Quantitative Characterization of Localized Hydrophobicity Surrounding Polymer Particle Surfaces for Aqueous Antibacterial Colloids

Aqueous antibacterial colloids are potential agents that kill bacteria via physical contact. Conventionally, antibacterial agents are designed to be small, cationic, or hydrophobic. However, hydrophobic materials easily aggregate in aqueous media, drastically inhibiting their activity. In this study, we developed cationic polystyrene (PSt) particles (zeta potential > +40 mV) with tunable localized hydrophobicity. PSt particles were synthesized using cationic initiators {2,2′-azobis-[2-(1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium-2-yl)]propane triflate (ADIP-TfO) or ADIP chloride (ADIP-Cl)}. A solvatochromic molecule, 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan), was employed for quantitative characterization of the localized hydrophobicity. To better understand the broad emission spectra of Laurdan in particle suspensions, a systematic deconvolution analysis was performed, resulting in two peak fractions: a low-polarity side (peak area = S_low, λ_low ∼ 430 nm) and a high-polarity side (S_high, λ_high ∼ 480 nm). The peak area ratio H = S_low/(S_low + S_high) was defined as the localized hydrophobicity, which indicated the distribution ratio of the hydrophobic (less polar) regions on the total particle surface. The H values were tuned by adding the cationic monomer (vinylbenzyl)trimethylammonium chloride. For sub-micrometer-sized PSt particles, a correlation between the antibacterial activity and defined localized hydrophobicity was observed against Staphylococcus epidermidis. Among the synthesized PSt particles, the higher antibacterial activity was attained in the cationic PSt particles with the highest H value. Therefore, the ADIP series is a suitable initiator for controlling localized hydrophobicity on the polymer particle surface.