Silica Monoliths with Embedded Silver Nanoparticles for Safe Point-of-Use Water Disinfection that Avoids Nanopollution

Abstract

Macroscopic silica monoliths, AgNP/SiO2 (d ∼ 1 cm, mass ∼ 100 mg), were synthesized with embedded silver nanoparticles (AgNPs, d = 36 nm). Differently from the published materials based on AgNPs intended for disinfecting water, in AgNP/SiO2, the AgNPs are not on the material surface but incorporated in the body of the monoliths, with the aim of avoiding the possible release to water even under mechanical solicitations. However, the synthesized silica materials are sufficiently porous to allow the diffusion of H2O, O2, and H+, obtaining the slow, sustained oxidation of the AgNPs and the release of trace quantities of Ag+. 1×, 10×, and 100× AgNP/SiO2 monoliths were prepared, corresponding to 0.01, 0.1, and 1.0% w/w Ag content, which release Ag+ in the 0.008−0.064 ppm range (after 24 h, 20 °C) when the monoliths are immersed in water. Increased Ag+ release is observed at 37 °C (0.17 ppm, 24 h) and in acidic-buffered pH, with 0.62 ppm at pH 4.0 after 24 h and 0.12 ppm at pH 5.5 after 24 h. The microbicidal effect of the monoliths was verified by immersion in planktonic suspensions of Escherichia coli and Staphylococcus aureus. Reduction of cell viability was observed after 5 h, which significantly increased at 24 h, correlating with the Ag concentration in the monoliths: 98.88, 99.61, and 99.78% viability reduction was found in the case of S. aureus with monoliths at 0.01, 0.1, and 1.0 % w/w Ag, respectively, and 99.96, 99.98, and 99.99% viability reduction in the case of E. coli with 0.01, 0.1, and 1.0% w/w Ag, respectively. Moreover, the AgNP/SiO2 monoliths also displayed similar antibacterial efficacy when sealed in small paper sachets, suggesting their use as handy point-of-use immersion devices, similar to tea bags.