Surface plasmon resonance tunable gold nanostar synthesis in a symmetric flow-focusing droplet device

Gold nanomaterials are known for their unique optical properties due to their localized surface plasmon resonance (LSPR). Controllable gold nanomaterials synthesis to tune LSPR of specific application demands is of great interest in the scientific community and is technically challenging to achieve in a conventional batch reactor. In this work, we tailored LSPR tunable gold nanostars (Au NS) using a symmetric flow-focusing droplet device after optimizing the flow parameters in 2D simulations. Simulations were used to represent the fluid interface within the device microchannels and helped to study the droplet evolution and their quantification. As nanomaterials synthesis in a droplet device demands the production of homogenous droplets and control over their size and production rate, we optimized the flow parameters from the simulation. By incorporating curved, sinusoidal geometries and semi-spiral microchannels, the mixing of reagents inside the droplet was improved for synthesizing heavily branched Au NS. The best performance was achieved when the flow rate of the shaping reagent was twice that of other reagents. As-synthesized Au NS are non-toxic, uniform and possess a small core diameter, large tip-to-tip length, negative surface zeta potential and red-shifted LSPR. These features are highly favourable for photothermal and photodynamic applications. Thus, our microfluidic platform can produce uniform droplets that provide sufficient mixing of reagents for the application-specific LSPR tunable synthesis of gold nanostars.