Fabrication of gold nanoparticles using luminol as a reductive and protective reagent

Article abstract of DOI:10.1134/S0020168508080062

An easy and simple reaction is employed to synthesize ligand-protected gold nanoparticles. In this reaction, luminol is used as a reductive and protective reagent to prepare gold nanoparticles in aqueous solution. It includes two steps: chemiluminescence and formation of gold nanoparticles. The formation processes were monitored by resonance light scattering spectroscopy, and the results were observed by transmission electron microscopy and x-ray photoelectron spectroscopy. Highly stable gold nanoparticles about 110 ± 2 nm in size can be readily obtained. A new insight into the design of water-soluble ligand-protected gold nanoparticles for biomedical, analytical, and catalytic applications has been provided.

Full text of DOI:10.1134/S0020168508080062

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 8, pp. 813–817. © Pleiades Publishing, Ltd., 2008.
Fabrication of Gold Nanoparticles Using Luminol as a Reductive
and Protective Reagent1
Baoqiang Lva, Xiaodong Sua, Yang Lib, Yi Lic, Jianfei Maob, and Dan Xiaoa,b
a College of Chemical Engineering, Sichuan University, Chengdu, 610065 People’s Republic of China
b Chemical College, Sichuan University, Chengdu, 610065 People’s Republic of China
c The Southwest Research & Design Institute of Chemical Industry, Chengdu, 610065 People’s Republic of China
e-mail: xiaodan@scu.edu.cn
Received June 22, 2007; in final form, November 27, 2007
Abstract—An easy and simple reaction is employed to synthesize ligand-protected gold nanoparti-
cles. In this reaction, luminol is used as a reductive and protective reagent to prepare gold nanoparticles in
aqueous solution. It includes two steps: chemiluminescence and formation of gold nanoparticles. The formation
processes were monitored by resonance light scattering spectroscopy, and the results were observed by trans-
mission electron microscopy and x-ray photoelectron spectroscopy. Highly stable gold nanoparticles about 110
2 nm in size can be readily obtained. A new insight into the design of water-soluble ligand-protected gold nano-
particles for biomedical, analytical, and catalytic applications has been provided.
DOI: 10.1134/S0020168508080062
1
INTRODUCTION
Though the RLS technique is proved to be an effective
method to investigate nanoparticulate systems, the
study on formation of gold nanoparticles using RLS
remains an unexplored area.
There has been much interest in the synthesis of dis-
persed gold nanoparticles for applications in catalysis
[1], nanoelectronics [2], and sensing probes for biolog-
ical systems [3, 4]. Gold is an excellent candidate by
virtue of its facile reductive preparation, high chemical
stability, and good biocompatibility. It has been well
established that gold nanoparticle surfaces can be
repeatedly functionalized with thiolated organic mole-
cules. Ligand place-exchange is a key step in gold
nanoparticle functionalization [5–7]. Functional groups
on the outermost portion of the nanoparticles ligand
sphere can undergo many synthetic transformations
directly [8, 9], such as gold-thiol chemistry can be used
as a platform for introducing ligands to manipulate
aggregation, solubility, and reactivity—all the while
maintaining the magnetic utility of the core, thus wid-
ening its applicability in the biomedical field signifi-
cantly [10]. To date, relatively simple thiocyanuric acid
[11] has been used to stabilize gold nanoparticles. To
our knowledge, there has been no report on synthesiz-
ing ligand-protected gold nanoparticle using a simple
luminol chemical reduction method at room tempera-
ture.
In this article, we report the synthesis of ligand-pro-
tected gold nanoparticles in aqueous state with luminol
by a simple two-step reaction. The formation processes
were monitored by RLS spectroscopy. Then these
nanoparticles were fully characterized by x-ray photo-
electron spectroscopy (XPS) and transmission electron
microscopy (TEM). Since the reaction system only
contains the gold precursor and luminol, it is helpful to
have an insight into the formation of gold nanoparti-
cles. Nanoparticles reduced by luminol offer high stab-
lility and good dispersion.
EXPERIMENTAL
Chemicals. Sodium tetrachloroaurate (NaAuCl4 ·
2H2O) and luminol were purchased from Sinopharm
Chemical Reagent Co. (Shanghai, China). All the
chemicals were analytical grade and were used without
further purification. All solutions were prepared with
doubly deionized water.
The formation of gold nanoparicles in aqueous solu-
tions was always monitored by UV/vis spectra [12].
However, this method shows some interference to mon-
itor the reductive process of gold nanoparticles,
because some kinds of reductive chemicals have stron-
ger UV/vis absorption. Solving this problem is to use a
resonance light scattering (RLS) technique [13–18].
Characterization. Chemiluminescence (CL) spec-
tra were obtained with an F-4500 spectrofluorophotom-
eter (Hitachi, Japan). CL was studied between 200 and
700 nm; the scan rate was 1200 nm/s.
The flow injection CL (FI-CL) system detection was
conducted on a model IFFM-E flow injection system
(Remex Electronic Science Co., Xi’an, China).
1
The text was submitted by the authors in English.
813

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BAOQIANG LV et al.
(a)
(b)
3500
3000
2500
2000
1500
1000
500
0
200 300 400 500 600 700
Wavelength, nm
10
20
30
40
50
Time, s
Fig. 1. CL spectra for luminol with aqueous NaAuCl : (a) wavelength scan, (b) time scan.
4
RLS spectra were recorded on an F-4500 spectrom- at 470 nm, which corresponds to the formation of gold
eter. In addition, RLS spectra were recorded with syn- nanoparticles.
chronous scanning at λex = λem (i.e., ∆λ = 0), and the
RLS intensities were measured successively by the
RESULTS AND DISCUSSION
spectrofluorophotometer. The intensities were plotted
against the incident light wavelength to make RLS
spectra.
CL generation in luminol with NaAuCl4 aqueous
solution. Once mixing the solution of sodium tetra-
chloroaurate with basic luminol reagent at room tem-
perature, intense blue CL is produced immediately
from the reaction of tetrachloroaurate with luminol.
Figure 1 shows a typical plot of the CL intensity vs.
wavelength and time. The CL intensity decreased
quickly, and the lifetime was kept for 1 s. The CL peak
is at 470 nm. In aqueous solution, it is presumed that the
luminescence results from the oxidation of luminol by
the terachloroaurate complex. The slope of the plot
obtained may be explained as the rate of formation of
an excited intermediate which fluoresces when return-
ing to its ground state.
The shape of the sample was observed by TEM on a
JEOL JEM 200CX operating at 200 kV. TEM images
were obtained from the solution without any purifica-
tion or centrifugation. For the TEM observation, sam-
ples were obtained by dropping solutions onto carbon-
coated copper grids placed on a filter paper for rapid
removal of the liquid.
In the XPS measurement (Kratos X-SAM800 spec-
trometer) we used AlKα (1486.6 eV) as the x-ray
source, operated at 12 kV and 15 mA. The correction of
the XPS measurement was done with the Au 4f peak at
84.0 eV and the Ag 3d at 386.3 eV. All measurements
were performed in vacuum.
CL method for the determination of AuCl4–
based on the luminol–NaAuCl4 reaction. Because the
CL of luminol–NaAuCl4 reactions has a short lifetime,
the FI configuration used for the determination of
Synthesis of gold nanoparticles. The experimental
method was a luminol reduction of NaAuCl4 in aqueous
solution. Luminol aqueous solution (5 ml, 3.0 mM) was
added to 5 ml of a 1.0 mM aqueous solution of
NaAuCl4. After stirring for 30 s, the solution was put at
room temperature. After the beginning of the reaction,
the solution was taken out at specific time intervals for
RLS spectroscopy.
AuCl–4 was designed to provide the best conditions for
reaction in order to measure the stable signals. The FI
manifold finally adopted is not shown. The sample and
luminol were injected simultaneously into theY-conflu-
ence with the aid of two rotary valves while the sample
Time-resolved measurement of procession of zone was mixed in the Y-confluence over the PMT
gold nanoparticles. The formation process of gold detector. The distance from theY-confluence to the inlet
nanoparticles in the solution phase was determined by point of the detector was set at about 6 cm, which is as
RLS. This was carried out by the following procedure: short as possible. The reagents and tubing were covered
the solution was directly measured with RLS. The with black box to prevent the loss of the feeble light
nanoparticles were confirmed from the absorption peak produced by the reaction.
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FABRICATION OF GOLD NANOPARTICLES USING LUMINOL
815
The proposed method provides a simple approach
350
300
250
200
150
100
50
for the determination of AuCl–4 in aqueous solution.
The dynamic range of the method is very wide, calibra-
tion curve for 1.92 × 10–7 to 1.21 × 10– M AuCl–4 (R2 =
0.9927). To examine the possible interferences pro-
duced by foreign ions and some complexing agents, the
concentration of AuCl–4 was fixed at 1.0 × 10–6M, and
the tolerable error was ≤ 5.0%. The results were as fol-
7
1
lows. Foreign ions and some complexing agents (toler-
ance ratio in mass): K+, Na+, NO–2, Ba2+, Co2+, Fe2+, Ni2+,
Cu2+, Zn2+ (1000). Therefore, we concluded that the
proposed method has good selectivity. The method
could use for determining AuCl–4 in some conditions.
The formation process of gold nanoparticles. The
reaction of luminol with NaAuCl4 in aqueous solution
was readily monitored by RLS spectroscopy. Figure 2
shows the time-dependent RLS spectrum of solution
containing luminol/NaAuCl4 (the luminol/NaAuCl4
molar ratio is 3.0), with a time interval 10 min in the
first 50 min and 5 min in the later 10 min. When the
reaction starts, we can observe an obvious scattering
band at around 470 nm, which corresponds to the for-
mation of gold nanoparticles. The scattering intensity
in this wavelength range increased rapidly for one hour
and then was almost constant. Besides, the color of the
solution changed from colourless to pale black within
several minutes with the addition of luminol to the
NaAuCl4 solution. Clearly, the formation of gold nano-
particles is due to the autoreduction of the luminol–
NaAuCl4 solution. To investigate the influence of the
formation of gold nanoparticles, the molar ratio of lumi-
nol/NaAuCl4 (denoted as R) 3.0 was selected. The intro-
duction of microwave and ultrasound can speed up the
autoreduction process. Because the molarity of luminol
usually exceeds that of NaAuCl4, and amine groups are
known to interact with gold surfaces strongly, the
amino ligands of luminol are expected to stabilize gold
nanoparticles via external capping. The presumed for-
mation mechanism of gold nanoparticles is depicted
schematically in Fig. 3. There are two steps for the for-
mation of gold nanoparticles. First, luminol-protected
gold nanoparticles were prepared by simple rapid lumi-
0
358.6
518.6
678.6
Wavelength, nm
Fig. 2. RLS spectra of gold nanoparticles: (1) 10, (2) 20,
(3) 30, (4) 40, (5) 50, (6) 55, (7) 60 min.
Figure 4 shows the TEM image of luminol-pro-
tected gold nanoparticles produced at room tempera-
ture. The gold nanoparticles have high dispersion. No
distinct particle aggregation was observed, indicating
that luminol can effectively stabilize gold nanoparti-
cles. The size distribution reveals that the average size
of the gold nanoparticles is 10
2 nm for lumi-
nol/NaAuCl4 = 3.0. Both morphologies and particle-size
distributions may depend on the reduction rate and
luminol content. In fact, the precipitate resulting from
this aggregation was observed several weeks later. It
should be pointed out that investigations are under way
to further improve the control of gold particle sizes and
shapes.
Gold nanoparticles were analyzed by XPS. We con-
cluded from the XPS spectra (Fig. 5) that gold nano-
paritcle had been prepared successfully. The appear-
ance of N and Au peaks in the XPS data reveals that
nol reduction of partly AuCl–4 with strong CL. Second,
the reduction of the remaining AuCl–4 leads to the
growth of gold nanoparticles to make gold nuclei at the
initial stage of the reaction. The amine group is known
to form coordination compounds with Au(III) [19].
According to the above proposal, the remaining amine
groups of luminol immediately cap the surface of the
nucleating gold moieties, leading to a controlled
growth of gold nanoparticles. The luminol capped on
the surface of gold nanoparticles maintains its shape.
The above discussion indicates that the dual role played
by luminol is not only the formation but also the stabi-
lization of gold nanoparticles.
–
AuCl
+
Step one
CL
Step two
4
nucleation
luminol
Fig. 3. Schematic illustration of the formation process of
gold nanoparticles by luminol reduction of NaAuCl : step
one, CL; step two, nucleation.
4
INORGANIC MATERIALS Vol. 44 No. 8 2008

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BAOQIANG LV et al.
50 nm
Fig. 4. TEM image of gold nanoparticles obtained using luminol reduction of NaAuCl .
4
92
88
84
80
404
400
396
Binding energy, eV
Binding energy, eV
Fig. 5. XPS spectra in the Au 4f and N 1s regions.
NH2 groups were adsorbed onto the gold nanoparticle prepare gold nanoparticles in aqueous solution. The
gold nanoparticles have been characterized by various
techniques. The results show that the gold nanoparticles
have good dispersion and stability. Our ongoing work
will focus on further exploring ligand-protected gold
nanoparticle applications in various areas.
surfaces. As expected, the binding energy at 83.8 eV of
Au 4f is similar to pure Au (84.0 eV) (see Fig. 5). The
N 1s signal from the Au nanoparticles shows the pres-
ence of chemical species with a binding energy of
398.98 eV. The decrease of the binding energy is due to
the presence of nitrogen in the capped gold nanoparti-
cles surfaces.
ACKNOWLEDGMENTS
This project was supported by the Student Innova-
tion Fund of Sichuan University (no. 2006G006) and
the National Natural Science Foundation of China
(no. 20575042). We thank Maoxun Zeng (The South-
west Research & Design Institute of Chemical Indus-
try) for taking the TEM measurements.
CONCLUSIONS
In this work, we have proposed a simple two-step
reaction to synthesize ligand-protected gold nanoparti-
cles. We demonstrate that luminol is used as a reductive
and protective reagent for the reduction of NaAuCl4 to
INORGANIC MATERIALS Vol. 44 No. 8 2008

FABRICATION OF GOLD NANOPARTICLES USING LUMINOL
817
Mechanism of Ligand Exchange, J. Am. Chem. Soc.,
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