Bactericidal Mechanisms Revealed for Rapid Water Disinfection by Superabsorbent Cryogels Decorated with Silver Nanoparticles

The authors have recently reported the fabrication of superabsorbent cryogels decorated with silver nanoparticles (PSA/AgNP cryogels) that demonstrate rapid water disinfection. This paper provides a systematic elucidation of the bactericidal mechanisms of AgNPs (silver nanoparticles), both generally and in the specific context of cryogels. Direct contact between the PSA/AgNP cryogel interface and the bacterial cells is required to accomplish disinfection. Specifically, the disinfection efficacy is closely correlated to the cell-bound Ag concentration, which constitutes >90% of the Ag released. Cells exposed to PSA/AgNP cryogels show a significant depletion of intracellular adenosine triphosphate (ATP) content and cell-membrane lesions. A positive ROS (reactive oxygen species) scavenging test confirms the involvement of ROS (·O₂^–, H₂O₂, and ·OH) in the bactericidal mechanism. Furthermore, exposed bacterial cells show an enhanced level of thiobarbituric acid reactive substances, indicating the occurrence of cell-membrane peroxidation mediated by ROS. In addition, this study reveals that both Ag⁺ and Ag⁰ are involved in the bactericidal mechanism of AgNPs via tests conducted using PSA cryogels with bound Ag⁺ ions (or PSA/Ag⁺ cryogels without reducing Ag⁺ to Ag⁰). Significantly, bacterial cells exposed to PSA/Ag⁺ cryogels did not show any cell-membrane damage even though the former had a higher cell-bound Ag concentration than that of the PSA/AgNP cryogels, thus indicating the differential action of Ag⁺ and Ag⁰.