Selective Cadmium Fluorescence Probe Based on Bis-heterocyclic Molecule and its Imaging in Cells

Fluorescence probes that selectively image cadmium are useful for detecting and tracking the amount of Cd²⁺ in cells and tissues. In this study, we designed and synthesized a novel Cd²⁺ fluorescence probe based on the pyridine-pyrimidine structure, 4-(methylsulfanyl)-6-(pyridin-2-yl)pyrimidin-2-amine (3), as a low-molecular-weight fluorescence probe for Cd²⁺. Compound 3 could successfully discriminate between Cd²⁺ and Zn²⁺ and exhibited a highly selective turn-on response toward Cd²⁺ over biologically related metal ions. The dissociation constant (K_d) and the limit of detection (LOD) of 5.4 × 10^- 6 mol L^- 1 and 4.4 × 10^- 7 mol L^- 1, respectively, were calculated using fluorescence titration experiments. Studies with closely related analogs showed that the bis-heterocyclic moiety of 3 acted as both a coordination site for Cd²⁺ and a fluorophore. Further, the methylsulfanyl group of compound 3 is essential for achieving selective and sensitive Cd²⁺ detection. Fluorescence microscopy studies using living cells revealed that the cell membrane permeability of compound 3 is sufficient to detect intracellular Cd²⁺. These results indicate that novel bis-heterocyclic molecule 3 has considerable potential as a fluorescence probe for Cd²⁺ in biological applications.