A fully automated pH measurement system for 96-well microplates using a semiconductor-based pH sensor

The pH values of solutions have been commonly measured by glass electrodes, devices which respond on a time scale of minutes and require complicated operations such as calibration at every measurement. The development of a fast, automated pH measurement system for analytes in small volumes would be quite useful in many fields. Yet the practical application of such a device has been hampered by the need for rapid measurement and precise calibration. Our group has constructed a fully automated, self-calibrating pH measurement system capable of consecutively measuring 96 samples. The system uses a flow-through-type differential pH sensor probe based on a tiny, quickly responding pH-sensitive field-effect transistor (pH-FET) technology. The system was confirmed to measure the pH values of 26 μl solutions over a 135 s cycle (60 s for wash, 20 s for movement, 3 s for suction, and 52 s for measurement). The measurement error of multiple sample measurements, including measurements of solutions with low buffering capacity, was −0.0116 ± 0.0481 pH (average error ± S.D.). The high accuracy and high reproducibility of the results suggest that this automatic pH measurement system based on pH-FET can be applied for versatile uses.