Optimizing the Concentration of Nile Red for Screening of Microplastics in Drinking Water

Increasing concern regarding the presence of microplastics in drinking water has led to a growing number of studies aimed at quantifying microplastics in water. In this work, we present an optimized procedure for the use of Nile red (NR) as a fluorescent staining agent for pre-screening of microplastics in bottled water. Positive and negative control experiments with NR concentrations ranging from 0.001 to 10 mg/L showed that the appropriate NR concentration is an important factor in obtaining representative particle counts. Non-optimized staining concentrations led to underestimation or overestimation of the particle count. In this study, the optimized NR staining concentration was found to be 0.1 mg/L. This method was successfully used to screen particles in seven different brands of bottled water, consisting of both still and carbonated water, in both plastic and glass bottles. Particles larger than 100 μm were chemically characterized using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Sixty-seven percent of these particles pre-screened with NR were confirmed to be polymers. Particles smaller than 100 μm were qualitatively analyzed using pyrolysis coupled with gas chromatography and mass spectroscopy (Py-GC-MS). Analysis of polymers between ∼5 and 100 μm using Py-GC-MS confirmed that this smaller fraction generally mirrors the FTIR results for particles larger than l00 μm.