Abstract
The synthesis of anisotropic metallic nanoparticles (NPs) has been a field of intense and challenging research in the past decade. In this communication, we report on the reproducible and highly controllable synthesis of monodisperse branched gold nanoparticles in a droplet-based microfluidics platform. The process has been automated by adapting two different bulk synthetic strategies to microdroplets, acting as microreactors, for NP synthesis: a surfactant-free synthesis and a surfactant-assisted synthesis. Microdroplets were generated in two different microfluidic devices designed to accommodate the requirements of both bulk syntheses. The epitaxial growth of AuNSTs inside the microdroplets allowed for a fine control of reagent mixing and local concentrations during particle formation. This is the first time branched gold NPs have been synthesised in a microfluidics platform. The monodispersity of the product was comparable to the synthesis in bulk, proving the potential of this technology for the continuous synthesis of high quality anisotropic NPs with improved reproducibility.
Highlights
Over the last two decades, the formation and properties of metallic NPs have been studied[1,2]
Ascorbic acid needs to be added as quickly as possible to obtain AuNSTs. Such limitations can be overcome by using droplet microfluidics, where reactants are pumped into the microfluidic chip at a constant flow rate, and a high control over addition steps and reagent mixing can be achieved
For the surfactant-free synthesis, microdroplets were cast on top of transmission electron microscopy (TEM) grids straight from the outlet tubing after generation on chip
Summary
Over the last two decades, the formation and properties of metallic NPs have been studied[1,2]. The presence of two plasmon bands, one corresponding to the cores, and the other to the spikes, and the consequent extra concentration of the electromagnetic field in their tips, make them extraordinary optical signal enhancers[12,13] Novel sensing technologies such as surface-enhanced Raman scattering (SERS) have focused efforts on establishing protocols for the synthesis and use of branched NPs in analytical science[14], biosensing[15], bioimaging[16,17] and nanomedicine[18,19]. The synthesis of more complex metallic nanoparticles, such as branched gold nanoparticles, has not been reported in the literature to the best of our knowledge We believe that the latter is due to the fact that most synthesis for branched gold nanoparticles involve more than one synthetic step, and a fine control over reaction times and reagents mixing. We report on the time effective, facile, automated and reproducible synthesis of branched gold nanoparticles in picoliter microdroplets by adapting two different bulk synthetic strategies, demonstrating the flexibility of this platform for anisotropic NP synthesis
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