Precise placement of gold nanorods by capillary assembly

Capillary assembly was explored for the precise placement of 25 nm × 70 nm colloidal gold nanorods on prestructured poly(dimethylsiloxane) template surfaces. The concentration of nanorods and cationic surfactant cetyltrimethylammonium bromide (CTAB), the template wettability, and most critically the convective transport of the dispersed nanorods were tuned to study their effect on the resulting assembly yield. It is shown that gold nanorods can be placed into arrayed 120-nm diameter holes, achieving assembly yields as high as 95% when the local concentration of nanorods at the receding contact line is sufficiently high. Regular arrays of gold nanorods have several benefits over randomly deposited nanorod arrangements. Each assembled nanorod resides at a precisely defined location and can easily be found for subsequent characterization or direct utilization in a device. The former is illustrated by collecting scattering spectra from single nanorods and nanorod dimers, followed by subsequent SEM characterization without the need for intricate registration schemes.     

Nanocontact electrification: patterned surface charges affecting adhesion, transfer, and printing

Contact electrification creates an invisible mark, overlooked and often undetected by conventional surface spectroscopic measurements. It impacts our daily lives macroscopically during electrostatic discharge and is equally relevant on the nanoscale in areas such as soft lithography, transfer, and printing. This report describes a new conceptual approach to studying and utilizing contact electrification beyond prior surface force apparatus and point-contact implementations. Instead of a single point contact, our process studies nanocontact electrification that occurs between multiple nanocontacts of different sizes and shapes that can be formed using flexible materials, in particular, surface-functionalized poly(dimethylsiloxane) (PDMS) stamps and other common dielectrics (PMMA, SU-8, PS, PAA, and SiO(2)). Upon the formation of conformal contacts and forced delamination, contacted regions become charged, which is directly observed using Kelvin probe force microscopy revealing images of charge with sub-100-nm lateral resolution. The experiments reveal chemically driven interfacial proton exchange as the dominant charging mechanism for the materials that have been investigated so far. The recorded levels of uncompensated charges approach the theoretical limit that is set by the dielectric breakdown strength of the air gap that forms as the surfaces are delaminated. The macroscopic presence of the charges is recorded using force-distance curve measurements involving a balance and a micromanipulator to control the distance between the delaminated objects. Coulomb attraction between the delaminated surfaces reaches 150 N/m(2). At such a magnitude, the force finds many applications. We demonstrate the utility of printed charges in the fields of (i) nanoxerography and (ii) nanotransfer printing whereby the smallest objects are ～10 nm in diameter and the largest objects are in the millimeter to centimeter range. The printed charges are also shown to affect the electronic properties of contacted surfaces. For example, in the case of a silicon-on-insulator field effect transistors are in contact with PDMS and subsequent delamination leads to threshold voltage shifts that exceed 500 mV.     

Synthesis and biological evaluation of prodrugs based on the natural antibiotic duocarmycin for use in ADEPT and PMT

Chemotherapy of malign tumors is usually associated with serious side effects as common anticancer drugs lack selectivity. An approach to deal with this problem is the antibody-directed enzyme prodrug therapy (ADEPT) and the prodrug monotherapy (PMT). Herein, the synthesis and biological evaluation of new glycosidic prodrugs suitable for both concepts are described. All prodrugs but one are stable in human serum and show QIC(50) values (IC(50) of prodrug/IC(50) of prodrug in the presence of the appropriate glycohydrolase) of up to 6500. This is the best value found so far for compounds interacting with DNA.     

Laccase-catalyzed phenol oxidation. Rapid assignment of ring-proton deficient polycyclic benzofuran regioisomers by experimental 1H-13C long-range coupling constants and DFT-predicted product formation

Laccase-catalyzed oxidation of substituted catechols followed by reaction with 4-hydroxy-pyrone/-benzopyrone afforded substituted benzofuran regioisomers whose structures with only two aromatic protons in total prevent a rapid structural assignment. Based on the evaluation of (1)H-(13)C long-range coupling constants a rule of thumb could be deduced for an easy and unambiguous differentiation between the possible regioisomers formed. DFT frontier orbital calculations of the reactants offer an interesting tool to explain the regioselectivity of the key reaction.     

Phase transitions of two-dimensional dipolar fluids in external fields

In this work, we study condensation phase transitions of two-dimensional Stockmayer fluids under additional external fields using Monte-Carlo (MC) simulations in the grand-canonical ensemble. We employ two recently developed methods to determine phase transitions in fluids, namely Wang-Landau (WL) MC simulations and successive-umbrella (SU) sampling. Considering first systems in zero field (and dipolar coupling strengths μ(2)∕εσ(3) ≤ 6), we demonstrate that the two techniques yield essentially consistent results but display pronounced differences in terms of efficiency. Indeed, comparing the computation times for these systems on a qualitative level, the SU sampling turns out to be significantly faster. In the presence of homogeneous external fields, however, the SU method becomes plagued by pronounced sampling difficulties, yielding the calculation of coexistence lines essentially impossible. Employing the WL scheme, on the other hand, we find phase coexistence even for strongly field-aligned systems. The corresponding critical temperatures are significantly shifted relative to the zero-field case.     

Catalytic deuteration of silanes mediated by N-heterocyclic carbene-Ir(III) complexes

The catalytic activity of a series of coordinatively unsaturated NHC-M(III) (M = Rh, Ir; NHC = N-heterocyclic carbene) complexes was tested in the deuteration of secondary and tertiary silanes. Among these, [IrCl(I(t)Bu')(2)] provides the highest conversions to the deuterated species. Mechanistic studies highlight the reversible nature of the ortho-metalation reaction.     

Self-assembly in the electrical double layer of ionic liquids

We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation-anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.     

Integral Menger curvature for surfaces

We develop the concept of integral Menger curvature for a large class of nonsmooth surfaces. We prove uniform Ahlfors regularity and a C1,λ-a priori bound for surfaces for which this functional is finite. In fact, it turns out that there is an explicit length scale R>0 which depends only on an upper bound E for the integral Menger curvature Mp(Σ) and the integrability exponent p, and not on the surface Σ itself; below that scale, each surface with energy smaller than E looks like a nearly flat disc with the amount of bending controlled by the (local) Mp-energy. Moreover, integral Menger curvature can be defined a priori for surfaces with self-intersections or branch points; we prove that a posteriori all such singularities are excluded for surfaces with finite integral Menger curvature. By means of slicing and iterative arguments we bootstrap the Hölder exponent λ up to the optimal one, λ=1−(8/p), thus establishing a new geometric ‘Morrey–Sobolev’ imbedding theorem.As two of the various possible variational applications we prove the existence of surfaces in given isotopy classes minimizing integral Menger curvature with a uniform bound on area, and of area minimizing surfaces subjected to a uniform bound on integral Menger curvature.