Enhancing and quenching functions of silver nanoparticles on the luminescent properties of europium complex in the solution phase

Article abstract of DOI:10.1021/jp035741b

Luminescent properties of the Eu(dinic) complex, where dinic is dinicotinic acid, were examined in the presence of silver nanoparticles in N,N-dimethylformamide. The formation of particle aggregates was observed upon mixing the particle solution with the complex solution, in which the complexes behaved like linker molecules. Luminescent intensity of the complexes was enhanced in the presence of silver nanoparticles, but at only a limited particle concentration region. The observed particle concentration dependences of the luminescent intensity were regarded as the result of a delicate balance between an enhancing and a quenching effect of the silver nanoparticles. In addition to the change in intensity, silver nanoparticles also affected the asymmetric ratio (or monochromaticity) of the europium luminescence. These changes in the luminescent properties of the complex were discussed in terms of the effects of particle aggregates and consequent modifications of the local electromagnetic field, refractive index, and the ligand field around europium ion.

Full text of DOI:10.1021/jp035741b

© Copyright 2003 by the American Chemical Society
VOLUME 107, NUMBER 35, SEPTEMBER 4, 2003
LETTERS
Enhancing and Quenching Functions of Silver Nanoparticles on the Luminescent Properties
of Europium Complex in the Solution Phase
Hideki Nabika and Shigehito Deki*
Department of Chemical Science & Engineering, Faculty of Engineering, Graduate School of
Science & Technology, Kobe UniVersity, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
ReceiVed: June 19, 2003; In Final Form: July 18, 2003
Luminescent properties of the Eu(dinic) complex, where dinic is dinicotinic acid, were examined in the presence
of silver nanoparticles in N,N-dimethylformamide. The formation of particle aggregates was observed upon
mixing the particle solution with the complex solution, in which the complexes behaved like linker molecules.
Luminescent intensity of the complexes was enhanced in the presence of silver nanoparticles, but at only a
limited particle concentration region. The observed particle concentration dependences of the luminescent
intensity were regarded as the result of a delicate balance between an enhancing and a quenching effect of
the silver nanoparticles. In addition to the change in intensity, silver nanoparticles also affected the asymmetric
ratio (or monochromaticity) of the europium luminescence. These changes in the luminescent properties of
the complex were discussed in terms of the effects of particle aggregates and consequent modifications of the
local electromagnetic field, refractive index, and the ligand field around europium ion.
Photoactive lanthanide complexes are of both fundamental
and technological interest because of their large Stokes shifts,
narrow emission bandwidths, and long emission lifetimes. Due
to these characteristics, they are suitable candidates for applica-
tions as light-emitting diode (LED),1 laser materials,2,3 and
fluoroimmunoassay.4-7 Selvin and co-workers reported that
luminescent (or lanthanide-based) resonant energy transfer
(LRET) can be a novel tool for measuring nanometer scale
distances, both in vitro and in vivo, in which LRET relies on
energy transfer from a donor lanthanide complex to an acceptor
organic dye via a nonradiative dipole-dipole (Fo¨rster-type)
interaction.8-10 A major problem concerning the use of lan-
thanide complex for fluoroimmunoassay is the quenching due
to a coupling with vibrational modes of solvent molecules. To
avoid this quenching effect, several kinds of chelating agents
have been synthesized.6,11,12 On the other hand, nanostructured
metallic surfaces would be promising materials to enhance the
luminescence of these lanthanide complexes. Metallic nano-
structures have caused a drastic increase in the intensity of
Raman scattering or luminescence of ions or molecules in close
proximity with the metallic surface.13 Recent investigations of
the surface enhancement effect have opened up a new meth-
odology for high-sensitivity detection such as single-molecule
surface-enhanced Raman spectroscopy (SM-SERS).14-16 La-
kowicz et al. reported the effects of silver island films on
fluorescence properties of DNA, with a quantum yield of 10-4
to 10-5, and they observed an intrinsic fluorescence from DNA
without any additional label used for the conventional DNA
detection method.17 These remarkable enhancement phenomena
stem from a resonant excitation of surface plasmon polaritons
confined within a metallic nanostructure and a consequent
enhancement in the induced dipole moment of the target
molecules.13
Pioneering experiments on the luminescence properties of the
Eu(III) ion located in close proximity to a metallic surface were
* Corresponding author. E-mail: deki@kobe-u.ac.jp. Phone: +81 (78)
803-6160. Fax: +81 (78) 803-6160.
10.1021/jp035741b CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/08/2003

9162 J. Phys. Chem. B, Vol. 107, No. 35, 2003
Letters
reaction was kept for 3 h and the obtained dark-brownish
solution was purified by centrifugation at 12 000 rpm for 45
min.
Preparation of rod-shaped silver nanoparticles was made
according to the previously reported seed-mediated method.23
The obtained ruby-red solution containing silver nanorods was
centrifuged (8000 rpm, 30 min), and precipitates were dissolved
to PVP aqueous solution (10 wt %), and kept at 30 °C for 1 h.
This solution was again centrifuged (8000 rpm, 30 min), and
precipitates were dissolved into a PVP-DMF solution (10 wt
%) and kept at 30 °C for 1 h. To remove excess PVP molecules,
purification was made by centrifugation at 6500 rpm for 30 min.
Eu(dinic), where dinic is dinicotinic acid (pyridine-3,5-
dicarboxylic acid), complexes were used as luminescent com-
plexes and obtained by dissolving the ligand into EuCl3 (DMF)
solution with a molar ratio of 2:1. The emission spectrum of
this solution (Figure 1) exhibits several well-defined emission
bands under ligand excitation (λex ) 267 nm), indicating the
formation of the complex that exhibits the energy transfer from
the ligand to the central Eu(III) ions.
Figure 1. Luminescent spectrum of Eu(dinic) complex dissolved in
DMF under ligand excitation (276 nm).
made by Drexhage18,19 and recently by Barnes.20,21 All these
experiments were carried out by depositing Eu(III) complex onto
the metallic thin films covered with an organic spacer. Never-
theless, lanthanide complexes are required to be used in the
solution phase as luminescent labels for biological applications.
In this regard, the use of metallic nanostructures in solution
phase as surface enhancers is an attractive challenge, and
nanosized metallic particles are promising candidates for this
purpose. In this work, we report on the fundamental aspects of
the role of metal nanoparticles on the luminescent properties
of europium complexes in solution phase using silver nanopar-
ticles of different sizes and shapes.
Spherical Ag nanoparticles of different sizes were synthesized
by reducing AgNO3 with N,N-dimethylformamide (DMF).22
Poly(vinylpyrrolidone) (PVP) aqueous solution (10 mL, 10 wt
%) was added to DMF (80 mL), followed by an addition of
AgNO3 aqueous solution (10 mL). The mean particle diameter
was tuned by changing the reaction temperature (90-110 °C)
and the concentration of AgNO3 solution (0.125-0.250 M). The
The morphology and size distribution of silver nanoparticles
were obtained by a transmission electron microscope (TEM,
JEOL JEM-2010, 200 keV). Optical absorption spectra were
measured with a U-3300 spectrophotometer (HITACHI) by
using a 10 mm optical path length quartz cuvette. Luminescence
measurements were performed using a FP-6500 spectrofluo-
rometer (JASCO) with a 90° configuration. The samples for
the optical measurements were prepared by mixing the complex
solution and the particle solution with keeping the final
concentration of Eu(III) ion at 250 µM.24
Three sets of spherical silver nanoparticles with a particle
diameter varying from 9.7 to 27.1 nm and rod-shaped silver
nanoparticles with an aspect ratio of 1.79 were prepared in the
present study. The representative TEM micrographs and size
distributions are presented in Figure 2, in which minor axis (a-
Figure 2. TEM micrographs, size distributions, and optical absorption spectra of spherical silver nanoparticles prepared at (a) 90 °C, [AgNO3] )
0.125 M, (b) 90 °C, [AgNO3] ) 0.250 M, and (c) 110 °C, [AgNO3] ) 0.250 M and (d) of the rod-shaped silver nanoparticle. The mean diameter,
d, was obtained by counting at least 500 particles in each sample, and the aspect ratio (AR) calculation for rod sample was done on only spheroidal
particles.

Letters
J. Phys. Chem. B, Vol. 107, No. 35, 2003 9163
Figure 4. Silver concentration dependence of asymmetric ratio (AS),
in which the AS value is normalized with the value in the absence of
Ag nanoparticles (0 nM): spherical particle with d ) 9.7 nm (O), 15.9
nm (×), and 27.1 nm (0); rod-shaped particle ([).
Figure 3. Silver concentration dependence of integrated luminescent
intensity, in which the intensity is normalized with the intensity in the
absence of Ag nanoparticles (0 nM): spherical particle with d ) 9.7
nm (O), 15.9 nm (×), and 27.1 nm (0); rod-shaped particle ([).
dence, including the quantitative treatment for g and Cmax. of
each sample.
axis) and major axis (b-axis) distribution are displayed for rod-
shaped sample (Figure 2d). The mean diameter was obtained
by counting at least 500 particles in each sample, and the aspect
ratio calculation was done on only spheroidal particles. DMF
solutions of both spherical and rod-shaped nanoparticles ex-
hibited a bright yellow-orange color, and a distinct optical
absorption band is observed around 400 nm in each sample
(Figure 2). Rod-shaped nanoparticles are known to exhibit two
distinct absorption maxima corresponding to transverse and
longitudinal modes of a localized surface plasmon resonance.25
However, the absorption spectrum of the rod-shaped sample
prepared in the present study shows a single peak at 428 nm,
along with a small shoulder around 400 nm because of the small
aspect ratio. Upon adding the complex solution into the
nanoparticle solution, the color of the solution immediately
changed to pinkish red, accompanying the appearance of a new
absorption or scattering band around 500-550 nm. The ap-
pearance of the new band at longer wavelength region indicates
the formation of particle aggregates in the solution.26 No color
or spectral change was observed when only either ligands or
europium ions were added into the particle solution, indicating
the function of Eu(dinic) complexes as linker molecules, and
the complexes are expected to be incorporated in the particle
aggregates.
In contrast to our results, only the quenching effect of metal
nanoparticles has been observed in the solution system where
gold nanoparticles and semiconductor nanoparticles29 or organic
fluorophores27 were co-dissolved. The origin of this difference
is attributable to the above-mentioned aggregates formation and
the difference in the metal used. The enhancement field around
metallic nanostructures is strongly enhanced when two or more
particles come into close proximity with each other, and the
aggregate effect concerning Raman scattering has been re-
ported.26 As mentioned above, Eu(dinic) complexes are expected
to be incorporated into particle aggregates, and thus the
fluorescent intensity was effectively enhanced in the present
system. Moreover, silver is a well-established material to exhibit
remarkable enhancement ability for Raman and fluorescent
intensity of molecules. The effectiveness of silver for enhance-
ment phenomenon is widely known and Lakowicz et al.
observed enhanced luminescence from [Ru(bpy)3]2+ com-
plexes,30 where bpy is 2,2′-bipyridyl, in the presence of silver
particle islands, whereas Au nanoparticles quench the fluores-
cence of [Ru(bpy)3]2+ following the Stern-Volmer relation.31
In addition to the changes in the luminescent intensity with
the particle addition, we found another effect arising from the
addition of Ag nanoparticles. Figure 4 shows the variation in
asymmetric ratio (AS) with the particle concentration, where
AS is defined as the integrated emission intensity ratio between
5D0 f 7F1 (magnetic dipole transition) and 5D0 f 7F2 (electric
The particle concentration dependence on the emission
intensity of the complexes is illustrated in Figure 3, in which
the intensity is normalized with the intensity in the absence of
Ag nanoparticles (0 nM). Luminescent intensity first increases
and then decreases with the increasing particle concentration,
and each sample shows a different maximum enhancement
factor (g) and the particle concentration at which the intensity
reaches the maximum (Cmax.). It is deduced from this concentra-
tion dependence that the silver nanoparticles act as both an
enhancer and a quencher for europium luminescence, because
the luminescent intensity should decrease monotonically when
the metal nanoparticles serve as only the quencher.27 The
obtained maximum enhancement factor for each sample is thus
regarded as the result of a delicate balance between the
enhancement and the quenching effect. The enhancement effect
is known to be strongly affected by the size and symmetry of
the metallic nanostructure,28 and a comprehensive expression
considering both the enhancing and the quenching effects will
give detailed information for the particle concentration depen-
5
7
5
7
dipole transition), AS ) ∫I D f F (ω) dω/∫I D f F (ω) dω. A
0
2
0
1
variation in the AS value describes two effects of the nanopar-
ticle addition, that is, the modifications of the ligand field and
the refractive index around europium ions. The increasing
tendency in AS values with the particle concentration indicates
the increase in the distortion of the ligand field arising from
the interaction with Ag nanoparticles during the aggregate
formation.32 The effect of refractive index modification is
explained on the basis of the local field effect, which gives the
refractive index dependence of the electric dipole transition
rate.33-35 The macroscopic variation in the refractive index upon
particle addition is almost negligible according to the Maxwell-
Garnett (MG) effective medium expression, because of quite a
low volume fraction of Ag nanoparticles (∼10-6).36 However,
considering that the Eu(dinic) complexes exist in close proximity
of Ag nanoparticles, the local refractive index around Eu(dinic)

9164 J. Phys. Chem. B, Vol. 107, No. 35, 2003
Letters
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Acknowledgment. This work was supported by Grant-in-
Aid for Scientific Research (B)(1) No. 14404014 and (C)(1)
No. 15635004 from the Japan Society for the Promotion of
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