Electron-Transfer Rate in Potential-Modulated Redox Reactions with Electro-Active Optical Waveguides

A novel methodology has been developed to determine electron-transfer rate in electrically driven redox reactions. Based on a widely adopted electrical circuit describing faradaic processes in an electrochemical cell, the approach uses a combination of impedance data from optical and electrical measurements that are simultaneously acquired in a spectroelectrochemical experiment. Once the consistency of our methodology was experimentally corroborated, it was put to practice for investigating electron-transfer rate of cytochrome c adsorbates at very low concentrations on an indium tin oxide electrode by using a highly sensitive, single-mode, electro-active, integrated optical waveguide platform. Different surface densities of redox species on the electrode interface and different ionic strengths in the electrolyte solution were studied. Higher surface densities and higher ionic strengths are shown to slow down the electron-transfer process between the redox molecules and the working electrode.

Optical impedance spectroscopy with single-mode electro-active-integrated optical waveguides

An optical impedance spectroscopy (OIS) technique based on a single-mode electro-active-integrated optical waveguide (EA-IOW) was developed to investigate electron-transfer processes of redox adsorbates. A highly sensitive single-mode EA-IOW device was used to optically follow the time-dependent faradaic current originated from a submonolayer of cytochrome c undergoing redox exchanges driven by a harmonic modulation of the electric potential at several dc bias potentials and at several frequencies. To properly retrieve the faradaic current density from the ac-modulated optical signal, we introduce here a mathematical formalism that (i) accounts for intrinsic changes that invariably occur in the optical baseline of the EA-IOW device during potential modulation and (ii) provides accurate results for the electro-chemical parameters. We are able to optically reconstruct the faradaic current density profile against the dc bias potential in the working electrode, identify the formal potential, and determine the energy-width of the electron-transfer process. In addition, by combining the optically reconstructed faradaic signal with simple electrical measurements of impedance across the whole electrochemical cell and the capacitance of the electric double-layer, we are able to determine the time-constant connected to the redox reaction of the adsorbed protein assembly. For cytochrome c directly immobilized onto the indium tin oxide (ITO) surface, we measured a reaction rate constant of 26.5 s(-1). Finally, we calculate the charge-transfer resistance and pseudocapacitance associated with the electron-transfer process and show that the frequency dependence of the redox reaction of the protein submonolayer follows as expected the electrical equivalent of an RC-series admittance diagram. Above all, we show here that OIS with single-mode EA-IOW's provide strong analytical signals that can be readily monitored even for small surface-densities of species involved in the redox process (e.g., fmol/cm(2), 0.1% of a full protein monolayer). This experimental approach, when combined with the analytical formalism described here, brings additional sensitivity, accuracy, and simplicity to electro-chemical analysis and is expected to become a useful tool in investigations of redox processes.