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Micrometre-sized particles covered with stiff, nanoscale spikes are shown to exhibit long-term colloidal stability in both hydrophilic and hydrophobic media, without the need for chemical coating, owing to the effect of the spikes on the contact area and, consequently, the force between the particles. A new way of forming colloidal suspensions The conventional way of persuading hydrophobic particles to form colloidal suspensions in water -- and hydrophilic particles to disperse in oil -- is to camouflage the particles chemically with surfactants, organic tethers, adsorbed polymers or other agents that impart affinity for the solvent. Joong Hwan Bahng et al. now show that surface corrugation provides a very effective alternative strategy for dispersing particles in a wide range of solvents. The team produce micrometre-sized 'hedgehog' particles that are coated with stiff nanospikes and exhibit long-term colloidal stability in both hydrophilic and hydrophobic media. This unusual dispersion behaviour challenges current theories and deepens our understanding of colloidal interparticle interactions. The technique may have technological uses, as it provides particles that function in both organic and aqueous media. Hydrophobic particles in water and hydrophilic particles in oil aggregate, but can form colloidal dispersions if their surfaces are chemically camouflaged with surfactants, organic tethers, adsorbed polymers or other particles that impart affinity for the solvent and increase interparticle repulsion.sup.1,2. A different strategy for modulating the interaction between a solid and a liquid uses surface corrugation, which gives rise to unique wetting behaviour.sup.3,4,5. Here we show that this topographical effect can also be used to disperse particles in a wide range of solvents without recourse to chemicals to camouflage the particles' surfaces: we produce micrometre-sized particles that are coated with stiff, nanoscale spikes and exhibit long-term colloidal stability in both hydrophilic and hydrophobic media. We find that these 'hedgehog' particles do not interpenetrate each other with their spikes, which markedly decreases the contact area between the particles and, therefore, the attractive forces between them. The trapping of air in aqueous dispersions, solvent autoionization at highly developed interfaces, and long-range electrostatic repulsion in organic media also contribute to the colloidal stability of our particles. The unusual dispersion behaviour of our hedgehog particles, overturning the notion that like dissolves like, might help to mitigate adverse environmental effects of the use of surfactants and volatile organic solvents, and deepens our understanding of interparticle interactions and nanoscale colloidal chemistry., Author(s): Joong Hwan Bahng [sup.1] , Bongjun Yeom [sup.2] , Yichun Wang [sup.1] , Siu On Tung [sup.3] , J. Damon Hoff [sup.4] , Nicholas Kotov [sup.1] [sup.2] [sup.3] [sup.5] [...]

This paper describes a simple microfluidic sorting system that can perform size profiling and continuous mass-dependent separation of particles through combined use of gravity (1 g) and hydrodynamic flows capable of rapidly amplifying sedimentation-based separation between particles. Operation of the device relies on two microfluidic transport processes: (i) initial hydrodynamic focusing of particles in a microchannel oriented parallel to gravity and (ii) subsequent sample separation where positional difference between particles with different mass generated by sedimentation is further amplified by hydrodynamic flows whose streamlines gradually widen out due to the geometry of a widening microchannel oriented perpendicular to gravity. The microfluidic sorting device was fabricated in poly(dimethylsiloxane), and hydrodynamic flows in microchannels were driven by gravity without using external pumps. We conducted theoretical and experimental studies on fluid dynamic characteristics of laminar flows in widening microchannels and hydrodynamic amplification of particle separation. Direct trajectory monitoring, collection, and postanalysis of separated particles were performed using polystyrene microbeads with different sizes to demonstrate rapid (< 1 min) and high-purity (>99.9%) separation. Finally, we demonstrated biomedical applications of our system by isolating small-sized (diameter

The high optical and chemical activity of nanoparticles (NPs) signifies the possibility of converting the spin angular momenta of photons into structural changes in matter. Here, we demonstrate that illumination of dispersions of racemic CdTe NPs with right- (left-)handed circularly polarized light (CPL) induces the formation of right- (left-)handed twisted nanoribbons with an enantiomeric excess exceeding 30%, which is ∼10 times higher than that of typical CPL-induced reactions. Linearly polarized light or dark conditions led instead to straight nanoribbons. CPL 'templating' of NP assemblies is based on the enantio-selective photoactivation of chiral NPs and clusters, followed by their photooxidation and self-assembly into nanoribbons with specific helicity as a result of chirality-sensitive interactions between the NPs. The ability of NPs to retain the polarization information of incident photons should open pathways for the synthesis of chiral photonic materials and allow a better understanding of the origins of biomolecular homochirality. [ABSTRACT FROM AUTHOR]

Concave nanoparticles (NPs) with complex angled and non-Platonic geometries have unique optical, magnetic, catalytic, and biological properties originating from the singularities of the electrical field in apexes and craters. Preparation of such particles with a uniform size/shape and core-shell morphology represents a significant challenge, largely because of the poor knowledge of their formation mechanism. Here, this challenge is addressed and a study of the mechanism of their formation is presented for a case of complex spiky morphologies that led us to the conclusion that NPs with concave geometries can be, in fact, supraparticles (SPs) produced via the self-assembly of smaller convex integrants. This mechanism is exemplified by the vivid case of spiky SPs formed via the attachment of small and faceted Au NPs on smooth Au-coated iron oxide (Fe3O4@Au) seeds. The theoretical calculations of energies of primary interactions-electrostatic repulsion and van-der Waals repulsion, elaborated for this complex case-confirm experimental observation and the self-limiting mechanism of SP formation. Besides demonstrating the mechanistic aspects of synthesis of NPs with complex geometries, this work also uncovers a facile approach for preparation of concave magnetoplasmonic particles. When combined with a spiky geometry, such bi-functional magnetoplasmonic SPs can serve as a unique platform for optoelectronic devices and biomedical applications. [ABSTRACT FROM AUTHOR]

This paper describes a fluidic actuation scheme that uses a combination of electrical modulation of surface wettability and spatially directed flow transitions to switch lateral positions of high-speed gas-liquid two-phase flows reversibly. Gas-liquid two-phase flow configurations are advantageous for development of various microfluidic systems such as bioreactors, electronic chip cooling systems, fuel cells, digitally controlled microflows, surface micropatterning, biochemical assays, and cell analysis. A key to success of two-phase processes is the ability to manipulate fluid flows in a rapid, reversible, and accurate manner to enhance mixing, to promote phase change, and to localize reagents and analytes stably.

6-mercaptopurine and its riboside derivatives are some of the most widely utilized anti-leukemic and anti-inflammatory drugs. Their short biological half-life and severe side effects limit their use. A new delivery method for these drugs based on 4-5 nm gold nanoparticles can potentially resolve these issues. We have found substantial enhancement of the antiproliferative effect against K-562 leukemia cells of Au nanoparticles bearing 6-mercaptopurine-9-beta-d-ribofuranoside compared to the same drug in typically administered free form. The improvement was attributed to enhanced intracellular transport followed by the subsequent release in lysosomes. Enhanced activity and nanoparticle carriers will make possible the reduction of the overall concentration of the drug, renal clearance, and, thus, side effects. The nanoparticles with mercaptopurine also showed excellent stability over 1 year without loss of inhibitory activity.