A Low Excitation Working Frequency Capacitively Coupled Contactless Conductivity Detection (C⁴D) Sensor for...

In this work, a new capacitively coupled contactless conductivity detection (C⁴D) sensor for microfluidic devices is developed. By introducing an LC circuit, the working frequency of the new C⁴D 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 C⁴D 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 C⁴D sensor can realize the conductivity measurement with low working frequency (less than 50 kHz). The LOD of the C⁴D sensor for conductivity measurement is estimated to be 2.2 µS/cm. Furthermore, to show the effectiveness of the new C⁴D sensor for the concentration measurement of other ions (solutions), SO₄^2- 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 C⁴D sensor for SO₄^2- and Li⁺ ion concentration measurement show good linearity with the LODs estimated to be 8.2 µM and 19.0 µM, respectively.