Insights into the disinfection enhancement of homojunction g-C3N4 photocatalyst from charge transfer regulation and cell-surface attachment

Photocatalytic bacterial inactivation has emerged as a promising approach for water disinfection. However, its efficacy is limited due to photocatalyst electron-hole (e+/h+) recombination and poor contact of photogenerated reactive oxygen species (ROS) with bacteria. Tremendous efforts have been made to improve photogenerated carrier utilization of photocatalyst, however, less attention has been given to the interaction of photocatalysts with bacteria, which is crucial for the effectiveness of inactivation because of the short half-life of ROS. Here, a homojunction g-C3N4 (HJ-CN) photocatalyst was rationally designed by in-situ growing g-C3N4 quantum dots (gCNQDs) on g-C3N4 nanosheets (g-CN). Compared with g-CN, the inactivation efficacy of the HJ-CN against methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli was increased by 3.79 and 3.23 times, respectively. It reduced MRSA and E. coli by 5.3 log (2 h-irradiation) and 3.1 log (4-h irradiation), respectively. The enhanced bactericidal activity of HJ-CN originated from the heterojunction engineering, which promoted the separation of e+/h+ pairs, resulting in a strong redox capacity. Interestingly, HJ-CN demonstrated specific absorption for MRSA, contributing to its superior effectiveness in inactivating MRSA than E. coli. The adhesion of MRSA to HJ-CN not only reduced the distance for photogenerated ROS to migrate and but enabled photogenerated h+ to better attack bacteria. By investigating the differences in surface charge, cell structure, and hydrophobicity of four bacteria (MRSA, Listeria monocytogenes, E. coli, and multidrug-resistant Salmonella Typhimurium), it was found that hydrophobicity played a significant role in driving the selective adhesion of MRSA to HJ-CN. This work offers valuable insights into the future development of metal-free photocatalysts with high antimicrobial efficacy.