Quantitative\nCharacterization of Localized Hydrophobicity\nSurrounding Polymer Particle Surfaces for Aqueous Antibacterial Colloids

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