In biological and environmental applications, it is desirable to be able to measure hydrogen and hydroxyl ion concentrations (pH levels). Conventionally, the measurement processes take a considerable amount of time involving several calibration steps and handling of fragile electrodes. Here we propose a new, more robust and theoretically reliable way of sensing pH. Following a similar approach to Dr. Qingjun Liu’s work on the light-addressable potentiometric sensor (LAPS) from the Zhejiang University in China, we fabricated a silicon pH sensor, with polydimethylsiloxane (PDMS) fluidic channels for solution delivery and testing. Particularly for biological and environmental applications, the range of pH sensing is confined to 6 to 8.The fabricated device used n-type silicon, so only five acidic solutions between pH 5 and pH 7 were tested. Device test results proved to be somewhat problematic. We were not able to obtain consistent capacitance measurements for a particular pH solution. For some solutions we had extremely large variances in capacitance, yielding “noisy” measurements. However, in retrospect, there are many improvements that could be made to our device and testing procedure in order to obtain more consistent capacitance readings, closer to the theoretical performance. Such changes include modifying the geometry of the electrode which supplies an AC signal for capacitance measurements, and ensuring airtight (PDMS) bonding with the device substrate, guaranteeing seamless solution delivery to the test chamber. It is our hope that future students are able to build upon these results and create a better, more reliable device.
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Silicon-based pH sensor for biological and environmental applications
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