Anion sensor arrays

Additional improvements can be achieved through the use of multilayers (based on different overlaid films). Such combination of the properties of different films has been documented with bilayers of Nation/CA (14) and Nafion/collagen (29). The former allows selective measurements of the neurotransmitter dopamine in the presence of the slightly larger epinephrine and the anionic ascorbic acid (Figure 5). In addition to bilayers, mixed (composite) films, such as PVP/CA (75) or polypyrrole/Eastman Kodak AQ (30) layers can offer additional permselectivity advantages, such composites exhibit properties superior to those of their individual components. Also promising are sensor arrays, based on electrodes coated with... 

Some recent advancement in this field, based on sensor arrays, is reported. An early report by Reinhoudt and Crego-Calama entailed the construction of a self-assembled hbrary of addressed anion receptors on glass, which could be of potential use in the development of microarray systems [67]. 

More recently, Anzenbacher et al. developed a supramolecular-based sensor array for detection of complex matrices of anions [68]. The arrays were prepared by implementing colorimetric sensors in polyurethane hydrogel. The sensor elements are reported in Fig. 20, based on calyxpyrrole, and anthraquinine moieties. 

An eight sensor-array was produced, showing selectivity for fluoride and pyrophosphate ions, and concomitant cross-reactivity for carboxylate, phosphate, and chloride. This was used to differentiate between 10 anions (Fig. 21). 

Fig. 21 Left. Typical eight-sensor array response to anion aqueous solution (200 nL, 5 mM NOs and HS04 concentration was 20 mM). Centre-. Net response profile of sensors 1-8 to the addition of aqueous AcO (200 nL). Right Pattern generated by the array in the green channel in the presence in the same conditions used in the left panel. Reprinted with permission from [68]...

Two types of potentiometric electrodes are usually described, the ISE, which was developed to selectively recognize ions and used to analyze ions (anions or cations) in a solution and the cross-sensitivity electrodes (CSE), used in sensor arrays for solution analysis, which due to their low selectivity gives, as output, a matrix fingerprint. E-tongue systems (multisensor systems with several low-selective sensors) are an example of this application. 

An automated electronic tongue consisting of an array of potentiometric sensors and an artificial neural network (ANN) was developed to resolve mixtures of anionic surfactants. The sensor array was formed by five different flow-through sensors for anionic surfactants, based on polyvinyl chloride membranes having cross-sensitivity features. 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

As mentioned for the previous libraries, these varied responses (especially the increases in fluorescence) across the library help decrease the chances of false-positives for the individual analytes. Additionally, amino-functionalized TO is an excellent sensor for HStTf not only is the magnitude of the fluorescence increase quite large (72%), but it is the only anion that induces such an increase. The addition of AcO induces a quenching of 34%, while N03 and P PCV result in virtually no response. These results support those found for the previous cation systems, wherein making a library of fluorophores and binding groups results in a unique array of responses to the anions. 

Anion detection at microelectrodes has not been studied widely. Amongst the first was the work of de Beer et al. [ 111 ] who manufactured a nitrite sensor with a tip just a few microns in diameter, which could detect nitrite ions down to 1 pM. This proved to be suitable for profiling the concentrations of nitrite anion within biofilms less than 1-mm thick inside water treatment plants. Other workers have found that use of an interdigitated microelectrode array [ 112] allows measurement of iodide via monitoring of its redox peak down to sub-micromole levels, making it a suitable technique for analysing mineral water. Carbon nanotubes coated onto Pt microdiscs have been utilised to make a nitrite sensor [113,114] with detection levels of 0.1 pM. Sulphide has also been detected at nickel microdiscs (50 pm diameter) [115]. 

The use of anion-centred polyhedra can be particularly useful in describing diffusion in fast ion conductors. These materials, which are solids that have an ionic conductivity approaching that of liquids, find use in batteries and sensors. An example is the high temperature form of silver iodide, a-Agl. In this material, the iodide anions form a body-centred cubic array, (Figure 7.20a). 

With precise design criteria established, supramolecular interactions have been exploited to design and synthesise new and functional mechanically interlocked molecules in high yields [9]. Over a relatively short period of time, mastery of controlled synthesis of mechanically interlocked molecules has led to an extraordinary array of functional materials and molecular machines [10]. Examples of these include logic gates, sensors for explosives, anions and cations, in addition to exquisite molecules with complex topologies (such as molecular knots) that have yet to be thoroughly exploited. 

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