On the effect of Fe oleate by-product in nano-stirbar synthesis.

We show that the by-product obtained from the preparation of Fe3O4 nanoparticles has dramatic effects on the synthesis of nano-stirbars. It is an oily substance likely resulting from the polymerization of oleic acid, followed by coordination/crosslinking with Fe ions. As such it is extremely difficult to remove it by conventional methods. By combining nonpolar organic solvent, prolonged swelling, and low-speed centrifugation, the by-product is successfully removed. Thus, various magnetic nanoparticles could be used for synthesizing nano-stirbars. Among them, the smallest nano-stirbars have reached a width of 21 nm and a length of ∼350 nm, setting a record. The nano-stirbars could be directly driven using a common hotplate stirrer, to facilitate mixing in tiny spaces.

Precise morphology control and fast merging of a complex multi-emulsion system: the effects of AC electric fields.

We showed that an AC electric field can be effectively used to control the full morphology of a multi-emulsion system (oil/water/oil, O/W/O and water/oil/water, W/O/W); specifically, the size of outer droplets and the number of inner droplets (from 5 to 0) could be controlled. In our system, such control was achieved by adopting non-contact type of electrodes together with double-flow-focusing geometry to apply an AC electric field during the formation of complex droplets. As such, the AC electric field could be used without contamination. In addition to morphology control, we also achieved both one-step and two-step merging of the core droplets in the W/O/W droplet system within 100 milliseconds, which is by far the fastest merging in double emulsion droplets ever reported. To the best of our knowledge, this paper is the first article to report the control of core droplets in an O/W/O system by matching the frequency of the AC electric field with that of the core production rate. In this article, we adopted the electric capillary number CaE to analyze the effectiveness of the AC electric field applied at a high frequency, which offers a guideline for practical applications. Furthermore, the merging phenomena among various droplet systems discovered could add extra dimensions for the manipulation of double emulsions. Our findings reveal new physical insights that bring about a better understanding of the interfacial phenomena and electrohydrodynamics of droplets, which is of great importance for practical applications involving the complex interactions of multiple droplets.

Nanoscrews: Asymmetrical Etching of Silver Nanowires.

World's smallest screws with helical threads are synthesized via mild etching of Ag nanowires. With detailed characterization, we show that this nanostructure arises not from the transformation of the initial lattice, but the result of a unique etching mode. Three-dimensional printed models are used to illustrate the evolution of etch pits, from which a possible mechanism is postulated.

Levelling the playing field: screening for synergistic effects in coalesced bimetallic nanoparticles.

Depending on the synthetic methods, bimetallic nanoparticles can have either core-shell, phase segregated, alloy, or partially coalesced structures, presenting different degrees of atomic mixing on their surface. Along with the variations of size and morphology, the structural differences make it difficult to compare the catalytic activity of bimetallic nanoparticles. In this article, we developed a facile screening method that can focus on the synergistic effects rather than structural differences. Prefabricated nanoparticles are mixed together to form linear aggregates and coalesced to form bimetallic junctions. Their hollow silica shells allow materials transport but prevent further aggregation. With a level playing field, this screening platform can identify the best bimetallic combination for a catalytic reaction, before optimizing the synthesis. This approach is more advantageous than the conventional approaches where structural difference may have dominant effects on the catalytic performance.

Understanding the phase emergence of mesoporous silica.

The cylinder-to-vesicle phase transition of mesoporous silica and the inter-dependence of the controlling factors are studied. (3-Mercaptopropyl)trimethoxylsilane (MPTMS) is used to alter the phase outcome of mesoporous silica from the cylindrical MCM-41 to the vesicular phase. Exploiting the phase selection at the critical time point of phase emergence allows investigation of the complex interactions among the ingredients. In this system, orderly cylindrical or vesicular phase directly emerges from the "clumps" of randomly mixed surfactant, co-surfactant, and silica precursor. The phase outcome depends on the exact ratio of the ingredients, provided that enough amounts of MPTMS can diffuse into the clumps before the hardening silica prevents the diffusion.

Thermodynamics versus kinetics in nanosynthesis.

One may discover a stone tool by chance but it takes more than luck to make a car or cell phone. With the advance of nanoscience, the synthesis of increasingly sophisticated nanostructures demands a rational design and a systems approach. In this Review, we advocate the distinction between thermodynamically and kinetically controlled scenarios, that is, whether a product forms because it is the most stable state or because the pathway leading to it has the lowest energy barrier. Great endeavours have been made to describe the multiple concurrent processes in typical nanosynthesis phenomena, so that the mechanistic proposals in the literature are brought into a common framework for easy contrast and comparison.

Homo- and co-polymerization of polysytrene-block-poly(acrylic acid)-coated metal nanoparticles.

Amphiphilic block copolymers such as polystyrene-block-poly(acrylic acid) (PSPAA) give micelles that are known to undergo sphere-to-cylinder shape transformation. Exploiting this polymer property, core-shell nanoparticles coated in PSPAA can be "polymerized" into long chains following the chain-growth polymerization mode. This method is now extended to include a variety of different nanoparticles. A case study on the assembly process was carried out to understand the influence of the PAA block length, the surface ligand, and the size and morphology of the monomer nanoparticles. Shortening the PAA block promotes the reorganization of the amphiphilic copolymer in the micelles, which is essential for assembling large Au nanoparticles. Small Au nanoparticles can be directly "copolymerized" with empty PSPAA micelles into chains. The reaction time, acid quantity, and the [Au nanoparticles]/[PSPAA micelles] concentration ratio played important roles in controlling the sphere-cylinder-vesicle conversion of the PSPAA micelles, giving rise to different kinds of random "copolymers". With this knowledge, a general method is then developed to synthesize homo, random, and block "copolymers", where the basic units include small Au nanoparticles (d = 16 nm), large Au nanoparticles (d = 32 nm), Au nanorods, Te nanowires, and carbon nanotubes. Given the lack of means for assembling nanoparticles, advancing synthetic capabilities is of crucial importance. Our work provides convenient routes for combining nanoparticles into long-chain structures, facilitating rational design of complex nanostructures in the future.

General methodology of using oil-in-water and water-in-oil emulsions for coiling nanofilaments.

Hydrophobic carbon nanotubes (CNTs) and hydrophilic nanofilaments such as oxidized CNTs, Pd nanowires (NWs), and MnO(2) NWs are transformed from wires to rings by a general methodology. We show that both oil-in-water and water-in-oil emulsions, so long as their droplet size is sufficiently small, can exert significant force to the entrapped nanostructures, causing their deformation. This effect can be easily achieved by simply mixing a few solutions in correct ratios. Even preformed oil droplets can take in CNTs from the aqueous solution converting them into rings, indicating the important role of thermodynamics: The question here is not if the droplets can exert sufficient force to bend the nanofilaments, because their random vibration may be already doing it. As long as the difference in solvation energy is large enough for a nanofilament, it would "want" to move away from the bulk solution and fit inside tiny droplets, even at the cost of induced strain energy. That said, the specific interactions between a droplet and a filament are also of importance. For example, when an oil droplet rapidly shrinks in size, it can compress the entrapped CNTs in multiple stages into structures with higher curvatures (thus higher strain) than that of a circular ring, which has minimal induced strain inside a spherical droplet.