Investigating optical microplastic detection methods using fluorescent staining through Nile Red

Abstract

Microplastics (MPs) are small pieces of plastic debris typically defined as smaller than 5mm. Given that the global environment faces a growing plastic pollution crisis, an urgent need exists for rapid, low-cost microplastic detection systems to characterize the health and environmental risk posed by MPs. Fluorescent tagging of microplastics using Nile Red (NR) has recently emerged as an accessible and popular detection method. However, robust, standardized methods of using Nile Red to identify between plastic and organic materials or distinguish between polymers are still being developed. This thesis pursued different optical microplastic detection methods using NR-based fluorescent staining with the ultimate goal of providing data that could be used towards building a polymer identification model that could be implemented into a low-cost detection system. Three different investigations are presented. First, the fluorescence emission spectra of various plastic and organic samples stained with Nile Red is presented. The motivation behind this study was to identify the strongest fluorescence emission peaks for NR-stained plastics under a series of different excitation wavelengths. The spectral results provide a preliminary basis to distinguish Nile Red-stained plastics based on their fluorescent emission spectra alone. Second, this thesis presents a low-cost imaging set-up for fluorescent samples. The system applies the same excitation wavelengths and optical filters used to collect the spectral data. The images are then combined with the spectral data to illustrate another basis for rapidly distinguishing between different plastic polymers. Finally, an optical method for detecting microplastics in liquid samples using photodiodes is explored and discussed. Overall, this thesis contributes to the development of accessible microplastic detection technologies by leveraging the fluorescent properties of NR-stained plastics. The findings highlight the challenges and potential solutions for distinguishing between plastics and organic materials and distinguishing between different plastic polymers.