**2. Synthesis of Schiff bases**

Any primary amine can produce Schiff bases under specified conditions when it interacts with an aldehyde or ketone. In terms of structure, a Schiff base is an aldehyde or ketone that has had the carbonyl group (C=O) substituted by an imine or azomethine group, creating a nitrogen analogue. The synthesis of Schiff bases has used a variety of synthetic methods. Only a few straightforward synthetic methods are described here for the benefit of beginners and others who are unfamiliar with organic synthetic chemistry. Hugo Schiff (1864) was the first to produce Schiff bases using an azeotropic distillation reaction between primary amines and an aldehyde or ketone (Compound a, **Figure 1**) [21]. Catalysts of many types, such as acetic acid, p-toluene sulfonic acids, montmorillonite, and acid resin. There have been reports of the synthesis of Schiff bases from primary amines and carbonyl compounds (aldehyde or ketone) using resin, HCl, H2SO4, TiCl4, Mg(ClO4)2, SnCl4, BF3Et2O, ZnCl2, MgSO4, etc. Additionally, under green chemistry circumstances, microwave irradiation of Schiff bases has been modified for solvent-free synthesis [22, 23]. In this approach, aldehyde or ketone-containing carbonyl compounds react with primary amines to produce Schiff bases with a high yield (Compound b, **Figure 1**). The process can proceed at ambient by the use of an appropriate catalyst. Without the use of any solvent, the reactants (primary amines and aldehyde/ketone) are ground in a mortar and pestle (Compound c, **Figure 1**) [24]. Additionally, primary amines and alcohol can be combined to create a Schiff base (Compound d, **Figure 1**). This approach works with a variety of alcohols and amines and does not need any additional equipment or supplements [25].

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

**Figure 1.** *The synthesis of Schiff base derivatives via several synthetic pathways.*

## **3. Chemosensors using Schiff bases to find hazardous metal ions**

For fluorescence turn-on/turn-off detection of different metal cations, a number of chemosensors based on Schiff bases have been created and employed (**Table 1**). Before employing these compounds as chemosensors, there are a few ideas to keep in mind. In order to create a heterogeneous system in an aqueous medium, bulky chains are preferred. This makes it possible to set up a system for quick and affordable separation. Additionally, spectroscopic techniques are used in the field of chemosensing; hence, the Schiff bases should preferably comprise a fluorophore for fluorescent spectroscopic studies [26]. Similar to this, a spacer (which can change the geometry of the system and control the electrical interaction between the receptor and chromophore unit) and a receptor (which is responsible for the selective analyte binding) are needed [27].
