Chemical vapor sensor

Chemical vapor sensors play an ever-increasing role in the environmental monitoring, homeland security, defense, and health care. The desirable characteristics of a chemical vapor sensor include ultrahigh sensitivity, specific and rapid response to certain vapor molecules, as well as the ability for on-the-spot chemical analysis, which usually requires the sensor to be small, portable, reusable, stable, robust, and cost effective. Toward this end, various sensing techniques have been studied... 

Theoretical Analysis of the OFRR Chemical Vapor Sensor... 

In this part, we demonstrate OFRR s capability as a rapid chemical vapor sensor. During experiments, ethanol and hexane vapors are used as a model system and represent polar and nonpolar analytes, respectively. 

Li, J. The Cyranose Chemical Vapor Sensor. Sensors Magazine Online, August 2000 visited 9/19/05 http //www.sensorsmag.com/articles/0800/56/main.shtml. 

Detecting Chemical Vapors with the FPW Sensor A practical FPW chemical vapor sensor typically employs a sorptive film on the plate, as do the TSM, SAW and APM devices (Figure 3.43, page 122). When calculating the mass sensitivity of the coated FPW sensor, we simply include the mass per unit area of... 

In chemical vapor sensor applications, it is often necessary to flow the vapors to be monitored over the AW sensor. Air-flow rate can affect both the sensor baseline signal and the apparent response time of the sensor. The baseline signal is affected because air flow over the device can cause cooling, which results in a small shift in the device temperature and, therefore, wave velocity. This effect has been exploited to construct an AW-based flow-rate sensor by heating a SAW device and using the device frequency as a measure of its temperature [37]. For... 

This section will broadly describe some of the practical system design strategies that are being used to minimize problems that sometimes plague all types of chemical vapor sensors such as baseline drift, inadequate sensitivity, and inadequate selectivity. While special emphasis will be given to acoustic sensors, the approaches described here are generally applicable to any vapor sensing device. 

Overall, the addition of a vapor-concentrator device to a chemical vapor sensor can produce dramatic enhancements in performance. Significantly lower vapor concentrations can be reliably detected from the combined effects of source concentration enrichment, which increases the apparent sensor signal, and baseline drift compensation, which reduces the apparent noise produced by the sensor. Unfortunately, these performance enhancements come with a fairly heavy price in the form of additional pumps, valves, traps, and increased energy consumption requirements. 

Piezoelectric bulk acoustic wave (BAW) chemical vapor sensor Glucose sensor... 

Fig. 3.9 (Left) Surface acoustic wave (SAW) resonator and (right) SAW dual delay line device (on penny). Quartz-based SAW sensors, coated with chemically selective films, can detect chemical vapors. Sensor arrays, with diverse coatings, can detect multiple chemical vapors, image courtesy of pacific northwest national laboratory...

It is noteworthy that the wetting effect caused PL quenching, as an extrinsic sensing mechanism, is likely extendable to other QD-polymer systems used as chemical vapor sensors for detection of different molecular species. The complexity arises from solid-state optical systems for vapor detections involving a combination of nanoparticles and polymer interaction with chemical species under illumination. This is an important point to which to pay attention in the studying and development of this type of dependable QD-based chemical sensor. 

The objectives of this paper are threefold. First, we will review the basic operating principles of SAW chemical vapor sensors including a theoretical model that predicts the sensitivity of SAW device response. Secondly, design considerations that affect the performance of SAW sensors will be presented. Finally, some recent results from various SAW sensors and an experimental four sensor SAW array will be discussed. 

A dual SAW delay line oscillator is illustrated in Figure 3. It consists of two SAW delay lines fabricated on the same ST-quartz substrate. One delay line is coated with the chemically sensitive film and the other is left uncoated. The frequencies of the two delay line oscillators are mixed and filtered to provide a frequency equal to the difference of the two oscillator frequencies. This scheme has several advantages when used in chemical vapor sensor applications. First, it helps to compensate for SAW frequency drift caused by ambient temperature and pressure fluctuations. Additionally, the difference frequency is much lower than the frequency of the oscillators themselves. This permits SAW vapor response measurements using inexpensive digital circuitry. 

This paper has dealt exclusively with SAW sensors that exploit the mass sensitivity of the device to achieve chemical vapor detection. Schemes to exploit the SAW sensitivity to coating conductance changes (17) or elastic modulus changes should afford new opportunities for imaginative chemical vapor sensor designs. Finally, the field of liquid phase chemical analysis may also yield to surface acoustic wave devices that utilize plate waves and horizontally polarized shear waves to minimize acoustic losses in the liquid (18). 

Wei C, Dai L M, Roy A and Tolle T B (2006) Multifunctional chemical vapor sensors of aligned carbon nanotube and polymer composites, J Am Chem Soc 128 1412-1413. 

Huanget A, Wong VTS, Ho C-M (2006) Silicone polymer chemical vapor sensors fabricated by direct polymer patterning on substrate technique (DPPOST). Sens Actuators B 116 2-10 Huo H, Ren H, Wang C, Shen M (2011) Highly sensitive gas sensors on low-cost nanostructured polymer substrates. Intern J Smart Nano Mater 2(1) 1-8... 

The sensitivity of piezoelectric detectors is directly proportional to the square of resonant frequency, and inversely proportional to the surface area. Therefore, a SAW device is considered to be an excellent transducer of chemical vapor sensor. Besides sensitivity, the SAW device could be made at relatively low cost, and miniaturized with precise and reproducible characteristics using photolithographic techniques. In addition, the lithographic fabrication capability easily permits a complex circuit to be present on the same crystal surface. 

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