DNA capture into a nanopore: interplay of diffusion and electrohydrodynamics

We present a detailed analysis of the process of voltage driven capture of DNA molecules by nanopores. We show that ionic current generates a nonuniform electric field that acts on both the DNA and on its counterions and that the response of DNA to the electric field is affected by its electroosmotic coupling to the mobile counterions. We calculate the voltage and molecular mass dependence of the radius of capture and of the capture rate in the diffusion limited regime. We argue that electroosmotic flow through the DNA coil is suppressed in the vicinity of the pore and present a tentative estimate of the capture rate in the barrier limited regime.     

Application of ion-impact energy measurement to electrospray ionization mass spectrometry of proteins and protein mixtures

We have used a normal metal-insulator-superconductor (NIS) microcalorimeter to measure the impact energy of protein ions produced by electrospray ionization (ESI) in a magnetic-sector mass spectrometer (MS). We have used these measurements to resolve spectral ambiguities and to analyze protein mixtures. Energy measurement may be useful for the direct MS analysis of complex biopolymer mixtures that normally would confound ESI-MS deconvolution algorithms.     

Fluorescence spectroscopy of silver clusters formed in rare gas droplets

The Pick Up technique allows continuous formation of high density metal cluster beams in the otherwise hardly accessible size range Me₂ to about Me₅₀. With an apparatus based on the use of cryo-condensation pumps, Ar as inert gas for production of the host clusters by adiabatic expansion and silver as metal we demonstrate how the various source parameters influence the composition and mean size of the resulting cluster distributions. The analysis in the gas phase by means fluorescence spectroscopy allows to conclude that an Ar shell encapsulates the metal clusters. Received 30 November 2000     

Inclusion of poly[bis(methoxyethoxyethoxy)phosphazene] into layered graphite oxide

The highly ionically conducting polymer poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) has been complexed with lithium triflate and intercalated between the layers of graphite oxide (GO). The resulting composite was characterized by powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and ionic conductivity measurements.     

Enhancement of weak anti-localization signatures in the magneto-resistance of bismuth anti-dot thin films

Anti-dot array thin films of bismuth were prepared by the e-beam deposition of this semi-metal on nano-porous substrates. The magneto-resistance measurements of bismuth thin films deposited under identical conditions on various substrates displayed signatures from both classical magneto-resistance and weak anti-localization effects. The relative intensity of the two effects could be altered by the choice of the substrate, with the anti-dot array morphology suppressing the classical magneto-resistance, and enhancing the weak anti-localization contribution to the measured magneto-resistance. As a result, the weak anti-localization effect can be traced to higher magnetic field strengths and higher temperatures than is possible in non-patterned films, improving the accuracy of the parameters extracted from the data. PACS 73.20.Fz; 73.23-b; 73.61.At     

The influence of CO on the surface plasmon absorption band of small silver clusters (D≤20 Å)

The surface plasmon absorption band of small Ag particles (diameter≤100 Å) in the gasphase as well as for clusters embedded in noble gas matrices is known to shift to higher energies and to broaden with decreasing diameter. This size effect depends sensitively on the microscopic surface properties of the clusters, which can be influenced by the interaction with the embedding matrix. Previous experiments with silver clusters in the size range 20 Å≤D≤100 Å proved that for SiO₂ and CO the matrix influence is strong enough to supersede the intrinsic size effect as observed in the noble gas matrices. The investigation of the influence of a CO matrix on smaller clusters with D≤20 Å reveals a novel feature: the position as well as the width of the surface plasmon absorption band become independent of the cluster size.     

Methylphenylphosphinate complexes with tri- and tetra-positive metal ions

Summary A series of methylphenylphosphinate {mpp; [-O-P-Ph(Me)-O-]^–} complexes of the forms M(mpp)₃ (M = Al, Ga, In, Sc, Y, Dy, Ti, V, Cr and Fe) and M(mpp)₄ (M = Th or U) were made from the appropriate metal chloride and a large excess of MeO(Me)PhP(O) at elevated temperatures. Structures involving linear, ligand-bridged polymeric species are suggested on the basis of the i.r. and magnetic data.     

Double negative: The necessity principle,commognitive conflict,and negative number operations

We use the DNR framework to analyze a classroom episode introducing negative integer exponents, comparing and contrasting our analysis with Sfard's recent commognitive analysis of a similar episode concerning multiplication of signed numbers. Students in both episodes objected to the standard rules for integer products or exponents, and they persisted in preferring their own rules even after the teacher justified the standard ones. We examine how pattern-based justifications may not address students’ intellectual needs, and we suggest other pedagogical strategies that promote student reasoning.