Adsorption and coadsorption of water and glycine on TiO2

Adsorption of water, ions, and biomolecules constitutes the first events oc curring at biomaterial biosystem interfaces. In this work, the adsorption a nd coadsorption of water and glycine on TiO2 were studied by thermal desorp tion spectroscopy (TDS). The first water monolayer desorbs in three peaks a round 180K, 300K, and 400K, which are assigned to water molecularly adsorbe d at oxygen sites, at Ti4+ sites, and to recombination of dissociated water , respectively. A fourth desorption peak (160K), appearing at coverages > 0 .8 monolayer, is attributed to water clusters and multilayers. The water-Ti O2 interaction is changed if the surface is annealed in vacuum, which leads to increased hydroxylation. Desorption spectra from glycine overlayers eva porated on TiO2 in situ show that around 40% of the first monolayer desorbs as intact molecules (-300-450 K) and the remainder as dissociation fragmen ts and surface reaction products around 600 K. At coverages > 0.6 monolayer s, intact molecules desorbing from cluster multilayers at 310 K are detecte d. The glycine desorption spectra are unaffected by coadsorbed water. In co ntrast, coadsorption of glycine displaces water from more strongly bound st ates in the monolayer to more weakly bound states and clusters, making the surface more hydrophobic. The study shows that TDS is a powerful method for characterizing biomaterial surfaces with regard to their interaction with biologically relevant molecules. (C) 1999 John Wiley & Sons, Inc.