Biomedical subjects

Benjamin M Ocko

Publications and source records attributed to Benjamin M Ocko.

12 recordsLinked to original sources

PubMed2006

Surface crystallization in a liquid AuSi alloy.

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.

Journal Article↗

PubMed2006

High resolution non-contact AFM imaging of liquids condensed onto chemically nanopatterned surfaces.

The wetting of ethanol and octane on chemically nanopatterned surfaces has been investigated using Atomic Force Microscopy (AFM) under controlled environmental conditions. The patterns were generated on a methyl-terminated, organic monolayer using an AFM electro-oxidation process. The subsequent wetting of the organic liquids was studied using non-contact mode AFM under equilibrium conditions with the vapor. This study of condensed nanoliquids provides the first reliable measurements of sub 100 nm liquid profile shapes. The derived contact angles give an estimate of the line tension.

Ethanol↗

PubMed2006

Liquid nanostripes.

Equilibrium wetting of ethanol, a volatile liquid, onto chemically patterned nanostripes has been investigated using noncontact atomic force microscopy (AFM). The chemical patterns, generated by a conducting AFM tip, are composed of COOH terminated "wetting" regions and CH3 terminated "nonwetting" regions. Controlled amounts of ethanol, from the vapor phase, condense on the COOH stripes and their shape is imaged in situ versus their width (70 < w < 300 nm). The measured profile shapes at saturation and their w(1/2) height dependence are well described by density functional theory with dispersive, nonretarded potentials.

Journal Article↗

PubMed2006

Direct structural observation of a molecular junction by high-energy x-ray reflectometry.

We report a direct angstrom resolution measurement of the structure of a molecular-size electronic junction comprising a single (or a double) layer of alkyl-thiol and alkyl-silane molecules at the buried interface between solid silicon and liquid mercury. The high-energy synchrotron x-ray measurements reveal densely packed layers comprising roughly interface-normal molecules. The monolayer's thickness is found to be 3-4 A larger than that of similar layers at the free surfaces of both mercury and silicon. The origins of this and the other unusual features detected are discussed in this article. Measurements of the bilayer junction with an applied potential did not show visible changes in the surface normal structure.

Journal Article↗

PubMed2005

Electro pen nanolithography.

We introduce Electro Pen Nanolithography (EPN), a nanoscale chemical patterning technique. In a single sweep of an ink-coated, biased conducting Atomic Force Microscope (AFM) probe-tip, an underlying thin organic film is oxidized, and the ink molecules are transferred directly to the oxidized regions. Results using trialkoxysilane inks with functional terminal groups are presented, where the same ink-covered tips are used to image the pattern with zero bias. This technique provides very fast--exceeding 10 microm/s--writing speeds with line-widths as small as 50 nm. Controlled multi-write operations permit the growth of an integer number of molecular layers. Multiple chemical patterns using a single probe-tip are demonstrated.

Journal Article↗

PubMed2005

Liquids on topologically nanopatterned surfaces.

We report here surface x-ray scattering studies of the adsorption of simple hydrocarbon liquid films on nanostructured surfaces-silicon patterned by an array of nanocavities. Two different regimes, filling and growing, are observed for the wetting film evolution as a function of the chemical potential offset from the bulk liquid-vapor coexistence. The strong influence of geometrical effects is manifested by a dependence of liquid adsorption in the nanocavities that is stronger than the van der Waals behavior for flat surfaces. The observed dependence is, however, much weaker than predicted for the infinitely deep parabolic cavities, suggesting that the finite-size effects contribute significantly to the observed adsorption behavior.

Journal Article↗

PubMed2005

Periodic arrays of interfacial cylindrical reverse micelles.

We report an approach for the fabrication of periodic molecular nanostructures on surfaces. The approach involves biomimetic self-organization of synthetic wedge-shaped amphiphilic molecules into multilayer arrays of cylindrical reverse micelles. The films were characterized by atomic force microscopy and X-ray reflectivity. These nanostructured films self-assemble in solution but remain stable upon removal and exposure to ambient conditions, making them potentially suitable for a variety of dry pattern transfer methods. We illustrate the generality of this approach by using two distinct molecular systems that vary in size by a factor of 2.

Letter↗

PubMed2005

Large-scale fabrication of protein nanoarrays based on nanosphere lithography.

Hexagonally patterned lysozyme nanoarrays have been assembled on silicon wafers by combining nanosphere lithography and surface silane chemistry using vapor and solution deposition processes. The patterned protein regions extend over cm sized regions, and the size of each island is approximately 120 nm for the solution-prepared template and approximately 60 nm for the vapor-prepared template. Antibody test indicates that the patterned lysozyme maintains its bioactivity on the surface. This new approach offers a fast and reliable method to fabricate protein arrays over large areas with feature sizes comparable to scanning-probe based techniques.

Biosensing Techniques↗

PubMed2005

Structure of mercaptobiphenyl monolayers on mercury.

The molecular-scale structure and phase behavior of single-component Langmuir films of 4'-methyl-4-mercaptobiphenyl (MMB) and 4'-perfluoromethyl-4-mercaptobiphenyl (FMMB) on mercury were studied using surface tensiometry, grazing incidence X-ray diffraction, and X-ray reflectivity. At low coverages, a condensed but in-plane disordered single layer of surface-parallel molecules is found for both compounds. At high coverages, both compounds exhibit in-plane-ordered phases of standing-up molecules. For MMB, the biphenyl core dominates the structure, yielding a centered-rectangular unit cell with an area A(x) of 21.8 A(2)/molecule, with molecules tilted by approximately 14 degrees from the surface normal in the nearest-neighbor direction, and a coherence length xi of >1000 A for the crystalline domains. For FMMB, the perfluoromethyl group dominates the structure, yielding a hexagonal unit cell with untilted molecules, an area A(x) of 24.2 A(2)/molecule, and a much smaller xi of approximately 110 A. The structure is discussed in comparison with self-assembled monolayers of MMB on crystalline Au(111) and similar-length alkanethiolate SAMs on Au(111) and on mercury. The differences in the structure are discussed and traced to the differences in the substrate's surface structure, and in the molecular cross section and rigidity.

Journal Article↗

PubMed2005

Surface layering in ionic liquids: an X-ray reflectivity study.

The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.

Journal Article↗

PubMed2005

Surface freezing of chain molecules at the liquid-liquid and liquid-air interfaces.

Surface freezing (SF) is the formation of a crystalline monolayer at the free surface of a melt at a temperature Ts, a few degrees above the bulk freezing temperature, Tb. This effect, i.e. Ts > Tb, common to many chain molecules, is in a marked contrast with the surface melting effect, i.e. Ts < or = Tb, shown by almost all other materials. Depending on chain length, n, the SF layer shows a variety of phases, in some cases tuneable by bulk additives. The SF behaviour of binary mixtures of different-length alkanes and alcohols is governed by the relative chain length mismatch, /delta n/n/2, yielding a quasi-"universal" behaviour for the freezing of both bulk and surface. While SF at the liquid air interface was studied rather extensively, Lei and Bain (Phys. Rev. Lett., 2004, 94, 176103) have shown only very recently that interfacial freezing (IF) can be induced also at the water: tetradecane interface by adding the ionic surfactant CTAB to the water phase. We present measurements of the interfacial tension of the water: hexadecane interface, as a function of temperature and the ionic surfactant STAB, revealing IF at a STAB-concentration-dependent temperature Ti > Tb. The measurements indicate that a single frozen monolayer is formed, with a temperature-existence range of up to 10 degrees C, much larger than the 1.2 degrees C found for SF at the free surface of the melt. We also find a new effect, where the IF allows tuning of the interfacial tension between the two bulk phases to zero for a range of temperatures, deltaT = Tmix - Tb < or = Ti - Tb by cooling the system below Ti. We discuss qualitatively the factors stabilizing the frozen layer and their variation from the liquid-air to the liquid-liquid interfaces. The surfactant concentration dependence of Ti is also discussed and a tentative theoretical explanation is suggested.

Journal Article↗

PubMed2002

Spectroscopic [correction of eSpectroscopic] and structural properties of valine gramicidin A in monolayers at the air-water interface.

Monomolecular films of valine gramicidin A (VGA) were investigated in situ at the air-water interface by x-ray reflectivity and x-ray grazing incidence diffraction as well as polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These techniques were combined to obtain information on the secondary structure and the orientation of VGA and to characterize the shoulder observed in its pi-A isotherm. The thickness of the film was obtained by x-ray reflectivity, and the secondary structure of VGA was monitored using the frequency position of the amide I band. The PM-IRRAS spectra were compared with the simulated ones to identify the conformation adopted by VGA in monolayer. At large molecular area, VGA shows a disordered secondary structure, whereas at smaller molecular areas, VGA adopts an anti-parallel double-strand intertwined beta(5.6) helical conformation with 30 degrees orientation with respect to the normal with a thickness of 25 A. The interface between bulk water and the VGA monolayer was investigated by x-ray reflectivity as well as by comparing the experimental and the simulated PM-IRRAS spectra on D(2)O and H(2)O, which suggested the presence of oriented water molecules between the bulk and the monolayer.

Air↗