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The one-step modification of a commercial RFID sensing tag is demonstrated using polydimethylsiloxane-based thin-film chemistry to construct reusable passive RFID sensors for changes in the dielectric properties of electrolyte solutions as a function of concentration. The effects of PDMS film thickness were characterized as a function of RFID sensor code value. The output sensor code of the RFMicron RFM2100-AER wireless flexible moisture sensor (taken between 800 and 860 MHz) was compared with the readings taken when the tag was dry and when the tag had a water deposition on the sensor area. The effect of the direct application of liquid water on the tag was to alter the capacitance presented to the integrated chip which autotunes to correct for the reactance. By varying the thickness of the PDMS film between the interdigitated sensor and deposited liquid, the sensitivity of the tag to a high dielectric medium could be controlled. Aqueous salt solutions were tested on a 500µm-thick film. It was found that the sensing platform could be used as a means of measuring the concentration of various salt solutions within the range 0-2 M and in turn could be used as a passive UHF RFID dielectric measuring tool. The measurement capability of the platform was subsequently demonstrated using a reduced frequency range (845-865 MHz).Index Terms-Radio frequency identification, RFID tags, chemical sensor, aqueous electrolyte.

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