Search PubMed⌕ Search

Biomedical subjects

J G Hou

Publications and source records attributed to J G Hou.

At least 37 records · Page 2Linked to original sources
PubMed2004

Dielectric properties and frequency response of self-assembled monolayers of alkanethiols.

This paper presents dielectric properties of alkanethiol self-assembled monolayers (SAMs) under an ac electric field. Using a Hg-SAM/SAM-Hg junction, we measured the ac impedance of alkanethiol SAMs using a sinusoidal perturbation of 30 mV (peak-to-peak) with frequency ranging from 1 Hz to 1 MHz at zero bias. Semicircles at higher frequencies and at middle frequencies along with Warburg lines at lower frequencies were observed in complex plane impedance plots, that is, Nyquist plots. The frequency response of SAMs was analyzed by modeling the junction using an equivalent circuit and fitting the Nyquist plots. The semicircles at higher frequencies are attributed to the effect of the SAM/SAM interfaces, and the ones at middle frequencies are attributed to the effect of alkanethiol SAMs. The comparison in the plots of the imaginary part of the impedance Z against frequency for the bare Hg electrodes (in pure ethanal) and the SAM-covered Hg electrodes (in alkanethiol solution) supports the analysis. The Warburg lines are attributed to a certain ionic impurity. The dielectric loss spectra are further analyzed. Chain-length-dependent peaks, which correspond to different relaxation mechanisms, at higher frequencies and middle frequencies were observed in the spectra of the dissipation factor (tan delta vs frequency). The peaks move to small frequency with the increase of chain length of alkanethiols. Using a correlation of peak position with the chain length, we then derived active energies of 39-99 meV for alkanethiol SAMs of C7-C18 under an ac electric field.

Journal Article↗
PubMed2004

Adsorption energies of molecular oxygen on Au clusters.

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.

Journal Article↗
PubMed2004

A first-principles study on quasi-1D alkali metal chains within zeolite channels.

We report first-principles studies on systems formed by alkali metal (Na, K, or Rb) added to zeolite ITQ-4. Geometric and electronic structures of the quasi-1D chains of intercalated alkali metal atoms at experimental loading (4 metal atoms per 32 Si) are studied. Clear differences between different kinds of alkali metal are found, with a general trend of decreased ionization and less metallic character for the lighter alkali metals. Within the zeolite channels, it is possible to form insulated and metallic alkali metal chains by doping Na or Rb. Agreeing with experiments, only Rb here is found to be a good candidate to generate inorganic electride. We also predict that a large quantity of Na can be doped into the zeolite channel, while no more than 4 Rb per 16 Si can be doped.

Journal Article↗
PubMed2004

Inorganic electrides.

Inorganic electrides are a novel kind of ionic compounds in which the anions are electrons confined in a complex array of cavities or channels and the cations are nanoscale arrays of alkali metal ions that provide charge balance. In electrides the donated electron behaves like a low-density correlated electron gas, whereby the dimensionality of the electron gas and its electronic and magnetic properties are determined by the topology of the cavities in the host matrix. Unlike traditional electrides, in which alkali cations are encapsulated within an organic cage, inorganic electrides are thermally stable. The current inorganic electrides based on alkali metal loaded zeolites can be designed as useful reduced-dimensionality materials. Inorganic electrides are powerful reducing agents, and they are able to reduce small aromatic molecules to the radical anions within the channels of the zeolite.

Journal Article↗
PubMed2004

High-resolution scanning tunneling microscopy for molecules.

Scanning tunneling microscopy (STM) can detect individual molecular configuration with its high spatial resolution ability, but some intrinsical and extrinsic factors result in the complexities of STM imaging of single molecules. By combining STM experimental work and theoretical simulation with the local density approximation based on Bardeen perturbation method, we have explored the atomic-scale configuration of the following molecular systems: C(60) molecules adsorbed on Si(111)-(7x7); alkanethiol self-assembly monolayers on Au(111); C(60) molecule imaged by STM tip adsorbed with another C(60) molecule; O(2) molecule adsorbed on Ag(110) and CO molecule adsorbed on Cu(111) imaged by CO chemically modified STM tip. Some related problems including: molecule-substrate interactions, STM imaging mechanism, chemically modified STM tip, etc., are discussed.

Journal Article↗
PubMed2003

Unveiling metal-cage hybrid states in a single endohedral metallofullerene.

The local structural and electronic properties of individual metallofullerenes are studied using scanning tunneling microscopy, scanning tunneling spectroscopy, and theoretical simulations. The energy-resolved metal-cage hybrid states of a single endohedral metallofullerene Dy@C82 isomer I have been spatially mapped, supporting a complex picture consisting of the orbital hybridization and charge transfer for the interaction between the cage and the metal atom. The relative position of the encapsulated Dy atom inside the cage and the molecular orientation on the surface have been inferred by comparing the experimental results with theoretical simulations. The combined technique provides promising applications in the fields of in situ characterization and diagnostics of metallofullerene-based nanodevices.

Journal Article↗
PubMed2003

Disorder and suppression of quantum confinement effects in Pd nanoparticles.

Size-selectable ligand-passivated crystalline and amorphous Pd nanoparticles (<4 nm) are synthesized by a novel two-phase process and verified by high-resolution transmission electron microscopy. Scanning tunneling spectroscopy preformed at 5 K on these two types of nanoparticles exhibits clear Coulomb blockade and Coulomb staircases. Size dependent multipeak spectral features in the differential conductance curve are observed for the crystalline Pd particles but not for the amorphous particles. Theoretical analysis shows that these spectral features are related to the quantized electronic states in the crystalline Pd particle. The suppression of the quantum confinement effect in the amorphous particle arises from the reduction of the degeneracy of the eigenstates and the level broadening due to the reduced lifetime of the electronic states.

Journal Article↗
PubMed2003

Inorganic electride: theoretical study on structural and electronic properties.

We report self-consistent ab initio calculations of structural and electronic properties for a kind of recently synthesized inorganic electride. The optimized geometry gives zigzag cesium chains within the sinusoidal channels of the zeolite. Among the wide energy gap of the zeolite, near the conduction bands, there are two interstitial electride bands mainly contributed by 6s electrons of Cs atoms, which have a delocalized real space distribution along the channels. For all different doping rates studied, we find that a finite density of states appears at the Fermi level, which predicts a metallic behavior of this material. Detailed electronic structure reveals all the essential properties of the electride model. The shift of Fermi level and the delocalization of the highest occupied bands cause this material to be a powerful reducing agent.

Journal Article↗
PubMed2003

Low-temperature orientationally ordered structures of two-dimensional C60.

Orientationally ordered structures of two-dimensional (2D) C(60) at low temperature have been investigated theoretically and experimentally. Using total energy optimization with a phenomenological potential, we find the ground state is a close packed hexagonal lattice in which all the molecules have the same orientation. Several local minima of the potential energy surface are found to be associated with other 1 x 1 lattices as well as 2 x 2 lattices. The energies of the orientational domain boundaries of the 1x1 lattices are also computed, and two kinds of which yield negative values. A majority of these theoretical findings are confirmed by our low-temperature scanning tunneling microscopy study of a 2D C(60) array supported on a self-assembled monolayer.

Journal Article↗
PubMed2001

Nonclassical behavior in the capacitance of a nanojunction.

The capacitance of a nanojunction formed by a scanning tunneling microscope (STM) tip and a two-dimensional gold cluster was measured through the single electron tunneling spectroscopy of a double-barrier tunnel junction. By decreasing the STM tip-cluster separation, it was observed that the capacitance first increases and then decreases at short separation. This characteristic clearly deviates from the classical behavior and provides evidence for potential quantum effects on the capacitance.

Journal Article↗
PubMed2001

Intraurethral brachytherapy for prevention of recurrent urethral stricture after internal urethrotomy or transurethral resection of scar.

BACKGROUND PURPOSE: Restricture after internal urethrotomy is the major limitation to the long-term success of the procedure. The objective of this study was to evaluate the effect of intraurethral brachytherapy after internal urethrotomy or transurethral scar resection on recurrent urethral stricture. PATIENTS AND METHODS: From January 1998 to June 1999, catheter-based intraurethral brachytherapy with 192-iridium was performed in 17 patients with recurrent urethral stricture to prevent restricture after internal urethrotomy or transurethral resection of scar. The radiation was repeated within 3 days after surgery to reach a total dosage of 1000 to 1500 cGy. RESULTS: During the follow-up (range 14-27 months; mean 20 months), two patients had dysuria, including one patient with an atonic detrusor muscle. The other patient needed self-dilation. Fifteen patients presented normal voiding. The stricture recurred 3 months later in only one patient, so the restricture rate is 7%. No significant complication was observed associated with brachytherapy during the follow-up. CONCLUSION: Intraurethral brachytherapy after internal urethrotomy or transurethral resection of scar is a safe and effective treatment for recurrent urethral strictures.

Adolescent↗
PubMed2001

Chiral separation of glycidyl selenide and glycidyl sulfide racemates on cellulose-based chiral stationary phases.

In recent years, a growing interest has been paid to glycidyl selenide and glycidyl sulfide racemic compounds for their importance in the life science field. In this study, cellulose-based chiral stationary phases are employed for the separation of glycerin selenium and glycerin sulfur racemates. Most analytes obtain satisfactory separation. In order to optimize the resolution of racemates, mixtures of n-hexane with different alcohols are used as mobile phases. The structural features of these racemic compounds affecting chiral discrimination are discussed in detail. The results in this study suggest that the chiral recognition mechanism for these racemic compounds involve two factors: (a) the substitution residue on a nonchiral atom can play a direct or indirect effect during chiral discrimination and (b) the competition between hydrogen-bonding and pi-pi interaction exists for compounds containing both the hydroxyl and aromatic group at the same time. The two interactions play an opposite role in the chiral discrimination process.

Carbamates↗
PubMed2001

[Preparation, characterization and evaluation of cellulose tris (3,5-dimethylphencarbamate) chiral stationary phase].

With the deposition of cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) on aminopropylated silica gel (mean particle size, 5 microns; pore size, 13 nm; surface area, 110 m2/g) by using two different methods (evaporation and precipitation), two chiral stationary phases (CSP1 and CSP2) characterized by elemental analysis and scanning electron micrography were obtained. They were also evaluated by using seven racemic compounds with n-hexane/ethanol(95/5, V/V) and n-hexane/2-propanol (90/10, V/V) as mobile phases. The results showed that the chiral stationary phase CSP1 obtained by the evaporation method had better efficiency and chiral resolution ability than CSP2 by the precipitation method.

Benzoin↗
PubMed1997

Basic fibroblast growth factor stimulation of glial cells protects dopamine neurons from 6-hydroxydopamine toxicity: involvement of the glutathione system.

Neurotrophic factors have been shown to support the survival and promote the recovery of injured neurons both in vivo and in vitro. Here, we investigated whether glial cell line-derived neurotrophic factor (GDNF) and basic fibroblast growth factor (bFGF) could modify the damage to dopamine (DA) neurons in mesencephalic cultures caused by the neurotoxin 6-hydroxydopamine (6-OHDA). The data show that bFGF, but not GDNF, effectively protected DA neurons from 6-OHDA toxicity. Because bFGF is a glial mitogen, whereas GDNF is not, we tested whether glial cells participated in bFGF neuroprotection. Inhibition of glial cell proliferation completely prevented the protective effect of bFGF. Because oxidative events have been associated with 6-OHDA-induced damage, we examined the levels of glutathione (GSH) in control and bFGF-treated cultures. Cultures treated with bFGF had higher levels of GSH, which increased even further in response to 6-OHDA exposure. Control cultures failed to up-regulate GSH levels after 6-OHDA, suggesting a relationship between increased GSH levels and protection from 6-OHDA. Inhibition of glial cell proliferation prevented the rise in GSH in bFGF-treated cultures and abolished the increase after 6-OHDA treatment. Protection from 6-OHDA by bFGF was also diminished when GSH levels were decreased by the GSH synthesis inhibitor L-buthionine sulfoximine. Our study shows that stimulation of glial cells by bFGF allows the up-regulation of antioxidant defenses and supports cell survival during oxidative stress.

Animals↗
PubMed1996

Secretion of GDNF by glial cells does not account for the neurotrophic effect of bFGF on dopamine neurons in vitro.

The neurotrophic effect of bFGF on dopamine (DA) neurons in vitro depends upon its mitogenic activity on glial cells. We examined whether glial secretion of the dopaminergic growth factor GDNF is responsible for the trophic activity of bFGF on mesencephalic DA neurons. When added with lower concentrations of bFGF, GDNF had an additive trophic effect, which disappeared at optimal bFGF concentrations. However, addition of GDNF-neutralizing antibodies could not modify the bFGF-induced survival and growth of DA neurons. These results indicate that although GDNF could be secreted by bFGF stimulated glia, its secretion alone does not account for the trophic effect of bFGF on mesencephalic DA neurons.

Animals↗
PubMed1996

Glial cell line-derived neurotrophic factor exerts neurotrophic effects on dopaminergic neurons in vitro and promotes their survival and regrowth after damage by 1-methyl-4-phenylpyridinium.

The effect of glial cell line-derived neurotrophic factor (GDNF) on the growth of mesencephalic dopaminergic neurons and on their survival following exposure to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) was examined in vitro. In cultures developing under normal conditions, GDNF at 1 ng/ml optimally improved the survival and stimulated the growth of dopaminergic neurons without affecting glial growth. In cultures treated with MPP+, GDNF could not prevent toxicity to dopaminergic neurons. The uptake of [3H]dopamine and the number of tyrosine hydroxylase-positive neurons were similarly reduced by MPP+ in the presence or absence of GDNF. However, after removal of MPP+, GDNF protected dopaminergic neurons from the continuous cell death and stimulated the regrowth of dopaminergic fibers damaged by MPP+. We conclude that GDNF supports the growth of normally developing dopaminergic neurons and stimulates their survival and recovery after damage. These findings suggest that GDNF could be useful in the development of therapeutic approaches to Parkinson's disease, which is characterized by dopaminergic cell loss.

1-Methyl-4-phenylpyridinium↗