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 sameZnO content (5 wt %; 4% mol) but different overall composi-tion have been synthesized and characterized, in comparisonwith a bioactive Zn-free reference glass. The role of ZnO inmodifying the bioactivity of sol-gel glasses was investigated bysoaking the glasses in a simple tris(hydroxymethyl)amino-methane-buffered solution (TRIS-BS), so as to maximize thesolubility and to minimize back-precipitation phenomena, which will depend only on the nature and concentration of dissolved glasscomponents. Glass dissolution/ions release in TRIS-BS was monitored by ion coupled plasma emission spectroscopy, whereasmodifications of surface composition upon reaction were checked by X-ray photoelectron spectroscopy (XPS). The deposition of aCa-P layer and the consequent crystallization to hydroxy-apatite (HA) and/or hydroxy-carbonate-apatite (HCA) at the glasssurface were investigated by X-ray diffraction and Raman, Fourier transform infrared (FTIR), and XPS spectroscopies. Glassdissolution rate, back-precipitation of silica gel, and formation/crystallization of an apatite-like layer on Zn-containing glasses werefound to be either inhibited or delayed, according to the overall glass composition, in that the presence of the network former ZnOcomponent enhances glass reticulation, with the consequent formation of Si-O-Zn units. The presence of a ZnO component hasno effect per se, but its influence depends on the overall composition of the glass and, in particular, on the CaO/SiO2 and ZnO/CaOratios, which determine the nature/structure of Zn and Ca surface species. Glass surface features were investigated by the combineduse of in situ FTIR spectroscopy and adsorption microcalorimetry. The role played by surface Ca species, thought to be the mosthydrophilic 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 bythe scarce population of Ca2þ cations in the role of network modifiers; and (ii) the amount of the latter species is, instead, muchlarger in the corresponding (moderately bioactive) high-Ca and low-silica glass, which dissolves more, although exhibiting a largerdurability in aqueous solution than the Zn-free glass.