**4. Conclusions**

In summary, uniform and small-size Cu2O/Ag nanocomposites (size of Cu2O particle <100 nm, size of Ag particle <20 nm) were synthesized successfully via a facile one-step process, and successfully used to fabricate an H2O2 sensor. The electrochemical experiment results reveal that the Cu2O/Ag/GCE exhibits outstanding electrochemical behavior and good electrocatalytic reduction performance towards H2O2. The linear range of the Cu2O/Ag/GCE is estimated to be 0.2–4000 μM with a sensitivity of 87.0 μA mM−<sup>1</sup> cm<sup>−</sup><sup>2</sup> and a low detection limit of 0.2 μM. The anti-interference capability experiment indicated that the Cu2O/Ag nanocomposites have good selectivity toward H2O2. Additionally, the H2O2 recovery test in the milk solution demonstrates the potential application of Cu2O/Ag/GCE in routine H2O2 analysis.

**Supplementary Materials:** The Supplementary Materials are available online at http://www.mdpi.com/2079- 4991/9/4/523/s1.

**Author Contributions:** Conceptualization and Methodology, K.Y. and Z.Y.; Formal Analysis and Investigation, K.Y., Z.Y., and L.M.; Data Curation, B.P. and Y.D.; Writing-Original Draft Preparation, K.Y.; Visualization, J.F.; Project Administration, K.Y.; Funding Acquisition, Z.Y.

**Funding:** This research was funded by Beijing Natural Science Foundation (2172005, Z180014), the National Natural Science Foundation of China (11674015, 91860202), and the "111" project under the gran<sup>t</sup> of DB18015.

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