Preparation and catalytic activity of poly(N-vinyl-2-pyrrolidone)-protected Au nanoparticles for the aerobic oxidation of glucose

Article abstract of DOI:10.1166/jnn.2014.8839

PVP-protected Au nanoparticles (NPs) for the aerobic oxidation of glucose were prepared by using NaBH₄ reduction method. The effects of processing parameters such as Au³⁺ ion concentration, reaction temperature, ratio of NaBH₄ or PVP to Au³⁺, and solvent composition on their particle sizes and catalytic activities were studied in detail and the synthesis conditions optimized. As-prepared Au NPs possessed a FCC structure, with an average size varying from about 100 to 2.6 nm depending on their preparation conditions. The size changes affected their catalytic activities in the aerobic oxidation of glucose. The Au NPs with the average size of 2.6 nm prepared under the optimal conditions showed a high instantaneous catalytic activity as well as a high long-time stability. Based on the kinetic study on the glucose oxidation over the PVP-protected Au NPs, the corresponding apparent activation energy was determined as 82 kJ mol^-1. Copyright

Full text of DOI:10.1166/jnn.2014.8839

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 5743–5751, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Preparation and Catalytic Activity of Poly(N-vinyl-2-
pyrrolidone)-Protected Au Nanoparticles for the
Aerobic Oxidation of Glucose
Haijun Zhang1 , Wenqi Li , Yajun Gu , and Shaowei Zhang
ꢀ2ꢀ∗
1ꢀ2
1ꢀ2
2ꢀ3
1College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan,
Hubei Province 430081, China
State Key Laboratory Breeding Base of Refractories and Ceramics, Wuhan University of Science and Technology,
Wuhan 430081, China
2
3
College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK
PVP-protected Au nanoparticles (NPs) for the aerobic oxidation of glucose were prepared by using
3
+
NaBH4 reduction method. The effects of processing parameters such as Au ion concentration,
3
+
reaction temperature, ratio of NaBH or PVP to Au , and solvent composition on their particle sizes
4
and catalytic activities were studied in detail and the synthesis conditions optimized. As-prepared Au
NPs possessed a FCC structure, with an average size varying from about 100 to 2.6 nm depending
on their preparation conditions. The size changes affected their catalytic activities in the aerobic
Delivered by Publishing Technology to: Adelaide Theological Library
oxidation of glucose. The Au NPs with the average size of 2.6 nm prepared under the optimal
IP: 31.184.194.81 On: Thu, 18 Feb 2016 09:11:54
conditions showed a high i nC s ot ap ny t ra i ng eh ot :u sA mc a et ar il yc tai cn a Sc tci vi ei t ny t ai f si cw Pe ul l ba l si s ah eh ri gs h long-time stability. Based
on the kinetic study on the glucose oxidation over the PVP-protected Au NPs, the corresponding
−
1
apparent activation energy was determined as 82 kJ mol
.
Keywords: Au, Nanoparticles, Catalytic Activity, Glucose Oxidation, Preparation.
1
. INTRODUCTION
atoms on the NP surface are coordinated to the polymer,
making the polymer-protected metal NPs rather stable
Intensified attention was paid to nanomaterials in recent
years.1 Because of their many unique properties, gold
nanoparticles (NPs) have attracted increasing attention
over the past two decades, and found wide applications in
electronics, optics,
involving biosensors, DNA hybridization, and catalysts
2
2
–8
in a dispersion and catalytically more active. A recent
study found that colloidal dispersions of poly(N-vinyl-2-
pyrrolidone) (PVP)-protected Au NPs were an active cat-
alyst for the aerobic oxidation of alcohol, in particular
9
ꢀ10
bio- and nano-technological fields
2
3ꢀ24
1
1
12
when their size was sufficiently small.
Our work also
1
3ꢀ14
found that PVP-protected Ag/Au bimetallic and Au/Pt/Ag
trimetallic NPs were highly active in the selective aerobic
in particular.
The great interest in using gold as a
catalyst was triggered by the finding that its nanoparti-
cles were surprisingly active for the low temperature CO
2
5ꢀ26
oxidation of glucose.
oxidation.1
3ꢀ14
Au NPs can be readily synthesized by using the well
Today a supported gold catalyst is known to
documented NaBH reduction method. Nevertheless, the
be active for a variety of reactions including oxidation of
4
3
+
alcohols15 and aldehydes, hydrochlorination of ethyne,
16
17
effects of processing parameters such as Au ion con-
3
+
centration, reaction temperature, NaBH (or PVP) to Au
4
1
8–20
carbon–carbon or carbon–silicon bond formation
epoxidation of propene.
Polymer-protected metal NPs can be prepared via reduc-
tion of the metal ions coordinated by polymers. The metal
and
2
1
ratio, and etc. on the morphologies/sizes and catalytic
activities of Au NPs for the glucose oxidation are still
not fully understood, and the synthetic conditions are
not optimized. In addition, several other important issues
remain unclear, e.g., the dependence of catalytic activities
of PVP-protected Au NPs on their particle sizes, and the
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 8
1533-4880/2014/14/5743/009
doi:10.1166/jnn.2014.8839
5743

Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
Zhang et al.
kinetics of the catalyzed glucose oxidation reaction as well
as the corresponding activation energy. To address these
issues, a systematic study was carried out by the authors,
and the main results and findings are now presented and
discussed in this paper.
(RIGAKU RINI-Ultim III/SAXS diffractometer). Patterns
ꢀ
ꢀ
were recorded between 10 and 90 (2ꢁ) at 40 kV and 40
mA using Ni filtered Cu Kꢂ radiation (ꢃ = 0ꢄ154187 nm).
2.4. Glucose Oxidation at Controlled pH
The glucose oxidation was used as a model reaction to
evaluate and compare the catalytic effects of Au catalysts,
and the product of the reaction is gluconic acid. It was
2
. EXPERIMENTAL PROCEDURE
2
.1. Materials
ꢀ
carried out at 60 C in a 50-mL glass beaker placed in
Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl ·
4
a ∼2000 mL thermostat. pH of the reaction suspension
4
H O, 99.9%), sodium borohydride (NaBH , 99.0%),
2 4
was controlled at 9.4 by an automatic potentiometric titra-
PVP (K35, molecular weight about 35,000) and glucose
99.9%) supplied by Wako Pure Chemical Industries, Ltd.,
−
1
tor using 1 mol L NaOH solution. Oxygen was bubbled
through the suspension magnetically stirred, at 100 mL
(
were used directly as the main raw materials without fur-
ther purification. All glassware and Teflon-coated magnetic
stirring bars were cleaned with aqua regia, followed by
repeated rinsing with pure water.
−
1
min under atmospheric pressure. The initial concentra-
−1
tion and volume of the glucose solution were 0.264 mol L
and 30 mL, respectively, and the catalyst charged was about
2
2
mg. The catalytic reactions automatically proceeded for
hour. The catalytic activity (based on the total amount of
2
.2. Preparation of Dispersions of PVP-Protected
Au Nanoparticles
the metal used, rather than on its total surface area) was cal-
culated from the slope of the linear plot of NaOH amount
vs reaction time (not shown here), and the average activity
over the 2 hour reaction period was taken for comparison.
The activity was calculated based on the NaOH consump-
tion as follows:
A HAuCl ·4H O solution was combined with a PVP solu-
4
2
tion at various temperatures for 15 min, followed by drop-
wise addition of a NaBH solution for about 1 h, under
4
continuous vigorous stirring. Upon 3 h further homogeniza-
tion stirring, a transparent brownish (or reddish) colloidal
dispersion of PVP-protected Au NPs was obtained. It
was filtered using an ultrafilter membrane with a cutoff
Activity = nNaOH/nAu
(1)
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molecular-weight of 10,000, prio IrP t :o 3t 1w . 1i c 8e 4 w. 1a 9t e4 r. 8w 1a sOh ni n: gThu, 18 Feb 2016 09:11:54
where n
in is the NaOH consumption in 1 hour (mol-
NaOH
−1
and once ethanol washing in nitrogen tCo or ep my roi gv he t a: n Ay me xe cr ei cs a- n Scientific Publishers
NaOH · h ) which can be calculated based on the slope
sive reagents and byproducts. After removal of the residual
ethanol in a rotary evaporator at 40 C and further vacuum
drying at 40 C for 24 hour, PVP-protected Au NPs were
finally obtained.
of the linear plot of NaOH amount vs reaction time, and
ꢀ
−1
nAu is the amount of the charged Au NPs (mol-Au ).
ꢀ
Since the reaction of glucose oxidation can be expressed
as follows,
Au NPs
2
.3. Characterization of PVP-Protected
Au Nanoparticles
2
C H O +O −− − −→ 2C H O
(2)
(3)
6
12
6
2
6
12
7
Ultraviolet and visible light (UV-vis) absorption spectra
of the resultant Au nanoparticles were recorded over the
range of 200–800 nm by a Shimadzu UV-2500PC record-
ing spectrophotometer equipped with a quartz cell with a
C H O +NaOH → C H O Na +H O
6
12
7
6
11
7
2
thus, the convention of the gluconic acid can be calculated
based on the consumption of the NaOH, and then the activ-
ity of the Au NPs can be obtained according to Eq. (1).
The selectivity to gluconic acid of the Au NPs is about
100%. Catalytic activities in all cases were measured
at least twice under identical conditions and the mean val-
ues were taken.
The long-time catalytic stability of the NPs was investi-
gated by repeating the aerobic oxidation batch. The activ-
ity in the second batch was immediately evaluated after
the first batch without isolation of the NPs, i.e., using the
same catalytic solution after the first batch. The molar ratio
of glucose to NPs was kept almost constant via adding a
small amount of solid glucose into the solution before the
second batch.
1
0-mm optical path length.
A colloidal ethanol solution of Au was ultra-sonicated
2
7
for 10 min to improve the dispersion, and one or two drops
of it were dropped onto a copper microgrid covered with
a thin amorphous carbon film, followed by evaporating the
ethanol at room temperature in the atmosphere. Microstruc-
tural morphologies of Au nanoparticles on the microgrid
were observed by using a transmission electron microscope
(TEM) (JEOL TEM 1230) at the accelerating voltage of
8
0 kV. For each sample, at least 200 particles from different
parts of the grid were generally analyzed by using the
iTEM software, so that their mean diameter and size dis-
tribution could be evaluated.
Phases and crystalline structure of Au NPs were
identified by using an X-ray diffractometer (XRD)
In addition, to determine the apparent activation
energy for the aerobic oxidation of glucose over the
5
744
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014

Zhang et al.
Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
PVP-protected Au NPs, reaction temperature was varied
from 50 and 80 C while the glucose and Au concentra-
tions were kept constant.
The Au atoms formed in this stage might collide mainly
with the existing nuclei, rather than forming new nuclei,
thus resulting in larger particles. With increasing RNaBH4
ꢀ
(up to 5), the reduction rate increased, generating many
more nuclei, thus leading to the formation of smaller par-
ticles. However, upon increasing RNaBH4 to >5, tiny Au
NPs would start to agglomerate together, forming large Au
3
. RESULTS AND DISCUSSION
3
+
3
.1. Effect of Molar Ratio of NaBH to Au
4
A series of colloidal dispersions of Au NPs were pre-
pared by dropwise addition of a NaBH solution into the
corresponding Au in the presence of PVP. Particle size
changes with the amount of reducing reagent were exam-
ined. Figure 1 shows representative TEM images and size
distribution histograms of Au NPs prepared with various
particles due to the destabilizing effect from the excessive
4
28
NaBH electrolyte. The prepared PVP-protected Au NPs
3
+
4
were used as the catalyst for the aerobic oxidation of glu-
ꢀ
cose in water at 60 C. Their catalytic activity increased
with their size reduction (The left Y axis, Fig. 2). The
−1
3
+
highest catalytic activity of 1, 230 mol-glucose·h ·mol-
molar ratios of NaBH to Au (RNaBH4 ). The average
4
−1
Au was achieved with the smallest Au NPs prepared
sizes (± standard deviation) of Au NPs corresponding to
using RNaBH4 = 5.
R
= 1, 2.5, 5 and 10 were estimated to be 54ꢄ8 ±
NaBH4
4
4ꢄ7 nm, 30ꢄ9 ± 12ꢄ0 nm, 3ꢄ7 ± 1ꢄ8 nm, and 25ꢄ1 ± 12ꢄ9
+
3
.2. Effect of Molar Ratio of PVP to Au3
nm, respectively, based on the TEM images, indicating
that the mean particle diameter initially decreased from
PVP concentration in the reaction solution is also an impor-
tant parameter greatly affecting the preparation of Au NPs
and their final sizes. Figure 3 presents TEM images and size
distribution histograms of Au NPs obtained with various
5
R
4.8 nm to the smallest (3ꢄ7 ± 1ꢄ8 nm) with increasing
from 1.0 to 5.0, but started to drastically increase
NaBH4
upon further increasing RNaBH4 . This can be attributed to
the influence of reduction rate on the nucleation. At a low
RNaBH4 , the reduction rate of Au was slow and only a few
Au nuclei were formed in the early stage of the reduction.
3
+
molar ratios of PVP to Au (RPVP). The average particle
3
+
size of Au NPs (2ꢄ9± 1ꢄ1 nm) prepared with R = 40 was
PVP
smaller than that in the case of R = 20 (3ꢄ6± 1ꢄ3 nm).
PVP
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5
4
3
2
1
0
0
0
0
0
0
D
av = 25.1nm
40
D = 3.7 nm
av
IP: 31.184.194.81 On: Thu, 18 Feb 2016 09:11:54
s = 1.8 nm
s = 12.9 nm
3
0
Copyright: American Scientific Publishers
2
0
0
0
1
7
–8 9–10 10–20 20–5050–100
1–2 2–3 3–4 4–5 5–6 6–7 7–8 8–910–20
Diameter/nm
Diameter/nm
5
00 nm
100 nm
(
a) RNaBH4 = 10
(b) RNaBH4 = 5
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
Dav = 30.9 nm
s = 12.0 nm
50
D
av = 54.8 nm
s = 44.7 nm
4
3
0
0
20
1
0
0
5
–6 10–20 20–50 50–100
Diameter/nm
8–9 10–20 20–50 50–100 100–200
Diameter/nm
5
00 nm
2 µm
(
c) RNaBH4 = 2.5
(d) RNaBH4 = 1
Figure 1. TEM images of Au NPs prepared with various amounts of NaBH (R = 40, [Au +ꢅ = 0ꢄ66 mM, [NaBH ꢅ = 66 mM, 25 C, NaBH →
3
ꢀ
4
PVP
4
4
HAuCl , water).
4
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014
5745

Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
Zhang et al.
6
5
4
3
2
1
0
0
0
0
0
0
0
3.3. Effect of the HAuCl Concentration
4
1
200
The average particle size of Au NPs is also closely related
3
+
to the Au concentration. Figure 4 illustrates typical TEM
images and size distribution histograms of Au NPs resultant
8
4
00
00
0
3
+
from various concentrations of Au . At the concentration
of 0.66 mM, Au NPs with average size of 3.6 nm were pro-
duced. Upon increasing the concentration to 1.32 mM, the
average particle size decreased to 2.7 nm. However, upon
further increasing the concentration to 1.98 mM, the aver-
age particle size increased again, to 3.1 nm. These results
suggested that a medium gold concentration is essential for
0
.0
2.5
5.0
7.5
10.0
RNaBH4, mol ratio
3
2
2
1
1
0
5
0
5
0
5
0
Dav = 3.6 nm
s = 1.3 nm
Figure 2. Catalytic activities of Au NPs prepared with various amounts
3
+
of NaBH4 for the glucose oxidation (R
= 40, [Au ꢅ = 0ꢄ66 mM,
4
PVP
ꢀ
[
NaBH ꢅ = 66 mM, 25 C, NaBH → HAuCl , water).
4
4
This was because in the former, the particles were fully
covered by PVP and thus effectively isolated by the steric
effect from the PVP, avoiding the particle agglomeration
and preventing the subsequent grain growth; whereas in
the latter, due to the incomplete coverage of the particles
with less PVP, the particle agglomeration and subsequent
growth occurred.
1
–2 2–3 3–4 4–5 5–6 6–7 7–8 9–10
Diameter/nm
1
00 nm
(
a) [Au3+] = 0.66 mM
3
2
2
1
1
0
5
0
5
0
5
0
Dav = 3.6 nm
s = 1.3
50
Dav = 2.7 nm
De
nmlivered by Publishing Technology to: Adelaide Theological Library
40
s = 0.8 nm
IP: 31.184.194.81 On: Thu, 18 Feb 2016 09:11:54
3
0
Copyright: American Scientific Publishers
20
10
0
1
–2 2–3 3–4 4–5 5–6 6–7 7–8 9–10
Diameter/nm
1
–2
2–3
3–4
4–5
Diameter/nm
1
00 nm
1
00 nm
(
a) RPVP = 20
b) [Au3+] = 1.32 mM
(
3
5
0
5
0
5
0
5
0
Dav = 2.9 nm
s = 1.1 nm
50
40
Dav = 3.1 nm
s = 1.9 nm
3
2
2
1
1
3
2
1
0
0
0
0
1
–2 2–3 3–4 4–5 5–6 6–7
Diameter/nm
1
–2 2–3 3–4 4–5 5–6 6–7
Diameter/nm
1
00 nm
100 nm
(
b) RPVP = 40
(c) [Au3+] = 1.98 mM
Figure 3. TEM images of Au NPs prepared with various amounts
Figure 4. TEM images of Au NPs prepared with various concentrations
3+
ꢀ
ꢀ
of PVP (RNaBH4 = 10, [Au ꢅ = 0ꢄ66 mM, [NaBH ꢅ = 33 mM, 0 C,
of HAuCl (R = 40, RNaBH4 = 10, [NaBH ꢅ = 33 mM, 0 C, NaBH →
4
4
PVP
4
4
NaBH → HAuCl , water).
HAuCl , water).
4
4
4
5
746
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014

Zhang et al.
Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
producing tiny Au NPs. In the present work, the small-
can affect the particle sizes of the final products. Figure 5
gives TEM images and size distribution histograms of the
3
+
est Au NPs were resulted from the Au ion concentra-
tion of 1.32 mM. This could be explained as follows:
final Au NPs obtained using various NaBH concentrations
4
3
+
when the Au concentration was lower than 1.32 mM
for example 0.66 mM), the concentration of metal atoms
in solution was below the threshold for reaching the so-
at room temperature while other parameters were fixed.
(
When the NaBH concentration was 6.6 mM, the average
4
size of Au NPs was 3ꢄ6± 1ꢄ1 nm (Fig. 5(a)). On increasing
the concentration to 16.5 mM, the average size became the
smallest, 3ꢄ3± 0ꢄ8 nm based on the TEM image shown in
Figure 5(b). However, upon increasing the concentration to
33 and 66 mM, the average particle size increased signifi-
cantly to 10ꢄ6± 11ꢄ9 nm and 25ꢄ1± 12ꢄ9 nm, respectively
(based on Figs. 5(c) and (d)). Comparison of catalytic
2
9
called ‘supersaturation.’ As a result, the nucleation pro-
cess could not finish in a short time and the nucleation
overlapped with the growth, leading to the formation of
“
large” particles. On the other hand, when the concen-
3
+
tration of Au was >1.32 mM (for example 1.98 mM),
more frequent collisions would occur between the nuclei
and the formed atoms, also resulting in “large” particle.
The smaller Au NPs prepared under the Au concentra-
tion of 1.32 mM were found to be catalytically more active
activities of Au NPs prepared at various NaBH concen-
4
3
+
trations revealed that Au NPs prepared under the NaBH4
concentration of 16.5 mM exhibited the greatest catalytic
activity.
−1
−1
(
2,060 mol-glucose·h ·mol-Au ) than the Au NPs pre-
3
+
pared respectively under the Au concentrations of 0.66
−
1
and 1.32 mM (1,630 and 1,950 mol-glucose · h · mol-
3.5. Effect of Reaction Temperature
−
1
Au , respectively). Based on the results and discussion
Shown in Figure 6 are TEM images and size distribution
3
+
above, 1.32 mM is considered to be the optimal Au con-
centration for preparing small catalytically active Au NPs.
ꢀ
histograms of product nanoparticles obtained at 0–25 C
while other conditions were identical, revealing the small-
ꢀ
est 2.8 nm particles formed at 0 or 8 C, and 3.6 nm
ꢀ
3
.4. Effect of the NaBH Concentration
particles at 25 C, respectively. The former arose from
4
NaBH concentration in the reaction solution is another
key parameter in the kinetically controlled process and
the quick generation of more Au nuclei whereas the lat-
ter from the increased collisions between the so-called
4
4
3
2
1
0
0
0
0
0
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5
0
D
= 3.6 nm
av
D = 3.3 nm
IP: 31.184.194.81 On: Thu, 18 Feb 2016 0av 9:11:54
4
0
s = 1.1 nm
s = 0.8 nm
Copyright: American Scientific Publishers
3
2
1
0
0
0
0
1
–2 2–3 3–4 4–5 5–6 6–7 7–8
Diameter/nm
1–2 2–3 3–4 4–5 5–6
Diameter/nm
1
00 nm
100 nm
(
a) [NaBH ] = 6.6 mM
(b) [NaBH ] = 16.5 mM
4
4
Dav = 10.6 nm
s = 11.9 nm
50
Dav = 25.1 nm
s = 12.9 nm
2
1
1
0
5
0
5
0
4
3
2
1
0
0
0
0
0
2
–3 4–5 6–7 8–9 10–20 50–100
Diameter/nm
7–8 9–10 10–20 20–50 50–100
Diameter/nm
2
00 nm
200 nm
(
c) [NaBH ] = 33 mM
(d) [NaBH ] = 66 mM
4
4
Figure 5. TEM images of Au NPs prepared with various concentrations of NaBH4 (RPVP = 40, RNaBH4 = 10, [Au +ꢅ = 0ꢄ66 mM, 0 ꢀC, NaBH →
3
4
HAuCl , water).
4
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014
5747

Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
Zhang et al.
3
0
Dav = 2.8 nm
s = 0.8 nm
Dav = 5.8 nm
s = 2.3 nm
4
3
2
1
0
0
0
0
0
25
20
1
1
5
0
5
0
1
–2 2–3 3–4 4–5 5–6
Diameter/nm
3–4
5–6
7–8 9–10 20–50
Diameter/nm
1
00 nm
200 nm
(
a) 0 ºC
(
a) HAuCl → NaBH
4 4
3
2
2
0
5
0
4
3
2
1
0
0
0
0
0
Dav = 2.8 nm
s = 0.8 nm
Dav = 3.9 nm
s = 1.7 nm
15
1
0
5
0
1
–2 2–3 3–4 4–5 5–6
Diameter/nm
2
–3
4–5
6–7
8–9 10–20
Diameter/nm
1
00 nm
2
00 nm
(
b) 8 ºC
Delivered by Publishing Technology to: Adelaide Theol(obg) iNcaaBlHLi b→ raHrAyuCl4
4
3
0
5
0
5
0
5
0
Dav = 3.6 nm
s = 1.3 nm
IP: 31.184.194.81 On: Thu, 18 Feb 2016 09:11:54
2
2
1
1
Copyright: American S cF i ieg un rt ei f i7 c. PT uE bM lisi mh ea gr es s of Au NPs prepared with different addition
3+
sequences (R
= 40, R
= 10, [Au ꢅ = 0ꢄ66 mM, [NaBH ꢅ =
PVP
NaBH
4
4
ꢀ
6ꢄ6 mM, 0 C, water).
in rather large-sized Au NPs (5.8 nm in average as shown
in Fig. 7(a)), whereas addition of NaBH to HAuCl led to
1
–2 2–3 3–4 4–5 5–6 6–7 7–8 9–10
Diameter/nm
4
4
the formation of much smaller Au particles with slightly
changed morphology (3.9 nm in average as shown in
Fig. 7(b)). The reason for this is considered to be similar
to that given in the case of RNaBH4
.
1
00 nm
(
c) 25 ºC
3.7. Effect of Solvent Composition
As shown in Figure 8, solvent composition markedly
affected the morphology of the product in the presence of
PVP while keeping the other conditions identical. When
water was used as the solvent, spherical Au NPs with
the average size of 3ꢄ6 ± 1ꢄ3 nm were formed. However,
when the mixed ethanol and water (in the volume ratio 8
to 2) was used as the solvent, the morphology of the final
product was completely changed. TEM images (Fig. 8)
show that Au nanorods were formed as the major prod-
uct along with a few spherical Au NPs, this suggests that
ethanol had facilitated the formation of the Au nanorods.
The catalytic activity of the prepared Au nanorods for
aerobic glucose oxidation is about 1040 mol-glucose ·
Figure 6. TEM images of Au NPs prepared at various reaction temper-
3+
atures (RPVP = 40, R
= 10, [Au ꢅ = 0ꢄ66 mM, [NaBH ꢅ = 33 mM,
NaBH
4
4
NaBH → HAuCl , water).
4
4
‘
sub-clusters.’ The former were found to be catalytically
−
1
−1
more active (1,760 and 1,740 mol-glucose·h ·mol-Au ,
respectively.) than the latter (1,630 mol-glucose·h ·mol-
Au ).
−
1
−
1
3
.6. Effect of Addition Sequence of
NaBH and HAuCl
4
4
It was found that the addition sequence of NaBH and
4
HAuCl had an important effect on the morphology of
4
−
1
−1
the final Au particles when the other conditions remained
identical. Dropwise addition of HAuCl to NaBH resulted
h
· mol-Au . Although this value is a little lower than
that of the Au NPs prepared by using water as solvent
4
4
5
748
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014

Zhang et al.
Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
3
2
2
1
1
0
5
0
5
0
5
0
3
.0
Dav = 3.6 nm
s = 1.3 nm
2.5
2.0
1
1
0
0
.5
.0
.5
.0
1
–2 2–3 3–4 4–5 5–6 6–7 7–8 9–10
Diameter/nm
1
00 nm
2
00
300
400
500
600
700
800
(
a) H O
Wavelength/nm
2
3
3
2
2
1
1
5
0
5
0
5
0
5
0
(a) UV-Vis spectrum
Dav = 12.0 nm
s = 7.2 nm
35
Dav = 2.6 nm
s = 1.1 nm
30
25
2
1
0
5
3
–4
5–6
7–8 9–10 20–50
10
5
Diameter/nm
0
0
–1 1–2 2–3 3–4 4–5 5–6
Diameter/nm
1
00 nm
(
b) E/W = 8/2
1
00 nm
Delivered by Publishing Technology to: Adelaide Theological Library
Figure 8. TEM images of Au NPs pIr Pe p: a 3r e 1d . 1w 8i t 4h . 1v a9r i4o .u 8s 1v oOl u nm :eThu, 18 Feb 2016 09:11:54
3
+
(b) TEM image
ratios of ethanol to water (R = 40, R
=C1 o0 ,p [yA r ui ghꢅt=: A0 ꢄ m6 6e mr iMc a, n Scientific Publishers
PVP
NaBH
4
ꢀ
[
NaBH ꢅ = 6ꢄ6 mM, NaBH → HAuCl , 0 C).
4
4
4
1
: Au
1
1,630 mol-glucose·h− ·mol-Au ), it should be point
1
−1
(
that, to the best of our knowledge, this is the first report to
date for the preparation of Au nanorods using such a sim-
ple method by just changing the solvent composition, and
it could provide a simple but novel technical route to the
large scale production of Au nanorods. The mechanism for
the formation of Au nanorods is still unclear, and we think
the different morphology of the formed Au nanomaterials
may be related with the various solubility of PVP in these
two kinds of solvent. Further work on this is ongoing and
the results will be reported in a future publication.
1
2
0
40
60
80
2Theta/Degree
c) XRD
Figure 9. UV-Vis spectrum, TEM image, and XRD of Au NPs prepared
(
3
+
under the optimal conditions (RPVP = 40, R
= 5, [Au ꢅ = 1ꢄ32 mM,
NaBH
4
ꢀ
[
NaBH ꢅ = 16ꢄ5 mM, NaBH → HAuCl , 0 C, water).
4
4
4
3
.8. Characterization and Catalytic Activity of Au
NPs Prepared Under the Optimal Conditions
The average particle size and the catalytic activity of Au
NPs prepared under various conditions were examined. To
having the average particle size of 2ꢄ6 ± 1ꢄ1 nm. XRD
(Fig. 9(c)) only detected a clear diffraction peak corre-
sponding to the (111) plane, suggesting that the prepared
particles had the face centered cubic (fcc) structure.
The catalytic activity of NPs generally increases with
decreasing in their sizes because of the increased total
surface area with the size reduction. In the present work,
the smallest Au NPs (2ꢄ6 ± 1ꢄ1 nm) prepared under the
optimal conditions were found to be catalytically most
synthesize small Au NPs, a NaBH aqueous solution was
4
−
added dropwise into a mixed aqueous solution of AuCl4
and PVP under the optimal conditions described and dis-
cussed above. UV-Vis absorption spectrum of the Au NP
aqueous dispersion (Fig. 9) exhibits a small absorbance
at 520 nm, arising from the surface plasmon resonance
absorption of metallic Au NPs. In fact, TEM images
−
1
−1
(Fig. 9(b)) reveal that the prepared Au NPs were small,
active (2,170 mol-glucose · h · mol-Au ) among all the
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014
5749

Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
Zhang et al.
prepared Au NPs. Prüße et al. synthesized Au colloids
2500
2000
1500
1000
by reducing tetrachloroauric acid (HAuCl ) with sodium
4
borohydride (NaBH ) in the presence of a polymer stabi-
4
3
0
lizing agent in an aqueous solution, and found that use
of three polymers, i.e., poly(diallyl dimethyl ammonium
chloride) (PDADMAC), chitosan and poly-2,2-trimethyl
ammonium methylmethachloride (PTAMM), could gener-
ate colloids with substantial catalytic activity for the glu-
−
1
500
cose oxidation. The highest activity (157 mol-glucose·h
·
−
1
mol-Au ) and selectivity (99.8%) for the glucose oxida-
tion were achieved with the Au-PDADMAC colloid. The
other two colloids led to lower catalytic reaction rates (70
and 50 mol-glucose · h · mol-Au for Au-chitosan and
Au-PTAMM, respectively) and thus, a lower selectivity,
i.e., 99.2%, due to the fructose formation via isomeri-
sation. Based on these, it can be considered that PVP-
protected Au NPs prepared with the present work could
be used as a very promising active catalyst for the aerobic
oxidation of glucose.
0
0
20
40
60
80
−1
−1
Particle size/nm
Figure 11. Relationship between average sizes and catalytic activities
(2 hour) of PVP-protected Au NPs for the glucose oxidation.
initial catalytic stage was reported as 18,043 mol-glucose·
−1
−1 31
h
·mol-Au . However, it needs to be pointed that the
catalytic activity of the ‘naked’ Au NPs decreased abruptly
due to the agglomeration after the initial stage. As men-
tioned above, the PVP-protected Au NPs prepared with
this work could retain their catalytic activity for at least
Nevertheless, the 2 h functional time is still not suffi-
ciently long for future industrial applications. Considering
difficulties in the evaluation of NPs reusability, the long-
term activity of the Au NPs for the glucose oxidation was
investigated, without isolating them from the tested glu-
cose solution (Fig. 10). The catalysts were continuously
tested in a successive batch immediately after the previ-
ous one. The results (Fig. 10) showed that the Au NPs did
1
0 h, exhibiting clearly a much longer catalytic stability
than their ‘naked’ counterparts.
The catalytic activities of metallic NPs were highly sen-
sitive to their size. As shown in Figure 11, the catalytic
Delivered by Publishing Technology at oc t: i vA i dt ye fl ao ir dt eh eT oh xe i od al ot igo i nc ao lf LD i b- gr al ur cy ose in an aqueous phase
not lose much of their activities after several runs, e.g.,
IP: 31.184.194.81 On: Thu,i 1n c8 r eF a es eb d2 g0 r1a 6d u 0a 9l l :y1 1w : 5i t 4h decreasing the Au NPs diameter
about 80% of the initial catalytic ac Ct i vo ipt yy r is gt ihl lt :r Ae mm ae i rn i ec da n Scientific Publishers
from 80 nm to 5 nm and then increased dramatically upon
further decreasing the diameters to below 5 nm. When the
activities are normalized by the surface atoms of the NPs,
the similar dependence can be also obtained for the pre-
pared Au NPs (the results are not shown here). As the
metal particle size decreases, the relative proportions of
surface atoms at the corners and edges increase, providing
more catalytically active sites for the aerobic oxidation of
glucose. It should be mentioned that the size dependence
of catalytic activities of PVP-protected Au NPs for the
aerobic oxidation of glucose has not been investigated in
detail yet by others.
even after five cycles, suggesting that the high catalytic
activity of the Au NPs could last for at least 10 h. In a
word, the as-prepared Au NPs exhibited not only a high
instantaneous catalytic activity but also a high catalytic
long-time stability.
The specific molar activity of ‘naked’ Au colloidal NPs
(with a mean diameter of 3.6 nm) during 200 second of an
100
8
6
4
2
0
0
0
0
0
3
.9. Kinetics of Au NPs Catalyzed Aerobic
Oxidation of Glucose
To the best of our knowledge, the apparent activation
energy (E ) for the aerobic oxidation of glucose over PVP-
protected Au NPs has not been calculated so far. There-
fore, it was determined in here by using the Arrhenius
method. The slope of the linear plot between the natu-
ral logarithm of the temperature-dependent rate constant
a
1
2
3
Run
4
5
(Fig. 12) and the inverse of temperature is –E /R, where
a
R is the universal gas constant. The value of the activation
Figure 10. Catalytic activities (during 2 hours) of first run and repeated
batches (2–5) of Au NPs for the glucose oxidation (i: run 1, run 2, run 3,
run 4, and run 5. Reaction conditions: [Glucose] 4.75 wt% (30 mL), Au
energy E is used because it is independent of concentra-
a
tion and the surface area of the nanocatalyst. In terms of
the Arrhenius plots within the temperature range from 50
catalysts 2 mg, O 100 mL/min. The reaction was carried out at pH = 9ꢄ4
2
ꢀ
controlled by titrating with NaOH aqueous solution).
to 80 C, the apparent activation energy was calculated to
5
750
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014

Zhang et al.
Preparation and Catalytic Activity of PVP-Protected Au Nanoparticles for the Aerobic Oxidation of Glucose
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4
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Received: 18 July 2013. Accepted: 17 August 2013.
J. Nanosci. Nanotechnol. 14, 5743–5751, 2014
5751