We present an approach to fabricate solid capsules with precise control of size, permeability, mechanical strength, and compatibility. The capsules are fabricated by the self-assembly of colloidal particles onto the interface of emulsion droplets. After the particles are locked together to form elastic shells, the emulsion droplets are transferred to a fresh continuous-phase fluid that is the same as that inside the droplets. The resultant structures, which we call "colloidosomes," are hollow, elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of these structures and their potential for cellular immunoisolation are demonstrated by the use of a variety of solvents, particles, and contents.Efficient encapsulation of active ingredients such as drugs, proteins, vitamins, flavors, gas bubbles, or even living cells is becoming increasingly important for a wide variety of applications and technologies, ranging from functional foods to drug delivery to biomedical applications (1-8). Increasingly sophisticated techniques are being developed to create physical structures that can meet the demanding requirements of these applications. A versatile technique should provide efficient encapsulation in structures whose size, permeability, mechanical strength, and compatibility can be easily controlled. Control of the size allows flexibility in applications and choice of encapsulated materials; control of the permeability allows selective and timed release; control of the mechanical strength allows the yield stress to be adjusted to withstand varying of mechanical loads and to enable release by defined shear rates; and control of compatibility allows encapsulation of fragile and sensitive ingredients, such as biomolecules and cells. Precise control of all these features would allow the strategic design of possible release mechanisms. Ideally, it should be feasible to construct these capsules from a wide variety of inorganic, organic, or polymeric materials to provide flexibility in their uses.A variety of techniques has been developed to address specific encapsulation requirements: Coacervation, or controlled gelation, of polymers at the surface of water drops can be used to fabricate nano-or microporous capsules (1-5, 9); other fluid extrusion methods can also be used to create the polymer coating (6, 7). Coating immiscible templates by electrostatic deposition of alternating layers of charged polymers or particles can be used to fabricate nanoporous capsules (10-18). Microfabrication technology can be used to create submillimeter-sized silicon capsules with exquisitely precise nanometer-scale holes for selective permeability and slow release (19). However, despite the enormous progress in encapsulation technologies, these methods can be limited in their applicability, in the range of materials that can be used, in the uniformity of pore sizes, in the accessible permeabilities and elasticities, or in the ease of synthesis, filling efficiency, and yield. We present a flexi...

We describe experimental investigations of the structure of two-dimensional spherical crystals. The crystals, formed by beads self-assembled on water droplets in oil, serve as model systems for exploring very general theories about the minimum energy configurations of particles with arbitrary repulsive interactions on curved surfaces. Above a critical system size we find that crystals develop distinctive high-angle grain boundaries, or scars, not found in planar crystals. The number of excess defects in a scar is shown to grow linearly with the dimensionless system size. The observed slope is expected to be universal, independent of the microscopic potential.Spherical particles on a flat surface pack most efficiently in a simple lattice of triangles, similar to billiard balls at the start of a game. Such six-fold coordinated triangular lattices [1] cannot, however, be wrapped on the curved surface of a sphere; instead, there must be extra defects in coordination number. Soccer balls and C 60 fullerenes [2,3] provide familiar realizations of this fact -they have 12 pentagonal panels and 20 hexagonal panels. The necessary packing defects can be characterized by their topological or disclination charge, q, which is the departure of their coordination number c from the preferred flat space value of 6 (q = 6 − c); a classic theorem of Euler [4,5] shows that the total disclination charge of any triangulation of the sphere must be 12 [6]. A total disclination charge of 12 can be achieved in many ways, however, which makes the determination of the minimum energy configuration of repulsive particles, essential for crystallography on a sphere, an extremely difficult problem. This was recognized nearly 100 years ago by J.J. Thomson [7], who attempted, unsuccessfully, to explain the periodic table in terms of rigid electron shells. Similar problems recur in fields as diverse as multi-electron bubbles in superfluid helium [8], virus morphology [9, 10, 11], protein s-layers [12,13] and coding theory [14,15]. Indeed, both the classic Thomson problem, which deals with particles interacting through the Coulomb potential, and its generalization to other interaction potentials remain largely unsolved after almost 100 years [16,17,18].The spatial curvature encountered in curved geometries adds a fundamentally new ingredient to crystallography, not found in the study of order in spatially flat systems. To date, however, studies of the Thomson and related problems have been limited to theory and computer simulation. As the number of particles on the sphere grows, isolated charge 1 defects are predicted to induce too much strain; this can be relieved by introducing additional dislocations, consisting of pairs of tightly bound 5-7 defects [19] which still satisfy Euler's theorem since their net disclination charge is zero. Dislocations, which are point-like topological defects in two dimensions, disrupt the translational order of the crystalline phase but are less disruptive of orientational order [19]. While they play an essential rol...

Nanometre- and micrometre-sized charged particles at aqueous interfaces are typically stabilized by a repulsive Coulomb interaction. If one of the phases forming the interface is a nonpolar substance (such as air or oil) that cannot sustain a charge, the particles will exhibit long-ranged dipolar repulsion; if the interface area is confined, mutual repulsion between the particles can induce ordering and even crystallization. However, particle ordering has also been observed in the absence of area confinement, suggesting that like-charged particles at interfaces can also experience attractive interactions. Interface deformations are known to cause capillary forces that attract neighbouring particles to each other, but a satisfying explanation for the origin of such distortions remains outstanding. Here we present quantitative measurements of attractive interactions between colloidal particles at an oil-water interface and show that the attraction can be explained by capillary forces that arise from a distortion of the interface shape that is due to electrostatic stresses caused by the particles' dipolar field. This explanation, which is consistent with all reports on interfacial particle ordering so far, also suggests that the attractive interactions might be controllable: by tuning the polarity of one of the interfacial fluids, it should be possible to adjust the electrostatic stresses of the system and hence the interparticle attractions.

We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles. These techniques reproducibly yield shells with controllable properties such as elastic moduli and breaking forces. To enable diffusive exchange through the particle shells, we transfer them into solvents that are miscible with the encapsulant. We characterize the mechanical properties of the shells by measuring the response to deformation by calibrated microcantilevers.

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For many biotechnological applications the label-free detection of biomolecular interactions is becoming of outstanding importance. In this Article we report the direct electrical detection of small peptides and proteins by their intrinsic charges using a biofunctionalized thin-film resistor. The label-free selective and quantitative detection of small peptides and proteins is achieved using hydrophobized silicon-on-insulator (SOI) substrates functionalized with lipid membranes that incorporate metal-chelating lipids. The response of the nanometer-thin conducting silicon film to electrolyte screening effects is taken into account to determine quantitatively the charges of peptides. It is even possible to detect peptides with a single charge and to distinguish single charge variations of the analytes even in physiological electrolyte solutions. As the device is based on standard semiconductor technologies, parallelization and miniaturization of the SOI-based biosensor is achievable by standard CMOS technologies and thus a promising basis for high-throughput screening or biotechnological applications.

Flexible pH sensors based on polysilicon thin film transistors and ZnO nanowalls Appl. Phys. Lett. 105, 093501 (2014); 10.1063/1.4894805 Multianalyte biosensor based on pH-sensitive ZnO electrolyte-insulator-semiconductor structuresWe characterize the recently introduced silicon-on-insulator based thin film resistor in electrolyte solutions and demonstrate its use as a pH sensing device. The sensor's response function can be tuned by a back gate potential, which is demonstrated by employing known changes of the pH of the solution. The highest sensitivity to pH changes is obtained when the charge carrier concentration at the back interface of the thin Si-film is low compared to the front interface. Calibration measurements with a reference electrode are used to relate the obtained resistance to the surface potential. Applying the site binding model to fit the measured data for variations of the pH gives excellent agreement. The sensors response can be related to a surface potential change of Ϫ50 mV/pH and from the obtained signal-to-noise ratio, the detection limit can be estimated to be 0.03 pH. For a ͑bio-͒molecular use of the sensor element, a passivation of the silicon oxide surface against this pH response can be achieved by depositing an organic layer of polymethyl-methacrylate ͑PMMA͒ onto the devices by spin coating. As expected, the pH response of the surface disappears after the deposition of PMMA. This passivation technique provides an easy and reliable way to obtain a biocompatible interface, which can be further functionalized for the detection of specific molecular recognition events.

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