A logic gate-based fluorogenic probe for Hg2+ detection and its applications in cellular imaging

• RN3 exhibits a selective response towards Hg2+ in a wide pH range.
• A fluorogenic method for the quantitative detection of Hg2+ is established by using RN3.
• RN3 has potential to construct an INHIBIT logic gate owing to the switchable fluorescence of RN3 with the alternative addition of Hg2+ and S2−.
• RN3 can be applied as a fluorescent probe for visualization of Hg2+ in living HeLa cells.

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.

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