New colorimetric rhodamine–helicene “OFF–ON” fluorescence sensor based on FRET for quantitative and qualitative Hg2+ determination

Abstract

A novel fluorescent sensor designated as DGM was synthesized by integrating rhodamine 6G with a [5]helicene derivative that served as the binding scaffold and tris(2-aminoethyl)amine as the linking agent. The DGM sensor exhibited a “turn-on” fluorescence response specific to Hg²⁺, which had a substantial Stokes shift of 176 nm and a detection limit of 21 ppb. This sensor functioned through the Hg²⁺-induced ring-opening of the rhodamine 6G spirolactam, triggering an efficient Förster resonance energy transfer process from the [5]helicene to rhodamine 6G. DGM demonstrated excellent selectivity toward Hg²⁺ compared to other metal ions, highlighting the sensor's suitability for detecting Hg²⁺ in real water samples. Moreover, the sensor enabled qualitative, on-site investigation through a distinct colorimetric change from pale yellow to pink, which is readily observable by the naked eye.