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• It was the first application of GaON in PEC immunosensors. • An effectively cascade-like energy band edge level was obtained, which can effectively amplified PEC response. • The BiVO 4 /GaON/CdS label-free PEC immunosensor was successfully developed for determination of procalcitonin. In this paper, GaON with excellent photoelectrochemical (PEC) performance was adopted in PEC immunosensor for the first time. Based on CdS sensitized BiVO 4 /GaON composite material, a label-free PEC immunosensor was successfully developed for determination of procalcitonin (PCT). Firstly, BiVO 4 nanoarray was prepared by electrodeposition. Then GaON was added to form the BiVO 4 /GaON composite material, due to the greatly increasing charge transfer efficiency, the PEC signal increased by about 3 times. At last, CdS grew in situ on BiVO 4 /GaON composite that further expanded the light absorption range and improved the light utilization, and the PEC signal further increased again. This PEC immunosensor has a good linear relationship of 0.1 pg mL−1 - 50 ng mL−1 and the detection limitation of 0.03 pg mL−1 is calculated. Importantly, the BiVO 4 /GaON/CdS immunosensor platform showed good specificity and stability, and serum sample analysis have been performed with satisfactory results. [ABSTRACT FROM AUTHOR]