Flash Colloidal Gold Nanoparticle Assembly in a Milli Flow System: Implications for Thermoplasmonic and for the Amplification of Optical Signals

The assembly and stabilization of a finite number of nanocrystals in contact in water could maximize the optical absorption per unit of the material. Some local plasmonic properties exploited in applications, such as photothermia and optical signal amplification, would also be maximized which is important in the perspective of mass producing nanostructures at a lower cost. The main lock is that bringing charged particles in close contact requires the charges to be screened/suppressed, which leads to the rapid formation of micrometric aggregates. In this article, we show that aggregates containing less than 60 particles in contact can be obtained with a milli-flow system composed of turbulent mixers and flow reactors. This process allows to stop a fast non-equilibrium colloidal aggregation process at millisecond times after the initiation of the aggregation process which allows to control the aggregation number. As a case study, we considered the rapid mixing of citrate-coated gold nanoparticles (NPs) and AlCl3 in water to initiate a fast aggregation controlled by diffusion. Injecting a solution of polycation using a second mixer allowed us to arrest the aggregation process after a reaction time (tQ) by the formation of “overcharged” cationic aggregates. We obtained within seconds stable dispersions of a few milliliters composed of particle aggregates. Our main result is to show that it is possible to master the average aggregation number (N̅agg) between ∼2 and ∼60 NP/aggregate by varying tQ between ∼10 ms and ∼1 s. The relationship between N̅agg and tQ is linear as expected for the so-called diffusion-limited cluster aggregation process. In particular, we were able to stabilize aggregates with N̅agg ∼ 10 NP/aggregate showing a strong plasmonic coupling giving rise to an optical absorption band whose maximum is located at 800 nm. Such aggregates of readily available compounds are interesting in the perspective of maximizing the optical absorption per unit of gold at the wavelength of commercial lasers used in the fields thermoplasmonic and surface-enhanced Raman scattering.