A surprising electrochemiluminescence (ECL) enhancement effect in graded-gap CdSeTe@ZnS-SiO2 quantum dot (QD) bilayers was observed and used to create an ultrasensitive immunoassay. CdSeTe@ZnS-SiO2 QDs of two different sizes were used as a donor-acceptor pair, owing to their tunable energy and low biotoxicity. The graded-gap CdSeTe@ZnS-SiO2 QD bilayers were fabricated by layer-by-layer assembly of differently sized CdSeTe@ZnS-SiO2 QDs on a glutaraldehyde-activated electrode. Benefiting from a short interlayer distance and perfect spectral overlap in the graded-gap QD bilayers, highly efficient ECL resonance energy transfer (ECLRET)-based energy funneling was observed, wherein excitons from trapped states could be effectively recycled. Consequently, the observed ECL enhancement for the bilayers was more than four times greater than that observed for reference samples. The graded-gap QD bilayers were utilized in an ECL biosensor for the detection of carcinoembryonic antigen (CEA). The proposed method featured a detection limit of 0.4 pg mL(-1) CEA with a linear calibration range from 1 pg mL(-1) to 200 ng mL(-1). This method represents a novel approach for versatile detection of biomolecules in research and clinical applications.