Image: Microwave sensor design
By Maziar Shafiei, PhD Student, Faculty of Engineering
There are several, well-established methods for detecting and monitoring microplastic particles – the most common being Fourier-transform infrared (FTIR) and Raman spectroscopy. Although these techniques are very accurate and can detect the types of plastic in the sample, they require intense training of the technician and bulky and expensive measurement equipment. They also use offline methods and require samples direct from water facilities for lab experiments. Overall, these approaches end up being time-consuming, labor-intensive, and expensive for researchers, especially when they have a large number of samples to process, such as is the case for UW’s Microplastics Fingerprinting project. As a result, we are working on new techniques for sensing microplastics that are online, low-cost, and user-friendly. An online monitoring method would help us avoid the expensive and time-consuming sampling process of detecting particles.
One of the more promising techniques that we are examining is the use of microwave sensors because they address the pitfalls mentioned above, while providing a reliable online platform for monitoring that requires a relatively simple measurement setup that takes comparably less time than the optical spectroscopic method. Microwave sensors can be installed and analyzed in situ, resulting in faster and more reliable detection.
Microwaves are highly sensitive to the properties of the materials with which they interact. Microwave sensors are based on the difference in permittivity of the host medium (e.g., water) and the microplastic contaminant. This contrast is high, enabling researchers to accurately count the number of particles present, even at low concentration levels. Microwave sensors also can be combined with planar technology to obtain compact, light, robust, and low-price fabrication. If we were to do this analysis using a method such as FTIR or Raman spectroscopy, it would take tens of hours, whereas our approach can be done in mere minutes. This makes planar microwave sensors a more practical option for continuous online measurements.
There are strengths and weaknesses associated with any detection methodology, including microwave sensing. One downside is that microwave sensors cannot detect particles smaller than 20 micrometers (in samples with concentrations less than 1000k particles/L or those with concentrations less than 50k particles/L). Another challenge with this method is that it cannot differentiate between two different plastics in a sample, but this could be addressed in the future as the technology improves.
Overall, microwave technology is a novel method for monitoring microplastics. It is a very promising approach for quickly determining the size and concentration of selective microplastic sensing. Although there are some limitations, we are working on ways to overcome the weaknesses by enhancing the microwave part of the device and improving the sensitivity of the sensor. This enhancement might allow us to detect microplastic particles as accurately as optical spectroscopic methods but would be much faster and cheaper.
References
- Malyuskin, O. (2020). Microplastic Detection in Soil and Water Using Resonance Microwave Spectroscopy: A Feasibility Study. IEEE Sens. J. Volume 20 (24): 14817–14826. https://doi.org/10.1109/JSEN.2020.3011311
- Shafiei, M., Abbasi, Z., & Ren, C. L. (2022). Passive Disposable Microwave Sensor for Online Microplastic Contamination Monitoring. IEEE/MTT-S International Microwave Symposium - IMS 2022. https://doi.org/10.1109/ims37962.2022.9865376.