Formation of dicarboxylic acid-terminated monolayers on silicon wafer surface

Dicarboxylic acid-terminated monolayers on hydroxylated silicon wafer were prepared via the chemisorption of 3-glycidoxypropyldimethylethoxysilane (GPDMES) molecules and subsequent reaction of the epoxy groups with iminodiacetic acid (IDA). The structure and surface composition of the monolayers were characterized by the means of contact-angle measurement, ellipsometric thickness measurement, reflectance FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Moreover, we found that the dicarboxylic acid-terminated monolayers on silicon wafer exhibit well-defined contact angle titration curve from which the surface acid dissociation constants were determined. The results were compared with the pKa values reported in the literature for IDA in aqueous solution. Small difference in the surface pKa values was attributed to variations of the microenvironment of the acid moieties. These experimental findings provide fundamental knowledge at the molecular level for the preparation of bioactive surfaces of controlled reactivity on silicon substrates.