Fluid Temperature Measurement in Aqueous Solution via Electrochemical Impedance

A novel temperature transduction method using the high-frequency electrochemical impedance between a pair of microelectrodes exposed to aqueous solution is reported. The solution resistance of an aqueous ionic solution is highly temperature dependent, and the electrochemical impedance between two microelectrodes is dominated by solution resistance when measured at the appropriate frequency. Therefore, precise measurements of electrochemical impedance at the proper frequency in a two-electrode system can be used to transduce solution temperature. To demonstrate this method, a temperature sensor composed of two thin-film platinum electrodes on a freestanding Parylene C substrate was designed and fabricated. A platinum resistance temperature detector was co-fabricated to provide a simple means of benchmarking against an existing standard. Transduction via electrochemical impedance was achieved by measuring the real part of impedance at the frequency where phase was minimized. Fluid temperature was transduced between 15C and 50C with high sensitivity (-1.21/C) and resolution (0.02C). A $4\times $ improvement in sensitivity and resolution over the conventional platinum resistance temperature detector was achieved. The sensor design described here features flexible construction with inert materials which facilitates future use in biomedical or microfluidic applications. [2019-0190]