Soft Hydrogel Nanoparticles Provide Foundation for Optically Tunable Photonic Crystals Via Controlled De-swelling
by John Toon
Researchers at the Georgia Institute of Technology have developed a family of
hydrogel-based nanoparticles that can be used to form photonic crystals whose
optical properties can be precisely tuned by thermally adjusting the particles'
The soft and conformable spherical particles could be the basis for a "photonic
fluid" that would be custom processed to form self-assembled periodic structures
able to transmit specific wavelengths of light. Potential applications include
optical switching and optical limiting in telecommunications and – by using
particles that respond to biological molecules – new types of medical
"We have a very simple and easy processing method for taking one type of particle and creating a whole host of optical materials from it, as opposed to having to synthesize a new particle for each optical material you would like," said Andrew Lyon, assistant professor of chemistry and biochemistry at Georgia Tech. "We have a polymer solution that can be processed in normal ways – spin coating, casting and molding – which typically cannot be done with other types of colloidal photonic materials."
Lyon presented details of the project at the 223rd national meeting of the
American Chemical Society April 8 in the
symposium on "Self-assembled Photonic Band Gap Materials."
Lyon and his colleagues have fabricated nearly 100 different types of
monodisperse hydrogel particles, in sizes ranging from 50 nanometers to 1 micron
in diameter. The temperature at which the particles transition to a crystalline
state can be controlled chemically during the synthesis process in a range from
10 degrees C to 60 degrees C.
The nanoparticles are synthesized from poly-N-isopropylacrylamide (pNIPAm)
lightly cross-linked with N,N-methylenebis(acrylamide)(BIS). After precipitation
polymerization in aqueous media, the particles are separated from the
surrounding water by simple centrifuging. The resulting glassy gelatinous
material, which has a faint blue, green or red hue, is more viscous than honey.
To give it desirable optical properties, the material must be annealed by
heating it past the volume phase transition temperature of the component
hydrogel particles, at which the photonic crystal loses its order and the
nanoparticles begin to give up water content. After removing small amounts of
water, the material is allowed to cool, re-absorb water and re-crystallize. This
thermal cycling process serves to pack the soft hydrogel particles into an
ordered 3-D hexagonal array, which produces the periodic dielectric structure
needed for optical properties.
The annealing step is repeated as many as 15 times until the resulting
crystalline structure has the desired optical properties. Crystals produced so
far by Lyon and collaborators Justin Debord, SaetByul Debord and Clinton Jones
reflect bright blue, green or red colors.
"While the assembly of colloidal crystals from such particles has been
reported previously, this represents the first report where the softness and
thermoresponsivity of the component particles is used to create a color-tunable
colloidal crystal via particle compression," Lyon said. "Such soft assemblies
may present new opportunities for the fabrication of particulate and templated
materials for photonic applications."
By closely controlling the hydration of the particles, the researchers can
tune the colors by one-nanometer steps over a wavelength range of more than 200
nanometers. "We have very good control, both with respect to the breadth of the
transition and the accuracy with which we can design the color of the material,"
When heated above the transition temperature, the material readily flows in a
liquid form and can be cast, molded or spin coated onto a surface using standard
polymer processing techniques.
Though practical applications may be a long way off, the researchers envision
uses in the telecommunications industry, where the precisely-tuned photonic
crystals could be used to extract information carried on optical fibers at
specific wavelengths. Sending signals coded at different wavelengths allows
fibers to carry large volumes of information in a process called muliplexing.
The tunable crystals produced through the Georgia Tech process would transmit
only a narrow range of wavelengths, allowing specific streams of data to be
retrieved from the optical fibers.
In addition to the temperature-responsive nanoparticles, Lyon's group has
also made particles that transition in response to pH levels and to the presence
of metal ions. They are working on particles that would respond to specific
proteins or other biological molecules that could be useful in medical
diagnostics to detect the markers of disease.
Though much work remains to be done on materials engineering issues, Lyon
envisions production of a nanoparticle powder that could be produced and
distributed with a "recipe" for producing crystals able to reflect specific
"Since these materials are self-assembled, they live in a deep thermodynamic
well so that their optical properties are the result of the thermodynamically
preferred arrangement of the particles," he explained. "That allows us to
produce materials that are intrinsically very stable. They form very fast, and
because we understand the annealing using thermal responsivity, that allows us
to make a material that is disordered and give it a temperature kick to help it
find its thermodynamically-preferred phase. The speed, the reproducibility and
the stability by which these can be assembled are huge advantages."
Development of the hydrogel nanoparticles has been funded by the Georgia Tech
Research Corporation. A paper on the work has been accepted for publication in
Advanced Materials, and earlier work was published in the July 13, 2000
issue of the Journal of Physical Chemistry.
Original article : http://gtresearchnews.gatech.edu/newsrelease/nanogel.pdf
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