Anion sensor molecules

Some analytes, such as riboflavin (vitamin B2)16 and polycyclic aromatic compounds (an important class of carcinogens), are naturally fluorescent and can be analyzed directly. Most compounds are not luminescent. However, coupling to a fluorescent moiety provides a route to sensitive analyses. Fluorescein is a strongly fluorescent compound that can be coupled to many molecules for analytical purposes. Fluorescent labeling of fingerprints is a powerful tool in forensic analysis.17 Sensor molecules whose luminescence responds selectively to a variety of simple cations and anions are available.18 Ca2+ can be measured from the fluorescence of a complex it forms with a derivative of fluorescein called calcein. 

Fabbrizzi and co-workers have demonstrated the use of bis-copper(II) cryptates to sense ambidentate anions [49]. On titrating molecule 71 with NaN3 in aqueous solution, the colour changed from pale blue to bright green and an anion-metal LMCT absorption appeared at 400 nm. X-ray diffraction studies have shown that the azide anion is held colinear with the two Cu(II) centres, coordinated through the two terminal sp2-hybridised nitrogen atoms. Stability constants for 71 with a variety of anions in aqueous solution were calculated and the selectivity of this anion sensor was found to be controlled by the bite distance between the two copper atoms (Fig. 1). 

One example of the cavitand-based anion sensor, an open-ended host molecule with a vase-shaped cavity, was synthesized by Rebek and coworkers [33]. In this study, cavitand-based dipyrrolyl quinoxaline compound 36, featuring four anion-detecting walls, was synthesized and showed a selective binding for F" and AcO" in toluene acetone (4 1 v/v). A dramatic color change from yellow to dark red was observed. A band at =420 nm disappeared and two new bands at A=350 and 490 nm emerged in the UV-vis spectrum. 

Ferrocene units appended with secondary amides have also been used for anion recognition (113-115) (231). Being neutral, unlike cobaltocenium based systems, these receptors have no inherent electrostatic attraction making the NMR stability constants much lower in magnitude than for the analogous cobaltocenium systems. Electrostatic interactions can, however, be switched on by oxidation of ferrocene to ferrocenium and consequently these molecules show interesting electrochemical effects and have a potential as amperometric anion sensors. Of interest to this development of sensor technology were the novel results of electrochemical competition experiments. These results demonstrated... 

Another lanthanide-based anion sensor 2 exploits a bipyridine phosphine oxide derivative as a pentadentate ligand to coordinate Eu cation. As anions bind to this complex, emission intensity was increased, presumably again due to the displacement of ligated water molecules. 

The set of eleven pH sensors had Nernstian slopes of 56.6-58.7 mV decade" while the resistances lying in the range 30kD-l MQ must relate to the incorporated anionic sites. Such favourable features, combined with the lipophilicity of the sensor molecules, have facilitated measurements with a miniprobe in gastric fluids (53). 

The design of indnced-fit anion sensors in not limited to the trialkylbenzene motif. Calixarenes with pendant binding arms have also been explored with pyrene derivatives typically nsed as the Unorophore. Pyrene excimer emission has been nsed as a Unorescent reporter which reqnires close contact between adjacent pyrene molecules for its formation. They are an ideal candidate for indnced-fit sensing as conformational changes can dramatically affect intramolecnlar pyrene-pyrene distances in both the gronnd and excited states. 

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. 

Colorimetric sensors are sensors in which binding of an anion results in a visible color change. They are a particularly attractive form of anion sensor as qualitative results can be achieved through naked eye sensing which does not require any equipment. There have been many examples of small molecule colorimetric sensors with a selection discussed below to give a flavor of the concepts which can be used when designing receptors. 

Abstract The click generation of a 1,4-disubstituted triazole from a terminal alkyne and a terminal azide has been widely used in recent years to prepare a plethora of sensor systems for the detectirai of cations, anions, small molecules and biomolecules. The focus of the chapter is oti sensors in which the triazole plays an intimate role in the sensing event, although some examples of its use as a means of ligation are also presented. 

Thioureas-binding site can be introduced to molecules like anthraquinone and is for use in various kinds of anion or fluoride sensors. Thiourea-based sensor are generally designed to recognize anion through an intermolecular hydrogen bonding.167 171... 

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