title: Silver nanoparticles: partial oxidation and antibacterial activities.
abstract: The physical and chemical properties of silver nanoparticles that are responsible for their antimicrobial activities have been studied with spherical silver nanoparticles (average diameter approximately 9 nm) synthesized by the borohydride reduction of Ag+ ions, in relation to their sensitivity to oxidation, activities towards silver-resistant bacteria, size-dependent activities, and dispersal in electrolytic solutions. Partially (surface) oxidized silver nanoparticles have antibacterial activities, but zero-valent nanoparticles do not. The levels of chemisorbed Ag+ that form on the particle's surface, as revealed by changes in the surface plasmon resonance absorption during oxidation and reduction, correlate well with the observed antibacterial activities. Silver nanoparticles, like Ag+ in the form of AgNO3 solution, are tolerated by the bacteria strains resistant to Ag+. The antibacterial activities of silver nanoparticles are related to their size, with the smaller particles having higher activities on the basis of equivalent silver mass content. The silver nanoparticles aggregate in media with a high electrolyte content, resulting in a loss of antibacterial activities. However, complexation with albumin can stabilize the silver nanoparticles against aggregation, leading to a retention of the antibacterial activities. Taken together, the results show that the antibacterial activities of silver nanoparticles are dependent on chemisorbed Ag+, which is readily formed owing to extreme sensitivity to oxygen. The antibacterial activities of silver nanoparticles are dependent on optimally displayed oxidized surfaces, which are present in well-dispersed suspensions.
authors.affiliation: The Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
authors.forename: Chun-Nam
authors.initials: CN
authors.lastname: Lok
authors.forename: Chi-Ming
authors.initials: CM
authors.lastname: Ho
authors.forename: Rong
authors.initials: R
authors.lastname: Chen
authors.forename: Qing-Yu
authors.initials: QY
authors.lastname: He
authors.forename: Wing-Yiu
authors.initials: WY
authors.lastname: Yu
authors.forename: Hongzhe
authors.initials: H
authors.lastname: Sun
authors.forename: Paul Kwong-Hang
authors.initials: PK
authors.lastname: Tam
authors.forename: Jen-Fu
authors.initials: JF
authors.lastname: Chiu
authors.forename: Chi-Ming
authors.initials: CM
authors.lastname: Che
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry* / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects; D053768:Metal Nanoparticles / Q000737:chemistry*; D010084:Oxidation-Reduction; D012834:Silver / Q000737:chemistry* / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D012834:Silver