**4. Conclusions**

In summary, the influence of Fe2O3/GO morphologies on electrochemical sensing performances was studied systematically. Cubic, thorhombic and discal Fe2O3 NPs with a uniform size and controllable structure were successfully prepared by a facile meta-ion mediated hydrothermal route. When coupled with GO nanosheets and then worked as sensing films for the simultaneous detection of DA and UA, the α-Fe2O3 NPs with discal morphology displayed the highest electrocatalytic activity. This remarkable electrocatalytic behavior was highly correlated with the discal shapes with more surface defects and a rougher surface. The synergistic effect from d-Fe2O3 NPs and GO nanosheets contributed to a significant enhancement in the response currents. As a consequence, two wide detection ranges (0.02–10 μM and 10–100 μM) were obtained for DA and UA, with very low limit of detection (LOD) of 3.2 and 2.5 nM for DA and UA, respectively. Moreover, the d-Fe2O3/GO nanohybrids showed good selectivity and reproducibility. The proposed d-Fe2O3/GO nanohybrids have become one of the most competitive candidates as sensing materials for the simultaneous determination of DA and UA in various real samples.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/6/835/s1, Figure S1: DPVs for the analysis of human serum samples with 100-fold dilution.

**Author Contributions:** Conceptualization, Z.C., Y.Y. and G.L.; methodology, Z.C., Y.Y. and X.W.; experiment, Z.C., Y.Y., X.W., and S.Y.; data analysis, Z.C., Y.Y., and Y.X.; writing—original draft preparation, Z.C., Y.Y.; writing—review and editing, G.L. and Q.H.; supervision, G.L. and Q.H.; funding acquisition, G.L., J.L. and Q.H.

**Funding:** This research was funded by the Undergraduates' Innovation Experiment Program of Hunan Province (No. 2018649), National Natural Science Foundation of China (No. 61703152), Natural Science Foundation of Hunan Province (No. 2019JJ50127, 2018JJ3134), Scientific Research Foundation of Hunan Provincial Education Department (18A273, 18C0522), Project of Science and Technology Plan of Zhuzhou (201707201806), and the Doctoral Program Construction of Hunan University of Technology.

**Acknowledgments:** We sincerely express our thanks to Zhuzhou People's Hospital for offering human serum samples.

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