Optical Sieve: A New Route to Detect Nanoplastics

Optical sieve microcavities shift color when they trap nanoplastics, enabling fast detection, sizing, and counting with a standard microscope.

In previous articles, we discussed microplastics and nanoplastics (MPs and NPs) and identified the lack of fast and reliable detection methods. This limitation affects the accuracy and precision of current and future results.

To address this issue, researchers from Germany and Australia collaborated to develop a simple, rapid method for NP detection. Unlike existing techniques—such as FTIR, SEM, AFM, Raman spectroscopy, and SRS microscopy—which are typically time-consuming and expensive, this new method promises rapid and cost-effective detection, sizing, and counting of nanoplastics.

You can also read: Bio-Based Media for Micro- and Nanoplastics Removal.

Using Color Physics to Detect Nanoplastics

This novel detection method, known as the optical sieve, uses a strip with micro-scale cavities and a conventional optical microscope.

Researchers fabricate the strip from a high-refractive-index material and incorporate multiple arrays of micro-holes, known as Mie voids. These cavities interact with light, producing a specific color depending on their size through a phenomenon known as optical resonance.

When a droplet of liquid containing NPs is placed on the optical sieve, cavities trap particles with diameters that match those of the cavities. Particles that are either too large or too small do not fit properly, and the washing step removes them.

A key advantage of this method is that empty cavities exhibit a characteristic color under illumination. When a particle enters a cavity, the reflected color changes immediately. Scientists observe this color difference using a standard optical microscope, avoiding the need for advanced detection techniques.

Detection, Sizing, and Counting in a Single Method

Beyond detecting nanoplastics in liquids, the optical sieve rapidly sizes and counts particles more simply than conventional methods.

To determine the size of NPs in a sample, researchers analyze the cavities with the highest particle occupancy. From this information, the particle size can be inferred. Additionally, a statistical occupancy analysis across multiple cavities improves accuracy. In the resulting histograms, peaks correspond to cavity sizes that closely match the actual particle diameter.

For particle counting, researchers only need to count the number of occupied cavities. However, larger cavities can trap multiple smaller particles simultaneously. In such cases, the number of particles inside a cavity alters the effective refractive index, producing different colors depending on particle count. Using CIE-based colorimetric analysis, researchers can distinguish empty cavities from those containing one, two, or multiple particles.

Revolutionizing the Detection of Nanoplastics

Overall, the optical sieve enables faster in situ detection and characterization of NPs in environmental and biological liquid systems. It does not require highly trained personnel and researchers can use it in the field with portable, low-cost equipment.

The method shows strong potential, and future studies will test liquid samples containing nanoplastics of different sizes and geometries.