A new electrode, comprising a dual working electrode (W1 and W2) in which the area of W1 was one-tenth the total working electrode area, was developed. It was successfully used to make a highly sensitive immunosensor based on a silver-nanoparticle-labeled sandwich-type immunoassay. After immunoreaction, the quantity of silver nanoparticles captured on W1 and W2 was measured, as follows. The silver nanoparticles were electrically oxidized to silver ions on W1 and W2, and then, the silver ions were electrodeposited on only W1. Finally, the amount of silver concentrated on W1 was determined using differential pulse voltammetry. Under optimized conditions, the detection limit for Hepatitis B surface (HBs) antigen as the model antigen was 0.09 IU mL(-1) (estimated from three times the standard deviation for 0 IU mL(-1) and the linear regression), which was nine times lower than the detection limit using a conventional immunosensor. The developed electrode in combination with the silver concentration technique holds promise for a wide variety of applications in electrochemical biosensing. (C) 2015 Elsevier Ltd. All rights reserved.