In brief, a biocompatible and biodegradable polyamidoamine (named AGMA1-SH), bearing 20%, on a molar basis, thiol-functionalized repeat units, is employed to stabilize and coat extra-small gold nanospheres of different sizes (2.5, 3.5, and 5 nm in gold core), and to generate a nanoplatform for the link with Trastuzumab monoclonal antibody for HER2-positive breast cancer targeting. Here, ERBB2 is linked to breast carcinoma.