Oxidation Of Ferrocenemethanol Research Articles

Toward the Detection Limit of Electrochemistry: Studying Anodic Processes with a Fluorogenic Reporting Reaction.

Recently, shot noise has been shown to be an inherent part of all charge-transfer processes, leading to a practical limit of quantification of 2100 electrons (≈0.34 fC) [ Curr. Opin. Electrochem. 2020, 22, 170-177]. Attainable limits of quantification are made much larger by greater background currents and insufficient instrumentation, which restricts progress in sensing and single-entity applications. This limitation can be overcome by converting electrochemical charges into photons, which can be detected with much greater sensitivity, even down to a single-photon level. In this work, we demonstrate the use of fluorescence, induced through a closed bipolar setup, to monitor charge-transfer processes below the detection limit of electrochemical workstations. During this process, the oxidation of ferrocenemethanol (FcMeOH) in one cell is used to concurrently drive the oxidation of Amplex Red (AR), a fluorogenic redox molecule, in another cell. The spectroelectrochemistry of AR is investigated and new insights on the commonplace practice of using deprotonated glucose to limit AR photooxidation are presented. The closed bipolar setup is used to produce fluorescence signals corresponding to the steady-state voltammetry of FcMeOH on a microelectrode. Chronopotentiometry is then used to show a linear relationship between the charge passed through FcMeOH oxidation and the integrated AR fluorescence signal. The sensitivity of the measurements obtained at different timescales varies between 2200 and 500 electrons per detected photon. The electrochemical detection limit is approached using a diluted FcMeOH solution in which no faradaic current signal is observed. Nevertheless, a fluorescence signal corresponding to FcMeOH oxidation is still seen, and the detection of charges down to 300 fC is demonstrated.

Investigating the cytotoxic redox mechanism of PFOS within Hep G2 by hyperspectral-assisted scanning electrochemical microscopy.

Perfluorooctane sulfonate (PFOS) is one of the most lethal per- and poly-fluoroalkyl substances (PFAS). Generally, exposure effects are studied through case-controlled studies, cohort studies, or cell assays. Unfortunately, most studies involving two-dimensional cell cultures require cell lysis or fixation. For in vitro studies, fluorescence microscopy has been useful, but methods to simultaneously discern phototoxic effects during an experiment are limited. Here, we use hepatocarcinoma (Hep G2) cells to examine the redox mechanism of PFOS cytotoxicity in vitro, while using hyperspectral-assisted scanning electrochemical microscopy (SECM) to differentiate between PFOS and redox mediator induced stress. Specifically, we correlate an increase in the electrochemical response of ferrocenemethanol oxidation with an increase in intracellular reactive oxygen species. Corresponding hyperspectral images of redox indicative-fluorophores implicate superoxide in the cytotoxic redox mechanism.

Revealing Dynamic Rotation of Single Graphene Nanoplatelets on Electrified Microinterfaces.

Nanoparticles interact with a variety of interfaces, from cell walls for medicinal applications to conductive interfaces for energy storage and conversion applications. Unfortunately, quantifying dynamic changes of nanoparticles near interfaces is difficult. While optical techniques exist to study nanoparticle dynamics, motions smaller than the diffraction limit are difficult to quantify. Single-entity electrochemistry has high sensitivity, but the technique suffers from ambiguity in the entity's size, morphology, and collision location. Here, we combine optical microscopy, single-entity electrochemistry, and numerical simulations to elucidate the dynamic motion of graphene nanoplatelets at a gold ultramicroelectrode (radius ∼5 μm). The approach of conductive graphene nanoplatelets, suspended in 10 μM NaOH, to an ultramicroelectrode surface was tracked optically during the continuous oxidation of ferrocenemethanol. Optical microscopy confirmed the nanoplatelet size, morphology, and collision location on the ultramicroelectrode. Nanoplatelets collided on the ultramicroelectrode at an angle, θ, enhancing the electroactive area, resulting in a sharp increase in current. After the collision, the nanoplatelets reoriented to lay flat on the electrode surface, which manifested as a return to the baseline current in the amperometric current-time response. Through correlated finite element simulations, we extracted single nanoplatelet angular velocities on the order of 0.5-2°/ms. These results are a necessary step forward in understanding nanoparticle dynamics at the nanoscale.

Determination of standard electrochemical rate constants from semi-circular sweep voltammetry: A combined theoretical and experimental study

Semi-circular potential sweep voltammetry has recently been proposed as a sensitive method to estimate electrochemical rate constants [J. Electroanal. Chem., 835 (2019) 60–66]. Herein, this approach is experimentally verified using microdisc electrode voltammetry. The reduction of hexaamineruthenium (III) chloride and the oxidation of ferrocenemethanol, both in aqueous solution, at carbon electrodes were studied. The rate constants obtained from semi-circular voltammetry for both investigated redox systems are in agreement with independent values obtained from the conventional Nicholson method at macroelectrodes. The method compares favourably in terms of simplicity and speed as it requires recording only a single voltammogram. This approach provides a novel, fast complementary method for the evaluation of the standard electrochemical rate constant.

Particle-modified electrodes: General mass transport theory, experimental validation, and the role of electrostatics

We present general mass transport theory for cyclic voltammetry at electrodes that are partially ‘blocked’ with electrochemically-inactive particles. It is broadly applicable covering reversible, quasi-reversible, and irreversible reactions and is experimentally validated using the hydrazine oxidation where theory follows experiment in quantitative detail. We extend our analysis to explore the role of electrostatics in three further reactions where the voltammetry is startlingly altered by electrostatic effects: the reduction of hexaammineruthenium (III), the oxidation of ferrocenemethanol, and the oxidation of ferrocyanide.

Photo-Scanning Electrochemical Microscopy on the Nanoscale with Through-Tip Illumination.

Scanning electrochemical microscopy (SECM) has previously been employed in probing photoelectrochemical processes at semiconductor surfaces. However, the spatial resolution of these studies has not yet matched the nanoscale SECM resolution attained without substrate illumination. Herein, we introduce nanoscale photo-SECM with a glass-sealed, polished tip simultaneously serving as a nanoelectrode and a light guide to produce a microscopic light spot on the substrate surface. The advantages of this approach are demonstrated by comparing current transients obtained using through-tip and global illumination of the sample. The spot of light on the substrate surface facing the nanotip was sufficiently bright to measure the diffusion-controlled positive feedback current in good agreement with the theory. We employed this approach for high-resolution photoelectrochemical mapping of ferrocenemethanol oxidation and oxygen evolution reactions at the Nb:TiO2 rutile (110) single crystal surface. The images obtained using 40-50 nm radius tips showed only minor and random variations in photoelectrochemical reactivity for both processes, pointing to essentially uniform distribution of the Nb dopant over the TiO2 surface and no measurable segregation on the ∼50 nm scale.

Unscrambling illusory inhibition and catalysis in nanoparticle electrochemistry: Experiment and theory

Recent theoretical work (Applied Materials Today15(2019), pp. 139-144) has demonstrated that the cyclic voltammetry of particle-modified electrodes bears a significant level of complexity: Altered mass transport inside the porous layer gives rise to effects that may falsely suggest positive or negative catalysis. This communication reports experimental work that validates this theory. Using the well-studied oxidation of ferrocenemethanol at glassy carbon, we describe experiments in the presence of non-conducting electrochemically-inert microspheres that reproduce trends in the theoretical predictions and illustrate how experimental results may mislead. In addition, we present experimental data of the reduction of oxygen at glassy carbon electrodes modified with Nafion microspheres and show that, unless compared with theoretical work, an electrocatalytic activity of Nafion may falsely be inferred.

Understanding electroanalytical measurements in authentic human saliva leading to the detection of salivary uric acid

The electroanalytical responses of several redox-active species are investigated in authentic human whole saliva. First, we show that ferrocenemethanol (FcCH2OH) and ferrocyanide ([Fe(CN)6]4−) display well-defined voltammetry in undiluted saliva at a carbon microdisc electrode without the addition of supporting electrolyte. Second, we demonstrate that dissolved oxygen is detectable in saliva. The ferrocenemethanol oxidation is shown not to be altered by the medium apart from the steady-state currents which change according to the viscosity of the solvent. In contrast, the voltammetry of ferrocyanide oxidation and oxygen reduction in authentic saliva are significantly different from those in synthetic saliva or aqueous electrolyte. It is demonstrated that the electrode is partially blocked by organic molecules or other electrochemically inert species when placed in authentic saliva samples. The distortion in voltammetry thus reflects the different surface sensitivity of the two processes. Ohmic drop is proved to be minimal at microelectrodes even without added electrolyte. Microelectrodes are thus suitable for use in electrochemical analysis of saliva samples. Finally, we demonstrate that uric acid which is a potential biomarker for gout, oral cavity cancer and other several diseases can be directly detected in authentic saliva at a carbon microelectrode with the interference of ascorbic acid and dopamine of no more than 10%.

Dual-Plate Gold-Gold Microtrench Electrodes for Generator-Collector Voltammetry without Supporting Electrolyte

A gold-gold dual-plate microtrench electrode system based on two oppositely placed gold surfaces with 5mm length, 17μm average depth, and 6μm inter-electrode gap is employed in generator-collector configuration in a four-electrode cell (counter electrode, reference electrode, and two independent working electrodes denoted “generator” – with scanning potential – and “collector” – with fixed potential). The dual-plate microtrench electrodes were investigated for (i) the reduction of Ru(NH3)63+, (ii) the oxidation of ferrocenemethanol, and (iii) the oxidation of iodide in aqueous media, all as a function of supporting electrolyte concentration. It is shown that due to the inter-electrode feedback character of the generator-collector currents, well-defined steady state sensor responses are obtained for the collector electrode even in the absence of added electrolyte. The variation in the mass transport limited steady state current (measured at the collector electrode) with addition/removal of supporting electrolyte remains low (compared to unexpectedly stronger effects caused by the switch between reduction and oxidation conditions at the collector electrode). Microtrench electrode systems are suggested for sensing applications without/with varying levels of supporting electrolyte.

Voltammetry on a Nanometer‐sized Electrode in Solution Containing Very Dilute Electroactive Species

AbstractA nanometer‐sized Pt electrode is becoming a routine tool in nano‐electrochemistry research, but, the dissolution of Pt in aqueous solution is quite common and the rate of the dissolution of Pt is related to the current density on the electrode. Measurement on a nanometer‐sized Pt electrode in solution containing very dilute electroactive species can effectively decrease the current on the electrode and thus decrease the extent of the dissolution of Pt electrode. So far, a commercial potentiostat which has a peak‐peak noise of less than 10 fA is not common. In this paper, a homemade potentiostat, which has a peak‐peak noise of less than 5 fA, has been made. By using the instrument, voltammograms with a limiting current of less than 20 fA can be obtained on a nanometer‐sized electrode in solution containing micro molar ferrocenemethanol. Our results show that the standard rate constant of the oxidation of ferrocenemethanol measured in micro molar ferrocenemethanol solution is a little greater than that measured in mili molar ferrocenemethanol solution, which indicates that kinetic parameter may be affected by the amount of electroactive molecules being measured.

Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition

High electrochemical reactivity is required for various energy and sensing applications of graphene grown by chemical vapor deposition (CVD). Herein, we report that heterogeneous electron transfer can be remarkably fast at CVD-grown graphene electrodes that are fabricated without using the conventional poly(methyl methacrylate) (PMMA) for graphene transfer from a growth substrate. We use nanogap voltammetry based on scanning electrochemical microscopy to obtain very high standard rate constants k(0) ≥25 cm s(-1) for ferrocenemethanol oxidation at polystyrene-supported graphene. The rate constants are at least 2-3 orders of magnitude higher than those at PMMA-transferred graphene, which demonstrates an anomalously weak dependence of electron-transfer rates on the potential. Slow kinetics at PMMA-transferred graphene is attributed to the presence of residual PMMA. This unprecedentedly high reactivity of PMMA-free CVD-grown graphene electrodes is fundamentally and practically important.

Electrochemical detection of hydrogen peroxide on platinum-containing tetrahedral amorphous carbon sensors and evaluation of their biofouling properties.

Hydrogen peroxide is the product of various enzymatic reactions, and is thus typically utilized as the analyte in biosensors. However, its detection with conventional materials, such as noble metals or glassy carbon, is often hindered by slow kinetics and biofouling of the electrode. In this study electrochemical properties and suitability to peroxide detection as well as ability to resist biofouling of Pt-doped ta-C samples were evaluated. Pure ta-C and pure Pt were used as references. According to the results presented here it is proposed that combining ta-C with Pt results in good electrocatalytic activity towards H2O2 oxidation with better tolerance towards aqueous environment mimicking physiological conditions compared to pure Pt. In biofouling experiments, however, both the hybrid material and Pt were almost completely blocked after immersion in protein-containing solutions and did not produce any peaks for ferrocenemethanol oxidation or reduction. On the contrary, it was still possible to obtain clear peaks for H2O2 oxidation with them after similar treatment. Moreover, quartz crystal microbalance experiment showed less protein adsorption on the hybrid sample compared to Pt which is also supported by the electrochemical biofouling experiments for H2O2 detection.

Observation of nanometer-sized electro-active defects in insulating layers by fluorescence microscopy and electrochemistry.

We report a method to study electro-active defects in passivated electrodes. This method couples fluorescence microscopy and electrochemistry to localize and size electro-active defects. The method was validated by comparison with a scanning probe technique, scanning electrochemical microscopy. We used our method for studying electro-active defects in thin TiO2 layers electrodeposited on 25 μm diameter Pt ultramicroelectrodes (UMEs). The permeability of the TiO2 layer was estimated by measuring the oxidation of ferrocenemethanol at the UME. Blocking of current ranging from 91.4 to 99.8% was achieved. Electro-active defects with an average radius ranging between 9 and 90 nm were observed in these TiO2 blocking layers. The distribution of electro-active defects over the TiO2 layer is highly inhomogeneous and the number of electro-active defect increases for lower degree of current blocking. The interest of the proposed technique is the possibility to quickly (less than 15 min) image samples as large as several hundreds of μm(2) while being able to detect electro-active defects of only a few tens of nm in radius.

Oxidation of ferrocenemethanol grafted to a hydrogel network through cysteine for triggering volume phase transition

New gels based on N-isopropylacrylamide and cysteine were prepared by free radical polymerization in aqueous solutions. To attach the amino acid to the polymer chain, its α-amino group was modified with acryloyl chloride and N,N′-bisacryloylcystine was obtained. Next, the disulfide bridges were reduced to thiol groups and the attachment of ferrocenemethanol to the chains was performed. The influence of the amount of electroactive groups and their degree of oxidation on the temperature of volume phase transition was investigated. The gel formed from the polymerization solution containing 2% N,N′-bisacryloylcystine could undergo the volume phase transition triggered by changing the ferrocene state of oxidation. The temperature window of the gel sensitivity to the oxidation state of its ferrocene component was 35–40 °C. During the transition the volume of the gel could be changed even by more than an order of magnitude. Thin layers of the electroactive and thermoresponsive gel could be anchored on the surface of a glassy carbon electrode. This was achieved by electroformation of radicals at the electrode surface. The influence of the volume phase transition triggered by a temperature change on redox processes of ferrocene moieties bound to the network was examined voltammetrically.

Attributes of Direct Current Aperiodic and Alternating Current Harmonic Components Derived From Large Amplitude Fourier Transformed Voltammetry Under Microfluidic Control in a Channel Electrode

The flow rate dependencies of the aperiodic direct current (dc) and fundamental to eighth alternating current (ac) harmonic components derived from large-amplitude Fourier transformed ac (FT-ac) voltammetry have been evaluated in a microfluidic flow cell containing a 25 μm gold microband electrode. For the oxidation of ferrocenemethanol ([FcMeOH]/[FcMeOH](+) process) in aqueous 0.1 M KNO(3) electrolyte, standard "Levich-like" dc behavior is observed for the aperiodic dc component, which enables the diffusion coefficient for FcMeOH to be obtained. In experimental studies, the first and second ac harmonic components contain contributions from the double layer capacitance current, thereby allowing details of the non-Faradaic current to be established. In contrast, the higher order harmonics and dc aperiodic component are essentially devoid of double layer capacitance contributions allowing the faradaic current dependence on flow rate to be studied. Significantly, flow rate independent data conforming to linear diffusion controlled theory are found in the sixth and higher ac harmonics at a frequency of 15 Hz and for all ac harmonics at a frequency of ≥ 90 Hz. Analysis of FT-ac voltammograms by theory based on stationary microband or planar electrode configurations confirms that stationary microband and planar electrode configurations and experimental data all converge for the higher order harmonics and establishes that the electrode kinetics are very fast (≥1 cms(-1)). The ability to locate, from a single experiment, a dc Faradaic component displaying Levich behavior, fundamental and second harmonics that contain details of the double layer capacitance, and Faradaic ac higher order harmonic currents that are devoid of capacitance, independent of the volume flow rate and also conform closely to mass transport by planar diffusion, provides enhanced flexibility in mass transport and electrode kinetic analysis and in understanding the performance of hydrodynamic electrochemical cells and reactors.

Effect of Thermal Treatment on the Electrocatalytic Activities and Surface Roughness of ITO Electrodes

ABSTRACT: The electrocatalytic activities and surface roughness of indium-tin-oxide (ITO) electrodes havebeen investigated after thermal treatment at 100, 150, or 200 o C for 30 min, 2 h, or 8 h. To checkelectrocatalytic activities, the electrochemical behavior of four electroactive species (p-hydro-quinone, Ru(NH 3 ) 63+ , ferrocenemethanol, and Fe(CN) 64– ) has been measured. The electron transferrate for p-hydroquinone oxidation and ferrocenemethanol oxidation increases with increasing theincubation temperature and the incubation period of time, but the rate for Ru(NH 3 ) 63+ is similar irre-spective of the incubation temperature and period because Ru(NH 3 ) 63+ undergoes a fast outer-spherereaction. Overall, the electrocatalytic activities of ITO electrodes increase with increasing the incu-bation temperature and period. The surface roughness of ITO electrodes increases with increasingthe incubation temperature, and the thermal treatment generates many towering pillars as high asseveral tens of nanometer. Keywords : Indium-tin-oxide electrode, Thermal treatment, Electrocatalytic activity, p-hydro-quinone, Ru(NH

On the diffusion of ferrocenemethanol in room-temperature ionic liquids: an electrochemical study

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes-Einstein equation did not apply to our data. The "correlation length" was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k(0) was at least 0.1 cm s(-1) in one of the least viscous RTILs studied.

Higher Harmonic Large-Amplitude Fourier Transformed Alternating Current Voltammetry: Analytical Attributes Derived from Studies of the Oxidation of Ferrocenemethanol and Uric Acid at a Glassy Carbon Electrode

An analytical evaluation of the higher ac harmonic components derived from large amplitude Fourier transformed voltammetry is provided for the reversible oxidation of ferrocenemethanol (FcMeOH) and oxidation of uric acid by an EEC mechanism in a pH 7.4 phosphate buffer at a glassy carbon (GC) electrode. The small background current in the analytically optimal fifth harmonic is predominantly attributed to faradaic current associated with the presence of electroactive functional groups on the GC electrode surface, rather than to capacitive current which dominates the background in the dc, and the initial three ac harmonics. The detection limits for the dc and the first to fifth harmonic ac components are 1.9, 5.89, 2.1, 2.5, 0.8, and 0.5 microM for FcMeOH, respectively, using a sine wave modulation of 100 mV at 21.46 Hz and a dc sweep rate of 111.76 mV s (-1). Analytical performance then progressively deteriorates in the sixth and higher harmonics. For the determination of uric acid, the capacitive background current was enhanced and the reproducibility lowered by the presence of surface active uric acid, but the rapid overall 2e (-) rather than 1e (-) electron transfer process gives rise to a significantly enhanced fifth harmonic faradaic current which enabled a detection limit of 0.3 microM to be achieved which is similar to that reported using chemically modified electrodes. Resolution of overlapping voltammetric signals for a mixture of uric acid and dopamine is also achieved using higher fourth or fifth harmonic components, under very low background current conditions. The use of higher fourth and fifth harmonics exhibiting highly favorable faradaic to background (noise) current ratios should therefore be considered in analytical applications under circumstances where the electron transfer rate is fast.

Electrochemical microdevice with separable electrode and antibody chips for simultaneous detection of pepsinogens 1 and 2

An electrochemical microdevice with separable electrode and antibody chips has been developed and applied to detect atrophic gastritis-related proteins, pepsinogen 1 (PG1) and pepsinogen 2 (PG2), based on sandwich-type enzyme-linked immunosorbent assays (ELISAs) with horseradish peroxidase (HRP)-labeled antibody. To fabricate the electrochemical device for simultaneous analysis of several proteins, the electrode chip with eight electrode elements was assembled along with an antibody chip with eight cavities containing immobilized anti-PG1 or anti-PG2. The immunoreactions occurring in the cavities of the device were detected simultaneously by amperometry. The labeled HRP in the cavity in the presence of hydrogen peroxide catalyzed the oxidation of ferrocenemethanol (FMA) to FMA+, which was detected electrochemically by the electrode chip. The amperometric responses of respective cavities in the device increased with increasing concentration of PG1 or PG2 of 0-50 ng/ml, ensuring the simultaneous detection of PG1 and PG2. The detection limits for both PG1 and PG2 were 0.6 ng/ml (S/N=2). The electrode chip was recovered easily by disassembling the electrochemical device; thereby, it was used repeatedly, whereas the antibody chip was discarded. No marked decrease in electrochemical responses was detected after repeated use. Reuse of the electrode chip is beneficial to reduce costs of protein analysis.

Cyclic voltammetry and scanning electrochemical microscopy of ferrocenemethanol at monolayer and bilayer-modified gold electrodes

Ferrocenemethanol was chosen to study quantitatively the kinetics of the heterogeneous electron transfer at mono- and bilayer-modified gold electrodes. The monolayer was prepared by tetradecanethiol adsorption on the gold surface and the bilayer was formed by phospholipid adsorption on the first layer. The apparent electron transfer rate constants at the modified gold electrodes were measured by cyclic voltammetry (Tafel analysis) and from scanning electrochemical microscopy approach curves, as well as by simulations of the cyclic voltammograms. The values obtained by simulation based on a CEC mechanism were close to the values obtained by both other techniques. The results show that the surface coverage was high and depended on the thickness of the layer (0.993 for the monolayer and 0.9998 for the bilayer). The major consequence of this high coverage is a large decrease in the apparent rate constant for ferrocenemethanol oxidation that depended on the thickness of the modifying layer. The results also indicate that ferrocenemethanol could permeate to a small extent into the monolayer. The electron transfer at the bilayer is mainly governed by diffusion of the mediator through pinholes with electron transfer occurring at the free sites on the electrode rather than permeation.