Hydrophobic Graphite Surface Research Articles

Direct real‐time imaging of protein adsorption onto hydrophilic and hydrophobic surfaces

Atomic force microscopy has been used to follow in real time the adsorption from solution of two of the gliadin group of wheat seed storage proteins onto hydrophilic (mica) and hydrophobic (graphite) surfaces. The liquid cell of the microscope was used initially to acquire images of the substrate under a small quantity of pure solvent (1% acetic acid). Continuous imaging as an injection of gliadin solution entered the liquid cell enabled the adsorption process to be followed in situ from zero time. For omega-gliadin, a monolayer was formed on the mica substrate during a period of approximately 2000 s, with the protein molecules oriented in parallel to the mica surface. In contrast, the omega-gliadin had a relatively low affinity for the graphite substrate, as demonstrated by slow and weak adsorption to the surface. With gamma-gliadin, random deposition onto the mica surface was observed forming monodispersed structures, whereas on the graphite surface, monolayer islands of protein were formed with the protein molecules in a perpendicular orientation. Sequential adsorption experiments indicated strong interactions between the two proteins that, under certain circumstances, caused alterations to the surface morphologies of preadsorbed species. The results are relevant to our understanding of the interactions of proteins within the hydrated protein bodies of wheat grain and how these determine the processing properties of wheat gluten and dough.

Kinetics of the Nanohydrogel Flattening at a Hydrophobic Interface

Hydrophobitized polysaccharides were designed to form the self-assembled nanohydrogels (hydrogel nanoparticles) in the aqueous conditions. For improving their biocompatibilities, they were decorated with the biomembrane-mimetic 2-methacryloyloxyethyl phosphorylcholine (MPC) polymers. The interfacial roles of the decorated membrane-mimetic nanohydrogels were investigated by choosing MPC branched choresteryl-bearing pullulan (CHP). Tapping-mode atomic force microscopy was used to study its adsorption mechanism on the hydrophobic highly oriented pyrolytic graphite (HOPG) surface in aqueous conditions. Dynamic observation at the interfaces revealed two distinctive patterns: the immobilized nanohydrogel particles and the flatten layers. The flattening (unfolding) kinetics with and without MPC branched nanohydrogel revealed that the flattening energy was at approximately 37 k(B)T. The flattening rate of the MPC decorated nanohydrogels was approximately 1.7 times faster than that without MPC decoration, corresponding to minor reduction of the flattening activation energy.

Ordering layers of [bmim][PF6] ionic liquid on graphite surfaces: Molecular dynamics simulation

Microscopic structures of room temperature ionic liquid (IL) [bmim][PF6] on hydrophobic graphite surfaces have been studied in detail by molecular dynamics simulation. It is clearly shown that both the mass and electron densities of the surface adsorbed ionic liquid are oscillatory, and the first peak adjacent to the graphite surface is considerably higher than others, corresponding to a solidlike IL bottom layer of 6 angstroms thick. Three IL layers are indicated between the graphite surface and the inner bulk IL liquid. The individually simulated properties of single-, double-, and triple-IL layers on the graphite surface are very similar to those of the layers between the graphite surface and the bulk liquid, indicating an insignificant effect of vapor-IL interface on the ordered IL layers. The simulation also indicates that the imidazolium ring and butyl tail of the cation (bmim+) of the IL bottom layer lie flat on the graphite surface.

Electrochemical catalysis and thermal stability characterization of laccase–carbon nanotubes-ionic liquid nanocomposite modified graphite electrode

The hydrophobic carbon nanotubes-ionic liquid (CNTs-IL) gel forms a stable modified film on hydrophobic graphite electrode surface. Laccase immobilized on the CNTs-IL gel film modified electrode shows good thermal stability and enhanced electrochemical catalytic ability. The optimal bioactivity occurs with increasing temperature and this optimum is 20 °C higher in comparison to free laccase. The improvement of laccase thermal stability may be due to the microenvironment of hydrophobic CNTs-IL gel on graphite electrode surface. On the other hand, the sensitive detection of oxygen has been achieved due to the feasibility of oxygen reduction by both of laccase and nanocomposite of CNTs-IL gel. Furthermore, the laccase hybrid nanocomposite also shows the fast electrochemical response and high sensitivity to the inhibitors of halide ions with the approximate IC 50 of 0.01, 4.2 and 87.5 mM for the fluoride, chloride and bromide ions, respectively. It implies the feasibility of laccase modified electrode as an inhibition biosensor to detect the modulators of laccase.

Stability of virus nanoparticles on substrates under applied load with atomic force microscope

Virus nanoparticles are supramolecular structures containing a nucleic acid (either DNA or RNA) core surrounded by a protein layer. In this article, we study the stability of cowpea mosaic virus (CPMV) nanoparticles on mica, Au and graphite surfaces under applied load by atomic force microscope in the tapping mode. The CPMV nanoparticles adsorbed on the hydrophilic mica and Au surfaces are intact, while on the hydrophobic graphite surface, the CPMV nanoparticles undergo the morphological changes from spherical to ellipsoidal and doughnut shapes with the increase in applied load. An understanding of the stability of the CPMV nanoparticles on substrates is critical in developing some of their applications in nanotechnology.

Adsorption of charged albumin subdomains on a graphite surface

We report some new molecular dynamics simulation results about the adsorption on a hydrophobic graphite surface of two albumin subdomains, each formed by three different alpha-helices, considering the correctly charged side groups at pH = 7 instead of the neutral ones as done in our previous exploratory paper (Raffaini and Ganazzoli, Langmuir 2003;19:3403-3412). We find that the presence of charges affects somewhat the initial adsorption stage on the electrostatically neutral surface, but not the final one. Thus, we recover the result that a monolayer of aminoacids is eventually formed, with a rough parallelism of distant strands to optimize both the intramolecular and the surface interactions. This feature is consistent with the adsorption on the hydrophobic surface being driven by dispersion forces only, and with the "soft" nature of albumin. Additional optimizations of the final monolayer carried out at pH = 3 and 11 do not modify appreciably this picture, suggesting that adsorption on graphite is basically independent of pH. The enhanced hydration of the final adsorption state due to the (delocalized) charges of the side groups is also discussed in comparison with similar results of the neutralized subdomains.

The effect of cosurfactants on sodium dodecyl sulfate micellar structures at a graphite surface

Sodium dodecyl sulfate (SDS) self assembles at hydrophobic graphite surfaces forming linear parallel hemicylinders with approximately 5 nm spacing between adjacent hemicylinders. In this paper, we report the effect of cosurfactants on the SDS surface micelle structures. It is known from the literature that cosurfactants influence the relaxation times of bulk phase micelles. Specifically, the relaxation times of SDS micelles increase dramatically when cosurfactants, such as non-ionic 1-dodecanol (C 12OH) or cationic dodecyl trimethyl ammonium bromide (C 12TAB) are added, resulting in the formation of stable micelles. Soft contact AFM imaging was used to determine if these increased relaxation times in bulk solution influence the formation of surface micelle structures at the graphite surface. Surface micelles formed by adsorption from SDS only solutions were significantly different from those surface micelles formed from mixed surfactant solutions. The addition of cosurfactants produced kinks and disrupted the otherwise highly ordered linear hemicylindrical SDS structures observed at graphite surfaces. In one case (SDS/C 12OH) swollen surface micelles with a spacing of more than 8 nm were observed. Finally, whereas surface micelles form instantaneously from SDS only solutions, the time taken for the equilibrium surface state to be established in the presence of the C 12OH cosurfactant was found to be more than 4 h.

Molecular Dynamics Simulation Study on Adsorption and Diffusion Processes of a Hydrophilic Chain on a Hydrophobic Surface

Molecular dynamics simulations are applied to investigate the adsorption and diffusion processes of a single hydrophilic poly(vinyl alcohol) (PVA) chain with different chain lengths on a hydrophobic graphite surface. It is expected that the chain and the surface "dislike" each other because one is hydrophilic and the other is hydrophobic. But surprisingly, a short PVA chain is well adsorbed on the surface, accompanied by large changes in the chain configuration. With increasing degree of polymerization (N), the chain turns gradually from two-dimensional adsorption to possessing certain height in the direction perpendicular to the surface. Moreover, the adsorption energy increases and the diffusion coefficient decreases with increasing N. In particular, for N = 20 in equilibrium, the hydroxyls of this short chain are close to the graphite surface in the stable adsorption configuration. In addition, we change the effective dielectric constant to 76.0 to mimic good solvent condition. The chain configurations and the diffusion coefficients both vary in contrast to the foregoing results.

Zeta-potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces

The zeta-potentials of the self-assembled surface ionic surfactants (sodium dodecyl sulfate—SDS and hexadecyltrimethyl ammonium bromide—CTAB) on graphite surfaces were determined both from streaming potential and electrophoretic mobility measurements. The adsorption of the surfactants at graphite–liquid interfaces has been studied using atomic force microscopy (AFM) soft-contact imaging which shows the formation of linear, parallel hemicylinders with headgroups oriented towards the solution. The magnitude of the zeta-potential increased with an increase in surfactant concentration, reaching a constant value at a concentration corresponding to the point of surface micelle formation as confirmed from AFM imaging. The streaming potential and electrophoretic mobility measurements showed that the zeta-potentials of SDS and CTAB surface micelles adsorbed at the graphite surface were about −75 and +70 mV, respectively, well in agreement with the values reported for bulk phase micelles in the literature.

Surface Ordering of Proteins Adsorbed on Graphite

The surface-induced rearrangement of distant protein strands adsorbed on a hydrophobic graphite surface is investigated through atomistic molecular dynamics simulations and energy minimizations. We show that fragments both of globular proteins consisting of either α-helices or β-sheets and of a fibrous protein containing a triple helix do form parallel strands on this surface, irrespective of their native structure. On the other hand, a new secondary structure consisting of β-sheets lying on the surface was never observed even after a long simulation time. The parallel ordering is characteristic of graphite being absent, for instance, on a hydrophilic poly(vinyl alcohol) surface. Our simulations indicate that this result is not related to the surface rigidity, and we suggest that it is due to a combination of the surface hydrophobicity, crystallinity, and smoothness.

Molecular Dynamics Simulation of the Adsorption of a Fibronectin Module on a Graphite Surface

We report atomistic simulations of the adsorption of a fibronectin type I module on a hydrophobic graphite surface. This module comprises only beta-sheets, unlike the albumin fragments previously investigated by us which contained only alpha-helices (Raffaini, G.; Ganazzoli, F. Langmuir 2003, 19, 3403-3412). As done in the latter case, most simulations are carried out in an effective dielectric medium by energy minimizations and molecular dynamics (MD). Further optimizations and MD runs in the explicit presence of water are also performed to assess the stability of the geometries found and to describe the solvation of the adsorbed fibronectin module. The initial adsorption is accompanied by local rearrangements of the strands in contact with the surface, but the overall molecular structure is largely preserved. Much larger rearrangements take place at longer times as found through the MD runs, with the molecule spreading as much as possible so as to maximize the surface coverage, hence the interaction energy, despite a significant strain energy. Energetic aspects of adsorption together with the concomitant size change are discussed in comparison with our previous results for two albumin fragments.

Simulation Study of the Interaction of Some Albumin Subdomains with a Flat Graphite Surface

We report atomistic computer simulations of some albumin subdomains on a hydrophobic graphite surface. The simulations are carried out in an effective dielectric medium by simple energy minimization and by long molecular dynamics (MD) runs. Further energy minimizations and shorter MD runs in the explicit presence of water are also performed to assess the stability of the geometries found and to describe the change of solvation of the adsorbed subdomains. We find that the initial adsorption is accompanied by significant rearrangements of the strands in contact with the surface, otherwise preserving the secondary structure and the overall globular shape. Much larger rearrangements take place at longer times during the MD runs, eventually yielding a thin layer of amino acids covering the surface as much as possible with complete denaturation. The interaction and strain energies of the adsorbed subdomains are discussed, together with their size and overall shape changes. The proposed adsorption mechanism is f...

Study of β-amyloid adsorption and aggregation on graphite by STM and AFM

The scanning tunneling microscopy (STM) and the atomic force microscopy (AFM) have been applied to the direct study of the adsorption and aggregation of β-amyloid(1–42)(Aβ42) on the hydrophobic graphite surface. It was found that Aβ42 were preferentially adsorbed on graphite defects such as the edges. Aβ42 peptides self-assembled into intermediate protofibrils, which in turn self-associated to form fibrils. Usually, two or more fibrils intertwined to form the helical structure. These results will provide an important clue to studying the aggregation process of β-amyloid.

Alkyl Group Dependence of the Surface-Induced Assembly of Nonionic Disaccharide Surfactants

N-Alkylmaltonamide nonionic diblock surfactants of varying hydrophobic segment lengths were synthesized, and their surface active properties on highly oriented pyrolytic graphite were characterized by atomic force microscopy (AFM). The N-alkylmaltonamide surfactants are composed of maltose, a hydrophilic disaccharide, with amide-linked alkyl groups of varying length, from octyl to octadecyl. All the surfactants readily adsorb uniformly onto the hydrophobic graphite surface from solution. Surfactants with alkyl lengths of 10 or more carbons exhibit epitaxial adsorption on graphite, forming ordered hemicylinders with a diameter that increases with increasing surfactant length. The solution concentration necessary for inducing surface ordering decreases with increasing alkyl chain length. N-Octadecylmaltonamide, which is insoluble in water, adsorbed from methanol solution without ordering and upon solvent replacement with water, assembled into hemicylinders. Once formed, the surfactant structures appeared st...

Imaging of polydiacetylene on graphite by scanning tunneling microscopy

Scanning tunneling microscopy has been applied to obtain atomically resolved images of polydiacetylene on graphite and to observe hydrogen bonding between polymers directly. Polydiacetylenes have a pseudo-one-dimensional conjugated pi-electron system which gives rise to nonlinear optical behavior of both fundamental and practical significance. Under low surface coverage, the images show a single polymer of CH3(CH2)11—C≡C—C≡C—(CH2)8—COOH. At high coverage, the images show a well-ordered monolayer of polymers with the nonpolar side groups pointing toward the hydrophobic graphite surface. The spatial electron distribution indicates substantial hydrogen bonding between polymers in the plane containing the polar side groups.