Direct voltammetry of cytochrome c at trace concentrations with nanoelectrode ensembles

Gold nanoelectrode ensembles (NEEs) are prepared by electroless plating of Au nanoelectrode elements within the pores of a microporous polycarbonate template membrane. The surfaces of these NEEs and also the inner morphology of the gold nanofibers inside the membrane pores are imaged by scanning and transmission electron microscopy. The use of NEEs in micromolar cytochrome c (cyt c) solutions reveals the possibility of observing the direct electrochemistry of the protein, without the need of any promoter or mediator. Cyt c detection limits at NEEs are 1.0 μM by cyclic voltammetry and 0.03 μM by differential pulse voltammetry. The main difference between the voltammetric signals recorded at NEEs in the absence and in the presence of the promoter 4,4′-bipyridyl is the more extended dynamic range obtained in the latter case. Quartz crystal microbalance measurements at gold-coated quartz crystals show that, in the absence of promoter, adsorption problems are responsible for the poisoning of the electrode surface. Such adsorption is, however, concentration dependent, so that in diluted solutions (cyt c concentration ⩽20 μM) it becomes negligible. Experimental evidence indicates that the capability of NEEs to detect the direct electrochemistry of cyt c even in the absence of promoters is related to their peculiar property of furnishing well-resolved voltammetric signals in diluted solutions, where this unwanted adsorption is eliminated.