**1. Introduction**

Silicon dioxide is a ubiquitous material in electronics, food, agriculture, and approved for use in pharmaceutical and drug delivery [1–3]. As a solid-state powder, silicon dioxide is used in solid or semi-solid formulations as a tabletting agent, carrier for active ingredients, thickener, or moisture scavenger. In core-shell structures silicon dioxide (SiO2) may provide a protective shell or barrier, regulating release profiles and conferring protection or stabilization of active core materials [4,5]. Controlled growth of spherical silica particles of uniform size by means of (1) hydrolysis of alkyl silicates and (2) subsequent condensation of silicic acid in alcoholic solutions, or the Stöber process, is commonly employed in a variety of fields [6–8]. Based on this process, a number of methods for silica dioxide core-shell have been prepared from alkyl silicates, moderating the stability and release of active ingredients. This strategy has been applied to the preparation of metal and weakly oxidizing metal oxide core silica dioxide-shell materials [4,7,9]. However, the utility of this process is limited for strong oxidizing agents due to precursor and by-product compatibly. Silver has been known for centuries to be an effective antimicrobial agen<sup>t</sup> [10–12]. Throughout these centuries the

foremost state or form of this antimicrobial agen<sup>t</sup> has been comprised of metallic and, or singly ionic silver: Ag (0) and Ag (I) [13–15]. As a weak oxidizing agent, with a standard electrode potential of +0.7996 V, strategies for fabrication of monodisperse Ag-SiO2 core-shell from Ag (I) materials have been identified [7,9]. These strategies utilize a seeded polymerization technique, based on the Stöber method, wherein sol-gel reactions of alkyl silanes generate silver nanomaterials with amorphous silica shell coatings. Protecting the silver nanoparticle cores from oxidation without hindering antimicrobial function. Recently, higher oxidation states of silver, Ag (II) and Ag (III), have come into light as a viable alternative for these predecessor lower oxidation silvers. Higher oxidation state silver compounds demonstrate superior broad-spectrum antimicrobial e fficacy against microbes in planktonic and biofilm states [16–18]. Exhibiting marked e fficacy against multi-drug resistant bacterium, while remaining safe and not impairing host tissue repair or function [18,19]. Ag (II) and Ag (III) are strong oxidizing agents with standard electrode potentials of +1.980 V and +1.9 V respectively [20]. Not unlike other transition metal compounds of higher oxidation states, the same properties of higher oxidation state silver compounds that elicit unique biological activity also make them susceptible to chemical reduction and thermal degradation; limiting their utilization in a wider variety of medical applications [21–23]. Furthermore, restricting their compatibility with silica core-shell processes such as the Stöber method or modified methods thereof.

Circumventing the first step in the Stöber process, the direct use of silicic acid may also be used to generate silicon dioxide as shown in Equation (1) [24,25]. However, silicic acid is known to be unstable at concentrations above 100 ppm, resulting in uncontrolled polymerization of silica gel or silicon dioxide. Silicic acid is therefore impractical as a starting material for controlled production of silicon dioxide. Higher concentration silicon solutions may be generated through hydroxide condensation of SiO2, forming stable alkaline solutions of silicate salts [26,27]. In these alkali silicate solutions, H2SiO4 2− and H3SiO4 − salts are believed to be the dominant ions and stable in alkali solution. Decreasing the pH of the solution may result in the formation of H3SiO4 − and H4SiO4 and subsequent uncontrolled formation of amorphous silica solids, as expressed in Equation (2). Approaches for controlled polymerization of silica in solution include reducing silica concentration, maintaining high pH, addition of polymerization inhibitors, and eliminating nucleation sites [5,24,27,28]. These strategies provide insight into the mechanisms for controlled silica polymerization from alkali silicate solutions, nevertheless they are also unconducive to highly oxidized silver due to the reactivity of silver with organic reagents and reaction by-products. Accordingly, methods for the preparation of silica-encapsulated highly oxidized silver compounds to enhance stability while facilitating antimicrobial e fficacy are needed.

$$\rm SiO\_2 + 2H\_2O \Leftrightarrow H\_4SiO\_4 \tag{1}$$

$$\mathrm{H\_3SiO\_4}^-\mathrm{(aq)} + \mathrm{H\_4SiO\_{4(aq)}} \to \mathrm{(OH)\_3Si-O-Si(OH)\_3(s)} + \mathrm{OH}^-\_{\text{(aq)}}\tag{2}$$

In this paper, we present a facile, one-pot method for the preparation of a higher oxidation state silver-silica gel, Ag7NO11:SiO2, framework based on the direct oxidation of silver nitrate from an oxidizing alkali silicate aqueous solution. The corresponding characterization, thermal stability, aqueous degradation, and antimicrobial e fficacy of the Ag7NO11:SiO2 framework are evaluated over a range of relative silica concentrations based upon the one-pot reaction described herein.

### **2. Materials and Methods**
