Materials and transducers toward selective wireless gas sensing.

Wireless sensors are devices in which sensing electronic transducers are spatially and galvanically separated from their associated readout/display components. The main benefits of wireless sensors, as compared to traditional tethered sensors, include the non-obtrusive nature of their installations, higher nodal densities, and lower installation costs without the need for extensive wiring.1–3 These attractive features of wireless sensors facilitate their development toward measurements of a wide range of physical, chemical, and biological parameters of interest. Examples of available wireless sensors include devices for sensing of pH, pressure, and temperature in medical, pharmaceutical, animal health, livestock condition, automotive, and other applications.4–7 Some implementations of wireless gas sensors can be already found in monitoring of analyte gases (e.g. carbon dioxide, water vapor, oxygen, combustibles) in relatively interference-free industrial and indoor environments.8,9 However, unobtrusive wireless gas sensors are urgently needed for many more diverse applications ranging from wearable sensors at the workplace, urban environment, and battlefield, to monitoring of containers with toxic industrial chemicals while in transit, to medical monitoring of hospitalized and in-house patients, to detection of food freshness in individual packages, and to distributed networked sensors over large areas (also known as wireless sensor networks, WSNs). Unfortunately, in these and numerous other practical applications, the available wireless gas sensors fall short of meeting emerging measurement needs in complex environments. In particular, existing wireless gas sensors cannot perform highly selective gas detection in the presence of high levels of interferences and cannot quantitate several components in gas mixtures. 1.1. Diversity Of Monitoring Needs Of Volatiles The monitoring of numerous gases of environmental, industrial, and homeland security concern is needed over the broad range of their regulated exposure concentrations. Figure 1 illustrates the relationships between several regulated exposure levels spanning several orders of magnitude of gas concentrations. Typical examples of concentrations of regulated exposure are presented in Table 110–14 for three groups of toxic volatiles such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), and chemical warfare agents (CWAs). These examples demonstrate the need for gas sensing capabilities with broad measurement dynamic ranges to cover 2 – 4 orders of magnitude in gas concentrations. Figure 1 Examples of regulated vapor-exposure limits established by different organizations: GPL: General Population Limit, established by USACHPPM – U.S. Army Center for Health Promotion and Preventative Medicine; PEL: Permissible Exposure Limit, established . Table 1 Examples of regulated concentration levels (in ppm by volume) from three representative classes of toxic gases: VOCs, TICs, and CWAs.10–14 Additional needs for detection of volatiles originate from medical diagnostics, food safety, process monitoring, and other areas.15–17 In those applications, the types and levels of detected volatiles can provide the needed information for further control actions.