Superhydrophobic polymer‐particle composite films produced using various particle sizes

Hydrophilic alumina (Al₂O₃) nanoparticles (25, 35, and 150 nm) are dispersed in different concentrations in solutions of a commercial hydrophobic poly(alkyl siloxane) (Silres BS‐290), and the suspensions are sprayed on glass surfaces. Static contact angles (θ_S), measured on surfaces of siloxane‐nanoparticle composite films that were prepared from dilute dispersions, increase rapidly with particle concentration. Composite films prepared from concentrated dispersions exhibit a maximum, constant θ_S (at saturation θ_S is 160°), which is not affected by the size of the particles. These films exhibit also very small contact angle hysteresis (5°), which is also independent of the particle size. Consequently, the same superhydrophobic character can be induced in siloxane films using nanoparticles, which can range from a few up to several tenths of nanometers. However, the particle size and more precisely the particle specific surface area affects dramatically the minimum critical particle concentration, which must be used in the dispersions to induce superhydrophobicity on the surface of the composite films, that is, to achieve θ_S = 150°. It is shown that critical particle concentration decreases exponentially with specific surface area. This result can be important for manufacturers of superhydrophobic surfaces who are interested in having a good control on the wettability of the composite films. Copyright © 2012 John Wiley & Sons, Ltd.     

Amyloid formation from an α-helix peptide bundle is seeded by 3(10)-helix aggregates

Transformation of proteins and peptides to fibrillar aggregates rich in β sheets underlies many diseases, but mechanistic details of these structural transitions are poorly understood. To simulate aggregation, four equivalents of a water‐soluble, α‐helical (65 %) amphipathic peptide (AEQLLQEAEQLLQEL) were assembled in parallel on an oxazole‐containing macrocyclic scaffold. The resulting 4α‐helix bundle is monomeric and even more α helical (85 %), but it is also unstable at pH 4 and undergoes concentration‐dependent conversion to β‐sheet aggregates and amyloid fibrils. Fibrils twist and grow with time, remaining flexible like rope (>1 μm long, 5–50 nm wide) with multiple strings (2 nm), before ageing to matted fibers. At pH 7 the fibrils revert back to soluble monomeric 4α‐helix bundles. During α→β folding we were able to detect soluble 3₁₀ helices in solution by using 2D‐NMR, CD and FTIR spectroscopy. This intermediate satisfies the need for peptide elongation, from the compressed α helix to the fully extended β strand/sheet, and is driven here by 3₁₀‐helix aggregation triggered in this case by template‐promoted helical bundling and by hydrogen‐bonding glutamic acid side chains. A mechanism involving α?α₄?(3₁₀)₄?(3₁₀)_n?(β)_n?m(β)_n equilibria is plausible for this peptide and also for peptides lacking hydrogen‐bonding side chains, with unfavourable equilibria slowing the α→β conversion.     

Folding–Unfolding Equilibrium of a Methylidene‐Substituted β‐Peptide

β‐Peptides possess the ability to fold into secondary structure elements, and this property, together with resistance to biodegradation, makes these compounds interesting for pharmaceutical applications. Recently, a novel class of β‐peptides containing methylidene moieties was described. The GROMOS 53A6 force field was used to simulate the folding equilibrium of a β³‐hexapeptide with methylidene (CH₂?) groups at all six CA‐atoms. Due to the rotational barriers induced by these methylidene groups, the helical secondary‐structure elements, normally found in β³‐peptides, are disfavored in this molecule. Simulations, started from fully extended and 3₁₄‐helical conformations, showed that the molecule adopts a complete 2₈‐helix for ca. 5% of the time and partial 2₈‐helical conformations for ca. 20% of the time. Yet, as suggested by experiments, the folding equilibrium is dominated by unfolded conformations.     

Colorimetric and fluorometric chemosensors for selective signaling toward Ca and Mg by aza-crown ether acridinedione-functionalized gold nanoparticles

The aza-crown ether acridinedione-functionalized gold nanoparticles (ACEADD-GNPs) 6 have been synthesized and investigated as a fluorescent chemosensor for metal ions. A blue shift along with an intensity enhancement of emission and color change are observed in the presence of both Ca²⁺ and Mg²⁺ ions. The enhanced fluorescence intensity is attributed to the photoinduced electron transfer (PET) suppression through space and color change of the suspension from red to blue due to shifted surface plasmon resonance (SPR) with aggregation of nanoparticles by the sandwich complexation.     

AuAg bimetallic nanoparticles film fabricated based on H2O2-mediated silver reduction and its application

A method to fabricate AuAg bimetallic nanoparticles film by H₂O₂-mediated reduction of silver was reported. Gold nanoparticles (Au NPs) were first adsorbed onto the surface of a self-assembled 2-aminoethanethiol monolayer-modified gold film or 3-aminopropyltriethoxysilane (APTES) monolayer-modified quartz slide. Upon further treatment of this modified film with the solution containing silver nitrate (AgNO₃) and H₂O₂, silver was deposited on the surface of Au NPs. The size of the AuAg bimetallic particles could be readily tuned by manipulating the concentration of H₂O₂. Surface plasmon resonance (SPR) was used to investigate the process, the deposition of silver on Au NPs modified gold film resulted in an obvious decrease of depth in the SPR reflectance intensity and minimum angle curves (SPR R-θ curves), which may be utilized for the quantitative SPR detection of the analyte, H₂O₂. Combination of the biocatalytic reaction that could yield H₂O₂ by using the enzyme, glucose oxidase, with the deposition of silver may enable the design of a glucose biosensor by SPR technique. Furthermore, we evaluated the AuAg bimetallic nanoparticles film for their ability to be an effective substrate for surface-enhanced Raman scattering (SERS).     

Infrared spectral observations of chlorophyll-a and ethyl chlorophyllide-a in the solid state

Abstract— Infrared spectra of ethyl chlorophyllide-a and chlorophlyll-a in Nujol mulls show a shift of the C₉ ketone carbonyl absorption band to higher frequencies when the pigments are subjected to drying processes. The ketone band can then be reversed back to lower frequencies by exposing the material to moisture. These observations indicate that the nature of chlorophyll aggregation in the solid state varies with the water content of the pigment.     

Formation of Silver Nanoparticles by Electron Transfer in Peptides and c-Cytochromes

The reduction of Ag⁺ ions to Ag⁰ atoms is a highly endergonic reaction step, only the aggregation to Ag_n clusters leads to an exergonic process. These elementary chemical reactions play a decisive role if Ag nanoparticles (AgNPs) are generated by electron transfer (ET) reactions to Ag⁺ ions. We studied the formation of AgNPs in peptides by photoinduced ET, and in c-cytochromes by ET from their Fe²⁺/hemes. Our earlier photoinduced experiments in peptides had demonstrated that histidine prevents AgNP formation. We have now observed that AgNPs can be easily synthesized with less-efficient Ag⁺-binding amino acids, and the rate increases in the order lysine<asparagine<aspartate<serine. The ability of Fe²⁺/hemes of c-cytochromes to reduce Ag⁺ to AgNPs was studied in an enzymatic experiment and with living bacteria Geobacter sulfurreducens (Gs).     

金纳米棒组装体表面等离子体共振耦合效应的FDTD模拟

贵金属纳米结构的光学性质与其尺寸、形貌、介质环境等因素的相关性是基础研究领域的重要内容. 本文利用时域有限差分（FDTD）方法,计算了不同构型二聚体和多聚体的表面等离子体共振（SPR）特性. 研究了金纳米棒结构和组装方式对SPR耦合效应的影响,模拟结果与实验规律比较吻合. 金纳米棒二聚体的光吸收结果表明：对于肩并肩（S-S）的组装体,随着间隙的减小,金纳米棒的横向SPR（SPR_T）峰有较小的红移,而纵向SPR（SPR_L）峰显著蓝移. 对于端对端（E-E）的组装体,随着组装体间隙的减小,金纳米棒的SPR_T峰无明显移动,而SPR_L峰显著红移,并在近红外较长波段范围内出现新的共振峰,其强度随着间隙的减小而增强;结合弹簧振子模型和纳米颗粒在外电场作用下的极化,对组装体共振吸收峰的移动和新的耦合共振峰的出现提出了初步的解释.     

Helical γ-Peptide Foldamers as Dual Inhibitors of Amyloid-β Peptide and Islet Amyloid Polypeptide Oligomerization and Fibrillization

Type 2 diabetes (T2D) and Alzheimer's disease (AD) belong to the 10 deadliest diseases and are sorely lacking in effective treatments. Both pathologies are part of the degenerative disorders named amyloidoses, which involve the misfolding and the aggregation of amyloid peptides, hIAPP for T2D and Aβ_1-42 for AD. While hIAPP and Aβ_1-42 inhibitors have been essentially designed to target β-sheet-rich structures composing the toxic amyloid oligomers and fibrils of these peptides, the strategy aiming at trapping the non-toxic monomers in their helical native conformation has been rarely explored. We report herein the first example of helical foldamers as dual inhibitors of hIAPP and Aβ_1-42 aggregation and able to preserve the monomeric species of both amyloid peptides. A foldamer composed of 4-amino(methyl)-1,3-thiazole-5-carboxylic acid (ATC) units, adopting a 9-helix structure reminiscent of 3₁₀ helix, was remarkable as demonstrated by biophysical assays combining thioflavin-T fluorescence, transmission electronic microscopy, capillary electrophoresis and mass spectrometry.     

银纳米粒子自组装结构的光谱性质研究

采用自组装方法在玻璃基底表面上构筑了银纳米粒子二维亚单层结构, 进而以对巯基苯胺为耦联分子进行银粒子的二次组装, 构成具有分子尺寸间隙的银粒子簇. 银粒子表面等离子体共振依赖于粒子间距、表面吸附分子和粒子组装方式. 同层内的银粒子相互间电磁偶合可导致银粒子偶极子等离子体共振蓝移; 对巯基苯胺的吸附则使得表面等离子体共振红移. 表面增强拉曼光谱结果表明, 具有分子尺寸间隙的银粒子簇对耦联于粒子间的对巯基苯胺分子的拉曼散射具有极大的增强效应, 同时也使耦联的对巯基苯胺与银粒子间产生更大程度的电荷转移.     

Multifunctional gold nanoparticles as signal transducers for fabrication of 1:2 molecular demultiplexer

A label-free and enzyme-free demultiplexer system for the fabrication of 1:2 molecular demultiplexer with luminol functionalized gold nanoparticles (Lum-AuNPs) as signal transducers was developed for the first time. The Lum-AuNPs had both chemiluminescence (CL) activity and surface plasmon resonance property. It was found that organothiols (RSH) could easily induce the aggregation of AuNPs via strong Au–S covalent interactions in the absence of hydrogen peroxide (H₂O₂), generating a red shift in the absorption band of AuNPs. However, the presence of H₂O₂ would readily oxidize RSH to disulfide (RS-SR), and the aggregation of Lum-AuNPs did not occur due to lack of the sulfhydryl group. Meanwhile, H₂O₂ could react with Lum-AuNPs, producing a strong CL emission owing to the enhancement effect of RSH on AuNPs-luminol-H₂O₂ CL system. Thus, RSH, H₂O₂, absorbance ratio, and CL intensity served as the signal input, address input, and two different signal outputs of the 1:2 molecular demultiplexer, respectively.     

Probing the Catalytic Activity of Reduced Graphene Oxide Decorated with Au Nanoparticles Triggered by Visible Light

Hybrid materials in which reduced graphene oxide (rGO) is decorated with Au nanoparticles (rGO–Au NPs) were obtained by the in situ reduction of GO and AuCl₄^?_(aq) by ascorbic acid. On laser excitation, rGO could be oxidized as a result of the surface plasmon resonance (SPR) excitation in the Au NPs, which generates activated O₂ through the transfer of SPR‐excited hot electrons to O₂ molecules adsorbed from air. The SPR‐mediated catalytic oxidation of p‐aminothiophenol (PATP) to p,p′‐dimercaptoazobenzene (DMAB) was then employed as a model reaction to probe the effect of rGO as a support for Au NPs on their SPR‐mediated catalytic activities. The increased conversion of PATP to DMAB relative to individual Au NPs indicated that charge‐transfer processes from rGO to Au took place and contributed to improved SPR‐mediated activity. Since the transfer of electrons from Au to adsorbed O₂ molecules is the crucial step for PATP oxidation, in addition to the SPR‐excited hot electrons of Au NPs, the transfer of electrons from rGO to Au contributed to increasing the electron density of Au above the Fermi level and thus the Au‐to‐O₂ charge‐transfer process.     

Discrete Dipole Approximation Analysis of Plasmonic Core/Alloy Nanoparticles

The surface plasmon resonance (SPR) properties of Au/Au_xAg_1?x core/alloy nanoparticles (NPs) have been investigated by means of the discrete dipole approximation. The core/alloy microstructure was varied by changing the shell alloy composition x, its thickness t_S, and the shell thickness to core radius ratio (t_S/r_C) in the range of 0.05–1.0. These changes resulted in a novel tuning of SPR shape, frequency, and extinction. These models were compared with experimental results for Au/Au_xAg_1?x NPs prepared by a microwave‐mediated hydrothermal processing method, which produces core/alloy NPs with SPR signatures closely resembling those of the models.     

NMR‐Solution Structures in Methanol of an α‐Heptapeptide,of a β3/β2‐Nonapeptide,and of an all‐β3‐Icosapeptide Carrying the 20 Proteinogenic Side Chains

The NMR‐solution structure of an α‐heptapeptide with a central Aib residue was investigated in order to verify that, in contrast to β‐peptides, short α‐peptides do not form a helical structures in MeOH. Although the central Aib residue was found to induce a bend in the experimentally determined structure, no secondary structure typical for longer α‐peptides or proteins was found. A β²/β³‐nonapeptide with polar, positively charged side chains was subjected to NMR analysis in MeOH and H₂O. Whereas, in MeOH, it folds into a 10/12‐helix very similar to the structure determined for a corresponding β²/β³‐nonapeptide with only aliphatic side chains, no dominant conformation could be determined in H₂O. Finally, the NMR analysis of a β³‐icosapeptide containing the side chains of all 20 proteinogenic amino acids in MeOH is described. It revealed that this 20mer folds into a 3₁₄‐helix over its whole length forming six full turns, the longest 3₁₄‐helix found so far. Together, our findings confirm that, in contrast to α‐peptides, β‐peptides not only form helices with just six residues, but also form helices that are longer than helical sections usually observed in proteins or natural peptides. The higher helix‐forming propensity of long β‐peptides is attributed to the conformation‐stabilizing effect of the staggered ethane sections in β‐peptides which outweighs the detrimental effect of the increasing macrodipole.     

Development and characterization of Au-YSZ surface plasmon resonance based sensing materials: high temperature detection of CO

Au-YSZ nanocomposite films exhibited a surface plasmon resonance absorption band around 600 nm that underwent a reversible blue shift and narrowed upon exposure to CO in air at 500 degrees C. A linear dependence of the sensing signal was observed for CO concentrations ranging between 0.1 and 1 vol % in an air carrier gas. This behavior of the SPR band, upon exposure to CO, was not observed when using nitrogen as the carrier gas, indicating an oxygen-dependent reaction mechanism. Additionally, the SPR band showed no measurable signal change upon exposure to CO at temperatures below approximately 400 degrees C. The oxygen and temperature-dependent characteristics, coupled with the oxygen ion formation and conduction properties of the YSZ matrix, are indicative of charge-transfer reactions occurring at the three-phase boundary region between oxygen, Au, and YSZ, which result in charge transfer into the Au nanoparticles. These reactions are associated with the oxidation of CO and a corresponding reduction of the YSZ matrix. The chemical-reaction-induced charge injection into the Au nanoparticles results in the observed blue shift and narrowing of the SPR band.     

Excited state polarizabilities from solvatochromic shifts

A new method is proposed to estimate the polarizability (α_e) of a molecule in an excited state using solvatochromic shift measurements and McRae's equation. In the earlier methods the contribution due to polarizability was not considered. In view of this, the proposed method is also expected to give a better estimation of excited state electric dipole moment (μ_e) and the (θ) angle between excited and ground state electric dipole moments, μ_e and μ_g apart from giving values of polarizability of the molecules in the excited state. This method has been applied in the case of the La band of p-nitro aniline and the results for all the parameters are found to be satisfactory and of right order in comparison with that reported in literature.     

Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers

The folding and aggregation behavior of a pair of oligo(phenylene ethynylene) (OPE) foldamers are investigated by means of UV/Vis absorption and circular dichroism spectroscopy. With identical OPE backbones, two foldamers, 1 with alkyl side groups and 2 with triethylene glycol side chains, manifest similar helical conformations in solutions in n‐hexane and methanol, respectively. However, disparate and competing folding and aggregation processes are observed in alternative solvents. In cyclohexane, oligomer 1 initially adopts the helical conformation, but the self‐aggregation of unfolded chains, as a minor component, gradually drives the folding–unfolding transition eventually to the unfolded aggregate state completely. In contrast, in aqueous solution (CH₃OH/H₂O) both folded and unfolded oligomer 2 appear to undergo self‐association; aggregates of the folded chains are thermodynamically more stable. In solutions with a high H₂O content, self‐aggregation among unfolded oligomers is kinetically favored; these oligomers very slowly transform into aggregates of helical structures with greater thermodynamic stability. The folded–unfolded conformational switch thus takes place with the free (nonaggregated) molecules, and the very slow folding transition is due to the low concentration of molecularly dispersed oligomers.     

Non-noble metal Bi/BiVO4 photoanode for surface plasmon resonance-induced photoelectrochemical biosensor of hydrogen peroxide detection

Non-noble metal for surface plasmon resonance (SPR)-induced nano-enzymatic photoelectrochemical (PEC) biosensors is a promising method for broad applications. Non-noble metal bismuth (Bi) is gradually being valued for their properties such as non-toxicity, abundant reserves, and good catalysis. Meanwhile, metallic Bi nanoparticles (NPs) have superior SPR properties similar to that of noble metal. Therefore, a non-noble metal Bi/BiVO₄ photoanode based on SPR was designed for the PEC detection of H₂O₂ in this work. The synergistic effect of BiVO₄ with suitable band gap (2.5 eV) and Bi NPs with superior SPR effectively reduces the recombination of photogenerated electrons and holes, which significantly improves the PEC activity of Bi/BiVO₄ electrode at low voltage. The in situ Bi/BiVO₄/FTO photoelectrode not only exhibits a more comprehensive linear range of 0.005–2.6 mM with a lower detection limit of 2.0 μM but also possesses excellent selectivity for the determination of H₂O₂ because the coexisting substances are difficult to be oxidized at 0 V bias voltage. The investigation of Bi/BiVO₄-based sensor has contributed to the development of non-noble metal for SPR-induced PEC biosensors.

     

Studies of molecular aggregation of a polybenzimidazole in solution by fluorescence spectroscopy

A purified, well characterized sample of polybenzimidazole (PBI) has been studied by fluorescence spectroscopy in order to obtain information concerning molecular aggregation in solution. Fluorescence and excitation spectra of PBI and a model compound have been measured in N,N-dimethylacetamide (DMA) and in formic acid (FA). The major emission band for PBI solutions is attributed an excited ¹L_b state, which consists of 0-0 and 0-1 vibrational energy bands. Concentration quenching is attributed to reabsorption of the emitted light, and self-quenching due to the formation of molecular aggregates. Therefore, the extent of molecular aggregation in solution is estimated from the self-quenching constant. In the course of the investigation, a new fluorescence band was found for the PBI-DMA solution; its intensity increased markedly due to overlapping of polymer coils. The formation of polyelectrolytes in PBI-FA solutions was confirmed by viscometry. The major fluorescence band was found to originate from the excited ¹L_b state and to shift to longer wavelengths upon addition of lithium bromide.     

Angle-dependent resonance of localized and propagating surface plasmons in microhole arrays for enhanced biosensing

The presence of microhole arrays in thin Au films is suited for the excitation of localized and propagating surface plasmon (SP) modes. Conditions can be established to excite a resonance between the localized and propagating SP modes, which further enhanced the local electromagnetic (EM) field. The co-excitation of localized and propagating SP modes depends on the angle of incidence (θ _exc) and refractive index of the solution interrogated. As a consequence of the enhanced EM field, enhanced sensitivity and an improved response for binding events by about a factor of 3 to 5 was observed with SPR sensors in the Kretschmann configuration for a set of experimental conditions (λ _SPR, θ _exc, and η). Thus, microhole arrays can improve sensing applications of SPR based on classical prism-based instrumentation and are suited for SP-coupled spectroscopic techniques.