Modification of a Nile Red Staining Method for Microplastics Analysis: A Nile Red Plate Method

Heejun Kang,Jaehwan Ahn,Bokjin Lee,Saerom Park,Seogku Kim

doi:10.3390/w12113251

Heejun Kang, Jaehwan Ahn + Show 3 more

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https://doi.org/10.3390/w12113251

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Abstract

Recently, environmental pollution from microplastics (MPs) has become a significant reason for increasing the number of studies to develop analysis methods. The Nile red staining method (NR-S), which is staining polymer particles with Nile red (NR) dye, has been widely used for the analysis of MPs in environmental samples. However, this method has several limitations, as it is difficult to stain MPs covered with organic matter residues. In this study, we modified the NR-S method into an NR plate method (NR-P), where the plate is coated with NR instead of staining MPs directly. The optimum concentration of NR solution was obtained (1000 mg/L), and the effectiveness of the NR-P method for the analysis of MPs was assessed using different types (polypropylene, polyethylene, polyethylene terephthalate, and polystyrene), sizes (100–1000 µm), and shapes (sphere, fiber, film, and flake) of plastic materials. The NR-P method demonstrated improved resolution in the overall types, shapes, and sizes of MPs and was better than the control (without NR plate method) and NR-S method. In particular, the NR-P method can effectively observe MPs covered with organic matter, which was a major limitation of the NR-S method. Finally, MPs in sewage field samples were analyzed by the NR-P method with an accuracy of 78% confirmed by FT-IR. We demonstrated that this method is a convenient and efficient alternative for identifying MPs, even for field samples.

Highlights

Microplastics (MPs), defined as plastics with a size below 5 mm, originate from plastic beads in cosmetic and personal care products, synthetic fiber laundering, tire abrasion, and the mechanical and photo-oxidative fragmentation processes of larger plastic items [1,2,3,4,5]
We assessed the performance of the Nile red (NR)-P method in detecting and quantifying MPs using various commercial MPs and field samples
For the size effect of the MP analysis, while no statistical difference in MP size integration in the range of 400 ~ 1000 μm was observed between the NR plate method (NR-P) method and the controls, all showing more than 90% accuracy, MPs smaller than 400 μm are more effectively observed using the NR-P method than the control, with 84 ± 21% and 53 ± 15% accuracy, respectively

Summary

Introduction

Microplastics (MPs), defined as plastics with a size below 5 mm, originate from plastic beads in cosmetic and personal care products, synthetic fiber laundering, tire abrasion, and the mechanical and photo-oxidative fragmentation processes of larger plastic items [1,2,3,4,5]. MPs have received great attention as emerging contaminants owing to their global distribution and association with potential ecotoxicological risk [6,7,8,9,10] As their distribution in various environmental matrices, including marine and aquatic environments, has exponentially increased, convenient and accurate analytical methods are required to monitor and understand the behavior of MPs [11,12,13,14,15,16]. MPs in environmental matrices are visually identified using microscopes after proper pretreatments, which allow for the determination of their number, sizes, and shapes [17,18] These techniques have limitations of slow speed and low reliability due to errors caused by analyst proficiency [19,20].

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