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Under most experimental conditions, a distinctly nonlinear background current is encountered in all forms of voltammetry which arises from the potential dependence of the capacitance. The nonlinear background current has been successfully modeled under large amplitude sinusoidal ac voltammetric conditions with a fourth order polynomial. The model was applied to a dummy cell containing a nonideal ceramic capacitor and commonly used electrodes. The nonlinearity in behavior of the background capacitance is particularly significant when considering the discrimination between the Faradaic and background contributions in the higher order harmonics resolved in ac voltammetry by Fourier transform--inverse Fourier transform approaches and in the simulation of the background current and hence double-layer capacitance as a function of potential. Typically, measurable background current under large amplitude conditions is detectable in the dc and fundamental to fourth harmonic components in large amplitude ac voltammetry. For analytical purposes, this background current can be corrected on a per harmonic basis without the need for any model. Background correction has been successfully applied to the first four harmonics for the oxidation of ferrocenemonocarboxylic acid over the concentration range of 5-500 [micro]M in aqueous 0.5 M NaCl solution. 10.1021/ac901318r

The analysis of dc cyclic voltammograms of surface-confined metalloproteins is complicated by large background currents, significant ohmic i[R.sub.u] drop, and frequency dispersion related to protein and electrode surface inhomogeneity. The use of large-amplitude Fourier transform ac voltammetry for the quantification of the electron-transfer properties of a thin film of redox-active protein azurin adsorbed onto edge-plane, basal-plane, and highly oriented pyrolytic graphite electrode surfaces has been evaluated and compared to results obtained by dc cyclic voltammetry. In principle, it has been established that fourth and higher harmonic sine-wave data are ideally suited for analysis of electron-transfer processes as they are almost completely devoid of background capacitance current contributions. However, uncompensated resistance has a higher impact on these components, as is the case with fast scan rate dc techniques, so strategies to include this term in the simulations have been investigated. Application of recommended strategies for the evaluation of the electron-transfer properties of azurin adsorbed onto three forms of graphite, each having different background or uncompensated resistance values, is described and compared to results obtained by traditionally used forms of cyclic voltammetry. The electron-transfer rate constant, [k.sup.0]', of azurin at a highly oriented pyrolytic graphite electrode surface was ~250 [s.sup.-1], compared with [greater than or equal to] 1000 [s.sup.-1] at edge-plane and basal-plane graphite electrodes. The significantly lower [k.sup.0]' value found at the highly oriented pyrolytic graphite electrode was related to the relatively low level of edge-plane defect sites present at the surface of this electrode. However, analysis of high ac harmonics suggests that frequency dispersion is substantial at all electrode surfaces. Such effects in these diffusionless situations are significantly enhanced relative to solution-phase voltammetry, where overlay of diffusion layers minimizes the impact of heterogeneity.

The technique of Fourier transform large-amplitude alternating current (ac) cyclic voltammetry has been applied to a study of the electrochemistry of surface-bound azurin (Az) at a paraffin-impregnated graphite electrode. With the methodology used, the total current, as well as the dc component, and the ac harmonics can be obtained from a single experiment. For the dc (and fundamental harmonic cases), the background current dominates the voltammetry, so that quantitatively useful data only can be obtained after employment of an empirical background correction procedure. In contrast, the capacitance current does not contribute to the second and higher harmonic voltammograms. Furthermore, the higher harmonic Faradaic currents are greatly amplified when large-amplitude sinusoidal perturbations are employed as an alternative to the traditionally used small-amplitude ac methods. Results obtained from numerical simulations are presented for a quasi-reversible process (Butler-Volmer model) in order to illustrate the advantages of the higher harmonic ac method, relative to dc cyclic voltammetry. The almost complete background current rejection and high sensitivity of large-amplitude techniques are confirmed experimentally in second- and third-harmonic ac studies with an adsorbed azurin thin film and the surface process, Az[[Cu(II)].sub.(surface)] + [e.sup.-] ?? Az[[Cu(I)].sub.(surface). However, as is the case with dc cyclic voltammetry, readily detected nonideal behavior places restrictions on the straightforward use of Butler-Volmer theory for quantitative evaluation of the kinetics and thermodynamics of this so-called model surface-confined process.

A combination of controlled annealing and characterization by scanning probe microscopy (SPM) is used to demonstrate that the refractive-index profile of a commercially available silica-based optical fiber can be accurately reconfigured for use as an evanescent field sensor. The process relies on the controlled relocation of the silica glass dopants across the fiber cross section through heat treatment and the accurate measurement of the resulting dopant redistribution with SPM and differential etching techniques. The effect of variable annealing along a length of fiber is to produce a mode transformer to couple light from a laser source into the sensing region of the fiber. OCIS codes: 060.2270, 060.2370, 180.5810, 290.1990, 290.3030.

The technique of Fourier transform large-amplitude alternating current (ac) cyclic voltammetry has been applied to a study of the electrochemistry of surface-bound azurin (Az) at a paraffin-impregnated graphite electrode. With the methodology used, the total current, as well as the dc component, and the ac harmonics can be obtained from a single experiment. For the dc (and fundamental harmonic cases), the background current dominates the voltammetry, so that quantitatively useful data only can be obtained after employment of an empirical background correction procedure. In contrast, the capacitance current does not contribute to the second and higher harmonic voltammograms. Furthermore, the higher harmonic Faradaic currents are greatly amplified when large-amplitude sinusoidal perturbations are employed as an alternative to the traditionally used small-amplitude ac methods. Results obtained from numerical simulations are presented for a quasi-reversible process (Butler-Volmer model) in order to illustrate the advantages of the higher harmonic ac method, relative to dc cyclic voltammetry. The almost complete background current rejection and high sensitivity of large-amplitude techniques are confined experimentally in second-and third-harmonic ac studies with an adsorbed azurin thin film and the surface process, Az[Cu(II)](surface) + e[SUP-] ⇄ Az[Cu(I)][SUB(surface)]. However, as is the case with dc cyclic voltammetry, readily detected nonideal behavior places restrictions on the straightforward use of Butler-Volmer theory for quantitative evaluation of the kinetics and thermodynamics of this so-called model surface-confined process. [ABSTRACT FROM AUTHOR]

Fourier-transformed alternating current voltammetry (FTacV) is a technique utilizing a combination of a periodic (frequently sinusoidal) oscillation superimposed onto a staircase or linear potential ramp. The advanced utilization of a large amplitude sine wave induces substantial nonlinear current responses. Subsequent filter processing (via Fourier-transformation, band selection, followed by inverse Fourier-transformation) generates a series of harmonics in which rapid electron transfer processes may be separated from non-Faradaic and competing electron transfer processes with slower kinetics. Thus, FTacV enables the isolation of current associated with redox processes under experimental conditions that would not generate meaningful data using direct current voltammetry (dcV). In this study, the enhanced experimental sensitivity and selectivity of FTacV versus dcV are illustrated in measurements that (i) separate the Faradaic current from background current contributions, (ii) use a low (5 μM) concentration of analyte (exemplified with ferrocene), and (iii) enable discrimination of the reversible [Ru(NH 3 ) 6 ] 3+/2+ electron-transfer process from the irreversible reduction of oxygen under a standard atmosphere, negating the requirement for inert gas conditions. The simple, homebuilt check-cell described ensures that modern instruments can be checked for their ability to perform valid FTacV experiments. Detailed analysis methods and open-source data sets that accompany this work are intended to facilitate other researchers in the integration of FTacV into their everyday electrochemical methodological toolkit., Competing Interests: The authors declare the following competing financial interest(s): Bryn Jones works for SciMed, the UK distributors of Gamry Instruments. Mark Dowsett works for Alvatek, the UK distributors of Ivium Instruments. The other co-authors declare no competing interests., (© 2024 The Authors. Published by American Chemical Society.)

The redox chemistry of the electron entry/exit site in Escherichia coli hydrogenase-1 is shown to play a vital role in tuning biocatalysis. Inspired by nature, we generate a HyaA-R193L variant to disrupt a proposed Arg-His cation-π interaction in the secondary coordination sphere of the outermost, "distal", iron-sulfur cluster. This rewires the enzyme, enhancing the relative rate of H 2 production and the thermodynamic efficiency of H 2 oxidation catalysis. On the basis of Fourier transformed alternating current voltammetry measurements, we relate these changes in catalysis to a shift in the distal [Fe 4 S 4 ] 2+/1+ redox potential, a previously experimentally inaccessible parameter. Thus, metalloenzyme chemistry is shown to be tuned by the second coordination sphere of an electron transfer site distant from the catalytic center.