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Interfacial Structure of Soft Matter Probed by SFG Spectroscopy Shen Ye,*[a] Yujin Tong,[a] Aimin Ge,[a] Lin Qiao,[a] and Paul B. Davies[b] Catalysis Research Center, Hokkaido University, Sapporo 001-0021 (Japan), E-mail: [email protected] [b] Department of Chemistry, Cambridge University, Cambridge, CB2 1EW (UK)

[a]

Received: April 22, 2014 Published online: ■■

ABSTRACT: Sum frequency generation (SFG) vibrational spectroscopy, an interface-specific technique in contrast to, for example, attenuated total reflectance spectroscopy, which is only interface sensitive, has been employed to investigate the surface and interface structure of soft matter on a molecular scale. The experimental arrangement required to carry out SFG spectroscopy, with particular reference to soft matter, and the analytical methods developed to interpret the spectra are described. The elucidation of the interfacial structure of soft matter systems is an essential prerequisite in order to understand and eventually control the surface properties of these important functional materials. DOI 10.1002/tcr.201402039 Keywords: interfaces, soft matter, sum frequency generation, surface analysis, vibrational spectroscopy

Introduction Soft matter is defined as condensed matter whose physical state can be deformed by weak forces at room temperature.[1,2] Soft matter includes a large number of molecular systems, such as polymers, surfactants, colloids, gels, liquid crystals, biocomposites, and biomimetic matter. Soft matter provides unique functionalities and has been applied in a wide range of fields.[1,2] The surfaces of soft matter are expected to have different properties and structures compared to those of the bulk phase and play key roles in many aspects of soft matter, such as wetting, adhesion, friction, and biocompatibility.[3–5] The elucidation and control of the tailored interfacial structure of soft matter at a molecular level under in situ conditions is crucial for developing novel functional soft materials for various purposes. Most existing surface techniques use electrons as probes and require the sample to be placed in an ultrahigh vacuum (UHV) environment.[6] This is usually unsuitable for investi-

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gating soft matter, which often contains water. Recently, novel surface analytical techniques, such as scanning probe microscopy (STM and AFM)[7,8] and surface X-ray scattering (SXS) using synchrotron radiation,[9] have been established to probe atomic and molecular arrangements on the surfaces of solids including soft matter. However, due to the low molecular specificity of these techniques, difficulties are often encountered when attempting to exactly account for the observations. In this regard, vibrational spectroscopy can provide profound information about the molecular structure. Unfortunately, traditional vibrational spectroscopy methods, such as infrared (IR) absorption spectroscopy and Raman scattering, are not intrinsically surface specific, and it is hard to distinguish the contribution of the surface from that of the bulk material. Furthermore, these measurements are usually not sensitive enough to investigate soft matter surfaces, where the number of molecules is much lower than that in the bulk. Recently, the

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spectroscopic sensitivity of these techniques on soft matter interfaces has been greatly improved by total internal reflection (TIR) spectroscopy.[10] Surface-enhanced Raman scattering (SERS) may show even higher surface sensitivity, but only works on several metals with intrinsic nanostructures.[11] Sum frequency generation (SFG) spectroscopy is a secondorder nonlinear optical technique that allows us to obtain vibrational spectra at surfaces and interfaces.[12] SFG is generated by two laser photons at frequencies ω1 and ω2, and observed as a single photon with a sum frequency at ω3 = ω1 + ω2. SFG does not occur in homogenous media with inversion symmetry under the electric dipole approximation and is active only at surfaces or interfaces where inversion symmetry is broken. For observing vibrational spectra, one of the frequencies is fixed in the visible (ωvis) and the other is usually scanned in the infrared region (ωIR). When ωIR is equal to a vibrational level of the molecule, the SFG signal is resonantly enhanced. SFG is attracting much attention in multidisciplinary research fields, including surface science, materials chemistry, biophysics and electrochemistry, due to its intrinsic surface specificity, extremely high sensitivity, and versatile applicability.[13–24] In the present Personal Account, our recent SFG studies on the molecular structures at interfaces of functional soft matter such as polymers,[25–27] self-assembled monolayers (SAMs)[28–34] and Langmuir–Blodgett (LB) ultrathin layers[21,35–42] and lipid bilayers[43–45] in various environments will be briefly reviewed. A general model dealing with the optical interference effect in thin layers of soft matter, which can dramatically affect the SFG spectral shape and intensity, will be introduced first.

Interference Effects in the SFG Spectra of Soft Matter Since many samples of soft matter are prepared as a thin film on a substrate, normally two interfaces are present, i.e., a free Dr. Shen Ye received his Ph.D. degree from the Division of Chemistry, Graduate School of Science, Hokkaido University, in March 1993. He currently works as an associate professor in the Catalysis Research Center (CRC), Hokkaido University. His current research interests are focused on the elucidation of surface and interface structures on soft matter and other functional materials at a molecular level, using many surface characterization techniques such as sum frequency generation and atomic force microscopy.

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interface in contact with the environment and a buried interface in contact with the substrate, and both are SFG active. More interfaces may exist when multiple component layers are sequentially deposited. Not only emitted SFG light but also pumped visible and IR light from these interfaces can optically interfere, especially when the film thickness is comparable to the wavelengths of the light.[46–49] In order to acquire the interfacial structural information from the SFG spectra of soft matter, one has to understand how these optical factors affect the spectral intensity and line shape. Although the interference effect is well known in optics, it was rarely considered in the early SFG works on soft matter. One of the reasons could be the complicated calculations involved. To overcome this problem, a general model was developed to quantitatively deal with the optical interference in SFG by taking into account the relative intensities and phases of the SFG signals from different interfaces.[38,39] SFG intensity (ISFG) is proportional to the intensities of the incident visible (Ivis) and IR (IIR) beams and to the square of the (2 ) ) of the interface: second-order nonlinear susceptibilities ( χijk 2

I SFG ∝

∑ ∑ Lii (ω SFG ) χijk(2)L jj (ωvis ) Lkk (ω IR ) I vis I IR i

(1)

j ,k

where i, j, k (=x, y or z) are the coordinates in the interface-fixed (2 ) is a third-rank tensor and carries surface reference frame; χijk structural information such as molecular density (Ns), orientation and conformation on the surface and can be related to the microscopic (molecular) properties of the molecules via:

( )

( )

(2 ) (2 ) χijk i ⋅ ξ ( j ⋅ η ) k ⋅ ς βξης ,q = N s ∑ ξ ,η ,ς

(2a)

( )( )( )

where iˆ ⋅ ξˆ ˆj ⋅ ηˆ kˆ ⋅ ςˆ are the Euler transformation matrices between the molecular frame (ξ, η, ζ) and the interface-fixed coordinate frame (i, j, k). The operator < > denotes the average (2 ) is the hyperpolarizability of over all possible orientations. βξης the molecules as: (2 ) βξης =

q βξης

ω IR − ω q + iΓ q

(2b)

q , ωq and Γq are the amplitude, frequency and where βξης damping constant of the qth vibrational mode, respectively. The q magnitude of βξης is directly related to the infrared and Raman properties of the vibrational mode by:

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q =− βξης

(1) 1 ∂α ξη ∂μς 2 ε 0ω q ∂Q q ∂Q q

(2c)

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Interfacial Structure of Soft Matter by SFG Spectroscopy

(1) where ∂α ξη ∂Qq and ∂μς/∂Qq are the (Raman) polarizability tensor and the (infrared) dipole derivatives, respectively. The Lii, Ljj and Lkk in Eq. (1) are the Fresnel coefficients or local field factors (L-factors) relating the input fields to the field in the polarization sheet at the interface.[49,50] For a single interface between medium 1 and 2, the L-factors in a reflected direction are given by:

LRxx ,12 = 1 − rp ,12

(3a)

LRyy ,12 = 1 − rs ,12 ⎛ n1 ⎞ LRzz ,12 = (1 + rp ,12 ) ⎜ ⎟ ⎝ nm ⎠

t s,12

rp,12

t p,12

2

(3c)

n1 cos θ1 − n2 cos θ 2 n1 cos θ1 + n2 cos θ 2

2n1 cos θ1 = n1 cos θ1 + n2 cos θ 2

(4b)

i

SFG

rs ,12 + rs ,23 exp (iΔ ) 1 + rs ,12 rs ,23 exp (iΔ )

(6b)

2

(6c)

The reflection and transmission coefficients for I and II take the same form as in Eq. (4), with appropriate changes in the subscripts. Δ is the phase factor accounting for the geometrical path difference between two neighboring successive reflected or transmitted beams in the multiple reflection process, and is a function of each laser wavelength(λ) and film thickness (d) and given by (there was a typo in our previous paper [38] where a factor of 2 was missing in Eq. 9 there):

2π Δ = 2 × ⎛ ⎞ n2 d cos θ 2 ⎝ λ ⎠

t p ,12 rp ,23 1 + rp ,12 rp ,23 exp (iΔ )

(8a)

t s ,12 rs ,23 1 + rs ,12 rs ,23 exp (iΔ )

(8b)

LII ωi , yy = exp (iΔ II ) (4d)

LII ωi , zz = exp (iΔ II )

t p ,12 rp ,23 ⎛ n3 ⎞ 1 + rp ,12 rp ,23 exp (iΔ ) ⎜⎝ nm,23 ⎟⎠

(5)

j ,k

2

(8c)

where exp(iΔII) is an additional phase factor to account for the relative phase differences generated from the beam propagating distance with respect to the coherence point and is given for all three beams as follows:

Δ II (SF ) =

(2 ), I I ) χijk L jj (ω vis ) LIkk (ω IR )

(7)

Similarly, the L-factors for the buried interface II between medium 2 and medium 3 can be expressed as:

(4c)

2n1 cos θ1 = n1 cos θ 2 + n2 cos θ1

I ii

LI yy = 1 −

LII ωi , xx = exp (iΔ II )

n2 cos θ1 − n1 cos θ 2 = n1 cos θ 2 + n2 cos θ1

∑ ∑ L (ω

(6a)

rp ,12 + rp ,23 exp (iΔ ) ⎞ ⎛ n1 ⎞ ⎛ LI zz = ⎜1 + ⎝ 1 + rp ,12rp ,23 exp (iΔ ) ⎟⎠ ⎜⎝ nm,12 ⎟⎠

(4a)

where n1 and n2 are the refractive indices of medium 1 and medium 2, respectively, and nm is the effective refractive index of the interfacial layer.[49–51] When a thin film (n2) with a thickness of d is introduced between two media (n1 and n3), two interfaces appear (i.e., the “free” interface I between 1 and 2, and the buried interface II between 2 and 3). The expressions for the SFG signals can be rewritten as:

I SFG (ω s ) ∝

rp ,12 + rp ,23 exp (iΔ ) 1 + rp ,12rp ,23 exp (iΔ )

(3b)

where rs/p,12 and ts/p,12 are the amplitude reflection and transmission coefficients for s- and p-polarized light at an interface between medium 1 and 2 and are given by:

rs,12 =

LI xx = 1 −

2π n2,SF d λSF cos θ 2,SFG

(9a)

2

+ ∑ ∑ L (ω SFG ) χijk L jj (ω vis ) L (ω IR ) I vis I IR II ii

i

(2 ), II II

II kk

j ,k

Δ II (vis ) =

L-factors at the “free” interface I can be obtained by introducing multiple reflection terms in the film into Eq. 3:

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2π n2,vis d λvis cos θ 2,vis 2π n1,vis d ( tan θ 2,SF + tan θ 2,vis ) sin θ1,vis (9b) − λvis

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Fig. 1. Simulated results of the modulus of the combined L-factors Lijk calculated according to the thin film model (inset) for the “free” air/film interface I with (a) ssp and (b) sps polarization combinations. See text for details.

Δ II ( IR ) =

2π n2, IR d λ IR cos θ 2, IR 2π n1, IR − d ( tan θ 2,SF + tan θ 2, IR ) sin θ1, IR (9c) λ IR

Based on the L-factors given above, it is then possible to (2 ) from the SFG spectra based on Eq. (5). get the unknown χijk The SFG intensities from the azimuthally isotropic monolayer with different polarization combinations (for example, ssp and sps) can be further simplified as:

I ssp ∝

2 ), I LIyy (ω SFG ) LIyy (ω vis ) LIzz (ω IR ) sin θ IR χ (yyz II II II 2 ), II + Lyy (ω SFG ) Lyy (ω vis ) Lzz (ω IR ) sin θ IR χ (yyz

2 ), I 2 ), II = LIyyz sin θ IR χ (yyz + LIIyyz sin θ IR χ (yyz

I sps ∝

(10a)

2

2 ), I LIyy (ω SFG ) LIzz (ω vis ) LIyy (ω IR ) sin θ vis χ (yzy II II II 2 ), II + Lyy (ω SFG ) Lzz (ω vis ) Lyy (ω IR ) sin θ vis χ (yzy

2 ), II = LIyzy sin θ vis χ (y2zy),I + LIIyzy sin θ vis χ (yzy

2

2

2

(10b)

The expressions above demonstrate that, in addition to the interference between individual vibrational modes, the SFG spectral intensity and shape are also considerably determined by the interference between the two interfaces modulated through L-factors, which are affected by the film thickness, d. In other words, to fully understand the observed SFG spectra, one has to take the optical effect into account. On the other hand, by tuning L-factors with the optical parameters such as d and incident angles, one should be able to find an optimum condition to get higher SFG intensity, which is technically helpful for measurement.

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Here we demonstrate the applications of the model calculation to a thin-layer composite of a dielectric film on a gold substrate (Figure 1, inset). Figure 1 shows the calculated moduli of (a) Lyyz (i.e., Lyy,SFG*Lyy,vis*Lzz,IR) and (b) Lyzy (i.e., Lyy,SFG*Lzz,vis*Lyy,IR) for the “free” interface I, corresponding to ssp and sps polarization combinations, as a function of dielectric film thickness (d, 0–600 nm). The wavelengths of the visible and IR are fixed at 800 nm and 3125 nm, respectively; hence SFG occurs at 650 nm. The L-factors vary significantly with d and polarization combination. No clear periodicities are observed, although the modulus of each component does oscillate periodically with d. Lyyz shows a maximum around 125 nm while Lyzy exhibits a maximum at 375 nm. It is interesting to note that L-factors show a value of zero at certain d. As shown in Eq. (6), the L-factors for the “free” interface I encompass two terms: one is the electric field that is directly reflected from the surface and the other is the electric field that is multiply reflected within the thin film. The magnitudes of the two terms are comparable to each other while the relative phases between the two terms oscillate periodically with d in exp(iΔ). Consequently, constructive and destructive coherence occur between the two terms with d, where the former will result in a maximum value for the moduli of the L-factors, while the latter will result in a zero point. Similar calculations for L-factors can also be performed for the buried interface II with Eq. (8). With the two sets of L-factors, a “real” SFG spectrum can (2 ) . Figure 2 shows (a) ssp- and (b) be simulated with a known χijk sps-SFG spectra (2800–3000 cm−1) simulated for the case of an ordered monolayer of long-chain fatty acid deposited on the dielectric film on a gold substrate (Figure 1, inset).[38] This can be regarded as a model for a thin layer of soft matter, such as polymer or LB multilayer, on the substrate. All three methyl (CH3) C–H stretching vibrational modes, symmetric (r+), Fermi resonance (r+FR) and asymmetric (r−) modes, with

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Interfacial Structure of Soft Matter by SFG Spectroscopy

Fig. 2. Simulated SFG spectra as a function of dielectric layer thickness (d) under the (a) ssp and (b) sps polarization combinations. See text for details.

frequencies set at 2878, 2940 and 2965 cm−1, are included. SFG spectra dramatically change with thickness although the source of the resonant SFG signal, i.e., the top monolayer, does not change its structure. In the case of ssp polarization, when the film is thin, the SFG signal is small (Figure 2a). r+ and r+FR appear as dips while r− appears weakly with a reversed phase. The ssp-SFG intensity and shape dramatically change with d. For example, SFG intensity at 125 nm is approximately two orders higher than the thinner film and r+ and r+FR change to Z-shaped bipolar peaks while r− still retains its peak shape. The ssp-SFG spectrum at 220 nm becomes weaker and shows a similar shape to that at 0 nm but changes to a spectrum with completely opposite peak directions at 270 nm. The simulated sps-SFG spectra mainly show the r− mode and their spectral intensities and shape of the resonances are also strongly affected by d (Figure 2b). It is known that the ssp- and sps-polarized SFG signals from a monolayer on the gold substrate are very weak. As shown in Figure 2, one is able to detect these weak SFG signals by adjusting the thickness of the dielectric film. The present simulation results have been tested and proven by experimental observations, indicating the validity of the present model calculation.[39,52,53] Recently, other groups have also realized the importance of the interference effects in the quantitative analysis of SFG observations.[54,55] However, the present calculations for the L-factors under various conditions are very time consuming. To overcome this problem, we developed a general model program, which can calculate these values if the optical parameters such as refractive index, film thickness, and polarization combinations, are avail-

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able.[38,39] The program source code by Maple® is in the public domain. This provides important insights for understanding the nonlinear optical responses from any thin film systems such as soft matter interfaces.

Structural Evaluation of Organic Monolayers SFG is known to have intrinsic surface selectivity and submonolayer sensitivity, and is especially useful for revealing the chain orientation and conformational ordering/disordering in thin organic films. We have carried out orientational and conformational evaluation on many functional monolayers. Dye-sensitized solar cells (DSCs) have received extensive development efforts over the past two decades and now stand out as one of the most promising alternatives for photovoltaic solar energy conversion.[56–58] Many new dye molecules have been synthesized, while their monolayer structures on TiO2 and the correlation with the conversion efficiency as well as electron transfer dynamics are still under investigation. Figure 3 shows ssp- and sps-SFG spectra (2150– 2300 cm−1) of TiO2 surfaces in air after modification by the Zn–porphyrin (ZnP) derivatives CNMP (black) and CNBP (red) in (a,b) MeOH and (c,d) t-BuOH:MeCN.[34] After removing the Fresnel L-factors and fitting processes, a peak was resolved around 2230 cm−1, which can be assigned to the C≡N stretching mode of the aryl nitriles in the molecules. The peak position is quite close to species dissolved in solution, while that of the COOH group was found to change to COO−

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Fig. 3. (a) ssp- and (b) sps-SFG spectra of CNMP-sensitized ( ) and CNBP-sensitized ( ) TiO2 films with sensitization solvent of MeOH. (c) ssp- and (d) sps-polarized SFG spectra of CNMP-sensitized ( ) and CNBP-sensitized ( ) TiO2 films with sensitization solvent of a t-BuOH:MeCN mixture. Sensitization time is 1 h. Solid traces are fitted SFG curves.

(results not shown). One therefore expects that the molecules are bound to the TiO2 surface via the carboxylate group. It is interesting to note that the SFG spectra for films modified in MeOH are much weaker than those in the mixed solvent of t-BuOH:MeCN. Since the amounts of ZnP adsorbed were comparable in both solvents, this difference cannot arise from differences in ZnP coverage. On the other hand, adsorption of

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the MeOH solvent on the TiO2 surface was clearly observed by SFG measurement while no adsorption of t-BuOH or MeCN occurs for mixed solvents. It is proposed that the co-adsorbed MeOH reduces the conformational ordering of the ZnP monolayers on the TiO2 surface. The tilt angle for the ZnP in the monolayer was estimated from the ssp- and sps-SFG spectra. After comparing with the results of ultrafast electron transfer

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Interfacial Structure of Soft Matter by SFG Spectroscopy

Fig. 4. (a) Chemical structures of cationic surfactants used. (b) ssp-SFG spectra of DOAC, QDA, and QDE monolayers prepared with a pure water subphase. (c) ssp-SFG spectra of DOAC monolayers prepared with pure water, 1 mM NaCl, 1 mM NaBr, and 1 mM NaI subphases. (d) ssp-SFG spectra of DOAC–dSA mixed monolayers with different chain fractions of dSA. All samples were deposited onto CaF2 substrates at 25 mN/m and measured in air. All spectra are offset for clarity. Solid symbols are SFG data. Solid curves are spectral fits.

dynamics obtained independently, we found that the smaller the tilt angle, the longer the lifetime of the electrons in the conduction band. This reduces the recombination for the excited electrons in the conduction band, thus improving the conversion efficiency for the DSCs. These results suggest that the electron transfer between ZnP and TiO2 occurs “through space” rather than “through the molecular spacer”.[34] Another type of functional monolayer system investigated systematically in our group is cationic surfactants of quaternary ammonium derivatives (Figure 4a),[40,41] which are widely used as fabric softeners, germicides, and in colloidal stabilizers. In comparison with the long-chain dialkyl dimethyl ammonium halide DOAC, QDA and QDE have additional amide and ester groups, respectively, on their alkyl chains (Figure 4a).[41] Figure 4b shows ssp-SFG spectra in the C–H stretching region (2800–3000 cm−1) for monolayers of these surfactants deposited on the CaF2 surface by the LB method. The peaks at 2850 and 2920 cm−1 are attributed to the C–H symmetric (d+) and asymmetric (d−) stretching modes of the methylene groups, while the peaks at 2880, 2940 and 2960 cm−1 can be assigned to the r+, r+FR, and r− modes of the methyl group on the alkyl chains, respectively. The relative peak intensities of the CH2 group seem to decrease in the sequence of DOAC, QDA and QDE. SFG is extremely sensitive to the local symmetry of the hydrocarbon chains of the monolayers.[13–23] In a densely packed organic monolayer in which all the hydrocarbon chains should take the all-trans conformation, d+ and d− modes for the CH2 groups are SFG inactive and only those for the terminal CH3 groups are SFG active. SFG peaks for CH2 groups only appear when the centrosymmetric environment is broken, such as the appearance of gauche defects in the alkyl chain (the ratio d+/r+ can be regarded as a disorder factor). The present SFG results indicate that the presence of amide or ester groups on the alkyl chains can increase the conformational ordering of the

Chem. Rec. 2014, ••, ••–••

alkyl chains, reducing the gauche defects. The hydrogen bonding of the amide groups in the QDA monolayer, confirmed by SFG measurements in both the N–H stretching and amide I/II regions, is expected to play an important role here. In the case of QDE, the flexibility of the ester moieties on the alkyl chains is believed to induce the dense packing of the monolayer. Due to this flexibility, the alkyl chains of QDE are anticipated to be close to each other resulting in a strong van der Waals interaction. Such a strong van der Waals interaction should overcome the repulsive interaction between the positively charged headgroups and keep the monolayer structure densely packed and stable.[41] Secondly, the influence of the anion in the subphase on the monolayer structure of DOAC was carefully studied. Figure 4c shows the SFG spectra of DOAC monolayers prepared in pure water as well as 1 mM halide subphases. The relative intensity of the d+ mode decreases in the sequence of water, Cl−, Br− and I−, indicating an increase in the packing ordering of DOAC monolayers. The present results demonstrate that halide ions adsorbed at the monolayer can improve the packing of surfactant molecules by decreasing the repulsive interaction between the positively charged headgroups of the cationic surfactants.[41] Thirdly, the effect of the lateral interaction between alkyl chains on the monolayer structure was also investigated.[40] Figure 4d shows the SFG spectra in the C–H stretching region of the mixed monolayers with different chain fractions of deuterated stearyl alcohol (dSA) and DOAC. Since dSA was used, all C–H signals must arise from the DOAC in the mixed monolayer. When a small amount of dSA was introduced into the monolayer, for example, chain fraction for dSA, χdSA=0.14, d+ decreased while r+ changed only a little. At χdSA=0.60, the peak for d+ became difficult to see and thus the disorder ratio of d+/r+ was very small, indicating an all-trans chain

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Fig. 5. SFG spectra of arachidate LB multilayers with different structures on gold substrates. The polarization combination was ppp. The H layers that give SFG signals in the C–H stretching region are shown by arrows. See text for details.

conformation. Thus, the mixing of dSA can also significantly reduce the repulsive interactions between DOAC and increase van der Waals interactions, resulting in an increase in the molecular packing and conformational ordering of the mixed monolayer.[40]

Molecular Structure of Buried Interfaces SFG can be employed to probe not only material surfaces but also buried hetero-interfaces, which is very difficult to achieve using traditional spectroscopy. Figure 5 shows SFG spectra in the C–H stretching region (2800–3000 cm−1) for 11-layered LB films of arachidate on gold substrates having different layered structures of (a) DHD9, (b) D10H, and (c) DH10, where H and D stand for perprotonated and perdeuterated arachidate monolayers, respectively.[21] The SFG spectrum of D10H (Figure 5b) gives three peaks assignable to the r+, r+FR, and r− modes of the terminal CH3 groups in the topmost H layer. The buried H layer in DHD9 (Figure 5a) also gives the corresponding peaks in the spectrum. The slight shifts in peak frequency in the spectra most likely reflect the difference in chemical environments of the topmost and buried H layers. The weak CH2 vibration at 2850 cm−1 indicates that the molecular arrangement in the buried H layer is slightly distorted.

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It is important to note that SFG spectra are greatly affected by the absolute orientation for the molecules probed in the film. The C–H vibrations of the buried H layer, in which CH3 groups (DH9, Figure 5a) are directed toward the surface, give positive peaks, while the topmost H layer with the opposite orientation gives negative peaks (D10H, Figure 5b). On the other hand, DH10 (Figure 5c) has two SFG-active interfaces (i.e., top air/H and bottom H/D interfaces) and hence gives the derivative-like features that can be given by the sum of spectra (5a) and (5b). (2 ) 2 , the sign Since the SFG intensity is proportional to χijk (2 ) of the complex χijk is lost in the conventional SFG spectra and, thus, difficulties are encountered in determining the absolute orientation of the molecules from the SFG spectra. As shown in Figure 5, by using substrates such as gold and silver, which can (2 ) ) under the SFG produce a certain nonresonant signal ( χ NR excitation conditions, one is able to obtain the phase informa(2 ) (2 ) (2 ) by analyzing the interference between χijk and χ NR tion of χijk [13,21,53,59] in the SFG spectra. However, this requires a special substrate, which is not always possible for the sample of interest. Shen and co-workers proposed a general method, phasesensitive SFG measurement using narrowband picosecond (2 ) by mixing the wave of interest lasers, to deduce the phase of χijk with a reference wave of known phase such as crystalline quartz.[19] Based on this method, they successfully obtained the (2 ) complex χijk spectra at different interfaces, from which the (2 ) absorptive part of χijk (Im χijk(2) ) was resolved.(1)This quantity is comparable to the linear susceptibility ( Im χijk ) obtained from infrared absorption spectroscopy but with the dipole direction indicated by the sign of the spectrum. They demonstrated the advantages of this technique in probing the up/down orientation of the molecules on the interface. Recently, Yamaguchi and Tahara further developed the phase measurement for a multiplex scheme using broadband femtosecond lasers (heterodynedetected SFG), which has advantages in the spectral acquisition time and signal-to-noise ratio over the narrowband system.[20] Most of these studies are still limited to the liquid/air interface.[20,60–62] It is also interesting to know how SFG spectra would look when several interfaces are present in the system. Figure 6a gives such an example, where LB films of arachidate (DH)n(DD)mD (n=1, 2, 3, or 4, and m+n=4) on gold substrates were evaluated in the C–H stretching region.[37] In the model system, n DH bilayers with their CH3 groups pointing towards the substrate (indicated by arrows in inset of Figure 6a) are deliberately introduced to provide n SFG-active heterointerfaces in the C–H stretching region. The three peaks can be attributed to C–H stretching resonances of the terminal CH3 groups in the H layers. The H layers in the multilayers show upward-pointing peaks, similar to SFG spectrum (a) in Figure 5. The SFG intensity increases with the number of DH hetero-interfaces. More quantitatively, as shown in Figure 6b,

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Interfacial Structure of Soft Matter by SFG Spectroscopy

cadmium arachidate nanoparticles.[36]

during

the

formation

of

CdS

Molecular Structure of Functional Polymer Surfaces

Fig. 6. (a) SFG spectra of arachidate LB multilayers with different structures of (DH)n(DD)mD on gold substrates as a function of the number of HD interfaces n. (b) Normalized SFG intensities of the CH3 group of the LB multilayers on gold (squares) and fused quartz (circles) substrates. The lines in the figure are provided as a guide to the eye.

the SFG intensities are almost proportional to n on gold (squares), while they are proportional to n2 on fused quartz (circles). The n2 dependence on the fused quartz substrate, which has little or no nonresonance background, is easily predicted from the second order nonlinear optical response. On the other hand, the n dependence on gold is explained by taking into account the interference from the nonresonant background. Note that the results shown above (Figures 5 and 6) are for multilayer systems with a thickness much smaller than the wavelength of visible light. For thicker films, an additional optical interference effect arising from multiple reflection of light within the films should also be taken into account as we discussed above.[37] The present method has been successfully employed to determine the structural change in the LB multilayer of

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One of the hottest topics in the SFG study of soft matter is polymer surfaces, which have wide applications in different fields. We are especially interested in the relationship between functionality and surface structure of the functional polymer materials used in biocompatibility and energy conversion. A series of polyacrylate polymer surfaces have been examined by SFG spectroscopy both in air and in water. For example, the surface of poly(2-methoxyethyl acrylate) (PMEA) was examined to find the origin of its excellent compatibility with blood.[63] The C=O stretching mode of the carbonyl group on the PMEA surface, observed at 1740 cm−1 in air, was found to be shifted to 1722 cm−1 after contact with water, while such a shift was not observed in bulk PMEA film by conventional IR spectroscopy.[26] The shift was ascribed to the hydrogen bonding between water and the carbonyl group on the PMEA surface. Since this kind of surface hydrogen bond was not observed on similar polyacrylate polymers such as poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), which do not show such good blood compatibility, the hydrogen bonding on the PMEA surface should play an important role in blood compatibility.[26] Organic solar cells made of organic semiconductors are promising for solar energy conversion. It is hard to evaluate and control the interfacial structure of organic semiconductors, which is a key factor for improving the efficiency and durability of solar cells. For example, by mixing the electron donor poly(3-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the bulk heterojunction can be formed to improve the charge separation. Recently, Tajima et al. reported that by adding a small amount of a C60 derivative with a fluorocarbon chain (FCn, n=1–7), a surface segregation occurs, which can reduce the probability of recombination of the hole and electron.[64] Figure 7a shows SFG spectra (1300–1600 cm−1) of polymer electrode surfaces prepared under different conditions.[27] A PCBM cast-film surface shows two peaks at 1470 and 1430 cm−1, which can be assigned to the Ag(2) and F1u(4) modes of the C60 moiety (Figure 7b). When a small amount of FC7 is added, the intensity of the SFG peak at 1470 cm−1 significantly increases while that at 1430 cm−1 is almost the same. Since the vibrational properties of the C60 moiety in FC7 and PCBM obtained by the IR and Raman measurements are very similar, the difference between the SFG spectra in Figure 7a-i and 7a-ii can be attributed to the different surface structures of the PCBM and FC7/PCBM films. The SFG

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Fig. 7. (a) The ssp-polarized SFG spectra of PCBM films, as-cast FC7/PCBM films, FC7/PCBM films after plasma etching, and FC7 LB films. (b) Optimized structure of PCBM by density functional theory (DFT) calculation. Arrows represent the dipole derivative vectors of the vibrational modes at 1470 cm−1 (Ag(2) mode) and 1430 cm−1 (F1u(4) mode). (c) SFG peak intensity for the peak at 1470 cm−1 as a function of the number of fluorocarbons in FCn.

spectrum after the removal of the surface FC7 layer by Ar+ plasma etching gave further support for this attribution. As shown in Figure 7a-iii, after ∼2 nm of the surface of the FC7/ PCBM film had been etched away, the intensity of the peak at 1470 cm−1 decreased substantially and the SFG spectrum became similar to that of the pure PCBM film. These results again confirm that the SFG signals from the FC7/PCBM film are mainly contributed by FC7 molecules segregated on the FC7/PCBM surface. The FC7 structure on the surface of FC7/ PCBM film that produced the higher intensity of the peak at 1470 cm−1 was removed by Ar+ plasma etching and the newly produced surface consisting of pure PCBM had a structure similar to that of the spin-coated film of PCBM. FC7 thin layer prepared by the LB method (Figure 7a-iv) only shows one peak at 1470 cm−1, indicating that it is attributed to the well ordered structure of FC7 molecules on the surface. These SFG spectral changes correspond well to the surface segregation process of FC7 molecules to the surface of the FC7/PCBM film. Furthermore, polarization dependences indicated that the C60 moiety of the FC7 monolayer aligned on the surface in an orientation with the Ag(2) mode more perpendicular to the surface. Considering the relative directions of the dipole moment of the Ag(2) mode and fluorocarbon chains, one can expect that the fluorocarbon chains point to the air.[27] Figure 7c shows the intensity of the peak at 1470 cm−1 as a function of the number of fluorinated carbons. It can be seen that a longer fluorocarbon chain gives a higher intensity of the SFG signal from the vibrational mode corresponding to the Ag(2) mode. The DFT calculations show that the length of fluorocarbon chain does not significantly affect the IR and Raman intensities for these vibrational modes. Therefore, the increase in the peak intensity with the chain length should be attributed to the better packing of molecules with long chains

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owing to the stronger van der Waals interactions between the fluorocarbon chains, which can further improve the packing and orientation of the C60 moiety in FCn molecules. On the other hand, the molecules with an even number of fluorinated carbons give a higher signal intensity than those with an odd number; the reason for this is still unknown.[27]

Probing Structural Changes at Biointerfaces Although linear IR spectroscopy has the sensitivity to detect monolayers on metallic surfaces, its application to lowreflective surfaces is difficult. On the other hand, SFG can easily be applied to transparent surfaces as well. Figure 8 shows such an example. The samples used here were bilayers prepared by successively depositing deuterated and normal stearate on the flat plane of a hemicylindrical fused quartz prism (which is transparent for both visible and IR). To examine the bilayer in solution, both visible and IR beams were introduced from the prism side in the internal reflection geometry, close to the total internal reflection condition. As mentioned above, the L-factors (i.e., the intensity of the local electric field) increase significantly when total internal reflection occurs, and SFG signals at the interface can be considerably enhanced. Furthermore, by using an internal reflection geometry, IR absorption loss due to water can be avoided, supplying another advantage for the measurement of the solid/solution interface.[38,39] Figure 8a shows ssp-SFG spectra in the C–H stretching region of the stearate bilayer D/H on a fused quartz surface in an aqueous solution containing 0.2 mM Cd2+ (pH=6.7).[43] The bilayer is isotopically asymmetric, and hence two intense peaks assigned to the r+ and r+FR modes of the terminal CH3 groups in the top H layer are observed. It is interesting to note

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Interfacial Structure of Soft Matter by SFG Spectroscopy

Fig. 8. The ssp-polarized SFG spectra of the LB bilayer of deuterated (D) and regular (H) stearate on a fused quartz surface in solutions with (a) 0.2 and (b) 0 mM Cd2+ for various immersion times as shown in the figure (in min). (c) A schematic model for the structural change of the bilayer induced by Cd2+.

that SFG peaks become weaker with time, which implies some changes in molecular arrangement. A possible interpretation of this result is the partial flip of molecules in the topmost layer to form a HH bilayer on the underlying D monolayer, as schematically illustrated in Figure 8c. The HH bilayer is SFG inactive, and hence the flip reduces the SFG signals. This interpretation was supported by AFM observation of the bilayer, where many steps with bilayer height were observed on the surface. On the other hand, such a phase transition of the top layer hardly occurs when Cd2+ is absent from the solution, as seen in Figure 8b. This observation suggests that the flip of stearate is induced by Cd2+ through strong electrostatic interaction of the carboxylate group with Cd2+ as shown in Figure 8c.[43] Such a phase transition is difficult to probe by conventional IR and Raman spectroscopy. Recently, Conboy et al. investigated the structural changes of supported phospholipid bilayers on a fused quartz surface

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using in situ SFG spectroscopy.[65,66] They demonstrated that the flip-flop rate for the lipid bilayer of perprotonated and perdeuterated phospholipids can be characterized by measuring the decay process of the SFG signals observed from the methyl groups of the alkyl chains of the lipids at a certain temperature.[65,67] They also reported that the SFG intensity gave an SFG signal maximum near the melting point (Tm) of the lipid bulk and suggested that SFG spectroscopy could be employed to probe the Tm of the supported lipid bilayer,[66] while the structural model is different from that based on AFM observations[68,69] and our combined SFG–AFM observations, in which a layer-by-layer melting process has been proposed. It is an exciting prospect to be able to probe the structural changes during biological and/or chemical reactions on biomembrane surfaces. We carried out the first in situ SFG spectroscopy mechanistic study on the hydrolysis reaction of a planar supported-lipid bilayer that is catalyzed by the bee venom enzyme phospholipase A2 (PLA2).[44,70] The PLA2 enzyme is known to stereoselectively catalyze the hydrolysis of the sn-2 ester linkage of l-phospholipids in biomembranes to produce a fatty acid and a lysophospholipid; importantly, it exhibits no catalytic activity on the enantiomeric d-lipids. The enzyme plays important roles in the functionality of cell membranes, such as cell metabolism, inflammation and signal transduction.[71–73] Based on the high catalytic selectivity of PLA2 toward l-enantiomer lipids, supported bilayers made of different combinations of l- and ddipalmitoylphosphatidylcholine (DPPC) were prepared. All of the lipid bilayers used in this study were prepared with fully deuterated lipid molecules in one of the leaflets. Figure 9 shows the SFG spectra in the (a) C–H and (b) C–D stretching region for a supported l-DPPC (distal)/lDPPC-d75 (proximal) bilayer on a CaF2 substrate in contact with a Tris buffer solution.[44] Before PLA2 was introduced (t≤0 min), the peaks in both the C–H and C–D stretching regions decreased slowly with time, which is ascribed to the flip-flop of the lipid molecules between the two leaflets. Soon after PLA2 was introduced into the solution, the SFG peaks started to decrease quickly with time. Two stages seemed to appear. In the first 10 min after PLA2 introduction, r− decreases quickly while r+ remains almost constant. After that, the intensities of the two modes decrease rapidly in a similar fashion for both leaflets and finally disappear after ca. 30 min. Similar SFG spectral changes and time profiles were observed in the lipid bilayer prepared in the reverse sequence, i.e., l-DPPC-d75/l-DPPC. The two-stage changes of the SFG signals should be related to the lag–burst processes as reported previously. In the lag stage, the gauche defects decrease quickly after adding the enzyme. At the same time, the tilt angle of the hydrocarbon chains of DPPC decreases from 20° to 5° by the end of the lag stage. These results suggest that ordering of the DPPC molecules in the bilayer increases during the lag

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Fig. 9. SFG spectra of (a) C–H and (b) C–D stretching regions of an l-DPPC (distal)/l-DPPC-d75 (proximal) supported bilayer in Tris buffer solution with 3.5 μm PLA2 for different reaction times. The spectra are offset for clarity. All the results were obtained using a ppp polarization combination. (c) Left: the normalized SFG intensities for the r+ mode of CH3 (black) or CD3 (blue) groups in the two bilayer systems of d-DPPC/l-DPPC-d75 (squares) and l-DPPC-d75/d-DPPC (circles) as a function of reaction time. As a reference, a time profile (purple) was obtained for a d-DPPC/l-DPPC-d75 bilayer in a solution without PLA2. The green region indicates the lag phase of the hydrolysis reaction. The straight lines are shown as a guide. Right: the schematic model for the two bilayer systems of d-DPPC/l-DPPC-d75 (top) and l-DPPC-d75/d-DPPC (bottom). The SFG intensities were normalized to the SFG intensity at t=0 for clarity.

stage and is constant during the burst stage. No SFG peak for product was observed, suggesting that hydrolysis products desorb from the surface. Kinetic analysis of the time-dependent SFG signals shows that the reaction can be approximated by pseudo-first-order kinetics.[44] To understand the reaction mechanism of each leaflet in the bilayer, SFG characterizations were carried out based on the enantioselectivity of PLA2.[44,70] Figure 9c summarizes the SFG intensities of the r+ mode of CH3 and CD3 groups as a function of reaction time for the two bilayer systems of d-DPPC/lDPPC-d75 and l-DPPC-d75/d-DPPC. An d-DPPC/l-DPPCd75 bilayer in pure Tris buffer solution without PLA2 is shown as reference (its time profile is only determined by the flipflop). In the case of the d-DPPC/l-DPPC-d75 bilayer, both the proximal (d-DPPC, blue squares) and distal leaflets (l-DPPCd75, black squares) decay slowly in the burst regime. On the other hand, l-DPPC-d75/d-DPPC exhibits a significantly different behavior: the l-DPPC-d75 distal leaflet (black circles) shows a quite fast decay, the SFG signals becoming indistinguishable after ca. 30 min, similar to the case of l-DPPC/lDPPC-d75 (Figure 9a). However, the SFG signal of the d-DPPC proximal leaflet (blue circles) also decreases rapidly in the first 30 min. Nevertheless, as soon as the hydrolysis of the l-enantiomeric distal leaflet is complete, the decay rate becomes significantly slower. These results demonstrate that the dynamic hydrolysis reaction catalyzed by PLA2 starts in the distal leaflet. The

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hydrolysis products quickly leave the surface. The lipids in the proximal layer immediately flip up to the distal layer to maintain the bilayer structure (much faster than in the normal flip-flop), which is energetically stable. However, if the distal layer is made up of the d-enantiomer, toward which PLA2 is inactive, even l-enantiomers in the proximal layer cannot be hydrolyzed before they can flip up to the distal layer through the normal flip-flop process.[44] The SFG results have been further supported by in situ AFM observation under similar conditions, providing complementary information on the morphological changes of these lipid bilayers during the enzymatic reaction.[70] The lipid bilayer used above is regarded as a model system for the cell membrane. The degree of unsaturation and the chain length of the phospholipids can significantly influence the density, fluidity and phase-transition behaviors of the membranes. An appropriate ratio of saturated and unsaturated phospholipids is regarded as an important feature of a particular membrane. Since the stabilities of the saturated and unsaturated lipids are different with respect to oxidation, the functionality of the membranes could be considerably affected, especially in an environment contaminated by oxidants such as ozone, which is a universal air pollutant. In fact, we failed to prepare a stable monolayer of unsaturated lipid in our lab environment in the initial stage. Therefore, the stability and structure of single and binary mixed monolayers of an unsaturated lipid, DOPC, and a

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Fig. 10. The ssp-polarized SFG spectra of DPPC-d75/DOPC mixed monolayers at different molar ratios observed in the C–H stretching region for the DOPC component (a) before and (b) after being exposed to ozone (10–30 ppb). Open symbols are SFG data, and solid traces are fitting results. (c) Schematic showing the oxidation of DOPC and DPPC-d75/DOPC monolayers upon exposure to low levels of ozone.

saturated lipid, DPPC-d75, on the water surface were explored carefully, in an ambient environment containing a trace amount of ozone (10–30 ppb). Figure 10 shows SFG spectra of a DOPC monolayer prepared in (a) nitrogen and (b) low-level ozone environments (black squares).[42] The SFG spectrum after exposure to ozone is significantly different from that in nitrogen. The vinyl C–H stretching mode from the cis C=C bond at 3017 cm−1 disappears completely after ozone exposure. This is clear evidence that the C=C bond itself is attacked and oxidized by the ozone. All the SFG peaks from the CH3 group significantly decrease, while those of the CH2 group are still clearly observed. The large intensity decrease for the CH3 group is attributed to the decrease of DOPC coverage, due to its decomposition induced by ozone. The C=C bonds of the DOPC are cleaved and most of the oxidation products dissolve into the subphase, with only a small proportion of them (randomly) staying on the water surface (Figure 10c, top). On the other hand, the saturated DPPC-d75 shows very high stability in both nitrogen and low-level ozone environments. Furthermore, we studied the structure and stability of mixed monolayers of DOPC and DPPC-d75 in different ratios (Figure 10, red circles and blue triangles).[42] The disorder factor, deduced from the d+/r+ ratio, for the DOPC in the mixed monolayer clearly decreases, indicating that conformational ordering of DOPC is largely improved by mixing with saturated lipids. It is interesting to note that the SFG spectra for DPPC-d75/DOPC (1:3 and 3:1) after exposure to ozone (Figure 10b, red circles and blue triangles) are quite similar to that of DPPC-d75/DOPC (3:1) in nitrogen (Figure 10a, blue triangles). This indicates that part of the unsaturated DOPC still remains on the water surface if mixed with saturated lipids,

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even after exposure to ozone (Figure 10c, bottom). It is anticipated that the saturated DPPC components in the mixed monolayer can partially protect the DOPC molecules from ozone oxidation. The present results suggest that the unsaturated phospholipids can be selectively oxidized by a trace amount of ozone in the ambient environment, while the oxidation of DOPC is partially inhibited by the presence of DPPC in the mixed monolayers.[42] We are continuing to study the structure and stability of different unsaturated lipid molecules in a variety of environments.

Summary and Outlook As shown above, SFG vibrational spectroscopy can provide valuable and unique information on the molecular structures at surfaces and interfaces of soft matter. Much novel structural information on material surfaces becomes available at a molecular level, which turns out to be vital for understanding and controlling the surface functionality of soft matter. Although the theoretical background of SFG has already been established, SFG processes are complicated and difficulties are often encountered in interpreting the observed spectra. A phenomenological explanation of SFG spectra derived from linear spectroscopy can lead to mistakes. Careful analysis based on simulation is required. A combination of experimental and theoretical approaches has been successfully demonstrated in the study of water structures at interfaces.[74,75] The combination of SFG with other techniques including IR and Raman spectroscopies as well as scanning probe microscopy (STM and AFM), can help us to achieve a comprehensive understanding of the relationship between surface structure and reactivity/functionality.

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It is also important to appreciate the limitations of SFG spectroscopy. For example, it is impossible to obtain SFG signals if there is no preferred orientation on the interface. Standard SFG measurements will not have surface specificity for chiral systems since their bulk phase lacks inversion symmetry. However, many biological systems including proteins show chiral properties. Fortunately, some recent work shows that the chiral structure of protein molecules can be successfully probed by SFG if one carefully utilizes accessible polarization combinations.[76–79] Furthermore, several novel techniques, such as phase-sensitive SFG and 2D-SFG approaches, have been developed for SFG studies. These techniques are expected to provide complementary or indeed completely novel information compared to traditional SFG methods, although in these cases the optical systems become even more technically complex and difficult.[19,20,24] It is anticipated that a bright future exists for understanding all the physical processes and chemical reactions that occur on soft matter interfaces at a molecular level with the help of SFG vibrational spectroscopy.

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Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination Program” 759 (24108701) and a Grant-in-Aid for Scientific Research (B) (23350058) from the Ministry of Education, Culture, Sports, Science & Technology (MEXT), Japan.

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