Surfactants and membrane lipids readily assemble into complex structures such as micelles, liposomes or hollow vesicles owing to their amphiphilic character-the fact that part of their structure is attracted to polar environments while another part is attracted to non-polar environments. The self-assembly of complex structures also occurs in polyoxometallate chemistry, as exemplified by the molybdenum blue solutions known for centuries. But while the presence of nanometre-sized metal oxide aggregates in these solutions has long been recognized, unravelling the composition and formation process of these aggregates proved difficult. Recent work has indicated that discrete, wheel-shaped mixed-valence polyoxomolybdate clusters of the type [Mo154] (refs 2-4) assemble into well-defined nanometre-sized aggregates, including spherical structures. Here we report light-scattering data and transmission electron microscopy images of hollow spherical structures with an average, almost monodisperse radius of about 45 nm and composed of approximately 1,165 [Mo154] wheel-shaped clusters. The clusters appear to lie flat and homogeneously distributed on the vesicle surface. Unlike conventional lipid vesicles, the structures we observe are not stabilized by hydrophobic interactions. Instead, we believe the polyoxomolybdate-based vesicles form owing to a subtle interplay between short-range van der Waals attraction and long-range electrostatic repulsion, with important further stabilization arising from hydrogen bonding involving water molecules encapsulated between the wheel-shaped clusters and in the vesicles' interior.

Large, hydrophilic polyoxoanions with high solubility in water and/or other polar solvents demonstrate unique solution behavior by self-assembling into single layer, hollow, spherical "blackberry" structures, which is obviously different from small, simple ions. These macroions cannot be treated as insoluble colloidal suspensions because they form stable "real solutions". Counterion-mediated attraction is considered as the main driving force for the self-assembly behavior. The size disparity between the macroions and their counterions results in macroion-counterion pairing which leads to the inter-macroanionic attraction. The blackberries, with robust membranes semi-permeable to cations, can adjust their size accurately and reversibly in response to the change of solvent polarity and charge density of individual macroions. The inorganic macroions with well-defined size, shape, mass, charge density, but no intramolecular interactions, are ideal model systems to study the intermolecular interactions in polyelectrolyte and bio-macromolecular solutions. The blackberry structures show certain similarities to spherical viral capsids, from the overall structure to the formation kinetics. More amazingly, these inorganic macroions demonstrate some features usually believed to belong only to complex biological molecules, such as the self-recognition in dilute solutions. Meanwhile, polyoxometalates-based organic-inorganic hybrid materials demonstrate amphiphilic properties by self-assembling into vesicles and reverse vesicles in polar and non-polar solvents, respectively, and form monolayer at the water/air interface. Different from conventional amphiphiles, these hybrids show pH-dependent and counterion-dependent self-assembly behaviors with controllable functionality, e.g. fluorescence and catalytic activity, due to the high and tunable charges and the functionalities of POM polar head groups.

The Pluronic polyol F127, PEO99PPO69PEO99 (PEO and PPO being poly(ethylene oxide) and poly(propylene oxide), respectively) has the potential to be used as an effective separation medium in capillary electrophoresis (CE) for the separation of biomacromolecules such as DNA fragments and proteins. Static light scattering (SLS), dynamic light scattering (DLS), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) were used to characterize the solution properties and the microstructures of F127 in the same electrophoresis buffer used in CE. In the dilute solution region, F127 in CE buffer forms micelles similar to that in water. At high solution concentrations, micelles tend to pack into some crystalline forms with relatively well-structured PPO centers. By using a combination of SANS and SAXS results, we are able to conclusively determine the gellike structure to have a facecentered cubic lattice. The effects of solvent, polymer concentration, temperature, and sample preparation procedure on the gel structure were studied. In the gellike region the aggregation number and also the micellar size are not sensitive to both the concentration change and the temperature change. The results of DNA electrophoretic migration in F127 gels also support these findings.

The solution behavior of the largest inorganic acid known thus far, the neutral, spherical iron/molybdenum/oxide nanocluster {Mo72Fe30} ([triple bond{(MoVI) MoVI5}12FeIII30 1a), including the pH-controlled deprotonation, is reported. The acidic properties are due to the 30 peripheral, weakly acidic FeIII(H2O) groups that form a unique Archimedean solid with all edges and dihedral angles being equal, the icosidodecahedron, and therefore an "isotropic" surface. Interestingly, the aqueous solutions are stable even for months because of the inertness of the spherical solutes and the presence of the hard FeIII and MoVI centers. The stability can be nicely proven by the very characteristic Raman spectrum showing, because of the (approximately) icosahedral symmetry, only a few lines. Whereas the {Mo72Fe30} clusters exist as discrete, almost neutral, molecules in aqueous solution at pH< 2.9, they get deprotonated and self-associate into single-layer blackberry-type structures at higher pH while the assembly process (i.e., the size of the final species) can be controlled by the pH values; this allows the deliberate generation of differently sized nanoparticles, a long-term goal in nanoscience. The average hydrodynamic radius (Rh) of the self-assembled structures decreases monotonically with increasing number of charges on the {Mo72Fe30} macroanions (from approximately 45 nm at pH approximately 3.0 to approximately 15 nm at pH approximately 6.6), as studied by laser light scattering and TEM techniques. The {Mo72Fe30} macroions with high-stability tunable charges/surfaces, equal shape, and masses provide models for the understanding of more complex polyelectrolyte solutions while the controllable association and dissociation reported here of the assembled soft magnetic materials with tuneable sizes could be interesting for practical applications.

The ability to construct size- and shape-controllable architectures using nanoparticles as building blocks is essential for the exploration of nanoparticle-structured properties. This paper reports findings of an investigation of a mediator-template strategy for the size-controllable assembly of nanoparticles. This strategy explores multidentate thioether ligands as molecular mediators and tetraalkylammonium-capped gold nanoparticles (5 nm) as templates toward the preparation of size-controllable and monodispersed spherical assemblies ( approximately 20-300-nm diameters). The combination of the mediation force of the multidentate thioether and the hydrophobic force of the tetraalkylammonium template establishes the interparticle linkage and stability. The morphological properties of the spherical assemblies have been characterized using TEM, AFM, and SAXS techniques. The finding of the soft-hard nature of the nanoparticle assemblies and their interactions with contacting substrates could form the basis of a new strategy for manipulating nanoscale linkages between nanoparticle assemblies, soldering nanoelectronics, and constructing nanosensor devices. The intriguing light scattering and optical absorption properties in response to assembly, disassembly, sizing, and interparticle spacing parameters have been characterized by dynamic light scattering and spectrophotometric measurements. The discovery of the controlled disassembly into individual nanoparticles and the size regulation by a third capping component could form the basis for applications in controlled drug delivery. The fundamental basis for the mediator-template strategy as a versatile assembly technique is further discussed in terms of experimental and theoretical correlations of the morphological and optical properties.

Mn-Anderson-C6 and Mn-Anderson-C16, A type of inorganic-organic hybrid molecules containing a large anionic polyoxometalate (POM) cluster and two C6 and C16 alkyl chains, respectively, demonstrate amphiphilic surfactant behavior in the mixed solvents of acetonitrile and water. The amphiphilic hybrid molecules can slowly assemble into membrane-like vesicles by using the POM clusters as polar head groups, as studied by laser light scattering and TEM techniques. The hollow vesicles have a typical bilayer structure with the hydrophilic Mn-Anderson cluster facing outside and long hydrophobic alkyl chains staying inside to form the solvent-phobic layer. Due to the rigidity of the POM polar heads, the two alkyl tails have to bend significantly for the vesicle formation, which makes the vesicle formation more difficult compared to some conventional surfactants. This is the first example of using hydrophilic POM macroions as polar head groups for a surfactant system.

A complete, continuous transition from discrete macroions to blackberry structures, and then back to discrete macroions, is reported for the first time in the system of {Mo132}/water/acetone, with {Mo132} (full formula (NH4)42[Mo132O372(CH3COO)30(H2O)72].ca.300H2O.ca.10CH3COONH4) as the C60-like anionic polyoxomolybdate molecular clusters. Laser light scattering studies reveal the presence of the self-assembled {Mo132} blackberry structures in water/acetone mixed solvents containing 3 vol % to 70 vol % acetone, with the average hydrodynamic radius (Rh) of blackberries ranging from 45 to 100 nm with increasing acetone content. Only discrete {Mo132} clusters are found in solutions containing <3 vol % and >70 vol % acetone. The complete discrete macroion (cluster)-blackberry-discrete macroion transition helps to identify the driving forces behind the blackberry formation, a new type of self-assembly process. The charge density on the macroions is found to greatly affect the blackberry formation and dissociation, as the counterion association is very dominant around blackberries. The transitions between single {Mo132} clusters and blackberries, and between the blackberries with different sizes, are achieved by only changing the solvent quality.

Large, hydrophilic inorganic ions (mostly polyoxometalate macroions and cationic metal-organic hybrid nanocages) with high solubility in water and/or other polar solvents demonstrate unique solution behaviors. In dilute solutions, they behave significantly different from small simple ions (as described by the Debye-Hückel theory) because the macroions cannot be treated as point charges or large, insoluble colloidal suspensions (usually described by the DLVO theory) because the macroions form homogeneous, stable "real solutions". The size disparity between the macroions and their counterions results in complex macroion-counterion interaction and leads to the self-assembly of macroions into single-layered, hollow, spherical "blackberry" structures. The blackberries, with robust and very stable structures mimicking biological membranes, can adjust their size accurately and reversibly in response to the change of solvent content, charge density on the macroions, or in some cases merely solution pH. The blackberry membrane is permeable to small cations. The inorganic macroions with well-defined size, shape, mass, charge density (even accurately tunable within certain range), and no intramolecular interaction can be treated as simple model systems to understand the intermolecular interaction in polyelectrolyte solutions. The blackberry structures show certain similarities to the spherical virus capsids, from the overall structure to the kinetic properties of formation.