Aggregation Kinetics of SERS-Active Nanoparticlesin Thermally Stirred SessileDroplets.

The aggregation kinetics of silvernanoparticles in sessile dropletswere investigated both experimentally and through numerical simulationsas a function of temperature gradient and evaporation rate, in orderto determine the hydrodynamic and aggregation parameters that leadto optimal surface-enhanced Raman spectroscopic (SERS) detection.Thermal gradients promote effective stirring within the droplet. Theaggregation reaction ceases when the solvent evaporates forming acircular stain consisting of a high concentration of silver nanoparticleaggregates, which can be interrogated by SERS leading to analyte detectionand identification. We introduce the aggregation parameter, Γa τevap/τa, whichis the ratio of the evaporation to the aggregation time scales. Fora well-stirred droplet, the optimal condition for SERS detection wasfound to be Γa,opt= kcNPτevap≈ 0.3, which is a product of the dimerizationrate constant (k), the concentration of nanoparticles(cNP), and the droplet evaporation time(τevap). Near maximal signal (over 50% of maximumvalue) is observed over a wide range of aggregation parameters 0.05< Γa< 1.25, which also defines the time windowduring which trace analytes can be easily measured. The results ofthe simulation were in very good agreement with experimentally acquiredSERS spectra using gas-phase 1,4-benzenedithiol as a model analyte. [ABSTRACT FROM AUTHOR]