Paulo B. Miranda

Interaction of water with a fatty acid Langmuir film

Abstract Using sum-frequency vibrational spectroscopy we have studied the structure of the fatty acid monolayer–water interface as a function of pH of water. At low pH values, the monolayer is neutral and the surface water structure is disordered by hydrogen bonding of water molecules with the acid head-groups. At high pH values, acid head-groups are ionized and the resulting surface field helps the surface water molecules form a more ordered hydrogen-bonding network. At pH ∼3, a small change in pH leads to a reorientation of the acid OH groups while the alkyl chains remain conformationally ordered at all pH values.

Surface crystallization of liquid n-alkanes and alcohol monolayers studied by surface vibrational spectroscopy

Abstract Infrared-visible sum-frequency generation (SFG) has been used to obtain the surface vibrational spectra of CH stretches for liquid n -eicosane and a monolayer film of 1-dodecanol in equilibrium with an excess drop on water. The SFG data for surface freezing of eicosane represent a unique study of ordering among nonpolar surface molecules and are consistent with the formation of a solid monolayer of nearly vertical, all-trans molecules at the free liquid surface. For the dodecanol film, the result indicates that the alcohol molecules are nearly all-trans and erect both before and after surface crystallization.

Cholesterol Mediates Chitosan Activity on Phospholipid Monolayers and Langmuir-Blodgett Films

The polysaccharide chitosan has been largely used in many biological applications as a fat and cholesterol reducer, bactericide agent, and wound healing material. While the efficacy for some of such uses is proven, little is known about the molecular-level interactions involved in these applications. In this study, we employ mixed Langmuir and Langmuir-Blodgett (LB) films of negatively charged dimyristoyl phosphatidic acid (DMPA) and cholesterol as cell membrane models to investigate the role of cholesterol in the molecular-level action of chitosan. Chitosan does not remove cholesterol from the monolayer. The interaction with chitosan tends to expand the DMPA monolayer due to its interpenetration within the film. On the other hand, cholesterol induces condensation of the DMPA monolayer. The competing effects cause the surface pressure isotherms of mixed DMPA-cholesterol films on a chitosan subphase to be unaffected by the cholesterol mole fraction, due to distinct degrees of chitosan penetration into the film in the presence of cholesterol. By combining polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), we showed that chitosan induces order into negatively charged phospholipid layers, whereas the opposite occurs for cholesterol. In conclusion, chitosan has its penetration in the film modulated by cholesterol, and electrostatic interactions with negatively charged phospholipids, such as DMPA, are crucial for the action of chitosan.

Stability of the Si-H bond on the hydrogen-terminated Si(111) surface studied by sum frequency generation

Stability of the Si-H bonds on the hydrogen terminated Si(III) surface has been investigated by sum frequency generation (SFG) spectroscopy in air at room temperature. The SFG observation showed that the Si(III)surface is terminated by a monolayer of monohydride (Si-H) after etching in a concentrated ammonium fluoride (NH4F)solution. The number of Si-H bonds decreased with laser irradiation time and the abstraction rate of hydrogen atoms on Si increased with the increase of input energy of

Molecular view of the interaction between iota-carrageenan and a phospholipid film and its role in enzyme immobilization.

Proteins incorporated into phospholipid Langmuir-Blodgett (LB) films are a good model system for biomembranes and enzyme immobilization studies. The specific fluidity of biomembranes, an important requisite for enzymatic activity, is naturally controlled by varying phospholipid compositions. In a model system, instead, LB film fluidity may be varied by covering the top layer with different substances able to interact simultaneously with the phospholipid and the protein to be immobilized. In this study, we immobilized a carbohydrate rich Neurospora crassa alkaline phosphatase (NCAP) in monolayers of the sodium salt of dihexadecylphosphoric acid (DHP), a synthetic phospholipid that provides very condensed Langmuir films. The binding of NCAP to DHP Langmuir-Blodgett (LB) films was mediated by the anionic polysaccharide iota-carrageenan (iota-car). Combining results from surface isotherms and the quartz crystal microbalance technique, we concluded that the polysaccharide was essential to promote the interaction between DHP and NCAP and also to increase the fluidity of the film. An estimate of DHP:iota-car ratio within the film also revealed that the polysaccharide binds to DHP LB film in an extended conformation. Furthermore, the investigation of the polysaccharide conformation at molecular level, using sum-frequency vibrational spectroscopy (SFG), indicated a preferential conformation of the carrageenan molecules with the sulfate groups oriented toward the phospholipid monolayer, and both the hydroxyl and ether groups interacting preferentially with the protein. These results demonstrate how interfacial electric fields can reorient and induce conformational changes in macromolecules, which may significantly affect intermolecular interactions at interfaces. This detailed knowledge of the interaction mechanism between the enzyme and the LB film is relevant to design strategies for enzyme immobilization when orientation and fluidity properties of the film provided by the matrix are important to improve enzymatic activity.

The role of electron photoemission in the `photoconductivity' of semiconducting polymers

We present the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hm ¼ 6:2 eV. An apparent increase in the ‘photoconductivity’ at hm > 3–4 eV arises from external current generated by electron photoemission (PE). After quenching the PE by adding a gas mixture of CO2 þ SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied. The single threshold for photoconductivity is spectrally close to the onset of p–p � absorption, a behavior that is inconsistent with a large exciton binding energy. 2001 Elsevier Science B.V. All rights reserved.

CONFORMATION OF SURFACTANT MONOLAYERS AT SOLID/LIQUID INTERFACES

Abstract Infrared-visible sum-frequency (SFG) vibrational spectroscopy is used to study chain conformation of cationic surfactant monolayers (dioctadecyl dimethyl ammonium chloride, DOAC), adsorbed at various quartz/liquid interfaces at low coverage. The results show that the chains can assume many different conformations: an all-trans form if the liquid is alkane with a similarly long chain length, a bent form with significant trans-gauche defects if the liquid is CCL 4 or CDCL 3 or short-chain alkanes, and a highly contracted form if the liquid is water or alcohol.

Amperometric Detection of Lactose Using β-Galactosidase Immobilized in Layer-by-Layer Films

A direct, low-cost method to determine the concentration of lactose is an important goal with possible impact in various types of industry. In this study, a biosensor is reported that exploits the specific interaction between lactose and the enzyme β-galactosidase (β-Gal) normally employed to process lactose into glucose and galactose for lactose-intolerant people. The biosensor was made with β-Gal immobilized in layer-by-layer (LbL) films with the polyelectrolyte poly(ethylene imine) (PEI) and poly(vinyl sufonate) (PVS) on an indium tin oxide (ITO) electrode modified with a layer of Prussian Blue (PB). With an ITO/PB/(PEI/PVS)1(PEI/β-Gal)30 architecture, lactose could be determined with an amperometric method with sensitivity of 0.31 μA mmol(-1) cm(-2) and detection limit of 1.13 mmol L(-1), which is sufficient for detecting lactose in milk and for clinical exams. Detection occurred via a cascade reaction involving glucose oxidase titrated as electrolytic solution in the electrochemical cell, while PB allowed for operation at 0.0 V versus saturated calomel electrode, thus avoiding effects from interfering species. Sum-frequency generation spectroscopy data for the interface between the LbL film and a buffer containing lactose indicated that β-Gal lost order, which is the first demonstration of structural effects induced by the molecular recognition interaction with lactose.

Molecular Ordering of Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy

The molecular arrangement in organic thin films is crucial for their increasing technological applications. Here, we use vibrational spectroscopy by sum-frequency generation (SFG) to study the ordering of polyelectrolyte layers adsorbed on silica for all steps of layer-by-layer (LbL) self-assembly. In situ measurements during adsorption and rinsing showed that the adsorbed polymer has a disordered conformation and confirmed surface charge overcompensation upon polyelectrolyte adsorption by probing the interfacial electric field. In dry films, the polymer chains acquired a net orientational ordering, which was affected, however, by the adsorption of subsequent layers. Such a detailed characterization may allow the control of LbL film structure and functionality with unprecedented power.

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces.

Investigation into nanostructured organic films has served many purposes, including the design of functionalized surfaces that may be applied in biomedical devices and tissue engineering and for studying physiological processes depending on the interaction with cell membranes. Of particular relevance are Langmuir monolayers, Langmuir-Blodgett (LB) and layer-by-layer (LbL) films used to simulate biological interfaces. In this review, we shall focus on the use of vibrational spectroscopy methods to probe molecular-level interactions at biomimetic interfaces, with special emphasis on three surface-specific techniques, namely sum frequency generation (SFG), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and surface-enhanced Raman scattering (SERS). The two types of systems selected for exemplifying the potential of the methods are the cell membrane models and the functionalized surfaces with biomolecules. Examples will be given on how SFG and PM-IRRAS can be combined to determine the effects from biomolecules on cell membrane models, which include determination of the orientation and preservation of secondary structure. Crucial information for the action of biomolecules on model membranes has also been obtained with PM-IRRAS, as is the case of chitosan removing proteins from the membrane. SERS will be shown as promising for enabling detection limits down to the single-molecule level. The strengths and limitations of these methods will also be discussed, in addition to the prospects for the near future.