Tandem assembly

Abstract Nanoparticles (NPs, diameter range of 1-100 nm) can have size-dependent physical and electronic properties that are useful in a variety of applications. Arranging them into hollow shells introduces the additional functionalities of encapsulation, storage, and controlled release that the constituent NPs do not have.This chapter examines recent developments in the synthesis routes and properties of hollow spheres formed out of NPs. Synthesis approaches reviewed here are recent developments in the electrostatics-based tandem assembly and interfacial stabilization routes to the formation of NP-shelled structures. Distinct from the well-established layer-by-layer (LBL) synthesis approach, the former route leads to NP/polymer composite hollow spheres that are potentially useful in medical therapy, catalysis, and encapsulation applications. The latter route is based on interfacial activity and stabilization by NPs with amphiphilic properties, to generate materials like colloidosomes, Pickering emulsions, and foams. The varied types of NP shells can have unique materials properties that are not found in the NP building blocks, or in polymer-based, surfactant-based, or LBL-assembled capsules. 

Keywords Hollow spheres Nanoparticles Layer-by-layer assembly Tandem assembly Nanoparticle assembled capsule Interfacial stabilization - Particle stabilized emulsion... 

Hollow Spheres from Electrostatic Assembly of Nanoparticles Through Tandem Assembly... 

Another recent development features the in situ formation of liquid colloidal templates. The assembly of NPs at the periphery of these templates is driven by electrostatics, resulting in the formation of robust NP-sheUed hoUow spheres, originally termed nanoparticle-assembled capsules (NACs). This scheme is called tandem assembly , nanoparticle-polymer tandem assembly , or polymer-aggregate tern-plating and presents an alternate, simple and non-destructive route for formation of NP-shelled hollow spheres [6,32-35,40,80,81]. 

Subsequent investigations have established the general nature of the tandem assembly method using a host of linear cationic polymers [such as PLL and poly(allylamine hydrochloride), PAH] and multivalent salt variants (monosodium... 

The unique selling proposition of the tandem-assembly process is its contribution to the field of green chemistry in obtaining closed-shell colloidal structures through an environmentally friendly route, as the entire synthesis is carried out in water, at near neutral pH, and at room temperature. Further, the size of the colloidal template can be controlled by the charge ratios of reactants, which results in structures with sizes ranging from 100nm to 2p,m [32-35,80,81]. 

Rapid and facile generation of capsules from tandem assembly in aqueous media is amenable to encapsulation of water-soluble compounds. Encapsulation of ICG dye within PAH/H2PO4 aggregates was shown by Yu et al. Enzyme encapsulation and the feasibility of capsules to serve as reaction vessels was demonstrated by Rana et al. In their study, they encapsulated acid phosphatase enzyme in PLL-citrate-silica sols and suspended the spheres in a solution containing fluorescein diphosphate. Fluorescence increased in intensity within the shell walls as fluorescein was formed by enzymatic cleavage of phosphate groups. This study showed that microcapsules could serve as reaction vessels that allow enzymatic action to take place in a protective environment and allow for reactants and/or products to diffuse through permeable shell walls. 

In more recent developments, Murthy et al. report a novel variant of tandem assembly leading to the formation of patchy or multicompartment anisotropic microspheres from NP/polymer assembly and demixing of polyamine. On sequentially mixing a blend of PEL and PAH with citric acid and a silica source, PLL/PAH solution was seen to phase-separate as heterogeneous domains. Addition of Si02 NPs... 

Kadali et al. demonstrated another useful application of NACs prepared by tandem assembly in the formation of catalyst support materials. NACs can be calcined to remove the polymer without collapsing the hollow sphere structure (Fig. 10). Such stability is more difficult to achieve with LBL-assembled capsules. On calcination,... 

This chapter describes the non-LBL approaches of tandem assembly and interfacial stabilization for the formation of closed shell structures, with an emphasis on ensembles in which NPs constitute the shell. Tandem assembly is a versatile and environmentally friendly route to the formation of useful NP-shelled capsules. In contrast to sacrificial core templating and LBL assembly methods, tandem assembly has the important differentiating feature that it avoids the incineration or solvent dissolution step to generate the hollow interior of the capsule. Enhancements in optical, mechanical, catalytic, and release properties of such materials hold great promise for their application in photoresponsive delivery systems, catalysis, and encapsulation. Interfacial stabilization routes are found to yield NP-shelled structures in the form of emulsions and foams that have enhanced stability over those from conventional, surfactant-based approaches. Unusual interactions of the NP with fluid interfaces have made possible new structures, such as water-in-air foams, colloidosomes, and anisotropic particles. 

CN to Pd species, tandem assembly ending on the capping with CN is attained for the three components of alkenyl or aryl halides, norbomene, and KCN (Schemes 31... 

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