A miniaturized microfluidic pH measurement device using ISFET sensors
We have succeeded in producing a new micro fluidic pH measurement device downsized from our previous model. The new device body has dimensions of ϕ15×5 mm and a channel section measuring 0.75×1.0 mm (width and height). The operability of the sensor probe and high-responsiveness and stability of the electrode are key preconditions to effective pH measurement. Our method combines a micro fluidic device made of polydimethylsiloxane (PDMS) and an ion-sensitive field-effect transistor (ISFET). The ISFET sensor is microscopic in scale and responds quickly to solutions with low buffering capacities. As the sample solution and baseline solution (BLS) flow in the channel of the micro fluidic device, a liquid junction forms between them. The flowing junction improves the stability of the measurement. The present device performed measurements at a rate of 120 sec/sample and consumed samples of 0.3 ml in volume. The measurement error of 9 sample solutions, including 3 diluted solutions with low buffering capacities, was 0.00992 ± 0.179.
Expansion of a linear range of the ion-concentration measurement system using microfluidic device with ISFET sensors
We have previously reported a pH measurement device using a microfluidic technology and ion-sensitive field-effect transistor (ISFET). The device has successfully measured pH values, however the range of linearity was limited within neutral range. In this paper we have evaluated the linearity of the sensor signal with a wide range of pH. The present device obtained high linearity in the range of pH 1-10 with the correlation factor of 0.9985. The averaged measurement error and standard deviation were -0.0083 and 0.0085 pH, respectively.
Automation of pH Measurement Using a Flow-Through Type Differential pH Sensor System Based on pH-FET
Automatic pH measurement system was developed by using a flow-through type differential pH-FET sensor. The system was operated by repeating 60 sec cycle consisting of 36 sec of washing period, 4 sec of moving period, and 20 sec of measuring period. In washing period, the differential pH-FET sensor was washed with baseline solution. In measuring period, about 50 μl of sample solution was withdrawn to the space around a measurement pH-FET, and the change of differential source potential was measured. Absolute values of error for nine sample solutions with weak buffering ability were less than 0.146 pH, and were 0.071 pH in average. From this experiment, it was suggested that high-speed and automated pH measurement will be possible by using pH-FET sensors even for sample solutions of weak buffer ability.