## 3 Results and Discussion  
### Structural Characterization of Si, Si-HNO3, SiQ5, and SiQ5-HNO3 Hybrid Materials  
The influence study of the acid catalyst on the synthesis of Si and SiQ5 materials was carried out by means of a structural analysis through FTIR. In our previous paper, the addition of nitric acid during the sol-gel synthesis of the SiO2 matrix to accelerate hydrolysis and polycondensation reactions contributed to the alteration of the quercetin structure. Indeed, it underwent an oxidation process with the alteration of the C-ring (Figure 1), resulting in the formation of a dihydroxybenzofuranone fraction because of the acidic environment experienced during synthesis [30]. The FTIR spectra of Si-HNO3, SiQ5-HNO3, and oxidated quercetin (Q Ox) are reported in Figure 4. In the spectrum of SiO2-HNO3, the bands at 796 and 460 cm\({}^{-1}\) correspond to the bending vibrations of the Si-O-Si structure, along with the peak at 1078 cm\({}^{-1}\) with a shoulder at 1210 cm\({}^{-1}\) attributed to the symmetric and asymmetric stretching of the Si-O-Si bond. Moreover, the use of HNO3 to accelerate the synthesis process was evident in the sharp peak at 1384 cm\({}^{-1}\), attributable to the NO3\({}^{-}\) group. The encapsulation of quercetin within the inorganic silicon matrix is highlighted in the SiQ5-HNO3 spectrum by the presence of a peak at 1165 cm\({}^{-1}\), attributed to the stretching of the ketone group C-CO-C and the appearance of the peak at 1757 cm\({}^{-1}\), due to the carbonyl group's vibration of the flavonoid in a hydroxybenzofuranone structure [38; 39], validated by NMR analysis conducted in our previous work [25]. The increase in the broadness of -OH stretching vibration at 3457 cm\({}^{-1}\) is indicative of the formation of H-bonds between the hydroxyl group of the silanol group from the silica matrix and the -OH groups of entrapped quercetin. This was also demonstrated using a dynamic molecular approach [26], in which the interaction between the silica matrix and two amounts of quercetin, 5 and 15%, respectively, was simulated and discussed. In Figure 5, the FTIR spectra of Si and SiQ5 synthesized via the sol-gel technique in the absence of nitric acid were reported using pure quercetin (Q in graph) for comparison. In the Si spectrum, unlike that synthesized in the acidic environment, sharp peaks indicate the residual presence of tetraethyl orthosilicate not completely hydrolyzed during the sol-gel process. Specifically, the peak at 1483 cm\({}^{-1}\) is attributed to the OCH group. The peaks at 1447 and 1395 cm\({}^{-1}\) could be attributed to the CH3 group and the OCH/CCH groups, respectively, of residual TEOS or ethanol in the material [40]. The peak at 1087 cm\({}^{-1}\) related to the Si-O-Si stretching exhibits a shift to higher wavenumbers, indicating a less advanced polycondensation process compared to that of Si-HNO3 (1078 cm\({}^{-1}\)). Furthermore, the peak at 1168 cm\({}^{-1}\), representing the CH2 rocking of TEOS, is still slightly visible, also indicating an incomplete hydrolysis of TEOS, confirmed by the absence of the shoulder at 1210 cm\({}^{-1}\) (which is present in Si-HNO3, as shown in Figure 4), which increases withthe advancement of the polycondensation process [41]. The spectrum of SiQ5, despite the absence of nitric acid, does not show traces of unhydrolyzed TEOS. Additionally, it appears to have completed the process of polycondensation of SiO\({}_{2}\), as evidenced by the presence of the shoulder at approximately 1200 cm\({}^{-1}\). It is also observable that in this case, the encapsulation of quercetin does not lead to significant structural modifications of the quercetin. In fact, in the spectrum of SiQ5, the band at 1165 cm\({}^{-1}\) is visible, overlapping with that of pure quercetin. Furthermore, there is a noticeable new broad band at 1740 cm\({}^{-1}\), which can be attributed to the stretching vibration of the carbonyl group within a six-membered-lactone structure [42].