NANO
LETTERS
Hydrophobic Anchoring of
Monolayer-Protected Gold Nanoclusters
to Carbon Nanotubes
2003
Vol. 3, No. 3
279-282
A. V. Ellis,† K. Vijayamohanan,†,§ R. Goswami,† N. Chakrapani,†
L. S. Ramanathan,‡ P. M. Ajayan,† and G. Ramanath*,†
Materials Science and Engineering Department, and Chemistry Department,
Rensselaer Polytechnic Institute, Troy, New York 12180
Received October 1, 2002; Revised Manuscript Received January 2, 2003
ABSTRACT
Creating hybrid nanostructures consisting of disparate nanoscale blocks is of interest for exploring new types of quantum device architectures.
Here, we demonstrate the novel anchoring of monolayer-protected gold nanoclusters of 1−3 nm diameter to sidewalls of carbon nanotubes
(CNTs) via hydrophobic interactions between octanethiols capping the nanoclusters and acetone-activated CNT surfaces. Such molecularly
interlinked hybrid nanoblocks are attractive for building biocompatible nanodevices.
Carbon nanotubes have remarkable electronic properties
owing to their unique molecular structure and are attractive
building blocks for creating next generation electronic
devices and networks.1 Examples of nanotube-based devices
include single-electron transistors,2,3 molecular diodes,4-6
memory elements,7 and logic gates.8,9 There is a great deal
of interest in devising strategies to individually address each
molecular unit, and interconnect them, without adversely
affecting the local electronic structure.10,11 One approach to
enable this is to attach metal- or semiconductor nanoclusters
to nanotubes. This approach is also attractive for creating
molecular-level hybrid units (e.g., in this case, a quantum
dot attached to a molecular wire) and will allow the
exploration of new properties and effects that arise from
electronic-structure-level interactions between the constituent
molecular units and applications such as active nanodevices
and heterogeneous nanocatalysts.
Since carbon nanotubes are chemically inert, activating
their surfaces is an essential prerequisite for linking nanoclusters to them. Chemical treatments such as wet oxidation
in HNO312,13 can functionalize nanotube surfaces with anchor
groups such as hydroxyl (-OH), carboxyl (-COOH), and
carbonyl (>CdO)14 that are necessary to tether metal ions
to the tube. Recent studies have utilized covalent interactions
in these groups to attach nanoclusters to single-walled
nanotubes (SWNTs). For example, Lordi et al.12 used in situ
reduction of K2PtCl4 and ion exchange to tether Pt clusters
on nanotubes via carboxyl groups. More recently, Azamian
* Corresponding author. E-mail: Ramanath@rpi.edu.
† Materials Science and Engineering Department.
‡ Chemistry Department.
§ On sabbatical from National Chemical Laboratories, Pune, India.
10.1021/nl025824o CCC: $25.00
Published on Web 02/05/2003
© 2003 American Chemical Society
et al.13 attached Au nanoclusters to SWNTs via thioamides
obtained by converting carboxyl groups on SWNT surfaces.
Other strategies including electrostatic deposition and physisorption of nanoparticles of Au, Pt, and Ag onto acid-treated
multiwalled carbon nanotubes (MWNTs) have also been
reported.14-16 These nanoclusters, however, tend to rapidly
coalesce on the nanotube templates resulting in wire-like
structures consisting of large polycrystals.17 Attachment of
compound semiconductor nanoclusters (e.g., CdSe)18,19 via
covalent linkages, and anchoring of metal complexes20,21 to
oxidized nanotube surfaces have also been reported.
Here we report a unique approach to connect monolayerprotected gold nanoclusters to non-oxidized carbon nanotubes
by means of relatively weak (e.g., ∼0.1 eV) hydrophobic,
rather than covalent, interactions. The anchorage is provided
by interdigitation of alkyl chains of self-assembled molecular
layers (SAMs) capping the nanoclusters and molecular
moieties adsorbed on nanotubes. Since the structure and
chemistry of SAMs are similar to many biomolecules such
as proteins, cluster attachment through SAMs will enable
the design and creation of new bio-inspired hybrid nanodevices. For example, creating metal-SAM-nanotube units
will open up possibilities for replicating quantum effects such
as single-electron hopping and coulomb blockade in nanotubes and nanoclusters functionalized with proteins. Moreover, it is conceivable to modulate such effects by adjusting
SAM chain length and structure, enabling molecular-level
design of nanodevices for switching, sensing, and information
storage.
MWNTs synthesized by the arc-discharge method were
treated with acetone in an ultrasonic bath for 1 h and dried
Figure 1. UV-visible absorption spectra of OT-capped Au
nanoclusters in toluene before (squares) and after (circles) attachment onto acetone-treated MWNTs. Spectra from acetone-treated
MWNTs (triangles) and pristine MWNTs (diamonds)-identical
before and after Au-OT treatment-are shown for reference.
in air for 15 h to remove the free acetone. These activated
tubes were dispersed in toluene by agitation in an ultrasonic
bath, prior to being mixed with OT-capped gold nanoclusters.
The OT-capped nanoclusters were prepared following the
Brust method,22,23 but without the use of phase transfer
agents. A 2 mM auric chloride solution in deionized water
was mixed with 5 mM OT in toluene in a 1:2 ratio and stirred
vigorously for 30 min. This two-phase mixture was reduced
by slowly adding a 0.1 M aqueous NaBH4. Stirring was
continued until all the metallic ions were completely
converted to OT-capped clusters, which migrate to the
toluene layer. The two layers were then isolated in a
separating funnel, and the resulting toluene layer was washed
five times with deionized water and dried in a N2 atmosphere.
To attach gold nanoclusters to MWNT surfaces, the dried
OT-capped nanoclusters were added to acetone-treated
MWNTs dissolved in toluene. The results reported here,
unless mentioned otherwise, were those obtained after
removing unattached nanoclusters by a 5-min agitation in
an ultrasonic bath, followed by decanting the supernatant
liquid, and subsequent washing six times with toluene. The
excess toluene was removed by drying in air for 12 h.
MWNTs-with and without Au attachment-dispersed in
a toluene medium were transferred to a carbon-coated Cu
grid for high-resolution transmission electron microscopy
(HRTEM) measurements in a JEOL-2010 TEM operated at
200 kV. Transmission-mode Fourier transform infrared
(FTIR) spectroscopy was carried out in a Bio-Rad Excalibur
series FTS 3000MX spectrophotometer. The samples for
FTIR consisted of 150 mg KBr disks containing 0.1 mg of
each dried sample. The results reported here were obtained
from 200 scans at a 4 cm-1 resolution, and were verified at
least four times with different nanocluster samples prepared
and stored under identical conditions.
Figure 1 shows UV-visible absorption spectra of OTcapped gold nanoclusters before and after attachment onto
280
acetone-treated carbon nanotubes. The Au-OT clusters
exhibit a strong surface plasmon peak at ∼400 nm, in
contrast with the well-known 520 nm peak characteristic of
>3 nm nanoclusters.24-26 We attribute this result to surface
plasmon excitation from ∼1-3 nm-diameter gold nanoclusters consisting of AuCl3 complexes and/or 3-13 Au atoms,
consistent with previous reports.27,28 The small peak at ∼310
nm corresponds to the ligand-to-metal charge-transfer absorption band of AuCl4-.29 Both these peaks remain, albeit
at a decreased intensity, in acetone-treated MWNTs mixed
with OT-capped Au nanoclusters, even after repeated
(6 times) washing and vigorous ultrasonication. This result
indicates that the gold nanoclusters are attached to the
nanotubes.
The retention of the optical absorption signatures of the
nanoclusters suggests that cluster attachment does not have
an adverse influence on the optical properties of the
nanoclusters due to phenomena such as coalescence. We note
that Au-OT nanoclusters do not attach to MWNTs that are
not treated with acetone; the nanoclusters get removed during
repeated washing in toluene. This is indicated by the absence
of surface plasmon peaks in UV-visible spectra obtained
from mixtures of Au-OT and pristine MWNTs. Thus,
acetone treatment is essential to enable the connection of
nanoclusters to nanotubes in our experiments.
Figure 2b is a representative TEM micrograph of a MWNT
with 1-3 nm nanoclusters, indicated by spheres of dark
contrast, attached to it. A micrograph of a nanotube prior to
cluster attachment is also shown in Figure 2a for comparison.
Many nanoclusters exhibit Moiré contrast arising from the
superimposition of the gold lattice planes with that of MWNT
shells, indicating that nanoclusters are either on, or close to,
the nanotube. An example of such contrast is seen in the
nanocluster marked by the arrow in Figure 2c. The gold
nanoclusters have an fcc structure, indicated by the spacing
of {111} lattice planes measured from HRTEM micrographs
(e.g., see Figure 2d), and that of (111) and (200) reflections
seen in electron diffraction patterns (see Figure 2e). In many
instances, we also observe assemblies of nanoclusters attached to the nanotubes, as shown in Figure 2f. The
intercluster spacings, measured by constructing Voronoi30
cells around each nanocluster, have discrete values with a
high frequency between ∼1.1-1.5 nm (see Figure 2g). This
spacing range correlates well with 1.5 to 2 times the length
of OT (∼0.7 nm), suggesting interdigitation between OT
molecules that cap adjacent clusters. Note that the lowest
intercluster spacing of ∼0.8 nm corresponds to the highest
degree of interdigitation between OT chains. Spacings higher
than 1.5 nm most likely arise from clusters belonging to
adjacent subassemblies that are not linked to each other.
To determine the mechanism by which the Au clusters
were attached to the nanotubes, we carried out transmissionmode FTIR measurements. Our results, shown in Figure 3
and Table 1 and described below, indicate that the nanoclusters are attached to nanotubes via hydrophobic interactions27 between CH3 groups in acetone (on MWNTs) and
the alkyl chains in OT molecules (on the nanoclusters).
Nano Lett., Vol. 3, No. 3, 2003
Figure 3. FTIR spectra of MWNTs (a) before and (b) after acetone
activation; and (c) OT-capped gold nanoclusters attached to acetonetreated MWNTs. (d) Reference spectrum from OT-capped Au
clusters before attachment on to the MWNTs. The dashed lines
show the positions of the asymmetric and symmetric modes of CH3
and CH2.
Figure 2. Representative HRTEM micrographs showing (a) lowmagnification of a pristine nanotube, (b) low-magnification of Au
nanoclusters attached on a nanotube, and (c) high-magnification
views of a 10-nm-diameter MWNT with Au nanoclusters attached
to it. (d) HRTEM image of a gold nanocluster showing {111} lattice
fringes. (e) Diffraction pattern showing (111) and (200) reflections
from fcc Au and (0002) reflections from MWNTs. (f) Example
HRTEM micrograph showing nanocluster assemblies attached to
a MWNT. Arrows indicate the tube walls. The lines drawn in (f)
illustrate the scheme used to measure intercluster spacings between
nearest neighbor nanoclusters, for two example nanoclusters. (g)
Histogram of spacings between nearest-neighbor nanoclusters.
Both pristine and acetone-activated MWNTs, prior to
cluster attachment, show no observable absorption signatures
corresponding to carbonyl groups >CdO (e.g., at 1750
cm-1). However, C-O stretch signatures expected at ∼1150
cm-1 could not be delineated due to peaks from CH wagging
vibrations in the 1350-1150 cm-1 range. Acetone treatment
increases the asymmetric CH3 stretching peak intensity a
2956 cm-1, indicating adsorption. This is also seen in Table
1, which lists the relative changes in CH3/CH2 intensity ratio
for MWNTs before and after acetone treatment, and for Au
clusters before and after attachment.
Spectra from the nanoclusters prior to attachment (see
spectrum d in Figure 3) show absorption peaks at 2871 and
2956 cm-1 corresponding to unhindered symmetric and
asymmetric stretching, respectively, of terminal CH3 groups
in the OT molecular caps. This indicates a high degree of
flexibility in the OT molecules that cap the nanoclusters, as
expected, and is corroborated by a series of discrete
equidistant twisting-rocking and wagging progression CH2
modes between 1150 and 1305 cm-1 (not shown).
Nano Lett., Vol. 3, No. 3, 2003
Table 1. Ratios of Asymmetric CH3 (2956 cm-1) to
Asymmetric CH2 (2926 cm-1) Stretching Vibrations for
MWNTs and OT-Capped Au Nanoclusters before and after
Various Treatments or Attachment
sample
pristine MWNTs
acetone-treated MWNTs
Au-OT attached to
acetone-activated MWNTs
pristine OT-capped Au nanoclusters
CH3/CH2 asymmetric
stretching peak ratio
0.5
0.63
0.30
0.87
Upon attaching nanoclusters to MWNTs both symmetric
and asymmetric CH3 stretching modes diminish by more than
25% (compare c and d in Figure 3, and see Table 1).
Furthermore, the CH3 to CH2 ratio for the asymmetric stretch
decreases by ∼52 and 66%, when compared with that of
acetone-treated tubes and unattached OT-capped nanoclusters, respectively (compare c and b in Figure 3, and see Table
1). We attribute both these results to the decreased flexibility
of OT molecules on the nanoclusters, and acetone molecules
on MWNTs, due to hydrophobic interchain interactions
between CH3 groups of acetone and alkyl chains of OT. This
constrained environment that enables nanocluster attachment
is consistent with the increased gauche defect concentration
indicated by the broadening, and decreased intensity of the
CH2 twisting rocking mode peaks, described earlier.
Based upon the above, we propose that OT-capped gold
nanoclusters are anchored to acetone-activated MWNTs by
attractive interactions resulting from interdigitation of the
alkyl chains of OT and the methyl termini of the acetone.
Figure 4 is a schematic sketch that illustrates the proposed
mode of attachment of OT-capped Au nanoclusters on
281
Figure 4. Schematic sketch (not to scale) illustrating the proposed
mechanism of attachment of OT-capped Au nanoclusters to acetoneactivated MWNTs. Interdigitation between OT molecules of
adjacent nanoclusters is also shown. Dashed lines in the acetone
molecules illustrate that acetone is not attached to nanotubes by
CdO type bonds.
acetone-activated MWNTs. The absence of CdO bonds in
acetone-treated nanotubes suggests that acetone is adsorbed
to the nanotube walls via C-O-C type bonds or resonance
structures, in configurations where the terminal methyl
groups point away from the nanotube surfaces. This view is
supported by ab initio calculations based on density functional theory showing that acetone attaches preferentially at
defects such as vacancies on nanotube sidewalls, via ∼0.903.3 eV interactions resulting from C(acetone)-O-C(nanotube) bonds, rather than CdO bonds, on nanotube surfaces.31
Preferred adsorption of acetone and cluster attachment at
defects are consistent with previous works32 which suggest
that ultrasonication creates defects on nanotube surfaces,
thereby facilitating functionalization.
In summary, we have shown that SAM-protected gold
clusters can be attached to acetone-activated carbon nanotubes through hydrophobic interactions. Such molecular
interactions are compatible with many biological systems and
will be important for exploring and creating a rich variety
of molecular nanostructures for device applications.
Acknowledgment. We gratefully acknowledge funding
from a seed grant from NSF- Nanoscale Science and
Engineering Center at RPI (G.R. and AE), a gift grant from
Philip Morris, USA (G.R., K.V. and PA), and G.R.’s NSFCAREER award DMR 994478.
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NL025824O
Nano Lett., Vol. 3, No. 3, 2003