Extreme biomimetic approach for development of novel ... · 1 Extreme biomimetic approach for...

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

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Nano Research

DOI 10.1007/s12274-015-0739-5

63



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

http://tu-freiberg.de/exphys/biomineralogy-and-extreme-biomimetics/gruppenleiter

1



properties





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]

mailto:[email protected]

mailto:[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


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


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


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


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

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Nano Res.

Electronic Supplementary Material



properties





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.

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