Gap-plasmon of Fe3O4@Ag core-shell nanostructures
for highly enhanced fluorescence detection of
Rhodamine B
Yunjia Wang, Xihong Zu*, Guobin Yi*, Hongsheng Luo,
Hailiang Huang
（School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, China）
Abstract: A novel gap-plasmon of Fe3O4@Ag core-shell nanoparticles for surface enhanced
fluorescence detection of Rhodamine B (RB) was developed. Fe3O4@Ag core-shell nanostructures with
Ag shell and Fe3O4 core were synthetized by self-assembled method with the assistance of
3-mercaptopropyl trimethoxy silane (MPTS). To study the RB fluorescence enhanced by gap-plasmon,
the fluorescence properties of RB on different nanogap densities substrates were systematically
investigated, and the results showed that the fluorescence intensity of RB on Fe3O4@Ag core-shell NPs
substrate were much stronger than that on bare glass substrate, and the fluorescence intensity was
further improved by using multilayer Fe3O4@Ag core-shell NPs substrate which had higher nanogap
density. Different from the mechanism that based on the maximum overlap of the surface plasmon
resonance (SPR) band and emission band, the mechanism of the fluorescence enhancement in our work
is based on the localized surface plasmon (LSP) and the gap plasmon near-field coupling with the
Fe3O4@Ag core-shell NPs. Besides, the detection limit obtained was as low as 1×10-7mol/L, and the
Fe3O4@Ag core-shell NPs substrate had high selectivity for RB fluorophores. It demonstrated that the
Fe3O4@Ag core-shell NPs substrate has active, good stability and selectivity for fluorescence detection
of RB. And the detection of RB by the surface plasmon enhanced fluorescence was more convenient and
rapidly than traditional detection methods in previous works.
Key words: Gap-plasmon; Self-assembled; Fe3O4@Ag nanoparticles, Core-shell, Fluorescence
Yunjia Wang(王运佳): Ph D; E-mail: [email protected];
*Corresponding author: Xihong Zu(俎喜红), associate professor, Ph D, e-mail: [email protected]; Guobin Yi(易国斌):
professor, Ph D, e-mail: [email protected]
Founded by the National Natural Science Foundation of China(NSFC) (Nos.:51273048; 51203025)and Natural Science
Foundation of Guangdong Province(No: S2012040007725).
enhancement
1 Introduction
Rhodamine B (RB) was widely used as a colorant in textiles and food stuffs, and was also a
well-known water tracer fluorescent[1]. It was harmful if swallowed by human beings and animals, and
causes irritation to the skin, eyes and respiratory tract[2]. Thus, it was considerated worthwhile to make
systematic efforts to detect the existence of RB from wastewater. However, the traditional method, such
as the chromatographic technique, was not only confined by the concentration of the dye, but also the
result was frequently interfered by other impurities in wastewater, which make the survey report
unreliable. The fluorescence technique, which limit of detection was far lower than chromatographic
technique, was a promising way for the detection of dyes. However, the weak signal due to the relative
lowly concentration had limited the further detection of RB. Therefore, increasing the signals of RB was
significantly critical and necessary.
Owing to the fluorophores coupling with metal nanoparticles, the detection sensitivity can be
significantly improved by using surface plasmon-enhanced fluorescence(SEF), which offered great
potential for a range of application. Recently, various methods[3-6] had been employed to enhanced the
fluorescence probes, such as changing the type or the component of nano antenna, employing core-shell
architectures and modulating the thickness of shell as a spacer, even coating an inorganic oxidation layer
as spacer to investigate the enhancement. Among these methods, surface–enhanced fluorescence
strategy had drawn considerable attention. These mechanism of fluorescence enhancement can be
mainly attributed to either increased excitation rate due to the local field enhancement effect, or
increased the emission rate since the surface plasmon coupling emission, leading to the decreased
lifetime of fluorescent probe.
Fluorescence enhanced by surface-plasmon was a complex process affected by a variety of
parameters, such as characteristics of fluorescence materials and coupling of metal nanoparticles [7-10].
Previous studies were performed by single nanoparticle to enhance fluorescence of RB, which was
mainly concentrated on silver nanoparticle and gold nanoparticle. Suslov A. et al
[11]
employed
monodispersed silver nanoparticles to enhance the fluorescence of RB, achieving 2-3 fold enhancement
higher than polydispersed silver nanoparticles. However, expensive equipment was needed for the
preparation of monodispersed silver nanoparticles, and the preparation process was critical for the
equipment condition. Yan et al
[12]
employed PMMA matrix to control the distance between silver
nanoparticles and RB molecules, obtaining maximum enhancement of 5 fold for RB. However, it was
difficult to modulated the space precisely with high resolution, and the experiment deviation should be
considered. Besides, other references have reported the fluorescence intensity of RB were enhanced by
gold nanoparticles, which mechanism was based on the maximum overlap of SPR band and the
emission band [13-14]. To the best of our knowledge, the emission band of RB is so narrow that the shape
and size of gold nanopartilce must be strictly controlled for the best enhancement.
Recently studies have shown that metal nanoparticles with a certain interdistance, for example,
10-20 nm, displayed excellent plasmonic absorption in the visible range, especially in the near-fared
[15-17]
. Such nanostructures can be considerated as gap plasmon resonators, where the electromagnetic
field was highly concentrated in the nanogaps due to constructively interfering gap plasmons which
were efficiently reflected at the resonator terminations
[18-20]
. The gap plasmon resonance wavelength
was therefore easily tuned by modifying the interdistance of the nanogaps, and the intensity of
resonance wavelength can be enhanced by increased the numbers of gaps. The field enhancement
provided by such gap plasmon can significantly exceed that of individual nanosphere resonators. Zhang
[21]
and his colleagues have used silver bialayer nanoparticile films to study the gap-plasmon effect on
fluorescence enhancement. Their results reflected the gap-plasmon was originated from different
dielectric layers, and fluorescence intensity of immunoassays achieved maximum enhancement of 15.4
fold. Lumdee[22]and his assistants have used gold nanoparticles coated on Al2O3 film to invesitigate the
gap-plasmon enhanced photoluminescence, and an inspired peak photoluminescence factor of 28000
was observed. Such results revealed an unusually strong photoluminescence signals, which unlike that
observed in the bulk metal material showed emssion peaks that correlate with plasmon-related peaks in
the extinction spectra, suggesting that surface plasmons were involved in the emssion enhancement.
In this paper, a novel method for fabricating Fe3O4@Ag core-shell NPs on the glass substrate was
developed to enhance the fluorescence intensity of RB by gap-plasmon effect, and systematically
investigate the fluorescence properties of RB on different substrates.
2 Experimental
2.1 Preparation of AgNPs
Silver NPs were synthesized by the following procedure. 0.19 mmol AgNO3 was dissolved in 50 mL
deionized water, and the mixture was heated to 90℃. After that, 0.05g sodium citrate was added to the
mixture as a reducer and stabilizer to react for 17 min under pure nitrogen, and then cooled to room
temperature. The solution was centrifuged and washed twice with ethanol, and silver nanoparticles
(AgNPs) were obtained.
2.2 Preparation of Fe3O4 NPs
Ferric chloride and Irion dichloride crystalline hydrate (mass ratio 2:1) were dissolved in deionized
water. 6 mL ammonia (28 wt%) was quickly added to the mixture solution when it was heated to 80℃.
Reaction was conducted at nitrogen atmosphere and kept for 20 minutes. After that, the mixture was
cooled down to room temperature, and Fe3O4 NPs was obtained by purifying with ethanol for twice and
centrifuging at 8000 r/min to remove the residuals. Finally, Fe3O4 NPs were dispersed in ethanol,
volumned adjust to 250mL before used.
2.3 Preparation of Fe3O4@Ag core-shell nanostructure
Firstly, Fe3O4 NPs were modified by 3-mercaptopropyl trimethoxy silane (MPTS) in a mixture
containing 30 mL isopropanol, 20 ml deionized water and 0.5 mL MPTS. The mixture was refluxed for
80 minutes to make Fe3O4 NPs coated by MPTS. After that, the Fe3O4-MPTS NPs solution was obtained
by centrifuging the mixture and dispersing in ethanol. Secondly, Fe3O4-MPTS NPs was mixed with Ag
NPs in a beaker to react for 5 h at 40℃ under stirring. After the process, Fe3O4@Ag core-shell
nanoparticles were obtained from the mixture by centrifuging at 10000 r/min.
2.4
Preparation of Fe3O4@Ag core-shell NPs on substrate
The glass substrate coated with MTPS was immersed into the solution of Fe3O4@Ag core-shell NPs
for 3 h or 5 h to adsorb the Fe3O4@Ag core-shell NPs on the substrate uniformly. Then, the sample
was taken out and dried in room temperature. The multilayered NPs were obtained by the spin-coating
method on the first layer. RB was employed as the fluorophores and spin-coated on the substrates with
different density of the Fe3O4@Ag core-shell NPs. Concentration of the RB solution was 4×
10-6mol/L, and the spin speed was 2000 rpm.
2.5 Measurement of samples
Transition electron microscope (TEM, 2200FS) was employed to characterize the structure of
Fe3O4@Ag core-shell nanostructures. Scanning electron microscope (SEM, Hitachi S-4800, 150kV)
was applied to characterize the morphology of different NPs. X-ray diffraction (XRD, Ultima-7000)
was employed to analyze the composition. Both steady state and time-resolved fluorescence
measurement were taken on a fluorescence spectroscopy (Horiba Jobin Yvon, Ltd.) to analyze the
fluorescence enhancement. For steady state measurement, the samples were excited by Xe lamp at 400
nm. Time-resolve measurement was carried out by using a pulse laser with 370 nm excitation
wavelength. UV-vis absorption spectra (UV-2450, Japan, wavelength ranged from 300 to 800nm) were
measured to demonstrate the surface plasmon resonance (SPR) which affect the fluorescence signals.
3 Results and discussion
The Fe3O4 NPs were prepared by the precipitation method which used ammonia as precipitant to
react with the mixture of FeCl3 and FeCl2 (mass ratio 2:1) at 90℃. And the Ag NPs were prepared by
the oxidation–reduction reaction of AgNO3 solution with sodium citrate. Fig.1 shows the morphologies
and size distributions of the prepared Fe3O4 and Ag NPs. It can be seen that the morphologies of the
Fe3O4 and Ag nanoparticles were both almost spherical, and dispersed well in solution. The average
diameter of Fe3O4 NPs was about 52 nm, and average diameter of Ag NPs were about 33 nm.
Fig.1. (a) SEM image and (b) size distribution histograms of Fe3O4 nanoparticles;
(c) SEM image and (d) size distribution histograms of Ag NPs.
In order to prepare Fe3O4@Ag core-shell NPs, the Fe3O4 NPs were modified by MPTS which had
mercaptopropyl groups to bond Ag NPs. Fig.2a and 2b show the high-resolution TEM micrograph of
the Fe3O4@Ag NPs. It can be seen clearly that the Fe3O4@Ag NPs were core-shell structures with
Fe3O4 core and Ag shell. The XRD pattern shown in Fig.2c clearly revealed the presence of both Fe3O4
peaks and Ag peaks corresponding to (220), (311) and (440) crystal planes of Fe3O4, and (111), (200),
(220) and (311) crystal planes of Ag. Fig.2d shows the EDX spectrum of the Fe3O4@Ag core-shell
NPs. Iron, oxygen and silver elements were observed clearly. It demonstrated that Fe3O4@Ag
core-shell NPs were prepared successfully.
Fig.2. (a) and (b) TEM images of the Fe3O4@Ag core-shell nanoparticle structures; (c)
XRD and (d) EDX spectra of the Fe3O4@Ag core-shell nanoparticle structures.
To study the RB fluorescence enhanced by gap-plasmon, Fe3O4@Ag core-shell NPs were adsorbed on glass
substrate with MPTS by immersing the substrate in Fe3O4@Ag NPs solution for different times. The glass substrate
modified by MPTS had mercaptopropyl groups to bond the Fe3O4@Ag core-shell NPs to form an uniform layer. Fig.
3a shows the SEM images by immersing the substrate in Fe3O4@Ag NPs solution for 3 h. Fe3O4@Ag NPs hadn’t been
adsorbed in partial area. When immersing for 5 h, an monolayer Fe3O4@Ag core-shell NPs formed (Fig.3b).
Furthermore, in order to obtain more nanogaps, multilayer Fe3O4@Ag core-shell NPs on glass substrate were prepared
by spin-coating the Fe3O4@Ag NPs solution on the monolayer Fe3O4@Ag core-shell NPs as shown in Fig. 3c. Some
nanoparticles can be observed to be located on top of the other nanoparticles, and the amount of nanogaps increased
greatly. Fig. 3d-f show the interdistance distribution histograms of Fe3O4@Ag core-shell NPs according to the SEM
images of Fig.3a-c, which were calculated by Image. J. software. The results indicated that the interdistances of
Fe3O4@Ag core-shell NPs were mostly ranged from 19 nm to 31 nm according to Fig. 3a&d, and 18 nm to 22 nm for
monolayer Fe3O4@Ag core-shell NPs according to Fig. 3b&e. As for the multilayer structure, the interdistances of
Fe3O4@Ag core-shell NPs were mostly ranged from 1 nm to 2 nm (Fig. 3c&f).
Fig.3 SEM images of the monolayer Fe3O4@Ag core-shell NPs on glass substrate with MPTS prepared
by immersing in the Fe3O4@Ag NPs solution for (a) 3 h and (b) 5 h, respectively; (c) multilayer Fe3O4@Ag
core-shell NPs on glass substrate with MPTS; (d), (e) and (f) are the interdistance distribution histograms of
Fe3O4@Ag core-shell NPs according to the SEM images of (a), (b) and (c).
To investigate the gap-plasmon effect on surface-enhanced fluorescence, RB molecules were
spin-coated on the Fe3O4@Ag core-shell NPs and on glass substrate. And the absorption spectra and
fluorescence emission spectra of RB on different substrate were both measured as shown in Fig.4. It can
be seen that the absorption of RB in UV-vis region was enhanced greatly by Fe3O4@Ag NPs substrate,
compared with that on bare glass substrate. And the absorption intensity further increased with the
nanogap density of the Fe3O4@Ag NPs substrate increasing (Fig.4a). The simular results were also
confirmed in fluorescence spectra of RB. Fig. 4b shows the steady-state fluorescence spectra of the RB
on bare glass substrate and different Fe3O4@Ag NPs substrates which were prepared by spin-coating the
RB solution on the bare glass substrate and the Fe3O4@Ag NPs on glass substrates. The emssion spectra
of RB was collected under the excitation of 400 nm laser light. The fluorescence intensity of the RB on
bare glass substrate was very weak. Compared to the bare glass substrate, the fluorescence intensity of
the RB on Fe3O4@Ag NPs substrate were improved significantly. The maximal enhancement factor was
about 6 on multilayer Fe3O4@Ag NPs substrate given by ( I S  I B ) / ( IG  I B ) , where IS and IG were the
maximal fluorescence intensity of emission spots on Fe3O4@Ag NPs and on bare glass substrate,
respectively, and IB was the background intensity taken from the dark area that could not be excited. The
results indicates that the prepared Fe3O4@Ag NPs substrates have significant fluorescence enhancement
effect for RB.
Fig.4. Surface enhanced fluorescence by gap-plasmon. (a)Absorption spectra of RB on monolayer
structure of Fe3O4@Ag NPs and multilayer structure of Fe3O4@Ag NPs compared with that on bare glass; (b)
Fluorescence emission intensity of RB on different substrates.
The emission intensity of the fluorophore on the noble metal nanostructures were affected by two
factors. On the one hand, an increase in intrinsic decay rate of fluorophore has influence on the quantum
yield and lifetime of the fluorophore. On the other hand, an enhanced local field provides stronger
excitation rate without changing the fluorescence lifetime of the molecules[23]. Fig. 5a shows the
fluorescence enhancement mechanism of RB on Fe3O4@Ag NPs substrates. When the RB molecules on
Fe3O4@Ag NPs substrates were excited by Xe lamp at 400 nm, the local electromagnetic field of each
Fe3O4@Ag core-shell NPs was enhanced based on the localized surface plasmon resonances around
each Fe3O4@Ag core-shell NPs, but the intensity of the local electromagnetic field decayed rapidly with
the distance increasing. Thus, the fluorescence properties of RB molecules near Fe3O4@Ag core-shell
NPs (molecules with green color) were enhanced greatly, but the fluorescence properties of RB
molecules far from Fe3O4@Ag core-shell NPs (molecules with red color) were almost not enhanced.
When the Fe3O4@Ag core-shell NPs were close enough, the induced electromagnetic fields of single
Fe3O4@Ag core-shell NPs coupled with each other and gave rise to the total field, resulting in great
fluorescence enhancement. Fig. 5b shows the local electromagnetic field around two near Fe3O4@Ag
NPs which was simulated by FDTD software. It indicates that the local electromagnetic field of the two
Fe3O4@Ag NPs will fold and couple with each other due to the gap plasmon near-field coupling,
resulting in the enhancement of the total electromagnetic field which will generate great fluorescence
enhancement.
To further understand the relation between local field and interdistance of nanodimers, formular(1)
was applied[24]:
E  e r / r
From formular (1), intensity of local field was enhanced in square when the interdistance was
decreased, which resulted in more free electrons oscillated and higher resonance absortance. In this case,
more energy was collected to improve the excitation rate, resulting in fluorescence enhanced greatly.
From other aspect, Fe3O4@Ag NPs can be regarded as collective nanoshell dimers in multilayer
structures, which concentrated incident electromagnetic field strongly in the intra gap region and also
around themselves result in enhanced electromagnetic field in their near field region. Dimer plasmon
oscillations can be well described by plasmon hybridization model[25], which explained dimer plasmons
as formed from individual, increasing monomer plasmons of same angular momentum. These
interaction was weaker for larger particle separation and as distance between interacting entries
decreased, plasmon oscillations generated were a combination of all angular momenta[26]. In general,
plasmonic coupling between the two nearby nanoshells was sensitive to the polarization of the incident
plane wave. This was because that plasmonic coupling of the dimer can be induced only by electric
component of the long-axis polarization of incident plane wave. The enhancement factor of multilayer
structure on overall fluorescence of a singular molecule was defined as the product of above two
parameters[27], which was shown in formular (2):
  , d ; 
e x , e m
 , d;  ex  ;d e m
(2)
where  , d was the incident angle of plane wave and the orientation angle of molecular dipole
moment, respectively; ex , em represented the excitation wavelength and emission wavelength. These
results sugguest that higher enhancement of fluorescence can be achieved by gap-plasmon with certain
incident.
Fig.5 a) Illustration of the fluorescence enhancement mechanism for RB on Fe3O4@Ag NPs substrates; b)
Simulated electromagnetic field enhancement for two near NPs due to the gap plasmon near-field coupling;
c) UV-vis absorption spectra of different NPs; d) Fluorescence spectra of RB on different substrates.
To demonstrate the influence of gap-plasmon and coupling between Ag and Fe3O4 on
surface-enhanced fluorescence, the properties of UV-vis absorption and fluorescence enhancement of
different NPs were investigated. Fig.5c shows the UV-vis absorption spectra of Ag NPs, Fe3O4 NPs and
Fe3O4@Ag NPs. It can be seen that the peak intensity of Fe3O4@Ag core-shell NPs was much stronger
than that of Fe3O4 NPs and AgNPs, and the peak position of the Fe3O4@Ag core-shell NPs was shifted
compared with Fe3O4 NPs and AgNPs, indicating more efficient energy were captured due to the
gap-plasmon coupling. In order to further demonstrate the gap-plasmon effect, the fluorescence
emission spectra of RB on different substrates were shown in Fig.5d. The fluorescence intensities of RB
on AgNPs and Fe3O4 NPs substrates were both improved greatly, compared with that on bare glass
substrate. Interestingly, the fluorescence intensity of RB on Fe3O4@Ag core-shell NPs substrate were
much stronger than that on AgNPs and Fe3O4 NPs substrates. The results indicated that the fluorescence
enhancement of RB on Fe3O4@Ag core-shell NPs substrate was not only due to the coupling of Ag and
Fe3O4, but also due to the gap-plasmon effect of NPs.
To further demonstrate the gap-plasmon effect on surface enhanced fluorescence, fluorescence
lifetime curves of RB on different substrates were recorded as shown in Fig.6. It can be seen that the
fluorescence lifetime had no observably decay compared with that on bare glass substrate. The result
sugguested that the fluorescence emission enhancement of RB on Fe3O4@Ag core-shell NPs substrate
was only due to the enhancement of the local electromagnetic field, which provided stronger excitation
rate without changing the fluorescence lifetime of the molecules.
Fig.6. Fluorescence lifetime curves of RB on different substrates.
Besides, detection limit was an important factor for a detection process. To evaluate the detection
limit of the proposed plasmon substrate system, fluorescence changes were detected with the
concentration of RB increasing. As shown in Fig.7, the fluorescence intensity was plotted as a function
of RB concentration. An obvious increase in the emission intensity was observed when the
concentration was ranged from 1×10-8mol/L to 2.5 ×10-6mol/L. An approximately linear relationship
between the fluorescence intensity and RB concentration over the range of 1×10-7mol/L to 1×
10-6mol/L was obtained, indicating that the detection limit was 1×10-7mol/L. Therefore, substantial
sensitivity for RB detection could be obtained with this convenient proposed plasmon substrate system.
Fig.7 (a) Plots of the fluorescence intensity of the Fe3O4@Ag NPs substrate as a function of the RB
concentration(1.0×10-8mol/L-2.5×10-6mol/L); (b)The linear fit plot of RB concentration was ranged from 1.0×
10-7mol/L to 1.0×10-6mol/L.
The stability was another important factor for detection process. For the well-established plasmon
substrate for RB determination, the interference of acid and alkali always complicated the detection
process due to the effect of PH on the fluorescence resonance energy transfer (FRET) system[28]. To
investigate whether the PH value had influence on the proposed substrate, the influence of the possible
protonic acid and alkali, such as H2SO4 and NaOH, were tested as shown in Table.1. When the PH value
was lower than 6.0, fluorophore was in acidic environment which could destroy the surface structure of
RB, resulting in the fluorescence intensity decreasing. When the PH value was too high, the
fluorescence intensity also decreased. Therefore, the preferable PH value is about 9 to 10 for the RB
detection.
Table.1 Effect of PH value on RB detection
Sample
PH
RB
4
6
9
10
12
QSD(%)
-8.8
-4.6
0.9
1.2
0.2
The selectivity of plasmon substrate system for detection of RB was further evaluated with the
addition of other interferential substances, including some common metal irons and possible foreign
substances, such as ethanol, glucose, and BSA. The tolerable concentration ratio, defined as the
concentration ratios of foreign species to RB causing less than±5% relative error, was examined. It can
be seen from the table.2 that the tolerable concentration ratios were about 300 times of the RB
concentration levels. The tolerable concentration ratios of BSA was lower than others because it can
combine to the plasmon substrate with the –SH bond and decrease the efficiency of FRET system.
Therefore, the plasmon substrate system was of high selectivity for RB fluorophores, except the
presence of BSA which had a slight effect.
Table.2 The interference of coexsiting substance on the determination of RB (10-6 mol/L)
Coexisting substance
Tolerable concentration ratios
+
Na
K+
SO42ClFe2+
ethanol
glucose
BSA
4
10
104
6×103
6×103
103
300
300
40
QSD(%)
2.1
1.2
1.8
1.5
-1.7
4.1
-2.2
-3.8
4 Conclusions
In summary, a novel approach for fabricating the active Fe3O4@Ag core-shell NPs on glass
substrates has been developed for the facile, sensitive and selective fluorescence detection of RB based
on the localized surface plasmon resonances and the gap plasmon near-field coupling effect. Fe3O4@Ag
core-shell NPs with Ag shell and Fe3O4 core were synthetized by bonding Ag NPs on Fe3O4 NPs with
MPTS. Furthermore, Fe3O4@Ag core-shell NPs substrates with different nanogap densities were
fabricated by using the glass modified by MPTS as substrates. The fluorescence intensities of RB on
Fe3O4@Ag core-shell NPs substrates were enhanced greatly compared with that on bare glass substrate,
and the maximal enhancement factor was about 6 on multilayer Fe3O4@Ag NPs substrate compared
with that on bare glass substrate. When the prepared Fe3O4@Ag NPs substrate was employed to detect
RB in waste water, the detection limit obtained was as low as 1×10-7mol/L. It was convenient and
rapidly than traditional detection method, and it showed better stability and selectivity for RB detection.
This work provides a promising potential application in many biological fields, such as biological
diagnose, analysis, and intracellular tracking.
Aknowlegements
This work was supported by the National Natural Science Foundation of China (Grant No.
51273048 and 51203025), the Natural Science Foundation of Guangdong province (Grant No.
S2012040007725).
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