Polyethyleneimine-protoporphyrin IX and its Zn(II) complex conjugates for photodynamic inactivation of Staphylococcus aureus on cucumber mesocarp surfaces

Microbial contamination of fresh produce remains a significant food safety challenge, particularly for minimally processed foods. In this work, two photodynamic polymer conjugates were synthesized by covalently attaching protoporphyrin IX (PPIX) and its zinc(II) complex (ZnPPIX) to branched polyethyleneimine (PEI), a polycationic polymer that promotes strong electrostatic interactions with bacterial surfaces. The resulting PEI-PPIX and PEI-ZnPPIX conjugates were characterized by FT-IR and ¹H NMR spectroscopy, while UV-visible absorption and fluorescence measurements confirmed successful coupling and preservation of the photophysical properties of the porphyrin macrocycle. Both conjugates efficiently generated reactive oxygen species under visible-light irradiation. PEI-ZnPPIX exhibited a higher singlet oxygen quantum yield, consistent with the heavy-atom effect of the coordinated Zn(II). In addition, both systems promoted NBT reduction to diformazan in the presence of NADH, and effectively photosensitized L-tryptophan photooxidation. Photobleaching studies showed greater stability in biological media than in organic solvents, although the Zn(II) conjugate photodegraded faster than PEI-PPIX. Photodynamic inactivation assays against Staphylococcus aureus demonstrated that both conjugates achieved > 6 log (> 99.9999%) reductions in planktonic cultures at micromolar concentrations after short irradiation times under white light. Furthermore, treatment of Cucumis sativus (cucumber) surfaces contaminated with S. aureus using 2.5 nmol of these conjugates, followed by 30 min of irradiation, resulted in a reduction of more than 99.99% in bacterial viability. The elimination of S. aureus on cucumber mesocarp surfaces was further assessed by detecting individual damaged cells after photodynamic treatment. These findings highlight PEI-PPIX and PEI-ZnPPIX as promising photodynamic agents for enhancing microbial safety and potentially extending the shelf life of fresh produce.