Indole Based Chemosensors for Bivalent and Trivalent Metal Ions Detection: At a Glance (2011-2025)

This review focuses on the development of Indole based colorimetric and fluorescent chemosensors for the detection of bivalent and trivalent cations including Cu (II), Hg (II), Zn (II), Co (II), Cd (II), Pb (II), Ni (II), Fe (II), Mg(II) and Fe (III), Al (III) ions as well as other metal cations detection. Various sophisticated methods like inductively coupled plasma mass spectroscopy, X-ray spectroscopy, fluorescence spectrometry, voltammetry, atomic absorption spectrometry (AAS), atomic fluorescence spectrometry, high-performance liquid chromatography (HPLC), flow injection analysis (FIA), electrothermal atomization atomic absorption spectrometry (ETAAS), neutron activation analysis, electrophoresis and ion-pair chromatography have been employed to recognize bivalent and trivalent cations in different matrixes like environmental, agricultural and biological. Despite having high sensitivity and selectivity, these methods have some drawbacks like time-consuming, high cost of equipment, required highly skilled technicians and tedious sample preparation. Colorimetric and fluorescent sensors are gaining popularity due to some inherent properties like cost-effectiveness, rapid response and various optical responses even at a trace level of analytes. Different photophysical mechanisms like photo-induced electron transfer (PET), intramolecular charge transfer (ICT) and chelation enhanced fluorescence (CHEF) play an important role in the detection of these metal cations. Indole derivatives exhibited a broad spectrum of biological activity, such as antioxidant, anti-HIV, antibacterial, antimalarial, antiviral, antitubercular, anti-inflammatory, anticancer, antidiabetic and anticholinesterase properties. In this review, we have focused on the applications of Indole based colorimetric and fluorescence chemosensors. Indole and its derivatives were found well-suited for the detection of bivalent and trivalent cations because these compounds exhibited significant photophysical properties due to the presence of ten π electrons, which move throughout the ring and also the ability to form metal complexes with these metal cations due to presence of N- donor atom in Indole ring. The work done on the applications of Indole fluorescent during the period 2011 to 2025 is presented in tabular for better understanding. This review also highlighted various methods of synthesis of Indole and its derivatives. Further, the sensing performances have been compared with some reported organic sensors as presented in tabular form.