Polymer nanoparticles have attracted attention as antibacterial materials, but the function of the polymer itself has not yet been clarified sufficiently. To estimate the essential surface properties of antibacterial polymer nanoparticles, herein, we synthesized cationic polystyrene (PSt) nanoparticles via soap-free emulsion polymerization using 2,2'-azobis-[2-(1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium-2-yl)]propane triflate (ADIP) as initiator. The conversion of total monomers was drastically increased through the addition of the commoner (vinylbenzyl)trimethylammonium chloride (VBTMAC), where unimodal size distributions (Cv ≤ 10%) were obtained at comonomer molar ratios between 0.0083 and 0.0323. The adsorption behavior of a solvatochromic anionic fluorescent dye revealed the surface charge density (σ) and affinity with anionic molecules (K) of PSt nanoparticles. The PSt nanoparticles with increased K values exhibited antibacterial activity against Staphylococcus epidermidis, with a minimum inhibitory concentration of at least 0.69 mg/mL. To determine a plausible mechanism for the antibacterial activity, the membrane damage induced by PSt nanoparticles was evaluated using an assay utilizing polydiacetylene vesicles as the model for negatively charged bilayer membranes. The PSt nanoparticles exhibiting large K values disturbed the bilayer structure of the model membrane system, suggesting that the synthesized PSt nanoparticles could be utilized as a contact-killing antibacterial agent.
Keywords: antibacterial material; cationic polymer nanoparticle; membrane damage; soap-free emulsion polymerization; surface charge density.