Colloidal metal nanocrystals are up-and-coming technologically important materials that might be ideal in applications such as catalysis, photonics and sensing. However, they are difficult to produce in large quantities.

Continuous-flow droplet reactors can be used to make nanocrystals in large quantities but they run into problems if the reaction solution does not contain a surfactant. This is understandable because the droplets have a much higher surface-to-volume ratio than the solution in a batch reactor and so the nanocrystals tend to concentrate at the water-oil interface of the droplets.

Passivating the water-oil interface

A team led by Younan Xia has now found that Triton X-100, a non-ionic surfactant, could help alleviate this problem. “Because it is a surfactant, the molecules in Triton-X tend to arrange at the water-oil interface with the hydrophilic chain and hydrophobic head in contact with the aqueous and oil phases respectively,” explains Xia. “We found that when the concentration of the surfactant added to an aqueous droplet reaches a level of 0.55 by weight, it completely ‘passivates’ the water-oil interface. The result is that the nanocrystals remain well dispersed in the aqueous phase of the droplet reactors during the entire synthesis process. We thus obtain uniform nanocrystals with a well controlled size and shape.”

The technique might be a simple way to help scale up the production of these materials, he tells nanotechweb.org.

Thanks to their experiments, Xia and colleagues say that they now better understand interfacial adsorption of nanocrystals in droplet-based reactors, as well as the importance of mixing in droplets and the carrier phase.

The researchers report their work in Nano Letters.