Dual-action antimicrobial surface coatings: methylene blue and quaternary ammonium cation conjugated silica nanoparticles†

The increasing prevalence of healthcare-associated infections from multidrug-resistant bacteria presents a growing challenge due to their high transmissibility, and resistance to traditional antimicrobial strategies. In this study, we introduce an innovative dual-mode antibacterial strategy through the development of novel surface coatings on glass substrates, offering a proof-of-concept solution for enhanced infection control. Our approach uniquely combines the light-active methylene blue silane (MBS1) dye with the potent antimicrobial compound dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (QAS) into silica nanoparticles (SNPs) to create multifunctional antibacterial surface coatings. The distinct use of silane-functionalized MB and QA enables strong covalent bonding with silica nanoparticles, while the robust silane chemistry ensures durable adhesion of SNPs to the glass substrates. While MBS1–SNP coatings generated highly hydrophilic (CA = 28°), light-active surfaces, combination of QAS (QA–MBS1–SNP) coating enhanced surface hydrophobicity (CA = 90°) without compromising photokilling efficiency. The antibacterial efficacy of these coatings was rigorously tested against the Gram-negative bacterium Escherichia coli. The synergistic action of MB and QA demonstrated exceptional photokilling performance achieving >99.999% (>5-log reduction) bactericidal activity under white light (∼500 lux, ∼0.0732 mW cm⁻²) and effectively inhibited biofilm formation by up to 80%. The demonstrated efficacy of these coatings highlights their potential for transformative applications in healthcare settings, providing a robust, multifaceted approach to infection control.