1) Nanoscale science

We discovered single-walled carbon nanotubes in 1993 during a joint research effort between the IBM Almaden Research Center and Caltech. Single-walled carbon nanotubes may be conductors or semiconductors, depending on their structures. Application in molecular computers has been suggested and the first nanotube transistor has been built. The large internal space in single-walled carbon nanotubes results in interesting properties and applications. For example, filling of nanotubes with molecular species by capillary forces has been predicted theoretically, and we have illustrated experimentally the absorption and desorption of molecular hydrogen in single-walled carbon nanotubes. The ability to fill and empty nanotubes with molecular species suggests the use of these nanotubes as a molecular pipet or a one-dimensional catalyst. We have synthesized crystalline metal nanowires inside single-walled carbon nanotubes for the first time.

Carbon nanotubes, when used as scanning probe microscope tips, have shown superior imaging capability compared to conventional tips. Chemically modified nanotube tips may be useful in chemical force microscopy. The non-toxic character of carbon nanotubes makes them a good candidate for biological applications, such as biosensors and membranes for controlled drug release.

Many theoretical predictions indicate that carbon nanotubes of various structures possess interesting properties, while experimental realization of these dreams has been hindered by the lack of progress in synthesis of these materials. Many electric and optical properties have been predicted to depend on the diameter and the helical structure of an individual tube. Verification of these predictions, however, awaits the availability of a variety of nanotubes. We are using the synthetic method that we have developed for producing large quantities of single-walled carbon nanotubes with wide diameter and helicity distributions. By combining chemical and mechanical methods, we can isolate significant amounts of size-specific, single-walled carbon nanotubes. and study their physical, chemical, and mechanical properties and test for potential applications such as molecular electronics for molecular nanotechnology, hydrogen storage media for vehicular fuel-cell applications, and scanning probe microscope tips for chemical probes and nanolithography.

2) Self-Assembly and Phase Transition in DNA-Gold Nanoparticle System

DNA melting and hybridization phenomena are of great importance in both fundamental biological science and biotechnology. Sequence-specific DNA recognition is important in detection, diagnosis of genetic diseases, and identification of infectious agents. Gene chips have been widely used for detecting specific nucleic acid sequences. More recently, DNA-modified gold particles have been used for DNA self-assembly. Organic molecules between gold particles have also been demonstrated to be useful as conducting molecular junctions. DNA-gold nanoparticle systems undergo a change of color upon network formation, which can be used for highly sensitive detection when specific nucleic acid sequences induce network formation. Indeed, such a system has been demonstrated to be useful to detect anthrax and other agents of biowarfare.

The DNA-gold nanoparticle system is a model for phase transitions. Melting of short, free DNA is not a phase transition. However, when short DNA are bound to gold particles, the system undergoes a phase transition because the DNA-gold particles form networks of micrometer size, which is approaching a bulk phase. Thus, the binding transition in the network is much sharper than that of free DNA in solution, due to the cooperative melting process. Many fundamental aspects of phase transitions may be investigated with this biomolecular system.

3) Structure / Function of Biological Macromolecular Assemblies

Three-dimensional structure determination remains a crucial step towards understanding biological function. Many crystal structures of proteins have been solved, but less is known about the structures of proteins that do not readily form crystals, such as membrane proteins, fibrous and structural proteins, and complexes of lipids and carbohydrates with proteins. Moreover, little direct information about dynamic interaction processes can be deduced from crystal structures. The recently developed cryoelectron microscopy and scanning probe microscopy are particularly suitable for studying non-crystalline macromolecular assemblies. We are developing new imaging techniques to solve the three-dimensional structure of biological macromolecular machines. We are studying the structure and dynamics of soft materials with cryoelectron microscopy and scanning probe microscopy.