Near-field and far-field competitive couplings between plasmonic nanodisk array and nanoparticles for rapid and facile heparin assay.

Plasmonic biosensors hold an enticing prospect for drug testing and disease diagnostics as they have demonstrated impressive superiority in ultrasensitive and label-free detection. In this paper, we demonstrate a plasmonic sensing strategy for the heparin assay by leveraging near-field and far-field competitive couplings between gold (Au) nanoparticles (NPs) and nanodisk (ND) array. Specifically, the near-field coupling of the ND array and Au NPs binding to its surface results in a spectral redshift. Meanwhile, the far-field coupling of dispersed Au NPs in mixed solution and ND array induces a spectral blueshift. As a result, heparin concentration-dependent near-field and far-field competitive couplings determine the direction and amount of the spectral shift. Compared with existing detection technologies, this sensing strategy also presents a balance point, enabling rapid assessment of heparin dosage safety. Additionally, an exceptionally wide dynamic range of 10^-4 μg/mL to 10³ μg/mL, a low detection limit of 29 pg/mL, and excellent selectivity are demonstrated. The analysis of heparin in serum further underscores this assay approach's potential for advancing disease diagnosis and therapeutic monitoring at the point of care.