Light emission of gold nanoparticles induced by the reaction of bis(2,4,6-trichlorophenyl) oxalate and hydrogen peroxide

Article abstract of DOI:10.1021/ac050882q

Light emission at ～415 nm was observed for gold particles with diameters of 2.6-6.0 nm dispersed in a solution containing bis(2,4,6- trichlorophenyl) oxalate and hydrogen peroxide. It was found that the light intensity was independent of the protecting reagents of the gold nanoparticles with similar size, the light intensity with gold nanoparticles of 5.0 and 6.0 nm in diameter was stronger than that with gold nanoparticles of 2.6 and 2.8 nm in diameter, and the light intensity increased linearly with the concentration of the gold nanoparticles using 6.0-nm gold nanoparticles. The gold nanoparticles were identified as emitting species, and the quantum yield was determined to be (2.8 ± 0.3) × 10^-5 using 6.0-nm gold nanoparticles. The light emission is suggested to involve a sequence of steps: the oxidation reaction of bis(2,4,6-trichlorophenyl) oxalate with hydrogen peroxide yielding an energy-rich intermediate 1,2-dioxetanedione, the energy transfer from this intermediate to gold nanoparticles, and the radiative relaxation of the as-formed exited-state gold nanoparticles. The observed luminescence is expected to find applications in the field of bioanalysis owing to the excellent biocompatibility and relatively high stability of gold nanoparticles.

Full text of DOI:10.1021/ac050882q

Anal. Chem. 2005, 77, 6402-6406
Light Emission of Gold Nanoparticles Induced by
the Reaction of Bis(2,4,6-trichlorophenyl) Oxalate
and Hydrogen Peroxide
Hua Cui,* Zhi-Feng Zhang, Ming-Juan Shi, Yang Xu, and Yun-Long Wu
Department of Chemistry, University of Science and Technology of China, Hefei 230026, P.R. China
Light emission at ∼415 nm was observed for gold
particles with diameters of 2.6-6.0 nm dispersed in a
solution containing bis(2,4,6-trichlorophenyl) oxalate and
hydrogen peroxide. It was found that the light intensity
was independent of the protecting reagents of the gold
nanoparticles with similar size, the light intensity with gold
nanoparticles of 5.0 and 6.0 nm in diameter was stronger
than that with gold nanoparticles of 2.6 and 2.8 nm in
diameter, and the light intensity increased linearly with
the concentration of the gold nanoparticles using 6.0-nm
gold nanoparticles. The gold nanoparticles were identified
as emitting species, and the quantum yield was deter-
two research groups observed that semiconductor nanoparticles
could induce the CL reactions as emitting species. Semiconductor
nanoparticles such as silicon, CdS, CdSe, CdSe/ZnSe, Ge, and
CdTe nanoparticles have been found to emit light in the presence
of a chemical reaction or an electrochemically initiated reaction.
Like semiconductor nanoparticles, metal nanoparticles of suf-
5-10
ficiently small size also possess discrete energy levels and thus
exhibit unique optical properties.1
1-17
For example, gold nano-
particles of 5.0-nm diameter separated by high-performance liquid
chromatography have been found to generate excited-state gold
nanoparticles upon exposure to ultraviolet light, emitting visible
light.17 There is a possibility that the excited-state gold nano-
particles can also be induced by a chemical reaction, and light
emission ensues. The reaction of bis(2,4,6-trichlorophenyl) oxalate
(TCPO) with hydrogen peroxide is known to be able to excite
fluorescent molecules to yield light emission.1 Here, we show
that gold nanoparticles can also be excited by reaction of TCPO
-
5
mined to be (2.8 ( 0.3) × 10
using 6.0-nm gold
nanoparticles. The light emission is suggested to involve
a sequence of steps: the oxidation reaction of bis(2,4,6-
trichlorophenyl) oxalate with hydrogen peroxide yielding
an energy-rich intermediate 1,2-dioxetanedione, the en-
ergy transfer from this intermediate to gold nanoparticles,
and the radiative relaxation of the as-formed exited-state
gold nanoparticles. The observed luminescence is ex-
pected to find applications in the field of bioanalysis owing
to the excellent biocompatibility and relatively high stabil-
ity of gold nanoparticles.
8,19
2 2
with H O and as a result light emission is achieved.
EXPERIMENTAL SECTION
Chemicals and Solutions. TCPO was obtained from Tokyo
Kasei Kogyo Co., Ltd. (Tokyo, Japan). Working solution of TCPO
was prepared by dissolving the solids in ethyl acetate. Working
2 2
solutions of H O were prepared fresh daily in acetone from 30%
Light emission induced by chemical reactions, known as
chemiluminescence (CL), has been intensively investigated for
many years. Originally, the study of CL was limited to molecular
(5) Ding, Z.; Quinn, B.; Haram, S.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science
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1
systems. Recently, the study has been extended to nanoparticle
systems. For example, we have found that gold nanoparticles of
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9) Myung, N.; Lu, X.; Johnston, K. P.; Bard, A. J. Nano Lett. 2004, 4, 183-
85.
(
4 2 3
different sizes could react with the KIO -Na CO /NaOH system
1
2
to yield CL as a nanosized reaction platform and could enhance
(
10) Poznyak, S. K.; Talapin, D. V.; Shevchenko, E. V.; Weller, H. Nano Lett.
3
the CL of the luminol-H
2
O
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system as catalyst. Pavlov and co-
2004, 4, 693-698.
(
(
(
11) Link, S.; El-Sayed, M. A. Annu. Rev. Phys. Chem. 2003, 54, 331-336.
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7781.
workers have reported the biocatalytic generation of chemi-
luminescence involving gold nanoparticles in the presence of
luminol and H
2
O
2
and its application for the amplified detection
4
(14) Link, S.; Beeby, A.; FitzGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten,
of DNA or telomerase activity. In these cases, gold nanoparticles
are not emitting species of the CL reactions. On the other hand,
R. L. J. Phys. Chem. B 2002, 106, 3410-3415.
(
(
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6986.
*
Corresponding author. Phone: +86-551-3606645. Fax: +86-551-3601592.
E-mail: hcui@ustc.edu.cn.
(17) Wilcoxon, J. P.; Martin, J. E.; Parsapour, F.; Wiedenman, B.; Kelley, D. F.
(
(
(
1) Robards, K.; Worsfold, P. J. Anal. Chim. Acta 1992, 266, 147-173.
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3) Zhang, Z. F.; Cui, H.; Lai, C. Z.; Shi, M. J. Anal. Chem. 2005, 77, 3324-
J. Chem. Phys. 1998, 108, 9137-9143.
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Yannotta, A. V.; Semsel, A. M.; Clarke, R. A. J. Am. Chem. Soc. 1967, 89,
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687.
(
1
6402 Analytical Chemistry, Vol. 77, No. 19, October 1, 2005
10.1021/ac050882q CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/27/2005

Figure 1. Schematic diagram of flow injection chemiluminescence
detection system.
Figure 2. CL intensity profiles via time in flow injection mode. (A)
Citrate-protected gold colloidal solutions and the corresponding blank
and (B) â-CD-protected gold colloidal solutions and the corresponding
blank were mixed with a solution of 5 mmol/L TCPO in ethyl acetate
and then a solution of 0.5 mol/L H O in acetone; the inset was the
(
v/v) H
China). HAuCl
Chemical Reagent Co. (Shanghai, China). A 1.0 g/L HAuCl
solution was prepared by dissolving 1 g of HAuCl in 1 L of triple-
distilled water and the resultant mixture was stored at 4 °C.
Trisodium citrate (Na ) (Sanpu Chemical Co. Ltd., Shanghai,
2
O
2
(Xinke Electrochemical Reagent Factory, Bengbu,
‚4H O (48% w/w) was obtained from the Shanghai
stock
4
2
4
2
2
CCD image of this chemiluminescent system in a laboratory-built static
CL cell.
4
3 6 7
C O
China), sodium hydroborate (Shanghai Chemical Reagent Co.),
and â-cyclodextrin (â-CD) (Shanghai Chemical Reagent Co.) were
directly dissolved in triple-distilled water to obtain the respective
solutions. All reagents were of analytical grade, and triple-distilled
water was used throughout.
photometer (Shimadzu), respectively. X-ray photoelectron spectra
were acquired on a model Escalab MK II electron spectrograph
(VG).
Chemiluminescence CCD Imaging. CCD images of the CL
reactions were taken from a laboratory-built static CL 2.0-mL
reaction cell with a Roper Scientific CCD (Princeton Instruments).
The CCD camera was cooled to -35 °C, and the exposure time
was 15 s. Moreover, a CCD array allowed us to take a full 2D
picture without scanning. The beam was finally focused on a CCD
matrix of 512 × 512 photodetector.
Synthesis of Gold Colloids. All glassware used in the
following procedures were cleaned in a bath of freshly prepared
3
3
:1 HNO -HCl, rinsed thoroughly in triple-distilled water, and
dried prior to use. All solutions were prepared using triple-distilled
water. When trisodium citrate was used as a protecting reagent,
colloidal gold nanoparticles of 2.6 and 6.0 nm in diameter were
synthesized by hydroborate reduction method2 and colloidal gold
nanoparticles of 16, 25, 38, 68, and 99 nm in diameter by citrate
Chemiluminescence Spectra Measurements. CL spectra
were measured by inserting high-energy cutoff filters from 300
to 700 nm between the flow CL cell and the PMT as described
elsewhere,23 because the chemiluminescence signals were not
high enough to be detected by a refitted fluorometer as other
systems.
X-ray Photoelectron Spectroscopy (XPS) Studies. The 6.0-
and 68-nm gold colloids before reaction and the reaction mixture
2 2
of 6.0-nm gold-H O -TCPO were rotatory evaporated to dryness
0
21
reduction method. When â-CD was used as a protecting reagent,
colloidal gold nanoparticles of 2.8 and 5.0 nm in diameter were
synthesized according to ref 22 with some modifications as
described in the Supporting Information.
TEM Measurements. The size and shape of the synthesized
gold nanoparticles were characterized by a model H-800 transmis-
sion electron microscope (Hitachi, Japan). Statistical analysis of
TEM data revealed that the average diameters of the gold colloids
were about 2.6 ( 2.0, 2.8 ( 1.5, 5.0 ( 2.1, 6.0 ( 3.5, 16 ( 1, 25 (
at temperature of 30 °C under vacuum condition. Then the solid
samples were used to acquire X-ray photoelectron spectra on a
model Escalab MK II electron spectrograph (VG).
2
, 38 ( 3, 68 ( 5, and 99 ( 7 nm, respectively.
Chemiluminescence Measurements. The chemilumines-
RESULTS AND DISCUSSION
Chemiluminescence of Gold Nanoparticles. When a solu-
tion of 5 mmol/L TCPO in ethyl acetate was first mixed with a
cence detection was conducted on a laboratory-built flow injection
chemiluminescence system, consisted of a model IFIS-C flow
injection system (Ruimai Electronic Science Co.), a model CR105
photomultiplier tube (PMT) (Bingsong Electronic Co.), a model
GD-1 luminometer (Ruimai Electronic Science Co.), and a
computer, as shown in Figure 1. Triple-distilled water was used
as a carrier for the colloidal solution of gold nanoparticles to mix
2
0
.6- or 6.0-nm gold colloidal solution, and then with a solution of
.5 mol/L H in acetone, light emission was observed as shown
2 2
O
in Figure 2A. The inset in Figure 2A is the CCD image showing
a uniform distribution of the lightening species in the reaction
solution for 6.0-nm gold colloids. Chemiluminescence signals
could also be obtained for â-CD-protected gold nanoparticles with
the size of 2.8 and 5.0 nm (Figure 2B). It was revealed that the
CL intensity was independent of the protecting reagents of the
gold nanoparticles with similar size, and the gold nanoparticles
of 5.0 and 6.0 nm in diameter were more emissive than the 2.6-
and 2.8-nm-diameter gold nanoparticles in this CL reaction.
However, no CL signal was obtained for the blank solutions
2 2
with TCPO and then with H O . The chemiluminescence signals
were monitored by the PMT adjacent to the flow CL cell.
Spectral Measurements. Fluorescence spectra and UV-
visible absorption spectra were measured on a model LS-55
spectrofluorometer (Hitachi) and a model UV-2401 PC spectro-
(
20) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154-
157.
1
(
21) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22.
(
22) Liu, Y.; Male, K. B.; Bouvrette, P.; Luong, J. H. T. Chem. Mater. 2003, 15,
4
172-4180.
(23) Cui, H.; Zou, G. Z.; Lin, X. Q. Anal. Chem. 2003, 75, 324-331.
Analytical Chemistry, Vol. 77, No. 19, October 1, 2005 6403

Figure 5. XPS of gold nanoparticles before and after the CL
reaction.
Figure 3. Relationship between the CL intensity and the concentra-
tion of 6.0-nm gold colloids. Different concentrations were obtained
via gradual dilution of gold colloids as synthesized.
Figure 6. (A) Excitation and emission spectra for blank solution
and 6.0-nm gold colloids before and after the CL reaction. (B) UV-
visible absorption spectra for the gold colloids (6.0 nm)-TCPO-H2O2
system before and after the CL reaction.
the binding energy of Au4f for either 6.0- and 68-nm gold
nanoparticles before the reaction or 6.0-nm gold nanoparticles after
the CL reaction were similar, indicating that the oxidation state
of gold did not change after the CL reaction. Therefore, no Au(I)
complex has been generated after the CL reaction. The origin of
chemiluminescence was thus assigned to the gold nanoparticles
themselves but not an Au(I) complex.
Figure 4. (A) Chemiluminescence spectra for Au colloids-TCPO-
H2O2 system: from top down, 5.0-, 6.0-, 2.8-, and 2.6-nm gold
colloids. (B) Fluorescence spectrum for colloidal solution of gold
colloids (excited at 230 nm): from top down, 5.0-, 6.0-, 2.8-, and 2.6-
nm gold colloids.
containing the precursor ionic chemicals without gold colloids.
For larger gold particles with diameters of g16 nm, no lumines-
cence was observed either.
Detailed information about UV-visible and FL spectra (Figure
6) also showed that there is no significant difference in the spectra
between the as-prepared gold nanoparticles and those after the
CL experiments. This indicates that the gold nanoparticles as an
emitting species did not decompose or aggregate and the surface
state of the gold nanoparticles remained unchanged after the CL
reaction.
The effect of the concentration of the 6.0-nm gold nanoparticles
on the CL intensity is shown in Figure 3. It can be seen that the
light intensity increases linearly with the concentration of gold
-
10
-8
nanoparticles over the range of 9.1 × 10 -3.3 × 10 mol/L.
-10
For the lower concentration of 4.6 × 10 mol/L, no light emission
Using the lucigenin-hydrogen peroxide system as a stan-
was detected. As the highest concentration synthesized in this
dard,25 the CL quantum yield of this system was determined to
-8
study was 3.3 × 10 mol/L, no attempt was made to measure
-5
be (2.8 ( 0.3) × 10 when 6.0-nm gold colloids were used, which
was comparable with the fluorescence quantum yield of gold
nanoparticles with similar size.17
the light intensity at higher concentrations.
Emitting Species Studies. The CL spectra for this system
were measured using high-energy cutoff filters of various wave-
2
3
Possible CL Mechanism. The mechanism for the TCPO-
lengths. As shown in Figure 4, the CL spectra with an emission
maximum of ∼415 nm were identical with the fluorescence (FL)
spectra of the colloidal gold solutions with the same diameter.
This reveals that the emitting species in the CL reaction were
probably the excited-state gold nanoparticles. However, it was
reported that Au(I) complex was emissive when excited by a
2 2
H O
CL reaction has been studied previously.2 It is suggested
6,27
2 2
that TCPO can be oxidized by H O to generate an energy-rich
intermediate, 1,2-dioxetanedione. For a fluorescent molecule, 1,2-
dioxetanedione could interact with the fluorescent molecule to
yield a charge-transfer complex,1
8,19
which decomposed with
2
4
electron transfer from the intermediate back to the fluorescent
proper light source, which might also be a possible origin of
the light emission reported here.
2
6,27
molecule, generating its excited state.
According to Cumber-
land and Strouse,28 gold nanoparticles could readily complex with
various ligands containing oxygen donor atoms. In the CL case,
To ascertain the emission species, XPS, UV-visible absorption
spectroscopy, and fluorescence spectroscopy studies were used
to characterize the gold nanoparticles before and after the CL
reaction. XPS results, as shown in Figure 5, demonstrated that
(25) Maskiewicz, R.; Sogah, D.; Bruice, T. C. J. Am. Chem. Soc. 1979, 101, 5347-
5354.
(26) Schuster, G. B. Acc. Chem. Res. 1979, 12, 366-373.
(
24) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P. J.
Am. Chem. Soc. 2001, 123, 11237-11247.
(27) Koo, J. Y.; Schuster, G. B. J. Am. Chem. Soc. 1977, 99, 6107-6109.
(28) Cumberland, S. L.; Strouse, G. F. Langmiur 2002, 18, 269-276.
6404 Analytical Chemistry, Vol. 77, No. 19, October 1, 2005

Scheme 1. CL Mechanism for TCPO-H
2
O
2
-Gold
that in both cases the electron transition occurred at the same
energy levels. Therefore, the chemiluminescence may arise from
the radiative interband recombination of excited electrons in the
sp-band and holes left in the low-lying d-band of the gold
nanoparticles similar to the fluorescence of gold nanoparticles.
For the smaller 2.6- and 2.8-nm-diameter gold nanoparticles, CL
and FL spectra resembled those for 5.0- and 6.0-nm-diameter gold
nanoparticles, but the light intensity decreased dramatically. It
was proposed that the electrostatic enhancement effects would
decrease with a decrease of nanoparticle size because of a
decrease in density of the surface plasma,31 which may lead to a
decrease in CL emission. If the size of the gold nanoparticles
further decreases to less than 2.0 nm (for example, the largest
emissive gold nanoparticles reported by Huang and Murray was
Colloid System
1
,2-dioxetanedione may interact with the gold nanoparticles and
15
of 1.8 nm in diameter ), the quantum size effects would take place,
causing an evident increase in the electronic gap between the
d-bands and the sp-bands. In this case, the electrons in the top
bind to the particle surfaces to form a similar charge-transfer
complex, followed by a transfer of energy from the surface-bound
1, 2-dioxetanedione to the gold core, resulting in the excited-state
d-band are readily excited to the sp-bands using excitation energy
gold nanoparticles. The CL reaction pathways are shown in
Scheme 1.
The light emission of gold nanoparticles was believed to relate
to their discrete energy levels. In fluorescence, for gold nanopar-
ticles or gold clusters smaller than 2.0 nm, their emissive
properties were often assigned to the electron interband transitions
lower than 2.75 eV (451 nm),1
4-16
giving rise to longer than 700-
nm light emission. Therefore, we deduce that gold nanoparticles
or clusters less than 2.0 nm may be chemically excited to generate
chemiluminescence with emission maximum longer than 700 nm.
Further work is underway.
For gold nanoparticles larger than 6.0 nm, the density of the
surface plasmon electrons also increased dramatically although
electrostatic fields of the surface plasma increased with an increase
in the particle scale. In this case, the surface plasmon electrons
but not the low-lying d-electrons would be more favored to interact
with incident photons during optical processes.32 Similarly, we
deduce that the surface plasmon electrons may be readily
thermalized by chemical energy released by the reaction. There-
fore, no emission could be found for gold nanoparticles larger
than 6.0 nm.
between the top d-band and the sp-conduction bands with enlarged
electronic gaps caused by quantum size effects.1
3-16
For gold
17
nanoparticles with a diameter of ∼5.0 nm, Wilcoxon et al.
proposed that absorption of light excites the electrons from the
d-band to the sp-band, leaving holes in the d-band, and the
recombination of the electrons and holes gives rise to light
emission. The light emission would be enhanced by electrostatic
fields of the surface plasma of the gold nanoparticles.29 However,
it seems that the luminescence was too high in energy to be only
assigned to the quantum-size effects of the gold nanoparticles,
and no further explanation for the origin of the energy has been
CONCLUSIONS
1
7
given in Wilcoxon et al.’s work. It was reported that the rough
metal surface could be regarded as a collection of randomly
oriented hemispheroids of nanometer-size dimension on a smooth
This work demonstrates that gold nanoparticles can be excited
chemically to give rise to light emission. Gold nanoparticles with
the size between 2.0 and 6.0 nm in diameter were found to exhibit
1
2
surface. Therefore, the optical properties for rough surface of
gold were supposed to be similar to those of the gold nano-
luminescence at ∼415 nm in the presence of the TCPO-H
2 2
O
chemical reaction. The observed CL is believed to involve a
sequence of steps: the transfer of energy to gold nanoparticles
from an energy-rich intermediate 1,2-dioxetanedione arising from
the oxidation of bis (2,4,6-trichlorophenyl) oxalate and radiative
decay of the as-formed excited state gold nanoparticles. The size
of the gold nanoparticles plays an important role in gold nano-
particle CL. At the present stage, the efficiency of gold nanoparticle
CL is lower than that of fluorescent molecules. However, there is
evidence that the CL efficiency can be improved substantially
1
1
30
particles. According to Boyd et al., at high excitation energy
of 4.67 eV, the electrons in the low-lying d-bands could be excited
to the sp-band, and the recombination of the excited electrons in
sp-band and holes in the low-lying d-bands would cause the
emission to peak at ∼3.1 eV, which was identified as the composite
emission of 6-4L and 6-3X. This emission energy was similar
to our results that the gold nanoparticles could emit at ∼2.98 eV
(415 nm). Therefore, we proposed that the electrons in the low-
lying d-bands could be excited to the sp-band by a high-energy
light source (230 nm), and the recombination of the excited
electrons in sp-band and holes in the low-lying d-bands was the
origin of the 2.98-eV emission of the gold nanoparticles. This
transition and recombination process could be enhanced by the
electrostatic enhancement mechanism, analogous to that in
surface-enhanced Raman scattering.
For the 5.0- and 6.0-nm-diameter gold nanoparticles, the CL
spectra were well matched with the fluorescence spectra, revealing
through optimizing the size and surface of the gold nano-
13-16
particles.
The relatively high stability of gold particles makes
it ideal as a model system for fundamental study of CL. Further-
more, the CL may also find applications in fields such as
biosensing, immunotechnologies, and biocatalytic generation of
chemiluminescence owing to the excellent biocompatibility and
ease of self-assembly of gold nanoparticles. The effect of biologi-
cally important compounds on the CL system, including CL
1
7,30
(31) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634-643.
(
29) Kerker, M. Acc. Chem. Res. 1984, 17, 271-277.
(32) Hwang, Y. N.; Jeong, D. H.; Shin, H. J.; Kim, D.; Jeoung, S. C.; Han, H. S.;
(
30) Boyd, G. T.; Yu, Z. H.; Shen, Y. R. Phys. Rev. B 1986, 33, 7923-7936.
Lee, J. S.; Cho, G. J. Phys. Chem. B 2002, 106, 7581-7584.
Analytical Chemistry, Vol. 77, No. 19, October 1, 2005 6405

inhibition and enhancement and other optical effects, and its
analytical applications are under investigation.
SUPPORTING INFORMATION AVAILABLE
Detailed procedure for the synthesis of gold nanoparticles of
various sizes and excitation and emission fluorescence spectra
for gold nanoparticles and their blank solutions. This material is
available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT
The support of this research by the National Natural Science
Foundation of P.R China (Grant 20375037) and the Overseas
Outstanding Young Scientist Program of China Academy of
Sciences are gratefully acknowledged. We also thank Prof. X. M.
Liu, Prof. S. Gao, Prof. M. R. Ji, Prof. H. Z. Zhao, and Mr. J. J.
Ding for measurements.
Received for review May 19, 2005. Accepted July 28,
2005.
AC050882Q
6406 Analytical Chemistry, Vol. 77, No. 19, October 1, 2005