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Thermosensitive CoreЦShell Particles as Carriers for Ag Nanoparticles Modulating the Catalytic Activity by a Phase Transition in Networks.

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DOI: 10.1002/anie.200502731
Thermosensitive Core–Shell Particles as Carriers
for Ag Nanoparticles: Modulating the Catalytic
Activity by a Phase Transition in Networks**
Yan Lu, Yu Mei, Markus Drechsler, and
Matthias Ballauff*
Metal nanoparticles have properties that are significantly
different from the bulk properties of the metals.[1–4] Moreover,
their high surface-to-volume ratio renders them ideal candidates for application as catalysts.[5–9] However, the pronounced tendency of nanoparticles to aggregate must be
overcome by using suitable carrier systems. Recently, a
number of systems have been discussed that are suitable for
applications in aqueous environments. These include polymers,[10–14] dendrimers,[15, 16] microgels,[17, 18] and other colloidal
systems.[19, 20] In all the cases studied so far, these carrier
systems only provide a suitable support for the nanoparticles
and prevent them from aggregating. In this way the carrier
system of, for example, dendrimers or microgels acts much in
the same way as a “nanoreactor” that immobilizes the
particles and leads to their more convenient handling.
Here we report on the first system that allows us to
modulate the activity of nanoparticles through a thermodynamic transition that takes place within the carrier system.
Figure 1 displays the principle. Metallic nanoparticles are
embedded in a polymeric network attached to a colloidal core
particle. In all the cases discussed here the core consists of
poly(styrene) (PS) while the network consists of poly(Nisopropylacrylamide) (PNIPA) cross-linked with N,N’-methylenebisacrylamide (BIS). The particles are suspended in
water, which swells the PNIPA at room temperature. The
PNIPA network, however, undergoes a phase transition
around 30 8C, during which most of the water is expelled.
Previous experiments[21] have demonstrated that this transition is perfectly reversible and the process of shrinking and
reswelling can be repeated without degradation or coagulation of the particles.
[*] Dr. Y. Lu, Y. Mei, Prof. Dr. M. Ballauff
Physikalische Chemie I
Universit8t Bayreuth
Universit8tsstrasse 30, 95440 Bayreuth (Germany)
Fax: (+ 49) 921-55-2780
Dr. M. Drechsler
Makromolekulare Chemie II
University of Bayreuth
95440 Bayreuth (Germany)
[**] We thank the Deutsche Forschungsgemeinschaft (SFB 481),
Bayreuth, BASF, and the Fonds der Chemischen Industrie for
financial support.
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2006, 45, 813 –816
Figure 1. PS-NIPA-Ag composite particles consisting of thermosensitive core–shell particles in which Ag nanoparticles are embedded. The
composite particles are suspended in water which swells the thermosensitive network attached to the surface of the core particles. In this
state the reagents can diffuse freely to the nanoparticles that act as
catalysts. At higher temperatures (T > 30 8C) the network shrinks and
the catalytic activity of the nanoparticles is strongly diminished.
Metallic nanoparticles embedded in such a network are
fully accessible to reactants at low temperature. Above the
phase transition, however, the marked shrinkage of the
network should be followed by a concomitant slowing down
of the diffusion of the reactants within the network. The rate
of reactions catalyzed by the nanoparticles should thus be
slowed down considerably. In this way, the network could act
as a “nanoreactor” that can be opened or closed to a certain
Herein we demonstrate that thermosensitive core–shell
networks may indeed be used as such a nanoreactor. The
activity of the catalyst can be modulated by temperature over
a wide range. As the model reaction we chose the reduction of
4-nitrophenol to 4-aminophenol by sodium borohydride. The
reaction was repeatedly performed to check the catalytic
activity of the metal nanoparticles, and the results obtained in
the present study can be directly compared to literature
data.[8, 22]
The carrier particles having a PS core and a PNIPA shell
were prepared as described recently.[23, 24] Figure 2 shows a
schematic representation of the silver nanoparticles being
Figure 2. Formation of silver nanoparticles in the PS–NIPA core–shell
system. The cross-linked PNIPA chains absorb Ag ions (step 1) which
are reduced to produce Ag nanoparticles immobilized in the thermosensitive network (step 2).
generated directly in the network of the carrier particles. To
this end, a dilute suspension of the carrier particles is treated
with a 0.1m AgNO3 solution and the reduction is achieved
through addition of an aqueous solution of NaBH4. The
suspension of the composite particles is subsequently cleaned
by ultrafiltration with pure water.
Figure 3 shows an image of several particles obtained by
cryogenic transmission electron microscopy (cryo-TEM;
additional micrographs can be found in the Supporting
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Hydrodynamic radii versus temperature plot for PS-NIPA-Ag
particles. &: hydrodynamic radii for the PS–NIPA particles, *: PS-NIPAAg particles.
Figure 3. Cryo-TEM image for the silver nanocomposite particles.
Information).[25] The formation of the Ag nanoparticles is also
seen from the suspension becoming yellow after the reduction
which corresponds to a maximum in the UV/Vis spectrum
(see Supporting Information) at 410 nm. The Ag nanoparticles generated by this method have diameters of 8.5 nm
(1.5 nm; Figure 3). Thermogravimetric analysis measurements indicate that 10.4 wt % Ag had been embedded.
Moreover, it shows that all the particles are immobilized
within the network and none are located outside of the
PNIPA shells of the carriers. This observation indicates a
strong localization of the Ag+ ions within the network, most
probably caused by a complexation of the Ag+ ions by the
nitrogen atoms of the PNIPA. This proposal was confirmed by
the slight shrinkage of the PNIPA network after the addition
of AgNO3 to the system, as shown by dynamic light scattering
measurements (see the Supporting Information). Figure 3
also indicates that the nanoparticles are truly immobilized in
the mesh of the PNIPA network, fixed to the surface of the
core particles. Hence, the composite particles consisting of the
carrier and the Ag nanoparticles are expected to present a
stable system; thus, no loss of Ag particles should occur
during the reaction.
Dynamic light scattering measurements of composite
particles at different temperatures indicate that the thermosensitive properties of the PNIPA network are not hindered
by the incorporation of silver particles into the network. The
silver nanoparticles do not disturb the phase transition within
the network. Figure 4 displays the hydrodynamic radii of the
carrier particles (squares) and the composite particles (circles). Both sets of data agree more or less, thus showing that
the thermosensitive shell undergoes a phase transition at
32 8C. This result is in excellent agreement with previous
findings.[21, 23]
Only small amounts of the composite particles are needed
to catalyze the reduction of 4-nitrophenol by NaBH4
(ca. 6 mg L 1). Hence, the adsorption of the composite
particles can be disregarded and the reduction can be
followed by monitoring the UV/Vis spectra as a function of
time (Figure 5). The degree of conversion can be directly read
Figure 5. Reduction of 4-nitrophenol by NaBH4 : UV/Vis spectra of
solutions of 4-nitrophenol measured at different times t.
off these curves, and the ratio of the concentration ct of 4nitrophenol at time t to its value c0 at t = 0 is given directly by
the ratio of the respective absorbance A/A0.
Since the concentration of borohydride largely exceeds
the concentration of 4-nitrophenol, the reaction should be of
first order with regard to this reactant. Figure 6 shows that this
is the case for temperatures ranging from 10 to 40 8C. Linear
relationships between ln[ct/c0] and the reaction time t are
obtained in all cases at low conversions. Deviations from
linearity is only seen for low temperatures and high degrees of
conversion. However, for the present purpose it suffices to
discuss the initial rates of reaction. It must be noted that there
is an induction period t0, which is obtained by extrapolation to
zero conversion (see Figure 6). This finding is in accord with
other studies on the catalysis of this reaction by metal
The rate constants k(T) obtained from these plots are
shown in Figure 7, and it is interesting to note that the
constants k obtained at different temperatures do not exhibit
the typical dependence on temperature. Instead of a simple
linear relationship between lnk and T 1, the change of k with
temperature can be divided into three regions. When the
reaction temperature is low, the PNIPA network is totally
swollen with water. In this case the silver nanoparticles which
have been embedded in the network are fully accessible to the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 813 –816
Experimental Section
Figure 6. Influence of temperature on the kinetic constant k measured
with PS-NIPA-Ag particles. The concentrations are as follows:
[4-nitrophenol] = 0.1 mm, [NaBH4] = 10 mm, [PS-NIPAAg] = 6.3 F 10 3 g L 1. *: 283 K; &: 293 K; ^: 305.5 K; ~: 313 K.
Figure 7. Arrhenius plot of the reaction rate k. &: reaction rate k
measured in the presence of PS-NIPA-Ag particles at different temperatures. *: volume change of the particles with temperature.
reactants for the catalytic reduction. Here the rate constant k
will exhibit a conventional Arrhenius-type dependence on
temperature. However, when the reaction temperature is
higher than 25 8C, the PNIPA network shrinks markedly with
an increase in temperature. The shrinking of the network
resulting from the expulsion of water is followed by a
concomitant slowing down of the diffusion of reactants
within the network. This process will in turn lower the rate
of the reaction catalyzed by the silver nanoparticles. It is clear
that the increase in k by the rise in temperature is overcompensated by the diffusional barrier. Hence, the reaction
rate must reach its minimum at the transition temperature.
Figure 7 demonstrates this through the plot of the cube of the
hydrodynamic radius RH (see Figure 4) as a function of 1/T.
RH3 is inversally proportional to the density of the network,
which increases considerably with temperature, thus impeding the diffusion within the network. If the temperature is
increased further, the density of the network stays constant;
the strong increase in k with T dominates and the reaction
rate rises again.
In conclusion, the thermosensitive PNIPA network can
act as a “nanoreactor” that can be opened or closed to a
certain extent: As shown in Figure 7, the catalytic activity of
the silver nanoparticles can be modulated by temperature in a
nonmonotonous way over a wide range.
Angew. Chem. Int. Ed. 2006, 45, 813 –816
N-Isopropylacrylamide (NIPA, Aldrich), N,N’-methylenebisacrylamide (BIS, Fluka), sodium dodecyl sulfate (SDS, Fluka), and
potassium peroxodisulfate (KPS, Fluka) were used as received.
Styrene (BASF) was destabilized on Al2O3 and stored in the
refrigerator. Silver nitrate (AgNO3, Aldrich), sodium borohydride
(NaBH4, Aldrich), and 4-nitrophenol (Aldrich) were used as
received. Water was purified by reverse osmosis (MilliRO, Millipore)
and ion exchange.
The core–shell-type PS–NIPA particles were synthesized and
characterized as described in Ref. [20, 22] The PS core latex was
prepared by conventional emulsion polymerization using styrene
(253.2 g) and NIPA (13.75 g) as the monomers, SDS (2.34 g dissolved
in 925 g water) as the surfactant, and KPS (0.50 g dissolved in 20 g of
water) as the initiator. The reaction was run at 80 8C for 8 h.
Purification was done by ultrafiltration with 10 times the volume of
The core–shell system was prepared by seeded emulsion polymerization. PS core latex (347.67 g) was diluted with water (500 g)
together with NIPA (40.07 g) and BIS (1.37 g). After this stirred
mixture had been heated to 80 8C, the reaction was started with the
addition of KPS (0.40 g, dissolved in 15 g water), and the reaction was
run for 4.5 h. The latex was purified by ultrafiltration against purified
water (membrane: cellulose nitrate with 100 nm pore size supplied by
Schleicher & Schuell).
The preparation of the silver nanoparticles in an aqueous solution
was carried out by addition of silver nitrate to a suspension of PS–
NIPA particles and subsequent reduction with sodium borohydride.
For a typical experiment, an aqueous solution of AgNO3 (0.1m,
0.575 mL) was added to an aqueous solution of PS–NIPA (2 g PS–
NIPA latex diluted with 98 g water), and the mixture was stirred for
30 minutes under N2. A solution of sodium borohydride (0.043 g) in
water (5 g) was then quickly added to the solution with stirring for one
hour. After that, silver nanocomposite particles were cleaned by
ultrafiltration against purified water (membrane: cellulose nitrate
with 100 nm pore size supplied by Schleicher & Schuell).
Catalytic reduction of 4-nitrophenol: An aqueous solution of
sodium borohydride (10 mm, 0.05 mL) was added to a solution of 4nitrophenol (0.1 mm, 2.95 mL). A definite amount of silver nanoparticles (6.3 G 10 3 g L 1) was then added to this solution. Immediately after the addition of silver nanoparticles, UV spectra of the
mixture were recorded with a UV/VIS spectrometer. The kinetic
study of the reaction was performed by measuring the change in
intensity of the absorbance at 400 nm with time. The spectra were
recorded every 1.5 minutes in the range 250–550 nm.
Cryo-TEM samples were prepared by vitrification of thin liquid
films supported on a TEM copper grid (600 mesh, Science Services,
Munich, Germany) in liquid ethane. The sample was inserted into a
cryotransfer holder (CT3500, Gatan, Munich, Germany) and transferred to a Zeiss EM922 EFTEM microscope (Zeiss NTS GmbH,
Oberkochen, Germany). Examinations were carried out at around
90 K. The electron microscope was operated at an acceleration
voltage of 200 kV. All images were recorded digitally by a bottommounted CCD camera system (UltraScan 1000, Gatan, Munich,
Germany) and processed with a digital imaging processing system
(Digital Micrograph 3.9 for GMS 1.4, Gatan, Munich, Germany).[25]
Dynamic light scattering measurements were performed with an
ALV 4000 goniometer (Peters) at different temperatures at 908.
The UV spectra were measured on a Lambda 25 spectrometer
(Perkin Elmer).
Received: August 3, 2005
Published online: December 19, 2005
Keywords: heterogeneous catalysis · nanoparticles ·
nanoreactors · thermochemistry
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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carrier, coreцshell, thermosensitive, network, catalytic, activity, transitional, modulation, particles, phase, nanoparticles
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