Photoactive methylene blue-functionalized polymer for antimicrobial activation under red light

This study presents the synthesis of a novel methylene blue acrylamide monomer and its incorporation into a diblock copolymer, PolyMB, which exhibits potent antimicrobial activity against Gram-negative (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria when photoirradiated under red light (λ = 630 nm). Mechanistic investigations revealed that singlet oxygen species, and not superoxides, are responsible for the antimicrobial activity, most likely by damaging cellular components such as proteins and DNA. The advantage of using red light as an external trigger because of its ability to penetrate skin and tissue is demonstrated here, where PolyMB is still active against E. coli when irradiated through a cover of chicken skin. In terms of biocompatibility, PolyMB is, significantly, 130 times more biocompatible than the original methylene blue dye. Overall, this study demonstrates the efficient modification of a red light-active photosensitiser into an antimicrobial macromolecule with improved biological properties for potential photodynamic applications in healthcare.