Highly sensitive and selective fluorescent cation sensors 1–3, based on heterocyclic derivatives, have been synthesized from the condensation reaction between their corresponding heterocyclic aldehydes and thiosemicarbazide. All of the receptors 1–3 can act as optical sensors for the detection of Cu²⁺ and Hg²⁺ ions, and as fluorescent sensors for the Cr³⁺ ion, via a chelation enhanced fluorescence effect in an aqueous environment. The structures of the receptors 1–3 were well characterized by IR, ¹H NMR, ¹³C NMR and HRMS techniques and their cation sensing abilities were studied by UV-vis and fluorescence spectroscopy.