Addition of Fe₃O₄ on Carbon Nitride Moves the ROS Formation from HO Radicals to Singlet Oxygen Leading to an Enhanced and Selective Disinfecting Action

Bacterial contamination poses serious challenges for public health in several scenarios, such as in developing antimicrobial surfaces for biomedical applications and clean water availability. To face this, in the current work we have incorporated Fe₃O₄ domains onto the graphitic carbon nitride framework (g-C₃N₄). Therefore, a series of iron-doped nanocomposites (g-C₃N₄, g-C₃N₄-25, g-C₃N₄-50, g-C₃N₄-100, and g-C₃N₄-500) were synthesized by controlling the adsorption of Fe on bare g-C₃N₄. These materials were rigorously characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), <i>Z</i>-potential (PZC), fluorescence, electrochemical impedance, and X-ray photoelectron spectroscopy (XPS), confirming the successful formation of Fe₃O₄ nanoparticles and the creation of abundant surface defects. Photocatalytic experiments using ciprofloxacin (1-cyclopropyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-quinoline-3-carboxylic acid) as probe compound, under UV-Vis irradiation, highlighted the g-C₃N₄-25 material as the one having the higher photocatalytic activity. Additional tests using chemical scavengers showed a shift from hydroxyl-radical-driven pathways in bare g-C₃N₄ to the more selective singlet-oxygen in the Fe-modified material. <i>E. coli</i> disinfection under indoor light produced a 6.0 log₁₀ CFU/mL reduction with g-C₃N₄-25, compared to only 2.8 log₁₀ with pristine g-C₃N₄. Comparative experiments with Co- and Cu-doped analogues further showed superior kinetics and ROS yields of the iron material. Finally, the best-performing Fe-g-C₃N₄ catalyst was embedded in an alginate matrix to produce a self-disinfecting surface that maintained a 5.9 ± 0.3 log₁₀ CFU/mL decrease in <i>E. coli</i> after five consecutives 24 h irradiation cycles. These results position Fe-doped g-C₃N₄ as a versatile platform for the sustainable photocatalytic removal of contaminants and durable antimicrobial coatings.