covalently functionalized graphene sheets with biocomp.pdf

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 service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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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

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Fig. 10

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Fig. 14

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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.

covalently functionalized graphene sheets with biocomp.pdf

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surface of graphene

graphene oxide

aliphatic amino acids

aromaticaliphatic amino

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natural graphite

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