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Materials Letters 59 (2

Formation of gold nanoparticles and self-assembly

into dimer and trimer aggregates

ShengTian Wanga,*, JiCang Yana, Li Chenb

aCollege of Chemistry, Northeast Normal University, Changchun 130024, ChinabCollege of Chemistry, Jilin University, Changchun 130021, China

Received 5 July 2004; accepted 13 December 2004

Available online 12 January 2005

Abstract

Gold nanoparticles synthesized by in situ chemical reduction of HAuCl4 with the use of polyelectrolyte serve as reductant and stabilizer. At

the same time, gold nanoparticles were observed self-assembly into dimer and trimer aggregates, which were evidenced by TEM. The

nucleation and growth kinetics was characterized by in situ UV–vis experiment. A model was proposed to illustrate the formation mechanism.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles; In situ; Polyelectrolyte; Dimer; Trimer

1. Introduction

Metal nanoparticles are certain to be important building

blocks of future nanoscale electronic and optical devices.

The preparation of metal nanoparticles has been extensively

studied. Metal nanoparticles protected by polymers can be

highly useful for a variety of applications [1]. Nanoparticles

tend to fairly unstable in solution due to small size, so it is

indispensable to take special precautions to avoid their

aggregation. The most common strategy is to protect

nanoparticles with protective agent that can absorb at the

particle surface to keep the nanoparticles suspended and

prevent their agglomeration by providing charge or sol-

ubility properties [2].

Polyelectrolytes have been widely used as building

blocks for the construction of functional multiplayer thin

films [3], and also used in industrial applications such as

water filtration, paper making and mineral processing due to

the fact that adsorption of charged polyelectrolytes to

oppositely charged colloids may turn intercolloidal repul-

sion into attraction and lead to flocculation [4]. However,

polyelectrolytes proved to be excellent stabilizer which are

0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2004.12.045

* Corresponding author. Tel.: +86 1359 600 1918.

E-mail address: Wangst706@nenu.edu.cn (S.T. Wang).

capable of combining both steric and electrostatic stabiliza-

tion resulting in electrosteric stabilization of colloids [5] and

the synthesis of polyelectrolytes stabilized nanoparticles

have been largely reported [6–12].

On the other hand, methods for assembling metal

nanoparticles into well-defined architectures must be found

because new properties often emerge from the particle

aggregates that are distinctly different from the correspond-

ing isolated nanoparticles. Much effort has focused on the

formation of organized arrays of nanoparticles by modifi-

cation of particle surface chemistry and application of

Langmuir–Blodgett [13,14] controlled solvent evaporation

[15], electrophoretic [16] etc. Won et al. reported that gold

nanoparticles could spontaneously form from aqueous

solution containing HAuCl4 and third/fourth generation

amine-terminated PAMAM dendrimers, a kind of amine-

containing polyelectrolyte, at room temperature [17].

Alivisatos and co-workers reported that DNA-mediated

aggregated gold nanoparticle into dimers and trimers [18].

Boal et al. have also demonstrated a polymer-mediated

strategy for the self-assembly of gold nanoparticles into

structured spherical and network aggregates [19].

Herein, we present a polyelectrolyte-based bsynthesisand linkQ strategy for the synthesis and self-assembly into

dimer and trimer aggregates of gold nanoparticles by

005) 1383–1386

S.T. Wang et al. / Materials Letters 59 (2005) 1383–13861384

heating aqueous solution of HAuCl4–polyelectrolyte com-

plex without the addition of extra reductant, with the use of

polyelectrolyte to serve as reductant and stabilizer and

linker. The dimer and trimer aggregates were evidenced by

TEM, and the nucleation and growth kinetics was charac-

terized by in situ UV–vis experiment. A model was also

proposed to illustrate the mechanism of the formation and

self-assembly of gold nanoparticles.

Fig. 1. Typical TEM image of as-prepared gold nanoparticles. Inset—

column distributions of uncoupled particles, dimer and trimer aggregates.

2. Experimental

Branched polyethylenmine (BPEI) shown in Scheme 1

and HAuCl4 were obtained from Aldrich and used

without further purification. The water used was purified

through a Millipore system (18MV cm). Solution con-

taining BPEI was prepared by dissolving 1 g of BPEI in

water. HAuCl4 solution was prepared by adding 1g of

HAuCl4 in water.

Gold nanoparticles were prepared as following: in brief,

200 Al of HAuCl4 (24.3 mM) and BPEI with the initial

molar ratio 4:1 of BPEI (repeat unit)/HAuCl4 were added

into a beaker followed by the addition of 20 ml of water,

then heated at 80 8C for an hour. As-prepared colloids were

stored in dark bottle at 4 8C.Transmission electron microscopy (TEM) image of as-

prepared colloids was acquired on a JEOL 2010 trans-

mission electron microscopy operated at 200 kV.

The X-ray diffraction analysis of the products was

carried out on a D/MAX 2500 V/PC X-ray diffractometer

using Cu (40 kV, 200 mA) radiation source. UV–vis

experiment was performed on CARY 500 Scan UV–vis–

near infrared (UV–vis–NIR) spectrophotometer.

3000

4500

6000

7500

(3 1 1)(2 2 0)

(2 0 0)

(1 1 1)

Rel

ativ

e In

tens

ity

3. Results and discussion

The structures shown in Fig. 1 are typical of those found

over the entire TEM grid. Counting ca. 80 structures for this

sample revealed yields of 41% and 28% for dimer and

trimer aggregates, respectively, and the remainder of the

sample consisted predominantly of uncoupled particles.

Inset shows the corresponding column distribution graph.

This image suggests that BPEI serves as stabilizer and

linker bridging gold nanoparticles into dimer and trimer

aggregates.

The XRD pattern of precipitates obtained from centrifug-

ing sample is given in Fig. 2. The peak located at 37.9, 43.8,

N CH2CH2NHCH2CH2

x yCH2CH2NH2

Scheme 1. Chemical structure of BPEI.

64.4, 77.38 are assigned to 111, 200, 220, and 311 faces of

an Au crystal, respectively, demonstrating the formation of

gold [20]. Spontaneous formation of gold nanoparticles can

be attributed to the direct redox reaction between BPEI and

HAuCl4, because there was no reductant in the present

experiment. Polyelectrolyte is water-soluble polymer having

high molecular weight and producing large chain-type ions

in solution. BPEI is a highly branched aliphatic polyamine

whose amine groups exist in primary, secondary, and

tertiary form. It is well known that BPEI can easily

protonate in acid, even neutral aqueous solution. Here it

was the amine groups of BPEI that reduced AuCl4� to

nucleate and gradually led to the formation of gold

nanoparticles. When mixing oxidative HAuCl4 solution

with BPEI, both the protonated amine and neighbouring

methylene of BPEI dehydrogenated to form a labile double

bond (–C=N–) intermediate, which transformed into amide

during the elevation of temperature. To gain an insight into

30 40 50 60 70 800

1500

2θ /degree

Fig. 2. Time-dependent UV–vis absorption spectra of gold colloids with

initial molar ratio 4:1 of BPEI (repeating unit) to HAuCl4 taken with time

interval 30 s at constant temperature 80 8C.

Scheme 2. Schematic representation for the formation and self-assembly of

gold nanoparticles during heating BPEI–HAuCl4 complex aqueous

solution.

S.T. Wang et al. / Materials Letters 59 (2005) 1383–1386 1385

the formation kinetics of gold nanoparticles, an in situ UV–

vis experiment was performed at constant temperature 80 8C(Fig. 3). The induce time is quite short (ca. 1.5 min) with the

appearance of a surface band centered at 515 nm. Evidently,

the height of the surface plasmon peak increases signifi-

cantly within the following 2 min and then increases more

slowly with the elapsed time, which can be assigned to the

increase of the amount of reduced gold. It is also evidenced

that the absorption band shifts continuously to longer

wavelength at longer reaction time, and ceases at the

absorption band centered at 535 nm within an hour.

Afterward, no apparent change in the absorption spectro-

scopy was noticed.

A model shown in Scheme 2 was proposed to illustrate

the mechanism of the formation and self-assembly of gold

nanoparticles. When HAuCl4 was mixed with BPEI, ion

pairs were formed between AuCl4� anions and protonated

amine cations of BPEI. With the elapsed time, zerovalent

gold atoms were formed from the AuCl4� precursor and

these gold atoms could collide with each other to form small

gold clusters. It is known that secondary amine has stronger

reducing ability than primary amine. So we speculate it is

the secondary amine groups that predominantly induce the

reduction of the precursor. On the other hand, it is the

primary amine groups of BPEI that associate with the

particle surface due to the electrostatic interaction between

the positively charged amine groups and absorbed Cl�

anions and result in the subsequent formation of uncoupled

particles and coupled aggregates. During the process of the

formation of gold nanoparticles, there were some different

situations. When some particles were fully coated with

polyelectrolyte, which prevented the attachment of another

polyelectrolyte due to the electrostatic repulsion, uncoupled

colloids were formed. However, when some particles were

partially coated with polyelectrolyte, which allowed the

attachment of amine groups of another polyelectrolyte to the

uncoated particle surface, the polyelectrolyte served as

300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

surface band position

time

abso

rban

ce

wavelength (nm)

Fig. 3. XRD pattern of the resulting precipitates. Inset—four peaks

corresponding to (111), (200, (222) and (311) faces of an Au crystal.

linker bridging different particles into dimer or trimer

aggregates. It has been reported that the more particles in

contact, the longer the range of the plasmon coupling and

that particles coupling can result in red-shifted absorbance.

From the in situ UV–vis absorption spectra (Fig. 3) a

continuous red shift was noticed, which could be attributed

to the particles coupling effect and the growth of the gold

nanoparticles with the elapsed time. It has been shown that a

low concentration of added polyelectrolyte causes particle

aggregation because of the electrostatic attraction of

partially coated and uncoated colloids. It is noticeable to

mention that the concentration of polyelectrolyte is fourfold

to HAuCl4 in our current experiment, so serious aggregates

were not noticed except for dimer and trimer aggregates.

4. Conclusions

We have provided a simple polyelectrolyte-based

bsynthesis and linkQ strategy for the facile formation self-

assembly into dimer and trimer aggregates of gold nano-

particles in aqueous solution. This method is very simple

Although current experiment cannot provide the well

controllable formation of dimer and trimer aggregates,

further work will be done to examine the most optimum

parameters.

Acknowledgements

This work was supported by the project of the science

committee of Jilin province (No. 2001101).

S.T. Wang et al. / Materials Letters 59 (2005) 1383–13861386

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