**4. Conclusions**

Photoelectrochemical biosensors have shown a very recent development, which came later than other biosensors due to their higher complexity and difficulty to achieve. Nevertheless, they provide better performance with respect to background signal suppression and the ability to switch the sensor ON and OFF. Moreover, the larger number of components opens the door to tailor on-demand sensors that are not only better suited to specific applications than regular biosensors, but have also shown new ways to create and analyze a sensing signal. Future work will focus on new interfaces electrode-semiconductor and semiconductor-macrobiomolecule—where the optimization can be carried out—while increasing the sensitivity and selectivity of the sensor. Another research trend that allows the inclusion of light-dependent sensors is the selection of the excitation wavelength, which opens plenty of possibilities of multi-sensing platforms by a mere decomposition of white light.

**Author Contributions:** This review has been conceptualized by M.P. and M.d.B. The selection of articles to review was performed by M.P. and M.d.B. Writing and original draft preparation was performed by M.P., M.d.B. and G.L.-L. Editing was performed by M.P., M.d.B. and G.L.-L. Project administration was performed by M.P. and M.d.B. Funding acquisition was performed by M.P. and M.d.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** M. del Barrio acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie gran<sup>t</sup> agreemen<sup>t</sup> No. 713366. G. Luna-López wishes to thank to "Comunidad de Madrid" and European Structural Funds for their financial support to FotoArt-CM project (S2018/NMT-4367). M. Pita thanks the Retos MCIU/AEI/FEDER, EU for funding project RTI2018-095090-B-I00.

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