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), Z-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. E. coli 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 E. coli 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.