**4. Conclusions**

In summary, we present a type of ratiometric luminescent nanoprobe (Ru-Tb NP) for intracellular oxygen, which was based on two metallopolymers branched with oxygen probe [Ru(bpy)3]2+ and reference dye [Tb(acac)3bpy]<sup>3</sup>+, respectively. The resultant Ru-Tb NPs have a hydrodynamic size of ~100 nm in diameter, and are easily swallowed by living cells due to PEGylation. Ru-Tb NPs exhibit a two-wavelength emission (545 nm/610 nm) under the excitation of 300 nm and 460 nm, respectively. The red emission from Ru-Poly is significantly quenched by oxygen, while the green emission from Tb-Poly is rather stable. The ratiometric luminescence towards oxygen is well fitted by a linear Stern–Volmer equation. Under the two-photon excitation of a 720 nm light, distribution of dissolved oxygen in MCTSs is successfully imaged with the ratiometric nanoprobes.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/8/1290/s1. Experimental detail on the chemical synthesis of Tb(acac)3 3H2O and Tb-Poly, temperature sensitivity, figures of optical properties, and stability.

**Author Contributions:** Methodology, C.Z.; writing—original draft preparation, W.-x.Z.; writing—review and editing, H.-s.P.; supervision, H.-s.P.

**Funding:** This research was funded by Natural Science Foundation of China (Grant number 61775245) and the MUC 111 project.

**Conflicts of Interest:** The authors declare no conflict of interest.
