Coating chemically selective

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

One of the insidious disadvantages of certain plastics is their tendency to absorb moisture from ambient air and then change their size and properties. There are protective measures that can be taken with these plastics such as coatings, chemical treatments, additives, and so on. To be practical, the best way to circumvent problems of this type is to select a plastic with the lowest possible absorption rate. 

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... 

Graphite composites Polished Wide solvent compatability, increased signal/background current, easy chemical modification Binder can coat carbon Selectivity can be varied with chemical modification 3... 

Research into chemically modified electrodes has led to a number of new ways to build chemical selectivity into films that can be coated onto electrode surfaces. Perhaps the simplest example is the use of the polymer Nafion (see Table 13.2) to make selective electrodes for basic research in neurophysiology [88]. Starting with the pioneering investigations by Ralph Adams, electrochemists have become interested in the electrochemical detection of a class of amine-based neurotransmitters in living organisms. The quintessential example of this class of neurotransmitters is the molecule dopamine, which can be electrochemically oxidized via the following redox reaction ... 

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. 

A number of coatings can be used for chemical selectivity, such as self-assembled monolayers (SAMs), polymers, metal oxides, and single-stranded DNA. The choice of coatings must take into account the response amplitude, response time, and recovery time. We have chosen a condition of combined detection and recovery time less than 1 min. Often, the response time is faster than the recovery time. For our case, a response time of 10 and a recovery time of 50 s were used. 

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). 

The relative importance of the mass-loading and viscoelastic contributions to the observed acoustic sensor response is an issue that has yet to be resolved. Capitalizing on these effects to improve chemical selectivity and detection sensitivity requires further characterization of sensor response, in terms of both velocity and attenuation changes, in addition to more accurate models describing how coating-analyte interactions affect relevant film properties. 

An attractive feature of acoustic-wave-based chemical sensors is that they impose relatively few constraints on the materials that can be used as chemically selective coatings. In brief, the film must be uniform, adherent, thin, chemically and physically stable when in contact with its working medium (gas or liquid), and it must not electrically short-circuit the IDTs. 

Figure 8.8. Selection for improved catalytic activity using single-turnover activity. Phage mixture contains many variants, and each fusion phage carries the DNA coding for its evolved protein that is attached to its coat protein.21 [Reprinted, with permission, from K. D. Janda, Lee-C. Lo, Chih-H. Lo, Mui-M Sim, R. Wang, Chi-H. Wong, and R. Lemer, Science 275, February 1997, 945-948. Chemical Selection for Catalysis in Combinatorial Antibody Libraries . Copyright 1997 by AAAS.]...

A sensor may be thought of as consisting of two parts the transducer which converts a chemical signal to an electronic signal, and a chemically selective layer which interacts directly with the sample. A transducer itself usually does not have the selectivity, dynamic range, or sensitivity required for many measurement applications. Just as a camera uses different lenses for telephoto, macro, or wide-angle shots, a transducer requires the cooperation of a chemically selective material in the form of a coating, membrane, or electrolyte to measure different species in different applications. These materials serve as a selective receptor to mediate between the outside environment and the sensor s transducer. 

Microcantilever sensors do not offer any intrinsic chemical selectivity. Chemical speciation is accomplished by coating the cantilevers with chemically selective layers. A number of chemically selective polymeric coatings have been developed for achieving chemical selectivity in SAW... 

By coating the internal pore surfaces of MCM-41 with functionalized self-assembled monolayers it is possible to impart excellent chemical selectivity for specific toxic metallic species (see Figure 2). For example, installation of a thiol-terminated monolayer provides unprecedented mercury sequestering capability [7,8]. Thiol-SAMMS also is effective for removing other soft heavy metals, such as Cd, Ag ami Au. In fact, the metallated thiols themselves (e.g. Hg and Ag) are also excellent sorbents for sequestering soft anions (e.g. radioiodide) as weU [9]. 

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...

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