E. Mielczarski

Quantitative and qualitative evaluation of adsorption/desorption of bovine serum albumin on hydrophilic and hydrophobic surfaces.

We studied the adsorption of bovine serum albumin (BSA) from phosphate-buffered saline (pH 7.4) to hydrophilic and hydrophobic surfaces. Attenuated total reflection Fourier transform infrared spectroscopy, supported by spectral simulation, allowed us to determine with high precision the amount of BSA adsorbed (surface coverage) and its structural composition. The adsorbed BSA molecules had an alpha-helical structure on both hydrophobic and hydrophilic surfaces but had different molecular conformations and adsorption strengths on the two types of surface. Adsorption of BSA was saturated at around 50% surface coverage on the hydrophobic surface, whereas on the hydrophilic surface the adsorption reached 95%. The BSA molecules adsorbed to the hydrophilic surface with a higher interaction strength than to the hydrophobic surface. Very little adsorbed BSA could be desorbed from the hydrophilic surface, even using 0.1 M sodium dodecyl sulfate, a strong detergent solution. The formation of BSA-phosphate surface complexes was observed under different BSA adsorption conditions on hydrophobic and hydrophilic surfaces. The formation of these complexes correlated with the more efficient blocking of nonspecific interactions by the adsorbed BSA layer. Results from the molecular modeling of BSA interactions with hydrophobic and hydrophilic surfaces support the spectroscopic findings.

Polyoxometalate-based layered structures for charge transport control in molecular devices.

Hybrid organic-inorganic films consisted of molecular layers of a Keggin-structure polyoxometalate (POM: 12-tungstophosphoric acid, H(3)PW(12)O(40)) and 1,12-diaminododecane (DD) on 3-aminopropyl triethoxysilane (APTES)-modified silicon surface, fabricated via the layer-by-layer (LBL) self-assembly method are evaluated as molecular materials for electronic devices. The effect of the fabrication process parameters, including primarily compositions of deposition solutions, on the structural characteristics of the POM-based multilayers was studied extensively with a combination of spectroscopic methods (UV, FTIR, and XPS). Well-characterized POM-based films (both single-layers and multilayers) in a controlled and reproducible way were obtained. The conduction mechanisms in single-layered and multilayered structures were elucidated by the electrical characterization of the produced films supported by the appropriate theoretical analysis. Fowler-Nordheim (FN) tunneling and percolation mechanisms were encountered in good correlation with the structural characteristics of the films encouraging further investigation on the use of these materials in electronic and, in particular, in memory devices.

Magnetron-sputtered Ag surfaces. New evidence for the nature of the ag ions intervening in bacterial inactivation.

DC-magnetron sputtering with an Ag target on textile surfaces produced Ag particles with sizes approximately 4.7 nm (+/-15%). Sputtering for 15 s led to Ag layers of 15-20 nm. The threshold sputtering time precluding airborne bacterial growth was about 60 s. In this case, the coating was approximately 40-50 nm thick and the cotton Ag loading was 0.0026 wt %. The Ag particle size did not vary significantly with sputtering time between 15 and 600 s. Only coatings above this thickness lead to bacterial inactivation. Ag/Pt targets with sputtering times<60 s did not increase the bactericide performance of the Ag cotton samples with respect to sputtering from an Ag target alone, as expected from the position of Pt respect to Ag in the electrochemical series (Galvanic effect). The Ag cotton deposition led to very thin metallic semitransparent gray color coatings. X-ray of the Ag cotton suggested the presence of amorphous and crystalline Ag species. By X-ray photoelectron spectroscopy (XPS), it was found that the amount of oxidized silver species on the cotton was similar for sputtering times of 60 and 600 s, but the total amount of Ag deposited was almost two times higher after 600 s sputtering. This suggests that the positive silver-ions were located mainly at the silver interface. The type of silver ions produced using the Ag/Pt sputtering was determined to be very similar at 15, 60, and 600 s with the silver ions produced with the Ag target. This explains the lack of an increased inhibitory effect of Pt during the inactivation of airborne bacteria when present in the Pt/Ag target with respect to the Ag target, because in both cases similar silver ionic species were found.

The surface-segregated nanostructure of fluorinated copolymer-poly(dimethylsiloxane) blend films.

Two fluorinated/siloxane copolymers, O5/19 and D5/3, carrying 6 and 8 CF(2) groups in the fluoroalkyl tail, respectively, were used as the surface-active components of cured poly(dimethylsiloxane) (PDMS) blends at different loadings (0.3-5.0 wt % with respect to PDMS). The surface chemical composition was determined by angle-resolved X-ray photoelectron spectroscopy at the takeoff angles theta of 0 degrees, 60 degrees, and 75 degrees. It was found that the fluorinated copolymer was surface-segregated, and in-depth segregation (approximately 5 nm) depended upon the chemical structure of the copolymer. The surface fluorine atomic percentage of the blends with D5/3 was up to 3 orders of magnitude higher than the theoretical value expected for ideal homogeneous samples. Moreover, small amounts of the copolymer in the blends were sufficient to saturate the outermost surface in fluorine content. The chemical composition of the surface-segregated nanostructure of the films was also proven to be affected by external environment, namely, exposure to water.

Influence of chain length on adsorption of xanthates on chalcopyrite

The direct characterization at a molecular level of the surface products formed by the interaction of aqueous solutions of ethyl and amyl xanthate with chalcopyrite polarized to different potentials were carried out by the infrared external reflection technique recently developed for detailed study of the adsorbed layer on mineral surfaces. The experimental spectroscopic data combined with the simulation of hypothetical adsorption layers have let us determine the type, structure and the surface distribution of the adsorbed species produced at different adsorption conditions, and to propose mechanisms of the interaction between the aqueous solutions of collectors and the surface of chalcopyrite. The spectroscopic results show striking differences in the composition of the outermost layer of chalcopyrite contacted with ethyl or amyl xanthate solutions, which obviously will produce differences in the flotation behavior of the mineral and in consequence will influence its separation from other ore components. These observations explain why different xanthate homologues should be used in flotation practice. A detailed discussion of flotation pulp solution conditions to produce different types of hydrophobic surface species for either collector and collectorless flotation of chalcopyrite is also presented.

Innovative UVC Light (185 nm) and Radio-Frequency-Plasma Pretreatment of Nylon Surfaces at Atmospheric Pressure and Their Implications in Photocatalytic Processes

Innovative pretreatment by UVC light (185 nm) and by radio-frequency (RF) plasma at atmospheric pressure to functionalize the Nylon surface, increasing its bondability toward TiO(2), is reported in this study. In the case of UVC light pretreatment in air, the molar absorption coefficient of O(2)/N(2) at 185 nm is very low and the air in the chamber absorbs very little light from the UVC source before reaching the Nylon sample. Nylon fabrics under RF plasma were also functionalized at atmospheric pressure because of the marked heating effect introduced in the Nylon by the RF plasma. This effect leads to intermolecular bond breaking and oxygenated surface groups in the topmost Nylon layers. Both pretreatments enhanced significantly the photocatalytic discoloration of the red-wine stain in Nylon-TiO(2) compared with samples without pretreatment. The UVC and RF methods in the absence of vacuum imply a considerable cost reduction to functionalize textile surfaces, suggesting a potential industrial application. Red-wine-stain discoloration under simulated sunlight was monitored quantitatively by diffuse-reflectance spectroscopy and by CO(2) evolution. X-ray photoelectron spectroscopy (XPS) was used to monitor the changes of the C, N, and S species on the Nylon topmost layers during the discoloration process. Significant changes in the XPS spectra of Ti 2p peaks were observed during discoloration of the wine spots. Wine stains attenuated the signal of the Ti 2p (458.4 eV) peak in the Nylon-TiO(2)-stained wine sample at time zero (from now on, the time before the discoloration process). Furthermore, a decrease of the wine-related O 1s signal at 529.7 eV and N 1s signal at 399.5 eV was observed during the discoloration process, indicating an efficient catalytic decomposition of the wine pigment on Nylon-TiO(2). X-ray diffraction detected the formation of anatase on the Nylon fibers. High-resolution transmission electron microscopy shows the formation of anatase particles with sizes between 8 and 20 nm.

Real time evaluation of composition and structure of concanavalin A adsorbed on a polystyrene surface.

In situ qualitative and quantitative evaluations of adsorbed submonolayers and multilayers of the protein Concanavalin A (ConA) on a polystyrene surface were carried out by attenuated total reflection infrared spectroscopy. The influence of pH and adsorption time on the composition and structure of the adsorbed protein layers was investigated by comparison of the experimental spectra with simulated spectra of hypothetical multilayer systems with the assumed composition, thickness, and structure. This methodology allows the differentiation of observed spectral changes that result from pure optical effects, associated with the passing of an incident beam through the multilayer system, from the chemical and structural changes caused by physicochemical interactions of proteins with polymer surfaces. This represents significant progress since small variations in the band positions or intensities of amide I and amide II infrared absorbance bands have an important interpretation consequence. The applied methodology significantly reduces the misinterpretation of recorded spectra of protein layers and is rewarded by a deep insight of the structure and composition of the samples. The composition, structure, and kinetics of the adsorption of ConA and hydration level of the adsorbed layers were evaluated in detail. Competitive adsorption of bovine serum albumin on pre-adsorbed ConA layers also was investigated to characterize the ConA surface distribution. Parallel studies using X-ray photoelectron spectroscopy support the conclusions drawn from infrared spectroscopic investigations on ConA molecular distributions at the polymer surface. Two-step models that describe ConA submonolayer formation at pH 4.8 and multilayer formation at pH 7.8 are proposed.

Influence of solution conditions and mineral surface structure on the formation of oleate adsorption layers on fluorite

Abstract Adsorption of oleate on two slab samples of fluorite obtained from two different sources was studied under acidic (pH 6) as well as basic (pH 10) conditions and with different pre-treatments using infrared external reflection spectroscopy. The ratio of intensities of two most favorable calcium oleate conformations on the fluorite surface, characterized by absorbance bands at 1536 and 1576 cm −1 corresponding to bidentate and unidentate configuration, respectively, varied significantly with adsorption/desorption treatment indicating sensitivity of the structure of the adsorbed layers of oleate to experimental conditions and the nature of the mineral surface. The alterations in the surface as a consequence of various pre-treatments were found to influence the structure of the adsorbed layer as observed by changes in the absorbance bands in the corresponding infrared spectra. The most stable surface structure is the compact well-organized monolayer of oleate adsorbed on fluorite. Our molecular modeling computations of oleate adsorption on fluorite {111} surface indicated bidentate configuration to be most favorable. The orientation of the optimized oleate molecule (angles) thus calculated also matched well with experiments. The optimized unidentate fluorite–oleate surface complex was also found to have comparable energy even though relatively lower than the bidentate configuration.

Surface modification at the molecular level in mineral beneficiation

Abstract Direct information at a molecular level about surface products, their structure and surface distribution is vitally important to perform an efficient separation processes for mineral beneficiation. This information is equally important for the modification of surface properties of industrial minerals for various specific applications such as: fillers, composites, weathering resistance, etc. The understanding of the mechanisms and kinetics of interaction of the first adsorbed organic molecules with mineral surfaces is the fundamental requirement to make possible the prediction, control and modification of the macroscopic surface properties that govern the efficiency of separation technologies or new material formulations. The developed infrared external reflection technique has a very unique ability to study interface phenomena at a molecular level on heterogeneous substrates. The variety, precision and reliability of information about interface phenomena delivered by this technique are superior to other single techniques. The experiments are fast and nondestructive. High sensitivity (starting from 20% of monolayer), in situ collected information in a multiphase system even in the region of a strong absorption of substrate, makes this technique a very valuable experimental tool. The complexity of the recorded reflection spectra, their sensitivity to any variations of the optical properties of all bulk and surface components and their spatial distribution in the system under investigation are in fact the major strength of the technique. In this paper, a few examples of application of this multidiagnostic technique for monitoring surface modifications of sulfide minerals in aqueous solutions will be discussed in detail.

Composition and structure of self-assembled layers at molecular level

Abstract The structure, orientation and chemical interaction of self-assembled submonolayers and monolayers at the interface of non-metallic (semiconductor, dielectric) and metallic substrates were studied by IR reflection spectroscopy. The reflection spectra of the adsorbed layers on different substrates recorded at different angles of incidence and for two polarizations are an excellent source of information about the composition and structure of the layers. The reflection spectra of the surface layer can be recorded for any type of substrate: transparent, opaque or non-transparent. The major problem is the interpretation of the recorded spectra which are modified by optical effects, compared with the corresponding transmission spectra of the same layer without a substrate support. The method developed allows one to interpret the experimental reflection spectra on the basis of spectral simulation data. This work demonstrates that it is possible to obtain information on chemical and structural changes in the adsorption layer and distinguish them from other observed spectral modifications due to optical effects. The high sensitivity of this method permits the investigation of ultrathin layers (submonolayers). In this presentation the experimental and calculated spectra of adsorption layers of surfactants on different types of substrate are discussed for various experimental conditions. By combination of the simulated and experimental data, the orientation angles of the transition moments resulting from major IR bands and the thickness of the adsorption layers were evaluated.