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Copper(II), Cu 2+ , ion plays not only a fundamental role to sustain important physiological processes in living organisms, but also an important environmental pollutant. A large number of chemosensors that employ the chromogenic, fluorogenic or electrochemical properties of molecules have been reported for selective sensing of copper ions with absorbance and emission in the visible region. Nevertheless, most of these chemosensors for Cu 2+ ions have some limitations including low water-solubility, tedious sample treatment, multistep synthetic routes, or unstable detection signal. Therefore, the challenge in the development of light-up chemosensors which are cost-effective, rapid, facile, biocompatible and applicable to the environmental and biological milieus is still a topical issue. In the context of optical sensing of Cu 2+ ion, the unique features offered by transition metal complexes over organic fluorophores have made them a suitable candidate to monitor Cu 2+ ions in biological systems. Encouraged by the importance of Cu 2+ ions, we herein summarize the recent development of transition metal complexes, particularly those of d 6 or d 10 complexes based on rhenium(I), ruthenium(II), iridium(III), zinc(II) and gold(I) complexes, for optical and electrochemical sensing or biosensing applications of Cu 2+ ion. [ABSTRACT FROM AUTHOR]