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

Abstract

Silver\nnanoparticles (AgNPs) are widely used in the health-care\nsector and industrial applications because of their outstanding antibacterial\nactivity. This bactericidal effect is mainly attributed to the release\nof Ag⁺ ions in an aqueous medium, the first step of which\nis the dissolution of the AgNP via the oxidation of its surface by\nO₂. With the aim of designing more durable and less toxic\nantibacterial devices, it is desirable to fine-tune the rate of Ag⁺ release into the surrounding environment. This can be achieved\nby choosing an adequate coating of the AgNPs, e.g., by embedding the\nnanoparticles in a silica matrix. In a previous work (Pugliara, A.;\net al. <i>Sci. Total Environ.</i> <b>2016</b>, <i>565</i>, 863), we have shown that the toxic effect on algae\nphotosynthesis of small AgNPs (size <20 nm) embedded in silica\nlayers is preserved, provided that the distance between the AgNPs\nand the silica free surface is below ≈6–7 nm. Better\ncontrol of the Ag⁺ release rate in these systems requires\na better understanding of the elementary mechanisms at play concerning\nboth the detachment of the Ag ions from the AgNPs and their diffusion\nthrough SiO₂. A first step in this direction consists in\ncharacterizing the interface between the AgNPs surface and the silica\nmatrix. In this context, periodic density functional theory calculations\nhave been performed on model systems representing the interfaces between\namorphous silica and the three crystalline facets of AgNPs, i.e.,\nAg(111), Ag(110), and Ag(100). Spontaneous breaking of the Si–O\nbonds and the formation of two O–Ag and one Si–Ag bonds\nare observed in 50% of the investigated interfaces, corresponding\nto 1.8 bonds/nm² on average. The covalent nature of the\nbonds between Ag and O and between Ag and Si is highlighted by analysis\nof the electronic structure of the interfaces.