Molecularly mediated processing and assembly of nanoparticles: exploring the interparticle interactions and structures.

The harnessing of the nanoscale properties of nanoparticles in most technological applications requires the abilities of controlled processing and assembly, which has been an important challenge because of the difficulty in manipulating interparticle properties. Molecularly mediated processing and assembly of nanoparticles have emerged as an important strategy for addressing this challenge. The capability of this strategy in manipulating size, shape, composition, and interparticle properties has significant implications for designing sensing, biosensing, nanoprobing, and many other functional nanostructures. This Account highlights some of the important findings in investigating both interparticle and collective properties as a forum for discussing new opportunities in exploiting nanoparticle-based designs and applications. The concept of mediator-template assembly of nanoparticles explores the combination of the forces from a mediator and a templating molecule for designing and controlling the interparticle interactions. The manipulation of the interparticle interaction properties and the detection of the molecular signatures are two of the key elements in this concept. A series of well-defined molecular mediators ranging from inorganic, organic, supramolecular, to biological molecules have been explored to ascertain how these two elements can be achieved in nanoparticle assemblies. The emphasis is the fundamental understanding of interparticle molecular interactions, such as covalent, electrostatic, hydrogen bonding, multidentate coordination, pi-pi interactions, etc. Each of these molecular interactions has been examined using specific molecules, such as multifunctional ligands, tunable sizes, shapes, or charges, well-defined molecular rigidity and chirality, or spectroscopic signatures, such as fluorescence and Raman scattering. Examples included thiols, thioethers, carboxylic acids, fullerenes, dyes, homocysteines, cysteines, glutathiones, proteins, and DNAs as molecular mediators for the assembly of gold, alloy, and magnetic nanoparticles. The understanding of these systems provided insights into how the unique electrical, optical, magnetic, and spectroscopic properties of the nanoparticle assemblies can be exploited for potential applications. This Account also highlights a few examples in chemical sensing and bioprobing to illustrate the importance of interparticle interactions and structures in exploiting these properties. One example involves thin-film assemblies of metal nanoparticles as biomimetic ion channels or chemiresistor sensing arrays by exploiting the nanostructured ligand framework interactions. Other examples explore the surface-enhanced Raman scattering signature as nanoprobes for the detection of protein binding or the enzyme-based cutting of interparticle DNAs. The detailed understanding of the design and control parameters in these and other systems should have a profound impact on the exploration of nanoparticles in a wide range of technological applications.