This paper proposes a Active Pixel Sensor (APS) structure for the BDJ photodetector in the X-FAB XH018 CMOS process. The pixel architecture will be used to design a matrix array of BDJ active pixels in order to develop a lab-on-chip Enzyme-linked immunosorbent assay (ELISA). A solution to solve the capacitance unbalance in the BDJ structure is proposed. It allows easy and accurate measurement of the BDJ photocurrents. Noise analysis has been carried out and our design reduces the follower noise to 60µVrms. The designed BDJ active pixel has a fill factor of 61% for an active area of 100µmx100µm.