Observation of Au-S Interface of L-Cysteine on Gold Surface

We studied the metal-molecular interface of Au-S bonds at the L-cysteine modified gold surface using X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS). Two kinds of L-cysteine layers, i.e., multi-layer and two-layers, were prepared on gold substrate. The formation of unique chemical bond was found at the sulfur-gold interface. The sulfur 1s core-level energy of two-layered L-cysteine on gold is higher by 8.2 eV than that of multi-layered sample. The large chemical shift was also observed in S K-edge NEXAFS spectra. Such remarkably large chemical shift in the case of two-layers has never been reported for any sulfur compounds. The direction of the S 1s energy shift for Au-S bond is opposite to that for the normal metal sulfides. This means that the electrons are donated from the sulfur atom to the gold substrate, and most likely the sulfur atoms are positively charged at likely [delta(+)] states. We conclude that the strong Au-S bonds that are recently used for the immobilization of molecules are due to the observed specific Au-S bond.