Y. Dan, H. Shi, X. Ma, W. Zhang, K. Liu, C. Stephan
Missouri University of Science and Technology,
Keywords: single particle-ICP-MS, enzymatic digestion, plant uptake of CeO2 nanoparticles, biotransformation, food safety, nanoparticle characterization
Summary:As one of the most commonly encountered engineered nanoparticles (ENPs) in industrial applications, cerium oxide nanoparticles (CeO2 NPs) have attracted significant attention on their environmental fate and transport. Plant uptake and accumulation of CeO2 NPs have been investigated as a potential pathway for human exposure to these NPs recently. However, the investigation is frequently hampered by the insufficiency of current analytical technologies to determine the quantity and properties of cerium-related compounds (particulate cerium, dissolved cerium…) in plant tissues. Therefore, it is important to have a technology that can detect particulate cerium and dissolved cerium to elucidate the uptake mechanism. A new capability to operate the ICP-MS in single particle mode (SP-ICP-MS) has emerged recently and shows a great potential to fill this gap. In SP-ICP-MS, particulate cerium (CeO2 NPs) will be detected as pulse/non-continuous signals and at the same time dissolved cerium will be detected as continuous signal. To fully harness SP-ICP-MS to characterize CeO2 NPs in plant tissue, the CeO2 NPs have to be extracted from those tissues without dissolving them before subjected to SP-ICP-MS analysis. This challenge is addressed using enzymatic digestion to extract CeO2 NPs within plant tissues. Macerozyme R-10 enzyme, a mixture of cellulase, hemicellulase and pectinase, was used to digest the plant tissues to extract the CeO2 NPs in this study and experimental results show that the enzymatic digestion process chemically broke down plant tissues to release the CeO2 NPs, but did not dissolve the CeO2 NPs. We successfully developed an enzymatic digestion-SP-ICP-MS technology to simultaneously detect CeO2 NPs size and size distribution, particle concentration, and dissolved cerium in four different plant species. The SP-ICP-MS was validated that it can accurately size CeO2 NPs and measure dissolved cerium in plant matrices. Tomato, cucumber, pumpkin and soybean were germinated and grown hydroponically, following an exposure to 7 mg/L of 30-50 nm CeO2 NPs for 19 days. After exposure the plant shoots were cut, homogenized and enzymatically digested for SP-ICP-MS analysis. The results indicated that both particulate NPs and dissolved Ce present in the plant shoots. The presence of dissolved cerium in plant shoots after exposure to CeO2 NPs hydroponically suggested that the plants might uptake CeO2 NPs through roots and translocated to plant shoots, where CeO2 NPs aggregation and possibly dissolution could occur. Differences on CeO2 NPs uptake and accumulation have been noticed between different plant species, requiring further investigation on the mechanisms. With these new advancements, several questions remain and future efforts will concentrate on determining the extent of CeO2 NPs dissolution on plant root surface and in plant tissues to understand why the uptake and accumulation differ among different plant species.