A hybrid plasmonic nanoprobe using polyvinylpyrrolidone-capped bimetallic silver-gold nanostars for highly sensitive and reproducible solution-based SERS sensing.

Practical solution-based assays using surface-enhanced Raman spectroscopy (SERS) with portable instrumentation are currently of particular interest for rapid, efficient, and low-cost detection of analytes. However, current assays still have limited applicability due to their poor sensitivity and reproducibility. Herein, we demonstrate highly stable polyvinylpyrrolidone (PVP)-capped bimetallic silver-coated gold nanostars (BGNS-Ag-PVP) as a solution-based SERS nanoprobe that is capable of producing a strong, uniform, and reproducible SERS signal using a portable Raman instrument. The developed hybrid BGNS-Ag-PVP nanostructure shows tunable optical properties with improved SERS sensitivity and reproducibility as compared to gold nanostars. We have synthesized bimetallic nanoprobes BGNS-Ag-PVP having three different silvers, referred to as BGNS-Ag-PVP-1, BGNS-Ag-PVP-2, and BGNS-Ag-PVP-3. The SERS performance of BGNS-Ag-PVP was studied using methylene blue (Meb) as a probe molecule, and we achieved a detection limit of up to 10 nM indicating the high sensitivity of the solution-based SERS platform. The application of such bimetallic nanoparticles is demonstrated the sensitive detection of the antithyroid drug methimazole (Mz) used as a model analyte system. We have achieved a detection limit of 1 nM for Mz spiked with human urine indicating three orders of magnitude lower than previously reported solution-based SERS detection methods. Furthermore, the SERS performance was reproducible over 3 months indicating excellent stability and repeatability. The result illustrates the potential of this solution-based SERS detection platform as a promising sensing tool for analytes such as illicit drugs, and biomarkers that have affinity to bind on nanoprobes.