Cooperation of ESIPT and ICT Processes in the Designed 2-(2'-Hydroxyphenyl)benzothiazole Derivative: A Near-Infrared Two-Photon Fluorescent Probe with a Large Stokes Shift for the Detection of Cysteine and Its Application in Biological Environments

A rationally designed near-infrared two-photon fluorescent probe (SDP-A) for selectively detecting cysteine (Cys) has been developed based on a newly designed conjugation-enhanced 2-(2'-hydroxyphenyl)benzothiazole derivative as the fluorophore, an acrylate moiety as the Cys reaction site, and an N-methylpyridinium scaffold as both the unit of organelle targeting and improving water solubility. The probe SDP-A alone essentially emitted no fluorescence, whereas it achieved a superb near-infrared fluorescence emission (713 nm) enhancement within 15 min with a significant Stokes shift (302 nm) in the presence of Cys. The photoluminescence mechanism of the probe SDP-A toward Cys was modulated by excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes. It exhibited high selectivity and sensitivity (LOD = 102 nM) for monitoring Cys over other analytes such as Hcy/GSH/H₂S owing to a specific conjugate addition-cyclization reaction between Cys and the acrylate moiety. More importantly, the released fluorophore SDP exhibits elevated quantum yields (1.52-18.17%) in different solvents and strong two-photon excited fluorescence with a sizeable two-photon action cross-section (Φ) of 213.5 GM at 820 nm in acetonitrile-PBS medium, which is highly desirable for two-photon fluorescence imaging of the living samples. Therefore, SDP-A was successfully applied to the imaging of Cys in live cells, zebrafish, mouse brain, and abdominal cavity down to a depth of more than 200 μm using a one/two-photon fluorescence microscope.