B
ac
p
ate
123,1
Morpho butterfly wing scales demonstr ate
highly selectiv e vapour response
RADISLAV A. POTYRAILO
1
*
, HELEN GHIRADELLA
2
, ALEXEI VERTIATCH IKH
3
, KAT HARINE DOVIDENKO
1
,
JAMES R. COURNOYER
1
AND ERIC OLSON
1
1
Materials Analysis and Chemical Sciences, General Electric Global Research Center, Niskayuna, New York 12309, USA
2
Department of Biology, University at Albany, Albany, New York 12222, USA
3
Micro and Nano Technologies, General Electric Global Research Center, Niskayuna, New York 12309, USA
*
e-mail: potyrailo@crd.ge.com
Published online: 1 February 2007; doi:10.1038/nphoton.2007.2
Tropical Morpho butterflies are famous for their brilliant iridescent colours, which arise from ordered arrays of scales on their
wings. Here we show that the iridescent scales of the Morpho sulkowskyi butterfly give a different optical response to different
individual vapours, and that this optical response dramatically outperforms that of existing nano-engineered photonic sensors.
The reflectance spectra of the scales provide information about the nature and concentration of the vapours, allowing us to
identify a range of closely related vapours–water, methanol, ethanol and isomers of dichloroethylene when they are analysed
individually. By comparing the reflectance as a function of time for different vapours, we deduce that wing regions with scale
structures of differing spatial periodicity give contributions to the overall spectral response at different wavelengths. Our optical
model explains the effect of different components of the wing scales on the vapour response, and could steer the design of new
man-made optical gas sensors.
Although nanotechnology attempts to mimic the partial
photonic bandgap structure and the iridescent features of the
Morpho butterfly scale
1–4
in materials with new visual effects
and functionality
5–8
, the exact combination of its
three-dimensional structure and cuticle complex refractive
index
9
[n* ¼ (1.55 + 0.05) þ i(0.06 + 0.01)] is still beyond
nanofabrication capabilities. Nanofabrication has been successful
in creating photonic structures for chemical and biological
detection
10–15
. Mechanisms of colour generation in existing
nanofabricated photonic-sensing materials include localized
plasmon resonance
12,16–18
, Bragg diffraction
10,19–21
, and Fabry–
Pe
´
rot interferometry
11,22
. An advantage of these photonic
structures over organic dyes is the elimination of photobleaching
problems. Unfortunately, the main limitation of existing
nanofabricated photonic-sensing materials is their low response
selectivity to different analytes. Thus, their selectivity is
conventionally enhanced by using chemically selective moieties
or layers
10,23–25
. There is only a limited commonality between
known biological and engineering solutions to challenging
problems
26
, which suggests that the biological world still has a lot
to teach us.
Here we show that by taking advantage of the hierarchical
nanostructure that leads to bright iridescence in butterfly scales,
nature provides a different design for selective response to diverse
vapours. This iridescence results from the combined effects of
diffraction and interference of light
27
and originates from the
cooperation of regularity and irregularity within a scale
28
.We
have found that, upon interaction with different vapours, such
photonic structures produce remarkably diverse differential
reflectance spectra, achieving a highly selective response to
individual vapours with a single photonic structure.
RESULTS
STRUCTURE-DETERMINED SCALE IRIDESCENCE
We worked with a highly iridescent butterfly species, Morpho
sulkowskyi. It has high visible reflectance, and higher ridge density
and a more regular lamellar structure (see below) than other
Morphos, and it lacks pigment that absorbs light and decreases
reflectance
1,27
. Butterfly and moth wings typically have, on each
surface, visible cover and obscured ground scales, which may
differ in form. On the wing obverse (top surface) of M.
sulkowskyi, both cover and ground scales are visible and are
similar nanostructurally, so we will treat them as a single layer of
obverse scales. The elaborate ridge structure of the obverse scales
(shown in Fig. 1 produces the iridescence; the simpler scales of
the wing reverse (bottom surface) are not iridescent. Each scale is
formed by a single epithelial cell that secretes the outer epicuticle
and the inner layers of cuticle to form lattices, pillars and other
internal scale structures. Mechanisms to shape these fine features
may include surface tension and elastic buckling
1
.
Reproducibility of scale reflectance is important for
reproducible vapour response. To examine the spatial uniformity
of the photonic structure in M. sulkowskyi scales, we scanned
several butterflies across two forewings and obtained a highly
reproducible (s.d. ¼ 3 nm) spectral reflectance peak due to the
uniform structure of the individual scales across the wing span.
The lamellae of the ridges act as multilayer interferometric
nanoreflectors, and the ridges act as a diffraction grating
28
,
together combining multilayer interference from individual
reflectors and diffraction from the array of these reflectors, a
radical departure from photonic structures based on separate
diffraction or interference effects
10,11,21
.
ARTICLES
nature photonics | VOL 1 | FEBRUARY 2007 | www.nature.com/naturephotonics 123