A Protocol to Characterize pH Sensing Materials and Systems

Although significant progress is made in identifying pH sensing materials and device configurations, a standard protocol for benchmarking performance of next-generation pH devices is still lacking. In particular, key properties of characterization systems, such as inherent component contributions, time plots for extended-gate field-effect transistor (EGFET) measurements, and the input resistance (R-in), often go unreported in studies of pH sensing systems. These properties strongly influence the characterization system and can lead to mistaken attribution of properties to the device. In this paper, a series of essential characterization tests and parameters are reported to evaluate pH systems, such as the zinc oxide EGFET, in a standardized protocol. This EGFET ZnO sensor has a sensitivity of -58.1 mV pH(-1), drift range from 2.5 to 14.2 mu A h(-1), and response time of 136 s. By using a ZnO sensing electrode, it is demonstrated that i) intrinsic contributions of reference electrode and commercial transistor (for EGFET) are not negligible; ii) time plots for EGFET configuration and defining a critical point at the onset of drift are essential for accurate sensitivity, response time, and drift reporting; and iii) the results of the pH sensing system are strongly dependent on the input resistance of the used characterization instruments.