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Graphene oxide nanofilm based surface acoustic wave sensor for fast measurements of water activity

Abstract:

Water activity (Aw) in different food products is normally measured by the measurement of the partial pressure of water vapor in the enclosed ambient with the sample to be evaluated placed inside. These measurements are done by monitoring the dew point temperature or by the measurement of the resistance of the special chemiresistor. We introduce the new method of Aw measurements for food industry based on the use of the surface acoustic wave sensor with the graphene oxide nanofilm as the sensitive element. The value of Aw is extracted from the time curve of desorption of water molecules from the product (kinetic curve) approximated by the sum of two exponents with different amplitudes and time factors, with one exponent (the fastest) having the saturation time much smaller than the other. It is proposed to use for calibration of the amplitude and time factor only of the fastest exponent after LSQ-fitting of the experimental curve to the sum of two exponents. This algorithm, omitting the use of the slow exponent, allows one not to wait until the complete thermodynamic equilibrium is reached. The proposed method gives the maximum discrepancy with the measurements by the commercial equipment on the order of 1-3%, having at the same time the average time of measurement 2-5 times smaller. This discrepancy can be adjusted to much smaller values introducing thermal stabilization of the measurement ambient. The results of experimental verification of the proposed method for large group of products are presented. Due to the high speed of measurements the proposed method could be used in the food industry for in-line Aw monitoring.