A Low Excitation Working Frequency Capacitively Coupled Contactless Conductivity Detection (C(4)D) Sensor for Microfluidic Devices

In this work, a new capacitively coupled contactless conductivity detection (C(4)D) sensor for microfluidic devices is developed. By introducing an LC circuit, the working frequency of the new C(4)D sensor can be lowered by the adjustments of the inductor and the capacitance of the LC circuit. The limits of detection (LODs) of the new C(4)D sensor for conductivity/ion concentration measurement can be improved. Conductivity measurement experiments with KCl solutions were carried out in microfluidic devices (500 µm × 50 µm). The experimental results indicate that the developed C(4)D sensor can realize the conductivity measurement with low working frequency (less than 50 kHz). The LOD of the C(4)D sensor for conductivity measurement is estimated to be 2.2 µS/cm. Furthermore, to show the effectiveness of the new C(4)D sensor for the concentration measurement of other ions (solutions), SO(4)(2−) and Li(+) ion concentration measurement experiments were also carried out at a working frequency of 29.70 kHz. The experimental results show that at low concentrations, the input-output characteristics of the C(4)D sensor for SO(4)(2−) and Li(+) ion concentration measurement show good linearity with the LODs estimated to be 8.2 µM and 19.0 µM, respectively.