Do you remember when you were a child producing paper
cubes by folding up a flat cross shape? Now two researchers have
applied the same technique at nanoscale, 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).
The researchers produced nanoscale boxes with letters carved in
each side or printed in gold
"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.