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Shape control of colloidal nanoparticles
Self assembled structures from colloidal particles have many applications in
biology, as chemical sensors and as photonic crystals. The control of shape
and valency of the colloidal particle is very important since it will
determine the 3D lattices of the assembled structure. There have been
several prior effort to fabricating particles with complex shapes. Most
particles with anisotropic shape are from the simple assembly of spheres or
the modification of spherical particles. Interference lithography is one of
the few techniques which can provide direct and systematic control over
symmetry and volume fraction of the 3D structure. It involves the simple
creation of interference patterns in a photoresist systems and subsequent
pinch off of the parent structure through a drying process. Researchers at
MIT have now presented a new facile and high-yield route for the fabrication
of highly nonspherical complex multivalent nanoparticles. This technique
exploits the ability of holographic interference lithography to control
network topology. These research results could lay the groundwork for
establishing and demonstrating control over particle shape in colloidal
nanoparticles.
"Compared to the previous techniques, such as microfilmed lithography and
assembling and sintering spherical particles in a lithographically defined
well (cavity), our approach gives us access to more complex and precisely
controlled shapes with much higher through-put since we have a 3-dimensional
yield" Dr. Edwin L. Thomas explains to Nanowerk.
Thomas is the Morris Cohen Professor of Materials Science and Engineering at
MIT and head of the department as well as the Founding Director of the MIT
Institute for Soldier Nanotechnologies. In a recent paper in Nano Letters ("
Shape Control of Multivalent 3D Colloidal Particles via Interference
Lithography"), he and his team report the use of holographic interference
lithography (HIL) as an easy and high-throughput fabrication method for
creating complex polymer particles with controlled symmetry, size, and
highly nonconvex shapes.
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HIL involves the formation of a stationary spatial variation of intensity
created by the interference of two or more beams of light. The pattern that
emerges out of the intensity distribution is transferred to a light-
sensitive medium, such as a photoresist, to yield structures.
"Importantly, by proper choice of beam parameters, one can control the
geometrical elements and volume fraction of the structures" Dr. Ji-Hyun Jang
, first author of the paper, points out. "Our multivalent particles are
fabricated by creating disconnected HIL structures in a negative photoresist
" she says. "Manipulation of the experimental parameters of intensity,
polarization, phase, and wave vectors of the interfering beams allows one to
target specific space group structures."
As a proof of concept, the MIT researchers demonstrate the fabrication of
two types of concave multivalent polymer particles, 4-valent particles from
a parent simple square lattice, and 6-valent particles from a parent simple
cubic structure, via interference lithography.
SEM images of 2D and 3D structures before and after UV/ozonolysis. (a and b)
Lightly connected structures after first strong development followed by CO2
supercritical drying. (c and d) Samples after UV/ozonolysis. Each lower
inset shows the single unit cell with the calculated light intensity
distributions corresponding to the SEM images. The upper insets in (c) and (
d) are magnified individual “4-valent” and “6-valent” particles very
similar to the theoretical model in the lower inset. The scale bars in the
insets are 300 nm. (Reprinted with permission from American Chemical Society)
"The control of the particle shape was hard" says Thomas. "Surface tension
always favors convex particles."
To get around this effect and to retain the pronounced nature of the “
valency” and a strongly concave particle shape, the researchers chose a dry
process. After an initial exposure and development, an isotropic etch using
a stronger solvent, followed by supercritical CO2 drying to prevent
distortion of the structure due to high surface tension forces, results in
particles with the desired shape. Finally, the particles can be fully
disconnected at the thinnest part of the arms between neighboring nodes to
obtain the discrete multivalent colloidal particles. The particles were
separated either by O2 plaa or UV/ozonolysis. Because the whole structure
has very thin connections, O2 plaa or UV/ ozonolysis does not affect the
final shape of the particle, but only decreases the size of the polymer
particle by 2-3%.
Thomas says that, besides the common application of colloidal particles in
self-assembled device, these "pointy" particles can be useful as a type of
biological sensor. For example, magnetic nanoparticles with specific
symmetry have been suggested as building blocks in supramolecular
architecture as nanosensors for rapid detection of viruses with specific
symmetry by providing the anchoring position. Another application has been
suggested in the coetic industry where the concave particles can confer
softness during their application and help to increase the adherence to the
skin.
"We are currently exploring multivalent particles out of inorganic materials
such as silica or iron oxide by the infiltration of the parent polymer
template" Thomas describes the direction of their ongoing research. "We
expect this will greatly increase the potential of our multivalent particles
."
By Michael Berger, Copyright 2007 Nanowerk LLC
