Click-Functionalization of Silanized Carbon Nanotubes: From Inorganic Heterostructures to Biosensing Nanohybrids.

Here we present an approach to functionalize silanized single-walled carbon nanotubes (SWNTs) through copper-free click chemistry for the assembly of inorganic and biological nanohybrids. The nanotube functionalization route involves silanization and strain-promoted azide-alkyne cycloaddition reactions (SPACC). This was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Fourier transform infra-red spectroscopy. Silane-azide-functionalized SWNTs were immobilized from solution onto patterned substrates through dielectrophoresis (DEP). We demonstrate the general applicability of our strategy for the functionalization of SWNTs with metal nanoparticles (gold nanoparticles), fluorescent dyes (Alexa Fluor 647) and biomolecules (aptamers). In this regard, dopamine-binding aptamers were conjugated to the functionalized SWNTs to perform real-time detection of dopamine at different concentrations. Additionally, the chemical route is shown to selectively functionalize individual nanotubes grown on the surface of silicon substrates, contributing towards future nano electronic device applications.

Dispersible SnO2 :Sb and TiO2 Nanocrystals After Calcination at High Temperature.

High-temperature treatment of functional nanomaterials, through postsynthesis calcination, often represents an important step to unlock their full potential. However, such calcination steps usually severely limit the preparation of colloidal solutions of the nanoparticles due to the formation of sintered agglomerates. Herein, a simple route is reported to obtain colloidal solutions of calcined n-conductive antimony doped tin oxide (ATO) as well as titanium dioxide (TiO2 ) nanoparticles without the need for additional sacrificial materials. This is achieved by making use of the reduced contact between individual nanoparticles when they are assembled into aerogels. Following the calcination of the aerogels at 500 °C, redispersion of the nanoparticles into stable colloidal solutions with various solvents can be achieved. Although a slight degree of sintering is inevitable, the size of the resulting aggregates in solution is still remarkably small with values below 30 nm.

Reactive Additive Capillary Stamping with Double Network Hydrogel-Derived Aerogel Stamps under Solvothermal Conditions.

Integration of solvothermal reaction products into complex thin-layer architectures is frequently achieved by combinations of layer transfer and subtractive lithography, whereas direct additive substrate patterning with solvothermal reaction products has remained challenging. We report reactive additive capillary stamping under solvothermal conditions as a parallel contact-lithographic access to patterns of solvothermal reaction products in thin-layer configurations. To this end, corresponding precursor inks are infiltrated into mechanically robust mesoporous aerogel stamps derived from double-network hydrogels. The stamp is then brought into contact with a substrate to be patterned under solvothermal reaction conditions inside an autoclave. The precursor ink forms liquid bridges between the topographic surface pattern of the stamp and the substrate. Evaporation-driven enrichment of the precursors in these liquid bridges, along with their liquid-bridge-guided conversion into the solvothermal reaction products, yields large-area submicron patterns of the solvothermal reaction products replicating the stamp topography. For example, we prepared thin hybrid films, which contained ordered monolayers of superparamagnetic submicron nickel ferrite dots prepared by solvothermal capillary stamping surrounded by nickel electrodeposited in a second orthogonal substrate functionalization step. The submicron nickel ferrite dots acted as a magnetic hardener, halving the remanence of the ferromagnetic nickel layer. In this way, thin-layer electromechanical systems, transformers, and positioning systems may be customized.

Intercalation-free, fast switching of mesoporous antimony doped tin oxide with cathodically coloring electrochromic dyes.

Mesoporous nanoparticle layers of transparent conductive oxides (TCOs) with anchored organic dyes are of great interest for electrochromic applications. Herein, we prepared mesoporous layers of antimony doped tin oxide (ATO) consisting of only 5 nm large particles with a low Sb concentration (2% antimony). The particles were prepared via a modified synthesis procedure based on hexahydroxostannate and pure Sb(v) hexahydroxoantimonate(v). We show that the ATO layers benefit from using a non-intercalating electrolyte such as tetrabutylammonium perchlorate (TBAP) compared to lithium perchlorate. Especially in the negative potential range, negative side effects, such as degradation due to lithium intercalation, are reduced. Furthermore, comparing the behavior of particles with varying antimony doping concentrations showed that the particles doped with 2% Sb are most suitable with respect to their conductivity and transparency. When modified with an electrochromic dye (viologen), the hybrid electrodes allow fully reversible (de)coloration with the non-intercalating electrolyte. Similar viologen/TiO2 electrodes on the other hand show severely restricted performance with the non-intercalating electrolyte as the oxidation of the dye is partially inhibited. Finally, we built a full electrochromic device composed of two ATO electrodes, each bearing a different electrochromic dye with TBAP as the electrolyte. Despite the dense morphology of the layers due to the small particle size as well as the large size of the electrolyte cation, the device displays remarkable switching times below 0.5 s.

Phenolic Resin Dual-Use Stamps for Capillary Stamping and Decal Transfer Printing.

We report an optimized two-step thermopolymerization process carried out in contact with micropatterned molds that yields porous phenolic resin dual-use stamps with topographically micropatterned contact surfaces. With these stamps, two different parallel additive substrate manufacturing methods can be executed: capillary stamping and decal transfer microlithography. Under moderate contact pressures, the porous phenolic resin stamps are used for nondestructive ink transfer to substrates by capillary stamping. Continuous ink supply through the pore systems to the contact surfaces of the porous phenolic resin stamps enables multiple successive stamp-substrate contacts for lithographic ink deposition under ambient conditions. No deterioration of the quality of the deposited pattern occurs, and no interruptions for ink replenishment are required. Under a high contact pressure, porous phenolic resin stamps are used for decal transfer printing. In this way, the tips of the stamps' contact elements are lithographically transferred to counterpart substrates. The granular nature of the phenolic resin facilitates the rupture of the contact elements upon stamp retraction. The deposited phenolic resin micropatterns characterized by abundance of exposed hydroxyl groups are used as generic anchoring sites for further application-specific functionalizations. As an example, we deposited phenolic resin micropatterns on quartz crystal microbalance resonators and further functionalized them with polyethylenimine for preconcentration sensing of humidity and gaseous formic acid. We envision that also preconcentration coatings for other sensing methods, such as attenuated total reflection infrared spectroscopy and surface plasmon resonance spectroscopy, are accessible by this functionalization algorithm.

Diffraction-Unlimited Photomanipulation at the Plasma Membrane via Specifically Targeted Upconversion Nanoparticles.

Engineered UCNP are used to trigger rapid photoconversion of the fluorescent protein Dendra2 with nanoscopic precision and over longer distances in mammalian cells. By exploiting the synergy of high-level thulium doping with core-shell design and elevated excitation intensities, intense UCNP emission is achieved, allowing fast photoconversion of Dendra2 with <10 nm resolution. A tailored biocompatible surface coating and functionalization with a derivate of green fluorescent protein (GFP) for recognition of antiGFP nanobodies are developed. Highly specific targeting of UCNP to fusion proteins of antiGFP on the surface of mammalian cells is demonstrated. UCNP bound to extracellular Dendra2 enable rapid photoconversion selectively in molecular proximity and thus unambiguous detection of cytokine receptor dimerization in the plasma membrane and in endosomes. Remarkably, UCNPs are also suited for manipulating intracellular Dendra2 across the plasma membrane. This study thus establishes UCNP-controlled photomanipulation with nanoscale precision, opening exciting opportunities for bioanalytical applications in cell biology.

The role of cations in hydrothermal synthesis of nonlinear optical sodium niobate nanocrystals.

The usability of the alkali niobates with their ferroelectric and photorefractive properties could be expanded if the development of synthesis methods would allow to obtain small, preferably monodispersed, crystals in the sub-µm to nanometer regime. Of all the possible synthesis methods, the most reliable is currently hydrothermal synthesis to generate small crystallite sizes of these materials. Although the products of sodium niobate are polydisperse and partially agglomerated, they show a significant SHG signal that is unexpectedly comparable to that of potassium niobate. A view on the hydrothermal synthesis of sodium niobate reveals that the incorporation of cations in the crystalline lattice of the niobium educt plays a part in the formation of the product. The occurrence of distinct different phases, as in the case of potassium niobate, is not observed. Instead, it is shown that a clear assignment of the crystalline phase cannot be made here. This indicates that crystallization of the alkali niobates in hydrothermal synthesis depends on the stoichiometry, the niobium starting material and the cation used.

Electrochromic graduated filters with symmetric electrode configuration.

Graduated optical filters are commonly used for spatial image control as they are capable of darkening the overexposed parts of the image specifically. However, they lack flexibility because each filter has a fixed transmission distribution. We herein present a fully controllable graduated filter based on the electrochromic device. Its graduated transmission distribution can be spatially controlled by the application of multiple electric potentials. In this way, the control of the gradient's position and its width, transmission and angular orientation is possible. Simulation of both the spatial potential distribution and the resultant optical absorption distribution are conducted to optimize the electrode configuration and furthermore to derive a control dataset that facilitates the adjustment and thus the application of the graduated filter. Based on three objective and quantitative criteria, we identify the electrode configuration with the highest flexibility in all four controls, manufacture the device using a gravure printing process for the nanoparticle electrodes and show its successful application.

Colloidal LaPO4:Gd3+ nanocrystals: X-ray induced single line UV emission.

Colloidal solutions of nearly monodisperse 5 nm LaPO4:Gd3+ nanocrystals are shown to strongly emit UV radiation upon excitation with tungsten Kα radiation (59.3 keV) or vacuum UV radiation (160 nm). The UV emission of the particles consists mainly of a single line at 311 nm corresponding to the 6P7/2-8S7/2 transition of Gd3+. The highest emission intensity is observed for LaPO4 nanocrystals with a Gd3+ concentration of 20%. Since the absorption cross section of biomaterials is low for X-rays but high for 311 nm radiation, the UV emission of particles embedded in the biological tissue can only affect the direct vicinity of the particles. Nanocrystals of LaPO4:Gd3+ could, therefore, be interesting for biomedical applications such as strongly localized drug release by X-ray triggered UV uncaging reactions.

Nonlinear optical potassium niobate nanocrystals as harmonic markers: the role of precursors and stoichiometry in hydrothermal synthesis.

Nanocrystals of alkaline niobates are currently being discussed for various applications because of their diverse and remarkable properties. Although the growth of bulk niobate crystals is well established, little is known about respective nanocrystals and the optical properties of niobates below 100 nm. A systematic view of the hydrothermal synthesis of potassium niobate with respect to the precursor species reveals the sensitive dependence of the resulting crystalline phases and sizes on the educt modifications. With a variation of stoichiometry of the procedure, the product modification and crystallite size can be changed. By means of second harmonic generation, nanocrystalline potassium niobate offers the possibility for use as an optical marker in high resolution nonlinear microscopy. Redispersed particles show a significant second harmonic generation signal throughout the visible spectral range.

NaYF4 :Yb,Er/NaYF4 Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield.

Upconversion core/shell nanocrystals with different mean sizes ranging from 15 to 45 nm were prepared via a modified synthesis procedure based on anhydrous rare-earth acetates. All particles consist of a core of NaYF4 :Yb,Er, doped with 18 % Yb3+ and 2 % Er3+ , and an inert shell of NaYF4 , with the shell thickness being equal to the radius of the core particle. Absolute measurements of the photoluminescence quantum yield at a series of different excitation power densities show that the quantum yield of 45 nm core/shell particles is already very close to the quantum yield of microcrystalline upconversion phosphor powder. Smaller core/shell particles prepared by the same method show only a moderate decrease in quantum yield. The quantum yield of 15 nm core/shell particles, for instance, is reduced by a factor of three compared to the bulk upconversion phosphor at high power densities (100 W cm-2 ) and by approximately a factor of 10 at low power densities (1 W cm-2 ).

Engineered Upconversion Nanoparticles for Resolving Protein Interactions inside Living Cells.

Upconversion nanoparticles (UCNPs) convert near-infrared into visible light at much lower excitation densities than those used in classic two-photon absorption microscopy. Here, we engineered <50 nm UCNPs for application as efficient lanthanide resonance energy transfer (LRET) donors inside living cells. By optimizing the dopant concentrations and the core-shell structure for higher excitation densities, we observed enhanced UCNP emission as well as strongly increased sensitized acceptor fluorescence. For the application of these UCNPs in complex biological environments, we developed a biocompatible surface coating functionalized with a nanobody recognizing green fluorescent protein (GFP). Thus, rapid and specific targeting to GFP-tagged fusion proteins in the mitochondrial outer membrane and detection of protein interactions by LRET in living cells was achieved.

Characterization of multifunctional ß-NaEuF4/NaGdF4 core-shell nanoparticles with narrow size distribution.

The properties of ß-NaEuF4/NaGdF4 core-shell nanocrystals have been thoroughly investigated. Nanoparticles with narrow size distribution and an overall diameter of ∼22 nm have been produced with either small ß-NaEuF4 cores (∼3 nm diameter) or large ß-NaEuF4 cores (∼18 nm diameter). The structural properties and core-shell formation are investigated by X-ray diffraction, transmission electron microscopy and electron paramagnetic resonance, respectively. Optical luminescence measurements and X-ray photoelectron spectroscopy are employed to gain information about the optical emission bands and valence states of the rare earth constituents. Magnetic characterization is performed by SQUID and X-ray magnetic circular dichroism measurements at the rare earth M(4,5) edges. The characterization of the core-shell nanoparticles by means of these complementary techniques demonstrates that partial intermixing of core and shell materials takes place, and a significant fraction of europium is present in the divalent state which has significant influence on the magnetic properties. Hence, we obtained a combination of red emitting Eu(3+) ions and paramagnetic Gd(3+) ions, which may be highly valuable for potential future applications.

Synthesis of 10†nm ß-NaYF4:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5†nm Particle Cores.

A new method is presented for preparing gram amounts of very small core/shell upconversion nanocrystals without additional codoping of the particles. First, ca. 5 nm ß-NaYF4:Yb,Er core particles are formed by the reaction of sodium oleate, rare-earth oleate, and ammonium fluoride, thereby making use of the fact that a high ratio of sodium to rare-earth ions promotes the nucleation of a large number of ß-phase seeds. Thereafter, a 2 nm thick NaYF4 shell is formed by using 3-4 nm particles of α-NaYF4 as a single-source precursor for the ß-phase shell material. In contrast to the core particles, however, these α-phase particles are prepared with a low ratio of sodium to rare-earth ions, which efficiently suppresses an undesired nucleation of ß-NaYF4 particles during shell growth.

Adiabatic burst evaporation from bicontinuous nanoporous membranes.

Evaporation of volatile liquids from nanoporous media with bicontinuous morphology and pore diameters of a few 10 nm is an ubiquitous process. For example, such drying processes occur during syntheses of nanoporous materials by sol-gel chemistry or by spinodal decomposition in the presence of solvents as well as during solution impregnation of nanoporous hosts with functional guests. It is commonly assumed that drying is endothermic and driven by non-equilibrium partial pressures of the evaporating species in the gas phase. We show that nearly half of the liquid evaporates in an adiabatic mode involving burst-like liquid-to-gas conversions. During single adiabatic burst evaporation events liquid volumes of up to 10(7) µm(3) are converted to gas. The adiabatic liquid-to-gas conversions occur if air invasion fronts get unstable because of the built-up of high capillary pressures. Adiabatic evaporation bursts propagate avalanche-like through the nanopore systems until the air invasion fronts have reached new stable configurations. Adiabatic cavitation bursts thus compete with Haines jumps involving air invasion front relaxation by local liquid flow without enhanced mass transport out of the nanoporous medium and prevail if the mean pore diameter is in the range of a few 10 nm. The results reported here may help optimize membrane preparation via solvent-based approaches, solution-loading of nanopore systems with guest materials as well as routine use of nanoporous membranes with bicontinuous morphology and may contribute to better understanding of adsorption/desorption processes in nanoporous media.

Ostwald-ripening and particle size focussing of sub-10 nm NaYF4 upconversion nanocrystals.

We have studied the growth behaviour of sub-10 nm NaYF4 upconversion nanocrystals of the hexagonal ß-phase and the cubic α-phase. Ostwald-ripening of such particles in oleic acid/octadecene solvent results in broadening of the particle size distribution if the colloid contains particles of one crystal phase only. Narrow size distributions are formed only if ß-phase particles grow in the presence of an excess of α-phase particles. Such binary mixtures of α-phase and ß-phase particles form intrinsically when colloids of α-phase particles are heated for a sufficiently long time, because seeds of the ß-phase nucleate in the solution after some time at high temperatures. Since the number of seeds determines the final size of the ß-phase product, control of the nucleation is crucial for controlling the final particle size. We show that the number of ß-phase seeds strongly depends on the composition of the α-phase known to form solid solutions Na1-xYF4-x in the range from x = 0 to x = 4/9. Sodium-deficient α-phase particles form a negligible number of ß-phase seeds whereas α-phase particles with high sodium content yield a very large number of seeds. By taking advantage of this dependence and modifying the synthesis of the α-phase particles accordingly, small phase-pure ß-NaYF4:Yb,Er particles with a size smaller than 6 nm can be prepared in oleic acid/octadecene just as well as much larger particles.

Intense up-conversion luminescence in Er3+/Yb3+ co-doped CeO2 powders.

The Er(3+) and Er(3+)/Yb(3+) co-doped CeO2 powders have been prepared by a urea combustion route. The structural, morphological, compositional and vibrational analysis of the Er(3+):CeO2 and Er(3+)/Yb(3+):CeO2 powders have been studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy. The optical and luminescence properties of Er(3+):CeO2 and Er(3+)/Yb(3+):CeO2 powders have been studied by using laser excited spectroscopy. The effects of Yb(3+) doping on up-conversion luminescence of Er(3+) co-doped CeO2 powders were studied. The ratio of red to green intensity is decreased in Er(3+):CeO2 whereas the ratio is increased in Er(3+)/Yb(3+):CeO2 powders with increase of power. The effect of co-doping with the Yb(3+) ions on the visible luminescence of Er(3+) and the energy transfer mechanism responsible for the variation in the green and red intensity are discussed. The results indicate that these materials may be suitable for display and light emitting devices.

Reversible adhesion switching of porous fibrillar adhesive pads by humidity.

We report reversible adhesion switching on porous fibrillar polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) adhesive pads by humidity changes. Adhesion at a relative humidity of 90% was more than nine times higher than at a relative humidity of 2%. On nonporous fibrillar adhesive pads of the same material, adhesion increased only by a factor of ~3.3. The switching performance remained unchanged in at least 10 successive high/low humidity cycles. Main origin of enhanced adhesion at high humidity is the humidity-induced decrease in the elastic modulus of the polar component P2VP rather than capillary force. The presence of spongelike continuous internal pore systems with walls consisting of P2VP significantly leveraged this effect. Fibrillar adhesive pads on which adhesion is switchable by humidity changes may be used for preconcentration of airborne particulates, pollutants, and germs combined with triggered surface cleaning.

Intrinsic focusing of the particle size distribution in colloids containing nanocrystals of two different crystal phases.

We have studied the Ostwald ripening of colloids containing nanocrystals of two different crystal phases of the same material. Ostwald ripening in such polymorphic systems is shown to result in an intrinsic focusing of the particle size distribution of the thermodynamically preferred phase while the particles of the less stable phase completely dissolve. Experimentally, a colloidal system of this kind was realized by mixing small NaEuF4 nanocrystals of the cubic α-phase with particles of the hexagonal ß-phase having the same mean size and size distribution. The temporal evolution of the particle sizes of both phases can be understood and numerically simulated within the framework of LSW theory. The simulations show that small differences in the bulk solubility or the surface energy of the two phases are sufficient to explain the experimentally observed narrowing of the particle size distribution.

NIR to visible frequency upconversion in Er3+ and Yb3+ co-doped BaZrO3 phosphor.

Phosphor powders of Yb(3+) co-doped BaZrO3:Er(3+) have been prepared by the urea combustion route. The formation of single-phase BaZrO3:Er(3+) and BaZrO3:Er(3+),Yb(3+) was confirmed by X-ray powder diffraction (XRD). Green and red luminescence along with a weaker blue emission is observed upon excitation at ~978 nm with a diode laser. These emissions are caused by frequency upconversion in the Er(3+)-doped BaZrO3 (BZO) phosphor. Both the green and red upconversion emissions of the Er(3+) are enhanced by about ~3 and ~43 times respectively when triply ionized ytterbium is incorporated as a co-dopant. The frequency upconversion processes responsible for the blue, green and red emissions are discussed on the basis of the experimental data.