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Thanh-Dinh Nguyen, Bernardo U. Peres, Ricardo M. Carvalho, and Mark J. MacLachlan*

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Photonic Hydrogels from Chiral Nematic Mesoporous
Chitosan Nanofibril Assemblies

wavelength that depends on the repeating
distance, or pitch, of the helicoidal
arrangement.[6]
The organization of chitin in the Bouligand structure is a solid-state manifestation of optically active chiral nematic
(cholesteric) lyotropic liquid crystals
(LCs).[7] Cellulose nanocrystals (CNCs)
and other cellulose derivatives, such as
hydroxypropyl cellulose, can also organize
into a lyotropic chiral nematic LC phase
above a critical concentration.[8] The LC
order of CNCs is preserved upon evaporation of solvent to yield left-handed chiral
nematic CNC films that are iridescent.[9]
The coloration of the films can be tuned
by changing the pitch of CNC assemblies
upon drying in response to certain stimuli.[10] Recently, we
produced photonic chiral nematic mesoporous cellulose films
from the self-assembly of a CNC dispersion with melamineurea-formaldehyde resins and subsequent alkali treatment.[11]
These cellulosic nanocrystal films are responsive photonic
materials whose reflected colors change upon swelling.
The exoskeletons of crustaceans are mainly composed of
chitin, minerals, and protein. Interestingly, chitin is arranged

into a Bouligand structure within the exoskeleton,[5] but most
likely arthropods first evolved the Bouligand-type organization
for structural reinforcement rather than for coloration. As crustacean shells are a waste by-product from seafood processing,
it is attractive to use the crustacean exoskeletons for the development of new advanced materials. Recently, we exploited the
Bouligand structure of chitin in endocuticles of crab shells[12]
directly as a template to produce photonic silica/chitin composites by hydrolyzing tetramethoxysilane within pores of the
chiral nematic mesoporous chitin network innate to the crab
cuticles. Calcination of these composites under different conditions recovered mesoporous solid replicas (silica and N-doped
carbon) that retained the Bouligand-twisted organization and
high surface area of the cuticle nanofibrils.[12] Similar to the
chiral nematic phase of CNCs, the twisted mesoporous chitinous cuticles could be useful for constructing hierarchical
photonic nanomaterials.[13]
Smart photonic hydrogels that can respond to external
stimuli with a change in color have received a great deal
of attention because they are useful for optical sensing.[14]
Chiral nematic structures with photonic properties can be
an intriguing platform to transfer into a hydrogel.[15] Stimuliresponsive photonic hydrogels were previously formed by
polymerizing suitable monomers (e.g., acrylamide, acrylic

Iridescence in animals and plants often arises from structural coloration,
which involves hierarchical organization of minerals and biopolymers over
length scales of the visible spectrum, leading to diffraction of light. In this
work, discarded crustacean shells that are not known for their structural
colors are used to produce photonic nanostructures of large, freestanding
chiral nematic mesoporous chitosan membranes with tunable iridescent
color. Bioinspired by colorful nanostructures in nature, photonic hydrogels
with Bouligand-type organization are fabricated from the twisted mesoporous
membranes, where the chitosan nanofibrils are a novel precursor for surface
acetylation and are also a biotemplate for polymerizing methyl methacrylate.
The colors of the hydrogels can be tailored by swelling as they show large

volume changes in response to changes in solvent environment.

1. Introduction
More than two billion years of evolution have led to remarkable natural materials, such as bone and nacre, with complex
3D hierarchical structures.[1] Diverse and beautiful examples of
iridescent colors in plants, insects, and other animals originate
from hierarchical nanostructured organization of substances,
often carbohydrates, which interact with light to create vibrant
colors. These structural colors have evolved by natural selection for warding off predators, attracting mates, and mimicry.[2]
Examples of structural colors include the blue feathers of peacocks, the metallic hues of dragonfly bodies and beetle elytra,
and the blue wings of the morpho butterfly.[3] The remarkable
natural architectures responsible for the coloration of these
organisms and others are inspiring scientists to create new
photonic materials.[4] Within the exoskeleton of many arthropods, for example, chitin fibrils organize into a left-handed
helical “twisted plywood” arrangement that is known as a Bouligand structure.[5] This long-range twisted order leads these
structures to selectively reflect circularly polarized light with a
Dr. T.-D. Nguyen, Prof. M. J. MacLachlan
Department of Chemistry
University of British Columbia
2036 Main Mall, Vancouver,
British Columbia V6T 1Z1, Canada
E-mail:
B. U. Peres, Prof. R. M. Carvalho
Department of Oral Biological and Medical Sciences
Faculty of Dentistry
University of British Columbia
2199 Wesbrook Mall, Vancouver, British Columbia V6T 1Z3, Canada

DOI: 10.1002/adfm.201505032


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acid) during the self-assembly of CNCs into
a chiral nematic phase.[16] There are many
reports of biocompatible chitosan-based
hydrogels,[17] but introducing the chiral
nematic order into photonic chitosan hydrogels has been virtually unexplored. Here we
report, for the first time, the preparation and
investigations of photonic hydrogels with
tunable polarized color from chiral nematic
mesoporous chitosan (CNMC) nanofibrils
derived from shells and endocuticles of
crustaceans.

2. Results and Discussion
Visibly, the original shells and endocuticles
of shrimps do not generally appear iridescent

(Figure S1a, Supporting Information). Until
now the chirality and coloration of shrimp
shells are unknown and this is not surprising
since they have an inhomogeneous layered
structure of chitin fibrils (Figure S2, Supporting Information). Protein and minerals
(mostly calcium carbonate) were removed
from the shrimp shells by sequentially
treating them with dilute alkali and acid solutions to obtain white chitin membranes. Even
after purification, chitin membranes obtained Figure 1. Photonic mesoporous chitosan nanofibrils with chiral nematic structure prepared
show neither structural colors nor chiroptical from shrimp shells. a) Photograph of dried membranes of iridescent CNMC obtained by
treating the shrimp shells twice with 50 wt% NaOH(aq) solution at 90 °C for 8 h. b) Solid-state
properties (Figure S1b, Supporting Informa- 13C CP/MAS NMR spectra of the purified chitin shrimp
shells before (red line) and after (blue
tion). We were thus surprised to find that line) repeated alkali treatment. c) POM image of the dried CNMC viewed under crossed polarhot alkali treatment of the purified shrimp izers at the top surface of the film. d) Photographs of dried CNMC observed under left-handed
shells affords photonic materials resembling (left) and right-handed (right) circularly polarized filters. e) CD spectra of dried (blue) and wet
crab endocuticles.[12] The coloration of the (red) CNMC.
deacetylated chitin membranes was evolved
main structure of the shrimp shells, but considerably decreased
by repeatedly treating the purified shrimp shells several times
the crystallinity of the chitin fibrils. This is consistent with the
with hot concentrated alkali solution. Interestingly, flexible
alkali-treated crab endocuticles previously reported.[12] These
membranes obtained after drying are more transparent, appear
violet-green iridescent in color, and retain the original shapes
results confirm that the materials obtained from the alkali treatof the shrimp shells with ≈100–150 µm thickness (Figure 1a).
ment of the purified shrimp shells are chitosan, the highly deaThe purified shrimp shells before and after alkali treatment
cetylated form of chitin. The alkali-treated membranes show
were analyzed using a variety of techniques. Elemental analyses
lower crystallinity and more transparency than pristine chitin.
confirm that the elemental C:N ratio in chitin decreases from

Reduced interchain hydrogen bonding between the chitosan
6.86 to 5.25 (wt/wt%) after alkali treatment, giving a ratio that
polymers may lead the originally inhomogeneous layered
is close to the theoretical C:N ratio of 5.14 in chitosan. Solidnanofibrils to significantly relax upon swelling into a structure
state 13C cross-polarization/magic-angle spinning (CP/MAS)
with improved order and photonic properties.
Mesoporosity was obtained within the colorful chitosan
NMR spectroscopy (Figure 1b) shows that the pristine chitin
membranes after removal of calcium minerals from the shrimp
has peaks at 163.5 and 13.5 ppm assigned to C7 carbonyl
shells, which makes them swell reversibly (Figure S4, Supand C8 methyl carbons, respectively; peaks at 46–96 ppm are
porting Information). Polarized optical microscopy (POM)
assigned to the resonances of C1-C6 on the N-acetyl-D-glucosaimages (Figure 1c) of the chitosan membranes show strong
mine unit of chitin.[18] The sharp peaks of the acetyl groups
birefringence with some regions showing fingerprint textures
mostly disappear in the chitin samples after alkali treatment,
composed of adjacent lines; the images look similar to those
demonstrating that chitin was strongly deacetylated by hot conof chiral nematic mesoporous cellulose films.[11] The iridescent
centrated alkali solution to yield chitosan.[18] Powder X-ray diffraction (PXRD) patterns (Figure S3b, Supporting Information)
violet-green colors of the membrane are clearly observed under
of the alkali-treated chitin show diffraction peaks at the same
a left-handed circularly-polarized filter and turn to a dull shade
positions as for pristine chitin, but with decreased diffraction
when viewed under a right-handed circularly-polarized filter
intensity, confirming that the alkali treatment preserved the
(Figure 1d). Circular dichroism (CD) spectra (Figure 1e) of the

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heating of the alkali treatment enhances the
green-blue polarized colors of CNMC and
consequently enhances the optical reflectance (Figure S6c,d, Supporting Information). Similar to the 13C CP/MAS NMR
spectroscopy of the shrimp shell-derived
chitosan membranes, the resonances of C7
carbonyl and C8 methyl carbons in N-acetylD-glucosamine unit are dramatically diminished after alkali treatment of the chitinous
crab endocuticles (Figure S7, Supporting
Information).[18] Furthermore, the peak
reflected wavelength of the pristine CNMC
red shifted from ≈550 nm to ≈770 nm when
aging the membrane in acid (pH 1.0) for
1 d (Figure S8, Supporting Information).
Treating CNMC with dilute acid may disrupt interchain hydrogen bonding between

nanofibrils, leading to swelling and an
increase in the pitch, and thus a red shift of
the iridescence. CNMC swells rapidly upon
immersion in solvents of different polarity,
resulting in a red shift of the reflectance peak
as the helical pitch lengthens (Figure S9,
Figure 2. SEM images of CNMC prepared from the chitin shrimp shells viewed along a) cutting Supporting Information). Soaking CNMC in
edges and b–d) fracture cross-sections with different magnifications, and e) Schematic of chiral water causes a large color change compared
nematic mesoporous chitosan nanofibril structure.
to soaking in anhydrous ethanol, where the
colors are mostly unchanged. By soaking
CNMC in water/ethanol, the reflected colors span from the UV
dried membrane show a very intense positive signal at ≈550 nm
to near-IR (infrared) regions. The mesoporosity, lower crystalthat red shifts to ≈650 nm upon swelling of the sample in
linity, and surface amphiphilicity likely result in fast swelling of
water. The position and shape of the CD spectrum of the chithe chitosan membranes in water within tens of seconds (see
tosan sample do not change significantly when rotating the
video in Supporting Information), which is comparable to that
membrane, indicating that that the signal is not dominated by
of photonic mesoporous cellulosic films.[11]
linear birefringence (Figure S5, Supporting Information). Scanning electron microscopy (SEM) images (Figure 2) of the chiPhotonic hydrogels based on natural polymers are of partosan membranes viewed along cross-sections show a repeating
ticular interest for applications in optical sensors for biomedilayered structure of the nanofibrils organized with a countercine.[14] As shown above, CNMC may be used directly as a
clockwise direction over micrometer distances. Together, these
swellable biomaterial with tunable chiroptical properties. To
results clearly confirm that the chitosan nanofibrils organize
further demonstrate the use of CNMC as a bioinspired platinto a left-handed chiral nematic structure in the membrane
form to develop photonic hydrogels, we investigated the surthat selectively reflects left-handed circularly polarized light
face acetylation of chitosan and the use of chitosan to template
to create the iridescent colors. Based on qualitative electronpoly(methylmethacrylate) (PMMA) photonic hydrogels with
microscopic observations, we suggest that the Bouligand-type

chiral nematic ordering. (The photonic membranes prepared
organization of the chitosan nanofibrils in the shrimp shells is
from both the crab endocuticles and shrimp shells can be used
slightly lower order than that in the crab shells.[12] This leads
for these experiments.)
Photonic hydrogels were directly prepared from CNMC by
the CNMC to appear less iridescent and to show less intense
controlled N-terminal acetylation and subsequent swelling in
reflectance peaks by UV–vis and CD spectroscopy. From a
water. Acetylation was carried out by simply treating CNMC
survey of the literature, most previous efforts to observe chiropwith pure acetic anhydride, giving partial acetamide groups
tical phenomena in regular shrimps have failed;[5b,c,d] this is the
on the chitosan fibrils. The mesoporous membranes retain
first demonstration of freestanding chiral nematic mesoporous
the original shapes and structural colors upon acetylation
chitosan membranes with tunable photonic properties from
(Figure 3a, left). The acetylated chitosan hydrogels (ACH) swell
common shrimp shells.
with a simultaneous change in the pitch of the chiral nematic
We previously reported iridescent CNMC prepared from crab
structure and the reflected color. Interestingly, we found
endocuticles,[12] but we found that the manual delamination
that ACH strongly absorbs water and undergoes substantial
of layers from inner sides of the shells leads to tearing of the
swelling with a several-fold increase in thickness to form transendocuticles into small pieces. We now successfully obtained
parent hydrogels within several minutes (Figure 3a-right and
the endocuticles with intact shapes of large-sized exoskeletons
Figure S10a of Supporting Information). This response is much
by peeling off outer sides of the crab shells (Figure S6a, Supmore dramatic than the pristine CNMC that only shows modest
porting Information). Also, we additionally found that extended


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Figure 3. Photonic hydrogels by swelling of the acetylated chitosan nanofibrils. a) Photographs of swelling behavior of ACH (prepared from crab endocuticles) in water. b) POM image of ACH after swelling viewed under crossed polarizers at a transverse cross-section of the film. c) SEM image viewed
along a cross-section of the dried ACH. d) CD and UV–vis spectra of ACH upon swelling in water. Because the CD spectrometer could not measure
beyond 900 nm (blue lines), the UV–vis spectrometer was used to confirm the reflection peaks of ACH in the swollen states at longer wavelengths
(red lines). e) Microtensile load-displacement curves for CNMC (red) and ACH dried (blue) and swollen (black) in water. Stress-strain curves were
calculated based on the original cross-sectional area of each tested specimen and assuming a gauge length of 20 mm.

swelling in water. The acetylation-induced swelling offers a
simple way to directly use the chitosan membranes for fabricating photonic hydrogels. Elemental analyses show that the
elemental C:N ratio in CNMC increases from 5.83 to 5.92 (wt/
wt%) after acetylation of chitosan. IR spectra (Figure S11, Supporting Information) of the acetylated CNMC show an increase
in the absorption intensity of the amide and alkyl stretching
bands, indicating that acetic anhydride predominantly reacetylated the primary amino groups of chitosan. PXRD patterns
(Figure S12, Supporting Information) of the deswollen acetylated chitosan confirm the retention of α-chitin structure with
slightly lower crystallinity than the pristine chitin.
The swollen acetylated chitosan hydrogels retain optically
anisotropic properties when viewed under crossed polarizers
and show a striated texture with a distance between lines corresponding to one half-pitch (Figure 3b). CD spectral measurements confirm that ACH before swelling shows a positive
signal at ≈550 nm, the same wavelength as the peak reflection
of CNMC, indicating the intact retention of the chiral nematic
structure during acetylation (Figure 3d). CD spectroscopy of

the hydrogels confirms the selective reflection of left-handed
circularly polarized light, and the wavelength of the reflectance peak red shifts as the material is swollen in water. The
fully swollen hydrogels appear colorless and transparent as a
result of extending the peak reflection into the near-IR region
at ≈1200 nm (Figure 3d). Upon deswelling of ACH, the positive CD signal blue shifts to ≈680 nm (Figure S14a, Supporting
Information), confirming that the left-handed twisted order
of the chitosan nanofibrils was preserved in the deswollen

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hydrogel structure. The acetylation likely results in reduction
of hydrogen bonding and crystallinity of the chitosan fibrils,
leading to large swelling of the acetylated membranes in water.
This is the cause of a drastic change in the pitch to induce
the reversible shift of the reflection of the swollen structure
of the acetylated chitosan hydrogels (Figure S14a, Supporting
Information). Tensile mechanical testing (Figure 3e) showed a
mean ultimate tensile strength (UTS) and elongation at break
(EB) of 23.6 ± 3.0 MPa and 7 ± 0.7% for the dried ACH; and
of 22.8 ± 1.2 MPa and 9 ± 0.2% for CNMC, respectively. While
both structures showed a typical viscoelastic behavior, there
was a clearly distinct strain-dependent shift of moduli between
them. ACH presented a higher modulus at low strain (0%–3%),
while CNMC presented a higher modulus only after reaching
6% strain. The combined information on the UTS and EB indicates that ACH is a stronger and more brittle material at low
strain, but CNMC is in an overall stronger and tougher material at high strain. These differences could be attributed to the
reduced hydrogen bonding between crystalline fibers resulting
from the surface acetylation of chitosan. Upon swelling in

water from dryness, the UTS of ACH dropped to 3.1 ± 0.7 MPa
at a strain of 1 ± 0.3%. After reaching this maximum stress, the
material did not sharply break, but seemed to gradually disintegrate as if the fibers were deforming permanently and breaking
nonuniformly as the strain increased. This behavior resembles
a gel-like structure and reflects the reduced internal cohesive
structure that is now filled with water.
The mesoporous structure of the twisted chitosan nanofibrils
makes the large-sized membrane useful as a template for the

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Figure 4. Biomimetic templating of twisted mesoporous chitosan nanofibrils with methyl methacrylate into photonic hydrogels. a) Photographs of
swelling response of PCH (prepared from crab endocuticles) at pH 1.4. b) POM image of PCH after swelling viewed under crossed polarizers at the
top surface of the film. c) SEM image viewed at cutting edges of the dried PCH. d) CD and UV–vis spectra of PCH upon swelling in acidic media at
pH 1.4. As the CD spectrometer that was used could not measure beyond 900 nm (blue lines), the reflection peaks of PCH in the swollen states beyond
this range were confirmed with complementary UV–vis spectroscopy (red lines). e) Microtensile load-displacement curves for PCH dried (red) and
swollen (blue) in acidic media at pH 1.4. Stress-strain curves calculated were based on the original cross-sectional area of each tested specimen and
assuming a gauge length of 20 mm.

construction of photonic materials. We improved the responsive swelling and mechanical properties of CNMC by incorporating PMMA into the mesopores of the chitosan nanofibril networks. PMMA was prepared by photo-induced polymerization
of methyl methacrylate monomer within the pores of CNMC
in the presence of a 2,2-diethoxyacetophenone photoinitiator.
PMMA/chitosan composite with slightly red shifted colors is

a true replica of the biotemplate and is more transparent and
flexible than the pristine and acetylated CNMC (Figure 4a, top).
The PMMA/chitosan composites strongly swell into hydrogels in acidic media over a wide range of pH from 1 to 4
(Figure 4a-bottom and Figure S10b of Supporting Information).
The photonic hydrogels show a change in color from green to
red and transparent during swelling with a several-fold increase
in thickness within several minutes, while retaining their original shapes and enhancing the flexibility relative to CNMC.
Elemental analyses confirmed 44.74 wt% carbon in the PMMA/
chitosan composite in comparison to 41.13 wt% carbon in the
pristine CNMC, consistent with the incorporation of PMMA
into the chitosan film. From these analysis results, the content
of PMMA in the PMMA/chitosan composites was found to be
≈8 wt%, in agreement with that determined from the dissolution of the chitosan template by immersing the composites in
an aqueous solution of 2 vol% acetic acid at room temperature
for 24 h. IR spectra (Figure S13, Supporting Information) of
PMMA/chitosan show amide I and II bands at 1560–1660 cm−1,
a C O stretching band at 1025 cm−1 characteristic of chitosan,
and a peak at 1720 cm−1 assigned to the ester carbonyl vibration of PMMA. This provides substantial evidence of grafting
PMMA on chitosan fibrils without altering the original chitosan

Adv. Funct. Mater. 2016, 26, 2875–2881

structure. The acidic pH-dependent response of the PMMA/
chitosan hydrogels (PCH) may result from the protonation of
the primary amino groups, which favors large expansion of the
hydrogel volume. PCH responds much faster to acidic pH than
to basic pH because the chitosan template strongly swells in
dilute acid solution but not in base.
POM images (Figure 4b) of the swollen PCH hydrogels show
optical birefringence. Optical spectroscopy shows photonic

response of the reversible swelling of PCH in acidic media.
CD spectra of the PMMA/chitosan composites before swelling
show a positive signal at ≈650 nm that is slightly red-shifted
relative to CNMC, which likely results from the expansion of
the helical structure by incorporating PMMA within the pores
of the chitosan nanofibril membrane (Figure 4d). The selective
reflection of left-handed circularly polarized light from films of
PCH was confirmed by CD spectroscopy, which shows a peak
with positive ellipticity that red shifts upon the swelling of the
material (Figure 4d). When fully swollen, PCH shows a reflectance peak at ≈1200 nm and appears colorless and transparent
as the reflection has moved into the near-IR region (Figure 4d).
Upon deswelling, PCH shows a slightly blue-shifted CD signal
with positive ellipticity at ≈720 nm (Figure S14b, Supporting
Information), indicating that the hydrogels preserved the lefthanded twisted organization of the chitosan template upon
drying of water from acidic media. The reversible shift of the
reflection originates from a drastic change in the pitch of the
highly swollen structure of PCH. The peak reflections of the
deswollen PCH slightly red shift relative to the pristine CNMC
before swelling, which is likely attributed to relaxation of the
organized nanofibril structure after deswelling. PCH shows

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reversible color changes and its chiroptical properties change
negligibly upon redrying and reswelling in acidic media
(Figure S14b, Supporting Information). Tensile mechanical
testing (Figure 4e) shows that the dried PMMA/chitosan
composites have a mean UTS of 27.2 ± 2.1 MPa at a strain of
13 ± 0.1% that are much higher than those of CNMC (22.8 ±
1.2 MPa, 9 ± 0.2%, see Figure 3e). Significantly lower UTS was
observed for the PMMA/chitosan composites (2.2 ± 0.5 MPa)
after swelling in acidic media. Elongation at break was also
reduced after swelling to 9 ± 0.3% (Figure 4e). Although these
values are lower than the dried specimens, the UTS is relatively
higher than that obtained with the swollen acetylated chitosan
hydrogels without PMMA (see Figure 3e). The enhanced properties, including toughness, of PCH are mainly attributed to
the combination of chitosan with polymer.
Retention of the chiral nematic organization of these hydrogels was confirmed by SEM (Figures 3c and 4c; Figures S15
and S16 of Supporting Information). ACH and PCH both
show a repeating layered structure with nanofibrils rotating in
a counter-clockwise direction at fracture cross-sections, which
is characteristic of left-handed chiral nematic order. These geometrical nanostructured features resemble those of CNMC,
confirming the preservation of the Bouligand-type organization of the hydrogels upon acetylation and PMMA polymerization. These observations also indicate that the chitosan acetylation predominantly occurred on the nanofibril surfaces while
PMMA homogeneously formed around the nanofibrils. The
interlayer separations measured from cross-sections perpendicular to the top surfaces of these deswollen hydrogels are slightly
higher than those of CNMC, which are consistent with their
relative reflectance peaks. The twisted layered structure of these
hydrogels resembles that of the mesoporous cellulosic films[11]
and the shells of Jewel beetles[6a] and Pollia fruits.[6b] Hydrogels prepared from chitosan[19a] and templating of PMMA by

chitosan butterfly wings[19b] were previously demonstrated, but
they do not have chiral nematic order and exhibit only modest
swelling. Photonic biological structures are frequently sought
by observing the visible coloration reflected on the organism’s
outer shells.[6] By starting from the popular crustacean exoskeletons without structural coloration, we exploited both their
endocuticles and shells to produce iridescent, freestanding
photonic mesoporous chitosan membranes and hydrogels with
chiral nematic structures.

3. Conclusions
In summary, we have used discarded crustacean exoskeletons
and shells to develop tunable photonic nanomaterials. The
Bouligand structure of chitin that is hidden in the crustacean
exoskeletons was exploited by deacetylation of the purified endocuticles and shells of the crustaceans to improve long-range
order of the Bouligand-type arrangement of chitosan nanofibrils and obtain large, intact mesoporous photonic membranes.
We demonstrated proof-of-concept for using the mesoporous
chitosan nanofibrils as a novel precursor for acetylation and
also as a platform for templating poly(methylmethacrylate) to
produce photonic hydrogels. Upon swelling, these hydrogels

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undergo large volume expansion that is responsible for their
tunable chiroptical properties. The introduction of the photonic
properties in various types of sustainable nanomaterials may
open new possibilities for optical sensing and templating.

4. Experimental Section

Preparation of Chiral Nematic Mesoporous Chitosan Nanofibrils: Shrimp
shells (12 g) were treated with NaOH(aq) (250 mL, 5 wt%) at 90 °C for
6 h to decompose protein and then treated with HCl(aq) (250 mL, 0.1 M)
at room temperature for 2 h to remove calcium minerals. The purified
chitin shells were collected and rinsed with copious water. Elemental
analysis: 6.19% N, 42.49% C, 6.47% H.
To gain a photonic structure, the resultant white chitin shells
(3.5 g) were treated with a concentrated NaOH(aq) solution (50 mL,
50 wt%) at 90 °C for 8 h and this deacetylation of chitin nanofibrils
was repeated at least twice to enhance the structural coloration. The
resulting shells were washed thoroughly with water and allowed to dry
at ambient conditions to obtain highly flexible, transparent chitosan
membranes that appear iridescent, retain the original shapes of the
shrimp shells, and lose ≈13 wt% compared to the purified chitin shells.
Elemental analysis: 6.93% N, 36.39% C, 6.82% H. Note that we found
that this photonic transformation can be extended to the most popular
crab species except the king crabs possibly due to rigid structures of
their outer shells. To produce intact shapes of the chitosan endocuticles
of large king crab exoskeletons, the chitin endocuticle membranes
were manually delaminated by carefully peeling off the outer sides of
the alkali-treated shells to obtain intact endocuticles. The pure chitin
membranes were obtained by treating the separated cuticles with a
dilute HCl(aq) solution (500 mL, 0.1 M) to completely remove calcium
minerals at room temperature within 2 h followed by washing with
copious water. We also found that the photonic twisted cuticles can
be obtained from different species of the crabs, but the chiral nematic
order of the nanofibrils organized in the king crabs is better than that of
other ones. The purified chitin crab endocuticles (7 g) were then treated
with a concentrated NaOH(aq) solution (100 mL, 50 wt%) at 90 °C with
various periods of time up to 8 h for controlled deacetylation of N-acetyl

groups. The reaction products (CNMC) were washed with copious
water to obtain intact chitosan membranes with enhanced transparency,
flexibility, and coloration. The CNMC samples were used to investigate
the tunable coloration by swelling the membranes in different solvents
and acidic media.
Fabrication of Photonic Hydrogels: The CNMC samples prepared
from the endocuticles and shells of the crustaceans were used to
investigate the production of photonic hydrogels. For the preparation
of ACH, the dried CNMC membranes as a starting material (300 mg)
were immersed in 20 mL pure acetic anhydride at room temperature.
The surface acetylation of chitosan was carried out for 30 min and then
the resulting membranes were collected from the reaction mixture by
filtration. The acetylated chitosan membranes were dabbed with tissue
paper to remove the absorbed acetic anhydride and then immersed in
water to swell into hydrogels. Elemental analysis of the dried acetylated
chitosan: 7.04% N, 41.64% C, 7.13% H.
For the preparation of PCH, the dried CNMC membranes
(300 mg) were immersed in 15 mL dimethylformamide solution
containing 5 mL methyl methacrylate (MMA) and 80 µL
2,2-diethoxyacetophenone photoinitiator. The reaction mixture was
exposed to UV light (from an 8 W, 300 nm UV-B light source) at
ambient conditions to polymerize MMA onto the chitosan nanofibril
template. The photo-induced polymerization was conducted within 1 h
to sufficiently incorporate PMMA into the mesoporous membranes to
obtain flexible, homogeneous PMMA/chitosan composites. Extending
the photopolymerization time beyond 1 h resulted in overall coating
of PMMA on the film surfaces of the chitosan template to form less
flexible, thicker composite membranes. The resulting PMMA/chitosan
composites were then taken out from the reaction mixture and dabbed


© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Funct. Mater. 2016, 26, 2875–2881


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[8]

Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.

Acknowledgements
The authors thank the Natural Sciences and Engineering Research
Council (NSERC) for funding (Discovery Grant) and a postdoctoral
fellowship for T.-D.N. R.M.C. thanks UBC Dentistry for start-up funds.

[9]

Received: November 23, 2015
Revised: February 4, 2016
Published online: March 15, 2016

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with tissue paper to remove absorbed monomer and solvent, and then
immersed in acidic media to swell into hydrogels. Elemental analysis of
a dried PMMA/chitosan composite: 6.85% N, 43.74% C, 6.70% H.


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