## 3 Results and Discussion
### Structural Characterization of Si, Si-HNO3, SiQ5, and SiQ5-HNO3 Hybrid Materials
The most plausible hypothesis would be the breakage of the double bond at positions 2-3 of the C-ring (see Q structure in Figure 1), resulting in the formation of a dihydroflavonol. [PERSON] et al. also proposed a mechanism for the reaction between [AuCl\({}_{4}\)]\({}^{-}\) and quercetin, highlighting that these reactions in the presence of water and alcohol lead to the formation of oxidation products where the double bond 2-3 in ring C is reduced [43]. To confirm the structural changes in quercetin encapsulated in the Si matrix in the presence of the acid catalyst and in the Si matrix synthesized without the catalyst, UV-vis spectra of quercetin extracted from the SiQ5-HNO\({}_{3}\) (Figure 6A) and SiQ5 (Figure 6B) systems were recorded. Several studies have demonstrated that quercetin can undergo oxidation in the presence of an aqueous environment and acidic pH [44]. This oxidation process alters the flavonoid skeleton, consequently affecting its antioxidant activity. It was therefore important to obtain further information about the structure of quercetin released from SiO\({}_{2}\) systems obtained through the sol-gel technique to assess if it maintains its biological activities. Pure quercetin, solubilized in a 2% ethanol physiological solution (SBF), shows two main characteristic bands: band I, located at 380 nm as the lambda maximum, generally attributed to the B-C rings system, which is a benzopyrone group, and band II at 250-260 nm, attributed to the cinnamoyl system [45]. Band I is due to a pure HOMO-LUMO transition, whereas band II results from a combination of several excitations [46]. During the oxidation of quercetin, as documented in various scientific studies, significant changes in the UV-visible spectrum occur. Initially, the characteristic absorption bands of quercetin at 250 nm and approximately 380 nm disappear, while a new band at 332 nm emerges, indicating the presence of the oxidation product after the removal of two electrons from the B-ring, forming a quinone structure. Additionally, there is an increase in absorption at 201 nm during  
Figure 4: FTIR spectra of Si-HNO\({}_{3}\) and SiQ5-HNO\({}_{3}\), along with oxidated quercetin extracted from SiQ5-HNO\({}_{3}\). Blue arrows indicate peak position explained in main text. oxidation. In an advanced stage of oxidation, a band at about 290 nm appears, with a shoulder at 320 nm. This product is consistent with that obtained through the sol-gel synthesis of SiQ5-HNO\({}_{3}\)[47]. This finding is consistent with what was observed in this study. The entrapped quercetin undergoes an oxidation process that causes the opening and recyclization of the C-ring, causing a displacement of the quercetin absorption maximum (380 nm), resulting in a new peak with maximum absorbance at 295 nm with a shoulder at 320 nm (Figure 6A). This oxidation product presents a spectrum similar to that of the oxidation product 3(2H)-benzofuranone [48]. This product is obtained through the formation of a quinone derivative and subsequent chemical reactions such as hydroxylation [49]. A different behavior is observed for SiQ5, where, despite the absence of an added acidic catalyst, the presence of tetraethyl orthosilicate as SiO\({}_{2}\) precursors, conferring the synthesis solution a slightly acidic pH of about 5.0 along with a hydroalcoholic environment, caused a change in the initial structure of quercetin, as observed in Figure 6B, confirming what has already been observed in the FTIR SiO5 spectrum. Specifically, a simultaneous decrease in band I (380 nm) and an increase in the 335 nm band are observed due to the breaking of the cinnamy system (B- and C-rings) [50]. This absorption band is associated with the formation of quercetin o-quinoids during the oxidation reaction of quercetin, even though a specific structure for these compounds has not been definitively assigned [51]. Figure 5: FTIR spectra of Si, SiQ5 and quercetin (Q). Arrows indicate peak position explained in the main text.