Optical determination of differential coverage-potential relations of redox-active species immobilized on electrode surfaces

Single-wavelength (HeNe laser, λ = 633 nm), normal incidence UV-visible reflectance spectroscopy was used to monitor the optical properties of the glassy carbon (GC)|0.2 M NaOH(aq) interface as a function of the applied potential, E. Whereas the electroreflectance coefficient for bare GC was found to be small and potential independent, surface functionalization by an irreversibly adsorbed layer of tetrasulfonated cobalt phthalocyanine (CoTSPc) yielded a clearly defined sigmoidally shaped normalized reflectance change (ΔR/R) vs E curve over the potential region in which the adsorbate displayed redox peaks. Assuming ΔR/R is proportional to the extent of redox conversion, as has been reported for macrocycles adsorbed on other types of carbon (e.g., Kim, S.; Xu, X.; Bae, I. T.; Wang, Z.; Scherson, D. A. Anal. Chem. 1990, 62, 2647-2650), differential coverage-potential relations were determined based purely on the optical data collected. A similar optical behavior was found for irreversible adsorbed CoPc and tetraamino CoPc (CoTAPc) adsorbed on GC, for which the voltammetric peaks were ill-defined, too small for coulometric analyses to be reliably performed, or both. No detectable changes in the ΔR/R vs E profiles of either bare or macrocyclic-functionalized surfaces were observed upon addition of hydrazine to the neat 0.2 M NaOH solution at potentials at which these surfaces display electrocatalytic properties for its oxidation. Possible factors responsible for this behavior are discussed.