04 November 2010
Single metal-organic framework (MOF) crystals can be used to separate mixtures of dyes just like a miniature chromatography column, say scientists in the US.
MOFs are highly porous structures made of metal oxide hubs joined by organic struts, and can adsorb high volumes of molecules onto the surface of the cavities within their structures. This makes them highly desirable for gas storage and separation applications, but now researchers at Northwestern University in Illinois have for the first time harnessed MOFs to create what could be the world's smallest chromatographic column.
Ordinarily, using MOFs instead of silica to separate liquids in a chromatography column would require columns up to tens of metres long, as the large pores between crystallites lower chromatographic resolution. Now, Bartosz Grzybowski and colleagues have managed to use a single, millimetre-sized MOF crystal made from terephthalic acid struts and octahedral carboxylato Zn4O(CO2)6 hubs to separate dyes on a much smaller scale.
The team placed the MOF crystal on an organogel that had been soaked in dimethylformamide (DMF) containing a mixture of two fluorescent dyes. Over a distance of just a few hundred micrometres, the mixture separated as it flowed up through the MOF. The team monitored the separations and measured the concentrations of each dye using fluorescence confocal microscopy - an optical imaging technique - and found that each of the dyes were clearly separated by the MOF.
The MOF column separates dyes over a distance of a few hundred micrometres
© J. Am. Chem. Soc.
Grzybowski can see the MOF chromatography approach being used more widely in the future. 'Putting the crystal on a microchip and separating compounds on the chip is what we would envisage for the system,' he says. 'In terms of cost, when this field matures, it shouldn't be any more expensive to make these crystals than growing crystals of salt.'
'What [the team] have shown is a proof of principle that a single crystal can act as a column, and that you are certainly able to get separation of a mixture, by one molecule diffusing through the crystal faster than another,' says Andrew Burrows, a MOF specialist at the University of Bath in the UK. 'For someone in the lab-on-a-chip area, this separation method could be quite attractive,' he adds.
Once further developed, Grzybowski believes his miniature MOF column system could separate compounds in a continuous flow arrangement, and allow for fractions to be collected once they have passed all the way through the crystal. 'I think that if we play it right, it could be used for isolation of strands of DNA of specific types,' he tells Chemistry World.
Grzybowski highlights that the MOF system could also be used as a single crystal reaction vessel. 'You could flow reagents in from two different ends of the MOF crystal, and when they meet in the middle a reaction would take place. As the crystal is transparent, you could observe what is going on with all sorts of microscopies,' he says.