A FRET-based ratiometric fluorescent probe for highly selective detection of cysteine based on a coumarin–rhodol derivative

Cysteine, as an important amino acid in the human body, plays a vital role in people's normal life activities. In this paper, a ratiometric fluorescent probe for detecting cysteine was designed and synthesized based on the fluorescence resonance energy transfer (FRET) process. In this FRET system, a coumarin derivative was used as the energy donor, a rhodol fluorophore was chosen as the energy receptor, and an acrylate group was utilized as a cysteine recognition unit. In the absence of cysteine, the rhodol receptor was in the non-fluorescent lactone state and the FRET process was inhibited. Upon addition of cysteine, the closed spirolactone form was converted to a conjugated fluorescent xanthene form to induce the occurrence of FRET which resulted in a fluorescent signal decrease at 470 nm and enhancement at 543 nm. The ratiometric fluorescent probe exhibited excellent selectivity to Cys over Hcy and GSH. In addition, I543nm/I470nm of the probe for cysteine displayed a good linear relationship in the range of 5.0 × 10−7–1.0 × 10−4 mol L−1, and the detection limit was 2.0 × 10−7 mol L−1. Furthermore, the probe showed low cell toxicity and was successfully applied to the confocal imaging of cysteine in HepG2 cells using dual emission channels.