Non-covalent monolayer-piercing anchoring of lipophilic nucleic acids: preparation, characterization, and sensing applications

Functional interfaces of biomolecules and inorganic substrates like semiconductor materials are of utmost importance for the development of highly sensitive biosensors and microarray technology. However, there is still a lot of room for improving the techniques for immobilization of biomolecules, in particular nucleic acids and proteins. Conventional anchoring strategies rely on attaching biomacromolecules via complementary functional groups, appropriate bifunctional linker molecules, or non-covalent immobilization via electrostatic interactions. In this work, we demonstrate a facile, new, and general method for the reversible non-covalent attachment of amphiphilic DNA probes containing hydrophobic units attached to the nucleobases (lipid-DNA) onto SAM-modified gold electrodes, silicon semiconductor surfaces, and glass substrates. We show the anchoring of well-defined amounts of lipid-DNA onto the surface by insertion of their lipid tails into the hydrophobic monolayer structure. The surface coverage of DNA molecules can be conveniently controlled by modulating the initial concentration and incubation time. Further control over the DNA layer is afforded by the additional external stimulus of temperature. Heating the DNA-modified surfaces at temperatures >80 °C leads to the release of the lipid-DNA structures from the surface without harming the integrity of the hydrophobic SAMs. These supramolecular DNA layers can be further tuned by anchoring onto a mixed SAM containing hydrophobic molecules of different lengths, rather than a homogeneous SAM. Immobilization of lipid-DNA on such SAMs has revealed that the surface density of DNA probes is highly dependent on the composition of the surface layer and the structure of the lipid-DNA. The formation of the lipid-DNA sensing layers was monitored and characterized by numerous techniques including X-ray photoelectron spectroscopy, quartz crystal microbalance, ellipsometry, contact angle measurements, atomic force microscopy, and confocal fluorescence imaging. Finally, this new DNA modification strategy was applied for the sensing of target DNAs using silicon-nanowire field-effect transistor device arrays, showing a high degree of specificity toward the complementary DNA target, as well as single-base mismatch selectivity.     

Microwave spectroscopy of electron solid and stripe phases in higher Landau levels

Our recent broadband microwave and RF spectroscopic measurements of two-dimensional electron systems at Landau level filling ν1 are reviewed. Resonances in the spectrum of diagonal conductivity in many discrete regions of ν allow us to identify and study the pinned electron solids.     

Kohn-Sham perturbation theory: simple solution to variational instability of second order correlation energy functional

The orbital-dependent correlation energy functional resulting from second order Kohn-Sham perturbation theory leads to atomic correlation potentials with correct shell structure and asymptotic behavior. The absolute magnitude of the exact correlation potential, however, is greatly overestimated. In addition, this functional is variationally instable, which shows up for systems with nearly degenerate highest occupied and lowest unoccupied levels like Be. In this contribution we examine the simplest resummation of the Kohn-Sham perturbation series which has the potential to resolve both the inaccuracy and the instability problem of the second order expression. This resummation includes only the hole-hole terms of the Epstein-Nesbet series of diagrams, which has the advantage that the resulting functional is computationally as efficient as the pure second order expression. The hole-hole Epstein-Nesbet functional is tested for a number of atoms and ions. It is found to reproduce correlation and ground state energies with an accuracy comparable to that of state-of-the-art generalized gradient approximations. The correlation potential, on the other hand, is dramatically improved compared to that obtained from generalized gradient approximations. The same applies to all quantities directly related to the potential, as, for instance, Kohn-Sham eigenvalues and excitation energies. Most importantly, however, the hole-hole Epstein-Nesbet functional turned out to be variationally stable for all neutral as well as all singly and doubly ionized atoms considered so far, including the case of Be.     

Biomimetic fabrication of 3D structures by spontaneous folding of tapes

This paper describes a biomimetic strategy for the fabrication of 3D structures-including an electrically functional light detector-modeled on the folding of biological macromolecules into globular shapes. The process started by fabricating precursors to 3D, millimeter-sized structures using flexible polymer tapes. These tapes were patterned with metal features supporting liquid solder, crimped into strings of 3D corrugations, and attached to flat polymer tapes to generate linear 3D structures. Capillary interactions between droplets of molten solder on adjacent faces of the crimped tapes resulted in folding of the precursors into quasi-3D and truly 3D structures.     

Smooth models of quiver moduli

For any moduli space of stable representations of quivers, certain smooth varieties, compactifying projective space fibrations over the moduli space, are constructed. The boundary of this compactification is analyzed. Explicit formulas for the Betti numbers of the smooth models are derived. In the case of moduli of simple representations, explicit cell decompositions of the smooth models are constructed.     

Integrated and compact fiber-coupled single-photon system based on nitrogen-vacancy centers and gradient-index lenses

A fiber-coupled single-photon system is presented. Gradient-index lenses are utilized for single-photon collection and fiber coupling of a nitrogen-vacancy defect center in a nanodiamond. Integrated filter technology separates excitation and laser light. Therefore, the system is ultracompact with 120 mm(3) in dimension as no bulky free beam optics are used. The commercial availability of all components and their simple assembly allows the implementation of a low-cost single-photon system, possibly approaching single-photon count rates of 500 kcts/s.     

Aufbau und Eigenschaften „ohmscher”︁ Kontakte an GaSb

After some general considerations on the concept of ohmic contacts measurements made of the contact resistances of Ag-, Au-; Ni-, Al-; In-, Sn- and Ag–In contacts are described. These contacts were made by evaporating films on n- and p-GaSb. The stages of this process were investigated according to the crystalline state of the materials. The four-point method was applied to the measurements of the contact resistivities. In the result of the experimental studies there was noticed a markable influence of the surface treatments and structures, of the contact metals and the temperature on the contact resistances R_K and ϱ_K. Sn/Ag/Au-contacts on n-GaSb and Ag- and Au-contacts on p-GaSb became ohmic contacts after an alloying process.     

Studies of monolayer/substrate adhesion as function of the monolayer headgroup charge: DMPE and DMPA

The variation of the work of adhesion between lipid monolayers and a plane silicon oxide surface in a typical LB-configuration is measured as function of the subphase pH. The adhesion energy is deduced via fluorescence microscopy from the equilibrium meniscus height. With increasing pH the negative headgroup charge of both, dimyristoylphosphatidylethanolamine (DMPE) and dimyristoylphosphatidic acid (DMPA) monolayers increases. The increasing charge of DMPE is reflected in a measured decrease of the work of adhesion at higher pH. The DMPA/SiO₂ interaction is not affected by increasing headgroup charges. These results are qualitatively understood in terms of an electrostatic double layer interaction between charged surfaces. It predicts decreasing adhesion for increasing, but low surface charge densities (DMPE). whereas the adhesion is constant for high surface charge densities (DMPA).