Real-time optical spectroscopy for in situ single-droplet analysis

In situ monitoring of chemical reactions, specifically reactions leading to the formation of plasmonic nanomaterials within droplets, has become a necessity for several applications in nanotechnology and sensing. In this study, we use a custom-designed optical transmission spectroscopy setup that can detect signals from a single anisotropic droplet over the entire visible spectral range (400-900 nm) without the use of external additives as reporters. We use data obtained from light scattered by the droplets to differentiate the 'drop only' regions from the 'oil only' regions and to extract information from within single droplets. We then load the droplets with anisotropic gold nanoparticles of different concentrations and show the variations in the optical signals based on their concentrations from single droplets and compare the data to the averaged data from several droplets, to demonstrate the validity of our technique. Finally, we employ the developed platform for monitoring in situ synthesis of gold nanoparticles within a microfluidic chip in real-time. Measurement at different locations along the channel enables us to track the reaction within a droplet at different time points, providing insights into the reaction kinetics. We also measure spectral data to understand the influence of reagent concentration on the synthesis. The developed technique can thus be employed for in situ monitoring of any chemical reaction within a single anisotropic droplet, provided it has absorption/transmission signatures, and will find wide applicability in the development of single-droplet analysis platforms.