Force Modulation Elasticity Mapping of Plastic-embedded, Thin-sectioned Skeletal Muscle

Microscopy and Microanalysis : the Official Journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada. Jan, 2001  |  Pubmed ID: 11246401

We have been researching the capability of atomic force microscopy to reveal nontopographic properties of tissue embedded in plastic and sectioned with standard electron microscopic techniques. We present topography and elasticity maps of plastic-embedded, thin sections of muscle tissue. The images show topography correlated with the normal repeating structure of the sarcomere. Elasticity mapping using force modulation revealed contrast between the actin- and myosin-rich areas. We attribute the observed contrast in elasticity to the difference in local concentrations of biological material in embedding plastic.

Noncovalent Engineering of Carbon Nanotube Surfaces by Rigid, Functional Conjugated Polymers

Journal of the American Chemical Society. Aug, 2002  |  Pubmed ID: 12148991

We report a new nonwrapping approach to noncovalent engineering of carbon nanotube surfaces by short, rigid functional conjugated polymers, poly(aryleneethynylene)s. Our technique not only enables the dissolution of various types of carbon nanotubes in organic solvents, which represents the first example of solubilization of carbon nanotubes via pi-stacking without polymer wrapping, but could also introduce numerous neutral and ionic functional groups onto the carbon nanotube surfaces.

Application of Scanning Force and Near Field Microscopies to the Characterization of Minimally Adhesive Polymer Surfaces

Biofouling. Apr, 2003  |  Pubmed ID: 14618710

This mini-review reports efforts to develop new scanning probe microscopies to characterize the function and aging of textured, minimally adhesive polymer surfaces used for antifouling applications in the marine environment. Novel atomic force and infrared near field microscopy techniques have been used to characterize the polymer surface adhesion and structural properties. These techniques may find promise for characterizing the deposition of the extracellular matrix of organisms as well as aging of the polymer coating itself. The reported work is part of a larger effort to reduce biofouling on ships' hulls through the development and use of improved coating materials.

Surface Elastic Modulus of Barnacle Adhesive and Release Characteristics from Silicone Surfaces

Biofouling. Dec, 2004  |  Pubmed ID: 15804712

The properties of barnacle adhesive on silicone surfaces were studied by AFM indentation, imaging, and other tests and compared to the barnacle shear adhesion strength. A multilayered structure of barnacle adhesive plaque is proposed based on layered modulus regions measured by AFM indentation. The fracture of barnacles from PDMS surfaces was found to include both interfacial and cohesive failure of barnacle adhesive plaque, as determined by protein staining of the substratum after forced barnacle release from the substrate. Data for freshly released barnacles showed that there was a strong correlation between the mean Young's modulus of the outermost (softest) adhesive layer (E<0.3 MPa) and the shear strength of adhesion, but no correlation for other higher modulus regions. Linear, quadratic, and Griffith's failure criterion (based on rough estimate of crack length) regressions were used in the fit, and showed significance.

Using the Adhesive Interaction Between Atomic Force Microscopy Tips and Polymer Surfaces to Measure the Elastic Modulus of Compliant Samples

Langmuir : the ACS Journal of Surfaces and Colloids. Jul, 2004  |  Pubmed ID: 16459598

An atomic force microscope (AFM) method for measuring surface elasticity based on the adhesive interactions between an AFM tip and sample surfaces is introduced. The method is particularly useful when there is a large adhesion between the tip and soft samples, when the indentation method would be less accurate. For thin and soft samples, this method will have much less interference from the substrate than is found using the indentation method because there is only passive indentation induced by tip-sample adhesion; in contrast, a large indentation with a sharp tip in the sample may break its stress-strain linearity, or even make it fracture. For the case where it is difficult to accurately locate the tip-sample contact point, which is problematic for the indentation method, the method based on adhesive interactions is helpful because it does not require locating the tip-sample contact point when fitting the whole retraction force curve. The model is tested on PDMS polymers with different degrees of cross-linking.

Conjugated Thiol Linker for Enhanced Electrical Conduction of Gold-molecule Contacts

The Journal of Physical Chemistry. B. Mar, 2005  |  Pubmed ID: 16851569

Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two pi-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts.

Force Microscopy Studies of Fibronectin Adsorption and Subsequent Cellular Adhesion to Substrates with Well-defined Surface Chemistries

Langmuir : the ACS Journal of Surfaces and Colloids. Apr, 2005  |  Pubmed ID: 15835980

Molecular force spectroscopy was used to study the mechanical behavior of plasma fibronectin (FN) on mica, gold, poly(ethylene glycol), and -CH(3), -OH, and -COOH terminated alkanethiol self-assembled monolayers. Proteins were examined at two concentrations, one resulting in a saturated surface with multiple intermolecular interactions referred to as the aggregate state and another resulting in a semiaggregate state where the proteins were neither completely isolated nor completely aggregated. Modeling of the force-extension data using two different theories resulted in similar trends for the fitted thermodynamic parameters from which insight into the protein's binding state could be obtained. Aggregated proteins adsorbed on hydrophobic surfaces adopted more rigid conformations apparently as a result of increased surface denaturation and tighter binding while looser conformations were observed on more hydrophilic surfaces. Studies of FN in a semiaggregate state showed heterogeneity in the model's thermodynamic parameters suggesting that, in the early stages of nonspecific adsorption, multiple protein conformations exist, each having bound irreversibly to the substrate. Proteins in this state all demonstrated a more rigid conformation than in the corresponding aggregate studies due to the greater number of substrate contacts available to the protein. Finally, the force spectroscopy experiments were examined for any biocompatibility correlation by seeding substrates with human umbilical vascular endothelial cells. As predicted from the models used in this work, surfaces with aggregated FN promoted cellular deposition while surfaces with FN in a semiaggregate state appeared to hinder cellular deposition and growth. The atomic force microscope's use as a means for projecting surface biocompatibility, although requiring additional testing, does look promising.

Ferrocenylundecanethiol Self-assembled Monolayer Charging Correlates with Negative Differential Resistance Measured by Conducting Probe Atomic Force Microscopy

Journal of the American Chemical Society. May, 2005  |  Pubmed ID: 15898817

Electrical and mechanical properties of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers (SAMs) of electroactive 11-ferrocenylundecanethiol (FcC(11)SH) and a Pt-coated atomic force microscope (AFM) tip have been measured using a conducting probe (CP) AFM in insulating alkane solution. Simultaneous and independent measurements of currents and bias-dependent adhesion forces under different applied tip biases between the conductive AFM probe and the FcC(11)SH SAMs revealed reversible peak-shaped current-voltage (I-V) characteristics and correlated maxima in the potential-dependent adhesion force. Trapped positive charges in the molecular junction correlate with high conduction in a feature showing negative differential resistance. Similar measurements on an electropassive 1-octanethiol SAM did not show any peaks in either adhesion force or I-V curves. A mechanism involving two-step resonant hole transfer through the occupied molecular orbitals (MOs) of ferrocene end groups via sequential oxidation and subsequent reduction, where a hole is trapped by the phonon relaxation, is proposed to explain the observed current-force correlation. These results suggest a new approach to probe charge-transfer involving electroactive groups on the nanoscale by measuring the adhesion forces as a function of applied bias in an electrolyte-free environment.

Viscoelastic Response of Poly(dimethylsiloxane) in the Adhesive Interaction with AFM Tips

Langmuir : the ACS Journal of Surfaces and Colloids. Sep, 2005  |  Pubmed ID: 16142950

As follow-up to a previous study,(1) the viscoelastic response of cross-linked poly(dimethylsiloxane) (PDMS) in the adhesive interaction with AFM tips is reported. The indentation of the AFM tip into the polymer caused by adhesion is monotonically dependent on the loading speed. During the unloading process, the adherence force between the AFM tip and the polymer has a turning point at a specific unloading speed. It is found that the viscoelastic relaxation processes in the bulk polymer, which may have more than one component, cause the monotonic rate dependence of the adhesion-induced indentation; the competition between the bulk relaxation and the interfacial relaxation results in the turning point of the adherence force at a specific ramp speed. Experiments at different dwell times provide additional evidence for the existence of material relaxation. Methyl- and hydroxyl-functionalized AFM tips were used to study PDMS samples and indicated strong dipolar attractions formed at the tip-sample interface.

Quantifying Adhesion Bond Parameters to Distinguish Interactions of Hydrophilic and Hydrophobic Blocks of Polystyrene-poly-2-vinylpyridine with a Silicon Nitride Surface

Journal of the American Chemical Society. Mar, 2005  |  Pubmed ID: 15783172

An analysis of the loading rate dependence of the forces required to rupture an AFM tip from a block copolymer surface has provided insight into the structure of the surface-macromolecule contact, differentiation of the block contacting the tip, a measure of the polymer-surface binding energy, and the rigidity of the contact. Polystyrene-poly-2-vinylpyridine block copolymers were studied adsorbing to silicon nitride. Polystyrene makes stiff van der Waals contact with the silicon nitride surface in aqueous solution, while the bond of poly-2-vinylpyridine to the surface is more flexible and may involve a bridging water.

Field Localization in Very Small Aperture Lasers Studied by Apertureless Near-field Microscopy

Applied Optics. Aug, 2006  |  Pubmed ID: 16892123

Localized surface plasmon polaritons (SSPs) have been observed on very small aperture lasers using apertureless near-field microscopy. Fields around multiple apertures are shown to result from interferences of SPP point sources at each aperture and optical fields. The near-field optical pattern around a single aperture indicates the interference of SPPs with their scattered counterparts. Near-field measurements also confirmed a preferred orientation of the rectangular aperture waveguide for the signal localization in very small aperture lasers.

Microfluidic Production of Biopolymer Microcapsules with Controlled Morphology

Journal of the American Chemical Society. Sep, 2006  |  Pubmed ID: 16967971

We report a microfluidic approach to generating capsules of biopolymer hydrogels. Droplets of an aqueous solution of a biopolymer were emulsified in an organic phase comprising a cross-linking agent. Polymer gelation was achieved in situ (on a microfluidic chip) by diffusion-controlled ionic cross-linking of the biopolymer, following the transfer of the cross-linking agent from the continuous phase to the droplets. Gelation was quenched by collecting particles in a large pool of cross-linking agent-free liquid. The structure of microgels (from capsules to gradient microgels to particles with a uniform structure) was controlled by varying the time of residence of droplets on the microfluidic chip and the concentration of the cross-linking agent in the continuous phase. We demonstrated the encapsulation of a controlled number of polystyrene beads in the microgel capsules. The described approach was applied to the preparation of capsules of several polysaccharides such as alginate, kappa-carrageenan, and carboxymethylcellulose.

Ordered CdSe Nanoparticles Within Self-assembled Block Copolymer Domains on Surfaces

Langmuir : the ACS Journal of Surfaces and Colloids. Feb, 2007  |  Pubmed ID: 17243744

Hierarchical, high-density, ordered patterns were fabricated on Si substrates by self-assembly of CdSe nanoparticles within approximately 20-nm-thick diblock copolymer films in a controlled manner. Surface-modified CdSe nanoparticles formed well-defined structures within microphase-separated polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) domains. Trioctylphosphine oxide (TOPO)-coated CdSe nanoparticles were incorporated into PS domains and polyethylene glycol-coated CdSe nanoparticles were located primarily in the P2VP domains. Nearly close-packed CdSe nanoparticles were clearly identified within the highly ordered patterns on Si substrates by scanning electron microscopy (SEM). Contact angle measurements together with SEM results indicate that TOPO-CdSe nanoparticles were partially placed at the air/copolymer interface.

Infrared Near-field Detection of a Narrow Resonance Due to Molecular Vibrations in a Nanoparticle

Langmuir : the ACS Journal of Surfaces and Colloids. Feb, 2007  |  Pubmed ID: 17309222

Di-iron nonacarbonyl particles on a gold surface have been imaged using an apertureless near-field scanning infrared microscopy. First and second harmonic detection, with and without an auto-homodyning option, have been used to collect the near-field spectrum of a single vibrational mode of the bridging carbonyls in di-iron nonacarbonyl nanocrystalline flakes on a gold surface. The experimental results have been compared to two theoretical models, a static image-dipole effective medium and an image dipole modified by a Fresnel coefficient for the appropriate observation angle. The calculations have taken into account the roughness of the gold film. The phase dependence of the near-field contrast has been investigated using broadband and tunable CO2 lasers. Particle size effects on contrast and spatial resolution have been studied to determine the limits of applicability of the half-space approximation.

Self-assembly of Metal-polymer Analogues of Amphiphilic Triblock Copolymers

Nature Materials. Aug, 2007  |  Pubmed ID: 17618291

Organized arrays of anisotropic nanoparticles show electronic and optical properties that originate from the coupling of shape-dependent properties of the individual nanorods. The organization of nanorods in a controllable and predictable way provides a route to the fabrication of new materials and functional devices. So far, significant progress has been achieved in the self-assembly of nanorod arrays, yet the realization of a range of different structures requires changing the surface chemistry of the nanoparticles. We organized metal nanorods in structures with varying geometries by using a striking analogy between amphiphilic ABA triblock copolymers and the hydrophilic nanorods tethered with hydrophobic polymer chains at both ends. The self-assembly was tuneable and reversible and it was achieved solely by changing the solvent quality for the constituent blocks. This approach provides a new route to the organization of anisotropic nanoparticles by using the strategies that are established for the self-assembly of block copolymers.

Force-induced Globule-coil Transition in Single Polystyrene Chains in Water

Journal of the American Chemical Society. Aug, 2007  |  Pubmed ID: 17665910

Pressure-induced Restructuring of a Monolayer Film Nanojunction Produces Threshold and Power Law Conduction

Langmuir : the ACS Journal of Surfaces and Colloids. Mar, 2008  |  Pubmed ID: 18278960

The electrical conduction of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers of structurally different 1-hexanethiol, 1-decanethiol, and ferrocenyl-1-undecanethiol and a Pt-coated atomic force microscope (AFM) tip has been measured under different compression forces using conducting-probe AFM. The observed junction resistance had two distinct power law scaling changes with the compression force. Different scaling regions were assigned to the change in the contact area, tunneling distance, number of conduction pathways, and structure of the film under compression.

Nanomechanical Fingerprints of Individual Blocks of a Diblock Copolymer Chain

Langmuir : the ACS Journal of Surfaces and Colloids. May, 2008  |  Pubmed ID: 18402472

Atomic force microscopy (AFM)-based single polymer chain pulling experiments have been used to study the structural transitions of individual homopolymer chains in water. Polystyrene (PS) showed a three-regime force-extension profile exhibiting a force plateau reminiscent of a first-order transition, as predicted theoretically, whereas poly(methyl methacrylate) (PMMA) showed a characteristic saw-tooth pattern reminiscent of multidomain disassembly behavior. The two distinct structural transtions provide fingerprints for the individual homopolymers, which can be used to identify individual blocks of symmetric and asymmetric PS- b-PMMA diblock copolymer chains.

Chemical Imaging of the Surface of Self-assembled Polystyrene-b-poly(methyl Methacrylate) Diblock Copolymer Films Using Apertureless Near-field IR Microscopy

Langmuir : the ACS Journal of Surfaces and Colloids. Jun, 2008  |  Pubmed ID: 18533693

The nanoscale chemical composition variations of the surfaces of thin films of polystyrene- b-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. The scattering of the incident infrared beam from a modulated atomic force microscopy (AFM) tip is probed using homodyne detection and demodulation at the tip oscillation frequency. An increase in the IR attenuation is observed in the PMMA-rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a small topography-induced artifact can be observed in the near-field images, the chemical signature of the sample is detected clearly.

Immuno-atomic Force Microscopy Characterization of Adsorbed Fibronectin

Langmuir : the ACS Journal of Surfaces and Colloids. Dec, 2008  |  Pubmed ID: 19360949

The fibronectin (Fn) binding conformation on mica and ultraflat poly(D,L-lactide-co-glycolide) (UPLGA) was characterized using atomic force microscopy (AFM). AFM topographic images showed that Fn was in an extended form on mica and in a compact structure on UPLGA. With immuno-AFM, an antibody (Ab(hep)) was used to characterize the Fn binding conformation. When Fn opens its binding site for an antibody upon adsorption to a surface, the resulting Fn-antibody complex creates an additional peak in the sample's height distribution. Immuno-AFM uses this change to detect antigen-antibody binding. In this letter, height histograms (distributions) were generated using the mean true height of molecules, which was measured by examining the histogram for each individual molecule and subtracting the mica background. Mean true height values were obtained from the histograms and showed that Fn and Ab(hep) formed complexes on mica, signifying that one of the heparin binding sites on Fn was open when Fn was adsorbed to mica. The mean true height of the Fn-antibody complex from the histogram is greater than expected, suggesting that the antibody had pulled the extended "arms" of Fn together and caused an Fn conformation change upon binding. The height histograms can illustrate the Fn binding conformation and other antigen-antibody binding.

Nanoscale Structures and Mechanics of Barnacle Cement

Biofouling. 2009  |  Pubmed ID: 19180351

Polymerized barnacle glue was studied by atomic force microscopy (AFM), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and chemical staining. Nanoscale structures exhibiting rod-shaped, globular and irregularly-shaped morphologies were observed in the bulk cement of the barnacle Amphibalanus amphitrite (=Balanus amphitrite) by AFM. SEM coupled with energy dispersive X-ray (EDX) provided chemical composition information, making evident the organic nature of the rod-shaped nanoscale structures. FTIR spectroscopy gave signatures of beta-sheet and random coil conformations. The mechanical properties of these nanoscale structures were also probed using force spectroscopy and indentation with AFM. Indentation data yielded higher elastic moduli for the rod-shaped structures when compared with the other structures in the bulk cement. Single molecule AFM force-extension curves on the matrix of the bulk cement often exhibited a periodic sawtooth-like profile, observed in both the extend and retract portions of the force curve. Rod-shaped structures stained with amyloid protein-selective dyes (Congo red and thioflavin-T) revealed that about 5% of the bulk cement were amyloids. A dominant 100 kDa cement protein was found to be mechanically agile, using repeating hydrophobic structures that apparently associate within the same protein or with neighbors, creating toughness on the 1-100 nm length scale.

Water-stable Diblock Polystyrene-block-poly(2-vinyl Pyridine) and Diblock Polystyrene-block-poly(methyl Methacrylate) Cylindrical Patterned Surfaces Inhibit Settlement of Zoospores of the Green Alga Ulva

Biomacromolecules. Apr, 2009  |  Pubmed ID: 19296654

Nanopatterned surfaces with hydrophobic and hydrophilic domains were produced using the diblock copolymer polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA). The PS-b-P2VP diblock copolymer, mixed with the cross-linker benzophenone and spin-coated onto silicon wafers, showed self-assembled cylindrical structures, which were retained after UV treatment for cross-linking. The thin films displayed cylindrical domains after immersion in water. This study shows that pattern retention in water is possible for a long period of time, at least for two weeks in pure water and three weeks in artificial seawater. The PS-b-PMMA diblock showed self-assembled cylindrical structures. PS-b-P2VP and PS-b-PMMA cylindrical patterned surfaces showed reduced settlement of zoospores of the green alga Ulva compared to unpatterned surfaces. The copolymers were investigated using atomic force microscopy and X-ray photoelectron spectroscopy.

Insights into the Composition, Morphology, and Formation of the Calcareous Shell of the Serpulid Hydroides Dianthus

Journal of Structural Biology. Feb, 2010  |  Pubmed ID: 19766190

To date, the calcareous tubes of serpulid marine worms have not been studied extensively in a biomineralization context. The structure and composition of the tube shell and adhesive cement of the marine tubeworm Hydroides dianthus were studied using a variety of characterization techniques, including powder XRD, FTIR, SEM, EDX, and AFM. The tube and cement were determined to be inorganic-organic composite materials, consisting of inorganic aragonite (CaCO(3)) and Mg-calcite ((Ca(0.8)Mg(0.2))CO(3)) crystals, and both soluble and insoluble organic matrices (SOM and IOM). SEM imaging revealed a variety of crystal morphologies. AFM nanoindentation of the inorganic components yielded Young's moduli of approximately 20GPa in the wet state, and approximately 50GPa in the dry state. Amino acid analysis of the SOM indicated substantial amounts of acidic and non-polar neutral amino acids. Part of the insoluble organic tube lining was identified as being composed of collagen-containing fibres aligned in a criss-crossed structure. The SOM and organic tube lining were found to contain carboxylated and sulphated polysaccharides. In an artificial seawater solution, the SOM and the organic tube lining mediated CaCO(3) mineralization in vitro.

The Role of Surface Energy and Water Wettability in Aminoalkyl/fluorocarbon/hydrocarbon-modified Xerogel Surfaces in the Control of Marine Biofouling

Biofouling. 2010  |  Pubmed ID: 19960390

Xerogel films with uniform surface topogrophy, as determined by scanning electron microscopy, atomic force microscopy (AFM), and time-of-flight secondary ion mass spectrometry, were prepared from aminopropylsilyl-, fluorocarbonsilyl-, and hydrocarbonsilyl- containing precursors. Young's modulus was determined from AFM indentation measurements. The xerogel coatings gave reduced settlement of zoospores of the marine fouling alga Ulva compared to a poly(dimethylsiloxane) elastomer (PDMSE) standard. Increased settlement correlated with decreased water wettability as measured by the static water contact angle, theta(Ws), or with decreased polar contribution (gamma(P)) to the surface free energy (gamma(S)) as measured by comprehensive contact angle analysis. The strength of attachment of 7-day sporelings (young plants) of Ulva on several of the xerogels was similar to that on PDMSE although no overall correlation was observed with either theta(Ws) or gamma(S). For sporelings attached to the fluorocarbon/hydrocarbon-modified xerogels, the strength of attachment increased with increased water wettability. The aminopropyl-modified xerogels did not follow this trend.

Detection of Chronic Lymphocytic Leukemia Cell Surface Markers Using Surface Enhanced Raman Scattering Gold Nanoparticles

Cancer Letters. Jun, 2010  |  Pubmed ID: 20042272

Selective targeting and detection of a hematologic malignancy, chronic lymphocytic leukemia, using surface enhanced Raman scattering (SERS) gold nanoparticles is reported. The functional nanoparticles were composed of a gold core onto which an optical reporter dye was adsorbed, protected from aggregation by grafted polyethyleneglycol, and targeted to CD19 antigen by antibodies. The signals were detected by dark-field microscopy and Raman spectrometry. The observation that the Raman signals are not disrupted by several traditional pathology stains indicates advantages over fluorescence methods.

Phase Segregation of Untethered Zwitterionic Model Lipid Bilayers Observed on Mercaptoundecanoic-acid-modified Gold by AFM Imaging and Force Mapping

Langmuir : the ACS Journal of Surfaces and Colloids. Jul, 2010  |  Pubmed ID: 20387821

Planar supported lipid bilayers (SLBs) are often studied as model cell membranes because they are accessible to a variety of surface-analytic techniques. Specifically, recent studies of lipid phase coexistence in model systems suggest that membrane lateral organization is important to a range of cellular functions and diseases. We report the formation of phase-segregated dioleoylphosphatidylcholine (DOPC)/sphingomyelin/cholesterol bilayers on mercaptoundecanoic-acid-modified (111) gold by spontaneous fusion of unilamellar vesicles, without the use of charged or chemically modified headgroups. The liquid-ordered (l(o)) and liquid-disordered (l(d)) domains are observed by atomic force microscopy (AFM) height and phase imaging. Furthermore, the mechanical properties of the bilayer were characterized by force-indentation maps. Fits of force indentation to Sneddon mechanics yields average apparent Young's moduli of the l(o) and l(d) phases of 100 +/- 2 and 59.8 +/- 0.9 MPa, respectively. The results were compared to the same lipid membrane system formed on mica with good agreement, though modulus values on mica appeared higher. Semiquantitative comparisons suggest that the mechanical properties of the l(o) phase are dominated by intermolecular van der Waals forces, while those of the fluid l(d) phase, with relatively weak van der Waals forces, are influenced appreciably by differences in surface charge density between the two substrates, which manifests as a difference in apparent Poisson ratios.

Interfacial Free Energy Governs Single Polystyrene Chain Collapse in Water and Aqueous Solutions

Journal of the American Chemical Society. May, 2010  |  Pubmed ID: 20405838

The hydrophobic interaction is significantly responsible for driving protein folding and self-assembly. To understand it, the thermodynamics, the role of water structure, the dewetting process surrounding hydrophobes, and related aspects have undergone extensive investigations. Here, we examine the hypothesis that polymer-solvent interfacial free energy is adequate to describe the energetics of the collapse of a hydrophobic homopolymer chain at fixed temperature, which serves as a much simplified model for studying the hydrophobic collapse of a protein. This implies that changes in polymer-solvent interfacial free energy should be directly proportional to the force to extend a collapsed polymer into a bad solvent. To test this hypothesis, we undertook single-molecule force spectroscopy on a collapsed, single, polystyrene chain in water-ethanol and water-salt mixtures where we measured the monomer solvation free energy from an ensemble average conformations. Different proportions within the binary mixture were used to create solvents with different interfacial free energies with polystyrene. In these mixed solvents, we observed a linear correlation between the interfacial free energy and the force required to extend the chain into solution, which is a direct measure of the solvation free energy per monomer on a single chain at room temperature. A simple analytical model compares favorably with the experimental results. This knowledge supports a common assumption that explicit water solvent may not be necessary for cases whose primary concerns are hydrophobic interactions and hydrophobic hydration.

Composite Nanoparticle Nanoslit Arrays: a Novel Platform for LSPR Mediated Subwavelength Optical Transmission

Optics Express. Apr, 2010  |  Pubmed ID: 20588611

Near infrared transmission of light through subwavelength slit arrays is shown to be significantly influenced by resonant metallic nanoparticles placed within the structure. Experimental and calculated transmission spectra show how the size, orientation of the nanoparticles, and the period of the nanoslit array influence the maximum transmission wavelength, the magnitude of the transmission, and width of the resonance. These findings suggest that the localized surface plasmon resonance (LSPR) of metallic nanoparticles and their subsequent near and far-field interactions can modulate the subwavelength transmission and bandwidth of nanoaperture array devices in optically thick metal films.

Cholesterol-dependent Nanomechanical Stability of Phase-segregated Multicomponent Lipid Bilayers

Biophysical Journal. Jul, 2010  |  Pubmed ID: 20643069

Cholesterol is involved in endocytosis, exocytosis, and the assembly of sphingolipid/cholesterol-enriched domains, as has been demonstrated in both model membranes and living cells. In this work, we explored the influence of different cholesterol levels (5-40 mol%) on the morphology and nanomechanical stability of phase-segregated lipid bilayers consisting of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/Chol) by means of atomic force microscopy (AFM) imaging and force mapping. Breakthrough forces were consistently higher in the SM/Chol-enriched liquid-ordered domains (Lo) than in the DOPC-enriched fluid-disordered phase (Ld) at a series of loading rates. We also report the activation energies (DeltaEa) for the formation of an AFM-tip-induced fracture, calculated by a model for the rupture of molecular thin films. The obtained DeltaEa values agree remarkably well with reported values for fusion-related processes using other techniques. Furthermore, we observed that within the Chol range studied, the lateral organization of bilayers can be categorized into three distinct groups. The results are rationalized by fracture nanomechanics of a ternary phospholipid/sphingolipid/cholesterol mixture using correlated AFM-based imaging and force mapping, which demonstrates the influence of a wide range of cholesterol content on the morphology and nanomechanical stability of model bilayers. This provides fundamental insights into the role of cholesterol in the formation and stability of sphingolipid/cholesterol-enriched domains, as well as in membrane fusion.

Barnacle Settlement and the Adhesion of Protein and Diatom Microfouling to Xerogel Films with Varying Surface Energy and Water Wettability

Biofouling. Aug, 2010  |  Pubmed ID: 20645195

Previous work has shown that organosilica-based xerogels have the potential to control biofouling. In this study, modifications of chemistry were investigated with respect to their resistance to marine slimes and to settlement of barnacle cyprids. Adhesion force measurements of bovine serum albumin (BSA)-coated atomic force microscopy (AFM) tips to xerogel surfaces prepared from aminopropylsilyl-, fluorocarbonsilyl-, and hydrocarbonsilyl-containing precursors, indicated that adhesion was significantly less on the xerogel surfaces in comparison to a poly(dimethylsiloxane) elastomer (PDMSE) standard. The strength of adhesion of BSA on the xerogels was highest on surfaces with the highest and the lowest critical surface tensions, gamma(C) and surface energies, gamma(S), and duplicated the 'Baier curve'. The attachment to and removal of cells of the diatom Navicula perminuta from a similar series of xerogel surfaces were examined. Initial attachment of cells was comparable on all of the xerogel surfaces, but the percentage removal of attached cells by hydrodynamic shear stress increased with gamma(C) and increased wettability as measured by the static water contact angle, theta(Ws), of the xerogel surfaces. The percentage removal of cells of Navicula was linearly correlated with both properties (R(2) = 0.74 for percentage removal as a function of theta(Ws) and R(2) = 0.69 for percentage removal as a function of gamma(C)). Several of the aminopropylsilyl-containing xerogels showed significantly greater removal of Navicula compared to a PDMSE standard. Cypris larvae of the barnacle B. amphitrite showed preferred settlement on hydrophilic/higher energy surfaces. Settlement was linearly correlated with theta(Ws) (R(2) = 0.84) and gamma(C) (R(2) = 0.84). Hydrophilic xerogels should prove useful as coatings for boats in regions where fouling is dominated by microfouling (protein and diatom slimes).

Self-assembly of Colloidal Quantum Dots on the Scaffold of Triblock Copolymer Micelles

ACS Applied Materials & Interfaces. Nov, 2010  |  Pubmed ID: 21033690

This paper describes the co-self-assembly of a polystyrene-poly(4-vinylpyridine)-poly(ethylene oxide) triblock copolymer with CdSe nanocrystals (quantum dots, QDs) and with a styrene compatible phenylenevinylene conjugated polymer (MEH-PPV) in mixtures of chloroform and 2-propanol. The polymer itself (PS(577)-P4VP(302)-PEO(852), where the subscripts refer to the number average degree of polymerization) forms worm-like micelles when 2-propanol is added to a solution of the polymer in CHCl(3). In the presence of increasing amounts of QDs, the structures become shorter and form only spherical hybrid micelles (with QDs bound to the surface) at 4:1 QD/polymer w/w, accompanied by free QDs. These structures retain their colloidal stability in 2-PrOH, suggesting that even the free QDs bear a surface shell of block copolymer. The presence of MEH-PPV has no affect on this self assembly. One of the most remarkable observations occurred when the samples in 2-PrOH were centrifuged to remove the free QDs accompanying the hybrid micelles. The micelles sedimented, but upon redispersion in 2-PrOH, rearranged to form colloidally stable long branched cylindrical structures including cylindrical networks.

Retrospective. Paul F. Barbara (1953-2010)

Science (New York, N.Y.). Nov, 2010  |  Pubmed ID: 21109661

The Control of Marine Biofouling on Xerogel Surfaces with Nanometer-scale Topography

Biofouling. Feb, 2011  |  Pubmed ID: 21213155

Mixtures of n-octadecyltrimethoxysilane (C18, 1-5 mole-%), n-octyltriethoxysilane (C8) and tetraethoxysilane (TEOS) gave xerogel surfaces of varying topography. The 1:49:50 C18/C8/TEOS xerogel formed 100-400-nm-wide, 2-7-nm deep pores by AFM while coatings with ≥3% C18 were free of such features. Segregation of the coating into alkane-rich and alkane-deficient regions in the 1:49:50 C18/C8/TEOS xerogel was observed by IR microscopy. Immersion in ASW for 48 h gave no statistical difference in surface energy for the 1:49:50 C18/C8/TEOS xerogel and a significant increase for the 50:50 C8/TEOS xerogel. Settlement of barnacle cyprids and removal of juvenile barnacles, settlement of zoospores of the alga Ulva linza, and strength of attachment of 7-day sporelings were compared amongst the xerogel formulations. Settlement of barnacle cyprids was significantly lower in comparison to glass and polystyrene standards. The 1:49:50 and 3:47:50 C18/C8/TEOS xerogels were comparable to PDMSE with respect to removal of juvenile barnacles and sporeling biomass, respectively.

Imaging Secondary Structure of Individual Amyloid Fibrils of a β2-microglobulin Fragment Using Near-field Infrared Spectroscopy

Journal of the American Chemical Society. May, 2011  |  Pubmed ID: 21524071

Amyloid fibril diseases are characterized by the abnormal production of aggregated proteins and are associated with many types of neuro- and physically degenerative diseases. X-ray diffraction techniques, solid-state magic-angle spinning NMR spectroscopy, circular dichroism (CD) spectroscopy, and transmission electron microscopy studies have been utilized to detect and examine the chemical, electronic, material, and structural properties of amyloid fibrils at up to angstrom spatial resolution. However, X-ray diffraction studies require crystals of the fibril to be analyzed, while other techniques can only probe the bulk solution or solid samples. In the work reported here, apertureless near-field scanning infrared microscopy (ANSIM) was used to probe the secondary structure of individual amyloid fibrils made from an in vitro solution. Simultaneous topographic and infrared images of individual amyloid fibrils synthesized from the #21-31 peptide fragment of β(2)-microglobulin were acquired. Using this technique, IR spectra of the amyloid fibrils were obtained with a spatial resolution of less than 30 nm. It is observed that the experimental scattered field spectrum correlates strongly with that calculated using the far-field absorption spectrum. The near-field images of the amyloid fibrils exhibit much lower scattering of the IR radiation at approximately 1630 cm(-1). In addition, the near-field images also indicate that composition and/or structural variations among individual amyloid fibrils were present.

Phospholipid Membrane Encapsulation of Nanoparticles for Surface-enhanced Raman Scattering

Langmuir : the ACS Journal of Surfaces and Colloids. Jun, 2011  |  Pubmed ID: 21528851

Lipid-encapsulated surface-enhanced Raman scattering (SERS) nanoparticles, with promising applications in biomedical diagnostics, were produced. Gold nanoparticles, 60 nm in diameter, were coated with a ternary mixture of DOPC, sphingomyelin, and cholesterol. The lipid layer is versatile for engineering the chemical and optical properties of the particles. The stability of the lipid-encapsulated particles is demonstrated over a period of weeks. The versatility of the layer is demonstrated by the incorporation of three different Raman-active species using three different strategies. The lipid layer was directly observed by TEM, and the SERS spectrum of the three dye species was confirmed by Raman spectroscopy. UV-vis absorption and dynamic light scattering provide additional evidence of lipid encapsulation. The encapsulation is achieved in aqueous solution, avoiding phase transfer and possible contamination from organic solvents. Furthermore, when fluorescent dye-labeled lipids were employed in the encapsulant, the fluorescence and SERS activity of the particles were controlled by the use of dissolved ions in the preparation solution.

Signature of Hydrophobic Hydration in a Single Polymer

Proceedings of the National Academy of Sciences of the United States of America. Oct, 2011  |  Pubmed ID: 21911397

Hydrophobicity underpins self-assembly in many natural and synthetic molecular and nanoscale systems. A signature of hydrophobicity is its temperature dependence. The first experimental evaluation of the temperature and size dependence of hydration free energy in a single hydrophobic polymer is reported, which tests key assumptions in models of hydrophobic interactions in protein folding. Herein, the hydration free energy required to extend three hydrophobic polymers with differently sized aromatic side chains was directly measured by single molecule force spectroscopy. The results are threefold. First, the hydration free energy per monomer is found to be strongly dependent on temperature and does not follow interfacial thermodynamics. Second, the temperature dependence profiles are distinct among the three hydrophobic polymers as a result of a hydrophobic size effect at the subnanometer scale. Third, the hydration free energy of a monomer on a macromolecule is different from a free monomer; corrections for the reduced hydration free energy due to hydrophobic interaction from neighboring units are required.