Molecular-level design of binary self-assembled monolayers on polycrystalline gold electrodes

Abstract

In this study, we followed up, by measuring the reductive desorption patterns, the time-dependent growth of the self-assembled monolayer (SAM) of a thiol compound (typically cysteine) formed on polycrystalline Au (poly-Au) electrode. Cysteine molecules appeared to adsorb preferentially and consecutively at the Au(1 1 0), Au(1 0 0) and then at the Au(1 1 1) surface domains of the poly-Au. A 95% surface coverage of cysteine ( Γ cysteine) was attained after 5 s, 120 s and more than 300 s for the Au(1 1 0), Au(1 0 0) and Au(1 1 1) domains, respectively, of the poly-Au electrode. The electrochemical reduction of molecular oxygen (O 2) in O 2-saturated 0.5 M KOH was utilized as a probing reaction for the extent of the compactness of the SAM. A binary SAM of two thiols (cysteine and cystamine) has been successfully designed over the poly-Au electrode at which a precise positioning of cysteine was achieved, i.e., cysteine was attached to the Au(1 1 1) domains, while cystamine to the Au(1 0 0) and Au(1 1 0) domains. SEM images for the electrodeposited Ag over the cysteine sub-SAM/Au electrode gave a possible mapping of the Au(1 1 1) domains of the poly-Au electrode.