In situ single particle characterization of the themoresponsive and co-nonsolvent behavior of PNIPAM microgels and silica@PNIPAM core-shell colloids.

Poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM colloidal shells attract continuous strong interest due to their thermoresponsive behavior, as their size and properties can be tuned by temperature. The direct single particle observation and characterization of pure, unlabeled PNIPAM microgels in their native aqueous environment relies on imaging techniques that operate either at interfaces or in cryogenic conditions, thus limiting the observation of their dynamic nature. Liquid Cell (Scanning) Transmission Electron Microscopy (LC-(S) TEM) imaging allows the characterization of materials and dynamic processes such as nanoparticle growth, etching, and diffusion, at nanometric resolution in liquids. Here we show that via a facile post-synthetic in situ polymer labelling step with high-contrast marker core-shell Au@SiO2 nanoparticles (NPs) it is possible to determine the full volume of PNIPAM microgels in water. The labelling allowed for the successful characterization of the thermoresponsive behavior of PNIPAM microgels and core shell silica@PNIPAM hybrid microgels, as well as the co-nonsolvency of PNIPAM in aqueous alcoholic solutions. The interplay between electron beam irradiation and PNIPAM systems in water resulted in irreversible shrinkage due to beam induced water radiolysis products, which in turn also affected the thermoresponsive behavior of PNIPAM. The addition of 2-propanol as radical scavenger improved PNIPAM stability in water under electron beam irradiation.

Repulsive van der Waals forces enable Pickering emulsions with non-touching colloids.

Emulsions stabilized by solid particles, called Pickering emulsions, offer promising applications in drug delivery, cosmetics, food science and the manufacturing of porous materials. This potential stems from their high stability against coalescence and 'surfactant-free' nature. Generally, Pickering emulsions require that the solid particles are wetted by both phases and as a result, the adsorption free energy is often large with respect to the thermal energy (kBT). Here we provide the first experimental proof for an alternative scenario: non-touching (effectively non-wetting), charged, particles that are completely immersed in the oil phase through a balance of charge induced attractions and repulsions caused by van der Waals forces. These particles nonetheless stabilize the emulsion. The main advantage of this novel adsorption mechanism is that these particles can easily be detached from the interface simply by adding salt. This not only makes the finding fundamentally of interest, but also enables a triggered de-emulsification and particle recovery, which is useful in fields like enhanced oil recovery, heterogeneous catalysis, and emulsion polymerization.

Charging of poly(methyl methacrylate) (PMMA) colloids in cyclohexyl bromide: locking, size dependence, and particle mixtures.

We studied suspensions of sterically stabilized poly(methyl methacrylate) (PMMA) particles in the solvent cyclohexyl bromide (CHB; εr = 7.92). We performed microelectrophoresis measurements on suspensions containing a single particle species and on binary mixtures, using confocal microscopy to measure the velocity profiles of the particles. We measured the charge of so-called locked PMMA particles, for which the steric stabilizer, a comb-graft stabilizer of poly(12-hydroxystearic acid) (PHSA) grafted on a backbone of PMMA, was covalently bonded to the particle, and for unlocked particles, for which the stabilizer was adsorbed to the surface of the particle. We observed that locked particles had a significantly higher charge than unlocked particles. We found that the charge increase upon locking was due to chemical coupling of 2-(dimethylamino)ethanol to the PMMA particles, which was used as a catalyst for the locking reaction. For particles of different size we obtained the surface potential and charge from the electrophoretic mobility of the particles. For locked particles we found that the relatively high surface potential (∼ +5.1 kBT/e or +130 mV) was roughly constant for all particle diameters we investigated (1.2 µm < σ < 4.4 µm), and that the particle charge was proportional to the square of the diameter.

A reduced SNARE model for membrane fusion.

Let's get together: A minimal model system was developed to mimic the SNARE-protein-mediated fusion of biological membranes (see picture). Fusion between two populations of liposomes is controlled by a pair of complementary lipidated oligopeptides that form noncovalent coiled-coil complexes and thereby force the membranes into close proximity to promote fusion. The model system displays the key characteristics of in vivo fusion events.