N-Methyl-2-pyrrolidone as a Reaction Medium for Gold(III)-Ion Reduction and Star-like Gold Nanostructure Formation.

The efficiency of a wet chemical route to synthesize gold nanostructures with tunable size and shape significantly depends on the applied solvent and the interaction of solvent molecules with other species such as gold ions. The ability of the organic solvent N-methyl-2-pyrrolidone (NMP) as a suitable medium for application in star-like gold nanostructure (AuNS) synthesis with a tunable morphology at ambient conditions has been investigated. The time-dependent analysis of the UV-vis absorption spectra of AuIIICl4 - in a pure NMP solution illustrates the role of NMP as simultaneous complexing and reducing agents. Kinetic studies indicate that AuIIICl4 - in NMP solution is reduced to AuICl2 -, with no need to use another reducing agent, any external energy sources, or solvent pretreatment. This is because AuI species stay stable in this solution unless poly(vinylpyrrolidone) (PVP) catalyzes their disproportionation. Morphological studies by transmission electron microscopy (TEM) specify the high-yield synthesis of AuNS with monocrystalline spikes in a concentrated NMP solution by PVP. This study illustrates that the presence of seeds, as another agent to catalyze the disproportionation of AuI species, makes it possible to synthesize AuNS in varying concentrations of PVP in this medium. The role of PVP concentration and the presence of seeds in the formation kinetics, morphology, and optical properties is systematically discussed. The results achieved through this study develop a straightforward and safe procedure for AuNS synthesis in high yield in a water-miscible organic polar solvent with tunable morphology and optical properties. Considering the high capability of NMP to dissolve various types of polymers and hydrophobic ligands, synthesizing AuNS in this solvent opens a window to a practical and easy way to fabricate gold-based nanomaterials with fascinating optical properties.

Simultaneous Nanolocal Polymer and In Situ Readout Unit Placement in Mesoporous Separation Layers.

Bioinspired solid-state nanopores and nanochannels have attracted interest in the last two decades, as they are envisioned to advance future sensing, energy conversion, and separation concepts. Although much effort has been made regarding functionalization of these materials, multifunctionality and accurate positioning of functionalities with nanoscale precision still remain challenging. However, this precision is necessary to meet transport performance and complexity of natural pores in living systems, which are often based on nonequilibrium states and compartmentalization. In this work, a nanolocal functionalization and simultaneous localized sensing strategy inside a filtering mesoporous film using precisely placed plasmonic metal nanoparticles inside mesoporous films with pore accessibility control is demonstrated. A single layer of gold nanoparticles is incorporated into mesoporous thin films with precise spatial control along the nanoscale layer thickness. The local surface plasmon resonance is applied to induce a photopolymerization leading to a nanoscopic polymer shell around the particles and thus nanolocal polymer placement inside the mesoporous material. As near-field modes are sensitive to the dielectric properties of their surrounding, the in situ sensing capability is demonstrated using UV-vis spectroscopy. It is demonstrated that the sensing sensitivity only slightly decreases upon functionalization. The presented nanolocal placement of responsive functional polymers into nanopores offers a simultaneous filtering and nanoscopic readout function. Such a nanoscale local control is envisioned to have a strong impact onto the development of new transport and sensor concepts, especially as the system can be developed into higher complexity using different metal nanoparticles and additional design of mesoporous film filtering properties.

3D NiCo-Layered double Hydroxide@Ni nanotube networks as integrated free-standing electrodes for nonenzymatic glucose sensing.

Nickel cobalt layered double hydroxide (NiCo-LDH)-based materials have recently emerged as catalysts for important electrochemical applications. However, they frequently suffer from low electrical conductivity and agglomeration, which in turn impairs their performance. Herein, we present a catalyst design based on integrated, self-supported nickel nanotube networks (Ni-NTNWs) loaded with NiCo-LDH nanosheets, which represents a binder-free, hierarchically nanostructured electrode architecture combining continuous conduction paths and openly accessible macropores of low tortuosity with an ultrahigh density of active sites. Similar to macroscale metallic foams, the NTNWs serve as three-dimensionally interconnected, robust frameworks for the deposition of active material, but are structured in the submicron range. Our synthesis is solely based on scalable approaches, namely templating with commercial track-etched membranes, electroless plating, and electrodeposition. Morphological and compositional characterization proved the successful decoration of the inner and outer nanotube surfaces with a conformal NiCo-LDH layer. Ni-NTNW electrodes and hydroxide-decorated variants showed excellent performance in glucose sensing. The highest activity was achieved for the catalyst augmented with NiCo-LDH nanosheets, which surpassed the modification with pure Ni(OH)2. Despite its low thickness of 20 µm, the optimized catalyst layer provided an outstanding sensitivity of 4.6 mA mM-1 cm-2, a low detection limit of 0.2 µM, a fast response time of 5.3 s, high selectivity and stability, and two linear ranges covering four orders of magnitude, up to 2.5 mM analyte. As such, derivatized interconnected metal nano-networks represent a promising design paradigm for highly miniaturized yet effective catalyst electrodes and electrochemical sensors.

In Situ Transmission Electron Microscopy Analysis of Thermally Decaying Polycrystalline Platinum Nanowires.

Owing to their large surface area, continuous conduction paths, high activity, and pronounced anisotropy, nanowires are pivotal for a wide range of applications, yet far from thermodynamic equilibrium. Their susceptibility toward degradation necessitates an in-depth understanding of the underlying failure mechanisms to ensure reliable performance under operating conditions. In this study, we present an in-depth analysis of the thermally triggered Plateau-Rayleigh-like morphological instabilities of electrodeposited, polycrystalline, 20-40 nm thin platinum nanowires using in situ transmission electron microscopy in a controlled temperature regime, ranging from 25 to 1100 °C. Nanowire disintegration is heavily governed by defects, while the initially present, frequent but small thickness variations do not play an important role and are overridden later during reshaping. Changes of the exterior wire morphology are preceded by shifts in the internal nanostructure, including grain boundary straightening, grain growth, and the formation of faceted voids. Surprisingly, the nanowires segregate into two domain types, one being single-crystalline and essentially void-free, while the other preserves void-pinned grain boundaries. While the single-crystalline domains exhibit fast Pt transport, the void-containing domains are unexpectedly stable, accumulate platinum by surface diffusion, and act as nuclei for the subsequent nanowire splitting. This study highlights the vital role of defects in Plateau-Rayleigh-like thermal transformations, whose evolution not only accompanies but guides the wire reshaping. Thus, defects represent strong parameters for controlling the nanowire decay and must be considered for devising accurate models and simulations.

Insights into the interplay of wetting and transport in mesoporous silica films.

The understanding and design of wetting-transport and wetting-charge-transport interplay in nanometer-sized pores is a still not fully understood key step in improving nanopore transport-related applications. A control of mesopore wettability accompanied by pore filling and ionic mesopore accessibility analysis is expected to deliver major insights into this interplay of nanoscale pore wetting and transport. For a systematic understanding, we demonstrate a gradual adjustment of nanopore ionic accessibility by gradually tuning silica nanopore wettability using chemical vapor phase deposition of 1H,1H,2H,2H-perfluorooctyl dimethylchlorosilane. The mutual influence of wetting on liquid imbibition, condensation, and molecular transport as well as on heat transfer were studied by ellipsometry, cyclic voltammetry and boiling experiments, respectively. A multi-methodical analytic approach was used to directly couple wetting properties of mesoporous silica thin films to ionic mesopore accessibility allowing us to determine two different ion transport mechanisms based on three defined wetting regimes as well as a threshold hydrophobicity suppressing pore accessibility. Furthermore, boiling experiments showed a clear increase in nucleation site density upon changing the wettability of the mesoporous surfaces from hydrophilic to hydrophobic. Hence, these results provide insights into the complex interplay of pore wall functionalization, wetting, and charge-dependent nanopore properties.

Conformal Solution Deposition of Pt-Pd Titania Nanocomposite Coatings for Light-Assisted Formic Acid Electro-Oxidation.

Many nanofabrication processes require sophisticated equipment, elevated temperature, vacuum or specific atmospheric conditions, templates, and exotic chemicals, which severely hamper their implementation in real-world applications. In this study, we outline a fully wet-chemical procedure for equipping a 3D carbon felt (CF) substrate with a multifunctional, titania nanospike-supported Pt-Pd nanoparticle (Pt-Pd-TiO2@CF) layer in a facile and scalable manner. The nanostructure, composition, chemical speciation, and formation of the material was meticulously investigated, evidencing the conformal coating of the substrate with a roughened layer of nanocrystalline rutile spikes by chemical bath deposition from Ti3+ solutions. The spikes are densely covered by bimetallic nanoparticles of 4.4 ± 1.1 nm in size, which were produced by autocatalytic Pt deposition onto Pd seeds introduced by Sn2+ ionic layer adsorption and reaction. The as-synthesized nanocomposite was applied to the (photo)electro-oxidation of formic acid (FA), exhibiting a superior performance compared to Pt-plated, Pd-seeded CF (Pt-Pd@CF) and commercial Pt-C, indicating the promoting electrocatalytic role of the TiO2 support. Upon UV-Vis illumination, the performance of the Pt-Pd-TiO2@CF electrode is remarkably increased (22-fold), generating a current density of 110 mA cm-2, distinctly outperforming titania-free Pt-Pd@CF (5 mA cm-2) and commercial Pt-C (6 mA cm-2) reference catalysts. In addition, the Pt-Pd-TiO2@CF showed a much better stability, characterized by a very high poisoning tolerance for in situ-generated CO intermediates, whose formation is hindered in the presence of TiO2. This overall performance boost is attributed to a dual enhancement mechanism (∼30% electrocatalytic and ∼70% photoelectrocatalytic). The photogenerated electrons from the TiO2 conduction band enrich the electron density of the Pt nanoparticles, promoting the generation of active oxygen species on their surfaces from adsorbed oxygen and water molecules, which facilitate the direct FA electro-oxidation into CO2.

Shape-Selective Electroless Plating within Expanding Template Pores: Etching-Assisted Deposition of Spiky Nickel Nanotube Networks.

Nano-objects are favored structures for applications such as catalysis and sensing. Although they already provide a large surface-to-volume ratio, this ratio can be further increased by shape-selective plating of the nanostructure surfaces. This process combines the conformity of autocatalytic deposition with the defined nucleation and growth characteristics of colloidal nanoparticle syntheses. However, many aspects of such reactions are still not fully understood. In this study, we investigate in detail the growth of spiky nickel nanotubes in polycarbonate template membranes. One distinctive feature of our synthesis is the simultaneous growth of nanospikes on both the inside and outside of nanotubes while the tubes are still embedded in the polymer. This is achieved by combining the plating process with locally enhanced in situ etching of the poylmer template, for which we propose a theory. Electron microscopy investigations reveal twinning defects as the driving force for the growth of crystalline nanospikes. Deposit crystallinity is ensured by the reducing agent hydrazine. Iminodiacetic acid is not only used as a complexing agent during synthesis but apparently also acts as a capping agent and limits random nucleation on the spike facets. Finally, we apply our synthesis to templates with interconnected pores to obtain free-standing spiky nickel nanotube networks, demonstrating its ability to homogeneously coat substrates with extended inner surfaces and to operate in nanoscale confinement.

Free-Standing Networks of Core-Shell Metal and Metal Oxide Nanotubes for Glucose Sensing.

Nanotube assemblies represent an emerging class of advanced functional materials, whose utility is however hampered by intricate production processes. In this work, three classes of nanotube networks (monometallic, bimetallic, and metal oxide) are synthesized solely using facile redox reactions and commercially available ion track membranes. First, the disordered pores of an ion track membrane are widened by chemical etching, resulting in the formation of a strongly interconnected pore network. Replicating this template structure with electroless copper plating yields a monolithic film composed of crossing metal nanotubes. We show that the parent material can be easily transformed into bimetallic or oxidic derivatives by applying a second electroless plating or thermal oxidation step. These treatments retain the monolithic network structure but result in the formation of core-shell nanotubes of altered composition (thermal oxidation: Cu2O-CuO; electroless nickel coating: Cu-Ni). The obtained nanomaterials are applied in the enzyme-free electrochemical detection of glucose, showing very high sensitivities between 2.27 and 2.83 A M-1 cm-2. Depending on the material composition, varying reactivities were observed: While copper oxidation reduces the response to glucose, it is increased in the case of nickel modification, albeit at the cost of decreased selectivity. The performance of the materials is explained by the network architecture, which combines the advantages of one-dimensional nano-objects (continuous conduction pathways, high surface area) with those of a self-supporting, open-porous superstructure (binder-free catalyst layer, efficient diffusion). In summary, this novel synthetic approach provides a fast, scalable, and flexible route toward free-standing nanotube arrays of high compositional complexity.

Fluid Flow Programming in Paper-Derived Silica-Polymer Hybrids.

In paper-based devices, capillary fluid flow is based on length-scale selective functional control within a hierarchical porous system. The fluid flow can be tuned by altering the paper preparation process, which controls parameters such as the paper grammage. Interestingly, the fiber morphology and nanoporosity are often neglected. In this work, porous voids are incorporated into paper by the combination of dense or mesoporous ceramic silica coatings with hierarchically porous cotton linter paper. Varying the silica coating leads to significant changes in the fluid flow characteristics, up to the complete water exclusion without any further fiber surface hydrophobization, providing new approaches to control fluid flow. Additionally, functionalization with redox-responsive polymers leads to reversible, dynamic gating of fluid flow in these hybrid paper materials, demonstrating the potential of length scale specific, dynamic, and external transport control.

Self-Supporting Metal Nanotube Networks Obtained by Highly Conformal Electroless Plating.

We present a versatile approach for the fabrication of well-defined networks of interconnected metal nanotubes, which applies electroless plating to ion-track-etched polymer templates that enclose designed pore networks. In order to obtain self-supporting structures, the deposition reactions must be optimized to yield conformal nanoscale metal films on microstructured substrates possessing extensive inner surfaces. Using this route, gold, copper, silver, nickel, and platinum nanotube networks are synthesized. The resulting structures can be handled macroscopically and combine a large surface area with continuous mass transport and conduction pathways, rendering them promising for application in, for example, electrocatalysis and sensing. This potential is demonstrated by employing a gold nanotube network for the amperometric detection of hydrogen peroxide, in which excellent sensitivity, catalyst utilization, and stability is achieved.

Metal nanotubes and nanowires with rhombohedral cross-section electrolessly deposited in mica templates.

Electroless plating is a facile wet-chemical process for the creation of metal thin films on arbitrary substrates, which can be used to produce intricate nanomaterials. In this study, we demonstrate how nanotubes and nanowires can be electrolessly deposited in the rhombohedral pores of ion-track etched muscovite mica templates. Mutual optimization of the activation and plating reactions proved to be essential for the fabrication of well-defined nanostructures of an aspect ratio (length-to-diameter) of up to approximately 70. By repeating the activation procedure utilizing the redox couple Sn(II) and Ag(I), a high density of Ag nanoparticle seeds could be deposited on the template surface, which was required to initiate metal film nucleation with nanoscale homogeneity. Furthermore, it was necessary to adapt the plating reaction to ensure sufficient diffusion of the reagents into the depth of the template pores. To prove the flexibility of the process and to evaluate the effect of the intrinsic film morphology on the shape of the resulting nanostructures, three different plating reactions were applied (Ag, Au, Pt). If the size of the deposited metal particles approached the dimension of the template pores, only wire-like structures of moderate shape conformity were obtained. Electroless plating protocols which yield homogeneous deposits consisting of small nanoparticles allowed exact replication of the pore shape. Under consideration of the above-mentioned requirements, electroless plating displays an effective and versatile route toward the fabrication of parallel arrays of angular metal nanotubes and nanowires in the chemically and thermally robust mica templates. By simply immersing the templates in aqueous plating solutions for an appropriate time, well-defined metal nanomaterials for application in, for example, plasmonics, catalysis, or molecular separation are obtained.

Synthesis, solid-state NMR characterization, and application for hydrogenation reactions of a novel Wilkinson's-type immobilized catalyst.

Silica nanoparticles (SiNPs) were chosen as a solid support material for the immobilization of a new Wilkinson's-type catalyst. In a first step, polymer molecules (poly(triphenylphosphine)ethylene (PTPPE); 4-diphenylphosphine styrene as monomer) were grafted onto the silica nanoparticles by surface-initiated photoinferter-mediated polymerization (SI-PIMP). The catalyst was then created by binding rhodium (Rh) to the polymer side chains, with RhCl3⋅x H2O as a precursor. The triphenylphosphine units and rhodium as Rh(I) provide an environment to form Wilkinson's catalyst-like structures. Employing multinuclear ((31)P, (29)Si, and (13)C) solid-state NMR spectroscopy (SSNMR), the structure of the catalyst bound to the polymer and the intermediates of the grafting reaction have been characterized. Finally, first applications of this catalyst in hydrogenation reactions employing para-enriched hydrogen gas (PHIP experiments) and an assessment of its leaching properties are presented.

A new route towards nanoporous TiO2 as powders or thin films from the thermal treatment of titanium-based hybrid materials.

Calcination of cyclopentadienyltitanium-based organic-inorganic hybrid materials at 450-500 °C led to the formation of anatase titanium dioxide as white powders consisting of a porous network of aggregated nanoparticles, the nanoporosity detected being related to the inter-particle space. Depending on the calcination temperatures, the surface area of the titanium dioxide particles varied from 65 to 158 m(2) g(-1).

Ligand-optimized electroless synthesis of silver nanotubes and their activity in the reduction of 4-nitrophenol.

A facile electroless plating procedure for the controlled synthesis of nanoscale silver thin films and derived structures such as silver nanotubes was developed and the products were characterized by SEM, TEM and EDS. The highly stable plating baths consist of AgNO(3) as the metal source, a suitable ligand and tartrate as an environmentally benign reducing agent. Next to the variation of the coordinative environment of the oxidizing component, the influence of the pH value was evaluated. These two governing factors strongly affect the plating rate and the morphology of the developing silver nanoparticle films and can be used to adapt the reaction to synthetic demands. The refined electroless deposition allows the fabrication of homogeneous high aspect-ratio nanotubes in ion track etched polycarbonate. Template-embedded metal nanotubes can be interpreted as parallelled microreactors. Following this concept, both the silver nanotubes and spongy gold nanotubes obtained by the use of the silver structures as sacrificial templates were applied in the reduction of 4-nitrophenol by sodium borohydride, proving to be extraordinarily effective catalysts.

4-(Dimethylamino)pyridine as a powerful auxiliary reagent in the electroless synthesis of gold nanotubes.

Gold nanotubes of small particle sizes down to 5 nm and high aspect ratios were synthesized in ion track etched polycarbonate following a rational reaction design. 4-(Dimethylamino)pyridine (DMAP) was employed to adjust the electroless deposition by interfering with the autocatalytically active gold surface. Modification of the pH value and DMAP concentration led to a wide range of products which were characterized by SEM, TEM, and EDS. Filigree nanotubes of 10-15 nm wall thickness and 5.0 ± 2.1 nm grain size were obtained as well as robust and free-standing structures proving homogeneous deposition along the whole template length of 30 µm. Template-supported gold nanotubes were applied in the UV-vis monitored reduction of 4-nitrophenol by sodium borohydride under pseudo-first-order conditions. They proved to be a reliable microfluidic system of excellent catalytic activity coming up with an apparent rate constant of 1.3 × 10(-2) s(-1). Despite a high flow rate, the reaction showed 99% conversion after a distance of just 60 µm.

Determination of the subcellular distribution of mercaptoundecahydro-closo-dodecaborate (BSH) in human glioblastoma multiforme by electron microscopy.

The subcellular distribution of mercaptoundecahydro-closo-dodecaborate (BSH) in glioblastoma multiforme tissue sections of several patients having received BSH prior to surgery was investigated by transmission electron microscopy (TEM) using antibodies against BSH and electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). These microscopic techniques show that BSH is associated with extracellular structures, the cell membrane as well as with the chromatin in the nucleus.