In the cage

A cage-like molecule containing 36 copper atoms and comprising 96 individual components has internal surface area of some 4000 square meters per gram. The material could be used in catalysis, gas storage, separation science or other applications. Wolfgang Schmitt of Trinity College Dublin, Ireland and AMBER, the Science Foundation Ireland-funded materials science research centre and colleagues suggest that their "molecular cages" with such an enormous surface to mass ratio could also be used as drug-delivery agents. [Schmitt et al, Nature Commun (2017): DOI: 10.1038/ncomms15268]

The new materials are not only cavernous on the molecular scale but also have solubility and as such offer great promise for energy conversion. The fundamental structure might be packed with a wide range of small molecules with specific functionality, the team suggests. Importantly, in these metal-organic polyhedra (MOP), the packing of its pores can be controlled so that it only occurs or leads to a reaction under very specific conditions. One example of such conditions might be exploited in sensing of biological molecules or in drug delivery. A biological or physiological cue would be necessary trigger an appropriate chemical reaction, for instance. By encapsulating a drug within the MOP one could be sure that it would only be released at the target site in the body, where a specific biological associated with the disease state would trigger its release.

The team discusses details of their new MOP in the journal Nature Communications. The structure uses Archimedean and Platonic bodies as the building blocks of principle and renders the supramolecular keplerates as a class of cages whose composition and topological aspects compare to characteristics of edge-transitive {Cu2}} MOFs with A3X4 stoichiometry, the team reports. They also hope to develop related materials as light-active porous, metal-organic materials for use in green energy. The ultimate goal would be to create a molecule that they could use to convert light energy directly into usable essentially mimicking the way plants produce energy through photosynthesis.

"We have essentially created a molecular 'flask' or better 'sponge' that can hold different molecules until a specific set of conditions spark them into life," explains Schmitt. "Hollow cage-type molecular structures have attracted a lot of scientific attention because of these features, but as the number of potential applications has grown and the target systems and environments become more complex, progress has been hampered by the lack of structures with sufficiently large inner cavities and surface areas.

He adds that this specific "MOP is among the largest ever made, comprising a number of internal sub-cages, with numerous solvent-bearing binding sites that are all distinct and can sequester small molecules. The nano-sized compartments can potentially change the reactivity and properties of molecules that are trapped within and, as such, these cages can be used as catalysts with the potential to emulate biological enzymes. "In future, we are hoping to use the molecule for catalysis using different guest molecules (substrates) that are encapsulated inside the different sub-cages," Schmitt told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.