Formation of efficient catalytic silver nanoparticles on carbon nanotubes by adenine functionalization

Article abstract of DOI:10.1002/anie.201102976

Stuck together: Adenine/carbon nanotube hybrids trigger the formation of controlled-size catalytic silver nanoparticles on the nanotube surface (see picture). The catalytic efficiency of the resulting species was assessed in the oxidation of 2-methylhydroquinone to its corresponding benzoquinone, with complete recovery and without loss of activity of the catalyst. Copyright

Full text of DOI:10.1002/anie.201102976

DOI: 10.1002/anie.201102976
Nanotubes
Formation of Efficient Catalytic Silver Nanoparticles on Carbon
Nanotubes by Adenine Functionalization**
Prabhpreet Singh, Giuseppe Lamanna, Cꢀcilia Mꢀnard-Moyon, Francesca Maria Toma,
Elena Magnano, Federica Bondino, Maurizio Prato, Sandeep Verma,* and Alberto Bianco*
Carbon nanotubes (CNTs) are currently receiving enormous
attention as advanced nanomaterials, mainly because of their
conductive, thermal, mechanical, and chemical properties.[1]
CNTs can be employed in composites, transistors, or sophis-
ticated electrodes, to mention just a few applications.[2,3] In
addition, because of their nanoscale dimensions and high
aspect ratio, CNTs have a huge surface area, which may be
exploited to achieve efficient catalytic processes.[4,5] However,
the difficult handling and the lack of complete control over
the CNTs inhibit their use on a large scale. In this respect,
extraordinary progress has been achieved in the field of
chemical functionalization of CNTs.[6,7] The modification of
the CNT surface by using a wide variety of chemical
approaches, and the combination of the different physico-
chemical properties have allowed the fast development of
CNTs as a support for growing catalytic metal nanoparti-
cles.[8–10]
endowed with a high surface area allow high efficiency of
catalyzed chemical reactions.[13] To prepare metal nanopar-
ticle/CNT hybrids, several approaches have been explored,[14]
including the reduction of metal salts,[15,16] the spontaneous
reduction of gold and platinum nanoparticles on nano-
tubes,[17,18] an electrochemical route to reduce metal
ions,[19,20] a chemical deposition method,[21] and the thermal
decomposition of metal salts.[22,23] Recently, N-doped multi-
walled CNTs (MWCNTs) have been used to anchor silver
nanoparticles.[24] However, appropriate CNT functionaliza-
tion to tailor catalytic molecule deposition has not yet been
explored, though it can become an attractive tool.
Herein, we describe a simple methodology to template the
growth of silver nanoparticles (NPs) on the surface of
MWCNTs, and the use of the resulting hybrids as catalyst
for the oxidation reaction of 2-methylhydroquinone to 2-
methylbenzoquinone. Benzoquinones are an important class
of molecules with antioxidant, anti-inflammatory, and anti-
cancer activities.[25] We have chosen the transformation of
hydroquinones as a model reaction to prove the efficiency of
the new CNT/NP hybrids. Indeed, heterogeneous catalysis to
produce quinones with high recovery yield and recycling
efficiency has not been extensively explored. We demonstrate
that it is possible to tailor the coordination of Ag NPs onto a
MWCNT surface by covalent functionalization of oxidized
MWCNTs with a modified adenine moiety. Indeed, the
presence of the adenine functional groups on MWCNTs
induces the formation of Ag NPs through metal/adenine
coordination. MWCNT/Ag complexes are efficient and
reusable heterogeneous catalysts for the oxidation of hydro-
quinones.
In the process of designing new CNT/NP hybrids, we
focused our attention on using functional groups that favor
the growth of NPs directly at the surface of functionalized
CNTs. Adenine, a purine nucleobase, is endowed with
suitable metal ion binding sites. Indeed, the exocyclic amino
group at C6 and the four imino groups at the N1, N3, N7, and
N9 positions have been exploited to build interesting
supramolecular structures as well as to support metal-assisted
catalytic transformations.[26] Based on these characteristics of
adenine, we covalently functionalized MWCNTs with the
nucleobase to decorate the surface of the nanotubes with
silver nanoparticles. We generated three different types of
functionalized CNTs from oxidized MWCNTs (ox-MWCNT;
Figure 1A) to probe the key role played by adenine. Adenine-
MWCNTs (Ad-MWCNT) 1a were prepared by activation of
ox-MWCNT by forming the corresponding acyl chloride, and
subsequent reaction with a monoprotected diaminotriethy-
lene glycol (TEG) spacer. After deprotection, 3-(9-adeni-
As catalysis represents a fundamental tool for improving
the efficiency of many processes, the design of novel building
blocks and nanoparticles will contribute to the development
of advanced catalysts.[11,12] In particular, solid catalysts
[*] Dr. P. Singh,[+] Dr. G. Lamanna,[+] Dr. C. Mꢀnard-Moyon,
Dr. A. Bianco
CNRS, Institut de Biologie Molꢀculaire et Cellulaire
Laboratoire d’Immunologie et Chimie Thꢀrapeutiques
67000 Strasbourg (France)
E-mail: a.bianco@ibmc-cnrs.unistra.fr
Dr. F. M. Toma, Prof. M. Prato
Dipartimento di Scienze Farmaceutiche, Universitꢁ di Trieste
34127 Trieste (Italy)
Dr. E. Magnano, Dr. F. Bondino
IOM CNR, Laboratorio TASC
34149 Trieste (Italy)
Prof. S. Verma
Department of Chemistry, Indian Institute of Technology-Kanpur
Kanpur-208016 UP (India)
E-mail: sverma@iitk.ac.in
[+] These authors contributed equally to this work.
[**] This work was supported by the French-Indian CEFIPRA/IFCPAR
collaborative project (project no. 3705-2). P.S. wishes to thank
CEFIPRA/IFCPAR for a postdoctoral fellowship. TEM images were
recorded at the RIO Microscopy Facility Platform of Esplanade
Campus (Strasbourg, France). HRTEM and ED were recorded at the
IPCMS facility in Cronenbourg (France). We wish to thank Dris
Ihiawakrim for helpful discussions, and Loredana Casalis and Pietro
Parisse for help on the preparation of XPS samples. G.L. wishes to
thank the European Union (ANTICARB program: HEALTH-2007-
201587) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102976.
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Communications
properties of CNTs (for comparison, see the TEM image of
ox-MWCNT in Figure S4 in the Supporting Information).
MWCNT/Ag nanohybrids were also analyzed by TEM
(Figure 2A). We observed a homogenous distribution of Ag
NPs over the surface of the nanotubes. An average size of
Figure 1. Chemical structures and TEM characterization of the different
adenine-functionalized carbon nanotubes. A) The three Ad-MWCNTs
1a, 1b, and 1c contain different spacers between the tubes and the
adenine moiety. B) TEM images of Ad-MWCNTs a) 1a, b) 1b, and
c) 1c allow observation of the morphology of the different Ad-
MWCNTs. Scale bars=100 nm.
nyl)propionic acid was coupled to the nanotubes. Similarly,
Ad-MWCNTs 1b and 1c were prepared by the same
activation reaction of ox-MWCNT and functionalization
with
6-amino-N-(2-(6-amino-9H-purin-9-yl)ethyl)hexan-
amide and 9-(2-aminoethyl)adenine, respectively (see the
Supporting Information). These three derivatives contain
spacers that, in principle, may modulate the solubility in
different solvents (i.e., hydrophilic triethylene glycol versus
hydrophobic hexanoic linker). In addition, by changing the
distance of adenine from the CNT surface, electron transfer
and catalytic processes could be accurately controlled. In this
context, the deposition of Ag NPs onto the surface of CNTs
was achieved by simply mixing a methanolic solution of
AgNO3 to the three different Ad-MWCNTs, so that Ag NPs
spontaneously grew on the nanotube surface.
The degree of functionalization of the oxidized MWCNTs
and the adenine-modified conjugates (1a, 1b, and 1c) were
initially evaluated by thermogravimetric analysis (TGA)
under a N2 atmosphere (see Figure S2 in the Supporting
Information). The TGA curves of Ad-MWCNTs 1a, 1b, and
1c show a weight loss of about 23.0%, 21.7%, and 20.0% at
5008C, respectively, as compared to 11.7% for ox-MWCNT.
On this basis, we estimated that the amount of functional
groups per gram (fw) of Ad-MWCNTs 1a, 1b, and 1c was
0.33, 0.34, and 0.48 mmolgÀ1, respectively, which correspond
to one adenine molecule every 222, 220, and 164 carbon
atoms, respectively.
Figure 2. Characterization of the silver/CNT nanohybrids. A) TEM
images of Ad-MWCNT hybrids a) 1a, b) 1b, and c) 1c complexed with
Ag NPs. Scale bars=100 nm. B) Ag 3d5/2 XPS spectra. a) Ad-MWCNT
1a (black line) and metallic silver (dark-gray line). The integrated
background was subtracted and the two spectra were normalized to
their maximum intensity. b) Ad-MWCNT 1a (black circles). The results
of the fitting procedure are reported in solid lines. The best fit of this
spectrum was obtained by using two components, a main component
(dark-gray line) centered at 368.2 eV BE (Gaussian width=0.4 eV,
Lorentzian width=0.27 eV, asymmetry=0.067) and a second compo-
nent (light-gray line) shifted 0.4 eV toward a lower binding energy
(Gaussian width=0.4 eV, Lorentzian width=0.27 eV, asymmetry=0).
C) HRTEM and ED patterns of a) Ag0 and b) Ag2O NPs on Ad-
MWCNT 1a and corresponding fast Fourier transformations. Scale
bars=5 nm.
about (22.6 Æ 0.5) nm (mean Æ SEM, n = 325) could be calcu-
lated from the statistical analysis of the NPs on Ad-MWCNT
1a (see Figure S3 in the Supporting Information). Similar
dimensions were also found when the other adenine-modified
nanotubes 1b and 1c were used, thus demonstrating that the
spacer between the tubes and the nucleobase does not
influence the NP diameter. In a control experiment, after
mixing the AgNO3 solution with ox-MWCNT, TEM images
The functionalized CNTs were then characterized by
transmission electron microscopy (TEM). The TEM images
of Ad-MWCNTs 1a, 1b, and 1c show rather isolated nano-
tubes with the presence of very small aggregates when
deposited from dispersions in 1:1 methanol/H2O (Figure 1B).
The CNTs appear debundled, thus confirming the important
role of adenine on the modulation of the dispersibility
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showed only physical adsorption of big clusters of silver on
the bundles of nanotubes (see Figure S4 in the Supporting
Information).[27]
parameters of 3.08 ꢀ and 4.94 ꢀ. The most intense electron
diffraction ring corresponds to the plane (101) with a spacing
of 2.33 ꢀ. Within the different NPs analyzed, we could see a
few silver oxide NPs composed of AgII and some amorphous
particles. From the detailed analysis of the HRTEM micro-
graphs and electron diffraction we can conclude that most of
the nanoparticles complexed to Ad-MWCNT 1a were
crystalline. The oxidation state of silver is variable as the
nanoparticles are either silver metal, silver(I) oxide, or
silver(II) oxide, with the majority of the NPs composed of
silver metal.
We analyzed the composition of these Ag NPs by using a
combination of energy dispersive X-ray (EDX), X-ray photo-
electron spectroscopy (XPS), and electron diffraction (ED)
techniques. The EDX spectrum of NPs clearly shows the peak
that corresponds to silver (see Figure S5 in the Supporting
Information). The analysis of several particles also allowed us
to observe the presence of peaks associated with oxygen.
EDX provides information about the chemical composition
of a sample, and only confirmed the presence of silver without
the possibility to assess its valence. Therefore, we extended
the characterization by using XPS in order to evaluate the
different oxidation states of silver. Ag 3d5/2 spectra measured
on a clean polycrystalline silver foil and on Ad-MWCNT 1a
are shown in Figure 2B (left-hand panel). For a better
comparison, an integrated background was subtracted and
the two spectra were normalized to their maximum intensity.
An evident energy shift of the Ad-MWCNT 1a/Ag 3d5/2 core
level toward a lower binding energy (BE) with respect to the
BE of metallic silver (368.2 eV), and a broadening of the peak
were observed. A fit procedure of the Ag 3d5/2 spectra
To evaluate the potential of these new silver NPs
coordinated to CNTs in heterogeneous catalysis, we inves-
tigated the catalytic activity of the Ag NPs in the oxidative
reaction of hydroquinones to benzoquinones (Figure 3A).
(Figure 2B, right-hand panel), carried out by using
a
Doniach–Sunjic line shape convoluted with a Gaussian
function, shows the simultaneous presence of two states of
silver oxidation, at BE values of (368.2 Æ 0.1) eVand (367.8 Æ
0.1) eV, which are ascribed to Ag0 and AgI, respectively. The
energy position of the first component corresponds to that of
the measured silver foil and shows an asymmetric line shape
typical of a metallic system. The assignment of the second
component to AgI ions is in agreement with the reported
data.[28] From this analysis, Ag0 represents the main contribu-
tion to the Ag 3d5/2 peak, a result that is consistent with the
results obtained by using ED (see below). By measuring an
XPS spectrum in a wide BE range (see Figures S6 and S7 in
the Supporting Information), a quantitative estimate of the
total amount of silver at the surface probing depth of around
1.5 nm could be obtained;[29] the amount of silver was about
1.3% of the amount of carbon. However, we have to point out
that this value is an underestimation of the total amount of Ag
NPs as assessed by TGA (see below).
Finally, to have a complete overview of the NP character-
ization, we used high-resolution TEM (HRTEM) coupled
with ED on selected areas to elucidate the structure of the
silver NPs complexed to Ad-MWCNT 1a at atomic resolu-
tion. We analyzed several NPs and found that most of them
have preferentially aligned crystalline planes. Figure 2C
shows a representative HRTEM image and the corresponding
ED patterns of two Ag NPs, and gives information on their
phase structure. In accordance with XPS, we found that most
of the NPs were constituted of Ag0. Figure 2C(a) indicates
that the NP is constituted of Ag0 with a face-centered cubic
structure and an interplane distance of 2.37 ꢀ, thus corre-
sponding to the most intense plane (111). This result is not
surprising as the conditions to generate the NPs allow the
reduction of AgI to Ag0.[30,31] However, we also observed
silver oxide NPs. The electron diffraction pattern shown in
Figure 2C(b) is consistent with hexagonal Ag2O with mesh
Figure 3. Catalytic reaction using silver/CNT nanohybrids. A) Oxidation
reaction of 2-methylhydroquinone to 2-methylbenzoquinone by Ag NPs
complexed to Ad-MWCNT 1a in the presence of hydrogen peroxide
under sonication. B) Percentages of conversion following three cycles.
R=reagent; P=product.
This reaction was selected as a model for testing the
MWCNT/Ag hybrids. Quinones can be obtained by using
various oxidants, but most of these reagents produce large
amounts of harmful waste, while others are not practical for
large-scale preparation because of their high cost.[25,32] The
development of recyclable solid-supported catalysts with low
environmental impact is becoming highly important.[33,34]
As a proof-of-concept, we explored the catalytic activity
of silver NPs complexed to Ad-MWCNT 1a on the oxidation
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Table 1: Catalytic efficiency of the MWCNT/Ag hybrid in comparison to
the control systems.
of 2-methylhydroquinone to 2-methylbenzoquinone. We
initially assessed the amount of Ag NPs on the nanohybrids
by using TGA in air. We measured a quantity of Ag NPs that
corresponded to 3% of the total amount of the nanohybrids
(see Figure S10 in the Supporting Information). This value
was used for the calculation of the amount of NPs added to
the catalytic reaction. To obtain the reference product, we
began with the oxidation of 2-methylhydroquinone in meth-
anol with 35% aqueous H2O2 in the presence of commercially
available Ag2O NPs, as previously reported.[35] The evolution
of the model reagent 2-methylhydroquinone to 2-methylben-
zoquinone could be easily followed by HPLC analysis. Since
most of the NPs on the nanotubes were constituted of metallic
silver, we also evaluated the efficacy of commercially
available Ag0 NPs of sizes below 100 nm. Indeed, both
silver oxide and metallic silver catalyzed the transformation
of the hydroquinone, although not to completion (see the
Supporting Information).
These control experiments allowed to rationalize the
heterogeneous catalytic reaction with our MWCNT/Ag NP
hybrids. The oxidation of 2-methylhydroquinone was carried
out in methanol in the presence of 35% aqueous H2O2 and
0.03 equivalents of NPs on Ad-MWCNT 1a. The resulting
reaction mixture was sonicated at room temperature to
maintain a good dispersion of the nanotubes at all stages of
the catalytic cycle. After 4 h, we observed the complete
disappearance of the starting material and the formation of
the desired 2-methylbenzoquinone (see Figure 3B, and Fig-
ure S11 in the Supporting Information). The catalytic oxida-
tion was completely selective, as we did not observe the
production of any substituted biaryl because of the compet-
itive oxidative homocoupling of 2-methylhydroquinone.[35,36]
At the end of the reaction, we easily recovered the nanotubes
by simple centrifugation and without any further purification.
The collected hybrids were reused under the same reaction
conditions for two additional cycles to give similar excellent
conversion (Figure 3B). After the third cycle, the NPs were
still present as assessed by TEM (data not shown). We also
confirmed the oxidation state of Ag NPs after the three
catalytic cycles by using XPS. We did not observe significant
variations on the composition of the nanoparticles (see
Figure S12 in the Supporting Information). In addition, as
an alternative control to assess the key role of adenine as a
ligand for the NP coordination, we prepared a hybrid by using
the nanotubes with only the ammonium groups (f-MWCNT 9,
precursors of Ad-MWCNT 1a; see Scheme S2 in the Sup-
porting Information). In this case, we could generate Ag NPs
on CNTs with a wider diameter distribution (see Figure S13 in
the Supporting Information). We then tested the catalytic
capacity of this complex on the oxidation of 2-methylhydro-
quinone without obtaining the desired product (see Fig-
ure S14 in the Supporting Information). After obtaining this
result, we observed the complex by TEM (results not shown),
and surprisingly, we did not find any NP on the grid, thus
confirming the important role of adenine in comparison to
primary amines for stabilizing Ag NPs on the tubes. To
compare the catalytic efficacy of the MWCNT/Ag nano-
hybrid with silver oxide and silver nanoparticles, we calcu-
lated the turnover number (TON) and turnover frequency
Catalyst
Turnover number
Turnover frequency
silver(I) oxide
silver(0) NPs
Ag/MWCNT 1a
10.3
26.3
33.3
2.6 hÀ1
6.6 hÀ1
8.3 hÀ1
(TOF) for all three types of catalysts. Table 1 shows the data
for the different systems.
The catalyst based on CNTs is clearly more efficient in the
conversion of the hydroquinone to benzoquinone than Ag2O
and Ag NPs alone. As shown in Table 1, the heterogeneous
MWCNT/Ag catalysts feature the characteristics that are
necessary for the reactions to run at higher rate in comparison
to the silver oxide or metal nanoparticles. Moreover, the
hybrid material can be easily recovered by simple filtration
and can thus be immediately recycled. CNTs can be
considered an efficient catalyst support because of their
high specific surface area and chemical inertness. Our
synthetic protocol proves that CNTs might become pliant
materials by tuning their functionalization to promote the
assembly of complex systems; this method is useful for the
controlled and cost-effective design of new selective catalysts
and for the production of advanced devices based on CNTs.
In conclusion, we have demonstrated the possibility to
shape CNT functionalization in order to induce catalytic
nanoparticle deposition. Adenine that is covalently bound to
the CNT surface is able to trigger the formation of catalytic
NPs with controlled sizes. This result provides the possibility
for the tailored design of novel catalytic materials based on
CNTs with full exploitation of their properties. Moreover, we
have offered a proof-of-concept protocol by using this novel
material as heterogeneous catalyst to promote the oxidation
of hydroquinones, and by proving the catalytic efficiency of
the nanohybrids on this model reaction and the complete
recycling without loss of activity over multiple cycles.
Experimental Section
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 mm, respectively). Details of the
synthesis of the ox-MWCNTs and the three Ad-MWCNTs 1a, 1b,
and 1c are reported in the Supporting Information.
Preparation of Ag NPs on Ad-MWCNTs 1a, 1b, and 1c: 0.1 mg
of the Ad-MWCNTs 1a, 1b, 1c, ox-MWCNT, and f-MWCNT 9 (used
as controls) were dispersed in methanol/water (1:1, 1 mL) by
sonication for 5 min. A solution of AgNO3 (1 or 5 equiv with respect
to the adenine loading) in methanol/water (1:1, 1 mL) was prepared
and added to the different functionalized carbon nanotubes. The
resulting dispersions of 1a, 1b, 1c, ox-MWCNT and f-MWCNT 9
were set aside for 10–12 h. After this period, some CNTs precipitated.
Catalysis: CNT-supported silver nanoparticles (1.00 mg of CNTs
contain 0.0003 mmol of silver) and 35% aqueous hydrogen peroxide
(4.0 mL, 0.05 mmol) were added to a solution of 2-methylhydroqui-
none (1.25 mg, 0.01 mmol) in methanol (0.50 mL). The resulting
suspension was sonicated at room temperature until complete
consumption of starting material and formation of 2-methylbenzo-
quinone was shown by HPLC. This analysis showed a selectivity of
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Angew. Chem. Int. Ed. 2011, 50, 9893 –9897

> 99% and a conversion of > 99%. The silver/carbon nanotubes were
recovered by centrifugation and recycled for successive catalytic
cycles.
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Keywords: adenine · heterogeneous catalysis · nanoparticles ·
nanotubes · supramolecular chemistry
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