Heterogeneous Electron Transfer Research Articles

Cyclic voltammetric studies of electron transfer reactions associated with bioactive iron–KA complexation at different biological pH

This study explores electrochemical interactions of a pharmaceutical drug, kojic acid (5-hydroxy-2-hydroxymethyl-4 H-pyran-4-one, KA), with bioactive iron Fe(III) at different pH systems by using cyclic voltammetric technique. For this study, buffers relevant to physiological pH of blood and digestive system have been selected. Bioactive iron was found to form an electroactive complex [Fe(III)L3] in the presence of KA at all studied pH, whereas electrochemical behavior of KA was also changed by complex formation. This electrochemical investigation reveals that under the experimental conditions, [Fe(III)L3] complex undergoes a diffusion controlled, one electron redox process that is quasi-reversible in nature. It was also found that [Fe(III)L3] complex undergoes coupled chemical reaction according to EC (an electron transfer reaction followed by a chemical reaction) mechanism, and this interaction is pH dependent. Modified Nicholson and Shain, Gileadi, and Kochi methods were used to determine standard heterogeneous electron transfer rate constant ( k0) of [Fe(III)L3] complex. It was found to be in the range of 10−3 to 10−5 cm/s.

Fabrication of an Organofunctionalized Talc-like Magnesium Phyllosilicate for the Electrochemical Sensing of Lead Ions in Water Samples.

A talc-like magnesium phyllosilicate functionalized with amine groups (TalcNH2), useful as sensor material in voltammetry stripping analysis, was synthesized by a sol–gel-based processing method. The characterizations of the resulting synthetic organoclay by scanning electron microscopy (SEM), X-ray diffraction, N2 sorption isotherms (BET method), Fourier transform infrared spectroscopy (FTIR), CHN elemental analysis and UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS) demonstrated the effectiveness of the process used for grafting of amine functionality in the interlamellar clay. The results indicate the presence of organic moieties covalently bonded to the inorganic lattice of talc-like magnesium phyllosilicate silicon sheet, with interlayer distances of 1568.4 pm. In an effort to use a talc-like material as an electrode material without the addition of a dispersing agent and/or molecular glue, the TalcNH2 material was successfully dispersed in distilled water in contrast to natural talc. Then, it was used to modify a glassy carbon electrode (GCE) by drop coating. The characterization of the resulting modified electrode by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed its charge selectivity ability. In addition, EIS results showed low charge transfer resistance (0.32 Ω) during the electro-oxidation of [Fe(CN)6]3−. Kinetics studies were also performed by EIS, which revealed that the standard heterogeneous electron transfer rate constant was (0.019 ± 0.001) cm.s−1, indicating a fast direct electron transfer rate of [Fe(CN)6]3− to the electrode. Using anodic adsorptive stripping differential pulse voltammetry (DPV), fast and highly sensitive determination of Pb(II) ions was achieved. The peak current of Pb2+ ions on TalcNH2/GCE was about three-fold more important than that obtained on bare GCE. The calculated detection and quantification limits were respectively 7.45 × 10−8 M (S/N = 3) and 24.84 × 10−8 M (S/N 10), for the determination of Pb2+ under optimized conditions. The method was successfully used to tap water with satisfactory results. The results highlight the efficient chelation of Pb2+ ions by the grafted NH2 groups and the potential of talc-like amino-functionalized magnesium phyllosilicate for application in electrochemical sensors.

Directly Imaging Dynamic Electronic Coupling during Electrochemical Oxidation of Single Silver Nanoparticles

The electronic coupling between a metal electrode and single nano-entities is of unfading significance which impacts the heterogeneous electron transfer. Herein, we demonstrated a simple optical technique for directly imaging the transient interfacial electronic coupling events during electrochemical oxidation of single Ag nanoparticles on Au electrode. The electronic coupling brings out a dramatic dip behavior of bright field imaging traces, and is conductive to cross the energy barrier of oxidation for single silver nanoparticles. This dip behavior is further verified by in situ vis-transmission spectroscopy, and the heterogeneity of the Au-Ag electronic coupling down to single-nanoparticle level is uncovered by unifying the morphology and size of individual silver nanoparticles. These results suggest the interfacial electronic coupling facilitates electron transfer of single nanoparticles, and provide important insight into understanding detailed mechanism of nanoelectrochemistry.

Directly Imaging Dynamic Electronic Coupling during Electrochemical Oxidation of Single Silver Nanoparticles.

The electronic coupling between a metal electrode and single nano-entities is of unfading significance which impacts the heterogeneous electron transfer. Herein, we demonstrated a simple optical technique for directly imaging the transient interfacial electronic coupling events during electrochemical oxidation of single Ag nanoparticles on Au electrode. The electronic coupling brings out a dramatic dip behavior of bright field imaging traces, and is conductive to cross the energy barrier of oxidation for single silver nanoparticles. This dip behavior is further verified by in situ vis-transmission spectroscopy, and the heterogeneity of the Au-Ag electronic coupling down to single-nanoparticle level is uncovered by unifying the morphology and size of individual silver nanoparticles. These results suggest the interfacial electronic coupling facilitates electron transfer of single nanoparticles, and provide important insight into understanding detailed mechanism of nanoelectrochemistry.

Janus 2D-carbon nanocomposite-based ascorbic acid sensing device: Experimental and theoretical approaches

Precisely functionalized and structurally well-ordered carbon-based nanocomposites are in high demand for developing economical molecular-recognition-based devices with solid efficacy. In this context, a flexible point-of-care sensor is developed for the highly specific detection of ascorbic acid (AA) down to picomolar level using a Janus carbon nanocomposite (JCC) consisting of asymmetric molecular engineered porous/highly ordered carbon nanomaterial. JCC is constructed via a new strategy of simultaneous covalent and non-covalent molecular grafting techniques by exploiting the guest-host molecule's interface chemistry. As the guest molecules are precisely linked with the host at the basal plane, it exhibits better charge mobility, heterogeneous electron transfer, energy density, and stability. The alterable interlayer distance of this unique JCC is also advantageous for biomolecule binding and dissemination. An integrated flexible device that exhibits a specific and selective response to AA down to 47 pM in the buffer and similar excellent performance in serum/artificial sweat-urine samples, over a broad linear range of 10˗12 M to 10˗6 M. The integrated sensor has high selectivity, durability (88%/60 days), stability (95.3%/10 cycles) and reproducibility (RSD 1.7%/5 devices). The rationale behind this simple design technique and the science underpinning sensor operation are being systematically explored to the state of the art, which will aid in the development of high-performance Janus 2D-nanocomposite-based sensors.

Synthesis and Application of Heteroatom-Doped Graphite as the Electrode Material: Influences of Defect Density and Heteroatom on Heterogeneous Electron Transfer Rate

Thermal method was applied for the synthesis of O-doped and N-doped graphite materials using melamine and glucose as the sources of O and N, respectively. Graphite paste electrodes were prepared with the synthesized O-doped and N-doped graphite materials. The route of synthesis was explored by cyclic voltammetry using the fabricated paste electrodes and [Fe(CN)6]3–/4– as a simple outer-sphere redox probe. The results of structural and electrochemical investigations clearly showed that doping of O and N as heteroatom in the synthesized materials not only increased the defect density, the ratio of edge to basal plane sites, of graphite materials but also enhanced the rate of heterogeneous electron transfer at their corresponding paste electrodes. The heterogeneous electron transfer rate constant (k0) for [Fe(CN)6]3–/4– at the fabricated O-doped and N-doped graphite paste electrodes were about 51 and 46 times that observed for graphite paste electrode, respectively. The values of electroactive surface area calculated for both O-doped and N-doped graphite materials were about 5 times that obtained for graphite paste electrode. Thereafter, electrochemical behaviors of isoniazid, ascorbic acid, dopamine, and uric acid as the models of biologically relevant molecules were investigated at the prepared heteroatom-doped graphite paste electrodes, and the obtained results were compared with those obtained using paste electrodes prepared by pristine graphite, pristine multiwalled carbon nanotubes, and oxidized multiwalled carbon nanotubes and also by glassy carbon electrode.

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

A nanoporous diamond particle microelectrode and its surface modification

Boron doped diamond electrode (BDDE) with dimensions of tens of or hundreds of μm has received widespread attention for electrochemical and biological applications because of exceptional features such as wide potential window for water stability, excellent electrochemical/chemical stability, and superior antifouling properties. This work reported a feasible way to fabricate a single nanoporous BDD particle microelectrode (sDPME) using a commercial single-crystal BDD particle (cBDD) as the substrate. The nanoporous surface of cBDD (pcBDD) was formed via thermal catalytic treatment with nickel as the catalyst, followed by further electrodeposition of gold nanoparticles (Au NPs) to fill the nanopores, increasing its electrocatalytic performance. As a comparison, the heavily-doped polycrystalline BDD film was deposited on cBDD via hot-filament chemical vapor deposition, increasing its electrical surface conductivity, followed by the similar thermal etching to form the nanoporous surface (prBDD) as well as Au NPs modification. We gradually characterized surface modification of cBDD using scanning electron microscopy, Raman, energy dispersive spectroscopy and various electrochemical techniques. The results demonstrated that among six sDPMEs, the Au NPs-modified nanoporous sDPMEs (Au/prBDD) have one order of magnitude higher heterogeneous electron transfer kinetic constant than the cBDD. This difference was attributed to synergistic effects of the improved electrical conductivity (with a higher doping level of few 1020 cm−3 versus 1018 cm−3), the improved capability for electron transfer (with lower charge transfer resistance of 1.3 kΩ versus 8.6 kΩ) as well as the increased electrochemically active surface area (1.12 mm2 versus 0.34 mm2).

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.

Determination of rSpike Protein by Specific Antibodies with Screen-Printed Carbon Electrode Modified by Electrodeposited Gold Nanostructures.

In this research, we assessed the applicability of electrochemical sensing techniques for detecting specific antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike proteins in the blood serum of patient samples following coronavirus disease 2019 (COVID-19). Herein, screen-printed carbon electrodes (SPCE) with electrodeposited gold nanostructures (AuNS) were modified with L-Cysteine for further covalent immobilization of recombinant SARS-CoV-2 spike proteins (rSpike). The affinity interactions of the rSpike protein with specific antibodies against this protein (anti-rSpike) were assessed using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. It was revealed that the SPCE electroactive surface area increased from 1.49 ± 0.02 cm2 to 1.82 ± 0.01 cm2 when AuNS were electrodeposited, and the value of the heterogeneous electron transfer rate constant (k0) changed from 6.30 × 10−5 to 14.56 × 10−5. The performance of the developed electrochemical immunosensor was evaluated by calculating the limit of detection and limit of quantification, giving values of 0.27 nM and 0.81 nM for CV and 0.14 nM and 0.42 nM for DPV. Furthermore, a specificity test was performed with a solution of antibodies against bovine serum albumin as the control aliquot, which was used to assess nonspecific binding, and this evaluation revealed that the developed rSpike-based sensor exhibits low nonspecific binding towards anti-rSpike antibodies.

Mechanistic Approach to Reveal Interaction of Uranyl Ions in Alkyltriphenylphosphonium Bromide-Based Deep Eutectic Solvent.

Speciation is known to control fundamental aspects of metal processing and electrochemical behavior such as solubility and redox potentials. Deep eutectic solvents (DESs) are an emerging class of green, low-cost and designer solvents and are being explored as alternatives for recycling nuclear fuel and critical materials. However, there is a lack of knowledge about the behavior of metals in them. Here, for the first time, we synthesized three new DESs based on alkyltriphenylphosphonium bromide (CnPPh3Br), with varied alkyl chain lengths (n), as the hydrogen-bond acceptor along with decanoic acid (DA) as the hydrogen-bond donor and explored the redox speciation of uranyl nitrate. The changes in the Fourier transform infrared and NMR spectra helped elucidate the formation of hydrogen bonds in DES. The absorption maxima of uranyl in DES was red-shifted by 10 nm compared to the free uranyl, with concomitant increase in intensity and luminescence lifetime, which suggested a strong interaction of uranyl nitrate with DES. Cyclic voltammetry was probed to understand the redox thermodynamics, transport properties, and heterogeneous electron transfer kinetics of the irreversible electron transfer of uranyl ions in the three DESs. Electrochemical and spectroscopic techniques together with density functional theory calculations unlocked microscopic insights into the solvation and speciation of UO22+ ions in three DESs and also the associated unusual trends observed in the physical properties of the DESs. The hydrogen-bonded structure of DES plays a crucial role in the redox behavior of the UO22+ ion due to its strong potent complexation with its components. The basic findings of the present work can have far-reaching consequences for the extraction, electrochemical separation, and future development of redox-based separation processes in the nuclear fuel cycle.

Theoretical-experimental synergy towards better understanding of interfacial electron transfer kinetics

Important questions at the molecular level of electrode kinetics remain open or controversial. This situation reflects the complexity of heterogeneous electron transfer events, as affected by classical and quantum factors of different nature that interplay along a region of highly spatially-dependent properties. Consequently, cumbersome, multi-parametric kinetic models arise, which are not always easily or accurately considered in experimental works, missing the chance of getting molecular insights and simultaneously assess the model assumptions. This calls for a joint theoretic-experimental effort for accessible theoretical tools, experimental protocols and data analysis procedures. Some recent advances and future perspectives in these aspects are here overviewed.

Deconvoluting Charge Transfer Mechanisms in Conducting Redox Polymer-Based Photobioelectrocatalytic Systems

Poor electrochemical communication between biocatalysts and electrodes is a ubiquitous limitation to bioelectrocatalysis efficiency. An extensive library of polymers has been developed to modify biocatalyst-electrode interfaces to alleviate this limitation. As such, conducting redox polymers (CRPs) are a versatile tool with high structural and functional tunability. While charge transport in CRPs is well characterized, the understanding of charge transport mechanisms facilitated by CRPs within decisively complex photobioelectrocatalytic systems remains very limited. This study is a comprehensive analysis that dissects the complex kinetics of photobioelectrodes into fundamental blocks based on rational assumptions, providing a mechanistic overview of charge transfer during photobioelectrocatalysis. We quantitatively compare two biohybrids of metal-free unbranched CRP (polydihydroxy aniline) and photobiocatalyst (intact chloroplasts), formed utilizing two deposition strategies (“mixed” and “layered” depositions). The superior photobioelectrocatalytic performance of the “layered” biohybrid compared to the “mixed” counterpart is justified in terms of rate (D app), thermodynamic and kinetic barriers (H≠, E a), frequency of molecular collisions (D 0) during electron transport across depositions, and rate and resistance to heterogeneous electron transfer (k 0, R CT). Our results indicate that the primary electron transfer mechanism across the biohybrids, constituting the unbranched CRP, is thermally activated intra- and inter-molecular electron hopping, as opposed to a non-thermally activated polaron transfer model typical for branched CRP- or conducting polymer (CP)-containing biohybrids in literature. This work underscores the significance of subtle interplay between CRP structure and deposition strategy in tuning the polymer-catalyst interfaces, and the branched/unbranched structural classification of CRPs in the bioelectrocatalysis context.

Inconsistency of BDD reactivity assessed by ferri/ferro-cyanide redox system and electrocatalytic degradation capability

Ferri/ferro-cyanide ([Fe(CN)6]3−/4−) redox couple has been extensively used as a benchmark to assess electrocatalytic degradation capability of boron-doped diamond (BDD) electrodes. However, the [Fe(CN)6]3−/4− is far more sensitive to the surface terminal groups of BDD surface than the other factors (e.g. surface morphology and electrode configuration) that are closely related to electrocatalytic degradation properties. Thus, inconsistency exists while correlating the degradation properties of BDD with electrochemical properties determined from ferri/ferro-cyanide redox couple. Herein, an exemplar pollutant, reactive blue 19 (RB-19), was electrochemically degraded using various terminated BDD electrodes, including hydrogen-terminated (H-BDD), oxygen-terminated (O-BDD) and porous oxygen-terminated BDDs (OE-BDD), obtained via cathodic and anodic polarization as well as oxygen plasma etching, respectively. Surprisingly, OE-BDD with the lower heterogeneous electron transfer rate constant for [Fe(CN)6]3−/4− showed a better electrocatalytic degradation capability toward RB-19, indicating the inconsistency for qualitatively evaluating degradation properties according to the kinetic parameters extracted from [Fe(CN)6]3−/4− redox system.

Graphene electrochemistry: ‘Adiabaticity’ of electron transfer

Graphene materials currently open new perspectives to electrochemical systems, providing high electron transfer rates, electrochemical stability in a wide potential range, and ability to control these properties, for instance, by introducing impurity atoms. One of the most important electrochemical characteristics of the graphene is the heterogeneous electron transfer rate (or electrochemical activity). However, contradictory results have been presented on the electrochemical activity of graphene, and how structural defects, the number of graphene layers, and the graphene substrate alter its activity. We found here that the question of an electron transfer regime on graphene, which is disputable and rarely considered, is critically important in understanding its electrochemical activity. We provide an evidence of different electron transfer regimes for O2/O2− and ferrocenium/ferrocene redox on the graphene electrode, that result in different structure-activity relationships. Our results contribute to further understanding of the graphene electrochemistry.

(Invited, Digital Presentation) Tafel Analysis Algorithm: Objective Identification of the Linear Region

Electrocatalysts for hydrogen evolution and oxidation (HER and HOR) reactions, oxygen reduction and evolution (ORR and OER) reactions, and carbon dioxide reduction reactions (CO2RR) are evaluated by Tafel analysis. The Tafel equation specifies the log-linear relationship between current and overpotential \U0001d6c8. Heterogenous electron transfer parameters of exchange current density j o and transfer coefficient \U0001d6c2 are found. Standard heterogenous electron transfer rate k 0 can be found from j o.Conventionally, Tafel analysis is an extension of the Butler-Volmer equation applied at high overpotentials but where mass transport is not significant and the reverse reaction rate is negligible. Applicable at high \U0001d6c8 when electron transfer rates are slow, kinetic parameters are extracted by linear regression. The conventional method is, however, subject to inaccuracies because the linear region is often determined subjectively, without attention to the constraints on overpotential range, no mass transport limitations, and low j o.An algorithm is developed to automate Tafel analysis with the objective to increase measurement accuracy and decrease subjective identification of the linear region. From linear sweep voltammograms (LSVs), j o and α are determined from Tafel slopes in the best fit, linear range.Comparisons of kinetic parameters between conventional and algorithmic Tafel analyses are made for the hydrogen evolution reaction (HER, 2H+ + 2e- ⇌ H2) on various unmodified electrodes and electrodes modified with Nafion® composites. The algorithmic Tafel analysis parameters correlate well with conventional Tafel analyses that respect constraints on mass transport, \U0001d6c8, and j o. Similar agreement is observed between literature and algorithmically fitted kinetic parameters for different electrochemical systems. The algorithm allows for straightforward, rapid Tafel analysis for improved measurement of rate parameters that is independent of user bias in selection of the linear region. Acknowledgments This work was supported by the Army Research Office.

Electrochemical Impedance Spectroscopic Investigations of Nanocomposite Thin-Film Formation at an Electrified Micro Liquid-Liquid Interface

Herein, the interface between two immiscible electrolyte solutions (ITIES) has been exploited as a pristine platform for electropolymerization of the 2D and 3D nanocomposite materials, i.e., metal nanoparticles (NPs) embedded within a conductive polymer matrix that are electrogenerated/electropolymerized simultaneously The free-standing conductive thin film can be prepared with an adjustable loading of metallic NPs, to be tailored for a variety of applications, including as catalytic electrodes in fuel cells, or act as a bioreceptor in a biosensor. Electrochemical impedance spectroscopy (EIS) has been used to investigate mechanistic aspects of film growth through two types of charge transfer processes, ion and electron transfer, that occur during electropolymerization. The thermodynamics of a heterogeneous electron transfer (HET) between redox centers, KAuCl4(aq) and terthiophene (TT, org), across the interface governs the electropolymerization reaction, including the rate, which is itself affected by the rate of diffusion of reactants toward the interface, ohmic resistance of the two electrolyte solutions, distance between reactants due to arrangement of ions at the interface, and reorganization energy of the involved species in the reaction. Electropolymerization is controlled through application of an applied external potential difference and monitored voltammetrically as well as by EIS. Curve fitting of EIS data was performed using an equivalent circuit incorporating a phase constant element (CPE) parallel to a resistor for non-ideal interfacial capacitance, and HET resistance across the film as it forms at the ITIES. Next, those elements are parallel to a resistor (R), and a Warburg element (W) to approximate ion transfer across the ITIES/porous thin-film as it grows (Fig. 1). Data suggest that the films electronic conductivity improves with increasing the [TT]:[KAuCl4] ratio. Increased capacitance was hypothesized to be linked to an increase in the effective surface area of the polymer film during its growth phase versus the molecularly smooth ITIES. Higher [TT]:[KAuCl4] ratios elicited higher capacitance as well as diffusion resistance which can be attributed to formation of a more compact film structure. Cyclic voltammetric and potential step methods are compared and discussed, highlighting the formed films final properties, including film thickness and conductivity. Figure 1

Voltammetric Analysis of the Ferrocenium/Ferrocene Redox Couple in Litfsi-Acetonitrile Highly Concentrated Electrolyte

Research on electrolytes have a defining role for the progression of current energy storage device and electrocatalysis. For instance, the electrolyte is often a limiting factor in the life span of Li-ions batteries due to slow decomposition at electrodes. Intensive research is therefore ongoing to increase the stability of the electrolyte. One example of such research is on highly concentrated electrolyte (HCE) which are obtained when the number of ions of a salt nears or equals that of the solvent molecules, while maintaining a liquid state. These highly concentrated electrolytes (HCE) or "solvent-in-salt" solutions differ in properties from conventional ones because all solvent molecules are occupying the ion solvation sphere where they are strongly coordinated to the cation. While on the application side, the attention is directed towards their use in batteries or supercapacitors, current research on HCE aims at increasing the fundamental understanding of their properties mostly the transport properties of the Li+ or the formation of the solid electrolyte interface (SEI). Interestingly, there are no reports on the study of heterogeneous electron transfer (ET) in HCE, yet the absence of "free" solvent molecule suggests that ET in HCE might be different from conventional electrolytes because of the importance of solvent reorganization during ET. We therefore investigate the mass transport and ET of dissolved ferrocene (Fc) and with ferrocene terminated self-assemble monolayer, (Fc-SAM). Ferrocene was selected as the redox center for its "ideal" outer shell electron transfer mechanism and well-defined electrochemical properties. To establish the effect of salt concentration, the electrochemical measurements were done in a series of electrolytes composed of Lithium bis(trifluoromethanesulfonyl)imide in acetonitrile with concentrations spanning from diluted (0.3 M) to highly concentrated (4.1 M) solutions. The diffusivity of Fc (Shoup-Szabo) is found to follow the trend with viscosity (η) expected from the Stokes-Einstein relation over the entire concentration range. The ET rate constant k0 variation with η on the other hand diverges from what is expected from a purely adiabatic ET. Possible explanations for deviation involve different dielectric properties, strong coordination of the solvent and particular double-layer structure of HCE, properties that are not fully established. This lack of knowledge highlights the importance of increasing fundamental research on the topic of electrochemistry in HCE.

Electrochemical Characterization and Application of Graphene Oxide Materials Obtained By Electrochemical Exfoliation of Graphite

Graphene oxide (GO) materials possess oxygen functional groups located at the edges and at the surface of the graphitic layers, confering them dispersibility in water, biocompatibility, and high affinity for specific molecules. These properties are highly appealing for their use in electrochemical sensors. However, the density and type of functional groups and defects in these materials also influence the heterogeneous electron transfer rate of redox processes occurring at the GO-based electrodes 1,2. In this respect, it is necessary to investigate the physicochemical and electrochemical properties of GO materials to choose the most suitable one for a target application.Here, we report a simple and facile platform for the electrochemical characterization of GO materials 3. The GO sheets are self-assembled on a glassy carbon electrode (GCE) through an aminophenyl-film linker (AP) through electrostatic interaction and pi-pi stacking, Figure 1a. Then, the modified electrodes are characterized by cyclic voltammetry with 1 mM [Fe(CN)6]3-/4- redox couple to determine the electrochemical surface area (ESA) through the Randles-Sevcik equation and to calculate the standard rate constant of electron transfer (k0) by the Nicolson method.In this work, series of GO materials were obtained by electrochemical exfoliation of graphite in 0.1 M H2SO4. The electrochemical exfoliation of graphite in aqueous solution is an easy and “green” method that allows the synthesis of large amounts (in the order of grams) of materials (EGO: electrochemically exfoliated graphene oxide) with tunable composition in a short time (few hours). The applied voltage and the distance between the graphite and the counter electrodes were varied, Figure 1b, to obtain EGOs with different physicochemical properties such as the number of layers, structural defects, type and content of oxygenated groups. Transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy analysis were used to investigate the physicochemical properties of the EGOs. As shown in Figure 1c, the measured ESA and k0 scale with each other and are sensitive to the physicochemical properties of EGOs. This confirms the suitability of the proposed platform to characterize the EGO materials 3.Finally, selected EGO materials were used to fabricate electrochemical aptasensors for the detection of cocaine. The influence of the EGO physicochemical properties on the performance of the aptasensors will be presented and discussed.References(1) Kampouris, D. K.; Banks, C. E. Chemical Communications 2010, 46, 8986-8988.(2) Ambrosi, A.; Bonanni, A.; Sofer, Z.; Cross, J. S.; Pumera, M. Chemistry – A European Journal 2011, 17, 10763-10770.(3) Lei, Y.; Ossonon, B. D.; Chen, J.; Perreault, J.; Tavares, A. C. Journal of Electroanalytical Chemistry 2021, 887, 115084. Figure 1

(Keynote, Digital Presentation) An Electrochemical Potential Perspective on Exchange Current Density and Work Function for Hydrogen Evolution Reaction (HER)

In 1972, Trasatti compiled the exchange current densities j0 and work functions \U0001d6bd for the hydrogen evolution reaction (HER) on 31 polycrystalline metals at pH 0. Exchange current density measures the HER rate and the work function measures the energy required to remove an electron from the surface of the metal to a point outside the metal. Trasatti showed a plot of log j0 vs \U0001d6bd linear for the so called d metals and for the sp metals. The slopes are statistically the same for the d and sp metals (6.44 and 6.6 excludes Hg) but the intercepts differ (-35.4 and -38).Here, a thermodynamic specification for the slope of log j0 versus \U0001d6bd is suggested as \U0001d6c2F/RT, where \U0001d6c2 is the transfer coefficient for heterogeneous electron transfer.Electrochemical potentials for species i in phase j have been used to derive the rates of heterogeneous electron transfer within a transition state context for Butler Volmer kinetics (Bard and Faulkner).The standard chemical potential, activity, and ion charge for species i are \U0001d741i 0,j, ai j, and zi is the electrical potential is \U0001d753j.Extrapolation of the electrochemical potential to include \U0001d6bd and derive a rate expression for j0 yield: The electrode potential and the work function group into a common term. The slope of log j0 versus \U0001d6bd as \U0001d6c2F/RT. An explanation of how Pt with the highest energetic cost to remove an electron \U0001d6bd yields the highest log j0 is presented. At 25 oC, F/RT = (0.05916 V)-1. Within the electrochemical potential model for log j0 versus \U0001d6bd, the slopes of 6.44 and 6.6 for the d and sp metals at room temperature yield \U0001d6c2 of 0.381 and 0.39. Across 30 metals, \U0001d6c2 is estimated the same. The value of 0.4 is common in measurements of \U0001d6c2 found for the potential dependent term in Butler Volmer kinetics. References Trasatti, S. Work Function, Electronegativity, and Electrochemical Behavior of Metals. III. Electrolytic Hydrogen Evolution in Acid Solution. Electroanalytical Chemistry and Interfacial Electrochemistry 39, 163-184 (1972).A.J. Bard and L. R. Faulkner, Electrochemical Methods, 1980, wiley and Sons, Chapter 2. Acknowledgments This work was supported by the National Science Foundation and the Army Research Office. The University of Iowa Obermann Center for Advanced Studies is acknowledged.