Influence of the Chemical Composition on Nature and Activity of the Surface Layer of Zn-Substituted Sol-Gel (Bioactive) Glasses

Two Zn-doped sol-gel glasses with the same ZnO content (5 wt %; 4% mol) but different overall composition have been synthesized and characterized, in comparison with a bioactive Zn-free reference glass. The role of ZnO in modifying the bioactivity of sol-gel glasses was investigated by soaking the glasses in a simple tris(hydroxymethyl)amino-methane-buffered solution (TRIS-BS), so as to maximize the solubility and to minimize back-precipitation phenomena, which will depend only on the nature and concentration of dissolved glass components. Glass dissolution/ions release in TRIS-BS was monitored by ion coupled plasma emission spectroscopy, whereas modifications of surface composition upon reaction were checked by X-ray photoelectron spectroscopy (XPS). The deposition of a Ca-P layer and the consequent crystallization to hydroxy-apatite (HA) and/or hydroxy-carbonate-apatite (HCA) at the glass surface were investigated by X-ray diffraction and Raman, Fourier transform infrared (FTIR), and XPS spectroscopies. Glass dissolution rate, back-precipitation of silica gel, and formation/crystallization of an apatite-like layer on Zn-containing glasses were found to be either inhibited or delayed, according to the overall glass composition, in that the presence of the network former ZnO component enhances glass reticulation, with the consequent formation of Si-O-Zn units. The presence of a ZnO component has no effect per se, but its influence depends on the overall composition of the glass and, in particular, on the CaO/SiO2 and ZnO/CaO ratios, which determine the nature/structure of Zn and Ca surface species. Glass surface features were investigated by the combined use of in situ FTIR spectroscopy and adsorption microcalorimetry. The role played by surface Ca species, thought to be the most hydrophilic sites, was found to be a decisive factor in both glass dissolution mechanism and formation of an apatite-like surface layer: (i) the scarce dissolution in aqueous media of a (non bioactive) low-Ca and high-silica glass is due to the high reticulation caused by the scarce population of Ca2+ cations in the role of network modifiers; and (ii) the amount of the latter species is, instead, much larger in the corresponding (moderately bioactive) high-Ca and low-silica glass, which dissolves more, although exhibiting a larger durability in aqueous solution than the Zn-free glass.