Simple Cubic Packing of Gold Nanoparticles through Rational Design of Their Dendrimeric Corona

The first simple-cubic liquid crystal was obtained by coating monodisperse Au nanoparticles (NPs) with a thick corona of amino-substituted organic dendrons. This unusual structure was determined by grazing-incidence diffraction and electron density reconstruction and explained by analyzing the radial density profile of the corona. Another novel structure is proposed for the phase preceding the cubic one: a hexagonal superlattice composed of alternating dense and sparse strings of Au NPs.

Inkjet Printing High-Resolution, Large-Area Graphene Patterns by Coffee-Ring Lithography

Taking advantage of the “coffee-ring” effect, graphene electrodes with channel lengths as low as 1−2 micrometers are patterned by inkjet printing. Organic thin film transistors and complementary inverters are also fabricated using these graphene electrodes and show excellent performance.

Cargo-Towing Fuel-Free Magnetic Nanoswimmers for Targeted Drug Delivery





Fuel-free nanomotors are essential for future in-vivo biomedical transport and drug-delivery applications. Herein, the first example of directed delivery of drug-loaded magnetic polymeric particles using magnetically driven flexible nanoswimmers is described. It is demonstrated that flexible magnetic nickel–silver nanoswimmers (5–6 μm in length and 200 nm in diameter) are able to transport micrometer particles at high speeds of more than 10 μm s−1 (more than 0.2 body lengths per revolution in dimensionless speed). The fundamental mechanism of the cargo-towing ability of these magnetic (fuel-free) nanowire motors is modelled, and the hydrodynamic features of these cargo-loaded motors discussed. The effect of the cargo size on swimming performance is evaluated experimentally and compared to a theoretical model, emphasizing the interplay between hydrodynamic drag forces and boundary actuation. The latter leads to an unusual increase of the propulsion speed at an intermediate particle size. Potential applications of these cargo-towing nanoswimmers are demonstrated by using the directed delivery of drug-loaded microparticles to HeLa cancer cells in biological media. Transport of the drug carriers through a microchannel from the pick-up zone to the release microwell is further illustrated. It is expected that magnetically driven nanoswimmers will provide a new approach for the rapid delivery of target-specific drug carriers to predetermined destinations.

Facile Fabrication of Metallic Nanostructures by Tunable
Cracking and Transfer Printing

A topographically patterned PDMS stamp was coated with thin
metal film and swelled under organic vapor to induce the tunable cracking of
the brittle film into metallic nanostructures (see SEM images, scale bars
1 μm). UV/Vis spectra, OLED efficiency, and SERS spectra demonstrate the fine
controllability of the metallic nanostructures, the well-ordered and highly
regulable surface plasmons, and the facile fabrication process.

Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces

Friction between solids is responsible for many phenomena such as earthquakes, wear or crack propagation^{1, 2, 3, 4}. Unlike macroscopic objects, which only touch locally owing to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel–Kontorova model, which considers a monolayer of interacting particles on a periodic substrate potential^{5, 6, 7, 8}. In addition to the well-known stick–slip motion, such models also predict the formation of kinks and antikinks^{9, 10, 11, 12}, which greatly reduce the friction between the monolayer and the substrate. Here, we report the direct observation of kinks and antikinks in a two-dimensional colloidal crystal that is driven across different types of ordered substrate. We show that the frictional properties only depend on the number and density of such excitations, which propagate through the monolayer along the direction of the applied force. In addition, we also observe kinks on quasicrystalline surfaces, which demonstrates that they are not limited to periodic substrates but occur under more general conditions.

Chiral Nematic Fluids as Masks for Lithography

Rolling Up Gold Nanoparticle-Dressed DNA Origami into Three-Dimensional Plasmonic Chiral Nanostructures

Construction of three-dimensional (3D) plasmonic architectures using structural DNA nanotechnology is an emerging multidisciplinary area of research. This technology excels in controlling spatial addressability at sub-10 nm resolution, which has thus far been beyond the reach of traditional top-down techniques. In this paper, we demonstrate the realization of 3D plasmonic chiral nanostructures through programmable transformation of gold nanoparticle (AuNP)-dressed DNA origami. AuNPs were assembled along two linear chains on a two-dimensional rectangular DNA origami sheet with well-controlled positions and particle spacing. By rational rolling of the 2D origami template, the AuNPs can be automatically arranged in a helical geometry, suggesting the possibility of achieving engineerable chiral nanomaterials in the visible range.

Coated Gas Bubbles for the Continuous Synthesis of Hollow Inorganic Particles

We present a microfluidic approach for the controlled encapsulation of individual gas bubbles in micrometer-diameter aqueous droplets with high gas volume fractions and demonstrate this approach to making a liquid shell, which serves as a template for the synthesis of hollow inorganic particles. In particular, we find that an increase in the viscosity of the aqueous phase facilitates the encapsulation of individual gas bubbles in an aqueous droplet and allows control of the thickness of a thin aqueous shell. Furthermore, because such droplets contain a finite amount of water, uncontrolled hydrolysis reactions between reactive inorganic precursors and bulk water can be avoided. We demonstrate this approach by introducing reactive inorganic precursors, such as silane and titanium butoxide, for sol–gel reactions downstream from the formation of the bubble in a droplet and consequently fabricate hollow particles of silica or titania in one continuous flow process. These approaches provide a route to controlling double-emulsion-type gas–liquid microstructures and offer a new fabrication method for thin-shell-covered microbubbles and hollow microparticles.

Designer Polymer-Based Microcapsules Made Using Microfluidics

Filled microcapsules made from double emulsion templates in microfluidic devices are attractive delivery systems for a variety of applications. The microfluidic approach allows facile tailoring of the microcapsules through a large number of variables, which in turn makes these systems more challenging to predict. To elucidate these dependencies, we start from earlier theoretical predictions for the size of double emulsions and present quantitative design maps that correlate parameters such as fluid flow rates and device geometry with the size and shell thickness of monodisperse polymer-based capsules produced in microcapillary devices. The microcapsules are obtained through in situ photopolymerization of the middle oil phase of water-in-oil-in-water double emulsions. Using polymers with selected glass transition temperatures as the shell material, we show through single capsule compression testing that hollow capsules can be prepared with tunable mechanical properties ranging from elastomeric to brittle. A quantitative statistical analysis of the load at rupture of brittle capsules is also provided to evaluate the variability of the microfluidic route and assist the design of capsules in applications involving mechanically triggered release. Finally, we demonstrate that the permeability and microstructure of the capsule shell can also be tailored through the addition of cross-linkers and silica nanoparticles in the middle phase of the double emulsion templates.

Microdrop Printing of Hydrogel Bioinks into 3D Tissue-Like Geometries

An optimized 3D inkjet printing process is demonstrated for structuring alginate into a tissue-like microvasculature capable of supporting physiological flow rates. Optimizing the reaction at the single-droplet level enables wet hydrogel droplets to be stacked, thus overcoming their natural tendancy to spread and coalesce. Live cells can be patterned using this process and it can be extended to a range of other hydrogels.

Unexpected Strength and Toughness in Chitosan-Fibroin Laminates Inspired by Insect Cuticle

A material inspired by natural insect cuticle and composed of chitosan and fibroin is created. The material exhibits the strength of an aluminum alloy at half its weight, while being clear, biocompatible, biodegradable, and micromoldable. The bioinspired laminate exhibits strength and toughness that are ten times greater than the unstructured component blend and twice that of its strongest constituent.

Bonding Assembled Colloids without Loss of Colloidal Stability

A facile method is demonstrated for bonding assembled colloids without loss of colloidal stability by thermal annealing. Examples include both close-packed and non-close-packed structures. The confocal microscopy image shows a cross-section of a 3D labyrinthine structure after it was made permanent. The 3D network is completely preserved after the annealing step.

Building Designed Granular Towers One Drop at a Time

A dense granular suspension dripping on an imbibing surface is observed to give rise to slender mechanically stable structures that we call granular towers. Successive drops of grain-liquid mixtures are shown to solidify rapidly upon contact with a liquid absorbing substrate. A balance of excess liquid flux and drainage rate is found to capture the typical growth and height of the towers. The tower width is captured by the Weber number, which gives the relative importance of inertia and capillary forces. Various symmetric, smooth, corrugated, zigzag, and chiral structures are observed by varying the impact velocity and the flux rate from droplet to jetting regime.

Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices

Understanding how polyhedra pack into extended arrangements is integral to the design and discovery of crystalline materials at all length scales^{1, 2, 3}. Much progress has been made in enumerating and characterizing the packing of polyhedral shapes^{4, 5, 6}, and the self-assembly of polyhedral nanocrystals into ordered superstructures^{7, 8, 9}. However, directing the self-assembly of polyhedral nanocrystals into densest packings requires precise control of particle shape¹⁰, polydispersity¹¹, interactions and driving forces ¹². Here we show with experiment and computer simulation that a range of nanoscale Ag polyhedra can self-assemble into their conjectured densest packings⁶. When passivated with adsorbing polymer, the polyhedra behave as quasi-hard particles and assemble into millimetre-sized three-dimensional supercrystals by sedimentation. We also show, by inducing depletion attraction through excess polymer in solution, that octahedra form an exotic superstructure with complex helical motifs rather than the densest Minkowski lattice¹³. Such large-scale Ag supercrystals may facilitate the design of scalable three-dimensional plasmonic metamaterials for sensing^{14, 15}, nanophotonics¹⁶ and photocatalysis¹⁷.

Physical ageing of the contact line on colloidal particles at liquid interfaces

Young’s law¹ predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle². This has been used to explain why colloids often bind to liquid interfaces^{3, 4}, and has been exploited in emulsification⁵, water purification⁶, mineral recovery⁷, encapsulation⁸ and the making of nanostructured materials^{9, 10}. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface^{11, 12}.

Drug Release from Electric-Field-Responsive Nanoparticles

We describe a new temperature and electric field dual-stimulus responsive nanoparticle system for programmed drug delivery. Nanoparticles of a conducting polymer (polypyrrole) are loaded with therapeutic pharmaceuticals and are subcutaneously localized in vivo with the assistance of a temperature-sensitive hydrogel (PLGA-PEG-PLGA). We have shown that drug release from the conductive nanoparticles is controlled by the application of a weak, external DC electric field. This approach represents a novel interactive drug delivery system that can show an externally tailored release profile with an excellent spatial, temporal, and dosage control.

Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent

Easy to prepare solid materials based on fumed silica impregnated with polyethylenimine (PEI) were found to be superior adsorbents for the capture of carbon dioxide directly from air. During the initial hours of the experiments, these adsorbents effectively scrubbed all the CO₂ from the air despite its very low concentration. The effect of moisture on the adsorption characteristics and capacity was studied at room temperature. Regenerative ability was also determined in a short series of adsorption/desorption cycles.

Directed Self-Assembly of Hybrid Oxide/Polymer Core/Shell Nanowires with Transport Optimized Morphology for Photovoltaics

An entirely bottom-up approach for the preparation of liquid crystalline suspensions of core-shell nanowires for ordered bulk heterojunction photovoltaics is demonstrated. Side-on attachment of polythiophene derivatives to ZnO nanowires promotes a co-axial polymer backbone-nanowire arrangement which favors high hole mobility. This strategy offers structural control over multiple length scales and a viable means of fabricating ordered films over large areas.

Encapsulation of Actives in Self-Sealing Microcapsules by Precipitation in Capsule Shells

Microcapsules with core–shell structures are excellent vehicles for the encapsulation of active ingredients; however, the actives often leak out of these structures over time, without observable damage to them. We present a novel approach to enhancing the encapsulation of active ingredients inside microcapsules. We use two components that can form solid precipitates upon mixing and add one each to the microcapsule core and to the continuous phase. The components diffuse through the shell in the same manner as the actives, but upon meeting, they precipitate to form solid particles within the shell; this significantly reduces leakage through the shell of the microcapsules. We show that the reduction in the leakage of actives is due to the blockage of channels or pores that exist in the shell of the capsules by the solid precipitates.

Nanoparticle-Stabilized Double Emulsions and Compressed Droplets

Stable double emulsions, both oil-in-water-in-oil and water-in-oil-in-water, stabilized by two types of nanoparticles residing at the o/w interfaces (see picture; red: CdSe quantum dots) were prepared in a simple fashion by shaking, and with narrow size distributions by microcapillary flow focusing. These double-emulsion droplets proved stable against coalescence throughout solvent evaporation, allowing for formation of nanoparticle foams and hexagonally arrayed structures.

Molecular Structure Encodes Nanoscale Assemblies: Understanding Driving Forces in Electrostatic Self-Assembly

Supramolecular nanoparticles represent a key field in recent research as their synthesis through self-assembly is straightforward and they often can respond to external triggers. A fundamental understanding of structure-directing factors is highly desirable for a targeted structure design. This contribution demonstrates a quantitative relation between the size of supramolecular self-assembled nanoparticles and the free energy of association. Nanoparticles are prepared by electrostatic self-assembly of cationic polyelectrolyte dendrimers as model macroions and oppositely charged di- and trivalent organic dye molecules relying on the combination of electrostatic and π–π-interactions. A systematic set of sulfonate-group carrying azo-dyes was synthesized. Light scattering and ζ-potential measurements on the resulting nanoparticles yield hydrodynamic radii between 20 nm < R_H < 50 nm and positive ζ-potential values indicating a positive particle charge. Studies on dye self-aggregation and dendrimer-dye association by isothermal titration calorimetry (ITC) and UV–vis spectroscopy allow for the correlation of the thermodynamic parameters of dendrimer-dye association with the size of the particles, showing that at least a free energy gain of ΔG ≈ – 32 kJ mol^–1 is necessary to induce dendrimer interconnection. Structural features of the azo dyes causing these to favor or prevent nanoparticle formation have been identified. The dye–dye-interaction was found to be the key factor in particle size control. A simple model yields a quantitative relation between the free energy and the particle sizes, allowing for predicting the latter based on thermodynamic measurements. Hence, a set of different molecular “building bricks” can be defined where the choice of building block determines the resulting assembly size.

Self-Assembly and Reconfigurability of Shape-Shifting Particles

Reconfigurability of two-dimensional colloidal crystal structures assembled by anisometric particles capable of changing their shape were studied by molecular dynamics computer simulation. We show that when particles change shape on cue, the assembled structures reconfigure into different ordered structures, structures with improved order, or more densely packed disordered structures, on faster time scales than can be achieved via self-assembly from an initially disordered arrangement. These results suggest that reconfigurable building blocks can be used to assemble reconfigurable materials, as well as to assemble structures not possible otherwise, and that shape shifting could be a promising mechanism to engineer assembly pathways to ordered and disordered structures.

Self-Powered Nanosensors and Nanosystems

Sensor networks are a key technological and economic driver for global industries in the near future, with applications in health care, environmental monitoring, infrastructure monitoring, national security, and more. Developing technologies for self-powered nanosensors is vitally important. This paper gives a brief summary about recent progress in the area, describing nanogenerators that are capable of providing sustainable self-sufficient micro/nanopower sources for future sensor networks.

PRINT: A Novel Platform Toward Shape and Size Specific Nanoparticle Theranostics

Nanotheranostics represents the next generation of medicine, fusing nanotechnology, therapeutics, and diagnostics. By integrating therapeutic and imaging agents into one nanoparticle, this new treatment strategy has the potential not only to detect and diagnose disease but also to treat and monitor the therapeutic response. This capability could have a profound impact in both the research setting as well as in a clinical setting. In the research setting, such a capability will allow research scientists to rapidly assess the performance of new therapeutics in an effort to iterate their designs for increased therapeutic index and efficacy. In the clinical setting, theranostics offers the ability to determine whether patients enrolling in clinical trials are responding, or are expected to respond, to a given therapy based on the hypothesis associated with the biological mechanisms being tested. If not, patients can be more quickly removed from the clinical trial and shifted to other therapeutic options. To be effective, these theranostic agents must be highly site specific. Optimally, they will carry relevant cargo, demonstrate controlled release of that cargo, and include imaging probes with a high signal-to-noise ratio.

There are many biological barriers in the human body that challenge the efficacy of nanoparticle delivery vehicles. These barriers include, but are not limited to, the walls of blood vessels, the physical entrapment of particles in organs, and the removal of particles by phagocytic cells. The rapid clearance of circulating particles during systemic delivery is a major challenge; current research seeks to define key design parameters that govern the performance of nanocarriers, such as size, surface chemistry, elasticity, and shape. The effect of particle size and surface chemistry on in vivo biodistribution of nanocarriers has been extensively studied, and general guidelines have been established. Recently it has been documented that shape and elasticity can have a profound effect on the behavior of delivery vehicles. Thus, having the ability to independently control shape, size, matrix, surface chemistry, and modulus is crucial for designing successful delivery agents.

In this Account, we describe the use of particle replication in nonwetting templates (PRINT) to fabricate shape- and size-specific microparticles and nanoparticles. A particular strength of the PRINT method is that it affords precise control over shape, size, surface chemistry, and modulus. We have demonstrated the loading of PRINT particles with chemotherapeutics, magnetic resonance contrast agents, and fluorophores. The surface properties of the PRINT particles can be easily modified with “stealth” poly(ethylene glycol) chains to increase blood circulation time, with targeting moieties for targeted delivery or with radiolabels for nuclear imaging. These particles have tremendous potential for applications in nanomedicine and diagnostics.

Flexible microfluidic cloth-based analytical devices using a low-cost wax patterning technique


This paper describes the fabrication of microfluidic cloth-based analytical devices (μCADs) using a simple wax patterning method on cotton cloth for performing colorimetric bioassays. Commercial cotton cloth fabric is proposed as a new inexpensive, lightweight, and flexible platform for fabricating two- (2D) and three-dimensional (3D) microfluidic systems. We demonstrated that the wicking property of the cotton microfluidic channel can be improved by scouring in soda ash (Na₂CO₃) solution which will remove the natural surface wax and expose the underlying texture of the cellulose fiber. After this treatment, we fabricated narrow hydrophilic channels with hydrophobic barriers made from patterned wax to define the 2D microfluidic devices. The designed pattern is carved on wax-impregnated paper, and subsequently transferred to attached cotton cloth by heat treatment. To further obtain 3D microfluidic devices having multiple layers of pattern, a single layer of wax patterned cloth can be folded along a predefined folding line and subsequently pressed using mechanical force. All the fabrication steps are simple and low cost since no special equipment is required. Diagnostic application of cloth-based devices is shown by the development of simple devices that wick and distribute microvolumes of simulated body fluids along the hydrophilic channels into reaction zones to react with analytical reagents. Colorimetric detection of bovine serum albumin (BSA) in artificial urine is carried out by direct visual observation of bromophenol blue (BPB) colour change in the reaction zones. Finally, we show the flexibility of the novel microfluidic platform by conducting a similar reaction in a bent pinned μCAD.

Ultralight Metallic Microlattices

Ultralight (<10 milligrams per cubic centimeter) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. We present ultralight materials based on periodic hollow-tube microlattices. These materials are fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit densities ρ ≥ 0.9 milligram per cubic centimeter, complete recovery after compression exceeding 50% strain, and energy absorption similar to elastomers. Young’s modulus E scales with density as E ~ ρ², in contrast to the E ~ ρ³scaling observed for ultralight aerogels and carbon nanotube foams with stochastic architecture. We attribute these properties to structural hierarchy at the nanometer, micrometer, and millimeter scales.

Silica-Like Malleable Materials from Permanent Organic Networks

Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non–cross-linked polymers and those with reversible cross-links are processable, but they are soluble. We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.

Chiral Self-Assembled Solid Microspheres: A Novel Multifunctional Microphotonic Device

Solid chiral microspheres with unique and multifunctional optical properties are produced from cholesteric liquid crystal-water emulsions using photopolymerization processes. These self-organizing microspheres exhibit different internal configurations of helicoidal structures with radial, conical or cylindrical geometries, depending on the physicochemical characteristics of the precursor liquid crystal emulsion.

Synthesis and Promising Properties of a New Family of High-Density Energetic Salts of 5-Nitro-3-trinitromethyl-1H-1,2,4-triazole and 5,5′-Bis(trinitromethyl)-3,3′-azo-1H-1,2,4-triazole

Salts of trinitromethyl-substituted triazoles, 5-nitro-3-trinitromethyl-1H-1,2,4-triazole and 5,5′-bis(trinitromethyl)-3,3′-azo-1H-1,2,4-triazole (5), form a new class of highly dense energetic materials. Single-crystal X-ray structuring supports the formation of the cocrystal of 5 with 3,5-diamino-1,2,4-triazole, which was found to be remarkably less impact-sensitive than the azo precursor. The compounds were fully characterized using IR and multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry. Based on heats of formation calculated with Gaussian 03 and combined with experimentally determined densities, detonation properties of the energetic materials obtained with the EXPLO5 program identify them as potentially explosive compounds. They exhibit high density, moderate to good thermal stability, acceptable oxygen balance, reasonable heat of formation, and excellent detonation properties, which in some cases are superior to those of 1,3,5,-trinitrotriazacyclohexane (RDX).

Conductive dense hydrogen

Molecular hydrogen is expected to exhibit metallic properties under megabar pressures. This metal is predicted to be superconducting with a very high critical temperature, T_c, of 200–400 K (ref. 1), and it may acquire a new quantum state as a metallic superfluid and a superconducting superfluid². It may potentially be recovered metastably at ambient pressures³. However, experiments carried out at low temperatures, T<100 K (refs 4, 5), showed that at record pressures of 300 GPa, hydrogen remains in the molecular insulating state. Here we report on the transformation of normal molecular hydrogen at room temperature (295 K) to a conductive and metallic state. At 200 GPa the Raman frequency of the molecular vibron strongly decreased and the spectral width increased, evidencing a strong interaction between molecules. Deuterium behaved similarly. Above 220 GPa, hydrogen became opaque and electrically conductive. At 260–270 GPa, hydrogen transformed into a metal as the conductance of hydrogen sharply increased and changed little on further pressurizing up to 300 GPa or cooling to at least 30 K; and the sample reflected light well. The metallic phase transformed back at 295 K into molecular hydrogen at 200 GPa. This significant hysteresis indicates that the transformation of molecular hydrogen into a metal is accompanied by a first-order structural transition presumably into a monatomic liquid state. Our findings open an avenue for detailed and comprehensive studies of metallic hydrogen.

Synthesis of Bifunctional Au/Pt/Au Core/Shell Nanoraspberries for in Situ SERS Monitoring of Platinum-Catalyzed Reactions

The synthesis of bifunctional Au/Pt/Au nanoraspberries for use in quantitative in situ monitoring of platinum-catalyzed reactions by surface-enhanced Raman scattering (SERS) is presented. Highly convolved SERS spectra of reaction mixtures can be decomposed into the contributions of distinct molecular species by multivariate data analysis.

Shape-anisotropic colloids: Building blocks for complex assemblies




Recent breakthroughs in colloidal synthesis allow the control of particle shapes and properties with high precision. This provides us with a constantly expanding library of new anisotropic building blocks, thus opening new avenues to explore colloidal self-assembly at a higher level of complexity. This article reviews the most recent advances in the preparation and self-assembly of colloids with well-defined anisotropic shapes. A particular emphasis is given to solution-based syntheses that provide micron-sized colloids in high yields, and to assembly schemes that exploit the shape anisotropy of the building blocks involved.

Using Stop-Flow Lithography To Produce Opaque Microparticles: Synthesis and Modeling


We report on modeling and experimental studies of the synthesis of opaque microparticles made via stop-flow lithography. Opaque magnetic beads and UV-absorbing dyes incorporated into hydrogel microparticles during synthesis changed the height and the degree of cross-linking of the polymer matrices formed. The effect of the concentration of these opaque materials on the particle height was determined experimentally and agreed well with model predictions based on the photopolymerization process over a wide range of UV absorbance. We also created particles with two independent anisotropies, magnetic and geometric, by applying magnetic fields during particle synthesis. Our work provides a platform for rational design of lithographic patterned opaque particles and also a new class of structured magnetic microparticles.

Replacing a Battery by a Nanogenerator with 20 V Output

Replacing batteries by nanogenerators (NGs) in small consumer electronics is one of the goals in the emerging field of self-powered nanotechnology. We show that the maximum measured output voltage of an NG optimized with pretreatments on the as-grown ZnO nanowire films reaches 20 V and the output current exceeds 6 μA, which corresponds to a power density of 0.2 W cm⁻³. The NG is also demonstrated to replace a battery for driving a electronic watch.

Non-equilibrium cluster states in colloids with competing interactions


Cluster formation and gelation are studied in a colloidal model system with competing short-range attractions and long-range repulsions. In contrast to predictions by equilibrium theory, the size of clusters spontaneously formed at low colloidal volume fractions decreases with increasing strength of the short-range attraction. Moreover, the microstructure and shape of the clusters sensitively depend on the strength of the short-range attraction: from compact and crystalline clusters at relatively weak attractions to disordered and quasi-linear clusters at strong attractions. By systematically varying attraction strength and colloidal volume fraction, we observe gelation at relatively high volume fraction. The structure of the gel depends on attraction strength: in systems with the lowest attraction strength, crowding of crystalline clusters leads to microcrystalline gels. In contrast, in systems with relatively strong attraction strength, percolation of quasi-linear clusters leads to low-density gels. In analyzing the results we show that nucleation and rearrangement processes play a key role in determining the properties of clusters and the mechanism of gelation. This study implies that by tuning the strength of short-range attractions, the growth mechanism as well as the structure of clusters can be controlled, and thereby the route to a gel state.

Transport of cargo by catalytic Janus micro-motors


Catalytically active Janus micro-spheres are capable of autonomous motion and can potentially act as carriers for transportation of cargo at the micron-scale. Focusing on the cases in which a single or a pair of Janus micro-motors is used as carrier, we investigate the complex dynamics exhibited by various active carrier–cargo composites.

Sedimentation and depletion attraction directing glass and liquid crystal formation in aqueous platelet/sphere mixtures



We present an experimental study on mixtures of the gibbsite platelets and silica nanospheres of the same charge sign. In these mixtures the spheres act as depletants changing the phase behaviour of the platelet system, for which two very different time scales can be distinguished. At short times as a result of a strong depletion attraction in the system a kinetically arrested phase was split off. Subsequently, on a time scale of months the top phase separated in an isotropic and a columnar phase on top of the arrested phase. We rationalize this gravity induced liquid crystal formation in terms of a platelet sedimentation and a strong depletion attraction. The sedimentation driven columnar phase is highly ordered as we show by microradian X-ray diffraction experiments.

Inkjet Printing of Conjugated Polymer Precursors on Paper Substrates for Colorimetric Sensing and Flexible Electrothermochromic Display

Printing of a diacetylene (DA)-surfactant composite ink on unmodified paper and photopaper, as well as on a banknote, enables generation of latent images that are transformed to blue-colored polydiacetylene (PDA) structures by UV irradiation. Both irreversible and reversible thermochromism with the PDA printed images are demonstrated and applied to flexible and disposable sensors and to displays.

Cool mechanical stuff!

http://vimeo.com/31262642

Assembly of Colloidal Nanoparticles Directed by the Microstructures of Polycrystalline Ice

We show that the microstructures of polycrystalline ice can serve as a confining template for one-dimensional assembly of colloidal nanoparticles. Upon simply freezing an aqueous colloid, the nanoparticles are excluded from ice grains and form chains in the ice veins. The nanoparticle chains are transferable and can be strengthened by polymer encapsulation. After coating with polyaniline shells, simple sedimentation is used to remove large aggregates, enriching single-line chains of 40 nm gold nanoparticles with a total length of several micrometers. When gold nanorods were used, they formed one-dimensional aggregates with specific end-to-end conforma- tion, indicating the confining effects of the nanoscale ice veins at the final stage of freezing. The unbranched and ultralong plasmonic chains are of importance for future study of plasmonic coupling and development of plasmonic waveguides.

Self-assembly of highly ordered conjugated polymer aggregates with long-range energy transfer

Applications of conjugated polymers (CP) in organic electronic devices such as light-emitting diodes and solar cells depend critically on the nature of electronic energy transport in these materials^{1, 2, 3, 4, 5}. Single-molecule spectroscopy has revealed their fundamental properties with molecular detail^{6, 7, 8, 9, 10, 11, 12, 13, 14}, and recent reports suggest that energy transport in single CP chains can extend over extraordinarily long distances of up to 75 nm (refs 13, 15, 16). An important question arises as to whether these characteristics are sustained when CP chains agglomerate into a neat solid². Here, we demonstrate that the electronic energy transport in aggregates composed of tens of polymer chains takes place on a similar distance scale as that in single chains. A recently developed molecular-level understanding of solvent vapour annealing has allowed us to develop a technique to control the CP agglomeration process¹⁷. Aggregates with volumes of at least 45,000 nm³ (molecular weight ≈ 21 MDa) maintain a highly ordered morphology and show pronounced fluorescence blinking behaviour, indicative of substantially long-range energy transport. Our findings provide a new lens through which the ordering of single CP chains and the evolution of their morphological and optoelectronic properties can be observed, which will ultimately enable the rational design of improved CP-based devices.

Controlling Water Capture of Bioinspired Fibers with Hump Structures

Geometrically engineered thin fibers that feature introduced hump structures similar to wetted spider capture silk greatly improve the adhesive ability to drops than uniform ones, which is attributed to an unusual three-phase contact line that extends axially along the fibers. The hump structures improve the stability of the contact line through a combination of “slope” and “curvature” effects, which creates sufficient capillary adhesion to pin drops.

Multiplexing superparamagnetic beads driven by multi-frequency ratchets

Here, we explore the single particle dynamics of superparamagnetic beads exposed to multifrequency ratchets. Through a combination of theory, simulation, and experiment, we determine the important tuning parameters that can be used to implement multiplexed separation of polydisperse colloidal mixtures. In particular, our results demonstrate that the ratio of driving frequencies controls the transition between open and closed trajectories that allow particles to be transported across a substrate. We also demonstrate that the phase difference between the two frequencies controls not only the direction of motion but also which particles are allowed to move within a polydisperse mixture. These results represent a fundamentally different approach to colloidal separation than the previous methods which are based on controlling transitions between phase-locked and phase-slipping regimes, and have a higher degree of multiplexing capabilities that can benefit the fields of biological separation and sensing as well as provide crucial insights into general ratchet behavior.

Does size matter? Elasticity of compressed suspensions of colloidal- and granular-scale microgels


We investigate the mechanics of dense packing of very small, colloidal-scale, and larger, granular-scale microgel particles. At low particle concentration, thermally induced Brownian motion of the particles is important for the colloidal-scale systems; in contrast, such Brownian motion is irrelevant at particle packing fractions beyond jamming. As a consequence, colloidal and granular systems behave very similarly under these conditions. At sufficiently high compression of the microgel particles, their polymeric nature sets the scale of the osmotic pressure and shear modulus of the whole packing, in direct analogy with macroscopic, continuous polymer gels. This observation suggests that the particulate nature of microgels is inconsequential for their linear elasticity in a highly packed state. In contrast, the particulate nature of the microgels does become essential when the packed suspensions are forced to yield and flow; here, the differences between colloidal- and granular-scale particles are marked.

Rate-dependent interaction between thin films and interfaces during micro/nanoscale transfer printing




This paper describes the mechanism of the rate-dependent adhesion for a transfer printing system. A cohesive element model accounting for the viscoelastic effect of the stamp reveals the evolution of the stress and the stress distributions as the peeling velocity increases. Experiments using Si ribbons separating from a wafer verify this prediction and demonstrate the influence of the rate-dependent effect on the transfer efficiency. The simulations show the relaxation time of the stamp has little effect on the transfer efficiency, and implies that it can be improved by adjusting the viscoelastic modulus.