Here, we present new antimicrobial nanoparticles based on silica nanoparticles (SNPs) coated with a quaternary ammonium cationic surfactant, didodecyldimethylammonium bromide (DDAB). Depending on the initial concentration of DDAB, SNPs immobilize between 45 and 275 mu g of DDAB per milligram of nanoparticle. For high concentrations of DDAB adsorbed to SNP, a bilayer is formed as confirmed by zeta potential measurements, thermogravimetry, and diffuse reflectance infrared Fourier transform (DRIFT) analyses. Interestingly, these nanoparticles have lower minimal inhibitory concentrations (MIC) against bacteria and fungi than soluble surfactant. The electrostatic interaction of the DDAB with the SNP is strong, since no measurable loss of antimicrobial activity was observed after suspension in aqueous solution for 60 days. We further show that the antimicrobial activity of the nanoparticle does not require the leaching of the surfactant from the surface of the NPs. The SNPs may be immobilized onto surfaces with different chemistry while maintaining their antimicrobial activity, in this case extended to a virucidal activity. The versatility, relative facility in preparation, low cost, and large antimicrobial activity of our platform makes it attractive as a coating for large surfaces.