# FT-IR Characterization of Antimicrobial Hybrid Materials through Sol-Gel Synthesis
## 3 Results and Discussion  
Gelation is the result of hydrolysis and condensation according to the following reactions, where M = Si and R = CH\({}_{3}\) for PCL/SiO\({}_{2}\); and M = Ti and R = C\({}_{4}\)H\({}_{9}\) for PCL/TiO\({}_{2}\). \[\text{M(OR)}_{4}+\text{nH}_{2}\text{O}\rightarrow\text{M(OR)}_{4\text{-n}}\text{ (OH)}_{\text{n}}+\text{nROH} \tag{1}\]  
\[\text{-MOH}+\text{RO-M}\rightarrow\text{-M-O-M- + ROH} \tag{2}\]  
\[\text{-M-OH}+\text{OH-M}\rightarrow\text{-M-O-M- + H}_{2}\text{O} \tag{3}\]  
(4)  
In Figure 2, FT-IR spectra of pure PCL and SiO\({}_{2}\) are reported, together with the one recorded for SiO\({}_{2}\)/PCL\({}_{6\text{ wt.\%}}\) gel in order to highlight spectral differences due to the interaction of the two phases. The PCL spectrum (Figure 2a) shows the asymmetric and symmetric stretching of polymer -CH\({}_{2}\)-groups, corresponding to the bands at 2945 and 2866 cm\({}^{-1}\), respectively, and two peaks at 1470 cm\({}^{-1}\) and 1360 cm\({}^{-1}\) related to their bending modes. The sharp and strong band at 1730 cm\({}^{-1}\) referred to the C=O stretching vibration. On the other hand, the hybrid's FT-IR spectrum (Figure 2b) showed a broad band in the range 3600-3200 cm\({}^{-1}\), the intensity of which almost completely covered the characteristic peaks of PCL methylene stretching. According to previous work, this referred to un-free O-H groups and could be explained considering the establishment of hydrogen bonds between the carbonyl groups of the polymer chains and the inorganic part [11]. The shift of the pure PCL signal at 1730 cm\({}^{-1}\) to lower wavenumbers (1720 cm\({}^{-1}\)) seemed to corroborate this hypothesis. Moreover, both in SiO\({}_{2}\)/PCL\({}_{6\text{ wt.\%}}\) and SiO\({}_{2}\) gel spectra (Figure 2b,c), the bands at 1080 and 460 cm\({}^{-1}\), corresponding to the stretching and bending modes of SiO\({}_{2}\) tetrahedra [20], were marked, as expected. This evidence indicates that the presence of the PLC in the hybrid composites does not inhibit the reticulation of the Si-O-Si 3D network. In Figure 3, the infrared spectrum of PCL, as discussed above, was compared to those recorded for the TiO2 + PCL\({}_{6\text{ wt.\%}}\) gel and the TiO2 gel. The signals detectable for the latter were the typical bands already described elsewhere [3]. In the hybrid's spectrum (Figure 3b), the broad and strong band in the region 3400-3200 cm\({}^{-1}\), due to -OH vibrations, obscured almost completely the polycaprolactone CH2 asymmetric and symmetric stretching modes (at 2945 and 2866 cm\({}^{-1}\), respectively). In addition, the band at 1715 cm\({}^{-1}\), attributable to the PCL ester bond, which shifted to low wavenumbers following the synthetic route, was not particularly evident. The scarce intensity could depend on the low amount of the polymer in the synthetized material. Furthermore, other signals typical of acetic-acid-containing titania sol-gel materials were detected. Indeed, the doublet at 1530 and 1445 cm\({}^{-1}\) likely corresponded to the asymmetric and symmetric stretching vibrations of the carboxylic group coordinated to Ti as a bidentate ligand. The band around 800 cm\({}^{-1}\) correlates with the vibrations of polyhedral TiOn with a coordination number less than 6. Figure 3: FT-IR spectra of (**a**) PCL, (**b**) TiO\({}_{2}\) + PCL\({}_{6\text{ wt. }\%}\) gel and (**c**) TiO\({}_{2}\) gel. Figure 2: FT-IR spectra of (**a**) PCL, (**b**) SiO\({}_{2}\) + PCL\({}_{6\text{ wt. }\%}\) gel and (**c**) SiO\({}_{2}\) gel. The nature and microstructure of the SiO2/PCL and TiO2/PCL hybrid materials were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In particular, based on the acquired diffractograms, hybrids exhibit amorphous features as their metal oxide precursors.