World Journal Of Engineering
Coupled Metal Nanoparticles with Enhanced Optical Properties and Their Applications
Zhenping Guan, Polavarapu Lakshminarayana, Cuifeng Jiang, and Qing-Hua Xu*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore 117543
Email: [email protected]
was formed as a result of the coupling of gold nanoparticles and
can be tuned from visible to near-infrared by adjusting the
polymer/Au molar ratio. The gold nanochains were used as a
SERS substrate and gave an enhancement factor of 8.4 × 109,
which is ∼400 times larger than that on the isolated gold
nanosphere substrate (Figure 2). The giant SERS enhancement is
ascribed to the large electromagnetic fields of coupled gold
nanoparticles.4
Introduction
Noble metals, such as Ag and Au, display unique optical
properties known as surface plasmon resonance (SPR).1 The SPR
bands are sensitive to the morphology of the metal NPs as well as
the surrounding dielectric media.2 The interparticle coupling of
SPR could result in many hotspots with greatly enhanced local
field.3 In our research, we have demonstrated that the field
enhancement induced by plasmon coupling could dramatically
enhance the linear and nonlinear optical properties such as
surface-enhanced Raman scattering (SERS) and two-photon
emission.
Enhanced two-photon emission
Enhanced SERS
A
B
Figure 3. TEM images of (A) isolated Ag NPs and (B)
aggregated NPs (127 pM AgNPs in the presence of 0.5 μM PFP).
(C) UV-vis extinction spectra of 127 pM Ag NPs in the presence
of PFP with concentrations of 0, 0.25, 0. 5, 1.25, 2.5 3.75 and 5.0
μM.
Figure 1. (A) TEM images of some 1-D nanochains formed with
a PFP/Au molar ratio of 9400:1. (B) UV-vis extinction spectra of
Au nanoparticles upon addition of different amounts of PFP with
PFP/Au molar ratios of (a) 0:1, (b) 4700:1, (c) 9400:1, (d) 14
100:1, (e) 18 800:1, and (f) 27 000:1.
Figure 4. (A) One photon excitation fluorescence spectra of PFP
(2.5 μMin repeat units) in the presence of different concentrations
(0, 4.6, 13.9, 32, 67, and 127 pM) of 37 nm Ag NPs (a-f). (B)
Two-photon luminescence of Ag NPs (127 pM) in the presence
of different concentraions of PFP (0, 0.25, 0.5, 1.25, 2.5, 3.75, 5.0
μM from a to g). h is for Ag NP aggregate g but under excitation
of a CW laser.
Figure 2. Raman spectra of (a) 100mMrhodamine 6G (R6G)
adsorbed on quartz, (b) 0.1 μM R6G adsorbed on a gold
nanosphere modified quartz substrate, and (c) 5.0 nM R6G
adsorbed on a gold nanochain modified quartz substrate.
Interactions between the conjugated polymers and metal
nanoparticles can induce the assembly of metal nanoparticles. We
have demonstrated formation of Au nanochains by assembly of
citrate-stabilized Au nanospheres induced by cationic conjugated
polymers (Figure 1).4 A longitudinal Plasmon resonance band
The interactions between the conjugated polymers and metal
nanoparticles can also enhance the two-photon emission of metal
nanoparticles. Metal nanoparticles were usually known to quench
the emission of fluorescent conjugated polymers.5 Ag and Au
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nanoparticles exhibited extraordinary quenching effects on the
fluorescence of cationic poly(fluorinephenylene) (Figure 3). The
quenching efficiency by 37-nm Ag nanoparticles is 9-10 orders of
magnitude more efficient than typical small molecule dyequencher pairs. However, the situation is totally different under
two-photon excitation. The cationic conjugated polymers induce
the aggregate formation and plasmonic coupling of the metal
nanoparticles. The two-photon emissions of Ag and Au
nanoparticles were found to be significantly enhanced upon
addition of conjugated polymers, by a factor of 51-times and 9times compared to the isolated nanoparticles for Ag and Au,
respectively (Figure 4). The origin of the enhancement was
ascribed to the formation of longitudinal plasmon absorption
band in the near-IR region. The longitudinal plasmon band, on
one hand, provides enhanced local electric field; on the other
hand, it provides an intermediate state for the two-photon
absorption processes. Both effects will greatly enhance the twophoton excitation efficiency and result in significantly enhanced
two-photon emission.5
PL/PL0
80
2+
[Hg ]
600
400
0M
200
100
0
A
20
0
0
400
500
600
Wavelength (nm)
700
20
60
2+
600
Cu
400
K+
200
Mg2+
0
500
600
700
Wavelength(nm)
Hg2+ Ni2+ Zn2+ Pb2+ Cu2+ K+ Mg2+ Al3+ Ca2+ Na+
B
C
References
1. Ehrenreich, H.; Philipp, H. R., Optical Propteries of Ag and Cu. Phys.
Rev. 1962, 128 (4), 1622-1629.
2. Noguez, C., Surface plasmons on metal nanoparticles: The influence of
shpae and physical environment. J. Phy. Chem. C 2007, 111 (10), 38063819.
3. Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S.,
Ultrasensitive Chemical Analysis by Raman Spectroscopy. Chemical
Reviews 1999, 99 (10), 2957-2976.
4. Polavarapu, L.; Xu, Q. -H., Water-soluble conjugated polymerinduced self-assembly of gold nanoparticles and its application to SERS.
Langmuir 2008, 24 (19), 10608-10611.
5. Guan, Z. P.; Polavarapu, L.; Xu, Q. -H., Enhanced Two-Photon
Emission in Coupled Metal Nanoparticles Induced by Conjugated
Polymers. Langmuir 2010, 26 (23), 18020-18023.
6. Jiang, C. F.; Guan, Z. P.; Lim, S.Y.R.; Polavarapu, L. ; Xu, Q. -H.,
Two-Photon Ratiometric Sensing of Hg2+ by Using Cysteine
Functionalized
Ag
Nanoparticles.
Nanoscale,
2011,
DOI:10.1039/C1NR10396B.
Y=0.677+2.41X
40
Pb
We have demonstrated that the plasmon coupling could
enhance the SERS signal of nearby molecule by a factor of ~400
times compared to the isolated ones. In addition the plasmon
coupling could also dramatically enhance the two-photon
emission of the metal nanoparticles, which has been utilized to
develop a new platform for two-photon ratiometric sensing of
Hg2+ with high sensitivity and selectivity. 6
0
0
Na+
800
2+
Conclusion
40
20
Zn2+
Figure 7. Two-photon excitation microscopy of cysteine modified Ag
NPs in absence (A) and presence of 0.5 M (B) and 10 M(C) of Hg2+.
60
40
Ca2+
1000
15m
[Hg2+](  M)
0 5 10 15 20 25
60
Al3+
Ni2+
Figure 6. Two-photon emission enhancement factors at 550 nm for cysAg in presence of various metal ions. Inset is the corresponding TPE
spectra. [metal ions] = 68 μM.
B
80
PL/PL 0
0M
8M
12  M
20  M
28  M
48  M
68  M
68  M
PL/PL0
Emission intensity (a.u.)
A
40
Hg2+
1200
400
Aggregation enhanced two-photon emission could be taken
advantaged to develop various two-photo sensors since many
biologically important species could cause the aggregation of
metal nanoparticles. We have demonstrated a practical twophoton ratiometric sensing scheme for detection of mercury ions
in aqueous media with high selectivity and sensitivity by using
cysteine modified silver nanoparticles (Cys-Ag NPs).6 We found
that up to 100-fold enhancement in two photon emission of the
cys-Ag NPs upon addition of Hg2+ (Figure 5). The phenomenon
could be utilized to detect Hg2+ with a detection limit of as low as
65 nM without interference from other environmentally relevant
metal ions. In addition, the sensitivity and sensing range are
tunable by adjusting the concentration of cysteine. Compared to
the conventional colorimetric or extinction spectrum based
methods, this scheme offers improved selectivity (Figure 6),
quantitative detection of Hg2+ with larger dynamic range and
allows detection deep into biological environments such as cells
and tissues where deep penetration is required. The sensitivity
could be further improved by using two-photon microscopy with
additional advantages of 3D detection and mapping. (Figure 7)
800
60
20
Application of enhanced two-photon emission in
biosensing
1000
Emission Intensity
100
80
[Hg2+]( M)
Figure 5. (A) Two-photon emission spectra of cys-Ag NPs in the absence
and presence of Hg2+, [Hg2+]:0, 8, 12, 20, 28, 48, 68 M; (B) Plot of TPE
enhancement versus [Hg2+]. The inset is the linear dependence in the low
Hg2+ concentration regime. [cysteine] = 35 M.
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