Monitoring Early-Stage Nanoparticle Assembly in Microdroplets by Optical Spectroscopy and SERS

Microfluidic microdroplets have increasingly found application in biomolecular sensing as well as nanomaterials growth. More recently the synthesis of plasmonic nanostructures in microdroplets has led to surface‐enhanced Raman spectroscopy (SERS)‐based sensing applications. However, the study of nanoassembly in microdroplets has previously been hindered by the lack of on‐chip characterization tools, particularly at early timescales. Enabled by a refractive index matching microdroplet formulation, dark‐field spectroscopy is exploited to directly track the formation of nanometer‐spaced gold nanoparticle assemblies in microdroplets. Measurements in flow provide millisecond time resolution through the assembly process, allowing identification of a regime where dimer formation dominates the dark‐field scattering and SERS. Furthurmore, it is shown that small numbers of nanoparticles can be isolated in microdroplets, paving the way for simple high‐yield assembly, isolation, and sorting of few nanoparticle structures. The assembly of gold nanoparticles is tracked in microfluidic droplets on the millisecond timescale. This reveals a regime where the formation of dimers governs the surface‐enhanced Raman scattering (SERS) response. This provides new insights for SERS optimization, as well as paving the way for high‐yield assembly of nanostructures within individual microdroplets.