Directed self-assembly in laponite/CdSe/polyaniline nanocomposites

Laponite films provide versatile inorganic scaffolds with materials architectures that direct the self-assembly of CdSe quantum dots (QDs or EviTags) and catalytic surfaces that promote the in situ polymerization of polyaniline (PANI) to yield novel nanocomposites for light emitting diodes (LEDs) and solar cell applications. Water-soluble CdSe EviTags with varying, overlapping emission wavelengths in the visible spectrum were incorporated using soft chemistry routes within Na-Laponite host film platforms to achieve broadband emission in the visible spectrum. QD concentrations, composition and synthesis approach were varied to optimize photophysical properties of the films and to mediate self-assembly, optical cascading and energy transfer. In addition, aniline tetramers coupled to CdSe (QD-AT) surfaces using a dithioate linker were embedded within Cu-Laponite nanoscaffolds and electronically coupled to PANI via vapor phase exposure. Nanotethering and specific host-guest and guest-guest interactions that mediate nanocomposite photophysical behavior were probed using electronic absorption and fluorescence spectroscopies, optical microscopy, AFM, SEM, powder XRD, NMR and ATR-FTIR. Morphology studies indicated that Lap/QD-AT films synthesized using mixed solvent, layer by layer (LbL) methods exhibited anisotropic supramolecular structures with unique mesoscopic ordering that affords bifunctional networks to optimize charge transport.     

Light-induced selective deposition of metals on gold-tipped CdSe-seeded CdS nanorods

We introduce a facile approach for the selective deposition of metals on Au-tipped CdSe-seeded CdS nanorods that exploits the transfer of electrons from CdS to the Au tips upon UV excitation. This light-induced deposition method was used for the deposition of Pd under mild conditions, which produced a Pd/Au alloyed tip while preserving the rest of the semiconductor nanoarchitecture. The highly site-selective deposition method was extended to the deposition of Fe, yielding monodispersed, structurally complex Au core/Fe(x)O(y) hollow shell-tipped semiconductor nanorods. These structurally well-defined rods were found to exhibit magnetic functionality. The synthetic strategies described in this work expand on the range of metals that can be deposited on heterostructured semiconductor nanorods, opening up new avenues for the hierarchical buildup of structural complexity and therefore multifunctionality in nanoparticles.     

Monodisperse CdSe nanorods at low temperatures

A new synthetic method is presented that allows the preparation of highly monodisperse CdSe nanorods (so called quantum rods) at relatively low temperatures (160 degrees C). This method is characterized by a high aspect ratio of the particles and affords good reproducibility. The morphology of the resulting nanorods was examined by means of transmission electron microscopy (TEM) and the electro-optic properties by means of fluorescence spectroscopy. The conditions of the reaction of nanoparticle growth were examined by varying the concentration of the organometallic precursors, the growth temperature, and the growth time. The experimental findings correspond well with previously published semiempirical pseudo-potential calculations.     

金纳米棒自组装的研究进展

金纳米棒具有独特的光电性能,其自组装形成的功能组装体能够展现出更加优异的整体协同性能,在纳米材料科学和生物医学领域中有广泛而重要的应用前景.本文从诱导自组装各种驱动作用力的角度,综述了金纳米棒自组装的最新研究进展,具体内容包括:表面张力引发的自组装、化学作用驱动如静电作用或氢键作用等引发的自组装以及生物分子识别作用引发的自组装.     

Solvent-based assembly of CdSe nanorods in solution

We demonstrate a purely solvent-based approach to assembling CdSe nanorods into vertically aligned, hexagonally packed monolayers in solution. Nanorods were dispersed in a mixture of good solvent with high vapor pressure and bad solvent with low vapor pressure, and preferential evaporation of the good solvent led to ordered assembly under conditions of continuously decreasing solvent quality. No applied external bias, extensive control of drying conditions, exceptionally monodisperse nanoparticles, or high concentrations of additives were required. This clean and facile method yielded ordered nanorod sheets of up to 7.5 μm wide with potential use as active materials in unique applications.     

Fabrication and self-assembly of hydrophobic gold nanorods

Hydrophobic gold nanorods were fabricated from hydrophilic gold nanorods coated with hexadecyltrimethylammonium bromide by treating with mercaptopropyltrimethoxysilane (MPS) and subsequently octadecyltrimethoxysilane (ODS). The fabrication of the hydrophobic shell went through the process of (1) binding MPS onto the nanorods, (2) hydrolysis of methoxysilanes, and (3) immobilization of ODS by dehydration condensation. The 2- or 3-D ordered structures of hydrophobic nanorods were self-assembled by the evaporation of solvent on a substrate. The aspects of 2-D assemblies were dependent on the concentration of the nanorods, as was seen in transmission electron microscopic images. At a low concentration, the nanorods assembled parallel to the substrate, whereas they stood on the substrate at a high concentration. On the other hand, in a solid of the gold nanorods, the formation of the 3-D assembly was confirmed by small-angle X-ray scattering. The assembly consisted of hexagonal arrays of the gold nanorods and their lamellar accumulation.     

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.     

Surface-functionalized CdSe nanorods for assembly in diblock copolymer templates

Poly(ethylene oxide)-covered CdSe nanorods were prepared and assembled in diblock copolymer templates by floating the block copolymer templates onto aqueous nanorod solutions. The assembly was enabled by consideration of the surface ligand coverage of the nanorods. Alkane-covered CdSe nanorods prepared by state-of-the-art techniques are not compatible with this assembly process. However, poly(ethylene oxide) (PEO)-functionalized CdSe nanorods were successfully used to assemble the nanorods into the channels and pores of diblock copolymer templates. Other water-dispersible CdSe nanorods, such as those covered with 11-mercaptoundecanoic acid (MUA), did not give the desired assemblies. These results are understood by considering the surface energies of the PEO-covered CdSe nanorods in this interfacial assembly process.     

Selective synthesis and characterization of CdSe nanorods and fractal nanocrystals

CdSe nanorods and dendritic fractals were synthesized through a novel controllable solution-phase hydrothermal method. Soluble selenite was employed to provide a highly reactive Se source in the synthesis. Both morphologies and phases of the CdSe products could be successfully controlled by choosing appropriate complexing agents to adjust the dynamics of the reaction process. Reaction temperature and Cd/Se ratio in raw materials were also important parameters influencing the morphologies and phases of the products. The phase structures, morphologies, and optical properties of the CdSe products were investigated by XRD, TEM, HRTEM, and UV-vis and photoluminescence spectroscopies. The formation mechanisms of the nanorods and fractals were investigated and discussed on the basis of the experimental results.     

Mechanism and origin of exciton spin relaxation in CdSe nanorods

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.     

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.     

Synthesis, characterization, and self-assembly of pencil-shaped CoO nanorods

We synthesized uniformly sized, pencil-shaped CoO nanorods by the thermal decomposition of a cobalt-oleate complex, which was prepared from the reaction of cobalt chloride and sodium oleate. The diameters and lengths of the CoO nanorods were easily controlled by varying the experimental conditions, such as the heating rate and the amount of Co-oleate complex. The X-ray diffraction pattern revealed that the CoO nanorods have an extraordinary wurtzite ZnO crystal structure. These uniformly sized nanorods self-assembled to form both horizontal parallel arrangements and perpendicular hexagonal honeycomb superlattice structures. Reduction of the nanorods by heating under a hydrogen atmosphere generated either hcp Co or Co(2)C nanorods. Characterization of the CoO nanorods using X-ray absorption spectroscopy, X-ray magnetic circular dichroism spectroscopy, and magnetic measurements showed that they contain a small fraction of ferromagnetic Co impurities.     

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.     

Directed self-assembly of monodisperse phospholipid bilayer Nanodiscs with controlled size

Using a recently described self-assembly process (Bayburt, T. H.; Grinkova, Y. V.; Sligar, S. G. Nano Letters 2002, 2, 853-856), we prepared soluble monodisperse discoidal lipid/protein particles with controlled size and composition, termed Nanodiscs, in which the fragment of dipalmitoylphosphatidylcholine (DPPC) bilayer is surrounded by a helical protein belt. We have customized the size of these particles by changing the length of the amphipathic helical part of this belt, termed membrane scaffold protein (MSP). Herein we describe the design of extended and truncated MSPs, the optimization of self-assembly for each of these proteins, and the structure and composition of the resulting Nanodiscs. We show that the length of the protein helix surrounding the lipid part of a Nanodisc determines the particle diameter, as measured by HPLC and small-angle X-ray scattering (SAXS). Using different scaffold proteins, we obtained Nanodiscs with the average size from 9.5 to 12.8 nm with a very narrow size distribution (+/-3%). Functionalization of the N-terminus of the scaffold protein does not perturb their ability to form homogeneous discoidal structures. Detailed analysis of the solution scattering confirms the presence of a lipid bilayer of 5.5 nm thickness in Nanodiscs of different sizes. The results of this study provide an important structural characterization of self-assembled phospholipid bilayers and establish a framework for the design of soluble amphiphilic nanoparticles of controlled size.     

Interface self-assembly to construct vertical peptide nanorods on quartz template

A rationally designed glycyl-glycine derivative containing a light cleaved pyrenylmethyl ester tail was covalently bound onto the surface of quartz template. The interface self-assembly of this dipeptide building block induced the formation of chemically bound vertically aligned nanorods (CBVANs) with light sensitivity on the template.     

Size effects and breakdown of the power-law blinking statistics of CdSe nanorods

In this study, the dependence of sample size and light intensity on the fluorescence intermittency of semiconductor nanorods is investigated. We present a model with diffusion-controlled electron-transfer reactions involving anomalous diffusion in energy configuration space. This model leads to a general formula t(-m) exp[-(Gammat)n] for the temporal behavior of blinking statistics, where m and n are related to the time dependence of the spectral diffusion. We reanalyze the experimental data of the long-time bending tail of CdSe nanorods and elucidate the size effects of the bending rates and activation energy.