DO YOU remember as a child producing paper cubes by folding up a flat cross shape? Now two researchers have applied the same technique to the nanoscale, in the process creating the first nanoparticles with precisely patterned surfaces. These patterns could form the basis of electronic nano-circuits or provide docking stations on targeted drug-delivery particles.
"At the macroscale, everything can easily be patterned in three dimensions," lead researcher David Gracias at Johns Hopkins University (JHU) in Baltimore, Maryland, told New Scientist. "However, nanoparticles with precisely patterned 3D surfaces simply do not exist."
The problem is that existing methods for patterning at the nanoscale only etch onto flat, two-dimensional surfaces. However, Gracias and his colleague Jeong-Hyun Cho realised that they could make a patterned 3D structure provided they could find a way to assemble it from a 2D one.
Using a narrow beam of electrons, Gracias and Cho carved into a silicon wafer the outline of six square panels, each 500 nanometres wide, to form a cross-shaped mould. Into this they deposited a film of nickel to form the sides of the cube, adding grains of tin between the panels to form hinges.
The researchers found two methods to create a pattern on the panels. They could either incorporate the pattern directly into the mould or deposit a quantity of another metal, such as gold, onto the wafer before coating it with nickel.
Getting the panels to then "self-assemble" into a box required the application of gases to etch away the silicon, while also heating the panels to melt the tin hinges. This caused the hinges to contract as the grains of molten tin merged together, pulling up the panels to form a nanoscale box.
Using this technique, Gracias and Cho produced boxes with the letters JHU carved in each side, as well as boxes with the letters J and U printed in gold (Nano Letters, DOI: 10.1021/nl9022176).
"Since we can build these particles with dissimilar materials [such as nickel and gold], we can construct electronic circuits on the faces to create 'smart' nanoparticles," predicts Gracias.
Chad Mirkin, director of the International Institute for Nanotechnology at Northwestern University in Evanston, Illinois, says such "tour de force" techniques could be the nanoparticle equivalent of understanding and controlling protein folding.