Quasi-reversible two-electron reduction of oxygen at gold electrodes modified with a self-assembled submonolayer of cysteine

The electrochemical reduction of molecular oxygen (O-2) has been performed at gold electrodes modified with a submonolayer of a self-assembly (sub-SAM/Au) of a thiol compound (typically cysteine (CYST)) in O-2-saturated 0.5 M KOH. At bare gold electrode O-2 reduction reaction proceeds irreversibly, while this reaction is totally hindered at gold electrodes with a compact structure of CYST over its surface. The partial reductive desorption of the compact CYST monolayer was achieved by controlling the potential of the CYST/Au electrode, leading to the formation of a submonolayer coverage of the thiol compound over the Au electrode surface (subSAM/Au), at which the CYST molecules selectively block the Au(1 0 0) and Au(1 1 0) fractions (the so-called rough domains) of the polycrystalline Au while the Au(1 1 1) component (the so-called smooth domains) remains bare (i.e., uncovered with CYST). This subSAM/Au electrode extraordinarily exhibits a quasi-reversible two-electron reduction of molecular oxygen (O-2) in alkaline medium with a peak separation (DeltaE(p)) between the cathodic and anodic peak potentials (E-p(c), E-p(a)) of about 60 mV. The ratio of the anodic current to the cathodic one is close to unity. The formal potential (E-o') of this reaction is found to equal -150 mV vs. Ag/AgCl/KCl(sat.). (C) 2003 Elsevier Science B.V. All rights reserved.