In this work, a novel and facile method for the synthesis of gold-platinum bimetallic nanoclusters (NCs) embedded in the structure of an IgG (Au/Pt NCs-IgG) is presented. Proteins have been widely used as scaffolds for the synthesis of atomic clusters. The harsh conditions usually required during the synthesis imply the denaturation of different proteins and the loss of their biological activity. We propose a strategy for the synthesis of NCs employing IgG as a scaffold, performed under physiological conditions in order to keep the IgG structure unaltered, allowing the resulting material to bind to Protein G and the antigen. NCs composed of two different types of atoms exhibit higher catalytic activity than monometallic NCs due to the synergistic effect of two diverse atoms. This peroxidase-like activity and the maintained affinity for its antigen make Au/Pt NCs-IgG a suitable material for its use as a detection antibody in a direct sandwich enzyme-linked immunosorbent assay (ELISA). The use of Au/Pt NCs-IgG as an alternative to IgG tethered to horseradish peroxidase in ELISA boosts the limit of detection (LOD) by 56 times.