Anions chemical sensors

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Conducting polymers based on polymer chains with conjugated double bonds are electroactive materials that have found widespread use also in the field of chemical sensors [11-41], Oxidation of the conjugated polymer backbone is accompanied by anion insertion or cation expulsion, as follows ... 

As with the majority of ISEs, all of the aforementioned receptors are immobilised within close proximity to the transducer element. However, conducting polymers (electroactive conjugated polymers) are now emerging rapidly as one of the most promising classes of transducer for use within chemical sensors. Here, the receptor can be doped within the polymer matrix, i.e. within the transducer element itself. This will facilitate the production of reliable, cost-effective, miniaturised anion-selective sensors, as it will be possible to move away from plasticiser-based membranes, but allow for ion recognition sites in conjunction with all-solid-state ion-to-electron transducers. 

Although, the main emphasis of this chapter lies on the fundamental aspects of calixpyrrole-fluoride complexation reactions, with the major part being devoted to the thermodynamic properties of these systems, calixpyrrole receptors seem to be promising for the development of chemical sensors and for the removal of fluorides from water. They also show promise for the separation of anion substrates. 

As far as chemical sensors are concerned, colorimetric chemosensors for anions based on calix[4]pyrrole (16-18, 22-31) showed strong binding to the fluoride anion. Receptors (29-31) are the first naked-eye detectable chemosensors that are able to discriminate between different anionic substrates as a result of detectable colour changes. On the other hand, the fluorescence of the receptors (16-18, 22-28) is quenched significantly in the presence of anionic guests. 

Bipyridinium cation 9 [58], phosphonium [59-62], guanidinium [63], and vina-midinium [64] cations were used to form thermotropic ionic liquid crystals. For example, the mixture of diheptyl and dioctyl viologens 9 (20 80 by wt%) with TFSI anion forms a smectic A phase between 22° and 132°C. It was reported that the viologens show fluorescence in both non-polar and polar organic solvents, which can be useful for the development of biological and chemical sensors [65]. 

Organotin Compounds as Anion-Selective Carriers in Chemical Sensors... 

It is the aim of this chapter is to present the efforts made worldwide for the development of chemical sensors based on the unique chemical recognition capabilities of organotin structures. In particular, we will examine in a time-based flowchart the progress of the design and application of Sn(IV)-based ionophores and their application in the development of anion selective chemical potentiometric sensors. 

One of the most important categories of ion selective chemical sensors is based on what are called liquid membranes. This term was flrst used in 196U to describe a matrix that is not water soluble it contains either anionic or cationic sites (liquid ion exchangers), which can selectivity facilitate the exchange of inorganic ions. In order to study the active carrier-mediated ion transport through these liquid membranes, a cell such as the one shown in Figure 3.4.4 has been employed. 

This section attempts to present a broad review of this technique in the light of recent research on fiber-optic chemical sensors (FOCS). A discussion on the advantages and performance of pH optodes will be broached by considering the various applications planned and the resulting pH measurements, including titration and ionic strength. Sensors for low molecular weight electrolytes, particularly optodes for anions and cations in solution, are also considered. 

The assembly of the nickel(ll)/H-atu cage 13, is accompanied by a dramatic colour change, from orange to green. Considering that in methanol only chloride acts as a template for the formation of cage 13, we have recently employed this anion-templated process to develop a colorimetric chemical sensor for chlorides [31]. 

Biihlmann, P. Amemiya. S. Nishizawa, S. Xiao. K.P. Umezawa, Y. Hydrogen-bonding ionophores for inorganic anions and nucleotides and their application in chemical sensors. J. Incl. Phenom. Mol. Recognit. Chem. 1998. 32, 151-163. 

These photophysical properties have been exploited to design chemical sensors for O2 [47], cations [46] and anions [48]. Several rhenium(I) polypyridine biotin compounds were shown to display up to a 3-fold enhancement of their emission intensity upon binding to the protein avidin [49]. 

The second type of CO2 fiber-optic chemical sensor is constructed by using ion pairs consisting of a pH indicator anion and an organic quaternary cation. First, a pH indicator dye (DH) and a quaternary ammonium hydroxide (Q OH ) are entrapped into a proton-impermeable but CO2-permeable polymer membrane, which is then immobilized onto the fiber s surface. The mechanism of this CO2 sensor is based on the interaction between the dye molecules (DH) and the quaternary cations (Q+OH ) to form hydrated ion pairs (Q D XH2O). The hydrated ion pair is dissolved in the polymer, where it reacts with CO2 according to the following reaction ... 

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