Tracking Molecular Aggregates at a Liquid Interface by Nonlinear

Article pubs.acs.org/JPCC

Tracking Molecular Aggregates at a Liquid Interface by Nonlinear Correlation Spectroscopy Pierre-Marie Gassin,†,‡ Gaelle Martin-Gassin,§ Emmanuel Benichou,*,‡ and Pierre-François Brevet‡ †

Institut de Chimie Séparative de Marcoule, UMR 5257 (CEA-CNRS-UM2-ENSCM), B.P. 17171, 30207 Bagnols sur Ceze Cedex, France § Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Agrégats, Interfaces et matériaux pour l’énergie, C. C. 1502, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France ‡ Institut Lumière Matière, UMR 5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne Cedex, France S Supporting Information *

ABSTRACT: An analytical model is developed to analyze the fluctuations in the surface second harmonic generation (SHG) intensity from a Langmuir film. From the SHG autocorrelation analysis, also called nonlinear correlation spectroscopy (NLCS), the characteristic times of the phenomena occurring at the interface are determined. This method is then applied to monolayers of the amphiphilic chromophore dye 4-(4dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA) at the air−water interface. This compound is known to spontaneously form aggregates at liquid interfaces. The aggregates characteristic size is estimated using the newly developed NLCS method and compared to Brewster angle microscopy (BAM) data for two experimental initial conditions leading to two different sizes, namely, a chloroform and a methanol DiA solution. This study combining the advantages of the surface specificity of SHG with the temporal analysis of intensity correlations demonstrates the potential of NLCS to investigate the dynamics of molecular aggregates at a liquid interface.

■

INTRODUCTION The formation of supramolecular assemblies is a promising way in nanotechnology to texture surfaces.1,2 In a “bottom-up” approach, the objective is to form well-defined supramolecular assemblies bearing different functionalities incorporated into molecular devices. Such molecular assemblies have been proven of utmost importance for many different applications ranging from organic electronics3 to photovoltaic devices.4 One way to perform this surface texturing is the Langmuir technique where supramolecular assemblies are formed at the air−water interface, either as two-dimensional (2D) or three-dimensional assemblies (3D), with films in collapsing conditions in the latter case.5 It has also been demonstrated that the technique allows the formation of supramolecular chiral structures at the air− water interface.6−8 However, in order to access quantitative information on these supramolecular assemblies, it is often necessary to study these aggregating structures in situ at the liquid interface. Therefore, it is mandatory to employ noninvasive experimental techniques sensitive to the organization and morphology of the molecular aggregates as well as their dynamics. Sensitive tools with local resolution such as scanning tunneling microscopy (STM)9−11 or scanning force microscopy (SFM)12,13 are particularly efficient to probe the molecular organization at the nanometer scale. However, they cannot be applied in a straightforward manner at the air−water interface in a Langmuir trough. Optical methods such as Brewster angle microscopy (BAM) can be used to investigate these interfaces © 2013 American Chemical Society

with successful examples, for example, the study of Haggregates morphology14 or the chiral character of molecular aggregates.15 This technique is nevertheless limited to the micrometer scale. Surface second harmonic generation (SHG) has also proven in the past to be a powerful surface sensitive tool to investigate molecular monolayers at the air−water interface.7,8,16 This technique, based on the conversion of two photons at a fundamental frequency ω into one photon at the harmonic frequency 2ω, is surface sensitive at interfaces between two centrosymmetric media. Indeed, within the electric dipole approximation, no second harmonic (SH) light can be generated in the bulk of media possessing inversion symmetry like gases and liquids. As a result, SH light can only be produced at the interface between those two media where the inversion symmetry is broken. The technique is noninvasive and surface specific and can be used to investigate both the structure and the dynamics displayed at these surfaces and interfaces.17−21 The dynamical studies entail in particular the use of time-resolved approaches only limited in time by the laser pulse duration. Besides air−liquid interfaces,22 buried interfaces, such as liquid−liquid23 or solid−liquid interfaces,24 can be investigated at the molecular level. Its combination with a Langmuir trough allows furthermore nonlinear optical studies with a precise control on the average surface density of the Received: November 19, 2013 Revised: December 18, 2013 Published: December 19, 2013 1135

dx.doi.org/10.1021/jp411373v | J. Phys. Chem. C 2014, 118, 1135−1141

The Journal of Physical Chemistry C

Article

amphiphilic compounds spread out at the liquid surface.7,16,25 However, the SHG technique does not provide any direct local information on the heterogeneity of the interface since the signal is averaged over the whole illuminated area. To reintroduce this ability, and in particular the possibility to access the aggregate size and dynamics at the interface, further refinement must be developed. One approach is to monitor the intensity fluctuations. The analysis of signal fluctuations is performed in the technique of fluorescence correlation spectroscopy (FCS), a well-known method to address a wide range of time scales, from nanoseconds up to seconds. With this broad range of time scales, information on processes such as molecular diffusion, molecular rotation, or photophysical processes can be inferred. Nowadays, numerous applications are found in biology, for example, notably for the study of the protein motion in cells,26 in chemistry for following chemical reaction kinetics,27,28 or in physics for monitoring flow profiles in microcapillaries.29,30 Based on this concept of the optical signal autocorrelation analysis, other methods have been developed such as twophoton fluorescence correlation spectroscopy,31 linear reflectance correlation spectroscopy,32 Raman correlation spectroscopy,33 or coherent anti-Stokes Raman scattering (CARS) correlation spectroscopy,34,35 for instance. Pioneering works have also aimed at coupling SHG with an intensity autocorrelation analysis to probe rotational dynamics36 or phase transition on a Langmuir film.37,38 It has been demonstrated very recently that SH correlation spectroscopy can be used to determine surface binding kinetics.39 Those studies have then lead to the extraction of the characteristic time of an interfacial molecular dynamics process, even though no theoretical framework was available yet to quantitatively analyze the SHG autocorrelation traces. Indeed, analytical models available in the case of FCS27,40 or two-photon FCS31 cannot be applied in a straightforward manner to the case of SHG, especially because the phenomenon involves three photons in a coherent way. In the present work, we provide a remedy to this lack and present the theoretical framework required to perform the quantitative analysis of the SHG autocorrelation traces. We thus demonstrate the potentiality of the correlation analysis technique applied to the SHG process with two related examples. Our work is somehow related to the recently published theory describing the case of coherent second and third harmonic generation applied to study the nanoparticles mobility in a bulk solution.41 However, here, we derive our theory to describe the case of surface SHG. The surface SHG autocorrelation trace is thus evaluated in the case of 2D displacements on the interface, taking into account surface diffusion and hydrodynamics. In this work, we focus on diffusion and convection and do not address other phenomena such as orientational changes, usually associated with phase transitions. This new theoretical framework is subsequently applied to the behavior analysis of the amphiphilic chromophore dye 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA) in a Langmuir film at the air−water interface. This molecular compound is known to form spontaneously chiral domains and aggregates at the air−water interface.42,43,7 By combining surface pressure measurements, Brewster angle microscopy (BAM) imaging, and therefore SHG autocorrelation analyses, the heterogeneity introduced by the dye aggregates is quantitatively studied at the liquid interface. The DiA dye is spread from a chloroform or a methanol

solution, and it appears that the solvents drive different heterogeneous states at the interface. A. Theoretical Basis. The general framework presented in this section is developed in order to analyze the SHG autocorrelation traces obtained from a fluctuating SHG intensity. This theoretical model only takes into account intensity fluctuations originating from surface density modifications under the laser spot. Two main phenomena are usually introduced to interpret these surface density fluctuations: the 2D diffusion of the SHG sources at the interface and the global flow of these sources induced by convection. This model is quite general and can be applied for various systems confined at a liquid interface, i.e., molecules, nanoparticles, or molecular aggregates. In the general case, C(r,⃗ t) is taken as the surface concentration of the species, here understood as pointlike SHG sources, generating an SHG signal at the position r ⃗ = uxex⃗ + uyey⃗ where (ux, uy) are the 2D coordinates in the plane of the interface and ex⃗ and ey⃗ are the two corresponding perpendicular unit vectors. A fluctuation at time t and location r ⃗ in this surface concentration is thus given by: δC( r ⃗ , t ) = C( r ⃗ , t ) − ⟨C⟩

(1)

where ⟨C⟩ is the average in space and time of this concentration. We then assume a global flow in the y direction, namely, V⃗ = Vey⃗ . The intensity profile of the laser beam is assumed to be Gaussian TEM00 and given by w(r)⃗ = I0 exp[−2(r2/r02)], where r0 is the laser beam waist. Following Geissbuehler et al., the SHG intensity is given by41 ISHG(t ) = K |

∫ χ (2) ( r ⃗ , t ) w( r ⃗) d r |⃗ 2

(2)

where K is a global constant containing all of the geometrical and optical constants related to the experimental geometry and the medium properties. χ(2) is the second-order electric susceptibility tensor depending a priori on space and time. With these initial assumptions, it is then possible to develop the analytical model (see the Supporting Information) yielding the following expression for the SHG autocorrelation function: τ 2

GSHG(τ ) = 1 +

(

N 1+ +

8e

τ 2

τ

4e−( τf ) [1/(1 + τD )] τ τD

)

+

(

N2 1 +

−2( ττf )2 [1/(1 + 2 ττf )]

(

3

N 1+

τ 2τ D

)

+

τ

8e−2( τf ) [1/(1 + τD )]

32e

τ τD

2

)

− 43 ( ττf )2 [1/(1 + 43 ττf )]

(

3N 2 1 +

4τ 3τD

) (3)

This correlation function depends only on three parameters, namely, N as the average number of aggregates in the spot size and τD and τf respectively as the characteristic diffusion time and the characteristic flow time. Figure 1 shows a typical evolution of this function for the case of a pure diffusive motion obtained when τD ≪ τf, a pure flow motion obtained when τD ≫ τf, and a mixed diffusive and flow motion of the molecular aggregates.

■

MATERIALS AND METHODS A. Molecular Film Preparation. The SHG experiments were performed with the amphiphilic molecular dye DiA (Molecular Probes) used without further purification. DiA was dissolved in chloroform or methanol, these two solvents inducing different aggregation states and therefore heterogeneity at the air−water interface. The Langmuir films were 1136



■

Article

RESULTS AND DISCUSSION

A. Surface Pressure Isotherm Analysis. An analysis of the DiA film surface pressure isotherm was first performed during its compression. Figure 2 shows the surface pressure isotherms for the two solvents used to spread the DiA molecules onto the air−water interface, namely, methanol and chloroform.

Figure 1. Theoretical SHG autocorrelation traces as a function of the time lag for tree cases: (a) a diffusive motion with τD = 0.1 s, τf = infinity, and N = 1 (red dashed line); (b) a flow motion with τD = infinity, τf = 0.1 s, and N = 1 (blue dotted line); and (c) a mixed diffusive and flow motion with τD = 0.1 s, τf = 0.1 s, and N = 1 (black line).

prepared using a standard Langmuir trough with a maximum surface area of 100 cm2 (Nima Technology, model 601). The trough was fitted with a Wilhelmy plate to record the pressure− area isotherms during the film compression at constant temperature. All experiments were carried out at 18 °C to reduce the evaporation rate and the subsequent laser beam focusing conditions loss. Ultrapure water (Millipore 18 MΩ· cm) was used as the subphase. A 10 μL glass syringe was used to disperse a known amount of DiA molecules uniformly on the water surface. In all cases, 10 nmol of DiA was spread onto the air−water interface. After a 5 min rest time allotted for solvent evaporation, the monolayer was slowly compressed at a rate of 5 cm2/min down to the minimum area of 15 cm2. During this compression procedure, 10 stops were applied at various surface coverages. The duration of each stop was 800 s during which time the SHG signal and the SHG autocorrelation traces were recorded. B. Brewster Angle Microscopy. The Langmuir film BAM images were recorded in the same experimental conditions as depicted in preceding text with a KSV-Nima apparatus, model microBAM, at the fixed angle of 53°. All images presented in this work are 2000 × 2000 μm2. C. Second Harmonic Generation. The SHG setup was built above the Langmuir trough. It was based on a femtosecond Ti−sapphire oscillator laser source providing pulses with duration of 70 fs at a repetition rate of 82 MHz (Spectra Physics, model Tsunami). After passing through a lowpass filter to remove any unwanted harmonic light generated prior to the interface, the fundamental beam set to a wavelength of 800 nm and an averaged power of about 500 mW was focused by a microscope objective (X20, NA 0.5) onto the air− water interface. The spot size on the interface was estimated to be about 1000 μm2. The SH beam was collected in reflection by the objective and was separated from the fundamental beam with a dichroic mirror and a spectrometer set on the SH wavelength. The SH light was detected with a photomultiplier tube. The photomultiplier tube was feeding a correlator to get the autocorrelation function of the SHG intensity (Flex02-12D, multiple Tau correlator). The correlator yielded both the intensity as a function of time and the autocorrelation function simultaneously.

Figure 2. DiA surface pressure isotherms obtained for two spreading solvents: methanol (dashed blue curve) and chloroform (continuous gray curve). The letters a, b, and c refer to the pressure where the BAM pictures were taken. The black curve corresponds to the surface pressure isotherm obtained during compression with several stops (each stop is about 800 s) performed to record the SHG intensity and autocorrelation traces.

The surface pressure isotherms of the DiA monolayers for different spreading conditions have been reported before.42 When methanol is used as the spreading solvent, the surface pressure increases at an area per molecule of about 50 Å2. This a value significantly shifted from the corresponding value of about 100 Å2 obtained with chloroform. This shift arises from the large micrometer-scale molecular aggregate formed with methanol as the spreading solvent. The formation of these large aggregates also explains why these measurements can be difficult to reproduce sometimes. These large molecular aggregates can be observed directly with the BAM technique. Figure 3 presents BAM pictures obtained at several stages of

Figure 3. BAM image of the DiA monolayer obtained at three different stages of the monolayer compression (see also Figure 2). DiA was initially spread in methanol. The size of each picture is 2 mm × 2 mm.

the monolayer compression when methanol is used as the spreading solvent. The molecular aggregates can be clearly identified in Figure 3a. Their size is about 10−100 μm in diameter, and they continuously grow during the monolayer compression; see Figure 3b. At high compression, a nearly 1137



Article

Figure 4. On the left, SHG intensity recorded during several stages of the SiA film compression, namely, 5, 15, 25, and 35 mN/m. On the right, the corresponding SHG autocorrelation traces (black line) and the adjustment using eq 3 (red line).

homogeneous DiA film is obtained at the interface; see Figure 3c. In the case of chloroform as the spreading solvent, no large molecular aggregates are observed in BAM even if, at this stage, it is not possible to demonstrate the lack or presence of nanometer-scale aggregates due to the limited spatial resolution of the technique. B. NLCS Analysis for Chloroform as Spreading Solvent. The SHG autocorrelation traces were first analyzed in the case of the chloroform solvent. The SHG intensity and its autocorrelations traces are presented in Figure 4 for several stages of the DiA film compression, as previously explained for Figure 2. Those experimental data were fitted with eq 3, and the value of the three parameters τD, τf, and N were obtained from the adjustment procedure. The extracted values for these parameters are give in Supporting Information and are discussed in the following paragraphs. First, the parameter N increases during the film compression. This parameter can be interpreted as the average number of molecular aggregates illuminated by the laser spot. The increase of this number simply indicates that the molecular aggregate

density increases under the laser spot. The value of the autocorrelation function at the origin of times, namely GSHG(0), scales inversely with this parameter N explaining the flattening of the SHG autocorrelation traces during the film compression; see Figure 4. This feature is rather standard in correlation analyses. The evolution with the film compression for the two other parameters, namely, τD and τF, is reported in Figure 5. The diffusion time is more or less constant with a value of about 0.3−0.5 s during the film compression except at very high surface densities, whereas the flow time presents an abrupt transition. This step-like transition in the flow time indicates the disappearance of the flow motion altogether at high surface densities, at areas per molecule lower than 60 Å2. As indicated in Figure 6, whereas it is necessary to introduce the two motions to adjust the experimental autocorrelation traces at low surface densities (see Figure 6a), the flow time is not required to correctly adjust the data at higher densities (see Figure 6b). Hence, we have arbitrarily fixed this flow time to 10 s, a value that can be considered as a lower limit for the neglect of the flow time as compared to the diffusion time. As a result, a flow 1138



Article

C. NLCS Analysis for Methanol as Spreading Solvent. The same analysis was performed using methanol instead of chloroform as the spreading solvent for DiA. Figure 7 shows

Figure 5. Diffusion time τD and flow time τF as a function of the DiA monolayer compression.

time of 100 s or more will give the same behavior. Thus, for an area per molecule larger than 60 Å2, the autocorrelation trace exhibits a mixed diffusive and flow behavior. Therefore, at areas per molecule lower than 60 Å2, the autocorrelation function exhibits a pure diffusive behavior. The origin of this flow motion and its disappearance at high density are not clearly established. It may be explained by the laser heating of the interface inducing the aggregate motion as proposed recently by Backus et al.44 These authors have also similarly reported the sudden freezing of the flow motion with the measurement of the sum frequency generation signal from phospholipid monolayers. Finally, it was also possible to estimate the size of the molecular aggregates from the values of the diffusion time using the 2D Stokes−Einstein relation. This equation links the diffusion coefficient D to the radius R of the aggregates as follow: D=

kBT 4πηR

Figure 7. SHG autocorrelation traces obtained for different DiA films using methanol as the spreading solvent. The letters a, b, and c refer to the pressure stops shown in Figure 3 and where the SHG intensity and the corresponding autocorrelation traces were recorded.

typical autocorrelation traces obtained at several stages of the monolayer compression. The extracted values of the three parameters are given in the Supporting Information. The main conclusion drawn, similarly to the case of chloroform, is that the flow motion is frozen at high surface densities. Nevertheless, a large shift is observed in the characteristic times obtained for both the diffusion and flow times. In these experimental conditions, the characteristic times for diffusion are about 100−1000 s. Applying the 2D Stokes− Einstein relation, a molecular aggregate size of about 10−100 μm is obtained. It is noticeable that, in this experiment, we measure aggregate gyration radii. This size corresponds to the size detection limit, which is typically the diameter of the laser spot. Beyond this limit, only orders of magnitudes can be provided mainly due to poor statistics. Those aggregate sizes are in good agreement with the size observed on the BAM images and presented in Figure 3.

(4)

where kB is the Boltzmann constant, η the viscosity of the water subphase, and T its temperature. The diffusion coefficient D is inversely proportional to τD (D = r02/4D, ro representing the waist of the Gaussian beam). As indicated previously, in the case of dilute surfaces, the diffusion time is more or less constant at about 0.3 s. Using the viscosity of water, a typical aggregate size is estimated to be about 10 nm.

■

CONCLUSION We have proposed a new NLCS method to investigate molecular dynamics at liquid interfaces by SHG in a quantitative way. We have applied the method to monitor

Figure 6. (a) Experimental autocorrelation trace recorded for at 5 mN/m (black line). The red line is an adjustment with a mixed diffusive and flow model. The dotted green line is a fit obtained with a purely diffusive motion and the dashed blue line is a fit obtained with a pure flow motion. (b) Same autocorrelation trace and analysis performed at 35 mN/m. 1139



Article

(4) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834−2860. (5) Rubia-Paya, C.; Jimenez-Millan, E.; Giner-Casares, J. J.; Brezesinski, G.; Martin-Romero, M. T.; Camacho, L. From TwoDimensional to Three-Dimensional at the Air/Water Interface: The Self-Aggregation of the Acridine Dye in Mixed Monolayers. Langmuir 2013, 29, 4796−4805. (6) Xu, Y.; Rao, Y.; Zheng, D.; Guo, Y.; Liu, M.; Wang, H. Inhomogeneous and Spontaneous Formation of Chirality in the Langmuir Monolayer of Achiral Molecules at the Air/Water Interface Probed by in Situ Surface Second Harmonic Generation Linear Dichroism. J. Phys. Chem. C 2009, 113, 4088−4098. (7) Martin-Gassin, G.; Benichou, E.; Bachelier, G.; Russier-Antoine, I.; Jonin, C.; Brevet, P. F. Compression Induced Chirality in Dense Molecular Films at the Air−Water Interface Probed by Second Harmonic Generation. J. Phys. Chem. C 2008, 112, 12958−12965. (8) Benichou, E.; Derouet, A.; Russier-Antoine, I.; Jonin, C.; Lascoux, N.; Liu, M.; Brevet, P. F. Supramolecular Chirality at the Air/Water Interface. Opt. Mater. Express 2011, 1, 17−26. (9) De Feyter, S.; Gesquière, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schyver, F. C. Homo- and Heterochiral Supramolecular Tapes from Achiral, Enantiiopure, and Racemic Promesogenic Formamides: Expression of Molecular Chirality in Two and Three Dimensions. Angew. Chem., Int. Ed. 2001, 40, 3217−3220. (10) Giancarlo, L. C.; Flynn, G. W. Raising Flags: Applications of Chemical Marker Groups to Study Self-Assembly, Chirality, and Orientation of Interfacial Films by Scanning Tunneling Microscopy. Acc. Chem. Res. 2000, 33, 491−501. (11) Zhang, J.; Gesquière, A.; Sieffert, M.; Klapper, M.; Müllen, K.; De Schyver, F. C.; De Feyter, S. Losing the Expression of Molecular Chirality in Self-Assembled Physisorbed Monolayers. Nano Lett. 2005, 5, 1395−1398. (12) Leveiller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Alsnielsen, J. Crystallinity of the Double-Layer of Cadmium Arachidate Films at the Water-Surface. Science 1991, 252, 1532−1536. (13) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Spontaneous Chiral-Symmetry Breaking by Achiral Molecules in a LangmuirBlodgett-Film. Nature 1994, 368, 440−443. (14) Jimenez-Millan, E.; Giner-Casares, J. J.; Munoz, E.; MartinRomero, M. T.; Camacho, L. Self-Assembly of Acridine Orange into H-Aggregates at the Air/Water Interface: Tuning of Orientation of Headgroup. Langmuir 2011, 27, 14888−14899. (15) Jiménez-Millan, E.; Giner-Casares, J. J.; Martín-Romero, M. T.; Brezesinski, G.; Camacho, L. Tuning of the Hydrophobic and Hydrophilic Interactions in 2D Chiral Domains. J. Phys. Chem. C 2012, 116, 19925−19933. (16) Martin-Gassin, G.; et al. Palmitateluciferin: A Molecular Design for the Second Harmonic Generation Study of Ion Complexation at the Air−Water Interface. J. Phys. Chem. C 2012, 116, 7450−7456. (17) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (18) Shen, Y. R. Surface Second Harmonic Generation: A New Technique for Surface Studies. Annu. Rev. Mater. Sci. 1986, 16, 69−86. (19) Heinz, T. F., Modern Problems in Condensed Matter Science; North Holland: Amsterdam, 1991. (20) Corn, R. M.; Higgins, D. A. Optical Second Harmonic Generation as a Probe of Surface Chemistry. Chem. Rev. 1994, 94, 107−125. (21) Brevet, P. F. Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications; Marcel Dekker: New York, 2001. (22) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Resonant Second Harmonic Generation Studies of p-Nitrophenol Adsorption at Condensed-Phase Interfaces. Langmuir 1992, 8, 1994− 2000. (23) Eisenthal, K. B. Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy. Chem. Rev. 1996, 96, 1343−1360.

the motion of DiA aggregates at the air−water interface for two different spreading conditions. At low molecular densities, it is shown two characteristic times coexist corresponding to a global flow on the surface and a diffusive motion of the aggregates. When the monolayers are compressed to higher densities, a transition is observed from this two-motion dynamics to a single-motion one where the global flow motion has disappeared and only the diffusion motion survives. The molecular aggregate size is also estimated from the NLCS analysis and a simple 2D Stokes−Einstein model. Aggregate sizes ranging from the nanometer to the micrometer scale are determined from the characteristic diffusion time. The micrometer sizes determined from NLCS are furthermore in agreement with the sizes observed with BAM imaging, giving a quantitative analysis of the heterogeneity of the interface. The NLCS technique applied to surface SHG thus opens up a new route to investigate the interfacial dynamics at liquid interfaces. The model presented in this work provides the necessary framework to quantitatively analyze the SHG autocorrelation traces. It is expected that other more complex phenomena present on liquid interfaces such as molecular reorientation,36 complexation,16 or supramolecular chirality appearance7,8 will be accessed similarly to diffusion and flow in future studies.

■

ASSOCIATED CONTENT

* Supporting Information S

Text describing the theoretical framework and fitting parameters and tables listing N, τD, and τF under different solvent conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge Olivier Diat and Pierre Bauduin for stimulating discussions, the support from the Centre of Nano-Optics NANOPTEC of the University Claude Bernard Lyon 1 to perform these studies, and the financial support of the French National Agency for Research under Project ILLA ANR-12-BS08-0021-02.

■

ABBREVIATIONS FCS, fluorescence correlation spectroscopy; BAM, Brewster angle microscopy; NLCS, nonlinear correlation spectroscopy; SHG, second harmonic generation; DiA, 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide.

■

REFERENCES

(1) Lehn, J.-M. Perspectives in Supramolecular Chemistry-from Molecular Recognition Towards Molecular Information Processing and Self-Organization. Angew. Chem., Int. Ed. 1990, 29, 1304−1319. (2) Vos, J. G.; Forster, R. J.; Keyes, T. E. I., Interfacial Supramolecular Assemblies. John Wiley & Sons: New York, 2003. (3) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. 1140



Article

(24) Zaera, F. Probing Liquid/Solid Interfaces at the Molecular Level. Chem. Rev. 2012, 112, 2920−2986. (25) Matar, G.; Duboisset, J.; Benichou, E.; Bachelier, G.; RussierAntoine, I.; Jonin, C.; Ficheux, D.; Brevet, P. F.; Besson, F. Second Harmonic Generation, a New Approach for Analyzing the Interfacial Properties of a Short Tryptophan-Rich Peptide. Chem. Phys. Lett. 2010, 500, 161−166. (26) Lippincott-Schwartz, J.; Snapp, E.; Kenworthy, A. Studying Protein Dynamics in Living Cells. Nat. Rev. Mol. Cell Biol. 2001, 2, 444−456. (27) Elson, E. L.; Magde, D. Fluorescence Correlation Spectroscopy. 1-Conceptual Basis and Theory. Biopolymers 1974, 13, 1−27. (28) Magde, D.; Webb, W. W.; Elson, E. Thermodynamic Fluctuations in a Reacting SystemMeasurement by Fluorescence Correlation Spectroscopy. Phys. Rev. Lett. 1972, 29, 705−708. (29) Lenne, P. F.; Colombo, D.; Giovannini, H.; Rigneault, H. Flow Profiles and Directionality in Microcapillaries Measured by Fluorescence Correlation Spectroscopy. Single Mol. 2002, 3, 194−200. (30) Lakowicz, J. Principles of Fluorescence Spectroscopy; Springer: New York, 2006; pp 798−840. (31) Berland, K. M.; So, P. T. C.; Gratton, E. 2-Photon Fluorescence Correlation SpectroscopyMethod and Application to the Intracellular Environment. Biophys. J. 1995, 68, 694−701. (32) Martin-Gassin, G.; El Harfouch, Y.; Benichou, E.; Bachelier, G.; Russier-Antoine, I.; Jonin, C.; Roux, S.; Tillement, O.; Brevet, P. F. Correlation Reflectance Spectroscopy of Heterogeneous Silver Nanoparticles Films Upon Compression at the Air/Water Interface. J. Phys.: Condens. Matter 2008, 20, 055228-8. (33) Schrof, W.; Klingler, J. F.; Rozouvan, S.; Horn, D. Raman Correlation Spectroscopy: A Method for Studying Chemical Composition and Dynamics of Disperse Systems. Phys. Rev. E 1998, 57, R2523−R2526. (34) Cheng, J. X.; Potma, E.; Xie, S. X. Coherent Anti-Stokes Raman Scattering Correlation Spectroscopy: Probing Dynamical Processes with Chemical Selectivity. J. Chem. Phys. 2002, 106, 8561−8568. (35) Hellerer, T.; Schiller, A.; Jung, G.; Zumbusch, A. Coherent AntiStokes Raman Scattering (Cars) Correlation Spectroscopy. ChemPhysChem 2002, 7, 630−633. (36) Yan, E. C. Y.; Eisenthal, K. B. Rotational Dynamics of Anisotropic Microscopic Particles Studied by Second Harmonic Generation. J. Phys. Chem. B 2000, 104, 6686−6689. (37) Zhao, X. L.; Eisenthal, K. B. Monolayer Orientational Fluctuations and a New Phase-Transition at the Air-Water Interface Detected by 2nd Harmonic Genaration. J. Chem. Phys. 1995, 102, 5818−5826. (38) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Orientational Fluctuations and Phase Transitions of Long Chain Molecules at the Air/Water Interface. Phys. Rev. Lett. 1991, 67, 2025−2028. (39) Sly, K. L.; Mok, S. W.; Conboy, J. C. Second Harmonic Correlation Spectroscopy: A Method for Determining Surface Binding Kinetics and Thermodynamics. Anal. Chem. 2013, 85, 8429−8435. (40) Magde, D.; Webb, W. W.; Elson, E. L. Fluorescence Correlation Spectroscopy. 3. Uniform Translation and Laminar Flow. Biopolymers 1978, 17, 361−376. (41) Geissbuehler, M.; Bonacina, L.; Shcheslavskiy, V.; Bocchio, N. L.; Geissbuehler, S.; Leutenegger, M.; Marki, I.; Wolf, J. P.; Lasser, T. Nonlinear Correlation Spectroscopy (NLCS). Nano Lett. 2012, 12, 1668−1672. (42) Lusk, A. L.; Bohn, P. W. Effect of Processing Conditions on the Formation of Aggregates and Phase Domains in Monolayers of the Hemicyanine Dye, 4-(4-(Dihexadecylamino)styryl)-N-methylpyridinium Iodide. J. Phys. Chem. B 2001, 105, 462−470. (43) Gonzalez-Delgado, A. M.; Giner-Casares, J. J.; Rubia-Paya, C.; Perez-Morales, M.; Martin-Romero, M. T.; Brezesinski, G.; Camacho, L. The Effect of the Reduction of the Available Surface Area on the Hemicyanine Aggregation in Laterally Organized Langmuir Monolayers. J. Phys. Chem. C 2011, 115, 9059−9067.

(44) Backus, E. H. G.; Bonn, D.; Cantin, S.; Roke, S.; Bonn, M. Laser-Heating-Induced Displacement of Surfactants on the Water Surface. J. Phys. Chem. B 2012, 116, 2703−2712.

1141


Tracking Molecular Aggregates at a Liquid Interface by Nonlinear

Recommend Documents