Inexpensive Adaptations of Basic Microscopes for the Identification of Microplastic Contamination Using Polarization and Nile Red Fluorescence Detection

Microplastic contamination of the environment is a major concern but detecting such contamination presents a challenge, particularly for particles <0.1 mm in size. Community scientists and students who participated in a plankton monitoring program and routinely examined samples of seawater using bright-field microscopy, often found fragments and filaments that were likely of anthropogenic origin. While in some cases this was clear from their structure and color, the origin of many particles remained questionable. To address this problem, optical components were added to microscopes for polarization and fluorescence detection. Plastics often show birefringence under crossed-polarizers due to alignment of the polymer chains within. They also bind Nile Red dye to give green, yellow, or red fluorescence emission, depending on the hydrophobicity of the polymer. Fluorescence excitation was achieved using a focused blue LED flashlight mounted externally to a microscope and emission was detected through a yellow plexiglass or gel filter. The required optical components cost a few tens of dollars and can be applied to any stereo (dissecting) microscope and most compound microscopes. These modifications were tested by community college students who set up their own microscopes and reported the presence of microplastics in the majority of their plankton samples. The equipment was also used at outreach events, where the observation of glowing microplastic particles suspended in natural water samples made an immediate impression with the public and demonstrated the magnitude of the environmental problem.