Plasmon Near-Field Coupling of Bimetallic Nanostars and a Hierarchical Bimetallic SERS "Hot Field": Toward Ultrasensitive Simultaneous Detection of Multiple Cardiorenal Syndrome Biomarkers.

Cardiorenal syndrome (CRS) has posed tremendous challenges in patient management, and the detection of serum biomarkers may provide opportunities for early diagnosis and effective treatment. Herein, we introduce a novel surface-enhanced Raman scattering (SERS)-based sandwich immunoassay platform to simultaneously detect cardiac troponin I (cTnI), N-terminal prohormone of brain natriuretic peptide (NT-ProBNP), and neutrophil gelatinase-associated lipocalin (NGAL) for the early diagnosis of CRS by using Raman reporter-molecule-labeled Ag-Au nanostars (Ag-Au NSs) as nanotags and a three-dimensional ordered macroporous (3DOM) Au-Ag-Au plasmonic array as substrate. The Ag-Au NSs prepared by galvanic replacement feature bimetallic composition and a multibranched structure so that high SERS stability and enhancement are exhibited. Meanwhile, a 3DOM Au-Ag-Au plasmonic array was fabricated through Au-assisted electrodeposition and was further covered by a protective Au layer; it is characterized by a large specific surface area and high homogeneity, serving as a "hot field". When the nanotags and substrate were combined, "hot spots" were generated from the plasmon near-field coupling, which greatly increased the SERS enhancement. The limits of detection (LODs) were 0.76, 0.53, and 0.41 fg mL-1 for cTnI, NT-ProBNP, and NGAL, respectively, and the Raman images indicated the approximate concentration ranges of the detected proteins for visual analysis. Taking advantage of the ultrasensitivity and multiplexing capability of this approach, we further analyzed clinical blood samples with high integrality, efficiency, and accuracy. Therefore, the presented SERS immunoassay platform holds promise as an ideal test method for point-of-care detection and a powerful tool for investigations into the complex CRS-related biological process.