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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: xwge@ustc.edu.cn;pstwmz@ustc.edu.cn; 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: jianan@mail.ustc.edu.cn;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.

This journal is ª The Royal Society of Chemistry 2011

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,

This journal is ª The Royal Society of Chemistry 2011

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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