Electrochemistry plays a major role in immunosensor development because of its higher sensitivity, simplicity, portability, and rapidity compared to more common enzymatic, fluorescent and radioactive labelling methods. Silver nanoparticles (AgNPs) have been identified as potential electrochemical labels for such immunoassays. Characterizing the composition, size, shape, and surface modifications of the AgNP is important to achieve expected analytical performance of the bio assay. Here we are interested in studying the effect of nanoparticle shape towards the electrochemical detection of a metallo-immunoassay using Au-Ag galvanic exchange (GE) reaction followed by anodic stripping voltammetry. We observe that during GE between electrogenerated Au³⁺ and the Ag labels, a thin shell of Au forms on the surface of the NP and prevent further exchange. This shell is more porous when GE proceeds on silver nanocubes (AgNCs) compared to spherical silver nanoparticles (sAgNPs), and therefore, more exchange occurs when using AgNCs. More interestingly, optimizing the ratio of the two types of AgNP labels, we could able to decrease the LOD of a biological assay designed to detect a heart failure biomarker (NT-proBNP) without compromising the dynamic range compared to using either of the two labels independently. This made it possible to achieve the clinically relevant range for NT-proBNP analysis used by physicians for heart failure risk stratification.