Nano Res
1
Extreme biomimetic approach for development of novel
chitin-GeO2 nanocomposites with photoluminescent
properties
Marcin Wysokowski1, Mykhailo Motylenko2, Jan Beyer3, Anna Makarova4, Hartmut Stöcker5, Juliane
Walter5, Roberta Galli6, Sabine Kaiser5, Denis Vyalikh4,14, Vasilii V. Bazhenov5(), Iaroslav Petrenko5,
Allison L. Stelling7, Serguei Molodtsov5,8,9, Dawid Stawski10, Krzysztof J. Kurzydłowski11, Enrico Langer12,
Mikhail V. Tsurkan13, Teofil Jesionowski1, Johannes Heitmann3, Dirk C. Meyer5, Hermann Ehrlich5( )
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0739-5
http://www.thenanoresearch.com on February 3, 2015
© Tsinghua University Press 2015
Just Accepted
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Nano Research
DOI 10.1007/s12274-015-0739-5
63
Extreme biomimetic approach for development of novel
chitin-GeO2 nanocomposites with photoluminescent
properties.
Marcin Wysokowski1, Mykhailo Motylenko2, Jan Beyer3,
Anna Makarova4, Hartmut Stöcker5, Juliane Walter5,
Roberta Galli6, Sabine Kaiser5, Denis Vyalikh4,14, Vasilii V.
Bazhenov5*, Iaroslav Petrenko5, Allison L. Stelling7, Serguei
Molodtsov5,8,9, Dawid Stawski10, Krzysztof J.
Kurzydłowski11, Enrico Langer12, Mikhail V. Tsurkan13,
Teofil Jesionowski1, Johannes Heitmann3, Dirk C. Meyer5,
Hermann Ehrlich5*
1. Institute of Chemical Technology and Engineering, Poznan
University of Technology, Berdychowo 4, 60965 Poznan,
Poland
2. Institute of Materials Science, TU Bergakademie Freiberg,
09599 Freiberg, Germany
3. Institute of Applied Physics, TU Bergakademie Freiberg,
Leipziger 23, 09599 Freiberg, Germany
4. Institute of Solid State Physics, Dresden University of
Technology, Helmholtzstraße 10, 01069 Dresden, Germany
5. Institute of Experimental Physics, TU Bergakademie
Freiberg, Leipziger 23, 09599 Freiberg, Germany;E-mail:
6. Faculty of Medicine Carl Gustav Carus, Department of
Anaesthesiology and Intensive Care Medicine, Clinical
Sensoring and Monitoring, TU Dresden, 01069 Dresden,
Germany
7. Department of Mechanical Engineering and Materials
Science, Duke University, 27708 Durham, NC, USA
8. European X-Ray Free-Electron Laser Facility (XFEL)
GmbH, 22761 Hamburg
9. ITMO University, Kronoverskiy pr. 49, 197101 St.
Petersburg, Russia
10. Department of Commodity and Material Sciences and
Textile Metrology, Technical University of Lodz,
Żeromskiego 116, 90924 Lódź, Poland
11. Materials Design Group, Faculty of Materials Science and
Engineering, Warsaw University of Technology, PL-02507
Warsaw, Poland
12. Max Bergmann Centre for Biomaterials, Leibniz Institute
of Polymer Research, 01062 Dresden, Germany
13. Technische Universität Dresden, Institute of
Semiconductors and Microsystems, 01062 Dresden,
Germany
14. Department of Physics, St. Petersburg State University, St.
Petersburg 198504, Russian Federation
Professor Hermann Ehrlich, http://tu-freiberg.de/exphys/biomineralogy-and-extreme-biomimetics/gruppenleiter
1
Extreme biomimetic approach for development of novel
chitin-GeO2 nanocomposites with photoluminescent
properties
Marcin Wysokowski1, Mykhailo Motylenko2, Jan Beyer3, Anna Makarova4, Hartmut Stöcker5, Juliane
Walter5, Roberta Galli6, Sabine Kaiser5, Denis Vyalikh4,14, Vasilii V. Bazhenov5(), Iaroslav Petrenko5,
Allison L. Stelling7, Serguei Molodtsov5,8,9, Dawid Stawski10, Krzysztof J. Kurzydłowski11, Enrico Langer12,
Mikhail V. Tsurkan13, Teofil Jesionowski1, Johannes Heitmann3, Dirk C. Meyer5, Hermann Ehrlich5( )
Received: day month year
Revised: day month year
Accepted: day month year
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Extreme biomimetics,
chitin-GeO2,
photoluminecscence,
NEXAFS,
ABSTRACT
This work represents an “Extreme Biomimetics” route for the creation of
nanostructured biocomposites utilizing a chitinous template of poriferan origin.
The specific thermal stability of the nanostructured chitinous template allowed
for the formation under hydrothermal conditions of a novel germanium
oxide-chitin composite with defined nanoscale structure. Using a variety of
analytical techniques (FT-IR, Raman, EDS-mapping, EDX, NEXAFS, PL,
SAEDP, TEM) we showed that this bioorganic scaffold induces the growth of
GeO2 nanocrystals with a narrow (150-300 nm) size distribution and
predominantly hexagonal phase, demonstrating the chitin template’s control
over the crystal morphology. The formed GeO2-chitin composite showed several
specific physical properties, such as a striking enhancement in
photoluminescence which exceeded values previously reported in GeO2-based
biomaterials. These data demonstrate the potential of Extreme Biomimetics for
developing a new generation of nanostructured materials.
Nano Research
DOI (automatically inserted by the publisher)
Research Article
2
1 Introduction
Extreme Biomimetics is a novel scientific direction of
modern biomimetics proposed for the first time in
2010 [1] and has shown several recent successes [2–7]. This powerful approach is a milestone for modern
biological materials inspired chemistry. In contrast to
traditional biomimetics, Extreme Biomimetics is
based on mineralization and metallization of various
biomolecules under conditions which mimic extreme
aquatic niches like hydrothermal vents, geothermal
pipelines, or hot springs with temperatures near the
boiling point. Consequently, the core principle of this
concept is to use biopolymers which are chemically
and thermally stable under these very specific
conditions in vitro. Thus, the goal of Extreme
Biomimetics is to bring together broad variety of
solvothermal and hydrothermal synthesis reactions
with templates of biological origin for the generation
of novel hybrid composites.
Nanoorganized aminopolysaccharide, chitin,
was originally known as the main structural
component within the cell walls of yeast, fungi and
skeletons of diatoms, protists, sponges, corals,
annelids, mollusks and arthropods [8]. Chitin is one
of the best candidates for Extreme Biomimetics.
Chemically, chitin is a linear polysaccharide
composed of oxygen-containing hexose rings with an
acetamido group at the second carbon position,
linked together by -1,4-glycosidic bonds [9, 10]. The
exceptional thermal stability of chitin (up to 360 °C)
[11–14] is the key factor for the practical application
of this biopolymer in hydrothermal synthesis [15].
For example, chitin has been recently reported as a
template for hydrothermal synthesis of nanocrystals
of ZnO, ZrO2, SiO2 and Fe2O3 (hematite) at
temperatures between 70 °C and 150 °C [2–7].
Although nanostructured Stöber silica can be
obtained using chitin at pH 14.0 and 120 °C [6, 7], to
our best knowledge, there are no reports concerning
the hydrothermal synthesis of germanium oxides
using chitin. Germanium oxide nanoparticles have attracted
much attention in nanotechnology and recent
materials science, as GeO2 possesses unique
physicochemical properties attractive for specific and
advanced applications. It exhibits high values for its
dielectric constant, refractive index, thermal stability
and mechanical strength [16]. GeO2 is also known to
display photoluminescence [16–19] and piezoelectric
[20] properties. Due to a combination of these
fascinating attributes, GeO2 possesses a special place
among various semiconductor nanomaterials; and is
widely used in optical waveguides for integrated
optical systems and nano-connections in optical
devices and systems [19]. Additionally, germanium
oxide has applications in the preparation of catalysts
for methanol steam reforming [21] and anodes for
Li-ion batteries [22].
However, this material exhibits several
polymorphs [23–26] and all these interesting
properties depend on the crystalline structure of
germanium oxide. Therefore, it is important to
develop synthesis methods which will allow control
over the morphology and crystallinity of GeO2. Thus
far, several different technologies have been
developed for synthesis of germanium oxide with
these desired properties; including sol-gel reactions
[18, 27–29], thermal evaporation [30], reverse micelle
method [31], direct precipitation followed by
calcination [32, 33] as well as thermal oxidation [34,
35]. Template-directed approaches including hard
templates using carbon nanotubes [36] or
soft-templates with surfactants [37], 2-methyl
pentamethylene diamine [38] and ionic liquids [39],
have been investigated for the ability to synthesize
specific crystalline phases of germanium oxide. All
these methods allow preparation of germanium
oxide with different crystal phases and morphology;
including nanowires [19, 35], nanorods [30],
nanotubes [30], spheres [40], cubes [18]. Many of
these techniques require harsh environments
including high temperatures, extreme pH, and
caustic conditions. Additionally, much attention is
paid to the synthesis of germanium oxide organized
into three dimensional networks formed from
nanofibers [41].
Intriguingly, there are only few references
regarding the use of biological templates for forming
the desired phases of germanium oxide. These only
relate to the formation of germania nanoparticles,
and, principally, no papers seem to exist on the
development of germania-based biocomposites on
Address correspondence to Hermann Ehrlich, [email protected]; Vasilii V. Bazhenov, [email protected]
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
3 Nano Res.
the micro- and macroscale. The first biomimetic
preparation of GeO2 was published by Patwardhan &
Clarson [42]. They proposed utilization of two
macromolecules - poly(allylamine hydrochloride)
and poly-L-lysine to facilitate synthesis of germania
particles under mild conditions with control over
product morphologies. On the basis of their results
these authors concluded that the macromolecules can
act as a catalyst/scaffold/template for the particulate
germania formation. Davis et al. [43] proposed
synthesis of germanium oxide by the sol-gel
technique, but with the presence of the amino acid
lysine. These researchers speculated that lysine may
stabilize germanium species through complexation,
observed via ESI-MS, giving rise to stable and
detectable germania nanoparticles. Such stabilization
may contribute to the delay in crystal nucleation and
growth, requiring additional (unstable) germania
species (i.e., higher germania concentrations) before
crystal nucleation can be initiated. From other side,
Sewell et al. [44] claimed that poly(amidoamine)
(PAMAM) dendrimers can be used for synthesis of
germanium oxide nanoparticles under ambient
conditions. Very recently, multicrystalline GeO2 was
prepared from germanium tetraethoxide (TEOG) in
the presence of different silk-based peptides [45]. It
was reported that these organic materials
incorporated into the mineral did not appear to affect
the unit cell dimensions. Moreover, the germania
binding peptide alone did not have any significant
effect on reaction rate, yield or the material's
properties compared to the control reference [45].
Here, we use our recently established Extreme
Biomimetics approach [2–4] for the synthesis of
nanostructured crystalline germanium
oxide-containing composite for the first time.
Principally, the established method is based on the
use of unique, tubular, three dimensional scaffolds
isolated from marine sponge skeletons as structural
templates for hydrothermal synthesis. In present
work, the synthesis of GeO2 was carried at 185 °C
over 24 h using germanium(IV) ethoxide as a
precursor. The prepared composites were
characterized with the use of various advanced
analytical techniques including fluorescent
microscopy, scanning electron microscopy (SEM),
NEXAFS, XPS, HR-TEM and electron diffraction,
FTIR and Raman as well as photoluminiscence
spectroscopy.
2 Experimental
2.1 Isolation of structural chitinous templates
The lyophilized skeletons of marine sponges
Aplysina cauliformis (Aplysinidae: Verongida:
Demospongiae: Porifera) (Fig. 1) were provided by
INTIB GmbH (Germany). The α-chitin standard from A. cauliformis was prepared according to a previously
described method [46]. Acetic acid, sodium
hydroxide and ammonia were obtained from VWR
(Germany). In brief, isolation was performed in the
three basic steps: (i) removal of water-soluble salts
and impurities by washing with distilled water; (ii)
deproteinization and removal of residual pigments
by treatment with 2.5 M NaOH solution; (iii) removal
of calcium and magnesium carbonates by treatment
with 20% acetic acid. The isolation procedure was
repeated a few times to obtain colorless tubular
chitin scaffolds (Fig.1c). These scaffolds were stored
in glass bottles with ultra-pure water at 4 °C.
2.2 Hydrothermal synthesis of chitin-GeO2 composites
For a typical synthesis, a piece of chitinous
scaffold (0.5 cm x 0.5 cm) and 1 ml germanium(IV)
ethoxide - TEOG (Sigma-Aldrich, no. 339180, 99.95%)
were added with rapid stirring to 10 ml of a
water/ethanol solution (70 % vol. water) with an
appropriate ammonium hydroxide concentration.
Stirring was continued for 15 minutes, and after that
time the solution was transferred to a Teflon-lined
hydrothermal reactor (Hydrion Scientific
Instruments, USA), closed and heated up to 185 °C
(Fig. 2). After 24 h, the reactor was cooled down,
opened and chitin-GeO2 composite was carefully
isolated and washed with anhydrous ethanol (three
times). To remove all unbound particles of
germanium oxide, the composite was washed with
ethanol in an ultrasound bath (Elmasonic GmbH,
Germany) for one hour. After this, it was dried in a
vacuum oven at 110 °C for 12 h, cooled at room
temperature in a desiccator and weighed using a
Mettler Toledo XP6 microbalance (for details see
ESM). The final chitin-GeO2 composite at macroscale
ideally represented the skeletal morphology of the
sponge chitin used (see Fig. S1 in the ESM). As a
control, GeO2 crystals (see Fig. S2 in the ESM) were
| www.editorialmanager.com/nare/default.asp
4 Nano Res.
also prepared within the same reaction system
without the presence of any chitin templates.
Figure 1. Dried fragments of marine sponge A. cauliformis with
the fingerlike bodies about 2 cm in diameter (a), are a renewable
source for isolation of the 3D skeletal chitinous scaffolds (b),
which, consequently, are source of transparent tube-like
structures isolated after the special alkali-based treatment (c). Bar
scales: 1 cm, 1cm and 100 μm, respectively.
2.3 Raman spectroscopy
Raman spectra were recorded using a Raman
spectrometer (RamanRxn1™, Kaiser Optical Systems
Inc., Ann Arbor, USA) coupled to a light microscope
(DM2500 P, Leica Microsystems GmbH, Wetzlar,
Germany). The excitation of Raman scattering was
obtained with a diode laser emitting at a wavelength
of 785 nm, propagated to the microscope with a
100 µm optical fibre and focused on the samples by
means of a 50x/0.75 microscope objective, leading to
a focal spot of about 20 µm. The Raman signal was
collected in reflection configuration and sent to the
f/1.8 holographic imaging spectrograph by using a
62.5 µm core optical fibre. The spectral resolution in
the range 150-3250 cm-1 was 4 cm-1. Raman spectra
were recorded using an integration time of 1 s and
averaging 50 spectra in order to improve the
signal-to-noise ratio. The laser power measured in
the focal spot was 15 mW. The spectra were analyzed
with MATLAB toolboxes (MathWorks Inc., Natick,
USA). In order to eliminate the background due to
the fluorescence, a variable baseline was calculated
for each spectrum applying the function “msbackadj”
within multiple windows of 200 cm-1 width shifted
with a 100 cm-1 step; a linear interpolation method
was chosen. Four spectra were acquired in different
positions of the samples to account for the small
composition of inhomogenities; the averages were
taken as representative spectra of each sample type
and are shown in the pictures.
2.4 Fourier transform infrared spectroscopy (FTIR)
Infrared spectra were recorded on a Nicolet 380
Fourier transform infrared spectrometer (USA).
About 3 mg of the sample was mixed with 400 mg
KBr and compressed. 100 scans were recorded at a
spectral resolution of 2 cm–1. All spectra were
baseline corrected with a two-point linear baseline (at
845 and 1890 cm–1)
2.5 Fluorescence and light microscopy
Samples of germanium oxide reference and
chitinous matrix prior to and after hydrothermal
synthesis were observed using Keyence BZ-9000
(Japan) microscope in light as well in fluorescence
microscopy modus.
2.6 Scanning electron microscopy (SEM)
The scanning electron micrographs were
observed using a FEI Helios NanoLab 600i
DUALBeam FIB/SEM equipped with a Schottky field
emission gun. Besides the usual Everhart Thornley
Detector (ETD) an in-lens “through-the-lens”
detector (TLD) served mainly for the detection of the
secondary electrons (SE) at a working distance (WD)
of 4.0 mm. Before testing, the samples were fixed in a
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
5 Nano Res.
sample holder by colloidal silver paste (PELCO®).
2.7 Transmission electron microscopy (TEM)
The sample for TEM study was prepared by
placing a drop of the water suspension with the parts
of chitin fibril with GeO2-particles on the carbon film
of a electron microscopy grid (Plano GmbH, Wetzlar,
Germany), followed by drying in air. The recording
of a selected area for the electron diffraction pattern
(SAEDP), high resolution TEM (HRTEM)
investigations and energy dispersive X-ray (EDX)
analysis were carried out on the TEM JEM-2200FS of
JEOL at an acceleration voltage of 200 kV by using a
ultra-high-resolution objective lens, Cs corrected
illumination system and an in-column filter. The
SAEDP and HRTEM images were taken using 2K x
2K CCD-camera of Gatan Inc. The determination of
orientation of local sample regions and the
estimation of phase composition was done by the
analysis of interplanar distances and angles by taking
into account the diffraction spots on SAEDP or
frequencies of local Fast Fourier Transformation
(FFT).
Figure 2. Schematic view of an experiment showing the hydrothermal synthesis of GeO2 nanoparticles using TEOG as the
precursor, and tube-like α-chitin of sponge origin as the template. SEM images: The initially smooth surface of chitin
fibres (a) became covered with germanium dioxide nanocrystals at different locations (b,c) after insertion in hydrothermal
reactor at 185 °C. Note that these images have been obtained after 1 h of ultrasound treatment of the composite product.
6
The chemical composition of the sample was
analysed with the recording of local EDX spectra and
elemental mapping in STEM mode by using EDX
analyser JED 2300 and corresponding analysis
software JED series "Analyse Station" of JEOL.
2.8 Powder X-ray diffraction (XRD)
XRD measurements were performed using a
Bruker D8 Advance diffractometer equipped with Cu
X-ray tube, Goebel mirror and silicon strip detector.
Samples were prepared on a glass plate without
adding further substances. Measurements used a
parallel beam with equal incident and emergent
angles within the ranges indicated in the figures.
Compositions and crystallite sizes were determined
by Rietveld refinement using the programme Bruker
Topas 4.2.
2.9 Near edge X-ray absorption fine structure
spectroscopy (NEXAFS)
NEXAFS measurements were performed at the
Helmholtz-Zentrum Berlin für Materialien und
Energie (HZB), the electron storage ring BESSY II,
using the facilities of the Russian-German beamline
[47]. NEXAFS spectra were recorded in a
total-electron yield mode and normalized to the
incident photon flux.
2.10 Photoluminescence analysis
Photoluminescence (PL) spectra were excited
using a HeCd laser emitting at 325 nm (3.815 eV). A
power of 10 µW was focused to a spot of 130 µm
diameter on the samples. Room-temperature
luminescence was detected by a liquid-nitrogen
cooled CCD camera (Princeton Instruments SPEC-10:
100BR_eXcelon) coupled to an Acton SP2560i
monochromator using an exposure time of 100 ms.
All PL spectra have been corrected by the spectral
response of the detection system.
3 Results and discussion
The presented SEM images (Fig. 2 a-c) indicate that using α-chitin from the A. cauliformis sponge as a
structural template during hydrothermal formation
of germanium oxide from the TEOG precursor leads
to the formation of chitin–GeO2 composites. Both the
inner space (Fig. 2c) and the surface of chitin fiber
(Fig. 2c) are homogeneously covered, with
nanoparticles of germanium oxide in crystalline form
as confirmed below using different analytical
methods. Additionally, the presented SEM shows
that GeO2 nanoparticles are tightly bonded to the
nanoporous chitin surface which could not be
removed even after 1 h of ultrasound treatment
(Fig.3). Moreover, images of mechanically fractured
fragments of the composite (Fig.4a and Fig.S5 in the
ESM) obtained from SEM measurements show that
nanofibrils of chitin about 17 nm in diameter
represent the nucleation sites for growth and
formation of GeO2 nanocrystals that are up to 300 nm
in size (Fig. 4b, see also Fig. S3 in the ESM).
Figure 3. SEM image obtained after 1 h of ultrasound treatment
of the chitin-GeO2 nanocomposite and shows the nanoporous
surface morphology of the material.
7
Figure 4. SEM images of the fracture region within chitin-GeO2 composite (a) definitively show that GeO2 nanocrystals are formed around nanofibrils (arrows) of the sponge chitin (b).
It is well known that germanium oxide has
different crystalline polymorphs; the best known
being tetragonal and trigonal/hexagonal. To obtain
evidence of the nature of the crystalline phase that is
growing on chitinous matrices studied, we used
different highly sensitive methods as represented
below. Initially we used XPS (see Fig. S4 in the ESM)
and NEXAFS techniques for the characterization of
the composite material. The last one is proven to be a
powerful tool to probe chemical bonding and
molecular structure. In Fig. 5 the Ge L3-edge
NEXAFS spectra of chitin-based composite as well as
the reference bulk hexagonal GeO2 sample are shown.
Note that the spectral pattern of the reference sample
is consistent with that obtained for full-density
hexagonal GeO2 in [48]. As for the rutile GeO2,
previously it was shown that the structure of its
unoccupied states differs from that of hexagonal
GeO2 [49]. Clearly, both spectra depicted in Fig. 5
look identical, implying that newly synthesized
chitin-based material contains hexagonal GeO2.
These data have been additionally confirmed
using infrared (Fig. 6) and Raman (Fig.7)
spectroscopy.
Figure 6 represents the FT-IR spectrum of the
chitin scaffold isolated from A. cauliformis sponge,
GeO2 reference, and our hydrothermally obtained
chitin-GeO2 nanocomposite. The spectrum of chitin
shows the bands characteristic of α-chitin polymorph
and corresponds to previously published data [50,
51]. The triplet band, characteristic for the GeO2
hexagonal phase, is well visible between 588 and 520
cm−1 and is attributed to the Ge–O–Ge υ4 vibrational
mode [24, 27, 45, 52–55]. The low frequency bands
around 555 cm−1 are equivalent with the longitudinal
optic (LO) bending mode observed in the Raman
spectrum at ~589 cm-1 (Fig. 7). Also peaks assigned to
Ge–O–Ge (the antisymmetric stretching mode of
hexagonal GeO2) were found at 892 and 960 cm−1 [25,
45, 56] (Fig. 6). These peaks are equivalent to a TO
and LO split asymmetric stretching of the bridging
oxygen, respectively [26, 27].
Figure 5. Ge L3-edge NEXAFS spectra of chitin-GeO2
composite and the reference bulk hexagonal GeO2 sample.
8
Figure 6. FTIR spectra of α-chitin standard (green line), germanium oxide reference (black line) and chitin-GeO2 nanocomposite
obtained (red line).
In the FTIR spectrum of chitin-GeO2
nanocomposite we can observe characteristic peaks
for hexagonal GeO2, however with some significant
changes. The main difference is that 1,4-β-glycosidic
bond peaks characteristic for α-chitin at 897 cm-1 [51]
is overwhelmed by the strong GeO2 mode at 892 cm-1.
However, the general view suggests that the lack of
bands at ~680 cm-1 and ~612 cm-1 indicates that
formation of Ge-O-C and Ge-C band can be excluded
[55], and we hypothesize that chitin interacts with
germanium oxide nanoparticles only by formation of
hydrogen bonds. Compared with chitin template the
absorption line of the O-H vibration is shifted from
3430 cm-1 to 3439 cm-1 in chitin-GeO2 composite due
to hydrogen bonding. Also the shift in the infrared
spectra from 1066 cm-1 that corresponds to C(3)-OH
stretch [57] in chitin to 1071 cm-1 in composite is well
visible. Qin et al. [58] reported that formation of
Ge-O-C bonds between germanium oxide and
graphene oxide can be also indicated by presence of a
band at 1380 cm-1. However, in the case of this
chitin-based composite, the absorbance at 1378 cm-1 is
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
9 Nano Res.
assigned to CH bending and asymmetric
deformation of CH3 from chitin.
The Raman spectra for GeO2 reference
nanoparticles, α-chitin standard from A. cauliformis,
and chitin-GeO2 composite materials are shown in
Fig. 7.
Figure 7. Raman spectra of α-chitin standard (green line), GeO2
reference (black line) and chitin-GeO2 nanocomposite obtained
(red line).
The Raman spectrum of GeO2 nanoparticles
show strong peaks characteristic for α-quartz-like
GeO2 hexagonal cells, including three symmetric
modes of A1 symmetry (at 259, 439 and 877 cm-1) and
four modes of E symmetry (at 514, 589, 855 and
957 cm-1) that split into transverse optic (TO) and
longitudinal optic modes (LO) [26, 34].
The Raman spectra of the obtained chitin-GeO2
composites show the presence of five intense peaks
characteristic of germanium oxide (vertical lines),
which indicate that the nanoparticles interacting with
the chitin surface are crystalline (hexagonal).
The TEM-investigations of the prepared sample
showed that the chitin-GeO2-composites are observed
in the form of GeO2-particles, which adhere to the
chitin fibers. The morphology of a typical composite
is shown in Fig. 8a. The TEM bright field
micrographs show a chitin lamella with the
dimensions 0.4 µm x 2 µm and crystallites of GeO2,
which have a diameter of about 150-300 nm.
The GeO2-nanoparticles are monocrystalline
and have a hexagonal structure P3121 (the fit to the
data used ICSD 637457), and is apparent from the
analysis of the SAEDP (Fig. 8b). In addition, a small
amount of GeO2 is in the form of thin nanolamellae
(width ca. 10 nm). The SAEDP of these sample areas
represents the ring diagrams involving only a small
number of reflections, or rather small randomly
oriented nanocrystallites (Fig. 8c). Here the
crystallites also have a hexagonal structure. For a
detailed investigation, the bright field TEM and
HRTEM micrographs were taken from areas with
nanolamellae (Fig. 8d-e). The visual analysis and the
analysis of local areas of HRTEM images with FFT
shows that the nanocrystallites of GeO2 have a
different orientation: an example (Fig. 8f) shows that
region 1 has the [13-1]-orientation of hexagonal GeO2.
The XRD pattern of the chitin-GeO2 sample (Fig. 9,
see also Fig. S6 and Fig. S7 in ESM) consists of
α-chitin, as verified by a reference measurement, and
hexagonal GeO2 (α-quartz type of structure) [59]. No
additional phases have been detected. However, the
XRD pattern of the GeO2 reference sample (prepared
under same conditions without any template)
contains both hexagonal GeO2 (α-quartz type of
structure) [59] as the main phase, and tetragonal
phase GeO2 (rutile type of structure) [60] 90.7% and
9.3% correspondingly (Fig. S7 and Fig. S8 in ESM).
10
Figure 8. The bright-field TEM image of the chitin-GeO2-composite with SAEDP from the region of spherical particles (b) and lamellae
(c) as well the TEM (d) and HRTEM micrograph (e) with corresponding FFT from local region of the lamellae (f) completed with the
results of elemental analysis by using of EDX point measurements (g) and EDS mapping of completely region of composite (h-i).
Figure 9. XRD pattern of α-chitin from A. cauliformis and obtained chitin–GeO2-composite in comparison with hexagonal
GeO2 (α-quartz type of structure) [59].
For verification of the diffraction analysis, the
chemical composition of chitin-GeO2 composites was
analyzed by measurement of the selected sample
regions by EDX in STEM mode. First, the local
regions of the sample underwent an EDX point
analysis (labeling in Fig. 8, a).
The results show the presence of Ge and O
signals in both thick crystallites (EDX1, Fig. 8g) as
well as in thin nanolamellae (EDX2, Fig. 8g). The
surface analysis by EDS mapping of regions of the
chitin-GeO2-composites shows high concentrations of
Ge and O in the crystallites (Fig. 8h-i), which are
grown on the nanostructured chitin lamellae.
The chitin-GeO2 composite shows stronger
autofluorescence (Fig. 10a) in comparison with
untreated chitin, and crystalline GeO2 sample
obtained under similar hydrothermal conditions
without the presence of any organic templates. The
observed phenomenon opens the way to practical
applications of chitin-GeO2 composite-based
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11 Nano Res.
3D-scaffolds in medicine, sensors and electronic
devices.
Figure 10. (a) Fluorescent microscopy images of (from left to
right): A. cauliformis chitin fibre, chitin-GeO2 composite, and germanium dioxide obtained without presence of organic template (Light exposure time1/4 s). (b) Room-temperature PL spectra of chitin-GeO2-nanocomposite (red line) show a 12 fold
increase in PL increase as compared to the GeO2 reference sample (blue line). For comparison, pure chitin (grey line) shows intermediate PL intensity with a different PL band shape. All PL bands can be fitted by two Gaussian peak components centered
around 2.6 eV and close to 3 eV. Here the two components shown as dotted green and dark-blue lines demonstrate the fitting of the chitin-GeO2 nanocomposite PL band, with the cumulative fit given as a thin solid line (R² > 0.999).
We also carried out measurements using
photoluminescence spectroscopy. Fig. 10b shows the
photoluminescence (PL) spectra for the
chitin-GeO2-nanocomposite together with those for
the α-chitin and GeO2 reference samples. A strong
enhancement in PL efficiency for the nanocomposite
is readily visible. For quantification, all spectra have
been fitted successfully by two Gaussian components;
one centered around 2.6 eV and the other one just
below 3 eV. Both components and the resulting
cumulative fit are shown in Fig. 10b; with PL1 as
dotted lines and thin full line for the chitin-GeO2
nanocomposite. For the other PL spectra, a similar
cumulative fit quality (R2 > 0.997) could be achieved
by only minor changes in peak component position
and FWHM.
The integrated PL of the nanocomposite is
found to be enhanced 12 times as compared to that of
the GeO2 reference. When compared to the α-chitin
reference, the enhancement factor is 4.3. The obvious
PL shape differences between the different samples
are reflected in a differing ratio between the two peak
components. For the GeO2 reference the ratio of 2.6
eV peak area to the 3 eV peak area is 3.2, whereas for
the α-chitin reference this value is only 0.6. The
chitin-GeO2-nanocomposite shows an intermediate
ratio of 2.2, in accordance with it being a mixture of
both materials.
The observed chitin luminescence spectrum
is in accordance with published data, as e.g. in
reference [61]. Reported luminescence spectra of
GeO2 nanoparticles differ widely, probably due to the
wide variety in preparation procedures, matrices and
shapes. Generally, luminescence in the 2-3 eV range
has been attributed to either surface/interface defects
or defects related to oxygen-deficiency in the GeO2
crystals. For GeO2 nanocrystals in an inorganic SiO2
matrix, a blue luminescence band close to 3.1 eV has
been related to the formation of these nanocrystals
[62]. As a possible origin, a Ge/O-related defect has
been proposed. For GeO2 nanowires, PL spectra
showed either a double-peak structure with
components centered around 2.45 eV and 2.91 eV [63,
64] or a single peak at 2.3 eV [64], all of which were
attributed to radiative recombination at defects in
GeO2, e.g. oxygen vacancies and oxygen-Ge-vacancy
centers. In the latter publication this attribution could
be confirmed using XEOL combined with XANES
[64]. Unusually sharp PL peaks, still in the same
spectral range and with the same attribution, have
been also found for GeO2 nanowires [65]. It is
suggested that in their case oxygen-rich preparation
conditions lead to improved crystallization, resulting
in the observed reduced inhomogeneous broadening
of the PL bands [66].
Studies of bulk rutile GeO2 crystals (4 mm³) have
been reported previously [67]. Their luminescence
showed two bands, centered around 2.3 eV and 3 eV.
By growing in either oxygen-rich conditions or
reducing conditions, a mechanistic correlation with
oxygen-deficient defect centers could be suggested.
Based on this short survey, we attribute our observed
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12 Nano Res.
PL to radiative recombination at intrinsic point
defects either at the surface or inside the GeO2
nanocomposite.
PL enhancement of GeO2 nanoparticles has
been reported before. It was observed when the
nanoparticles were capped with
polyvinylpyrrolidone (PVP) [55]. In contrast, here we
report a PL enhancement by incorporation of the
GeO2 nanocrystals into an organic matrix directly
during their growth.
4 Conclusions
The results presented and discussed in this
paper have shown the new hydrothermal route for
development of novel germanium oxide-chitin
composites. The specific thermal stability of
nanostructured chitin of poriferan origin allowed us
to synthesize a crystalline phase of hexagonal GeO2
from the precursor germanium ethoxide at 185 °C in
the form of a centimeter large sponge for the first
time. Nanocrystals of GeO2 grew exclusively within
and on the surface of this unique tube-like chitinous
matrix which signifies a typical morphology of the
sponge skeleton (see Fig. S1 in ESM). From this point
of view, we developed a solid GeO2-chitin-based
composite of sponge structure with functionalized
surfaces that shows interesting fluorescence and
photoluminescence properties. Another crucial
property of the material obtained is the high
resistance of the composite to ultrasound treatment.
Nanolayers of GeO2-crystals, which grow on
chitinous nanofibrils were observed using electron
microscopy methods, even after 1 hour treatment in
an ultrasound bath. Clearly, the future elucidation in
growth and PL mechanisms of nanoorganized
GeO2-chitin-based composite represents a scientific
task of major interest. Corresponding experiments
are in progress in our Lab now.
This work demonstrates that the chitin-based
sponge scaffolds are of potential interest in
bioinspired materials science since the processing of
chitin from other sources like fungi or crustaceans
into sponge-like materials or foams is technologically
difficult and needs chemical modifications [68]. Since
Verongida sponges can be grown in marine ranching
stations [5], their scaffolds may provide a natural
renewable source for such materials with
applications, e.g. in biomedicine and technology.
Acknowledgements
This work was partially supported by Research
Grants for Doctoral Candidates and Young
Academics and Scientists up to 6 months – DAAD
Section 323-Project No. 50015537 and the following
research grants: PUT research grant no.
03/32/443/2014-DS-PB, DFG Grant EH 394/3-1, BHMZ
Programme of Dr.-Erich-Krüger-Foundation
(Germany) at TU Bergakademie Freiberg, BMBF
within the project CryPhys Concept (03EK3029A).
This work was also partially supported by the
Federal Ministry for Environment, Nature
Conservation, Building and Nuclear Safety within
the joint research project “BaSta” (0325563D) and by
the Cluster of Excellence “Structure Design of Novel
High Performance Materials via Atomic Design and
Defect Engineering (ADDE)” that is financially
supported by the European Union (European
regional development fund) and by the Ministry of
Science and Art of Saxony (SMWK).
Authors are thankful to Mrs. Heidrun Hahn, Mr.
Thomas Behm and Dr. Claudia Funke for great
technical assistance.
Electronic Supplementary Material: Supplementary
material (stereomicroscope images of chitin-GeO2
composite, SEM of reference GeO2 sample, XPS data,
SEM of chitin-GeO2 nanocomposite) is available in
the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-* References
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67. Trukhin, A.; Kink, M.; Maksimov, Y.; Jansons, J.; Kink, R.
Luminescence of GeO2 glass, rutile-like and α-quartz-like
crystals. J. Non. Cryst. Solids 2006, 352, 160–166.
68. Narayanan, D.; Jayakumar, R.; Chennazhi, K. Versatile
carboxymethyl chitin and chitosan nanomaterials: a review.
WIREs Nanomed. Nanobiotechnol. 2014, 6,574–598.
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Electronic Supplementary Material
Extreme biomimetic approach for development of novel
chitin-GeO2 nanocomposites with photoluminescent
properties
Marcin Wysokowski1, Mykhailo Motylenko2, Jan Beyer3, Anna Makarova4, Hartmut Stöcker5, Juliane
Walter5, Roberta Galli6, Sabine Kaiser5, Denis Vyalikh4,14, Vasilii V. Bazhenov5(), Iaroslav Petrenko5,
Allison L. Stelling7, Serguei Molodtsov5,8,9, Dawid Stawski10, Krzysztof J. Kurzydłowski11, Enrico Langer12,
Mikhail V. Tsurkan13, Teofil Jesionowski1, Johannes Heitmann3, Dirk C. Meyer5, Hermann Ehrlich5( )
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
1 Experimental
1.1 Stereomicroscopy
Samples of chitinous matrix after hydrothermal synthesis were observed using stereomicroscope Di-Li
(Germany).
1.2 Scanning electron microscopy
The scanning electron micrographs were observed using a FEI Helios NanoLab 600i DUALBeam FIB/SEM
equipped with a Schottky field emission gun. Besides the usual Everhart Thornley Detector (ETD) an in-lens
“through-the-lens” detector (TLD) served mainly for the detection of the secondary electrons (SE) at a working
distance (WD) of 4.0 mm.
1.3 X-ray photoelectron spectroscopy
XPS analyse
x-ray source (1486.6 eV). The x-ray source has a spot size of 650 µm and operates at a power of 14.8 kV and 19.2
mA. The spectra were taken with a pass energy of 20 eV and an energy step width of 0.1 eV. The base pressure
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was 2•10-10 mbar but during the measurement the pressure increased to 3•10-7 mbar due to the ion gas flow
from the flood gun, which was used for charge compensation.
1.4 Gravimetric measurements
Sample of chitin-GeO2 was weighted before and after sonication with use METTLER TOLEDO XP6
microbalance. The percentage of particles bounded to chitin was calculated by following formula:
C =M1 − Ms
M1 ∙ 100%
where: C- percentage of unbounded particles; M1 – mass of sample before sonication; Ms – mass of sample after
ultrasound treatment.
M1= 0.300 g
Ms= 0.291 g
Obtained results indicate that percentage of GeO2 nanoparticles bounded to chitin is ~97% and unbounded
~3%.
2 Results
Figure S1. Optical microscopy image of the obtained chitin-GeO2 nanocomposite in the form of a sponge-like material.
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Figure S2. SEM image of GeO2 nanoparticles with hexagonal plate like morphology obtained hydrothermally as a reference sample,
without the presence of any chitinous scaffold.
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Figure S3. SEM image of the inner space of the chitin-GeO2 nanocomposite. Arrows indicate nanocrystals of GeO2 which are grown on
chitinous nanofibers.
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Figure S4 XPS measurements of chitin (green line), GeO2 reference (black line) and chitin-GeO2 composite (red line) - survey spectra.
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Figure S5. SEM images of the fracture region within chitin-GeO2 composite (a) definitively show that GeO2 nanocrystals are formed
around nanofibrils (arrows) of the sponge chitin (b).
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Figure S6. XRD pattern of α-chitin standard from A. cauliformis and obtained chitin–GeO2-composite in comparison with hexagonal GeO2 (α-quartz type of structure) [59].
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Figure S7. XRD pattern of the GeO2 reference sample (prepared under same conditions without any template) in comparison with hexagonal GeO2 (α-quartz type of structure) [59] and tetragonal GeO2 (rutile type of structure) [60].
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Figure S8. XRD pattern of the chitin-GeO2 composite in comparison GeO2 reference sample (prepared under same conditions without any template). The most intensive line for tetragonal phase of GeO2 (arrow) (rutile type of structure) [60] is not presented in pattern of chitin-GeO2 sample.