Rapid Screening of Aquatic Toxicity of Several Metal-Based Nanoparticles Using the Metplate™ Bioassay

Current understanding of potential toxicity of engineered nanomaterials to aquatic microorganisms is limited for risk assessment and management. Here we evaluate if the MetPLATE™ test can be used as an effective and rapid screening tool to test for potential aquatic toxicity of various metal-based nanoparticles (NPs). The MetPLATE bioassay is a heavy metal sensitive test based on β-galactosidase activity in Escherichia coli . Five different types of metal-based NPs were screened for toxicity: (1) citrate coated nAg (Citrate-nanosilver), (2) polyvinylpyrrolidone coated nAg (PVP-nAg), (3) uncoated nZnO, (4) uncoated nTiO ₂ and (5) 1-Octadecylamine coated CdSe Quantum Dots (CdSe QDs); and compared with their corresponding ionic salt toxicity. Citrate-nAg was further fractionated into clean Citrate-nAg, unclean Citrate-nAg and permeate using a tangential flow filtration (TFF) system to eliminate residual ions and impurities from the stock Citrate-nAg suspension and also to differentiate between ionic- versus nano-specific toxicity. Our results showed that nAg, nZnO and CdSe QDs were less toxic than their corresponding ionic salts tested, while nano- or ionic form of TiO ₂ was not toxic as high as 2.5 g L ^{− 1} to the MetPLATE™ bacteria. Although coating-dependent toxicity was noticeable between two types of Ag NPs evaluated, particle size and surface charge were not adequate to explain the observed toxicity; hence, the toxicity appeared to be material-specific. Overall, the toxicity followed the trend: CdCl ₂ > AgNO ₃ > PVP-nAg > unclean Citrate-nAg > clean Citrate-nAg > ZnSO ₄ > nZnO > CdSe QDs > nTiO ₂ /TiO ₂ . These results indicate that an evaluation of β-galactosidase inhibition in MetPLATE™ E. coli can be an important consideration for rapid screening of metal-based NP toxicity, and should facilitate ecological risk assessment of these emerging contaminants.