Herein, we describe the development of a new, highly efficient label for immunosensing comprising an antibody/PEGylated gold-nanoparticle (AuNP) conjugate in which the antibody molecules are bound to the AuNP surface in a fixed orientation. Our method exploits the high density of positive charges on the major plane of antibodies that exists when the pH of the solution is lower than the isoelectric point of the antibody; the antibody molecules interact with the negatively charged AuNP surface through their major plane, enabling the antigen binding sites to move freely and therefore to reach maximum accessibility. This directed ionic interaction is reinforced by the formation of a peptide bond between the amino group of the Lys residues in the antibodies and the carboxylic groups of the PEGylated-AuNP surface via EDC chemistry. Electrochemical analyses revealed that the fixed-orientation conjugate offers a limit of detection that is 1 order of magnitude lower than that of a randomly oriented label. The performance of the new conjugate as an immunosensing label was assessed for the quantitative detection of IgG in human serum.