## 4 Conclusions  
The aim of this work was to synthesize a biomaterial in an eco-friendly manner, avoiding the use of acids and minimizing the generation of hazardous waste, while still retaining the desired characteristics of the material obtained with the acid catalyst. Our research findings demonstrate that it is possible to successfully synthesize an organic-inorganic hybrid system without the use of nitric acid as a catalyst, as confirmed by FTIR spectra. However, structural alterations affected the entrapped drug in both cases. In the presence of nitric acid, the quercetin oxidation forms 3(2H)-benzofuranone, while in its absence, oxidation in the hydroalcoholic synthesis environment resulted in the formation of a six-membered lactone ring. Both synthesized hybrid materials effectively promoted hydroxyapatite formation (as confirmed by FTIR analysis and SEM/EDS analysis), essential for osteointegration, with no significant differences observed between them. In vitro release studies demonstrated pH-dependent quercetin release, with a higher release capacity observed for the SiQ5 system compared to the SiO5-HNO\({}_{3}\) system. This difference in release capacity was influenced by a different SiO\({}_{2}\) surface. However, the SiQ5-HNO\({}_{3}\) exhibited more effective antibacterial activity compared to SiQ5, likely due to the release of residual nitrate ions as well as the oxidated quercetin structure. These results highlight the  
Figure 11: Measured inhibition halos after Si, Si-HNO\({}_{3}\), SiQ5, and SiQ5-HNO\({}_{3}\) incubation with the microbial strains.  
possibility of achieving a biomaterial with the desired properties without the use of acids. Further studies are necessary to improve the drug stability with any structural alteration (for example, Brunauer-Emmett-Teller (BET) theory could be useful to determine the specific surface area of the synthesized solid materials based on gas adsorption measurements, or magnetic nuclear resonance (NMR) could be used to strengthen molecular changes in quercetin's structure), as well as to understand the possible cytotoxicity and loss in antioxidant properties of modified quercetin. This ongoing research will contribute to the development of innovative biomaterials with improved performance and reduced environmental impact, ultimately benefiting various biomedical fields.  
Conceptualization, M.C. and A.D.; methodology, M.F. and A.D.; software, M.F. and V.V.; validation, M.F., A.D., V.V. and L.V.; formal analysis, M.F., A.D., V.V. and L.V.; investigation, M.F., V.V. and A.D.; resources, L.V.; data curation, M.F. and A.D.; writing--original draft preparation, A.D., M.F. and V.V.; writing--review and editing, all authors; visualization, L.V.; supervision, M.C.; project administration, M.C.; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in this article; further inquiries can be directed to the corresponding author.  
The authors thank the PRIN 2022 PNRR project #P2022S4 TK2 GLASS-based TREAtments for Sustainable Upcycling of inorganic RESidues for the support in the antimicrobial tests. The authors also thank Ecoricerche Srl (Capua, Italy) for the SEM/EDS analysis.  
The authors declare no conflicts of interest.