Osmotically induced deformations (invaginations) of polyelectrolyte capsules were observed in poly(styrene sulfonate, sodium salt) (PSS) solution since PSS of Mw 70 000 is excluded from the capsule interior. It was found that there is a critical osmotic pressure difference at which the initial spherical capsule shape becomes unstable and invaginations are formed. This critical osmotic pressure was obtained as a function of the wall thickness and the capsule size. A theoretical model is provided which describes the relationship between the critical osmotic pressure, the elasticity modulus, the capsule wall thickness, and the capsule radius. The model was verified by measuring the invagination onset as a function of particle radius and wall thickness. The elasticity modulus of the PSS/PAH (polyallylamine hydrochloride) polyelectrolyte multilayer was measured as a function of wall thickness and capsule diameter. The modulus ranges between 500 and 750 MPa, which indicates a relatively strongly interconnected polyelectrolyte multilayer structure. With higher molecular weight PAH the elasticity modulus of the PSS/PAH multilayer was slightly enhanced.PACS. 46.32.+x Static buckling and instability -68.60.Bs Mechanical and acoustical properties

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Ammonia borane hydrolysis is considered as a potential means of safe and fast method of H production if it is efficiently catalyzed. Here a series of nearly monodispersed alloyed bimetallic nanoparticle catalysts are introduced, optimized among transition metals, and found to be extremely efficient and highly selective with sharp positive synergy between 2/3 Ni and 1/3 Pt embedded inside a zeolitic imidazolate framework (ZIF-8) support. These catalysts are much more efficient for H release than either Ni or Pt analogues alone on this support, and for instance the best catalyst NiPt@ZiF-8 achieves a TOF of 600 mol·mol·min and 2222 mol·mol·min under ambient conditions, which overtakes performances of previous Pt-base catalysts. The presence of NaOH boosts H evolution that becomes 87 times faster than in its absence with NiPt@ZiF-8, whereas NaOH decreases H evolution on the related Pt@ZiF-8 catalyst. The ZIF-8 support appears outstanding and much more efficient than other supports including graphene oxide, active carbon and SBA-15 with these nanoparticles. Mechanistic studies especially involving kinetic isotope effects using DO show that cleavage by oxidative addition of an O-H bond of water onto the catalyst surface is the rate-determining step of this reaction. The remarkable catalyst activity of NiPt@ZiF-8 has been exploited for successful tandem catalytic hydrogenation reactions using ammonia borane as H source. In conclusion the selective and remarkable synergy disclosed here together with the mechanistic results should allow significant progress in catalyst design toward convenient H generation from hydrogen-rich substrates in the close future.

We have studied the changes in physical and chemical properties of cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) brushes after collapse driven by ion-pairing interactions in the presence of ClO 4anions. Results derived from the quartz crystal microbalance technique, atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle goniometry indicate that ion-paired collapsed polyelectrolyte brushes suffer a dramatic loss of water accompanied by conformational changes leading to markedly different mechanical properties. This scenario is completely different from polyelectrolyte brushes whose collapse is simply driven by pure Coulombic screening, for example, in the presence of Clanions. In addition, wetting measurements indicated that ion-pairing interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a reversible manner. The immediate implications of these experimental results are related to the promising use of polyelectrolyte brushes as biolubricants and the design of "smart" surfaces exhibiting ion-sensitive reversible changes in interfacial properties.

Dipalmitoyldiphosphatidic acid (DPPA), dipalmitoyldiphosphatidylcholine (DPPC), and sphingosine were adsorbed onto polyelectrolyte coated colloids and capsules forming composite lipidpolyelectrolyte layers. The stepwise coating was performed either by adsorption of preformed vesicles onto the capsule surface or by a solvent exchange protocol. The lipid assembly was monitored by ζ-potential measurements. Single particle light scattering, flow cytometry, and fluorescence studies of Fo ¨rster energy transfer have been used to quantify the lipid coating. Confocal microscopy images of capsules coated with fluorescent lipids demonstrated a homogeneous coverage of the capsule surface. Differential scanning calorimetry shows a phase transition temperature characteristic for lipid layer structures. It was concluded that the adsorbed DPPA form bilayers while DPPC may form multilayers. It was further shown that on top of the lipid layers further polyelectrolyte layers could be assembled. The permeability of 6-carboxyfluorescein (6-CF) through the composite layer structure was studied by means of confocal laser scanning microscopy. The permeation time through the composite layer was of the order of 10 1 -10 2 min, while in the absence of lipids 6-CF equilibrated faster than the time resolution of the technique.

Non-noble metal nanoparticles are notoriously difficult to prepare and stabilize with appropriate dispersion, which in turn severely limits their catalytic functions. Here, using zeolitic imidazolate framework (ZIF-8) as MOF template, catalytically remarkably efficient ligand-free first-row late transition-metal nanoparticles are prepared and compared. Upon scrutiny of the catalytic principles in the hydrolysis of ammonia-borane, the highest total turnover frequency among these first-row late transition metals is achieved for the templated Ni nanoparticles with 85.7 mol mol min at room temperature, which overtakes performances of previous non-noble metal nanoparticles systems, and is even better than some noble metal nanoparticles systems. Mechanistic studies especially using kinetic isotope effects show that cleavage by oxidative addition of an O-H bond in HO is the rate-determining step in this reaction. Inspired by these mechanistic studies, an attractive and effective "on-off" control of hydrogen production is further proposed.

Capsules composed of poly(styrene sulfonate, sodium salt) (PSS)/poly(diallyldimethyl ammonium) chloride (PDADMAC) were prepared by layer-by-layer deposition of the polyelectrolytes on melamine formaldehyde colloidal templates followed by the decomposition of the cores by hydrochloride. A yield of more than 90% of intact capsules was achieved if (i) the core diameter was equal to or less than 3.8 µm and (ii) not more than five pairs of layers were adsorbed. When the cores were larger or the layers were thicker, the osmotic pressure difference caused by core dissolution led to an increased frequency of wall rupture. The elasticity modulus of a multilayer consisting of five pairs of PSS/PDADMAC was about 140 MPa as measured by osmotic pressure induced capsule deformation.

Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Möhwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.