Nitrene addition to exfoliated graphene: A one-step route to highly functionalized graphene

Article abstract of DOI:10.1039/c001488e

We demonstrate a high yield method of functionalizing graphene nanosheets through nitrene addition of azido-phenylalanine [Phe(N₃)] to exfoliated micro-crystalline graphite (μG). This method provides a direct route to highly functionalized grap

Full text of DOI:10.1039/c001488e

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Nitrene addition to exfoliated graphene: a one-step route to highly
functionalized graphenew
T. Amanda Strom, Eoghan P. Dillon, Christopher E. Hamilton and Andrew R. Barron*
Received 22nd January 2010, Accepted 22nd April 2010
First published as an Advance Article on the web 10th May 2010
DOI: 10.1039/c001488e
We demonstrate a high yield method of functionalizing graphene
nanosheets through nitrene addition of azido-phenylalanine
[Phe(N3)] to exfoliated micro-crystalline graphite (lG). This
method provides a direct route to highly functionalized graphene
sheets. TEM analysis of the product shows few layer (n o 5)
graphene sheets. The product was determined to have 1 phenyl-
alanine substituent per 13 carbons.
desirable. In this regard, we report a new, high yield method of
covalently functionalizing graphene sheets.
It has been shown that both CNTs and fullerenes are easily
functionalized by nitrene addition, generated in situ by thermal
decomposition of azido starting materials.5 The result is high
density functionalization of the nanostructure surface through
a [2+1] cycloaddition to the C–C double bonds forming an
aziridino-ring linkage, however; this methodology has only
been applied to curved or strained (i.e. reactive) carbon
nanostructures. The alkyl and aryl azido starting materials
are easily synthesized on the gram scale and azides offer a wide
range of functional group tolerance.6 For our purposes, the
azido moiety resides on the R side chain of the amino acid
phenylalanine (Phe) allowing incorporation of two new
chemical handles on the graphene scaffold for subsequent
reaction. In this regard, the graphene sheet could be used as
a solid support for solid phase peptide synthesis (SPPS) of a
sequence on the graphene surface. The addition of sequences
of amino acids to the graphene surface should impart water
solubility and some level of biocompatibility.
Recent years have brought about a renewed interest in graphite or
specifically individual or few-layer (n o 5) sheets of graphite
called graphene. The unique physical and electronic properties
of graphene make it attractive as a substitute for other
more costly carbon nanostructures.1 Due to its similarities
and differences with other carbon nanostructures, graphene
(2 dimensional, 2D) is no longer the underappreciated parent
of fullerenes (0D) and carbon nanotubes (1D). The ability to
individualize sheets of graphene has brought the material to
the forefront of carbon research.2 And while graphene does
provide similar scale, properties, and reaction pathways as
fullerenes and CNTs, it does so at a fraction of the cost.
Regardless of the novel electronic properties of graphene,
it also presents an inexpensive, large surface area scaffold
for the covalent attachment of organic molecules. Covalent
functionalization provides a robust material for use as a
scaffold and as a means of incorporating additional function-
ality. Incorporation of functionalized graphene sheets into
composite materials may enhance mechanical and electronic
properties. The majority of reports of covalently functionalized
graphene use graphene oxide (GO) as the starting material.3
While the structure of GO has been shown to include regions
of unoxidized benzene rings, the presence of islands of
epoxides and hydroxyl groups on the basal plane and carboxylic
acids around the perimeter of the sheet make GO extremely
hydrophilic. In addition, graphene oxide contains a large
percentage of water requiring more than four weeks to dry
post synthesis.4 While this method does result in highly
functionalized graphene sheets, the drawbacks are that reactions
using GO as the starting material are limited to aqueous or
polar conditions and more important a second reduction step
is necessary to remove the excess oxides still present.3
A simpler route to widespread functionalization is therefore
In a typical reaction,z microcrystalline graphite (mG, o20 mm,
50 mg) was exfoliated without homogenization in o-dichloro-
benzene (ODCB, 20 mL) to make a clear gray solution of
graphene sheets without the use of harsh oxidizing conditions.7 It
is worth noting that this reaction was not reproducible using
thermally expanded graphite intercalation compounds (GIC)8
as the starting material. Given that GICs use a variety of
reactive molecules to intercalate between graphene layers,4
side reactions with reactants are inevitable, thus limiting their
use as graphitic starting materials as well. The exfoliated mG
was reacted with Boc–Phe(4-N3)–OHy at refluxing temperatures.
The reaction was filtered over a 0.2 mm PTFE filter paper, and
washed to remove any unreacted Phe(N3) (Scheme 1, yield 1
110 mg). The product 1 was found to make an amber colored
solution in DMSO, averaging 0.06 mg mLꢀ1 after 10 min of
probe sonication and centrifugation (3400 rpms for 15 min). The
product also proved sparingly soluble in other organic solvents
such as DMF, toluene, and chloroform in comparison to the mG
starting material. To confirm the chemical modification FTIR
spectroscopy, Raman spectroscopy, and XPS were utilized.
Richard E. Smalley Institute for Nanoscale Science and Technology,
Center for Biological and Environmental Nanotechnology,
and Department of Chemistry, Rice University, Houston,
Texas 77005, USA. E-mail: arb@rice.edu;
Web: www.rice.edu/barron; Tel: +1-713-348-5610
w Electronic supplementary information (ESI) available: Details of the
synthesis; XPS, IR, TGA, and additional TEM, AFM, and STEM
images. See DOI: 10.1039/c001488e
Scheme 1 Nitrene addition to exfoliated mG in refluxing ODCB.
Chem. Commun., 2010, 46, 4097–4099 | 4097
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The elemental composition of 1 was determined by XPS,
confirming the presence of the phenylalanine (Fig. S1, ESIw).
The oxygen (16.1%) and nitrogen (13.5%) content are
consistent with the presence of the phenylalanine substituent
with the expected loss of the Boc protecting group which can
occur at 180 1C.9z However, the FTIR of 1 (Fig. S3, ESIw)
showed the characteristic absorption of a secondary carbamate
carbonyl stretch at 1655 cmꢀ1, indicating there was only
partial loss of the Boc protecting group. In addition, the
spectrum shows absorption in the carboxylic acid (B3200 cmꢀ1
stretch and 1438 cmꢀ1 bend) and the amide C–N regions
(1319 cmꢀ1) relative to the mG starting material. Also present
in the spectrum are the NH stretch (3426 cmꢀ1) of the Boc
amino protecting group and the prominent C–N amine stretch
at 1022 cmꢀ1 of the aziridino linkage.
Fig. 2 AFM image and associated height profiles of Phe–N–mG (1),
spin coated on a cleaved mica substrate from CHCl3, showing a large
graphene sheet surrounded by smaller graphene flakes.
Scanning TEM (STEM) images of 1 (Fig. S6, ESIw) deposited
from DMSO on a lacey carbon grid show few-layer sheets, the
majority of which had lengths of over 1 mm and widths
of about 500 nm. However, some sheets were as small as
150 ꢂ 300 nm, in agreement with TEM and AFM measurements.
The extent of functionalization is obtained from thermo-
gravimetric analysis (TGA) over the temperature range of
130–850 1C in agreement with the decomposition of phenyl-
alanine (Fig. S7, ESIw). Assuming no mass loss due to mG and
taking into account the carbonaceous material remaining from
phenylalanine (Fig. S7–S8 and discussion, ESIw), the final
product 1 was determined to be 69% phenylalanine (wt%).
This corresponds to one Boc–Phe substituent per 10 mG
carbons (XPS data show 1 : 13 C : Phe, Fig. S1, ESIw). It
should be noted, however, that each Phe substituent is bonded
to two carbon atoms (Scheme 1). It has been shown previously
that graphene is preferentially functionalized along its edges.11
However, even with the smallest sheets of 1 (150 ꢂ 300 nm by
STEM), if the functionalization is limited to the edges, only
2–3% by mass of the sample is attributable to Phe. Assuming
the entire mass loss by TGA is due to covalently bonded Phe,
to achieve 69% by mass functionalization of 1, the nitrene
addition could not occur solely at the edges of the sheets.
STEM images (Fig. S6, ESIw) show dark spots along the basal
plane of the sheets but EDX analysis of the spots could not
definitively confirm that they are Phe. We conclude, therefore,
that by our route, functionalization is not limited to the edges
and the resulting structure is analogous to that of GO.4
Raman spectroscopy (514 nm excitation) of 1 (Fig. 3)
showed a D : G ratio of 0.78 in comparison to that of
exfoliated mG at 0.66 (Fig. 3a and b). While there is not a
significant change, this could be due to the distribution of
functional groups on the surface and/or an effect of the size of
the sheets.12 The most noticeable feature in the spectrum of 1
is the loss of the 2D peak (ca. 2700 cmꢀ1). Most previous
reports of functionalized graphene omit mention of this
peak.11,13 The only material reported to show a similar loss
of the 2D peak is graphene oxide (GO),14 in which the
graphitic structure is almost completely oxidized with some
unoxidized regions remaining. Based on the similar spectra of
1 to GO (Fig. 3b and c), the absence of the 2D peak indicates
the lack of sp2 carbons present and, therefore, the high degree
of functionalization. Thermolysis of 1 at 600 1C for 12 h
produced a black powder with a D : G ratio of 0.67 and a
partially restored 2D peak (Fig. 3d) similar to that of the
High resolution TEM images show graphene sheets layered
over each other, with an average length of 300–500 nm and
with varying widths (Fig. 1). This is in agreement with STEM
measurements (Fig. S6, ESIw), in which large, few-layer
graphitic domains are visible. The formation of few- or
multi-layer sheets is further supported by selected area electron
diffraction (SAED, Fig. 1 inset and Fig. S4, ESIw) and fast
Fourier transform (FFT, Fig. S4, ESIw) in selected areas.
The AFM analysis confirmed the presence of graphene
sheets with a wide variety of lateral dimensions from 150 nm
to >5 mm. The thickness of the observed sheets ranged from
0.5 nm to B2.5 nm (Fig. 2). The theoretical height of our Phe
functional group is B0.75 nm, correlating to the addition of
1.5 nm to the thickness of the graphene sheet if functionalization
occurs on both sides. Several groups have reported a height of
B1 nm for GO sheets with some variability.10–12 Our analysis
of 1 shows a variety of sheet thicknesses, however, we surmise
that the thinnest sheets, 0.5 nm, are only edge functionalized
while the thickest at 2.5 nm are basal plane functionalized on
both sides. Based on both the TEM and AFM characterization,
we conclude that few-layer (n o 5) graphene sheets are
produced by this reaction method.
Fig. 1 TEM image of Phe–N–mG (1) showing stacked, few layer,
sheets with SAED inset confirming few-layer graphene sheets.
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was added NEt3 (1.39 mL, 10.0 mmol). After stirring at room
temperature for 12 h, it was diluted with Et2O (200 mL) and washed
with AcOH (10%, 3ꢂ) and brine (1ꢂ). Drying with MgSO4 and
concentration, followed by flash chromatography yielded the pure
product. Yield = 1.2 g, 80%. 1H NMR (500 MHz, CDCl3): d 7.07
[2H, d, J(H–H) = 8.97 Hz, CH], 6.97 [2H, d, J(H–H) = 8.97 Hz, CH],
4.93 [1H, d, J(H–H) = 8.63 Hz, NH], 4.58 [1H, dd, J(H–H) = 6.64
Hz, J(H–H) = 14.92 Hz, CH], 3.12 [1H, dd, J(H–H) = 5.12 Hz,
J(H–H) = 13.92 Hz, CH2], 3.05 [1H, dd, J(H–H) = 6.41 Hz, J(H–H)
t
= 13.64 Hz, CH2], 1.42 [9H, s, Bu]. IR (cmꢀ1): 2115 (nN3).
z To determine if polymerization of Phe on the graphene surface was
occurring, we repeated the reaction with a methyl ester of Boc–
azido–Phe. Protection at both carboxy and amino terminus should
eliminate the possibility of peptide formation. Details of the reaction
and characterization are included in the ESI.w
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Fig. 3 Raman spectra of (a) exfoliated mG, D : G 0.66; (b) 1, D : G
0.78; (c) graphene oxide, D : G 0.77 and (d) annealed 1, D : G 0.67.
exfoliated mG sample. It should be noted that the bulk mG
material prior to exfoliation had a D : G ratio of 0.25
indicating possible modification to the electronic framework
simply through sonication in ODCB.15
3 A. B. Bourlinos, D. Gournis, D. Petridis, T. Szabo, A. Szeri and
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´ ´
In conclusion, we have reported a high yield method of
covalently functionalizing exfoliated graphene by nitrene
addition in ODCB. The addition results in few-layer, DMSO
soluble graphene sheets. Given the extensive range of organic
azides accessible by a simple diazo-transfer reaction,6 this
method offers a route to a wide variety of functional addends.
The advantages of our method are as follows: (1) no final
reduction step is required, (2) a wide variety of functional
addends are accessible, (3) reactions are not limited to aqueous/
polar conditions, and (4) highly functionalized, few-layer
graphene sheets are produced. The control over the extent of
functionalization is currently under investigation, as well as
the feasibility of 1 as a solid phase support for peptide
synthesis on the surface of a graphene scaffold.
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Financial support for this work is provided by the Robert
A. Welch Foundation. We thank Dr Noe Alvarez for the TEM
images and Dr Dmitry Kosynkin for the graphene oxide
sample.
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Notes and references
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z Boc–Phe(4-N3)–OH (250 mg in ODCB) was added to the exfoliated
mG (Sigma Aldrich) solution in a 100 mL round bottom flask fitted
with a condenser and a stir bar. Refluxing temperatures were
maintained for 4 days after which the brown suspension was filtered
over a 0.2 mm PTFE filter paper. The filter cake was washed copiously
with MeOH, CHCl3, and DMF to remove any unreacted Phe(N3). The
fine brown powder was dried in an oven overnight at 125 1C. Final
yield: 110 mg.
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J. M. Tour, J. Am. Chem. Soc., 2008, 130, 16201.
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y In a 500 mL Schlenk flask, imidazole-1-sulfonylazideꢃHCl (1.02 g,
6 mmol), Boc–Phe(4-NH2)–OH (1.40 g, 5 mmol), and ZnCl2 (27.3 mg,
0.20 mmol) was dissolved in MeOH : MeCN (3 : 2, 300 mL) to this
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