Continuous and automatic hexavalent chromium sensing using an ion-selective membrane deposited ion-sensitive field-effect transistor device integrating a microfluidic control system.

Heavy metal ion testing in water is vital in various applications, including domestic water quality, agricultural, and industrial wastewater discharge monitoring. However, current water testing methods require tedious and prolonged sample processes with bulky equipment operated by well-trained technicians. To address the above problems, we propose an ion-sensitive field-effect transistor (ISFET) deposited with an ion-selective membrane (ISM) for real-time and continuous hexavalent chromium (Cr(VI)) detection. Compared to previously reported Cr(VI) modifiers requiring complicated fabrication processes, the ISM made of a polyvinyl chloride (PVC) material can be drop-cast on the specific area of the ISFET sensor, allowing the differential signal readout and repeat ISFET sensor usage. To improve the sensitivity and specificity of Cr(VI) detection, we systematically optimized the Cr(VI) ISM parameters and the buffer condition to stabilize the pH value of the sensing environment. To further achieve the continuous and automatic Cr(VI) sensing in the actual water sample, we integrated a microfluidic control system with the Cr(VI) ISM-ISFET device, allowing the precise mixture of the water sample and buffer into the microchannel, thereby facilitating the continuous operation of the sensing device. Our results demonstrate that the Cr(VI) concentration at 10^-6-10^-4 M in ditchwater can be successfully differentiated and meet the guideline values (GVs) of industrial discharge water. Based on the above features, this fully integrated yet portable Cr(VI) ISM-ISFET system can be applied in various Cr(VI) detection environments, such as households, factories, farms, and orchards, requiring continuous and long-term on-site water quality monitoring.