H2S-induced reorganization of mixed monolayer of carboxylic derivatives on silver surface

The structure of self-assembled monolayers ofp-terphenyl-4-carboxylic acid and the mixed monolayers of this acid with n-hexadecanoic acid on silver surface were studied by reflection-IR spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurement, X-ray photoemission electron microscopy (X-PEEM), and atomic force microscopy. Exposure of the p-terphenyl-4-carboxylate monolayer to H2S vapor resulted in reorganization of the film structure into clusters of the corresponding free acids, in tens of nanometer dimension. Exposure of the mixed monolayer to H2S resulted in reorganization of the mixed monolayer film into phase-separated clusters of respective component molecules. The saturated aliphatic acid formed clusters of submicrometer size, whereas the p-terphenyl-4-carboxylic acid formed clusters of tens of nanometer size, presumably due to different surface mobility and/or intermolecular interaction of the two types of molecule. Restoration of the monolayer film from the clusters, driven by the reaction between the free acid molecules and the basic surface sites, proceeded at different speeds for the two types of molecules. The saturated acid monolayer was restored much faster than the p-terphenyl-4-carboxylic acid monolayer. A domain-separated monolayer in several micrometers scale was obtained. The process was imaged by tapping mode atomic force microscopy.     

Interaction between organophosphorous hydrolase and paraoxon studied by surface chemistry in situ at air-water interface

Subphase conditions have been optimized to obtain stable organophosphorous hydrolase (OPH-EC 3.1.8.1) as Langmuir films. The Langmuir film was characterized by surface pressure and surface potential-area isotherms and UV-Vis spectroscopy in situ. The interaction of an organophosphorous compound, namely Paraoxon, with the OPH film was investigated for various surface pressures. The stability of the monolayer and the evidence of the enzyme activity at air-water interface support the use of enzyme LB films as biosensor.     

Building a novel vitronectin assay by immobilization of integrin on calixarene monolayer

Membrane proteins possess significant hydrophobic domains and are likely to deplete their native activity immobilized on the solid surface relative to those occurring in a membrane environment. To investigate an efficient immobilization method, calix[4]crown-ether monolayer as an artificial protein linker system was constructed on the gold surface and characterized by Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS), atomic force microscopy (AFM) and cyclic voltammetry (CV). Integrin alpha(v)beta3 was functionally immobilized onto the monolayer and the integrin-vitronectin interaction was investigated by surface plasmon resonance (SPR). It was found that calix[4]crown-ether was assembled as a monolayer on the gold surface. Orientation and accessibility of integrin alpha(v)beta3 was assessed by sensitive binding of its natural ligand, vitronectin at pg mL(-1) level. Moreover, surface coverage of integrin layer and thickness calculated through SPR curve simulation verified that integrin layer was a monolayer in activated form. In combination with the SPR method, this calix[4]crown monolayer provided a reliable and simple experimental platform for the investigation of isolated membrane proteins under experimental conditions resembling those of their native properties.     

Self-assembled supramolecular array of polymeric phthalocyanine on gold for the determination of hydrogen peroxide

This Article describes for the first time the formation of a supramolecular self-assembled monolayer of polymeric phthalocyanine (poly(CuPc)) onto a gold substrate. The latter is established through the interaction of the cyano group, belonging to the poly(CuPc), with the metal substrate. The functionalized gold substrate was characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical methods. Results clearly demonstrated the interaction between gold and the nitrogen atom of cyano group and showed at the same time the formation of a completely covered polymeric monolayer on the gold surface. In addition, the modified gold surface seems to exhibit a reversible redox behavior and is found to act as an electronic conductor, which allows rapid electron transfer. Electrochemical impedance spectroscopy (EIS) analyses in the presence of [Fe(CN)(6)](3-/4-) as a redox couple revealed that the modified electrode showed a much lower electron transfer resistance compared with bare gold. In addition, the modified electrode is found to catalyze the H(2)O(2) reduction very effectively, showing a catalytic current that varies linearly with the peroxide concentration in the range of 0.35 to 70 μM with a detection limit of 0.25 μM.     

Rose bengal dye on thiol-terminated bilayer for molecular devices

Recently, it has become increasingly important to control molecular layers, especially with regard to the formation of bilayers, in order to avoid electrical shorts in molecular electronics. In this paper, we report on the characterization of an in situ thiol-terminated bilayer that is formed by hydrogen bonding between the amine group of an aminoalkanethiol monolayer on a gold surface and the free amine group of aminoalkanethiolates in a bulk solution. We also report on the use of a rose bengal (RB) monolayer on a thiol-terminated bilayer for the purpose of application in a molecular memory device. Using surface-sensitive techniques such as grazing angle Fourier transform infrared (FT-IR) spectroscopy, quartz crystal microbalance (QCM) measurement, ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV), we characterized a thiol-terminated bilayer (TUA-AUT) and an RB functionalized monolayer on a bilayered surface (RB-TUA-AUT). For a control experiment, we prepared a single RB monolayer attached by an ethanethiol group to a gold surface. In order to assess the feasibility of the present approach with respect to application in molecular electronics, we tested the switching property of the self-assembled monolayers (SAMs) using conducting-probe atomic force microscopy (CP-AFM). The RB monolayer on the bilayered surface exhibited hysteresis, while a single RB monolayer gave an electrical short.     

In situ evaluation of anticancer drug methotrexate-DNA interaction using a DNA-electrochemical biosensor and AFM characterization

An in situ evaluation of the dsDNA-methotrexate (MTX) interaction was performed by voltammetry using a DNA-electrochemical biosensor and characterized by atomic force microscopy (AFM) at a highly oriented pyrolytic graphite (HOPG) surface. Electrochemical experiments in incubated solutions showed that the interaction of MTX with dsDNA leads to modifications to the dsDNA structure in a time-dependent manner. The AFM images show reorganization of the DNA self-assembled network on the surface of the HOPG electrode upon binding methotrexate and the formation of a more densely packed and slightly thicker MTX-dsDNA lattice with a large number of aggregates embedded into the network film. The intercalation of MTX between complementary base pairs of dsDNA lead to the increase of purine oxidation peaks due to the unwinding of the dsDNA. The dsDNA-electrochemical biosensor and the purinic homo-polynucleotide single stranded sequences of guanosine and adenosine, poly[G] and poly[A]-electrochemical biosensors, were used to investigate and understand the interaction between MTX and dsDNA.     

A model system to study the insertion of cholesterol into a phospholipid monolayer

Colloidal probe atomic force microscopy (AFM) was used to study the interaction between a surface bearing tethered cholesterol groups and an egg phosphatidylcholine (egg-PC) monolayer. The cholesterol bearing surface was comprised of a mixed self-assembled monolayer comprised of O-cholesteryl N-(8'-mecapto-3',6'-dioxaoctyl)carbamate (CPEO3) molecules and beta-mercaptoethanol formed on a 20 mum diameter gold-coated silica particle. The egg-PC monolayer was adsorbed onto an octadecylthiol monolayer formed on template-stripped gold. The force between the surfaces, as a function of separation, was measured for surface concentrations of CPEO3 from 0 to 100 mol %. At all concentrations there was a long-range repulsive double-layer force due to weak surface charges. At surface concentrations of CPEO3 from 1 to 29 mol % the interaction on the approach of the surfaces showed a maximum in the repulsive force, followed by a small (2-5 nm) jump into a force minimum corresponding to adhesion of the surfaces. On separation, a normalized pull-off force of 1.0-1.6 mN m(-1) was measured. Over the same concentration range, the calculated interaction energy per CPEO3 molecule decreased from 1.1 +/- 0.2 kT to 0.04 kT. At surface concentrations of 35 mol % and above there was no reproducible adhesion between the cholesterol-bearing surface and the phospholipid monolayer. We attribute the occurrence of short-range attraction and adhesion in the 1-29 mol % regime to the insertion of (some) cholesterol groups into the phospholipid monolayer. At higher surface concentrations the efficiency of insertion is reduced due to steric effects. We discuss the experimental results in the light of the energetics of the insertion of a cholesterol molecule into a lipid bilayer.     

Direct photochemical functionalization of Si(111) with undecenol

Direct UV photochemical functionalization of H-terminated Si(111) with bifunctional 10-undecen-1-ol was achieved with selective attachment via its vinyl end, resulting in the formation of a compact monolayer with free terminal alcohol groups. This is due to the faster radical propagation mechanism in hydrosilylation with alkene compared to the nucleophilic attack mechanism of alcohol, which is impeded by intermolecular hydrogen bonding present at room temperature. Evidence from X-ray photoelectron spectroscopy, infrared spectroscopy, and resistance to fluoride etching shows that Si-C is the interfacial bond, and atomic force microscopy shows the presence of a smooth, uniform monolayer conforming to the atomic terraces of the Si(111) surface. The application of such a hydroxyl-terminated monolayer was demonstrated by tethering a bromoinitiator through surface esterification and thereafter subjecting the surface to the surface-initiated atom-transfer radical polymerization of butyl methacrylate. The poly(butyl methacrylate) brushes formed were found to be smooth (R(a) < 0.3 nm) and uniform even for a thin film of 4.0 nm.     

Enzyme-octadecylamine Langmuir-Blodgett membranes for ENFET biosensors

Langmuir-Blodgett (LB) films containing butyrylcholinesterase (BuChE) are fabricated to realise an enzymatic field effect transistor (ENFET) for the detection of organophosphorus pesticides in water. Trichlorfon as a common pesticide is examined in our work. The BuChE-immobilised LB films are formed by adsorbing the enzyme molecules onto a stearylamine monolayer using the electrostatic force. Enzyme/stearylamine mixed LB films are immobilized onto a pH-ISFET surface and treated by glutaraldehyde vapour to improve the LB film's stability. The ENFET thus obtained worked as a potentiometric biosensor for trichlorfon detection on the basis of enzyme inhibition. The detection limit for trichlorfon can reach 10(-7) M (26 ppb). The surface characteristics of BuChE/stearylamine LB films obtained under various conditions of the dipping surface pressures are analysed qualitatively by atomic force microscopy (AFM) and analysed quantitatively by FTIR spectroscopy.     

Surface characterization and efficiency of a matrix-free and flat carboxylated gold sensor chip for surface plasmon resonance (SPR)

We report the preparation and characterization of a matrix-free carboxylated surface plasmon resonance (SPR) sensor chip with high sensing efficiency by functionalizing a bare gold thin film with a self-assembled monolayer of 16-mercaptohexadecanoic acid (SAM–MHDA chip). The self assembled monolayer surface coverage of the gold layer was carefully evaluated and the SAM was characterized by infrared reflection absorption spectroscopy, X-ray photoemission spectroscopy, atomic force microscopy, X-ray reflectivity-diffraction, and SPR experiments with bovine serum albumin. We compared the SPR signal obtained on this chip made of a dense monolayer of carboxylic acid groups with commercially available carboxylated sensor chips built on the same gold substrate, a matrix-free C1 chip, and a CM5 chip with a ~100 nm dextran hydrogel matrix (GE Healthcare). Two well-studied interaction types were tested, the binding of a biotinylated antibody (immunoglobulin G) to streptavidin and an antigen–antibody interaction. For both interactions, the well characterized densely functionalized SAM–MHDA chip gave a high signal-to-noise ratio and showed a gain in the availability of immobilized ligands for their partners injected in buffer flow. It thus compared favourably with commercially available sensor chips.     

Mixed alkaline phosphatase/sphingomyelin monolayer at the air-buffer interface: phase behavior and morphology

The glycosylphosphatidyl inositol(GPI)-anchored proteins are localized on the outer of the plasma membrane and clustered in membrane microdomain known as lipid rafts. Among them, mammalian alkaline phosphatase(AP) is an enzyme widely distributed. So, it has important biological significance to study the combination of AP with lipid monolayer. In our work, the interaction between AP and sphingomyelin has been studied at the air-buffer interface as a biomimetic membrane system by the Langmuir film technique and atomic force microscopy. The surface pressure-area isotherm for the mixed alkaline phosphatase/sphingomyelin monolayer shown the presence of a transition from a liquid-expanded phase to the liquid-expanded/liquidcondensed coexist phase. And the surface compressional modulus suggested the mixed alkaline phosphatase/sphingomyelin monolayer has larger compressibility compared with the pure sphingomyelin monolayer. Besides, according to the micrographs, we inferred when combined with lipid monolayer at the air-buffer interface, the AP molecules formed polymer not multilayer or micelle. And, according to the limiting molecules area of AP, we inferred that 12 AP molecules formed a hexagon polymer unit.     

Dynamic information for cardiotoxin protein desorption from a methyl-terminated self-assembled monolayer using steered molecular dynamics simulation

Dynamic information, such as force, structural change, interaction energy, and potential of mean force (PMF), about the desorption of a single cardiotoxin (CTX) protein from a methyl-terminated self-assembled monolayer (SAM) surface was investigated by means of steered molecular dynamics (SMD) simulations. The simulation results indicated that Loop I is the first loop to depart from the SAM surface, which is in good agreement with the results of the nuclear magnetic resonance spectroscopy experiment. The free energy landscape and the thermodynamic force of the CTX desorption process was represented by the PMF and by the derivative of PMF with respect to distance, respectively. By applying Jarzynski's equality, the PMF can be reconstructed from the SMD simulation. The PMFs, calculated by different estimators based upon Jarzynski's equality, were compared with the conventional umbrella sampling method. The best estimation was obtained by using the fluctuation-dissipation estimator with a pulling velocity of v = 0.25 nm/ns for the present study.     

Covalent attachment of TAT peptides and thiolated alkyl molecules on GaAs surfaces

Four TAT peptide fragments were used to functionalize GaAs surfaces by adsorption from solution. In addition, two well-studied alkylthiols, mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) were utilized as references to understand the structure of the TAT peptide monolayer on GaAs. The different sequences of TAT peptides were employed in recognition experiments where a synthetic RNA sequence was tested to verify the specific interaction with the TAT peptide. The modified GaAs surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). AFM studies were used to compare the surface roughness before and after functionalization. XPS allowed us to characterize the chemical composition of the GaAs surface and conclude that the monolayers composed of different sequences of peptides have similar surface chemistries. Finally, FT-IRRAS experiments enabled us to deduce that the TAT peptide monolayers have a fairly ordered and densely packed alkyl chain structure. The recognition experiments showed preferred interaction of the RNA sequence toward peptides with high arginine content.     

Electrostatic orientation of enzymes on surfaces for ligand screening probed by force spectroscopy

In this letter, we show that electrostatic immobilization provides a simple but effective approach for the immobilization and orientation of carbonic anhydrase onto charged surfaces. The enzyme is oriented differently on oppositely charged surfaces, with the majority of active sites facing upward on a positively charged surface and downward on a negatively charged surface. An array of negatively charged microscale surface patterns within a positively charged background was prepared by microcontact printing and used as the substrate to immobilize the enzymes. This enabled the probing of the enzyme orientations on the two differently charged surface regions by force spectroscopy with the same atomic force microscopy (AFM) probe modified with a thiolated sulfonamide inhibitor. The unbinding forces between the inhibitor tip and the enzyme immobilized on the two differently charged surfaces were measured. Two control experiments, blocking of the enzyme active site with a competitive inhibitor and removal of the zinc ion from the enzyme catalytic center, were employed to distinguish between specific and nonspecific interactions and to further verify the differences in enzyme orientation. Autocorrelation analysis of the force histograms was carried out to evaluate the specific single enzyme-inhibitor interaction force.     

Solvent-controlled organization of self-assembled polymeric monolayers on gold: an easy approach for the construction of protein resistant surfaces

Protein resistant surfaces based on poly(ethylene glycol) (PEG) coatings are extensively applied in the fields of biosensors, tissue engineering, fundamental cell-surface interaction research, and drug delivery systems. The structural organization of the PEG film on the surface has a significant effect on the performance of the film to resist protein adsorption. In this paper, we report an approach using solvent to control the organization of the polymeric monolayer on gold. A water soluble copolymer with grafted PEG side chains and alkyl disulfide side chains was synthesized. A polymeric monolayer was fabricated on a gold surface from different solutions (water- and toluene-based) of the copolymer. The organization of the polymeric monolayers was characterized by means of ellipsometry, cyclic voltammetry, contact angle, X-ray photoelectron spectroscopy, and atomic force microscopy. It was proven that the structural organization of the polymeric monolayer on a gold surface could be controlled by the solvent. A polymeric monolayer with PEG enriched at the outer level is obtained when water is used as the solvent. Various types of proteins, including fibrinogen, albumin, and normal human serum, were used to test the protein resistance of the gold surfaces modified by the polymeric monolayers. The polymeric monolayer formed from a water solution of the copolymer showed excellent protein resistance. In addition, by using water as the solvent, patterning of the polymeric monolayer could easily be achieved through a combination of lift-off and self-assembly. We believe that the approach reported here provides an easy, fast, and efficient way to fabricate a robust protein resistant surface.     

Interaction between dendrons directly studied by single-molecule force spectroscopy

In this article, we have investigated the interaction between two poly(benzyl ether) dendrons directly by single-molecule force spectroscopy. For this purpose, one dendron was immobilized on an AFM tip through a poly(ethylene glycol) (PEG) spacer, and the other dendron was anchored on a gold substrate as a self-assembled monolayer. Two dendrons approached and then interacted with each other when the AFM tip and the substrate moved close together. The rupture force between dendrons was measured while the AFM tip and the substrate separated. PEG as a flexible spacer can function as a length window for recognizing the force signals and avoiding the disturbance of the interaction between the AFM tip and the substrate. The interaction between two first-generation dendrons is measured to be about 224 pN at a force loading rate of 40 nN/s. The interaction between second- and first-generation dendrons rises to 315 pN at the same loading rate. Such interactions depend on the force loading rate in the range of several to hundreds of nanonewtons per second, indicating that the rupture between dendrons is a dynamic process. The study of the interaction between surface-bound dendrons of different generations provides a model system for understanding the surface adhesion of molecules with multiple branches. In addition, this multiple-branch molecule may be used to mimic the sticky feet of geckos as a man-made adhesive.     

Fishing of beta-amylase with a SAM of alpha-cyclodextrin-poly- (ethylene glycol) conjugate

Alpha-cyclodextrin (alpha-CD) with an amino group was conjugated to an alpha, omega-dicarboxylated poly(ethylene glycol) (PEG). The inhibition constant (Ki) of the alpha-CD-PEG conjugate for the catalysis by beta-amylase was larger than that of alpha-CD, due to a steric obstruction of the PEG moiety to the binding of alpha-CD moiety to beta-amylase. alpha-CD-PEG was further modified with cystamine (CD-PEG-Cys) or cysteamine methyl disulfide (CD-PEG-MDS), and the disulfide-carrying alpha-CD-PEG was accumulated on a gold surface as a self-assembled monolayer (SAM). The binding of beta-amylase to the alpha-CD-PEG SAM was followed by a decrease in cathodic peak current in the voltammogram of hydroquinone as a probe using a cyclic voltammetry (CV). The beta-amylase bound to the alpha-CD-PEG SAM was desorbed by the addition of free alpha-CD, and the ratio of desorbed beta-amylase from the SAM of alpha-CD-PEG-Cys to the total amount of the enzyme bound to the SAM was 40% whereas that from the alpha-CD-PEG-MDS SAM was 83-85%. The percentage of desorption was increased to 100% by the treatment of the alpha-CD-PEG-MDS SAM-carrying electrode with 2-hydroxyethyldisulfide prior to the immersion in the enzyme solution. Adsorption and desorption processes of beta-amylase to the surface of alpha-CD-PEG-MDS SAM were clearly observed using localized surface plasmon resonance absorption spectroscopy. The binding constant of the enzyme to the surface-confined alpha-CD-PEG was much larger than that to free alpha-CD, probably due to a large local concentration of the alpha-CD moiety on the gold surface.     

Investigation on the orderly growth of thick zinc phthalocyanine films on Ag(100) surface

The growth of zinc phthalocyanine (ZnPc) on Ag(100) surface from monolayer to multilayer was investigated by low-energy electron diffraction, x-ray diffraction, and high-resolution electron energy loss spectroscopy (HREELS). At monolayer coverage, ZnPc molecules form an ordered film with molecular planes parallel to the substrate. The same structure is maintained as the film thickness increases. HREELS analysis shows that intermolecular π-π interaction dominates during the film growth from monolayer to multilayer. The π-d interaction between the adsorbates and the substrate is only applicable in the first adlayer. Stronger intermolecular-layer interaction is observed at higher coverages.     

Surface-initiated growth of poly d(A-T) by Taq DNA polymerase

In this paper, we report surface-initiated d(A-T) polymerization by Taq DNA polymerase as a method for constructing DNA-tethered surfaces using an enzyme. The enzymatic polymerization was conducted successfully via two steps: tethering of oligo d(A-T)s onto the surface presenting carboxylic acids by amide coupling and surface-initiated polymerization using Taq DNA polymerase. In this enzymatic polymerization process, the design and construction of carboxylic acid-presenting surfaces were found to be an important factor: DNA growth did not occur on the gold surface coated only with the self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid (MHDA), but effectively proceeded on the surfaces presenting mixed SAMs of MHDA and 1-pentadecanethiol. The coupling of oligo d(A-T)s and the subsequent DNA polymerization reaction were characterized by polarized infrared external reflectance spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy.     

The role of surface free energy on the formation of hybrid bilayer membranes

The interaction of small phospholipid vesicles with well-characterized surfaces has been studied to assess the effect of the surface free energy of the underlying monolayer on the formation of phospholipid/alkanethiol hybrid bilayer membranes (HBMs). The surface free energy was changed in a systematic manner using single-component alkanethiol monolayers and monolayers of binary mixtures of thiols. The binary surfaces were prepared on gold by self-assembly from binary solutions of the thiols HS-(CH(2))(n)()-X (n = 11, X = CH(3) or OH) in THF. Surface plasmon resonance (SPR), electrical capacitance, and atomic force microscopy (AFM) measurements were used to characterize the interaction of palmitoyl,oleoyl-phosphatidylcholine (POPC) vesicles with the surfaces. For all surfaces examined, it appears that the polar part of surface energy influences the nature of the POPC assembly that associates with the surface. Comparison of optical, capacitance, and AFM data suggests that vesicles can remain intact or partially intact even at surfaces with a contact angle with water of close to 100 degrees. In addition, comparison of the alkanethiols of different chain lengths and the fluorinated compound HS-(CH(2))(2)-(CF(2))(8)-CF(3) that characterize with a low value of the polar part of the surface energy suggests that the quality of the underlying monolayer in terms of number of defects has a significant influence on the packing density of the resulting HBM layer.