Structural studies of excimer laser-induced percolative phase transition on polyimide thin film surfaces

The behavior of electrical conductivity for excimer laser irradiated polyimide films in the vicinity of the critical number of laser shots was described by three-dimensional percolative phase transition model. It is: found that electrical conductivity changed more rapidly than that predicted by the percolation model. Thus, the change in microstructure with increasing number of laser shots was analyzed by FT-IR Raman spectrometry and laser desorption time-of-flight mass spectrometry. It is demonstrated that not only the number but also the average size of graphite particles on the irradiated polyimide film surfaces increased with increasing number of laser shots. These results were helpful to better understand the critical change in electrical conductivity on the irradiated polyimide film surfaces. (C) 2001 Elsevier Science B.V. All rights reserved.

Characteristics of the conductive polyimide film surfaces induced by ultraviolet laser beam

A conducting layer with the conductivity of 1.2 Omega(-1)cm(-1) stripped in a solvent from KrF-laser-irradiated polyimide thin film is taken as a sample to determine the microstructure of the conducting layer. Fourier-transform infrared and X-ray photoelectron spectroscopies show the formation of the carbon-rich clusters after irradiation. The element analysis gives the atomic ratio of C:H:N:O for the carbon-rich cluster as 60:20:3:1. Wide-angle X-ray diffraction indicates that the conducting layer is mainly amorphous carbon with a small amount of the short-range ordered carbon-rich clusters. This study suggests a structural model with three-layer carbon sheets linked together in a random fashion for the short-range ordered carbon-rich clusters. The interplanar spacing is 3.87 Angstrom and the layer diameter 25 Angstrom. The transport model of variable-range hopping in three dimensions is used to explain the conducting behavior of the conducting layer. In our case, the short-range ordered carbon-rich clusters are assumed to be conducting islands dispersed in the amorphous carbon-rich cluster matrix.

Supported phospholipid membranes: Comparison among different deposition methods for a phospholipid monolayer

Supported lipid membranes consisting of self-assembled alkanethiol and lipid monolayers on gold substrates could be produced by three different deposition methods: the Langmuir-Blodgett (L-B) technique, the painted method, and the paint-freeze method, By using cyclic voltammetry, chronoamperometry/chronocoulometry and a.c. impedance measurements, we demonstrated that lipid membranes prepared by these three deposition methods had obvious differences in specific capacitance, resistance and thickness. The specific capacitance of lipid membranes prepared by depositing an L-B monolayer on the alkanethiol alkylated surfaces was 0.53 mu Fcm(-2), 0.44 mu Fcm(-2) by the painted method and 0.68 mu Fcm(-2) by the paint-freeze method. The specific conductivity of lipid membranes prepared by the L-B method was over three times lower than that of the painted lipid membranes, while that of the paint-freeze method was the lowest. The difference among the three types of lipid membranes was ascribed to the influence of the organic solvent in lipid films and the changes in density of the films. The lipid membranes prepared by the usual painted method contained a trace amount of the organic solvent. The organic solvent existing in the hydrocarbon core of the membrane reduced the density of the membrane and increased the thickness of the membrane. The membrane prepared by depositing an L-B monolayer containing no solvent had higher density and the lowest fluidity, and the thickness of the membrane was smaller. The lipid membrane prepared by the paint-freeze method changed its structure sharply at the lower temperature. The organic solvent was frozen out of the membrane while the density of the membrane increased greatly. All these caused the membrane to exist in a ''tilted'' state and the thickness of this membrane was the smallest. The lipid membrane produced by the paint-freeze method was a membrane not containing organic solvent. This method was easier in manipulation and had better reproducibility than that of the usual painting method and the method of forming free-standing lipid film. The solvent-free membrane had a long lifetime and a higher mechanical stability. This model membrane would be useful in many areas of scientific research.

Orientation and electrocatalysis of riboflavin adsorbed on carbon substrate surfaces

Based on scanning tunnelling microscopy and electrochemical measurements, orientation and electrocatalytic function of riboflavin adsorbed on carbon substrates have been described for the first time. Scanning tunnelling micrographs show clearly that tip induction may result in an orientation change of the adsorbed riboflavin molecule on highly oriented pyrolytic graphite from the initially vertical orientation to the stable flat form. The adsorbed riboflavin as an effective mediator can accelerate the reduction of dioxygen which accepts two electrons from the reduced riboflavin to generate hydrogen peroxide. The rate constants of the electrocatalytic reaction in various pH solutions were determined using a rotating disc electrode modified with riboflavin. The pH effect and possible catalytic mechanism are discussed in detail.

A COMPARATIVE-STUDY ON STM IMAGING AND ELECTROCATALYTIC ACTIVITY OF DIFFERENT SURFACES MODIFIED WITH FLAVIN ADENINE-DINUCLEOTIDE

Flavin adenine dinucleotide (FAD) was modified onto the highly oriented pyrolytic graphite (hopg) and glassy carbon electrode (gee) surfaces with three methods, respectively. Corresponding image analysis for FAD-modified hopg surfaces has been performed by scanning tunnelling microscope (STM) for the first time. The molecular resolution STM image of FAD adsorbed on the freshly-cleaved hopg was obtained, the quantitative size determination suggests that the FAD molecules adsorb side lying on the substrate surface. The anodization treatment of hopg surface yields many pits, which were clearly observed under STM. These pits provide active sites on the hopg surface for modification and the treated hopg can strongly adsorb FAD molecules, the latter exhibiting an irregular cluster structure on such a surface. When FAD was electrochemically deposited on the substrate surface, a chain structure was successfully observed. The adsorbed FAD on anodized glassy carbon electrode (gee) surface can effectively catalyze the reduction of glucose oxidase, hemoglobin and myoglobin, with a large decrease in the overvoltage, whereas the deposited FAD film exhibits excellent electrocatalysis towards dioxygen reduction.

MARINE ORGANISMS ATTACHED TO SEAWEED SURFACES IN JIAOZHOU BAY, CHINA

Scanning electron microscopy of the surfaces of the seaweeds Laminaria japonica, haploid Porphyra yezoensis, Ulva pertusa and the diploid conchocelis of P. yezoensis and P. haitanensis revealed Vibrio and Micrococcus to be abundant on the surfaces of U. pertusa and P. yezoensis. Vibrio, Flavobacterium, Pseudomonas, Staphylococcus, Bacillus, Corynebacterium and other genera were isolated from the surfaces of L. japonica.

Isolation and identification of bacteria associated with the surfaces of several algal species

We conducted this study to assess the diversity of bacteria associated with the surfaces of algae based on 16S rDNA sequence analyses. Twelve strains of bacteria were obtained from the surfaces of the following four species of algae: Gracilaria textorii, Ulva pertusa, Laminaria japonica, and Polysiphonia urceolata. The isolated strains of bacteria can be divided into two groups: Halomonas and Vibrio, in physiology, biochemical characteristics and 16S rDNA sequence analyses. The phylogenetic tree constructed based on 16S rDNA sequences of the isolates shows four obvious clusters, Halomonas venusta, Vibrio tasmaniensis, Vibrio lentus, and Vibrio splendidus. Isolates from the surface of P. urceolata are more abundant and diverse, of which strains P9 and P28 have a 16S rDNA sequence very similar (97.5%-99.8%) to that of V. splendidus. On the contrary, the isolates from the surfaces of G textorii, U. pertusa and L. japonica are quite simple and distribute on different branches of the phylogenetic tree. In overall, the results of this study indicate that the genetic relationships among the isolates are quite close and display a certain level of host species specificity, and alga-associated bacteria species are algal species specific.

Characteristics of sulfide corrosion products on 316L stainless steel surfaces in the presence of sulfate-reducing bacteria

It has been found that microbial communities play a significant role in the corrosion process of steels exposed in aquatic and soil environments. Biomineralization influenced by microorganisms is believed to be responsible for the formation of corrosion products via complicated pathways of electron transfer between microbial cells and the metal. In this study, sulfide corrosion products were investigated for 316L stainless steel exposed to media with sulfate-reducing bacteria media for 7 weeks. The species of inorganic and organic sulfides in the passive film on the stainless steel were observed by epifluorescence microscope, environmental scanning electron microscope combined with energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The transformation from metal oxides to metal sulfides influenced by sulfate-reducing bacteria is emphasized in this paper. (c) 2005 Elsevier B.V. All rights reserved.

Addition reactions of nitrones on the reconstructed C(100)-2 x 1 surfaces

In this paper, the reactions of nitrone, N-methyl nitrone, N-phenyl nitrone and their hydroxylamine tautomers (vinyl-hydroxylamine, N-methyl-vinyl-hydroxylamine and N-phenyl-vinyl-hydroxylamine) on the reconstructed C(100)-2 x 1 surface have been investigated using hybrid density functional theory (B3LYP), Moller-Plesset second-order perturbation (MP2) and multi-configuration complete-active-space self-consistent-field (CASSCF) methods. The calculations showed that all the nitrones can react with the surface "dimer" via facile 1.3-dipolar cycloaddition with small activation barriers (less than 12.0 kJ/mol at B3LYP/6-31g(d) level). The [2+2] cycloaddition of hydroxylamine tautomers on the C(100) surface follows a diradical mechanism. Hydroxylamine tautomers first form diradical intermediates with the reconstructed C(I 00)-2 x I surface by overcoming a large activation barrier of 50-60 kJ/mol (B3LYP), then generate [2+2] cycloaddition products via diradical transition states with negligible activation barriers. The surface reactions result in hydroxyl or amino-terminated diamond surfaces, which offers new opportunity for further modifications. (C) 2007 Elsevier B.V. All rights reserved.