Tuning the aggregation of polyimide thin films by modification of their molecular interactions

Thin films of an organo-soluble polyimide based on 1,4-(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA) and 2,2'-dimethyl-4,4'-methylene dianiline (DMMDA) have been studied. A prism coupler was used to measure the refractive indices. The average refractive indices of thin films prepared by annealing at different temperatures and times were chosen to characterize the condensation states of thin films. Thin films annealed at 200 degrees C show irreversible changes in physical properties, eg solubility. FTIR spectroscopy showed that the chain structures of the above thin films remained unchanged. It is proposed that specific molecular interactions induce the irreversible changes revealed by fluorescence spectroscopy. (C) 2000 Society of Chemical Industry.

A comparative study of insoluble and soluble polyimide thin films

Physical properties of thin films of soluble and insoluble aromatic polyimides were compared by d.s.c., u.v.-visible and fluorescence spectroscopy, and prism coupler technique. D.s.c. results showed that the thermal properties of insoluble polyimides are superior to those of soluble ones, owing to the specific molecular interactions of insoluble polyimides, revealed by fluorescence spectroscopy. However, the specific molecular interactions sacrifice the transparency in their thin films, shown by u.v.-visible transmission spectra. Negative birefringence of thin films, not only for soluble polimides but also for polyamic acids of insoluble polyimides, was confirmed by prism coupler, while thin films of insoluble polyimides thermally imidized lack high levels of negative birefringence. It is regarded that thin films of insoluble polyimides thermally imidized are likely to be isotropic because of molecular relaxation above their glass transition temperatures (T-g) in the course of thermal imidization, according to Flory's principle. On the contrary, thin films of insoluble poyimide chemically imidized keep the same levels of negative birefringence as those of the relevant polyamic acid thin films. Judging from the opposite cases, it is recognized that molecular relaxation above T-g weakens the optical anisotropy of polyimide thin films. (C) 1998 Elsevier Science Ltd. All rights reserved.

Characterization of the structural and functional changes of hemoglobin in dimethyl sulfoxide by spectroscopic techniques

Circular dichroism (CD), fourier transform infrared (FTIR), and fluorescence spectroscopy were used to explore the effect of dimethyl sulfoxide (DMSO) on the structure and function of hemoglobin (Hb). The native tertiary structure was disrupted completely when the concentration of DMSO reached 50% (v/v), which was determined by loss of the characteristic Soret CD spectrum. Loss of the native tertiary structure could be mainly caused by breaking the hydrogen bonds, between the heme propionate groups and nearby surface amino acid residues, and by disorganizing the hydrophobic interior of this protein. Upon exposure of Hb to 52% DMSO for ca. 12 h in a D2O medium no significant change in 1652 cm(-1) band of the FTIR spectrum was produced, which demonstrated that alpha-helical structure predominated. When the concentration of DMSO increased to 57%: (1) the band at 1652 cm(-1) disappeared with the appearance of two new bands located at 1661 and 1648 cm(-1); (2) another new band at 1623 cm(-1) was attributed to the formation of intermolecular beta-sheet or aggregation, which was the direct consequence of breaking of the polypeptide chain by the competition of S=O groups in DMSO with C=O groups in amide bonds. Further increasing the DMSO concentration to 80%, the intensity at 1623 cm(-1) increased, and the bands at 1684, 1661 and 1648 cm(-1) shifted to 1688, 1664 and 1644 cm(-1), respectively. These changes showed that the native secondary structure of Hb was last and led to further aggregation and increase of the content of 'free' amide C=O groups. In pure DMSO solvent, the major band at 1664 cm(-1) indicated that almost all of both the intermolecular beta-sheet and any residual secondary structure were completely disrupted. The red shift of the fluorescence emission maxima showed that the tryptophan residues were exposed to a greater hydrophilic environment as the DMSO content increased. GO-binding experiment suggested that the biological function of Hb was disrupted seriously even if the content of DMSO was 20%. (C) 1998 Elsevier Science B.V. All rights reserved.

天然有机质与多环芳烃相互作用的荧光光谱学研究—以苝、菲和蒽为例

自然界水体（如地下水、湖泊、河流及海洋）中的天然有机质（NOM）因具有显著的生态及环境功能而受到人们的广泛关注。NOM作为全球碳循环的主要组成部分，是水体中异养型微生物生长所需碳及能量的重要来源。此外，NOM对湖泊生态系统中的多种物理、化学过程具有不同程度的影响：NOM中的有机酸对淡水体系的酸度具有控制作用而且对酸沉降有一定的缓冲作用；NOM通过减弱可见光及紫外线照射从而影响淡水系统的光化学环境，这一方面降低了水体中自养生物的繁衍速度，同时保护水生生物免受有害的紫外线辐射。更为重要的是，NOM能够影响环境中污染物的命运，比如重金属离子和疏水性有机污染物与天然有机质结合后，其迁移途径、溶解度、生物可利用性及毒性受到明显的控制，这样其对环境中生物的危害性减弱。多环芳烃（PAHs）是一类对环境危害极大的疏水性有机污染物，多数PAHs对有机体具有毒性和致癌性而被美国环保署（EPA）列为优先处理的污染物。因此，确定NOM-PAHs反应的强弱，即PAHs在NOM中的分配系数（Kdoc）以及NOM的物化特性对Kdoc的影响对于预测PAHs毒性和生物可利用性大小、了解NOM-PAHs相互作用的机理及确立污染预测模型具有十分重大的意义。前人的研究结果表明，多环芳烃与天然有机质相互作用的分配系数Kdoc的与PAHs的疏水度（即Kow大小）、NOM的物化特性（如芳香度、脂肪碳含量、分子量及极性等）和水化学参数（pH、离子强度等）有很大的相关性。然而，许多学者在NOM的物化特性对分配系数的影响方面存有较大分歧。前人在研究天然有机质与多环芳烃之间的相互作用时所用的NOM多为腐殖质，即水体NOM中的疏水性组分，而其中的亲水性组分与PAHs的结合作用文献中却鲜见提及，因此人们对其结合作用的特性如反应机理和结合作用强弱等的了解十分有限。 本论文利用XAD树脂分离技术把红枫湖水体中天然有机质分成了疏水性酸、碱、中性物质和亲水性酸、碱、中性物质等六种有机组分。同时运用元素分析、有机碳分析仪、紫外－可见分光光度，高效液相体积排阻色谱、三维荧光光谱和荧光偏振技术等现代分析方法，对各有机组分的物理、化学特性进行了表征。荧光光谱技术被用于研究水化学参数对腐殖质荧光特性和分子构型的影响以及测定Amherst腐殖酸（Amherst HA）、垃圾渗滤液富里酸（LF FA）和红枫湖NOM有机组分与多环芳烃苝、菲和蒽的结合系数（分配系数、条件稳定常数）并探讨了影响分配系数的各种因素和NOM与多环芳烃相互作用的机理。本论文的获得的主要结果如下： 1. 在水化学参数对腐殖质荧光特性及分子构型的影响方面： 在NOM与PAHs相互作用过程中，腐殖质的分子构型起着关键性的作用，而其分子构型又受到其本身的浓度、溶液的pH和离子强度等水化学参数的影响。本论文运用三维荧光光谱和荧光偏振技术研究了水化学参数对腐殖质荧光特性的影响，并由此推断其分子构型变化情况。实验结果表明随着腐殖质浓度、溶液的pH和离子强度的改变，腐殖质荧光特性（如荧光强度、荧光峰位和荧光偏振值等）出现不同程度的变化，揭示了腐殖质分子构型的改变。因此，本实验结果有利于深入了解腐殖质的分子构型及其对吸附PAHs的重要影响，对理解NOM的环境行为具有一定的理论价值。 2. 运用荧光猝灭滴定法研究Amherst HA和红枫湖NOM有机组分对多环芳烃苝、菲和蒽的吸附作用，结果表明NOM对多环芳烃的吸附能力与其中芳香结构单元有很强的相关性，logKdoc值与NOM在280 nm处的摩尔吸收（ε280）和分子量有线性度较高的正比关系，而C/H原子比值对不饱和碳含量（芳香度）并没有很好的指示作用。同时，脂肪碳（0－50 ppm）中的聚亚甲基碳对吸附PAHs也有很大的贡献，而有机组分的极性与分配系数有明显的负相关性。不同种类的多环芳烃与NOM有机组分的结合能力有很大的差异，这取决于不同PAHs的疏水度大小。本研究结果有利于弥合前人在NOM物化特性对PAHs吸附能力的影响方面的分歧，揭示了NOM的组成和结构在与PAHs等疏水性有机污染物相互作用方面的重要影响。 3. 在结合机理方面，苝与NOM有机组分结合的Stern－Volmer图具有较高的线性度，这表明其结合机理以疏水反应（或分配）为主，而菲和蒽与有机组分相互作用的吸附等温线显示较多的非线性特征表明极性反应为主要反应机理，结合过程以吸附为主。在红枫湖NOM有机组分中，疏水性组分显示对PAHs较高的结合能力，同时疏水性组分与PAHs相互作用的吸附等温线具有较高的线性度。这一结果揭示了不同种类的PAHs和各NOM有机组分之间结合机理的差异，加深了人们对NOM与PAHs相互作用机理的认识。 4. 前人在研究pH、离子强度等水化学参数对分配系数的影响方面得到的结果多有矛盾之处，同时，对实验结果的解释也不尽相同。本实验结果表明分配系数随pH增加而下降；离子强度对分配系数的影响比较复杂，就总体趋势而言，增加离子强度有利于对PAHs吸附能力的提高，这一结果初步揭示了水化学参数对PAHs分配系数的影响，丰富了人们对其影响机理的认识。 5. 运用荧光偏振技术测定了蒽与Amherst HA、LF FA和红枫湖NOM有机组分的条件稳定常数，结果如下：随着NOM浓度增加，蒽的荧光偏振值不断下降，这表明蒽的分子构型由平面线性向圆柱状转变。在所有NOM样品中，土壤源的Amherst HA与蒽结合的条件稳定常数最大（pH 4时logK=5.6，pH 6时 logK=5.4）；就NOM有机组分而言，疏水性组分的logK变化范围在pH 6时为（4.4－5.2），而亲水性组分为（4.3－4.8），这表明疏水性组分具有相对较高的蒽吸附能力。实验结果有利于了解PAHs与NOM相互作用前后的分子构型变化情况，揭示了不同的NOM有机组分在吸附PAHs过程中的差异。

水环境中的溶解有机质及其与金属离子的相互作用——荧光光谱学研究

本论文通过选择这一国际前沿科学领域,主要运用三维荧光光谱仪、紫外-可见分光光度仪、溶解有机碳/氮分析仪、高效体积排阻色谱、荧光猝灭滴定等分析化学方法和手段,对云贵高原湖泊红枫湖、百花湖、洱海沉积物孔隙水以及一些河流等当中的DOM及其水化学特征进行了深入细致的研究,并且运用三维激发发射荧光光谱研究DOM中不同组分与Hg的相互作用,在溶解有机质的化学结构及其与金属汞的相互作用机理等方面取得了一些进展.

Direct synthesis and characterization of titanium-substituted mesoporous molecular sieve SBA-15

Ti-substituted mesoporous SBA-15 (Ti-SBA-15) materials have been synthesized by using a new approach in which the hydrolysis of the silicon precursor (tetramethoxysilane, TMOS) is accelerated by fluoride. These materials were characterized by powder X-ray diffraction patterns (XRD), X-ray fluorescence spectroscopy (Y-RF), N-2 sorption isotherms, diffuse-reflectance UV-visible (UV-vis) and UV-Raman spectroscopy, Si-29 MAS NMR, and the catalytic epoxidation reaction of styrene. Experiments show that Ti-SBA-15 samples of high quality can be obtained under the following conditions: F/Si greater than or equal to 0.03 (molar ratio), pH less than or equal to 1.0, aging temperature less than or equal to 80 degreesC, and Ti/Si less than or equal to 0.01. It was found that the hydrolysis rate of TMOS was remarkably accelerated by fluoride, which was suggested to play the main role in the formation of Ti-SBA-15 materials of high quality. There is no stoichiometric incorporation of Ti, and the Ti contents that are obtained are quite low in the case of the approach that is proposed. The calcined Ti-SBA-15 materials show highly catalytic activity in the epoxidation of styrene.

Improvement of direct methanol fuel cell performance by modifying catalyst coated membrane structure

A five-layer catalyst coated membrane (CCM) based upon Nation 115 membrane for direct methanol fuel cell (DMFC) was designed and fabricated by introducing a modified Nafion layer between the membrane and the catalyst layer. The properties of the CCM were determined by SEM, cyclic voltammetry, impedance spectroscopy, ruinous test and I-V curves. The characterizations show that the modified Nation layers provide increased interface contact area and enhanced interaction between the membrane and the catalyst layer. As a result, higher Pt utilization, lower contact resistance and superior durability of membrane electrode assembly was achieved. A 75% Pt utilization efficiency was obtained by using the novel CCM structure, whereas the conventional structure gave 60% efficiency. All these features greatly contribute to the increase in DMFC performance. The DMFC with new CCM structure presented a maximum power density of 260 MW cm(-2), but the DMFC with conventional structure gave only 200 mW cm(-2) under the same operation condition. (c) 2005 Elsevier B.V. All rights reserved.

Multiplex binding modes of toluidine blue with calf thymus DNA and conformational transition of DNA revealed by spectroscopic studies

It is noteworthy to understand the details of interactions between antitumor drugs and DNA because the binding modes and affinities affect their antitumor activities. Here, The interaction of toluidine blue (TB), a potential antitumor drug for photodynamic therapy of tumor, with calf thymus DNA (ctDNA) was explored by UV-vis, fluorescence, circular dichroism (CD) spectroscopy, UV-rnelting method and surface-enhance Raman spectroscopy (SERS). The experimental results suggest that TB could bind to ctDNA via both electrostatic interaction and partial intercalation.

Electrochemical preparation of silver nanostructure on the planar surface for application in metal-enhanced fluorescence

We introduce a fast and simple method, named the potentiostatic electrodeposition technique, to deposit metal particles on the planar surface for application in metal-enhanced fluorescence. The as-prepared metallic surfaces were comprised of silver nanostructures and displayed a relatively homogeneous morphology. Atomic force microscopy and UV-visible absorption spectroscopy were used to characterize the growth process of the silver nanostructures on the indium tin oxide (ITO) surfaces. A typical 20-fold enhancement in the intensity of a nearby fluorophore, [Ru(bpy)(3)](2+), could be achieved on the silvered surfaces. In addition, the photostability of [Ru(bpy)(3)](2+) was found to be greatly increased due to the modification of the radiative decay rate of the fluorophore. It is expected that this electrochemical approach to fabricating nanostructured metallic surfaces can be further utilized in enhanced fluorescence-based applications.

Fabrication and characterization of DNA/QPVP-Os redox-active multilayer film

Calf thymus DNA was immobilized on functionalized glassy carbon, gold and quartz substrates, respectively, by the layer-by-layer (LBL) assembly method with a polycation QPVP-Os, a quaternized poly(4-vinylpyridine) partially complexed with osmium bis(2,2'-bipyridine) as counterions. UV-visible absorption and surface plasmon resonance spectroscopy (SPR) showed that the resulting film was uniform with the average thickness 3.4 nm for one bilayer. Cyclic voltammetry (CV) showed that the total surface coverage of the polycations increases as each QPVP-Os/DNA bilayer added to the electrode surface, but the surface formal potential of Os-centered redox reaction shifts negatively, which is mainly attributed to the intercalation of redox-active complex to DNA chain. The electron transfer kinetics of electroactive QPVP-Os in the multilayer film was investigated by electrochemical impedance experiment for the first time. The permeability of Fe(CN)(6)(3-) in the solution into the multilayer film depends on the number of bilayers in the film. It is worth noting that when the multilayer film is up to 4 bilayers, the CV curves of the multilayer films display the typical characteristic of a microelectrode array.

Interaction of echinomycin with guanine: electrochemistry and spectroscopy studies

The interaction of antitumor antibiotic, echinomycin (Echi) with guanine (Gua) was thoroughly investigated by adsorptive transfer stripping cyclic voltammetry, ultraviolet and visible adsorption spectra (UV/Vis) and Fourier-transform infrared spectroscopy (FTIR). Electrochemistry provided a simple tool for verifying the occurrence of interaction between Echi and Gua. Echi could be accumulated from the solution and give well-defined electrochemical signals in 0.1 M phosphate buffer solution (pH 7.0) only when Gua was present on the surface of the electrochemically pretreated glass carbon electrode (GCE), suggesting a strong binding of Echi to Gua. All the acquired spectral data showed that a new adduct between Echi and Gua was formed, and two pairs of adjacent intermolecular hydrogen bonds between the Ala backbone atoms in Echi and Gua (Ala-NH to Gua-N3 and Gua-NH2 to Ala-CO) played a dominating role in the interaction. Electrochemistry coupled with spectroscopy techniques could provide a relatively easy way to obtain useful insights into the molecular mechanism of drug-DNA interactions, which should be important in the development of new anticancer drugs with specific base recognition.

Application of impedance spectroscopy for monitoring colloid Au-enhanced antibody immobilization and antibody-antigen reactions

We used colloidal An to enhance the amount of antibody immobilized on a gold electrode and ultimately monitored the interaction of antigen-antibody by impedance measurement. Self-assembly of 6 nm (diameter) colloidal An onto the self-assembled monolayers (SAMs) of 4-aminothiophenol modified gold electrode resulted in an easier attachment of antibody. The redox reactions of [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) on the gold surface were blocked due to the procedures of self-assembly of 4-aminothiophenol and antibody immobilization, which were investigated by cyclic voltammetry and impedance spectroscopy. The interaction of antigen with grafted antibody recognition layers was carried out by soaking the modified electrode into a phosphate buffer at pH 7.4 with various concentrations of antigen at 37 degreesC for 30 min. The antibody recognition layers and their interactions with various concentrations of antigen could be detected by measurements of the impedance change. The results show that this method has good correlation for detection of Hepatitis B virus surface antigen in the range of 0.5-200 mug/l and a detection limit of about 50 ng/l.

Effect of DNA flanking sequence on charge transport in short DNA duplexes

Several factors can influence charge transport (CT)-mediated DNA, such as sequence, distance, base stacking, base pair mismatch, conformation, tether length, etc. However, the DNA context effect or how flanking sequences influence redox active drugs in the DNA CT reaction and later in DNA enzymatic repair and synthesis is still not well understood. The set of seven DNA molecules in this study have been characterized well for the study of flanking sequence effects. These DNA duplexes are formed from self-complementary strands and contain the common central four-base sequence 5'-A-G-C-T-3', flanked on both sides by either (AT)(n) or (AA)(n) (n = 2, 3, or 4) or AA(AT)(2). UV-vis, fluorescence, UV melting, circular dichroism, and cyclic voltammetry experiments were used to study the flanking sequence effect on CT-mediated DNA by using daunomycin or adriamycin cross-linked with these seven DNA molecules. Our results showed that charge transport was related to the flanking sequence, DNA melting free energy, and ionic strength. For (AA)(n) or (AT)(n) species of the same length, (AA)(n) series were more stable and more efficient CT was observed through the (AA)(n) series. The same trend was observed for (AA)(n) and (AT)(n) series at different ionic strengths, further supporting the idea that flanking sequence can result in different base stacking and modulate charge transport through these seven DNA molecules.