Unveiling the potent antimicrobial photodynamic therapy in Gram-positive and Gram-negative bacteria – Water remediation with monocharged chlorins

Water pollution is a significant concern worldwide, and it includes contaminants such as antibiotic-resistant pathogens. Antimicrobial photodynamic therapy (aPDT) offers a non-invasive and non-toxic alternative for the inactivation of these microorganisms. So, this study reports the synthesis, structural characterisation, photophysical properties, and aPDT efficacy of cationic free-base and zinc(II) chlorin (Chl) derivatives bearing N,N-dimethylpyrrolydinium groups (H₂Chl 1a and ZnChl 1b). The aPDT assays were performed against two bacterial models: Staphylococcus aureus (Gram-(+)) and Escherichia coli (Gram-(-)). The H₂Chl 1a and ZnChl 1b distinct's solubility profile, coupled with their ability to generate singlet oxygen (¹O₂) under light exposure, (H₂Chl 1a, Ф_Δ = 0.58 < TPP, Ф_Δ = 0.65 < ZnChl 1b, Ф_Δ = 0.83) opens up their potential application as photosensitizers (PS) in aPDT. The effectiveness of H₂Chl 1a and ZnChl 1b at 1.0 and 5.0 μM in aPDT against S. aureus and E. coli at 500 W m^-2 (total exposure time: 60-120 min) showed a viability reduction >6.0 log₁₀ CFU mL^-1. Additionally, KI was used as a coadjuvant to potentiate the photoinactivation of E. coli, reaching the method's detection limit (>4.0 log₁₀ RLU). As most of the PS developed to inactivate Gram-negative bacteria are cationic with three or more charges, the fact that the H₂Chl 1a and ZnChl 1b with only one cationic charge photoinactivate E. coli at low concentrations and with a reduced light dose, it is an importing discovery that deserves further exploration. These monocharged chlorin dyes have the potential for water remediation.