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Biomedical subjects

Sheng Dai

Publications and source records attributed to Sheng Dai.

At least 19 recordsLinked to original sources
PubMed2007

Rheological properties of a telechelic associative polymer in the presence of alpha- and methylated beta-cyclodextrins.

The viscosity of hydrophobic ethoxylated urethane (HEUR) solution decreased in the presence of alpha-CD or m-beta-CD; however their interactions were quite different. When the alpha-CD/hydrophobe molar ratio exceeded 5.0, the viscosity was close to that of a PEO solution of similar molecular weight. Oscillatory shear indicated that the mechanically active chains in HEUR solution decreased with the addition of alpha-CD. This agreed with the hypothesis that alpha-CD formed an inclusion complex with the hydrophobic moiety of the HEUR polymer, thereby destroying the transient hydrophobic associative network. The viscosity/temperature relationship of the alpha-CD/HEUR system (for HEUR with 70% of the PEO chains capped at both ends) did not obey the Arrhenius relationship for alpha-CD/hydrophobe molar ratio in the range 0.8-5.0. The low shear viscosity increased with increasing temperature at molar ratio of 1.0, and this was attributed to the competitive complexation of the alpha-CD/hydrophobe and the alpha-CD/PEO chain. Increasing temperature favored alpha-CD/PEO complexation. Comparison between the behavior of alpha-CD/HEUR and m-beta-CD/HEUR resulting from the different binding characteristics was discussed.

Hydrophobic and Hydrophilic Interactions↗
PubMed2006

How do aryl groups attach to a graphene sheet?

How aryl groups attach to a graphene sheet is an experimentally unanswered question. Using first principles density functional theory methods, we shed light on this problem. For the basal plane, isolated phenyl groups are predicted to be weakly bonded to the graphene sheet, even though a new single C-C bond is formed between the phenyl group and the basal plane by converting a sp2-carbon in the graphene sheet to sp3. However, the interaction can be strengthened significantly with two phenyl groups attached to the para positions of the same six-membered ring to form a pair on the basal plane. The strongest bonding is found at the graphene edges. A 1,2-addition pair is predicted to be most stable for the armchair edge, whereas the zigzag edge possesses a unique localized state near the Fermi level that shows a high affinity for the phenyl group.

Letter↗
PubMed2006

Metastable tetragonal phase CdWO4 nanoparticles synthesized with a solvothermal method.

CdWO4 has only previously been reported in the monoclinic, or wolframite, phase. Here we report the first metastable, tetragonal or scheelite, CdWO4 nanopowder. The tetragonal CdWO4 was synthesized by a propylene glycol solvothermal method. The scheelite phase is stabilized by a combination of high surface area and surface complexation by the propylene glycol. The CdWO4 is stable at 1 bar to 300 degrees C, and converts back to the monoclinic wolframite phase between 300 and 500 degrees C. The nanopowder exhibits cubic morphology and the average particle size of the nanopowder is around 50 nm.

Journal Article↗
PubMed2006

Effect of cosolvents on the binding interaction between poly(ethylene oxide) and sodium dodecyl sulfate.

The micellization of sodium dodecyl sulfate (SDS) in different glycol-water solvent mixtures was studied using the isothermal titration calorimetric (ITC) technique. At the same time, microcalorimetric titrations were also carried out to monitor the binding interaction of SDS and poly(ethylene oxide) (PEO) in the presence of different cosolvents. The demicellization of SDS in mixtures of water and cosolvents is different from that in water due to the reduction in solvent polarity and charge interaction of surfactants. The critical micelle concentration (cmc) first decreases with the addition of a small amount of cosolvents and then increases at higher cosolvent concentrations. The thermodynamics of surfactant micellization can be analyzed using the solubility parameters of solvent mixtures. For the binding interaction between SDS and PEO in different solvent mixtures, the dehydration process at low SDS concentrations is replaced by the chain solubilization process with decreasing solvent polarity. With further reduction in the solvent polarity, the binding interaction between SDS and PEO becomes weak and no aggregates can be formed beyond a certain glycol concentration. The binding interaction between SDS and PEO in different solvent mixtures was analyzed and ascribed to the effects of PEO solubility and hydrophobicity of SDS.

Colorimetry↗
PubMed2006

Electrosorption capacitance of nanostructured carbon-based materials.

The fundamental mechanism of electrosorption of ions developing a double layer inside nanopores was studied via a combination of experimental and theoretical studies. A novel graphitized-carbon monolithic material has proven to be a good electrical double-layer capacitor that can be applied in the separation of ions from aqueous solutions. An extended electrical double-layer model indicated that the pore size distribution plays a key role in determining the double-layer capacitance in an electrosorption process. Because of the occurrence of double-layer overlapping in narrow pores, mesopores and micropores make significantly different contributions to the double-layer capacitance. Mesopores show good electrochemical accessibility. Micropores present a slow mass transfer of ions and a considerable loss of double-layer capacitance, associated with a shallow potential distribution inside pores. The formation of the diffuse layer inside the micropores determines the magnitude of the double-layer capacitance at low electrolyte concentrations and at conditions close to the point of zero charge of the material. The effect of the double-layer overlapping on the electrosorption capacitance can be reduced by increasing the pore size, electrolyte concentration, and applied potential. The results are relevant to water deionization.

Adsorption↗
PubMed2006

Olefin adsorption on silica-supported silver salts--a DFT study.

Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin pi states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO3- were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.

Journal Article↗
PubMed2006

Preparation of highly active silica-supported Au catalysts for CO oxidation by a solution-based technique.

Although Au catalysts can be readily prepared on titania via the deposition-precipitation (DP) method, the direct application of the method similar to the preparation of silica-supported Au catalysts only results in diminished success. This paper reports a novel, efficient method to synthesize highly active Au catalysts supported on mesoporous silica (SBA-15) through a gold cationic complex precursor [Au(en)2]3+ (en = ethylenediamine) via a wet chemical process. The gold cationic precursor was immobilized on negatively charged surfaces of silica by a unique DP method that makes use of the deprotonation reaction of ethylenediamine ligands. The resulting mesoporous catalyst has been demonstrated to be highly active for CO oxidation at room temperature and even below 273 K, the activity of which is much superior to that of silica-supported Au catalysts previously prepared by various solution techniques. The pH value of the gold precursor solution plays a key role in determining the catalytic activity through the regulation of [Au(en)2]3+ deprotonation reaction and the surface interaction of silica with the gold precursor. This mesoporous gold silica catalyst has also been shown to be highly resistant to sintering because of the stabilization of Au nanoparticles inside mesopores.

Journal Article↗
PubMed2006

Structure and bonding between an aryl group and metal surfaces.

Modifying solid surfaces with aryl groups has many potential applications. Using first principles density functional theory methods, we investigated the trend of the structure and bonding of the phenyl group (C6H5, the simplest aryl group) on selected transition metals across the periodic table. We found that the bond between C6H5 and metal surfaces is chemical in nature. Decreasing bond strength is found from left to right, concurrent with a switching of the preferred orientation for C6H5 from the flat-lying configuration to the upright configuration. This switching is attributed to the increasing of d-electrons; that is, early transition metals, lacking d-electrons, favor the carbon-metal pi-bond and therefore the flat-lying configuration, while late transition metals rich in d-electrons prefer the carbon-metal sigma-bond and thus the upright fashion. C6H5 is also found to undergo beta-dehydrogenation on early transition metals. This work invites further theoretical and experimental research on the aryl-solid interface.

Journal Article↗
PubMed2006

Salt effects on aggregation of O-carboxymethylchitosan in aqueous solution.

The effects of salt with different valences (NaCl, CaCl2 and CrCl3) on the aggregation of O-carboxylmethylchitosan (OCMCS) in dilute aqueous solution were investigated using viscometry, dynamic laser light scattering (DLS) and atomic force microscopy (AFM). With increasing OCMCS concentration beyond a critical aggregation concentration (cac) of approximately 0.045 g/l, the aggregation of OCMCS appears in solution. The driving forces of the OCMCS aggregation are intermolecular hydrogen bond, hydrophobic interaction and electrostatic repulsion. The OCMCS aggregation behavior strongly depends on the valence of salt. When NaCl is added, the aggregate size increases with NaCl concentration. When CaCl2 or CrCl3 is added to a given OCMCS concentration, there exists a critical concentration each of Ca2+ and Cr3+. Before the critical concentration, the aggregates decrease in size with increasing salt concentration due to the intra-aggregate complexation; while after the critical concentration, the size of the aggregates increases with salt concentration due to the inter-aggregate complexation. Moreover, the effect of Cr3+ on the OCMCS aggregation is greater than that of Ca2+. The formation of the intra-aggregate complexation is found to be a kinetic process and the aggregate size decreases with time; the formation of the inter-aggregate complexation is also kinetic where the aggregate size increases with time. The aggregates dominated by the intra-aggregate complexation are small, compact and spherical, while the aggregates dominated by the inter-aggregate complexation show the big, compact and spherical morphology.

Calcium Chloride↗
PubMed2005

Surface functionalization of ordered mesoporous carbons--a comparative study.

Hexagonally structured mesoporous carbons C15 and CMK-5 and cubically structured carbon C48 were synthesized using ordered silica SBA-15 and MCM-48 as templates and carbon precursors of different structures. The surfaces of these ordered carbons were chemically functionalized by employing an approach, in which the selected diazonium compounds were in situ generated and reacted with the carbon frameworks of the mesoporous carbons. The aromatic organic molecules containing chlorine, ester, and alkyl groups were covalently attached to the surface of these ordered mesoporous carbons. The presence of functional groups on the modified carbons was confirmed with Fourier transform infrared spectroscopy, thermogravimetric analysis, and nitrogen adsorption. The BET-specific surface area and the pore width of ordered carbons were significantly reduced, whereas the primary structure of these ordered carbons and their unit cells were intact. Basically, the density of grafted functional groups is related to the specific surface area of the sample, particularly the surface area of mesopores. The surface functionalization reaction takes place only on the external surface of carbon C15, while it occurs on both of the internal and external surface of CMK-5 carbon with the nanopipe structure. The presence of the micropores in CMK-5 carbon should be responsible for its lower grafting density because the small micropores are inaccessible in the reaction. It was also proposed that the preferred adsorption/reaction in C48 may be related to the observed unsymmetrical degradation of the XRD patterns for the functionalized C48 samples. The chemical modification process considerably reduced the primary mesopores in these ordered carbons by approximately 1-1.5 nm, affording carbons with micropores in the cases of C15 and C48, and mixed micropores and small mesopores in the case of CMK-5. A grafting density of approximately 0.9-1.5 micromol/m(2) was achieved under current research.

Adsorption↗
PubMed2005

Effect of supporting surface layers on catalytic activities of gold nanoparticles in CO oxidation.

The surfaces of fumed silica materials were modified with a surface sol-gel process for catalysis applications. This surface-modification approach allows not only a monolayer growth of TiO(2) or Al(2)O(3) but also a stepwise double-layer growth of TiO(2)/TiO(2), Al(2)O(3)/Al(2)O(3), TiO(2)/Al(2)O(3), or Al(2)O(3)/TiO(2) on the surfaces of the silica materials with a monolayer precision. XRD analyses revealed that the coated monolayers and double layers of TiO(2) and Al(2)O(3) were amorphous. Gold nanoparticles were successfully deposited on the above six surface-modified silica materials via a deposition-precipitation method. The catalytic activities of these six gold catalysts for CO oxidation are highly dependent on the structures of their surface monolayers or double layers. The gold catalyst supported on the silica material functionalized with a TiO(2) monolayer (Au/TiO(2)) is the most active in both as-synthesized and oxidized forms, while the gold catalyst supported on the silica material functionalized with an Al(2)O(3)/TiO(2) double layer (Au/Al(2)O(3)/TiO(2)/SiO(2)) is the most active in the reduced form among the six catalysts. Surprisingly, the gold catalyst supported on the silica material functionalized with a TiO(2)/Al(2)O(3) double layer (Au/TiO(2)/Al(2)O(3)/SiO(2)) has much less activity than Au/Al(2)O(3)/TiO(2)/SiO(2) under all various treatments, underscoring the sensitivity of the catalytic activity to the structure of the supporting surfaces.

Journal Article↗
PubMed2005

Ultrastable Au nanocatalyst supported on surface-modified TiO2 nanocrystals.

The surfaces of TiO2 nanocrystals were modified with amorphous aluminum-oxide layers using a surface sol-gel process to control the interaction between supports and metal particles. Ultrastable Au nanocatalysts were prepared by the deposition of Au nanoparticles on the surface-modified TiO2 nanocrystals using a deposition-precipitation (DP) method. The TEM analysis showed that the Au nanoparticles on the surface-modified nanocrystal supports were highly stable with a sinter-resistant capability during high-temperature calcination. The HRTEM analysis revealed that the surface of the TiO2 nanocrystals was covered by an amorphous aluminum-oxide layer and the Au nanoparticles were primarily anchored to this amorphous layer. This amorphous aluminum-oxide layer played an extremely important role in the stabilization of the supported Au nanoparticles without affecting catalytic activities. The surface modification of nanocrystal supports highlights new opportunities in tailoring the stability and activity of supported nanocatalyst systems.

Journal Article↗
PubMed2005

Microstructure of un-neutralized hydrophobically modified alkali-soluble emulsion latex in different surfactant solutions.

At low pH conditions and in the presence of anionic, cationic, and nonionic surfactants, hydrophobically modified alkali-soluble emulsions (HASE) exhibit pronounced interaction that results in the solubilization of the latex. The interaction between HASE latex and surfactant was studied using various techniques, such as light transmittance, isothermal titration calorimetry, laser light scattering, and electrophoresis. For anionic surfactant, noncooperative hydrophobic binding dominates the interaction at concentrations lower than the critical aggregation concentration (CAC) (C < CAC). However, cooperative hydrophobic binding controls the formation of mixed micelles at high surfactant concentrations (C > or = CAC), where the cloudy solution becomes clear. For cross-linked HASE latex, anionic surfactant binds only noncooperatively to the latex and causes it to swell. For cationic surfactant, electrostatic interaction occurs at very low surfactant concentrations, resulting in phase separation. With further increase in surfactant concentration, noncooperative hydrophobic and cooperative hydrophobic interactions dominate the binding at low and high surfactant concentrations, respectively. For anionic and cationic surfactant systems, the CAC is lower than the critical micelle concentration (CMC) of surfactants in water. In addition, counterion condensation plays an important role during the binding interaction between HASE latex and ionic surfactants. In the case of nonionic surfactants, free surfactant micelles are formed in solution due to their relatively low CMC values, and HASE latexes are directly solubilized into the micellar core of nonionic surfactants.

Journal Article↗
PubMed2005

Laser light scattering and isothermal titration calorimetric studies of poly(ethylene oxide) aqueous solution in presence of sodium dodecyl sulfate.

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration.

Lasers↗