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The antimicrobial capability and recyclability of two conjugates that combines the versatility of iron oxide magnetic nanoparticles (MNPs) with the high photosensitizing proficiency of boron-dipyrromethene (BODIPY) dyes are assessed. By a relatively simple synthetic pathway, two conjugates were obtained. The first one, MNP-B1, contains a highly fluorescent dye for bioimaging and suitable inactivating properties. The other one, MNP-B2, is optimized to improve the production of cytotoxic reactive oxygen species (ROS) by incorporating heavy atoms in the BODIPY core. In vitro experiments in bacterial cell suspensions and at the single bacterium level reveal that both conjugates can inactivate either Gram-positive (methicillin-resistant Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. By means of fluorescence microscopy, not only cellular uptake of the conjugates but also recyclability and sustained performance over the cycles of photodynamic inactivation (PDI) are demonstrated. This is the first time that MNPs functionalized with BODIPY dyes are utilized to obtain fluorescent images of bacterial cells and photoinactivate pathogens.

A nanoplatform concept was developed to synthesize accessible photoactive magnetic nanoparticles (MNPs) of Fe 3 O 4 coated with silica. This approach was based on the covalent binding of 5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin (TPPF 20 ) to aminopropyl-grafted MNPs by nucleophilic aromatic substitution reaction (S N Ar) to obtain conjugate MNP− P1. After in situ modification, the remaining pentafluorophenyl groups of TPPF 20 attached to MNPs were substituted by dimethylaminoethoxy groups to form MNP−P2. The basic amine group of these conjugates can be protonated in aqueous media. In addition, MNP−P1 and MNP−P2 were intrinsically charged to produce cationic conjugates MNP + −P1 and MNP + −P2 + by methylation. All of them were easily purified by magnetic decantation in high yields. The average size of the MNPs was ∼15 nm, and the main difference between these conjugates was the greater coating with positive charges of MNP + −P2 + , as shown by the zeta potential values. Absorption spectra exhibited the Soret and Q bands characteristic of TPPF 20 linked to MNPs. Furthermore, these conjugates showed red fluorescence emission of porphyrin with quantum yields of 0.011−0.036. The photodynamic effect sensitized by the conjugates indicated the efficient formation of singlet molecular oxygen in different media, reaching quantum yield values of 0.17−0.34 in N,N-dimethylformamide. The photodynamic activity of the conjugates was evaluated to inactivate the Gram-positive bacteria Staphylococcus aureus, the Gram-negative bacteria Escherichia coli, and the yeast Candida albicans. The modified cationic MNP + −P2 + was the most effective conjugate for photodynamic inactivation (PDI) of microorganisms. Binding of this conjugate to bacteria and photoinactivation capability was checked by means of fluorescence microscopy. Also, sustainable use by recycling was determined after three PDI treatments. Therefore, this methodology is a suitable scaffold for the in situ modification of conjugates, and in particular, MNP + −P2 + represents a useful photodynamic active material to eradicate microorganisms.