Concurrent Imaging of Surface-Enhanced Raman and Mie Scattering from Built-in Nanogap Plasmonic Particles.

We report a bimodal imaging method that can spatially resolve and concurrently correlate SERS and background-free Mie scattering signals. By examining two types of nanoparticle assemblies with different types of plasmonic junctions, namely raspberry-like metamolecules (raspberry-MMs) containing intraparticle nanogaps and groups of Au nanocubes forming interparticle gaps, we were able to rapidly screen SERS-active particles among the entire population of nanoparticles. Ratiometric analysis of SERS/Mie scattering revealed distinct behaviors for these intra- and interparticle nanogaps. In particular, raspberry-MMs showed a high fraction of SERS-active particles with the SERS intensity essentially insensitive to the nanoparticle aggregation state and a predictable environmental dependence. In comparison, nanocube clusters exhibited highly heterogeneous SERS/Mie scattering ratios and unpredictable intensity fluctuations but higher maximum SERS intensity. This dual-imaging approach constitutes an in situ visualization tool that enables simultaneous and stoichiometric analysis of dual signals consisting of elastic and inelastic scattering, which can significantly improve the reliability of SERS measurements.