Upcycling rice husk ash into antiseptic-encapsulated ordered mesoporous silica materials for antimicrobial applications

Abstract

Rice husk ash (RHA), an abundant agricultural and industrial waste, was upcycled into functional mesostructured silica encapsulating cetylpyridinium chloride (CPC), an FDA-approved antiseptic. Sodium silicate was efficiently extracted from RHA and leveraged as a biosourced silica precursor for the aqueous sol–gel synthesis of mesostructured hybrid silica at 30 °C. Micelles of CPC simultaneously acted as structure-directing agents and as integrated antimicrobial cargo, enabling the direct formation of ordered hybrids in one-pot. The condensation pH influenced the synthesis yield, the bonding configuration of silicon, the encapsulation of CPC and the nanostructure of the hybrids. An optimal condensation pH of 11 with a 10 : 1 Si : CPC ratio enabled highly ordered mesostructured hybrids comprising around 40 wt% of CPC and a large proportion of silanolate sites promoting electrostatic interactions. Calcination of these materials unveiled mesoporous silica with well-ordered 2D hexagonal mesophases of cylindrical pores (∼2.3 nm in diameter) and high surface areas up to 520 m² g⁻¹. Increasing the CPC content of the hybrid materials was possible by adjusting the Si : CPC ratio. The mesostructured hybrids exhibited limited CPC release (4–5%) under physiological pH conditions, highlighting their potential for slow and burst-free release. Consequently, they exhibited potent broad-spectrum antimicrobial efficacy with a CPC dose-dependent effect (evaluated by inhibition zones, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC)) against a representative panel of pathogens: aerobic (Staphylococcus aureus, Enterococcus faecalis) and anaerobic (Streptococcus mutans, Porphyromonas gingivalis) bacteria, as well as the fungal pathogen Candida albicans. Notably, the mesostructured hybrids exerted antiseptic effects not only through direct contact with microorganisms but also via CPC diffusion. These results established RHA-derived CPC-loaded mesoporous silica as a sustainable, high-value platform for next-generation antimicrobial applications, contributing to circular economy efforts, healthcare, and environmental sectors.