Directed self-assembly of gold-tipped CdSe nanorods

Gold-tipped CdSe rods (nanodumbbells) were solubilized in an aqueous phase and self-assembled in a head-to-tail manner using biotin disulfide and avidin. The disulfide end of the biotin molecule attaches to the gold tip of the nanodumbbell, and the biotin end of the molecule is able to conjugate to an avidin protein. The avidin can strongly conjugate up to four biotin molecules. Changing the ratios of biotin to nanodumbbells leads to the formation of dimers, trimers, and flowerlike structures. To further improve the distribution of chain lengths, a separation method based upon weight was applied using a concentration gradient. The gold tips provide effective anchor points for constructing complex nanorod structures by self-assembly.     

Synthesis and self-assembly of monodisperse silver-nanocrystal-doped silica particles

A simple method based on the St?ber reaction was developed to prepare silver-nanocrystal-doped silica composite particles. The silane coupling agent N-[3-(trimethoxysilyl)dropyl]ethylene diamine (TSD) incorporated Ag+ into a siloxane framework and a further chemical reducer reduced Ag+ to silver nanoparticles. TEM images showed that, in the presence of TSD, silver nanocrystals (fcc) of 2-8 nm were homogeneously doped in the silica particles, which showed a typical surface plasmon resonance (SPR) peak. The as-prepared Ag/SiO2 composite particles can be self-assembled into long-range ordered lattices (or photonic crystals) over large areas.     

Directed self-assembly of inorganic redox complexes with artificial peptide scaffolds

An ongoing challenge in the construction of supramolecular systems is controlling the relative geometry of functional redox species for molecular electronics devices, including wires, switches, and gates. This review focuses on the use of artificial peptide strands to assemble inorganic complexes that are redox active. These approaches toward macromolecular assembly use varying oligoamide backbones and assembly motifs that grew from earlier reports of single oligolysine or proline chains containing pendant redox species that undergo photoinduced charge separation. Recently, peptide nucleic acid chains that form double-stranded duplexes analogous to DNA by hydrogen bonding of complementary base pairs have been modified to contain metal complexes. In these structures, hydrogen bonding and metal coordination combine to form crosslinks between the PNA strands. Finally, a family of structures is described that is based on an aminoethylglycine scaffold with pendant metal coordination sites, but without intervening nucleic acid base pairs. These structures form multimetallic complexes that are either single- or double-stranded, or that form hairpin loop structures. These motifs for using artificial peptide strands for self-assembly hold electron donors and acceptors in relative positions that provide structural connectivity and permit electron transfers between linked metal complexes. This is a new approach for creating polyfunctional redox architectures that could ultimately enable the construction of potentially large and complex molecular electronics devices.     

Reversible self-assembly of patchy particles into monodisperse icosahedral clusters

We systematically study the design of simple patchy sphere models that reversibly self-assemble into monodisperse icosahedral clusters. We find that the optimal patch width is a compromise between structural specificity (the patches must be narrow enough to energetically select the desired clusters) and kinetic accessibility (they must be sufficiently wide to avoid kinetic traps). Similarly, for good yields the temperature must be low enough for the clusters to be thermodynamically stable, but the clusters must also have enough thermal energy to allow incorrectly formed bonds to be broken. Ordered clusters can form through a number of different dynamic pathways, including direct nucleation and indirect pathways involving large disordered intermediates. The latter pathway is related to a reentrant liquid-to-gas transition that occurs for intermediate patch widths upon lowering the temperature. We also find that the assembly process is robust to inaccurate patch placement up to a certain threshold and that it is possible to replace the five discrete patches with a single ring patch with no significant loss in yield.     

Directed formation of DNA nanoarrays through orthogonal self-assembly

We describe the synthesis of terpyridine modified DNA strands which selectively form DNA nanotubes through orthogonal hydrogen bonding and metal complexation interactions. The short DNA strands are designed to self-assemble into long duplexes through a sticky-end approach. Addition of weakly binding metals such as Zn(II) and Ni(II) induces the formation of tubular arrays consisting of DNA bundles which are 50-200 nm wide and 2-50 nm high. TEM shows additional long distance ordering of the terpy-DNA complexes into fibers.     

Directed self-assembly of surfactants in carbon nanotube materials

The self-assembly of surfactant molecules on crossing carbon nanotubes has been investigated using a bead-spring model and implicit solvent dissipative particle dynamics simulations. Adsorption is directed to the nanotube crossing by its higher hydrophobic potential which is due to the presence of two surfaces. As a consequence of the tendency of surfactant molecules to self-assemble into micelles, the adsorbed molecules form a "central aggregate" at the crossing, thus, confining the molecules to the immediate vicinity of the crossing. Adsorption on the remaining nanotube surface becomes significant only at higher surfactant concentrations, where the molecules self-assemble to hemimicelles which grow continuously to full micelles upon increase of the (bulk) surfactant concentration. Our results allow two conclusions for the rational design of nanostructured materials: (i) the size of the central aggregate can not be much larger than that of a bulk micelle and (ii) control of the adsorbed structures is conveniently possible via the (bulk) surfactant concentration.     

Directed self-assembly of colloidal crystals by dielectrophoretic ordering

In this Article, we report the dielectrophoretic assembly of colloidal particles and show how the kinetics of assembly and degree of ordering depend on the particle size, charge, solution ionic strength, and field strength and frequency. A special dielectrophoresis (DEP) sample cell is constructed and validated to quantitatively measure directed self-assembly via sequential light scattering and optical microscopy measurements. Our results confirm the recently established scaling for the order-disorder transition and extend it to higher scaled frequencies. The limiting scaling of the order-disorder transition and particle electrophoretic mobility are correctly predicted by the standard electrokinetic model (SEKM). In particular, the order-disorder transition line is predicted from the particle properties using a recently proposed empirical scaling law and the SEKM over an order of magnitude in particle size.     

One-pot synthesis, optical property and self-assembly of monodisperse silver nanospheres

High-quality spherical silver (Ag) nanocrystals have been synthesized by using a one-pot approach, in which pre-synthesis of organometallic precursors is not required. This reaction involves the thermolysis of a mixed solution of silver acetate and n-dodecanethiol in a non-coordinating organic solvent. The size of the as-obtained Ag nanospheres can be controlled by adjusting the reaction time, reaction temperature and the amount of silver acetate added. The growth and nucleation process of the resultant Ag nanospheres have been studied by employing UV-vis absorption spectra and transmission electron microscopy (TEM) images. Furthermore, these Ag nanospheres have good self-assembly behaviors, and they are easily self-assembled into two- or three-dimensional superlattice structures due to the bundling and interdigitation of thiolate molecules adsorbed on Ag nanospheres. This one-pot synthetic procedure is simple and highly reproducible, which may be extended to prepare other noble-metal nanocrystals.     

Directed self-assembly of microcomponents enabled by laser-activated bubble latching

This article introduces a method for microscale assembly using laser-activated bubble latching. The technique combines the advantages of directed fluidic assembly and surface tension-driven latching to create arbitrarily complex and irregular structures with unique properties. The bubble latches, generated through the laser degradation of the tile material, are created on the fly, reversibly linking components at user-determined locations. Different phases of latching bubble growth are analyzed, and shear force calculations show that each bubble is able to support a tensile force of approximately 0.33 μN. We demonstrate that by exploiting the compressibility of bubbles, assembled objects can be made to switch between rigid and flexible states, facilitating component assembly and transport. Furthermore, we show reconfiguration capabilities through the use of bubble hinging. This novel hybrid approach to the assembly of microscale components offers significant user control while retaining a simplistic design environment.     

Dendronized protein polymers: synthesis and self-assembly of monodisperse cylindrical macromolecules

Monodisperse dendronized protein polymers (DPPs), cylindrical dendrimers containing protein core, can be efficiently produced through a combined modular biosynthetic strategy. These DPP materials possess predictable size, shape, and solubility. In organic solutions, the DPPs self-assemble to form highly ordered liquid crystalline structures with nanoscale order controlled by their exact molecular dimensions.     

Large-scale synthesis and self-assembly of monodisperse hexagon Cu2S nanoplates

One simple high-temperature solution phase method has been developed to synthesize large-scale, monodisperse, hexgon beta-Cu2S nanoplates. The hexgon nanoplates have edge lengths of 9 +/- 0.5 nm and thicknesses of 4.5 +/- 0.2 nm and self-assemble closely into three-dimensional superlattices. The present results suggest that the simple method might be useful for the synthesis ofmonodispese hexagon nanoplates for many other chalcogenide semiconductors with hexagonal symmetry structure.     

Microfluidic fabrication of monodisperse biocompatible and biodegradable polymersomes with controlled permeability

We describe a versatile technique for fabricating monodisperse polymersomes with biocompatible and biodegradable diblock copolymers for efficient encapsulation of actives. We use double emulsion as a template for the assembly of amphiphilic diblock copolymers into vesicle structures. These polymersomes can be used to encapsulate small hydrophilic solutes. When triggered by an osmotic shock, the polymersomes break and release the solutes, providing a simple and effective release mechanism. The technique can also be applied to diblock copolymers with different hydrophilic-to-hydrophobic block ratios, or mixtures of diblock copolymers and hydrophobic homopolymers. The ability to make polymer vesicles with copolymers of different block ratios and to incorporate different homopolymers into the polymersomes will allow the tuning of polymersome properties for specific technological applications.     

Synthesis and self-assembly of nearly monodisperse nanoparticles of a naturally occurring polymer

Nearly monodisperse nanoparticles have been synthesized based on a naturally occurring polymer of hydropropylcellulose (HPC) using precipitation polymerization method. It is found that when the polydispersity index value of the HPC nanoparticles is less than 1.10, the HPC particles can self-assemble into an ordered structure, displaying bright colors. UV-visible spectroscopy reveals that the color shifts to a lower wavelength as the interparticle distance decreases following the Bragg diffraction equation. The HPC nanoparticle assembly in water has been further stabilized by covalently bonding neighboring particles to form a three-dimensional network. This network contains a large amount of water similar to a conventional bulk gel but displays colors as a result of its ordered structure.     

Enzymatic lithography of phospholipid bilayer films by stereoselective hydrolysis

The stereoselective phospholipase A2-catalyzed hydrolysis of patterned phospholipid bilayers consisting of the l- and d-isomers of alpha-dilauroylphosphatidylcholine (DLPC) and alpha-dipalmitoylphosphatidylcholine (DPPC) is reported. The stereochemically directed enzyme lithography demonstrated herein allows the parallel modification of large surface areas and constitutes a potentially useful method to structure biomimetic films, given the stereospecific action of many enzymes.     

Chemically controlled self-assembly of protein nanorings

The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase (DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2)) are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.     

Detecting cross talk between two halves of a phospholipid bilayer

One of the most challenging questions that relates to the structure and function of biological membranes is whether the two halves of the bilayer "talk" to each other. In this letter, we show how the perturbation of the lateral organization of one leaflet of a fluid phospholipid bilayer by an external agent also alters the lateral organization of the adjoining leaflet. In addition, we show that the energy involved in such "cross talk" corresponds to ca. 100 cal/mol of phospholipid. These findings provide a basis for expecting similar cross talk to exist in cell membranes.     

Influence of nanotopography on phospholipid bilayer formation on silicon dioxide

We have investigated the effect of well-defined nanoscale topography on the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid vesicle adsorption and supported phospholipid bilayer (SPB) formation on SiO2 surfaces using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Unilamellar lipid vesicles with two different sizes, 30 and 100 nm, were adsorbed on pitted surfaces with two different pit diameters, 110 and 190 nm, as produced by colloidal lithography, and the behavior was compared to results obtained on flat surfaces. In all cases, complete bilayer formation was observed after a critical coverage of adsorbed vesicles had been reached. However, the kinetics of the vesicle-to-bilayer transformation, including the critical coverage, was significantly altered by surface topography for both vesicle sizes. Surface topography hampered the overall bilayer formation kinetics for the smaller vesicles, but promoted SPB formation for the larger vesicles. Depending on vesicle size, we propose two modifications of the precursor-mediated vesicle-to-bilayer transformation mechanism used to describe supported lipid bilayer formation on the corresponding flat surface. Our results may have important implications for various lipid-membrane-based applications using rough or topographically structured surfaces.     

Kinetically controlled self-assembly of pseudorotaxanes on crystallization

[reaction: see text] Mixing of equimolar amounts of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) with a bis-4-methylphenyl ether (MPE twice) of a 1,5-dioxynaphthalene (DNP) derivative in MeCN/CH(2)Cl(2) (3:1) results in the formation of a [2]pseudorotaxane which, on crystallization, yields a [4]pseudorotaxane in the solid state that is stabilized by multiple [C-H...F] interactions: a mixture of the same components in a 1:3 ratio affords a crystalline [2]pseudorotaxane after vapor diffusion of methyl-tert-butyl ether into a solution of these components in MeCN/CH(2)Cl(2) (3:1).