Large area, molecularly smooth (0.2 nm rms) gold films for surface forces and other studies

Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 +/- 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.     

Extraction of CO2 from air samples for isotopic analysis and limits to ultra high precision delta18O determination in CO2 gas.

The determination of delta18O values in CO2 at a precision level of +/-0.02 per thousand (delta-notation) has always been a challenging, if not impossible, analytical task. Here, we demonstrate that beyond the usually assumed major cause of uncertainty - water contamination - there are other, hitherto underestimated sources of contamination and processes which can alter the oxygen isotope composition of CO2. Active surfaces in the preparation line with which CO2 comes into contact, as well as traces of air in the sample, can alter the apparent delta18O value both temporarily and permanently. We investigated the effects of different surface materials including electropolished stainless steel, Duran glass, gold and quartz, the latter both untreated and silanized. CO2 frozen with liquid nitrogen showed a transient alteration of the 18O/16O ratio on all surfaces tested. The time to recover from the alteration as well as the size of the alteration varied with surface type. Quartz that had been ultrasonically cleaned for several hours with high purity water (0.05 microS) exhibited the smallest effect on the measured oxygen isotopic composition of CO2 before and after freezing. However, quartz proved to be mechanically unstable with time when subjected to repeated large temperature changes during operation. After several days of operation the gas released from the freezing step contained progressively increasing trace amounts of O2 probably originating from inclusions within the quartz, which precludes the use of quartz for cryogenically trapping CO2. Stainless steel or gold proved to be suitable materials after proper pre-treatment. To ensure a high trapping efficiency of CO2 from a flow of gas, a cold trap design was chosen comprising a thin wall 1/4" outer tube and a 1/8" inner tube, made respectively from electropolished stainless steel and gold. Due to a considerable 18O specific isotope effect during the release of CO2 from the cold surface, the thawing time had to be as long as 20 min for high precision delta18O measurements. The presence of traces of air in almost all CO2 gases that we analyzed was another major source of error. Nitrogen and oxygen in the ion source of our mass spectrometer (MAT 252, Finnigan MAT, Bremen, Germany) give rise to the production of NO2 at the hot tungsten filament. NO2+ is isobaric with C16O18O+ (m/z 46) and interferes with the delta18O measurement. Trace amounts of air are present in CO2 extracted cryogenically from air at -196 degrees C. This air, trapped at the cold surface, cannot be pumped away quantitatively. The amount of air present depends on the surface structure and, hence, the alteration of the measured delta18O value varies with the surface conditions. For automated high precision measurement of the isotopic composition of CO2 of air samples stored in glass flasks an extraction interface ('BGC-AirTrap') was developed which allows 18 analyses (including standards) per day to be made. For our reference CO2-in-air, stored in high pressure cylinders, the long term (>9 months) single sample precision was 0.012 per thousand for delta13C and 0.019 per thousand for delta18O.     

Rotational invariance and double frustration in the structures of gold clusters growing around the F(s)-defected MgO (100) surface

The interaction of small gold clusters (Au(n), n = 1-4, 20) and a gold monolayer with the MgO (100) surface surrounding a neutral oxygen vacancy (F(s) center) is investigated using density-functional (DF) calculations. It is found that the presence of the defect modifies the interaction of gold not only with the vacancy itself, but also with the oxygen and magnesium atoms around it by increasing both the adhesion energy and the equilibrium bond distances. This is at variance with the interaction of metal atoms with the regular MgO (100) surface or the F(s) defect itself, in which an increase of the adhesion energy is associated with a shortening of the metal-surface distance. The resulting double frustration and cylindrical invariance of the metal-surface interaction cause small gold clusters growing around an F(s) nucleation center to be highly fluxional in terms both of rotational freedom and of multiple competing structural motifs. Fragmentation energies of the gold clusters are also discussed, finding that the lowest-energy pathway corresponds to the detachment of a dimer.     

Study on degradation characteristics of chitosan by pepsin with piezoelectric quartz crystal impedance analysis technique.

Piezoelectric quartz crystal impedance analysis technique was applied to study the chitosanolytic activity of pepsin. The method is based on the viscosity-density reduction of chitosan solution during the enzymatic degradation process. Experiments examined the time courses of the variations of motional resistance (deltaR1) for a quartz crystal. By comparing the deltaR1 response curves under different degradation conditions, the effects of pH, temperature, enzyme and substrate concentration on the chitosanolytic activity of pepsin was investigated in detail. The results suggest that the optimum pH and temperature were 4.6 and 55 degrees C, respectively. Increasing aptly the enzyme or substrate concentration was in favor of the degradation of chitosan. Moreover, the influence of the degree of deacetylation (DD) on the enzymatic degradation was studied. The result indicates that chitosan with a lower DD was easier to be degrade compared with chitosan with a higher DD. Also, it was found that there was a good linear relationship between the deltaR1 response and the DD value. The regression equation was deltaR1 = 0.058 x DD-6.795 and the correlation coefficient was 0.987.     

Assessment of 4-(dimethylamino)pyridine as a capping agent for gold nanoparticles

Gold nanoparticles stabilized with 4-(dimethylamino)pyridine (DMAP) were prepared by ligand exchange and phase transfer (toluene/water) of functionalized gold nanoparticles. DMAP-protected gold nanoparticles are water-soluble, positively charged, and fairly monodisperse (6.2 +/- 0.9 nm). To understand the scope of this interesting system, the details of the binding of DMAP to gold nanoparticles were investigated. The adsorption of DMAP onto gold surfaces was studied by electrochemistry and surface plasmon resonance. It is concluded that of the three most likely binding modes, the one involving the pyridine nitrogen binding to the gold surface, as suggested previously (Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001), is consistent with experimental data. Other 4-substituted pyridines were also assessed as capping agents. The solubility in toluene and basicity of the incoming ligand, as well as the ability to form charged nanoparticles, determine whether ligand exchange and subsequent phase transfer of the nanoparticles occur. The solubility and stability of the DMAP-protected gold nanoparticles were studied as a function of pH using UV-visible spectroscopy and transmission electron microscopy (TEM). These nanoparticles are soluble and stable over a wide pH range (5.0-12.8). It was found that excess DMAP is necessary for both the preparation and the stability of the DMAP-protected gold nanoparticles.     

Quantification of fluorine density in the outermost atomic layer

The outermost atomic layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Absolute quantification of fluorine density in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine density of monolayers of a C8F17-thiol on gold was 1.48 x 10(15) F atoms/cm2, whereas for PTFE a value of 1.24 x 1015 F atoms/cm2 was observed. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a molecular interpretation on the perfluorinated thiol monolayer on gold are given.     

19F Magic angle spinning NMR reporter molecules: empirical measures of surface shielding, polarisability and H-bonding

Magic Angle Spinning (MAS) (19)F NMR spectra have been obtained and chemical shifts measured for 37 molecules in the gas phase and adsorbed on the surfaces of six common materials: octadecyl- and octyl-functionalised chromatography silicas, Kieselgel 100 silica, Brockmann neutral alumina, Norit activated charcoal and 3-(1-piperidino)propyl functionalised silica. From these six surfaces, octadecyl-silica is selected as a non-polar reference to which the others are compared. The change in chemical shift of a fluorine nucleus within a molecule on adsorption to a surface from the gas phase, Deltadelta(gas)(surface), is described by the empirical relationship: Deltadelta(gas)(surface) = delta(s) + (alpha(s)+pi(s))/alpha(r) (Deltadelta(gas)(reference) - delta(r)) + delta(HBA) + delta(HBD), where delta(s) and delta(r) are constants that describe the chemical shift induced by the electromagnetic field of the surface under investigation and reference surface, alpha(s) and alpha(r) are the relative surface polarisability for the surface and reference, pi(s) is an additional contribution to the surface polarisabilities due to its ability to interact with aromatic molecules, and delta(HBA) and delta(HBD) are measurements of the hydrogen acceptor and donor properties of the surface. These empirical parameters are measured for the surfaces under study. Silica and alumina are found to undergo specific interactions with aromatic reporter molecules and both accept and donate H-bonds. Activated charcoal was found to have an extreme effect on shielding but no specific interactions with the adsorbed molecules. 3-(1-Piperidino)propyl functionalised silica exhibits H-bond acceptor ability, but does not donate H-bonds.     

Thermosensitive copolymer networks modify gold nanoparticles for nanocomposite entrapment

The core-shell gold nanoparticles and copolymer of N-isopropylacrylamide (NIPAM) and N,N'-methylenebisacrylamide (MBAA) hybrids (Au@copolymer) were fabricated through surface-initiated atom-transfer radical polymerization (ATRP) on the surface of gold nanoparticles in 2-propanol/water mixed solvents. The surface of citrate-stabilized gold nanoparticles was first modified by a disulfide initiator for ATRP. The slight cross-linking polymerization between NIPAM and MBAA occurred on the gold surface and resulted in the formation of core-shell Au@copolymer nanostructures that were characterized by TEM, and FTIR and UV-visible spectroscopy. Such synthesized Au@copolymer hybrids possess clearly thermosensitive properties and exhibit "inspire" and "expire" water behavior in response to temperature changes in aqueous solution. Because of this property, we enable to trap and encapsulate smaller nanoparticles by using the free space of the copolymer-network scaffold anchored at the gold surface.     

Structure of the Langmuir monolayers with fluorinated ethyl amide and ethyl ester polar heads creating dipole potentials of opposite sign

This study experimentally checks our previous hypothesis (Petrov, J. G.; Polymeropoulos, E. E.; Moehwald, H. Langmuir 2007, 23, 2623) that different conformations of the fluorinated heads of RCONHCH(2)CF(3) and RCOOCH(2)CF(3) monolayers cause the opposite signs and the striking difference of 1.480 V between their surface potentials Delta V. In situ X-ray diffraction at grazing incidence (GIXD) shows that both monolayers form orthorhombic lattices with closely packed chains tilted to the next-nearest neighbors in the RCONHCH(2)CF(3) film and upright in the RCOOCH(2)CF(3) monolayer. The packing of the chains in the plane perpendicular to them, which excludes the effect of the tilt, shows the same distance between the next-nearest neighbors, but significantly closer nearest neighbors in the RCONHCH(2)CF(3) film. This difference implies a specific anisotropic attraction between the adjacent amide heads. IR reflection absorption spectroscopy (IRRAS) shows that the -CONHCH(2)CF(3) heads have trans conformation and participate in H-bonds forming a -NH...O=C- lateral network. We speculate that such structure hinders the energetically optimal orientation of the hydrophobic -CH(2)CF(3) terminals toward air, so that the (delta+)C-(F (delta-))(3) dipoles at the monolayer/water boundary yield a strong positive contribution to Delta V. In contrast, most of the unbounded by H-bonds -COOCH(2)CF(3) heads statistically orient their hydrophobic (delta+)C-(F (delta-))(3) dipoles toward air, yielding a negative average dipole moment at the monolayer/water boundary and negative surface dipole potential.     

Differentiation Between Anti-Epidermal Growth Factor Receptors Antibody Conjugated and Unconjugated Gold Nanoparticles Using Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) along with UV-Vis was used to distinguish between structural differences between a normal human breast cell line (MCF-12 F) and a cancerous breast cell line (MCF-7) after both being treated with conjugated gold nanoparticles and unconjugated gold nanoparticles. The MCF-7 cell line possessed a small amount of epidermal growth factor receptors (EGFR) on its cell surface whereas the MCF-12 F cell line did not. The anti-EGFR antibodies blocked the binding of epidermal growth factors to this receptor which plays an important role in the regulation of cell growth, proliferation, and differentiation. The infrared (IR) spectra for both cell lines yield several differences between normal biological samples and cancerous samples. As the malignancy of the sample increases, the peak intensity and wavenumber positions decrease. The same conclusion was drawn from thin-layers of conjugated nanoparticles and nonconjugated nanoparticles. The presence of conjugated nanoparticles increased the peak intensity of the MCF-7 cell line whereas it decreased the peak intensity for the MCF-12 F cell line. UV-Vis was used to show the presence of the anti-EGFR antibodies on the surface of the gold nanoparticles which was 5 nm red shifted compared to the gold nanoparticles solution.     

Preliminary study on application of impregnated synthetic peptide TLC stationary phases for the pre-screening of 5-HT1A ligands

Chromatographic parameters (deltaR(f)), defined as a difference in the migration of tested compound on the control and peptide impregnated silica gel TLC plates, were determined for 42 arylpiperazine derivatives. An amino acid sequence of the peptide used for impregnation was derived from the III transmembrane segment of the 5-HT(1A) receptor in the close vicinity of aspartic acid (Asp 166) residue. It was found that the deltaR(f) values obtained in a model employing tetrapeptide P4LA (ADVL), as well as the calculated logP correlate with 5-HT(1A) receptor affinity of the studied compounds.     

Water microdroplets on molecularly tailored surfaces: correlation between wetting hysteresis and evaporation mode switching

The evaporation of water microdroplets from solid surfaces was studied using digital contact angle analysis techniques. An inclusive trend for the evaporation process, that is, a switch from the initial constant contact area to the subsequent constant contact angle mode was observed for all surfaces examined, including mixed self-assembled monolayers (SAMs) on gold and "conventional" surfaces such as silicon wafers, polycarbonate, and Teflon. More importantly, it has been shown that the change in contact angle during the evaporation process (i.e., evaporation hysteresis, delta theta(evap), the difference between the initial and "equilibrated" contact angle) correlates well with the wetting hysteresis determined directly (i.e., measuring the advancing and receding contact angles on these surfaces by changing the drop volume). The comparison between mixed SAM surfaces and conventional solids revealed that the evaporation/wetting hysteresis is dominated by the roughness (from nanometer to micrometer scale) rather than the chemical heterogeneity of the surface. The evaporation rates of water microdroplets on these surfaces were also monitored and modeled.     

Water monomer interaction with gold nanoclusters from van der Waals density functional theory

We investigate the interaction between water molecules and gold nanoclusters Au(n) through a systematic density functional theory study within both the generalized gradient approximation and the nonlocal van der Waals (vdW) density functional theory. Both planar (n = 6-12) and three-dimensional (3D) clusters (n = 17-20) are studied. We find that applying vdW density functional theory leads to an increase in the Au-Au bond length and a decrease in the cohesive energy for all clusters studied. We classify water adsorption on nanoclusters according to the corner, edge, and surface adsorption geometries. In both corner and edge adsorptions, water molecule approaches the cluster through the O atom. For planar clusters, surface adsorption occurs in a O-up/H-down geometry with water plane oriented nearly perpendicular to the cluster. For 3D clusters, water instead favors a near-flat surface adsorption geometry with the water O atom sitting nearly atop a surface Au atom, in agreement with previous study on bulk surfaces. Including vdW interaction increases the adsorption energy for the weak surface adsorption but reduces the adsorption energy for the strong corner adsorption due to increased water-cluster bond length. By analyzing the adsorption induced charge rearrangement through Bader's charge partitioning and electron density difference and the orbital interaction through the projected density of states, we conclude that the bonding between water and gold nanocluster is determined by an interplay between electrostatic interaction and covalent interaction involving both the water lone-pair and in-plane orbitals and the gold 5d and 6s orbitals. Including vdW interaction does not change qualitatively the physical picture but does change quantitatively the adsorption structure due to the fluxionality of gold nanoclusters.     

Concentration of gold nanoparticles in the QCM sensitive film as a factor of adsorption properties controlling

The adsorption properties of sensitive films formed by gold nanoparticles, dropped from solutions with different nanoparticles content are investigated using mass-sensitive (QCM) sensors. The dependence of surface adsorption capacity on nanoparticles concentration is shown. The decrease of surface concentration of nanoparticles leads to the increase of sensors sensitivity to different alcohols and water. The adsorption character of water (response amplitude and kinetics) substantially differs form that of alcohols. The sensitivity of gold nanoparticles layers to water is more then one order higher then to alcohols. The kinetics of water adsorption runs in two stages.     

Polymer-templated self-assembly of a 2-dimensional gold nanoparticle network

We here report on the formation of well-ordered 2D gold nanostructures at the air/water interface. Spreading a mixture of alkanethiol-capped gold nanoparticles (AuNPs) and an amphiphilic poly(p-phenylene) on a water surface and compressing the mixture to a surface pressure of 40 mN/m lead to the formation of a network of well-ordered gold nanostructures. The structures are transferred horizontally (Langmuir-Sch?fer) onto a solid substrate and investigated with TEM, AFM, and X-ray reflectivity, showing a pattern that is repeating over several micrometers. AFM and X-ray reflectivity data at different surface pressures reveal that the polymer is lifting the AuNPs 1.5-2 nm in the vertical direction, away from the polymer layer, when the pressure is increased from 20 to 40 mN/m.     

PEGylated gold nanorod separation based on aspect ratio: characterization by asymmetric-flow field flow fractionation with UV-Vis detection

The development of highly efficient asymmetric-flow field flow fractionation (A4F) methodology for biocompatible PEGylated gold nanorods (GNR) without the need for surfactants in the mobile phase is presented. We report on the potential of A4F for rapid separation by evaluating the efficiency of functionalized surface coverage in terms of fractionation, retention time (t _R ) shifts, and population analysis. By optimizing the fractionation conditions, we observed that the mechanism of separation for PEGylated GNRs by A4F is the same as that for CTAB stabilized GNRs (i.e., according to their AR) which confirms that the elution mechanism is not dependent on the surface charge of the analytes and/or the membrane. In addition, we demonstrated that A4F can distinguish different surface coverage populations of PEGylated GNRs. The data established that a change in M_w of the functional group and/or surface orientation can be detected and fractionated by A4F. The findings in this study provide the foundation for a complete separation and physicochemical analysis of GNRs and their surface coatings, which can provide accurate and reproducible characterization critical to advancing biomedical research.

Electrochemical growth of two-dimensional gold nanostructures on a thin polypyrrole film modified ITO electrode

Two-dimensional gold nanostructures have been fabricated by electrochemical deposition of gold nanoparticles onto indium tin oxide (ITO) glass substrate modified with thin polypyrrole film. By controlling the electrodeposition conditions, gold nanoparticles with dendritic rod, sheet, flower-like (consisting of staggered nanosheets), and pinecone-like structures were generated. The flower-like gold nanoparticles showed high catalytic activity on electrochemical reduction of oxygen, and its activity was measured to be approximately 25 times that of gold pinecones and 10(4) times that of gold nanosheets in terms of gold weight. The pinecone-like nanoparticles can form a compact film with nano-/microscale binary structure like a lotus leaf surface. After modification with n-dodecanethiol, the surface showed superhydrophobic properties with a water contact angle of 153.4 degrees and a tilt angle of 4.4 degrees (5 microL droplet).     

Water-soluble gold nanoparticles protected by fluorinated amphiphilic thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.     

DFT and in situ EXAFS investigation of gold/ceria-zirconia low-temperature water gas shift catalysts: identification of the nature of the active form of gold

A combined experimental and theoretical investigation of the nature of the active form of gold in oxide-supported gold catalysts for the water gas shift reaction has been performed. In situ extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) experiments have shown that in the fresh catalysts the gold is in the form of highly dispersed gold ions. However, under water gas shift reaction conditions, even at temperatures as low as 100 degrees C, the evidence from EXAFS and XANES is only consistent with rapid, and essentially complete, reduction of the gold to form metallic clusters containing about 50 atoms. The presence of Au-Ce distances in the EXAFS spectra, and the fact that about 15% of the gold atoms can be reoxidized after exposure to air at 150 degrees C, is indicative of a close interaction between a fraction (ca. 15%) of the gold atoms and the oxide support. Density functional theory (DFT) calculations are entirely consistent with this model and suggest that an important aspect of the active and stable form of gold under water gas shift reaction conditions is the location of a partially oxidized gold (Audelta+) species at a cerium cation vacancy in the surface of the oxide support. It is found that even with a low loading gold catalysts (0.2%) the fraction of ionic gold under water gas shift conditions is below the limit of detection by XANES (<5%). It is concluded that under water gas shift reaction conditions the active form of gold comprises small metallic gold clusters in intimate contact with the oxide support.     

Water-dispersible nanoparticles via interdigitation of sodium dodecylsulphate molecules in octadecylamine-capped gold nanoparticles at a liquid-liquid interface

This paper describes the formation of water-dispersible gold nano-particles capped with a bilayer of sodium dodecylsulphate (SDS) and octadecylamine (ODA) molecules. Vigorous shaking of abiphasic mixture consisting of ODA-capped gold nanoparticles in chloroform and SDS in water results in the rapid phase transfer of ODA-capped gold nanoparticles from the organic to the aqueous phase, the latter acquiring a pink, foam-like appearance in the process. Drying of the coloured aqueous phase results in the formation of a highly stable, reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-Vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of an ODA primary monolayer directly coordinated to the gold nanoparticle surface and a secondary monolayer of SDS, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle transfer into water and rendering them water-dispersible. Dedicated to Professor C N R Rao on his 70th birthday