Soluble 2-D Supramolecular Frameworks

Large, supramolecular complexes that use non-covalent bonding to retain their structure are gaining popularity within the nanotechnology field. One of the primary areas of focus has been on MOF's (metal-organic frameworks), which are useful because of their structure, selectivity, and sensitivity. They can be used as high contrast agents in biomedical imaging applications such as CT (computed tomography) and MRI (magnetic resonance imaging) scanners, and may be tailored for chemical sensing applications.

A downside of this technology is their relative insolubility in useful solvents, making solution-based processing difficult, if not impossible. This increases the manufacturing costs and limits practical and economic uses in devices.

Scientists at the Lawrence Berkeley National Laboratory have begun to address this issue by creating SOF's-supramolecular organic frameworks. These polymeric compounds are designed to be soluble and capable of self-assembly (forming a structure through interactions with itself) in solution. This research was summarized in a recently published paper in the Journal of the American Chemical Society.

A solution-based 2-dimensional self-assembly process can provide single-layer coatings with a wide variety of useful applications. For example, chemical receptors selective to a particular toxin could be integrated in the supramolecular structure to act as a highly sensitive sensor.

Supramolecular structures exist in nature-the double-helix structure of DNA molecules is one example-but the challenge for the researchers was to create these structures with the right mix of functionality to allow them to self-assemble in solution while keeping them from precipitating out.

The approach was to use a macrocyclic molecular structure (to allow the honeycomb-like base similar to MOF's) combined with a three-armed structure to provide the desired stability. Cucurbituril molecules (monomers of more common glycoluril structures connected via methylene bridging) were used for the macrocycle, and the three-armed structures were composed of aromatic bipyridine molecules that were functionalized with bulky hydrophilic groups. The cucurbitril formed a stable base, while the functionalized bipyridines provided the repelling forces needed to resist stacking of the structures that could lead to aggregation, and eventually precipitation.

The result is a water-soluble structure held together by non-covalent, reversible bonds. While solubility in general is useful for industrial processes, water-solubility should make this sort of SOF very useful in the biological field, and may open up new possibilities for biomimetics (the field of imitating nature through human-made devices, systems, and substances).

Overall, the SOF framework provides a useful template that can hold guest molecules and amplify chemical signals. However, this SOF chemistry was a proof-of-concept design. The challenge for researchers will be to modify the chemical structures to provide desired functionality or sensitivity for a given application without destroying the properties that allow the structure to stay stable in solution or correctly form the desired framework. The Berkeley Labs research team is already at work on 3D soluble SOF compounds, which provides an even greater solubility challenge.

At least, this research gives another nanotechnology development tool for scientists to use in emerging technology fields. At best, it could be the core technology to convert theoretical applications into economically viable processes.