Nonlinear Spectroscopy of Bio-Interfaces

S. Roke: Nonlinear spectroscopy of bio-interfaces

Sylvie Roke Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), Stuttgart, Germany Nonlinear spectroscopy of bio-interfaces

In the past five years the Max Planck Research Group for di str ib ut ion . Nonlinear spectroscopy of bio-interfaces has worked at the intersection of surface science, nonlinear optics and soft and biological matter. For hard matter there is molecular level understanding of the surface region. This has enabled researchers to develop the highly complex structures that are nowadays found in (e.g.) computers, and mobile phones. Although biological and soft systems (such as liq- N o t fo r elec tr onic uid droplets, nanoparticles, liposomes, and viruses) are ubi-

i t e s. quitous in our daily lives, our understanding is often limited to macroscopic theories. Consequently, the level of control of soft and biological matter is largely empirical and far behind that of hard matter. To initiate a change, we have in the Fig. 1. Unequal numbers in bulk and surface. Illustration of the num- past period worked on three main themes: (i) The investiga- ber of atoms situated at the interface and in the bulk of spherical particles with radius R. tion of structure and properties of biologically and medically relevant interfaces (supported lipid bilayers, biopolymer interfaces, water, protein-surface interactions). (ii) their exceptional fluorescence. A small change in the interfa- n t e r ne o in tr ane ts Development of nonlinear light scattering techniques as cial structure, or an increase in interfacial material (brought ni probes for molecular structure at the interfaces of small par- about by a reduction of size) can dramatically alter the color

s ei ticles. (iii) The study of molecular interfacial structure, ki- and intensity of the fluorescent light. The stability of emul- netics and dynamics of water, surfactants, phospholipids, sions is determined by the interactions (van der Waals forces, sugars and peptides on small objects (nano-droplets, vesi- electrostatic interaction, etc.) that occur in the interfacial re- cles) in suspensions. The result of our work on these themes gion. Surface modified beads phase separate readily upon N o t fo ru is summarized for this special issue article. the addition of certain proteins and enzymes and can thus be used as sensitive biochemical sensors. Although we some- Keywords: Non-linear spectroscopy; Interfaces; Liquids; times have a clear picture of what happens on the macro- Emulsions; Light scattering and microscopic length and time scales, we do not (yet) understand the underlying molecular picture. ww. ijm r. de In this review of the research of the Max-Planck Research 1. Introducing Group “Nonlinear spectroscopy of bio-interfaces”, we start with a description of nonlinear optical surface spectroscopy. Interfaces play a key role in many processes. They play a We then explain some limitations of the method and the steps regulating role in transport and structural phenomena in bi- we have taken towards improvement. Using those improve- ological cells, they can determine the chemistry and phase ments we have started to study biologically relevant surfaces G e r man yw behavior of colloidal systems, they are important for the such as poly-sugar films and biological samples consisting of mechanical properties of amorphous solids and they can de- self assembled monolayers and proteins. We then move on to termine the electrical properties of electronic devices. A a description of developments in the field of nonlinear light

M u nich, large portion of natural matter exists on a micro- and nano- scattering (NLS). We summarize the theoretical advance- scopic scale. Living cells, organelles, colloidal systems, ments made by our research group and show how surface–sol- emulsion, micelles, and many other systems are composed vent effects can be important. Finally, we describe first results of (sub)micron sized parts. For such systems, the relative obtained from probing the liquid/liquid interface of nano- interfacial area – and consequently the importance of inter- scopic oil droplets in water. This article is meant as an over- face atoms and molecules – increases. view of the activities at the MPI for Metals Research. For The relative number of atoms and molecules present on complete referencing we refer to the original publications. the interface of a material increases dramatically when the

C a r l H an s e Ve lag, material is reduced in size (see Fig. 1). For micro- and nano- 2. Probing planar substrates scopic systems it is well-known that the interfacial region is a

20 11 dominating factor in determining the physical and chemical 2.1. Nonlinear optical spectroscopy – an interface

properties of a material. For such nanoscopic systems it is specific tool known that a change in the interfacial properties results in profound changes in the physical and/or chemical properties. During recent decades several successful attempts have For example, semiconductor quantumdots are known for been made to study planar interfaces on the molecular level

906 Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 FFeature S. Roke: Nonlinear spectroscopy of bio-interfaces

and under ambient conditions. One particularly successful 3. Poly-sugars and proteins at interfaces development is based on the use of second-order nonlinear optical techniques. Since the second-order susceptibility Since the energies of the vibrational modes lie in the infra- vanishes in isotropic media (see Ref. [1] for a more detailed red region of the electromagnetic spectrum, infrared explanation), second-order nonlinear optical spectroscopy photons are needed to excite vibrational resonances. Also, can be used as an extremely sensitive surface probe. high electromagnetic field strengths are required to achieve A molecular picture of an interface can be obtained by enough response from the second-order susceptibility using vibrational sum frequency generation (SFG). Here, (which is nonlinear and only occurs when pulsed lasers are an infrared (IR) and a visible (VIS) electromagnetic field, used). In recent decades femtosecond laser sources have with frequencies x2 and x1, are reflected from an interface. been built that can be used to generate infrared femtosecond At the interface a second-order polarization can be created laser pulses (with enough output energy for nonlinear opti-

di str ib ut ion . that oscillates at the sum frequency x0 of the incoming cal spectroscopy) with wavenumbers in the range 4000– waves: 1500 cm –1 or 2.5–7 lm. In this spectral range resonances of the localized surface chemical groups are probed, such ð2Þ ð2Þ as CH and OH groups, so that the structure, orientation Pi ðx0 ¼ x1 þ x2Þ¼vijk ðx0 ¼ x1 þ x2ÞE1;jðx1ÞE2;kðx2Þ 3 and order of small molecules can be mapped. Larger mole- ð1Þ cules such as polymers, proteins and peptides however, are still a challenge as they have complicated three dimensional whose size and spectral shape is determined by the second-

N o t fo r elec tr onic structures that cannot be deduced from measuring only lo- order susceptibility tensor vð2Þ. If the energy of the IR beam calized subgroups of a larger structure. is tuned around the resonance of the vibrational modes of i t e s. Three dimensional structures can be deduced by probing the interfacial molecules the surface response is resonantly molecular vibrational modes that are comprised of the dis- enhanced. Since vð2Þ is a physical property of the material placement of a large number of atoms. These modes vibrate it must also reflect the spatial symmetry properties of that with lower frequencies, from 1500–500 cm –1 (7–20 lm). material (Neumann’s principle). As a consequence (in the ð2Þ In order to generate femtosecond infrared pulses with en- electric-dipole approximation) v vanishes in bulk media ijk ough energy for nonlinear optical surface spectroscopy ex- [2]. This means that SFG can often be applied as a surface periments in the low frequency region of the infrared spec- specific technique, with which only the first 1–3 molecular trum, we developed a modified Ti:Sapphire laser with a n t e r ne o in tr ane ts layers situated at the interface are probed. The surface re- ð2Þ cryogenically cooled additional amplifier that pumps an op- ni sponse, identified as v , is usually split up in a frequency s tical parametric amplifier combined with difference fre- dependent resonant (res) and a frequency independent s ei quency generation stage (see Ref. [6] for details). Using non-resonant (nr) response: the pulses from this system we have measured the three di- X mensional surface structure of a polysugar and the binding ð2Þ ð2Þ ð2Þ iDu NshTiaTjbTkciRn;ab ln;c N o t fo ru vs;ijk ¼ vs;nr;ijk þ vs;res;ijk / Anre þ of a protein to a self assembled surface. n hðx2 x0n þ i Ç0nÞ Figure 2 shows the surface localized (right panel) and de- ð2Þ localized (left panel) modes of two forms of a poly-(lactic acid), here named form A and B. See Ref. [7] for more de- where n refers to a specific vibrational mode, with reso- tails. This biodegradable and bio-compatible poly-sugar nance frequency x0n, and damping constant Ç0n. This has unique characteristics that heavily depend on the three

ww. ijm r. de ð2Þ equation relates the resonant surface susceptibility vs;res dimensional structure of the molecule. It is an ideal candi- ð2Þ to the hyper-polarizability tensor bn which is the product date for use in systems and devices for drug delivery and of the vibrational dipole moment ln with the Raman po- implants. Thorough knowledge of the three dimensional larizability Rn through a transformation matrix T that surface structure is thus of great importance, since the sur- transforms from the molecular coordinate frame (a, b, c) face constitutes a barrier to the surroundings and provides to the surface coordinate frame (i, j, k). Ns is the density an adsorption site for chemicals. Form A and B are chemi- G e r man yw of surface molecules. Anr is the amplitude of the fre- cally identical but composed of monomers with different quency independent non-resonant response that can be chirality. Form A consists of only L-lactic acid monomers, phase lagged with a factor Du with respect to the resonant while form B is composed of an even number but random

M u nich, response. The values of x0n, Ç0n, ln, and Rn depend on mixture of D and L monomers. the type of molecules, their orientation, chirality and or- It is therefore logical to expect that the molecules at the der. Since this type of information is often exclusively in- surface of the A and B films have different three dimen- terface specific and can be obtained non-invasively and sional structures. Since the chemical bonds are identical, label-free, SFG experiments can be a useful addition to however, chiral molecules are very difficult to distinguish methods that are sensitive to (e.g.) average structure and spectroscopically using linear optics. With nonlinear optics mass changes. however, it is possible. A further interesting feature of nonlinear optical methods Figure 2 shows surface SFG spectrums of films A and B.

C a r l H an s e Ve lag, is that they are specific towards chirality [3–5]. In chemis- It shows that try and biology most compounds consist of chiral building 1. there is only a difference in spectral shape in the low fre-

20 11 blocks (e.g., 21 of the 22 amino acids in the human body quency region of the spectrum, so that this part clearly

are chiral). A change in chirality can completely change reveals the three dimensional structure of a complex the chemical and biological function of a molecule, as it polymer. strongly influences the three dimensional structure of a 2. From these low frequency spectra (combined with simu- compound. lation data) we can determine that the surface chains of

Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 907 S. Roke: Nonlinear spectroscopy of bio-interfaces

Fig. 2. Vibrational sum frequency generation spectra in reflection mode of the first few atomic layers of the interfaces of chemically identical biopolymer (poly-(lactic acid)) films A (red trace) and B (blue trace). The black lines are fits to the data in which all contribu- tions to the reflected electrical sum frequency field are added. Left: The delocalized surface modes of A and B. The modes indicated with a * are representative of helices. Middle: Illus- tration of chiral mirror images. Molecular di str ib ut ion . model of the biopolymer. Right panel: The localized surface modes of A and B.

form A adopt a helical conformation, while those of in opposite leaflets is needed. This will result in a certain N o t fo r elec tr onic form B reside in a disordered heterogeneous state. amount of ordering in the protein orientation. Thus, delocalized vibrations are indeed very sensitive to the This oriented binding behavior of L1 was mimicked by i t e s. three dimensional structure of a large molecule and can be binding of a crucial piece of the L1 protein to a surface used to unravel such complex structures at interfaces. See structure made of gold and a self-assembled monolayer Refs. [7, 8] for more information. (SAM) of nitrilotriacetic acid (NTA). To detect the binding To test the current ability of SFG to detect protein bind- and to determine whether significant ordering takes indeed ing in a surface structure used by biologists, we have inves- place we used reflection mode SFG to follow the preferen- tigated the interaction of the cell-adhesion protein L1 with a tial surface order in combination with a method that detects self-assembled surface that is used as a template. L1 is surface viscosity and mass change (quartz crystal microbal-

n t e r ne o in tr ane ts thought to be crucial for a variety of processes during the ance), and a method sensitive to the change of the average ni formation of the nervous system such as cell adhesion, cell chemical film structure (reflection absorption infrared spec- migration, cell differentiation, neurite formation, and re- troscopy, RAIRS). These methods are complementary and s ei generation. L1 interacts mainly by homophilic interactions allow one to probe non-invasively the average structural with proteins from other cell membranes. This is schemati- changes as well as the structural changes in the surface cally illustrated in Fig. 3. Like all proteins, L1 has an N-ter- binding centers. N o t fo ru minus (amine group) on one side and a C-terminus (car- Our measurements showed that NTA-SAMs are formed boxilic group) on the other end. The current understanding but with less preferential density than a grown n-dodecan- of L1 facilitated cell–cell interactions is that specific inter- ethiol C12-SAM. Subsequent addition of histidine tagged actions between the extracellular domains of L1 proteins L1 in quantities used to obtain biological functionality re- ww. ijm r. de

Fig. 3. Protein surface interaction under biological conditions. Left: Model of cell–cell in- G e r man yw teraction mediated by the extracellular domains of proteins. Bottom: Schematic picture of the protein interaction that facilitates cell– cell adhesion. Top: Illustration of the model M u nich, for the extracellular protein domain on a func- tionalized surface. Right: Surface spectroscopy of proteins. Bottom: RAIRS spectra of self-assembled NTA films on silver (lower trace) and self-assembled NTA films with subsequent binding of the his-tagged extracellular domain of the L1 protein (upper trace). Top: Reflection mode SFG spectra in the C–H stretching region of self-assembled monolayers on (111)-oriented silver linked with a C a r l H an s e Ve lag, self-assembled monolayer of C12H25SH (gray), silver films with NTA (red) and silver

20 11 films with NTA and L1 protein (blue).

908 Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 FFeature S. Roke: Nonlinear spectroscopy of bio-interfaces

sults in the transfer of only a small amount of L1 bound to the surface. We found that the binding mechanism in aqueous solution of L1 to the surface occurs by replacing cap- ping molecules of ethanol, which are linked to the carboxy- late groups of the NTA complex. From RAIRS data (not shown) we find that L1 links to the NTA film, whereby L1 retains its hydration shell, even in air. This could be the most important factor for the observed biological functionality, which is still present after exposure to air. Although the RAIRS spectrum is rich in features in the amide and fin- gerprint regions, we do not find much evidence of that in

di str ib ut ion . the SFG spectra, which indicates that the protein is amorphous and that the present a helices and b sheets do not play a (directional) role in the protein surface interaction. It therefore seems that the directionality and preferred ordering as concluded from biological experiments is not ob- Fig. 4. Nonlinear scattering spectroscopy data of 300 nm and 100 nm served in a study of the chemical surface structure. See (top trace) glass particles covered with carbohydrate chains, dispersed in three different apolar solvents (hexadecane, benzene and CCl4). Ref. [9] for more information. Large differences in the spectra and thus the surface structure are ob-

N o t fo r elec tr onic served. 4. Non-linear light scattering spectroscopy i t e s. 4.1. Model systems vent is changed. As can be seen from Fig. 4 the surface structure at room temperature for the three solvents is very differ- Since a large portion of naturally occurring matter exists in ent. This difference in structure was used to explain the form of small objects, such as cells, liposomes, droplets difference in colloidal gelation [10] and aging [11]. (emulsions) or colloids that are embedded in solid or liquid, Thanks to continuing improvements in the experimental it is desirable to obtain similar detailed molecular informa- setup we have been able to improve the signal to noise sign- tion about interfaces of small dispersed particles. Even icantly [12]. n t e r ne o in tr ane ts more so, since the relative surface area of particles is orders Apart from a spectrum there is also a scattering pattern. ni of magnitude larger than that of planar interfaces. In order In order to understand the mechanism of nonlinear light s ei to accomplish this aim, second-order nonlinear light scat- scattering we have done a considerable amount of theoreti- tering methods have been developed. The vibrational spec- cal modeling. trum of a particle or droplet surface can be obtained using

N o t fo ru vibrational sum frequency scattering, a form of nonlinear 4.2. Theoretical modeling light scattering that utilizes the interaction of an infrared and visible laser pulse with a colloidal dispersion. The pro- To understand the interaction of two laser beams with a sin- cesses that occur depend not only on the nonlinear optical gle particle we need to understand the generation of a sum process on the surface but also on light scattering. frequency source polarization and how this acts as a source In an SF scattering experiment, IR and VIS pulsed laser for light scattering. To this end we have made models using ww. ijm r. de beams are overlapped inside a cuvette containing dispersed a varying degree of assumptions: particles in a liquid or solid matrix. At the droplet interface, (i) Electrostatic approximation (for small particles), a second-order nonlinear polarization is created which oscil- where it is assumed that the electric field can be as- lates at the sum of IR and VIS frequencies. This polarization sumed constant and static across the particle [13]. is small but does not vanish. Just as in linear light scattering, (ii) The Rayleigh–Gans–Debye (RGD) approximation, there is a phase difference between the polarization generated which assumes that the electric fields are dynamic but G e r man yw on different parts of the particle surface. Therefore, sum fre- not modified by the particle [14]. quency (SF) photons can be emitted. Interference of the SF (iii) The Wentzel–Kramers–Brilliouin (WKB) approxima- field that is generated on different positions on the surface of tion [13], for spheres and cylinders, where it is as-

M u nich, the droplet generates a scattering pattern in the far-field. sumed that the distortion of the incoming dynamic The SF intensity contains surface structural information plane waves occurs only due to mismatch in velocity and is again resonantly enhanced when the energy of the IR of the incoming beams. photons equals the energy of a vibrational transition. The re- (iv) Mie theory for spheres, where no approximations are sulting scattered spectrum represents the average vibrational made [15]. spectrum of the interface of the particles that are present in A summary of various other nonlinear light scattering phe- the spatial region where both laser pulses overlap. Figure 4 nomena can be found in Ref. [12]. shows sum frequency scattering (SFS) spectra of a dispersion The RGD approximation has proven to be most useful for

C a r l H an s e Ve lag, of small glass particles covered with chemically linked car- NLSS experiments if we are looking at particles, droplets or bohydrate chains dispersed in three different apolar solvents domains with small refractive index differences with the sur-

20 11 (CCl4, benzene and hexadecane). These particles are well rounding medium [10–14]. Using the RGD method, we

documented and it is known that in apolar solvents the inter- have also developed models for NLS from chiral surfaces action is determined by van der Waals interactions. It is [16], determining the molecular orientation [17], higher or- therefore expected that the interaction and also the surface der scattering [16], scattering from surface charges [18], structure of these particles will not change if the polar sol- and scattering from a particle with an arbitrary shape [19].

Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 909 S. Roke: Nonlinear spectroscopy of bio-interfaces

We have also studied the effects of particle clustering and In the following we will describe briefly how surface examined the possibility of dynamic nonlinear light scatter- chirality appears in the scattering pattern. As it turns out, ing [12]. To make our work accessible we have developed a thanks to the scattering process, the scattering angle allows software package that can be used to calculate scattering pat- us to distinguish molecular properties that are otherwise terns for different molecular orientations and surface sym- difficult to find. metry (Fig. 5, see: http://www.mf.mpg.de/en/abteilungen/roke/ index.html to download the software package). 4.3. Chiral surfaces

For spherical particles with chiral surfaces the surface sus- ð2Þ ceptibility has non-zero elements of type vxyz. It can be seen from Fig. 5 that the angular distribution of a chiral surface di str ib ut ion . pattern is different from a non-chiral surface pattern. Since the patterns are so clearly different it should be possible to separately measure chiral and non-chiral surface responses from colloids. Having developed a system to perform nonlinear light scattering spectroscopy we proceeded to investigate systems that are relevant for society: A new type of medication for li- N o t fo r elec tr onic ver cancer treatment, and emulsions consisting of nanoscopic droplets in water. i t e s.

5. Microspheres for liver cancer treatment

The chiral response was also apparent when we studied the composition of solid biodegradable poly-(lactic acid) microspheres. The 20–50 micron sized spheres, which are key to a promising new treatment for an incurable form of liver cancer, n t e r ne o in tr ane ts interacted with the laser beams in an atypical fashion, giving ni rise to the scattering patterns in Fig. 6 [20]. By modeling we

s ei were able to deduce that the microspheres are composed of buried nanoscopic crystalline domains in an amorphous sur- rounding. This finding explains the apparent structural robust- ness of the microspheres that seems to be crucial in the under- N o t fo ru standing of the working mechanism of the treatment: In the proposed medicine the crystalline domains can form a host Fig. 5. Modeling of nonlinear light scattering. Top: The available soft- matrix for the necessary medicinal complex. The ability to ware package, used to calculate scattering patterns for different orienta- look inside a solid matrix without cutting open the material tions. Bottom: Three dimensional image of calculated sum frequency opens up new avenues for the study of heterogeneities in scattering pattern of a 200 nm particle (which e. g. could be a virus cap- ww. ijm r. de side) containing both chiral (blue) and non-chiral (red) groups. It can be chemistry and physics, such as the monitoring of crystal nu- seen that both groups generate a distinctly different scattering pattern that cleation and growth, or the detection of small amounts of bio- points in different directions, so that they can be measured separately. logical crystals (such as proteins and biopolymers). G e r man yw M u nich,

Fig. 6. Nonlinear light scattering from biodegradable microspheres. The scattering patterns (left) obtained for different polarization com- binations showed that the 20–50 micron sized spheres (see electron microscopy image in the inset) consist of embedded nanoscopic domains. Spectroscopic measurments revealed the chemical link between the medically active C a r l H an s e Ve lag, compound and the polymer matrix. 20 11

910 Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 FFeature S. Roke: Nonlinear spectroscopy of bio-interfaces di str ib ut ion .

N o t fo r elec tr onic Fig. 7. Top: SF amplitude in ssp and ppp polarization combination of the sulfate stretch vibrational mode at 1070 cm –1, obtained from measuring SF spectra of hexadecane droplets prepared with constant droplet size distribution and different total concentration of SDS surfactant. The SF am- i t e s. plitude is proportional to the SDS surface excess (Ns). Bottom: The corresponding upper limit for Ns, derived from the amplitude data. The solid blue line is a fit to the modified Langmuir adsorption model.

6. Applications to nano-droplets and vesicles metric stretch scattered SF signal in multiple polarization combination. The measurement is shown in Fig. 7 (left pa- Emulsions consist of one liquid dispersed as droplets in an- nel). The resultant spectra display the vibrational signature other liquid, such as butter and milk. Despite the impor- of the sulfate stretch mode in the head group of the surfac- n t e r ne o in tr ane ts tance of emulsions in our daily lives, protocols for control tant. For an emulsion series prepared with constant droplet ni of properties and stability are very often empirical, and the concentration and size we found that the SF amplitudes s ei molecular mechanism behind emulsion formation, stability (and therefore also the number density of molecules at the and dynamics remains unknown. Emulsion properties are surface) change only by a factor of three when the total to a large extent determined by their molecular interfacial SDS concentration is varied from 50 lM to 10 mM. From

N o t fo ru properties. Since the (oil) droplet interfaces of emulsions the change in surface density as a function of total surfac- are surrounded by a liquid (water) phase they are difficult tant concentration in the aqueous phase we can obtain the to access for molecular surface-specific probes. Molecular change in interfacial tension by means of the curvature cor- interfacial properties are therefore generally inferred from rected Gibbs equation [21]. From this we concluded that the experiments on planar interfaces. However, since the sur- interfacial density of adsorbed SDS is at least one order of face to volume ratio of an emulsion is typically 4 to 7 orders magnitude lower than the interfacial density at a corre- ww. ijm r. de of magnitude larger than that of a planar interface, it could sponding planar interface. From Fig. 7 it can be seen that be asked if changing one of the key physical parameters of the derived maximum decrease in interfacial tension was a system by such a large amount would not result in differ- only 5 mN m –1 [21]. Thus, these first sum frequency spec- ences in chemical and physical behavior. troscopic investigations of the oil droplet/water interface In a common planar oil/water system such as bulk n-hexa- show that the interface of small droplets stabilized by SDS decane in contact with bulk water, the surfactant sodium do- (R*100 nm) harbor a very low density of surfactant, even G e r man yw decylsulfate (SDS) will reduce the interfacial tension from at the cmc. Other experiments in which we probed the con- 52 mN m –1 to 10 mN m –1 by populating the interface. The formation of surfactant and oil indicated that the SDS alkyl apolar SDS tail resides in the oil phase, and the polar sulfate chains have an extremely high disorder, which starkly con-

M u nich, head group is immersed in the water. At SDS concentrations trasts with the relatively well-ordered alkane oil chains approaching the critical micelle concentration (c.m.c.) of [22]. Further analysis shows that it seems feasible that the 8 mM the interface is fully occupied by SDS molecules giv- oil at the interface adopts a distribution that is mostly paral- ing rise to a surface coverage of 3.3·10 –6 mol m –2 (corre- lel oriented with respect to the interface. A consequence of sponding to an occupied molecular interfacial area of this is that the surfactant is residing almost completely in *40–50 Å2 ). It is widely assumed that similar behavior the water phase. can be expected for SDS adsorption on 80 nm n-hexadecane Comparing results from planar oil/water interfaces with oil droplets dispersed in water. droplet interfaces, it appears that quite a few differences ex-

C a r l H an s e Ve lag, The experimental proof for this assumption, however, is ist. Further experiments will be needed to determine what difficult since accessing the liquid/liquid interface of a are the existing differences and why the occur.

20 11 nanoscopic droplet interface is a great challenge.

We have therefore conducted SFS experiments on the oil 7. Outlook droplet–water interface stabilized by SDS, in which we fol- lowed the change in surface density as a function of SDS In the past five years much progress has been made in our surfactant concentration by monitoring the sulfate sym- ability to study fundamental processes at soft matter inter-

Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7 911 S. Roke: Nonlinear spectroscopy of bio-interfaces

faces. Many advances have also taken place on the theoret- [14] S. Roke, W.G. Roeterdink, J.E.G.J. Wijnhoven, A.V. Petukhov, ical level. We have begun to study interfaces with biologi- A.W. Kleyn, M. Bonn: Phys. Rev. Lett. 91 (2003) 258302. DOI:10.1103/PhysRevLett.91.258302 cal and medical relevance and have shifted the frontier of [15] A.G.F. de Beer, S. Roke: Phys. Rev. B 79 (2009) 155420. science so that we are now able to preform vibrational spec- DOI:10.1103/PhysRevB.79.155420 troscopy and nonlinear light scattering even at liquid/liquid [16] A.G.F. de Beer, S. Roke: Phys. Rev. B. 75 (2007) 245438. interfaces of nanoscopic droplets in water. This opens up DOI:10.1103/PhysRevB.75.245438 [17] A.G.F. de Beer, S. Roke: J. Chem. Phys. 132 (2010) 234702. the road to study surface water at hydrophobic interfaces, DOI:10.1063/1.3429969 and in confined geometry. We can study the behavior of [18] A.G.F. de Beer, R.K. Campen, S. Roke: Phys. Rev. B 82 (2010) surfactants, co-surfactants, and ions and biologically rele- 235431. DOI:10.1103/PhysRevB.82.235431 vant molecules at such interfaces. Vesicles [23], liposomes [19] A.G.F. de Beer, S. Roke, I.J. Dadap: J. Opt. Soc. Am. B (submitted). and the formation of membranes can be studied in situ. Pro- [20] A.G.F. de Beer, H.B de Aguiar, J.W.F. Nijsen, S. Roke: Phys. Rev. Lett. 102 (2009) 095502. DOI:10.1103/PhysRevLett.102.095502 di str ib ut ion . cesses such as the formation of a sugar coat around a cell, [21] H.B. de Aguiar, A.G.F. de Beer, M.L. Strader, S. Roke: J. Am. the interaction of glucose with the cell membrane and pep- Chem. Soc. 132 (2010) 2122. DOI:10.1021/ja9095158 tide binding that leads to cell adhesion can be studied. [22] H.B. de Aguiar, M.L. Strader, A.G.F. de Beer, S. Roke: J. Phys. Chem. B 113 (2011) 2970. This work is part of the research programme of the Max-Planck So- [23] M.L. Strader, H.B. de Aguiar, A.G.F. de Beer, S. Roke: Soft Mat- ciety. We thank the German Science Foundation (grant num- ter 7 (2011) 4959. ber 560398) and the European Research Council (Startup grant number 240556).

N o t fo r elec tr onic (Received February 3, 2011; accepted May 6, 2011) References i t e s.

[1] S. Roke: Chem. Phys. Chem. 10 (2009) 1380. Bibliography DOI:10.1002/cphc.200900138 [2] P.N. Butcher, D. Cottor: The Elements of Nonlinear Optics (Cam- DOI 10.3139/146.110535 bridge University Press, 1990). Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 7; page 906–912 [3] S. Sioncke, T. Verbiest, A. Persoons: Mater. Sci. Eng. 42 (2003) # 115. DOI:10.1016/j.mser.2003.09.002 Carl Hanser Verlag GmbH & Co. KG [4] G.J. Simpson: Chem. Phys. Chem. 5 (2004) 1301. ISSN 1862-5282 DOI:10.1002/cphc.200300959 n t e r ne o in tr ane ts [5] M.A. Belkin, Y.R. Shen: Int. Rev. Phys. Chem. 24 (2005) 257. ni DOI:10.1080/01442350500270601 Correspondence address [6] A.B. Sugiharto, C.M. Johnson, H.B. de Aguiar, L. Aloatti, S. Roke: s ei Appl. Phys. B. 91 (2008) 315. DOI:10.1007/s00340-008-2993-7 Dr. Sylvie Roke [7] C.M. Johnson, A.B. Sugiharto, S. Roke: Chem. Phys. Lett. 449 Max Planck Institute for Intelligent Systems (2007) 191. DOI:10.1016/j.cplett.2007.10.061 (formerly Max Planck Institute for Metals Research) [8] A.B. Sugiharto, C.M. Johnson, I.E. Dunlop, S. Roke: J. Phys. Chem. Heisenbergstr. 3, D-70569 Stuttgart, Germany N o t fo ru C 112 (2008) 7531. DOI:10.1021/jp801254y Tel.: +49 711 6893679 [9] J. Fick, T. Wolfram, F. Belz, S. Roke: Langmuir 26 (2010) 1051. Fax: +497116893612 DOI:10.1021/la902320b E-mail: [email protected] [10] S. Roke, J. Buitenhuis, J.C. van Miltenburg, M. Bonn, A. van Blaa- deren: J. Phys. Condens Matter 17 (2005) 3469. DOI:10.1088/0953-8984/17/45/036 [11] S. Roke, O. Berg, J. Buitenhuis, A. van Blaaderen, M. Bonn: Proc. ww. ijm r. de Nat. Acad. Sci. 103 (2006) 13310. DOI:10.1073/pnas.0606116103 [12] J.I. Dadap, H.B. de Aguiar, S. Roke: J. Chem. Phys. 130 (2009) You will find the article and additional material by enter- 214710. DOI:10.1063/1.3141383 ing the document number MK110535 on our website at [13] S. Roke, M. Bonn, A.V. Petukhov: Phys. Rev. B 70 (2004) 115106. www.ijmr.de DOI:10.1103/PhysRevB.70.115106 G e r man yw M u nich, C a r l H an s e Ve lag, 20 11

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