Density Functional Theory Study of the Spontaneous Formation of Covalent Bonds at the Silver/Silica Interface in Silver Nanoparticles Embedded in SiO₂: Implications for Ag⁺ Release

Silver nanoparticles (AgNPs) are widely used in the health-care sector and industrial applications because of their outstanding antibacterial activity. This bactericidal effect is mainly attributed to the release of Ag+ ions in an aqueous medium, the first step of which is the dissolution of the AgNP via the oxidation of its surface by O2. With the aim of designing more durable and less toxic antibacterial devices, it is desirable to fine-tune the rate of Ag+ release into the surrounding environment. This can be achieved by choosing an adequate coating of the AgNPs, e.g., by embedding the nanoparticles in a silica matrix. In a previous work (Pugliara, A.; et al. Sci. Total Environ. 2016, 565, 863), we have shown that the toxic effect on algae photosynthesis of small AgNPs (size <20 nm) embedded in silica layers is preserved, provided that the distance between the AgNPs and the silica free surface is below ≈6–7 nm. Better control of the Ag+ release rate in these systems requires a better understanding of the elementary mechanisms at play concerning both the detachment of the Ag ions from the AgNPs and their diffusion through SiO2. A first step in this direction consists in characterizing the interface between the AgNPs surface and the silica matrix. In this context, periodic density functional theory calculations have been performed on model systems representing the interfaces between amorphous silica and the three crystalline facets of AgNPs, i.e., Ag(111), Ag(110), and Ag(100). Spontaneous breaking of the Si–O bonds and the formation of two O–Ag and one Si–Ag bonds are observed in 50% of the investigated interfaces, corresponding to 1.8 bonds/nm2 on average. The covalent nature of the bonds between Ag and O and between Ag and Si is highlighted by analysis of the electronic structure of the interfaces.