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[en] A sensitive and selective photoelectrochemical (PEC) method is described for the detection of protein kinase A (PKA) activity based on the use of graphite-like carbon nitride (g-C3N4) and the CdS quantum dots (QDs). Firstly, a complex was synthesized from g-C3N4 and gold nanoparticles (AuNPs). It was employed as both the PEC-active material and as a support for immobilization of peptides. The latter were assembled on an ITO electrode modified with g-C3N4-AuNPs and subsequently phosphorylated by PKA in the presence of adenosine 5′-[γ-thio]triphosphate (ATP-S). Finally, CdS quantum dots (QDs) were introduced on the ITO in order to increase the PEC response of g-C3N4 based on the Cd-S binding between the QDs and thiol groups. Under the optimal conditions and a typical working voltage of −0.3 V, the method has a dynamic range that extends from 0.05 to 50 unit·mL−1, with a 0.017 unit·mL−1 lower detection limit. The method was successfully applied to the quantification of the inhibitory effect of ellagic acid on the activity of PKA, and to monitor enzyme activity in cell lysates. .