**4. Conclusions**

It was proved that the fCNTs and ZrO2-fCNT improved the electron transfer behavior of the GC electrode. The changed surface area of fCNTs by in situ zirconia nanoparticles synthesis induces changes in the electron transfer behavior. Furthermore, the fCNTs and ZrO2-fCNTs layers exhibit good compatibility and affinity to the PB layer. The applicability of the sensor for detection of hydrogen peroxide at the ZrO2-fCNTs/GC electrode was demonstrated. The best electrochemical detection and linear range detection is shown by the ZrO2-fCNTs/GC modified electrode. Based on these advantages, the fabricated sensor exhibits good electrochemical sensibility, reversibility, and excellent linear relationship, nonetheless, the detection limit of the electrode can be improved, especially enhancing the conditions of hydrogen peroxide detection. Finally, the zirconia doped carbon nanotubes can be used in the development of enzyme based biosensors, which is the basis for future studies in our laboratory.

**Author Contributions:** Conceptualization, P.J.E.-M., L.F., and G.G.; methodology, P.J.E.-M., L.F. and G.G.; validation, M.D.J.-M. and M.M.-J.; formal analysis, G.G.; investigation, M.D.J.-M., G.G., L.F., M.M.-J., and P.J.E.-M.; resources, P.J.E.-M. and G.G.; data curation, M.D.J.-M. and M.M.-J.; writing—original draft preparation, M.D.J.-M., L.F., G.G., and P.J.E.-M.; writing—review and editing, L.F., G.G. and P.J.E.-M.; visualization, G.G. and P.J.E.-M.; supervision, L.F., G.G., and P.J.E.-M.; project administration, P.J.E.-M.; funding acquisition, G.G. and P.J.E.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by CEDIA (Corporación Ecuatoriana para el Desarrollo de la Investigación y la Academia) through the project "Biosensores basados en Nanotubos de carbono modificados" code CEPRA XII-2018-14, Biosensores and Pontificia Universidad Católica del Ecuador, Quito-Ecuador; Elemento PEP: QINV0126.

**Acknowledgments:** The authors would like to thank to Pontificia Universidad Católica del Ecuador, CEDIA (Corporación Ecuatoriana para el Desarrollo de la Investigación y la Academia), and Universidad Yachay Tech for the financial support.

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