A logic gate-based fluorogenic probe for Hg detection and its applications in cellular imaging

A new colorimetric and fluorogenic probe (RN₃) based on rhodamine-B has been successfully designed and synthesized. It displays a selective response to Hg²⁺ in the aqueous buffer solution over the other competing metals. Upon addition of Hg²⁺, the solution of RN₃ exhibits a ‘naked eye’ observable color change from colorless to red and an intensive fluorescence with about 105-fold enhancement. The changes in the color and fluorescence are ascribed to the ring-opening of spirolactam in rhodamine fluorophore, which is induced by a binding of the constructed receptor to Hg²⁺ with the association and dissociation constants of 0.22 × 10⁵ M⁻¹ and 25.2 μM, respectively. The Job's plot experiment determines a 1:1 binding stoichiometry between RN₃ and Hg²⁺. The resultant “turn-on” fluorescence in buffer solution, allows the application of a method to determine Hg²⁺ levels in the range of 4.0–15.0 μM, with the limit of detection (LOD) calculated at 60.7 nM (3σ/slope). In addition, the fluorescence ‘turn-off’ and color ‘fading-out’ happen to the mixture of RN₃-Hg²⁺ by further addition of I⁻ or S²⁻. The reversible switching cycles of fluorescence intensity upon alternate additions of Hg²⁺ and S²⁻ demonstrate that RN₃ can perform as an INHIBIT logic gate. Furthermore, the potential of RN₃ as a fluorescent probe has been demonstrated for cellular imaging.