Successful Differentiation of Mouse Neural Stem Cells on Layer-by-Layer Assembled Single-Walled Carbon Nanotube Composite

The same properties that made carbon nanotube (CNT) composites interesting for electronics, sensing, and ultrastrong structural materials also make them an asset for biomedical engineering. The combination of electron conductivity, corrosion resistance, and strength are essential for neuroprosthetic devices. All of the studies in this area demonstrating cellular adhesion and signal transduction activity on CNT matrixes were conducted, so far, with terminally differentiated primary cells and cancerous cell lines. Neural stem cells are very plastic neural precursors capable of adapting to environmental conditions and recreating signal transduction pathways. Their intrinsic biological functionality not only makes the transition to stem cell cultures a difficult-to-avoid step but also implies several fundamentally important challenges. Here we demonstrate that mouse embryonic neural stem cells (NSCs) from the cortex can be successfully differentiated to neurons, astrocytes, and oligodendrocytes with clear formation of neurites on layer-by-layer (LBL) assembled single-walled carbon nanotube (SWNT)−polyelectrolyte multilayer thin films. Biocompatibility, neurite outgrowth, and expression of neural markers were similar to those differentiated on poly-l-ornithine (PLO), one of the most widely used growth substratums for neural stem cells

Thermodynamic and Structural Insights into Nanocomposites Engineering by Comparing Two Materials Assembly Techniques for Graphene

Materials assembled by layer-by-layer (LBL) assembly and vacuum-assisted flocculation (VAF) have similarities, but a systematic study of their comparative advantages and disadvantages is missing. Such a study is needed from both practical and fundamental perspectives aiming at a better understanding of structure–property relationships of nanocomposites and purposeful engineering of materials with unique properties. Layered composites from polyvinyl alcohol (PVA) and reduced graphene (RG) are made by both techniques. We comparatively evaluate their structure, mechanical, and electrical properties. LBL and VAF composites demonstrate clear differences at atomic and nanoscale structural levels but reveal similarities in micrometer and submicrometer organization. Epitaxial crystallization and suppression of phase transition temperatures are more pronounced for PVA in LBL than for VAF composites. Mechanical properties are virtually identical for both assemblies at high RG contents. We conclude that mechanical properties in layered RG assemblies are largely determined by the thermodynamic state of PVA at the polymer/nanosheet interface rather than the nanometer scale differences in RG packing. High and nearly identical values of toughness for LBL and VAF composites reaching 6.1 MJ/m3 observed for thermodynamically optimal composition confirm this conclusion. Their toughness is the highest among all other layered assemblies from RG, cellulose, clay, etc. Electrical conductivity, however, is more than 10× higher for LBL than for VAF composites for the same RG contents. Electrical properties are largely determined by the tunneling barrier between RG sheets and therefore strongly dependent on atomic/nanoscale organization. These findings open the door for application-oriented methods of materials engineering using both types of layered assemblies

Semiconductor Nanoparticles on Solid Substrates: Film Structure, Intermolecular Interactions, and Polyelectrolyte Effects

Citrate-stabilized CdSe or CdSe/CdS core−shell nanoparticles (NPs) were adsorbed on the standard silicon wafers bearing either a short-chain covalently bound adsorption promoter (3-aminopropyl)triethoxylsilane (APTES) or macromolecular adsorption promoterspolyethylenimine (PEI) or poly(diallydimethylammonium) chloride (PDDA). The aim of this study is 2-fold:  (1) to compare different methods of NP processing into thin films and (2) to elucidate the effect of the long-chain dynamics on the NP film structure. Systematic atomic force microscopy study of the films revealed that both types of NPs produced densely packed films on PDDA, while rarified films with significant clustering formed on PEI and APTES. The difference in NP layer morphologies was rationalized on the basis of intermolecular NP−polyelectrolyte interactions. Importantly, we observed that the adsorption layer of the weak polyelectrolyte PEI could alter its chain distribution by partial wrapping around the NPs, while no disturbance in APTES and PDDA monolayer by NP was observed. This was attributed to more labile binding of PEI to the solid substrate than for other adsorption promoters

Two Modes of Linear Layer-by-Layer Growth of Nanoparticle−Polylectrolyte Multilayers and Different Interactions in the Layer-by-layer Deposition

The structure of the multilayer assemblies of yttrium iron garnet nanoparticles (YIG) with polyelectrolytes was investigated with the emphasis on the control of the particle density in the adsorption layers. It was found that the growth of YIG films prepared by the layer-by-layer assembly can occur via two deposition modes:  (1) sequential adsorption of densely packed adsorption layers (normal growth mode) and (2) in-plane growth of isolated particle domains (lateral expansion mode). Importantly, the dependence of the optical density on the number of deposition cycles remains linear in both cases. Microscopy results indicate that the origin of the lateral growth is in the interplay of particle/particle and particle/polyelectrolyte interactions rather than in a substrate effect. The lateral expansion mode is a general attribute of the layer-by-layer deposition and can be observed for various aqueous colloids. For the preparation of sophisticated multifunctional assemblies on nanoparticles, the film growth via domain expansion should be avoided, and therefore, one must be able to control the growth pattern. The switch from lateral to normal growth mode can be effected by grafting charged organic groups to YIG nanoparticles. Hydrophobic interactions between the hydrocarbon groups of the modified YIG and polyelectrolyte significantly increase the attractive component of the particle/polyelectrolyte and particle/particle interactions. The films from modified YIG display densely packed nanoparticle layers with a greatly reduced number of defects

Electrical Stimulation of Neural Stem Cells Mediated by Humanized Carbon Nanotube Composite Made with Extracellular Matrix Protein

One of the key challenges to engineering neural interfaces is to minimize their immune response toward implanted electrodes. One potential approach is to manufacture materials that bear greater structural resemblance to living tissues and by utilizing neural stem cells. The unique electrical and mechanical properties of carbon nanotubes make them excellent candidates for neural interfaces, but their adoption hinges on finding approaches for “humanizing” their composites. Here we demonstrated the fabrication of layer-by-layer assembled composites from single-walled carbon nanotubes (SWNTs) and laminin, which is an essential part of human extracellular matrix. Laminin-SWNT thin films were found to be conducive to neural stem cells (NSC) differentiation and suitable for their successful excitation. We observed extensive formation of functional neural network as indicated by the presence of synaptic connections. Calcium imaging of the NSCs revealed generation of action potentials upon the application of a lateral current through the SWNT substrate. These results indicate that the protein−SWNT composite can serve as materials foundation of neural electrodes with chemical structure better adapted with long-term integration with the neural tissue

Traversing Material Scales: Macroscale LBL-Assembled Nanocomposites with Microscale Inverted Colloidal Crystal Architecture

Traversing Material Scales: Macroscale LBL-Assembled Nanocomposites with Microscale Inverted Colloidal Crystal Architectur

Simple Preparation Strategy and One-Dimensional Energy Transfer in CdTe Nanoparticle Chains

One-dimensional aggregates of CdTe nanoparticles were prepared by an exceptionally simple method of self-assembly initiated by partial removal of the stabilizing shell. The driving force of the particle self-organization is likely to be the dipole−dipole attraction between the nanoparticle cores. The obtained nanoparticle chains can be easily transferred on any substrate. Steady-state and time-resolved luminescent spectroscopes revealed strong Forster resonance energy transfer (FRET) between the particles, which led to the migration of excitation along the chain similarly to the waveguiding of light observed for chains of metal nanoparticles. The efficiency of FRET quenching in this one-dimensional system is comparable to that in three-dimensional packed nanoparticle solids despite a substantially smaller number of adjacent particles. The strong coupling of the donor and acceptor excited states is likely due to short inter-nanoparticle distances and partial ground-state dipole alignment taking place during the chain formation

Spontaneous CdTe → Alloy → CdS Transition of Stabilizer-Depleted CdTe Nanoparticles Induced by EDTA

CdTe nanoparticles stabilized by l-cysteine are chemically transformed into CdS nanoparticles of the same diameter via an intermediate CdTeS alloy without any auxiliary source of sulfur. The reaction is induced by ethylenediaminetetraacetic acid dipotassium salt dehydrate (EDTA), which was demonstrated experimentally to act as a catalyst by partially removing thiol stabilizers from the nanoparticle surface. It is hypothesized that addition of EDTA facilitates Te2- release, and oxidation of Te2- drives the nanoparticle transition process. Unlike many reports on reactions catalyzed by nanocolloids, this is likely to be the first observation of a catalytic reaction in which nanoparticles function as a substrate rather than a catalyst. It opens new pathways for the synthesis of novel nanoscale II−VI and other semiconductors and represents an interesting case of chemical processes in nanocolloids with reactivity increased by depletion of the surface layer of thiol stabilizers. This includes but is not limited to accurate control over the particle composition and crystallization rate. The slow rate of the CdTe → alloy → CdS transition is important for minimizing defects in the crystal lattice and results in a substantial increase of the quantum yield of photoluminescence over the course of the transition

Resonance Tunneling Diode Structures on CdTe Nanowires Made by Conductive AFM

Variation of band gap across the nanowire length would be an exceptionally attractive property for the fabrication of on-nanowire devices such as resonance tunneling diodes (RTD). Band gap variation can be achieved by selective thinning of semiconductor wires by scanning probe lithography (SPL) technique. The external bias applied to a conductive AFM tip during scanning of CdTe nanowires was chosen so as to exceed the threshold of electric field-assisted evaporation of CdTe, estimated to be 5.5 V. Relatively high external voltages of 10−11 V cause fast and complete disintegration of a nanowire portion under the tip. In this way the nanowire can be cut to a desired length. Selection of a voltage between 5.5 and 10 V allows one to control the speed of CdTe evaporation. Thus, one can modulate the thickness of the semiconductor with angstrom scale precision along the nanowire length. Smaller diameter of the nanowire results in increase of quantum confinement in selected areas. The double barrier quantum well valence band profile necessary for the manufacturing of RTD Esaki diodes was demonstrated

Layered Nanocomposites from Gold Nanoparticles for Neural Prosthetic Devices

Treatments of neurological diseases, diagnostics of brain malfunctions, and the realization of brain–computer interfaces require ultrasmall electrodes that are “invisible” to resident immune cells. Functional electrodes smaller than 50 μm are impossible to produce with traditional materials due to high interfacial impedance at the characteristic frequency of neural activity and insufficient charge storage capacity. The problem can be resolved by using gold nanoparticle nanocomposites. Careful comparison indicates that layer-by-layer assembled films from Au NPs provide more than 3-fold improvement in interfacial impedance and 1 order of magnitude increase in charge storage capacity. Prototypes of microelectrodes could be made using traditional photolithography. Integration of unique nanocomposite materials with microfabrication techniques opens the door for practical realization of the ultrasmall implantable electrodes. Further improvement of electrical properties is expected when using special shapes of gold nanoparticles