Synthesis and Catalytic Applications of Multi-Walled Carbon Nanotube–Polyamidoamine Dendrimer Hybrids

Article abstract of DOI:10.1002/chem.201802301

Polyamidoamine (PAMAM) dendrimers were covalently immobilized on multi-walled carbon nanotubes (MWNT) by two “grafting to” strategies. We demonstrate the existence of non-covalent interactions between the two components but outline the superiority of our

Full text of DOI:10.1002/chem.201802301

A Journal of
Accepted Article
Title: Synthesis and Catalytic Applications of Multi-Walled Carbon
Nanotube-Polyamidoamine Dendrimer Hybrids
Authors: Antonin Desmecht, Timothy Steenhaut, Florence Pennetreau,
Sophie Hermans, and Olivier Riant
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Synthesis and Catalytic Applications of Multi-Walled Carbon
Nanotube-Polyamidoamine Dendrimer Hybrids
Antonin Desmecht, Timothy Steenhaut, Florence Pennetreau, Sophie Hermans* and Olivier Riant*[a]
We dedicate this work to the memory of Pr. István E. Markó (1956-2017).
Abstract: Polyamidoamine (PAMAM) dendrimers were covalently
immobilized on multi-walled carbon nanotubes (MWNT) via two
‘grafting to’ strategies. We demonstrate the existence of non-covalent
interactions between the two components but outline the superiority
of our two grafting approaches, namely xanthate and click chemistry.
MWNT surfaces were functionalized with activated ester and
propargylic moieties prior to their reaction with PAMAM or azido-
PAMAM dendrimers, respectively. The grafting of PAMAM
dendrimeric architectures like polyamidoamines (PAMAM)
dendrimers display high solubility, reactive peripheral amino
groups and unlike most polymers, are accurately defined.[22]
These structures offer for instance precise spots for catalytic sites,
i.e. in the dendrimer core or at its terminations.[23] Those locations
are able to host either metallic nanoparticles (NPs) or
organometallic complexes for homogeneous-like catalytic
applications.[23–29] Dendrimers can also fill in
a role of
generations
0
to
3
was evaluated with X-ray photoelectron
encapsulating vectors in the field of nanomedicine and are
intensively studied in drug delivery systems.[26,30–38]
spectroscopy (XPS), thermogravimetric analysis (TGA) and
transmission electron microscopy (TEM). The versatility of our hybrids
was demonstrated by post-functionalization sequences involving
copper alkyne-azide cycloaddition (CuAAC). We synthesized
homogeneous supported iridium complexes at the extremities of the
dendrimers. In addition, our materials were used as template for the
encapsulation of Pd nanoparticles (NP), validating our
nanocomposites for catalytic applications. The palladium-based
catalyst was active for carbonylative coupling during 5 consecutive
runs without loss of activity.
The combination of NCs and PAMAM gives various advantages.
First of all, dendrimers significantly increase the dispersibility of
the carbonaceous material and therefore favor its handling and
processability. Secondly, dendrimers face the usual drawbacks of
homogeneous catalytic systems. They are tedious to recover and
recycle. This is especially true for small size dendrimers. However,
once grafted on NCs, dendrimeric systems are easily separable
from the medium and can therefore be recovered and used again.
Thirdly, the use of carbon nanotubes (CNTs) in nanomedicine is
still discussed as their toxicity is yet to be fully understood. It is
commonly accepted that pristine CNTs (p-CNTs) are cytotoxic
and cause inflammation to human organs. However, the
modification of nanotubes surfaces alters their behavior towards
biological systems and reduces their toxicity.[39–41] Grafting
dendrimers on CNTs surfaces enables such phenomenon.[33,34] In
addition, dendrimer’s anchoring is highly valuable as the amount
of surface reactive sites is therefore considerably augmented
depending on the size of the dendrimer.
Introduction
Throughout the last decades, nanocarbon-based hybrids piqued
scientific community’s curiosity.[1–7] This interest can easily be
explained by the fact that each component of the material is able
to bring up peculiar properties and, together, form a composite
with a combination of unique functionalities. By extension,
composites can even exhibit new properties resulting from the
synergy of their constituents. In this context, nanohybrids
composed of nanocarbons (NCs) and dendrimeric structures
have proven to be of great interest.[8–21] NCs such as single-/multi-
walled carbon nanotubes (S-/MWNTs) and graphene (G) have
very high physical and chemical stabilities, exceptional thermic
and electric conductivities and relatively high surface areas.
However, NCs are prompt to stack together through - stacking.
The resulting aggregates show very low solubility or dispersibility
in most organic solvents and especially in water. On the opposite,
Numerous examples of nanohybrids CNT-PAMAM have already
been studied but few propose
a
systematic study of
functionalization efficiency, especially when comparing covalent
with non-covalent approaches. Two main strategies are
commonly used for the immobilization of dendrimers on CNTs,
namely the ‘grafting from’ and ‘grafting to’ approaches. In the first
case, the dendrimers are grown sequentially in situ on the
surface.[14,34,42–56] The main drawback of this methodology is that
the conversion at each step of dendrimer growth is not complete
and therefore a mixture of dendrimers is present on the final
surface. In the latter case, a fully grown dendrimeric structure is
attached directly on the nanotube.[57–65] In most cases, this
immobilization is achieved through amidation reactions between
carboxylic acid groups on the carbonaceous surface and the
amino groups of the PAMAM. However, an oxidative pre-
treatment of the support is needed to increase the number of such
carboxylic functions. This preliminary step often results in the
shortening or the collapsing of CNTs, together with introduction of
many defect sites, inducing therefore a partial or complete loss of
electric conductivity that is essential in many applications. In
addition, carboxylic groups are thermally-sensitive functions,
[a] A. Desmecht, T. Steenhaut, Dr. F. Pennetreau, Prof. S. Hermans,
Prof. O. Riant
Institute of Condensed Matter and Nanosciences, Molecules, Solids
and Reactivity (IMCN/MOST)
Université catholique de Louvain
Place Louis Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
E-mail: olivier.riant@uclouvain.be
sophie.hermans@uclouvain.be
Supporting information for this article is given via a link at the end of
the document.
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easily removed upon heating. Although Prato et al. reported
another strategy using disulfides for the radical grafting of
dendrimers,[63] alternatives to amidation are still scarce in the
literature. In addition to covalent immobilization, dendrimers can
also be adsorbed on NC in a non-covalent fashion.[66–70] Non-
covalent interactions are known to occur through ionic interplays
between the positively charged terminal amines of the dendrimer
and the negatively charged carboxylates from the support.
Moreover the macromolecule wrap around nanotubes via
hydrophobic, π−π or CH−π interactions.
We propose hereby the study of covalent grafting of
ethylenediamine core PAMAM dendrimers on CNTs by the
means of two different ‘grafting to’ approaches and systematic
comparison with non-covalent immobilization. The grafted
macromolecules will prove their utility as they significantly
increase the number of active sites per nanotube. Their primary
amino groups will be derivatized into azido moieties to undergo
CuAAC click reaction leading to triazole-functionalized
terminations. Although this transformation is well-known in
organic synthesis, Jacobson et al. were the only ones to our
knowledge to report this transformation on unsupported PAMAM
dendrimers.[71,72] We could successfully immobilize a significant
amount of metal thanks to the dendrimers and the resulting
composites were evaluated for catalytic applications in both
homogeneous supported (i.e. organometallic complexes) and
heterogeneous (i.e. metallic nanoparticles) catalysis.
Results and Discussion
Scheme 1. A) Covalent functionalization of MWNTs with xanthate (left) or
propargylic (right) moieties, B) PAMAM dendrimers of generation 0 to 3.
We reported in 2013 the covalent functionalization of
nanocarbons with xanthates using peroxides as radical
initiators.[73,74] Following this procedure, we were able to graft an
activated ester on p-MWNT and the fluorine content of the
resulting material 1 was evaluated by XPS to be 2.28 at.%. More
recently, we reported the efficient grafting of propargylic moieties
on NC surface and emphasized their use as precursors for
CuAAC post-functionalization.[75] Support 2 was synthesized
according to this procedure (Scheme 1A).
Commercial PAMAM dendrimers (20 wt.% in methanol) of
different generations (0 to 3) were used in this work (Scheme 1B).
The primary amines were able to react with the activated esters
of 1, leading to the formation of an amide bond and the materials
3-G0-3 as an homogeneous black suspension (Scheme 2A).
Products 3-G0-3 were characterized by the means of X-ray
photoelectron spectroscopy (XPS) and thermogravimetric
analysis (TGA). XPS shows a significant decrease in fluorine
content after dendrimer grafting, going from 2.28 at.% to 0.84 at.%
(Figure 1A) which indicates the effective reaction between 1 and
dendrimers. The amount of nitrogen increases with the dendrimer
generation (Table 1).
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Scheme 2. Grafting of generation 0-3 PAMAM dendrimers on A) xanthate or B)
propargylic functionalized MWNTs.
Figure 1. XPS spectra of A) F1s region (xanthate strategy) and B) N1s region
(click strategy).
Table 1. XPS analysis of 3-G0-3, 4-G0-3 and 5-G0-3 (at.%).
3-G0-3
4-G0-3
5-G0-3
Beside the covalent immobilization of those macromolecules on
our supports, non-covalent interactions were also considered and
blank experiments were carried out. Those control experiments
were run by incubation of nanotubes with PAMAM dendrimers in
the same conditions as above. p-MWNTs were incubated with
Gen
C1s
O1s
N1s
C1s
O1s
N1s
C1s
O1s
N1s
0
1
2
3
94.37
93.24
90.38
87.85
3.17
3.14
5.26
6.92
1.26
2.35
3.21
3.68
92.62
88.85
87.58
81.57
4.78
6.43
6.05
7.30
2.60
4.72
96.96
95.74
92.73
90.56
1.86
2.55
3.76
4.87
1.19
1.71
3.51
4.57
generation 0-3 PAMAM dendrimers, leading to the hybrids 5-G0-3
.
The resulting composites were carefully and repeatedly washed
before analysis to make sure that PAMAM dendrimers are
strongly immobilized on nanotubes. Those compounds were
studied by XPS and TGA (Table 1). The quantity of nitrogen was
determined and compared for all those samples by the means of
XPS. For the comparison to be reliable, one should carefully pay
attention to the specific amount of nitrogen in the hybrids. 3-G0-3
and 5-G0-3 can be compared one by one easily as each generation
of dendrimer has the same number of N. However, 4-G0-3 has
three N atoms on each extremity whereas 3-G0-3 and 5-G0-3 have
only one. Therefore, the quantity of N presented in Table 1 was
normalized according to the relative abundance of nitrogen on the
extremities of the dendrimers (Figure 2). More detail on the
calculations can be found in the supporting information. The
increase of nitrogen contents can be observed for each series of
hybrids. Whereas N content is very similar for series 3-G0-3 and 5-
G0-3, one can clearly see that samples 4-G0-3 have the highest
6.37
11.13
Prior to their reaction with chemical platform 2, dendrimers were
turned into azido-PAMAM thanks to the imidazole-1-sulfonyl
azide sulfuric acid salt (Scheme 2B).[76] The resulting azido-
PAMAM were not isolated and directly contacted in solution with
2. The azide synthesis is catalyzed by a copper (II) species
whereas CuAAC is catalyzed by a copper (I) species. Therefore,
sodium ascorbate was added at the same time in order to reduce
Cu(II) into Cu(I). This strategy enables the direct transformation
of primary amines into triazole click products in a one-pot process
in a similar way to the one described by Stonehouse et al. in
2009.[77] Azido PAMAM and 2 were reacted in suspension for a
very short time to avoid the grafting of the dendrimer through
multiple extremities leading to a multipodal composite. Azido
hybrids 4-G0-3 were investigated by XPS and TGA as well. The
XPS spectrum of 4-G0-3 in the N1s region revealed the presence
of remaining azide moieties at 405 eV (Figure 1B). Again, the
nitrogen content increases with the size of PAMAM dendrimer
(Table 1).
nitrogen content values, which indicates
a more efficient
immobilization of PAMAM on MWNTs. In order to have a better
insight on our methodologies, TGA was used as a complementary
analytical tool. When hybrids of generation 0 (3-G0, 4-G0 and 5-
G0) are compared, a lower amount of immobilized dendrimer is
observed for non-covalent interactions (6.7 wt.%). However, we
notice an identical loading of 11.4 wt.% for 3-G0 and 4-G0 (Figure
3A). This trend is confirmed by TGA of generation 3 hybrids (3-
G3, 4-G3 and 5-G3) (Figure 3B). Non-covalent stacking leads to
18.0 wt.% whereas covalent binding enables the grafting of
roughly 30 wt.%. We can therefore conclude that the covalent
strategies significantly increase the amount of dendrimers which
can be immobilized on MWNTs. A significant difference of loading
is observed by XPS analysis while the loading seems identical
using TGA for 3-G0,3 and 4-G0,3. This difference can be
rationalized simply by the fact that XPS is a surface analysis
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Figure 3. Comparative TGA graphs recorded under N2 atmosphere: A)
generation 0, B) generation 3 and C) generation 0-3 from click strategy. Mass
losses correspond to functionalization degree.
technique and that the increasing thickness of the polymer layer
on the hybrids surface is such that XPS is not able to probe the
whole layer anymore. Loading determination by means of TGA
shall hence be favored. Each generation of clicked dendrimers (4-
G0-3) were submitted to TGA as well. We observe regular loading
increase as the dendrimer expands (Figure 3C).
Going
a
step further, we exploited the presence of
polyamidoamines on our supports for post-functionalization
through click chemistry. The amino extremities of 3-G0 were
turned into azides with imidazole-1-sulfonyl azide sulfuric acid salt
as described previously to give compound 6-G0 (Scheme 3A).
Those
azides
were
brought
in
presence
of
4-
chlorophenylacetylene, copper iodide and N,N,N’,N’’,N’’-
pentamethyldiethylenetriamine (PMDETA) to yield triazoles 7-G0.
XPS analysis of 7-G0 indicates the presence of 0.28 at.% of
chlorine. A closer look at Cl/N ratios obtained by XPS indicates
the conversion of all free extremities of the generation 0
dendrimer into triazoles. In fact, we observe a small excess of
chlorine compared to nitrogen which could result from the
adsorption of the acetylene derivative on the nanotube Csp2
surface through - stacking. We also reacted the remaining
azide functions of 4-G0-3 with 3-trifluoromethylphenyl acetylene,
CuI and PMDETA to yield 8-G0-3. Their XPS analysis revealed
fluorine that increased with dendrimer’s generation, as clearly
seen on Figure 4. The comparison of the experimental N/F ratio
with the theoretical one (2 for each generation) suggests that all
the free extremities of each dendrimer generation are transformed
into clicked products. In the case of 8-G3, the N/F ratio is lower
than expected which can be explained by the adsorption or the
trapping of phenylacetylene on nanotube or inside dendrimers.
Those transformations outlined the utility of grafting dendrimers
on carbon nanotubes as we dramatically increased the quantity of
functions that can be grafted on MWNTs surfaces by the
conventional methods. Although the F/C ratios can reach up to
0.51 for direct fluorination of SWNTs,[78] the F/C ratio of
fluorophenyl-functionalized SWNTs obtained from aryl diazonium
salts do not exceed 0.037.[79] The F/C ratio of 8-G3 could be as
high as 0.082 (3 F atoms per function) which is considerably high
taking into account the fact that the support is MWNT (higher C
content) and not SWNT. For comparison, the F/C ratio value of 1
is 0.024 (5 F atoms per function).
Figure 2. XPS data comparison of the hybrids obtained by the xanthate, click
and non-covalent strategies.
Scheme 3. Post-functionalization of PAMAM hybrids into triazole adducts.
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Table 2. XPS analysis of post-functionalized dendrimers 8-G0-3 (at.%).
Gen
C1s
O1s
N1s
F1s
N/F
0
1
2
3
91.53
88.10
83.57
77.19
4.68
5.58
7.25
6.79
2.66
4.18
6.05
9.67
1.13
2.14
3.14
6.35
2.35
1.95
1.93
1.52
Figure 5. TEM images of A) p-MWNTs, B) 3-G3, C and D) 4-G3.
We further demonstrated the utility of our grafted materials
through the synthesis of two composites with catalytic activity
exploiting the presence of PAMAM dendrimers on carbon
nanotubes. Starting from 4-G3, we clicked 2-ethynyl-4-
methoxypyridine in the presence of CuI and PMDETA to afford 9-
G3 (Scheme 4A). The formation of the triazole-pyridine moiety
enables the use of such composite as bidentate ligand, for the
complexation of various metallic centres.[80–82] The use of the
dimer [IrCp*Cl2]2 as precursor leads to supported iridium
complexes 10-G3. This composite was characterized by XPS and
inductively coupled plasma atomic emission spectroscopy (ICP-
AES). It is noteworthy that for the sake of dendrimer economy, 0.2
equivalents of PAMAM were employed for the synthesis of 4-G3
instead of 2 equivalents as described above. Although the amount
of introduced dendrimer is 10 times less important, we observe a
nitrogen content of 7.03 at.% which is only 27 % lower and really
appreciable (Table 3). The XPS analysis of 10-G3 revealed the
presence of 0.79 at.% of Ir (Figure 6A) whereas bulk ICP-AES
indicates a content in Ir of 7.71 wt.%. Looking at the structure of
the functionalized dendrimer, 124 out of 217 N atoms (57 %)
come from the triazole-pyridine ligand which means that 3.69 at.%
N are dedicated to Ir complexation. This amount can be divided
by 4 as 4 atoms of N are present for 1 atom of Ir. We can therefore
conclude that the theoretical maximum loading of Ir would be 0.92
at.%. We were pleased to see the matching between the
calculated and the experimental value of 0.79 at.% which means
that our strategy and the reactions involved are reliable and
efficient. As a proof of concept, a preliminary experiment with the
iridium-based composite was carried out for catalytic reduction
reactions. 10-G3 could successfully reduce an imine into the
corresponding amine in presence of sodium formate as hydride
source with 31 % conversion after 4 hours at 80 °C (Scheme 4B).
Figure 4. General XPS survey of hybrids 8-G0-3
.
p-MWNTs, hybrids 3-G3 and 4-G3 were also characterized by
transmission electron microscopy (TEM). No significant difference
was observed between pristine nanotubes and composite 3-G3
(Figure 5A,B). However, one can observe the formation of an
external layer of dendrimers on hybrid 4-G3 (Figure 5C,D). This
external layer has a thickness of approximatively 8 to 10 nm and
seems to be present as randomly dispersed patches on
nanotubes. This surface structure difference between 3-G3 and 4-
G3 is supported by the differences observed in the XPS
measurements of 3-G3 and 4-G3, whereas the TGA
measurements do not show significant variations.
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This preliminary experiment remains to be optimized but shows
the possibility of high loading of metal complexes on CNTs for
potential applications in organometallic catalysis.
Pd in proportions of 65 % of Pd(0) and 35 % of Pd(II) (Figure 6C).
The asymmetry of metallic Pd peaks was not taken into account
and therefore the amount of Pd(0) is underevaluated. Thermal
treatment of 12-G1 in air leads to the formation of 15.5 wt.% of
palladium oxide residue. The first weight loss observed on the
TGA curve is due to the combustion of dendrimers (Figure 6D).
Scheme 4. A) Synthesis of bidentate supported ligand 9-G3 and supported Ir
complex 10-G3 and B) its catalytic application for imine reduction.
Table 3. XPS analysis of post-functionalized dendrimers (at.%).
Gen
C1s
O1s
N1s
Ir4f
-
Cl2p
Pd3d
9-G3
10-G3
11-G1
12-G1
86.10
84.85
91.23
88.43
6.82
6.58
5.99
7.84
7.03
6.46
2.64
2.47
-
-
0.79
-
1.68
-
-
Scheme 5. A) Synthesis of Pd nanoparticles encapsulated in immobilized
PAMAM dendrimers on MWNTs and B) their catalytic application for
carbonylative coupling.
-
-
-
1.25
In addition to the post-functionalization of the PAMAM extremities,
we also demonstrated the ability of the supported dendrimers to
host nanoparticles. The azido moieties of 4-G1 were reduced into
amines using the Staudinger reaction to give nanocomposite 11-
G1 in order to favour the interactions between free extremities and
metallic precursors (Scheme 5A). As described previously, the
quantity of introduced PAMAM was reduced from 2 to 0.2 eq. The
quantity of immobilized dendrimer was here (for G1 generation)
only reduced by 19
% according to TGA analysis. The
disappearance of N azido component on the XPS spectrum at 405
eV was noticed for 11-G1 (Figure 6B). The N content was reduced
from 3.94 at.% to 2.64 at.% which is in agreement with the loss of
2 N atoms per extremity. An aqueous solution of Na2PdCl4 was
stirred with a suspension of 11-G1 and the resulting mixture was
filtered, washed and finally reduced with a solution of sodium
borohydride to yield 12-G1. Compound 12-G1 was investigated by
XPS and TGA. XPS spectrum of Pd region revealed 1.25 at.% of
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Figure 7. Recycling of 12-G1 in the carbonylative coupling between iodotulene
and 4-(2-aminoethyl)morpholine.
A closer look was taken on 12-G1 before catalysis and after the
fifth run with TEM. The encapsulating ability of 12-G1 is
demonstrated through the preferential immobilization of Pd NPs
inside dendrimer layers rather than on nanotubes bare surface
(Figure 8A,B). The presence of dendrimers templates the
formation of small NPs (2 nm) with a narrow size distribution (see
inset). However, the size of Pd NPs increases after catalysis to
reach about 6-8 nm (Figure 8C,D). This difference in NPs sizes
before and after catalysis explains the difference in Pd content
determined with XPS. This variation of size suggests that some
catalytic species are leached away during the process due to
Pd(0)/Pd(II) mechanism before they redeposit on immobilized
NPs, thus increasing their size.
Figure 6. XPS spectra in A) Ir region of 10-G3, B) N region of 4-G1 and 11-G1,
C) Pd region of 12-G1, D) TGA graph of 12-G1 recorded under aerobic
atmosphere.
We investigated the activity of 12-G1 for carbonylative coupling
with ex situ generated CO (Scheme 5B). In a double-chamber
reactor using the technology developed by the group of
Skrydstrup,[83] carbon monoxide is produced by decomposition of
formic acid in presence of triethylamine and mesyl chloride
according to the procedure described by De Borggraeve et al.[84]
The use of such device enables the handling of this deadly gas in
a safe environment. The generated CO goes up in the second
chamber where 12-G1 is in presence of iodotoluene and 4-(2-
aminoethyl)morpholine and is consumed to produce the desired
amino-carbonylation product 13, which we obtained with 49 %
NMR yield. Residual iodotoluene was also present in the reaction
mixture. Our catalyst was recycled by mere filtration and reused
4 times. We observed an increased reactivity after the first run to
reach about 80 % conversion after the fifth run (Figure 7). XPS
analysis of 12-G1 after the first and the fifth runs indicated the
decrease of Pd surface content from 1.25 at.% to 0.36 at.% and
then 0.25 at.%. This decrease can be partially explained by the
leaching of residual Pd(II) from the catalyst synthesis. However,
this leaching had no negative effect on the activity of our
composite, quite the opposite.
Figure 8. TEM images of 12-G1 before catalysis (A,B) and after the fifth run
(C,D).
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Conclusions
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To summarize, we presented the immobilization of PAMAM
dendrimers on MWNTs by the means of radical and click
chemistry. We outlined the existence of non-covalent interactions
between dendrimers and support but demonstrated the
superiority of covalent functionalization. Hybrid materials were
investigated through TGA, XPS and TEM analyses. We propose
a convenient PAMAM post-functionalization sequence based on
the synthesis of azido dendrimers and subsequent CuAAC
reaction with alkynyl derivatives. This process was proven
efficient as a high loading of functionalized PAMAM was obtained.
The synthesized nanocomposites were exploited either by
grafting iridium complexes as supported homogeneous catalysts
at the extremities of the dendrimers or by encapsulating palladium
nanoparticles within their branches for carbonylative coupling.
Our approach enables the use of such hybrids in various
applications like catalysis or drug delivery.
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The authors wish to thank the Fonds de la Recherche Scientifique
(F.R.S.-FNRS), the Fonds pour la Formation à la Recherche dans
l’Industrie et dans l’Agriculture (FRIA), as well as the Université
catholique de Louvain for funding. We are grateful to Jean-
François Statsyns and François Billard for technical assistance.
We are also thankful to Tommy Haynes for TEM images.
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Entry for the Table of Contents (Please choose one layout)
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We report and compare the covalent
and non-covalent immobilization of
PAMAM dendrimers on MWNTs. The
Antonin Desmecht, Timothy Steenhaut,
Florence Pennetreau, Sophie Hermans*
and Olivier Riant*
resulting
derivatized,
hybrids
to
were
further
Page No. – Page No.
immobilize
homogeneous-like iridium complexes
or Pd NPs. These were demonstrated
efficient for catalytic applications.
Synthesis and Catalytic Applications
of Multi-Walled Carbon Nanotube-
Polyamidoamine Dendrimer Hybrids
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