DNA-nanotube artificial ion channels

There is considerable interest in developing chemical devices that mimic the function of biological ion channels. We recently described such a device, which consisted of a single conically shaped gold nanotube embedded within a polymeric membrane. This device mimicked one of the key functions of voltage-gated ion channels: the ability to strongly rectify the ionic current flowing through it. The data obtained were interpreted using a simple electrostatic model. While the details are still being debated, it is clear that ion-current-rectification in biological ion channels is more complicated and involves physical movement of an ionically charged portion of the channel in response to a change in the transmembrane potential. We report here artificial ion channels that rectify the ion current flowing through them via this "electromechanical" mechanism. These artificial channels are also based on conical gold nanotubes, but with the critical electromechanical response provided by single-stranded DNA molecules attached to the nanotube walls.     

Rheology of PVC plastisol--VI: criteria for yielding and fracture of an immobilized layer

PVC plastisol exhibits pseudo-plastic flow in steady shear; that is, viscosity decreases with the increasing shear rate. At higher shear rates viscosity reaches a minimum and then increases, i.e., dilatant behavior. Previously, pseudo-plastic behavior was explained by a mechanism in which the suspended particles partition into an immobilized layer and a mobile phase. The development of the immobilized layer with the increase in shear rate was shown to quantitatively account for pseudo-plastic behavior. In higher shear rates dilatation of the immobilized layer was shown to be the cause of dilatacy. At even higher shear rates the immobilized layer fractures. In this paper the viscosity minimum was interpreted as the yielding of the immobilized layer. Subsequently, data in the literature were analyzed to find criteria for the yielding and fracture of the immobilized layer. Yielding was found to obey Coulomb's criterion, from which the coefficient of friction and the cohesive strength of the immobilized layer were evaluated. These properties were controlled by the nature of particle assembly in the immobilized layer and the plasticizer type had only a minor effect. The value of the coefficient of friction was on the lower side and within the range of values found in the literature for other materials. There were two modes of fracture of the immobilized layer, one with low strength, low strain to break, and the other with high strength, high strain to break. The former is analogous to the brittle fracture of solids and the latter ductile failure. The strength of brittle fracture was somewhat higher than cohesive strength, which was evaluated from yielding data. This is akin to Griffith's criterion for brittle fracture of a solid. Ductile failure occurred when the shear stress exceeded normal stress.     

Development of the continuously variable volume reactor for flow injection analysis: Part 1. Design, capabilities and testing

A new apparatus for mixing sample and reagent in flow injection analysis (FIA) is described. The continuously variable volume reactor (CVVR) replaces the conventional mixing coil in a flow injection (FI) manifold to provide mixing and dilution. A linear actuator motor allows control of the chamber volume via LabVIEW software. The chamber volume can be incremented in steps of 1 μl over the range 68-1704 μl. In addition, the chamber has an integral variable-speed stirring unit that is also under computer control. Experiments were performed to evaluate the dispersion characteristics of this new device, evaluate the volume reproducibility, and understand the mixing characteristics. Use of the chamber is shown in the determination of iron(II) in pond water, and in NIST SRM 1643d with excellent results and a detection limit of 3.7 μg/l iron(II). Advantages of the CVVR and future research activities using the device are discussed.     

Characterization of gem-difluoropropargyl synthons through HF loss from protonated molecules in methane chemical ionization mass spectra

Electron impact and methane chemical ionization mass spectra were obtained following gas chromatography/mass spectrometry for several gem-difluoropropargyl compounds, which had been synthesized as potential intermediates for synthesis of gem-difluoromethylene-containing C-3 acetylenes. EI spectra were variable with respect to the presentation of molecular ions, depending on substituent functional groups present. Methane-CI spectra were characterized by loss of 19 mass units from molecular weight with all compounds examined. These [M − 19]⁺ ions often presented as base peaks of the CI spectra, and were more reliably present and abundant than [M + 1]⁺ ions for these compounds. These ions could have been formed by elimination of HF from the protonated molecules under conditions of methane chemical ionization.     

Narrow band quantitative and multivariate electroencephalogram analysis of peri-adolescent period

ABSTRACT: BACKGROUND: The peri-adolescent period is a crucial developmental moment of transition from childhood to emergent adulthood. The present report analyses the differences in Power Spectrum (PS) of the Electroencephalogram (EEG) between late childhood (24 children between 8 and 13 years old) and young adulthood (24 young adults between 18 and 23 years old). RESULTS: The narrow band analysis of the Electroencephalogram was computed in the frequency range of 0--20 Hz. The analysis of mean and variance suggested that six frequency ranges presented a different rate of maturation at these ages, namely: low delta, delta-theta, low alpha, high alpha, low beta and high beta. For most of these bands the maturation seems to occur later in anterior sites than posterior sites. Correlational analysis showed a lower pattern of correlation between different frequencies in children than in young adults, suggesting a certain asynchrony in the maturation of different rhythms. The topographical analysis revealed similar topographies of the different rhythms in children and young adults. Principal Component Analysis (PCA) demonstrated the same internal structure for the Electroencephalogram of both age groups. Principal Component Analysis allowed to separate four subcomponents in the alpha range. All these subcomponents peaked at a lower frequency in children than in young adults. CONCLUSIONS: The present approaches complement and solve some of the incertitudes when the classical brain broad rhythm analysis is applied. Children have a higher absolute power than young adults for frequency ranges between 0-20 Hz, the correlation of Power Spectrum (PS) with age and the variance age comparison showed that there are six ranges of frequencies that can distinguish the level of EEG maturation in children and adults. The establishment of maturational order of different frequencies and its possible maturational interdependence would require a complete series including all the different ages.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Symbolic Drawings Reveal Changes in Preservice Teacher Mathematics Attitudes After a Mathematics Methods Course

A new method of analyzing mathematics attitudes through symbolic drawings, situated within the field of Jungian‐oriented analytical psychology, was applied to 52 preservice elementary teachers before and after a mathematics methods course. In this triangulation mixed methods design study, pretest images related to past mathematics experiences drawn by prospective teachers were 63.2% negative in tone, and listed associated emotions were 60.4% negative; on the posttest these changed significantly to 72.1% positive images, with 70.5% positive associated emotions. The qualitative analysis of images and preservice teacher interpretations of them indicate that mathematics anxiety decreased and motivation changed from extrinsic to intrinsic as a result of the course. Pretest images and interpretations focused primarily on grades, unhappiness, time and pressure, struggle, and lack of success. Posttest images and interpretations revealed (a) greater understanding of mathematical concepts through use of concrete materials; (b) greater engagement in mathematics through interesting activities and discourse with peers; and (c) a sense of accomplishment from teaching practicum lessons. Because the drawing exercise helped students connect with their previously unconscious images of mathematics, thereby helping to shift the mathematics anxiety complex toward a more positive affective state, it is recommended that these activities be part of mathematics methods courses.