On the structure of Ag-containing sol-gel bioactive glasses: A surface crystal growth of metallic silver removes its network modifier role in the glass structure

Ag-containing bioactive glasses (Ag-BG) have been extensively studied as a bactericidal biomaterial, produced mainly by the sol-gel method over the last decades. According to the literature, sol-gel-derived Ag-BG may show metallic or ionic silver species in the glass structure. However, most of these works lack detailed information about how the addition of silver affects the glass network. In this study, we systematically produced sol-gel derived 58S glasses containing different quantities of Ag2O (1, 5, and 10 wt%), yielding glasses with different proportions of silver species (Ag0 or Ag+1). Results from X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) evidenced the progressive formation of silver crystals due to the addition of silver in the glass structure. 29Si and 31P nuclear magnetic resonance and photoelectronic X-ray spectroscopies evidenced a glass depolymerization caused by Ag1+ addition in the glass structure. On the other hand, the presence of silver crystals (Ag0) removes the network modifier role of silver, yielding glasses with higher network connectivity. A possible mechanism to explain the formation of silver crystals was proposed based on surface nucleation and growth. Finally, we suggest understanding Ag1+ or Ag0 formation as a solution to design glasses with tuned bactericidal properties.