Sensor based devices

Roman C (2008) Modeling the properties of carbon nanotubes fin- sensor-based devices. In Hierold C (ed) Carbon nanotube devices, 2nd edn. Wiley VCH, Weinheim... 

PIM-l thin films exhibit excellent optical sensing properties by changing their color on adsorbing organic vapor, even at very low concentration (Figure 10.12). Although the performance of a fiber-optic spectrometer is better than this, for PIM-l the hindrance effect due to humidity is less due to the hydrophobicity. The microporous nature, as well as the behavior towards solvent, is the key to fabricate sensor-based devices with PIMs. ... 

A new chemical sensor based on surface transverse device has been developed (99) (see Sensors). It resembles a surface acoustic wave sensor with the addition of a metal grating between the tranducer and a different crystal orientation. This sensor operates at 250 mH2 and is ideally suited to measurements of surface-attached mass under fluid immersion. By immohi1i2ing atra2ine to the surface of the sensor device, the detection of atra2ine in the range of 0.06 ppb to 10 ppm was demonstrated. 

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

For example, Novasina S.A. (www.novasina.com), a Swiss company specializing in the manufacture of devices to measure humidity in air, has developed a new sensor based on the non-synthetic application of an ionic liquid. The new concept makes simple use of the close correlation between the water uptake of an ionic liquid and its conductivity increase. In comparison with existing sensors based on polymer membranes, the new type of ionic liquid sensor shows significantly faster response times (up to a factor of 2.5) and less sensitivity to cross contamination (with alcohols, for example). Each sensor device contains about 50 pi of ionic liquid, and the new sensor system became available as a commercial product in 2002. Figure 9-1 shows a picture of the sensor device containing the ionic liquid, and Figure 9-2 displays the whole humidity analyzer as commercialized by Novasina S.A.. 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

A wide variety of solid-state sensors based on hydrogen-specific palladium, metal oxide semiconductor (MOS), CB, electrochemical, and surface acoustic wave (SAW) technology are used in the industry for several years. Microelectromechanical systems (MEMS), and nanotechnology-based devices for the measurement of hydrogen are the recent developments. These developments are mainly driven by the demands of the fuel cell industry. Solid-state approaches are gaining rapid popularity within the industry due to their low cost, low maintenance, replacements, and flexibility of multiple installations with minimal labor. 

There are more issues and complexity to be considered if various micro-electromechanical (MEMS)-type devices are included in the macroelectronics tool kit. As described previously, the materials and devices required for TFTs and circuits can provide adequate electromagnetic (visible and RF) sensitivity for many image-type applications. These materials may also provide satisfactory performance in pressure and strain sensors. Nanotube/nanowire-based devices look promising for various chem-bio sensors.85 However, there is little that is known about the ability to integrate printed microfluidic devices (and other such devices with moving parts) into a roll-to-roll-type process. 

The number of chemical sensors based on LPGs coated with a chemo-sensitive overlay is still quite limited and even lower is the number of those sensors for which the phenomenon of the modal transition is knowingly used to enhance the performances of the devices. Some of them are based on overlays with low refractive index and/or on thick overlays, which have the drawback of slow diffusion limited response times. One application that seems to attract particular interest is the measurement of the relative humidity (RH). 

The device has an all-glass structure and does not involve assembly of multiple components. As a result, we expect that the device will have very small temperature dependence. In addition, the open micronotch FP cavity allows prompt access to gas or liquid samples for direct refractive index measurement, making it possible to be used as an ultracompact chemical sensor based on refractive index measurement. 

The goal of this chapter will be to provide an overview of the use of planar, optically resonant nanophotonic devices for biomolecular detection. Nanophotonics23 24 represents the fusion of nanotechnology with optics and thus it is proposed that sensors based on this technology can combine the advantages of each as discussed above. Although many of the issues are the same, we focus here on optical resonance rather than plasmonic resonance (such as is used in emerging local SPR and surface-enhanced Raman spectroscopy-based biosensors). 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

The limit for the operating temperature of CMOS-microhotplates can be extended by using the microhotplate that was presented in Sect. 4.3. We now detail high-temperature microhotplates with Pt-resistors that have been realized as a single-chip device with integrated circuitry. While the aluminum-based devices presented in Sect. 4.1 were limited to 350 °C, these improved microhotplates can be heated to temperatures up to 500 °C. As the typical resistance value of the Pt-resistor is between 50 and 100 Q, a chip architecture adapted to the low temperature sensor resistance was developed. The system performance was assessed, and chemical measurements have been performed that demonstrate the full functionality of the chip. 

The microsystems may also serve potential applications in material science and in the growing field of nanotechnology. Microhotplates can be used for material processing, and, at the same time, for the monitoring of material properties such as the electrical resistance [10]. Moreover, the microsystems can be applied to determine thermal properties of new materials such as the melting point, especially when only small quantities of material are available [145], so that monolithic microhotplate-based devices are not only powerful sensor systems for a broad range of applications, but also new research tools for sensor science and nanotechnology. 

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