Solubility and toxic effects of AgNPs and Ag+ ions in aquatic plant Lemna minor

Engineered nanotechnology growth has enabled the use of nanoparticles in several areas, such as cosmetics and food industry. To ensure that these nanoparticles are safe to our health and the environment, the high mobility and reactivity of nanoparticles call for the development of studies to assess their risks. The aquatic plant Lemna minor is relevant to many aquatic environments, including lakes, streams, effluents, rain, and sediment, and can acts as environmental indicator of pollution. This study investigates the solubility and toxicity of Ag nanoparticles (AgNPs) and Ag+ ions in the model aquatic plants Lemna minor, which is a genera of Duckweed plant. The AgNPs were synthesized by the Quin et al. [1] procedure in different sizes (30, 90 and 120 nm). A TEM (transmition electronic microscope) furnished the images for characterization of the nanoparticles size. All determinations of Ag were made using flame atomic absorption spectroscopy (FAAS). AgNPs were added in different concentration (10, 25 and 50 mg L-1) to the culture of the plant. The number of leaves was counted every week during a month (n=5); the growth rate showed that the toxicity of AgNPs with 30 nm is higher than the other sizes. In comparison with the other sizes (90 and 120 nm), the AgNPs measuring 30 nm releases about 2 times more Ag+, the toxic form. Cloud point was used to difference AgNPs and silver ions. Applying PCA (principal components analysis) Scores plot and PCA loadings plots in growth rate data, it was demonstrated that the concentration of 50 mg L-1 causes high mortality independent of AgNPs size, while the concentrations of 10 and 25 mg L-1 affect the growth rate in sizes of 90 and 120 nm, respectively. In order to evaluate the difference of AgNPs and Ag+ toxicity, in the sample with 3.5, 5, 6.5, 8 and 9.5 mg L-1 of AgNO3 the number of live leaves was counted after 1, 3, 5, 7, 15 and 30 days of exposure respectively according to the results from of Ag+ release from cloud point extraction, and in samples with AgNPs the number of leaves was counted in the same periods. It has been observed a difference of the toxicity between the AgNPs and AgNO3: after 30 days all the leaves dead in sample with AgNO3, on the other hand, 8 leaves were alive after the same period in sample with AgNPs. These results can stimulate studies in this area by giving a better understanding of the AgNPs effects in plant uptake and its environmental effects.