Ion-sensitive polymers

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

ISFETs Sensitive to Other Ions. Deposition of an ion-selective membrane on top of the gate insulator opens the way to measurement of ions other than H . Most of the sensitive materials for ISE applications described in Section 28,2.3.1,1 have been used in conjunction with ISFETs, providing sensors covering a wide variety of species. Thus, ISFETs with solid membranes have been described with AgCl-AgBr membranes sensitive to Ag . Cr, and Br [149], for example, or with LaFs membranes sensitive to fluoride [150], Ion-sensitive polymer-matrix membranes (liquid membranes with a polymer matrix) have also been used as sensitive membranes for ISFETs. The first experiments involved mainly PVC membranes [151]. [152], but these membranes show poor adhesion and poor mechanical strength. To improve membrane adhesion, modified PVC was utilized as a matrix material [153], The use of silicones as matrix materials has made it possible to prepare very durable ISFETs with polymer-matrix membranes [154], Other reports describe the use of photopolymerized polymers as matrix materials [155], [156],... 

Stimuli-responsive polymers and blends thereof for ophthalmic drug delivery systems are reviewed. These include polyaci ylic acid, temperature sensitive polymers, which are convertible into gels at body temperature, dual responsive polymers, ion-sensitive polymers, such as alginates, and enzyme-sensitive polymers, such as xanthan gum. 26 refs. 

As in the case of bulk optodes, plasticizers can be added to modify the properties of polymers (e.g., gas permeability). Plasticizers are mainly used to design ion-sensitive nanobeads. 

Rininsland F, Xia W, Wittenburg S, Shi X, Stankewicz C, Achyuthan K, McBranch D, Whitten D (2004) Metal ion-mediated polymer superquenching for highly sensitive detection of kinase and phosphatase activities. Proc Natl Acad Sci 101 15295-15300... 

In a similar way, many acidic and basic ion-exchange polymers have been used to quench and work-up reactions as an alternative to aqueous conditions. This is especially valuable if the compound in question is moisture sensitive or hydroscopic, or if a rapid quench and work-up procedure is required. 

As we mentioned before, G(s) is a structure dependent constant that describes the number of scissions per unit absorbed dose and in that sense can be compared to a photochemical quantum efficiency. The G(s) of a radiation-sensitive polymer is a figure of merit that can be used in comparing one material with another. There is a very high correlation between G(s) values to gamma radiation (the radiation most commonly used for determining G(s)) and high sensitivity for lithographic materials used in either electron beam, ion beam or x-ray exposure. 

S. P. Pogprelova, M. Zayats, T. Bourenko, A. B. Kharitonov, O. Lioubashevski, E. Katz, and L Willner, Analysis of NAD(P) /NAD(P)H Cofactors by Imprinted Polymer Membranes Associated with Ion-Sensitive Field-Effect Transistor Devices and Au-Quartz Crystals, Anal. Chem. 2003, 75, 509. 

Electrostatic binding [11] may provide another very useful approach to preconcentration analysis. Enhancement of the redox ion concentration in the ion-exchange polymer volume should permit very sensitive analysis when combined with an appropriate electroanalyti-cal method [12,13]. However, the sensitivity of the ion-exchange equilibrium to the sample solution electrolyte composition and concentration and the necessity of having a multiply charged analyte ion may limit the usefulness of the electrostatic binding approach. 

Our choice for an Ion Sensitive Field Effect Transistor (ISFET) as a transducing element was based on the fact that the SiO surface contains reactive SiOH groups for the covalent attachment of organic molecules and polymers. In addition the FET has fast response times and can be made very small with existing planar IC technology. FIGURE 1... 

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 transistor-based chemical sensor used in aqueous environment must be electrically insulated. An ordinary FET sensor produced with a silicon wafer as starting material has four bare lateral edges, which are made when scribing a processed silicon wafer into FET chips. It has bonding pads on its surface for wires that are connected for the electrical operation of a FET (see Fig. la). It is necessary to insulate the bare lateral sides and bonding pad region, and this is usually done by polymer encapsulation. Because a FET chip is very small and the areas to be insulated are closely located to its ion-sensitive gates, which must... 

Two major improvements in the fabrication of an ion-sensitive FET that avoid most of the tedious polymer encapsulauon process have been reported. Matsuo and his coworkers (4, 37) fabricated a probe-type FET with a three-dimensional silicon nitride passivation layer around most of its surface, as shown in Fig. 2. The probe-type FET has one disadvantage Its fabrication requires a three-dimensional process that is uncommon for semiconductor construction facilities. An alternative approach utilizes a silicon-on-sapphire (SOS) wafer for FET fabrication (38, 39). The structure of a SOS-FET is depicted in Fig. 3. It has an island-like silicon layer on a sapphire substrate, in which an ion-sensitive FET is fabricated. The bare lateral sides do not need encapsulation because of the high insulation property of sapphire. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

Metal ion-imprinted polymers can be applied to the pre-concentration and the sample clean-up stages for metal ion determinations. Most elemental techniques such ICP-AES and ICP-MS suffer from the difficulties imposed by complex matrices that produce high dissolved salt concentrations. The use of imprinted resins for selective extraction of metal ions allows these methods to be used with greater flexibility and can significantly lower detection limits. The selectivity of some imprinted resins has been sufficient to allow selective and sensitive analyses of metal ions at ultra-trace levels using simpler and less expensive detection methods. By reducing the detection step to a simple colorimetric method, economy and simplicity are assured. The combination of imprinted polymer clean-up and colorimetric detection are attractive as the basis of an FIA system for the ultra-trace analysis of a specific metal or combination of metals. 

The ion-sensitive surface of the ISFET is naturally sensitive to pH changes, but the device may be modified so that it becomes sensitive to other species by coating the silicon nitride gate insulator with a polymer containing molecules that tend to form complexes with species other than hydronium ion. Furthermore, several ISFETs may be fabricated on the same substrate so that multiple measurements may be made at the same time. All the ISFETs may detect the same species to enhance accuracy and reliability, or each ISFET may be coated with a different polymer so that measurements of several different species may be made. Their small size (about 1 to 2 mm ), rapid response time relative to glass electrodes, and ruggedness suggest that ISFETs may be the ion detectors of the future for many applications. 

As a model to understand and to describe the processes during the response of a smart gel on changes of enviromnental properties, a two-step mechanism can be assumed (Fig. 8). In a first step, the stimulus which triggers the swelling/shrinking must permeate the gel. Heat transfer for temperature-sensitive polymers or mass transfer (ions, organic solvents) determine the rate of the first step. 

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