# FT-IR Characterization of Antimicrobial Hybrid Materials through Sol-Gel Synthesis
## 1 Introduction  
Since the 1990s, organic/inorganic nanocomposites networks have become a growing field of investigation [1; 2; 3]. The sol-gel route represents an interesting approach, which can be effectively applied to the preparation of the inorganic phases within the inorganic/organic hybrid materials, due to the fact that it can occur in liquid solutions at room temperature. These materials are considered to be biphasic, where the organic and inorganic phases are mixed at the nanometric or submicronic scales. The smaller the scale of mixing, the more unexpected the properties are, since these materials may show properties that are not the sum of the individual contributions from both phases. Additionally, it has been proved that the role of the inner interfaces is predominant; in particular, the nature of the interface is the critical parameter used to distinguish two different classes [4]: class I collects those hybrid materials with weak bonds (hydrogen, van der Waals or ionic bonds), and class II gathers materials where the phases are linked together through strong chemical bonds (covalent or ionic-covalent bonds). In class I, materials, obtained combining the sol-gel process with polymer chemistry, can be enclosed with a large number of different applications [5; 6; 7], from non-linear optical materials [8] to mesoporous materials [9]. The basic chemical reactions of the sol-gel process are the hydrolyses of metal alkoxides in the proper solvent, usually an alcohol/water mixture, followed by their polycondensation. In the first stage, substituting the alkoxide groups (OR) with the hydroxyl groups (OH) in the metal alkoxide offers a large number of opportunities, due to the presence of such reactive OH groups, to match with organic monomers. Thus, a variety of organic polymers have been introduced into inorganic networks. The final material derives from the condensation of two OH groups with the elimination of a water molecule. The two adjacent OH groups may belong to one organic and one inorganic monomer to afford the formation of a hybrid or composite material with covalent bonds between the polymer and inorganic elements. The gel reaction might also give rise to weaker bonds, such as a hydrogen bond, in the organic/inorganic hybrid bulky materials. It is well known that many parameters, such as type and concentration of reactants, solvents, catalysts, reaction temperature and removal rate of by-products and solvents, influence the sol-gel process [10; 11]. Hence, it is evident that the presence of organic elements modifies the morphology and physical properties of the final sol-gel products. The difference in products that are obtained through the base-catalyzed sol-gel reaction and through the carefully controlled acid-catalyzed one is evident, since from the first process, translucent or opaque products with visible organic/inorganic phase separation are obtained, while from the second, transparent and monolithic materials are realized. A key topic that is persistently challenging in these organic-altered materials is the degree of mixing of the organic/inorganic elements, i.e., the phase homogeneity. The high optical transparency to visible light suggests that the organic/inorganic phase separation, if any, occurs on a scale of \(\leq\)400 nm [12]. Two of the most-used techniques to investigate phase mixing in hybrid materials, at micrometric scale, are atomic force microscopy (AFM) [13] and scanning electron microscopy (SEM). In this work, we propose a combination of bioactive organic and inorganic phases to obtain a single and homogeneous hybrid composite using the sol-gel route for the inorganic three-dimensional (3D) network. In particular, we chose SiO\({}_{2}\) and TiO\({}_{2}\)-containing bioactive materials that are often produced in the amorphous state, typical of glasses, via the sol-gel route. Both of these oxides are known to bond to living bone [14].