Date of Award
Master of Science (MS)
Electrical and Computer Engineering
In this work, RFID tags are investigated as a liquid-phase chemical sensing platform. A dual-layer of selective coating film with polyaniline (PANi) on top of poly(epichlorohydrin) (PECH) is used for the detection of trace amounts of organic compounds (toluene and ethylbenzene) in aqueous solutions. Various experiments were performed to evaluate the coated RFID tag sensors. Shifts in the impedance and resonant frequency due to analyte sorption are measured. Details of the responses of the dual-layer film to different pH baseline solutions are shown. The results show that this dual-layer film can be used to detect toluene and ethylbenzene with high sensitivity. The conductive nature of PANi when protonated with an acidic solution allowed for increased sensitivity.
To describe the observed changes that occur during detection, an equivalent circuit model of the RFID tag chemical sensor was developed. The derived equations could be used to describe the observed response due to chemical sorption. The addition of PANi provided added sensitivity as the traces of the planar inductor began to effectively short out with increasing conductivity of the polymer. This is due to the different level of protonation of PANi that occurs. Lower pH solutions result in an increase in the conductivity of PANi to a level that drastically changes the characteristics of the coil (number of turns and width of the traces), thus shifting the operating resonant frequency. The large shift in frequency is related to a decrease in both the inductance and the capacitance of the coil. However, this shift would cause the tag to no longer operate with the reader. The results with the aqueous baseline solution of pH = 1 provides the largest sensitivity; however, the results with the aqueous baseline solution of pH = 3.5 also provides highly sensitive responses. Partial selectivity of the coated sensor was also found to be a function of the solution pH, thus the protonation of the PANi. For the present study, a limit of detection with a baseline solution of pH = 3.5 was found to be 6.24 ppb and 8.3 ppb for toluene and ethylbenzene, respectively.