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

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 (<i>Staphylococcus aureus</i>, <i>Enterococcus faecalis</i>) and anaerobic (<i>Streptococcus mutans</i>, <i>Porphyromonas gingivalis</i>) bacteria, as well as the fungal pathogen <i>Candida albicans</i>. Notably, the mesostructured hybrids exerted antiseptic effects not only through direct contact with microorganisms but also <i>via</i> 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.