Size and concentration characterization of microplastic particles in aqueous samples using sensitivity-enhanced coupled planar microwave resonators

This study presents a novel microwave sensing platform for real-time monitoring of microplastic (MP) particle size and concentration in liquid media, with enhanced sensitivity achieved through the integration of an interdigital capacitor with the traditional split-ring resonator structure. A disposable sample holder (< $1) allows multiplex testing without cross-contamination. The sensing principle relies on electromagnetic interactions between suspended MP particles in liquid media and the microwave resonator, which is coupled with a planar microwave reader. Initially, MP particles are homogeneously distributed, but as they settle over time, they result in a dynamic shift in the resonance frequency that depends on MP size. The resonance frequency shift plateaus once all particles have settled, providing a measure of MP concentration. The sensor was designed and optimized using HFSS simulations and tested at three temperatures (10-30°C) in four host media (DI water, tap water, NaCl and urea solutions). Its performance was evaluated by detecting MPs of varied sizes (20, 70, and 275 µm) at concentrations of 100k, 1,000k, and 10,000k particles/L. The average detection slopes across the tested concentrations were 8.64 kHz for 20 µm, 38.52 kHz for 70 µm, and 110.78 kHz for 275 µm. This novel sensor demonstrates strong potential for on-site MP size and concentration monitoring, offering a cost-effective solution for environmental applications.