"The idea of using liquid interfaces as scaffolds is tremendously useful since researchers can tailor or modify the nanoparticles from both sides of the interface," said team member Anthony Dinsmore. "We have much more surface area to work with for adding or removing specific particles."

The researchers used fluorescent cadmium selenide (CdSe) nanoparticles coated with tri-n-octylphosphine oxide (TOPO) and dispersed in the solvent toluene. Introducing a water droplet into the toluene-nanoparticle mix caused the nanoparticles to organize into a monolayer at the interface of the water and toluene. This stabilized the water droplet, preventing it from coalescing with other droplets. In this way, the scientists produced nanoparticle-coated water droplets with diameters of 10-100 μm.

The scientists say such ligand-stabilized colloidal nanoparticles are ideal for hierarchical self-assembly because the nanoparticle core dictates optical, electronic or magnetic properties, while the surface-bound ligands define the particle's interactions with its surroundings - factors such as solubility, miscibility and charge transport properties.

"What's really key is that you attach ligands that extend from the nanoparticles like hairs, in order to preserve the nanoscopic integrity of the particles and prevent them from clustering," explained Dinsmore's colleague Todd Emrick. "Changing the nature of these organic ligands can really modify the behaviour of the particles. You can endow the particles, and thus the capsules that they form upon interfacial assembly, with a wide range of properties based on which ligands are attached."

Applications for the technique could include targeted pharmaceutical therapies, nutraceuticals - nutrition-enhanced foods - and nanoscopic sensors for medical imaging and diagnostics.

The researchers also found they could make the oil-soluble nanoparticles soluble in water merely by shining a light onto them in the presence of sulphorhodamine-B dye. Repeatedly scanning a water droplet with 488 nm light resulted in more and more of the CdSe nanoparticles crossing from the surface of the droplet to its interior. Water-soluble nanoparticles could be very useful in biosensors and medical imaging.

What's more, the scientists found that if nanoparticles of different sizes (namely 2.8 nm diameter and 4.6 nm diameter) were competing for assembly at the surface of a water droplet, the larger particles would win, segregating into patches on the drop's surface. This could enable researchers to build nanoscopic capsules with different properties in specific areas, for example regions with permeability, magnetism or conductivity - a capability that might ultimately lead to controlled-release encapsulation and delivery.

The researchers reported their work in Science.