Sum Frequency Generation Signal Research Articles

Membrane orientation of MSI-78 measured by sum frequency generation vibrational spectroscopy.

Antimicrobial peptides (AMPs) selectively disrupt bacterial cell membranes to kill bacteria whereas they either do not or weakly interact with mammalian cells. The orientations of AMPs in lipid bilayers mimicking bacterial and mammalian cell membranes are related to their antimicrobial activity and selectivity. To understand the role of AMP-lipid interactions in the functional properties of AMPs better, we determined the membrane orientation of an AMP (MSI-78 or pexiganan) in various model membranes using sum frequency generation (SFG) vibrational spectroscopy. A solid-supported single 1,2-dipalmitoyl-an-glycero-3-[phospho-rac-(1-glycerol)] (DPPG) bilayer or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) bilayer was used as a model bacterial cell membrane. A supported 1,2-dipalmitoyl-an-glycero-3-phosphocholine (DPPC) bilayer or a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was used as a model mammalian cell membrane. Our SFG results indicate that the helical MSI-78 molecules are associated with the bilayer surface with ∼70° deviation from the bilayer normal in the negatively charged gel-phase DPPG bilayer at 400 nM peptide concentration. However, when the concentration was increased to 600 nM, MSI-78 molecules changed their orientation to make a 25° tilt from the lipid bilayer normal whereas multiple orientations were observed for an even higher peptide concentration in agreement with toroidal-type pore formation as reported in a previous solid-state NMR study. In contrary, no interaction between MSI-78 and a zwitterionic DPPC bilayer was observed even at a much higher peptide concentration (∼12,000 nM). These results demonstrate that SFG can provide insights into the antibacterial activity and selectivity of MSI-78. Interestingly, the peptide exhibits a concentration-dependent membrane orientation in the lamellar-phase POPG bilayer and was also found to induce toroidal-type pore formation. The deduced lipid flip-flop from SFG signals observed from lipids also supports MSI-78-induced toroidal-type pore formation.

Wave mixing in nonlinear magnetic metacrystal

We present experimental measurements of three- and four-wave mixing phenomena in an artificially structured nonlinear magnetic metacrystal at microwave frequencies. The sum frequency generation signal for the varactor-loaded split-ring resonator (VLSRR) metamaterial agrees quantitatively with that predicted using an analytical effective medium model describing the VLSRR medium. A resonant enhancement of the nonlinear response is observed near the metamaterial resonance.

Time-Resolved Measurements of Sum-Frequency Generation Strongly Enhanced in (113)B GaAs/AlAs Coupled Multilayer Cavity

Time-resolved measurements of sum-frequency generation (SFG) of two cavity modes, which were realized in a GaAs/AlAs coupled multilayer cavity structure grown on a (113)B-oriented GaAs substrate, were performed. Strongly enhanced SFG and second-harmonic generation (SHG) signals have been demonstrated by the simultaneous excitation of two cavity modes using 100 fs laser pulses. In the time-resolved measurements, the sample was irradiated by two 100 fs laser beams with a time delay and the SFG and SHG signals generated by combining two beams were detected using a small slit. The delay-time-dependent measurements have revealed that the SFG and SHG signals decay with the photon lifetime (0.6 ps) of the coupled multilayer cavity, and the oscillating behavior with the period (0.3 ps) corresponding to the optical frequency difference (3.3 THz) between two cavity modes is significant only for the SFG. The experimental results were well explained by the simulated light electric field inside the coupled cavity under two Gaussian pulse excitation.

An SFG Study of Interfacial Amino Acids at the Hydrophilic SiO2 and Hydrophobic Deuterated Polystyrene Surfaces

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids--L-phenylalanine, L-leucine, glycine, L-lysine, L-arginine, L-cysteine, L-alanine, and L-proline--in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid-liquid interface between both hydrophobic d(8)-polystyrene (d(8)-PS) and SiO(2) model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d(8)-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C-H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO(2) surface, no C-H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO(2) interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO(2)/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid-liquid interface and under physiological conditions.

Sensibility to Changes of Vibrational Modes of Excited Electron: Sum Frequency Signals Versus Difference Frequency Signals

In this paper, we investigate a two electronic level system with vibrational modes coupled to a Brownian oscillator bath. The difference frequency generation (DFG) signals and sum frequency generation (SFG) signals are calculated. It is shown that, for the same model, the SFG signals are more sensitive than the DFG signals to the changes of the vibrational modes of the electronic two-level system. Because the SFG conversion efficiency can be improved by using the time-delay method, the findings in this paper predict that the SFG spectrum may probe the changes of the microstructure more effectively.

Investigation of sub-monolayer, monolayer, and multilayer self-assembled semifluorinated alkylsilane films

Semifluorinated self-assembled (FAS SA) films fabricated from trifunctional precursors are frequently used in myriad applications, yet an understanding of the effects of fabrication conditions, including deposition time, on adsorption mechanisms and molecular architectures is still being developed. In this work we prepared SA films based on the F(CF(2))(8)(CH(2))(2)SiCl(3) (FAS-17) precursor and characterized these films using a suite of surface analytical techniques. Contact angle, sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ellipsometry results are consistent with the formation of disordered sub-monolayer structures at short deposition times, well-ordered monolayers at intermediate deposition times, and inhomogeneous multilayers at long deposition times. Correlation of SFG and XPS results demonstrates a change in FAS-17 chain orientation as the deposition time increases from 2 s to 5 min. Group theory-based calculations, SFG studies, and Fourier-transform infrared (FTIR) results also afford additional evidence in support of the assignment of the SFG signals at ~1345 and ~1370 cm(-1) to the asymmetric stretching mode of the semifluorinated silane chain's terminal CF(3) group rather than to its axial CF(2) stretches. To our knowledge, this is the first report of SFG studies on semifluoroalkyl silane self-assembled films in the C-F stretching frequency region.

When the Solute Becomes the Solvent: Orientation, Ordering, and Structure of Binary Mixtures of 1-Hexanol and Cyclohexane over the (0001) α-Al2O3Surface

Mixtures of 1-hexanol in cyclohexane over the (0001) α-Al(2)O(3) surface were probed in the CH stretching region using vibrational broadband sum frequency generation (SFG). Below 10 mol % 1-hexanol, the alcohol adsorbs to the surface through interactions that result in observed free adsorption energies of around 14-15 kJ/mol obtained from the Langmuir adsorption model. Polarization-resolved SFG spectra indicate ordering of the alkyl tails with increasing surface coverage. Highly correlated with the ordering of 1-hexanol is an orientational change of the cyclohexane solvent from flat to most likely tilted, suggesting that cyclohexane mediates the adsorption of 1-hexanol via intercalation. Above 10 mol %, the SFG signals for 1-hexanol and cyclohexane decrease with increasing concentration of 1-hexanol, consistent with the notion that cyclohexane is excluded from the interfacial region while 1-hexanol becomes increasingly disordered. At the interface, the alcohol solute becomes the solvent at a mole fraction of only 10%, i.e., five times below what is considered the solute-to-solvent mole fraction transition in the bulk. These results provide an important benchmark for theory, inform reaction design, and demonstrate that bulk thermodynamic properties of binary mixtures are not directly transferable to interfacial environments.

Interference effects in the sum frequency generation spectra of thin organic films. I. Theoretical modeling and simulation

A general theoretical calculation is described for predicting the interference effect in the sum frequency generation (SFG) spectra from a model thin-film system as a function of film thickness. The calculations were carried out for a three-layer thin film consisting of an organic monolayer, a dielectric thin film of variable thickness, and a gold substrate. This system comprises two sources of SFG, namely, a resonant contribution from the monolayer/dielectric film interface and a nonresonant contribution from the dielectric film/gold interface. The calculation shows that both the spectral intensity and the shape of the SFG spectra vary significantly with the thickness of the dielectric layer due to interference effects in the thin film. The intensity changes at a particular frequency were explained in terms of the changes in the local field factors (L factors) as a function of the dielectric film thickness. The L factor for each beam changes periodically with the thickness of the dielectric film. However, the combined L factor for the three beams shows complicated thickness dependent features and no clear periodicity was found. On the other hand, if the susceptibilities of both the resonant and nonresonant terms are fixed, changes in the spectral shape will be mainly due to changes in the phase differences between the two terms with the film thickness. The interference behavior also depends strongly on the polarization combinations of the sum frequency, visible, and infrared beams. A general method is provided for predicting changes in the spectral shapes at different film thicknesses by taking into account the relative intensities and phases of the SFG signals from the two interfaces. The model calculation provides important insights for understanding the nonlinear optical responses from any thin-film system and is an essential tool for quantitatively revealing the nonlinear susceptibilities, which are directly related to the actual structure of the interfacial molecules from the observed SFG spectra after quantitative removal of the L factors.

In Vitro Characterization of Surface Properties Through Living Cells

The ability to probe an interface beneath a layer of living cells in situ without the need for labeling and fixation has the potential to unlock some of the key questions in cell biology and biointerfacial phenomena. Here, we show that vibrational sum frequency generation (SFG) spectroscopy can be used to detect alkanethiol self-assembled monolayers (SAMs) buried underneath a layer of living erythrocytes (ECs). SFG spectra with and without ECs showed the spectral signatures typical of these SAMs, indicating that the signal was being generated solely by the SAM and was not influenced by the presence of cells. Direct comparison of infrared spectroscopy to SFG measurements of cells adhered on a fibronectin layer showed that the SFG signal emanated solely from this layer. These results have important implications for the characterization of surfaces in biomedical, environmental, and industrial applications.

Surface Structures of an Amphiphilic Tri-Block Copolymer in Air and in Water Probed Using Sum Frequency Generation Vibrational Spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate surface structures of an amphiphilic surface-active block copolymer (SABC) film deposited on a CaF(2) substrate, in air and in water in situ. Developed as a surface-active component of an antifouling coating for marine applications, this amphiphilic triblock copolymer contains both hydrophobic fluorinated alkyl groups as well as hydrophilic ethoxy groups. It was found that surface structures of the copolymer film in air and in water cannot be probed directly using the SFG experimental geometry we adopted because SFG signals can be contributed from the polymer/air (or polymer/water) interface as well as the buried polymer/CaF(2) substrate interface. Using polymer films with varied thicknesses, structural information about the polymer surfaces in air and in water can be deduced from the detected SFG signals. With SFG, surface restructuring of this polymer has been observed in water, especially the methyl and methylene groups change orientations upon contact with water. However, the hydrophobic fluoroalkyl group was present on the surface in both air and water, and we believe that it was held near the surface in water by its neighboring ethoxy groups.

Surface organization of aqueous MgCl 2 and application to atmospheric marine aerosol chemistry

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl(2) surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl(2) concentration. At low concentrations (< 1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M). Finally, the MgCl(2) concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg(2+) to the PA headgroup.

Surface-Segregated Monolayers: A New Type of Ordered Monolayer for Surface Modification of Organic Semiconductors

We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C(61)-buryric acid 1H,1H-perfluoro-1-alkyl ester or FC(n)) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C(60) moiety in the surface FC(n) layers. The intensity of the SFG signals from FC(n) on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.

Directly Probing Molecular Ordering at the Buried Polymer/Metal Interface

We developed a methodology to directly probe molecular ordering at the buried polymer/metal interface. Using sum frequency generation (SFG) vibrational spectroscopy, we observed ordering of ester methyl groups at the buried poly(methyl acrylate) (PMA)/silver (Ag) interface. In order to directly probe the PMA/Ag interface, we collected SFG signal from a thin PMA film sandwiched between a fused silica substrate and a silver surface. It was found that the observed SFG signal intensity does not depend on the PMA film thickness. According to the calculated Fresnel coefficients of the fused silica/PMA and PMA/Ag interfaces with PMA films of different thicknesses, it was shown that SFG signals are solely contributed by the PMA/Ag interface. Further studies indicated that the ester methyl groups at the PMA/Ag interface tilt away from the Ag surface.

Ultrafast Excitation of Molecular Adsorbates on Flash-Heated Gold Surfaces

An ultrafast flash-thermal conductance technique is used to study energy transfer from a flash-heated polycrystalline Au(111) surface to adsorbed thiolate self-assembled monolayers (SAMs). The focus is on understanding energy transfer processes to parts of SAM molecules situated within a few carbon atoms of the Au surface, by probing specific SAM functional groups with vibrational sum-frequency generation (SFG) spectroscopy. The SFG intensity drop after flash-heating for benzenethiol (BT) CH-stretch transitions shows a substantial overshoot lasting several tens of picoseconds before BT and Au equilibrate at a higher temperature estimated at 600 degrees C. The thermal redshift of BT CH-stretch transitions also shows an overshoot. Other aromatic molecules and aliphatic molecules such as cyclohexanethiol (CHT) and hexanethiol (C6) have an overshoot as well. A model is proposed where the overshoot is primarily the result of hot surface electrons existing only during the flash-heating pulses. The intensity overshoot is caused by electron excitation of the probed vibrations and the redshift overshoot is caused by electron excitation of lower-energy vibrations anharmonically coupled to the probed vibration. Although electron excitation causes a substantial perturbation, up to 50% in some cases, of the SFG signal, the total amount of energy deposited into SAMs by electrons is much smaller than the heat transferred by Au surface phonons. Studies of a variety of molecular structures including substituted benzenes, biphenyl and terphenyl, and benzene rings connected to the Au surface by alkane linkers show that the likelihood of electron excitation becomes small for distances of 4-5 carbon atoms above the surface.

Molecular interactions between magainin 2 and model membranes in situ.

In this paper, we investigated the molecular interactions of magainin 2 with model cell membranes using sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Symmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-[Phospho-rac-(1-glycerol)] (POPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, which model the bacterial and mammalian cell membranes, respectively, were used in the studies. It was observed by SFG that magainin 2 orients relatively parallel to the POPG lipid bilayer surface at low solution concentrations, around 200 nM. When increasing the magainin 2 concentration to 800 nM, both SFG and ATR-FTIR results indicate that magainin 2 molecules insert into the POPG bilayer and adopt a transmembrane orientation with an angle of about 20 degrees from the POPG bilayer normal. For the POPC bilayer, even at a much higher peptide concentration of 2.0 microM, no ATR-FTIR signal was detected. For this concentration on POPC, SFG studies indicated that magainin 2 molecules adopt an orientation nearly parallel to the bilayer surface, with an orientation angle of about 75 degrees from the surface normal. This shows that SFG has a much better detection limit than ATR-FTIR and can therefore be applied to study interfacial molecules with a much lower surface coverage. This magainin 2 orientation study and further investigation of the lipid bilayer SFG signals support the proposed toroidal pore model for the antimicrobial activity of magainin 2.

The molecular surface conformation of surface-tethered polyelectrolytes on PDMS surfaces

Surface properties of polymer materials are important for many applications such as biomedical materials, marine antifouling coatings, polymer membranes for biological and chemical molecule separations, and polymer adhesives. Surface properties are highly dependent on molecular surface structures. Presently, surface polymerization is one of the most effective methods to tailor and optimize molecular surface structures of many polymers. Herein, the surface structure of polydimethylsiloxane (PDMS) is modified by tethering polyelectrolytes (PEs) through surface-initiated ultraviolet (UV) polymerization. Using dimethylacrylamide, acrylic acid, and [2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium as monomers, cationic, anionic, and zwitterionic PEs are grafted onto PDMS surfaces using the “graft-from” method. Successful grafting of PEs onto PDMS surfaces has been verified by several analytical techniques, including Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurement. In particular, sum-frequency generation (SFG) vibrational spectroscopy has been employed to probe the molecular structure of modified PDMS surfaces in both the dry and hydrated states. It was found that in air, the surface dominating –Si–CH3groups of unmodified PDMS segregate to the PDMS surface along with the grafted PEs. These methyl groups have similar orientations to those on the unmodified PDMS surface: they more or less stand up on the surface, along the surface normal. Upon exposure to water, only SFG signals from the surface-tethered PE chains on the modified PDMS surface are observed, showing a substantial surface restructuring behavior. More details regarding such surface restructuring behavior can be deduced through SFG data analysis. With numerous PDMS applications in aqueous environments, it is of particular importance to modify its surface structures and characterize such surface structures in aqueous environments in situ.

Imaging and quantitative analysis of atherosclerotic lesions by CARS-based multimodal nonlinear optical microscopy.

The purpose of this study was to assess the ability of label-free multimodal nonlinear optical (NLO) microscopy to characterize, and thus enable quantitative in situ analyses of, different atherosclerotic lesion types, according to the original scheme suggested by the AHA Committee. Iliac arteries were taken from 24 male Ossabaw pigs divided into lean control and metabolic syndrome groups and were imaged by multimodal NLO microscopy where sum-frequency generation (SFG) and 2-photon excitation fluorescence (TPEF) were integrated on a coherent anti-Stokes Raman scattering (CARS) microscope platform. Foam cells, lipid deposits, matrices, and fibrous caps were visualized with submicron 3D resolution. Starting from the adaptive intimal thickening in the initial stage to the fibrous atheroma or mineralization in the advanced stages, lesions were visualized without labels. Histological staining of each lesion confirmed the lesion stages. Lipid and collagen contents were quantitatively analyzed based on the CARS and SFG signals. Lipid accumulation in thickened intima culminated in type IV whereas the highest collagen deposition was found in Type V lesions. Luminal CARS imaging showed the capability of viewing the location of superficial foam cells that indicate relatively active locus in a lesion artery. We have demonstrated the capability of CARS-based multimodal NLO microscopy to interrogate different stages of lesion development with subcellular detail to permit quantitative analysis of lipid and collagen contents.

Ultrafast Nonlinear Coherent Vibrational Sum-Frequency Spectroscopy Methods To Study Thermal Conductance of Molecules at Interfaces

It is difficult to study molecules at surfaces or interfaces because the total number of molecules is small, and this is especially problematic in studies of interfacial molecular dynamics with high time resolution. Vibrational sum-frequency generation (SFG) spectroscopy, where infrared (IR) and visible pulses are combined at an interface, has emerged as a powerful method to probe interfacial molecular dynamics. The nonlinear coherent nature of SFG helps overcome the sensitivity issues, especially when femtosecond IR pulses are used. With femtosecond pulses, a range of vibrational transitions can be probed simultaneously and high time resolution can be achieved. Ultrafast SFG experiments use three pulses, a pump pulse to generate nonequilibrium conditions with a pair of probe pulses, and two time delay parameters. Mapping SFG intensity as a function of the two time delays creates a two-dimensional surface, where one axis (t(1)) provides information about molecular dynamics driven by the pump pulses, and the other axis (t(2)) about the dynamics of the SFG probing process. We present examples of ultrafast SFG measurements drawn from our studies of heat transport through interfacial molecules that are models for molecular wires in electronic circuits. In these flash-heating experiments, a self-assembled monolayer (SAM) of long-chain molecules adsorbed on a metal surface is subjected to a large amplitude (up to 800 K) temperature jump. Specific vibrational reporter groups on the SAM molecules probed by SFG serve as tiny ultrafast thermometers approximately 1.5 A thick with a approximately 1 ps response time. These SFG thermometers can monitor ultrafast heat transport through the SAM molecules. By varying the lengths of the molecular wires we can tell if the heat is propagating ballistically along the chains, at constant speed, or diffusively. In our analysis of 2D SFG methods, we first describe a simpler situation where the visible probe pulse is effectively infinite in duration. This is the usual way time-resolved SFG measurements are made, and the SFG experiment then becomes a function of a single time delay, the pump-IR probe delay t(1). Unfortunately, in this case the SFG signals have a large contribution from the nonresonant (NR) background generated by the metal surface, which adds a great deal of noise to the data, and the time resolution is limited by the molecule's vibrational dephasing time constant T(2), which is often 1 ps or more. We have recently shown that the NR background can be suppressed using a time delay t(2) between IR and visible probe pulses. In this now 2D SFG method, one would expect that information about the molecular response to the pump pulses would be contained in slices along the t(1) axis, but by simulating the experiment we show that the t(1) and t(2) parameters interact. Changing t(2) to suppress the NR background causes t(1) slices to shift in time. We also show how to improve the time resolution of ultrafast SFG experiments while maintaining NR suppression using femtosecond visible pulses at appropriate t(2) delay values.

An Optical Voltmeter for Studying Cetyltrimethylammonium Interacting with Fused Silica/Aqueous Interfaces at High Ionic Strength

Electrostatics and counterions play important roles in many supramolecular processes, including surfactant adsorption and aggregation at interfaces. Here, we assess their influence on how the common surfactant cetyltrimethylammonium (CTA) interacts with fused silica/aqueous interfaces by determining thermodynamic, kinetic, and electrostatic parameters for CTA adsorption across a range of NaCl concentrations (10-500 mM NaCl) using second harmonic generation (SHG). Using vibrational sum frequency generation (SFG), we demonstrate that vibrationally resonant contributions and nonresonant background contributions to the SFG signal intensity that depend on the interfacial potential can be quantified simultaneously during the adsorption process, which provides insight into the nonequilibrium dynamics of CTA adsorption. By analyzing the adsorption free energies as a function of interfacial potential at these four salt concentrations, the charge density per adsorbate is determined, indicating that CTA coadsorbs with counterions at a ratio of approximately 4 to 3 (i.e., 4 CTA(+) ions for every 3 Cl(-) ion). The chemical (i.e., non-Coulombic) portion of the free energy is found to dominate the overall free energy of adsorption, indicating that CTA adsorption at these ionic strengths is primarily driven by the favorable hydrophobic interactions between interdigitated surfactant hydrocarbon chains in the adsorbed aggregate. By applying Gouy-Chapman-Stern theory to our data, an average charge density of 2.8(3) x 10(13) charges/cm(2), which corresponds to 0.7 to 1.7 molecules/nm(2), was obtained for the four NaCl concentrations.

Enhanced detection of thiophenol adsorbed on gold nanoparticles by SFG and DFG nonlinear optical spectroscopy

Sum frequency generation (SFG) and difference frequency generation (DFG) are applied to study vibrational resonance of the thiophenol molecule adsorbed on two different gold samples. One sample is made of 17 nm gold nanoparticles (AuNPs) fixed on a silicon substrate that has been previously functionalized with a silane monolayer (aminopropyltriethoxysilane, APTES). This sample is fully characterized through visible reflection spectroscopy and AFM. The second sample is a gold monocrystal also covered with thiophenol molecules. From their comparison, an enhancement factor of 21 is deduced for the SFG signal on AuNPs with respect to the Au(111), related to the surface plasmon resonance (SPR). From a combined analysis of the SFG and DFG spectra, we demonstrate that SFG/DFG spectroscopy is able to identify the nature of the substrate where the molecules are adsorbed. This opens new perspectives for this nonlinear spectroscopy by adding to its well-known intrinsic surface specificity, the ability to selectively probe the chemical layer capping the AuNPs.