Effect of the ethanol-to-water ratio on the properties of silica-carbon core-shell materials for prolonged antibacterial activity of thymol

Porous silica (SiO2) nanoparticles have been widely used as drug-delivery materials; however, it is difficult to control drug release at desirable rates when using only SiO2. Thus, a facile synthesis of SiO2-carbon core-shell (SiO2@C) nanoparticles with tunable sizes and shapes for efficient controlled release was developed to prolong the antibacterial activity of thymol (T). The physicochemical properties, and surface bonding between thymol and the carbon shell were determined with XPS, NEXAFS, and DFT studies. An in vitro thymol release study was also applied, and the prolonged antibacterial performance was evaluated with gram-positive Staphylococcus aureus. The optimal conditions for producing spherical SiO2@C core-shell nanostructures with a uniform size distribution included an ethanol-to-water ratio of 20:80 (E20W80). This produced a SiO2@C core-shell material with a high porous carbon content and a large pore volume. Moreover, SiO2@C-T exhibited the highest thymol release profile. DFT studies demonstrated that the thymol was physically adsorbed on the carbon surface. This adsorption mode was advantageous for prolonged release. Consequently, the bacteriostatic agent SiO2@C-T E20W80 showed sustained thymol release levels of up to 40% over 48 h. The developed SiO2@C material shows potential for use as a nanocarrier of drug/active ingredients.