Light induced toxicity reduction of silver nanoparticles to Tetrahymena Pyriformis: effect of particle size.

As a result of the extensive application of silver nanoparticles (AgNPs), their potential hazards, once they are released into the natural environment, are of great concern to people. Since silver is very sensitive to light, the toxicity of AgNPs released into the natural environment will be inevitably affected by light. The aim of this study was to investigate the relationship between light, toxicity, and particle size of AgNPs and deduce the possible mechanism of any interaction. Our study revealed that there was negative correlation between the particle size and the toxicity: small AgNPs (5-10nm) had higher toxicity than large AgNPs (15-25nm) to Tetrahymena pyriformis (T. pyriformis) under dark condition. Comparing the size dependent AgNPs toxicity under dark and light conditions, the effect of light to size dependent AgNPs toxicity was ascertained. The results indicated that AgNPs toxicity was decreased by light and the most important discovery was that the change of size dependent AgNPs toxicity had significant difference under light irradiation. The decrease of small AgNPs toxicity induced by light was more notable than large AgNPs. The decreased level of cell toxicity for small AgNPs was 32±0.7%, whereas it was only 10.6±5.2% for large AgNPs kept 24h under light irradiation. The further investigation indicated that the above changes induced by light can be attributed to the decrease of released silver ions, particle growth and aggregation of AgNPs under light irradiation. The obtained results showed that the light irradiation can promote the rapid growth of small AgNPs and result in the obvious increase of particle size and serious aggregation. These phenomena would induce the decrease of silver ions released from small AgNPs, which is responsible for the remarkable decrease of toxicity for small AgNPs.