Size effects of gold nanaoparticles on plasmon-induced photocurrents of gold-TiO2 nanocomposites

An ITO electrode was coated with a nanoporous TiO2 film, and the film was loaded with Au nanoparticles with different diameters (15, 40 and 100 nm). The electrode exhibited plasmon-induced photocurrents in the presence of Fe(2+/3+) couple under visible light (lambda > 420 nm). The quantum efficiency increased with increasing particle size, whereas the maximum photocurrent density decreased.     

Early reactions of light-induced protochlorophyllide and chlorophyllide transformations analyzed in vivo at room temperature with a diode array spectrofluorometer

The steps of protochlorophyllide (Pchlide) photoreduction and subsequent chlorophyllide (Chlide) transformations which occur in the seconds to minutes time-scale were studied using a diode array spectrofluorometer in dark-grown barley leaves. The intensity of the excitation light was varied between 3 and 2,500 micromol m(-2) s(-1) and a series of fluorescence spectra were recorded at room temperature in the seconds and minutes time scales. In certain experiments, 77-K emission spectra were measured with the same equipment. The high quality of the spectra allowed us to run spectral resolution studies which proved the occurrence, at room temperature, of multiple Pchlide and Chlide forms found previously in 77-K spectra. The comparison of the 77-K and room-temperature spectra showed that the fluorescence yields of the nonphotoactive 633-nm Pchlide form and of the Chlide product emitting at 678 nm were temperature independent. The fluorescence intensity of aggregated NADPH-pigment-POR complexes (photoactive 656-nm Pchlide and 693-nm Chlide forms) were strongly increased at 77 K, while that of the NADP(+)-Chlide-POR (684-686-nm Chlide form) was much less affected by temperature. Information was obtained also about the dynamics of the transformation of pigment forms in the light at different photon densities. At low light intensities, the phototransformation of the 642-644-nm Pchlide form was faster than that of the 654-656-nm form. The relative amplitudes of Gaussian components related to different Chlide forms found after exposure to a constant amount of photons strongly depended on the light intensity used. Strong quenching of all Chlide components occurred upon prolonged exposure to high intensity light. These effects are discussed by considering the interconversion processes between different forms of the pigment-protein complexes, their relative fluorescence yields and energy migration processes.     

含不同链长的三个卟啉LB膜修饰电极的光电响应研究

利用LB (Langmuir-Blodgett)技术将含不同链长的卟啉化合物(C4Py, C6Py和C8Py)单层膜转移到ITO (indium-tin oxide)导电玻璃上, 发现其具有良好的光电转换性质. 卟啉化合物修饰后的紫外吸收光谱与光电流工作谱重叠, 表明卟啉化合物起到了敏化光电流产生的效果; 而且电子给体、电子受体和偏压对其敏化效果的实验结果表明: 光诱导电子转移是产生光电响应的主要原因. 而且, 这三个卟啉化合物的光电响应性质与碳链长度相关, 其中含有六个碳链的C6Py表现出最佳的光电转化效果.     

Photophysical properties of hydroxylated amphiphilic poly(p-phenylene)s

A homologous series of polyhydroxylated poly(p-phenylene)s with different alkoxy groups (C6PPPOH, C12PPPOH, and C18PPPOH) were synthesized with use of the Suzuki polycondensation reaction. Comparative studies of the structure correlation between their photophysical properties and film morphology is described. The absorption and emission spectra of polymers in solution and thin films showed similar features indicating that the electronic properties in solution were retained in the film state. Compared to the polymer with the short alkoxy chains (C6PPPOH), the polymers with long alkoxy groups (C12PPPOH and C18PPPOH) showed improved film forming properties with continuous and smooth film morphology. The absorption properties of the C12PPPOH showed an enhanced effective conjugation length and high quantum yield implying planarization of the backbone through alkoxy chain packing (C12H25O-) and potential hydrogen bonds. No overlap in the absorption and emission spectra was observed, which indicated minimized excimer formation or excitation energy transfer in the films. Time-resolved fluorescence measurements showed that the decay times increased from 43 ps (C6PPPOH) to 78 ps (C12PPPOH) and 99 ps (C18PPPOH). Electrochemical studies were performed for all polymers and the observed oxidation potential for C6PPPOH was higher than that of C12PPPOH and C18PPPOH. In addition, the C12PPPOH has the lowest band gap of DeltaE = 2.59 eV when compared to the 3.1 (C6PPPOH) and 2.61 eV (C18PPPOH) gaps. The optical band gaps estimated from the absorption onset of the polymers are significantly higher than those obtained from electrochemical data. C12PPPOH was chosen for investigating the charge carrier mobility by the time-of-flight (TOF) technique. The observed results also showed negative field dependent values of the drift mobility for the polymer C12PPPOH.     

Ballistic energy transport along PEG chains: distance dependence of the transport efficiency

Dual-frequency relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy was used to investigate energy transport in polyethylene glycol (PEG) oligomers of different length, having 0, 4, 8, and 12 repeating units and end-labeled with azido and succinimide ester moieties (azPEGn). The energy transport initiated by excitation of the N≡N stretching mode of the azido group in azPEGn in CCl(4) at ca. 2100 cm(-1) was recorded by probing the C=O stretching modes (reporters) of the succinimide ester moiety. Sensitive to the excess energy delivered to the reporter modes, RA 2DIR permits observation of both the through-bond and through-solvent energy transport contributions. The cross-peak data involving the reporter modes with different thermal sensitivity and the data for mixtures of compounds permitted concluding that through-bond energy transport is the dominant mechanism for most cross peaks in all four azPEGn compounds. The through-bond energy transport time, evaluated as the waiting time at which the cross peak maximum is reached, was found to be linearly dependent on the chain length of up to 60 ?, suggesting a ballistic energy transport regime. The through-bond energy transport speed determined from the chain-length dependence of T(max) in CCl(4) is found to be ca. 450 m s(-1). The cross-peak amplitude at the maximum decays exponentially with the chain length; a characteristic decay distance is found to be 15.7 ± 1 ?. The cross-peak amplitude at zero waiting time, determined by the end-to-end distance distribution, is found to decay with the chain length (L) as ～L(-1.4), which is close to predictions of the free flight chain model. The match indicates that the end-group interaction does not strongly perturb the end-to-end distribution, which is close to the ideal random coil distribution with the Gaussian probability density.     

表面活性剂疏水链长对高温下泡沫稳定性的影响

选用不同疏水链长的α-烯烃磺酸盐(AOS)形成泡沫, 分别用泡沫衰减法和泡沫岩芯封堵法测定不同温度下的泡沫稳定性, 并采用动态表面张力、界面流变、分子模拟等方法研究了表面活性剂在气/液界面的吸附行为和界面吸附层的性质, 分析了高温下泡沫的稳定机制. 实验结果表明, 在高温下, 极性头的“锚定作用”减弱, 表面活性剂疏水链难以在气液界面保持以直立状态吸附, 疏水链碳数大于20的表面活性剂分子难以分立吸附, 其疏水链相互交叉缠绕, 增强了泡沫膜的强度, 减缓了气体通过液膜的扩散, 形成的泡沫在高温下具有较好的稳定性.     

Preparation of nitrogen-substituted TiO2 thin film photocatalysts by the radio frequency magnetron sputtering deposition method and their photocatalytic reactivity under visible light irradiation

Nitrogen-substituted TiO2 (N-TiO2) thin film photocatalysts have been prepared by a radio frequency magnetron sputtering (RF-MS) deposition method using a N2/Ar mixture sputtering gas. The effect of the concentration of substituted nitrogen on the characteristics of the N-TiO2 thin films was investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The absorption band of the N-TiO2 thin film was found to shift smoothly to visible light regions up to 550 nm, its extent depending on the concentration of nitrogen substituted within the TiO2 lattice in a range of 2.0-16.5%. The N-TiO2 thin film photocatalyst with a nitrogen concentration of 6.0% exhibited the highest reactivity for the photocatalytic oxidation of 2-propanol diluted in water even under visible (lambda > or = 450 nm) or solar light irradiation. Moreover, N-TiO2 thin film photocatalysts prepared on conducting glass electrodes showed anodic photocurrents attributed to the photooxidation of water under visible light, its extent depending on wavelengths up to 550 nm. The absorbed photon to current conversion efficiencies reached 25.2% and 22.4% under UV (lambda = 360 nm) and visible light (lambda = 420 nm), respectively. UV-vis and photoelectrochemical investigations also confirmed that these thin films remain thermodynamically and mechanically stable even under heat treatment at 673 K. In addition, XPS and XRD studies revealed that a significantly high substitution of the lattice O atoms of the TiO2 with the N atoms plays a crucial role in the band gap narrowing of the TiO2 thin films, enabling them to absorb and operate under visible light irradiation as a highly reactive, effective photocatalyst.     

Computational studies on structural and excited-state properties of modified chlorophyll f with various axial ligands

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll f (Chlide f), Chlide a, Chlide b, and axial ligated (with imidazole, H(2)O, CH(3)OH, CH(3)COOH, C(6)H(5)OH) Chlide f molecules. The computed differences among the Q(x), Q(y), B(x), and B(y) band absorbance wavelengths of Chlide a, b, and f molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl f molecules. Our computations provide evidence that the red shift in the Q(y) band of chl f is due to the extended delocalization of macrocycle chlorin ring because of the presence of the -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (lowest unoccupied molecular orbital (LUMO) of the Chlide f and LUMO-1 of the Chlide b). All the absorption bands of Chlide f shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide f-imidazole acts as a good electron donor and Chlide f-CH(3)COOH acts as a good electron acceptor.     

A modular molecular photovoltaic system based on phospholipid/alkanethiol hybrid bilayers: photocurrent generation and modulation

Monolayer quantities of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), incorporated with either fullerenes or ruthenium tris(bipyridyl) (Ru(bpy)(3)(2+)) complexes, were formed on ferrocene-terminated C11-alkanethiol self-assembled monolayers (SAMs) through lipid fusion. Thus formed hybrid structures are characterized by quartz crystal microbalance, UV-vis spectroscopy, cyclic voltammetry and impedance analysis. In comparison to lipid monolayers deposited on C12-alkanethiol SAMs, photocurrent generation from these ferrocene-based structures is significantly modulated, displaying attenuated anodic photocurrents and enhanced cathodic photocurrents. While a similar trend was observed for the two photoagents studied, the degree of such modulations was always found to be greater in fullerene-incorporated bilayers. These findings are evaluated in the context of the film structure, energetics of the involved photo(electrochemical) species and cross-membrane electron-transfer processes.     

Enhanced Photocurrent Generation in Self-Assembled Monolayers Formed at Plasmonic Gold Nanostructures

The effect of localized surface plasmon resonance (SPR) appearing at the surfaces of gold nanoparticles (AuPs) was successfully applied for the enhancement of photocurrents from porphyrin ( Po ) immobilized on AuPs. The electrode consisting of AuPs with different sizes (∼15 or ∼35 nm) was prepared using the AuP film formed at the liquid/liquid interface, and then Po was self-assembled on the gold surface. The SPR effect for the AuP film was verified from the attenuated total reflection SPR and absorption measurements. Photocurrents from the modified electrodes were compared with the corresponding planar electrode. Appreciable enhancement of photocurrents was observed.     

Biomolecular electronics: observation of oriented photocurrents by entrapped platinized chloroplasts

Electrical contact of the electron transport chain of photosynthesis has been achieved by precipitating colloidal platinum onto the surface of thylakoid membranes. This composite metal-biological membrane material was immobilized on filter paper and sandwiched between metal gauze electrodes. Upon irradiation, oriented photocurrents were observed. The direction of the flow of photocurrent was consistent with the vectorial model of photosynthesis, which predicts that the electric potential of the external surface of photosynthetic membranes swings negative with respect to the internal surface. Control experiments indicated that the oriented nature of the observed photocurrents could not be attributed to structural asymmetries associated with electrodes or to light gradients associated with the direction of illumination.     

TiO2 photoanodes prepared by cathodic electrophoretic deposition in 2-propanol: effect of the electric field and deposition time

This paper evaluates the influence of electric field and deposition time applied on cathodic electrophoretic formation of TiO₂ films in organic medium (2-propanol). The film morphology was tracked by measuring the deposited mass and film thickness. The variation in film porosity was correlated with the apparition of surface states distribution in the cyclic voltammetric characterization in the dark, due to grain boundaries defects generated in the contact of the TiO₂ particles. The open-circuit voltage decay curves showed that there is no formation of deep energy states inside the band gap of the TiO₂. The photopotential of the films increased until a critical thickness but the photocurrents showed to be dependent on operational variables, due to the fact that anodic polarization in thin films increases the electric field generated by the illumination at the ITO/TiO₂ interface, favoring the transport of the photogenerated electrons to the rear contact.     

原子力显微镜中化学成键对力的贡献

迄今非接触原子力显微镜已经成为一个非常强大的工具. 它不仅能够得到表面的原子周期结构,还能给出分子内部的化学键信息. 针尖和样品之间的相互作用是原子力显微镜的有效信号,主要包括三种,即范德瓦尔斯相互作用、静电相互作用和化学键相互作用. 本文在生长于Si(111)-7×7 的铅薄膜上测量了针尖和样品之间的化学键相互作用. 通过获取该相互作用随偏压的变化,并且利用抛物线拟合有效局域接触势的位置,我们发现它是随着针尖和样品之间距离的增大而减小的. 这种趋势来自于针尖和样品之间波函数的交叠. 从而可以得到电子的衰减长度. 我们还测量到了该衰减长度随着铅薄膜厚度的变化会发生振荡,这种振荡归因于平顶楔形铅岛内电子的量子尺寸效应.     

Analysis of Fluorescence Lifetime of Protochlorophyllide and Chlorophyllide in Isolated Etioplast Membranes Measured from Multifrequency Cross-correlation Phase Fluorometry

The fluorescence decays of protochlorophyllide (Pchlide) and of chlorophyllide (Chlide) in wheat etioplast membranes were analyzed using a multiexponential fluorescence decay model. Using different excitation wavelengths from 430 to 470 nm, we found that a triple-exponential model at 14°C and a double-exponential model at — 170°C were adequate to describe the Pchlide fluorescence decay. We discuss the origin of the three fluorescence lifetime components at 14°C on the basis of the dependence of their fractional intensities on the excitation wavelength and by correlating the fractional intensities with integrated fluorescence intensities of different Pchlide forms in steady-state fluorescence spectra. The fluorescence decay of the main Pchlide form, photoactive Pchlide-F657, is shown to have a complex character with a fast component of 0.25 ns and a slower component of about 2 ns. Two lifetime components of 2 ns and 5.5–6.0 ns are ascribed to the second photoactive form, Pchlide-F645, and to nonphotoactive Pchlide forms, respectively. In etioplast membranes preilluminated by a short saturating light pulse, we found a single 5.0 ns component for Chlide-F688 (the Chlide-NADPH: protochlorophyllide oxidoreductase [PORJ-NADP⁺complex) and an additional 1.6 ns component when the formation of Chlide-F696 (the Chlide-POR-NADPH complex) was promoted by exogenous NADPH. From the fluorescence lifetime results we evaluated the quantum yield of the primary photoreaction by Chlide-F696 as being 70%.     

Ceramide N-acyl chain length: a determinant of bidimensional transitions, condensed domain morphology, and interfacial thickness

Several lipids of biological interest are able to form monomolecular surfaces with a rich variety of thickness and lateral topography that can be precisely controlled by defined variations of the film composition. Ceramide is one of the simplest sphingolipids, consisting of a sphingosine base N-linked to a fatty acid, and is a membrane mediator for cell-signaling events. In this work, films of ceramides N-acylated with the saturated fatty acids C10, C12, C14, and C16 were studied at the air-aqueous interface. The dipole moment contribution (from surface potential measurements) and the surface topography and thickness (as revealed by Brewster angle microscopy) were measured simultaneously with the surface pressure at different molecular areas. Several surface features were observed depending on the asymmetry between the sphingosine and the N-linked acyl chains. At 21 °C, the C16:0 and C14:0 ceramides showed condensed isotherms and the film topography revealed solid film patches (17.3-15.7 ? thick) that coalesced into a homogeneous surface by further compression. On the other hand, in the more asymmetric C12:0 and C10:0 ceramides, liquid expanded states and liquid expanded-condensed transitions occurred. In the phase coexistence region, the condensed state of these compounds formed flowerlike domains (11.1-13.3 ? thick). C12:0 ceramide domains were larger and more densely branched than those of C10:0 ceramide. Both the film thickness and the surface dipole moment of the condensed state increased with ceramide N-acyl chain length. Bending of the sphingosine chain over the N-linked acyl chain in the more asymmetric ceramides can account for the variation of the surface electrostatics, topography, and thickness of the films with the acyl chain mismatch.     

Adsorption of chain molecules into a thin film structure and solvation interaction versus molecular flexibility

The influence of molecular flexibility on the properties of thin fluid films formed by linear chain molecules is studied by means of a singlet level of inhomogeneous integral equation theory. The considered m-mer chain molecules are formed through the polymerization of m hard-sphere beads with two sticky bonds randomly placed inside each bead core. Different molecular flexibility, from totally flexible up to almost completely rigid is reached by varying the interbead bonding length. The homogeneous properties of the same model that is necessary input to the singlet approach are extracted from the Wertheim’s theory of polymerization. The adsorption, local density distribution, disjoining pressure and solvation force of the chain molecule films confined by attractive and repulsive surfaces are analyzed. The obtained results indicate significant influence of the molecular flexibility on the film layering that is the origin of oscillations of solvation interaction arising between film surfaces. The oscillations of solvation pressure and force become more pronounced with restriction of molecular flexibility and with increase of bulk volume fraction of chain molecules. The decay of the oscillations across the film depends on the chain length and on the physical nature of the film surfaces, i.e. whether they are lyophilic or lyophobic. The partitioning of chain molecules from the bulk into the film strongly depends on the chain flexibility and this effect is more pronounced for the lyophilic surfaces.     

1H pulsed NMR study of bombyx mori silk fibroin: Dynamics of fibroin and of absorbed water

The dynamical behavior of the Bombyx mori silk fibroin chain and of absorbed water in silk fiber, film, and powder has been studied by ¹H pulsed nuclear magnetic resonance (NMR). Segmental motions do not occur and only the rapid rotation of the methyl groups of alanine residues is observed from ?120 to 130°C. This is independent of the conformation or form of the silk fibroin samples. Magnetization of dry silk fibroin by the solid-echo method shows a single Gaussian decay, while two components are observed in the solid-echo signals of films containing 6–10 w/w% water. An immobile component with a T₂ value of 11 μs is attributed to silk fibroin, and the mobile component to bound water. The T₂ of the latter varies from 50 to 200 μs, depending on the sample. The dynamical behavior of water trapped in the film is discussed on the basis of these T₂ values.     

On the adsorption and condensed film formation of dodecyl-, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromides at the mercury/electrolyte interface

The adsorption and condensed film formation of dodecyl (DTAB)-, tetradecyl (TTAB)-, hexadecyl (CTAB)-, and octadecyl (OTAB)-trimethylammonium bromides on the hanging mercury electrode is studied in KBr as supporting electrolyte, at various temperatures from 5 to 45 degrees C. A condensed film is formed at negative potentials and at room temperature only in the presence of CTAB. The decrease of the temperature favors the formation of the condensed film. A transition temperature is observed for the film formation. Capacity-time curves at the potentials where the film is formed show a nucleation and growth mechanism, with induction time depending not only on the final potential but also on the initial potential range, although it is in the desorption region. In this temperature range no film is observed for DTAB and TTAB. However, the film is observed for OTAB, but only at higher temperatures, and is more easily formed with increasing temperature. The film is formed in a certain potential region and the nucleation rate increases while moving toward more negative potentials. Hysteresis phenomena are observed during changes of scan direction. The capacity vs time curves for OTAB, where condensed film is formed, are treated using an Avrami plot formulation and have been explained as progressive one-dimensional nucleation with a decrease of the nucleation rate during the overall film formation. The results show a marked effect of the chain length of the alkyl chain on the film formation.     

CHLOROPHYLEPHOTOSENSITIZED ELECTRON TRANSFER BETWEEN CYTOCHROME c AND A LIPID-MODIFIED TRANSPARENT INDIUM OXIDE ELECTRODE

Both direct and chlorophyll-photosensitized electron transfer reactions between cytochrome c and a lipid-modified indium oxide electrode have been studied by cyclic voltammetry and photoelectrochemistry. The lipid films consisted of egg phosphatidylcholine (PC), either alone or in combination with the negatively charged surfactant dihexadecylphosphate (DHP-), and contained varying amounts of chlorophyll. The photocurrents were either ca-thodic, when cytochrome c was used in its oxidized form in the supporting electrolyte, or anodic when the reduced form of cytochrome c was present, with a magnitude that depended on the PC concentration and the applied voltage. Quantum efficiencies of the anodic and cathodic photocurrents reached their maxima (5% and 10/o, respectively), at an intermediate PC concentration (5 mg/mL in the membrane-forming solution), conditions under which the diffusion-controlled electrochemical response of cytochrome c was also maximal. Incorporation of DHP^- into the PC film increased both the direct and chlorophyll-photosensitized currents, presumably due to electrostatic binding of the positively charged cytochrome c to the electrode surface.     

On the adsorption and condensed film formation of binary mixtures of dodecyl-, tetradecyl-, hexadecyl-, and octadecyl-trimethylammonium bromides at the mercury–electrolyte interface

The adsorption and condensed film formation on mercury at the negative potential region for binary mixtures of dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), octadecyltrimethylammonium bromide (OTAB) is studied in KBr at various temperatures from 5 to 45 °C. The formation of the CTAB condensed film is hindered with the addition of DTAB and TTAB. There are interactions between unlike hydrophobic chains. The strong interactions between the CTAB molecules do not take place when DTAB or TTAB is present above a certain concentration. This hindering is more pronounced in the case of TTAB compared to the same DTAB concentration, i.e. the increase of the chain length hinders the film formation. The initially adsorbed molecules play a templating role in the kinetics of the film formation and in the self-assembling of the molecules. The initial induction time strongly depends on the temperature. The less surface active CTAB can hinder the OTAB film formation in binary mixtures. Also, increased interaction between OTAB and CTAB can be observed, indicating synergy effects in the film formation in some cases. The temperature range that the film is formed can be changed using mixtures of surfactants. Thus, the development of the film can become impossible, more difficult or even easier. Hysteresis phenomena are observed. The capacity versus time curves in the case that condensed film is formed are treated with the Avrami plot formulation, giving values between 1.5 and 2 indicating a progressive one dimensional nucleation with constant growth rate or a decrease of the nucleation rate during the overall film formation. There is generally a marked effect of the chain length of the alkyl chain on the film formation.