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C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7
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Nucleobase-pairing triggers the self-assemblyof uracil-ferrocene on adenine functionalizedmulti-walled carbon nanotubes
Prabhpreet Singh a,b, Cecilia Menard-Moyon a, Jitendra Kumar c, Bruno Fabre d,Sandeep Verma c,*, Alberto Bianco a,*
a CNRS, Institut de Biologie Moleculaire et Cellulaire, Laboratoire d’Immunologie et Chimie Therapeutiques, 67000 Strasbourg, Franceb Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, Indiac Department of Chemistry, Indian Institute of Technology-Kanpur, Kanpur 208016, UP, Indiad CNRS/Universite de Rennes 1, UMR 6226 Sciences Chimiques de Rennes, Matiere Condensee et Systemes Electroactifs MaCSE,
Campus de Beaulieu, 35042 Rennes, France
A R T I C L E I N F O
Article history:
Received 21 September 2011
Accepted 22 October 2011
Available online 28 October 2011
0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.10.037
* Corresponding authors.E-mail addresses: sverma@iitk.ac.in (S. V
A B S T R A C T
Shortened and oxidized multi-walled carbon nanotubes (MWCNTs) were functionalized
with adenine using the amidation strategy. The adenine functionalized MWCNTs
(Ad-MWCNTs) were complexed with a uracil substituted ferrocene and characterized by
transmission electron microscopy (TEM), high resolution TEM (HRTEM), electron diffraction
X-ray spectroscopy (EDX), and atomic force microscopy (AFM). The electrochemical proper-
ties of these novel nanohybrids were studied by cyclic voltammetry. The favorable supra-
molecular interaction of the electroactive species with the functionalized nanotubes
through the efficient adenine–uracil base-pairing can be exploited for the design of new
electronic devices.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Nucleic acids consist of five natural nucleobases, namely
adenine (A), guanine (G), cytosine (C), thymine (T) and uracil
(U) [1,2]. These nucleobases are versatile building blocks that
can provide hydrogen bonding aided molecular recognition
for the formation of duplexes, triplexes, tetraplexes and
higher order architectures [1–3]. Hydrogen bonding interac-
tions involving nucleobases highlighted the tremendous
importance of base-pairing in critical areas such as protein
synthesis, genetic coding, biological information storage, to
name a few [4]. The most famous base-pairing is the Wat-
son–Crick model, that involves A–T(U) and C–G base pairs
which strongly held together two anti parallel strands of
DNA [4–7].
er Ltd. All rights reservederma), a.bianco@ibmc-cn
Based on the capacity of nucleobases to generate well-
defined 3D structures, the concept of base-pairing has shifted,
in the last decade, from the biological realm to the field of
supramolecular chemistry [6]. Starting from self-assembled
systems containing two or more nucleobases, this field has
evolved towards the design and the development of new archi-
tectures based on low and high molecular weight polymers
modified with nucleobases, stabilized by H-bonding interac-
tions [4]. In this context, nucleobases have been recently com-
bined with other types of new materials including carbon
nanotubes (CNTs) [8–11].
CNTs are quasi 1D nanomaterials and due to their good
mechanical, electrical and thermal properties [12–15], they
are becoming useful candidates in nanoscience and nanotech-
nology. In the development of CNT-based composites and
.rs.unistra.fr (A. Bianco).
C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7 3171
nanoscale devices [16–19], it is crucial to find a procedure to sol-
ubilize and prevent them from forming bundles. Among the
different functionalization methods, the covalent functionali-
zation involves oxidation at defect sites followed by amidation
and esterification reactions, fluorination, addition of diazo-
nium salts or radicals, and 1,3-dipolar cycloaddition reactions
[20]. In many developed modifications of functionalized CNTs,
besides coating with nanoparticles [10,21–23], conducting
polymers [24,25] and ceramics [26,27], their decoration with
electroactive species has widened the scope of CNT-based
(bio)analytical electrochemistry [28,29]. As a matter of fact, fer-
rocene is one of the most exploited organometallic molecules
in the development of electrochemical sensor devices owing
to its well behaved redox chemistry [30]. The attractive electro-
chemical characteristics exhibited by ferrocene (i.e. fast elec-
tron-transfer rate, low oxidation potential, and stability of
two redox states [31,32]) are usually retained after its immobi-
lization on conducting surfaces, including doped semiconduc-
tors, metals and carbonaceous materials. In comparison to the
large number of reports devoted to the covalent binding of fer-
rocene to surfaces [33–37], the anchoring of this electroactive
molecule using non-covalent interactions has not been devel-
oped extensively [38]. In a pioneering work, Prato and Guldi
developed a system where ferrocene was covalently linked to
single-walled carbon nanotubes [39]. An efficient photoin-
duced intramolecular electron transfer was measured and
considered extremely promising in light of the possibility of
further developments of this type of donor–acceptor systems
in solar-energy conversion. Using a different approach, a
supramolecular assembly of multi-walled carbon nanotubes
(MWCNTs) and ferrocene branched chitosan has displayed a
good electrocatalytic activity [40]. More recently ferrocene
was bound to a linear polyethyleneimine and combined with
the glucose oxidase and the CNTs [41]. The resulting modified
electrode showed an enzymatic response significantly en-
hanced due to the presence of the nanotubes. Such multicom-
ponent assemblies can be envisaged for the construction of
miniaturized biosensors or enzymatic biofuel cells.
In this work, we describe an efficient decoration of adenine
functionalized MWCNTs with a ferrocene uracil derivative
Fig. 1 – Schematic illustration of the assembly of electroactive fe
supramolecular non-covalent interactions of adenine–uracil bas
driven by the nucleobase-pairing (Fig. 1). The approach in-
volves the covalent modification of oxidized nanotubes with
an adenine moiety followed by the complexation with the elec-
troactive molecule under aqueous/methanolic conditions. The
non-covalent supramolecular assembly of CNTs and ferrocene
was characterized by several techniques including transmis-
sion electron microscopy (TEM), high resolution TEM (HRTEM),
atomic force microscopy (AFM), and energy dispersive X-ray
spectroscopy (EDX). The electrochemical properties of the
complexes were measured by cyclic voltammetry. These new
self-assembled conjugates could find applications in advanced
photovoltaic and optoelectronic devices, and electrochemical
sensing.
2. Experimental
2.1. General
MWCNTs, produced by the catalytic carbon vapor deposition
(CCVD) process, were purchased as purified from Nanocyl
(Thin MWCNT 95 + % C purity, Nanocyl 3100� batch No.
071119, average diameter and length: 9.5 nm and 1.5 lm,
respectively). Adenine derivatives 6–8 were synthesized
according to modified literature procedures [8,10]. The synthe-
sis and the crystallographic characterization of ferrocene
substituted uracil derivative 9 are reported in the Supporting
Information [42].
2.2. Sample preparation
One gram of pristine MWCNTs was sonicated in a water bath
(20 W, 40 kHz) for 24 h in 150 mL of sulfuric acid/nitric acid
(3:1 v/v, 98% and 65%, respectively) at room temperature
[43,44]. Deionized water was then carefully added and the
MWCNTs were filtered (Omnipore� membrane filtration,
0.45 lm), re-suspended in water, filtered again until pH
became neutral and dried to obtain oxidized MWCNTs (ox-
MWCNT 4) (Fig. 2). The synthesis of the adenine functional-
ized MWCNTs (Ad-MWCNTs) 1–3 was previously described in
details in Ref. [10].
rrocene onto functionalized MWCNTs by exploiting the
e pairs.
Fig. 2 – Synthesis of MWCNT precursors 4, 5 and Ad-MWCNT hybrids 1–3.
3172 C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7
2.2.1. Preparation of the complexes of Ad-MWCNTs 1–3 withuracil substituted ferrocene0.1 mg of the Ad-MWCNTs 1–3 and ox-MWCNT 4 (used as con-
trol) were dispersed in 0.1 mL of 1:1 methanol/water by ultra-
sonication in a water bath for 5 min. Similarly, solutions of
ferrocene substituted uracil 9 (1 or 5 eq vs the adenine load-
ing; 0.5 mmol/g) were prepared. The uracil substituted ferro-
cene was added to Ad-MWCNTs 1–3 and ox-MWCNT 4. After
addition of the solution, the mixtures of Ad-MWCNTs 1–3
and ox-MWCNT 4 were shaken thoroughly and ultrasonicated
for 5 min, then kept for 10–12 h at rt. After this period, some
CNTs precipitated.
2.2.2. Preparation of the MWCNT-coated surfaces for cyclicvoltammetryAbout 0.3 mg of Ad-MWCNT 1 or 3 were dispersed in a metha-
nol (VWR, HPLC grade)/water (ultrapure, 18.2 MX cm) mixture
(1:1 v:v) and ultrasonicated for 10 min to obtain a homoge-
neous black dispersion. Considering a 1:1 complexation be-
tween adenine units and uracil substituted ferrocene 9, the
stoichiometric quantity of uracil substituted ferrocene was
then added into this mixture from a solution containing 1 mg
of uracil substituted ferrocene in 2 mL of MeOH. The mixture
was again sonicated for 5 min and kept at room temperature
overnight. An aliquot of 10 lL of the dispersion was deposited
onto a previously polished glassy carbon disk electrode for
electrochemical measurements or previously cleaned oxidized
silicon(111) (n-type, phosphorus doped, from Siltronix) surface
for AFM characterization. The coated surfaces were then dried
at rt for 3 h prior to characterization. The oxidized silicon sur-
face was produced by cleaning a native oxide-coated silicon
shard in a 3:1 v/v concentrated H2SO4/30% H2O2 (piranha solu-
tion) at 100 �C for 30 min, followed by abundant rinsing with
ultrapure water. Caution. The concentrated H2SO4:H2O2 (aq)
piranha solution is very dangerous, particularly in contact with
organic materials, and should be handled extremely carefully.
2.3. Characterization
The thermogravimetric analyses were performed using a TGA
Q500 TA instrument with a ramp of 10 �C/min under N2 from
100 �C to 800 �C, and already reported in Ref. [10]. Transmis-
sion electron microscopy was performed on a Hitachi H600
microscope with an accelerating voltage of 75 kV and at differ-
ent magnifications. Ten micro liters of the dispersion of ferro-
cene substituted uracil 9 with Ad-MWCNTs 1–3 or ox-MWCNT
4 were deposited onto a carbon-coated TEM grid after sonica-
tion for 5 min in a water bath. In the case of HRTEM and EDX
characterization, 5 eq. of uracil substituted ferrocene were
used. After 6 h a drop of the supernatant of the dispersion
with Ad-MWCNT 1 or ox-MWCNT 4 was deposited onto a
holey-carbon TEM grid (diameter of the holes below 1 lm).
HRTEM images of ferrocene decorated CNTs were taken with
a TOPCON microscope at an accelerating voltage of 200 kV
and a resolution of 0.18 nm. Linear potential sweep cyclic vol-
tammetry (CV) experiments were performed with an Autolab
PGSTAT 20 potentiostat from Eco Chemie B.V., equipped with
General Purpose Electrochemical System (GPES) software. Tet-
ra-n-butylammonium hexafluorophosphate, Bu4NPF6 (Fluka,
puriss, electrochemical grade, 99%, stored in a dessicator) at
0.2 M in dichloromethane (distilled over hydride calcium, Car-
lo Erba, HPLC grade) was used as the electrolytic medium. The
working electrode was a 3-mm-diameter glassy carbon disk
(area 0.07 cm2) and was polished successively with 5 lm
C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7 3173
silicon carbide paper (Struers, FEPA P No. 4000) and 0.25 lm
alumina slurry (Struers). The counter electrode was a glassy
carbon rod. Potentials were relative to aqueous saturated cal-
omel electrode (SCE). Electrochemical measurements were
carried out at room temperature (20 ± 2 �C) and inside a
homemade Faraday cage under a constant flow of argon.
AFM images were recorded in intermittent contact mode with
a PicoSPM II microscope from Molecular Imaging using n+-
type silicon tips (AC mode, FM, 65–90 kHz resonance fre-
quency) from ScienTec-Nanosensors.
3. Results and discussion
Ad-MWCNT derivatives 1–3 were obtained by following the
procedure we previously reported in the literature (Fig. 2).
Oxidized MWCNT 4 obtained by strong acid treatment [43,44]
were derivatized by amidation with Boc mono-protected
diaminotriethylene glycol. The Boc group was removed with
HCl solution in dioxane to afford functionalized MWCNT 5 con-
taining 0.5 mmol/g of amino groups as assessed by Kaiser test
[45]. Adenine nucleobase was modified, in turn, with reactive
functionalities like amine or acid carboxylic groups for conju-
gation to CNTs [46,47]. 3-(9-Adeninyl) propionic acid 8 was acti-
Fig. 3 – TEM images of Ad-MWCNT hybrids 1–3 alone (a)–(c) or co
correspond to 500 nm. Panels (g)–(i) show the magnification of t
corrugations. Scale bars correspond to 100 nm.
vated using standard coupling reagents and condensed with
functionalized MWCNT 5 to afford Ad-MWCNT 1. Ad-MWCNTs
2 and 3 were prepared by acyl chloride activation of ox-MWCNT
4 and subsequent amidation with adenine derivatives 7 and 6,
respectively.
Ad-MWCNTs 1–3 were characterized by thermogravimetric
analysis (TGA) and TEM. Ad-MWCNTs 1–3 showed a weight
loss of 23.0%, 21.7%, and 20.0% at 500 �C, respectively, as com-
pared to 11.7% for ox-MWCNT 4 [10]. The adenine-MWCNT
hybrids, deposited from a 1:1 methanol/H2O solution onto a
carbon coated grid, were then observed by TEM (Fig. 3). TEM
images display small aggregates of Ad-MWCNTs 1–3, assess-
ing the role of functional groups and spacers in modulating
dispersibility of the nanotubes.
We then explored the possibility to cover the surface of
nanotubes with ferrocene as electroactive species via a novel
supramolecular assembly approach. For this purpose we syn-
thesized the ferrocene substituted uracil molecule 9 to favor
deposition of ferrocene on the nanotube surface via H-bond-
ing interactions between adenine and uracil moieties. The
dispersions of Ad-MWCNTs 1–3 and ox-MWCNT 4 (as control)
were mixed with a solution of ferrocene substituted uracil 9
and after 12 h one drop of the soluble CNTs in the superna-
mplexed to the ferrocene uracil derivative 9 (d)–(f). Scale bars
he nanotubes to better evidence the presence of the
3174 C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7
tant was deposited on a TEM grid (Fig. 3(d)–(i)). We observed
that the surface of Ad-MWCNTs 1–3 became very rough, cer-
tainly due to non-covalent base-pairing between adenine
and uracil. As control, we repeated the same experiment with
ox-MWCNT 4. We did not observe any roughness of the sur-
face of oxidized nanotubes, thus allowing excluding a non-
specific interaction between the ferrocene uracil 9 and the
nanotube surface. It is clear that the presence of adenine
bound to CNTs is fundamental to induce the supramolecular
complexation with the substituted uracil nucleobase.
To confirm the presence of ferrocene on the CNT surface,
we characterized the Ad-MWCNT 2 complexed to ferrocene
conjugate 9 by HRTEM coupled with EDX. Similarly to the data
obtained with TEM (Fig. 3(e)), HRTEM images of Ad-MWCNT 2
complexed to ferrocene substituted uracil show that the
nanotube surface is highly rough (Fig. 4(a) and (b)) in compar-
ison to the surface of ox-MWCNT 4 mixed with ferrocene ura-
cil in similar conditions (Fig. 4(d) and (e)). The HRTEM image
of an isolated single MWCNT shows many dark kinks or
amorphous structures in close proximity to the outer surface
of the nanotube walls (Fig. 4(a),(b)). From the observation of
this roughness throughout the entire sample, we can attri-
bute these features to the presence of non-covalently linked
ferrocene uracil molecules on the nanotube surface.
We used EDX spectroscopy to detect the presence of iron
on the nanotube surface of Ad-MWCNT 2 complexed with fer-
rocene substituted uracil. A significant amount of iron was
measured on the nanotubes after complexation (Fig. 4(c)),
whereas no iron was found on areas of the TEM grid where
Fig. 4 – HRTEM images of Ad-MWCNT 2 (a, b) and ox-MWCNT 4
20 nm for panel (a), (b) and (d); scale bar is 100 nm for panel (e)];
with ferrocene-uracil derivative 9. The area of the EDX analysis
references to colour in this figure legend, the reader is referred
CNTs were missing. A control experiment was performed
with ox-MWCNT 4 mixed with ferrocene substituted uracil
in similar conditions. In this case, no iron was detected on
the nanotube surface of ox-MWCNT 4 (Fig. 4(f)) which resulted
smoother than that of Ad-MWCNT 2. Taken together, all these
observations confirmed the specific interactions between Ad-
MWCNTs and ferrocene substituted uracil through adenine–
uracil base-pairing complexation.
Further characterization by AFM provided clear evidence
for complexation of uracil substituted ferrocene 9 by adenine
units attached to MWCNTs. The AFM images show relatively
well separated tubes in the case of Ad-MWCNT complexed
with uracil substituted ferrocene while the presence of the
triethylene glycol (TEG) linker leads to the formation of aggre-
gates (Fig. 5). Consistent with that, the mean diameter mea-
sured from cross-section profiles of four isolated tubes was
15 ± 4 nm and 28 ± 5 nm for the complexed Ad-MWCNT 3
and Ad-MWCNT 1, respectively.
We finally decided to study the electrochemical properties
of the complexes between Ad-MWCNTs and the ferrocene
uracil derivative 9. The presence of ferrocene on the surface
of CNTs was confirmed by measuring the electrochemical re-
sponses of the electroactive molecule. Typical cyclic voltam-
mograms of the adenine-functionalized MWCNT-coated
glassy carbon electrodes after complexation with uracil
substituted ferrocene in CH2Cl2 and 0.2 M Bu4NPF6 are shown
in Fig. 6. A single reversible redox system at 0.66 V vs SCE was
observed. Since the adenine-functionalized MWCNTs do not
show any oxidation peak within the same potential range,
(d, e) mixed with ferrocene-uracil derivative 9 [scale bar is
EDX analysis of Ad-MWCNT 2 (c) and ox-MWCNT 4 (f) mixed
is represented by a red circle. (For interpretation of the
to the web version of this article.)
Fig. 5 – AFM images and corresponding cross-section profiles of Ad-MWCNT 1 (a, c) and Ad-MWCNT 3 (b, d) samples after
complexation with uracil-substituted ferrocene. The samples were deposited onto an oxidized silicon surface. The cross-
section profiles of four isolated tubes were taken from the lines 1 and 2 drawn in the 2 · 2 lm2 images.
Fig. 6 – Cyclic voltammograms of Ad-MWCNT 1 (A) and Ad-
MWCNT 3 (C) modified glassy carbon (3 mm diameter)
electrodes after complexation with uracil-substituted
ferrocene in CH2Cl2 and 0.2 M Bu4NPF6 at 0.1 V s�1.
Corresponding Ipa–v plots of the modified electrodes (B, D).
The inset in (A) corresponds to the electrochemical response
of uracil-substituted ferrocene at 2 mM in the same
electrolytic medium.
C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7 3175
this system can be undoubtedly ascribed to the one-electron
reversible oxidation of complexed ferrocene and the mea-
sured formal potential E� 0 (average of anodic and cathodic
peak potentials) is approximately similar to that observed
for the ferrocene derivative in solution, namely 0.67 V vs
SCE. Moreover, the presence of adenine units anchored to
CNTs accounts for the complexation phenomenon because
the cyclic voltammogram of acid-functionalized MWCNTs,
devoid of adenine, (ox-MWCNT 4) after contact with uracil
substituted ferrocene did not show any redox signature char-
acteristic of immobilized ferrocene. Moreover, we have no-
ticed that the addition of acetonitrile led to a decrease of
the electrochemical currents assigned to the complexed fer-
rocene as anticipated from the greater polarity of this solvent,
which weakens the hydrogen bonding interactions [48]. Inter-
estingly, the surfaces showed an electroactivity loss of only
10% after 10 cycles upon a repetitive voltammetric scanning,
demonstrating that ferrocene was durably retained in the
MWCNT networks through the adenine–uracil complexation.
As expected for surface-confined redox species [49], the
anodic peak current intensities Ipa corresponding to the ferro-
cene/ferrocenium couple are found to be directly proportional
to the potential scan rates v (Fig. 6(B) and (D)) according to the
following equation:
Ipa ¼n2F2
4RTvAC ð1Þ
where n is the number of exchanged electrons (n = 1), F the Far-
aday constant, R the gas constant, T temperature, A the surface
area of glassy carbon (7 · 10�2 cm2) and C the surface coverage
of complexed electroactive units. From the slope of the Ipa vs v
plots, the surface coverage of complexed ferrocene units can be
estimated at (5.1 ± 0.2) · 10�11 mol cm�2 with no significant ef-
fect of the nature of the linker bearing the adenine units (with
or without TEG). The magnitude of this coverage is perfectly
suitable for electrocatalysis and bioelectroanalytical sensing
devices using ferrocene as the redox mediator. Moreover, it also
indicates that a proportion of CNT-bound adenine sites are
3176 C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7
uncomplexed by ferrocene uracil and therefore remains avail-
able for further complexation with other nucleobases, such as
biotinylated functional species.
4. Conclusions
We report an original approach for the decoration of CNTs
with electroactive species. Oxidized MWCNTs were function-
alized with adenine nucleobase via different linkers which
can modulate the dispersibility of the conjugates. The three
Ad-MWCNTs were mixed with a uracil substituted ferrocene.
Nucleobase-pairing between adenine and uracil triggered the
self-assembly of the electroactive ferrocene on Ad-MWCNTs.
The ferrocene-MWCNT hybrids were characterized by various
techniques such as electron and atomic force microscopy,
EDX, as well as cyclovoltammetry. Electron microscopy re-
vealed the presence of corrugations on the nanotube surface
confirming the complexation between CNT-bound adenine
and uracil. By means of EDX coupled with HRTEM, we con-
firmed the presence of iron from ferrocene on the nanotube
surface. Finally, we assessed the redox behavior of ferrocene
by cyclic voltammetry and showed that the electrochemical
properties of ferrocene were preserved after immobilization
on the nanotube surface. This simple supramolecular assem-
bly strategy could find interesting applications in the field of
photovoltaic and optoelectronics. For instance, it would be
relevant to compare the photoinduced electron transfer prop-
erties of our redox-active assemblies with similar CNT-based
systems covalently bound to ferrocene [39] and determine the
resulting implications in solar-energy conversion. Moreover,
this type of modified electrodes could find novel opportuni-
ties in biomolecular recognition using the free CNT-bound
adenine sites as the molecular receptor and the complexed
ferrocene centers as the electrochemical transducer.
Acknowledgements
This work was supported by the French-Indian CEFIPRA/IFC-
PAR collaborative project (Project No. 3705-2). P.S. wishes to
thank CEFIPRA/IFCPAR for a post-doctoral fellowship. TEM
images were recorded at the RIO Microscopy Facility Plate-
form of Esplanade Campus (Strasbourg, France). The authors
are grateful to F.M. Toma for helping with TGA.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.carbon.2011.10.037.
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