Molecular nanoparticles (MNPs) have gained increased attention recently due to their unique structures and properties. However, their applications remain largely unexplored. Herein, we present an Ag MNPs-based multimodal plasmonic assay. This assay relied on changes in optical properties due to stimuli-responsive state trans-formation from MNPs to plasmonic nanoparticles (PNPs). As a proof-of-concept, naked-eye colorimetric assay, spectrophotometric assay and "turn-on" Raman assay of Cu(2+) were developed. The feasibility of this approach for real-world applications was demonstrated with the determination of Cu(2+) in human serum. This multimodal plasmonic assay exhibited several significant advantages, including selectivity, sensitivity, label-free nature, and multimodal capability. Because of these merits, Ag MNPs could be promising nanosensors for wide important applications such as diagnostics and environmental analysis.