Mesoscale CdTe helices with near-unity enantiomeric excess offer insight into design rules for chiroptical semiconductor materials.

Solutions for nanoparticle solutions

Nanoparticle interactions in solution affect their binding to biomolecules, their electronic properties, and their packing into larger crystals. However, the theories that describe larger colloidal particles fail for nanoparticles, because the interactions do not add together linearly. Nanoparticles also have complex shapes and are closer in size to the solvent molecules. Silvera Batista et al. review approaches that can treat the nonadditive nature of nanoparticle interactions, resulting in a more complete understanding of nanoparticles in solution.

Science, this issue p. 10.1126/science.1242477

Abstract

A novel nanostructured composite film (see figure) that takes advantage of two different natural materials—layered nacre and the marine adhesive of mussels—is prepared. L-3,4-Dihydroxyphenylalanine (DOPA) molecules impart unusual adhesive strength to the clay composite and the hardening mechanism found in the natural “cement” plays an equally important role in the strengthening of the “nanostructured nacre”.

Abstract

Nanoscale building blocks are individually exceptionally strong because they are close to ideal, defect-free materials. It is, however, difficult to retain the ideal properties in macroscale composites. Bottom-up assembly of a clay/polymer nanocomposite allowed for the preparation of a homogeneous, optically transparent material with planar orientation of the alumosilicate nanosheets. The stiffness and tensile strength of these multilayer composites are one order of magnitude greater than those of analogous nanocomposites at a processing temperature that is much lower than those of ceramic or polymer materials with similar characteristics. A high level of ordering of the nanoscale building blocks, combined with dense covalent and hydrogen bonding and stiffening of the polymer chains, leads to highly effective load transfer between nanosheets and the polymer.

Abstract

In their physical dimensions, surface chemistry, and degree of anisotropic interactions in solution, CdTe nanoparticles are similar to proteins. We experimentally observed their spontaneous, template-free organization into free-floating particulate sheets, which resemble the assembly of surface layer (S-layer) proteins. Computer simulation and concurrent experiments demonstrated that the dipole moment, small positive charge, and directional hydrophobic attraction are the driving forces for the self-organization process. The data presented here highlight the analogy of the solution behavior of the two vastly different classes of chemical structures.

The design of advanced, nanostructured materials at the molecular level is of tremendous interest for the scientific and engineering communities because of the broad application of these materials in the biomedical field. Among the available techniques, the layer-by-layer assembly method introduced by Decher and co-workers in 1992 has attracted extensive attention because it possesses extraordinary advantages for biomedical applications: ease of preparation, versatility, capability of incorporating high loadings of different types of biomolecules in the films, fine control over the materials' structure, and robustness of the products under ambient and physiological conditions. In this context, a systematic review of current research on biomedical applications of layer-by-layer assembly is presented. The structure and bioactivity of biomolecules in thin films fabricated by layer-by-layer assembly are introduced. The applications of layer-by-layer assembly in biomimetics, biosensors, drug delivery, protein and cell adhesion, mediation of cellular functions, and implantable materials are addressed. Future developments in the field of biomedical applications of layer-by-layer assembly are also discussed.

Te nanowires made from stabilizer- depleted CdTe nanoparticles are assembled in thin films following a layer-by-layer assembly protocol. The thin film has a distinctively metallic mirror-like appearance and displays a strong photoconductance effect characteristic of narrow-bandgap semiconductors. In situ reduction of gold results in the formation of Au nanoparticles adhering to Te nanowires, which leads to the disappearance of photoconductivity of the Te thin film.

The first example of neural stimulation through a SWNT material assembled by layer-by-layer deposition is described (see figure). Excitation by lateral currents in the nanotube coating results in the opening of voltage-dependent Na⁺ ion channels. The experimental observations indicate the possibility of engineering of implantable biomedical devices from SWNT/polymer multilayers to be used for neuron prosthesis, pain management, and muscle stimulation.

Glänzendes Silber: In einer Überstruktur aus Ag-Nanopartikeln und CdTe-Nanodrähten, die durch ein Streptavidin(SA)-D-Biotin(B)-Affinitätspaar verbunden sind, ist die Lumineszenz des Nanodrahtes verdoppelt. Der beteiligte optische Prozess könnte auch in anderen Metamaterialien ablaufen, und er könnte als Grundlage für Anwendungen in einer Vielzahl optoelektronischer Funktionseinheiten dienen.

Shiny silver: A superstructure consisting of Ag nanoparticles and CdTe nanowires connected by a streptavidin (SA) and D-biotin (B) affinity pair displays twofold enhancement of the nanowire luminescence. The optical process involved could also be operative in other metamaterials and could serve as a basis for applications in a variety of optoelectronic devices.

Se and Te nanowires (NWs) are spontaneously formed in solution through chemical decomposition of stabilizer-depleted nanoparticles (NPs) in the presence of a strong complexing agent. Highly crystalline Se and Te NWs with monodisperse lateral sizes, in the range 10–100 nm, are produced (see Figure and cover). This work provides a new approach to nanoscale synthesis, which takes advantage of the transformation of NPs to NWs induced by stabilizer shell depletion and ensuing chemical reactions.

The cover image shows a transmission electron microscopy image of selenium nanowires from the self-reorganization process of stabilizer-depleted CdSe nanoparticles in the presence of a complexing agent. On p. 358, Kotov and co-workers report the preparation of semiconductor nanowires with tightly controlled diameter using stabilizer-depleted nanoparticles rather than typical molecules as reaction precursors.

Nanoparticle (NP) assemblies are of considerable interest for both fundamental research and applications, since they provide direct bridges between nanometer-scale objects and the macroscale world. Unlike two-dimensional or three-dimensional NP assemblies, which have been extensively studied and reviewed, reports on one-dimensional (1D) NP assemblies are rather rare, even though these assemblies are likely to play critical roles in the improvement of the efficiencies of various electronic, optoelectronic, magnetic, and other devices based on single NPs or their composites. Additionally, 1D assemblies of NPs, i.e., chains, can significantly help in the understanding of a number of biological processes and fundamental quantum mechanics of nanometer-scale systems. The difficulties are very evident when one wants to realize anisotropic 1D assemblies from presumably isotropic, zero-dimensional NPs. In this context, the authors present a systemic review of current research on 1D NP assemblies. Their preparation methods are classified and novel characteristics of NP chains, such as collective properties and directional transfer of photons, electrons, spins, etc., are elucidated. Current problems underlying the fundamental research and practical applications of 1D NP assemblies are also addressed.

Successful engineering of functional tissues requires the development of three-dimensional (3D) scaffolds that can provide an optimum microenvironment for tissue growth and regeneration. A new class of 3D scaffolds with a high degree of organization and unique topography is fabricated from polyacrylamide hydrogel. The hydrogel matrix is molded by inverted colloidal crystals made from 104 μm poly(methyl methacrylate) spheres. The topography of the scaffold can be described as hexagonally packed 97 μm spherical cavities interconnected by a network of channels. The scale of the long-range ordering of the cavities exceeds several millimeters. In contrast to analogous material in the bulk, hydrogel shaped as an inverted opal exhibits much higher swelling ratios; its swelling kinetics is an order of magnitude faster as well. The engineered scaffold possesses desirable mechanical and optical properties that can facilitate tissue regeneration while allowing for continuous high-resolution optical monitoring of cell proliferation and cell–cell interaction within the scaffold. The scaffold biocompatibility as well as cellular growth and infiltration within the scaffold were observed for two distinct human cell lines which were seeded on the scaffold and were tracked microscopically up to a depth of 250 μm within the scaffold for a duration of up to five weeks. Ease of production, a unique 3D structure, biocompatibility, and optical transparency make this new type of hydrogel scaffold suitable for most challenging tasks in tissue engineering.

Eine temperaturabhängige Emission zeigen CdTe-Nanopartikel, die über flexible PEG-Ketten, die wie molekulare Federn wirken, mit Au-Nanopartikeln verknüpft sind (siehe Bild). Die Länge der PEG-Federn ändert sich mit der Temperatur, was zu Änderungen in der CdTe-Emission als Folge von Plasmon-Exciton-Wechselwirkungen führt. Ein theoretisches Modell der Plasmon-Exciton-Kopplung erklärt die optischen und thermischen Effekte.