Sensing material

Construction In tin dioxide semiconductor sensors, the sensing material is small sintered particles. For the sensor current flow, particle boundaries form potential energy barriers, which act as a random barrier netw ork. Different types t)f semiconductor gas sensors are shown in Fig. 13..54. 

One of the major themes of boundary lubrication is to transfer the shear stress at the interface of direct solid contact to somewhere inside the lubricating layer, to achieve low friction and high wear resistance. In this sense, materials with low shear strength, such as liquid films, soft metals, and lamella solids, can be employed as candidate lubricants. 

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

A very recent example of the first case is presented by Vilaseca et al. [71] where an LTA coating on a micromachined sensor made the sensor much more selective to ethanol than methane. Moos et al. [72, 73] report H-ZSM5 NH3 sensor based on impedance spectroscopy using the zeolite as active sensing material. At elevated temperatures (>673 K) NH3 still adsorbs significantly in contrast to CO2, NO,... 

However, there seems to be some drawback in the solubility or dispersibility of ion-sensing material in silicone rubber. This is mainly because silicone rubber does not contain a large quantity of plasticizer as the membrane solvent, in which neutral carriers can be dissolved easily, unlike in plasticized-PVC ion-sensing membranes. This issue is serious, especially with silicone-rubber membranes containing neutral carriers that show high crystallinity. Valinomycin, a typical ionophore, seems applicable to silicone-rubber-based K" -selec-tive electrodes [7,8,12-14]. Conventional crown-ether-based neutral carriers are also quite soluble in silicone rubber. 

Nanocrystalline gamma iron oxide (y-Fe203) recently been studied as a gas sensing material, has been synthesised at 70°C using sonication-assisted precipitation technique [21]. The synthesised material was then used for fabricating the... 

Wolfbeis O.S., Leiner M.J.P., Posch H.E., A new sensing material for optical oxygen measurement, Mikrochim. Acta (Vienna) 1986 HI 359. 

Holtz J.H. and Asher S.A., Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials, Nature 1997 389 829-832. 

The scientists from Hong Kong reported83 on a sol-gel derived molecular imprinted polymers (MIPs) based luminescent sensing material that made use of a photoinduced electron transfer (PET) mechanism for a sensing of a non-fluorescent herbicide - 2,4-dichlorophenoxyacetic acid. A new organosilane, 3 - [N,V-bis(9-anthrylmethyl)amino]propyltriethoxysilane, was synthesized and use as the PET sensor monomer. The sensing MIPs material was fabricated by a conventional sol-gel process. 

The 02-sensing materials based on spin-coated n-octyl-triethoxysilane (Octyl-triEOS) / tetraethylorthosilane (TEOS) composite xerogel films was synthesized and investigated99. These sensors were based on the O2 quenching of tris(4,7-diphenyl-l,10-phenanthroline)ruthenium (II)... 

Leung M., Chow C., Lam M., A sol-gel derived molecular imprinted luminescent PET sensing material for 2,4-dichlorophenoxyacetic acid, J. Mater. Chem. 2001 11 2985-2991. 

As opposed to conventional analytical techniques, optical sensors and biosensors, particularly those employing absorption and fluorescence-based sensing materials potentially allow for measurement through transparent or semi-transparent materials in a non-destructive fashion4, 5> 9 10. Chemical sensor technology has developed rapidly over the past years and a number of systems for food applications have been introduced and evaluated with foods. 

At the same time, many practical issued associated with the use of optical oxygen sensors in food packs still remain. These have to be addressed to adapt the existing sensing materials and prototype systems for real-life applications, achieve the required sensor specifications, operational performance and safety. Considerable technological developments and effort in eliminating current problems and bottlenecks are required, to facilitate widespread use of the oxygen sensors by food and packaging industry. 

In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications as health disorders are usually correlated to ion activity. While most ISEs are used in vitro, the possibility to perform measurements in vivo and continuously with implanted sensors could arm a physician with a valuable diagnostic tool. In-vivo detection is still a challenge, as sensors must meet two strict requirements first, minimally perturb the in-vivo environment, which could be problematic due to injuries and inflammation often created by an implanted sensor and also due to leaching of sensing materials second, the sensor must not be susceptible to this environment, and effects of protein adsorption, cell adhesion, and extraction of lipophilic species on a sensor response must be diminished [13], Nevertheless, direct electrolyte measurements in situ in rabbit muscles and in a porcine beating heart were successfully performed with microfabricated sensor arrays [18],... 

Size-related problems may become important for all microsensors. Leakage of sensing materials from a small membrane may lead to rapid deterioration of sensor properties [104], While the lipophilicity of membrane components cannot be increased infinitely, immobilization of ionophore and ion exchanger in the polymer by covalent attachment or molecular imprinting along with utilization of plasticizer-free membranes could help solve the leakage problem. 

Table 4.1 Examples of mechanisms of color generation in nanostructured photonic sensing materials...

Potyrailo, R. A. Mirsky, V. M., Combinatorial and high throughput development of sensing materials The first ten years, Chem. Rev. 2008, 108, 770 813... 

Drift (D) represents a slow, unpredictable fluctuation of the output signal. It has no statistical meaning. Its presence can sometimes be reduced by a careful design of all the individual sensor parts, but cannot be eliminated. It is a rather complex phenomenon, probably due to the aging effects of the microscopic constituents of the sensing material. 

Passive heat-transfer enhancement techniques, retrofitted, 13 267 Passive mixers, in microfluidics, 26 966, 967 Passive noise detectors, 11 673 Passive nondestructive tests, 17 416, 425 Passive reactors, 17 555 Passive sensing materials, 22 706-707 Passive smart textiles, 24 625 Passive solar collection, silica aerogel application, 1 761-762 Pasta products, 26 278 Paste-extrusion process, 18 301-302 Paste forming, ceramics, 5 651 Paste inks, 14 315-316... 

M. Ahmad, N. Mohammad and J. Abdullah, Sensing material for oxygen gas prepared by doping sol-gel film with tris(2,2 -bipyridyl)dichlororuthe-nium complex, J. Non-Crystal. Solids, 290(1) (2001) 86-91. 

Comes M, Marcos MD, Martmez-Manez R, Millan MC, Ros-Lis JV, Sancenon F, Soto J, Villaescusa LA (2006) Anchoring dyes into multidimensional large-pore zeolites a prospective use as chromogenic sensing materials. Chem Eur J 12 2162-2170... 

Abstract Dye-doped polymeric micro- and nanobeads represent smart analytical tools that have become very popular recently. They enable noninvasive contactless sensing and imaging of various analytical parameters on a nanoscale and are also widely employed in composite sensing materials, in suspension arrays, and as labels. This contribution gives an overview of materials and techniques used for preparation of dye-doped polymeric beads. It also provides examples of bead materials and their applications for optical sensing and imaging. 

Dye-doped polymeric beads are commonly employed in different formats (Fig. 5), namely as water-dispersible nanosensors, labels and in composite materials (DLR-referenced and multianalyte sensors, sensor arrays, magnetic materials, etc.). The sensing properties of the dye-doped beads are of little or no relevance in some more specific materials, e.g., the beads intended for photodynamic therapy (PDT). The different formats and applications of the beads will be discussed in more detail in the following section, and the relative examples of sensing materials will be given. 

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