Microfluidics for Programmable Self-Assembly of Gold Nanorods

Self-assembly of nanomaterials is an effective tool for developing functional assemblies and nano devices that can be used for a multitude of electrical, biomedical, and mechanical applications to name a few. We focus on the self-assembly of anisotropic gold nanomaterials, gold nanorods (GNR) in our work, since gold nanorods possess unique optical properties that can be tailored based on their aspect ratios. The side-to-side, end-to-end and self-assembly of GNRs have been a topic of intense research due to both the challenges involved in achieving such assemblies and their application in surface enhanced Raman spectra (SERS) signal enhancements. Several studies show the assembly of GNR using solvents (Ethylene diamine tetra acetic acid (EDTA), tetra hydrofuran (THF), dimethyl formamide (DMF)), pH, temperature, surfactants, co-polymers and other conditions. However, a systematic analysis and thereby understanding of the mechanism of assembly/dis-assembly and the resultant influence of several contributing parameters is a limitation in these studies. The lack of reproducibility and kinetic control over the reaction make such analysis challenging in the methods used in the studies.<br/>In the current work, we use droplet microfluidics to analyze the solvent based (EDTA) self-assembly of GNR. The GNR are encased in a ‘micro-vessel’ droplet within which reaction with EDTA happens is automated and proceeds reproducibly and controllably. We vary the concentrations of EDTA, change the drop size and also the ratio of aqueous to continuous flows and look at the shift in the resonance spectral wavelength of the clusters hence formed. The spectral shifts in these studies show the individual interaction of GNR with EDTA with respect to the aspect ratio of the GNR, size and shape of the individual particles. The study also shows the effect of mixing, leading to different results in each scenario. We performed a systematic analysis of the interaction and are currently working on programming and optimizing a general assembly strategy for GNR.