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Janus particles have built momentum in the last years owing to their unique structure and properties. In this review, we present an overview of the advances in the field of non-centrosymmetric Janus particles and discuss in detail the synthesis, self-assembly behavior, and physical properties as well as applications in various fields.

We study the active dynamics of self-propelled asymmetrical colloidal particles (Janus particles) fueled by an AC electric field. Both the speed and direction of the self-propulsion, and the strength of the attractive interaction between particles can be controlled by tuning the frequency of the applied electric field and the ion concentration of the solution. The strong attractive force at high ion concentration gives rise to chain formation of the Janus particles, which can be explained by the quadrupolar charge distribution on the particles. Chain formation is observed irrespective of the direction of the self-propulsion of the particles. When both the position and the orientation of the heads of the chains are fixed, they exhibit beating behavior reminiscent of eukaryotic flagella. The beating frequency of the chains of Janus particles depends on the applied voltage and thus on the self-propulsive force. The scaling relation between the beating frequency and the self-propulsive force deviates from theoretical predictions made previously on active filaments. However, this discrepancy is resolved by assuming that the attractive interaction between the particles is mediated by the quadrupolar distribution of the induced charges, which gives indirect but convincing evidence on the mechanisms of the Janus particles. This signifies that the dependence between the propulsion mechanism and the interaction mechanism, which had been dismissed previously, can modify the dispersion relations of beating behaviors. In addition, hydrodynamic interaction within the chain, and its effect on propulsion speed, are discussed. These provide new insights into active filaments, such as optimal flagellar design for biological functions.

The sinking of organic particles produced in the upper sunlit layers of the ocean forms an important limb of the oceanic biological pump, which impacts the sequestration of carbon and resupply of nutrients in the mesopelagic ocean. Particles raining out from the upper ocean undergo remineralization by bacteria colonized on their surface and interior, leading to an attenuation in the sinking flux of organic matter with depth. Here, we formulate a mechanistic model for the depth-dependent, sinking, particulate mass flux constituted by a range of sinking, remineralizing particles. Like previous studies, we find that the model does not achieve the characteristic ‘Martin curve’ flux profile with a single type of particle, but instead requires a distribution of particle sizes and/or properties. We consider various functional forms of remineralization appropriate for solid/compact particles, and aggregates with an anoxic or oxic interior. We explore the sensitivity of the shape of the flux vs. depth profile to the choice of remineralization function, relative particle density, particle size distribution, and water column density stratification, and find that neither a power-law nor exponential function provides a definitively superior fit to the modeled profiles. The profiles are also sensitive to the time history of the particle source. Varying surface particle size distribution (via the slope of the particle number spectrum) over 3 days to represent a transient phytoplankton bloom results in transient subsurface maxima or pulses in the sinking mass flux. This work contributes to a growing body of mechanistic export flux models that offer scope to incorporate underlying dynamical and biological processes into global carbon cycle models.

Coffee rings have hidden depths When a drop of coffee dries, a halo of particles accumulates at the drop's edge. This 'coffee-ring effect', first described formally in a Nature paper in 1997, is a common occurrence when a solution of suspended colloidal particles evaporates. Far from being just a household curiosity, it has turned out to have relevance for many applications in which a uniform particle deposition is required, such as inkjet printing, assembly of photonics components and manufacture of DNA chips. In this issue, Peter Yunker and colleagues show that ellipsoidal particles suppress the coffee-ring effect. Attractive interparticle interactions between ellipsoids are sufficiently strong to counteract the forces that drive spherical particles towards the drop's edge as the drop evaporates. The coffee-ring effect can be restored for ellipsoids in solution containing surfactant, and 'designed' mixtures of spheres and ellipsoids can lead to uniform deposition. When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect 1 , 2 , 3 , is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids 1 , 4 , 5 to nanoparticles 6 and individual molecules 7 . In fact—despite the many practical applications for uniform coatings in printing 8 , biology 9 , 10 and complex assembly 11 —the ubiquitous nature of the effect has made it difficult to avoid 6 , 12 , 13 , 14 , 15 , 16 . Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions 17 , 18 , 19 , 20 , 21 , 22 , 23 . Thus, after the ellipsoids are carried to the air–water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air–water interface 17 , 18 , 21 , 24 . These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.

The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable and efficient systems for the conversion and storage of renewable energy sources. The production of hydrogen through water splitting seems a promising and appealing solution. We found that a robust nanoparticulate electrocatalytic material, H 2 –CoCat, can be electrochemically prepared from cobalt salts in a phosphate buffer. This material consists of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte and mediates H 2 evolution from neutral aqueous buffer at modest overpotentials. Remarkably, it can be converted on anodic equilibration into the previously described amorphous cobalt oxide film (O 2 –CoCat or CoPi) catalysing O 2 evolution. The switch between the two catalytic forms is fully reversible and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode. After deposition, the noble-metal-free coating thus functions as a robust, bifunctional and switchable catalyst. Innovative solutions for the design of sustainable and efficient systems for the conversion and storage of renewable energy sources are needed, and one promising option is the production of hydrogen through water splitting. A nanoparticulate electrocatalytic material consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer is now found to exhibit active hydrogen evolution, and can also be converted into a cobalt oxide film catalysing oxygen evolution.

Molecular mass (MM) is one of the key structural parameters obtained by small-angle X-ray scattering (SAXS) of proteins in solution and is used to assess the sample quality, oligomeric composition and to guide subsequent structural modelling. Concentration-dependent assessment of MM relies on a number of extra quantities (partial specific volume, calibrated intensity, accurate solute concentration) and often yields limited accuracy. Concentration-independent methods forgo these requirements being based on the relationship between structural parameters, scattering invariants and particle volume obtained directly from the data. Using a comparative analysis on 165,982 unique scattering profiles calculated from high-resolution protein structures, the performance of multiple concentration-independent MM determination methods was assessed. A Bayesian inference approach was developed affording an accuracy above that of the individual methods, and reports MM estimates together with a credibility interval. This Bayesian approach can be used in combination with concentration-dependent MM methods to further validate the MM of proteins in solution, or as a reliable stand-alone tool in instances where an accurate concentration estimate is not available.

The behaviour of a non-spherical osmotic motor – an axisymmetric catalytic particle self-propelling in a dilute dispersion of reactant particles – is considered. In contrast to a conventional osmotic motor that creates differences in concentration, and hence in osmotic pressure, due to asymmetry in reaction rate along its surface (e.g. a Janus particle with reactive and non-reactive patches), a non-spherical particle is able to move even with uniform chemical activity on its surface. For small departures from a sphere the velocity of self-propulsion is proportional to the square of the non-sphericity or distortion of the particle shape. It is shown that the inclusion of hydrodynamic interactions (HI) may drastically change the self-propulsion. Except for very slow chemical reactions, even the direction of self-propulsion changes with and without HI. Numerical calculations at finite non-sphericity suggest that the maximum velocity of self-propulsion is obtained by a sail-like motor shape, leading to the name ‘chemical sailing’. Moreover, no saturation in the speed of propulsion is found; the motor velocity increases as the area of this ‘sail’ grows and its thickness decreases. The self-propulsion of a non-spherical particle releasing products of a chemical reaction – a constant flux motor – is also considered.

Highly active metal nanoparticles are desired to serve in high-temperature electrocatalysis, for example, in solid oxide electrochemical cells. Unfortunately, the low thermal stability of nanosized particles and the sophisticated interface requirement for electrode structures to support concurrent ionic and electronic transport make it hard to identify the exact catalytic role of nanoparticles embedded within complex electrode architectures. Here we present an accurate analysis of the reactivity of oxide electrodes boosted by metal nanoparticles, where all particles participate in the reaction. Monodisperse particles (Pt, Pd, Au and Co), 10 nm in size and stable at high temperature (more than 600 °C), are uniformly distributed onto mixed-conducting oxide electrodes as a model electrochemical cell via self-assembled nanopatterning. We identify how the metal catalysts activate hydrogen electrooxidation on the ceria-based electrode surface and quantify how rapidly the reaction rate increases with proper choice of metal. These results suggest an ideal electrode design for high-temperature electrochemical applications.The impact of metal nanoparticles on the reactivity of mixed-conducting oxide fuel cell electrodes is identified and quantified.

The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate our ability to engineer the open-system dynamics through the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions, and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation. Opening the way to quantum computation Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics. However, engineering the dynamics of many particles by a controlled coupling to an environment (in an 'open' quantum system) remains largely unexplored. Barreiro et al . demonstrate an approach based on ion-trap technology for simulating an open quantum system with up to five qubits. By adding controlled dissipation to coherent operations, the work offers novel prospects for open-system quantum simulation and computation. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics. However, engineering the dynamics of many particles by a controlled coupling to an environment (in an 'open' quantum system) remains largely unexplored. Here, an approach is demonstrated based on ion-trap technology for simulating an open quantum system with up to five qubits. By adding controlled dissipation to coherent operations, the work offers novel prospects for open-system quantum simulation and computation.

The search for active semiconductor photocatalysts that split water directly under visible-light irradiation remains challenging for solar applications. An orthophosphate semiconductor, Ag 3 PO 4 , which is capable of harnessing visible light to oxidize water as well as decompose organic contaminants in aqueous solution is now reported. The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization 1 , 2 , 3 , 4 , 5 , 6 . Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In 1− x Ni x TaO 4 and (V-,Fe- or Mn-)TiO 2 (refs  7 8 ), non-metal-ion substitution as in TiO 2− x N x  and Sm 2 Ti 2 O 5 S 2 (refs  9 10 ) or solid-solution fabrication as in (Ga 1− x Zn x )(N 1− x O x ) and ZnS–CuInS 2 –AgInS 2 (refs  11 12 ). Here we report a new use of Ag 3 PO 4 semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.