Real-time and hazard-free water quality monitoring based on microwave planar resonator sensor

Abstract This manuscript presents the development of a microwave microfluidic sensor for monitoring organic contaminants in water. This method offers a contactless, non-intrusive, and real-time monitoring system. The sensor is comprised of a compact double-ring resonator integrated with a 3D-printed microfluidic channel, operating at 4.5∼4.6 GHz with a quality factor of 120. The channel configuration on the resonator ensures maximum interaction between the electromagnetic field and the liquid sample and provides high sensitivity and resolution. The developed sensor was used to monitor organics (glucose, acetate and glucose-acetate mix) and the chemical oxygen demand (COD) standard (potassium hydrogen phthalate) in concentrations ranging from 50 – 800 mg/L as COD. The feasibility, reproducibility, and accuracy of the sensing platform was further validated by analyzing the S21 characteristic of the sensor. A monotonic decrease in the resonant amplitude of S21 was observed as the concentration of organics increased, with the sensitivity of 0.603 dB/COD [g/L], 0.087 dB/COD [g/L], 0.099 dB/COD [g/L] and 0.077 dB/COD [g/L] for potassium hydrogen phthalate, sodium acetate, glucose, and a glucose- acetate mixture, respectively. The findings support the capability of the microwave microfluidic sensor to detect dielectric properties variation associated with a minimum concentration of 17 mg/L glucose in water in real time. The response of the sensor was compared with conventional COD measurements using potassium dichromate digestion to compare the efficiency of the developed sensor with the spectrophotometric method to measure COD.