Analysis of Selective Fluorescence for the Characterization of Microplastic Fibers: Use of a Nile Red-Based Analytical Method to Compare between Natural and Synthetic Fibers

The scientific community has been focusing on studying and understanding the extent of damage caused by microplastics (MPs) to flora, fauna, and humans, including the environmental and health risks associated with them. MPs with different morphologies have been described in different environments, with fibers being the most common type regardless of the environment. Various methods have been used to analyze MPs. Analytical methodologies such as visual inspection, spectroscopic methods, and others currently used to study MPs are time-consuming, and only subjective results are obtained when these methods are used for sample analysis. Researchers have used various dyes, such as Nile Red (NR), a selective fluorescent stain, to differentiate the polymers from the other sample components and address these problems. Using such dyes helps distinguish polymer particles from other contaminants present in the samples. We aimed to study the analytical process, morphology, and wettability of synthetic (such as polyethylene and polypropylene) and natural (such as linen and cotton) fibers using NR to characterize the fibers. The fibers were fragmented manually, and the samples were prepared using a cryomicrotome. The prepared samples were subjected to different NR incubation times of 30 min, 24 h, and 168 h, and characterized under ultraviolet light using optical microscopy. We investigated the effect of NR on different fibers, and the samples selection using the fluorescence properties generated when the fibers adsorbed the NR dye. The wettabilities of the samples indicated that polyethylene and polypropylene were hydrophobic, while linen and cotton were hydrophilic. Both synthetic and natural fibers exhibited fluorescence properties in the presence of NR. This increased the complexity of executing the MP characterization process, indicating that combined methodologies and optical and chemical identification processes should be used to characterize plastic specimens efficiently. We summarize and discuss the results and findings and provide recommendations for future laboratory research on microplastic fibers focusing on (I) microplastic selection, (II) stain preparation, and (III) microplastic characterization.