Current function for irreversible electron transfer processes in linear sweep voltammetry for the reactions obeying Marcus kinetics

More Accurate Measurement of Return Peak Current in Cyclic Voltammetry Using Diffusional Baseline Fitting

The difficulty associated with accurately measuring the height of the back peak (Ipb) in cyclic voltammetry (CV) has long plagued electrochemists. Most commonly, Ipb is measured by extrapolating a linear fit from a selected region of a voltammogram after the switching potential (Eλ), but without substantial separation between the peak potential (Ep) and Eλ, this approach always overestimates the background current and so underestimates Ipb. Moreover, experimental conditions can present challenges for this method as an appropriate region for linear fitting is often lacking due to neighboring peaks or solvent electrolysis current. Here, we present a new method for finding the baseline current for the back peak in CV experiments. By examining the CV data as a function of time rather than potential, it is possible to fit a generalized Cottrell or Shoup-Szabo equation to the current decay of the forward peak and extrapolate this function as a baseline for the return peak. This approach was tested by using simulated and experimental data in a variety of conditions, including data demonstrating linear and radial diffusional control. We found that the method allows for more accurate determination of back peak currents, especially when linear fits are complicated by narrow electrochemical windows or radial diffusion. A user-friendly Python program was written to automatically find an appropriate fitting range for this analysis and measure peak currents. We have made this program available to the electrochemical community at large.

Cyclic and Linear Sweep Voltammetric Studies of a Modified Carbon Paste Electrode with Nickel Oxide Nanoparticles toward Tamoxifen: Effects of Surface Modification on Electrode Response Kinetics

Due to the serious adverse futures of some anticancer drugs, the determination of trace amounts of these drugs by simple analytical techniques is of great interest. In this regard, knowing about the mechanism of the analyte with the sensing material plays an important role. Nickel oxide nanoparticles (NiO NPs) modified by a carbon paste electrode (NiO-CPE) showed an irreversible cyclic voltammetric (CV) behavior in the NaOH (pH 13) supporting electrolyte based on the peak separation of 311 mV. Its peak current was decreased by adding tamoxifen (TAM), confirming that TAM molecules can consume NiO before participating in the electrode reaction. For this goal, TAM can be oxidized or reduced, and the corresponding mechanisms are schematically illustrated in the text. This study focused on the kinetic aspects of the process. Based on the CV results, a surface coverage (Γ) value of 2.72 × 10^-5 mol NiO per cm² was obtained with charge transfer coefficients α_a and α_c of 0.317 and 0.563, respectively. α_a and α_c values were changed to 0.08 and 0.72 in the presence of TAM. Further, the rate constant (k _s) value was 0.021 ± 0.01 s^-1 in the presence of TAM. In linear sweep voltammetry (LSV), an α value of about 0.636 ± 0.023 and an exchange rate constant (k _o) value of about 0.097 ± 0.031 s^-1 were obtained in the absence of TAM, which changed to 0.62 ± 0.081 and 0.089 ± 0.021 s^-1 in the presence of TAM, respectively. Despite more published papers, when the TAM analyte was added to the NaOH supporting electrolyte, both anodic and cathodic peak currents of the modified NiO-CPE decreased. We suggested some reasons for this decreased peak current, and four mechanisms were illustrated for the electrode response in the presence of TAM.