Encapsulation of a Single Metal Nanoparticle with Tunable Size in a Monodisperse Polymer...

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188

Encapsulation of a Single Metal Nanoparticlewith Tunable Size in a Monodisperse PolymerMicrocapsule

Haiqing Li, Chang-Sik Ha, Il Kim*

A versatile approach to fabricate monodisperse poly[styrene-co-(divinyl benzene)] (PS-co-DVB)microcapsules that contain a single gold nanoparticle (AuNP) has been demonstrated. Usingthe PS-co-DVB microcapsule as a microreactor, aqueous HAuCl4 and NaBH4 solutions aresubsequently infiltrated. The size of the result-ing AuNP inside of the PS-co-DVBmicrocapsulesis easily tunable by controlling the repeatedinfiltration cycles of aqueous HAuCl4 andNaBH4. PS-co-DVB microcapsules that containa single silver and palladium nanoparticle arealso obtained by following a similar protocol.

Introduction

Spherical polymer microcapsules have received consider-

able attention because of their diverse technological

applications as microreaction vessels, drug delivery

vehicles, photonic crystals, and biomedical implants.[1] A

variety of chemical and physicochemical methods, such as

the template approach, self-assembly of diblock copoly-

mers, the inverse water-in-oil emulsion process, and the

swelling–evaporation strategy, have been employed to

produce polymer capsules.[2]

In general, the as-prepared hollow polymer capsules

have relatively limited functionalization surfaces, which

limit their applications greatly. To further extend the

applications of the resulting polymer capsules, it is

significant to modify their surface with specific functions.

An important aspect concerning the modification methods

is that the functionalization of the interface of polymer

H. Li, C.-S. Ha, I. KimDepartment of Polymer Science and Engineering, Pusan NationalUniversity, Pusan 609-735, KoreaFax: þ82-51-513-7720; E-mail: [email protected]

Macromol. Rapid Commun. 2009, 30, 188–193

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

capsules can be achieved by encapsulating guest species,

which could endow these capsules with diverse properties.

As an important type of guest species, novel nanoparticles

such as metal nanoparticles, metal oxide nanoparticles,

and quantum dots have attracted interest because of their

remarkable optoelectronic, catalytic, and biomedical

properties.[3] To date, several studies concerning the

synthesis of polymer capsules that encapsulate these

novel nanoparticles have been explored. Most of the

reported protocols are based on a template-assistant

approach. First, the preformed nanoparticles (e.g., Au

nanoparticles (AuNPs)) are coated with a silica shell. This

sheath is further functionalized with certain reactive

groups to grow a polymer layer. Upon removal of the

middle silica layer using hydrofluoric acid, polymer

capsules that contain single nanoparticles are formed.

For instance, Kamata et al.[4] synthesized poly(benzyl

methacrylate) capsules that contained a single AuNP by

means of conformally coating a silica shell and a

poly(benzyl methacrylate) sheath by atom transfer radical

polymerization, followed by the removal of the silica shell.

Kim et al.[5] included divinyl benzene and initiator into the

mesoporous silica coating of the AuNP. After polymeriza-

DOI: 10.1002/marc.200800629

Encapsulation of a Single Metal Nanoparticle with . . .

tion and removal of silica, spherical poly(divinyl benzene)

capsules encapsulating AuNPs were fabricated. In addi-

tion, Cheng et al.[6] developed a relatively simple photo-

reduction process to synthesize silver-encapsulated hollow

polypyrrole–chitosan spheres.

Although the template-assisted approach and other

protocols mentioned above offer promising ways to create

polymer capsules that encapsulate a single metal nano-

particle, they involved rather complex synthesis proce-

dures and rigorous control of reaction conditions. It is also

not convenient to control the size and optical properties of

nanoparticles because of use of preformed nanoparticles.

Therefore, it is significant to develop a facile method to

prepare polymer capsules that contain single nanoparti-

cles with tunable size.

Herein, we report a versatile protocol to fabricate

monodisperse poly[styrene-co-(divinyl benzene)] (PS-co-

DVB) microcapsules that bear a single AuNP inside of them

using the PS-co-DVB microcapsule as a microreactor. In

addition we show that the size of the resulting AuNP inside

the PS-co-DVB microcapsules is tunable by simply

controlling the reaction parameters. To the best of our

knowledge, this is the first case of the fabrication of highly

monodisperse polymer capsules that contain a single

novel nanoparticle using these facile procedures. Owing to

the high monodispersity in size of the polymer capsules

and the optical properties derived from the encapsulated

metal nanoparticles, the corresponding polymer micro-

capsules that contain a single novel nanoparticle are

promisingly functional colloidal crystal materials after

ordered packing.[1b]

Experimental Part

Materials

Tetraethylorthosilicate (TEOS), absolute ethanol, ammonium

hydroxide, poly(vinyl pyrrolidone) (PVP, MW 55 000), 3-(triethoxy-

silyl) propyl isocyanate (TPI), 3-buten-1-ol, hydrofluoric acid (HF),

tetrachloroauric(III) acid (HAuCl4), sodium borohydride (NaBH4),

silver nitrate (AgNO3), and palladium chloride (PdCl2) purchased

from Aldrich, were used as received. N,N-Dimethylformamide

(DMF), styrene, and divinyl benzene (DVB) were obtained from

Aldrich and purified by distilling under reduced pressure before

use. 2,20-Azoisobutyronitrile (AIBN) commercially obtained from

Fluka was recrystallized in methanol before use.

Synthesis of H2C––CH(CH2)2OC(O)NH(CH2)3

Si(OCH2CH3)4 (M1)

To modify the silica surface with C––C groups, the modifier M1 was

synthesized. In a typical procedure, 3-buten-1-ol (0.02 mol, 4.9 mL)

was added dropwise to a 250 mL flask that contained a mixture of

DMF (20 mL) and TPI (0.02 mol, 1.7 mL) under gentle stirring. The

reaction was carried out at 60 8C for 20 h. After evaporating the

solvent under reduced pressure, M1 was obtained. 1H NMR



(CDCl3): d¼0.56 (m, 2H, SiCH2), 1.15 (t, 9H, CH3), 1.55 (m,

2H, CH2CH2CH2), 2.24 (t, 2H, OCH2CH2), 3.10 (m, 2H, NHCH2),

3.74 (q, 6H, OCH2CH3), 4.04 (t, 2H, OCH2), 4.99 (m, 2H, ––CH2), 5.75

(m, 1H, CH2––CH), 7.22 (br, NH).

Preparation of Modified Monodisperse Silica Spheres

The monodisperse silica particles with an average diameter of

550 nm and relative standard deviation of 4% were synthesized

according to reported procedures.[7] The resulting silica particles (1

g) were redispersed in absolute ethanol (20 mL) that contained

ammonium hydroxide (7.5 mL), followed by adding 0.8 mL of M1

gradually under vigorous stirring. The mixture was heated to 40 8Cand kept stirring for another 24 h. The surface-modified silica

particles were collected by centrifugation and cleaned by repeated

centrifugation and ultrasonic dispersion in ethanol.

Fabrication of Silica@PS-co-DVB CompositeMicrospheres and PS-co-DVB Microcapsules

By means of the C––C groups on the functionalized surface of silica,

a PS-co-DVB could be generated on the surface of the silica

particles by a dispersion polymerization. In a typical procedure,

the surface-modified silica particles (1 g) were dispersed in ethanol

(50 mL), and then subsequently mixed with PVP (0.1 g), AIBN (0.01

g), styrene (0.04 g), and DVB (0.04 g). The mixture was

continuously stirred for 4 h at 60 8C and then cooled to room

temperature. The resulting silica@PS-co-DVB composite particles

were cleaned by repeated centrifugation and ultrasonic dispersion

in ethanol. The monodisperse PS-co-DVB microcapsules were

produced by etching out the silica core with a 10 vol.-% HF ethanol

solution over 30 min. After cleaning the resulting microcapsules

by repeated centrifugation and ultrasonic dispersion in ethanol,

PS-co-DVB microcapsules bearing ethanol solvent were generated.

Preparation of PS-co-DVB Microcapsules that Contain

a Single AuNPPS-co-DVB was first dispersed in 1.5 mL of aqueous HAuCl4

solution (0.1 M) by ultrasonication and then placed overnight to

allow the HAuCl4 to diffuse into the PS-co-DVB microcapsules. The

unwanted HAuCl4 outside of the PS-co-DVB capsules was removed

by three cycles of centrifugation–redispersion in deionized water.

The resulting PS-co-DVB capsules that contained HAuCl4 were

then redispersed in 5 mL of deionized water, followed by addition

of 1.0 mL of aqueous NaBH4 solution (0.1 M) under vigorous

stirring. The mixture was continuously stirred at ambient

temperature for 12 h, and then purified by three cycles of

centrifugation–ultrasonic redispersion in deionized water. In this

simple procedure, PS-co-DVB microcapsules that contain a single

AuNP were prepared. To further increase the size of the AuNPs, the

above filling cycles of aqueous HAuCl4 and NaBH4 were repeated

several times.

Following a similar procedure, PS-co-DVB microcapsules that

contain a single silver (AgNP) and palladium (PdNP) nanoparticle

were also fabricated successfully by using 1.5 mL of AgNO3 (0.1 M)

and PdCl2 (0.1 M) instead of HAuCl4, respectively.

www.mrc-journal.de 189

H. Li, C.-S. Ha, I. Kim

190

Sample Characterization

Samples for transmission electron microscopy (TEM) were

deposited onto carbon-coated copper electron microscope grids

and dried in air. TEM analysis was performed using a JEOL 1200 EX

at 120 keV. All the UV-vis absorption spectra of the PS-co-DVB

microcapsule suspensions (in absolute alcohol) were recorded at

room temperature on a UV-1650PC apparatus (Shimadzu).

Infrared spectra of the samples were obtained at a resolution of

1 cm�1 with a Bruker FT-IR spectrophotometer over a wavenum-

ber range of 4 800 to 400 cm�1. IR measurement of the powder

samples was performed in the form of KBr pellets.

Results and Discussion

The facile but efficient protocol developed in this study is

illustrated in Scheme 1. The first step involves the

modification of monodisperse silica microspheres with

polymerizable vinyl groups (M1), followed by the encap-

sulation of the modified silica surface by a PS-co-DVB shell

through a disperse polymerization. After the resultant PS-

co-DVB-coated silica (silica@PS-co-DVB) composites are

subjected to removal of the silica cores using hydrofluoric

acid (HF), monodisperse PS-co-DVB capsules are created.

The resulting PS-co-DVB capsules are dispersed in aqueous

HAuCl4 to allow Au3� ions to diffuse into the PS-co-DVB

capsules. Subsequent diffusion of aqueous NaBH4 into the

PS-co-DVB capsules produces a single AuNP (AuNP@PS-co-

DVB). By repeating the filling cycles of HAuCl4 and NaBH4,

the size of the resulting AuNPs can be controlled. This

protocol for making AuNP@PS-co-DVB has the following

advantages over the reported methods: 1) The preparation

Scheme 1. Schematic illustration of the preparation of poly[styrene-conanoparticles (AuNP@ PS-co-DVB). M1 is the surface-modifier of the



procedures are simple and readily controllable, 2) the size

of the resultant AuNPs are tunable, which endows the

PCAs with controllable optical absorption, 3) the resulting

PCAs are highly monodisperse.

Figure 1a and 1b show TEM images of silica micro-

spheres and the corresponding silica@PS-co-DVB core–

shell microspheres, respectively. It is observed that the

size of the composite microspheres increases from

550 nm (neat silica) to 800 nm. In addition, the

silica@PS-co-DVB particles exhibit good monodispersity

that is around 4% in size polydispersity. The functional

silica surface and the PS-co-DVB coating, the as-prepared

surface-modified silica microspheres, and the correspond-

ing silica@PS-co-DVB composites were characterized by

infrared spectroscopy (Figure 1c). The absorption peak

centered at 1 645 cm�1 (curve i) is assigned to the

characteristic peak of hydrogen-bonded urethane carbo-

nyl groups,[8] which confirms M1 is grafted onto the silica

surface successfully. The characteristic peaks of polystyr-

ene at 1 600 and 1 450 cm�1, which belong to the C––C

bands of the phenyl ring (curve ii),[9] evidence the

formation of the PS-co-DVB shell. The monodisperse PS-

co-DVB microcapsules are produced by the removal of the

silica core using HF. The TEM image (Figure 1d) shows a

clear contrast between the dark ring and the pale center

of the spherical particles, which indicates the formation

of a hollow interior within the PS-co-DVB. By investigat-

ing over 100 microcapsules, it is found that most of the

microcapsules exhibit fairly regular shapes and high

monodispersity in both outer and inner diameter. The

average shell thickness of the PS-co-DVB microcapsules is

about 125 nm.

-(divinyl benzene)] (PS-co-DVB) microcapsules that contain single Ausilica microsphere, H2C––CH(CH2)2OC(O)NH(CH2)3Si(OCH2CH3)4.

DOI: 10.1002/marc.200800629


Figure 2. TEM images of PS-co-DVB microcapsules that contain Au nanoparticles(AuNP@PS-co-DVB) of varied size prepared by repeat filling cycles of aqueous HAuCl4and NaBH4: a) one cycle, b) two cycles, and (c) three cycles. (d) UV-Vis spectra of theresulting AuNP@PS-co-DVBs obtained by repeated filling cycles of aqueous HAuCl4 andNaBH4: i) one cycle, ii) two cycles, and iii) three cycles.

Figure 1. TEM images of a) silica microspheres and b) poly[styrene-co-(divinyl benzene)](PS-co-DVB)-coated silica microspheres (silica@PS-co-DVB). c) FT-IR spectra of i) H2C––CH(CH2)2OC(O)NH(CH2)3Si(OCH2CH3)4-modified silica particles and ii) silica@PS-co-DVBmicrospheres. d) TEM image of PS-co-DVB microcapsules obtained by removal of thesilica core from silica@PS-co-DVB composite microspheres.



Figure 2 shows the TEM images of PS-

co-DVB microcapsules with a diverse size

of AuNPs. When the PS-co-DVB micro-

capsules were subject to the first filling

cycle of HAuCl4 and NaBH4, AuNPs of

30 nm average diameter are formed

(Figure 2a). It is interesting to note that

more than 90% of the PS-co-DVB capsules

contain a single AuNP inside of them,

which demonstrates the versatility of

this protocol. When the filling cycle is

repeated two and three times, the

average diameter of the AuNPs increases

to 45 and 67 nm, respectively. No more

newly formed AuNPs are observed. In

addition, the spherical shape of the

capsules is maintained after the incor-

poration of AuNPs.

The mechanistic pathway for the for-

mation of a single AuNP within the PS-co-

DVB microcapsule might be explained in

terms of a confined nucleation and

growth process. When the reducing agent

NaBH4 diffuses into the cavity of the PS-

co-DVB microcapsules filled with HAuCl4,

the AuCl�4 ions are reduced and form

many Au nuclei, which distribute into the

solution within the cavity of the PS-co-

DVB capsule (see Scheme 1). These nuclei

tend to aggregate to form bigger particles.

Once the larger particles form, the remain-

ing nuclei within the cavity of the capsule

will be successively absorbed onto the

surface of the preformed particles, which

results in the growth of Au particles. Even

though several tiny AuNPs could form at

the same time, the confined space within

the capsule also allows them to aggregate,

which may also be responsible for the

formation of a single AuNP. The limited

amount of precursor (AuCl�4 ions) accom-

modated in the confined cavity of the PS-

co-DVB capsule avoids further growth of

the nanoparticle. In addition, the PS-co-

DVB shell also acts as a protective shell to

stabilize the resulting nanoparticles.

When HAuCl4 and NaBH4 solutions are

further introduced into the cavity of the

PS-co-DVB capsule, the preformed AuNPs

act as nuclei and keep growing into larger

nanoparticles.

AuNPs usually exhibit strong surface

plasmon resonance owing to the inter-

action with the external electromag-


H. Li, C.-S. Ha, I. Kim

Figure 3. TEM images of PS-co-DVB microcapsules that contain single silver (a) andpalladium (b) nanoparticles. c) UV-vis spectrum of PS-co-DVB microcapsules thatcontain single silver (i) and palladium (ii) nanoparticles. The scale bars in (a) and (b)are 1 mm.

192

netic field induced by light.[10] Moreover, the plasmon

band of the AuNPs is tunable depending on the size of the

nanoparticles.[11] In our current study, the formation of

AuNPs with varied size encapsulated in the PS-co-DVB

capsules would endow the polymer capsules with

different optical absorptions. This can be evidenced by

investigating the resulting AuNP@PS-co-DVB by UV-vis

spectroscopy. Figure 2d shows that the AuNP@PS-co-DVB,

obtained from the first, the second, and the third filling

cycles of HAuCl4 and NaBH4, exhibit broad optical

absorption peaks centered at 525, 544, and 556 nm

(curves i, ii, and iii), respectively. It is evident that this

red-shift in the optical absorption is induced by the

increase of the size of the resulting AuNPs. It is also found

that the absorption intensity increases with increasing

AuNP size because of light scattering in the presence of

larger nanoparticles.

To extend the current protocol to fabricate PS-co-DVB

microcapsules that contain other metal nanoparticles, Ag

and Pd nanoparticles were chosen as candidate materials

because of their extensive applications in the field of

biomedicine and catalysis.[12] According to the TEM images

in Figure 3, more than 90% of the PS-co-DVB microcapsules

encapsulate a single Ag or Pd nanoparticle. The PS-co-DVB

microcapsules were subjected to three filling cycles of

metal precursor (AgNO3 or PdCl2) solutions and aqueous

NaBH4. The average diameters of the resulting AgNPs and

PdNPs are 60 and 62 nm, respectively. The optical

absorption of the resulting PS-co-DVB capsules that

contain Ag and Pd nanoparticles were also investigated.

As is shown in Figure 3c, the two composites, AgNP@PS-

co-DVB and PdNP@PS-co-DVB, exhibit absorption peaks

centered at 435 (curve i) and 337 nm (curve ii), which are

derived from the surface plasmon resonances of Ag and Pd

nanoparticles, respectively. These results demonstrate that



the versatile protocol developed in this

study might be utilized to fabricate a

variety of polymer–metal nanoparticle

composites, by using different types of

polymers and metal nanoparticles or

quantum dots. Since this will endow

the polymer capsules with diverse prop-

erties and thus extend their applications

greatly, related studies that include

searching for suitable applications are

in progress.

Conclusion

In summary, we present a simple yet

efficient way to fabricate monodisperse

PS-co-DVB microcapsules that contain-

ing a single AuNP. The size of the

resulting AuNPs can be controlled by simply changing

the infiltration cycles of the aqueous HAuCl4 and NaBH4

solution. This tunability in the size of the AuNPs endows

the resulting monodiperse AuNP@PS-co-DVB with tunable

optical absorption. Following a similar procedure, PS-co-

DVB microcapsules that bear a single Ag or Pd nanoparticle

were also prepared successfully, which demonstrates the

versatility of the protocol developed in this study.

Acknowledgements: This work was supported by grants-in-aidfor the National Core Research Center Program from MOST/KOSEF(R15-2006-022-01001-0), Brain Korea 21 program (BK-21), and theCenter for Ultramicrochemical Process Systems (CUPS).

Received: October 7, 2008; Revised: November 11, 2008; Accepted:November 14, 2008; DOI: 10.1002/marc.200800629

Keywords: disperse polymerization; gold nanoparticles; micro-encapsulation; microreactors; monodisperse microcapsules;silicas

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[2] [2a] S. A. Johnson, P. J. Ollivier, T. E. Mallouk, Science 1999, 283,963; [2b] M. S. Wong, J. N. Cha, K. Choi, T. J. Deming, D.Stucky, Nano Lett. 2002, 2, 583. [2c] L. Song, M. Wang, Y.Cong, W. Liu, X. Ge, Z. Zhang, Polymer 2007, 48, 150. [2d] J.Han, G. Song, R. Guo, J. Polym. Sci., Part A: Polym. Chem. 2007,45, 2638.

[3] A. N. Shipway, E. Katz, I. Willner, CHEMPHYSCHEM 2000, 1, 18.[4] K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 2003, 125, 2384.[5] M. Kim, K. Sohn, H. B. Na, T. Hyeon, Nano Lett. 2002, 2, 1383.

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[6] D. Cheng, X. Zhou, H. Xia, H. Z. O. Chan, Chem.Mater. 2005, 17,3578.

[7] H. Li, S. Abraham, W. Yan, C.-S. Ha, I. Kim, Macromol. RapidCommun. 2007, 28, 1534.

[8] C. Chen, S. A. Dai, H. Chang, W. Su, T. Wu, R. Jeng, Polymer2005, 46, 11849.



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