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

Anomalously Fast Diffusion of Targeted Carbon Nanotubes in Cellular Spheroids

Understanding transport of carbon nanotubes (CNTs) and other nanocarriers within tissues is essential for biomedical imaging and drug delivery using these carriers. Compared to traditional cell cultures in animal studies, three-dimensional tissue replicas approach the complexity of the actual organs and enable high temporal and spatial resolution of the carrier permeation. We investigated diffusional transport of CNTs in highly uniform spheroids of hepatocellular carcinoma and found that apparent diffusion coefficients of CNTs in these tissue replicas are anomalously high and comparable to diffusion rates of similarly charged molecules with molecular weights 10000× lower. Moreover, diffusivity of CNTs in tissues is enhanced after functionalization with transforming growth factor β1. This unexpected trend contradicts predictions of the Stokes−Einstein equation and previously obtained empirical dependences of diffusivity on molecular mass for permeants in gas, liquid, solid or gel. It is attributed to the planar diffusion (gliding) of CNTs along cellular membranes reducing effective dimensionality of diffusional space. These findings indicate that nanotubes and potentially similar nanostructures are capable of fast and deep permeation into the tissue, which is often difficult to realize with anticancer agents

Streptavidin Inhibits Self-Assembly of CdTe Nanoparticles

Nanoparticles (NPs) exhibit strong tendency to self-assemble. It is important to understand how the presence of other macromolecular compounds, affects this ability. The interaction between standard thiol-capped cadmium telluride (CdTe) NPs and streptavidin (STAV)the essential component of many NP applicationswas examined at different molar ratios and pH values. The central observation of this study is that STAV strongly inhibits the self-assembly of CdTe NPs into nanowires. The underlying mechanism of inhibition was attributed to the formation of a STAV corona and surface layer that precludes attachment of NPs to each other. Instead of nanowires, we observed a spectrum of agglomerates containing both CdTe and STAV of different geometries depending on the molar ratios of the reagents in NP synthesis and pH values of the media

Nanoparticle Assemblies into Luminescent Dendrites in Shrinking Microdroplets

The self-assembly of nanoparticles (NPs) is essential for emerging dispersion-based energy-conscious technologies. Of particular interest are micro- and macro-scale self-organizing superstructures that can bridge 2D/3D processing scales. Here we report the spontaneous assembly of CdTe NPs within an aqueous microdroplet suspended in soybean oil. The gradual diffusion of the water into the surrounding medium results in shrinking of the microdroplet, and a concomitant formation of branched assemblies from CdTe NPs that evolve in size from ∼50 μm to ∼1000 μm. The fractal dimension of NP assemblies increases from ∼1.7 to ∼1.9 during the assembly process. We found that constituents of the soybean oil enter the aqueous solution across the microdroplet interface and affect NP assembly. The obtained NP dendrites can be further altered morphologically by illumination with light that results in the disassembly of the NP dendrites. The use of this microheterogeneous dispersion platform with partially soluble hydrophilic and hydrophobic solvents highlights the sensitivity of the NP assembly process to environment and presents an opportunity to explore droplet-confined NP assembly

Omnidispersible Hedgehog Particles with Multilayer Coatings for Multiplexed Biosensing

Hedgehog particles (HPs) replicating the spiky geometry of pollen grains revealed surprisingly high dispersion stability regardless of whether their hydrophobicity/hydrophilicity matches that of the media or not. This property designated as omnidispersibility is attributed to the drastic reduction of van der Waals interactions between particles coated with stiff nanoscale spikes as compared to particles of the same dimensions with smooth surfaces. One may hypothesize but it remains unknown, however, whether HPs modified with polymers or nanoparticles (NPs) would retain this property. Surface modifications of the spikes will expand the functionalities of HPs, making possible their utilization as omnidispersible carriers. Here, we show that HPs carrying dense conformal coatings made by layer-by-layer (LBL/LbL) assembly maintain dispersion stability in environments of extreme polarity and ionic strength. HPs, surface-modified by multilayers of polymers and gold NPs, are capable of surface-enhanced Raman scattering (SERS) and overcome the limited colloidal stability of other SERS probes. The agglomeration resilience of HPs leads to a greater than one order of magnitude increase of SERS intensity as compared to colloids with smooth surfaces and enables simultaneous detection of several targets in complex media with high ionic strength. Omnidispersible optically active colloids open the door for rapid multiplexed SERS analysis in biological fluids and other applications

Pseudonegative Thermal Expansion and the State of Water in Graphene Oxide Layered Assemblies

Unraveling the complex interplay between thermal properties and hydration is a part of understanding the fundamental properties of many soft materials and very essential for many applications. Here we show that graphene oxide (GO) demonstrates a highly negative thermal expansion (NTE) coefficient owing to unique thermohydration processes related with fast transport of water between the GO sheets, the amphiphilic nature of nanochannels, and close-to-zero intrinsic thermal expansion of GO. The humidity-dependent NTE of GO layered assemblies, or “pseudonegative thermal expansion” (PNTE), differs from that of other hygroscopic materials due to its relatively fast and highly reversible expansion/contraction cycles and occurrence at low humidity levels while bearing similarities to classic NTE. Thermal expansion of polyvinyl alcohol/GO composites is easily tunable with additional intricacy of thermohydration effects. PNTE combined with isotropy, nontoxicity, and mechanical robustness is an asset for applications of actuators, sensors, MEMS devices, and memory materials and crucial for developing methods of thermal/photopatterning of GO devices

Pushing the Limits: 3D Layer-by-Layer-Assembled Composites for Cathodes with 160 C Discharge Rates

Deficiencies of cathode materials severely limit cycling performance and discharge rates of Li batteries. The key problem is that cathode materials must combine multiple properties: high lithium ion intercalation capacity, electrical/ionic conductivity, porosity, and mechanical toughness. Some materials revealed promising characteristics in a subset of these properties, but attaining the entire set of often contrarian characteristics requires new methods of materials engineering. In this paper, we report high surface area 3D composite from reduced graphene oxide loaded with LiFePO₄ (LFP) nanoparticles made by layer-by-layer assembly (LBL). High electrical conductivity of the LBL composite is combined with high ionic conductivity, toughness, and low impedance. As a result of such materials properties, reversible lithium storage capacity and Coulombic efficiency were as high as 148 mA h g^–1 and 99%, respectively, after 100 cycles at 1 C. Moreover, these composites enabled unusually high reversible charge–discharge rates up to 160 C with a storage capacity of 56 mA h g^–1, exceeding those of known LFP-based cathodes, some of them by several times while retaining high content of active cathode material. The study demonstrates that LBL-assembled composites enable resolution of difficult materials engineering tasks

Shape-Dependent Biomimetic Inhibition of Enzyme by Nanoparticles and Their Antibacterial Activity

Enzyme inhibitors are ubiquitous in all living systems, and their biological inhibitory activity is strongly dependent on their molecular shape. Here, we show that small zinc oxide nanoparticles (ZnO NPs)pyramids, plates, and spherespossess the ability to inhibit activity of a typical enzyme β-galactosidase (GAL) in a biomimetic fashion. Enzyme inhibition by ZnO NPs is reversible and follows classical Michaelis–Menten kinetics with parameters strongly dependent on their geometry. Diverse spectroscopic, biochemical, and computational experimental data indicate that association of GAL with specific ZnO NP geometries interferes with conformational reorganization of the enzyme necessary for its catalytic activity. The strongest inhibition was observed for ZnO nanopyramids and compares favorably to that of the best natural GAL inhibitors while being resistant to proteases. Besides the fundamental significance of this biomimetic function of anisotropic NPs, their capacity to serve as degradation-resistant enzyme inhibitors is technologically attractive and is substantiated by strong shape-specific antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), endemic for most hospitals in the world

Template-Free Hierarchical Self-Assembly of Iron Diselenide Nanoparticles into Mesoscale Hedgehogs

The ability of semiconductor nanoparticles (NPs) to self-assemble has been known for several decades. However, the limits of the geometrical and functional complexity for the self-assembled nanostructures made from simple often polydispersed NPs are still continuing to amaze researchers. We report here the self-assembly of primary ∼2–4 nm FeSe₂ NPs with puck-like shapes into either (a) monocrystalline nanosheets ∼5.5 nm thick and ∼1000 nm in lateral dimensions or (b) mesoscale hedgehogs ∼550 nm in diameter with spikes of ∼250 nm in length, and ∼10–15 nm in diameter, the path of the assembly is determined by the concentration of dodecanethiol (DT) in the reaction media. The nanosheets represent the constitutive part of hedgehogs. They are rolled into scrolls and assembled around a single core with distinct radial orientation forming nanoscale “needles” approximately doubling its fractal dimension of these objects. The core is assembled from primary NPs and nanoribbons. The size distribution of the mesoscale hedgehogs can be as low as 3.8%, indicating a self-limited mechanism of the assembly. Molecular dynamics simulation indicates that the primary FeSe₂ particles have mobile edge atoms and asymmetric basal surfaces. The top-bottom asymmetry of the puck-like NPs originates from the Fe-rich/Se-rich stripes on the (011) surface of the orthorhombic FeSe₂ crystal lattice, displaying 2.7 nm periodicity that is comparable to the lateral size of the primary NPs. As the concentration of DT increases, the NPs bind to additional metal sites, which increases the chemical and topographic asymmetry and switches the assembly pathways from nanosheets to hedgehogs. These results demonstrate that the self-assembly of NPs with non-biological surface ligands and without any biological templates results in morphogenesis of inorganic superstructures with complexity comparable to that of biological assemblies, for instance mimivirus. The semiconductor nature of FeSe₂ hedgehogs enables their utilizations in catalysis, drug delivery, optics, and energy storage

Universal Synthesis of Single-Phase Pyrite FeS₂ Nanoparticles, Nanowires, and Nanosheets

Nanoscale pyrite FeS₂ is considered to be one of few potentially transformative materials for photovoltaics capable of bridging the cost/performance gap of solar batteries. It also holds promise for energy storage applications as the material for high-performance cathodes. Despite prospects, the synthesis of FeS₂ nanostructures and diversity of their geometries has been hardly studied. Moreover, the state-of-the-art aqueous dispersions of nanoscale pyrite, which have special significance for solar energetics, are particularly disappointing due to low quality. There are no known methods to produce well-crystallized nanoparticles and other geometries of nanoscale pyrite in water or mixed aqueous solvents. Here, we describe a successful synthesis of single-phase pyrite nanoparticles with a diameter of 2–5 nm in polar solvent and aqueous dispersions. The particles display high uniformity and crystallographic purity. Moreover, the synthetic approach developed for nanoparticles was proven to be quite universal and can be modified to produce both nanowires and nanosheets, which also display high crystallinity. The diameter of the pyrite nanowires was 80–120 nm with the length exceeding 5 μm. The nanosheets displayed lateral dimensions of 100–200 nm with the thickness of 2 nm. Availability of single-phase FeS₂ nanoscale aqueous dispersions is expected to stimulate further studies of these materials in green energy conversion technologies and drug delivery applications