Optical and structural properties of poly(vinyl alcohol) films embedded with citrate-stabilized

gold nanoparticles

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 44 (2011) 205105 (8pp) doi:10.1088/0022-3727/44/20/205105

Optical and structural properties ofpoly(vinyl alcohol) films embedded withcitrate-stabilized gold nanoparticlesSuman Mahendia1, A K Tomar1,2, Rishi Pal Chahal1, Parveen Goyal1 andShyam Kumar1

1 Department of Physics, Kurukshetra University, Kurukshetra - 136 119, India2 Department of Physics, Kurukshetra Institute of Technology and Management, Kurukshetra - 136 119,India

E-mail: mahendia@gmail.com

Received 28 January 2011, in final form 5 March 2011Published 28 April 2011Online at stacks.iop.org/JPhysD/44/205105

AbstractHydrosol of Au nanoparticles was prepared by citrate reduction of chloroauric acid. Thesynthesized nanoparticles were characterized through transmission electron microscopy (TEM)and UV–Visible spectroscopy. The prepared nanoparticles were almost spherical in shape withtheir mean diameter ∼6 nm and possessed face-centred-cubic (fcc) structure. The absorptionspectrum of the as-prepared nanoparticles shows the SPR peak at 530 nm in agreement withthat predicted from calculations based on Mie theory. These nanoparticles were dispersed inpoly(vinyl alcohol) (PVA) using the sol–gel method to prepare PVA–Au nanocomposite filmswith different concentrations of Au. Optical and structural properties of these nanocompositeswere studied using UV–Visible spectroscopy, x-ray diffraction (XRD) and FTIR spectroscopy.The value of optical band gap deduced from the UV–Visible absorption spectroscopy is foundto be reduced from 4.98 eV (for pure PVA) to 3.85 eV after embedding 0.074 wt% of Aunanoparticles. Further, the refractive index behaviour for pure PVA and PVA–Aunanocomposite films was studied through transmission and reflection behaviour. The inducedstructural changes, revealed through XRD and FTIR spectroscopy, are responsible for theobserved changes in optical behaviour of PVA after embedding Au nanoparticles in it.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

In recent days, polymers embedded with nanoparticlesof inorganic nature have high potential in a variety oftechnological applications [1–4] such as optical devices,biosensors, biomedical science, energy storage and coatingmaterials etc. This is because of the fact that incorporation ofinorganic nanoparticles within a polymer matrix significantlymodifies the properties of host matrix [5, 6]. The preparednanocomposites exhibit the improved optical, thermal,electrical, mechanical properties, etc [7, 8]. However, theseproperties are controlled by the size, concentration anddistribution of embedded nanoparticles in the host matrix[9, 10]. Further, for application in the desired technologicalsector, the choice of proper inorganic nanoparticles is also

very crucial. For example, noble metal nanoparticles suchas silver and gold are of great significance in the fieldof optics and opto-electronics due to their inherent size-dependent optical and electrical properties [11–14]. In theliterature, different procedures have been reported for thesynthesis of polymer–metal nanocomposites such as electro-spinning, ion-implantation and suspension polymerization[15–17]. However, the embedding of nanoparticles in thepolymer matrix via sol–gel method is generally adoptedbecause this method is quite simple and of great interest inapplications where large area coatings are required [18].

In this paper, we have reported the ex situ synthesisof poly(vinyl alcohol)–gold (PVA–Au) nanocomposites. Wehave chosen PVA as base matrix for a couple of reasons. At aglance, it is water soluble, easily processable, having good film

0022-3727/11/205105+08$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Table 1. Optical band gap values and various optical dispersion constants for pure PVA and different PVA–Au nanocomposite films.

Optical (e2/πc2)

band gap Ed E0 λ0 S0 × 1013 (N/m∗)Sample Eg (eV) (eV) (eV) n∞ (nm) (m−2) ε1 ×10−8 (nm−1)2

Pure PVA 4.98 ± 0.01 10.32 ± 0.02 8.45 ± 0.02 1.50 ± 0.01 138 ± 0.6 7.29 ± 0.05 2.34 ± 0.01 3.06 ± 0.01PVA + 0.024 wt% Au 4.69 ± 0.03 11.67 ± 0.01 8.97 ± 0.01 1.53 ± 0.01 140 ± 0.4 6.69 ± 0.02 2.51 ± 0.03 2.22 ± 0.03PVA + 0.035 wt% Au 4.53 ± 0.01 11.82 ± 0.04 9.53 ± 0.04 1.55 ± 0.01 142 ± 0.2 6.92 ± 0.01 2.56 ± 0.02 1.06 ± 0.02PVA + 0.074 wt% Au 3.85 ± 0.02 11.13 ± 0.01 8.58 ± 0.01 1.48 ± 0.03 144 ± 0.5 5.76 ± 0.03 2.38 ± 0.04 2.27 ± 0.04

forming and adhesive nature for applications in optical coatingand opto-electronic devices. Further, it is also considered as agood host matrix for metal nanoparticles [19–21]. Regardingthe PVA/gold nanocomposites, reports are available in theliterature related to their synthesis and characterization [22–26]but, to the best of our knowledge, systematic studies relatedto the effect of embedding gold nanoparticles on the optical,electrical, mechanical behaviour of the PVA matrix are scarce.In this endeavour, we have carried out a systematic studyon the effect of embedding of different concentrations of Aunanoparticles on the optical properties of PVA, to explore itsapplications in optical coating, graded index waveguides, etc.

2. Experimental details

For the preparation of Au nanoparticles, we have followedthe procedure laid down by Xie et al [27]. Accordingly,1.8mM aqueous solution of hydrochloroauric acid (HAuCl4)was prepared as the stock solution. The prepared solution(250 ml) was then kept for heating at 70 ◦C under continuousstirring. As soon as the solution started boiling, 10 ml of(1.8 wt%) trisodium citrate solution (aq.), which acts as bothreducing and stabilizing agent, was added quickly. The colourof the solution, now, gradually started changing from lightyellow to faint purple and then to dark purple. After 20 minof addition of citrate solution, the heating was stopped and thesolution was left for cooling at room temperature. Finally, thecolour of the solution was changed to red-wine indicating theformation of gold nanoparticles.

PVA–Au nanocomposite films with varying concentrationof Au nanoparticles were prepared by mixing different amountsof as-prepared gold hydrosol to PVA solution. For this purpose,2 g of PVA was dissolved in de-ionized water and then differentvolumes of the gold hydrosol were added under continuousstirring followed by ultrasonication. In order to quantify theconcentration of Au nanoparticles, these nanocomposites weresubjected to atomic absorption spectroscopy (AAS) using aPerkinElmer Atomic Absorption Spectrometer-Analyst 800.From this technique, the concentration of an analyte in asample can be determined. The technique requires standardswith known concentration of analyte. In our study, wehave used 1000 ppm gold solution as standard. Taking1 ml of prepared nanocomposite solution and equal volumeof standard gold solution as reference, the concentrationsof gold in the prepared nanocomposite solutions have beendetermined. The measured concentrations of Au (by wt%) inthese nanocomposite solutions have been tabulated in table 1.

In order to convert the PVA and its nanocompositesolutions with different concentrations of Au (wt%) into films,

the respective solution was casted to plastic petri-dish. Afterevaporation of solvent at ambient temperature; film was peeledoff and rinsed in benzene to remove any volatile material. Thethickness of these films was found to be ∼70 µm as measuredusing the digital micrometre with a least count of 1 µm.

In order to ensure the formation of Au nanoparticles withinthe hydrosol and for optical characterization of nanocompositefilms, Shimandzu Double Beam Double monochromatorUV–Visible Spectrophotometer (UV–Visible 2550), operatedin the wavelength range 190–900 nm with the resolution of0.5 nm, equipped with integrating sphere assembly ISR-240A,was used. In order to record the transmission (T ) spectra andreflection (R) spectra, the sample was placed at the requisitepositions in the integrated sphere assembly. The recorded T

and R spectra take into account the reflection from interfacebetween air and sample at the top and bottom surfaces. Fromthe measured T and R values directly from the instrument, wededuced transmission coefficient (t) for light during single passthrough the sample and reflection coefficient (r) at sheet–airinterface, through the following expressions using the iterativeprocess [28]:

T = (1 − r)2t

1 − r2t2, (1)

R = r +r(1 − r2)t2

1 − r2t2. (2)

The absorption coefficient ‘α’ was determined from ‘t’ usingthe expression

α = 1

dln

(1

t

), (3)

where d is the sample thickness.Transmission electron microscope (Hitachi ‘H-7500’)

operated at 60 kV was used to record the selective area electrondiffraction (SAED) image and for size distribution of as-prepared Au nanoparticles. To record the TEM images, a fewdrops of hydrosol were taken on the carbon coated copper gridwhile for nanocomposite samples, a small part of the respectivefilms were cut and re-dissolved in double distilled water andthen, dropped on to the grid. For structural characterizationof nanocomposite films, x-ray diffraction (XRD) and FTIRstudies were carried out employing an X’Pert ProAnalyticalX-ray diffractometer with a filtered 0.154 nm Cu Kα radiationsand PerkinElmer ABB FTIR spectrophotometer, respectively.

3. Results and discussion

3.1. Au hydrosol

Figure 1 presents the recorded absorption spectrum of theprepared Au hydrosol. It is evident from this figure that

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J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Figure 1. UV–Visible absorption spectra of Au nanofluid: (a)experimentally recorded and (b) theoretically predicted using Mietheory.

there is a strong absorption band peaking at around 530 nm,which corresponds to the surface plasmon resonance (SPR)absorption band of Au nanoparticles [29]. In addition, thereexists strong absorption at wavelengths below 300 nm whichmay be attributed to the presence of surfactant, i.e. CTABwhich was used as a reducing agent for the synthesis of Aunanoparticles. Reproduction of the peak position of the SPRband of as-prepared Au hydrosol (curve ‘a’) was tried onthe basis of Mie theory [30]. According to Mie theory, forspherical metal particles having r � λ where r is the radius ofthe particle and λ is the wavelength of the incoming light),the extinction coefficient Cext approximates to absorptioncoefficient Cabs and is given by the expression [30]

Cabs = 24π2r3ε3/2m

λ

ε2

(ε1 + 2εm)2 + ε2, (4)

where ε1 and ε2 are, respectively, real and imaginary part of thedielectric function of the metal particle and εm is the dielectricconstant of the surrounding medium. At the light frequencyωsp where the condition ε1 = −2εm is satisfied, the metalnanoparticle interacts very strongly with the light, resulting inthe collective coherent oscillations of the conduction electrons(with respect to the positive metallic lattice) in resonancewith the electromagnetic field of the light. These oscillationsbring the resonance known as the SPR in cross-section givenby equation (4) at ωsp and this frequency is known as SPRfrequency. The SPR occurs in the visible frequency regionfor the noble metal nanoparticles, making them opticallyinteresting metals [30]. It has been noticed from figure 1 thatthe predicted peak position (curve ‘b’) from Mie theory forAu nanoparticles with a diameter of 6.7 nm and εm = 1.3(dielectric constant of the surrounding medium) correspondsto the observed SPR peak position (curve ‘a’). The broadenednature of the SPR peak observed experimentally (curve ‘a’) incomparison with that predicted by Mie theory (curve ‘b’), maybe due to the size distribution of as-prepared Au nanoparticleswhile the prediction of the Mie theory is based on the fixedvalue of the particle diameter.

Figure 2. (a) TEM photograph of as-prepared Au nanoparticles(inset shows the SAED pattern) and (b) particle size distribution.

Figures 2(a) and (b) present the TEM image of as-preparedAu nanoparticles and their size distribution, respectively,which indicates the almost spherical shape of these particleswith their average diameter of 6 ± 2 nm in agreement withthat predicted by Mie theory. The analysis of the SAEDpattern, shown as the inset in figure 2(a), indicates thed-spacing = 2.48, 2.14,1.67 Å, which on comparison withthat of standard data available in Joint Committee PowderDiffraction Standards (JCPDS), File No 4-0783 indicates thecorresponding hkl parameters as [1 1 1], [2 0 0], [2 2 0] andupshot the face-centred-cubic (fcc) structure of prepared Aunanoparticles.

3.2. PVA–Au nanocomposite films

3.2.1. Structural characterization. Figure 3 presents theXRD patterns for pure PVA and PVA–Au nanocompositefilms with varying concentrations of Au nanoparticles. Thediffraction pattern of pure PVA (curve ‘a’) indicates adiffraction band around 2θ = 19.4◦, depicting its semi-crystalline nature. This may be due to the presence of strongintra-molecular hydrogen bonding in individual monomerunit of PVA (as shown below) and inter-molecular hydrogenbonding between its different monomer units [4].

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J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Figure 3. XRD patterns for pure PVA and different PVA–Aunanocomposite films.

Monomer unit of PVA

Further, it is evident from this figure that the relativeintensity of this peak decreases after the embedding of Aunanoparticles in PVA matrix and two new peaks [25] at2θ = 38.4◦ and 44.6◦ start emerging at 0.035 wt% of Auwith significant increased intensities as a result of increasingconcentration of embedded Au nanoparticles to 0.074 wt%.The appearance of new peaks with increasing intensity justifiesthe existence of Au nanoparticles in the crystalline phase withthe fcc crystal structure [25] corresponding to hkl parameters[1 1 1] and [2 0 0], respectively. This features out the increasein the crystalline phase of the PVA matrix after embedding ofAu nanoparticles.

Figure 4(A) presents the FTIR spectra of pure PVAand PVA–Au nanocomposite films in the wavenumber range600–3600 cm−1. In order to differentiate different peaksclearly, this spectrum has been presented in two parts, fromthe wavenumber range 600–1800 cm−1 in figure 4(B) and2800–3600 cm−1 in figure 4(C). The peaks around 614 cm−1

and 846 cm−1 (curve ‘a’) correspond to out-of-plane vibrationsof O–H and C–H groups of PVA, respectively [24, 26]. Ithas been observed that the intensity of peak at 846 cm−1

decreases with an increase in concentration of embedded Aunanoparticles (curves ‘b’ to ‘d’). The existence of broadband in the region 1000–1150 cm−1 in pure PVA (curve ‘a’)is attributed to the stretching vibrations of C–O and C–O–Cgroups. On dispersing citrate stabilized-Au nanoparticles, thisband gets split into sharp peaks with rise in their intensitieson increasing concentration of embedded Au nanoparticles.This shows the interaction of the C–O bond of ester groupof trisodium citrate with molecules of PVA. The peak at1250 cm−1, which is absent in pure PVA (curve ‘a’) startsemerging with an increase in its intensity as the concentration

of Au nanoparticles increases, this is also due to the C–Obond of ester group of trisodium citrate. The splitting ofbands in the region 1270–1480 cm−1 after embedding ofAu nanoparticles (curves ‘c’–‘d’) may imply the couplingof O–H in-plane vibrations and C–H wagging vibrationsof PVA with embedded citrate stabilized-Au nanoparticles[24, 26]. The new peak in the region 1700–1750 cm−1 (curves‘c’–‘d’) contributes towards the stretching vibration of carbonbackbone c—

c —c==o: carboxylate group of citrate component[22]. Further, the broad band in the region 3000–3600 cm−1

(curve ‘a’) corresponds to the stretching vibration of the O–Hgroup of PVA. The decreasing intensity of this band andsplitting into peaks (curves ‘b’–‘d’) shows the interactionbetween O–H groups of citrate with O–H of PVA, afterincreasing concentration of embedded citrate stabilized Aunanoparticles. Thus, it can be inferred that the embeddedcitrate stabilized-Au nanoparticles interact with the H and Oatoms of the PVA backbone chains.

3.2.2. Optical characterization. Figure 5(A) presents theoptical absorption spectra obtained through expression (3) forpure PVA and its composites at varying concentrations of Aunanoparticles. For pure PVA (curve ‘a’), there is a smallabsorption peak at 276 nm, which may be attributed to the n-π*transitions of the C=O group of PVA [31] while it remainstransparent in the complete visible region. For PVA–Aunanocomposite films (curves ‘b’–‘d’), in addition to this smallpeak, another absorption band having peak position at 537 nmstarts emerging with its intensity increasing continuously withan increase in the content of Au nanoparticles embeddedin PVA. This band corresponds to the SPR absorption ofembedded gold nanoparticles. The red shift in the SPR band ofthe prepared Au nanoparticles (figure 1) from 530 to 537 nm,after their embedding in PVA (figure 5(A), curves ‘b’–‘d’),is due to an increase in dielectric constant of the surroundingmedium [29].

In order to study the effect of embedding of Aunanoparticles on the optical transition behaviour of PVA,optical band gap for pure PVA and its nanocomposite filmsat different concentrations of Au nanoparticles has beendetermined using Tauc’s relation [32]. For this purpose, thevalues of (αhν)1/2, extracted from the UV–Visible absorptionspectra (where α is the absorption coefficient correspondingto the fundamental absorption edge) have been plotted asa function of hν (photon energy). These plots have beenpresented in figure 5(B). The intercepts of the fitted straightlines in these plots on the hν axis correspond to the optical bandgap values (table 1). For pure PVA, the value of optical bandgap which has been found to be 4.98 eV decreases to 3.85 eVafter embedding of 0.074 wt% of gold nanoparticles. Such adecrease in the value of optical band gap may be explained onthe basis of the fact that the incorporation of a small amount ofgold nanoparticles forms the bonding with the basic functionalgroups of PVA (as revealed through the FTIR analysis), whichprovoke the formation of charge transport complexes (CTCs)as trap levels between HOMO and LUMO bands in the hostlattice. These CTCs may enhance the lower energy transitionsand lead to the observed change in optical band gap values [33].

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J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Figure 4. FTIR spectra for pure PVA and different PVA–Au nanocomposite films in the wavenumber range: (A) 600–3900 cm−1;(B) 600–1800 cm−1 and (C) 2800–3600 cm−1.

Further, figures 6(A) and (B), respectively, present thetransmission (T ) and reflection (R) spectra recorded for purePVA and its nanocomposite films. As observed, these figuresdemonstrate the decrease in transmittance and increase inreflectance in the visible and near-IR range, with the increasein concentration of embedded gold nanoparticles (by wt%).In order to study the refractive index behaviour of PVA andits nanocomposite films, the values of r and t have beendetermined from equations (1) and (2) [28] as already discussedin experimental section. From the values of r and t sodetermined, the refractive index n(λ) has been obtained usingthe relation [34]

n(λ) =[

4r

(r − 1)2− k2

]1/2

− r + 1

r − 1; (5)

where k = αλ/4π is the extinction coefficient with α as theabsorption coefficient.

Figure 7 presents the variation of refractive index n(λ)

as a function of wavelength in the region of maximumtransparency, for pure PVA and PVA–Au nanocomposite films,as determined using equation (5). It is evident from thisfigure that initially the refractive index decreases slightly withincreasing wavelength and finally almost saturates for purePVA as well as for its nanocomposite films, suggesting normaldispersion behaviour in the region of maximum transparency.Further, it has been noticed that the values of n(λ) increasecontinuously with the increase in concentration of Auembedded in PVA up to 0.043 wt%. However, for the filmwith 0.074 wt% of Au nanoparticles, n(λ) decreases. Thus, itcan be noticed that PVA–Au nanocomposites exhibit refractive

index tunability based on the variation in concentration ofembedded gold nanoparticles. This glimpses the usefulnessof such nanocomposites to make the graded index opticalfibres used in waveguides by depositing the layers of PVA–Aunanocomposites with different concentrations of embeddednanoparticles.

The increase in refractive index n of the PVA–Aunanocomposites in comparison with pure PVA are in line withthe predictions of the Bhar and Pinto model [35] developedthrough simulation of Lorimer’s theory [36]. However, thedecrease in refractive index of the composite film observed at0.074 wt% concentration may be due to the agglomeration ofAu nanoparticles within the polymer matrix [37].

Further, the observed normal dispersion behaviour ofrefractive index in the wavelength region of maximumtransparency can be explained on the basis of Sellmeierdispersion theory [38, 39]. According to this theory, thematerial is treated as a collection of atoms, which under theinfluence of the oscillating electric field of the light beam formspolarized dipoles which oscillate. This theory presumes that athigher wavelengths, these oscillating atomic dipoles become inresonance with the incident frequency and, therefore, possessalmost no absorption. Thus, dielectric response can bemodelled as one or more Lorentz oscillators with almost zerobroadening; due to which n shows the normal dispersionbehaviour with wavelength (λ) in the region of maximumtransparency. The refractive index at wavelength (λ) is relatedto the energy parameter Ed, which is a measure of the strengthof interband optical transition and single oscillator energy E0

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J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Figure 5. (A) UV–Visible absorption spectra and (B) (αhν)1/2

versus hν plots for pure PVA and different PVA–Au nanocompositefilms.

(E0 ∼ 2Eg [42]) through the following expression [34, 42]:

n2 − 1 = E0Ed

(E20 − (hν)2)

. (6)

The values of Ed and E0 can be obtained from the interceptand slope of the linear fitted lines by plotting (n2 − 1)−1

against (hν)2 (figure 8(A)). Further, to yield the longwavelength refractive index (n∞), average interband oscillatorwavelength (λo) together with the average oscillator strength(So) for the pure PVA and its nanocomposite films, the singleterm Sellmeier oscillator model equation (equation (7)) isconsidered [34, 38],

n2∞ − 1

n2 − 1= 1 −

(λo

λ

)2

. (7)

From this equation, it is evident that the values of n∞ andλo can be determined from the slope and intercept of linearfitted line of the plots of (n2 − 1)−1 versus λ−2 (figure 8(B)).From the obtained values of n∞ and λo, the values of averageoscillator strength So can be obtained using the relation

So = (n2∞ − 1)/λ2

o. (8)

In addition to this, the value of lattice dielectric constant (ε1)and the ratio of carrier concentration to the electron effective

Figure 6. UV–Visible (A) transmission and (B) reflection spectrafor pure PVA and different PVA–Au nanocomposite films.

Figure 7. The variation in refractive index (n) w.r.t wavelength (λ)for pure PVA and different PVA–Au nanocomposite films.

mass (e2/πc2)(N/m∗) can be calculated, by considering thedependence of n2 on λ2 (figure 8(C)) in the light of the anotherdispersion relation [34]

n2 = ε1 −(

e2

πc2

) (N

m∗

)λ2. (9)

The values of all the parameters (Ed, E0, n∞, λo, So, ε1

and (e2/πc2)(N/m∗)) determined from the analysis of the

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J. Phys. D: Appl. Phys. 44 (2011) 205105 S Mahendia et al

Figure 8. (A) (n2 − 1)−1 versus (hν)2 plots, (B) (n2 − 1)−1 versus(λ)−2 plots and (C) n2 versus λ2 plots for pure PVA and differentPVA–Au nanocomposite films.

above equations for PVA and PVA–Au nanocomposites atdifferent concentrations of Au (wt%) have been listed intable 1. The quantitative measurements of these parametersmay help in tailoring and modelling of the properties of suchnanocomposites for their use in optical devices.

4. Conclusion

After synthesizing gold nanoparticles, PVA–Au nanocompos-ite films with varying concentrations of Au nanoparticles were

prepared through the ex situ method. These films were charac-terized through the UV–Visible spectroscopic, XRD and FTIRtechniques. We have attempted to understand the observedchange in optical band gap of PVA after doping with differ-ent concentrations of Au nanoparticles in terms of inducedstructural changes. The refractive index ‘n’ showed normaldispersion behaviour in the region of maximum transparencyand we have attempted to explain this through the concept ofthe single oscillator model. This study clearly indicates thatthe optical properties of PVA can be tuned to the desired valuesafter embedding gold nanoparticles for possible applicationsin optical coating and optical wave guiding purpose.

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