Synthesis of dendrimer-encapsulated silver nanoparticles and its catalytic activity on the reduction of 4-nitrophenol

Article abstract of DOI:10.1002/jccs.201100734

Anew type of water soluble PEG core dendrimer having hydroxyl groups at the periphery was synthesized and used to prepare silver nanoparticles. The dendrimer and the dendrimer encapsulated nanoparticles (DENs) were characterized by spectroscopic technique

Full text of DOI:10.1002/jccs.201100734

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Synthesis of Dendrimer-Encapsulated Silver Nanoparticles and Its Catalytic Activity
on the Reduction of 4-Nitrophenol
I. V. Asharani* and D. Thirumalai
Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, India
(Received: Dec. 16, 2011; Accepted: Apr. 3, 2012; Published Online: ??; DOI: 10.1002/jccs.201100734)
Anew type of water soluble PEG core dendrimer having hydroxyl groups at the periphery was synthesized
and used to prepare silver nanoparticles. The dendrimer and the dendrimer encapsulated nanoparticles
(DENs) were characterized by spectroscopic techniques. The kinetics of catalytic activity of the prepared
silver nanoparticle on the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 as a reductant was stud-
ied using UV-Visible spectrophotometer.
Keywords: Dendrimer; Nanoparticles; Reduction; Kinetics.
INTRODUCTION
The emergence of nanotechnology for making metal
metal ions and thus avoiding cross-linking of dendrimers,
which in turn eliminates precipitation (ii) both hydroxyl-
terminated dendrimers and metal-ion complexes are water
soluble.12 Palladium nanoparticles were prepared from
Pd(OAc)2 and poly ethylene glycol (PEG) in the absence of
other chemical agent, where PEG appeared to act as both
reducing agent and stabilizer.13 Recently, synthesis of silver
nanoparticles using hydroxyethyl cellulose polymer as sta-
bilizer was also reported.14
nanoparticles is providing new opportunities for the poten-
tial revolution in various fields that includes catalysis,1 op-
tical,2 electronic3 biological and molecular sensors.4 Among
these potential applications of metal nanoparticles, cataly-
sis stands out as one of the most promising, since metal
nanoparticles have very high surface area which could en-
hance the catalytic activity. Although a variety of nanopar-
ticle synthetic strategies can be employed, the most widely
used method utilizes a simple metal salt, a reducing agent
and a colloid stabilizer.5 Dendrimers are a special class of
hyper branched polymer which can act as host for metal
nanoparticle since they have internal cavity capable of en-
capsulating the metal particles.6 The preparations of den-
drimer encapsulated nanoparticles were reported in the late
1990s. After that several reports have been published on
the preparation of mono-metallic as well as bi-metallic
nanoparticles having passinating applications in many ar-
eas.
The reduction of aromatic nitro compounds to the
corresponding anilines is one of the most important trans-
formations in synthetic organic chemistry, since substituted
anilines serve as starting material in the synthesis of impor-
tant industrial products such as dyes, pharmaceuticals,
agrochemicals etc. Despite the extensive availability of
conventional catalysts15 for the reduction of nitro com-
pounds, even the best of the current catalysts is yet to be
identified. However, metal nanoparticles are shown to ex-
hibit better catalytic activity and selectivity towards the re-
duction of nitro compounds. Hence, in this paper, we report
the synthesis of a hydroxyl terminated dendrimer, den-
drimer-encapsulated silver and its catalytic activity on the
reduction of 4-nitrophenol.
R. M. Crooks and his co-workers have reported the
synthesis of gold and palladium nanoparticles using func-
tionalized PAMAM dendrimer as a stabilizer7-10 and they
proved that the catalytic activity of dendrimer-encapsu-
lated nanoparticles could be controlled by changing the
dendrimer generation. K. Esumi et al have reported the
preparation of PAMAM and PPI dendrimer-metal (silver,
platinum, and palladium) nanocomposites and studied their
catalytic activity on the reduction reaction of 4-nitrophe-
nol.11 Hydroxyl terminated PAMAM dendrimers are pre-
ferred for the synthesis of metal nanoparticles, since (i)
hydroxyl groups are non complexing towards most of the
EXPERIMENTAL
Polyethylene glycol (PEG) (M.Wt. 3400), dicyclo-
hexylcarbodiimide (DCC), 4-dimethylaminopyridine
(DMAP), 4-nitrophenol (4-NP), Sodium borohydride
(NaBH4) were purchased from Aldrich and used as such.
Double-distilled water and AR grade solvents were used
throughout the investigation.
UV-Visible spectra were recorded on “Shimadzu”
* Corresponding author. E-mail: ivasharani@gmail.com
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Asharani and Thirumalai
UV-Visible spectrophotometer (UV-1601) by using a
matching pair of Teflon stopper quartz cells of path length
1 cm. Infrared spectra were recorded using Bruker-Tensor
27 FTIR spectrophotometer in the region 4000-400 cm-1 as
KBr pellets. NMR spectra were recorded on Bruker-400
TX (400 MHz) instrument with TMS as internal standard
and CHCl3-d as solvent. MALDI-TOF mass spectrum was
recorded on Bruker Biflex III MALDI-TOF MS instrument
using 2,4,6-THAP (2,4,6-trihydroxyacetophenone) matrix
by following ‘Dry Droplet Click method’. Transmission
electron microscopy (TEM) studies of the particles were
carried out at an accelerating voltage of 120 kV using a
Hitachi, H-7500 TEM. Samples for TEM were prepared on
300 mesh copper grids coated with carbon. A drop of the
nanoparticle solutions in water was carefully placed on the
copper grid surface and dried. The size distributions of the
particles were measured from enlarged photographs of the
TEM images.
hydroxyl groups (2.0 g, 0.47 mmol) in DCM (50 mL) was
added benzylidene-protected 3,5-dihydroxybenzoic acid 1
(1.27 g, 2.26 mmol) followed by DCC (0.585 g, 2.83 mmol).
The reaction mixture was stirred at room temperature for
48 h under nitrogen atmosphere. Then, the separated DCC
urea was filtered off and the filtrate was added drop wise
into a beaker containing diethyl ether (1 L). The separated
1
white solid was filtered and dried. Yield: 75%. H NMR
(400 MHz, CDCl3): d 0.94 (s, 6H), 1.03 (s, 24H), 3.64 (bs),
4.57 (m), 5.41 (s, 8H, benzylidene CH), 7.13-7.58 (m,
52H).
Synthesis of G1 generation of the PEG-dendrimer 3
[PEG-G1-(3,5-DHB-OH)16]
The benzylidene protected dendrimer [PEG-G1-(3,5-
DHBA-O2Bn)4] 2 (1.5 g) with protecting groups was dis-
solved in a mixture of methylene chloride and methanol
(1:2, 60 mL). To this, 0.075 g of Pd/C was added and the
mixture was stirred in the presence of H2 gas for 24 h. The
Pd/C was filtered through short celite pad and the filtrate
was poured into diethyl ether (1 L). The white powder was
filtered off quickly and dried under vacuum. Yield: 65%.
1H NMR (400 MHz, CDCl3): d 0.94 (s, 6H), 1.02 (s, 24H),
3.62 (bs), 4.27-4.60 (m), 7.15 (m, 12H). MALDI-TOF:
M.Wt. 5003.7 (M.Wt. calculated = 5104).
Dynamic light scattering (DLS) was measured using
MALVERN Nano series (Nano-ZS) particle size analyzer.
XPS spectrum for the nanoparticle was collected with a
Physical Electronics ESCA PHI 1600 spectrometer, with
Mg Ka as the X-ray source.
Synthesis of benzylidene-2,2-bis(oxymethyl)propionic
acid and its anhydride
Synthesis of Dendrimer Encapsulated Nanoparticles
The method of preparation of silver nanoparticles de-
scribed below is similar to the procedure reported earlier.17
Silver nanoparticle was synthesized by dissolving hydrox-
yl terminated dendrimer 3 (7.6 mg, 1.47 mmol) in doubly
distilled water (10 mL) followed by the addition of silver
nitrate (1.0 mg, 5.9 mmol). The reaction mixture was heated
at 80 °C for 1 hour with vigorous stirring and cooled to
room temperature. The pale pink coloured solution was
made up to the original volume using doubly-distilled wa-
ter (10 mL) to get the colloidal solution of silver nanoparti-
cle.
Benzylidene-2,2-bis(oxymethyl)propionic anhydride
and the PEG-hybrid with four hydroxyl terminals were pre-
pared according to literature method.16
Synthesis of benzylidene-protected 3,5-dihydroxyben-
zoic acid 1
A mixture of 3,5-dihydroxybenzoic acid (DHBA)
(1.0 g, 6.48 mmol), benzylidene-2,2-bis(oxymethyl)pro-
pionic anhydride (8.29g, 19.4 mmol) and DMAP (0.158 g,
1.29 mmol) in CH2Cl2 (50 mL) was stirred at room temper-
ature for 24 h under nitrogen atmosphere. Methanol (10
mL) was added to quench the excess anhydride and the re-
action mixture was stirred for another 2 h. The reaction
mixture was poured into excess of diethyl ether (500 mL)
under vigorous stirring. A white powder separated was fil-
tered and dried. Yield: 85%. 1H NMR (400 MHz, CDCl3): d
1.09 (s, 6), 3.67 (d, 4H, J = 11.0 Hz, -OCH2), 4.62 (d, 4H, J
= 14.8 Hz, -OCH2), 5.48 (s, 2H, benzylidene CH), 7.23-
7.34 (m, 6H, Ar-H), 7.47 (m, 4H, Ar-H).
RESULTS AND DISCUSSION
It was of our interest to synthesize an environment-
friendly dendrimer, dendrimer encapsulated nanoparticles,
and to use these dendrimer encapsulated nanoparticles as
catalyst to study the kinetics of reduction of nitro aro-
matics. For this purpose, we have chosen poly ethylene
glycol (PEG) core dendrimer since PEG is non-toxic and
water-soluble. PEG hybrid anchored with four –OH
groups at the terminal were prepared according to litera-
ture procedure starting from poly ethylene glycol core
Synthesis of benzylidene-protected G1 generation of
the PEG-DHBA-dendrimer 2 [PEG-G1-(3,5-DHBA-
O2Bn)4]
To a solution of PEG-hybrid having four terminal
2
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Hydroxyl-terminated Dendrimer, Silver Nanoparticles
(M.Wt. 3400). Then, 3,5-dihydroxybenzoic acid was re-
acted with benzylidine-bis(oxymethylene)propionic anhy-
dride (2 equiv.) in dichloromethane in the presence of
DMAP to afford the BOP-protected benzoic acid derivative
1. In the proton NMR spectrum of 1, the methyl protons ap-
peared as a singlet at d 1.09, the oxymethylene protons ap-
peared as two doublets at d 3.67 and 4.62, the benzylidene
proton appeared as a singlet at d 5.48 and the aromatic pro-
tons appeared as two multiplets between d 7.23-7.34 and d
7.47.
Scheme I
Structure of BOP-protected dendrimer 2
As a next step, the PEG-hybrid having 4-OH groups
was coupled with four equivalence BOP-protected benzoic
acid 1 in the presence of dehydrating agent, N,N¢-dicyclo-
hexylcarbodiimide (DCC) using dichloromethane as a sol-
vent. After stirring the reaction mixture at room tempera-
ture for the stipulated time, the dicyclohexyl urea separated
as a white solid was filtered and the reaction mixture was
concentrated. The purified product 2 was obtained by the
drop wise addition of concentrated reaction mixture in to
excess of diethyl ether with vigorous stirring followed by
filtering the resulted solid.
The 1H NMR spectrum of 2 showed two singlets at d
0.94 and d 1.03 due to the methyl protons, a broad singlet at
d 3.64 due to PEG-protons, a multiplet at d 4.57 due to
PEG-terminal methylene protons, a singlet at d 5.41 due to
benzylidene protons and a multiplet between d 7.13-7.58
due to aromatic protons of benzoic acid residue and the
benzylidiene phenyl group. Finally, the deprotection of
benzylidene group in 2 was carried out using H2 gas in the
presence of Pd/C. The required product, the G1 dendrimer
3 was isolated from the reaction mixture by the usual proce-
dure. The formation of the dendrimer 3 was confirmed by
the proton NMR and MALDI-TOF spectra. The proton
NMR spectrum of the dendrimer 3 showed two singlets at d
0.94 and d 1.02 corresponding to methyl protons, a broad
singlet at d 3.62 corresponding to PEG protons, a multiplet
between d 4.27-4.60 corresponding to terminal methylene
protons of PEG and a multiplet at d 7.15 corresponding to
the aromatic protons of benzoic acid residue. The MALDI-
TOF mass spectrum showed a peak at m/z 5003 which is in
well-agreement with the theoretically calculated value m/z
5104. The complete reaction sequence involved in the syn-
thesis of target dendrimer is shown in Scheme I.
Structure of the dendrimer 3
Synthesis of Silver Nanoparticles
The preparation of nano-Ag was very straightfor-
ward. The nano-Ag preparation process is “green” and very
simple. Silver nitrate was added in to the PEG-DHBA den-
drimer 3 at 80 °C. The resulted solution was stirred at this
temperature for one hour. The appearance of pale pink col-
our indicated the formation of silver nanoparticles.
For the synthesis of metal nanoparticles, the most ac-
cepted mechanism involves a two-step process, i.e., nucle-
ation and successive growth of the particles. In the first
step, a part of the metal ions in solution is reduced by a suit-
able reducing agent. Thus, the atoms produced act as nucle-
ation centres and catalyze the reduction of the remaining
As a consequence of the three dimensional structure
and multiple internal and external functional groups, den-
drimers are able to function as hosts for a range of ions and
molecules.
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Article
Asharani and Thirumalai
metal ions present in the bulk solution and this stage has an
autocatalytic nature. The coalescence of atoms leads to the
formation of metal clusters and these are effectively stabi-
lized by ligands, surfactants or polymers. The terminal pri-
mary alcoholic group is responsible for the reduction of
metal ions into zero-valent metals during the nanoparticle
formation. The newly synthesized dendrimer 3 has 16
oxidisable primary alcoholic groups and during the nano-
particle formation, these primary alcoholic groups may re-
duce the metal salts into their zero-valent metal nanoparti-
cles there by getting oxidized themselves.
sizes measured by TEM and DLS is due to the fact that
TEM is sensitive to electron-dense metal particle, where as
DLS measures the hydrodynamic volume of the nanoparti-
cles.
XPS was used to determine the oxidation state of the
metal in the dendrimer encapsulated nanoparticles. Figure
2 shows the XPS spectrum of Ag nanoparticles. In the spec-
trum, the 3d5/2 and 3d3/2 peaks for metallic silver appear at
367.6 and 373.6 eV, which are coincident with the reported
values (368 and 374 eV) for metallic silver.19
Kinetics of the reduction of nitro compounds using
NaBH4 as reductant in the presence of dendrimer-
encapsulated silver nanoparticle as catalyst
Characterization of Nanoparticles
The UV-Visible absorption spectrum of Ag nanopar-
ticle solution (0.5 ´ 10-5 mol dm-3) showed an intense,
broad absorption peak around 433 nm (lmax) due to surface
plasmon excitation of silver particles and indicates dis-
persed Ag(0) in solution. The wavelengths of absorption of
the prepared Ag nanoparticle is in good agreement with the
reported value.18
Catalytic activity of the silver nanoparticles on the re-
duction of 4-nitrophenol by varying the [NaBH4] was in-
vestigated. In a typical kinetic experiment, an aqueous so-
lution of NaBH4 (20-100 fold) (2 ´ 10-3 to 1 ´ 10-2 mol
dm-3) and the catalyst, (1/20 fold) (0.5 ´ 10-5 mol dm-3)
were added to the substrate (1 ´ 10-4 mol dm-3) and the
change in the absorbance was monitored by a UV-Visible
spectrosphotometer at regular intervals of time.
The size and shape of nanoparticles were obtained by
transmission electron microscopy (TEM). Fig. 1 presents
the TEM image and size distribution graph of nano-Ag
which indicates that the average particle size of silver is
about 20 nm and it demonstrated that the prepared nano-
particles were separated widely with narrow size distribu-
tion and did not agglomerate. In addition, the particle size
of the prepared nanoparticle was also determined using a
dynamic light scattering instrument, which gave the parti-
cles size of Ag as 68 nm. The difference in the particles
4-Nitrophenol has an intense absorption at 377 nm
and in NaBH4 medium (pH = 12.0), is red shifted due to the
formation of 4-nitrophenolate ion (lmax 400 nm). In the ab-
sence of nano-Ag catalyst, the peak due to 4-nitrophenol at
400 nm remains unaltered. The addition of catalyst to the
reaction mixture initiates the reduction reaction of 4-nitro-
phenol to 4-aminophenol.
The reduction can be visualized by the disappearance
of 400 nm peak with a concomitant appearance of a new
peak at 300 nm. This peak has been attributed to 4-amino-
phenol (Fig. 3).20 The UV spectrum of commercially avail-
able 4-aminophenol is in good agreement with the spec-
trum of the above reduction product. Here, it was found
Fig. 2. X-ray photoelectron spectrum of silver nano-
particle.
Fig. 1. TEM image and size distribution graph of
dendrimer-encapsulated silver nanoparticles.
4
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Hydroxyl-terminated Dendrimer, Silver Nanoparticles
Table 1. Effect of [NaBH4] on the rate constants for the reduc-
tion of 4-nitrophenol at a [4-NP] of 1 ´ 10-4 mol dm-3
and [catalyst] of 0.5 ´ 10-5 mol dm-3 at pH 12 and
measured at l = 400 nm
that with the increase in concentration of NaBH4 (20-100
fold), the induction time, IT (time require to observe visual
change, i.e., the start of the reduction of 4-NP), decreased
and consequently the initial rate increased. A little NaBH4
is always decomposed in the reaction medium during the
course of reaction. So an excess amount of NaBH4 was al-
ways employed. As soon as the catalyst was added, the
metal particles started the catalytic reaction by relaying
[NaBH4] / 10-3
mol dm-3
kobs / 10-3, s-1
S. No.
in Ag nano particles
1.
2.
3.
4.
5.
6.
7.
2.0
3.0
0.77
1.55
1.90
2.35
-
4.0
-
5.0
electron from the donor BH4 to the substrate 4-NP (accep-
6.0
tor) only after the adsorption of both onto the particle sur-
7.0
3.07
3.92
faces.
10.0
-
As the concentration of BH4 is very high, it remains
essentially constant during the reaction. Accordingly, pseu-
do-first-order kinetics with respect to 4-NP could be used
in this case to evaluate the catalytic rate. A good linear cor-
relation with time, i.e, log At vs time plot is observed. Here
At stands for absorbance at any time t. The values of
pseudo-first-order rate constants (k) determined from the
plots shown in Fig. 4, are given in the Table 1.
tion mixture using ethyl acetate, a broad band at 3361 cm-1
was appeared due to both –NH2 and –OH stretching and the
bands at 1497 and 1341 cm-1, the characteristic bands for
nitro group, were disappeared. Although the band at 3326
cm-1 could not be explained due to the merging of both
–NH2 and –OH stretching, the disappearance of bands
characteristic for nitro group strongly indicated the reduc-
tion of nitro group to an amino group.
Product Analysis by Infrared Spectroscopy (IR)
IR spectroscopy is a versatile tool used to identify the
appearance or disappearance of certain functional groups
during the course of the organic functional group transfor-
mation. Hence, IR spectroscopy has been used to identify
the product formed from the reduction of 4-nitrophenol by
sodium borohydride in the presence of prepared nanoparti-
cles. In general, nitro aromatic compounds show bands
near 1550-1500 and 1360-1290 cm-1 due to N-O stretching.
The IR spectrum of the commercially available 4-NP is
shown in Fig. S10, in which the nitro groups showed char-
acteristic bands at 1497 and 1341 cm-1 and the –OH group
absorbed at 3326 cm-1. In the IR spectrum of the product,
4-aminophenol, which was obtained by extracting the reac-
CONCLUSIONS
A new type of water soluble PEG core dendrimer 3
having hydroxyl groups at the periphery was synthesized
starting from polyethylene glycol (PEG). Dendrimer en-
capsulated silver nanoparticles were prepared by heating a
mixture of silver nitrate and the dendrimer 3 in water at 80
°C in a suitable ratio. The prepared dendrimer 3 and the
dendrimer encapsulated nanoparticles were characterized
Fig. 3. UV-Visible spectrum for the reduction of 4-NP
by Ag nanoparticle as catalyst. [4-NP] = 1 ´
10-4 mol dm-3 [NaBH4] = 5 ´ 10-3 mol dm-3 and
[Ag catalyst] of 0.5 ´ 10-5 mol dm-3. Time inter-
val is 70 seconds. The peak around 400 nm de-
creases due to 4-nitrophenolate ion formation
and peak at 295 nm increases due to the forma-
tion of 4-amino phenol.
Fig. 4. Plot of log absorbance against time for the re-
duction of 4-NP in the presence of variable con-
centration of NaBH4. [4-NP] = 1 ´ 10-4 mol
dm-3 [NaBH4] = 2 ´ 10-3 mol dm-3 (a), 3 ´ 10-3
mol dm-3 (b), 4 ´ 10-3 mol dm-3 (c) 5 ´ 10-3 mol
dm-3 (d) 6 ´ 10-3 mol dm-3 (e) 7 ´ 10-3 mol dm-3
(f) 10 ´ 10-3 mol dm-3 (f) and [catalyst] of 0.5 ´
10-5 mol dm-3.
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Article
Asharani and Thirumalai
7. Zhao, M.; Suh, L.; Crooks, R. M. Angew. Chem., Int. Ed.
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by UV, IR, MALDI-TOF, NMR, TEM and XPS. The cata-
lytic activities of the prepared DENs were tested by follow-
ing the reaction kinetics of the reduction of 4-nitrophenol
to 4-aminophenol by NaBH4 as a reductant. From the ki-
netic data, it was found that silver nanoparticles are more
efficient in catalyzing the reduction reaction.
8. Kim, Y. G.; Oh, S. K.; Crooks, R. M. Chem. Mater. 2004, 16,
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ACKNOWLEDGEMENTS
Authors gratefully acknowledge Prof. V. R.
Vijayaraghavan, University of Madras and the manage-
ment of VIT University, Vellore for encouragement and
providing necessary facilities for carrying out this work.
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