Porous graphene as an atmospheric nanofilter

The fabrication of nanoscale membranes exhibiting high selectivity is an emerging field of research. The possibility to use bottom-up approaches to fabricate a filter with porous graphene and analyze its functionality with first principle calculations is investigated. Here, the porous network is produced by self-assembly of the hexaiodo-substituted macrocycle cyclohexa-m-phenylene (CHP). The resulting porous network exhibits an extremely high selectivity in favor of H(2) and He among other atmospheric gases, such as Ne, O(2), N(2), CO, CO(2), NH(3), and Ar. The presented membrane is superior to traditional filters using polymers or silica and could have great potential for further technological applications such as gas sensors or fuel cells.

Virus-like particles templated by DNA micelles: a general method for loading virus nanocarriers

DNA amphiphile particles template formation of virus capsids and enable their loading.

2D analogues of the inverted hexagonal phase self-assembled from 4,6-dialkoxylated isophthalic acids at solid-liquid interfaces

Self-assembly of organic molecules at solid-liquid interfaces is a route for developing novel functional materials on surfaces and modeling assembly phenomena in 3D. 5-Alkoxylated isophthalic acids (ISA) are known to self-assemble into two-dimensional (2D) lamellae at the interface between a surface of Au(111) or HOPG (highly oriented pyrolytic graphite) and a solvent. Presently, the self-assembly of 4,6-dialkoxylated isophthalic acid derivatives with variable alkyl chain length is investigated at Au(111)-water, Au(111)-tetradecane and HOPG-tetradecane interfaces with a particular focus on the first one. The main aspect of this study is to evaluate the role of the molecular geometry and different interactions in the 2D assembly of amphiphilic molecules. In contrast to 5-alkoxylated ISA, 4,6-dialkoxylated ISA derivatives self-assemble preferentially into arrays of cyclic pentameric/hexameric structures, which appear as 2D analogues of the inverted hexagonal phase of lipids. As a general trend, the derivatives bearing shorter alkyl chains show a higher level of ordering at Au(111)-liquid interfaces. In particular, at the Au(111)-water interface, the 4,6-diheptyloxy ISA derivative forms exclusively pentamers, which are arranged in a quasi-hexagonal lattice. Moreover, the cyclic pentameric features are not empty but host a single isophthalic acid residue which is found to be dynamic. Finally, the packing of the diheptyloxy derivative shows a distinct potential dependence: while at more negative potentials the pentameric arrangement is converted into lamellae, at more positive potentials a loosely packed zig-zag pattern is formed. The present results show that at different solid-liquid interfaces 4,6-dialkoxylated ISA derivatives tend to form cyclic structures that are 2D analogues of an inverted hexagonal phase, akin to lipids having two hydrophobic alkyl chains and a small polar head group. Moreover, the substrate potential at the Au(111)-water interface can tune the 2D molecular arrangement.

Quantification of the singlet-singlet annihilation times of individual bichromophoric molecules by photon coincidence measurements

Singlet-singlet annihilation (SSA) times in individual bichromophoric molecules have been quantified by time-resolved photon coincidence measurements. An analytical expression has been derived to obtain the SSA times from the coincidence histograms. The results have been confirmed by Monte Carlo simulations. SSA was found to be about three times faster than the fluorescence lifetime of the chromophores. Considering the spectral overlap for SSA and for energy transfer from an excited to a ground state chromophore, we conclude that in the weak coupling limit for any arrangement of the two chromophores both processes occur on similar time scales.

Electronic properties and supramolecular organization of terminal bis(alkylethynyl)-substituted benzodithiophenes

Benzodithiophene (BDT) was symmetrically bisubstituted in the terminal positions with five different alkynes C≡C-(C(n)H(2n+1)) with n = 4, 6, 8, 10, 12. The materials were characterized as potential materials for field-effect transistor applications. Electrochemical measurements in solution and photophysical measurements in solution and in the solid state, together with UV photoelectron spectroscopy in air and quantum-chemical calculations, elucidate the nature of the frontier orbitals and of the excited states as well as their deactivation pathways. Structural information on the molecular assembly in the solid state, both at room temperature and at elevated temperatures, is obtained by a combination of DSC, polarized optical microscopy, and 2D-WAXS, which point to the crystallinity of the compounds in all phases and reveal π-stacking arrangements independently of the length of the alkyl side chains.

Fluorescence enhancement from individual plasmonic gap resonances

We studied the fluorescence enhancement of a dye-loaded polyphenylene dendrimer in a gap of 2-3 nm between a silver film and single silver particles with an average diameter of 80 nm. This sphere-on-plane geometry provides a controllable plasmonic resonator with a defined dye position. A strong fluorescence signal was seen from all particles, which was at least 1000 times stronger than the signal from the plane dye-coated metal surface. The fluorescence emission profile varied between the particles and showed light emission at higher energies than the free dye, which we assigned to hot luminescence. The maximum fluorescence emission peak shifted along with the scattering maximum of the plasmonic resonance. Two classes of scattering resonators could be distinguished. Up to a significant line-broadening, the response of the "sphere-on-plane"-like cases resembled the theoretical prediction for a perfect sphere-on-plane geometry. Resonators which deviate strongly from this ideal scenario were also found. Electron microscopy did not show significant differences between these two classes, suggesting that the variations in the optical response are due to nanoscale variations of shape and roughness in the gap region. The strong modifications of the dye emission spectrum suggested the presence of physical mechanisms at very small metal/dye separations, which are beyond a simple wavelength-dependent enhancement factor.

Density-dependent reorientation and rehybridization of chemisorbed conjugated molecules for controlling interface electronic structure

The adsorption of the molecular acceptor hexaazatriphenylene-hexacarbonitrile on Ag(111) was investigated as function of layer density. We find that the orientation of the first molecular layer changes from a face-on to an edge-on conformation depending on layer density, facilitated through specific interactions of the peripheral molecular cyano groups with the metal. This is accompanied by a rehybridization of molecular and metal electronic states, which significantly modifies the interface and surface electronic properties, as rationalized by theoretical modeling.

Influence of lipid heterogeneity and phase behavior on phospholipase A2 action at the single molecule level

We monitored the action of phospholipase A(2) (PLA(2)) on L- and D-dipalmitoyl-phosphatidylcholine (DPPC) Langmuir monolayers by mounting a Langmuir-trough on a wide-field fluorescence microscope with single molecule sensitivity. This made it possible to directly visualize the activity and diffusion behavior of single PLA(2) molecules in a heterogeneous lipid environment during active hydrolysis. The experiments showed that enzyme molecules adsorbed and interacted almost exclusively with the fluid region of the DPPC monolayers. Domains of gel state L-DPPC were degraded exclusively from the gel-fluid interface where the buildup of negatively charged hydrolysis products, fatty acid salts, led to changes in the mobility of PLA(2). The mobility of individual enzymes on the monolayers was characterized by single particle tracking. Diffusion coefficients of enzymes adsorbed to the fluid interface were between 3.2 microm(2)/s on the L-DPPC and 4.9 microm(2)/s on the D-DPPC monolayers. In regions enriched with hydrolysis products, the diffusion dropped to approximately 0.2 microm(2)/s. In addition, slower normal and anomalous diffusion modes were seen at the L-DPPC gel domain boundaries where hydrolysis took place. The average residence times of the enzyme in the fluid regions of the monolayer and on the product domain were between approximately 30 and 220 ms. At the gel domains it was below the experimental time resolution, i.e., enzymes were simply reflected from the gel domains back into solution.