Environmental effects of nanosilver: impact on castor seed germination, seedling growth, and plant physiology

The rapid advancement of nanotechnology has created a vast array of engineered nanomaterials (ENMs) which have unique physical (size, shape, crystallinity, surface charge) and chemical (surface coating, elemental composition and solubility) attributes. These physicochemical properties of ENMs can produce chemical conditions to induce a pro-oxidant environment in the cells, causing an imbalanced cellular energy system dependent on redox potential and thereby leading to adverse biological consequences, ranging from the initiation of inflammatory pathways through to cell death. The present study was designed to evaluate size-dependent cellular interactions of known biologically active silver nanoparticles (NPs, Ag-15 nm, Ag-30 nm, and Ag-55 nm). Alveolar macrophages provide the first defense and were studied for their potential role in initiating oxidative stress. Cell exposure produced morphologically abnormal sizes and adherence characteristics with significant NP uptake at high doses after 24 h. Toxicity evaluations using mitochondrial and cell membrane viability along with reactive oxygen species (ROS) were performed. After 24 h of exposure, viability metrics significantly decreased with increasing dose (10-75 microg/mL) of Ag-15 nm and Ag-30 nm NPs. A more than 10-fold increase of ROS levels in cells exposed to 50 microg/mL Ag-15 nm suggests that the cytotoxicity of Ag-15 nm is likely to be mediated through oxidative stress. In addition, activation of the release of traditional inflammatory mediators were examined by measuring levels of cytokines/chemokines, including tumor necrosis factor (TNF-alpha), macrophage inhibitory protein (MIP-2), and interleukin-6 (IL-6), released into the culture media. After 24 h of exposure to Ag-15 nm nanoparticles, a significant inflammatory response was observed by the release of TNF-alpha, MIP-2, and IL-1beta. However, there was no detectable level of IL-6 upon exposure to silver nanoparticles. In summary, a size-dependent toxicity was produced by silver nanoparticles, and one predominant mechanism of toxicity was found to be largely mediated through oxidative stress.

Nanomaterial properties differ from those bulk materials of the same composition, allowing them to execute novel activities. A possible downside of these capabilities is harmful interactions with biological systems, with the potential to generate toxicity. An approach to assess the safety of nanomaterials is urgently required. We compared the cellular effects of ambient ultrafine particles with manufactured titanium dioxide (TiO2), carbon black, fullerol, and polystyrene (PS) nanoparticles (NPs). The study was conducted in a phagocytic cell line (RAW 264.7) that is representative of a lung target for NPs. Physicochemical characterization of the NPs showed a dramatic change in their state of aggregation, dispersibility, and charge during transfer from a buffered aqueous solution to cell culture medium. Particles differed with respect to cellular uptake, subcellular localization, and ability to catalyze the production of reactive oxygen species (ROS) under biotic and abiotic conditions. Spontaneous ROS production was compared by using an ROS quencher (furfuryl alcohol) as well as an NADPH peroxidase bioelectrode platform. Among the particles tested, ambient ultrafine particles (UFPs) and cationic PS nanospheres were capable of inducing cellular ROS production, GSH depletion, and toxic oxidative stress. This toxicity involves mitochondrial injury through increased calcium uptake and structural organellar damage. Although active under abiotic conditions, TiO2 and fullerol did not induce toxic oxidative stress. While increased TNF-alpha production could be seen to accompany UFP-induced oxidant injury, cationic PS nanospheres induced mitochondrial damage and cell death without inflammation. In summary, we demonstrate that ROS generation and oxidative stress are a valid test paradigm to compare NP toxicity. Although not all materials have electronic configurations or surface properties to allow spontaneous ROS generation, particle interactions with cellular components are capable of generating oxidative stress.

Plants need to be included to develop a comprehensive toxicity profile for nanoparticles. Effects of five types of nanoparticles (multi-walled carbon nanotube, aluminum, alumina, zinc, and zinc oxide) on seed germination and root growth of six higher plant species (radish, rape, ryegrass, lettuce, corn, and cucumber) were investigated. Seed germination was not affected except for the inhibition of nanoscale zinc (nano-Zn) on ryegrass and zinc oxide (nano-ZnO) on corn at 2000 mg/L. Inhibition on root growth varied greatly among nanoparticles and plants. Suspensions of 2000 mg/L nano-Zn or nano-ZnO practically terminated root elongation of the tested plant species. Fifty percent inhibitory concentrations (IC50) of nano-Zn and nano-ZnO were estimated to be near 50mg/L for radish, and about 20mg/L for rape and ryegrass. The inhibition occurred during the seed incubation process rather than seed soaking stage. These results are significant in terms of use and disposal of engineered nanoparticles.