Characterization of surface-confined alpha-synuclein by surface plasmon resonance measurements

Urea-driven denaturation and renaturation of surface-bound alpha-synuclein are monitored by surface plasmon resonance (SPR) spectroscopy. The differential SPR angle shift (Delta Theta(SPR))(Net) enables us to estimate the Gibbs free energy change (DeltaG(o)) for the denaturation of the supported alpha-synuclein. DeltaG(o) for the denaturation of the supported alpha-synuclein, which is indirectly related to its biological activity can be increased significantly by the mixed self-assembled monolayers of 11-mercaptoundecanoic acid and 1,6-hexanedithiol. These SPR measurements of surface-bound biomolecules suggested herein can be further utilized to design effective biological scaffold for biosensor, biocatalyst, and possible diagnosis.     

Characterization of aluminium surface treatments by means of gas adsorption measurements

Adsorption measurements were used to determine the specific surface area of a.c. electrolytically grained aluminium and the porosity distribution diagram of porous anodized aluminium. Alternating current electrolytic graining of aluminium is used to increase the specific surface area for the preparation of litho sheets or capacitor foil. The specific surface area S (calculated by the BET procedure from the nitrogen adsorption isotherm) was studied as a function of the graining frequency between 0.1 and 1000 Hz. The results were related to the graining morphology by comparing them with scanning electron microscopy micrographs. It is concluded that in contrast to the more common surface investigation techniques the method can be used to measure quantitatively the gain in surface area after such a treatment as a.c. electrolytic graining. In the second part of the paper the pore size distribution diagrams are calculated from the adsorption isotherm for porous sulphuric acid films. It is shown that the dominant pore diameter corresponds rather well to the pore diameter observed in transmission electron microscopy sectional views. This means that the adsorption method can be applied to study the porosity of the aluminum oxide films. The method was then used to study the influence of the pretreatment on the porous oxide film morphology.     

Characterization of surface coatings by the scratch adhesion test and by indentation measurements

Summary Functional coating and surface modified layers can be characterized by engineering related criteria such as the ultramicrohardness test and the scratch adhesion test. The ultramicrohardness test is based on penetration depth measurements during a loading and unloading cycle. The depth sensing indentation provides a means of evaluating the elastic and plastic deformations of the tested material, from which, by calculation, the microhardness, HV, and the elasticity modulus, E, can be derived as illustrated on a number of examples of hard and soft materials and coating composites. The scratch test (REVETEST^R) and microscratch test (MST^R) has been applied to determine the adhesive and/or cohesive strength of coatings deposited on softer or tougher substrates. A Rockwell C diamond cone is used to scratch the surface of the coated or surface-modified specimen at a constant speed and under a continuously increasing load. The smallest load at which the coating is damaged, called the Critical Load, L_c, is determined by optical, electron optical, acoustic emission detection and/or by frictional force measurement, i.e. by the sudden increase of the driving force. This is illustrated on hard, brittle, and on soft coatings.     

Characterization of film surface treated with ECR plasma by Doppler broadening

Doppler broadened positron annihilation measurements were carried out using the positron beam technique on plasma treated polyethylene films as a function of incident positron energy. In addition, surface properties of the treated films also have been measured using other conventional techniques such as FT-IR, SEM and AFM. The surface tension of the films was also determined using sessile drop method. The S-parameter is seen to decrease on the surface upon plasma treatment that introduces polar groups such as hydroxyl and carbonyl on the surface. The results are discussed.     

The characterization of polymer interfaces by fluorescence decay measurements

Fluorescence decay measurements of energy transfer between donor (D) and acceptor (A) chromophores covalently attached to polymers can provide rich information about a polymer system. Here we use these techniques to study two quite different polymer phenomena. The first involves the diffusion of polymer molecules across the interfaces created during latex film formation. We are able to determine diffusion coefficients for this process. The second involves the phase separation which occurs in mixtures of homopolymers and graft copolymers. Here we are able to show that where a component of the graft is highly incompatible with the homopolymer matrix, interconnected domains of very small size can form. Under certain circumstances fluorescence decay measurements in conjunction with the recent model for “energy transfer in restricted geometrics” can be used to map out the size and shape of these domains.     

Characterization of metallized ceramic interfaces

Using a variety of surface analytical tools, the interfaces of several metal-ceramic composites have been characterized. Three processes that lead to the bonding of a metallizing to a ceramic substrate are illustrated. When a pure refractory metallizing is deposited onto a 94% Al₂O₃, bonding is achieved by glass migrating from the ceramic into the metallizing during firing. During cooling, the glass forms a mechanical-chemical bond between the ceramic and metallizing. In order to achieve bonding to a 99⁺% Al₂O₃ or 99⁺% BeO, the metallizing itself must contain a sufficient quantity of glass for wetting the ceramic, or be capable of forming a direct chemical bond to the ceramic.     

Polysaccharides at interfaces 1. Adsorption of cholesteryl-pullulan derivatives at the solution-air interface. Kinetic study by surface tension measurements

The surface properties of a series of cholesteryl-pullulan (CHP) derivatives have been assessed by surface tension measurements at the solution-air interface. The results reveal that these properties are related to the nature of the hydrophobic cholesteryl group substituted in pullulan, and that the unsubstituted polysaccharide does not display any surface activity. The adsorption kinetics of such an amphiphilic macromolecule has been shown to be diffusion controlled, obeying the Ward and Tordai¨diffusional model only at low solution concentrations. In the 2 × 10⁻⁷–5 × 10⁻⁶ mol l⁻¹ concentration range for which this model is verified, the calculated diffusion coefficients are concentration dependent. The non-ideality of the system at higher concentrations may be explained both by the presence of solute/solute interactions in solution and in adsorbed monolayers, and by the existence of an adsorbed layer, even at time t₀, which prevents the process of adsorption from being governed only by diffusion.     

Communication: enhancement of dopant dependent x-ray photoelectron spectroscopy peak shifts of Si by surface photovoltage

Binding energies measured by x-ray photoelectron spectroscopy (XPS) are influenced by doping, since electrons are transferred to (p-type) and from (n-type) samples when they are introduced into the spectrometer, or brought into contact with each other (p-n junction). We show that the barely measurable Si2p binding energy difference between moderately doped n- and p-Si samples can be enhanced by photoillumination, due to reduction in surface band-bending, which otherwise screens this difference. Similar effects are also measured for samples containing oxide layers, since the band-bending at the buried oxide-Si interfaces is manifest as photovoltage shifts, although XPS does not probe the interface directly. The corresponding shift for the oxide layer of the p-Si is almost twice that of without the oxide, whereas no measurable shifts are observable for the oxide of the n-Si. These results are all related to band-bending effects and are vital in design and performance of photovoltaics and other related systems.     

Electrochemical mass spectroscopic and surface photovoltage studies of catalytic water photooxidation by undoped and carbon-doped titania

Carbon-doped TiO2, demonstrated as an efficient photocatalyst in visible light photooxidation of organic compounds, was prepared with different doping concentrations and investigated via differential electrochemical mass spectroscopy (DEMS) and capacitive surface photovoltage (SPV) measurements in the form of thin layer electrodes. In all cases the total photocurrent as well as the surface photovoltage of the doped materials decreased markedly in relation to the undoped one. No detectable oxygen evolved from the doped electrodes in acidic solution under UV-light excitation. Since an oxidation of formic acid is still apparent, it is concluded that this oxidation occurs via isolated, catalytically poorly active trap states within the forbidden energy region. The existence of these states is confirmed by capacitive SPV measurements.     

Controlled silicon surface functionalization by alkene hydrosilylation

Immobilization of indene ligands onto two types of hydrogen-terminated surfaces, oxide-free Si [H/Si(111)] and oxidized Si [H/SiO2/Si], has been investigated by infrared absorption spectroscopy. The activity of a common catalyst (H2PtCl6) is shown to depend critically on the nature of the solvent. For instance, 2-propanol preferentially reacts with the surface, preventing any ligand attachment. Chlorobenzene is more stable, allowing some ligand attachment, but the H2PtCl6 catalyst also fosters silicon oxidation. In contrast, UV irradiation on oxide-free surfaces promotes a cleaner and more efficient reaction, leading to ligand attachment without substrate oxidation. The complete inactivity of H-terminated surfaces with a thin oxide layer [H/SiO2/Si] suggests that the UV-induced immobilization is mediated solely by the excitation of electron-hole pairs (excitons) in the substrate and is not the result of direct Si-H bond breaking.     

Ion-transfer voltammetry at silicon membrane-based arrays of micro-liquid-liquid interfaces

Microporous silicon membranes, fabricated by lithographic patterning and wet and dry silicon etching processes, were used to create arrays of micro-scale interfaces between two immiscible electrolyte solutions (muITIES) for ion-transfer voltammetry. These membranes served the dual functions of interface stabilization and enhancement of the rate of mass-transport to the interface. The pore radii were 6.5 microm, 12.8 microm and 26.6 microm; the pore-pore separations were ca. 20- to 40-times the pore radii and the membrane thickness was 100 microm. Deep reactive ion etching (DRIE) was used for pore drilling through the silicon, which had been previously selectively thinned by potassium hydroxide etching. DRIE produces hydrophobic fluorocarbon-coated internal pore walls. The small pore sizes and large pore-pore separations used resulted in steady-state voltammograms for the transfer of tetramethylammonium cation (TMA(+)) from the aqueous to the organic phase, whereas the reverse voltammetric sweeps were peak-shaped. These asymmetric voltammograms are consistent with the location of the ITIES at the aqueous side of the silicon membrane such that the organic phase fills the micropores. Comparison of the experimental currents to calculated currents for an inlaid disc micro-interface revealed that the interfaces were slightly recessed, up to 10 microm (or 10% of the pore length) in one case. Facilitated ion transfer, with an organic-phase ionophore, confirmed the location of the organic phase within the pores. These microporous silicon membranes offer opportunities for various analytical operations, including enhancing the rate of mass transport to ITIES-based sensing devices and stabilization of the ITIES for hydrodynamic applications.     

Characterization of solar grade silicon by Neutron Activation Analysis

An analytical method using neutron activation was developed in order to orientate and check different silicon elaboration processes either as solid ingots or ribbon shaped. This method without chemical separation after irradiation implies the use of a high efficiency semiconductor detector. A particular attention was paid to different causes of error and to the detection limits really obtained. These limits range from 10⁹ to 10¹⁵ at.cm^–3 for about 30 elements systematically locked for after a 72-h irradiation.     

Determining the mechanical response of particle-laden fluid interfaces using surface pressure isotherms and bulk pressure measurements of droplets

The mechanical response of particle-laden fluid interfaces is determined by measuring the internal pressures of particle-coated drops as a function of the drop volume. The particle monolayers undergoing compression-expansion cycles exhibit three distinct states: fluid state, jammed state, and buckled state. The P-V curves are compared to the surface pressure isotherms Pi-A that are measured using a Langmuir trough and a Wilhelmy plate on a flat water-decane interface covered with the same particles. We find that in the fluid and jammed states, the water drop in decane can be described by the Young-Laplace equation. Therefore in these relatively low compression states, the bulk pressure measurements can be used to deduce the interfacial tension of the droplets and yield similar surface pressure isotherms to the ones measured with the Wilhelmy plate. In the buckled state, the internal pressure of the drop yields a zero value, which is consistent with the zero interfacial tension measured with the Wilhelmy plate. Moreover we find that the compressibility in the jammed state does not depend on the particle size.     

Dye-sensitized solar cells based on nanocrystalline TiO2 films surface treated with Al3+ ions: photovoltage and electron transport studies

Nanocrystalline TiO2 films, surface modified with Al3+, were manufactured by depositing a TiO2 suspension containing small amounts of aluminum nitrate or aluminum chloride onto conducting glass substrates, followed by drying, compression, and finally heating to 530 degrees C. Electrodes prepared with TiO2 nanoparticles coated with less than 0.3 wt % aluminum oxide with respect to TiO2 improved the efficiency of the dye sensitized solar cell. This amount corresponds to less than a monolayer of aluminum oxide. Thus, the Al ions terminate the TiO2 surface rather than form a distinct aluminum oxide layer. The aluminum ion surface treatment affects the solar cell in different ways: the potential of the conduction band is shifted, the electron lifetime is increased, and the electron transport is slower when aluminum ions are present between interconnected TiO2 particles.     

Observation of diffusion and tunneling recombination of dye-photoinjected electrons in ultrathin TiO2 layers by surface photovoltage transients

Surface photovoltage transients were used to monitor both the short time dynamics (>10 ns) and the spatial distribution of electrons photoinjected in thin (2-20 nm) TiO2 layers from dye molecules adsorbed at the surface. At low temperatures (100-250 K), the dynamics are governed exclusively by spatially dependent tunneling recombination, with a rate that varies with the distance from the surface x as exp(-2x/a), and an initial exponential distribution of photoinjected electrons, n0 exp(-x/b). This model is confirmed by the observation of power law decay in time t(-a/2b) with a ratio a/b = 0.28 +/- 0.04. The stability of cis-di(isothiocyanato)-N-bis(2,2'-bipyridine-4,4'-dicarboxy) ruthenium(II) (N3) dye molecules on TiO2 during treatment in a vacuum at high temperatures was proven. For high temperatures (250-540 K), the thickness dependence of the decays indicates that the dynamics of surface recombination are retarded by the diffusion of electrons toward the interior of the film. The implications for thin layer coating in dye-sensitized solar cells are discussed.     

Characterization of buried interfaces by multiple internal reflection spectroscopy (MIRS)

Multiple internal reflection spectroscopy (MIRS) was applied to analyse atomic bonds at deep (500 m) interfaces of directly bonded Si/Si wafer pairs. It is shown that under the conditions used the polarized spectra contain information only about the interface layer a few nanometers thick. Examples are given of analysing the Si-H- and SiO-H vibration modes and bands of undissociated water in interfaces of bonded hydrophobic and hydrophilic wafer pairs, respectively, after annealing at temperatures between 200 and 1100 °C. Variations of the bonding behaviour (especially caused by alterations of the Si-H bonds) are discussed.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Model aluminum-poly(p-phenylenevinylene) interfaces studied by surface raman spectroscopy

Polymeric organic light-emitting diodes (OLEDs) have emerged as inexpensive and versatile alternatives to traditional inorganic-based display technologies. Further advances in this field depend on extending device lifetimes and improving electroluminescence efficiencies, both of which are strongly coupled to the integrity of the electrode/organic contacts. Therefore, the deposition of low-work function metals on organic materials has been extensively studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and near-edge X-ray adsorption fine structure (NEXAFS) to ascertain information about the chemical and electronic states within the interfacial region. However, to date, the only molecular structural information about metal-organic species formed in these regions has come from fits of theoretical models to existing XPS and UPS data. Very little direct structural information exists about the potential multitude of metal-organic species formed during cathode deposition. We report the use of surface Raman spectroscopy to study the interactions between aluminum and trans-stilbene, a model for poly(p-phenylenevinylene) (PPV). The Raman spectral data suggest preferential formation of covalent Al-C bonds at the vinylene carbons as opposed to the phenyl carbons of PPV.     

Determination of surface area of copolymer latex particles by surface tension measurements

The surface tension γ of four copolymer latices, of their respective serums, and of aqueous solutions of dispersant alone were measured at various dilutions. By extrapolating the surface excess of dispersant (calculated by the Gibbs adsorption equation) both at the aqueous solution surface and at the serum surfaces to 1/c = 0 (c being the bulk concentration of dispersant) the same limiting site area A_lim per adsorbed molecule was determined. Amounts of dispersant adsorbed by copolymer particles at various dilutions were determined from differences between the known total concentrations of the dispersant in latex and in serum at the same γ. These values were then extrapolated to the maximum adsorption at 1/c = 0 in latex. The surface area of copolymer particles was determined therefrom by using A_lim. The average particle radius calculated this way agrees reasonably well with electron microscope measurements. Thus it appears that the method for determining latex particle surface area by surfactant titration may be calibrated by means of the Gibbs adsorption equation, provided one uses A_lim and not the site area at the critical micelle concentration of dispersant.