Gilbert C. Walker

Signature of hydrophobic hydration in a single polymer.

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.

Approaches in designing non-toxic polymer surfaces to deter marine biofouling

In the past years, research into surfaces that inhibit marine biofouling has focused on designing environmentally friendly materials due to current bans on toxic coatings. This Review focuses on recent progress in the development and fabrication of surfaces that have antifouling and/or foul release properties by looking at selected examples of materials. In addition, we also review the antifouling and/or fouling release properties of these surfaces based on using marine organisms. A summary comparison of materials performance against algal spore settlement is reported: the efficacy of surface amphiphilicity and nanoscale phase segregation is noteworthy.

Barnacle settlement and the adhesion of protein and diatom microfouling to xerogel films with varying surface energy and water wettability

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, γC and surface energies, γS, 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 γC and increased wettability as measured by the static water contact angle, θWs, 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 θWs and R 2 = 0.69 for percentage removal as a function of γC). 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 θWs (R 2 = 0.84) and γC (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).

Cholesterol-dependent nanomechanical stability of phase-segregated multicomponent lipid bilayers.

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.

The role of surface energy and water wettability in aminoalkyl/fluorocarbon/hydrocarbon-modified xerogel surfaces in the control of marine biofouling

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. Youngs 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, θWs, or with decreased polar contribution (γP) to the surface free energy (γS) 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 θWs or γS. 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.

Protein adsorption resistance of anti-biofouling block copolymers containing amphiphilic side chains

Surface active block copolymers (SABCs) with amphiphilic side chains containing ethoxylated fluoroalkyl groups have previously demonstrated advantageous properties with regard to marine fouling resistance and release. While it was previously postulated that the ability of the block copolymer surface to undergo an environment-dependent transformation in surface structure aided this behaviour, protein adsorption characteristics of the surface were never explored. This study aims to expand our knowledge of protein interaction with the amphiphilic surface active block copolymer in an aqueous environment through experiments with bovine serum albumin (BSA), a widely utilized test protein. Fluorescence microscopy analysis using BSA labelled with fluorescein isothiocyanate (BSA–FITC) was performed on a SABC test surface to establish the polymers protein adsorption resistance. Additionally, atomic force microscopy (AFM) based chemical force microscopy (CFM) was utilized to examine the force of adhesion of an AFM tip functionalized with strands of BSA protein with the SABC. No measurable force of adhesion was detected for 58% of the measurements of adhesion force taken for a BSA coated AFM tip interacting with the surface of the amphiphilic SABC in a PBS buffer. Furthermore, no measurements of force of adhesion were made in excess of 0.15 nN. This was in contrast to the non-zero mean adhesion force seen for several control surfaces in PBS buffer.

Detection of chronic lymphocytic leukemia cell surface markers using surface enhanced Raman scattering gold nanoparticles

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.

Insights into the composition, morphology, and formation of the calcareous shell of the serpulid Hydroides dianthus.

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 Youngs 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.

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

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.

Structural and Optical Properties of Self-Assembled Chains of Plasmonic Nanocubes

Solution-based linear self-assembly of metal nanoparticles offers a powerful strategy for creating plasmonic polymers, which, so far, have been formed from spherical nanoparticles and cylindrical nanorods. Here we report linear solution-based self-assembly of metal nanocubes (NCs), examine the structural characteristics of the NC chains, and demonstrate their advanced optical characteristics. In comparison with chains of nanospheres with similar dimensions, composition, and surface chemistry, predominant face-to-face assembly of large NCs coated with short polymer ligands led to a larger volume of hot spots in the chains, a nearly uniform E-field enhancement in the gaps between colinear NCs, and a new coupling mode for NC chains due to the formation of a Fabry-Perot resonator structure formed by face-to-face bonded NCs. The NC chains exhibited stronger surface-enhanced Raman scattering in comparison with linear assemblies of nanospheres. The experimental results were in agreement with finite difference time domain simulations.