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Colloidal silver deposition onto functionalized polystyrene microspheres
Jianan Zhang,ab Xuewu Ge,*a Mozhen Wang,a Jianjun Yang,b Qingyun Wu,b Mingyuan Wub and Dandan Xub
Received 27th September 2010, Accepted 4th January 2011
DOI: 10.1039/c0py00320d
This paper presents a facile method for the preparation of silver/polystyrene (Ag/PS) composite
microspheres. PS microspheres with carboxyl and nitrile groups on the surfaces were synthesized via
a two-step dispersion copolymerization of styrene, itaconic acid, and acrylonitrile in ethanol–water
media. Ag/PS composite microspheres were prepared successively by addition of AgNO3 aqueous
solution to the dispersion, absorbing to the surfaces of functional PS microspheres, and then reduction
of Ag+ ions to silver nanoparticles by aqueous hydrazine hydrate. The results showed that Ag
nanoparticles with size of about 50 nm were located on the shell of PS microspheres due to the
combined interactions between the carboxyl and nitrile groups of PS microspheres and the in situ
formed silver nanoparticles. The as-prepared Ag/PS microspheres showed good catalytic properties.
Introduction
In recent years, Ag/polymer composites have attracted much
interest in view of their practical applications or potential
applications in the fields of sensors,1 optical,2,3 and catalysis.4,5
Polystyrene (PS) as a kind of inert support has widely been used
to prepare nanocomposite microspheres. There exit several
methods for the deposition of colloidal metal particles onto the
surface of functionalized PS microspheres, such as simple
adsorption of preformed colloids,6–8 in situ metal reduction and
electrostatic deposition,9–12 and layer-by-layer (LbL) assembly.13
Among these methods, electrostatic deposition has been proved
to be an effective method to attaching metal nanoparticles onto
polymer microspheres.
However, electrostatic repulsion between the nanoparticles
and the absence of affinity between nanoparticles and polymer
microspheres has proven to be the major difficulties in producing
Ag/polymer composites. In order to improve the affinity between
Ag nanoparticles and polymer microspheres, various functional
groups such as carboxyl, amine, hydroxy, nitrile, sulfate, and
thiol groups, have been used to improve the interactions between
nanoparticles and polymer microspheres.8–12 Kim et al.8 synthe-
sized silver/polymer colloidal composites from surface-func-
tional porous polymer microspheres by taking advantage of the
interactions between nitrile group and Ag nanoparticles. Akash
et al.9 fabricated Ag/polymer composite microspheres from
poly(N-isopropylacrylamide)-coated PS microspheres by taking
aCAS Key Laboratory of Soft Matter Chemistry, Department of PolymerScience and Engineering, University of Science and Technology of China(USTC), Hefei, 230026, PR China. E-mail: [email protected];[email protected]; Fax: +86 551 3601592; Tel: +86 551 3607410bSchool of Chemistry and Chemical Engineering of Anhui University, theKey Laboratory of Environment-friendly Polymer Materials of AnhuiProvince, Hefei, 230039, PR China. E-mail: [email protected];Fax: +86 551 5107614; Tel: +86 551 5108701
970 | Polym. Chem., 2011, 2, 970–974
advantage of the interactions between the amide groups of
poly(N-isopropylacrylamide) and silver particles. Attractive
interactions among carboxylic acid groups and silver cations
have been widely used to the adsorption of Ag+ onto functional
microspheres and the in situ formation of silver particles to
prepare silver/polymer composite microspheres.10 Owing to the
electrostatic interaction between Ag+ and carboxyl groups on
polymer substrate, Cheng et al.10 fabricated Ag/polymer
composite microspheres. To the best of our knowledge, func-
tional PS microspheres with carboxylic acid and nitrile groups on
the surfaces have not been used as polymer substrates to fabri-
cate Ag/polymer composite microspheres. Inspired by the above
interesting and valuable results, we investigated the feasibility of
preparation of functional PS microspheres with carboxyl and
nitrile groups on the surfaces, and further anchoring Ag nano-
particles onto the shell of final composite microspheres by their
affinity interaction with carboxyl and nitrile groups.
Herein, we report the synthesis of functional PS composite
microspheres via a two-step copolymerization of styrene, acry-
lonitrile, and itaconic acid. Then Ag/PS composite microspheres
were obtained by taking full advantage of the carboxyl–Ag+
electrostatic adsorption and nitrile–silver nanoparticles affinity
effects. The catalytic properties were studied as well.
Experimental
Raw materials
Styrene (St) and acrylonitrile (AN) were purified by distillation
under reduced pressure before use. 2,20-Azobis(isobutyronitrile)
(AIBN) was purified by recrystallization in ethanol and kept
refrigerated until use. Silver nitrate (AgNO3, 99.5%), poly-
vinylpyrrolidone (PVP, MW z 30,000), itaconic acid (ITA), and
hydrazine hydrate (85 wt%) were purchased in their reagent and
used as received. NaBH4 and methylene blue (MB) were used as
This journal is ª The Royal Society of Chemistry 2011
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received. Other reagents of analytical grade were utilized without
further purification. All reagents were purchased from Shanghai
Chem. Reagent Co. (China). Deionized water was used for all
experiments.
Scheme 1 Schematic illustration for the preparation of Ag/PS micro-
spheres using a two-step dispersion copolymerization. (a) Carboxyl
modified PS microspheres were prepared with St and ITA as monomers.
(b) Carboxyl and nitrile groups modified PS microspheres were formed
after St and acrylonitrile were added dropwise. (c) Ag+ was adsorbed on
the shells after the addition of AgNO3 solution because of their electro-
static interactions with carboxyl groups. (d) Ag nanoparticles were
obtained by in situ reduction of Ag+ ions at room temperature and Ag/PS
microspheres were fabricated by the assistance of electrostatic interac-
tions between Ag and nitrile groups.
Preparation of Ag/PS composite microspheres
PS microspheres with carboxyl and nitrile groups on the surfaces
were synthesized via a two-step dispersion polymerization in
ethanol–water media using PVP as dispersant. The PS micro-
spheres were prepared with all the ingredients kept unchanged
but the variation of ITA as shown in Table 1, and were used as
template particles to prepare Ag/PS composite microspheres.
Briefly, 5.0 g of St, 0.10 g of AIBN, 2.0 g of PVP, 0.05–0.20 g of
ITA, 4 g of water, and 76 g of ethanol were charged into a
250 mL four-necked vessel. The reaction system was deoxygen-
ated by bubbling with nitrogen gas for about 30 min at room
temperature, and the polymerization took place at 75 �C.
Carboxyl modified PS microspheres were obtained after the
polymerization was carried out for 3.5 h (Scheme 1a). The
mixtures of St/AN (5.0/5.0, g/g) were added dropwise within 1 h
and then the reaction was carried out for another 6.0 h to obtain
PS microspheres with carboxyl and nitrile groups on the surfaces
(Scheme 1b). The deposition of Ag nanoparticles onto functional
PS microspheres was carried out by the reduction of AgNO3.
After the addition of AgNO3 solution at room temperature
(Scheme 1c), Ag+ ions were absorbed on the shells due to
the affinity interaction with carboxyl and nitrile group.
Hydrazine solution (22 wt%, 10.0 g) was dropped slowly into the
above aqueous dispersion under vigorous stirring. The in situ
reduction reaction of Ag+ was carried out for 2 h to yield Ag/PS
microsphere dispersion with a slightly dark yellow appearance
(Scheme 1d).
Catalytic properties of Ag/PS microspheres
To investigate the catalytic properties of the Ag/PS composite
microspheres, 5 mL of as-prepared Ag/PS suspension was puri-
fied by centrifugation three times and redispersed in 200 mL
water. 10 mL dye MB solution was added in the above aqueous
suspension and then injected with 1 mL of NaBH4 solution under
stirring. The catalytic properties of Ag/PS composite micro-
spheres were investigated by monitoring the variation in optical
absorption of the dye with a UV-vis spectrometer.
Table 1 Synthetic conditions of Ag/PS composite microspheres
Entry
PS microsphere
St/g ITA/g AIBN/g PVP/g H2O/g
1 5.0 0.05 0.10 2.0 4.02 5.0 0.10 0.10 2.0 4.03 5.0 0.15 0.10 2.0 4.04 5.0 0.20 0.10 2.0 4.05a 5.0 0.20 0.10 2.0 4.06 5.0 0.20 0.10 2.0 4.0
a One-step dispersion copolymerization of St, ITA, and AN.
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Characterization
Samples for FT-IR characterization were dried at 60 �C under
vacuum for 24 h, dispersed in KBr matrices, and measured in the
wavenumber range from 4000 to 400 cm�1 at a resolution of
4 cm�1 using a Nicolet Nexus-870 FT-IR spectrophotometer.
Samples for transmission electron microscopy (TEM) were
prepared by drying a drop of the dilute dispersion of composite
microspheres onto a carbon-coated copper grid. Analysis was
conducted using a Hitachi H-800 electron microscope operating
at 100 kV. UV-vis transmittance spectra were recorded over the
range 200–1000 nm on a Shimadzu U-3101PC spectrophotom-
eter, equipped with a custom-built sample holder for angle-
resolved UV-vis transmittance measurements. The average
particle size and size distribution of Ag nanoparticles were
determined using light scattering equipment (Zetasizer 3000
HSA, Malvern Company). UV-vis transmittance spectra were
recorded over the range 200–1000 nm on a Shimadzu U–3101PC
Functional PSmicrospheres
Ag/PS compositemicrospheres
Ethanol/g St/g AN/g AgNO3/g Hydrazine/g
76.0 5.0 5.0 0.3 10.076.0 5.0 5.0 0.3 10.076.0 5.0 5.0 0.3 10.076.0 5.0 5.0 0.3 10.076.0 — 5.0 — —76.0 5.0 — 0.3 10.0
Polym. Chem., 2011, 2, 970–974 | 971
Fig. 1 (a) Typical IR spectra of PS (1), functional PS (2), and Ag/PS
composite microspheres (3); (b) a partial enlarged view of (a).
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spectrophotometer, equipped with a custom-built sample holder
for angle-resolved UV-vis transmittance measurements in
aqueous media. The powder X-ray diffraction (XRD) was
performed on Rigaku Geigerflex diffractometer with Cu-Ka
radiation (l ¼ 0.15418 nm), and the 2q angle varied from 16� to
80� at a scanning rate of 1�/min.
Results and discussion
Synthesis of functional PS and Ag/PS composite microspheres
Itaconic acid (ITA) and acrylonitrile (AN) were used as func-
tional monomers to prepare functional PS microspheres via
dispersion polymerization. Traditional one-step dispersion
polymerization often leads to particles with very narrow size
distribution. However, dispersion polymerization was actually
highly sensitive to small changes in the reaction system even
when only a small amount of a second monomer is added.14–16
Yang et al.17 studied the dispersion copolymerization of styrene
with acrylic acid (AA). They found that with the increasing
amount of AA in the monomer mixture, the microsphere size
increased and the microsphere size distribution became broader.
Winnik and co-workers18–20 successfully prepared different
functional monodispersions by delaying the addition of various
comonomers to the reaction for dispersion copolymerization of
styrene, and they called their methodology as ‘‘two-stage’’
dispersion polymerization. To confirm the important role of two-
step dispersion polymerization on the dispersity of final PS
composite microspheres, we have tried the one-step synthesis of
functional microspheres as a control experiment. The other
components including AN were kept unchanged but using a one-
step dispersion copolymerization process as shown in Entry 5 of
Table 1. Compared with that of Entry 4 in Table 1, the size of the
resulting PS microspheres prepared from Entry 5 is about 1.0 mm
and the obtained microspheres were polydisperse (by TEM
observation). So we adopted the two-stage dispersion polymer-
ization method by delaying the addition of AN monomers.
Structure of Ag/PS composite microspheres
After the copolymerization of ITA and styrene in the first stage,
the carboxylic acid groups were mostly located on the surface of
PS microspheres because of their hydrophilic nature. Monomers
of St and AN were added dropwise in the second stage of poly-
merization, the hydrophobic St and AN tended to diffuse into
seeded functional PS microspheres and polymerized. The
carboxylic acid may be buried in the interior, located on the
surface, or remain in the serum phase, which can be distinguished
by conductometric titration.21 The amount of carboxyl groups
was determined by conductometric titration by the aqueous
solution of NaOH. Quantitative information of the carboxyl
contents was acquired from the titration plots.21 The results
showed that with the increase of ITA amount from 0.05, 0.10,
0.15, to 0.20 g, the carboxyl contents on the surface of micro-
spheres increased with nearly the same proportion, i.e., from
0.109, 0.202, 0.311, up to 0.362 mmol g�1.
The incorporation of nitrile groups onto the surfaces of PS
microspheres was confirmed by FT-IR spectroscopy measure-
ments. Curves 1, 2, and 3 in Fig. 1a show FT-IR spectra of PS,
functional PS, and Ag/PS microspheres, respectively. Compared
972 | Polym. Chem., 2011, 2, 970–974
with the IR spectrum of PS microspheres in curve 1, the presence
of typical peak of nitrile groups (2237 cm�1) in curve 2 of func-
tional PS microspheres confirmed the copolymerization of St and
AN. The spectral features of the silvered microspheres (curve 3)
were overall very similar to those of unsilvered ones (curve 2).
The partial enlarged view of Fig. 1a is shown in Fig. 1b, which
gives more detailed information. The shape of the amide
carbonyl stretch absorption of PVP is centered at 1662 cm�1 for
un-silvered PS microspheres (curves 1 and 2) and shifted to 1672
cm�1 (curve 3) for silvered ones.22 The increase in intensity of the
N–H stretching vibration band (near 3434 cm�1) and the N–H
bending vibration band (1672 cm�1) appeared in the spectrum of
the silvered microspheres as shown in Fig. 1b, which was due to
the interactions between the metal atoms and nitrogen functions
of the polymer support.9,22 Therefore, we can confirm the
presence of silver nanoparticles in the composite microsphere.
Morphology of composite microspheres
The dispersion polymerization is an attractive method for
producing micron-size polymer microspheres in a single batch
process.23 Traditional one-step dispersion polymerization often
leads to microspheres with very narrow size distribution.
However, dispersion polymerization was highly sensitive to small
changes in the reaction system when even only small amount of
functional monomer is added.15,17 We have tried the one-step
synthesis of functional microspheres and the size distribution of
the obtained microsphere became broader. Fig. 2a and b shows
the TEM images of the functional PS microspheres and Ag/PS
composite microspheres, respectively. The functional PS micro-
spheres are spherical in shape and nearly monodisperse in size
with a diameter of about 1.0 mm (Fig. 2a). After the addition of
monomers of St and AN in the second step of dispersion copo-
lymerization, the functional microspheres had an increased
average size of 1.7 mm with lower monodispersity. The as-
prepared PS particles were negatively charged due to the func-
tional comonomer, ITA. The positively charged Ag+ ions were
attracted onto the surface of PS particles by electrostatic inter-
action at first, and then the Ag+ ions were in situ reduced to
zerovalent sliver nanoparticles by hydrazine. Due to the affinity
effects of carboxyl groups, Ag nanoparticles grew and were
located on the shell of functional PS microspheres. Furthermore,
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Fig. 2 TEM images of typical functional PS microspheres (a), Ag/PS
composite microspheres (b), and a magnified TEM image (c) showing the
shell details of Ag/PS composite microspheres.
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the polar head groups (nitrile groups) on the surface of the
polymer microspheres had a high affinity for metal nano-
particles,which provided preferentially sufficient deposition sites
for Ag particles.8,24,25 Controlled experiments without the addi-
tion of acrylonitrile were carried out to confirm the effects of
nitrile groups as shown in Entry 6 of Table 1. The results showed
that Ag particles were located not only on the surfaces of func-
tional PS microspheres, but isolated from surfaces of polymer
microspheres and existed as free particles in the aqueous
medium. Without adding acrylonitrile, some Ag+ ions nucleated
in the aqueous medium and grew into free triangular nanoplates.
Due to the combined interactions of carboxyl and nitrile groups,
Ag/polymer microspheres were obtained successfully with silver
particles of about 50 nm being immobilized on the shell of PS
microspheres as shown in Fig. 3. The result is in good agreement
with conventional observations.26,27
Fig. 3 Size and size distribution of Ag nanoparticles. Ag nanoparticles
was obtained by dissolving polymer component of Ag/PS microspheres
with toluene and purified by centrifugation twice before determination.
This journal is ª The Royal Society of Chemistry 2011
UV-vis absorption spectrum
An obvious silver plasmon absorption band at ca. 414 nm28 was
observed for the dispersion of silvered composite microspheres
(Fig. 4b). However, the supernatant obtained from the dialyzed
microsphere suspension was colorless and showed no absorption
band at ca. 414 nm (Fig. 4a). These results indicate that all the
formed Ag nanoparticles were adsorbed on the surfaces of PS
microspheres.
XRD patterns of Ag/PS composite microspheres
The typical XRD patterns of the as-prepared composite micro-
spheres were shown in Fig. 5. The exhibited peaks at 2q angles of
38.3�, 44.4�, 66.4�, 77.7�, and 81.6� corresponding to the reflec-
tions of (111), (200), (220), (311), and (222) crystal plane of the
fcc structure of Ag (JCPDS No. 04–0783), respectively. It
confirmed that Ag nanoparticles with crystalinity could be
obtained successfully by reducing AgNO3 using hydrazine
hydrate as reducing agent.
Catalytic properties of Ag/PS composite microspheres
The Ag/polymer composite microspheres have been used as
catalysts10,29 and antibacterial materials because of easy recycling
and access to the active site of the noble metal particles.30,31 Ag
nanoparticles can serve as an electron relay in the oxidant–
reductant system, which can find use in catalytic reducing of
methylene blue by NaBH4.10
Fig. 6 shows the catalytic properties of Ag/PS composite
microspheres under ambient conditions. The characteristic peaks
of MB are located at 665 nm and 613 nm. With the redox reaction
proceeding, the concentration of MB can be monitored by the
absorbance maximum (lmax) by using UV-vis spectrometer. As
curve g in Fig. 6 displayed, the reduction of MB with NaBH4
finished after 30 min. It should be noted that without the addition
of Ag/PS microspheres, the color of NaBH4 and MB solution
remained unchanged. After the addition of Ag/PS microspheres,
the color of mixture became pale gradually and vanished even-
tually. The nucleophile NaBH4 can donate electrons to silver
particles, and the electrophile dyes would capture electrons from
silver nanoparticles. Thus the silver nanoparticles acted as an
electron relay for catalytic reducing of MB by NaBH4.32,33
Fig. 4 UV-vis absorption spectra of the supernatant (a) and Ag/PS
composite microspheres (b).
Polym. Chem., 2011, 2, 970–974 | 973
Fig. 5 XRD patterns of Ag/PS composite microspheres.
Fig. 6 UV-vis spectra of MB reduced by NaBH4 and a catalyst of Ag/PS
as functions of reaction time. (a)–(g) corresponds to MB solution with
Ag/PS microspheres for 0, 5, 10, 15, 20, 25, and 30 min, respectively.
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Conclusions
In summary, surface-functionalized PS microspheres were
synthesized by a two-step dispersion copolymerization of
styrene, itaconic acid, and acrylonitrile. Ag/PS composite
microspheres were obtained by taking full advantage of
adsorption interactions between carboxyl and nitrile groups of
PS microspheres and the in situ formed Ag nanoparticles. It was
found that the surface characteristics of the supporting micro-
spheres played an important role in the deposition of Ag nano-
particles on the composite microspheres and Ag/PS microspheres
showed the catalytic abilities in the oxidation-reduction reaction
of methylene blue by NaBH4.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 50573070, 21044006, 50773073, and
974 | Polym. Chem., 2011, 2, 970–974
51073146) and the program for Changjiang Scholars and Inno-
vative Research Team at the University of Science and Tech-
nology of China.
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