In the present work, a rapid and ultrasensitive electrochemiluminescent (ECL) competitive immunoassay for mercury (II) based on CdSe quantum dots (QDs), gold nanoparticles (GNPs) and specific monoclonal antibody (mAb) against Hg2+ has been developed. GNPs were creatively applied as substrate and electron transfer accelerator, which would load a large number of coating antigen and amplify the electrochemical signal. Taking advantage of the specific mAb against Hg2+, a cost-effective and selective GNPs/Ovalbumin-MNA-CH3Hg/mAb-QDs immunosensor was fabricated. A competitive immunoassay format was selected as a way of quantifying Hg2+, where Hg2+ in a sample would compete with the corresponding coating antigen for the limited QDs labeled mAb. With the coreactant S2O82-, a strong ECL signal was observed and the decrease of the ECL intensity is linear to the logarithm of the mercury (II) concentration within the range of 0.01 to 50 ng mL(-1), with a detection limit of 2.6 pg mL(-1), much lower than previous methods. To our knowledge, this ECL immunoassay applied GNPs as substrate combined with specific monoclonal antibody detecting Hg2+ is presented for the first time. The application in real samples with favorable results and the highly sensitivity and selectivity further indicate that this ECL immunosensor is feasible for Hg2+ detection and enlarge the application of GNPs and monoclonal antibody in monitoring heavy metals. (C) 2014 The Electrochemical Society. All rights reserved.