A review of recent developments in fluorescent sensors for the selective detection of palladium ions

Palladium is one of the important elements used in various industries, such as fuel cells, automobile, dental crowns and catalysts in drug synthesis. Its extensive use and waste produced have raised pollution emissions to water sources and soils, and have severe effects on plants, animals and humans. As a result, many methods have been developed to assess the levels of palladium in drug chemicals, food and crops. The dietary intake per person is limited to <15 μg day−1, as reported by the World Health Organization (WHO). This issue has led many researchers developing optical probes, hereafter fluorescent sensors, for the sensitive and selective detection of palladium, especially in the environmental and biological settings. The design of functionalized receptors that serve as palladium-selective chemosensors has gained great interest despite the difficulties associated with the similarities among the same group elements (especially platinum). This review provides a general overview of the recent research developments in fluorescent sensors, emphasizing on the structural features for designing fluorescent sensors for the selective detection of palladium ions. Because the number of chemosensors for palladium detection has grown in the past few years, this review was written to provide a comprehensive discussion of these types of sensors reported from 2000 to the present day. This review categorizes the palladium probes based on their sensing mechanisms or mode of interaction towards palladium ions, such as coordination, deallylation, depropargylation, hydrolysis, palladium induced aggregation, photo-induced electron transfer, Claisen rearrangement, CCN bond cleavage, oxidative cyclization and cross coupling reactions. The basic principles involved in the design of chemosensors for the selective analysis of palladium, problems and challenges in the field as well as possible future research directions are presented.