Contactless Conductivity Detection For The Microfluidic Chip

Contactless conductivity detection (CCD) is a simple and universal detection method, and it has several advantages because of the insulation of the electrode from the solution, such as the elimination of electrode fouling, bubble generation and so on. In the last ten years, CCD is widely applied into microfluidic chip, and shows expansive application prospects.In this paper, a new equivalent circuit model for CCD is introduced according to the structure of detection cell in the microfluidic chip.The stray capacitance is calculated with a back-calculating method, and all influences of materials rounding the electrodes are considered in this method.In order to improve the accuracy of the model, the solution facing the electrode is divided into hundreds of segments with uniform width, and each segment contains a film capacitance and an impedance of the solution in the model.Finally, the influences of the film capacitance and the stray capacitance on the detection sensitivity are discussed.A signal detection circuit is designed according to the principle of CCD, this circuit can generate a variable frequency and amplitude sinusoidal signal, and extract the weak current on the pick-up electrode.In order to achieve continuous sampling in a small space, a microfluidic chip with a probe is designed. The microfluidic chip consists of a poly (methyl methacrylate)(PMMA) plate with membrane microelectrodes, a polydimethylsiloxane (PDMS) film as the insulating layer and a PDMS plate with microchannel. According to the principle of peristaltic pump, a small and exquisite vacuum instruction is designed. In order to realize the portability of the detector, a handheld bracket for installation of the microfluidic chip and the vacuum instruction is carved.A home-made portable CCD detector was tested with KC1solution, and the influences of excitation frequency and electrode gap on the detecting signal were analyzed. On the basis of the experimental results, the impacts of the signal conditioning circuit and the stray capacitance were also analyzed. At last, the theoretical results of the detector were calculated according to the equivalent model, and compared with the experimental results. The comparison results indicate that the outputs of the new model in this paper are much closer to the experimental results than that of a simple model.