Rapid vertical flow immunoassay on AuNP plasmonic paper for SERS-based point of need diagnostics.

SERS based immunoassays for point-of-care diagnostics are a promising tool to facilitate biomarker detection for early disease diagnosis and disease control. The technique is based on a sandwiched system in which antigen is first captured by a selective plasmonic paper substrate and then labeled by an extrinsic Raman label (ERL), consisting of a 60 nm gold nanoparticle (AuNP) functionalized with a mixed monolayer of detection antibody and 4-nitrobenezenethiol (NBT) as a Raman reporter molecule. Here, we report on the use of AuNP modified filter paper as a novel capture membrane in a vertical flow format. This vertical flow configuration affords reproducible flow of sample and label through the capture substrate to overcome diffusion limited kinetics and significantly reduced assay time. The filter paper was selected due to its affordability and availability, while the embedded AuNPs maximized plasmonic coupling with the ERLs and SERS enhancement. Additionally, the embedded AuNP served as a scaffold to immobilize capture antibody to specifically bind antigen. In this work, a SERS-based rapid vertical flow (SERS-RVF) immunoassay for detection of mouse IgG was developed to establish proof-of-principle. Optimization of assay conditions led to a limit of detection of 3 ng/mL, which is comparable to more traditional formats carried out in multi-well plates, and significantly reduced assay time to less than 2 min. Additionally, IgG was accurately quantified in normal mouse serum to validate the SERS-RVF assay for application to the analysis of biological samples. These results highlight the potential advantages of the SERS-RVF platform for point-of-need testing.