Quantifying Microplastic Litter using Fluorescent Tagging

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Litter in marine environments is a well-known, global issue. Microplastics can be particularly damaging to marine life as their size allows them to pass through sewage treatment plants unfiltered and be ingested by marine organisms, allowing them to enter and accumulate in the food chain.

Microplastics are typically defined as fragments of plastics below 5 mm in diameter. Microbeads, which have commonly been added to cosmetics such as facial cleansers and toothpaste, have now been banned in a number of countries due to their detrimental effects on the marine environment. In the US, the Microbead-Free Waters Act of 2015 prohibits the manufacture of rinse-off cosmetics containing intentionally-added plastic microbeads from July 1, 2017. Besides microbeads, microplastic fragments can also originate from synthetic fibers and larger plastic items that have been broken down in the environment by UV radiation, oxidation, or mechanical force.

To enable the concentrations of microplastics in marine environments to be monitored, and investigate the fate of microplastics, simple, fast, and cost-effective methods to detect and quantify microplastic fragments are required. Previously, microplastics have been quantified by filtration and visual sorting, which can be time-consuming, require expert knowledge, and can be error-prone. Other approaches have included chemical or physical characterization, spectroscopy and/or microscopy. However, such techniques are often slow, costly, and face a number of technical limitations.

A group of scientists has now designed a new technique for the quantification of microplastics based on fluorescent staining followed by extraction and filtration. Fluorescent dyes are more commonly used in medical applications to highlight specific biological structures. The team tested a number of fluorescent and dyes and found that Nile Red, a lipid soluble dye, was able to absorb to plastics and provide sufficient fluorescence intensity to allow the quantification of microplastics in marine sediments.

Marine sediment samples were incubated with Nile Red, then ZnCl₂ was added to allow plastic particles to float, and the samples were centrifuged to extract the plastics. The extracted and filtered microplastics were collected and exposed to blue light, making them clearly visible due to their fluorescent tags. Images were collected using photography through an orange filter and image analysis allowed quantification of the microplastic concentration in the marine sample.  Nile Red displays solvatochromic behavior; i.e. its fluorescence emission spectrum changes depending on the environment. This property allowed the detected microplastics to be grouped by polarity, which may allow weathering and biofouling of microplastics during environmental exposure to be tracked in future, though this needs further validation.

The newly developed method was verified by comparing the results with results obtained using infrared microscopy. The team concluded that the new method is able to identify the majority of microplastic particles currently used in Europe. Particles as small as a few micrometers were detected, with the magnification and optical resolution of the camera defining the detection limits. Detection of microplastics as small as a few micrometers has rarely been reported due to analytical issues and detection limits. However, the newly developed technique overcomes these difficulties, and the results suggested that microplastic abundance in marine sediments may have been underestimated previously, as previously developed techniques are not able to detect very small microplastic particles.

Fluorescent staining combined with separation provides a simple and sensitive approach to quantify the most commonly used microplastics in marine sediments and is able to detect very small microparticles. On-going improvements to the technique include automated image recognition, microparticle counting, and RGB characterization algorithms, to automate the quantification and grouping processes. With further development and optimization, this technique could provide a powerful analytical approach for the quantification of microplastics in marine samples.

Source:

Maes T., Jessop R., Wellner N., Haupt K., Mayes A.G., Scientific Reports, 2017, 7, 44501.

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