Uptake and Translocation of ZnO Nanoparticles in Rice Tissues Studied by Single Particle-ICP-MS

Foliar spay of zinc oxide nanoparticles (ZnO-NPs) is a common practice to provide the necessary Zn element for crops, which may also be a potential source for human expose to NPs with possible adverse health effects. Presently little is known about the forms and distributions of ZnO-NPs in plants after it is applied by foliar spay due to the lack of reliable quantification method. In this work, a complexed enzymatic extractant combined with single particle-ICP-MS system (SP-ICP-MS) was developed for simultaneously determining the concentrations and size distributions of ZnO-NPs in plant tissues. The measuring conditions were optimized and the method was applied to quantify ZnO-NPs uptake and distributions in rice plants after foliar application of ZnONPs. The results showed that the established method could well quantify the NP concentration in plant tissues with a detection limit of 16 nm; foliar spray of ZnO-NPs increased the Zn concentration (22.66. 0.51 mg center dot kg(-1)) in rice grains by 447%, which was still within the limits of the food safety standard of China (NY 861-2004, 50 mg center dot kg(-1) for Zn). ZnO-NPs larger than 60 nm cannot be directly absorbed by the leaves and may instead fall into the soil. ZnONPs with a size between 30-60 nm in the leaves could be retained due to the hindering effect of the cellulose-hemicellulose network. The smaller ZnO-NPs with size less than 30 nm could only be detected in stems but not in the grains, indicating that ZnO-NPs were dissociated or resolved as ionic form before they were transported into grains. The xylem and phloem were believed to be the organs responsible for the translocation and transformation of ZnO-NPs after being uptake by rice leaves. The study demonstrated the feasibility of the proposed method in characterizing NP behaviours in plants and proved the effectiveness and safety of foliar application of ZnO-NPs to rice.