Metal–organic frameworks have been assembled in water from natural, and even edible, products derived from renewable sources.
A wide variety of metal–organic frameworks (MOFs) — built from metal centres and organic linkers — have lent their structures and porous domains to many different applications, including gas absorption and drug delivery. These crystalline porous materials, however, have rarely been prepared from natural products, probably because those are seldom symmetrical, and thus not prone to such crystallization. Now, a team in the USA and the UK, led by Fraser Stoddart at Northwestern University, has circumvented1 this issue by using potassium ions and γ-cyclodextrins — symmetric macrocycles consisting of eight sugar residues — as building blocks.
On slow diffusion of methanol vapour into an aqueous mixture of the components, the pendant hydroxyl groups of each cyclodextrin coordinated to the potassium centres to form cubic motifs. These further assembled into a highly porous MOF that features both straight channels and spherical cavities. The material dissociates in water, but recrystallizes on vapour diffusion of methanol — a promising characteristic in terms of recyclability. Isostructural assemblies were obtained by replacing potassium by rubidium or caesium centres.
The materials showed good thermal stability and retained their crystallinity on evacuation of any molecules present in the pores. This enabled small guests to be encapsulated, as demonstrated by the uptake of the dye Rhodamine B by the rubidium analogue. By choosing food-grade cyclodextrins, salt substitute KCl and neutral grain spirit instead of methanol for the synthesis, the researchers succeeded in preparing a MOF that is potentially safe to eat.
Smaldone, R. A. et al. Metal–organic frameworks from edible natural products. Angew. Chem. Int. Ed. 10.1002/anie.201002343 (2010).