As a process by which molecular subunits spatially organize into well-defined supramolecular structures through noncovalent interactions, self-assembly is becoming a powerful synthesis approach for generating advanced materials out of nanoparticle (NP) building blocks.[1] Highly structured NP assemblies, such as wires, rings, and superlattices, can be prepared on flat surfaces. [2] However, ordered NP-based structures remain difficult to prepare in the unsupported colloidal form, [3±5] limiting the prospects of the latter for practical applications (such as drug delivery and catalysis). Here, we identify the solution conditions under which inorganic NPs effectively self-assemble into hierarchically ordered closed-shell structures in the presence of polymer. We demonstrate that in a tandem, two-step process, cationic polyamines form supramolecular aggregates with multivalent counteranions via ionic crosslinking, and negatively charged NPs deposit around these aggregates to form a multilayer-thick shell. The resulting organic/inorganic hybrid microcapsules contain polymer or water in the core interior, depending on the multivalent anion used.In this work, we analyze the polymer aggregation step and present a new preparative route that we term ªtandem self-assemblyº or ªpolymer aggregate templatingº. Rotello and co-workers showed that appropriately functionalized gold NPs could be induced to self-assemble into micrometer-sized spherical aggregates by using polymer chains specially designed with complementary hydrogen-bonding pendant groups.[4] Stucky and co-workers later showed that diblock copolypeptides could mediate NP self-assembly to form hollow microspheres, in which cysteine and lysine polymer blocks bind to gold and silica NP surfaces, respectively.[5]Hollow-sphere (alternatively, capsule or shell) structures function as encapsulation, protection, and delivery agents, and are used in application areas as diverse as medicine, foods, cosmetics, and paints. [6,7] Further research indicated that Au NP/polymer aggregates were important intermediates in the formation of the hollow spheres. [8] In the course of studying this NP self-assembly process, we observed that poly(L-lysine) (PLL) chains undergo counterion condensation in certain salt solutions to form polymer aggregates, and discovered that these ionically crosslinked polymer aggregates lead to the rapid formation of ordered microcapsule structures.The room-temperature synthesis of microcapsules (or nanoparticle-assembled capsules, NACs) is illustrated by using PLL conjugated to fluorescein isothiocyanate (FITC) dye. In a typical preparation, 21 lL of an FITC-tagged PLL solution (2 mg mL ±1 , 68 kDa, HBr salt; 1 Da = 1 g mol ±1 ) was gently mixed for 10 s with 125 lL of a tetrasodium ethylenediamine tetraacetate solution (Na 4 EDTA, 4.02 mM) or with a trisodium citrate solution (Na 3 Cit, 5.36 mM) (Fig. 1a). The overall charge ratio, R, of total negative charge of the added salt to total positive charge of the polymer (R = [anion] ½z ± ½/[polymer] ½z + ½...

There is current interest in developing new synthesis strategies for multifunctional hollow spheres with tunable structural properties that would be useful in encapsulation and controlled release applications. A new route was reported recently, in which the sequential reaction of polyamines, multivalent anions, and charged nanoparticles leads to the formation of polymer-filled and water-filled organic/inorganic micron-sized structures known as nanoparticle-assembled capsules. This technique is unique among other capsule preparation routes, as it allows the rapid and scalable formation of robust shells at room temperature, in near-neutral water, and with readily available precursors. This nanoparticle assembly synthesis route involves two steps: the formation of polymer aggregates and the subsequent deposition of particles around the aggregates. The purpose of this paper is to understand in greater detail the noncovalent chemistry of the polymer-salt aggregation step. With poly(allylamine hydrochloride) (PAH) as the model polymer, aggregate formation was investigated as a function of charge ratio, pH, and time through dynamic light scattering, electrophoretic mobility measurements, chloride ion measurements, and optical microscopy. PAH formed aggregates by the cross-linking action of divalent and higher-valent anions above a critical charge ratio and in a pH range defined by the pKa values of PAH and the anion. The aggregates grew in size through coalescence and with growth rates that depended on their surface charge. Controlling polymer aggregate growth provided a direct and simple means to adjust the size of the resultant capsule materials.

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We describe here a methodology to immobilise/stabilise metallic Pd on surface-functionalised ferrite nanoparticles leading to a magnetically separable catalyst, which exhibits efficient catalytic activity in various hydrogenation and ligand-free Suzuki-Miyaura reactions. Importantly, the use of highly branched polyethylenimine to entrap Pd nanoparticles on the surface of Fe 3 O 4 results in structurally stable catalytic sites, which makes them recyclable without any loss in activity.