Colloid-templated nanoparticle assemblies

Figure 6.8 Illustration of colloid-templated nanoparticle assemblies. The process involves the layer-by-layer adsorption of charged polymers and oppositely charged nanoparticles onto the surfaces of the colloidal template. The colloidal core particles may then be removed to generate a hollow sphere of nanoparticles, held together by electrostatic interactions with the linear polymer glue ...

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]. 

Particularly in 2D systems, control over the self-assembly of colloidal templates has offered a versatile way to produce patterned surfaces or arrays with a precision of few nanometres. Diblock copolymer micellar nanolithography (dBCML) is a versatile method that uses homopolymers or block copolymers for the production of complex surface structures with nanosized features [69], In contrast to other approaches like electron-beam lithography (EBL) and photolithography, dBCML does not require extensive equipment. In fact, it is commonly used in the fabrication of data storage devices and photonic crystals, in catalyses [70], and for the design of mesoporous films and nanoparticle arrays [71]. 

A similar process for preparing functional cyclodextrin nanospheres has been reported by the Harada group [25], )-Cyclodextrins were exhaustively thiolated at the lower rim and assembled around colloidal gold nanoparticles. Following assembly around the gold template the cyclodextrins were linked through the formation... 

Fig. 11 Process of the DNA-based colloidal nanoparticle assembly, a Base-pairing interactions induced assembly in ID template, b Immobilization by DNA hybridization onto 2D surface, c 3D assembly by duplex DNA interconnects...

Ethayaraja, M., Dutta, K. Bandyopadhyaya, R. 2006 Mechanism of nanoparticle formation in self-assembled colloidal templates population balance model and Monte Carlo simulation. Journal of Physical Chemistry B 110, 16471-16481. 

In Fig. 6, we illustrate some different ways that the core-shell topology could be varied for silica and gold. So far we have considered the two normal core-shell structures. We now focus on the third example the assembly of Au Si02 nanoparticles onto spherical polystyrene latex colloids. The resulting spheres are also essentially different to continuous metal shells grown on colloid templates, which have been reported by Halas and colleagues [17] and by van Blaaderen and coworkers [18]. Such continuous shells display optical properties associated with resonances of the whole shell, and are therefore extremely sensitive to both core size and shell thickness, while in the system presented here... 

LbL capsules are spherical compartments with walls formed by adsorption of alternating layers of positively and negatively charged polyelectrolytes [83]. Assembly of LbL capsules is achieved by consecutive deposition of complementary polymers onto colloidal particles (LbL deposition technique), followed by removal of the colloidal template by addition of acids [16,84-86]. CaC03 nanoparticles are commonly used as templates because they have low toxicity and are biodegradable. LbL capsules are an interesting platform for the design of nanoreactors because their permeability can be selectively modified... 

F. Caruso, M. Spasova, V. Salgueiriilo-Maceira, and L.M. Liz-Marzan, Multilayer assemblies of silica-enciqrsulated gold nanoparticles on decomposable colloid templates, Adv. Mater. 13,1090-1095 (2001). 

Other interesting application of PEMs in dmg delivery is the fabrication of multicapsules [76, 230, 231]. Multicapsules are multicompartmental cargo systems based in the LbL assembling of multiple subunits such as polyelectrolyte layers, liposomes, and nanoparticles. The typical example of these systems consists in the coating of a colloidal template using multiple layers formed by polymers and intact vesicles. Once the supramolecular architecture is obtained, the initial template is removed by dissolution. 

A scaled-up version of this central template-concentric sphere surface assembly approach has been demonstrated for the growth of multi-layer core-shell nano- and microparticles, based upon the repeated layer-by-layer deposition of linear polymers and silica nanoparticles onto a colloidal particle template (Figure 6.8) [60]. In this case, the regioselective chemistry occurs via electrostatic interactions, as opposed to the covalent bond formation of most of the examples in this chapter. The central colloidal seed particle dictates the final particle... 

Increasingly chemists are contributing to the synthesis of advanced materials with enhanced or novel properties by using colloidal assemblies as templates. Colloid chemistry is particularly well suited to this objective since nanoparticles, by definition, are colloidal and since processing of advanced materials involve reactions at solid-solid, solid-liquid or solid-gas interfaces (3-5). 

Most of the time, metal/dielectric nanocomposites are studied in the form of solutions or thin solid films on a substrate Colloids, doped and annealed glasses, sol-gels, surfactant-stabilized nanoparticles, micelles, two- or three-dimension self-assembled nanocomposites, self-organized mesoporous oxides filled with metals, electrochemically-loaded template membranes, metal-ion implanted crystals, nanocomposite films elaborated by laser ablation, cluster-beam deposition, radio-frequency sputtering, or nanolithography. 

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

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