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Xuefeng Guo

Publications and source records attributed to Xuefeng Guo.

18 recordsLinked to original sources
PubMed2007

Mesoporous nanotubes of iron phosphate: synthesis, characterization, and catalytic property.

Iron phosphate nanotubes with mesoporous walls are solvothermally synthesized using sodium dodecyl sulfate (SDS) as a template. With different template concentrations, various shapes of nanosized iron phosphates can be obtained. When the concentration of SDS is set at the transition regions between the lamellar and the hexagonal mesophases, according to its phase diagram, the coassembly of iron phosphate precursor and SDS forms a flake-type mesoporous iron phosphate. Otherwise, nanoparticles or bulky sheets of iron phosphates are obtained. The followed solvothermal treatments on the mesoporous iron phosphate flakes produce iron phosphate nanotubes with mesoporous walls. The removal of the surfactant by acetate exchange and heat treatment results in the clean mesoporous nanotubes of iron phosphate with diameters of 50-400 nm and lengths of several microns. The nanotubular and mesoporous iron phosphate possesses a specific surface area of 232 m2/g and a bimodal distribution of pore sizes, corresponding to the size of mesopores in the walls and the diameter of the nanotubes, respectively. The novel nanotubular iron phosphate with composite meso-macroporous structure, in favor of the diffusion of reactive molecules, has been tested for direct hydroxylation of benzene with hydrogen peroxide and has shown better catalytic performance compared with the conventional particulate mesoporous iron phosphate.

Journal Article↗
PubMed2006

Mechanistic investigations of PEG-directed assembly of one-dimensional ZnO nanostructures.

Mechanistic investigation on spherical assembly of the unique one-dimensional ZnO nanorods, solid nanocones, or hollow prisms with the closed -c end, directed by poly ethylene glycol (PEG) with different molecular weights, has been carried out using spectroscopic methods. The single crystalline ZnO nanoprisms, hollow along the c axis but closed at the -c end, aggregate to urchin-type globules in the microscale when PEG 2000 is used as directing reagent, while spherical aggregates of single crystalline ZnO nanocones are obtained under the direction of PEG 200. Studies reveal that both the PEG molecules aggregate to globules by interacting with zinc species in suitable solvents and englobe the zinc species. By the short time of ultrasonic pretreatment on the solution, a kind of flagellum structure is induced around the globules, in long tubular shapes for PEG 2000 but as shorter wedges for PEG 200. The globules with flagellums are templates for the assembly of the ZnO nanotubes or ZnO nanocones in the hydrothermal treatment. The tiny ZnO crystallites, produced in the hydrothermal process, stack to the templates and amalgamate to single crystalline nanotubes or nanocones, similar to the oriented attachment mechanism. The PEG 2000 template is included in the ZnO cavity of nanotubes, while PEG 200 is excluded from the ZnO nanocones due to the different intertwist properties between the two PEG molecules. Both the urchin-type assemblies, possessing the same external crystalline plane, compose a isotropic powder and emit very strong yellow light, centered at approximately 2.1 eV, under the excitation of the He-Cd laser at 325 nm, which has been correlated to the specific crystal plane. The special powders will be easily coated onto any type of surface for the decoration of a large area of surfaces for future applications.

Journal Article↗
PubMed2006

Colloids seeded deposition: growth of titania nanotubes in solution.

The formation of titania nanotubes through CeO2 colloids seeded deposition process in solution has been demonstrated. The interactions among colloidal particles play important roles for the solution "VLS"-type growth of nanotubes. The observation has significance for understanding solution nucleation and for developing new methods for nanomaterials preparation via the disclosed colloids seeded deposition (CSD) mechanism.

Journal Article↗
PubMed2006

Transferring self-assembled, nanoscale cables into electrical devices.

This study details a new derivative of the contorted HBCs that self-organizes into one-dimensional, single-crystalline fibers. X-ray diffraction, transmission electron microscopy, and electron diffraction studies show that they have an orthorhombic unit cell with dimensions of 5.8 nm x 4.5 nm x 0.45 nm. Each fiber is composed of a few thousands columns. A method is put forth that utilizes elastomer stamps to manipulate and position isolated fibers in organic field effect transistors.

Journal Article↗
PubMed2006

Chemoresponsive monolayer transistors.

This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors.

Biosensing Techniques↗
PubMed2006

Microsphere organization of nanorods directed by PEG linear polymer.

We demonstrate the sphere organization of ZnO, Bi2S3, MnO2, and La(OH)3 nanorods directed by PEG linear polymer. Our study shows that zinc, bismuth, manganese, or lanthanum species added to PEG solutions, in which PEG molecules are well dissolved in a coil state, convert the polymer coils to aggregate structures, which further aggregate into micrometer-sized M(n+)-PEG globules. The concentration of metallic species is higher in the globules than in bulk solutions. The surfaces of the globules act as soft templates for the initial nucleation and thereafter the growth of the nanorods. Finally, echinus-type assemblies of single-crystalline nanorods form by the metallic species hydrolyzing or reacting with deposition agents. This approach opens the possibility of using polymers as soft templates to control the organization of nano building units into designed structures.

Journal Article↗
PubMed2006

Covalently bridging gaps in single-walled carbon nanotubes with conducting molecules.

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid-terminated electrodes separated by gaps of </=10 nanometers. These point contacts react with molecules derivatized with amines to form molecular bridges held in place by amide linkages. These chemical contacts are robust and allow a wide variety of molecules to be tested electrically. In addition to testing molecular wires, we show how to install functionality in the molecular backbone that allows the conductance of the single-molecule bridges to switch with pH.

Journal Article↗
PubMed2005

Stable, reproducible nanorecording on rotaxane thin films.

Stable, reversible conductance transitions and memory effects were observed in solid-state rotaxane-based Langmuir-Blodgett (LB) thin films. Taking advantage of the switching property, we have achieved reproducible nanometer-scale recording dots on rotaxane LB films via voltage application using a scanning tunneling microscope probe. The stable and easily controlled conductance switching makes the rotaxanes a hopeful candidate for nanorecording.

Journal Article↗
PubMed2005

Directing and sensing changes in molecular conformation on individual carbon nanotube field effect transistors.

This study explores how to populate the surface of the carbon nanotubes with functional molecules that can be toggled back-and-forth between different molecular conformations. The molecules synthesized for this study are tagged with a photoswitchable headgroup and a functional group which directs the assembly on the surface of the carbon nanotubes. Single-walled carbon nanotube field effect transistors switch between high and low conductance as the molecules are switched with light between open and closed conformations. These devices detect the photoswitching of approximately 104 molecules.

Journal Article↗
PubMed2004

7-trifluoromethylquinoline-functionalized luminescent photochromic spiropyran with the stable merocyanine species both in solution and in the solid state.

A new spiropyran (SP2) with the stable merocyanine form (MC2) both in solution and in the solid state at room temperature was designed and synthesized. The stability of MC2 is believed to be due to the electron-withdrawing effect of both the quinoline and the trifluoromethyl groups. (1)H NMR spectra indicate that the ratio of the open form vs the closed form of SP2 is dependent on the polarity of solvents. Single crystals composed of only the open form (MC2) were successfully obtained. X-ray structural analysis indicates that except trifluoromethyl and two methyl groups MC2 is completely planar with an s-trans,s-cis conformation. It should be noted that this is the first report of the X-ray crystal structure of the pure open form of spiropyran. MC2 can be slowly transformed into SP2 at -30 degrees C or lower temperature, and the process is accelerated by visible light irradiation. This special photochromic behavior can be explained by the calculated thermodynamic data. The spectral properties of SP2/MC2 in the presence of different metal ions are also studied, and the results show the potential application of SP2/MC2 in sensing metal ions.

Journal Article↗
PubMed2004

Concatenation of two molecular switches via a Fe(II)/Fe(III) couple.

[reaction: see text] Modulation of the fluorescein fluorescence in the presence of spiropyran and ferric ion by light was observed. Such fluorescence modulation was due to the low oxidation potential of complex MC.Fe(2+), which made the electron transfer from MC.Fe(2+) to Flu(+)()(*)() thermodynamically favorable. As a result, the communication between two molecular switches based on fluorescein and spiropyan, respectively, was realized via the reversible Fe(III)/Fe(II) redox couple. The communicating behavior corresponds well to the function of an INHIBIT logic gate.

Journal Article↗
PubMed2004

Novel coassembly route to Cu-SiO2 MCM-41-like mesoporous materials.

A series of mesostructured Cu-SiO2 composites have been synthesized with sodium metasilicate (Na2SiO3) and cuprammonia nitrate (Cu(NH3)4(NO3)2) respectively used as Si and Cu sources. The synthetic procedures were conducted at room temperature, and cetyltrimethylammonia bromide was used as a template. Under our experimental conditions, ordered mesoporous Cu-SiO2 composites could be obtained with a copper content up to 16.8 wt %. Average pore diameters (2.80-3.15 nm), wall thickness (1.30-2.20 nm), and specific surface area (1020-690 m2/g) are found to vary linearly with copper content (0-16.8 wt %). Results of thermal gravimetry-differential thermal analysis reveal the collapse temperature of the order structure starts at approximately 1250 K for mesoporous Cu-SiO2 with 16.8 wt % copper content. As indicated by the outcomes of inductively coupled plasma and X-ray photoelectron spectroscopy studies, copper is mainly incorporated inside the pore wall rather than embedded on the wall surface. Copper species strongly interact with silica, and calcination at high temperatures cannot cause phase separation between silica and copper oxide. Cu status in mesoporous Cu-SiO2 composites is similar to that in copper silicate in neighboring structures. Based on the results, a S+ I- I+ I- mechanism is proposed in which copper entities are surrounded by silicon species during synthesis of the mesostructured composite.

Copper↗
PubMed2004

Synthesis, biodistribution and quantitative structure-activity relationship studies of new 99mTc labeled pseudo-peptide complexes.

Twelve new peptide or pseudo-peptide chelators have been synthesized in the course of our continuing investigation of 99mTc-labelled peptides for application for renal imaging agents. All compounds were characterized on the basis of, IR, 1HNMR, 13CNMR spectroscopy, as well as MS and elemental analysis. All peptides yield stable 99mTc complexes using Sn(II) reactive coupling and exhibit renal uptake. Linear regression analysis between Logarithm of renal uptake value (RU) and the parameters obtained by the ZINDO/1 method was performed. Some equations were obtained which showed that molecular polar and charge have some relationship with their renal uptake.

Animals↗
PubMed2003

Synthesis and spectral investigations of a new dyad with spiropyran and fluorescein units: toward information processing at the single molecular level.

A new dyad 1 with two spiropyran units as the photochromic acceptors and one fluorescein unit as the fluorescent donor was synthesized and characterized. External inputs (ultraviolet light, visible light, and proton) induce the reversible changes of the structure and, concomitantly, the absorption spectrum of dyad 1 due to the presence of two spiropyran units. Only the absorption spectrum of the ME form of the spiropyran units in dyad 1 has large spectral overlap with the fluorescence spectrum of the fluorescein unit. Thus, the fluorescence intensity of dyad 1 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the ME form and the fluorescein unit. Based on the fact that dyad 1 could "read out" three external input signals (ultraviolet light, visible ligh,t and proton) and "write" a compatible specific output signal (fluorescence intensity), dyad 1 described here can be considered to perform an integrated circuit function with one OR and one AND interconnected logic gates. The present results demonstrate an efficient strategy for elaborating and transmitting information at the single molecular level.

Journal Article↗