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

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.