A novel wireless, passive CO₂sensor incorporating a surface acoustic wave reflective delay line

Abstract

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed for CO2 detection. The developed SAW gas sensor is composed of single phase unidirectional transducers (SPUDTs), three shorted grating reflectors, and CO2-sensitive polymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted design parameters, the SAW device was fabricated. Teflon AF 2400 was used as the sensitive film because it provides high CO2 solubility, permeability and selectivity. In wireless device testing using a network analyzer, four sharp reflection peaks with high signal-to-noise (S/N) ratio, small signal attenuation, and few spurious peaks were observed in the time domain. The time positions of the reflection peaks were well matched with the predicted values from the simulation. Infusion of CO2 into the chamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observed for a CO2 concentration of 0–450 ppm. The obtained sensitivity was 1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluation process.

Materials and Transducers Toward Selective Wireless Gas Sensing

“…acid/base) reactions between analytes and sensing polymers. 141 Transducers implemented for wireless gas sensing with dielectric polymers include capacitors, 139 and TSM, 46 SAW, 64 magnetoelastic, 45 and RF 71 resonators.…”

“…In capacitance-based sensors, water exerted a 30 times stronger response than CO 2 145 while smaller water effects were found for SAW transducers. 64 It was suggested to use either a complete water removal before CO 2 measurements 145 or to incorporate a separate humidity sensor to mitigate this issue. 64 An example of a wireless SAW sensor response to CO 2 at different humidity levels is presented in Figure 10b.…”

Nanocrystalline SnO2 film prepared by the aqueous sol–gel method and its application as sensing films of the resistance and SAW H2S sensor

“…It is possible to realize both wireless and passive measurement by optimizing the sensor structures [3]. Except for optimizing the device structure, one of the basic technologies is preparing of the sensing films of the gas sensor.…”

Radio-frequency tag with optoelectronic interface for distributed wireless chemical and biological sensor applications

“…Wireless sensor networks (WSNs) provide the technological infrastructure for realising such distributed systems, and the underlying topological theories, enabling hardware and software technologies, and diverse applications of WSNs have been widely reported [8][9][10]. In addition to wireless RFID-based sensors, other types of passive, contactless chemical sensor have been reported that encompass a wide range of different transduction methods, including surface acoustic wave (SAW) devices [11][12][13][14], magnetoelastic devices [15][16][17], and tuned resonant electric circuit devices based on variable capacitor-inductor (LC) resonant tuned circuits. The LC tags resonate when exposed to an applied radio-frequency (RF) field, and the resonant frequency varies with analyte concentration, either because of interaction with a chemically sensitive dielectric material [18], or because of a conformational change in the gap separating the capacitive plates [19].…”