Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

22.4 Fullerene-Rich Dendrimers 709

signals of the starting carboxylic acid precursors 4 – 6 , as well as the expected

additional resonances arising from the n - butyl chains. Importantly, the spectra

clearly showed that all the peripheral fullerene subunits were equivalent in 4 - 6 ,

as might be expected for a sixfold symmetric assembly with a drum - shaped

organostannoxane core. In addition, a single resonance was observed at approxi-

mately − 480 ppm in the

119

Sn NMR spectra of 4 – 6 recorded in C

6

D

6

. This charac-

teristic signature of tin - drum clusters provided deﬁ nitive evidence for the formation

of 4 – 6 .

The absorption spectra obtained from CH

2

Cl

2

solutions of compounds 4 – 6 were

identical to those recorded for the corresponding starting carboxylic acid precur-

sors; this demonstrated an absence of any signiﬁ cant inﬂ uence of the stannoxane

core on the electronic properties of the fullerene moieties. To further conﬁ rm that

the characteristics of the fullerene subunits would be maintained for 4 – 6 , their

electrochemical properties were investigated using cyclic voltammetry ( CV ). For

the sake of comparison, electrochemical measurements were also carried out with

3 and G1CO

2

t Bu. All of these experiments were conducted at room temperature

Figure 22.8 Top: Preparation of compound 3 . Bottom: X - ray

crystal structure of 3 (C = pale gray, O = black, Sn = gray) and

detailed view of the Sn

6

O

6

core. Selected bond lengths:

Sn(1) - O(1): 2.084(3) Å , Sn(2) - O(1): 2.075(4) Å , Sn(2) - O(2):

2.097(3), Sn(2) - O(3): 2.093(3) Å , Sn(3) - O(1): 2.101(3) Å .

710 22 Fullerene-Rich Nanostructures

Figure 22.9 Structural formulae of compounds 4 and 5 .

22.4 Fullerene-Rich Dendrimers 711

Figure 22.10 Structural formula of compound 6 .

in CH

2

Cl

2

solutions containing tetra - n - butylammonium tetraﬂ uoroborate (0.1 M )

as the supporting electrolyte, with a Pt wire as the working electrode and a satu-

rated calomel electrode ( SCE ) as a reference. The potential data for all of these

compounds are collected in Table 22.1 .

In the anodic region, all of the studied compounds presented at least one irre-

versible peak at approximately +1.7 – 1.9 V versus SCE that could likely be attributed

to oxidation of the dialkyloxyphenyl and/or alkyloxyphenyl units. Whereas, model

compound 3 was found to be electrochemically silent in the cathodic region, the

fullerene - substituted stannoxane derivatives 4 – 6 revealed the typical electrochemi-

cal response of fullerene derivatives [28] . Indeed, the electrochemical behavior

of 4 – 6 appeared to be similar to that of the model compound G1CO

2

t Bu. This

712 22 Fullerene-Rich Nanostructures

Table 22.1 Electrochemical data on the reduction of 4 – 6 and

G1CO

2

t Bu determined by cyclic voltammetry on a Pt working

electrode in CH

2

Cl

2

+ 0.1 M n Bu

4

NBF

4

at room temperature.

Values shown are for ( E

pa

+ E

pc

)/2 (in Volts) versus SCE and

Δ E

pc

(in milliVolts) (in parentheses) at a scan rate of 0.1 V s

− 1

.

E

1

E

2

E

3

G1CO

2

t Bu

− 0.51 (75) − 0.89 (75) − 1.34 (65)

4

− 0.53 (85) − 0.91 (80) − 1.35 (110)

5

− 0.54 (75) − 0.92 (90) − 1.33 (110)

6

− 0.54 (80) − 0.92 (90) − 1.34 (110)

indicated that all methanofullerene moieties in 4 – 6 behaved as independent redox

centers, and that their electrochemical properties were not affected by the stan-

noxane core.

22.5

Conclusions

The preparation of covalent, fullerene - rich dendrimers is difﬁ cult and involves

a signiﬁ cant number of synthetic steps, thus limiting their accessibility and

therefore their applications. Recent obtained results on the self - assembly of fuller-

ene - containing components, by using supramolecular interactions rather than

covalent bonds, have shown clearly that this strategy represents an attractive alter-

native for their preparation. Indeed, fullerene - rich derivatives are thus easier to

produce, and the range of systems that can be prepared is not severely limited by

the synthetic route. In this way, in - depth investigations of their properties are

possible, and the use of fullerene - rich materials for speciﬁ c applications can be

envisaged. Despite some remarkable recent achievements, it is clear that the

examples discussed in this chapter represent only the ﬁ rst steps towards the design

of fullerene - rich molecular assemblies that can display functionality at the macro-

scopic level. Clearly, further research is required in this area to fully explore the

possibilities offered by these materials, for example, in nanotechnology or in

photovoltaics.

Acknowledgments

These research studies were supported by the CNRS. The authors warmly thank

all of their coworkers and collaborators for their outstanding contributions; their

names are cited in the references.

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Interactions of Carbon Nanotubes with Biomolecules:

Advances and Challenges

Dhriti Nepal and Kurt E. Geckeler

715

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

23

23.1

Introduction

Carbon nanotube s ( CNT s) are allotropes of carbon, composed up of rolled - up

graphene sheets with a unique combination of properties that include excellent

electrical, optical, and mechanical properties [1] . Such properties have opened up

a great range of new possibilities for these fascinating materials in a wide range

of research, including electronics, optics, high - performance ﬁ bers, composites,

and biotechnology, as well as other ﬁ elds of materials science.

23.2

Structure and Properties

The chemical bonding of nanotubes is composed entirely of sp

2

bonds, similar

to those of graphite. The strength of the sp

2

carbon – carbon bonds gives the

CNTs amazing mechanical properties, as these bonds are much stronger than

the sp

3

bonds found in diamonds. Such properties, when coupled with the low

speciﬁ c weight of CNTs, provides these materials with great potential over a

wide range of applications. The extraordinary electronic properties of CNTs are

due to the quantum conﬁ nement effect, which is based on a unique structure

that allows electrons to propagate only along the nanotube axis. Depending on

how the two - dimensional ( 2 - D ) graphene sheet is “ rolled up, ” the CNTs may

be either metals or semiconductors. Three types of nanotubes are possible,

referred to as armchair , zigzag , and chiral nanotubes.

Despite the great potential that CNTs offer, many fundamental challenges

remain to be met before such potential can be fully utilized. This includes the

puriﬁ cation, dispersion, debundalization, and chiral - selective separation of nano-

tubes. The major hurdle results from the natural tendency of CNTs to remain

in highly aggregated states, due to their intrinsic van der Waals interactions. In

716 23 Interactions of Carbon Nanotubes with Biomolecules: Advances and Challenges

fact, under normal conditions this property not only makes it practically impos-

sible to solubilize the materials in any type of solvent but also hampers their

electrical, optical, and mechanical properties. Consequently, the functionalization

of nanotubes holds much promise as it provides processability and also helps

when studying the materials ’ properties using solution - based techniques. In

order to explore the potential of nanotubes, they have been functionalized in

many ways, including direct covalent reaction with a variety of reagents, indirect

reaction with different functional moieties starting from oxidized precursors, via

organometallic routes, and noncovalent interactions. The reactions of nanotubes

with polymers, whether by grafting or by using the nanotubes as an anchor to

initiate polymerization, has led to the production of a range of nanotube - based

polymer composites [2, 3] . Functionalization with well - deﬁ ned chemistry has

also helped in the design of nanotube - based “ smart ” devices, by modifying their

properties.

Today, single - walled carbon nanotube s ( SWNT s) are attracting increasing

attention in biomedical research [4 – 6] . Because of their unique physical and

chemical properties, they offer the promise for the development of novel diag-

nostic and therapeutic methods, high - sensitivity biosensors, and biofuel cells.

When SWNTs absorb energy from near - infrared ( NIR ) light they emit heat – a

property which can play important role in the design of novel biomedical devices

or novel therapeutic agents. In developing such applications, the challenges of

in vivo detection and biocompatibility are the most essential. The functionaliza-

tion of biomolecules not only offers biocompatibility, but also provides new

possibilities in the material world for the advancement of future technology.

DNA and proteins represent major classes of biopolymers with extensive poten-

tial in both the natural as well as the synthetic world. In this chapter, an attempt

has been made to summarize recent developments on the functionalization of

CNTs using these polymers, with special attention on noncovalent functiona-

lization. The aim also is to highlight some of the key challenges, and future

prospects.

23.3

Debundalization

Due to their inherent van der Waals interactions and their special geometric

structures, CNTs normally aggregate to form large bundles. Unfortunately, this

process of “ bundalization ” causes the most outstanding characteristics of

CNTs – namely their electrical [7] , mechanical [8] , and optical [9] properties – to be

diminished. As an example, it has been shown experimentally that an individual

SWNT has a tensile strain of ∼ 280% and can undergo superplastic deformation

by reducing its diameter 15 - fold before breaking [10] . Yet, bundled SWNTs [11]

and multi - walled carbon nanotube s ( MWNT s) [12] will break at strains of less than

∼ 6% and 12%, respectively. This provides a clear picture of the importance of

including debundalized nanotubes in nanocomposites. Moreover, the develop-

23.3 Debundalization 717

ment of efﬁ cient techniques to produce stable dispersions of debundalized

nanotubes is a clear prerequisite to many applications, ranging from molecular

electronics to nanocomposites.

In an effort to overcome these problems, many research groups worldwide have

investigated the debundalization or individualization of SWNTs. The debundaliza-

tion of SWNTs has been achieved via two routes, namely covalent [13] or noncova-

lent [14] modiﬁ cation/functionalization [3] , with the former approach causing

inevitable damage to the sidewalls of the nanotubes that altered their chemical

and physical properties [15] . As a consequence, a range of studies has focused on

noncovalent approaches to obtaining individual nanotubes by using different

agents and polymers.

Previous studies have been focused on using synthetic polymers or sur-

factants. For example, Smalley and coworkers [9] obtained individual nanotubes

by encasing each one in cylindrical micelles. This was made possible by ultra-

sonically agitating an aqueous dispersion of raw SWNTs in sodium dodecyl

sulfate ( SDS ), followed by ultracentrifugation to remove tube bundles and resid-

ual catalyst. The absorbance spectra hold the key role to determining the degree

of debundalization. The quasi - one - dimensionality of the SWNTs produces sharp

van Hove peaks in the density of electronic states [16] . In this respect, for

example in the case of HiPCO - SWNTs, the optical absorption spectrum consists

of a series of transitions: ﬁ rst van Hove transitions of met - SWNT s ( metallic -

SWNT ) (M

11

) ( ∼ 400 and ∼ 600 nm); second van Hove transitions of sem - SWNT s

( semimetallic - SWNT ) (S

22

) ( ∼ 550 to 900 nm); and ﬁ rst van Hove transitions of

sem - SWNTs (S

11

) ( ∼ 800 to ∼ 1600 nm). The well - resolved absorbance feature is a

key indication of the debundalization, as otherwise the spectra are broadened

(Figure 23.1 ).

In the bulk state, the individualization can be further conﬁ rmed by recording

ﬂ uorescence spectra, which have been observed directly across the band gap of

sem - SWNTs. A ﬂ uorescence spectrum can only be obtained in the debundalized

state, because otherwise the presence of met - SWNTs will quench the electronic

excitation on adjacent sem - SWNTs in the bundle, which prevents luminescence.

This approach of direct ﬂ uorescence observation from individual SWNTs in

solution has great importance for the development of SWNT - based optical d

evices. Different surfactants [17] , synthetic polymers, and also biopolymers [3]

were each found to be excellent dispersing agents for SWNTs, especially

for making them disperse individually in aqueous media. A study was conducted

to identify the best dispersing agent for SWNTs by comparing various anionic,

cationic, and nonionic surfactants and polymers [17] . Each of these agents was

compared with respect to their ability to suspend individual SWNTs, and the

quality of the absorption and ﬂ uorescence spectra. For ionic surfactants, sodium

dodecylbenzene sulfonate ( SDBS ) was found to provide well - resolved spectral

features, and this correlated directly with the most favorable ionic surfactant for

debundalization. For the nonionic systems, surfactants with a higher molecular

mass could suspend more SWNT material and had more pronounced spectral

features.

718 23 Interactions of Carbon Nanotubes with Biomolecules: Advances and Challenges

23.4

Noncovalent Functionalization

Covalent functionalization weakens the strength of all interband transitions in

both the semiconducting and metallic SWNTs, because the saturated bonds func-

tion as defects. This perturbs the periodicity of the pseudo - one - dimensional ( 1 - D )

lattice and eventually destroys the electronic band structure altogether. However,

the largest effects of covalent functionalization occur in metallic SWNTs, because

the metallic state should be extremely sensitive to the occurrence of defects intro-

duced by saturation of the delocalized electronic structure. In particular, these

effects should be most strongly manifested in the transitions at E

F

because these

transitions are characteristic of a metal, which is a species without a gap in the

energy band spectrum.

On the other hand, the noncovalent functionalization of CNTs represents one

of the best alternatives for preserving the sp

2

- structure of nanotubes and, thus,

their electronic characteristics. The advantage is the possibility of attaching chemi-

cal functionalities to the surface, without disrupting the bonding network of the

Figure 23.1 Schematic diagram of the dispersion - dependent

energy transfer in nanotubes, and evidence of isolated

nanotubes by optical spectroscopy. (a) Model of energy -

transfer quenching in a matrix with bundled nanotubes;

(b) Model of ﬂ uorescent emission, if nanotubes are

individually isolated; (c) Absorbance and emission spectra of

individually isolated nanotubes in a protein solution.

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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