Biomedical Engineering Reference
In-Depth Information
always an easy task. Structural colors require
the design and fabrication of nanostructures
because the lengthscales of these structures
must be on the same order as the wavelengths
of visible light, but translation to even the
centimeter scale appears to be a Herculean
task. At the same time, the promise of avoid-
ing organic paints and fumes is so attractive
and the brilliance of structural colors is so
alluring that researchers continue to strive for
progress.
In particular, numerous attempts have been
made to replicate the brilliant blue of the
Morpho
butterflies, most successfully by biomimetic
techniques
[28, 29, 86-88]
. Furthermore, biorep-
lication techniques
[89]
based on nanocasting
[46, 90]
, nanoimprinting
[47, 91]
, physical vapor
deposition
[48, 92]
, atomic layer deposition
[45]
,
and hybrid techniques
[93]
are being developed.
Some of these approaches and practical indus-
trial realizations were recently summarized in a
topical review by Saito
[94]
.
The metallic reflection of koi fish was repli-
cated using biogenic guanine crystals, which
were collected from the skin beneath their scales.
Guanine crystals were also grown in dimethyl
sulfoxide
in vitro
. Although the two sets of crys-
tals show resemblance in form, the biogenic
crystals are exceptionally thin (
∼
50 nm) with
well-defined crystal faces, whereas the crystals
grown
in vitro
are much thicker and irregularly
stepped
[64]
.
The ideal structure of precious opals inspired
the synthesis of silica photonic crystals by pre-
cipitation of 10-nm grains
[95]
as well as by
chemical self-assembly
[96]
. Inverse opals
found on the wings of many beetles have been
nanoimprinted by pressing a wing against liq-
uid gallium that is then quickly frozen below
its melting point (28.78
o
C)
[55]
. Although this
nanoimprinting technique was aimed toward
the high-resolution identification of surface
details, it has potential for industrial-scale pro-
duction of surfaces endowed with structural
color
[93]
.
11.5.1 Structural Colors from Particle
Arrays
It has long been known that beautiful and
intense colors are observed in dispersions of
small particles. Iridescence due to the Bragg
phenomenon exhibited by three-dimensional
arrays (i.e., colloidal crystals) formed in some-
what concentrated suspensions of latex particles
of diameters between 150 and 500 nm has been
observed, the diffraction pattern often being
used for characterizing the properties of the col-
loidal crystals
[97]
.
Particle diameter is an important factor for
the different color-display mechanisms exhib-
ited by multilayered stacks formed by small
spheres. Latex spheres with diameters consider-
ably smaller than the visible wavelengths
(<400 nm) self-assemble in two-dimensional
arrays comprising monolayers and multilayers.
When illuminated with polychromatic light, a
stack of nanospheres reflects intense and uni-
form pastel colors.
Figure 11.18
a shows the jux-
taposition of several stacks, each of a different
color in reflection
[71]
. These colors result from
multilayer constructive interference. In contrast,
colors in transmission are almost unnoticeable
because each stack is just a few spherical diam-
eters in thickness.
The interference colors in
Figure 11.18
a are
enhanced by a gold-coated glass substrate. Each
color corresponds to a stack of a specific thick-
ness, given in
Table 11.1
. If monochromatic illu-
mination is used instead of white light, the
multilayers look like monocolored stripes of dif-
ferent intensity. By varying the monochromatic
light wavelength and observing the changes in
stripes' intensity, one can notice that at a given
wavelength, the intensities of two neighboring
stripes coincide. The value of this particular
wavelength is a key factor in a theoretical model
that suggests that arrays of self-assembled nano-
particles have primarily hexagonal close-pack-
ing structure. This prediction was confirmed by
scanning electron microscopy and atomic force
