Sensors, chemically selective

The signal from continuous chemical sensors is continuous in time. It follows changes in the concentration of the analyte up and down. The signal often originates from the interaction of the analyte with a chemically selective layer... 

The interdisciplinary nature of optical chemical sensors opens a variety of new directions in sensor development. The issue of chemical selectivity is... 

Grate, J. W. Abraham, M. H., Solubility interaction and design of chemically selective sorbent coatings for chemical sensor and arrays, Sens. Actuators B 1991, 3, 85 111... 

In addition to sensitivity, an important feature for any biochemical sensor is selectivity, i.e., the ability to response to a specific chemical or agent. In additional to the obvious significance (reducing the false alarm rate, etc.), this ability is... 

Hence, by using the above design it is possible to sensitively (parts per million -few parts per billion) detect any changes in the resonators unperturbed response (Fig. 15.3). The remaining task is to find specificity in these changes. These changes can be due to any foreign material until we are able to specify what material, the sole purpose of the sensor remains half complete. The next phase of the experiment is to find out how to eliminate the role of the vacuum only and to introduce chemical selectivity of the resonator for a specific toxin. 

The key to achieving chemical selectivity using microcantilevers is the ability to functionalize one surface of the microcantilever with receptive molecules so that explosive molecules will preferentially bind to the treated surface. Choosing receptor molecules that can provide highest affinity, therefore, can control the selectivity of detection. Another important requirement for a sensor system is fast regeneration (recovery), so that the sensor can be used repetitively. 

The federal government must take the lead in supporting the research and development effort for ocean instrumentation because the government, through its agencies, is the beneficiary and ultimate consumer of most ocean measurements. Regardless of the approaches taken, it is clear that a considerable investment of time and resources will be required to develop the arsenal of chemically selective and stable host compounds that will be required to measure all the key oceanic species via in situ chemical sensor technology. 

Angel, S.M. 1987. Optrodes Chemically selective fiber-optic sensors. Spectroscopy 2(4) 38-48. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Development of this sensor was based on a concept very different from that of conventional chemical sensors, which selectively detect specific chemical substances such as glucose or urea. However, taste cannot be measured in those terms even if all the chemical substances contained in foodstuffs are measured. Humans do not distinguish each chemical substance, but express the taste in itself the relationship between chemical substances and taste is not clear. It is also not practical to arrange so many chemical sensors with respect to the number of chemical substances, which amounts to over 1000 in one kind of foodstuff. Moreover, there exist interactions between taste substances, such as the synergistic effect or the suppression effect. A taste sensor should measure these effects the intention is not to measure the amount of each chemical substance but to measure the taste itself, and to express it quantitatively. The recently developed sensor satisfies this request. In fact, this sensor could detect the interactions between saltiness and sourness. 

Clearly, a mass-related signal will be obtained only if the species-sensor interaction results in a net change of mass of the chemically selective layer attached to the device. Thus, an equilibrium binding will yield a measurable signal. On the... 

Various thin selective layers, as discussed in Chapter 1, can be used to provide chemical selectivity. As with piezoelectric crystals, the acoustic properties of these films affect the performance of the SAW sensor in different ways by the change of... 

Because enzymes present such an attractive possibility for achieving chemical selectivity, enzyme electrodes were the first enzymatic chemical sensors (or first biosensors) made. The early designs used any available method of immobilization of the enzyme at the surface of the electrode. Thus, physical entrapment using dialysis membranes, meshes, and various covalent immobilization schemes have been... 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

Egorov, O. B., Fiskum, S. K., O Hara, M. J., and Grate, J. W., Radionuclide sensors based on chemically selective scintillating microspheres Renewable column sensor for analysis of 99Tc in water, Anal. Chem., 71, 5420-5429, 1999. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

This detector is based on the collective oscillations of the free electron plasma at a metal surface. Typically a prism is coated with a metal film and the film coated with a chemically selective layer. The surface is illuminated by a laser and the amount of material adsorbed by the coating affects the angle of the deflected beam. This platform is theoretically similar in sensitivity to a quartz crystal microbalance. This is another platform whose selectivity is based on the coating. The typical coating is using bound antibodies thus, this device becomes a platform for immuno-sensors (12). 

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