Nanocrystalline Silver Supported on Activated Carbon Matrix from Hydrosol: Antibacterial Mechanism Under Prolonged Incubation Conditions

Nanocrystalline silver-supported activated carbon (AC) was fabricated by directly loading silver nanoparticles into the porous AC matrix from a preformed nanosilver hydrosol. Silver-AC composites were also synthesized using a conventional thermal impregnation method. While XRD calculation indicated the presence of Ag crystallites in nanometer range, silver nanoparticle hydrosol-treated AC having the finest crystallite size CS (<14.4 nm), SEM images clearly revealed that Ag crystals coalesced significantly with increasing temperature resulting in much larger particle size in thermally impregnated silver-AC composities. To clarify the antibacterial mechanism of silver nanoparticles impregnated into AC under prolonged incubation conditions the antibacterial activity was investigated against Gram-negative Escherichia coli. The kinetics of bacterial inactivation, in presence of hydroxyl radical (OH) scavengers, and superoxide anion radical (O⁻ ₂) inducer suggest the contribution of the reactive oxygen species (ROS) to antibacterial effect. However, these ROS scavengers did not show any inhibition of bactericidal activity after ∼1 h, suggesting that generated ROS are responsible for E. coli inactivation only during the initial 1 h of the incubation time. This study clearly indicates the plausible implication of eluted Ag⁺ as major lethal species responsible for the E. coli inactivation over extended process time. The antibacterial process was found to be highly promoted at higher temperature which was ascribed to the enhanced ROS formation and Ag⁺ elution at higher temperature. SEM images revealed considerable differences in the morphology of E. coli cells contacting with the virgin AC and that contacting with silver-supported AC. The strong antibacterial ability of formaldehyde-modified silver-supported AC further provided the indirect evidences for catalytic oxidation by ROS, and for the synergistic antibacterial effects of nanocrystalline silver and adsorbed formaldehyde. Comparison of the antibacterial activities of the silver-supported materials prepared by silver colloid deposition and by conventional thermal impregnation technique indicates that former is more efficient in controlling microorganism.