Nanomaterials synthesis, using nanocrystals

The most prominent nanomaterials for bioanalysis at present are semiconductor QDs. Rare-earth doped upconverting nanocrystals and precious metal nanoparticles are becoming increasingly popular, yet they are still far from reaching the level of use of QDs. Other luminescent nanoparticles like carbon-based nanoparticles start to appear, but the synthesis and application of these materials are still in their infancy and not significant for practitioners in the field of bioanalysis. 

Synthesis forms a vital aspect of the science of nanomaterials. In this context, chemical methods have proved to be more effective and versatile than physical methods and have therefore, been employed widely to synthesize a variety of nanomaterials, including zero-dimensional nanocrystals, one-dimensional nanowircs and nanotubes as well as two-dimensional nanofilms and nanowalls. Chemical synthesis of inorganic nanomaterials has been pursued vigorously in the last few years and in this article we provide a perspective on the present status of the subject. The article includes a discussion of nanocrystals and nanowires of metals, oxides, chalcogenides and pnictides. In addition, inorganic nanotubes and nanowalls have been reviewed. Some aspects of core-shell particles, oriented attachment and the use of liquid-liquid interfaces are also presented. 

Nanomaterial growth can be controlled by selective microwave heating as exemplified by the synthesis of CdSe and CdTe in microwave transparent alkane solvents. The high microwave absorptivity of the chalcogenide precursors allowed instantaneous activation and subsequent nucleation and is a clear example of the specific microwave effect. Regardless of the desired size, narrow dispersity nanocrystals could be isolated in less than 3 min. The reaction did not require a high temperature injection step, a problem encountered with conventional approaches. In addition, the use of a stop-flow reactor allowed for automation of the process for scale-np. 

An emerging direction in nanomaterials research concerns the synthesis of multifunctional nanocrystals [62]. Seeded growth [63] of a second material on already preformed nanocrystals has given access to nano-objects associating discrete, nonconcentric, and chemically different domains. Anisotropic nanocrystals due to their distinct reactivity along different facets offer the possibility to position the second domain on a specific facet [64]. Using as seeds cobalt nanorods we have... 

The hot-soup method has also been applied to the production of semiconductor core/shell nanostructures, where a semiconductor nanocrystal is coated with a second semiconductor of wider bandgap. Shown in Figure 23 is a scheme for the synthesis of CdSe/CdS core/shell nanocrystals. These nanostructures were synthesized via a high-temperature route in a mixture of TOP and TOPO (223,224) and via a low-temperature route in pyridine (225). Among the semiconductor core/shell nanomaterials produced were CdSe/ZnS, CdSe/CdS, InAs/InP, InAs/CdSe, InAs/ZnSe, and InAs/ZnS (223,225-228). In these structures, the shell type and thickness allow further control of the optical, electronic, and other properties of semiconductor nanocrystals. For example, the shell may be used to passivate the imperfect surface of the core semiconductor, resulting in significantly improved luminescence efficiency. 

Ceria has a cubic phase with a fluorite structure. Therefore, common as-prepared ceria nanocrystals are isotropic in shape. On the other hand, the fluorite structure has a different atomic arrangement in different facets, which leads to distinct catalytic activity. Controlling the shape of nanocrystals can be used to tune the catalydc properties of such materials. The size of nanocrystals is another important factor in determining catalytic properties because of the surface effect. Various methods have been developed to control the size and shape of ceria nanomaterials. In this section, we will describe recent advances in the synthesis of this kind of nanomaterial and present some typical examples to introduce the characteristics of the various methods. 

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