A Visible/NIR-II\nRatiometric Fluorescent Nanoprobe\nfor Tissue Depth Analysis

Fluorescence\nimaging is a promising approach for in vivo real-time\ntwo-dimensional (2D) visualization of biological structures. However,\nit is still a challenging problem to acquire depth information conveniently\nfrom 2D images. Herein, we developed a visible/second near-infrared\n(NIR-II) ratiometric fluorescence strategy to acquire tissue depth\nfrom 2D fluorescence imaging. A ratiometric fluorescent probe TAC\nNSs that emitted red and NIR-II fluorescence was prepared through\nelectrostatic interactions between poly(ethylene-imine)-modified Ti<sub>3</sub>C<sub>2</sub> MXene nanosheets and CdTe and Ag<sub>2</sub>S quantum dots (QDs). The visible/NIR-II fluorescence ratio decreased\nexponentially with increasing tissue depth, and its natural logarithm\n(ln) values shows good linear relationship with tissue depth in the\nrange of 0–1.0 mm and 1–2.5 mm, respectively. Notably,\nthe ratiometric fluorescence at a fixed tissue depth is not affected\nby the concentration of TAC NSs, indicating that the tissue depth\nanalysis is not interfered by the nonuniform tissue distribution of\nTAC NSs. Moreover, the TAC NSs exhibited efficient photothermal conversion\nability for tumor ablation. This work can provide a strategy for acquiring\nthree-dimensional information from <i>in vivo</i> fluorescence\nimaging.