Electrochemistry of ferrocene derivatives on highly oriented pyrolytic graphite (HOPG): quantification and impacts of surface adsorption

Abstract

Cyclic voltammetry of three ferrocene derivatives – (ferrocenylmethyl)trimethylammonium (FcTMA⁺), ferrocenecarboxylic acid (FcCOOH), and ferrocenemethanol (FcCH₂OH) – in aqueous solutions shows that the reduced form of the first two redox species weakly adsorbs onto freshly cleaved surfaces of highly oriented pyrolytic graphite (HOPG), with the fractional surface coverage being in excess of 10% of a monolayer at a bulk concentration level of 0.25 mM for both compounds. FcCH₂OH was found to exhibit greater and stronger adsorption (up to a monolayer) for the same bulk concentration. The adsorption of FcTMA⁺ on freshly cleaved surfaces of high quality (low step edge density) and low quality (high step edge density) HOPG is the same within experimental error, suggesting that the amount of step edges has no influence on the adsorption process. The amount of adsorption of FcTMA⁺ is the same (within error) for low quality HOPG, irrespective of whether the surface is freshly cleaved or left in air for up to 12 hours, while – with aging – high quality HOPG adsorbs notably more FcTMA⁺. The formation of an airborne contaminating film is proposed to be responsible for the enhanced entrapment of FcTMA⁺ on aged high quality HOPG surfaces, while low quality surfaces appear less prone to the accumulation of such films. The impact of the adsorption of ferrocene derivatives on graphite for voltammetric studies is discussed. Adsorption is quantified by developing a theory and methodology to process cyclic voltammetry data from peak current measurements. The accuracy and applicability, as well as limits of the approach, are demonstrated for various adsorption isotherms.