Membranes ion-sensitive

As early as 1937, Kolthoff and Sanders32) used silver halide discs as ion-sensitive membranes for the corresponding anion. They found that interferences through redox-systems were negligible with those membranes compared to electrodes of... 

U. Teravaninthorn, Y. Miyahara and T. Moriizumi, The suitability of Ta205 as a solid-state ion-sensitive membrane, Jpn. J. Appl. Phys., 26(12)(1987) 2116-2120. 

P. Gimmel, K.D. Schierbaum, W. Gopel, H.H. van den Vlekkert and N.F. de Rooij, Microstructured solid-state ion-sensitive membranes by thermal oxidation of Ta, Sens. Actuators B Chern., 1(1-6) (1990) 345-349. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

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. 

By conditioning of the poIyHEMA layer in a buffer of fixed pH and primary ion concentration prior to use, the activity of the ions (u,) in the polyHEMA layer is constant. This fixes the inner boimdary potential of the ion sensitive membrane Fot an accurate measurement of an unknown activity of the primary ion in the sample solution based on Nemst law, it is essential that the primary ion activity in the membrane phase (a,) remains constant. This can be achieved by the introduction of proper amounts of primary ion-selective receptor molecules (ligands, L) in the membrane. The extraction of a particular ion in the membrane phase is then determined by the product of the partition of the ion over membrane and aqueous phase (/fp) and the association constant of the ion ionophore complex in the membrane (, ). The association constant /I, determines the free cation activity in the membrane ... 

An ISE produces a potential that is proportional to the concentration of an analyte. Making measurements with an ISE is therefore a form of potentiometry. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the hydrogen ion concentration in a solution. The potential difference across an ion-sensitive membrane is as follows ... 

Errors observed in the use of ISEs fall into three categories. First are errors caused by lack of selectivity. For instance, many CE electrodes lack selectivity against other halide ions. Second are errors introduced by repeated protein coating of the ion-sensitive membrane, or by contamination of the membrane or salt bridge by ions that compete or react with the selected ion and thus alter electrode response. These necessitate periodic changes of the membrane as part of routine maintenance. Finally, the electrolyte exclusion effect, which applies only to indirect methods and is caused by the solvent-displacing effect of lipid and protein in the sample, results in falsely decreased values (see the section on the electrolyte exclusion effect later in this chapter). 

Other ion-sensitive electrodes are not nearly as effective in screening out interfering ions. Selectivity ratios are as low as 1, meaning that the electrode is as sensitive to the interfering ion as to the test ion. Such measurements are valid only when the concentrations of interfering ions are considerably lower than that of the test ion. Ion-sensitive membranes are being continually improved and hold considerable promise for soil chemical analysis. 

With some modifications, an electrode system can be made responsive to enzyme levels in test solution by surrounding the ion-sensitive membrane with substrate molecules. For example, an electrode that will measure the enzyme activity of cholinesterases in blood fractions has been described. GeneraUy, such electrodes must be designed to replenish the substrate since it is consumed in the reaction. 

The construction of ISEs used in clinical measurements is of the membrane electrode type, i.e., the ion-sensitive membrane separates the sample from an internal reference electrolyte, which is the site of the internal reference element, usually a silver wire covered by silver chloride. The membrane can be shaped to different forms such as flat, convex, tubular, etc. Sodium sensitive membranes are made from special composition glass, the other ion-sensitive membranes from a polymer matrix such as plasticized polyvinylchloride (PVC) or silicon rubber. The particular selectivity of polymer membranes is first of all due to a small percentage of active material, e.g., valinomycin, dissolved in the polymer. Important secondary effects have been attributed to the type and permittivity of the polymer. The useful lifetime of the sensors also depends on the polymer. The time response [13] may again depend on membrane composition. 

Tab. 1. solid state thin film ion sensitive membranes... 

R Becht, M Bruns W Hoffmann. H J Ache, Sodium Ion Sensitive Membranes for Microsensors Fabricated by R F sputtering technique Abstract of 44th ISEMeeting, BcAm. 99 )... 

An ionometric system for the analysis of electrolyte solutions has been developed Planar chip structures of ion sensitive field effect tansistors (ISFET s) and ion selective electrodes (ISE s) are used as sensors Their layout allows an easy preparation of the ion sensitive membrane and also a very simple electrical contacting The sensor chips can be clipped to small volume flow-through cells for dynamic measurements An advanced electronic device was developed for measuring both, the ISFET and the ISE signals This system is usefial for basic investigations of ion sensitive materials and can be integrated comfortably into electroanalytical sensor/actuator microsystems... 

The over all chip size is (5 x 8) mm which enables easy chip handling. ISE s prepared on chips of the same dimensions can be used alternatively. This size enables individual preparation of ion sensitive membranes on single chips without lithographic processes. For example, solid state membranes can be deposited on top of the basic pH gates to modify their sensitivity. This was demonstrated by evaporation [9] and sputtering [10], [11] methods. Furthermore, spin-on techniques wd lithographic pattering of polymeric membrane materials are possible at this chip dimension. 

Harrison, D.J., A. Teclemariam, and L.L. Cunningham. 1989. Photopolymerization of plasticizer in ion-sensitive membranes on solid-state sensors. Anal. Chem. 61 246-251. 

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