The effect of iodide and temperature on enhancing antibacterial properties of nanoparticles with an encapsulated photosensitizer

[Display omitted] •Stable nanoparticles for the photodynamic inactivation of bacteria were developed.•The antibacterial effect was enhanced at higher temperatures (37 °C) and/or after the addition of iodide.•Photophysical measurements were used for prediction of the antibacterial properties.•The formation of singlet oxygen was not influenced by the environment of the nanoparticles. Aqueous dispersions of sulfonated polystyrene nanoparticles (average diameter: 30 ± 14 nm) with encapsulated 5,10,15,20-tetraphenylporphyrin (TPP) are promising candidates for antibacterial treatments due to the photogeneration of cytotoxic singlet oxygen species O2(1Δg) under physiological conditions using visible light. The antibacterial effect on gram-negative Escherichia coli was significantly enhanced after the addition of nontoxic potassium iodide (0.001–0.01 M) because photogenerated O2(1Δg) oxidized iodide to I2/I3−, which is another antibacterial species. The improved antibacterial properties were predicted using luminescence measurements of O2(1Δg), transient absorption of TPP triplets and singlet oxygen-sensitized delayed fluorescence (SODF). In contrast to a solution of free photosensitizers, the aqueous dispersion of photoactive nanoparticles did not exhibit any quenching of the excited states after the addition of iodide or any tendency toward aggregation and/or I3−-induced photo-aggregation. We also observed a decrease in the lifetime of O2(1Δg) and a significant increase in SODF intensity at higher temperatures, due to the increased oxygen diffusion coefficient in nanoparticles and aqueous surroundings. This effect corresponds with the significantly stronger antibacterial effect of nanoparticles at physiological temperature (37 °C) in comparison with that at room temperature (25 °C).