Butterfly wings, peacock feathers, opals and pearls are some of nature's jewels
that use nanostructures to dazzle us with color. It's accomplished through the
way light reaches our eyes after passing through the submicroscopic mazes within
these materials.
The seemingly effortless way that nature creates this effect is now rivaled
by a rapid and simple method developed through a collaboration of researchers
from North Carolina State University (NCSU), Arizona
State University (ASU) and the Universidad Complutense de Madrid (UCM).
Professor Orlin Velev and graduate student researcher Vinayak Rastogi in the
Department of Chemical Engineering at NCSU have shown how colloid chemistry
methods originally used to form particle aggregates from nanoparticles can be
used to quickly make particles with dazzling colors simply by letting a suspension
of nanoparticles dry on a superhydrophobic surface.
Superhydrophobicity is a property of a material that repels water like ducks'
feathers or lotus leaves. It has been used commercially in textiles, coatings
and building materials.
The basic idea behind the process is akin to stacking round fruits or vegetables
in a supermarket produce bin in high, neat rows to keep the produce from falling
to the floor as customers pick them out. Doing this with nanoscale particles
of different sizes leads to opalescence, since some colors of light are reflected
differently depending on the size of the holes between the nanoparticles and
the angle from which they are viewed.
Normally, carefully arranging the nanoparticles in neat rows requires a complex
series of steps with oily solvents and water mixtures requiring extensive washing
afterwards to remove the solvents.
Now, with the help of researchers at ASU, this process has been made as simple
as placing a drop on a superhydrophobic surface and letting it dry for one to
two hours.
The researchers call these one- to two-millimeter particles "nanojewels."
Velev and Rastogi of NCSU developed the method with help of several colleagues,
including: Manuel Marquez, an adjunct professor in the Harrington Department
of Bioengineering in ASU's Ira A. School of Engineering, and Antonio Garcia,
a professor in the bioengineering department and director of the Laboratory
for Personalized Molecule Measurement; and professors Sonia Melle and Oscar
Calderon in the School of Optics at UCM.
Rastogi's presentation at the 82nd American Chemical Society Colloid &
Surface Science Symposium on June 18, 2008 titled "Synthesis of Light-Diffracting
Colloidal Crystal Assemblies from Microspheres and Nanoparticles in Droplets
on a Superhydrophobic Surface" and a paper just published in the journal
Advanced Materials (published Online: July 28, 2008), authored by these researchers,
describes how for the first time superhydrophobic surfaces are shown to play
an important role in making new materials.
In the paper, they describe how different nanoparticles of various sizes can
produce "nanojewels" of various colors that display different optical
properties.
"These nanojewels can potentially find application in photonics, drug
delivery, special coatings, sensors and microfluidics," Velev explains.
Indeed, many researchers around the world are working on ways to make similar
two-dimensional and three-dimensional photonic crystals to fabricate light-emitting
diodes, optical fibers for communications, submicroscopic lasers, ultrawhite
pigments, antennas and reflectors, and optical integrated circuits.
The biggest stumbling blocks in making these materials is finding ways of making
photonic crystals with uniform properties in very large quantities and in minimizing
imperfections in structure that reduce the quality of the final product.
This new process is certainly easy to replicate to make large quantities, and
superhydrophobic surfaces lead to structures that naturally form ordered structures.
Superhydrophobic surfaces allow nanojewels to be created from a single drop
of water containing nanoparticles, because of several effects.
First, the drop stays in the shape of a ball because water does not spread
on it while the nanoparticles are held in the drop due to the surface tension
of water.
Compared to drying the drop in air, which is a fast evaporation process that
causes the water in the drop to distort and flow, the drop gently dries on the
superhydrophobic surface. This lets the nanoparticles get as close to each other
as possible, swirling in a slow circular motion until all of the water evaporates.
When nanoparticles of two different sizes are used in the same drop, the smaller
ones move to the surface of the drop while the bigger ones stay in the middle.
This is because the smaller ones have more Brownian motion and are elevated
to the surface with the water molecules that are subsequently evaporating at
the surface, leaving all of the nanoparticles behind to form the nanojewels.
"Besides the dazzling look of these nanojewels, the most exciting thing
about this work is that it opens up many interesting possibilities in quickly
and inexpensively making new materials with nanoparticles", Marquez says.
"By understanding how different particle sizes determine the colors produced,
these nanojewels can be designed for applications in optical communication systems,"
Melle adds.
As more nanoparticles and nanostructures come into the marketplace, technologies
that can quickly assemble the structures so that their unique size and properties
can be employed in new devices will be important to the growth of nanotechnology
and related industries.
"Synthesis of Light-Diffracting Assemblies from Microspheres and Nanoparticles
in Droplets on a Superhydrophobic Surface", Vinayak Rastogi, Osca G. Calderon,
Antonio A. Garcia, Manuel Marquez, and Orlin Velev, Advanced Materials, (Early
View) Published Online 28 Jul 2008 (DOI 10.1002/adma.200703008).
