Photoactive electrodes with high photon-to-electron conversion efficiency are key to achieving sensitive photoelectrochemical sensors. Among all the photoactive materials, titanium dioxide (TiO2) nanoparticles have attracted much attention due to their unique electronic and optical properties. However, the large bandgap of TiO2 results in limited photocurrent signal generation under visible irradiation, which is important for its use in many applications including sensing. Herein, we modified TiO2 nanoparticles with both pyrocatechol violet and graphene quantum dots to obtain high photocurrents at visible light excitation while also improving TiO2 nanoparticle dispersion and film-forming properties. This material system enhances photocurrent by 5 times compared to TiO2 nanoparticles that are modified with only pyrocatechol violet and 60 times compared to TiO2 nanoparticles modified with graphene quantum dots. Additionally, the optimized photoelectrodes were used to detect hexavalent chromium (Cr(VI)), which has been reported as a toxic carcinogen. Under visible light irradiation, the fabricated sensor offered a low limit-of-detection of 0.04
μM for Cr(VI), with selectivity against Na, Mg, Cu, and Cr (III) ions, paving the route toward photoelectrochemical Cr(VI) sensing.