In Situ Formation of Gold Nanoparticles within a Polymer Particle and Their Catalytic Activities in Various Chemical Reactions

Article abstract of DOI:10.1002/cphc.201800891

Composite materials consisting of nanoscale gold particles and protective polymer shells were designed and tested as catalysts in various chemical reactions. Initially, the systematic incorporation of multiple gold nanoparticles into a poly(N-isopropylacrylamide) particle was achieved by an in situ method under light irradiation. The degree of gold nanoparticle loading, along with the structural and morphological properties, was examined as a function of the amount of initial gold ions and reducing agent. As these gold nanoparticles were physically-embedded within the polymer particle in the absence of strong interfacial interactions between the gold nanoparticles and polymer matrix, the readily-accessible surface of the gold nanoparticles with a highly increased stability allowed for their use as recyclable catalysts in oxidation, reduction, and coupling reactions. Overall, the ability to integrate catalytically-active metal nanoparticles within polymer particles in situ allows for designing novel composite materials for multi-purpose catalytic systems.

Full text of DOI:10.1002/cphc.201800891

Accepted Article
Title: In Situ Formation of Gold Nanoparticles within a Polymer Particle
and Their Catalytic Activities in Various Chemical Reactions
Authors: Wongi Jang, Richard Taylor IV, Pascal N. Eyimegwu, Hongsik
Byun, and Jun Hyun Kim
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To be cited as: ChemPhysChem 10.1002/cphc.201800891
Link to VoR: http://dx.doi.org/10.1002/cphc.201800891
A Journal of
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
In Situ Formation of Gold Nanoparticles within a Polymer Particle
and Their Catalytic Activities in Various Chemical Reactions
Wongi Jang,[a, b] Richard Taylor IV,[a] Pascal N. Eyimegwu,[a] Hongsik Byun,*[b] and Jun-Hyun Kim*[a]
Abstract: Composite materials consisting of nanoscale gold particles
and protective polymer shells were designed and tested as catalysts
in various chemical reactions. Initially, the systematic incorporation of
multiple gold nanoparticles into a poly(N-isopropylacrylamide) particle
was achieved by an in situ method under light irradiation. The degree
of gold nanoparticle loading, along with the structural and
morphological properties, was examined as a function of the amount
of initial gold ions and reducing agent. As these gold nanoparticles
were physically embedded within the polymer particle in the absence
of strong interfacial interactions between the gold nanoparticles and
polymer matrix, the readily-accessible surface of the gold
nanoparticles with a highly increased stability allowed for their use as
recyclable catalysts in oxidation, reduction, and coupling reactions.
Overall, the ability to integrate catalytically-active metal nanoparticles
within polymer particles in situ allows for designing novel composite
materials for multi-purpose catalytic systems.
because the catalytically-active sites of the embedded metal
particles are readily hindered by the strongly-bound functional
groups of the outer polymer materials.
To avoid the aforementioned problems, the utilization of
functional-group-free and hydrophilic polymeric materials may
serve as attractive templates to effectively host nanoscale metal
particles.
For example, temperature-responsive poly(N-
isoprpolyacrylamide), poly(NIPAM), particles can possess a
capability to effectively load many inorganic materials in situ in the
absence of strong interfacial interactions (e.g., electrostatic
interactions or chemical bonds). In addition, given their useful
properties such as an excellent biocompatibility/stability and
4]
temperature-responsiveness,[1d,
poly(NIPAM)-derived host
materials can provide an enhanced stability to the guest
substances and exhibit swelling and deswelling properties as a
function of the temperature. Our previous work successfully
demonstrated a reliable in situ approach to physically encapsulate
multiple gold nanoparticles within bare poly(NIPAM) particles at
room temperature.[4e] Here, we took one further step to
demonstrate a simple process to control the number of embedded
metal nanoparticles within the polymer particles, and the resulting
composites were tested as effective catalysts in various chemical
reactions, including oxidation, reduction, and carbon-carbon
Introduction
Incorporating nanoscale metal particles within and/or around
polymeric materials can result in composites that exhibit
enhanced chemical and physical properties compared to their
individual components, as well as highly stable characteristics.[1]
Typically, metal core-polymer shells (e.g., polymer particles filled
with metal nanoparticles) or polymer core-metal shells (polymer
particles covered with metal nanoparticles) are prepared by
utilizing the electrostatic interactions or covalent bonds between
the functional groups of a polymer and the surface of metal
nanoparticles.[1b-d, 1f] These composites have an improved long-
term stability and good dispersity in solution; they can be used in
coupling reactions.
As the surface of embedded gold
nanoparticles is free from strong interactions from the protective
polymer layers, the readily-available catalytic sites can improve
their activities in these chemical reactions.
Bare poly(NIPAM) particles ~600 nm in diameter were initially
prepared by radical polymerization and served as templates to
physically incorporate gold nanoparticles.
Unlike the
conventional synthetic method that often requires a high reaction
temperature, our in situ formation of gold nanoparticles within
poly(NIPAM) particles was performed at room temperature under
light irradiation because these polymer particles readily exist as
swollen structures below the lower critical solution temperature
(LCST) of ~32 oC.[4b, 5] We then carefully compared the formation
and stability of gold nanoparticles in the presence and absence of
poly(NIPAM) particles. The size of the gold nanoparticles was
systematically controlled by the initial concentration (or ratio) of
reactants in the absence of the polymer particles. However, the
size of the embedded gold nanoparticles within the polymer
particles was relatively small (≤ 10 nm in diameter), regardless of
the reaction conditions (e.g., the amount of reactant). The
restricted size distribution was presumably due to the limited
degree of diffusion of the gold ions within the polymer network
a range of practical applications, including electrical, biological,
1d, 1f, 2]
catalytic, and optical systems.[1c,
However, most
approaches to prepare these types of composite materials require
multiple modification and purification steps. Even after the
successful formation of composites, the utilization of implanted
metal nanoparticles (particularly as catalysts in chemical
reactions) may not result in superior performance,[3] presumably
[a]
[b]
R. Taylor, P. Eyimegwu, W. Jang, Prof. J.-H. Kim
Department of Chemistry
Illinois State University
4160 Campus Box, Normal, IL 61790, U.S.A.
E-mail: jkim5@ilstu.edu
W. Jang, Prof. H. Byun
during the growth step.
These small gold nanoparticles
Department of Chemical Engineering
Keimyung University
2800 Dalgubeol Daero, Daegu 42601, South Korea
E-mail: hsbyun@kmu.ac.kr
embedded within the polymer particles could provide advantages
as catalysts in various chemical reactions because the optimal
size for catalytically-active gold nanoparticles has been reported
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
to be 2-7 nm.[6] It is also important to remember that the surface
of these embedded gold nanoparticles was absent from strong
interfacial interactions, possibly allowing for them to maintain their
catalytic properties while the polymer shell rendered a highly
increased stability and dispersity.
Radical
polymerization
NIPAM
BIS
APS
Poly(NIPAM)
HAuCl4
Sodium
citrate
0.2 m
Gold nanoparticles
within poly(NIPAM)
NIPAM: N-isopropylacrylamide
BIS: N,N′-methylenebis(acrylamide)
APS: ammonium persulfate
As catalytically-active nanomaterials in many chemical
reactions often require a high stability in acidic or basic media to
improve their reactivity and selectivity,[1a, 3a, 6a, 7] the stability of our
composite particles was examined at various pHs, and the results
were compared to the stability of bare gold nanoparticles.
Apparently, the embedded gold nanoparticles exhibited a greatly
increased stability due to the hosting poly(NIPAM) particles.
These composites were then tested in three chemical reactions
(the oxidation of benzyl alcohol, reduction of 4-nitrophenol, and
homocoupling of phenylboronic acid) in basic conditions, which
resulted in significantly higher yields and better recyclability than
those of bare gold nanoparticle systems. The improved catalytic
activities in three different reactions using our composites were
clearly attributed to the readily-available nanoparticle surfaces
without chemical modifiers and/or the increased stability. Overall,
exploring a strategy to design these types of stable composite
materials and evaluating their versatile catalytic activities in
various chemical reactions can allow for the development of multi-
purpose chemical reaction systems.
3 m
1 m
Figure 1. Preparation process and the corresponding SEM/TEM images of
before and after the formation of gold nanoparticles within the poly(NIPAM)
particles using 0.5 mM gold salt.
Figure 2 shows the absorption patterns of bare poly(NIPAM)
particles and a series of gold-poly(NIPAM) composite particles as
a function of the gold ion concentration. The use of a small
amount of initial gold salt resulted in the formation of a fewer
number of gold nanoparticles within the polymer particles,
confirmed by a very weak and broad absorption band at 520 nm
and the TEM image in Figure 2b. As the amount of HAuCl4
increased, the number of gold nanoparticles gradually increased
within the poly(NIPAM) particles, which was verified by the slight
increase of the absorption peak at 520 nm as well as the TEM
images shown in Figure 1. When the amount of HAuCl4 further
increased (e.g., ≥ 0.75 mM), the size of embedded gold
nanoparticles became gradually larger with a slightly higher
polydispersity (Figure 2c), exhibiting slightly broader absorption
bands. These composites also possessed the presence of
partially aggregated gold nanoparticles.
Based on this
Results and Discussion
observation, the use of 0.50 mM of gold salt under our reaction
conditions allowed for the formation of relatively small and
somewhat uniform gold nanoparticles without notable
aggregations, which were speculated to be suitable for catalytic
reactions.
Poly(NIPAM) particle templates ~600 nm in diameter (PDI:
0.15) were initially prepared by conventional radical
polymerization. An aliquot of gold ions was then fully suspended
in this poly(NIPAM) solution, followed by the reduction with
trisodium citrate under light irradiation. The temperature of the
reaction mixture was kept below 25 °C using a water-jacketed
beaker throughout the reaction to prevent the collapse of
poly(NIPAM) particles via light-induced heating, allowing for the
formation of physically embedded gold nanoparticles within the
polymer particles in situ. The resulting composite particles
displayed the relatively broad distribution of small gold
nanoparticles (3-9 nm in diameter) within the polymer particles
(Figure 1). Some of the gold nanoparticles were freely formed on
the outside of the polymer particles, but were still relatively small
in size, probably due to the interfacial interactions between the
gold nanoparticles and polymer particles and/or residual free
polymers during growth.[4c] Upon the removal of the free gold
nanoparticles by centrifugation, most of the gold nanoparticles
were shown to be around and/or within the poly(NIPAM) particles,
implying the successful formation of composite particles. Unlike
the typical light-induced synthesis of gold nanoparticle without
poly(NIPAM) particles (vide infra), controlling the final size of the
embedded gold nanoparticles was apparently limited regardless
of varying the amount of the mild reducing agent, trisodium citrate,
at a fixed amount of the gold salt solution. This limited size
distribution was presumably caused by the poly(NIPAM) particle
network preventing the full growth of the embedded gold
nanoparticles under our reaction conditions.
(c)
(b)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
0.2 m
0.2 m
Aggregation
Composites prepared
with increase of HAuCl4
Poly(NIPAM)
300 400 500 600 700 800 900 1000
Wavelength (nm)
Figure 2. Absorption patterns (a) of bare and various composite particles
prepared as a function of the gold salt amount (from 0.25 mM to 1.0 mM) and
the representative images of the composites prepared with 0.25 mM (b) and 1.0
mM (c) of the gold salt.
The absorption band patterns generally elucidated the trend of
gold nanoparticle formation in the presence of poly(NIPAM)
particles as a function of the gold concentration, but the image
analysis by microscope did not evidently indicate if the gold
nanoparticles were embedded within the poly(NIPAM) particles.
As such, three additional control experiments were performed to
confirm the successful formation of the polymer particles filled
with multiple gold nanoparticles by the in situ method. First, the
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
bare gold nanoparticles were formed under the same reaction
conditions in the absence of poly(NIPAM) particles, and the size
of the resulting gold nanoparticles was gradually increased by
decreasing the amount of trisodium citrate (representative
examples are shown in Figure 3a and 3b). The gradual increase
in the size of the bare gold nanoparticles implied the absence of
any interference during the growth of the gold nanoparticles,
which was easily confirmed by measuring the absorption band
changes from 520 to 530 nm (data not shown). Controlling the
size of the gold nanoparticles (≤ ~10 nm in diameter), as we
mentioned the above, was limited in the presence of the
poly(NIPAM) particles regardless of the ratios of the gold salt and
trisodium citrate because of the interfacial interactions in the
(c)
(a)
(b)
100 nm
100 nm
100 nm
(d)
(f)
(e)
Gold embedded
polymer
Gold embedded
polymer
Bare gold
nanoparticles
Bare gold
nanoparticles
1 m
as is
with base
Figure 3. Formation of gold nanoparticles using (a) 0.50 mL citrate, (b) 0.3 mL
citrate without poly(NIPAM) particles, (c) 0.3 mL citrate with poly(NIPAM)
particles, (d) a simple mixture of poly(NIPAM) particle and preformed gold
nanoparticles, and (e) before and (f) after the addition of the base to the
composite and bare gold nanoparticles.
nanoscale environment.
Second, the preformed gold
nanoparticles (prepared by light-induced synthesis in the absence
of polymer particles) was mixed with bare poly(NIPAM) particles.
This simple mixing process could allow for the gold nanoparticles
to be physically adsorbed onto the surface of the polymer particles
via van der Waals and hydrogen bonding interactions.[4c] The
resulting composite particles only showed the random distribution
of gold nanoparticles adsorbed around the surface of the polymer
particles with some partial aggregations (Figure 3d). As such, the
inefficient interactions between the preformed gold nanoparticles
and poly(NIPAM) clearly supported that our in situ synthetic
approach allowed for the formation of gold nanoparticles within
the polymer particles. Last, the stability of the gold embedded
polymer particles was tested upon treatment with a strong acid
and/or base, and then these results were compared to those of
the bare gold nanoparticles. The bare gold nanoparticles easily
underwent irreversible aggregations under basic conditions. This
rapid aggregation process was simply observed by the solution
color changes upon the dropwise addition of the base. However,
the solution of the composite nanoparticles remained the same
under acidic and basic conditions (examples shown in Figure 3e
and 3f). In the case where the composite nanoparticle solution
contained free gold nanoparticles, the resulting solution was
speculated to undergo pH-dependent color changes due to the
instability of the free gold nanoparticles. Thus, these three control
experiments strongly supported the idea that our in situ method
allowed for the successful incorporation of multiple gold
nanoparticles within the poly(NIPAM) particles. In addition to the
enhanced stability at various pHs, these physically embedded
gold nanoparticles were expected to be free from strong
interactions originating from any surface modifiers (e.g.,
molecules containing primary amine or thiol groups), rendering
them highly active and readily-available metal surfaces to use as
catalysts. As such, these speculations withdrew our attention for
their possible use in various catalytic reactions under acidic or
basic conditions. Given the highly enhanced stability and readily-
available catalytic sites, our composite nanoparticles could
provide improved catalytic functions (e.g., efficiency and
recyclability), allowing for the development of practical,
environmentally-friendly catalytic systems. As a proof of concept,
our composite particles were employed in three different chemical
reactions including oxidation, reduction, and coupling reactions in
an aqueous solution under ambient conditions.
First, the oxidation of benzyl alcohol (BA) to benzoic acid (BZA)
was carried out using our composite particles in a basic solution
at room temperature overnight because developing the selective
oxidation of alcohols under aerobic conditions is important to
supporting industrial mass production at a low cost.[8] The GC
chromatograms in Figure 4a show the reduction of the BA peak
intensity around 5.5 min and the appearance of a broad new BZA
peak at 6.4 min, clearly indicating that there was a successful
oxidation reaction. The % yield of the reaction was calculated
based on the standard curve (Figure 4b) prepared by the peak
areas of BZA as a function of the concentration where octane was
used as the internal standard for all samples. The bare gold
nanoparticles typically showed low catalytic activities due to their
poor dispersity in the reaction mixture,[1e, 7a, 9] but our composite
particles resulted in relatively high yields, presumably due to the
greatly increased stability in a basic solution throughout the
reaction. Detectable amounts of additional products such as
benzaldehyde (BZ) and benzyl benzoate were also observed.
Particularly, the formation of a notable amount of benzyl benzoate
under the basic conditions has been explained by the prompt
reaction of BA and newly formed BZA.[10] Interestingly, the use of
excess H2O2 in the reaction readily reduced the formation of BZA,
but notably increased the formation of benzyl benzoate. This
observation could be due to the extra formation of the benzyloxy
species under a sufficient oxygen concentration, resulting in a
high tendency of undergoing nucleophilic attack onto BZ.[11] In a
separate study we performed, the use of molecular oxygen under
1 atm pressure instead of H2O2 generally led to the low conversion
of BA to BZA, but selectively formed BZ. Although the molecular
oxygen source is sometimes ideal for various oxidation
reactions,[12] BZ was found to be the main product and BZA was
considered to be an overoxidation product under our reaction
conditions. More thorough studies are underway to optimize the
reaction conditions and to fully understand their oxidation
properties in various reaction conditions.
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
observation clearly explained the importance of the stability of the
embedded gold nanoparticles as catalysts, affecting the overall
reaction efficiency. An earlier study reported that recycling gold
nanoparticles in the reduction of nitro groups containing
derivatives could induce partial aggregations, thereby slowing
their reactivity.[13] It is also worth noting that the baselines of the
absorption bands slightly roughened during the reaction, which
was caused by the generation of hydrogen bubbles.[14]
3.0E+07
BA and BZA references
Bare AuNPs
(a)
Composites
2.5E+07
2.0E+07
1.5E+07
1.0E+07
5.0E+06
0.0E+00
Composites with excess peroxide
Internal
standard:
octane
solvent
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4-NP
(a)
4-NP
(b)
10 sec
1.5 min
3 min
4 min
5 min
10 sec
1.5 min
3 min
4 min
5 min
2.5
3.5
4.5
5.5
6.5
7.5
Time (min)
1.0E+08
8.0E+07
6.0E+07
4.0E+07
2.0E+07
0.0E+00
(b)
Using composites
Using bare AuNPs
y = 1.31E+07x + 4.52E+04
R² = 9.93E-01
250
350
450
550
650
250
350
450
550
650
Wavelength (nm)
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(c)
0
1
2
3
4
5
6
7
Using composites
Benzoic acid (µg/mL)
Figure 4. GC chromatograms of benzyl alcohol (BA) oxidation with bare gold
nanoparticles and composite particles (a) and a calibration curve using benzoic
acid (BZA) peak areas (b).
Using bare AuNPs
0
1
2
3
4
5
6
7
8
9
Batches
Second, the catalytic reduction of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP) was performed using the composite and bare
gold nanoparticles. The reduction of the nitro groups into amines
is a very useful transformation strategy in organic and medicinal
chemistry, and developing increasingly efficient and recyclable
catalysts is an essential field of research. The catalytic reduction
process was easily monitored by two distinctive peaks at 300 nm
for 4-AP and 400 nm for 4-NP using UV-vis spectroscopy. No
reduction of 4-NP by the excess reducing agent, NaBH4, was
observed in the absence of the gold nanoparticle catalysts.
However, both the bare gold and composite nanoparticles
generally catalyzed the fast reduction rate of 4-NP. To precisely
compare both systems, an aliquot of 4-NP (0.02 mL of 1.0 mM in
water) was added to the nanoparticle solution, and the absorption
band (peak to peak) at 400 nm was recorded after 5 min. After
the completion of the reduction, another aliquot of 4-NP was
continuously added to the mixture and the absorbance patterns
were recorded (Figure 5c). The bare gold nanoparticles exhibited
a slightly faster reduction of 4-NP in a few early batches, but then
slowed down the reduction rates in the later batches. However,
the composite nanoparticles exhibited somewhat steady and
constant reduction rates regardless of the batches, presumably
due to the improved stability of the nanoparticles in the solution.
The absorption bandwidth of the bare gold nanoparticles after the
complete reduction of 4-NP was found to slightly broaden, while
the bandwidth of the composites remained almost the same. This
band broadening could be due to the excess adsorption of 4-AP
and/or 4-NP at high concentrations, possibly causing the partial
aggregation of the bare gold nanoparticles in the solution, which
could be responsible for the slow reduction of 4-NP. This
Figure 5. Catalytic reduction of 4-nitrophenol (4-NP) with bare gold
nanoparticles (a) and composite particles (b) and their peak intensity reduction
after 5 min as a function of the reaction cycle (c).
Last, the homocoupling of phenylboronic acid to biphenyl was
performed using the nanocomposites and bare gold nanoparticles
in a basic solution under aerobic conditions. The formation of
biphenyl was monitored by GC chromatograms after the
extraction of the reaction mixture with diethyl ether. The use of
bare gold nanoparticles in the reaction resulted in the formation of
a small amount of biphenyl (< 10%) and a trace amount of phenol,
regardless of the reaction time. However, the use of composite
particles led to the formation of a noticeable amount of the main
product, biphenyl (~37%), and the side product, phenol (≤ ~6 %)
after 24 h of reaction time at ambient conditions, which were
monitored by a strong biphenyl peak (3.45 min) and a phenol peak
(2.1 min), respectively. As the starting material, phenylboronic
acid, is not GC active, the % yield of biphenyl was estimated by
using a calibration curve prepared from a series of biphenyl
standard solutions containing octane as the internal standard in
ether. Unlike other studies utilizing very small gold nanoparticles
(≤ 5 nm in diameter) or gold ions in the C-C coupling reactions,[1e,
15]
our embedded gold nanoparticles with a slightly large size
exhibited comparable catalytic activities, although many reaction
conditions, including time, concentration, and temperature, were
not optimized in this study. This improved catalytic property was
also caused by the highly increased stability and easily accessible
surface of the gold nanoparticles, which were not blocked by
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
strong organic modifiers via covalent bonds or electrostatic
interactions.
polydispersity. The noticeable growth of the embedded gold
nanoparticles could be explained by the absence of strong
surface modifiers. Although previous work demonstrated the
possible growth of gold nanoparticle catalysts during chemical
reactions,[7b, 15c] the insignificantly minimal growth often occurred
as a function of recycling due to the strongly bound surface
modifiers (e.g., PVP and thiols) via electrostatic and covalent
interactions.[15b, 15c, 16] The small increase in the diameter of these
nanoparticles (e.g., 2-4 nm in diameter) resulted in the slight
reduction of their catalytic properties.[1e, 17] It was also reported
that gradually increasing the size of gold nanoparticles (> ~5 nm
in diameter) often leads to the notable reduction of their catalytic
properties, causing them to eventually become inactive. However,
our nanoparticles still exhibited moderate catalytic activities, even
after much growth. As such, the absence of strong surface
modifiers around relatively large gold nanoparticles allowed for
somewhat different catalytic functions and gave us interesting
contradicting interpretations; the relatively large growth of the
embedded gold nanoparticles as catalysts was directly
responsible for the decreased catalytic activities, but the overall
catalytic properties in this size regime were still relatively high due
to the readily-available surfaces of the stable gold nanoparticles.
Our ongoing tasks involve the improvement of reaction conditions
(e.g., various solvents and bases) and the examination of possible
loss of catalysts (i.e., leaching) during the coupling reaction. The
ability to design these types of composite materials and
understand their catalytic functions in diverse chemical reactions
can facilitate the development of smart materials with multi-
purpose properties.
8.0E+07
Phenol and biphenyl references
Bare AuNPs-24h
(a)
Solvent
7.0E+07
6.0E+07
5.0E+07
4.0E+07
3.0E+07
2.0E+07
1.0E+07
0.0E+00
Composites-5 h
Octane
Composites-24 h
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
Time (min)
60
(b)
Bare AuNPs
Composites
Composites
50
40
30
20
10
0
Bare AuNPs
1st
2nd
Recycle number
3rd
Figure 6. GC chromatograms after the homocoupling reaction of phenylboronic
acid with the bare gold and composite nanoparticles (a) and their recyclability
(b).
(b)
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2 m
(a)
To examine the recyclability, the bare gold and composite
nanoparticles were recovered by centrifugation after the
extraction step and used in the same coupling reaction a few more
times. The recycled bare gold nanoparticles showed negligible
catalytic activities due to the severe irreversible aggregations (i.e.,
black precipitates) during the first cycle. However, the composites
still yielded the moderate formation of biphenyl, and their overall
Recycle 3
Initial
1 m
(c)
λmax
0.2 m
catalytic activities gradually decreased.
These diminished
catalytic properties could be explained by the morphological (e.g.,
shape and size) changes of the embedded gold nanoparticles, as
the coupling reaction required a long reaction time in the basic
solution.[7b, 15c] This speculation was confirmed by examining the
size of embedded gold nanoparticles after the recycling test
(Figure 7). Specifically, the initial diameter of the embedded gold
nanoparticles underwent notable growth after the first coupling
reaction (~15 nm in diameter). These enlarged particles, however,
did not notably grow, even after the third coupling reaction.
In addition, the diameter increase of the embedded gold
nanoparticles was easily observed by the solution color change
from light to dark pink/red and was also examined by the gradual
shift of the absorption peaks from ~520 to ~525 nm. These
absorption bands were slightly broadened, which possibly
indicated the growth of nanoparticles with an increased
400 450 500 550 600 650 700
Wavelength (nm)
1 m
Figure 7. UV-vis absorption patterns of the composite particles before and after
the coupling reaction (a) and their representative TEM images [before the
reaction (b) and after recycling three times (c)].
Conclusions
We developed an in situ strategy to prepare physically
embedded gold nanoparticles within functional-group-free
polymer particles. Uniform poly(NIPAM) particles were reliably
prepared by radical polymerization to serve as stable templates
to physically incorporate catalytically-active gold nanoparticles.
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
While the size of bare gold nanoparticles was easily controlled by
the initial concentration of the reactants in the absence of
poly(NIPAM) particles, the limited growth of the gold
nanoparticles was observed within the poly(NIPAM) particles.
The embedded gold nanoparticles exhibited a high stability
regardless of the pH and salt concentration, presumably due to
the protective polymer layers. The subsequent use of these
stable composite nanoparticles as catalysts in three different
chemical reactions resulted in notably high and recyclable
catalytic functions because of the enhanced stability and readily-
available surface of the embedded gold nanoparticles.
Developing a strategy to incorporate catalytically-active metal
nanoparticles into functional-group-free polymer materials and
understand their functions in various chemical reactions can help
design practical and smart composite materials exhibiting multi-
purpose catalytic properties.
nanoparticles. The mixture was then centrifuged at 4000 rpm for 20 min
twice to remove the free gold nanoparticles, resulting in polymer particles
loaded with multiple gold nanoparticles. Under the same reaction
conditions, bare gold nanoparticles were prepared in the absence of
poly(NIPAM) particles.
Oxidation of benzyl alcohol[15c, 17]
The oxidation of benzyl alcohol was carried out in the presence of gold
nanoparticle-embedded poly(NIPAM) particles. An aliquot (2 mL) of the
purified composite particles in water was mixed with 10 µL of benzyl
alcohol and 22.5 mg of K2CO3 in a glass vial. After a brief sonication,
hydrogen peroxide (100 µL, 30 v/v%) was added to the reaction mixture
every 1 h, three times. After stirring the final mixture overnight, 1 M HCl
(0.5 mL) was added to the final solution, followed by the extraction of
benzoic acid with 2.0 mL of diethyl ether. The ether layer (1.0 mL) was
subjected to the GC analysis after adding octane (5 µL) as an internal
standard. Similarly, the bare gold nanoparticles were used under the same
reaction conditions as the control experiment.
Reduction of 4-nitrophenol (4-NP)[2c, 3a, 3b]
Experimental Section
4-NP (0.02 mL of 1.0 mM) and NaBH4 (0.02 mL, 0.1 M) were added to
a quartz cuvette containing water (2 mL) at room temperature, resulting in
a bright yellow solution. An aliquot of composite nanoparticles (0.1 mL)
were then introduced into the mixture. Immediately, the UV-vis spectra of
the mixture were recorded as a function of time where the intensities of the
absorption peaks at 400 nm were recorded before and after 5 min upon
the continuous addition of 4-NP. The reaction progress was also
examined by the color changes from yellow to light pink. The bare gold
nanoparticles were used under the same reaction conditions.
Materials
Ammonium persulfate, N,N'-methylenebisacrylamide, nitric acid,
hydrochloric acid, hydrogen peroxide (30%), trisodium citrate, potassium
carbonate, 4-nitrophenol, phenylboronic acid, sodium borohydride, benzyl
alcohol, octane, diethyl ether, biphenyl, phenol, and hydrogen
tetrachloroaurate(III) trihydrate were purchased from Fisher Scientific; all
of the chemicals were used without purification. N-isopropylacrylamide
(NIPAM, Aldrich) was recrystallized in hexanes and dried under vacuum
prior to use. The water used in all reactions was purified to a resistance
of 18 MΩ (Nanopure water system; Barnstead/Thermolyne) and filtered
through a 0.2 µm membrane. Glassware was cleaned with an aqua regia
solution or a base bath, and then rinsed with water.
Homocoupling of phenylboronic acid[2b, 15b, 17a]
The homocoupling of phenylboronic acid was performed using the bare
gold and composite nanoparticles in an aqueous solution. Specifically, an
aliquot (2 mL) of the composite particles was mixed with phenylboronic
acid (21.0 mg, 0.17 mmol) and K2CO3 (67.0 mg, 0.48 mmol) in a glass vial.
After vigorous stirring at ambient conditions overnight, the mixture was
centrifuged at 3000 rpm for 20 min. The water layer was gently extracted
with 2 mL of diethyl ether. 1 mL of the ether solution was then subjected
to the GC analysis after adding octane as an internal standard. The
recyclability of the composite nanoparticles was tested upon recovering
the precipitated composite particles after the removal of ether and water
layers. The purified bare gold nanoparticles (e.g., absence of gold ions)
were also used under the same reaction conditions.
In situ synthesis of gold nanoparticles in the presence of a polymer
particle template
The polymer particles were initially prepared by radical polymerization
of N-isopropylacrylamide monomer (1.0 g) in the presence of N,N’-
methylenebisacrylamide cross-linker (0.10 g) and ammonium persulfate
initiator (0.12 g) in a 500 mL round bottom flask containing 200 mL water.
The mixture was purged with argon gas for 45 minutes and then heated to
70 °C in an oil bath for 5 h (the color of the solution develops from clear to
milky white within an hour). The final polymer solution was cooled to room
temperature and filtered through filter paper (Whatman #1) prior to use.
Multiple gold nanoparticles were then formed in the presence of
Characterization
A combination of scanning electron microscopy (SEM), transmission
electron microscopy (TEM), and dynamic light scattering (DLS) was used
to characterize the overall structure of the samples. The diameter,
uniformity, and morphology were examined by SEM (FEI-Quanta 450
instrument operating at a voltage of 20 kV) and TEM (Hitachi H8100
microscope operating at an accelerating voltage of 200 kV). All
nanoparticles for SEM analysis were deposited on silicon wafers and
completely dried at room temperature overnight. The samples were then
coated with a thin layer of Au film using a sputter coater (DESKII). All
samples for the TEM analysis were deposited on 300 mesh carbon-coated
copper grids and then dried completely before analysis.
poly(NIPAM) particles in situ under light irradiation.[4e]
1.0 wt%
HAuCl4·3H2O solution (e.g., 0.1 – 0.4 mL) was thoroughly mixed in 10 mL
of the polymer particle solution (0.25 mM – 1.0 mM) in a series of vials for
30 min. 1 wt% trisodium citrate solution (0.5 mL) was then added into the
mixture, which was placed in a water jacketed beaker and exposed to a
solar-simulated light (100 mW/cm2). The formation of gold nanoparticles
was easily monitored by the color changes of the solution within 1 h. After
stirring for 3 h, the reaction mixture was stirred for additional 5-6 h to
complete the reaction. It is important to note that the temperature of the
solution flask was kept under 25 °C throughout the formation of the gold
nanoparticles because the light-induced heating causes the deswelling of
poly(NIPAM) particles, minimizing the efficient loading of gold
The hydrodynamic diameter and polydispersity of the bare gold and
composite particles were examined by dynamic light scattering (DLS,
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
6237; c) P. Herves, M. Perez-Lorenzo, L. M. Liz-Marzan, J. Dzubiella, Y.
Lu, M. Ballauff, Chem. Soc. Rev. 2012, 41, 5577-5587.
ZetaPALS, Brookhaven Instruments Corp., Holtsville, New York) equipped
with a 35 mW solid-state laser (with angular measurements of 90o). The
nanoparticles (20 µL) were suspended in 2 mL water. The data were
collected using an average of five measurements (100 sec).
[3] a) K. Esumi, R. Isono, T. Yoshimura, Langmuir 2004, 20, 237-243; b) K.
Kuroda, T. Ishida, M. Haruta, J. Mol. Catal. A: Chem. 2009, 298, 7-11; c)
T. Ma, W. Yang, S. Liu, H. Zhang, F. Liang, Catalysts 2017, 7, 38.
[4] a) D. E. Bergbreiter, B. L. Case, Y.-S. Liu, J. W. Caraway,
Macromolecules 1998, 31, 6053-6062; b) A. Doring, W. Birnbaum, D.
Kuckling, Chem. Soc. Rev. 2013, 42, 7391-7420; c) J. Hu, C. A.
Brackemyer, H. Byun, J.-H. Kim, J. Mater. Sci. Technol. 2014, 30, 441-
448; d) S. T. Jones, Z. Walsh-Korb, S. J. Barrow, S. L. Henderson, J. del
Barrio, O. A. Scherman, ACS Nano 2016, 10, 3158-3165; e) J.-H. Kim, B.
W. Boote, J. A. Pham, J. Hu, H. Byun, Nanotechnology 2012, 23, 275606-
275702; f) J.-H. Kim, T. R. Lee, Langmuir 2007, 23, 6504-6509.
[5] a) Y.-G. Guo, J.-S. Hu, H.-P. Liang, L.-J. Wan, C.-L. Bai, Chem. Mater.
2003, 15, 4332-4336; b) T. Hoare, R. Pelton, J. Phys. Chem. B 2007, 111,
1334-1342; c) C. D. Jones, L. A. Lyon, Macromolecules 2000, 33, 8301-
8306.
The absorption patterns of the nanoparticles and 4-NP reduction
processes were examined by an Agilent UV-visible spectrometer over the
wavelength range of 200 to 1100 nm. All samples were placed in a quartz
cell.
The catalytic reactions were examined by gas chromatography (Thermo
Focus GC equipped with an FID detector, Thermo Scientific) equipped with
an autosampler, a flame ionization detector, and a fused silica capillary
column (30 m). Two different temperature programming methods were
used for the coupling reaction (constant flow rate of 10 psi, ramping rate
of 25 °C/min from 130 °C to 250 °C) and the oxidation reaction (constant
flow rate of 5 psi from 90 °C to 250 °C with ramping rate of 30 °C/min and
equilibrating the initial and final temperatures for 1 min each), respectively.
The temperature of the inlet and detector for both analyses was set to
250 °C. The conversion yields were calculated by measuring the peak
area of the products (biphenyl, phenol, benzoic acid, benzaldehyde,
benzyl benzoate) using octane as the internal standard.
[6] a) P. Haider, B. Kimmerle, F. Krumeich, W. Kleist, J.-D. Grunwaldt, A.
Baiker, Catal. Lett. 2008, 125, 169-176; b) J. Jia, K. Haraki, J. N. Kondo,
K. Domen, K. Tamaru, J. Phys. Chem. B 2000, 104, 11153-11156; c) S.
Sreedhala, S. C. P. Vinod, Catal. Today 2015, 244, 177-183.
[7] a) A. Corma, H. Garcia, Chem. Soc. Rev. 2008, 37, 2096-2126; b) A.
Moragues, F. Neatu̧ , V. I. Pꢀrvulescu, M. D. Marcos, P. Amorꢁs, V.
Acknowledgements
Michelet, ACS Catal. 2015, 5, 5060-5067.
[8] a) R. Citriminna, E. Falletta, C. D. Pina, J. H. Teles, M. Pagliaro, Angew.
Chem. Int. Ed. 2016, 55, 14210-14217; b) S. K. Klitgaard, A. T. DeLa Riva,
S. Helveg, R. M. Werchmeister, C. H. Christensen, Catal. Lett. 2008, 126,
213-217.
We gratefully acknowledge the financial support from Illinois State
University. Acknowledgment is also made to the donors of The
American Chemical Society Petroleum Research Fund (PRF#
57743-UR7) for the partial support of this research. In addition,
this work is supported by the National Research Foundation of
Korea Grant funded by the Korean Government (NRF-
2018R1A2B6008854).
[9] M. B. Cortie, E. van der Lingen, Mater. Forum 2002, 26, 1-14.
[10] V. R. Choudhary, A. Dhar, P. Jana, R. Jha, B. S. Uphade, Green Chem.
2005, 7, 768-770.
[11] J. C. F. Rodriguez-Reyes, C. M. Friend, R. J. Madix, Surf. Sci. 2012, 606,
1129-1134.
[12] a) R. Noyori, M. Aoki, K. Sato, Chem. Commun. 2003, 1977-1986; b) N.
Narkhede, A. Patel, S. Singh, Dalton Trans. 2014, 43, 2512-2520.
[13] V. Ramtenki, V. D. Anumon, M. V. Badiger, B. L. V. Prasad, Coll. Surf., A
2012, 414, 296-301.
Keywords: In situ formation • gold nanoparticles • poly(N-
isopropylacrylamide)
stability
• multi-purpose catalysts • enhanced
[14] S. Wunder, F. Polzer, Y. Lu, Y. Mel, M. Ballauff, J. Phys. Chem. C 2010,
114, 8814-8820.
[1] a) G. Agrawal, M. P. Schurings, P. van Rijn, A. Pich, J. Mater. Chem. A
2013, 1, 13244-13251; b) R. Bleach, B. Karagoz, S. M. Prakash, T. P.
Davis, C. Boyer, ACS Macro Lett 2014, 3, 591-596; c) H. M. Chem, R.-S.
Liu, J. Phys. Chem. C 2011, 115, 3513-3527; d) R. Contreras-Caceres, J.
Pacifico, I. Pastoriza-Santos, J. Perez-Juste, A. Fernandez-Barbero, L. M.
Liz-Marzan, Adv. Funct. Mater. 2009, 19, 3070-3076; e) J. Han, Y. Liu, R.
Guo, J. Am. Chem. Soc. 2009, 131, 2060-2061; f) Y. Lu, Y. Mei, M.
Drechsler, M. Ballauff, Angew. Chem. Int. Ed. 2006, 45, 813-816; g) K.
Rahme, M. T. Nolan, T. Doody, G. P. McGlacken, M. A. Morris, C.
O'Driscoll, J. D. Holmes, RSC Adv. 2013, 3, 21013-21024.
[15] a) S. Carrettin, J. Guzman, A. Corma, Angew. Chem. Int. Ed. 2005, 44,
2242-2245; b) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T.
Tsukuda, Langmuir 2004, 20, 11293-11296; c) Y. Yuan, N. Yan, P. J.
Dyson, Inorg. Chem. 2011, 50, 11069-11074.
[16] H. Sakurai, H. Tsunoyama, T. Tsukuda, J. Organomet. Chem. 2007, 692,
368-374.
[17] a) H. Tsunoyama, N. Ichikuni, H. Sakurai, T. Tsukuda, J. Am. Chem. Soc.
2009, 131, 7086-7093; b) H. Tsunoyama, H. Sakurai, Y. Negishi, T.
Tsukuda, J. Am. Chem. Soc. 2005, 127, 9374-9375.
[2] a) S. K. Ghosh, T. Pal, Chem. Rev. 2007, 107, 4797-4862; b) J. Zheng, S.
Lin, X. Zhu, B. Jiang, Z. Yang, Z. Pan, Chem. Commun. 2012, 48, 6235-
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10.1002/cphc.201800891
ChemPhysChem
ARTICLE
Entry for the Table of Contents
ARTICLE
Physical integration of gold
nanoparticles within a polymer particle
highly impacts their stability and
catalytic function
Wongi Jang,[a, b] Richard Taylor IV,[a]
Pascal N. Eyimegwu, [a] Hongsik
Byun,*[b] and Jun-Hyun Kim*[a]
Page No. – Page No.
Reduction
In Situ Formation of Gold
Coupling
Oxidation
Nanoparticles within a Polymer
Particle and Their Catalytic Activities
in Various Chemical Reactions
Physically embedded gold
nanoparticles within poly(NIPAM)
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