Anisotropic particles assembly

When instead assemblies of helices are taken into account, it is well known that for many aspects DNA duplexes in solution can be treated as a charged anisotropic particle [2]. Accordingly, steric, electrostatic, and Van der Waals interactions, together with the mechanical properties of the helix (bending and torsional rigidity), play a major role in the formation of DNA mesophases. In addition, all these different kinds of interactions combine in a subtle and still poorly understood way to generate other forces relevant for the case of DNA. A notable example is the helix-specific, chiral interaction, whose importance for DNA assemblies will be discussed below. 

The previous sections reviewed recent advancements in sequential electrostatic assembly to form NP-shelled structures. An alternate route to NP assembly arises from interfacial activity and stabilization of NPs. Colloidal particles with partial hydrophilic and hydrophobic character are known to behave like surface-active molecules (surfactants), particularly when adsorbed to a fluid-fluid interface. The assembly of small particles at interfaces is of relevance to advance fields that traditionally feature emulsions, foams, and flotation systems. It is also of pertinence to the development of new fields such as the synthesis of novel materials that include Janus particles, colloidosomes, porous solids, and anisotropic particles, all recently prepared by particle assembly at interfaces [36,38]. 

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. 

Mesocrystals are a very interesting form of colloidal crystal, as they extend the so far known colloidal crystals with spherical building units to those with non-spherical building units. This offers new possibilities of superstructme formation due to the anisotropic particle shape of the nanoparticle building units [113]. Thus, mesocrystals are colloidal crystals but with extended possibihties for their self-assembled superstructure, offering new handles for crystal morphology control. 

The possibility of assembling anisotropic particles is an interesting feature and has been mainly investigated in capillary assembly. Complex particles such as tetrahedrally shaped CdTe nanotetrapods were successfully assembled. The assembly of nanorods and nanowires was also reported. " ... 

Anisotropic particles, such as rods, are fabricated using two different methods. The first are solution based methods where a surfactant assembles soft templates in solution and promotes growth of particles in one direction" A variety of particles with extremely large aspect ratios can be produced with this method. An alternative method is to fabricate particles within the pores of a template (e.g. nanopore alumina or polycarbonate filters) that have well defined pore sizes Metal is deposited into the... 

The hydrogen-bond mediated self-assembly of nanoparticles and polymers provides a versatile and effective method to control interparticle distances, assembly shapes, sizes, and anisotropic ordering of the resultant nanocomposites. This approach presents the bottom-up strategy to fabricate nanomaterials from molecular building blocks, which have great potential for assembling and integrating nanoscale materials and particles into advanced structures, systems, and devices. 

Many fibers composed of natural polymers contain uniaxially oriented anisotropic hollow spaces in which metal particles can crystallize under appropriate conditions. As a consequence, the impregnated fibers contain anisotropic assemblies of metal nanoparticles that induce anisotropic optical properties... 

Assembly of silicon chips onto substrates with anisotropically conductive adhesives uses specialized equipment, initially developed for ffip-chip solder and TAB inner lead bonding. Heat and pressure are transmitted to the adhesive through a thermode attached to a robotic arm or a high-precision linear translator. Equipment requirements are more demanding than for solder assembly, as no self-alignment can occur. A minimum placement accuracy of 0.0005 in. is required. Coplanarity between the substrate and die is critical one study reports maintaining coplanarity to within 0.00004 in. [19]. The pressure required to achieve interconnection depends on the size of the die, the type of conductive particle used, and the viscosity of the adhesive at the bonding temperature. 

Along this line of using amphiphilic features of particles to drive assembly using a hydrophobic effect, there has been a recent surge of interest in the fabrication and behavior of anisotropic patchy or Janus-type colloidal particles as a promising route to innovative nanocomposite materials [40, 41]. Whereas a thorough review lies outside our scope, we would like to highlight a few examples. Muller and coworkers prepared disc-like polymer Janus particles from assem-... 

Velegol used a so-called particle lithography technique in which colloids were deposited on a flat solid surface, after which heterocoagulation of macromolecules and or particles could take place on the exposed areas [132-135]. Anisotropic assemblies of colloids can be manufactured via this route. 

We have seen in this review that there are a vast array of physical methods that we can make use of in the design of nanocomposite polymer colloids. The classical approach of heterocoagulation can undergo a renaissance by exploring driving forces such as the hydrophobic effect and secondary molecular interactions. Self-assembly of complex anisotropic colloidal particles is already creating a whole new direction in the fabrication of supracolloidal structures. 

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