Comparison of the self-chemisorption of azurin on gold and on functionalized oxide surfaces

The formation of extended uniform protein monolayers by single- or multiple-step self-chemisorption was investigated on gold and on functionalized oxide surfaces. These protein layers were characterized by spectroscopic ellipsometry , x-ray photoelectron spectroscopy (XPS) and scanning force microscopy (SFM). Chemisorption on oxygen-terminated surfaces (SiO2, mica) was preceded by exposure of the surface to 3-aminopropyltriethoxysilane and glutaric dialdehyde. On both substrates (gold and insulator) the protein layers appeared flat and robust under SFM imaging conditions. The thickness of the three-layer structure (4.5 nm on mica), measured by engraving the layer with the SFM tip, matched fairly well with the value expected for the supramolecular architecture. The thickness could be verified after each chemisorption step by ellipsometry and XPS. The elements expected for the azurin layer (Cu and S in particular) were confirmed by XPS at both substrates. The multiple self-chemisorption approach reported herein could be applicable to similar proteins, because it relies on the general principles of protein assembling.