Modeling of Infrared–Visible Sum Frequency Generation Microscopy Images of a Giant Liposome

The article explores the theory of Infrared-Visible Sum Frequency Generation microscopy of phospholipid envelopes with dimensions larger than the wavelength of the nonlinear emission. The main part of the study concerns derivation and accounting for the contributions of effective nonlinear responses specific to sites on the surfaces of a bilayer envelope and their dependence on polarization condition and experimental geometry. The nonlinear responses of sites are mapped onto the image plane according to their emission directions and the numerical aperture of a sampling microscope objective. According to the simulation results, we discuss possible approaches to characterize the shape of the envelope, to extract molecular hyperpolarizabilities, to anticipate possible heterogeneity in envelope composition and anisotropy of the environment proximal to the envelope. The modeling approach offers a promising analytic facility to assist connecting microscopy observations in engineered liposomes, cellular envelopes and sub-cellular organelles of relatively large dimensions to molecular properties and hence to chemistry and structure down to available the spatial resolution

Secondary Structure Transition and Critical Stress for a Model of Spider Silk Assembly

Spiders spin their silk from an aqueous solution to a solid fiber in ambient conditions. However, to date the assembly mechanism in the spider silk gland has not been satisfactorily explained. In this paper, we use molecular dynamics simulations to model N. clavipes MaSp1 dragline silk formation under shear flow and determine the secondary structure transitions leading to the experimentally observed fiber structures. While no experiments are performed on the silk fiber itself, insights from this polypeptide model can be transferred to the fiber scale. The novelty of this study lies in the calculation of the shear stress (300-700 MPa) required for fiber formation and identification of the amino acid residues involved in the transition. This is the first time that the shear stress has been quantified in connection with a secondary structure transition. By study of molecules containing varying numbers of contiguous MaSp1 repeats we identified the smallest molecule size that gives rise to a 'silk-like' structure contains six poly-alanine repeats. Through a probability analysis of the secondary structure we identify specific amino acids that transition from α-helix to β-sheet. In addition to portions of the poly-alanine section these amino acids include glycine, leucine and glutamine. Stability of β-sheet structures appears to arise from a close proximity in space of helices in the initial spidroin state. Our results are in agreement with the forces exerted by spiders in the silking process and the experimentally determined global secondary structure of spidroin and pulled MaSp1 silk. Our study emphasizes the role of shear in the assembly process of silk and can guide the design of microfluidic devices that attempt to mimic the natural spinning process and predict molecular requirements for the next generation of silk-based functional materials

Plastron properties of a superhydrophobic surface

Most insects and spiders drown when submerged during flooding or tidal inundation, but some are able to survive and others can remain submerged indefinitely without harm. Many achieve this by natural adaptations to their surface morphology to trap films of air, creating plastrons which fix the water-vapor interface and provide an incompressible oxygen-carbon dioxide exchange surface. Here the authors demonstrate how the surface of an extremely water-repellent foam mimics this mechanism of underwater respiration and allows direct extraction of oxygen from aerated water. The biomimetic principle demonstrated can be applied to a wide variety of man-made superhydrophobic materials

Porous materials show superhydrophobic to superhydrophilic switching

Switching between superhydrophobicity and superhydrophilicity in porous materials was predicted theoretically and demonstrated experimentally with the example of thermally induced contact angle change; tunability of this system was also demonstrated

The nature of the silicaphilic fluorescence of PDMPO

PDMPO (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole), has unique silica specific fluorescence and is used in biology to understand biosilicification. This ‘silicaphilic’ fluorescence is not well understood nor is the response to local environmental variables like solvent and pH. We investigated PDMPO in a range of environments: using UV-vis and fluorescence spectroscopy supported by computational data, (SPARC, molecular dynamics simulations, density functional theory calculations), dynamic light scattering and zeta potential measurements to understand the PDMPO–silica interaction. From absorption data, PDMPO exhibited a pKa of 4.20 for PDMPOH22+ to PDMPOH+ . Fluorescence emission measurements revealed large shifts in excited state pKa* values with different behaviour when bound to silica (pKa* of 10.4). PDMPO bound to silica particles is located in the Stern layer with the dye exhibiting pH dependent depolarising motion. In aqueous solution, PDMPO showed strong chromaticity with correlation between the maximum emission wavelength for PDMPOH+* and dielectric constant (4.8–80). Additional chromatic effects were attributed to changes in solvent accessible surface area. Chromatic effects were also observed for silica bound dye which allow its use as a direct probe of bulk pH over a range far in excess of what is possible for the dye alone (3–5.2). The unique combination of chromaticity and excited state dynamics allows PDMPO to monitor pH from 3 to 13 while also reporting on surface environment opening a new frontier in the quantitative understanding of (bio)silicification

Directional Wetting in Anisotropic Inverse Opals

Porous materials display interesting transport phenomena due to the restricted motion of fluids within the nano- to micro-scale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy.Chemistry and Chemical Biolog

Aqueous peptide-TiO2 interfaces: iso-energetic binding via either entropically- or enthalpically-driven mechanisms

A major barrier to the systematic improvement of biomimetic peptide-mediated strategies for the controlled growth of inorganic nanomaterials in environmentally benign conditions lies in the lack of clear conceptual connections between the sequence of the peptide and its surface binding affinity, with binding being facilitated by non-covalent interactions. Peptide conformation, both in the adsorbed and non-adsorbed state, is the key relationship that connects peptide-materials binding with peptide sequence. Here, we combine experimental peptide–titania binding characterization with state-of-the-art conformational sampling via molecular simulations to elucidate these structure/binding relationships for two very different titania-binding peptide sequences. The two sequences (Ti-1: QPYLFATDSLIK and Ti-2: GHTHYHAVRTQT) differ in their overall hydropathy, yet via quartz-crystal microbalance measurements and predictions from molecular simulations, we show these sequences both support very similar, strong titania-binding affinities. Our molecular simulations reveal that the two sequences exhibit profoundly different modes of surface binding, with Ti-1 acting as an entropically-driven binder while Ti-2 behaves as an enthalpically-driven binder. The integrated approach presented here provides a rational basis for peptide sequence engineering to achieve the in-situ growth and organization of titania nanostructures in aqueous media and for the design of sequences suitable for a range of technological applications that involve the interface between titania and biomolecules

Isolation of a wide range of minerals from a thermally treated plant: Equisetum arvense, a Mare’s tale

Silica is the second most abundant biomineral being exceeded in nature only by biogenic CaCO3. Many land plants (such as rice, cereals, cucumber, etc.) deposit silica in significant amounts to reinforce their tissues and as a systematic response to pathogen attack. One of the most ancient species of living vascular plants, Equisetum arvense is also able to take up and accumulate silica in all parts of the plant. Numerous methods have been developed for elimination of the organic material and/or metal ions present in plant material to isolate biogenic silica. However, depending on the chemical and/or physical treatment applied to branch or stem from Equisetum arvense; other mineral forms such glass-type materials (i.e. CaSiO3), salts (i.e. KCl) or luminescent materials can also be isolated from the plant material. In the current contribution, we show the chemical and/or thermal routes that lead to the formation of a number of different mineral types in addition to biogenic silica

The use of high aspect ratio photoresist (SU-8) for super-hydrophobic pattern prototyping

In this work we present a reliable technique for the production of large areas of high aspect-ratio patterns and describe their use as model super-hydrophobic systems. The high thickness and straight sidewalls possible with SU-8 were used to generate dense patterns of small pillars. These photoresist patterns could be used directly, without the need for micromoulding. A method is given allowing resist thickness to be varied over a wide range and a bottom antireflective layer was used to simplify patterning on reflective substrates. This patterning technique allows rapid testing of wetting theories, as pattern size and depth can be varied simply and samples can be produced in sufficient numbers for laboratory use. We show how the static contact angle of water varies with pattern height for one sample-pattern and how static and dynamic contact angles vary with dimension using high aspect-ratio patterns

Preparation of hexagonal GeO2 particles with particle size and crystallinity controlled by peptides, silk and silk-peptide chimeras

We demonstrate the use of silk based proteins to control the particle/crystallite size during GeO2 formation, using a bio-mimetic approach at circumneutral pH and ambient temperature. Multicrystalline GeO2 was prepared from germanium tetraethoxide (TEOG) in the presence of different silk-based proteins: Bombyx mori silk (native silk) and two chimeric proteins prepared by linking a germania binding peptide (Ge28: HATGTHGLSLSH) with Bombyx mori silk via chemical coupling at different peptide loadings (silk-Ge28 10% and silk-Ge28 50%). The mineralisation activity of the silk-based proteins was compared with that of peptide Ge28 as a control system. GeO2 mineralisation was investigated in water and in citric acid/bis-tris propane buffer at pH 6. Morphology, particle size, crystallinity, water and organic content of the materials obtained were analysed to study the effect of added biomolecules and mineralisation environment on material properties. In the presence of silk additives well-defined cube-shape hybrid materials composed of hexagonal germania and up to ca. 5 wt% organic content were obtained. The cubic particles ranged from 0.4 to 1.4m in size and were composed of crystalline domains in the range 35-106 nm depending on the additive used and synthesis conditions