Light induced toxicity of silver nanoparticles produced by laser ablation

Full text: Silver is one of the most studied metals in the biomedical field and has been used for centuries in various forms as an antimicrobial agent dating back to Chaldean dynasty c.a. 4,000 B.C.E. Silver nanoparticles (AgNP) are of particular interest due to their physical properties, which have been shown to strongly influence antimicrobial activity. Our lab used laser ablation in liquid to synthesize AgNPs, giving us the ability to make "bare" particles, free from precursors that are typically associated with chemical synthetic methods. A Nd:YAG laser at the fundamental wavelength (λ=1064 nm) was used to ablate a pure silver target immersed in a 60 mM sodium dodecyl sulfate solution. As some pathogenic bacteria form resistances to antibiotics, understanding the mechanisms behind AgNPs antimicrobial activity is paramount. A major problem that is preventing the universal application of AgNPs is their possible toxicity to higher organisms. Our current work supports the hypothesis that colloidal suspensions of silver nanoparticles produced by laser ablation, when irradiated with visible light, release a higher concentration of silver ions. Currently the consensus is that AgNPs are toxic due to their ability to release Ag⁺ ions. Therefore, an increase in ion release will cause an increase in antimicrobial activity that may allow for lower levels of AgNPs required when treating bacterial infections, thus, limiting off-target toxicity. Our group showed that ablating a pure silver target immersed in a SDS solution produced very concentrated colloidal silver nanoparticle solutions, with hydrodynamic radii of approximately 40 nm. We also showed that this antimicrobial activity was greatly enhanced when the solutions were exposed to visible light at power of approximately 0.7 W. Using the optical properties of the DZ-Ag⁺ complex, it was shown that particles exposed to light release ions at a much higher rate relative to particles kept in the dark. The reduced antimicrobial activity of the AgNP in the presence of L-cysteine showed that ions play a major role in the antibacterial effectiveness of AgNPs. As stated above we hope that paired with light, these silver nanoparticles can be employed at a low enough concentration that no adverse effects to mammalian cells will occur, or at the very least the mammalian cell death will remain localized. (author)