Characterization of the surface redox process of adsorbed cercosporin (CER) at glassy carbon electrodes by anodic stripping square-wave voltammetry

The adsorptive accumulation of cercosporin (CER) at glassy carbon electrodes is studied by square-wave voltammetry (SWV). The Freundlich adsorption isotherm resulted in being the best one to describe the specific interaction of CER with glassy carbon electrodes by using a fitting procedure of experimental fractional surface coverage vs. the CER bulk concentration (c*(CER)). SWV was also used to generate Q vs. c*(CER) and I-p,I-n. vs. c*(CER) calibration plots from pure commercial reagent solutions. Theoretical detection limits of 1.8 x 10(-7) and 9.7 x 10(-8) M were calculated from Q. vs. c*(CER) and I-p,I-n vs. c*(CER) plots, respectively. The lowest concentration value measured experimentally from calibration plots performed at a f = 40 Hz for a signal to noise ratio of 2:1 was 3.7 x 10(-8) M, being this value two orders of magnitude smaller than that obtained previously by us from the diffusion controlled CER reduction peak. I-p,I-n./f vs. f plots from SW voltammograms performed at different c*(CER) as well as different accumulation times showed the so-called "quasi-reversible maxima". A splitting of the voltammetric peak was also observed by increasing the SW amplitude at a given frequency. A value of ( - 0.260 +/- 0.011) V was determined for the formal potential of the adsorbed redox couple from the split voltammetric peak. A full characterization of the surface redox process was obtained by applying the methods of the "quasi-reversible maximum" and the "split SW peak". In 1 M HClO4 aqueous solution, the formal rate constant and the anodic transfer coefficient were (3.5 +/- 0.5) x 10(2) s(-1) and (0.50 +/- 0.03), respectively. Besides, the number of electrons exchanged during the redox reaction was calculated as n approximate to 1.