CAF@ZIF-8: One-Step Encapsulation of Caffeine in MOF

Two strategies for encapsulating caffeine in ZIF-8 were
carried
out in this work: (1) one-step, in situ encapsulation where caffeine
is added to a ZIF-8 synthesis solution and the MOF structure is formed
around the entrapped molecule; and (2) ex situ encapsulation whereby
caffeine is put into contact with previously synthesized or purchased
ZIF-8. The products obtained were analyzed with XRD, TGA, Vis-UV,
GC-MS, FTIR, 13C NMR, and N 1s XPS to compare both encapsulation
methods. Chemical and structural evidence indicated that the preferential
adsorption site of caffeine molecules inside the ZIF-8 structure is
near the methyl and CH groups of 2-methylimidazole ligand. These two
groups interact with caffeine by van der Waals forces with methyl
groups and via CH···O hydrogen bonds with CO
groups, respectively. In addition, the one-step encapsulation of caffeine
in ZIF-8 produced high guest loading (ca. 28 wt % in only 2 h at 25 °C)
and controlled release (during 27 days).