Gold nanoparticles stabilized by amphiphilic hyperbranched polymers for catalytic reduction of 4-nitrophenol

Article abstract of DOI:10.1016/j.jcat.2016.01.014

Amphiphilic hyperbranched polymers (Hx-PEI-PEG) with polyethylene glycol (PEG) chains and low-molecular-weight polyethylenimine (PEI) conjugated to commercial aliphatic hyperbranched polyesters (Boltorn Hx) were used to stabilize gold nanoparticles (AuNPs) at the interlayer between the core and the shell. It was found that low generations of Hx-PEI-PEG such as H20-PEI-PEG and H30-PEI-PEG cannot effectively stabilize AuNPs. However, H40-PEI-PEG was both the reduction agent and the stabilizer for the formation of AuNPs, which was confirmed from the solution color change and UV-vis absorption spectra. Spherical nanosized AuNPs (below 10 nm) with narrow particle size distributions were observed from the TEM images. The H40-PEI-PEG-stabilized AuNPs solution could be shelf-stored at room temperature for more than 1 month. In addition, H40-PEI-PEG-stabilized AuNPs showed high catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).

Full text of DOI:10.1016/j.jcat.2016.01.014

Journal of Catalysis 337 (2016) 65–71
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Gold nanoparticles stabilized by amphiphilic hyperbranched polymers
for catalytic reduction of 4-nitrophenol
b
Yu Dai a, Peng Yu a, Xiaojin Zhang b, , Renxi Zhuo
⇑
a Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
b Key Laboratory of Biomedical Polymers of Ministry of Education, Department of Chemistry, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Amphiphilic hyperbranched polymers (Hx-PEI-PEG) with polyethylene glycol (PEG) chains and
low-molecular-weight polyethylenimine (PEI) conjugated to commercial aliphatic hyperbranched polye-
sters (Boltorn Hx) were used to stabilize gold nanoparticles (AuNPs) at the interlayer between the core
and the shell. It was found that low generations of Hx-PEI-PEG such as H20-PEI-PEG and H30-PEI-PEG
cannot effectively stabilize AuNPs. However, H40-PEI-PEG was both the reduction agent and the
stabilizer for the formation of AuNPs, which was confirmed from the solution color change and UV–vis
absorption spectra. Spherical nanosized AuNPs (below 10 nm) with narrow particle size distributions
were observed from the TEM images. The H40-PEI-PEG-stabilized AuNPs solution could be shelf-stored
at room temperature for more than 1 month. In addition, H40-PEI-PEG-stabilized AuNPs showed high
catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).
Received 17 December 2015
Revised 18 January 2016
Accepted 18 January 2016
Keywords:
Hyperbranched polymers
Chemical reduction
Gold nanoparticles
Catalytic reaction
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
assemble in water or organic solvents and prevent the agglomera-
tion of AuNPs to obtain more uniform and stable AuNPs [12–15].
Sub-10 nm gold nanoparticles (AuNPs) show high catalytic
activity and good selectivity for organic reactions such as reduc-
tion, oxidation, and coupling [1]. However, AuNPs in colloidal solu-
tion tend to aggregate, resulting in poor performance [2].
Therefore, the stabilizers of AuNPs face the very challenging task
of obtaining stable AuNPs with good size and shape [3]. The stabi-
lizers mainly include inorganic ligands, small organic molecules,
linear polymers, and dendrimers [4]. Among these, dendrimers
having a spherical structure and a large molecular bore show sig-
nificant advantages in the synthesis of monodisperse and stable
AuNPs [5–7]. However, dendrimers need a tedious multistep syn-
thesis, leading to high cost. Moreover, due to their open structure
at low generations, the reactants escape from the interior before
contact with AuNPs. The high generations inhibit movement of
substances into the interior of the dendritic structure [8].
Hyperbranched polymers are a class of alternatives to den-
drimers and can be synthesized by a one-step reaction from mono-
mers [9]. Use of hyperbranched polymers in the synthesis of AuNPs
has also been attempted [10]. However, either hydrophobic or
hydrophilic hyperbranched polymers affect their stability in water
or organic solvents. Amphiphilic modification is a workable way to
solve this problem [11]. Amphiphilic hyperbranched polymers self-
Conventional amphiphilic hyperbranched polymers also have
steric hindrance and the reactants have difficulty in entering the
interior to undergo the catalytic reaction. It has been one of the
most important topics in the research of AuNPs catalysts to both
effectively disperse AuNPs and easily contact the reactants with
AuNPs [16–19].
Here we describe the use of unique amphiphilic hyperbranched
polymers (Hx-PEI-PEG) to support AuNPs, which are dispersed at
the interlayer. More importantly, the reactants easily reach the
interlayer and contact AuNPs for catalytic reaction. Hx-PEI-PEG
were synthesized through conjugating polyethylene glycol (PEG)
chains and azide groups to commercial aliphatic hyperbranched
polyesters (Boltorn Hx) after altering hydroxyls to carboxyls with
succinic anhydride and then connecting low-molecular-weight
polyethylenimine PEI to the polymer via click chemistry
(Scheme 1). We hypothesized that metal precursors (AuClꢀ4 ) can
be adsorbed by PEI and then reduced to AuNPs at the interlayer.
2. Experimental
2.1. Materials
Polyethylenimine (PEI, Mw 800 Da), polyethylene glycol methyl
ether (PEG, Mw 2000 Da), hyperbranched bis-MPA polyester (H20:
16 hydroxyl, H30: 32 hydroxyl, and H40: 64 hydroxyl), and 4-
⇑
Corresponding author.
E-mail address: zhangxj@whu.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.jcat.2016.01.014
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

66
Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
3
C O O H
O H
COOH
3
N
OH
N
C O O H
O H
O H
O H
COOH
COOH
COOH
OH
C O O H
C O O H
3 N
OH
OH
N3
COOH
C O O H
O H
COOH
N
OH
3
Hx-PEI-PEG
NH2
O
OH
H2N
O
HN
n
PEG
NH2
HN
NH
N
N
N
N
NH2
H
N
N
N
PEI
NH
H2N
Hyperbranched polyester (Hx)
NH2
H2N
NH2
Scheme 1. Schematic illustration of the synthesis of amphiphilic hyperbranched polymers Hx-PEI-PEG.
nitrophenol (4-NP) standard solution (10 mM) were purchased
from Sigma-Aldrich and used as received. Hydrogen tetrachloroau-
rate(III) trihydrate (HAuCl4ꢁ3H2O) was purchased from Shaanxi
Kaida Chemical Engineering Co., Ltd. in China and used as received.
Other reagents were purchased from Sinopharm Chemical Reagent
Co., Ltd. in China and used as received. Hx-PEI-PEG was synthe-
sized as described elsewhere [20].
spectrophotometer in scanning range 200–700 nm at room
temperature.
3. Results and discussion
3.1. Synthesis of hyperbranched polymer-stabilized AuNPs
2.2. Synthesis of hyperbranched polymer-stabilized AuNPs
In previous work, Hx-PEI-PEG were synthesized and studied as
an effective miRNA carrier. Interestingly, miRNA was combined at
the interlayer between the core and shell to form a sandwiched
structure, which was confirmed by TEM [20]. As we know, the
amino groups of the polymers have the capability of combining
metal ions and metal nanoparticles [21]. Therefore, PEI units of
Hx-PEI-PEG will tend to absorb AuClꢀ4 and AuNPs after chemical
reduction [22]. AuNPs are dispersed at the interlayer between
the core and the shell. The reactants easily contact AuNPs for cat-
alytic reaction (Scheme 2).
The in situ formation of AuNPs stabilized by hyperbranched
polymers Hx-PEI-PEG was carried out by the addition of HAuCl4
solution into an aqueous polymer solution and subsequent reduc-
tion with NaBH4. In a typical experiment, an aqueous solution of
HAuCl4 (5.0 ꢂ 10ꢀ4 M, 2.5 mL) was added to 2.0 mL of an aqueous
solution of the polymer (the ratio of nitrogen in the polymer to Au
atoms was in the range 0.25–10) with desired pH (4.0, 7.0, and
10.0). The mixture was left at room temperature for 1 h. Subse-
quently,
a
fresh aqueous solution of NaBH4 (2.5 ꢂ 10ꢀ2 M,
Hx-PEI-PEG series have three different generations of cores
with similar composition ratios. The concentration of Au in all
the solutions is 250 lM. The concentrations of Hx-PEI-PEG for C1,
0.5 mL) was added to this mixture at once. The mixture was kept
at room temperature for at least 4 h before it was employed in
the catalytic reaction.
C2, C3, and C4 are based on the ratio of nitrogen in the polymer
to Au atoms, which is 0.25, 1, 5, and 10, respectively. Amino resi-
dues of Hx-PEI-PEG exhibit the ability of Au-ion reduction [21].
The solution color change is observed (Line B in Fig. 1) and the
characteristic absorption of AuNPs to the mixture solution of
HAuCl4 and H40-PEI-PEG at low pH is found at wavelengths ca.
536 nm (red line in Fig. 2) in the absence of NaBH4, which proves
the reduction potential of PEI in Hx-PEI-PEG for Au ions at room
temperature. The H40-PEI-PEG solution at pH 4.0 exhibits the best
reduction capacity. The absence of obvious color change at pH 10.0
indicates that the reduction capacity of PEI in H40-PEI-PEG for Au
ions decreases with increased pH. At low pH, amino functional
groups are protonated and the acidified ammonium has much
greater electron-donating ability for the reduction of Au ions than
the unprotonated amino groups at high pH [23]. However, the rate
of reduction of Au ions by H40-PEI-PEG at room temperature is
low. After the addition of NaBH4 to the above solution, the mixture
immediately takes on a wine red color (Line C in Fig. 1), and the
characteristic absorption of AuNPs (blue line in Fig. 2) is observed
at wavelengths ca. 536 nm for all pH’s, which indicates the forma-
tion of AuNPs after the reduction of Au ions. When the solution is
The Au colloids were characterized by transmission electron
microscopy (TEM). TEM was performed on a JEOL-3011 HREM
transmission electron microscope operated at accelerating voltage
200 kV. The samples for TEM measurements were prepared by
placing a drop of the freshly prepared colloidal solution on a
carbon-coated 300-mesh copper grid and allowing it to dry in air
naturally. Particle size was calculated by measuring the diameters
of samples from the corresponding TEM micrographs and particle
distribution was obtained using a Gaussian fit.
2.3. Catalytic reaction of 4-nitrophenol using hyperbranched polymer-
stabilized AuNPs as catalysts
The catalytic reduction occurred in a standard quartz cell with a
1 cm path length and about 2 mL volume. A fresh aqueous solution
of NaBH4 (0.1 M, 1.0 mL) was mixed with an aqueous solution of 4-
nitrophenol (2.0 ꢂ 10ꢀ4 M, 1.0 mL) in the quartz cell; this led to a
change of color from light yellow to yellow green. Immediately
after the addition of 2
lL of the prepared AuNPs, the absorption
spectra were recorded by a Perkin–Elmer Lambda Bio 40 UV–vis

Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
67
Mn+
Reduction
N
M0
nanoparticle
Reactant
N
N
N
N
N
N
Product
Hx-PEI-PEG
N
Scheme 2. Schematic illustration of the formation and catalytic reaction of hyperbranched polymer-stabilized AuNPs.
Fig. 1. Photographs of a mixed aqueous solution of HAuCl4 and H40-PEI-PEG (A), the mixture left to stand at room temperature for 1 h (B), addition of a fresh aqueous
solution of NaBH4 to the mixture (C), after 4 h (D), and after 24 h (E). The concentration of Au in all the solution is 250
and C4 are based on the ratio of nitrogen in the polymer to Au atoms, which are 0.25, 1, 5, and 10, respectively.
lM. The concentrations of H40-PEI-PEG for C1, C2, C3,
kept at room temperature for 4 h (Line D in Fig. 1), there is an obvi-
ous black precipitate at low concentrations of H40-PEI-PEG (espe-
cially such as C1) and the solution is still a beautiful wine red color
at high concentrations of H40-PEI-PEG (especially at C4). There-
fore, no symmetrical peak at wavelengths greater than 500 nm
could be observed for C1 concentration, and the characteristic
absorption of AuNPs is still remarkable for C4 concentration from
UV–vis absorption spectra (magenta line in Fig. 2). After being left
to stand at room temperature, the color (Line E in Fig. 1) and UV–
vis absorption (olive line in Fig. 2) of the solution do not show a
clear change, suggesting that 4 h of duration is enough for the
reduction of Au ions by NaBH4 within the H40-PEI-PEG solution.
These results confirm that H40-PEI-PEG is both the reduction agent
and the stabilizer for the formation of AuNPs.
Interestingly, Hx-PEI-PEG with low generations such as H20-
PEI-PEG (Fig. 3) and H30-PEI-PEG (Fig. 4) cannot effectively stabilize
AuNPs, and all the solutions at different concentrations and
different pH’s exhibit the black precipitate. It is suggested that
H20-PEI-PEG and H30-PEI-PEG have much a looser space structure
than H40-PEI-PEG, resulting in the escape of AuNPs from the
stabilizer and then the aggregation of AuNPs. This implies that
H40-PEI-PEG with the dense hyperbranched structure is a superior
stabilizer for AuNPs. Therefore, only H40-PEI-PEG-stabilized AuNPs
were used in the study of stability, morphology, and catalytic
applications.
TEM images of H40-PEI-PEG-stabilized AuNPs at different pHs
are presented in Fig. 5. The concentration of H40-PEI-PEG based
on the ratio of nitrogen in the polymer to Au atoms is 10. The
reduction time is 4 h at room temperature. Spherical nanosized
AuNPs with narrow particle size distributions are observed from
the TEM images. The corresponding particle size distribution his-
tograms of H40-PEI-PEG-stabilized AuNPs at different pHs are
shown in Fig. 5. The mean particle diameters statistically calcu-
lated are 8.07 1.42 nm, 7.63 0.93 nm, and 6.85 0.82 nm for
H40-PEI-PEG-stabilized AuNPs at pH 4.0, pH 7.0, and pH 10.0,
respectively. With the increase of pH, more uniform AuNPs with
smaller size are formed within the H40-PEI-PEG solution. At pH
4.0, PEI in H40-PEI-PEG exhibits the ability to form Au seeds from
Au ions, but the rate of reduction of Au ions by H40-PEI-PEG is low
and the residue Au ions continue to grow from Au seeds after the
addition of NaBH4, resulting in bigger AuNPs. At pH 10.0, H40-
PEI-PEG has no significant reduction capacity to form Au seeds,
and Au ions in the solution are reduced to AuNPs with time.
In our chemical reduction system, only water is used as a sol-
vent and the experiment is easily handled without heating and
stirring. More importantly, the H40-PEI-PEG-stabilized AuNPs
solution can be shelf-stored at room temperature for more than
1 month and the solution color has no perceptible change
(Fig. 6). PEI in H40-PEI-PEG plays a major role in absorbing AuCl4ꢀ
and can partially reduce AuClꢀ4 to AuNPs at pH < 7.0. Since

68
Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
Fig. 2. UV–vis absorption spectra of the solution at pH 4.0 (A–D), 7.0 (E–H), and 10.0 (I–L) in Fig. 1 at a scanning range of 200–700 nm.
Fig. 3. Photographs of a mixed aqueous solution of HAuCl4 and H20-PEI-PEG (A), the mixture left to stand at room temperature for 1 h (B), addition of a fresh aqueous
solution of NaBH4 (C), after 4 h (D), and after 24 h (E). The concentration of Au in all the solution is 250
on the ratios of nitrogen in the polymer to Au atoms, which are 0.25, 1, 5, and 10, respectively.
lM. The concentrations of H20-PEI-PEG for C1, C2, C3, and C4 are based
H40-PEI-PEG is an amphiphilic polymer, which exhibits a strong
tendency to self-assemble into core–shell micelles in aqueous solu-
tion [20], AuNPs are entrapped at the interlayer between the core
and shell, thus resulting in stable AuNPs.
3.2. Catalytic activity of hyperbranched polymer-stabilized AuNPs
Catalytic reduction is one of the important applications of
AuNPs [24]. Here, catalytic activity of H40-PEI-PEG-stabilized

Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
69
Fig. 4. Photographs of a mixed aqueous solution of HAuCl4 and H30-PEI-PEG (A), the mixture left to stand at room temperature for 1 h (B), addition a fresh aqueous solution
of NaBH4 (C), after 4 h (D), and after 24 h (E). The concentration of Au in all the solutions is 250
ratios of nitrogen in the polymer to Au atoms, which are 0.25, 1, 5, and 10, respectively.
lM. The concentrations of H30-PEI-PEG for C1, C2, C3, and C4 are based on the
Fig. 5. TEM images and the corresponding particle size distribution histograms of AuNP samples obtained within H40-PEI-PEG at pH 4.0 (A and D), 7.0 (B and E), and 10.0 (C
and F). The ratio of nitrogen in the polymer to Au atoms is 10. The reduction time is 4 h at room temperature. Scale bar: 20 nm.
AuNPs is tested by the reduction of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP), which is a well-known catalytic reaction in
industry [25]. After the addition of a fresh aqueous solution of
NaBH4, the absorption peak of 4-NP exhibits a red shift from 317
to 400 nm due to the formation of 4-nitrophenolate ions. The solu-
tion color has a significant change from light yellow to yellow-
green [26]. Without the addition of H40-PEI-PEG-stabilized AuNPs,
the mixture of 4-NP and NaBH4 sustains the solution color and the
absorption strength at 400 nm for a long time, which indicates that
NaBH4 itself cannot reduce 4-nitrophenolate ions. However, after
the addition of H40-PEI-PEG-stabilized AuNPs, the mixture under-
goes fading from yellow-green to colorless within about 15 min.
The kinetics of 4-NP reduction was investigated by UV–vis spec-
troscopy. Since the doses of H40-PEI-PEG-stabilized AuNPs used in
the catalytic reduction is very low, the absorption spectra are not
affected by the H40-PEI-PEG-stabilized AuNPs. Time dependent
UV–vis spectra and the corresponding plot of ln(Ct/C0) vs. time
are shown in Fig. 7. The concentration of H40-PEI-PEG, based on
the ratio of nitrogen in the polymer to Au atoms, is 10. The absorp-
tion peak of 4-nitrophenolate ion at 400 nm immediately
decreases with time after the addition of H40-PEI-PEG-stabilized
AuNPs. Two new peaks at 230 and 300 nm appear in the absorption
spectra of reduction products, indicating the formation of 4-AP.
The catalytic reduction of 4-NP is complete within 15 min, con-
firmed from the strength of the absorption peak at 400 nm and
the solution color change. As desired, a good linear correction of
ln(Ct/C0) vs. time is gained and the kinetic reaction rate constants
(k) at pH 4.0, 7.0, and 10.0 are calculated to be 0.153, 0.196, and

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Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
Fig. 6. Photographs of H40-PEI-PEG-stabilized AuNPs in aqueous solution after shelf storage for 2 days (A), 7 days (B), and 1 month (C).
Fig. 7. Time-dependent UV–vis spectra for the reduction of 4-NP and the corresponding plots of ln(Ct/C0) vs. time using AuNPs samples obtained within H40-PEI-PEG at pH
4.0 (A and D), 7.0 (B and E), and 10.0 (C and F). The ratio of nitrogen in the polymer to Au atoms is 10.
0.256 minꢀ1, respectively. k increases with the increase of pH. The
reason is probably that smaller particles at low pH are more active
owing to a larger number of surface atoms available for catalysis.
Therefore, H40-PEI-PEG is a good stabilizer in the preparation of
AuNPs and shelf-storage H40-PEI-PEG-stabilized AuNPs have high
catalytic activity for the reduction of 4-NP.
The previous studies on the reduction of 4-NP using polymer-
stabilized AuNPs catalysts are summarized together with our work
in Table 1. Compared with other reported polymer-stabilized
AuNPs catalysts for the reduction of 4-NP, the concentration of
significant reduction capacity for Au ions, the catalytic activity of
H40-PEI-PEG-stabilized AuNPs for the reduction of 4-NP at high
pH is very good. AuNPs prepared at different pHs exhibit different
Au particle radii (rAu) and the quality of AuNPs used in the catalytic
reduction of 4-NP is identical, so that Au areas (SAu) are different at
different pH. In order to examine the quantitative relation between
catalytic activity (k) and rAu, excluding the impact of SAu, SAu is cal-
culated as follows:
CAu ꢂ Vsol ꢂ MAu
3 ꢂ CAu ꢂ Vsol ꢂ MAu
SAu
¼
ꢂ 4
p
r2Au
¼
43 pr3Au
ꢂ
qAu
AuNPs (0.25 lM) in our work is very low and the as-prepared
rAu
ꢂ
qAu
H40-PEI-PEG-stabilized AuNPs can catalyzed the fast chemical
reduction of 4-NP to 4-AP. For similar catalytic activity, the number
of AuNPs that can effectively catalyze the reduction of 4-NP is
lower in this work (100-fold or more) than others. Thus, H40-
PEI-PEG-stabilized AuNPs exhibit good catalytic performance for
the chemical reduction of 4-NP. At low pH, H40-PEI-PEG has much
greater absorption and reduction ability for Au ions. But the parti-
cle size distribution of AuNPs obtained at low pH is not so good,
resulting in unsatisfactory catalytic activity for the reduction of
4-NP. Although H40-PEI-PEG at pH higher than 7.0 does not exhibit
where CAu is 0.25
lmol/L, Vsol is 2 mL, MAu is 197 g/mol, and qAu is
19.32 g/cm3. Thus, SAu = 0.153 ꢂ 10ꢀ7 (cm3)/rAu. The quantitative
relation between rAu and SAu with k at different pHs is listed in
Table 2. The rAu decreases with the increase of pH, resulting in an
increase of SAu. Higher SAu exhibits higher catalytic activity. After
the impact of SAu is excluded, the k/SAu increases with increased
pH. Therefore, the concentration of AuNPs and Au particle size,
including particle size distribution, are the most important factors
influencing the catalytic reduction of 4-NP.

Y. Dai et al. / Journal of Catalysis 337 (2016) 65–71
71
Table 1
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applications of dendrimer-encapsulated nanoparticles, J. Phys. Chem. B 109
(2005) 692–704.
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characterization, and kinetic evaluation of dendrimer-derived bimetallic Pt-
Ru/SiO2 catalysts, J. Catal. 269 (2010) 376–387.
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Matsumoto, C.T. Williams, M.D. Amiridis, Dendrimer-mediated synthesis of
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342.
[8] R.M. Crooks, M.Q. Zhao, L. Sun, V. Chechik, L.K. Yeung, Dendrimer-encapsulated
metal nanoparticles: synthesis, characterization, and applications to catalysis,
Acc. Chem. Res. 34 (2001) 181–190.
Comparison of the reduction of 4-NP using polymer-stabilized AuNPs catalysts.
Entry Polymer
Size of Au
(nm)a
CAu
M)
k
Ref.
(
l
(minꢀ1
)
1
2
3
PAMAMb
2.3
4.2
3.3
40
436
20
0.120
0.192
0.036
[27]
[28]
[29]
PDMAEMA-PSc
PNIPAM-co-
P4VPd
4
5
Caseine
2.6
7.6
50
0.25
0.233
0.196
[26]
This work
H40-PEI-PEGf
a
b
c
Observed by TEM.
Poly(amidoamine)-based hollow capsules.
Poly(2-(dimethylamino)ethyl methacrylate) grafted onto a solid polystyrene
core.
Poly(N-isopropylacrylamide)-co-poly(4-vinyl pyridine).
A protein thought of as amphiphilic block copolymers.
Aliphatic hyperbranched polyester–polyethylenimine–polyethylene glycol.
[9] Y.C. Zheng, S.P. Li, Z.L. Weng, C. Gao, Hyperbranched polymers: advances from
synthesis to applications, Chem. Soc. Rev. 44 (2015) 4091–4130.
[10] K. Kim, H.B. Lee, J.W. Lee, H.K. Park, K.S. Shin, Self-assembly of poly
(ethylenimine)-capped Au nanoparticles at
a toluene-water interface for
d
e
f
efficient surface-enhanced Raman scattering, Langmuir 24 (2008) 7178–7183.
[11] J. Su, L. Zhang, Q. Wu, Amphiphilic dendritic polymers, Prog. Chem. 20 (2008)
1980–1986.
[12] Q. Tang, F. Cheng, X.L. Lou, H.J. Liu, Y. Chen, Comparative study of thiol-free
amphiphilic hyperbranched and linear polymers for the stabilization of large
gold nanoparticles in organic solvent, J. Colloid Interface Sci. 337 (2009) 485–
491.
Table 2
[13] N. Hu, J.Y. Yin, Q. Tang, Y. Chen, Comparative study of amphiphilic
hyperbranched and linear polymer stabilized organo-soluble gold
nanoparticles as efficient recyclable catalysts in the biphasic reduction of 4-
nitrophenol, J. Polym. Sci. Polym. Chem. 49 (2011) 3826–3834.
[14] S. Skaria, R. Thomann, C.J. Gomez-Garcia, L. Vanmaele, J. Loccufier, H. Frey, S.E.
Stiriba, A convenient approach to amphiphilic hyperbranched polymers with
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stabilizers rendering gold nanoparticles with an unprecedentedly long lifetime
in the catalytic reduction of 4-nitrophenol, J. Mater. Chem. 22 (2012) 21173–
21182.
Quantitative relation of Au particle size (rAu) and Au area (SAu) with catalytic activity
(k) at different pH’s.
pH
4.0
7.0
10.0
rAu (nm)
SAu (cm2)
k (minꢀ1
)
k/SAu (minꢀ1 cmꢀ2
)
4.035 0.71
3.815 0.46
3.425 0.41
0.039 0.0069
0.041 0.0052
0.045 0.0054
0.153
0.196
0.256
3.9 0.6
4.8 0.5
5.7 0.6
4. Conclusions
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In summary, we describe a facile preparation of H40-PEI-PEG-
stabilized AuNPs at room temperature. The preparation route is
mild and low-cost. The particle sizes of AuNPs are sub-10 nm,
which is adjusted by changing the solution pH. PEI in H40-PEI-
PEG can reduce Au ions, and the reduction ability decreases with
increased pH. AuNPs are stabilized at the interlayer between the
core and the shell due to the absorption of PEI to Au ions and
AuNPs. H40-PEI-PEG-stabilized AuNPs at high pH have superior
catalytic activity for the reduction of 4-NP. H40-PEI-PEG will hope-
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Acknowledgments
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This work was supported by the Natural Science Foundation of
Hubei Province (2015CFB697) and the State Key Laboratory of
Advanced Technology for Materials Synthesis and Processing
(Wuhan University of Technology).
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A
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