Microplastic detection in saline water utilizing a microfluidic sensor with MXene-coated electrodes and a Wheatstone bridge

In situ detection of microplastics in aquatic environments remains challenging, with factors such as variable salinity significantly impacting sensor performance-particularly for systems that rely on electrical signal transduction. Here, we present an electro-microfluidic sensor incorporating a Wheatstone bridge and MXene-coated microwires for enhanced in situ detection of microplastics in salty water. The sensor significantly improves upon state-of-the-art technology by addressing key challenges, including reduced sensitivity and detection limits in saline water, clogging, reliance on sophisticated fabrication methods, and the need for baseline blank solutions. Using a simple DC current, the sensor effectively detects polystyrene microplastics (1-10 µm) within a concentration range 1-25 ppm, in the presence of 0-1000 ppm NaCl, representative of freshwater salinity. As a DC current is applied between two MXene-coated microwire electrodes in a microchannel, an electrophoretic force is generated, concentrating microplastics at the anode electrode. This leads to a reduction in electrical resistance, which enables the quantification of microplastics using the sensor as one of the arms of a Wheatstone bridge. Without the Wheatstone bridge and microwire MXene coating, the sensor's sensitivity significantly diminished at salinity levels above 650 ppm NaCl, especially for larger microplastics (10 µm). Bridge balancing suppressed baseline drift and raised the signal-to-noise ratio, retaining full sensitivity at 1000 ppm NaCl. The optimized sensor demonstrated a power-law response at 650 ppm NaCl and achieved limits of detection and quantification of 0.825 and 3.38 ppm, respectively. This work lays the foundation for low-cost, portable, and sensitive microplastic detection in saline aquatic samples, paving the way towards developing real-time, in situ sensors for environmental monitoring.