**4. The sensing mechanism of the Schiff base chemosensor, including its selectivity and sensitivity**

The electron configuration of both the ligand and the metal ion, as well as the structural stiffness and the capacity of the Schiff base chemosensor to attach to the metal ion, is a parameter that controls the selectivity and high sensitivity of Schiff base chemosensors [28, 29]. Therefore, various mechanisms, such as chelationenhanced fluorescence (CHEF), photo-induced electron transfer (PET), intra/intermolecular charge transfer (ICT), hydrolysis, chelation enhancement quenching effect



 *Different analytical parameters for the analysis of heavy metal ions using Schiff bases.*

**Table 1.**
