Atomic force microscopy nanomanipulation of shape persistent, spherical, self-assembled aggregates of gold nanoparticles.

Gold (Au) nanoparticles have been synthesized that are stabilized by an organic ligand bearing a dithiolane functional group for binding to Au, an oligo(p-phenylene vinylene) (OPV) chromophoric group to drive self-assembly via π-π interactions, and a hydroxy functionality for interparticle hydrogen bonding. The OPV-Au particles reversibly self-assemble in n-heptane solution, forming shape persistent, spherical, nanometer-sized aggregates that do not collapse on a substrate. Optical absorption and transmission electron microscopy tomography studies show that the size and shape persistency can be tuned by modification of the ligands, adjustment of the core size, and variation of the concentration. The spherical assemblies can be manipulated with the tip of an atomic force microscope: an aggregate can be pushed over the surface for at least 20 times with nanometer precision and without substantial loss of material.

Patterns of diacetylene-containing peptide amphiphiles using polarization holography.

The polarization dependence of a diacetylene polymerization was studied. For this purpose, peptide amphiphile fibers with a diacetylene moiety, which could only polymerize in the direction of the fiber, were employed. If nonaligned samples were illuminated with polarized light, only the fibers parallel to the polarization direction of the light were polymerized. With magnetically aligned fibers, spatially selective polymerization was accomplished using polarization holography.

Aggregation kinetics of macrocycles detected by magnetic birefringence.

We have used magnetic-field-induced birefringence as a new sensitive technique to probe the aggregation kinetics of macrocyclic molecules in solution. We have found three consecutive aggregation stages: disordered objects, ordered fibers, and a network. The transition from disordered objects to ordered fibers is found to be slow, taking days or weeks to complete. We attribute this to the molecular tails of the macrocycles, which hamper fiber formation. We anticipate that linking aggregation kinetics to molecular properties will lead to a better understanding of the mechanisms by which molecules self-assemble, allowing for a more rational design of the molecular building blocks.

Molecular organization of cylindrical sexithiophene aggregates measured by X-ray scattering and magnetic alignment.

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

Insights into templated supramolecular polymerization: binding of naphthalene derivatives to ssDNA templates of different lengths.

We report on two diaminotriazine-equipped naphthalene derivatives that bind reversibly to a single-stranded DNA template or "tape-measure molecule" via hydrogen bonding, yielding monodisperse double-stranded DNA hybrids with one strand consisting of a supramolecular naphthalene backbone. These assemblies have been investigated extensively, both experimentally and theoretically. The structure and the templated self-assembly process of the complex have been characterized with UV-vis spectroscopy, circular dichroism spectroscopy, molecular dynamics simulations, cryo-transmission electron microscopy, liquid atomic force microscopy, electrospray ionization mass spectrometry, light scattering, and 1H NMR and infrared spectroscopy. We have found that the DNA hybrid complexes have a right-handed helical arrangement stabilized by pi-pi interactions and hydrogen bonds. The hydrophilic hydroxyl group at the end of the ethylene glycol of the guest molecule suppressed both the nontemplated self-assembly of the naphthalene guest molecules and the further aggregation of the entire DNA hybrid complex. Through the use of a theoretical mass-action model for the templated self-assembly, the host-guest and guest-guest interaction energies were estimated by fitting to the spectroscopic data. The differently estimated values of the interaction energies and thermodynamic parameters vary within experimental error, showing the self-consistency of the model. From the obtained correlation between the positions of the guest molecules bound on the template, we have obtained a qualitative theoretical picture of the way in which the guests are physically distributed on the templates. For short templates, the templates are filled one-by-one, even at moderate fractions of bound sites. For larger templates, the templates first have alternating sequences of filled and empty sections, after which, at large fractions of bound sites, virtually all of the binding sites for all template lengths are filled.

Using magnetic birefringence to determine the molecular arrangement of supramolecular nanostructures.

Supramolecular aggregates can be aligned in solution using a magnetic field. Because of the optical anisotropy of the molecular building blocks, the alignment results in an anisotropic refractive index of the solution parallel and perpendicular to the magnetic field. We present a model for calculating the magnetic birefringence, using solely the magnetic susceptibilities and optical polarizabilities of the molecules, for any molecular arrangement. We demonstrate that magnetic birefringence is a very sensitive tool for determining the molecular organization within supramolecular aggregates.

Oligo(p-phenylenevinylene)-peptide conjugates: synthesis and self-assembly in solution and at the solid-liquid interface.

Two oligo(p-phenylenevinylene)-peptide hybrid amphiphiles have been synthesized using solid- and liquid-phase strategies. The amphiliphiles are composed of a pi-conjugated oligo(p-phenylenevinylene) trimer (OPV) which is coupled at either a glycinyl-alanyl-glycinyl-alanyl-glycine (GAGAG) silk-inspired beta-sheet or a glycinyl-alanyl-asparagyl-prolyl-asparagy-alanyl-alanyl-glycine (GANPNAAG) beta-turn forming oligopeptide sequence. The solid-phase strategy enables one to use longer peptides if strong acidic conditions are avoided, whereas the solution-phase coupling gives better yields. The study of the two-dimensional (2D) self-assembly of OPV-GAGAG by scanning tunneling microscopy (STM) at the submolecular level demonstrated the formation of bilayers in which the molecules are lying antiparallel in a beta-sheet conformation. In the case of OPV-GANPNAAG self-assembled monolayers could not be observed. Absorption, fluorescence, and circular dichroism studies showed that OPV-GAGAG and OPV-GANPNAAG are aggregated in a variety of organic solvents. In water cryogenic temperature transmission electron microscopy (cryo-TEM), atomic force microscopy (AFM), light scattering, and optical studies reveal that self-assembled nanofibers are formed in which the helical organization of the OPV segments is dictated by the peptide sequence.

Disk micelles from amphiphilic Janus gold nanoparticles.

Janus gold nanoparticles are synthesised via a simple preparation method and behave as amphiphiles self-assembling in water into disk-shaped micelles.

Polarized emission of individual self-assembled oligo(p-phenylenevinylene)-based nanofibers on a solid support.

We have prepared 5 nm diameter, micrometer long tetra(p-phenylenevinylene) (OPV)-based nanofibers on a graphite surface. The fluorescence emission of an individual fiber shows a profound polarization over its entire length that directly corresponds to its orientation on the substrate. Quantitative analysis of the fluorescence polarization, including the depolarizing effect of the underlying graphite, evidences the high degree of organization within chiral fibers with the OPV molecules perpendicular to the fiber axis. The control of the internal order within self-assembled fibers, and the ability to measure it, is a crucial step to obtain uniform organic fibers that can be applied in nanosized electronics at room temperature.