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 μM, 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 μM. 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.