A highly sensitive and selective rectilinearly π-extended NIR fluorescent rhodamine probe for Cu 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 Cu. Its structure was characterized by H NMR, C 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 Cu, with a 268-fold increase in fluorescence intensity upon recognition. In the concentration range of 1-5 μM, the ratio of fluorescence intensity to Cu 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 Cu, 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 μM. Using colorimetric test strips, Cu was successfully detected on the surfaces of cherry tomatoes, cucumbers, apples, and oranges. Notably, NRTB exhibited excellent biocompatibility and enabled Cu imaging in living 4T1 cells and mice, with animal imaging signal intensities reaching 80-fold greater than controls.