**4. Conclusions**

New voltammetric sensors based on combinations of gold nanoparticles and sulfur substituted zinc phthalocyanines have been developed and used as electrochemical sensors for the detection of catechol. The electron transfer process, as well as the existence of synergistic effects between both components in the absence and presence of covalent links has been analyzed.

It has been demonstrated that the electrocatalytic properties and the kinetic parameters depend on the type of interaction between both components. The AuNPtOcBr/ZnPcRS and the AuNPtOcBr/ZnPcR-S-ZnPcR mixtures enhance the electron transfer rate of the catechol reduction. Both modifiers showed similar LODs of 10−<sup>6</sup> mol·L−1. As in the dimeric phthalocyanine the sulfur group is blocked, it can be inferred that the sulfur group does not play a role in the electrocatalytic process. In turn, the AuNPtOcBr-S-ZnPcR covalent adduct facilitates the oxidation of catechol, showing an enhanced charge transfer rate, and an LOD of 10−<sup>7</sup> mol·L−1.

Under the light of these results, combining covalently nanoparticles and phthalocyanines can be considered a good strategy to improve the charge transfer rate and the limits of detection of catechol. Future works should be dedicated to analyzing the effect of the interaction between electrocatalytic materials in other systems different than the nanoparticle-phthalocyanine system.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/11/1506/s1.

**Author Contributions:** Conceptualization, M.L.R.-M., Á.S.-S. and C.G.-C.; methodology, M.L.R.-M.; investigation, Á.I.R.-C., J.O. and C.G.-H.; resources, Á.S.-S. and M.A.R.-P.; data curation, A.I.R.-C. and F.M.-P.; writing—original draft preparation, A.I.R.-C.; writing—review and editing, M.L.R.-M. and Á.S.-S.; funding acquisition, M.L.R.-M. and Á.S.-S.

**Funding:** Financial support by MINECO-FEDER (RTI2018-097990-B-I00 and CTQ2017-87102-R) and the Junta de Castilla y Leon FEDER (VA275P18) is gratefully acknowledged.

**Acknowledgments:** Celia Garcia-Hernandez would also like to thank Junta de Castilla y León for a gran<sup>t</sup> (BOCYL-D-4112015-9).

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