Harnessing light for antimicrobial action: light-driven inactivation of Gram-positive bacteria by boronic acid-based zinc(II) phthalocyanines

Photodynamic inactivation (PDI) has emerged as an efficient approach to combat resistant pathogens, particularly Gram-positive bacteria such as Staphylococcus aureus . In this study, we report the synthesis, characterization, and antimicrobial evaluation of three novel zinc(II) phthalocyanines (Pcs) functionalized with 4-mercaptophenylboronic acid units (tetra-substituted ZnPc 1 , octa-substituted ZnPc 2 , and octa-substituted fluorinated ZnPc 3 ). Their PDI performance was investigated against S. aureus at two concentrations (5.0 and 10.0 μM) under white light irradiation at 100 mW cm −2 (up to 60 min; total light dose 360 J cm −2 ), both in the absence and presence of potassium iodide (KI). The ZnPc 1 demonstrated potent activity against S. aureus , reducing bacterial counts by approximately 4 log units at 5 μM and nearly 7 log units at 10 μM after 60 min of light irradiation. Remarkably, upon addition of KI, inactivation of S. aureus to the detection limit of the method was achieved within 30 min at both concentrations. The ZnPc 2 exhibited the most potent effect, leading to inactivation to the detection limit of the method within only 15 min with KI at both concentrations, and without KI at 30 min (10 μM) or 45 min (5 μM). The ZnPc 3 required KI to achieve bacterial inactivation, with reductions observed after 60 min at 5 and 10 μM. These results demonstrate that structural modification with functional units strongly influences the antimicrobial efficacy of ZnPcs 1 – 3 , with the ZnPc 2 emerging as the most promising drug. The synergistic role of KI further highlights the potential of these boronic acid-functionalized phthalocyanines as efficient photosensitizers for photodynamic therapy against Gram-positive pathogens. • Photoinactivation (PDI) of S. aureus bacteria using a phthalocyanines dyes (ZnPcs 1 – 3 ). • ZnPcs 1 – 3 generates singlet oxygen under white light irradiation. • Potassium iodide (KI) enhances PDI efficiency, reducing time for bacterial inactivation. • ZnPc 2 achieved a bacterial viability reduction >8 Log CFU/mL reduction in ≤15 min.