Alicia M. Jackson et al. have looked at nanotechnology in the round, and describe what they believe to be a promising new class of structured material (Nature Mater. 3, 330–336; 2004). They have examined nanoparticles that consist of a metal centre surrounded by a mixed-ligand shell. Using scanning tunnelling microscopy backed up by X-ray diffraction, they show that the shell is separated into ordered phases at the unprecedented scale of fractions of nanometres.

The particles were produced in a one-step synthesis involving the spontaneous assembly of ligands on nanosized gold cores. The ligand shell — a mixture of 1-octanethiol and mercaptopropionic acid — separates into chemically distinct domains that are as small as 0.5 nm. These computer-generated representations show how the domains can form ripples that encircle the nanoparticles, and reveal the different ordering that stems from making the ligand shell with different molecular proportions of 1-octanethiol (yellow lobes) and mercaptopropionic acid (red). The nanoparticle diameter is 3.7 nm.

The principle works with cores of silver as well as gold. From this and other evidence, Jackson et al. argue that the phase separation that produces the different domains is not controlled by the characteristics of the core. Rather, after experimenting with ligand assembly on small gold hemispheres of different diameters, they conclude that the ordering is primarily driven by surface curvature — how is not clear. But production of coated nanoparticles with tailored characteristics could involve varying the core curvature as well as the ligand type and mixture.

In looking into the properties of their nanoparticles, Jackson and colleagues tested the binding of three proteins — cytochrome c, lysozyme and fibrinogen — to the ligand shell. The proteins didn't bind at all, and the authors consider that this failure to stick is probably down to the unique size and patterns of hydrophobic and hydrophilic areas on the shell. This ‘nonspecific protein resistance’ could be a major virtue in bioengineering applications.