Detection of engineered nanoparticles in aquatic environments: current status and challenges in enrichment, separation, and analysis

There is an ever-increasing amount of engineered nanoparticles (ENPs) released into aquatic environments due to human activities. The sophisticated and accurate detection of ENPs is imperative to assess the threat of ENPs to the environment and human health, as well as to establish the benchmarks for controlling ENP discharge. The present review aims at summarizing and evaluating the current situation of ENP detection in water systems. Attention has been focused on the development of ENP detection processes and technologies. Given their extremely small size, low mass concentration, high activities, and complex coexisting components in the environmental matrixes, the online or in situ analysis of aquatic ENPs is proposed to be achieved via enrichment–separation–detection. Multiple ENP enrichment methods have been adopted, including coagulation, flotation, filtration (membrane processes), and extraction, followed by microfluidic separation and sorting. The enriched ENPs are then qualitatively and/or quantitatively measured under controlled conditions (by microfluidic devices) in terms of size (via nanoparticle tracking analysis, NTA), concentration (via NTA, inductively coupled plasma mass or Raman spectrometry), and species (via electrochemical and fluorogenic methods). In spite of the technical development, most of the research is carried out in a laboratory or on the bench scale. The lack of online and in situ measurement systems, as well as the implementation in different worldwide regions, have led to the unavailability of a single database of aquatic ENPs.