A Fluorescein Derivative Chemosensor Combined with Triplet–Triplet Annihilation Upconversion System for Ratiometric Sensing of Cysteine

A derivative of fluorescein, fluorescein O,O-diacrylate (FODA), was utilized in a triplet–triplet annihilation upconversion (TTA-UC) system to develop a composite ratiometric chemosensor capable of detecting cysteine (Cys). FODA acted as both the probe for Cys and the energy acceptor for upconversion (UC) emission, thereby making UC a responsive signal to Cys. In addition, the phosphorescence signal of the sensitizer in the TTA-UC system remained constant and did not respond to Cys, making it an ideal internal reference signal for constructing a ratiometric sensor. Through this simple strategy, traditional fluorescent probes can be combined with TTA-UC system to establish a ratiometric sensing platform, which can be applied in more scenarios due to the longer wavelength excitation.