Sample preparation for scanning tunneling microscopy imaging of proteins: Characterization of gold surfaces chemically modified with a disulfide reagent

A protocol for sample preparation that may offer a good tool for the study of proteins with the scanning tunneling microscope is described. Gold surfaces have been mechanically polished and then incubated overnight with 4,4'-dipyridyl disulfide (DPDS) dissolved in ethanol. This mixed disulfide is able to produce an ordered hydrophilic monolayer above the gold surface, with the pyridine group exposed to the outer surface. Furthermore, the treatment with DPDS significantly improves the wettability of the gold substrates, as seen by taking measurements of the solid/liquid contact angles. Human liver ferritin (HLF) molecules have successively been deposited. Samples have also been characterized under ultrahigh vacuum conditions by means of Auger and photoemission spectroscopy. With regard to the untreated gold surfaces, we found the usual structures of the valence band of gold, as well as almost contaminant free outer layers. Samples treated with DPDS and later with the HLF showed their fingerprints in the ''new'' valence band together with a good stability under the Auger beam.