We present a near-field microwave sensor based on a hairpin resonator for detecting aqueous ethanol and aqueous NaCl concentrations in microfluidics. A hairpin sensor with a high-quality (high-Q) dielectric resonator allows the detection of a small variation in the aqueous NaCl concentration by measuring the scattering parameter (S-21) responses at resonance frequencies. The responses caused by the test liquids with various dielectric constants in aqueous ethanol solution were observed at operating frequencies of 1.9 and 2.1 GHz. The detection of various concentrations of NaCl in solution was also achieved. The changes in S-21 at resonance frequencies are directly related to changes in the aqueous NaCl concentration because of the electromagnetic interaction between the resonator and the NaCl solution. The stable linear relationships arc advantageous for detecting and analyzing aqueous ethanol and aqueous NaCl concentrations. It shows that the sensor is reliable for both dielectric constant and conductivity detection in liquid. The hairpin resonator, which is easily fabricated by standard printed circuit board (PCB) technology, has great potential for microfluidic sensing applications.
