Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube p-i-n photodiodes

Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V(SD), and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.     

On the Positive Mass Theorem for Manifolds with Corners

We study the positive mass theorem for certain non-smooth metrics following P. Miao’s work. Our approach is to smooth the metric using the Ricci flow. As well as improving some previous results on the behaviour of the ADM mass under the Ricci flow, we extend the analysis of the zero mass case to higher dimensions.     

Best Partial Covering of a Convex Domain by Congruent Circles of a Given Total Area

Generalizing results of L. Fejes Toth [3], [5], we prove the following theorem. Let R be a convex domain of area |R| and let S be a finite family of at least two congruent circles of total area t. Then for the area |F| of the part of R covered by the circles of S, the inequality |F|< tf(|R|/t) holds, where f(x) is the area of the intersection of a circle of unit area and a regular hexagon of area x concentric with the circle.     

Determination of mitoxantrone with a fiber-optic device

A major problem in therapeutic use of the cytostatic agent mitoxantrone is the occurrence of side effects. Estimation of the drug concentration in or next to the tumor can lead to a reduction of the toxicity through accurate dose adjustment. In this paper a method for the determination of mitoxantrone by a fibre-optic device is described. The advantage of the system is the possibility of remote sensing at the working site of the cytostatic agent by a method that is simple in handling and sufficient in sensitivity. The measuring principle is based on the blue colour of mitoxantrone. The function of the device was tested byin vitro assays; linear calibration curves in a concentration range between 2 and 10g of mitoxantrone per ml of sample solution were obtained.     

The role of oxoammonium cation in the SOD-mimic activity of cyclic nitroxides

Cyclic nitroxides (RNO(*)) mimic the activity of superoxide dismutase (SOD) and demonstrate antioxidant properties in numerous in vitro and in vivo models. Their broad antioxidant activity may involve the participation of their reduced and oxidized forms, that is, hydroxylamine (RNO-H) and oxoammonium cation (RNO(+)). To examine this possibility we studied the reactions of RNO(*) and RNO(+) with HO(2)(*)/O(2)(*)(-) and with several reductants by pulse radiolysis and rapid-mixing stopped-flow techniques. The oxoammonium cations were generated by electrochemical and radiolytic oxidation of 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidinoxyl (3-CP). The rate constant for the reaction of RNO(*) with HO(2)(*) to form RNO(+) was determined to be (1.2 +/- 0.1) x 10(8) for TPO and (1.3 +/- 0.1) x 10(6) M(-)(1) s(-)(1) for 3-CP. The kinetics results demonstrate that the reaction of RNO(*) with HO(2)(*) proceeds via an inner-sphere electron-transfer mechanism. The rate constant for the reaction of RNO(*) with O(2)(*)(-) is lower than 1 x 10(3) M(-)(1) s(-)(1). The rate constant for the reaction of RNO(+) with O(2)(*)(-) was determined to be (3.4 +/- 0.2) x 10(9) for TPO(+) and (5.0 +/- 0.2) x 10(9) M(-)(1) s(-)(1) for 3-CP(+). Hence, both nitroxides catalyze the dismutation of superoxide through the RNO(*)/RNO(+) redox couple, and the dependence of the catalytic rate constant, k(cat), on pH displayed a bell-shaped curve having a maximum around pH 4. The oxoammonium cation oxidized ferrocyanide and HO(2)(-) by a one-electron transfer, whereas the oxidation of methanol, formate, and NADH proceeded through a two-electron-transfer reaction. The redox potential of RNO(*)/RNO(+) couple was calculated to be 0.75 and 0.89 V for 3-CP and TPO, respectively. The elucidated mechanism provides a clearer insight into the biological antioxidant properties of cyclic nitroxides that should permit design of even more effective antioxidants.     

Synthesis of 8-amino-9-β-D-ribofuranosylpurin-6-thione and related 6-substituted-8-aminopurine nucleosides

Acetylation of 8-amino-9-β-D-ribofuranosylpurin-6-one (III), followed by chlorination of the tetraacetyl derivative 8-acetamido-9-(2,3,5-tri-O-aeetyl-β-D-ribofuranosyl)purin-6-one (IV) with phosphorus oxychloride yielded 8-aeetamido-6-ehloro-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)-purine (V). The 6-chloro substitutent of V was readily displaced with thiourea to give, after treatment with sodium methoxide 8-acetamido-9-β-D-ribofuranosylpurine-6-thione (VIII). Chlorination of 8-bromo-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)purin-6-one (IX) yielded 6,8-dichloro-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)purine (X), which underwent nucleophilic displacement with ethanolic ammonia selectively in the 8 position. The resulting 8-amino-6-chloro-9-β-D-ribofuranosylpurine (VII) was converted to 8-amino-9-β-D-ribofuranosylpurine-6-thione (I), 8-amino-6-methylthio-9-β-D-ribofuranosylpurine (II), and to 8-amino-6-hydrazino-9-β-D-ribofuranosylpurine (XI).     

Methane dissociative adsorption on the Pt(111) surface over the 300-500 K temperature and 1-10 Torr pressure ranges

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.