-
Accepted Manuscript
Title: Covalently functionalized graphene sheets
withbiocompatible natural amino acids
Author: Shadpour Mallakpour Amir Abdolmaleki
SedighehBorandeh
PII: S0169-4332(14)00834-4DOI:
http://dx.doi.org/doi:10.1016/j.apsusc.2014.04.070Reference: APSUSC
27670
To appear in: APSUSC
Received date: 11-12-2013Revised date: 7-4-2014Accepted date:
9-4-2014
Please cite this article as: S. Mallakpour, A. Abdolmaleki, S.
Borandeh, Covalentlyfunctionalized graphene sheets with
biocompatible natural amino acids, Applied SurfaceScience (2014),
http://dx.doi.org/10.1016/j.apsusc.2014.04.070This is a PDF file of
an unedited manuscript that has been accepted for publication.As a
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Revised
Covalently functionalized graphene sheets with biocompatible
natural
amino acids
Shadpour Mallakpour a,b,, Amir Abdolmaleki a,b,, Sedigheh
Borandeh a
aOrganic Polymer Chemistry Research Laboratory, Department of
Chemistry, Isfahan
University of Technology, Isfahan, 84156-83111, I. R. Iran.
bNanotechnology and Advanced Materials Institute, Isfahan
University of Technology,
Isfahan, 84156-83111, I. R. Iran.
Corresponding author: Tel.; +98-311-391-3249; Fax: +98
3113912350. E-mail addresses: [email protected],
[email protected] (A. Abdolmaleki); [email protected],
[email protected], [email protected] (S.
Mallakpour)
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Abstract
Graphene sheets were covalently functionalized with
aromatic-aliphatic amino acids,
(phenylalanine and tyrosine), and aliphatic amino acids
(alanine, isoleucine, leucine,
methionine and valine) by simple and green procedure. For this
aim, at first natural graphite
was converted into graphene oxide (GO) through strong oxidation
procedure; then, based on
the surface-exposed epoxy and carboxylic acid groups in GO
solid, its surface modification
with naturally occurring amino acids, occurred easily throughout
the corresponding
nucleophilic substitution and condensation reactions. Amino acid
functionalized graphene
demonstrates stable dispersion in water and common organic
solvents. Fourier transform
infrared, Raman and X-ray photoelectron spectroscopies, X-ray
diffraction, field emission
scanning electron microscopy and transmission electron
microscopy were used to investigate
the nanostructures and properties of prepared materials. Each
amino acid has different
considerable effects on the structure and morphology of the pure
graphite, from increasing
the layer spacing to layer scrolling, based on their structures,
functional groups and chain
length. In addition, therogravimetric analysis was used for
demonstrating a successful
grafting of amino acid molecules to the surface of graphene.
Keywords: Graphite sheet; Amino acid functionalization; Graphene
scrolling; Graphene
oxide.
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1. Introduction
In nanoscience field, carbon-based nanomaterials play an
important role and have
attracted the scientific community since their discovery [1].
Among these materials, graphene
is one of the most exciting materials which has fascinated great
interest in the past several
years owing to its potential applications. Graphene, a single
layer of sp2-bonded carbon
atoms, is a two-dimension (2D) honeycomb nanostructure [2-6].
The outstanding properties
of graphene as well as their functionalized forms render it an
ideal candidate in a wide range
of applications; including composite materials, gas sensors,
transparent electrodes and
transistors [7-11]. Due to characteristic structures of
carbon-based nanomaterials, they can
interact with wide variety of organic molecules by covalent or
noncovalent forces (hydrogen
bonding, stacking, electrostatic forces, van der Waals forces
and hydrophobic
interactions) [12-15].
However, the existence of high stacking and van der Waals forces
due to
attraction between adjacent layers facilitates graphene forming,
irreversible aggregation or
even restacking to graphite which greatly limits the
applications of graphene in several areas,
as well as fabrication of graphene based polymer nanocomposites,
biosensors, drug delivery
systems, solar cells, nanomechanical and transistor devices
[13,16-21]. Along with numerous
approaches that developed to address this obstacle, the most
reliable techniques are
functionalization of graphene [22-24]. Several methods have been
used for the modification
of graphene which can be divided into two common categories:
covalent and non-covalent
functionalization. Utilizing of graphene oxide (GO) as a
precursor, has received great
attention of a significant number of researchers these days. GO
is an oxygen-containing
graphene derivative with partial breakage of sp2sp2 bonds into
sp3sp3 bonds for inserting
some pendent groups like; hydroxy, epoxy, and carboxylic acid.
These functional groups
facilitate the interaction between the host materials and GO,
also they lead to good dispersion
of GO in aqueous solutions thanks to the hydrophilic nature of
surface functionalities [25-29].
On the other hand, despite GO sheets readily swell and disperse
in aqueous media but
they cannot be readily dispersed in most common organic solvents
[3,12,30]. So, in order to
increase the dispersibility of graphene in various solvents
(aqueous and organic), further
functionalization (amidation, esterification, sulfonation and
etc) is needed [13,15,31-33].
Inserting different kinds of functional groups onto graphene
layers leads to various changes
on graphene structure such as increasing the interlayer spacing
or layer scrolling that possess
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the structure distinct from graphite and multi-walled carbon
nanotubes [34-37]. In addition,
nowadays, the biological applications of graphene have also been
interested [38-42]. Aiming
this goal, biocompatible equipments may be rewarded while using
multi-functional natural
metabolites such as amino acids. These materials are
environmentally friendly and naturally
occurring compounds, that make graphene a good candidate for
biological activities [43-50].
In this context, bio-functionalized graphene sheets with
aromatic-aliphatic and
aliphatic amino acids have been prepared through easy and green
procedure. For this propose,
at first GO was synthesized using the simplified Hummers method
[51], then amino acid
functionalized graphene materials were synthesized by
condensation and nucleophilic
addition reactions between NH2 groups of amino acid and
carboxylic acid or epoxy groups
on the GO sheets. All synthesized amino acid functionalized
graphene were characterized by
several techniques including Fourier transform infrared
spectroscopy (FT-IR), X-ray
diffraction (XRD) and one of them was structurally characterized
by Raman spectroscopy
and X-ray photoelectron spectroscopy (XPS). The thermal
properties of graphite, GO and
functionalized graphene materials were examined through
thermogravimetric analysis
(TGA). Moreover their morphology was investigated by field
emission scanning electron
microscopy (FE-SEM). Furthermore, two of them were examined as
representative by
transmission electron microscopy (TEM) analysis.
2. Experimental
2.1. Materials
Natural graphite powder (diameter 510 m, thickness 4-20 nm,
layers < 30 and
purity >99.5 wt%), was purchased from Neutrino Co. (Iran).
Other chemicals used in this
study were obtained from Fluka Chemical Co. (Switzerland) and
Merck Chemical Co.
(Germany) and were used without further purification. Sulfuric
acid (H2SO4 98%), hydrogen
peroxide (H2O2), and hydrochloric acid (HCl), from Merck were
used for the synthesis of
mediators.
2.2. Instrumentation
FT-IR spectra of the composites were recorded with a Jasco-680
(Japan) spectrometer
at 4 cm1 resolution and they were scanned at wavenumber range
4004000 cm1. XRD was
used to characterize the crystalline structure of the
composites. XRD patterns were collected
using a Bruker, D8 Advanced diffractometer with a copper target
at the wave length of
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CuK = 1.5406 and a tube voltage of 40 kV and tube current of 35
mA, in the range of 5
100 at the speed of 0.05 /min. Raman spectroscopy was recorded
from 500 to 3500 cm-1 on
a Almega Thermo Nicolet Dispersive Raman Spectrometer using a
Nd:YLF laser source
operating at wavelength of 532 nm. X-ray photoelectron
spectroscopy (XPS) was utilized to
investigate chemical states variations of the functionalized
graphene using twin anode
XR3E2 X-ray source system operating at a vacuum by X-ray
8025-BesTec spectrometer. The
XPS peaks were deconvoluted by using Voigt function which is
combination of Lorentzian
and Gaussian. Thermogravimetric analysis (TGA) is performed with
a STA503 win TA
(Bahr-Thermoanalyse GmbH, Hllhorst, Germany) at the heating rate
of 10C/min from 25 C to 800 C under nitrogen atmosphere. The
morphology of several amino acid
functionalized graphene were observed using FE-SEM (HITACHI
S-4160, Japan). TEM
image was obtained using Philips CM 120 operated (Netherlands)
at voltage of 150 kV.
2.3. Synthesis of graphene oxide
Graphene oxide (GO) was exfoliated compared to natural graphite
through Hummers
technique [51]. Graphite powder (0.5 g) was poured into cold (0
C) solution of concentrated
H2SO4 (12 mL) and NaNO3 (0.25 g). KMnO4 (1.5 g) was gradually
added with stirring and
cooling, so that the temperature of the mixture kept below 20 C.
The mixture was then
stirred at 35 C for 30 min and as the reaction progressed, the
mixture gradually became
pasty, and the color turned from black into light brownish.
Then, distilled water (25 mL) was
added and the temperature was raised to 98 C and maintained at
this temperature for 15
minutes. The reaction was terminated by addition of a large
amount of distilled water (70
mL) followed by treated with 30% H2O2 (2 mL) and the mixture
changed into brilliant yellow
color. The mixture was filtered and washed with distilled water
and 10% HCl solution in
order to remove metal ions. The obtained graphite oxide powder
was dispersed in deionized
water. The resulting yellow brownish suspension was centrifuged
at 3000 rpm for 1 h to
eliminate unexfoliated graphitic plates. Finally, an aqueous
suspension containing GO sheets
was obtained through exfoliation of the filtered graphite oxide
suspension through its
sonication at frequency of 2.25 104 Hz and power of 100 W for 1
h. Finally the GO powder
was dried at 60 C.
2.4. Functionalization of GO with different aromatic-aliphatic
and aliphatic amino acids
GO powder (0.1 g) was dispersed in distilled water (10 mL) and
different amino acids
(0.3 g) (phenylalanine, tyrosine, alanine, leucine, isoleucine,
methionine and valine) and an
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equimolar amount of NaOH in distilled water (10 mL) were added.
The mixture was stirred
for 24 h at room temperature. At the end of the reaction the
colloidal dispersion was treated
with ethanol, and the resulting precipitate was centrifuged,
washed well with H2O/EtOH
mixture and finally dried at 60 C.
3. Results and discussion
3.1. Preparation of amino acid functionalized graphene
Amino acids were chosen to fabricate bio-functionalized
graphene. At first, chemical
oxidation technique fabricates GO by introducing carboxylic
acid, hydroxy and epoxy
functional groups between carbon layers of graphene through
strong oxidation process (Fig.
1). By means of these functional groups, the van der Waals bond
between the carbon layers
reduces which cause graphene to peel off layer by layer. The
resulting GO can be further
functionalized with numerous compounds. Amino acids are cheap
and environmentally
friendly, therefore, they are appropriate nucleophilic reagents.
As shown in Fig. 1,
nucleophilic and condensation reactions occurred between the
amine groups of amino acids
with epoxy and carboxylic acid groups of the GOs surface. The
surface and morphology of
the prepared materials were analyzed by FT-IR, Raman, XPS
spectroscopy, XRD, FE-SEM,
and TEM methods.
Fig. 1.
3.3. Amino acid functionalized graphene Characterization
3.3.1. FT-IR analysis
FT-IR measurement was employed to investigate the bonding
interactions in natural
graphite before and after the oxidation and functionalization
processes. In the GO spectrum
(Fig. 2) comparing to the neat graphite, the presence of a peak
at 1046 cm-1 attributed to the
C-O bond, confirming the presence of oxide functional groups
after the oxidation process. In
addition, the presence of different types of functionalities in
GO cause the appearance of
absorption band centered at 3430 cm-1, which is attributed to
the O-H stretching bands of
hydroxy and carboxylic acid moieties. Aliphatic sp3 C-H
stretching around 2922 cm-1 and the
C=O stretching groups of carboxylic acids at 1719 cm-1, that
prove the oxidation process,
while no significant peak was found in the pure graphite.
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In different amino acid functionalized graphene spectra, the
existence of C=O
stretching groups of amide and carboxylate salts at the range of
1615-1630 cm-1 and the peak
attributed to the OH and NH stretching groups at 3450 cm-1
confirm the covalent
functionalization of the neat graphite by amino acids molecules.
In addition, for better
comparisons, the FT-IR spectra of phenylalanine and tyrosine
functionalized garaphene were
compared with corresponding amino acids. Appearance of amino
acid absorption peaks in the
FT-IR spectrum of functionalized garaphene, confirmed successful
functionalization process.
Fig. 2.
3.3.2. Raman spectroscopy
Raman spectroscopy was used as a powerful technique for
obtaining an indication of
the structure and quality of carbon materials. Therefore, Raman
spectroscopy was utilized to
examine the carbon structure of the GO and phenylalanine
functionalized graphene as shown
in Fig. 3. Generally, the well-known and main characteristics of
graphite Raman spectrum are
the D and G bands. The G band (strong band) at 1575 cm1 is
assigned to the first order
scattering of the E2g phonon from sp2 carbon atoms and the D
band (very weak band) at 1355
cm1 is attributed to a breathing mode of -point photons of A1g
symmetry which is related to
the local sp3 disorder bands formation through oxidation process
especially the ones located
at the edges of graphite sheets [41,52,53]. So the intensity
ratio of the D and G band (ID/IG) is
a useful parameter for determining the sp2 domain size of a
carbon structure containing sp3
and sp2 bonds. Compared with pure graphite, GO exhibited the G
band at 1593 cm1 and the
D band at 1352 cm1 (Fig. 3a). In the GO Raman spectrum, the
intensity of the D band was
increased compared with that of graphite but the G band is still
prominent and the ID/IG (an
indication of sp3/sp2 carbon ratio) for GO is 0.90. According to
the Fig. 3b, it is found that by
functionalization of GO with phenylalanine, the G and D bands
are shifted to 1594 and 1349
cm1, respectively and the D band becomes more prominent. Higher
ID/IG ratio of
functionalized graphene (1.06) compared to GO (0.90), approve
the introduction of sp3
defects after functionalization of GO with phenylalanine.
Furthermore, Raman spectra of carbon materials possess a famous
feature (2D band)
which is sensitive to stacking of graphene sheets. Based on 2D
band shape, intensity and
position, the formation and the layer numbers of graphene sheets
can be comprehended [54-
56]. According to the Raman spectrum of phenylalanine
functionalized graphene, a broader
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and symmetrical 2D band in the 25003200 cm-1 region, proves that
bi-layer graphene sheets
with less than five layers were formed by functionalization of
GO [56-60].
Fig. 3.
3.3.3. XRD studies
Fig. 4 shows the XRD patterns of graphite and GO. The noticeable
peak of graphite
(002) at 26.6 has an interplanar distance, d002, of 3.26 . This
signifies that graphite is a
highly oriented carbon material. Whereas, the XRD pattern of GO
shows a strong and sharp
peak centered at 11.6 which corresponds to the layer-to-layer
distance of 7.7 [61,62]. This
outstanding shift signifies that the abundant functional groups
are involved and increased the
interlayer spacing of GO due to the intercalation and the
bonding effect of O-containing
functional groups.
Fig. 4.
Figs. 5 and 6 show the XRD patterns of variety of
aromatic-aliphatic and aliphatic
amino acid functionalized graphene. As it can be seen, there are
noticeable differences within
the XRD patterns of functionalized GO with aromatic-aliphatic
and aliphatic amino acids. In
contrast to aliphatic amino acids, the insertion of aromatic
ones, especially tyrosine,
decreasing the layer-to-layer distance and increasing the 2
position was observed; in fact,
after functionalization of GO with tyrosine molecule, the peak
shifted back to the original
002 peak at 26. This XRD pattern reveals that tyrosine based
graphene demonstrates a
particular scrolled structure between graphene and multi-walled
carbon nanotubes that comes
from rolling single layer of graphene. Along with numerous
researches, a theoretical study on
the interaction of aromatic amino acids with graphene has been
examined by Rajesh et al.
within linear combination of atomic orbitals-molecular orbital
approach [63]. As they
revealed, the aromatic rings of the amino acids prefer to orient
in parallel with the benzene
rings plane of graphene, which is characteristics of the -
stacking interactions. They have
been also proved that the binding strength or interaction energy
between the aromatic ring of
tyrosine and graphene is higher than the one between the
aromatic ring of phenylalanine and
graphene by theoretical calculation.
While the mechanism for this unpredicted phenomenon is unknown,
we suspect that
the shift in the interlayer spacing has been attributed to the
nature of tyrosine molecules to
adopt a flat orientation in the interlayer zone of GO,
presumably due to the - stacking
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interaction between the aromatic ring of tyrosine and graphene
and also H-bonding
interactions between the phenolic OH groups of guest molecules
and O-containing groups of
GOs layers. So, based on above explanations, these factors cause
the enhancing attraction
between adjacent layers and forming irreversible aggregation or
even restacking to graphite.
It is also proposed that the interactions in tyrosine based GO,
are intramolecular interactions
but in phenylalanine based GO, both intra and intermolecular
interactions could existed, since
in GO+Phenylalanine pattern both increasing and decreasing in
the layers distance were
observed. Furthermore, in aromatic-aliphatic amino acids
functionalized graphene, as a result
of - stacking and H-bonding interactions, the crystalline
structure of graphene decreases
and becomes somewhat amorphous.
Fig. 5.
As shown in Fig. 6, the crystallographic orientations of the
intercalation compounds
were found to vary significantly based on the structre of each
aliphatic amino acid and their
properties. While GO exhibited the interplanar distance of 7.7
(2= 11.6), after
functionalization of GO with aliphatic amino acids, an
increasing in d-spacing was monitored
from valine to methionine. These observations could be justified
by amino acids structure,
functional groups and their chain length. The screening effect
due to aliphatic amino acids,
that reduce the - stacking interaction between GO layers, plays
an important role for the
exfoliation of GO. These enhancements are due to decreased
interlayer interactions.
Collectively, it is attained that, the aliphatic amino acids can
be successfully intercalated into
GO and facilitating the formation of GO-amino acid intercalation
compounds.
Fig. 6.
3.3.4. XPS spectroscopy
A more quantitative analysis has been done based on XPS spectra
to investigate the
presence of functional groups on the amino acid functionalized
GO. Fig. 7 displays the
survey XPS spectrum in the region of 0 to 1200 eV (a) and
deconvoluted XPS spectra of the
C1s and O1s regions (b and c) of phenylalanine functionalized
graphene. According to the
survey spectrum (Fig. 7a), three peaks centered at 284, 401 and
532 eV are observed which
can be assigned to C1s, N1s and O1s signals. In Fig. 7b, the C1s
peak of functionalized
graphene located at 284 eV can be deconvoluted into four fitting
curves with the binding
energy located at 284.1, 286.2, 287.5 and 289.03 eV which are
assigned to C-C, C-N, amidic
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C=O and O-C=O, respectively [43,57,64,65]. Compared with GO XPS
spectrum in
literatures, a high intensity peak at 286.7 eV is observed which
is attributed to the epoxy
groups on GO surface [14,57,66], but here after
functionalization of GO with phenylalanine,
a new peak at 286.2 eV is appeared which is due to nucleophilic
interaction of phenylalanine
NH2 groups and GO epoxy groups which causes to formation of C-N
bands. Furthermore,
Fig. 7c presents the O1s peak of functionalized graphene with
two fitting curves. There are
binding energies located at 531.1 and 532.4 eV which are
assigned to O-C=O and C-OH,
respectively. As it can be seen, the intensity of O-C=O groups
is higher than that of C-OH
groups. This can be due to the inserted amino acid
functionalities which contain O-C=O
groups. In another word, the higher intensity of O1s relative to
C1s can be due to high degree
of O-containing groups in phenylalanine.
Fig. 7.
3.3.5. Dispersion stability in water
As a result of amino acid functionalization of graphene,
dispersibility of the hybrids
was improved dramatically. Fig. 8 shows digital photographs of
graphite, GO and
phenylalanine functionalized graphene which were dissolved in
distilled water. Due to
graphite poor hydrogen bonding and van der Waals interaction, it
does not disperse in water
and has tendency to aggregate or rope. In the case of amino acid
functionalization of
graphene, homogeneous and stable dispersion was observed which
is more stable than GO.
Fig. 8.
3.3.6. Morphology studies
For observing the sheet structure and morphology of graphite, GO
and its
functionalized form, microscopically and also to investigate the
effect of functionalization on
the graphite surface by different amino acids, FE-SEM and TEM
measurements were
performed. The results from FE-SEM images are in a good
agreement with XRD analysis.
Fig. 9 displays the FE-SEM images of graphite and GO in
different magnifications. As it can
be seen, the morphology and layer thickness of graphite sheets
were completely changed after
oxidation. FE-SEM images of GO reveal a large increase in the
thickness of graphene layers
throughout oxidization process. The observed outstanding
increase in the thickness of
graphene sheets during the oxidization can be attributed to the
formation of oxygen groups in
the basal plane of graphene.
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Fig. 9.
Fig. 10 demonstrates the FE-SEM micrographs of graphite and
different types of
aliphatic amino acid functionalized graphene. The graphites
image presents the sheet-like
structure with smooth surface. The pure graphite exhibits
layered structures and becomes
very thin. As it was reported in the XRD section, the graphene
was more exfoliated and the d-
spacing was further increased in the presence of methionine,
leucine and isoleucine. Their
FE-SEM micrographs display flake-like structure with the large
thickness, wrinkled edge and
crumble graphene sheet structure. The tiny particles on the
surface of functionalized graphene
materials may be the proofs of amino acid existence on the
graphene sheets.
Fig. 10.
Fig. 11 shows the FE-SEM images of aromatic-aliphatic amino acid
based graphene.
As it can be seen, the FE-SEM micrographs of tyrosine based
graphene certify the presence
of tubular structure that is as a result of scrolling effect of
graphene layers. These images are
a complement to XRD data and show their nanotube-like structure.
In Fig. 12 different
schematic kinds of graphene scrolling are shown.
Fig. 11.
Fig. 12.
Fig. 13 shows the TEM micrographs of phenylalanine
functionalized graphene. As
shown, we can distinctly find that it has a typical shape
resembling the exfoliated crumpled
thin flake that its wrinkled structure is due to the presence of
phenylalanine moieties between
layers after covalent functionalization.
Fig. 13.
Fig. 14 displays TEM images of tyrosine functionalized graphene.
As it was observed
in XRD and FE-SEM sections, when GO was functionalized with
tyrosine, the graphene
layers scrolled around each other which exhibit a unique
structure between graphite and
multi-walled carbon nanotube. Accordingly, in TEM micrographs of
tyrosine functionalized
graphene, it is obvious that the scrolled graphene was
fabricated and it is also seen that the
graphene layers are more wrinkled than phenylalanine
functionalized graphene. These images
revealed the presence of the tubular structure with different
diameter ranging which the size
and shape of scrolled graphene are not uniform.
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Fig. 14.
3.3.7. Thermal stability
For further investigation of functionalized graphene properties,
TGA analysis was
used as an effective instrument to determine the quantity of the
grafted amino acid materials,
because the forming covalent bonds between graphene sheet and
amino acid moieties are
thermally stripped off in the temperature range of 150600 C.
Fig. 15 shows TGA curves of
graphite, GO, tyrosine functionalized graphene and methionine
functionalized graphene
which indicate the thermal stability of surface functionalized
graphene materials. As it can be
seen, pure graphite did not show any weight loss until 800 C.
For GO, a slight weight loss
below 100 C is attributed to the removal of adsorbed water by
O-containing functional
groups on GO sheets which is due to tightly bounded H2O
molecules into GO stacked
structure. In addition, the weight loss around 200 C is
attributed to decomposition of O-
containing functional groups from the GO surface, yielding CO,
CO2, and steam [3,67-69].
For comparison, one of the aliphatic functionalized graphene
substances (methionine) and
one of the aromatic-aliphatic functionalized graphene materials
(tyrosine) were choose. In
both cases, the weight loss of functionalized graphene is much
smaller owing to decreased
amount of oxygen functional groups on their surfaces. For
tyrosine functionalized graphene
and methionine functionalized graphene, the total weight loss is
23% and 54% respectively,
which is related to the pyrolysis of organic moieties on the
graphene sheets, demonstrating a
successful grafting of amino acid molecules to the graphene
layers. As it observed, graphene
is more thermal stable when is functionalized with tyrosine
amino acid than with methionine.
This phenomenon may be due to the existence of - stacking
interactions between the
aromatic ring of tyrosine and graphene which cause to form more
stacked structure.
Fig. 15.
4. Conclusions
An easy and effective approach was utilized for synthesis of
different kinds of aliphatic and
aromatic-aliphatic amino acid functionalized graphene. The
prepared materials were
characterized by FT-IR, Raman, XPS, XRD, FE-SEM, TEM and TGA
analysis. Based on the
amino acid structure, functional groups and chain length,
different changes on the structure
and morphology of graphene were observed. According to the XRD
and FE-SEM results,
aliphatic amino acids cause an increase in interlayer spacing of
graphene which the d-spacing
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is depend on their chain length. On the other hand,
aliphatic-aromatic amino acid
functionalized graphene display distinct manner especially
tyrosine based graphene. The
XRD patterns of tyrosine based graphene reveals a particular
scrolled structure between
graphene and multi-walled carbon nanotubes that comes from
rolling single layer of
graphene. FE-SEM and TEM images also confirm this
occurrence.
Acknowledgements
We gratefully acknowledge the partial financial support from the
Research Affairs Division
Isfahan University of Technology (IUT), Isfahan. Further partial
financial support of Iran
Nanotechnology Initiative Council (INIC), National Elite
Foundation (NEF) and Center of
Excellency in Sensors and Green Chemistry (IUT) is also
gratefully acknowledged. Useful
help from Dr. H. farrokhpour is also gratefully
acknowledged.
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Figure captions
Fig. 1. Synthesis of GO and functionalized graphene with
different types of amino acids.
Fig. 2. FT-IR spectra of the pure graphite, GO and different
amino acid functionalized
graphene and FT-IR comparing of phenylalanine and tyrosine
functionalized graphene with
their attributed amino acids.
Fig. 3. Raman spectra of a: GO and b: phenylalanine
functionalized graphene.
Fig. 4. XRD patterns of graphene and GO.
Fig. 5. XRD patterns of GO and different aromatic-aliphatic
amino acid based graphene.
Fig. 6. XRD patterns of GO and different aliphatic amino acid
based graphene.
Fig. 7. a: The survey XPS spectrum and deconvoluted XPS spectra
of the C1s and O1s
regions (b and c) of phenylalanine functionalized graphene.
Fig. 8. Digital photograph of graphite, GO and phenylalanine
functionalized graphene. All
the samples were dispersed in distilled water.
Fig. 9. FE-SEM images of graphite and GO.
Fig. 10. FE-SEM micrographs of pure graphite and different
aliphatic amino acid
functionalized graphene.
Fig. 11. FE-SEM micrographs of different aromatic-aliphatic
amino acid functionalized
graphene.
Fig. 12. Graphene scrolling types.
Fig. 13. TEM micrographs of phenylalanine functionalized
graphene.
Fig.14. TEM micrographs of tyrosine functionalized graphene.
Fig. 15. TGA curves of graphite, GO, tyrosine functionalized
graphene and methionine
functionalized graphene.
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Fig. 1
Fig. 2
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Fig. 3
Fig. 4
Fig. 5
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Fig. 6
Fig. 7
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Fig. 8
Fig. 9
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Fig. 10
Fig. 11
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Fig. 12
Fig. 13
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Fig. 14
Fig. 15
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Highlights Bio-functionalized graphene materials were
synthesized through a simple technique. Biocompatible aliphatic and
aromatic amino acids were used for graphene
functionalization.
Based on amino acid structure different changes on graphene
structure and morphology were observed.
Graphene scrolling was observed by tyrosine functionalized
graphene.