Highly Sensitive Quartz-Based Sensing System for the Detection of Subpercentage Changes in the Relative Permittivity of Liquids Flowing in Microchannels

We describe a highly sensitive quartz sensor for measuring changes in the relative permittivity of liquids flowing in microchannels. The proposed method uses a highly stable oscillator and capacitance-dependent quartz crystal together with a capacitance-sensitive element attached along the microchannel. A change in the relative permittivity of the fluid induces a change in the capacitance of the sensitive element in the aF range, which is detected as a change in the resonant frequency. The advantages of the proposed measurement technique are the extreme sensitivity (changes in the relative permittivity as low as 0.01% can be detected), the temperature independence of the setup between 10 and 40 °C, the stability (the frequency reading fluctuates within 0.025 Hz), and the low cost compared with the methods that use impedance analyzers or lock-in amplifiers. We present the use of the method to detect changes in mixtures of liquids if the temperature, volume fractions, or properties of one liquid change. The method presents a useful tool for applications in biology, chemistry, pharmacy, and technology in general wherever accurate monitoring of compositions of fluids is required and where changes, for example, due to temperature variation or mixture aging, need to be detected in real time.