Synthesis and photophysical properties of the nanoprobe. A, Design of the RHyLI nanoprobe and its hypoxia sensing mechanism. B, A representative TEM image of the nanoprobe. C, Size distribution of the nanoprobe under TEM. D, Hydrodynamic diameter of the nanoprobe in distilled water by DLS. E, Stability test of the nanoprobe in PBS (1×, pH 7.4). Results are presented as mean ± SD (N = 3). F, Emission spectra of the RHyLI nanoprobe in distilled water (5 μg/mL) at various oxygen levels (0-21%). Excitation: 630 nm. G, The ratio between the emission intensities at 795 and 685 nm at various oxygen levels. The region of oxygen level related to tumor resistance to RT is highlighted. H, Reversible response of the nanoprobe to the change of oxygen level.