A series of poly(2-hydroxyethyl methacrylate) (PHEMA) thin films entrapping photosensitizer Rose Bengal (RB) and tetrabutylammonium iodide (TBAI) have been synthetized. The materials have been characterized by means of Thermogravimetric Analysis (TGA), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and UV-vis Absorption spectroscopy. Irradiation of the materials with white light led to the generation of several bactericidal species, including singlet oxygen (1O2), triiodide anion (I3-) and hydrogen peroxide (H2O2). 1O2 production was demonstrated spectroscopically by reaction with the chemical trap 2,2'-(anthracene-9,10-diylbis(methylene))dimalonic acid (ABDA). In addition, the reaction of iodide anion with 1O2 yielded I3- inside the polymeric matrix. This reaction is accompanied by the formation of H2O2, which diffuses out the polymeric matrix. Generation of both I3- and H2O2 was demonstrated spectroscopically (directly in the case of triiodide by the absorption at 360 nm and indirectly for H2O2 using the xylenol orange test). A series of photodynamic inactivation assays were conducted with the synthesized polymers against Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa. Complete eradication (7 log10 CFU/mL) of both bacteria occurred after only 5 min of white light irradiation (400-700 nm; total energy dose 24 J/cm2) of the polymer containing both RB and TBAI. The control polymer without embedded iodide (only RB) showed only marginal reductions of ca. 0.5 log10 CFU/mL. The main novelty of the present investigation is the generation of three bactericidal species (1O2, I3- and H2O2) at the same time using a single polymeric material containing all the elements needed to produce such a bactericidal cocktail, although the most relevant antimicrobial activity is shown by H2O2. This experimental approach avoids multistep protocols involving a final step of addition of I-, as described previously for other assays in solution.
Keywords: E. coli; P. aeruginosa; antibacterial materials; bactericidal polymers; hydrogen peroxide; iodide anion; photodynamic inactivation; singlet oxygen; triiodide anion.