Rapid and accurate automatic temperature calibration of disposable screen-printed heated gold electrodes

Abstract

Joule-heated electrodes have been used to enhance electrochemical analysis. Due to such direct heating, a steep temperature gradient is created near the electrode surface. The heating device that provides the high-frequency AC (50 kHz or more) has to be calibrated, in order to apply the desired temperature during analysis. The applied temperature of the working electrode influences both its electrical resistance and the electrochemical potential of a redox couple. Open circuit potentiometric (OCP) measurements were performed automatically with screen-printed gold loop electrodes (Au-LE), while applying 50 kHz AC heating pulses of increasing intensity provided by a ThermaLab® AC generator. Potentiometric temperature calibrations were performed using 5 mM equimolar ferri/ferrocyanide in 0.1 M of potassium chloride at 20 °C bulk temperature. Potential differences produced during each heat pulse were used to automatically calculate the electrode temperature using the temperature coefficient of this redox couple (-1.6 mV/K). The electrode resistance values at each heating pulse were obtained by measuring the heating voltage and heating current. The automatic temperature calibration experiments with five Au-LEs were shown to be highly reproducible and precise, with an RSD for the temperature of 0.24% and 4% for resistance. The average margin error of OCP temperatures were ±0.66 K at a 95% confidence level. The temperature coefficient (α) of electrical resistivity of the screen-printed gold layers was found to be 0.0025 °C-1, which is 27% lower than the theoretical value for gold metal. These findings were confirmed by DC resistance measurements using a potentiostat. Comparing the OCP temperature with the resistivity method, the temperature difference was about 0.94 °C (2.8%). Both methods enable quick, reproducible and accurate temperature calibration for disposable Au-LE, which were also used for trace mercury detection in lake water samples