Photoelectrochemical (PEC) sensors are promising for monitoring the environment and in clinical diagnosis, but their point-of-use application requires portable systems and materials with high photocatalytic activity. In this work, we extend the use of a recent miniaturized PEC system to detect the breast cancer biomarker carbohydrate antigen 15-3 (CA15-3) with the enhanced catalytic activity of graphitic carbon nitride (CN) decorated with gold nanoparticles (Au NPs). The performance of the immunosensor was highly dependent on the size of the nanoparticles and on the film architecture. Optimized sensing of CA15-3 was reached with electrodeposited 13.1 nm Au NPs linked to a long-chain mercapto (11-mercaptoundecanoic acid), which allowed for an effective adsorption of antibodies. CA15-3 could be detected from 0.1 fg mL^-1 to 10 ng mL^-1, with detection limit of 0.04 fg mL^-1. As a demonstration of the viability of the PEC immunosensor, the biomarker CA15-3 was detected in human serum with recovery values between 98 and 109%.