Direct Metallization of Gold Nanoparticles on a Polystyrene Bead Surface using Cationic Gold Ligands

Communication

634

Direct Metallization of Gold Nanoparticleson a Polystyrene Bead Surface usingCationic Gold Ligands

Jun-Ho Lee, Dong Ouk Kim, Gyu-Seok Song, Youngkwan Lee,Seung-Boo Jung, Jae-Do Nam*

Gold nanoparticles are formed to cover the surface of sulfonated-polystyrene (PS) beads by thein-situ ion-exchange and chemical reduction of a stable cationic gold ligand, which makes itdifferent from thephysical adsorption ormultiple electrolessmetallizationmethods. PS beads aresynthesized by dispersion polymerization with a diameter of 2.7 mm, and their surface ismodified by introducing sulfonic acid groups (SO�

3 ) to give an ion exchange capacity of up to2.25mequiv. � g�1,which provides 1.289�1010 SO�

3 per bead. Subsequently, the anionic surface ofthe PS beads is incorporated with a cationic gold ligand, dichlorophenanthrolinegold(III) chloride([AuCl2(phen)]Cl), through an electrostatic interaction in the liquid phase to give gold nano-particles (ca. 1–4 nm in diameter) formed on the PSo surface. Assuming that approxi-mately three SO�

3 groups interact with one [AuCl2(phen)]þ ion in the ion-exchange process,

the gold coverage on a PS bead is estimated as 12.0wt.-%, which compares well with the 16.8 wt.-% of goldloading measured by inductively coupled plasma–massspectrometry. Because of the adjustable IEC values of thepolymer surface and the in-situ metallization of Au inthe presence of S atoms, both of which are of a softnature, the developed methodology could provide asimple and controllable route to synthesize a robustmetal coating on the polymer bead surface.

J.-H. Lee, D. O. Kim, G.-S. Song, J.-D. NamDepartment of Polymer Science and Engineering, SAINT, Sung-kyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon440-746, South KoreaE-mail: [email protected]. LeeDepartment of Chemical Engineering, Sungkyunkwan University,300 Chunchun-dong, Jangan-gu, Suwon 440-746, South KoreaS.-B. JungDepartment of Advanced Materials Engineering, SungkyunkwanUniversity, 300 Chunchun-dong, Jangan-gu, Suwon 440-746,South Korea

Macromol. Rapid Commun. 2007, 28, 634–640

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Introduction

Core-shell particles, which are often composed of metal

coating and dielectric core materials, have been inten-

sively investigated in various applications such as optical

resonance,[1] multiplexed optical bar coding,[2] crypto-

graphy,[3] flow cytometry,[4] surface-enhanced Raman

scattering,[5] photonic crystals,[6,7] photocatalyst,[8] and

biosensors.[9] For example, while the miniaturization of

portable and mobile electronics has become a key issue in

microelectronic packaging, flip-chip technology has

focused on the use of an anisotropic conductive film

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Direct Metallization of Gold Nanoparticles . . .

(ACF) in place of solder and underfill for the attachment of

electronic chips to the electrodes on the glass or organic

substrate.[10] In this technology, the conductive beads,

which are composed of monodispersed polymer beads

with a coverage of a metal layer, establish electrical

contacts between the interconnecting areas. ACF flip-chip

bonding provides finer pitch, higher package density,

reduced package size, and improved environmental

compatibility.[10] In this technology, the metal-coated

polymer beads, most commonly coated with gold, are

currently produced by multiple electroplating steps,

usually composed of Pd-catalyst, Ni plating, and Au

plating. These often induce difficulties in controlling bead

coagulation and metal loading and, thus, additional

fractionation and filtering steps are used to obtain the

desired sizes of metal-coated polymer beads.

Recently, various approaches have been proposed to

formmetallic layers on a polymer bead, including thermal

evaporation, sputtering, and sol-gel methods, to produce

conductive particles.[11] However, these methods often

incur high fabrication costs of high-vacuum systems, and

technical difficulties in exposing all sides of the bead to the

sputter source. Furthermore, the coating thickness ob-

tained by these methods is usually too thin.[12] More

recently, several metal-coating methods have been repor-

ted: seeding with electroless plating,[1] chemical surface

functionalization of the polymers followed by nanoparti-

cle adsorption,[13] layer-by-layer (LbL) self-assembly of

metal nanoparticles with polyelectrolytes,[7] and metal

nanoparticle infiltration into polyelectrolyte-coated poly-

mer beads.[14,15] For example, Kaltenpoth et al.[16] success-

fully prepared a metallic layer on a polymer surface by

treating polystyrene (PS) beadswith a poly(ethyleneimine)

(PEI) polyelectrolyte in phosphate-buffered solution, and

then negatively charged gold nanoparticles (3 nm) were

absorbed on the positively charged PS beads by electro-

static interactions. The fabrication of core-shell particles

has been reported on the mesoscopic scale by the

self-assembly of the LbL adsorption of different materials

like gold, iron oxide, and titania.[7,14,17] Aminoalkylalk-

oxysilane-treated silica beads have been coated with gold

nanoparticles by the functionalization of gold nanoparti-

cles by dithiols.[18]

However, it still remains difficult to control the loading

content of metals and induce a robust adhesion of nano-

particles on heterogeneous surfaces, which is usually

required in the quality control and part assembly of

biomedical and optoelectronic devices.[19] In particular, the

metal coating on polymer beads in the flip-chip bonding

technique should be strong enough to sustain several

physicochemical downstream processes, which include

binder mixing, film forming, thermal bonding, etc. A fur-

ther difficulty is that the charged gold nanoparticles

usually have electrostatic repulsion and, thus, excessive



electrophoretic force is needed to form a close-packed,

two-dimensional, metallic layer.[20]

Accordingly, we propose a simple and controllable route

to coat the polymer beads with gold nanoparticles by an

in-situ direct metallization reaction of a cationic gold

ligand on a sulfonated PS surface. In this method, a strong

bond between the polymer surface and the gold nano-

particle is expected because of the soft nature of gold and

sulfur.[21] Since the cationic gold ligands are incorporated

on the anionic sulfonic acid groups by electrostatic inter-

actions, most possibly in the ratio of one gold-complex ion

to three sulfonate groups based on a geometric considera-

tion the gold loading is controllable as desired to ensure a

uniform gold coating on the PS bead surface.

Experimental Part

Materials

The styrene monomer, poly(vinylpyrrolidone) (PVP, Mn: 40 000),

hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 � 3H2O), chlor-

osulfonic acid, and sodium borohydride (NaBH4) were obtained

from Aldrich Chemicals Co. 2,20-Azoisobutyronitrile of analytical

grade (AIBN; Junsei Chemical) was used as an initiator without

further purification. 1,10-Phenanthroline was purchased from

Junsei Chemical.

PS Bead Synthesis

The PS beads were synthesized using a previously reported proce-

dure.[22] Briefly, styrene monomer (75 mL) and PVP (7.5 g) were

charged into a 700 mL, four-necked reaction vessel, and then

dissolved in 70/30 (vol.-%) ethanol/water solutions. The AIBN

concentration was 1 wt.-% relative to the total amount of

monomer. After charging 370mL of the polymerizingmixture into

a reactor, it was purged with nitrogen for 10 min. The poly-

merization temperature was fixed at 60 8C in an oil bath. The

agitation speed was 120 rpm. After completion of the polymer-

ization, the resulting particles were obtained by centrifugation at

3 000 rpm and washed repeatedly with methanol and water. The

molecular weight, molecular weight distribution, and size of the

synthesized PS beads were 209 000, 1.5, and 2.7 mm, respectively.

Sulfonation of PS Beads

The synthesized PS beads were sulfonated by using chlorosulfonic

acid. The sulfonation reaction was allowed to proceed for 250 min

at 0 8C and was then terminated by adding ethanol. The ion

exchange capacity (IEC) of the sulfonated PSwas determined by an

acid-base titration method. Sulfonated PS particles were im-

mersed into 50mL of saturated NaCl solution and themixturewas

stirred for 24 h to allow the Hþ and Naþ ions to exchange. The

released Hþ ions were titrated with 0.1 N NaOH solution. From the

amount of consumed NaOH, the IEC of the PS beads was

calculated.[23]

www.mrc-journal.de 635

J.-H. Lee, D. O. Kim, G.-S. Song, Y. Lee, S.-B. Jung, J.-D. Nam

636

Metallization of PS Beads

The AuIII cationic complex, dichlorophenan-

throlinegold(III) chloride ([Au(phen)Cl2]Cl),

was synthesized by the reaction of 1,10-

phenanthroline and hydrogen tetrachloro-

aurate(III) hydrate in ethanol.[24,25] As

shown in Scheme 1, the sulfonated PS parti-

cles (10 g) were immersed into the aqueous

solution of [Au(phen)Cl2]Cl (20mmol) at 60 8Cfor 3 h. The cationic gold ligands absorbed on

the sulfonated PS particles were subse-

quently reduced by sodium borohydride.

After the reduction reaction, the polymer/

metal beads were filtered, washed with

water, and finally dried using a freeze-dryer.

The metallization process (step III and IV in

Scheme 1) was repeated three times until all

the ion-exchange sites on PS were consumed.

Scheme 1. Preparation of the metallized PS beads. Monodispersed PS beads are synthes-ized by dispersion polymerization (step I). The PS beads are then functionalized by

þ

Characterization

A scanning electron microscope (SEM, JEOL,

JSM 890) was used to analyze the size of the

PS beads and the morphology of the gold-

coated PS beads. The size, structure, and ele-

mental composition of the gold-coated PS

beads were investigated by high-resolution

transmission electron microscopy (HR-TEM,

JEOL, 300KV). Inductively coupled plasma

mass spectrometry (ICP-MS, Agilent, agilent

7500i) was used for the quantification of total

gold present in the repeatedly coated (three

times) PS beads. The gold-coated PS beads

(0.018 g) synthesized in this study were dis-

solved in a mixture of HCl/HNO3 in a 3:1

weight ratio to give a 1.063 g solution, which

was diluted subsequently with HNO3 to give

a solution of 49.94 g for the ICP-MS measure-

ment.
chlorosulfonic acid (step II). Au(phen)Cl2 ligands are adsorbed onto the negativelycharged PS beads (step III), and subsequently reduced by NaBH4 to give gold nanopar-ticles formed on the PS bead surface (steps IV).
Results and Discussion

Figure 1A compares the FT-IR spectra of sulfonated PS and

pristine PS beads. The peaks at 2 924 and 2850 cm�1 arise

from the C–H stretching vibration of the CH2 and CH

groups on the main PS chains. The sharp peaks at 700 and

780 cm�1 can be ascribed to the –C–H out of plane

deformation in the pristine PS.[26] After sulfonation, the

disappearance of these peaks and the occurrence of a new

peak at 576 cm�1, which represents a para substitution of

sulfonated PS, indicates the attachment of sulfonic acid

groups at the para position. The peak identified in the

spectra at 1 375 cm�1 corresponds to the asymmetric

stretching of the S––O bond. The symmetric vibration of

this bond produces the characteristic split bands at



1 150–1 185 cm�1. There was no significant change in

the peaks at 2 925 and 3000 cm�1, which represent C–C

and C–H bonds.[27]

The SEM microphotographs in Figure 1B and C compare

pristine PS beads and the sulfonated PS beads, respectively,

both of which maintain their spherical shape, and the

surfaces of both cases are smooth without wrinkles or

slits on the bead surface. Figure 1C shows the degree of IEC

(mequiv. of SO3H per gram) of the sulfonated PS beads as a

function of sulfonation treatment time. It is clear that the

increase in the reaction time leads to a gradual increment

in the IEC of the PS beads. Assuming that the sulfonic acid

groups are formed as a mono-layer on the PS bead surface,

DOI: 10.1002/marc.200600757


Figure 1. FTIR spectra (A) and SEM images (B and C) comparing PS beads before and aftersulfonation. Ion-exchange capacity (IEC) measured as a function of time at 24 8C is shownin D.

Figure 2. SEM micrographs of gold-coated PS beads after a single metallization (A) andrepeated (three times) metallization (B). The EDX spectra is shown in C.

the number of SO�3 ions formed on the

PS bead surface may be esti-

mated.[25,28] For example, for a PS bead

with IEC at 2.25 mequiv. � g�1 (2.7 mm

in diameter and a density of 1.02), the

number of SO�3 groups can be esti-

mated as 1.289� 1010 SO�3 per bead.

When the sulfonated PS beads are

immersed in a solution that contains a

cationic gold source, three anionic sites

may interact with one gold cation

because the distance of three repeating

units of sulfonated PS (7.5 A) is

comparable to the size of the gold

complex ion (6.0 A).

Figure 2A and B show the SEM

images of the resulting particles

prepared by the direct metallization

process. The PS beads after a third

metallization process show a subs-

tantial increase in the surface coverage

of the gold particles (Figure 2B), which

can be compared with the PS beads

after a single metallization process



(Figure 2A). It is clear that the gold

loading increases with a repeated

metallization process. Although the

number of Au particles increase, the

gold particle size seems to be main-

tained at a diameter of 1–4 nm. The

energy dispersive X-ray spectrometry

(EDS) results confirm the presence of

gold and sulfur on the PS particle

(Figure 2C).

In the formation of gold nano-

particles, the diameter of a spherical

gold nanocrystal (D), which is com-

posed of NAu gold atoms, may be

estimated as:[29]

D ¼ 23VAuNAu

4p

� �1=3(1)

where VAu¼ 17 A3 and is the same as

that observed in the bulk fcc lattice. For

example, the number of gold atoms

needed to form a gold nanoparticle

with a diameter of 4 nm can be calcul-

ated as 1 970 gold atoms. Furthermore,

assuming that three sulfonate groups

(SO�3 ) interacts with one [AuCl2(phen)]

þ

ion, the number of gold nanoparticles

formed on a single PS bead can be



Figure 3. TEM images of a cross-section of a metallized PS bead (A, B, and C) and the X-ray diffraction pattern of the selected area of C (D).The fringe spacing (�0.2 nm) observed in C agrees well with the separation between the (200) lattice plane in D. E) The particle sizedistribution of gold nanoparticles counted from the TEM image.

638

estimated by dividing 1.289� 1010 atom per bead by 1 970

atoms to give 2.181� 106 gold nanoparticles. Accordingly,

the weight of the gold nanoparticles can be estimated as

1.41� 10�12 g �bead�1. The weight ratio of the coated gold

nanoparticles to a single sulfonated PS may be estimated

by the weight of a PS bead (1.050� 10�11 g �bead�1) and

the weight of gold nanoparticles on a PS bead (1.41�10�12 g �bead�1). Subsequently, the gold loading in a single

PS bead is estimated as 12.0 wt.-%. For comparison, the

gold loading after the thirdmetallizationwasmeasured by

ICP-MS to give a reliable result of 16.8 wt.-%, which

compares well with the estimated gold content of

12.0 wt.-%. In fact, the total number of gold atoms, which

are available to form gold nanoparticles, are limited by the

IEC value of the sulfonated PS beads. Consequently, it

should be addressed that the estimated gold loading is

independent of the gold nanoparticle size and the gold

loading density can possibly be controlled by the IEC value

of the sulfonated PS beads.

The TEM image in Figure 3A shows a cross-section of a

gold-coated PS bead. It can be seen that the gold nano-

particles are well distributed on the periphery of the PS

bead to give a uniform coverage without forming a

continuous phase of thin film. At a high magnification of



the selected area of Figure 3A, the size of the gold nano-

particles seems to range from 1 to 4 nm in diameter, and a

few exceptionally big gold particles can also be observed.

When nanoparticles are formed and grow in the vicinity of

a heterogeneous phase (the PS bead surface in this study),

the solubility of the smaller particles will be higher than

that of the larger particles. Consequently, there would be a

net diffusion of solute from the proximity of the small

particle to the proximity of the large particle along the

surface of the heterogeneous phase. In order to maintain

the equilibrium, solute will be deposited onto the surface of

the large particle, whereas the small particle has to continue

dissolving so as to compensate for the amount of solute

diffused away.[30,31]We speculate that this Ostwald ripening

effect[30] takes place in our metallization process and,

furthermore, it may be magnified by the facile surface

diffusion of solute along the heterogeneous PS surface to

give a few exceptionally large gold particles (Figure 3B). The

fringe spacing (�0.2 nm) observed in Figure 3C and theX-ray

diffraction (XRD) patterns in Figure 3D compared well with

the literature results for gold.[32] Bragg gold reflections are

evident in the XRD patterns at 38.2 8 (111), 44.4 8 (200), 64.6 8(220) and 77.5 8 (311) in the crystal planes of the fcc structure.

The separation between the (200) lattice plane agrees with

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the fringe spacing. Figure 3E shows the particle size

distribution of the gold nanoparticles counted from the

TEM image. The major peak is centered at 1.8 nm and

the particle size ranges around 1–4 nm.

The developed methodology demonstrates that the gold

nanoparticles can be selectively formed on the PS surface

to give a uniform coverage and the gold loading can be

controlled as desired by adjusting the IEC of the polymer

surface. Although further research is needed, the adhesion

of gold nanoparticles on the sulfonated PS may be im-

proved by the in-situ metallization of gold in the pre-

sence of sulfur, both of which have soft characteristics.

Apparently, this technique is simpler and more control-

lable than other techniques to ensure an advanced route in

quality control and material development in various fields

of optoelectronic and biomedical device fabrication.

Conclusion

Monodisperse PS beads 2.7 mm in size have been syn-

thesized by dispersion polymerization. The PS beads are

sulfonated and successfully metallized by using a cationic

gold ligand through ion-exchange and liquid-phase redox

reactions. The gold loading is likely to be controlled by the

IEC of the sulfonated PS beads. This fabrication method

showed a great deal of potential to control the gold loading

on the PS bead surface with a uniform coverage in a simple

procedure.

Acknowledgements: This work was supported by No.R0120060001034802006 from the Basic Research Program of theKorea Science & Engineering Foundation. We also appreciate theinstrumental and technical support from the ‘‘Local GovernmentInitiated R&D Program (Project No. 2004-0693-200)’’ of the KoreaMinistry of Commerce, Industry and Energy, in conjunction withthe Gyeonggi Province.

Received: November 1, 2006; Revised: December 18, 2006;Accepted: December 18, 2006; DOI: 10.1002/marc.200600757

Keywords: cationic gold ligand; core-shell polymers; beads;direct metallization; electrostatic interaction; gold nanoparticles;polystyrene

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DOI: 10.1002/marc.200600757

Direct Metallization of Gold Nanoparticles on a Polystyrene Bead Surface using Cationic Gold Ligands

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