Self-assembled monolayers of cystamine and cysteamine on gold studied by XPS and voltammetry

The formation of self-assembled chemisorbed layers of cystamine, cysteamine , and 4-aminothiophenol on gold has been studied by XPS and voltammetry. Th ese compounds, often used in the preparation of biosensors and modified ele ctrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethan olic solutions. The results of the XPS experiments reveal that a shoulder o n the S 2p(3/2) peak (situated at 162.1 eV) develops at 161.3 eV upon incre asing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. T his shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption ti mes also give rise to increased amounts of oxidized carbon and sulfur in th e films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments wit h different takeoff angles. The main N 1s peak for cystamine is shifted tow ard higher binding energies for increasing adsorption times while two promi nent nitrogen peaks are generally seen for cysteamine. For cysteamine, incr easing adsorption times result in an increase of the main nitrogen componen t at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations fo r these experimental findings are discussed. Cystamine, cysteamine, and 4-a minothiophenol films on gold are shown to be irreversibly oxidized in the g old oxide formation region. On the basis of evaluation of the oxidation cha rge, surface coverages of approximately 1 x 10(-9) mol/cm(2) were obtained for adsorption times between 5 min and 1 week.