Conventional laboratory microscopes vary in their configuration, require dedicated space, are not field-ready, and can be prohibitively expensive to many groups interested in microplastics. Thus, relying on conventional microscopes for microplastics research and monitoring can lead to issues of inequitable and inaccessible science. Hand-held, digital microscopes could resolve these challenges. These are compact, rugged, versatile, relatively inexpensive, single-objective microscopes with integrated digital cameras, which provide research-grade imaging.
We evaluated three different digital light microscopes sold on Amazon.com, which ranged in price from $25-160. Resin pellets were used as reference microplastics to evaluate the digital microscope imaging capabilities because they are common marine debris detected by beach and oceanic surveys, differences in their size, shape, and color have been documented, and changes in these properties have been identified as relevant factors in monitoring their degradation and pollutant accumulation.[3] Images were analyzed using a standardized method in ImageJ to quantify the morphometrics and colorimetrics of the microplastics. To promote standardization of colorimetric and morphometric analyses by this approach, a detailed step-by-step protocol was prepared as well as guidelines for the microscopy. Metric precision was evaluated by imaging the same pellet ten times using each microscope. There were slight differences in the measured morphometrics between the microscopes, but overall, the coefficient of variation for them were less than 5%. There were differences in the colorimetrics caused by the white balance of the microscopes, requiring white balance correction for the less expensive digital light microscopes. Pellet-to-pellet variability (assessed by imaging 25 different pellets) was greater than the measurement precision for all metrics across all digital light microscopes, indicating that these microscopes can adequately quantify environmental microplastics.
An additional challenge for monitoring microplastics in environmental samples is that often they contain inorganic particulates alongside them. To identify microplastics in this situation, the collected material is stained with Nile red,[1] a lipophilic stain specific for plastic that fluoresces red. We next demonstrated that digital epifluorescence microscopes could resolve and detect Nile red stained microplastics. The pellets were stained for 20 hours with a 1 µg/mL methanolic solution of Nile red. Ten pellets were characterized to evaluate the pellet-to-pellet variability. Images were captured using a DinoLite digital epifluorescence microscope with excitation/emission filters for green and red fluorescence. Camera settings for the microscope were integral to successful morphometric analysis; the resolution and gain were adjusted to enhance image quality. The coefficient of variation for the morphometrics were <10%. While the digital epifluorescence microscopes are more expensive (~$1000) than the digital light microscopes, compared to conventional epifluorescence microscopes they are significantly cheaper. Collectively, digital microscopes are positioned to standardize the visualization and characterization of microplastics and broaden accessibility for conducting microplastic research.
[1] Kotar et al. Chemosphere 2022, 308, 5, 136449.
[2] James et al. ACS Environ. Au 2022, 2, 5, 467–479.
[3] de Vos et al. ACS Environ. Au 2022, 2, 2, 128–135.
[4] Erni-Cassola et al. Environ. Sci. Technol. 2017, 51, 23, 13641–13648.