Magnetic Nanoplatforms for <i>in Situ</i> Modification of Macromolecules: Synthesis, Characterization, and Photoinactivating Power of Cationic Nanoiman–Porphyrin Conjugates

Abstract

A nanoplatform concept was developed to synthesize accessible photoactive magnetic nanoparticles (MNPs) of Fe₃O₄ coated with silica. This approach was based on the covalent binding of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF₂₀) to aminopropyl-grafted MNPs by nucleophilic aromatic substitution reaction (S_NAr) to obtain conjugate MNP-P1. After <i>in situ</i> modification, the remaining pentafluorophenyl groups of TPPF₂₀ 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 TPPF₂₀ 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 <i>N</i>,<i>N</i>-dimethylformamide. The photodynamic activity of the conjugates was evaluated to inactivate the Gram-positive bacteria <i>Staphylococcus aureus</i>, the Gram-negative bacteria <i>Escherichia coli,</i> and the yeast <i>Candida albicans</i>. 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 <i>in situ</i> modification of conjugates, and in particular, MNP⁺-P2⁺ represents a useful photodynamic active material to eradicate microorganisms.