kapton® substrate. Gold electrodes consisting of four stacked printed layers exhibited a conductivity of (1 ± 0.1) × 107 S/m, close to that of Au bulk, after low-temperature thermal annealing. The electrochemical behaviour with respect to heterogeneous electron transfer reactions and the reactivity towards thiol chemisorption are identical to those obtained when a conventional polycrystalline Au electrode is used.

Enhancing Electrogenerated Chemiluminescence on Platinum Electrodes through Surface Modification

AbstractIncreasing the light emission of electrogenerated chemiluminescence is an important goal for enhancing the sensitivity for potential practical applications. Electrogenerated chemiluminescence is primarily triggered by a heterogeneous electron transfer reaction, for which the electrode material plays a pivotal role. We investigated how a platinum electrode, one of the most used but poorly efficient noble metal electrode materials in electrogenerated chemiluminescence, can be modified to enhance light emission. A polypyrrole layer was deposited on the platinum electrode through electrochemically induced polymerization, and subsequently pyrolyzed with the formation of a carbonaceous film. Electrochemiluminescence of the [Ru(bpy)3]2+/tri‐n‐propylamine system on such carbon film electrodes showed an enhancement of up to a 4 times increase, as compared with the bare platinum electrode.

A Mexacarbate Electrochemical Biosensor on Carbon Materials Based on a Functionalized Multiwalled Carbon Nanotube Modified Glassy Carbon Electrode

The commercial multi-walled carbon nanotube (MWCNT) was carboxylated using sulphuric and nitric acid and characterized by various techniques such as scanning eletron spectroscopy, Infra-red spectroscopy and atomic force microscopy. The acid functionalized multiwalled carbon nanotube serves as the effective matrix for the sensing of carbamate pesticide namely mexacarbate. The electrochemical oxidation of mexacarbate at GCE and MWCNT/GCE follows heterogeneous electron transfer reaction. Different experimental parameters such as the effect of scan rate and pH were studied. Rotating disk electrode voltammetry studies confirms the one-electron transfer reaction. The sensor calibration plot for mexacarbamate was constructed by means of pulse technique. The acid functionalized multiwalled carbon nanotube sensor was applied to real water sample with the recovery percentage of 85-89%.

Electrode Stimulation: Heterogeneous Electron Transfer Reactions Induced By Modulation of Electrostatic Potentials in the Electrolyte

Introduction The rates of electrode reactions are frequently expressed in terms of the surface overpotential, ηs, defined as “the potential of the working electrode relative to a reference electrode of the same kind placed in the solution adjacent to the surface of the working electrode.”1 Changes in ηs are most commonly effected by means of a potentiostat, a device that allows accurate control of the potential of the working electrode with respect to a reference electrode in solution.2 This communication describes a method that makes it possible to promote heterogeneous electron transfer reactions at a working electrode, WE, polarized at a fixed potential, EWE, by changing the electrostatic potential in the electrolyte “just outside its double layer”, ϕdl, and thus ηs. Experimental The body of the electrochemical cell used in our experiments was a polypropylene centrifuge tube with its bottom end cut off to expose a circular hole ca. 5 mm in diameter. A commercial Au disk microelectrode (12.5 µm diameter), Au µ-el, inserted into the hole of the cell facing upwards, was used as WE, and a carbon rod and a Ag/AgCl as counter and reference electrodes, respectively. The stimulating electrode, SE, was a Pt disk (1.6 mm diameter) placed directly above the WE. The potentials of the WE and SE were controlled independently using a Metrohm Autolab potentiostat. The cell was filled with 0.1 M HClO4 and then purged with Ar prior to the measurements. EWE was fixed at a prescribed value, while the potential of the SE, ESE, was scanned at a rate ν = 50 V/s, starting at 1.2 V, down to -0.25 V and back to 1.2 V for all experiments. Results The cyclic voltammogram of the Au µ-el, WE, recorded at a scan rate ν = 50 V/s in deaerated 0.1 M HClO4 (see insert, Fig. 1), displayed characteristic features ascribed to the formation of Au oxide and its subsequent reduction. ESE was set at 1.2 V for ca. 1 s, and then scanned at ν = 50 V/s down to -0.25 V, and back again to 1.2 V, yielding the “unfolded” voltammogram in the upper panel, Fig. 1. The current response of the Au working µ-el, iWE, during this scan, when fixing EWE= 1.15 V, is shown in green in the lower panel, Fig. 1. As clearly indicated, for ESE < 0.5 V during the scan in the negative direction, both |iSE| and |iWE| markedly increased, albeit in opposite directions, reaching a maximum at ca. 22 ms. Whereas |iSE| decreased by ca. 30% toward the end of the scan in negative direction, |iWE| dropped to a very small value similar to that found prior to the stimulation. These results are consistent with the formation of Au oxide stimulated by a shift in ηs toward positive values induced by iSE. In fact, the sudden drop in iWE signals the end of Au oxide formation at the maximum applied ηs. During the entire scan toward positive potentials, iWE was very small, indicating that the value of ηs achieved was insufficient to effect the reduction of the Au oxide formed. This is in agreement with the cyclic voltammogram of Au, for which the onset of Au oxide reduction is ca. 0.9 V (see insert, Fig. 1). In striking contrast, very small iWE was observed in similar experiments in which EWE = 0.4 V (see black, Fig. 1) regardless of the direction of the scan. We ascribe this behavior to changes in the structure of the interface, which may involve a redistribution of charge in the diffuse double layer, as well as a possible reorientation of water dipoles at the WE surface. Strong evidence that the response of the WE is indeed derived from changes in ϕdl was provided by experiments in which the distance between WE and SE, δ, was varied. As shown in Fig. 2, increasing δ led to a decrease in ηs, thus, in a corresponding decrease in the magnitude of the peak current associated with Au oxide formation. This behavior, as well as the lack of proportionality between the peak current and δ, are both in agreement with theoretical predictions. Acknowledgments This work was supported by a grant from NSF, CHE-1412060.

Local studies of photoelectrochemical reactions at nanostructured oxides

Abstract The conversion of photon energy to chemical energy and vice versa requires the close arrangement of absorber/emitters and (electro)chemical reactions sites. This review considers local measurement techniques aiding in the design of efficient oxide systems for the utilization of light as energy source and as efficient detection principle. Artificial photoelectrochemical systems are often build on oxides as they are abundant and have semiconducting properties. However, no single oxide fulfills all requirements for an efficient conversion of sunlight to chemical energy and thus complex oxides are explored. These oxides might be obtained by doping oxides with other metal cations or by combining different oxides for absorbance and catalyzing the desired reaction, mainly water splitting. Due to the enormous amount of possible combinations combinatorial search for new material systems has been pursued and accelerated around the world making use of local photoelectrochemical characterization techniques in the screening step. Local detection schemes based on scanning electrochemical microscopy and scanning electrochemical cell microscopy also provide details about the kinetics for heterogeneous charge transfer and the release of soluble reaction products. During the recent years the scanning probe methods have been complemented by local detection of fluorescent reaction products that are formed by heterogeneous electron transfer reactions from and non-fluorescent precursor molecules. Such detection is possible with single molecule sensitivity and spatial resolution exceeding the diffraction limit (superresolution). Such approaches enabled the discovery of population within ensembles of metal oxide nanoparticles that are distinguished by the location and reactivity of their reaction sites. Optical techniques for measuring Faradaic currents hold great promise for the measurement of very low currents beyond the study of photoelectrochemistry of metal oxides.

Limitations in Estimation of E1/2 from Cyclic Voltammetric Data

It is common practice to estimate the standard (E0) or formal (E0’) potential from cyclic voltammetric data. A frequent estimate of E0 or E0’ is taken from either the peak potential (Ep), the half wave potential (E1/2), or the average of the cathodic and anodic peak potentials (Epc+Epa)/2. Often, this suffices to identify the standard or formal potential within 100 mV, but confidence in such estimates is not always justified. Mechanisms that include both electron transfer (E) and chemical (C) steps can impact the quality of estimated standard and formal potentials. In some systems, limitations to the estimated standard and formal potentials depend on the rates of heterogeneous electron transfer and homogenous reactions. Here, the quality of estimated standard and formal potentials based on peak potentials, half-wave potentials, and average potentials for the cathodic and anodic sweeps are described for various mechanisms. Cases where the standard and formal potentials are well estimated from measured cyclic voltammetric data are identified and the quality of the estimated standard and formal potentials are characterized. That is, specify whether the estimate is within 100 mV, 50 mV, or 10 mV for a given mechanism. Situations where the estimate fails are identified and alternative means to estimate the standard and formal potential are provided. Several reaction schemes that include EC, EE, CE, and the various ECE mechanisms are characterized.

Synergistic effect of iron diselenide decorated multi-walled carbon nanotubes for enhanced heterogeneous electron transfer and electrochemical hydrogen evolution

Iron diselenide (FeSe2) has been of recent interest as a potentially useful electrode material. Here we demonstrate, for the first time, the synergistic effect between multi-walled carbon nanotubes (MWCNT) and FeSe2 in a form of MWCNT/FeSe2 composite, for the heterogeneous electron transfer (HET) reaction with the [Ru(NH3)6]3+/2+ redox probe and the electrochemical hydrogen evolution reaction (HER) in acidic media. Plain FeSe2 and the MWCNT/FeSe2 composite were synthesized using a one-pot hydrothermal method. Although all electrode materials (pristine MWCNT, FeSe2, and MWCNT/FeSe2) show a quasi-reversible electrode behavior, MWCNT/FeSe2 has a smaller peak separation potential and exhibits an enhanced HET rate constant, k0 = 5.4 × 10−3 cm/s, which is 3.2 and 1.6 times higher than that of FeSe2 and MWCNT, respectively. Similarly, the HER performance of the composite demonstrates a smaller Tafel slope of 70 mV/dec and a reduced overpotential. We anticipate that such promising outcome can be further improved by employing different morphologies of FeSe2 and engineered interaction with other conductive supports suitable for their applications in electrochemical sensing and energy conversion.

Collision Dynamics during the Electrooxidation of Individual Silver Nanoparticles.

Recent high-bandwidth recordings of the oxidation and dissolution of 35 nm radius Ag nanoparticles at a Au microelectrode show that these nanoparticles undergo multiple collisions with the electrode, generating multiple electrochemical current peaks. In the time interval between observed current peaks, the nanoparticles diffuse in the solution near the electrolyte/electrode interface. Here, we demonstrate that simulations of random nanoparticle motion, coupled with electrochemical kinetic parameters, quantitatively reproduce the experimentally observed multicurrent peak behavior. Simulations of particle diffusion are based on the nanoparticle-mass-based thermal nanoparticle velocity and the Einstein diffusion relations, while the electron-transfer rate is informed by the literature exchange current density for the Ag/Ag+ redox system. Simulations indicate that tens to thousands of particle-electrode collisions, each lasting ∼6 ns or less (currently unobservable on accessible experimental time scales), contribute to each experimentally observed current peak. The simulation provides a means to estimate the instantaneous current density during a collision (∼500-1000 A/cm2), from which we estimate a rate constant between ∼5 and 10 cm/s for the electron transfer between Ag nanoparticles and the Au electrode. This extracted rate constant is approximately equal to the thermal collisional velocity of the Ag nanoparticle (4.6 cm/s), the latter defining the theoretical upper limit of the electron-transfer rate constant. Our results suggest that only ∼1% of the surface atoms on the Ag nanoparticles are oxidized per instantaneous collision. The combined simulated and experimental results underscore the roles of Brownian motion and collision frequency in the interpretation of heterogeneous electron-transfer reactions involving nanoparticles.

Double layer effects in voltammetric measurements with scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) operating as a variable gap ultra-thin layer twin-working electrode cell, has long been recognised as a powerful technique for investigating fast kinetics (heterogeneous electron transfer and homogeneous reactions coupled to electron transfer) as a consequence of high mass transport rates between the working electrodes when biased to promote redox shuttling. Recently, SECM has advanced technically and nanogap cells with dimensions on the 10s of nm scale have been reported. In this paper, we consider double layer effects on voltammetric measurements in this configuration, outlining a comprehensive model that solves the Nernst-Planck equation and Poisson equation with charged interfaces. For supporting electrolyte concentrations that have been used for such measurements (50mM and 100mM), it is shown that for typical electrode charges and charge on the glass insulator that encases the ultramicroelectrode (UME) tip used in SECM, there are profound effects on the voltammetric wave-shapes for redox reactions of charged redox couples, in the common modes used to study electron transfer kinetics, namely the tip-voltammetry (feedback) mode and substrate-voltammetry (substrate-generation/tip-collection and competition) modes. Using the reduction and oxidation of a singly charged redox species to a neutral and doubly charged species, respectively, as exemplar systems, it is shown that the charge on the electrodes can greatly distort the voltammetric wave-shape, while charge on the glass that surrounds the UME tip can affect the limiting current. Analysis of SECM voltammograms using methods that do not account for double layer effects will thus result in significant error in the kinetic values derived and tip-substrate distances that have to be estimated from limiting currents in SECM. The model herein provides a framework that could be developed for further studies with nanogap-SECM (e.g. consideration of alternative models for the electrical double layer, other supporting electrolyte concentrations, potential of zero charge on the electrodes and charges on the redox couples). The model results presented are shown to qualitatively match to SECM voltammetric features from experimental data in the literature, and are further supported by experimental data for redox processes of tetrathiafulvalene (TTF), namely the TTF/TTF+ and TTF+/TTF2+ redox couples. This serves to demonstrate the immediate practical application of some of the ideas presented herein. For future applications of SECM, the use of different supporting electrolyte concentrations and a range of tip-substrate separations may allow the determination of both electron transfer kinetics and double layer properties.

The electrochemical reduction of di-(2-ethylhexyl) phthalate (DEHP) in acetonitrile

The redox behavior of the extensively utilized plasticizer and toxicant, di-(2-ethylhexyl) phthalate (DEHP), was studied in acetonitrile solutions using a combination of cyclic voltammetry (at varied scan rates, temperatures, and concentrations) and controlled potential electrolysis techniques. Overall, DEHP was found to be reducible by two one-electron steps at moderately large negative potentials to form a radical anion (Epred≈−2.47 vs. (Fc/Fc+)/V, where Epred=cathodic peak potential and Fc/Fc+=ferrocene/ferrocenium) and dianion (Epred≈−2.65 vs. (Fc/Fc+)/V), respectively. The results revealed that the cathodic reactions were only moderately chemically reversible, even when measured at high scan rates or lowered temperatures, as the reduced forms of DEHP were involved in several chemical transformations (dimerization and decomposition) following their generation. Based on the voltammetric and electrolytic responses gathered, a mechanism was proposed for the electrochemical reduction of DEHP, and the heterogeneous electron transfer and homogeneous reaction steps were modeled by digital simulations to determine the associated thermodynamic and kinetic parameters.

Towards Understanding the Solvent-Dynamic Control of the Transport and Heterogeneous Electron-Transfer Processes in Ionic Liquids.

The impact of temperature-induced changes in solvent dynamics on the diffusion coefficient and standard rate constant k0 for heterogeneous electron transfer (ET) of ethylferrocene (EFc) in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6 ]) is investigated. The results are analysed to understand the impact of solvent-dynamic control, solute-solvent interactions and solvent friction on the transport of redox probes and k0 . Concentration dependence of the diffusion coefficient of EFc in [BMIM][PF6 ] is observed. This is attributed to the solute-induced enhancement of the structural organisation of the ionic liquid (IL), which is supported by the concentration-dependent UV/Vis absorption and photoluminescence responses of EFc/[BMIM][PF6 ] solutions. Similar values of the activation energies for mass transport and ET and a linear relationship between the diffusion coefficient and the heterogeneous ET rate is observed. The ratio between the diffusion coefficient and the heterogeneous rate constant allows a characteristic length Ld , which is temperature-independent, to be introduced. The presented results clearly establish that mass transport and heterogeneous ET of redox probes are strongly correlated in ILs. It is proposed that the apparent kinetics of heterogeneous ET reactions in ILs can be explained in terms of their impact on thermal equilibration, energy dissipation and thermal excitation of redox-active probes.

Toward More Reliable Measurements of Electron-Transfer Kinetics at Nanoelectrodes: Next Approximation.

Steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) have recently been used to measure kinetics of several rapid heterogeneous electron transfer (ET) reactions. One problem with those experiments was that the dependence of the shape of the steady-state voltammogram on kinetic parameters becomes weak when the reaction rate approaches the diffusion limit. The possibility to fit the same experimental voltammogram using different combinations of the standard rate constant, transfer coefficient, and standard potential results in significant uncertainties in extracted parameter values. In this article, the reliability of the kinetic analysis was improved by obtaining steady-state voltammograms with both oxidized and reduced forms of redox species initially present in solution. Additional improvements were attained by characterizing the nanoelectrode geometry with the atomic force microscope and using water with a very low level of organic contaminants (TOC ≤ 1 ppb). This approach was used to re-evaluate the ET rate constants measured for several electroactive species, including ferrocene, ferrocenemethanol, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ferrocyanide at Pt electrodes. The obtained standard rate constants are higher than the values measured earlier at Pt and Au nanoelectrodes but comparable to those obtained in recent nanogap/SECM experiments.

Investigating Nanoscale Electrochemistry with Surface- and Tip-Enhanced Raman Spectroscopy.

The chemical sensitivity of surface-enhanced Raman spectroscopy (SERS) methodologies allows for the investigation of heterogeneous chemical reactions with high sensitivity. Specifically, SERS methodologies are well-suited to study electron transfer (ET) reactions, which lie at the heart of numerous fundamental processes: electrocatalysis, solar energy conversion, energy storage in batteries, and biological events such as photosynthesis. Heterogeneous ET reactions are commonly monitored by electrochemical methods such as cyclic voltammetry, observing billions of electrochemical events per second. Since the first proof of detecting single molecules by redox cycling, there has been growing interest in examining electrochemistry at the nanoscale and single-molecule levels. Doing so unravels details that would otherwise be obscured by an ensemble experiment. The use of optical spectroscopies, such as SERS, to elucidate nanoscale electrochemical behavior is an attractive alternative to traditional approaches such as scanning electrochemical microscopy (SECM). While techniques such as single-molecule fluorescence or electrogenerated chemiluminescence have been used to optically monitor electrochemical events, SERS methodologies, in particular, have shown great promise for exploring electrochemistry at the nanoscale. SERS is ideally suited to study nanoscale electrochemistry because the Raman-enhancing metallic, nanoscale substrate duly serves as the working electrode material. Moreover, SERS has the ability to directly probe single molecules without redox cycling and can achieve nanoscale spatial resolution in combination with super-resolution or scanning probe microscopies. This Account summarizes the latest progress from the Van Duyne and Willets groups toward understanding nanoelectrochemistry using Raman spectroscopic methodologies. The first half of this Account highlights three techniques that have been recently used to probe few- or single-molecule electrochemical events: single-molecule SERS (SMSERS), superlocalization SERS imaging, and tip-enhanced Raman spectroscopy (TERS). While all of the studies we discuss probe model redox dye systems, the experiments described herein push the study of nanoscale electrochemistry toward the fundamental limit, in terms of both chemical sensitivity and spatial resolution. The second half of this Account discusses current experimental strategies for studying nanoelectrochemistry with SERS techniques, which includes relevant electrochemically and optically active molecules, substrates, and substrate functionalization methods. In particular, we highlight the wide variety of SERS-active substrates and optically active molecules that can be implemented for EC-SERS, as well as the need to carefully characterize both the electrochemistry and resultant EC-SERS response of each new redox-active molecule studied. Finally, we conclude this Account with our perspective on the future directions of studying nanoscale electrochemistry with SERS/TERS, which includes the integration of SECM with TERS and the use of theoretical methods to further describe the fundamental intricacies of single-molecule, single-site electrochemistry at the nanoscale.

Heterogeneous versus homogeneous electron transfer reactions at liquid–liquid interfaces: The wrong question?

The exact mechanism of the electron transfer reactions at liquid–liquid interfaces still remains a source of interrogation. The purpose of this paper is to revisit this topic using a finite element simulation approach to analyze cyclic voltammograms for some previously published systems. Also, we compare the voltammograms obtained in the absence or presence of an adsorbed gold nanoparticle film. The current results indicate that the electron transfer between ferrocene in the organic phase and hexacyanoferrate(III) in the aqueous phase takes place by a potential independent homogeneous reaction in the aqueous phase, while the observed potential dependence stems from that of the concomitant ion transfer reaction of ferrocenium. In the presence of the interfacial gold nanofilm the electron transfer takes place by a bipolar mechanism where the electrons are shuttled through the metallic nanofilm.