Gold Nanoclusters: Structural Disorder and Chirality
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One of the main goals of researchers working on Nanoscience and Nanotechnology is the design and the fabrication of nanostructured materials with novel and perhaps unexpected properties. These systems are defined as materials constructed from structural elements (clusters, crystallites, or molecules) with dimensions in the range of 1–100 nm. The case of gold-based nanostructured materials has been especially relevant because of the potential applications in nanoelectronics and in biological diagnostics. An important contribution in this area was the self-assembly of two- and three-dimensional superlattices of nanometer-diameter gold particles linked to each other by organic interconnects. Thiol-passivated gold nanoclusters, assembled in closed-packed arrays, showed interesting electronic transport properties, such as single-electron tunneling at room temperature, and are expected to be useful for the development of nanoscale electronics. It was also shown that although the self-organization of nanoparticles is a powerful route to grow these materials, imperfections in the superlattice can result from incorrect chemical recognition between the constituents. This can be a serious limitation in making nanostructured materials for electronic applications, where long-range order is important. On the other hand, biological systems are able to solve complex recognition problems. In particular, DNA transmits well-defined chemical information through the pairing properties of nucleotide bases. A major advance in controlling the self-assembly of metal particles was achieved by using oligonucleotides to organize colloidal gold nanoclusters into superlattices, allowing for the controlled growth of hybrid DNA-gold nanostructured materials.
Although the main mechanisms to synthesize and to isolate hybrid DNA-gold nanostructured materials have been discovered, further studies continue to fully characterize the structural, electronic, optical, transport, and other physical and chemical properties. This research represents a challenge for future investigations in Nanoscience and Nanotechnology because of the complexity of these materials, which are a complex mixture of inorganic and biological structures. The first investigations toward a full characterization of gold-based nanostructured materials have already begun. In a first stage, valuable information on the properties of each subsystem, metal particles and DNA, is being obtained. These separate properties will be fundamental in understanding the behavior of the hybrid materials. Along this direction, theoretical and experimental information on the shape and morphology of bare and passivated gold nanoclusters will be fundamental to fully predict and understand their electronic, optical, and other physical and chemical properties. This information is essential to optimizing their utilization as building blocks of the new molecular nanostructured materials.