A highly sensitive and selective rectilinearly π-extended NIR fluorescent rhodamine probe for Cu2+detection in real food samples and fluorescence bioimaging in living cells and mice

A novel near-infrared fluorescent probe (NRTB) was designed and synthesized to detect Cu2+. Its structure was characterized by 1H NMR, 13C NMR and ESI-HRMS. In addition, the fluorescence properties were improved by introducing an ethyl phenothiazine group into the rhodamine skeleton, resulting in a near-infrared (NIR) emission at 765 nm with a large Stokes shift of 165 nm. The probe demonstrates exceptional specificity and sensitivity for Cu2+, with a 268-fold increase in fluorescence intensity upon recognition. In the concentration range of 1-5 mu M, the ratio of fluorescence intensity to Cu2+ concentration showed a good linear correlation, with a limit of detection (LOD) of 22.6 nM. Furthermore, NRTB demonstrated a 1:1 coordination interaction with Cu2+, inducing ring opening and generating a high-fluorescence state. The recognition mechanism was further elucidated by ESI-HRMS analysis and Density Functional Theory (DFT) calculations. Based on the colorimetric properties of the probe, we developed a smartphone-integrated visual sensing platform with an LOD of 1.52 mu M. Using colorimetric test strips, Cu2+ was successfully detected on the surfaces of cherry tomatoes, cucumbers, apples, and oranges. Notably, NRTB exhibited excellent biocompatibility and enabled Cu2+ imaging in living 4T1 cells and mice, with animal imaging signal intensities reaching 80-fold greater than controls.