Adsorption and bioactivity of protein A on silicon surfaces studied by AFMand XPS

The adsorption of protein A on silicon surfaces was studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. The deposition was made statically from various concentrations of protein A in water solution. The biological activity was checked by the immobilization of rabbit immuno globulin G. The protein adsorption occurs in least two different phases and leads to a multilayer film. The first monolayer of proteins is rapidly ads orbed on the surface. The adsorption of the second layer of proteins occurs much more slowly (a thousand times slower) and also involves the third mon olayer. The protein A of the first monolayer is denaturated and biologicall y inactive. On the contrary, the proteins of the second monolayer keep thei r natural diameter and remain biologically active. AFM artifacts such as th e convolution with small objects and the resulting estimation of the covera ge ratio are discussed. (C) 2001 Academic Press.