C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7

.sc iencedi rect .com

Avai lab le at www

journal homepage: www.elsev ier .com/ locate /carbon

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.

R E F E R E N C E S

[1] Sinden RR. DNA structure and function. San Diego,CA: Academic Press; 1994.

[2] Neidle S. Oxford handbook of nucleic acid structure. NewYork: Oxford University Press; 1999.

[3] Suhnel J. Beyond nucleic acid base pairs: from triads toheptads. Biopolymers 2001–2002;61(1):32–51.

[4] Fathalla M, Lawrence CM, Zhang N, Sessler JL,Jayawickramarajah J. Base-pairing mediated non covalentpolymers. Chem Soc Rev 2009;38(6):1608–20.

[5] Sessler JL, Lawrence CM, Jayawickramarajah J. Molecularrecognition via base-pairing. Chem Soc Rev2007;36(2):314–25.

[6] Sivakova S, Rowan SJ. Nucleobases as supramolecular motifs.Chem Soc Rev 2005;34(1):9–21.

[7] Verma S, Mishra AK, Kumar J. The many facets of adenine:coordination, crystal patterns and catalysis. Acc Chem Res2010;43(1):79–91.

[8] Singh P, Kumar J, Toma FM, Raya J, Prato M, Fabre B, et al.Synthesis and characterization of nucleobase-carbonnanotube hybrids. J Am Chem Soc 2009;131(37):13555–62.

[9] Singh P, Toma FM, Kumar J, Venkatesh V, Raya J, Prato M,et al. Carbon nanotube-nucleobase hybrids: nanorings fromuracil-modified single-walled carbon nanotubes. Chem Eur J2011;17(24):6772–80.

[10] Singh P, Lamanna G, Menard-Moyon C, Toma FM, Magnano E,Bondino F, et al. Formation of efficient catalytic silvernanoparticles on carbon nanotubes by adeninefunctionalization. Angew Chem Int Ed 2011;50(42):9893–7.

[11] Quintana M, Prato M. Supramolecular aggregation offunctionalized carbon nanotubes. Chem Commun2009;40:6005–7.

[12] Iijima S. Helical microtubules of graphitic carbon. Nature1991;354(6348):56–8.

[13] Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1 nmdiameter. Nature 1993;363(6430):603–5.

[14] Bethune DS, Klang CH, de Vries MS, Gorman G, Savoy R,Vazquez J, et al. Cobalt-catalysed growth of carbonnanotubes with single-atomic-layer walls. Nature1993;363(6430):605–7.

[15] Monthioux M, Kuznetsov VL. Who should be given the creditfor the discovery of carbon nanotubes? Carbon2006;44(9):1621–3.

[16] Jorio A, Dresselhaus G, Dresselhaus MS. Carbon nanotubes:advanced topics in the synthesis, structure, properties andapplications. Berlin, Heidelberg: Springer-Verlag; 2008.

[17] Reich S, Thomsen C, Maultzsch J. Carbon nanotubes: basicconcepts and physical properties. Weinheim,Germany: Wiley-VCH; 2004.

[18] Kreupl F. Carbon nanotubes in microelectronicapplications. Weinheim: Wiley-VCH, Verlag GmbH & Co.KgaA; 2008.

[19] Byrne MT, Gun’ko YK. Recent advances in research on carbonnanotube-polymer composites. Adv Mater2010;22(15):1672–88.

[20] Singh P, Campidelli S, Giordani S, Bonifazi D, Bianco A, PratoM. Organic functionalisation and characterisation of singlewalled carbon nanotubes. Chem Soc Rev 2009;38(8):2214–30.

[21] Xue B, Chen P, Hong Q, Lin J, Tan KL. Growth of Pd, Pt, Ag andAu nanoparticles on carbon nanotubes. J Mater Chem2001;11(9):2378–81.

[22] Lin Y, Watson KA, Fallbach MJ, Ghose S, Smith Jr JG, DelozierDM, et al. Rapid, solventless, bulk preparation of metalnanoparticle-decorated carbon nanotubes. ACS Nano2009;3(4):871–84.

[23] Choi HC, Shim M, Bangsaruntip S, Dai H. Spontaneousreduction of metal ions on the sidewalls of carbonnanotubes. J Am Chem Soc 2002;124(31):9058–9.

[24] Bliznyuk VN, Singamaneni S. Polymer/carbon nanotubescomposites in electronics and sensor application. In: Lee K-P,Gopalan AI, Marquis FDS. Functional composites of carbonnanotubes and applications, 2009. p. 137–91

[25] Hecht DS, Hu L, Irvin G. Emerging transparent electrodesbased on thin films of carbon nanotubes, graphene, andmetallic nanostructures. Adv Mater 2011;23(13):1482–513.

C A R B O N 5 0 ( 2 0 1 2 ) 3 1 7 0 – 3 1 7 7 3177

[26] Arsecularatne JA, Zhang LC. Carbon nanotube reinforcedceramic composites and their performance. Recent PatNanotechnol 2007;1(3):176–85.

[27] Boccaccini AR, Cho J, Subhani T, Kaya C, Kaya F.Electrophoretic deposition of carbon nanotube-ceramicnanocomposites. J Eur Ceram Soc 2010;30(5):1115–29.

[28] Vashist SK, Zheng D, Al-Rubeaan K, Luong JH, Sheu FS.Advances in carbon nanotube based electrochemical sensorsfor bioanalytical applications. Biotechnol Adv2011;29(2):169–88.

[29] Collin JP, Gavina P, Heitz V, Sauvage JP. Construction of one-dimensional multicomponent molecular arrays: control ofelectronic and molecular motions. Eur J Inorg Chem1998:1–14.

[30] van Staveren DR, Metzler-Nolte N. BioorganometallicChemistry of Ferrocene. Chem Rev 2004;104(12):5931–85.

[31] Martic S, Labib M, Shipman PO, Kraatz HB. Ferrocene-peptidoconjugates: from synthesis to sensory applications. DaltonTrans 2011;40(28):7264–90.

[32] Floris B, Illuminati G, Jones PE, Ortaggi G. Electron-donorproperties of ferrocene. Coord Chem Rev 1972;8(1-2):39–43.

[33] Finklea HO. Electrochemistry of organized monolayers ofthiols and related molecules on electrodes. In: Bard AJ,Rubinstein I, editors. Electroanalytical chemistry, 19. NewYork: Marcel Dekker; 1996. p. 109–335.

[34] Chidsey CED, Bertozzi CR, Putvinski TM, Mujsce AM.Coadsorption of ferrocene-terminated and unsubstitutedalkanethiols on gold: electroactive self-assembledmonolayers. J Am Chem Soc 1990;112(11):4301–6.

[35] Fabre B. Ferrocene-terminated monolayers covalently boundto hydrogen-terminated silicon surfaces. Toward thedevelopment of charge storage and communication devices.Acc Chem Res 2010;43(12):1509–18. and refs therein.

[36] Decker F, Cattaruzza F, Coluzza C, Flamini A, Marrani AG,Zanoni R, et al. Electrochemical reversibility ofvinylferrocene monolayers covalently attached on H-terminated p-Si(100). J Phys Chem B 2006;110(14):7374–9.

[37] Hauquier F, Ghilane J, Fabre B, Hapiot P. Conducting FerroceneMonolayers on Nonconducting Surfaces. J Am Chem Soc2008;130(9):2748–9.

[38] Fabre B, Ababou-Girard S, Singh P, Kumar J, Verma S, BiancoA. Noncovalent assembly of ferrocene on modified goldsurfaces mediated by uracil-adenine base pairs. ElectrochemCommun 2010;12(6):831–4.

[39] Guldi DM, Marcaccio M, Paolucci D, Paolucci F, TagmatarchisN, Tasis D, et al. Single-wall carbon nanotube-ferrocenenanohybrids: observing intramolecular electron transfer infunctionalized SWNTs. Angew Chem Int Ed Engl2003;42(35):4206–9.

[40] Zhou H, Yang W, Sun C. Amperometric sulfite sensor basedon multiwalled carbon nanotubes/ferrocene-branchedchitosan composites. Talanta 2008;77(1):366–71.

[41] Tran TO, Lammert EG, Chen J, Merchant SA, Brunski DB, KeayJC, et al. Incorporation of single-walled carbon nanotubesinto ferrocene-modified linear polyethylenimine redoxpolymer films. Langmuir 2011;27(10):6201–10.

[42] Kumar J, Purohit CS, Verma S. Directing spatial disposition offerrocene around homoadenine tetrads. Chem Commun2008:2526–8.

[43] Liu J, Rinzler AG, Dai H, Hafner JH, Bradley RK, Boul PJ, et al.Fullerene pipes. Science 1998;280(5367):1253–6.

[44] Li S, Wu W, Campidelli S, Sarnatskaıa V, Prato M, Tridon A,et al. Adsorption of carbon nanotubes on active carbonmicroparticles. Carbon 2008;46(7):1091–5.

[45] Samorı C, Sainz R, Menard-Moyon C, Toma FM, Venturelli E,Singh P. Potentiometric titration as a straightforward methodto assess the number of functional groups on shortenedcarbon nanotubes. Carbon 2010;48(9):2447–54.

[46] Soulere L, Sturm JC, Nunez-Vergara LJ, Hoffmann P, Perie J.Synthesis, electrochemical, and spectroscopic studies ofnovel S-nitrosothiols. Tetrahedron 2001;57(33):7173–80.

[47] Weisser M, Kashammer J, Menges B, Matsumoto J, NakamuraF, Ijiro K, et al. Adenine–uridine base-pairing at the water-solid-interface. J Am Chem Soc 2000;122(1):87–95.

[48] Sun H, Steeb J, Kaifer AE. Efficient electronic communicationbetween two identical ferrocene centers in a hydrogen-bonded dimer. J. Am. Chem. Soc. 2006;128(9):2820–1.

[49] Bard AJ, Faulkner LR. Electrochemical methods.fundamentals and applications. New York, NY: Wiley; 1980.p. 522.