A simple ion concentration measurement system using a microfluidic device with ISFET sensors

We present a newly developed measurement system using a microfluidic device designed to measure hydrogen ion concentrations (pH) in a relatively simple procedure that consumes only small amounts of sample solution. The device takes the form of a channel consisting of two inlets and one outlet connected together with a Y-shaped junction. Two ion-sensitive field-effect transistors (ISFETs) and an Ag/AgCl pseudo reference electrode are fitted into the device as a differential measurement circuit. The small size and high sensitivity of the ISFET sensors enable the measurement of small volumes of sample solution within a short time. The design principle and operation of the measurement system are very simple: a baseline solution (BLS) and sample solution are supplied into the device by a gravity-driven pump and the inflows of solutions are switched by adjusting the respective heights of the solution reservoirs. The measurement accuracy of a trial device was evaluated using standard solutions and diluted solutions after calibration. The maximum measurement errors for the standard solutions and 100-fold diluted solutions were 0.04 pH and 0.16 pH, respectively.