New Synthetic Receptors for Molecular Recognition of Anions and Their Practical Applications

Fluoride is beneficial to human health in small quantities; it plays vital roles in preventing dental decay and in treating osteoporosis. In contrast, excess fluoride consumption can cause dental fluorosis or mottled enamel and skeletal fluorosis, a bone disease that is a major problem in India. It has been reported that an excess of fluoride can cause the inhibition of over 100 different enzyme systems. Acute fluoride exposure can cause collagen breakdown, decreased thyroid activity, bone cancer, immune-system disturbance, and anaemia. Recently, fluoride has also been suspected of playing a role in terminal diseases, including those that arise due to damage in the hippocampal area of brain. Despite this knowledge, introduction of fluoride to the environment by excessive usage of fertilizers and industrial wastes continues. Given the balance between positive and negative effects of fluoride, its detection has attained significance. In addition, the detection of organic anions, such as carboxylate, is also of interest as these play vital roles in numerous metabolic processes, such as the glyoxalate cycle, generation of high-energy phosphate bonds, and in the dicarboxylate cycle for autotrophic carbon dioxide fixation. Among the widespread organic anions, the discrimination of geometric isomeric dicarboxylates, such as cis/trans isomers (maleate and fumarate ions), has acquired significance because of the different biological behaviours of these isomers. For example, fumarate is produced during the Krebs cycle, whereas maleate is an inhibitor of the Krebs cycle. Owing to the similar physical and chemical properties, it is difficult to discriminate between isomeric dicarboxylates by using conventional analytical methods. While the discrimination of these geometrical isomeric dicarboxylates has previously been achieved by using a colorimetric/fluorometric detection approach, the receptors employed are complicated organic molecules that require skilled processes and difficult synthetic procedures. Therefore, the development of new ‘easy-to-synthesis’ receptors for the discrimination of geometrical isomers, such as maleate and fumarate, remains a challenging task that needs to be addressed. In general, anion receptors are the molecules that can recognize or sense the anions. While instrumental methods, such as anion monitor probes, are available for the detection of anions, they are time consuming and require skilled efforts to operate. In contrast, colorimetric methods of detection are widely employed due to their low cost, instantaneous results, easy/safe to handle format, and their selectivity and sensitivity, which is typically equal to that of instrumental methods. Therefore, designing novel synthetic receptors for the colorimetric detection of anions has attracted considerable attention in the last two decades, and as a result, colorimetric anion receptor chemistry has been studied extensively.