Photodynamic anti-biofilm PHEMA/Rose Bengal hydrogels enhanced by iodide: evidence for hydrogen peroxide and triiodide formation.

Biofilm-associated infections remain a major challenge for biomedical devices, motivating the development of light-activated antimicrobial materials. Here we report the antibiofilm activity (against Staphylococcus aureus and Pseudomonas aeruginosa) of a photodynamic hydrogel consisting of Rose Bengal (RB) physically entrapped within a crosslinked poly(2-hydroxyethyl methacrylate) (PHEMA) matrix. Under visible-light irradiation, RB-PHEMA alone produced only modest reductions (ca. 1 log), whereas addition of iodide markedly enhanced antibiofilm performance, yielding a 4.2-log reduction for S. aureus and a 1.7-log reduction for P. aeruginosa. To elucidate the basis of iodide potentiation, we conducted a systematic spectroscopic study under irradiation with varying concentrations of KI and NaI. Long-lived products consistent with iodide oxidation by singlet oxygen were detected, including triiodide (I3-) and hydrogen peroxide (H2O2). I3- was observed within the PHEMA hydrogel but not in the supernatant, consistent with electrostatic retention in the polymer network, while H2O2 was detected both at the hydrogel surface and in the surrounding solution, indicating diffusion away from the material. These findings support a mechanism in which diffusible H2O2, together with polymer-retained iodine species, contributes to biofilm inactivation beyond short-lived reactive intermediates. Practically, 10 mM iodide was sufficient to reach the maximum I3- signal, and NaI performed comparably to KI. The transparency and flexibility of the hydrogel support applications requiring conformal contact and back-illumination. Finally, photobleaching of RB within the hydrogel is effectively eliminated in the presence of iodide, providing an additional practical advantage of iodide as a potentiating additive.