# Infection Control with Antimicrobial Solid-State ZnO Nanoparticles on Silk Fiborin Gauze
## 3 Results and Discussion
While the exact toxicity mechanism of ZnO towards bacteria is still controversial, the potential antibacterial effects can be attributed to the following three main mechanisms [52]: (1) ROSs production resulting in the disintegration of the bacterial membrane; (2) Zn\({}^{2+}\) release disrupting critical processes including DNA replication and protein synthesis [53]; and (3) direct interaction between ZnO NPs and bacteria causing antimicrobial effects [54; 55]. A significant factor in the antibacterial properties of ZnO is ROSs production, i.e., hydroxyl radicals, H\({}_{2}\)O\({}_{2}\), and singlet oxygen [56]. Although the precise mechanism remains unclear, nanoparticle size-dependent ROSs production appears to impact bacterial death. Indeed, smaller particles exhibit a greater surface area, which facilitates interactions with cellular constituents and elevated ROSs generation. Additionally, smaller particles can show increased reactivity because of their elevated surface energy, leading to greater ROSs production. In this work, the results demonstrated insufficient ROS production, probably caused by the large ZnO nanoparticle size. Indeed, several studies (Table 1) have studied the antibacterial properties of ZnO NP (\(<\)100 nm) on different substrates [57; 58; 21; 59; 60]. ZnO NP with sizes \(<\)100 nm is known to exhibit better antibacterial properties than larger sizes [59]. Figure 4: **(A)** Total ROSs production after 24, 48, and 72 h of incubation in PBS (C, control), SF gauze (SF), and ZnO-SF gauze (ZnO). **(B)** Zn\({}^{2+}\) release for ZnO-SF gauze after 24 h, 48 h, and 72 h (\(n\) = 3, \(p\)-value \(<\) 0.05, significant difference between time points) and **(C)** Zn\({}^{2+}\) release ratio from zinc particles. The release ratio is the amount of zinc ions released to the total amount of zinc on the SF gauze. This size-dependent toxicity is further accentuated by the increased internalization of smaller particles within bacteria, thereby enhancing their antibacterial efficiency compared to larger particles at the same concentration [53]. Based on the obtained results, the larger size of the ZnO nanoparticles causing lower internalization, and weak ROSs production, it can be inferred that the significant release of Zn\({}^{2+}\) ions in the medium is the logical cause of ZnO toxicity toward _S. aureus_ and _P. aeruginosa_. The main proposed antibacterial mechanism for Zn\({}^{2+}\) uptake is linked to active transport inhibition, amino acid metabolism, and enzyme system disruption [38]. However, these aspects, i.e., ROSs, Zn, and internalization, also apply to cells, thereby causing great cytotoxicity [53]. The gain of antibacterial activity is, however, negatively correlated with biosafety and cell viability [61; 62]. Consequently, a greater particle size (with a smaller specific surface area) was chosen to minimize the cytotoxicity of ZnO particles. This approach slows down the dissolution kinetics of the ZnO particles and reduces the number of particles internalized by cells, resulting in decreased cytotoxic effects [63]. Indeed, with FDA approval for the use of ZnO in bulk and with a size exceeding 100 nm [64], our material (230 \(\pm\) 45 nm) is considered safe. The size of nanoparticles (NPs) plays a key role in dictating their unique interactions with cells. It was demonstrated that smaller ZnO nanoparticles (30 nm) had a higher internalization effect compared to larger nanoparticles