Confinement effect on thermodynamic and structural properties of water in hydrophilic mesoporous silica

The adsorption of water on porous silica surfaces at 300 K, has been qualitatively reproduced by Grand Canonical Monte Carlo simulations (GCMC) without any adjustment of adsorbate/substrate potential parameter. The simulated adsorption isotherm and isosteric differential enthalpy of adsorption compare well to experimental data for Vycor, showing the ability of the model in describing hydrophilic properties of silica surfaces. The analysis of fluid structure in the mesoporous glass gives detailed insights into confinement and disorder effects on water adsorbed on the hydrophilic surface of a porous glass. It is shown that hydrophilic properties are not simply related to surface hydroxyl density but are also related to local structure of the silica surface.     

Hybrid surface from self-assembled layer and its effect on protein adsorption

We synthesized hybrid self-assembled monolayer (SAM) with short chain hydrophobic and hydrophilic groups on the same molecule. The physical characteristics such as surface roughness and surface energy of the synthesized hybrid SAM were compared with mono SAMs of amine, octyl and mixed amine-octyl SAM. We also compared the response of the surfaces towards adsorption of bovine serum albumin (BSA) using quartz crystal microbalance (QCM). We determined adsorbed amount (Γ) of BSA on the various surfaces from its various bulk concentrations. It follows the Langmuir adsorption isotherm in the concentration range of our study. The strength of adsorbed protein was characterized from the dissipation factor (ΔD). The highest ΔD value of adsorbed BSA was observed for the adsorption on hybrid surface. The arrangement of BSA on hybrid surface such that it leaded to soft layer, corresponded to the highest ΔD value. These findings suggest that the hybrid surface is a potential surface modifying agent of biomaterials.     

A new approach to immobilize poly(vinyl alcohol) on poly(dimethylsiloxane) resulting in low protein adsorption

The hydrophobic characteristics of PDMS and non-specific protein adsorption are major drawbacks for its application in biosensing. Here we have combined surface oxidation by plasma and chemical binding of polyvinyl alcohol (PVA) to obtain long-term stability of hydrophilic PDMS surfaces. Mercaptopropyltrimethoxisilane and aminopropyltrimethoxisilane were used as adhesives between the plasma-oxidized PDMS surface and the PVA, immobilized at room temperature. This approach has allowed for fast, uniform, and very stable modification of the PDMS surface, which maintained a hydrophilic character for as long as 30 days. In addition, the modified hydrophilic surface presented minimized protein adsorption when compared to pristine PDMS. The results obtained in this work are important contributions to the growing field of integrated microfluidic biosensors.     

Theoretical study of the effect of spin-selective adsorption of water molecules on MgO surface

The effect of spin-selective adsorption of water molecules on the surface of MgO crystal is theoretically studied. The study is performed using two different approaches, i.e., quantum-chemical simulation and an analytical calculation in a quasiclassical approximation. The adsorption energy is calculated using the B3LYP density functional and the 6–311G* basis set. The calculated value of the adsorption energy 0.70 eV agrees well with an experimental value of 0.65 eV. It is established that the energy difference of adsorbed ortho- and para-water molecules is negligible and, thus, the difference of the adsorption energies is completely determined by the energy difference of free molecules in ortho- and para-states. It follows from the analytical calculation that this result is essentially general and is related not only to an MgO surface, but to any other surface on which the energy barrier for rotation of the adsorbed molecule is much larger than the corresponding rotational constant. Based on this, the conclusion is reached that the effect of spin-selective adsorption on these surfaces should not be observed under normal conditions.     

Mössbauer spectroscopic studies of ferrocene adsorbed on silica gel

Mössbauer spectra of ferrocene adsorbed on silica gel were measured in order to study the state of adsorption. Ferrocene adsorbed on silica gel tended to oxidize in air and form ferricenium ion. It is assumed that the oxidation of adsorbed ferrocene was caused by surface hydroxyls on the silica gel and O₂ in air. It was observed that ferrocene adsorbed weakly at 293 K, although the ferricenium ion adsorbed strongly at 293 K. Thus the adsorption states of ferrocene depend on the experimental condition.     

Cooperative binding of drugs on human serum albumin

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer–Emmet–Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheim's theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [Varela, L. M., García, M., Pérez-Rodríguez, M., Taboada, P., Ruso, J. M., and Mosquera, V., 2001, J. chem. Phys., 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process.     

Dynamics of nonspecific adsorption of insulin to erythrocyte membranes

Molecules may arrive at targets (receptors, enzymes, etc.) localized on a membrane surface by first adsorbing onto the surface and then surface diffusing to the targets. The flux rate of molecules arriving at targets via this mechanism depends on the surface diffusion coefficient of the molecules and, in some circumstances, on the adsorption/desorption kinetics. The technique of total internal reflection with fluorescence recovery after photobleaching (TIR-FRAP) was used here to study these rate parameters of fluorescein-labeled insulin (f-insulin) interacting with erythrocyte ghosts. Ghosts were adhered to polylysine coated slides for TIR illumination. Some ghosts became flattened and unsealed on the polylysine so that both extracellular and cytoplasmic sides of the membrane were openly exposed to the solution. An aluminum thin film between the polylysine and the fused silica of a slide quenched background fluorescence from f-insulin adsorbed directly onto the polylysine. An interference fringe pattern from two intersecting and totally internally reflecting laser beams provided surface-selective excitation with a spatial variation of illumination intensity across a ghost for surface diffusion measurements. Measured characteristic values of desorption rate constants ranged from 0.043 to 270 s^–1. According to a preexisting theoretical model, the largest desorption rate constant in this range would result in some increase in the total flux rate to a perfect sink target due to capture from the surface, provided that the surface diffusion coefficient was about 10^–8 cm²/s. However, based on TIR-FRAP measurements on our system, we estimate that the surface diffusion coefficient is less than about 5×10^–10 cm²/s. The combination of novel techniques presented here may prove valuable to other workers seeking to make diffusive and chemical kinetic rate parameter measurements of biomolecules at biological cell membranes.     

水分子在氢化锂表面的吸附行为

运用理论分析方法计算研究了水分子在氢化锂表面的吸附行为,分析了氢化锂表面改性对其疏水性能的影响。结果表明,在LiH-111面和LiH-100面上构建槽结构、柱状结构后,水分子在其上的吸附力比完整表面更强,说明表面微结构的引入的确改变了势能分布。壁相交处存在势能叠加,加强了吸附水分子的能力,但是没有引起表面的亲水性能变化。水分子可以稳定的吸附在完美的LiH(001)表面,其解离能垒仅为0.386 eV,这一解离反应在室温下完全可以进行。水分子极易在具有结构缺陷的LiH表面解离,这是LiH在一定湿度的空气和水环境中极易分解的根本原因。     

Efficient Entrapment of Dye in Hollow Silica Nanoparticles: Direct Evidence Using Fluorescence Spectroscopy

Using hollow silica nanoparticles we demonstrate a simple and highly efficient way of removing hydrophilic dye (Rhodamine B) from water by encapsulation within these hollow spheres. The hollow silica spheres were obtained by using a surfactant templated procedure. Using fluorescence spectroscopy, we also show the evidence of the dye being absorbed within the hollow core of the silica shell (which is crucial for many applications) and differentiate from the adsorption of dye on the surface of the silica shell. It was found that that up to 94 % of the hydrophilic dye could be entrapped using these hollow shells within 72 h of exposure. Fluorescence spectroscopy shows a red shift in the dye encapsulated in the hollow silica which is due to aggregation of the dye and enables us to follow quantitatively the uptake of the dye molecules by the silica shells with time. The evidence for the encapsulation of the dye in these hollow spheres was reinforced by carrying out a comparative study, using solid silica particles.

Novel method for immobilization of enzymes to magnetic nanoparticles

The value of coupling biological molecules such as enzymes to solid materials has long been recognized. To date, protein immobilization onto such surfaces often involves covalent coupling, encapsulation, or non-specific adsorption techniques. Here we demonstrate the feasibility of specifically attaching a haloalkane dehalogenase enzyme to silica-coated or uncoated iron oxide superparamagnetic nanoparticles using affinity peptides. The enzyme was cloned from Xanthobacter autotrophicus strain GJ10 into Escherichia coli to produce fusion proteins containing dehalogenase sequences with C-terminal polypeptide repeats that have specific affinity for either silica or iron oxide. The fusion proteins serve dual functions, allowing for specific inorganic surface binding and for enzymatic activity. The degree of fusion protein adsorption to nanoparticle surfaces was found to exceed that of enzymes that had not been activated with affinity sequences, particularly for iron-oxide nanoparticles. The ability to specifically adsorb cloned affinity-tagged dehalogenase was further demonstrated by selectively adsorbing dehalogenase fusion proteins containing an iron-oxide affinity tripeptide directly from cell lysate. The retention of enzymatic activity was found to be dependent upon the surface chemistry of the nanoparticles. An increase in activity was observed after adsorption of fusion proteins onto the surface of nanoparticles modified by treatment with hydrophilic polyethylene glycol or 3-glycidoxypropyltrimethoxysilane molecules. As a result of this work, it is possible to tag an active enzyme with specific peptides that bind to inorganic nanoparticle surfaces. Because the conjugates self assemble, the novel surface-specific conjugate formation procedure is highly efficient and easily scalable for use in large-scale applications.     

Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces

It has been shown that the flow of a simple liquid over a solid surface can violate the so-called no-slip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers. Received 21 December 2001 and Received in final form 3 August 2002 RID="a" ID="a"e-mail: ccottin@dpm.univ-lyon1.fr RID="b" ID="b"Present address.     

Nanodroplets on rough hydrophilic and hydrophobic surfaces

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root-mean-square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces.     

Enhancement of the fluorescence of triphenylmethane dyes caused by their interaction with nanoparticles from β-diketonate complexes

We have studied the absorption and fluorescence spectra of Malachite Green and Crystal Violet in aqueous and alcoholic-aqueous solutions in which nanoparticles from Ln(III) and Sc(III) diketonates are formed at concentrations of complexes in a solution of 5–30 μM. We have shown that, if the concentrations of the dyes in the solution are lower than 0.5 μM, dye molecules are incorporated completely into nanoparticles or are precipitated onto their surface. The fluorescence intensity of these incorporated and adsorbed Malachite Green and Crystal Violet molecules increases by several orders of magnitude compared to the solution, which takes place because of a sharp increase in the fluorescence quantum yields of these dyes and at the expense of the sensitization of their fluorescence upon energy transfer from β-diketonate complexes entering into the composition of nanoparticles. We have shown that, if there is no concentration quenching, the values of the fluorescence quantum yield of the Crystal Violet dye incorporated into nanoparticles and adsorbed on their surface vary from 0.06 to 0.13, i.e., are close to the fluorescence quantum yield of this dye in solid solutions of sucrose acetate at room temperature. The independence of the fluorescence quantum yield of Crystal Violet on the morphology of nanoparticles testifies to a high binding constant of complexes and the dye. The considerable fluorescence quantum yields of triphenylmethane dyes in nanoparticles and sensitization of their fluorescence by nanoparticle-forming complexes make it possible to determine the concentration of these dyes in aqueous solutions by the luminescent method in the range of up to 1 nM.     

Changes in wetting properties of silica surface treated with DPPC in the presence of phospholipase A2 enzyme

Wetting properties of silica plates contacted with dipalmitoylphosphatidylcholine (DPPC) or DPPC/enzyme (phospholipase PLA₂) in NaCl solution were determined by thin layer wicking and with a help of Washburn equation. The wicking experiments were performed both for bare plates and the silica plates precontacted overnight with the probe liquid saturated vapors the silica plates, as well as untreated and DPPC (or DPPC/enzyme) treated. Adsorption of DPPC on original silica plates increases a bit hydrophobic character of silica surface in such a way that hydrocarbon chains are directed outwards and the polar part towards the silica surface. However, after the enzyme action the products of DPPC hydrolysis by PLA₂ (palmitic acid and lysophosphatidylcholine) increase again hydrophilic character of silica surface (an increase in acid-base interactions, ).The changes of silica surface wettability are evidently dependent on the time of enzyme contacting with DPPC in NaCl solution. Although, the changes of total surface free energy of silica after treatment with DPPC/enzyme solution are minor about 2-6 mJ/m², the changes of the electron-donor () and Lifshitz-van der Waals () component of the surface free energy are noticeable. Despite, these results are somehow preliminary, it seems that thin layer wicking method is an interesting tool for investigation of the effect of adsorbed DPPC on hydrophobicity/hydrophilicity of silica surface and influence of enzyme PLA₂ action.     

A high throughput approach to quantify protein adsorption on combinatorial metal/metal oxide surfaces using electron microprobe and spectroscopic ellipsometry

Although metallic biomaterials are widely used, systematic studies of protein adsorption onto such materials are generally lacking. Combinatorial binary films of Al_1−xTi_x and Al_1−xNb_x (0  x  1) and corresponding pure element films were produced on glass substrates using a unique magnetron sputtering technique. Fibrinogen and albumin adsorption amounts were measured by wavelength-dispersive spectroscopy (WDS) and spectroscopic ellipsometry (SE) equipment, both high throughput techniques with automated motion stage capabilities. X-ray diffraction revealed that the binary films have crystalline phases present near the ends of the compositional gradient with an amorphous region throughout the interior of the gradient. X-ray photoelectron spectroscopy provided the surface chemistry along the binary films and showed that Al₂O₃ preferentially formed at the surface. Protein adsorption onto these films was found to be closely correlated to the alumina surface fraction, with high alumina content at the surface leading to low amounts of adsorbed fibrinogen and albumin. Protein adsorption amounts obtained with WDS and SE were in excellent agreement for all films. This suggests that this combinatorial materials approach combined with these state-of-the-art, automated high throughput instruments provides a novel way to accurately monitor protein adsorption taking place at the surfaces of these metal/metal oxide materials.     

The study of vicinal GaAs( 100) surfaces by PAX

The adsorption of xenon at low temperatures on both GaAs(100) and vicinal surfaces has been studied using ultraviolet photoemission spectroscopy. The Xe 5p peaks show a characteristic shift to lower binding energy with surface As-depletion. Additional weak emission features seen on the vicinal planes, shifted by around 0.45 eV to higher binding energy, are attributed to Xe adsorbed at the step sites on such surfaces.     

Abnormal adsorption behavior of dimethyl disulfide on gold surfaces

Adsorption of dimethyl disulfide (DMDS) on gold colloidal nanoparticle surfaces has been examined to check its binding mechanism. Differently from previous results, DMDS molecules adsorbed on the gold surface at high concentration showed the S–S stretching band at 500 cm⁻¹ in surface-enhanced Raman scattering (SERS) spectra, which indicates the presence of intact adsorption of DMDS molecules. However, it was found that the S–S bond of disulfides was easily cleaved on the gold surface at low concentration. These behaviors were not observed for diethyl disulfide (DEDS) or diphenyl disulfide (DPDS). Our results indicate that DMDS molecules with the shortest alkyl chains on the gold surface can be inserted into self-assembled monolayers (SAMs) without the S–S bond cleavage during self-assembly due to insufficient lateral van der Waals interaction and the low adsorption activity of disulfides, whereas DEDS with longer alkyl chains or DPDS with the weak disulfide bond dissociation energy would not. These unusual DMDS adsorption behaviors were examined by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). We also compared the bonding dissociation energy of the S–S bonds of various disulfides by means of a density functional theory (DFT) calculation.     

Site-specific core level spectroscopy of CO and NO adsorption on Pt(110)(1×2) and (1×1) surfaces

The adsorption of CO and NO on the (1×2) and (1×1) modifications of the Pt(110) surface was studied by x-ray photoemission spectroscopy, LEED and work-function change measurements. The O(1s) binding energy of adsorbed CO is site-specific and differentiates between on-top and bridge adsorbed species. CO adsorption on Pt(110)(1×2) at 120 K occurred sequentially into on-top and bridge sites yielding an orderedc(8×4) layer at the maximum coverage. At 300 K only on-top bonded CO was present after CO adsorption on the (1×2) surface. CO adsorption on the (1×1) surface at 120 K showed a transient bridge adsorbed CO and on-top CO at saturation, with an ordered (2×1)p1g1 LEED pattern. Heating the (2×1)p1g1 CO layer to 400 K also showed this transient bridge CO species. Work function changes generally correlated with the appearance of different CO species but were complex in detail. The findings for CO adsorption are consistent with the missing row model of the (1×2) surface.Parallel data for NO adsorption on (1×2) and (1×1) surfaces at 120 K were less informative than those for CO because O(1s) spectra showed single broad peaks. Peak contributions due to bridge and on-top bonded NO could be estimated.     

The platelet integrin alpha(IIb) beta(3) imaged by atomic force microscopy on model surfaces

The platelet membrane receptor alpha(IIb) beta(3) binds to adsorbed protein ligands including fibrinogen, von Willebrand factor and fibronectin, and is critically important in mediating platelet adhesion to damaged subendothelium and to synthetic biomaterial surfaces. This receptor is a member of the integrin family, a highly prevalent class of heterodimeric molecules consisting of a single alpha and beta subunit. In an ongoing effort to understand the mechanisms underlying platelet adhesion events, high-resolution atomic force microscopy (AFM) under dynamic conditions was used to obtain images of alpha(IIb) beta(3) molecules as well as aggregates of the protein. Images of integrin molecules were obtained by tapping mode AFM under aqueous buffer conditions following adsorption on a series of ultrasmooth model surfaces. On a model hydrophobic surface, detergents stabilizing the protein in solution competed for surface adsorption sites. When this detergent was removed from the system, the protein was predominantly seen as aggregates with head groups pointing outward. A limited number of individual integrin molecules were observed, and were found to have dimensions consistent with those reported previously by electron microscopy studies. Integrin molecules showed weak adhesion to the two hydrophilic surfaces used in the study, although formation of a lipid bilayer around surface-adsorbed molecules improved the resolution. At longer time periods, the integrin molecules embedded in this lipid bilayer exhibited sufficient mobility to form molecular aggregates. The structural measurements described in this study not only reveal three-dimensional features of the molecule, they represent an important step towards dynamic adsorption experiments and visualizing the integrin interacting with surface-adsorbed proteins as in biomaterial-induced thrombogenesis.     

Adsorption isotherms for fluids with strong interparticle repulsions

We derive adsorption isotherms for an adsorbate of hard-sphere particles with sticky interactions at any fluid density being adsorbed onto a plane, sticky surface. The theory is based on the Percus-Yevick theory for bulk fluids and explicitly includes the equilibrium between the adsorbed fluid and the bulk adsorbate. The theory predicts a surface condensation at low temperatures and low bulk densities in good agreement with surface condensations found in experimental studies of adsorption of gases onto graphite. An approximate law of corresponding states for these transitions is developed. At higher bulk densities and room temperatures, the adsorption isotherms can show a maximum, in accord with recent experimental work.Supported by the Australian Research Grants Commission.