Molecular self-assembly is widely used in biological systems to form higher-order structures. An example is the formation of the tobacco mosaic virus (TMV). The TMV viral particle consists of a single strand of RNA encased in a protein sheath. The protein sheath is formed from 2130 identical protein monomers and the isolated components may be reconstructed in vitro to reform the intact virus. Chemists have taken the paradigm of molecular self-assembly to construct well-defined molecular nanostructures. These nanostructures are formed spontaneously by molecules which assemble under the influence of inter-molecular forces.
Dr Richard Gillard, author of this article, studied for a PhD under the supervision of Fraser Stoddart. The research looked into the interactions between π-electron deficient aromatic units and π-electron rich aromatic rings in the template-directed synthesis of mechanically interlocked compounds, specifically so-called catenanes and rotaxanes. Catenanes comprise a molecular ring that is mechanically interlinked with a different ring. Rotaxanes comprise a molecular ring threaded onto a molecular axle.
Supramolecular Structures Exemplified by a Nanoelevator
More recently, exploiting such technology Stoddart et al. reported in the journal ‘Science’ an artificial molecular machine that functions like a nanoelevator. The nanoelevator is a rig-like construct with three legs embracing an interlocked deck-like component which can be made to move between two levels. It is about 3.5 nanometres in diameter and 2.5 nanometres in height. The elevator has been made to go up and down 10 times by the consecutive addition of acid and base, respectively. Such nanoscale robotic devices could find use in slow-release drug delivery systems and in the control of chemical reactions within nanofluidic systems conducted in laboratories on a chip.
The Formation of Molecular Switches
The same technology has also been used to form molecular switches. A molecular switch is a nanoscale machine which switches reversibly between two or more positions. The goal of this research is to fabricate molecular electronic components by “engineering up” from a molecule to a functioning electronic device. The so-called “bottom up” approach. Molecular electronics require switchable devices as well as the interfaces to exchange information with the outside world. Furthermore, these assemblies must be controllable, reversible and readable at the molecular level. One approach involves stringing together dozens of molecular switches and nanowires into logic circuits and memory circuits in which the molecular switches are able to communicate with one another. The molecular switches which Stoddart et al. are working towards involve redox-active catenanes and rotaxanes as mentioned above. In both structures, the ring can assume two different positions that represent the digital states "1" and "0", and it can be switched between those states using applied voltages.
Use of Silicon Nanowires and Carbon Nanotubes
To connect molecular switches, Stoddart et al. are exploring the use of silicon nanowires and carbon nanotubes arranged in a grid. This architecture is derived from that used in a unique silicon-based computer known as Teramac, which was built at Hewlett-Packard a few years ago. At each junction of this grid, the nanowires are connected by a monolayer of the molecular switches. This technology has been patented, see, for example, US patent no. 6,314,019.
Incorporating Molecular Switches into Logic and Memory Circuits
Recently, Stoddart et al. demonstrated that rotaxane-based molecular switches could be strung together to form logic gates, although the state of the switches could only be changed one time, see ‘Science’ 285, 391 (1999). In August, the group took the next step and reported on catenane-based molecular switches that can be reconfigured, i.e. switched on and off, many times, see ‘Science’ 289, 1172 (2000). Although the difference between the "on" and "off" states is much too small (in terms of resistance) to be useful for logic circuits, the switches could be useful for memory. Importantly is that it is the first time that a solid-state molecular switch has been shown to work repeatedly under ambient conditions. The challenge now is to incorporate such structures into logic and memory circuits.