Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy Metal Ions

*Sabeel M. Basheer, Puthiyavalappil Rasin, Vipin Manakkadan, Vishnunarayanan Namboothiri Vadakkedathu Palakkeezhillam and Anandaram Sreekanth*

## **Abstract**

The Schiff base was first synthesized by Hugo Schiff through the condensation reaction of primary amines with carbonyl compounds (aldehyde or ketone) in 1864. Schiff bases exhibit many structural and electrical characteristics that enable their use in a variety of fields, including medical and chemosensing. Schiff bases generate stable complexes when they bind with different metal ions. Schiff bases are employed as fluorescent turn-on/turn-off chemosensors for the detection of various metal cations, such as Hg2+, Cd2+, Cr3+, Pd2+, and As3+ in various materials due to their outstanding coordination ability. This chapter examines a variety of Schiff bases that are employed in chemosensing procedures for various metal ions (such as divalent and trivalent cations) in various biological, agricultural, and environmental settings.

**Keywords:** thiosemicarbazones, chemosensor, fluorescence, copper, fluoride ion

## **1. Introduction**

Various fields use metallic cations, some of which control countless biological processes necessary for life. However, since they are not biodegradable and can accumulate in the food chain, the excess of these ions can lead to serious environmental issues. Even at low concentrations, they constitute a substantial threat to both the environment and human health. Numerous health issues, such as allergies, lung damage, anemia, kidney failure, neurotoxicity, genotoxicity, oxidative toxicity, steroidogenic toxicity, sperm toxicity, apoptotic toxicity, and axillary toxicity, can be brought on by these ions [1–10]. So it is crucial to architect an effective method for identifying these ions in various samples. In order to detect metallic cations, different techniques have been developed, including liquid chromatography [11], electrochemistry [12], voltammetry [13], reversed-phase high-performance liquid chromatography [14], and inductively coupled plasma-mass spectrometry [15]. Even though these methods are

quite effective, they nevertheless have certain drawbacks, such as being expensive, difficult to use, requiring a lot of sample preparation, and providing ambient conditions for their use. As an alternative to the aforementioned drawbacks of conventional techniques, fluorescent-based Schiff bases have been developed for the detection of metallic cations. They serve as stabilizers for polymers, catalysts, pigments, and dyes, intermediates in organic synthesis, and pigments and dyes [16]. Additionally, a variety of biological activities involving antifungal, antibacterial, antimalarial, antiproliferative, anti-inflammatory, antiviral, and antipyretic effects have been linked to Schiff bases [17, 18]. There are imine or azomethine groups in a variety of natural, natural-derived, and synthetic substances. It has been demonstrated that these compounds' imine group is essential for their biological actions. Schiff bases with great sensitivity and selectivity to a variety of species (cations and anions) were demonstrated by the spectrofluorometric method. In particular, chemosensors based on Schiff bases have shown exceptional performance for the identification of different metallic cations due to their simple and affordable production as well as their capacity to coordinate with practically all metal ions and stable them in a variety of oxidation states. Additionally, these Schiff bases have demonstrated a wide range of biological uses, such as anti-ulcerogenic, analgesic, antifungal, anti-inflammatory, anti-viral, antioxidant, and anticancer effects. In various processes, many of these compounds exhibit outstanding catalytic efficiency [19, 20]. Various Schiff bases have been examined in this review article as fluorescent turn-on/turn-off chemosensors for the detection of different metal ions in various matrices. Additionally, many Schiff base chemosensors' synthetic processes and sensing mechanisms have been studied.
