Structure-Function Relationships for Surface-Enhanced Raman Spectroscopy-Active Plasmonic Paper

Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for the specific, rapid, and nondestructive detection of small molecules. For detection applications, it is desirable to develop means for synthesizing SERS substrates that provide large and consistent Raman enhancement, are inexpensive and simple to synthesize, and are active for a wide variety of molecules. Here we demonstrate that nanoparticle colloids (ranging in size from 5 to 100 nm) that are purposely aggregated using electrolytes can be stabilized in the solid state using common filter paper. The SERS substrate enhancement factor (EF) is characterized using trans-1,2-bis(4-pyridyl)ethylene, and it is observed that an intermediate aggregation state provides the largest substrate EF of ∼3 × 10⁴. Generalized Mie theory is used to investigate the relationship between nanoparticle aggregation extent, cluster geometry, and plasmonic enhancement. Overall, the electrodynamics simulations are in good agreement with experimental EF values and provide a framework for understanding which aggregate structures provide the best enhancement (linear aggregates). The approach outlined here for stabilizing and characterizing deliberately aggregated plasmonic nanoparticles is general and provides a consistent means for testing the SERS substrate properties of nanoparticle colloids that are stabilized in the solid state.