A Transmission Line Sensor With Sensitivity Improved for Detection of Ionic Concentration in Microfluidic Channel

Microwave measurements of minute dielectric property variations have many advantages such as noninvasive test, rapid sensing capability, and being label-free. Several researches including concentration detection are promoted by measuring the minute dielectric property variations of samples. Highly sensitive and accurate device is required for measurement of electrolyte concentration in biological fluids, especially when measuring samples in nanoliter or less scale. In this paper, a miniaturized broadband coplanar waveguide (CPW) transmission-line (TL) sensor with enhanced sensitivity is proposed. The sensitive device is realized by introducing interdigital capacitance (IDC) at the central conduction band of the CPW TL. The sensor is fabricated using print circuit board (PCB) technology with a microfluidic channel designed on it. A calibration based on microwave network theory is used to obtain the characteristic impedance and propagation constant of the sensing part. And accordingly, variations in magnitude and phase of S-parameters are employed to express the changes in concentration of the sample. Furthermore, an extracted equation between S-parameters and electrolyte concentration of the sample is first proposed. Experiments show the proposed sensor can detect the minimum concentration of 0.05 g/l for the three electrolytes (NaCl, KCl, and GaCl2 in deionized (DI) water)). Compared with a corrugator coplanar transmission line and a split ring resonator senor, the proposed sensor has a much higher sensitivity. Moreover, the device is robust in front of cross sensitivity caused by external factors (such as temperature variations, moisture, etc.). Thus, the proposed sensor is well suited for low-cost, real-tine, and CMOS compatibles sensing technologies.