Molecular Tools for the Manipulation of Size, Surface Chemistry and Assemblies of Metal Nanoparticles

Nanoscale particles have been envisioned to be the building blocks of a wide variety of future technologies including catalysis, electronic, optical and information technologies.As the nanoparticles have enhanced surface activity a layer of organic molecules are often used as passivating or capping agents. The surface functioanlization assumes significance not just for their stability in diverse solvent media but defines the way nanoparticles interact either with themselves or with the environment/biological systems. For example, water dispersibility is an essential criterion to realize bio-applications of nanoparticles.On the other hand, dispersions in organic media can be utilized to obtain interesting assemblies. In this connection, we have been working on a procedure called “digestive ripening” process in which a colloidal metal suspension in a solvent is refluxed at or above the solvent boiling temperature in the presence of the surface active agent like thiols resulting in the conversion of a highly polydisperse colloid into a monodispersed one (<5%). It is hypothesized that the thiols bind and remove reactive surface atoms/clusters from big nanoparticles and redeposit them on smaller nanoparticles. In this way, large particles become smaller, while small particles become larger and eventually, an equilibrium size is obtained that is specific to each of the digestive ripening agent used. While the original work on digestive ripening has been largely carried out with gold nanocrystals, it has recently been extended to several other nanoparticle systems. Once again, the mechanism of this process is not understood, which is extremely important to generate nanocrystals with controlled and desired size distributions. In this presentation we will review the state of the art in digestive ripening and some of our recent experimental results that we hope will help in developing a mechanistic model for digestive ripening. 
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