Reliable detection of microplastics in water

| Hans Wolkers

Microplastics pose an increasing threat to human health and the environment. The absence of a reliable detection methods is an obstacle for legislation and restriction of these new contaminants. Associate Professor Alvaro Marin, Chair of Physics of Fluids, and his team are developing an efficient detection technology for microplastics in water. This is a first step towards better microplastic management.

‘Oh wow, this looks so cool!’ using a simple magnifying device, MSc student Eva Krolis checks out the agglomerated residue of an evaporated fluid droplet containing microplastics: a tiny, reddish-colored sphere-shaped island in a sea of grey matter. To create such an agglomerate, a fluid droplet containing microplastics, is pipetted on a special surface and evaporated in a controlled way. ‘Because the surface repels the droplet, it keeps a very round shape, minimizing any surface interactions with the droplet,’ Marin explains. ‘We want to study the evaporation as if the droplet was floating in the air, without any other effects; this is how close we can get.’

Marin and his team study the controlled evaporation of the tiny microplastic-containing fluid droplet in a chamber, where they can manipulate temperature and humidity. Depending on the different evaporation conditions, but also the microplastic concentration in the droplet, the particles may congregate in different ways, for example depending on their size.

Different sources

Microplastics are tiny, undegradable pieces of plastic, that are present literally everywhere: in the food we eat, water we drink, the air we breathe, and also in ecosystems. There is still no clear definition for these group of substances. The current classification is that they include all plastics between five millimeters and 0,1 micrometers in size.

They originate from different sources, for example, when plastic waste degrades over time, large plastic pieces are worn down into smaller pieces and eventually become microplastics. Another important source of microplastics is washing of synthetic clothes. As single polyester fleece jacket may shred hundreds of thousands of microplastic particles, that end up in the sewage, and eventually in the environment. These synthetic fibers have been shown to be an important source of oceanic microplastic pollution. But these substances are also added intentionally to cosmetics, like creams, shampoos and toothpaste to improve the quality, feel and touch of the product. Water containers and liners in food packaging also contain microplastics, that may end up in food products, and are ingested by people. Only a few studies address their potential health effects, but there is increasing concern about the long-term impact on people and planet.

Potential threat

On average, humans ingest about 12 milligrams of microplastics during their lifetime, of which only a limited amount is actually taken up by the body. Nevertheless, the ongoing microplastic pollution is a reason for concern, since it poses a potential threat to human and ecosystem health. Micropollutants, like Bisphenol A, PFAS, heavy metals, and pharmaceutical may adhere to microplastics, resulting in an increased chance of toxic effects on the immune and endocrine systems.

‘Industries see microplastics in their products as a problem of the future’

However, microplastic research is still in its infancy and there are still many unknowns. Even current technologies to detect them are still insufficient. This is one of the reasons that there is still no legislation regarding allowable microplastic amounts in food products. ‘We know bottled water and milk in cartons contain microplastic, but we have no clue how much,’ Marin says. ‘Industries see microplastics in their products as a problem of the future, so they have limited interest in dealing with it before laws are put into place.’ Nevertheless, the necessary future reductions in the amounts of these tiny pollutants, poses a huge challenge for government and industries, and it all starts with their reliable detection.

Hard to detect

To speed up legislation on microplastic pollution in food products, a quick and reliable method to quantify and qualify these substances is essential. Current detection techniques are based on microscopy and spectrometry, but these methods are time consuming and require specially trained people. In addition, microplastics smaller than five micrometers are very hard to detect using these methods.

In 2021, Marin’s colleagues from UT’s Nanobiophysics (MESA+ Institute for Nanotechnology and Technical Medical Centre) set an important step towards quantifying smaller microplastics. They combined fluorescence microscopy with so-called single particle tracking to count and size small microplastics in fluids. The scientists managed to quantify plastic particles as low as 45 nanometers, however, the type of microplastic could not be determined.

‘This research, led by Christian Blum and Mireille Claessens from the department of Nano Biophysics, inspired me to apply my fundamental knowledge on the physics of evaporating droplets and the behavior of small particles to this microplastic detection issue,’ Marin says. ‘During my research I discovered that the evaporation of a small droplet containing microparticles resulted in a specific assembly and aggregation, based on their size and chemical properties. So, I started working on using this principle to develop an improved detection method for microplastics.’

Unique fingerprint

Using the controlled evaporation process, Marin and his team managed to control the particle agglomeration in such a way that they were able to effectively organize and segregate microplastics based on their size and composition. They subsequently analyzed the particle conglomerates using a spectrometer to identify their chemical build-up. The resulting spectrogram, a graph containing different peaks, gives a unique fingerprint of the chemical composition, identifying the microplastic. Marin: ‘By using spectrometry, we can identify the chemical composition of the different aggregates and determine the type of plastic.’

‘By using spectrometry, we can determine the type of plastic’

Quantify and identify

The newly developed method solves a lot of problems regarding the detection of microplastics. It does not require to scan large volumes, and it will be able to both quantify and identify microplastics in fluids. Currently, the scientists are filing a patent for their methodology, that will need some time to finish.

In the meantime, Marin and his team are applying the technique to consumable water sources, and hope to be able to use it soon in more complex liquid samples, like sea water. In the longer run, he plans to form a consortium with the industry to further develop and apply his method to different food products. Marin: ‘This is just the beginning; the possibilities of the method to detect, quantify and qualify microplastics are almost endless.’

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