## 3 Results and Discussion
### Encapsulation Efficiency and In Vitro Release Study  
The initial concentration of quercetin incorporated into the SiO\({}_{2}\) system was 5% with respect to the total amount of SiO\({}_{2}\) for both SiQ5 and SiQ-HNO\({}_{3}\). The encapsulation efficiency for both hybrid materials was evaluated to understand the real amount incorporated into the SiQ5 and SiQ5-HNO\({}_{3}\) systems and available for release. The results of encapsulation efficiency (EE%) are shown in Figure 9. The overall EE (%) was estimated to be 71.7 \(\pm\) 2.15 for SiQ5, whereas for SiQ5-HNO\({}_{3}\), this value was 82.7 \(\pm\) 3.9. These results suggest a high grade of drug incorporation for both systems, with a higher capacity for the silica system synthesized in the presence of the acid catalyst. Successively, the drug release study was conducted for the SiQ5 and SiQ5-HNO\({}_{3}\) hybrid materials to investigate their capacity to act as drug delivery systems. For this purpose, the hybrid systems were incubated in simulated body fluid (SBF) at 37 \({}^{\circ}\)C, mimicking both a healthy physiological condition at pH 7.4 and a cancer-like condition at pH 5.0. The released quercetin was followed for 24 h, and the results were expressed as a percentage of the cumulative drug released relative to the maximum amount of quercetin incorporated into the matrix, as reported in Figure 10. \begin{table}
\begin{tabular}{c c} \hline
**Samples** & **EDS Measured Ca/P Ratio** \\ \hline SiO5 & 1.73 \\ \hline SiQ5-HNO\({}_{3}\) & 1.67 \\ \hline \end{tabular}
\end{table} 
Table 1: EDS data on Ca, O, and P weight percentages and calculated Ca/P ratios in SiQ5 and SiQ5-HNO\({}_{3}\) after 21 days of soaking in SBF. Figure 8: FTIR spectra of SiQ5 (orange line) and SiQ5-HNO\({}_{3}\) (green line) recorded in the range of 1100–400 cm\({}^{-1}\) after 21 days of soaking in SBF and their respective SEM images. For all materials, the release kinetics occur in two phases: an initial phase with a rapid release within the first two hours, regardless of the pH used for the test, corresponding to the desorption of drugs weakly interacting at the surface (burst release), and a second and slower phase corresponding to the diffusion of the flavonoid entrapped within the inner part of the clusters from the matrix to the physiological solution, as reported for other flavonoid drug delivery systems [44,59]. Curves in Figure 10 reveal that both the synthesized hybrid system and the pH employed for controlled release exert significant influence on the total percentage of released encapsulated quercetin. Specifically, at pH 7.4, the SiO5 and SiO5-HNO3 systems demonstrate release rates of up to 69.05 and 49.78%, respectively. Conversely, under acidic conditions at pH 5.0, the cumulative percentage of quercetin released diminishes, reaching 27.46% for SiO5. The SiO5-HNO3 system does not  
Figure 10: In vitro release study of SiO5 and SiO5-HNO3 in SBF at pH 7.4 and 5.0. Figure 9: Encapsulation efficiency of quercetin inside SiO5 and SiO5-HNO3 systems. show a significative difference between pH 7.4 and 5.0 release, indicating that the acidic environment does not substantially affect the release profile in this system. For pH 7.4, both systems reach a plateau after about 8 h of release, whereas for pH 5.0, it is reached after only 2 h. The release efficiency of quercetin can be controlled by pH, with a decrease in release as the pH value decreases, as reported in other studies [38,39]. At pH 5.0, the predominantly protonated functional groups of quercetin might closely interact with the silanol inorganic matrix, potentially slowing down drug release by limiting quercetin's availability to diffuse from the matrix. Conversely, at pH 7.4, the deprotonated functional groups of quercetin make the molecule less prone to interact with the inorganic matrix, thereby facilitating quercetin diffusion and increasing the drug release rate [60,61].