A novel label-free electrochemical immunoassay based on core/shell AgCa-)Au nanoparticles monolayer as sensing interface has been developed for probing IgG. Several coupling techniques, such as Ag nanoparticles, Au nanoparticles, and the core/shell AgCa)Au nanoparticles with L-Cysteine (Cys) cross-linking, were investigated for the determination of IgG and a very good result was obtained with the core/shell AggAu nanoparticles coupling. With the core/shell AgC@Au nanoparticles coupling method, the effects of the incubation time and pH on amperometric responsesofthe immunoassaywere studied. The strong attachment of the cross-linked complex to the core/shell AggAu nanoparticles surface resulted in an excellent storage lifetime of 33 days. A dynamic concentration range of 2.3 to 960 ng/mL with a detection limit of 10 ng/mL was observed. Analytical results of 30 human serum samples obtained using the developing technique are in satisfactory agreement with those given by ELISA. In addition, it presents some superior advantages over the traditional sandwich format in that the analyzing performances are direct, rapid, and simple without multiple separation and labeling steps. (c) 2006 Wiley Periodicals, Inc.