Fluorescent molecular sensors of anions

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. 

The methods of anion detection based on fluorescence involve quenching, complex formation, redox reactions and substitution reactions (Fernandez-Gutierrez and Munoz de la Pena, 1985). This chapter will be restricted to anion molecular sensors based on collisional quenching (in general, they exhibit a poor selectivity) and on recognition by an anion receptor linked to a fluorophore (fluoroionophore). 

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Kovalchuk A, Bricks JL, Reck G et al (2004) A charge transfer-type fluorescent molecular sensor that lights up in the visible upon hydrogen bond-assisted complexation of anions. Chem Commun 1946-1947... 

The design of fluorescent sensors is of major importance because of the high demand in analytical chemistry, clinical biochemistry, medicine, the environment, etc. Numerous chemical and biochemical analytes can be detected by fluorescence methods cations (H+, Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Pb2+, Al3+, Cd2+, etc.), anions (halide ions, citrates, carboxylates, phosphates, ATP, etc.), neutral molecules (sugars, e.g. glucose, etc.) and gases (O2, CO2, NO, etc.). There is already a wide choice of fluorescent molecular sensors for particular applications and many of them are commercially available. However, there is still a need for sensors with improved selectivity and minimum perturbation of the microenvironment to be probed. Moreover, there is the potential for progress in the development of fluorescent sensors for biochemical analytes (amino acids, coenzymes, carbohydrates, nucleosides, nucleotides, etc.). 

Many fluorescent molecular sensors for halide ions (except F ) are based on collisional quenching of a dye. In particular, the determination of chloride anions in living cells is done according to this principle. Examples of halide ion sensors are given in Figure 10.29. 

The drawback of these molecular sensors is their lack of selectivity, as shown by the Stern-Volmer constants (Table 10.4). For instance A-l, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) is mainly used as a Cl -sensitive fluorescent indicator, but its fluorescence is also quenched by several other anions (I-, Br and SCN-, but not by NO ). 

Boronic acids RB(OH)2, boronic esters RB(OR)2, and triorganoboranes R3B have been studied extensively for the selective recognition of anions. " Moreover, arylboronic acids have proven highly useful for sensing of saccharides based on the facile formation of esters with diols, thus providing an excellent alternative to more commonly employed synthetic molecular receptors based on hydrogenbonding interactions. Absorption and fluorescence spectroscopy, circular dichroism, and cyclic voltammetry have all been successfiilly used as detection methods for organoboron sensors. 

Fluorescence Sensing of Anions, p. 566 Fluorescent Sensors, p. 572 Molecular Logic Gates, p. 893 Molecular Squares, Boxes, and Cubes, p. 909 Molecular Switches, p. 917 Molecular Wires, p. 925 Molecular-Level Machines, p. 931 Photochemical Sensors, p. 1053 Rotaxanes and Pseudorotaxanes, p. 1194 Self-Assembling Catenanes, p. 1240 Self-Assembly Definition and Kinetic arid Thermodynamic Considerations, p. 1248... 

The squaraine rotaxanes based on the macrocycle 16b exhibit intense NIR absorption and emission maxima, and it should be possible to develop them into molecular probes for many types of photonic and bioimaging applications. In contrast, the squaraine fluorescence intensity is greatly diminished when the dye is encapsulated with macrocycle 18. The fluorescence is restored when a suitable anionic guest is used to displace the squaraine dye from a pseudorotaxane complex, which indicates that the multicomponent system might be applicable as a fluorescent anion sensor. 

Small molecule anion sensors have been an active field of research for the last 20 years. Fluorescent sensors have received significant attention and have been reviewed exten-sively. They offer advantages such as high sensitivity and simple instrumentation. The incorporation of a fiuo-rophore such as the molecular clip, indolocarbazoles, and indoles described previously not only provide a reporter group but can also provide a structural element to the receptor. 

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