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Adsorbed Dyes Onto Nanoparticles: Large Wavelength Dependence in Second Harmonic Scattering Pierre-Marie Gassin, Sarah Bellini, Jerzy Zajac, and Gaelle Martin-Gassin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00703 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017
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Adsorbed Dyes onto Nanoparticles: Large Wavelength Dependence in Second Harmonic Scattering Pierre-Marie Gassin*a, Sarah Bellinib, Jerzy Zajacb, Gaelle Martin-Gassinb *
[email protected] AUTHOR ADDRESS a) Ecole Nationale Supérieure de Chimie Montpellier, 240 avenue du Professeur Emile Jeanbrau 34090 MONTPELLIER CEDEX 5 b) Institut Charles Gerhardt de Montpellier, UMR-5253 CNRS-UM-ENSCM, C.C. 1502 Place Eugene Bataillon, Montpellier F-34095 Cedex 5, France
ABSTRACT
Spectrally and polarization resolved Second Harmonic Scattering (SHS) has been used to probe the adsorption of an hemicyanine dye onto polystyrene nanoparticles. The polarization resolved SHS measurements in both the vertically and horizontally output polarization configuration demonstrate the ability of this technique to monitor simultaneously the amount of free dye in solution and the amount of dye species adsorbed onto nanoparticles. Indeed, those two contributions can be discriminated when changing the laser incident polarization angle.
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Moreover, a large modification appears for the horizontally polarization resolved pattern when the laser excitation wavelength is tuned from 790 nm to 810 nm. This result is discussed within the framework of nonlinear Mie theory. Introduction Separation processes based on sorption equilibrium at the solid/liquid interface are widely used for pollutant removal
1-2
or in sample enrichment and purification by solid phase extraction
(SPE)3-4 especially in Quick Easy Cheap Effective Rugged and Safe (QuEChERS) methods.5 In these approaches, particles with a strong and selective affinity towards the target molecules are added to the solution in order to adsorb them. To better understand the adsorption process between a solid surface and a specific water-soluble pollutant, such microscopic level information as the nature and number of active sites at the solid surface is needed. Surface specific tools are thus necessary to investigate such systems. Second order nonlinear optical techniques, like Second Harmonic generation (SHG) or sum frequency generation (SFG), have demonstrated their ability to provide the missing information,6-8 mostly for flat solid/liquid interface.9 To investigate a real SPE system in situ, the Second Harmonic Scattering (SHS) technique, firstly developed by Eisenthal,10 has recently been proposed to study the surface of colloidal systems in solutions.11-13 This approach has been used, in particular, to study dye adsorption onto nano or micro-particles14-15 or to probe interactions of dyes with micelles.16-17 Two main theoretical models18, namely Nonlinear Rayleigh Gans Debye (NLRGD)19 and Nonlinear Mie (NLM) theory,12, 20-24 have been developed to interpret such experiments and to understand the relationship between the emission pattern and the surface properties. To go beyond these previous works, we investigate here both polarization and spectral dependency of the SHS signal coming from the adsorption of an hemicyanine dye, named 4-(4-
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(diethylamino)styryl)-N-methylpyridinium iodide and hereinafter referred to as sDiA, onto small polystyrene nanoparticles of 50 nm in diameter. Firstly, we shall show that the experimental polarization-resolved analysis permits to distinguish directly the free dye contribution from that produced by the adsorbed dye. In this case, the methodology is proven particularly well adapted to investigate the adsorption process in situ. Secondly, a large modification appears for the polarization resolved pattern when the laser excitation wavelength changes from 790 nm to 810 nm. This result is discussed within the framework of NLM theory including or not a contribution due to the Electric Field-Induced Second Harmonic Generation (EFISH).25-27
1. Experimental and Theoretical Methods 1.1. Experimental Optics setup: Figure 1 presents the experimental setup and the notation used in the following discussion. The laboratory frame is defined with XYZ axes, the particle frame with the spherical coordinates (r, θ, φ), the particle surface frame with X’Y’Z’ axes and the dye molecular frame with the x’’y’’z’’ axes. The angle γ defines the laser incident light polarization direction. The second harmonic light is detected in the 90° direction and can be analyzed in two polarization states: vertical (Γ = 0°) further named I(γ,V), or horizontal (Γ = 90°), referred to I(γ,H).
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Figure 1. Scheme of the experimental set up and definition of the different frames: laboratory frame XYZ, particle frame (r, θ, φ), surface particle frame (X’Y’Z’) and molecular frame (x’’y’’z’’)
The SHS setup was based on a femtosecond Ti-sapphire oscillator laser source providing pulses with a duration of about 100 fs at a repetition rate of 80 MHz (coherent, model Chameleon ultra II). After passing through a long-pass filter, the fundamental beam, set to a fixed wavelength varying from 790 to 810 nm and an averaged power of 800 mW, was focused by a microscope objective (Ealing x10, numerical aperture 0.25) onto the sample at 2 mm close to the output cell window. The SH light was collected at 90° by a 10 cm focal length lens and separated from its
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fundamental counterpart by a short-pass filter. The SH light was detected with a water-cooled CCD camera (Andor, Newton) placed after a spectrometer (Andor, Shamrock 193i) and the background signal was systematically subtracted when recording the SH light. The fundamental input beam was linearly polarized and the input polarization angle γ was selected with a rotating half-wave plate. An analyzer, placed in front of the spectrometer, was used to separate the vertical or horizontal SH intensities named, respectively, I(γ,V) and I(γ,H). Each single data point reported in the present work is recorded during 15 seconds, and the water HRS intensity was removed systematically (except for Figure 2A and B). The second harmonic intensity was also systematically corrected by the linear absorbance of the solution at the harmonic frequency, because the UV visible spectrum of the solution exhibited a weak absorption at the harmonic wavelength. A2 ω I2ω (γ, Γ) = I 2ω measured (γ, Γ)×10
(1)
Imeasured is the second harmonic light effectively detected and A2ω is the linear absorbance at 2ω. 1.2. Zeta potential measurements: The zeta (ζ) potential of the particles was determined by electrophoretic measurements using the Malvern Zetasizer 3000 HSA (Malvern Instrumentation, UK). For two different samples, the measurement was repeated 20 times, with values averaged to give final result. 1.3. Chemistry The probe molecular compound used in the present experiments was 4-Di-1-ASP (4-(4(Dimethylamino)styryl) -N-Methylpyridinium Iodide) (sDiA, Molecular Probes Inc.), a dipolar push-pull type hydrophilic compound (see Figure 1). The wavelength of the maximum of the one
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photon absorption band for sDiA dispersed in water was located around 480 nm, and, therefore, the harmonic wavelength fixed around 400 nm was weakly resonant with this transition. Millipore water (resistivity 18 MΩ.cm-1) was used throughout the experiments. The colloidal solutions were prepared with polystyrene microspheres (Polysciences, Inc.) in deionized water under neutral pH=6,8. The microspheres used represented “plain” polystyrene microspheres with a diameter 0,049 (±0,005) µm. In all experiments, the particles concentration was kept at the same value of 5.1011 part/ml. The solution was systematically sonicated before each experiment and kept continuously stirred. The temperature was maintained constant at 18°C. In the experiment performed at a high ionic strength, a 0.1 M KNO3 solution (Merck) was employed. 1.4. Theoretical model used to described experiment The second harmonic light detected in all experiment includes three main contributions as specified in Eq. 2, namely the water Hyper-Rayleigh Scattering (HRS), dye HRS and an SHS signal, which comes from the dye species adsorbed onto the particles. The discussion about the nature of those different contributions in terms of the coherence or incoherence of the sources, can be found elsewhere.13 λ I2ω (γ, Γ) = I λwater,HRS (γ, Γ) + IλDye,HRS (γ, Γ) + ISHS (γ, Γ)
(2)
Here λ is the incident laser wavelength and γ and Γ are defined above. In the consideration presented further, the water HRS contribution has been systematically removed (except for Figure 2A and B) so that the second harmonic light detected can be written as the sum of the dye HRS contribution and the SHS one. The HRS term can be described by a dipolar emission28:
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IλHRS,i (γ, V) = K λHRS,i ( cos2 (γ) + D λi sin 2 (γ)) IλHRS,i (γ, H) = K λHRS,i × D iλ
(3a,b)
The subscript i refers to the water or dye contribution; KλHRS,i is a constant and Dλ i is defined as the depolarization ratio for the i species at the λ excitation wavelength. The SHS term is expressed using the Dadap’s derivation23 (more details are given in Supporting Information, section 4): λ ISHS (γ, V) = K SHS sin 2 (2γ) λ ISHS (γ, H) = K SHS × aλ
2
(4a,b)
KλSHS and aλ are functions that depend on the different nonlinear susceptibility components χ(2)I’J’K’ . In particular, if a homogeneous medium with no dispersion is considered (see Supporting Information, section 4) aλ can be expressed as:
aλ =
(2) χ (2Z' )Z' Z' + 4χ (2) Z ' X' X' − 2χ X' X' Z ' (5) (2) (2) χ (2) Z ' Z 'Z ' − χ Z 'X 'X ' + 3χ X 'X ' Z '
2. Results and discussion
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Figure 2. Polarization resolved experiment: I2ω(γ,V) (red) and I2ω(γ,H) (blue) for pure water (A), water with nanoparticles (B), 15 µM sDiA solution in water (C), 15 µM sDiA solution containing nanoparticles at a ionic strength of 10-5 M (D, E, F) or 10-1 M (G, H). The results reported in panels A, B, C, D, G have been recorded at 790 nm excitation wavelength, the results in panels E at 800 nm, and those in panels F and H at 810 nm. The solid lines in panel A, B, C represent a global fit with Eqs 3 a-b, and those in panels D, E, F, G, H refer to a global fit with the use of Eq. (6) and Eq. (8). All graphs have the same radius scale. 2.1. Pure water Figure 2A shows the polarization resolved HRS for pure water. The data are well described by a dipolar emission28, in line with Eqs (3a,b). Here, Kwater is constant and the water depolarization ratio Dλ water takes a value of Dλ = 0.12 ± 0.01 insensitive for the three excitations wavelengths studied here, i.e., 790, 800 or 810 nm. (see complementary measurement in the Supporting Information, section 1). 2.2. Particles in water
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Figure 2B presents the polarization resolved second harmonic light detected from the water phase containing nanoparticles. This graph is very similar to that in Figure 2A. The second harmonic light detected seems thus to be dominated by water HRS contribution and the particles SHS contribution is not significant. The slight difference observed when particles are added may be attributed to the EFISH signal given by the water molecules organized in the electric double layer induced by the surface charge of particles. Indeed, the zeta potential of particles in water was measured to be equal to -28 ± 4 mV. 2.3. Dyes in water Figure 2C presents the polarization resolved second harmonic light of the sDiA molecules in solution containing no particles. As explained before, the water HRS contribution has been removed, so that only the HRS contribution of the dye, IDye,HRS, is detected. The result is yet well described by a dipolar emission which is expected for this kind of 1D linear push-pull molecules28. Here KλDye, is proportional to the dye concentration, as shown in Figure 3, i.e., KλHRS,Dye =Kλ0*[Dye]. The following values were obtained: K7900= 39 ±1 cps/µM and K8100= 43 ± 0.5 cps/µM.
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Figure 3. Evolution of the KλHRS parameter with the sDiA concentration in solution. The red symbols and lines correspond to the 810 nm excitation and the green ones to the 790 nm excitation. The dye depolarization ratio DλDye was found to be more or less constant, namely D790Dye = 0.19 with a standard deviation of 0.02 and D810Dye = 0.18 with a standard deviation of 0.02 (see section 2 in Supporting Information). This depolarization parameter is consistent with the 1D rod like topology of the molecule, which leads to a conclusion that βz’’z’’z’’ is the predominant component of the hyperpolarisability tensor.14, 29 2.4. Dyes and particles in water When polystyrene nanoparticles are added together with dye molecules in the solution, polarization pattern strongly changes because of the adsorption of the dye units onto the nanoparticles (Fig. 2 D, E, F, G, H). This adsorption can also be inferred from the decrease in the zeta potential from -28±4 mV (particles in water) to -18 mV±4 mV (particles in the sDia solution). To rationalize the data obtained, we shall first detail the analysis of the I(γ,V) polarization pattern and then that of I(γ,H). 2.4.1. Discussion about the polarization resolved I(γ,V) pattern The I(γ,V) patterns in Figure 2 D, E, F, G, H exhibit four “lobes” at 45°, 135°, 225° and 315° and a weak wavelength dependence in the excitation range of 790-810 nm. Moreover, no ionic strength effect has been observed since the same patterns are obtained for low and high ionic strengths.25 For this reason, the EFISH contribution is supposed to be negligible compared to the
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χ(2) one. Thus, the following model equation deduce from equation 3a and 4a is adequate to interpret the I(γ,V) data: λ I2ω ( γ, V) = IλDye,HRS ( γ, V) + K SHS sin 2 (2γ)
(6)
Here, KλSHS is basically proportional to the square of χ(2) (see equation S1 and S2 in Supporting Information), which in turn is proportional to the amount of adsorbed molecules, Ns. Equation (6) shows that, at γ=0° or 90°, only the HRS of the free dye in solution contributes to the signal, whereas at γ=45°, both the HRS and SHS give their contributions. This property can be employed to evaluate the amount of free dye in solution versus the absorbed dye units. In the top left part of Figure 4 is presented the evolution of I(γ,V) pattern as a function of the dye concentration in the sample corresponding to 5.1011 particles per ml. The fit of these data to equation 6 permits to deduce KλDye,HRS and KλSHS as a function of the sDiA dye present in the system. A complete table of the resulting KλDye,HRS and KλSHS values is given in SI section 3. The top right part of Figure 4 shows the way to decompose the global second harmonic light into a HRS of the free dye contribution and a SHS of the adsorbed dye.
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Figure 4. Top Left: I2ω(γ,V) signal as a function of the sDiA concentration: red (5 µM), orange (15 µM), green (30 µM) and blue (50 µM) at 810 nm excitation. Bottom Left: I2ω(γ,H) signal as a function of the sDiA concentration: red (5 µM), orange (15 µM), green (30 µM) and blue (50 µM) at 810 nm excitation. Right: the overall second harmonic signal decomposes into HRS and SHS parts (equations 6 and 8).
Figure 5 presents the evolution of √KλSHS as a function of KλDye,HRS which can be directly converted into the concentration of free sDiA units in solution, as exemplified by the calibration curve in Figure 3.
A Langmuir-type equation may be used to describe the adsorption
phenomenon30:
λ K SHS ∝ Ns = N∞
[sDiA] a +[sDiA]
(7)
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where N∞ is the maximum adsorption quantity at saturation, and a is the Langmuir-Szyszkowski constant. We can see that measurements at 790 or 810 nm excitation give similar results and they lead to the same conclusion: at concentrations close to 30 µM the particle surface in solution attains the state of full coverage. The best fit Langmuir-Szyszkowski constant appears to be 8 +/2 µM.
Figure 5. Sqrt(KλSHS) versus KλHRS (top axis) or free sDiA concentration in solution (bottom axis). The red full circles come from the measurement at 810 nm and the green squares refer to that taken at 790 nm. The solid line represents a fit of data using a Langmuir–type equation 7.
2.4.2. Discussion about the polarization resolved I(γ,H) pattern: I(γ,H) curves recorded at the 790 nm wavelength excitation and presented in Figures 2 D and G follow the same behavior pattern. The effect of ionic strength is hardly detectable since only a small increase in the second harmonic scattered signal is observed when the ionic strength decreases. These data are well fitted by a constant term, as expected from the analysis of Eq. 4b. For a 800 nm and especially a 810 nm excitation wavelength, a clear deviation in the I(γ,H) pattern is observed with the appearance of four lobes centered at 0°, 90°, 180° and 270° (see
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Figure 2 E, F, G). Equation 4b fails to describe correctly this result and an additional term depending on cos(2γ) is introduced in equation 8 to better adjust the patterns obtained:
λ I2ω (γ, H) = K λHRSD λ + K SHS ( a λ + bλ × cos(2γ))
2
(8)
The KλSHS and KλDye,HRS values, previously introduced in equation 6 and deduced from the I(γ,V) fit presented in Figure 3 top, are kept fixed here and only aλ and bλ parameters are free in the I(γ,H) fit. A complete table of the best-fit aλ and bλ values obtained at a low ionic strength is given in Supporting Information. 3.4.2.1 Discussion about the polarization resolved I(γ,H) pattern at 790 nm With a 790 nm excitation, a790 is found to be around 1.1 and b790 is around zero. Equation 5 shows that aλ parameter can be simply expressed as a function of the χ(2) components. In particular, aλ is around 1 if the surface nonlinearity is dominated by χ(2)Z’Z’Z’; aλ is around 16 (respectively 0.4) if the surface nonlinearity is dominated by χ(2)Z’X’X’ (respectively χ(2)X’X’Z’). An a790 numerical value around 1.1 is thus consistent with a surface nonlinearity dominated by the χ(2)Z’Z’Z’ component. At the molecular level, the interpretation of the dye orientation11 at the particle surface is based on the relationship between the molecular hyperpolarizability β and the Surface Second-Order Susceptibility χ(2) given by the following equation.
(9)
where
stands for the orientational average of the transformation tensor
from the molecular
reference frame to the surface particles frame and Ns is the average surface density of the adsorbed molecules. The dye molecule studied here has only one no-vanishing element, β z’’z’’z’’,
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as previously demonstrated from the depolarization ratio DDye value. If χ(2)Z’Z’Z’ is predominant, the transformation of tensor T is thus immediate in this case and the axes z” and Z’ are parallel to each other which means that the dye is more or less vertically oriented with respect to the surface. 2.4.2.2 Discussion about the polarization resolved I(γ,H) pattern at 810 nm For 810 nm, bλ is predominant and has a value about 1, regardless the dye concentration, whereas aλ is nearly zero. This shows that the appearance of the 4 lobes is possible even at a low dye density, as can be seen in Figure 4 (the bottom patterns). Increasing strongly the ionic strength diminishes the I(γ,H) intensity, even if the global shape of the pattern remains unchanged. Within the framework of the theory developed in the present work (see Eq. 2 and 4b), none of the terms can explain a cos(2γ) evolution in the polarization resolved pattern. An octupolar contribution31,29 firstly considered, has been rejected because of its minor importance compared to the quadrupolar effect. Another derivation of the NLM theory in the quadrupolar approximation21 predicts a cos(2γ) evolution in the I(γ,H) pattern associated with an χ(2)X’X’Z’ contribution (see equation S10 in Supporting Information). This calculation does not fall within the framework of the Dadap’s theory developed here. A discussion about the discrepancies between those two NLM calculations is out of the scope of this work. Nevertheless, the Dadap’s calculation is adequate within the limit of small particles, whereas the Pavlyukh’s calculation is more general. It may be possible that important solute aggregation at the particule surface results in shell-like structures with an increased radius compared to the initial particles, thereby contributing to the scattering pattern. Furthermore, other nonlinear susceptibility contributions, associated with an anisotropic and/or chiral surface, may also be involved to explain such cos(2γ) evolution. Indeed, it is known that these dyes can formed H-aggregates in solutions
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or some chiral supramolecular structures at liquid surface.34-35 Such reorganization may also occur within the electric double layer as the effect is enhanced at low ionic strengths. In any case, it seems unclear why such a small wavelength variation in the probe induces so large differences in the emission pattern. It may be also argued that the reorganization of the system upon the change of the wavelength excitation may come from the local thermal effect appearing in the vicinity of the dye resonance. No temperature change has been detected in the solution by varying the excitation wavelength from 790 to 810 nm. Nevertheless, it is possible that locally (at the molecular scale) a thermal dissipation occurring in the vicinity of the resonance induces a kind of molecular reorganization onto particles.36 Much more theoretical efforts are needed to better understand this behavior and in particular other dyes will be tested with the same particles in future works. 3. Conclusion A complete model to describe the polarization resolved second harmonic light scattering taking into accounts both HRS and SHS origin is proposed. This approach and specially the analysis of the I(γ,V) show the ability of this technique to distinguish simultaneously information about the free molecules in solution and those adsorbed onto particles. The I(γ,H) polarization analysis also indicates an unexpected very strong wavelength dependence pattern. This behavior may be attributed to a reorganization of the dye units in the vicinity of polystyrene nanoparticles induced by local thermal effect changing with the excitation wavelength. Altogether, such studies are intended to spread the use of SHS to probe the interactions of molecular species with colloidal particles and to promote the development of new models and simulation works to better assist the interpretation of such SHS experiments.
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Supporting Information. water HRS experiments; sDiA HRS experiments; Polarization resolved SHS fit: Table of the parameters; Derivation of the qualitative NLM theory; Two photons fluorescence spectrum. This material is available free of charge via the Internet at http:// pubs.acs.org. Acknowledgment. The financial support of this work by the ANR project CAMOMILS (ANR15-CE21-0002) is greatly acknowledged. The authors would also like to thank the reviewers for their insightful comments on the paper and very fruitful discussion. Abbreviations HRS, Hyper Rayleigh Scattering; SHS, Second Harmonic Scattering; NLRGD, Non Linear Rayleigh Gans Debye Theory; NLM, Non Linear Mie Theory; SPE, Solid Phase Extraction; SHG, Second Harmonic Generation; SFG, Sum Frequency Generation. Reference List 1. Sorrenti, V.; Di Giacomo, C.; Acquaviva, R.; Barbagallo, I.; Bognanno, M.; Galvano, F., Toxicity of Ochratoxin A and Its Modulation by Antioxidants: A Review. Toxins 2013, 5 (10), 1742-1766. 2. Aksu, Z., Application of biosorption for the removal of organic pollutants: a review. Process Biochemistry 2005, 40 (3–4), 997-1026. 3. Camel, V., Solid phase extraction of trace elements. Spectrochimica Acta Part B: Atomic Spectroscopy 2003, 58 (7), 1177-1233. 4. Kataoka, H.; Lord, H. L.; Pawliszyn, J., Applications of solid-phase microextraction in food analysis. Journal of Chromatography A 2000, 880 (1–2), 35-62. 5. Payá, P.; Anastassiades, M.; Mack, D.; Sigalova, I.; Tasdelen, B.; Oliva, J.; Barba, A., Analysis of pesticide residues using the Quick Easy Cheap Effective Rugged and Safe (QuEChERS) pesticide multiresidue method in combination with gas and liquid chromatography and tandem mass spectrometric detection. Analytical and Bioanalytical Chemistry 2007, 389 (6), 1697-1714. 6. Eisenthal, K. B., Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy. Chemical Reviews 1996, 96 (4), 1343-1360. 7. Shen, Y. R. Optical second harmonic generation at interfaces Annual Review of Physical Chemistry Vol. [Online], 1989, p. 327. 8. Verbiest, T.; Clays, K.; Rodriguez, V., Second-order Nonlinear Optical Characterization Techniques: An Introduction. Taylor and Fancis Group, 2009.
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