**4. Conclusions**

The UCNPs@R-TiO2 nanocomposite, an effective antibacterial material, was prepared with the electrostatic assembly strategy. The reduced TiO2 nanoparticles with a bandgap of 2.8 eV and the ability to absorb visible light were successfully assembled onto the surface of Gd-enhanced β-NaYF4:Yb,Er fluorescent nanorods. The UCNPs@R-TiO2 composite with an MIC of 40 μg/mL can kill more than 98.1% of *E. coli* within 12 min under 980 nm NIR light irradiation (1 W). The good antibacterial properties are mostly attributed to the efficient light energy transfer from UCNPs to R-TiO2 nanoparticles. The low-toxicicity UCNPs@R-TiO2 nanocomposite shows great potential for creating efficient NIR-responsive photocatalysts.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-4 344/11/2/184/s1: Figures S1–S6: EDX, FT-IR, XRD, UCL and Antibacterial efficiency analysis of the prepared nanomaterials, Table S1: Antibacterial efficiency of UCNPs@R-TiO2 nanocomposite under different preparation conditions, Table S2: Comparison of the performance of UCNPs@R-TiO2 photocatalytic sterilization system with that of some antibacterial agents.

**Author Contributions:** H.Z.: the acquisition and analysis of data for the work; Drafting the work; Final approval of the version to be published; Agreement to be accountable for all aspects of the work. F.H.: the conception or design of the work; revising the work; Final approval of the version to be published; Agreement to be accountable for all aspects of the work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research funded by National Natural Science Foundation of China, grant number 21275042.

**Institutional Review Board Statement:** The study was conducted and approved by the Ethics Committee of University of South China (SYXK(湘)2020-0002 and January, 2020).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** There are no conflict of interest to declare.

### **References**


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