Silver nanoparticles: partial oxidation and antibacterial activities

  • Chun-Nam Lok
  • Chi-Ming Ho
  • Rong Chen
  • Qing-Yu He
  • Wing-Yiu Yu
  • Hongzhe Sun
  • Paul Kwong-Hang Tam
  • Jen-Fu ChiuEmail author
  • Chi-Ming CheEmail author
Original Paper


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.


Silver nanoparticles Silver ions Antibacterial agents Surface plasmon resonance absorption Oxidation 



Bovine serum albumin


N-(2-Hydroxyethyl)piperazine-N′-ethanesulfonic acid


Minimum inhibitory concentration


Silver nanoparticles


Surface plasmon resonance



We thank Simon Silver, Xian-Zhi Li, and Keith Poole for providing the bacterial strains, and Rory Watt for his help in editing the manuscript. This work was supported by the Area of Excellence Scheme (AoE/P-10/01) established under the University Grants Committee of the Hong Kong Special Administrative Region, People’s Republic of China, the Strategic Research Themes on Bionanotechnology, and the Research Support Programs (to C.M.C., J.F.C., and C.N.L) and the University of Hong Kong.

Supplementary material


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Copyright information

© SBIC 2007

Authors and Affiliations

  • Chun-Nam Lok
    • 1
    • 3
  • Chi-Ming Ho
    • 1
    • 2
  • Rong Chen
    • 1
    • 2
  • Qing-Yu He
    • 1
    • 2
  • Wing-Yiu Yu
    • 1
    • 2
  • Hongzhe Sun
    • 1
    • 2
  • Paul Kwong-Hang Tam
    • 4
  • Jen-Fu Chiu
    • 1
    • 3
    Email author
  • Chi-Ming Che
    • 1
    • 2
    Email author
  1. 1.The Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and SynthesisThe University of Hong KongHong KongChina
  2. 2.Department of ChemistryThe University of Hong KongHong KongChina
  3. 3.Department of AnatomyThe University of Hong KongHong KongChina
  4. 4.Department of SurgeryThe University of Hong KongHong KongChina

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