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.