Tannic acid-induced interfacial ligand-to-metal charge transfer and the phase transformation of Fe₃O₄ nanoparticles for the photothermal bacteria destruction

An iron oxide (Fe₃O₄)-mediated photothermal treatment has been revealed as the next generation of noninvasive and nontoxic theranostic nanoagents compared with other inorganic nanoparticles. Nevertheless, iron oxide with NIR-mediated activity is limited by its inability to be mass produced and its long-lasting photon-to-thermal conversion. Herein, we develop using a green reagent, tannic acid (TNA), which assisted hydrothermal reaction to generate black Fe₃O₄ nanoparticles by using commercially available nanoptical γ-Fe₂O₃ NPs as a starting material. The phase transformation from γ-Fe₂O₃ to the reduced form of Fe₃O₄ is assisted by TNA in the solution phase. Based on the formation of the interfacial TNA-Fe chelation and the delicate phase transformation from γ-Fe₂O₃ to Fe₃O₄ structures, the colloidal black Fe₃O₄ nanoparticles exhibit broad absorption that covers the visible and NIR wavelengths. Specific interfacial ligand-to-metal charge transfers between the TNA and iron ions at the surface of iron oxide nanoparticles, improves the absorbance and leads to the highest photon-to-thermal conversion (η = 35.7%) at 808 nm compared with other iron oxide nanomaterials. After modifying d-mannose (MA) onto the surface of the Fe₃O₄@TNA nanoparticles, the local heat can efficiently transfer from the Fe₃O₄@TNA nanoparticles to the vicinity of the bacterial FimH adhesion molecule, causing extensive photothermal injury to O157:H7 and ESBL strain bacteria with over 99% cell death at 200 ppm_[Fe] with 808 nm light at 2.25 mW/cm². Based on its robust photostability, Fe₃O₄@TNA@MA shows photothermal bactericidal recyclability through magnetic collection, adhesion, and photothermal processes.