# FT-IR Characterization of Antimicrobial Hybrid Materials through Sol-Gel Synthesis
## 3 Results and Discussion
In fact, taking into account Figure 4, it is observable that sharp peaks, typical of a crystalline material, can be detected on the diffractogram of pure polycaprolactone (Figure 4a), whereas the TiO2 gel exhibits broad humps characteristic of amorphous materials (Figure 4b). On the other hand, the XRD spectrum of the TiO2/PCL hybrid material gel was in line with an amorphous material, such as TiO2 gel (Figure 4c). SEM micrographs of TiO2 gel and TiO2/PCL gel samples, reported in Figure 5, evidence a more homogenous morphology for the first one, as expected. With the introduction of the organic phase (Figure 5b), the unstructured TiO2 gel presents a micrometric feature. Neither of the two morphologies, though, exhibit a regular geometric morphology indicating crystalline structures or phase separation. This result confirms the XRD evidence, i.e., the crystalline nature of the organic element was lost during the synthesis. A similar behavior was detected on XRD patterns and SEM micrographs of a SiO2 gel sample and a SiO2/PCL gel sample. Figure 4: XRD spectra of (**a**) PCL, (**b**) TiO2 gel and (**c**) TiO2/PCL gel. Figure 5: Scanning electron microscopy (SEM) micrograph of (**a**) TiO2 gel and (**b**) TiO2/PCL6 wt.% gel. The degree of mixing of the elements in the hybrid material was investigated with Atomic Force microscopy (AFM). An AFM contact mode image can be measured in the height mode or in the force mode. In force images (z range in nN), differences appear sharper and richer and the contours of the nanostructure's essentials are clearer. In contrast, height images (z range in nm) provide a more exact reproduction of the height itself [21]. In this study, the height mode was adopted to estimate the degree of homogeneity of the hybrid materials. AFM topographic images of SiO\({}_{2}\)/PCL and TiO\({}_{2}\)/PCL gel samples are shown in Figure 6, where it can be observed that the average domain size is less than 130 and 40 nm. This answer confirms that the synthesized SiO\({}_{2}\)/PCL and TiO\({}_{2}\)/PCL gels can intrinsically be organic/inorganic hybrid materials [22]. The bioactivity of the SiO\({}_{2}\)/PCL gel was ascertained by FT-IR measurements on a sample soaked in simulated body fluid for 7, 14 and 21 days as shown in Figure 7. The formation of a hydroxyapatite deposit was recognized by the appearance of the 587 and 550 cm\({}^{-1}\) bands generally assigned to P-O stretching [23]. After soaking for 7 days, the splitting of the 580 cm\({}^{-1}\) band into two others at 587 and at 550 cm\({}^{-1}\) can be ascribed to the shape of crystalline hydroxyapatite [23]. These spectra modifications could be ascribable to the formation of the hydroxyapatite precipitate and, in particular, to the stretching of the hydroxyapatite -OH groups and the vibrations of PO\({}_{4}\)\({}^{3}\)- groups, respectively [24]. Moreover, a slight upshift of the Si-OH band (from 955 cm\({}^{-1}\) to 960 cm\({}^{-1}\)) suggested the interaction of the hydroxyapatite layer with the -OH groups of the silica matrix. Finally, the band at 800 cm\({}^{-1}\) can be assigned to the Si-O-Si band vibration among two adjacent tetrahedra characteristic of silica gel [24]. The results obtained are in line with the mechanism of the formation of a hydroxy apatite deposit proposed in the literature [25]. Figure 6: AFM images showing the microstructure of (**a**) SiO\({}_{2}\)/PCL\({}_{6\text{ wt. }\%}\) gel and (**b**) TiO\({}_{2}\)/PCL\({}_{6\text{ wt. }\%}\) gel. The numbers on the axis are in \(\upmu\)m (micrometers). Figure 8a shows the SEM micrographs of a TiO\({}_{2}\)/PCL gel sample soaked in SBF for 21 days. Figure 8b confirms that the surface layer observed in the SEM micrographs is composed of calcium phosphate. Figure 7: FT-IR spectra of SiO\({}_{2}\)/PCL gel samples after exposure to simulated body fluid (SBF) for different times. (**a**) not exposed; (**b**) 7 days; (**c**) 14 days; (**d**) 21 days.