The spread of infectious diseases is closely linked to the presence of pathogenic microorganisms in contaminated water sources. Therefore, the development of cost-effective and biocompatible methods for microbial eradication is of great importance. In this study, we synthesized magnetic nanoparticles (MNPs) of magnetite covalently functionalized with the photosensitizer 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF₂₀). These were further modified with polyethyleneimine (MNP-PEI) or spermine (MNP-SPM), which confer positive surface charges, enhancing interaction with bacterial membranes.

All nanoconjugates showed characteristic porphyrin absorption and fluorescence in aqueous media and were able to generate reactive oxygen species under irradiation. In vitro photodynamic inactivation (PDI) studies were performed on Staphylococcus aureus and Escherichia coli. Both MNPs were non-toxic in the dark and exhibited strong antibacterial activity upon light exposure. Complete eradication of S. aureus (>99.999%) was achieved after 30 min of irradiation in the presence of both MNP conjugates (2 µM), while MNP-PEI at 5 µM concentration also reduced E. coli viability by 99.5% under the same conditions.

Recycling experiments showed effective reuse. Both MNPs conjugates maintained antibacterial activity over three multiple PDI cycles in S. aureus cells, whereas MNP-PEI managed to reduce E. coli population by 99.7% in the first PDI cycle and 97.5% in the second cycle. Furthermore, in a continuous-flow spiral tube reactor, both MNPs (5 µM) eradicated ~99.7% of S. aureus, and MNP-PEI (10 µM) reduced E. coli viability by 99.8% after 30 min of irradiation. These results highlight the potential of these photoactive MNPs as recyclable, light-activated materials for scalable bacterial water disinfection.