## 4 Discussion
It is widely recognized that inorganic NPs possess a significant surface area which enhances the adhesion capacity, improving bond strength. It was also revealed that silver phosphate NPs are capable of forming apatite crystals as well as causing noticeable alterations in the calcium/phosphate ratio of dentin substrate [37]. Furthermore, an expected synergistic effect arises from ZrO\({}_{2}\) nanoparticles, and this can be reasonably elucidated by the heightened wettability of the adhesive resin. Zirconium-based nanoparticles showcase excellent dispersion characteristics within a material, thereby enhancing its biocompatibility. The authors anticipate that these alterations collectively contribute to the augmentation of resin tags and the formation of a hybrid layer within the irregularities induced by acid etching. Consequently, this improvement is presumed to positively impact bond integrity scores [30; 39]. The results have also demonstrated that an increase in the concentration of ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) nanoparticles in SE adhesive leads to a linear improvement in both antimicrobial potency and micro-tensile bond strength (uTBS). It is critical to emphasize that the release rate of silver ions is determined by both the size and surface properties of nanoparticles. The dimensions of nanoparticles impact the extent of contact and interaction with the surrounding medium, while the charge and surface composition determine the stability of the nanoparticles. Smaller nanoparticles have been found to dissolve more quickly in different substances, resulting in the release of silver ions. This phenomenon has the potential to make a substantial contribution to the antibacterial efficacy of nanoparticles [40; 41; 42]. Existing works in the literature have often suggested that increasing nanoparticle concentrations in adhesives may have a detrimental effect on the physical and mechanical properties of materials [21; 43]. However, in the current study, it is evident that a higher concentration of ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) nanoparticles has a positive impact on the adhesive's bond strength. Nonetheless, it is important to note that the influence of ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) nanoparticles on the mechanical properties at higher concentrations, beyond those investigated in this research, remains a subject for future investigation. Concerning outcomes associated with DC, one plausible explanation for the observed decrease in DC following the inclusion of ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) nanoparticles is that it interferes with the propagation of curing light within the adhesive substance. This interference hinders the transformation of monomers into polymers during the curing process [24]. The potential consequence of reduced light penetration is a drop in DC, as it may impede the efficient polymerization of all monomers during the curing phase [44]. Regarding failure mode, it was observed that ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) NP-modified adhesives displayed a cohesive type of failure, whereas unmodified bonding agents predominantly reported adhesive failure patterns. Several reasons for cohesive failure can be attributed to factors such as stress concentration, material properties, material aging and defects, chemical reactions, fatigue, thermal change, etc. [26; 45]. The results of the current study show promise; however, readers are advised to interpret them cautiously, as the prospective implications are confined to the specific adhesive used in this investigation. It is important to note that different proportions of ZrO\({}_{2}\)/Ag\({}_{3}\)PO\({}_{4}\) nanoparticles, not identical to those employed in this study, may exhibit distinct characteristics. Additional studies should encompass an assessment of how these nanoparticles influence other mechanical properties, including flexural strength, microhardness, and surface roughness.