## 3 Results and Discussion  
### FT-IR Analysis  
Recently, it has been demonstrated that in the hybrid silica/polyethylene glycol/quercetin biomaterials, obtained using the acid-catalyzed sol-gel route, the phenolic drug compound undergoes an oxidizing phenomenon, which led to the formation of a hybrid that still stores the antioxidant and antimicrobial activity [20]. Starting from this finding and the previous results based on silica and caffeic acid hybrid materials, the occurrence of chemical modification was investigated using the comparison of the FT-IR spectra of pure caffe acid (Figure 3A) and the oxidized one (Figure 3B) obtained by using the sol-gel procedure without adding TEOS and P inside the solutions, followed by the solvent removal. In the pure caffeic acid spectrum, Bands of about 3424 and 3233 cm\({}^{-1}\) are attributed to the -OH stretching vibration. Bands at 2982 and 2912 cm\({}^{-1}\) are ascribed to the C-H vibration modes. The intense band at 1645 cm\({}^{-1}\) is assigned to the stretching of carbonyl group C=O [40; 41; 42]. Moreover, the intense bands at 1625 cm\({}^{-1}\) and 1450 cm\({}^{-1}\) are attributed to olefinic C-C stretching modes. Thus, the signal at 1217 cm\({}^{-1}\) is imputed to aromatic bending C-H modes. Frequencies lower than 1120 cm\({}^{-1}\) might be due to the C-C-C bending modes of the aromatics system except for the signals at 815 and 648 cm\({}^{-1}\) that are assigned to bending modes of the carbonyl group [34; 41; 42]. As a confirmation of oxidation occurrence, there are some shifts of the main signals both to lower and higher wavenumbers, followed by the formation of new absorption bands. Indeed, the two sharp absorption peaks in the range of 3500-3200 cm\({}^{-1}\) (Figure 3A) changed into a main broad peak at 3431 cm\({}^{-1}\), with a shoulder located at 3250 cm\({}^{-1}\) (Figure 3B), probably due to the increase in -OH and  
Figure 2: Calibration curve of Caffeic acid. H-bonds [43] in the oxidized structure. Moreover, because of the oxidization, the signals at 2982 and 2912 cm\({}^{-1}\) (Figure 3A), attributed to -CH\({}_{3}\) sp3 stretching, shifted at 2987 and 2908 cm\({}^{-1}\). Furthermore, there was also a shift of the C=O vibration from 1645 cm\({}^{-1}\) to 1745 cm\({}^{-1}\). This shift to higher wavenumbers could be explained by the stopping of \(\pi\) electron delocalization of the C=O group [40; 41], because of the oxidization phenomenon. This latter band within the bands at 860 cm\({}^{-1}\), attributed to C-H alkene bending, and the band at 1635 cm\({}^{-1}\), attributed to C=C alkene stretching, may suggest that the oxidized caffe acid still contains the active part of its molecule. To better understand the chemical influence of the inorganic and organic components of the synthesized hybrid materials, in Figure 4A, the spectra of the silica and the caffe acid are compared to the SP12 Ay systems, whereas in Figure 4B, the spectra of the silica and polyethylene glycol are compared to SPxA20 systems. SiO\({}_{2}\) showed the -OH stretching and bending at 3450 cm\({}^{-1}\) and 1640 cm\({}^{-1}\). The shoulder at 3700-3500 cm\({}^{-1}\) could be ascribable to the -OH of the silanol groups [44; 45; 46]. The Si-O-Si asymmetric stretching is assigned to the band at 1077 cm\({}^{-1}\) with the shoulder at 1200 cm\({}^{-1}\)[44; 47; 48]), whereas the peak absorption at 800 and 460 cm\({}^{-1}\) are attributed to Si-O-Si and Si-OH bending vibrations [45]. In addition, the band at 1385 cm\({}^{-1}\) is assigned to residual HNO\({}_{3}\)[42]. In all the spectra shown in Figure 4A, at a fixed amount of silica and polyethylene glycol (12 wt%), the increment in the caffe acid amount corresponds to an increase in the intensity of the band detected at 1745 cm\({}^{-1}\) (C=O vibration [36]). This is further proof that caffe acid is present in its oxidized form in all the synthesized systems.