The time evolution of the resistances and inductances of thedischarges in a pulsed gas laser through its current waveforms

In the present work, the time-dependent resistances and inductances of the electric discharges in a pulsed gas laser are revealed through the current waveforms of the circuit. This can be achieved combining step-by-step the experimental current waveforms with the current differential equations of the system. Thus, digitizing the signal, the derivative is calculated through a computer. For a certain time instant, substituting the values of the current and its derivative into the integrodifferential equations describing the performance of the circuit loops, we form relationships which connect the values of the resistance and inductance for this particular time instant. Combining relationships originating from very close adjacent time instants, the values of the resistances and inductances can be found. Scanning the entire time region of the discharges, the time dependence of the resistances and inductances of the discharges are revealed. Their behavior shows an abrupt drop for the resistances and a sharp peak for the inductances, both during the “formation phase”. After that, the above characteristic quantities fluctuate slowly around constant values. The sharp drop of the resistances was expected, bearing in mind that the number of the charges increases dramatically through the electron avalanche multiplication during the first few nanoseconds, causing the abrupt reduction of the resistances. On the other hand, the sharp peak of the inductances was unexpected. A plausible explanation for this phenomenon is that the plasma undergoes a temporary constriction which is due to the predominant attractive magnetic forces during the “formation phase” of the discharge     

Study of the lipophilicity of selenium species

In this work the lipophilicity of different selenium species occurring in environmental matrices and food, Se(IV), Se(VI), selenomethionine (Se-Met), selenocystamine (Se-CM), selenocystine (Se-Cyst), and dimethyl diselenide (CH₃)₂Se₂, was investigated in the octanol–water system, using the shaking flask method and detection with inductively coupled plasma–atomic emission spectrometry (ICP–AES), in order to assess their environmental fate and tendency to bioaccumulate. Polarography was also used for the electrochemically active Se species, Se(IV), Se-Cyst, Se-CM and (CH₃)₂Se₂, and the results were compared with those measured by ICP–AES. Furthermore, the influence of pH was studied by determining the logarithm of the distribution coefficient, log D, at three pH values, 5, 7, and 9, as was the impact of the marine environment on the lipophilicity profile of the six investigated Se species. The results were compared with those estimated approximately by use of PrologD software, based on the Ghose-Crippen log P (P: partition coefficient) calculation system, the only system which incorporates values—even though approximate—for the atom type of Se. Finally, from our experimental data an indicative value of the Se–Se fragment for log P prediction, for use in drug design, was estimated.     

Nonlinear dynamics of a spinning shaft with non-constant rotating speed

Research on spinning shafts is mostly restricted to cases of constant rotating speed without examining the dynamics during their spin-up or spin-down operation. In this article, initially the equations of motion for a spinning shaft with non-constant speed are derived, then the system is discretised, and finally a nonlinear dynamic analysis is performed using multiple scales perturbation method. The system in first-order approximation takes the form of two coupled sets of paired equations. The first pair describes the torsional and the rigid body rotation, whilst the second consists of the equations describing the two lateral bending motions. Notably, equations of the lateral bending motions of first-order approximation coincide with the system in case of constant rotating speed, and considering the amplitude modulation equations, as it is shown, there are detuning frequencies from the Campbell diagram. The nonlinear normal modes of the system have been determined analytically up to the second-order approximation. The comparison of the analytical solutions with direct numerical simulations shows good agreement up to the validity of the performed analysis. Finally, it is shown that the Campbell diagram in the case of spin-up or spin-down operation cannot describe the critical situations of the shaft. This work paves the way, for new safe operational ‘modes’ of rotating structures bypassing critical situations, and also it is essential to identify the validity of the tools for defining critical situations in rotating structures with non-constant rotating speeds, which can be applied not only in spinning shafts but in all rotating structures.     

Characterization of molecular disorder in vapor-deposited thin films of aluminum tris(quinoline-8-olate) by one-dimensional 27Al NMR under magic angle spinning

Aluminum tris (quinoline-8-olate) (Alq3) is used as an electron-transport layer in organic light-emitting diodes. The material can be obtained in a wide range of different solid phases, both crystalline and amorphous, by deposition from the vapor phase or from solution under controlled conditions. While the structure of the crystalline polymorphs of Alq3 has been investigated thoroughly by x-ray diffraction as well as solid-state NMR, very little information is currently available on the amount of structural disorder in the amorphous forms of Alq3. In the present contribution, we report the use of 27Al NMR spectroscopy in the solid state under magic angle spinning to extract such information from amorphous vapor deposits of Alq3. The NMR spectra obtained from these samples exhibit different degrees of broadening, reflecting distributions of the electric-field gradient tensor at the site of the aluminum ion. These distributions can be obtained from the NMR spectra by solving the corresponding inverse problem. From these results, the magnitude of structural disorder in terms of molecular geometry has been estimated by density-functional theory calculations. It was found that the electric-field gradient anisotropy delta follows a bimodal distribution. Its majority component is centered around delta values comparable to the meridianal alpha crystal polymorph and has a width of about 10%, corresponding to distortions of the molecular geometry of a few degrees in the orientation of the ligands. Alq3 samples obtained at higher deposition rates exhibit higher degrees of disorder. The minor component, present at about 7%, has a much smaller anisotropy, suggesting that it may be due to the facial isomer of Alq3.     

The mechanism of interaction between focused ultrasound and microbubbles in blood-brain barrier opening in mice

The activation of bubbles by an acoustic field has been shown to temporarily open the blood-brain barrier (BBB), but the trigger cause responsible for the physiological effects involved in the process of BBB opening remains unknown. Here, the trigger cause (i.e., physical mechanism) of the focused ultrasound-induced BBB opening with monodispersed microbubbles is identified. Sixty-seven mice were injected intravenously with bubbles of 1-2, 4-5, or 6-8 μm in diameter and the concentration of 10(7) numbers/ml. The right hippocampus of each mouse was then sonicated using focused ultrasound (1.5 MHz frequency, 100 cycles pulse length, 10 Hz pulse repetition frequency, 1 min duration). Peak-rarefactional pressures of 0.15, 0.30, 0.45, or 0.60 MPa were applied to identify the threshold of BBB opening and inertial cavitation (IC). Our results suggest that the BBB opens with nonlinear bubble oscillation when the bubble diameter is similar to the capillary diameter and with inertial cavitation when it is not. The bubble may thus have to be in contact with the capillary wall to induce BBB opening without IC. BBB opening was shown capable of being induced safely with nonlinear bubble oscillation at the pressure threshold and its volume was highly dependent on both the acoustic pressure and bubble diameter.     

Carbon nanotube amplification strategies for highly sensitive immunodetection of cancer biomarkers

We describe herein the combination of electrochemical immunosensors using single-wall carbon nanotube (SWNT) forest platforms with multi-label secondary antibody-nanotube bioconjugates for highly sensitive detection of a cancer biomarker in serum and tissue lysates. Greatly amplified sensitivity was attained by using bioconjugates featuring horseradish peroxidase (HRP) labels and secondary antibodies (Ab(2)) linked to carbon nanotubes (CNT) at high HRP/Ab(2) ratio. This approach provided a detection limit of 4 pg mL(-)(1) (100 amol mL(-)(1)), for prostate specific antigen (PSA) in 10 microL of undiluted calf serum, a mass detection limit of 40 fg. Accurate detection of PSA in human serum samples was demonstrated by comparison to standard ELISA assays. PSA was quantitatively measured in prostate tissue samples for which PSA could not be differentiated by the gold standard immunohistochemical staining method. These easily fabricated SWNT immunosensors show excellent promise for clinical screening of cancer biomarkers and point-of-care diagnostics.     

Photoswitchable PEG‐CA hydrogels and factors that affect their photosensitivity

The photoswitching behavior of polyethylene glycol‐based hydrogels was determined by monitoring the changes in their swellability and absorption spectra upon exposure to alternating wavelengths of irradiation. Highly hydrophilic PEG‐based hydrogels were prepared by the irradiation of a cinnamylidene acetate‐terminated PEG solution (PEG‐CA). The degree of swelling of the PEG‐CA hydrogels was predictably modulated by alternating the wavelength of exposing irradiation. Exposing the PEG‐CA gels to >300 nm irradiation resulted in the decrease of their swellability, while short exposure to 254 nm led to more hydrophilic gels. That is, the physical properties of the PEG‐CA gels can be controlled by the selection of wavelength of irradiation. Interestingly, it was found that 254 nm irradiation could not only lead to the photoscission of the PEG‐CA gels, but also initiate a crosslinking reaction between PEG‐CA monomers. On the contrary, only photocrosslinking via a cyclobutane ring formation was observed under >300‐nm irradiation. Finally, the factors that could affect an efficient photoswitching behavior of the PEG‐CA gels were investigated. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1466–1476, 2000     

Synthesis and redox behavior of flavin mononucleotide-functionalized single-walled carbon nanotubes

In this contribution, we describe the synthesis and covalent attachment of an analogue of the flavin mononucleotide (FMN) cofactor onto carboxylic functionalities of single-walled carbon nanotubes (SWNTs). The synthesis of FMN derivative (12) was possible by coupling flavin H-phosphonate (9) with an aliphatic alcohol, using a previously unreported N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride coupling. We found that the flavin moiety of 12-SWNT shows a strong pi-pi interaction with the nanotube side-walls. This leads to a collapsed FMN configuration that quenches flavin photoluminescence (PL). The treatment of 12-SWNT with sodium dodecyl sulfate (SDS) overcomes this strong nanotube/isoalloxazine interaction and restores the FMN into extended conformation that recovers its luminescence. In addition, redox cycling as well as extended sonication were proven capable to temporally restore PL as well. Cyclic voltammetry of FMN onto SWNT forests indicated profound differences for the extended and collapsed FMN configurations in relation to oxygenated nanotube functionalities that act as mediators. These findings provide a fundamental understanding for flavin-related SWNT nanostructures that could ultimately find a number of usages in nanotube-mediated biosensing devices.     

Total synthesis of psammaplysenes A and B, naturally occurring inhibitors of FOXO1a nuclear export

[reaction: see text] Two inhibitors of FOXO1a-mediated nuclear export, psammaplysenes A and B, have been synthesized by a flexible and efficient route. A common starting material, 4-iodophenol, was used to prepare both halves of these pseudosymmetric dibromotyrosine-derived metabolites.