Spectroscopic probe to contribution of physicochemical transformations in the toxicity of aged ZnO NPs to Chlorella vulgaris: new insight into the variation of toxicity of ZnO NPs under aging process

Abstract

Zinc oxide nanoparticles (ZnO NPs) are one of the most abundantly applied nanomaterials in nanotechnology-based industries and they may cause unexpected environmental and health risks with their physicochemical transformations in the environment. Currently, there is still a lack of the in-depth understanding of the toxicity of aged ZnO NPs to aquatic organisms, particularly demanding quantitative analysis of the physicochemical transformations to distinguish their contributions in the toxicity assessment. For this purpose, therefore, we initiated the study of the toxicity of aged ZnO NPs to the model aquatic microalga, i.e. Chlorella vulgaris, and with the aid of spectroscopic tools for characterization and quantification of the physicochemical transformations, we scrutinized the toxicity variations for ZnO NPs with different aging times. As a result, we found that the toxicity altered in an abnormal manner with the aging time, i.e. the toxicity of aged ZnO NPs for 30 days showed the higher toxicity to the green alga than the fresh ZnO NPs or the ZnO NPs aged for longer time (e.g. 120 and 210 days). Through spectroscopic tools such as XRD, FTIR and Raman spectroscopy, we made both the qualitative and quantitative assessments of the physicochemical changes of the ZnO NPs, and confirmed that in the early stage, the toxicity mainly stemmed from the release of zinc ions, but with longer aging time, the neoformation of the nanoparticles played the critical role, leading to the overall reduced toxicity due to the less toxic hydrozincite and zinc hydroxide in the transformed compounds.

• Algal cells
• nanotoxicology
• physicochemical transformation
• Raman spectroscopy
• ZnO nanoparticles

Acknowledgements

We would like to thank Dr. Nanjing Zhao for providing the water-PAM to detect the photosynthesis activity of C. vulgaris. This work was supported in part by grants from Major National Scientific Research Projects (Grant No. 2014CB932002, No.2013CB934304), the Natural Science Foundation of China (No. 21207137 and No. 11475217), and the CASHIPS director’s fund (No. YZJJ201501).

Declaration of interest

The authors declare no competing financial interest. The authors alone are responsible for the content and writing of the paper.

Supplementary material available online

Supplementary Tables S1–S2 and Figures S1–S9