Dissolution kinetics of zinc oxide nanoparticles: real-time monitoring using a Zn²⁺-specific fluorescent probe

Zinc oxide nanoparticles (nano-ZnO) are emerging pollutants that have raised concerns of potential environmental risk. Previous studies demonstrated that the toxic effects of nano-ZnO towards aquatic organisms were mainly contributed by the dissolved Zn²⁺. Thus, identifying the dissolution kinetics is important in ecotoxicological research regarding nano-ZnO. Most currently available methodologies (e.g. inductively coupled plasma mass spectrometry-ICP-MS analysis) for studying the dissolution of nano-ZnO require the separation of the dissolved Zn²⁺ from nano-ZnO by ultrafiltration, which are time-consuming and may cause inaccurate results. To overcome these limitations, this study reported a fluorescence method for monitoring the rapid dissolution kinetics of nano-ZnO in aquatic environments. A Zn²⁺ sensor, namely, (9-anthrylmethyl)bis(2-pyridylmethyl)-amine (AMBPA), was used for the quantification of Zn²⁺ dissolved from nano-ZnO. AMBPA showed high specificity towards Zn²⁺ relative to nano-ZnO. Also, the small effect of interfering cations and anions on the fluorescence emission of Zn²⁺-AMBPA proved the selectivity of AMBPA towards Zn²⁺. The measurement of Zn²⁺ dissolved from four types of nano-ZnO by the proposed fluorescence method was compared with that by ICP-MS analysis following ultrafiltration. The proposed fluorescence method showed higher capability in terms of monitoring the rapid dissolution kinetics of nano-ZnO than the ICP-MS method, given that the former could monitor the real-time dissolution kinetics of nano-ZnO. This is important for understanding the dynamic behaviors of nano-ZnO at the early stage of reaction. The proposed fluorescence method described in this study is a suitable tool for studying the dissolution kinetics of nano-ZnO in aquatic environments.