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Zeolitic Imidazolate Frameworks (ZIFs) are crystalline materials
that comprise of metal nodes and Imidazole derivatives as
linkers. ZIF-67 is often used in polymer composite materials
e. g. for gas separation membranes. Post-synthesis treatment of
ZIF-67 crystals with ultrasound leads to unforeseen plasticity
that resulted in sintered ZIF-67 and ZIF-67 nano-flakes.
Consequently, ultrasound increases the external surface area of
ZIF-67 which might improve e.g. blending with polymers in
composite materials. These new morphologies of ZIF-67 were
characterized by transmission electron, scanning electron, and
atomic force microscopy. The ultrasound treatment of ZIF-67
did not result in the formation of an amorphous framework or
a meta-stable crystal structure as indicated by powder x-ray
diffraction. In addition, ultra-sonicated ZIF-67 retained the high
gas adsorption capacity and pore size compared to synthesized
ZIF-67. The morphological changes are hard to detect with
standard analytical methods that are usually utilized for MOF
characterization. These findings also suggest that sonochemical
treatment of ZIFs leads to structural effects beyond increasing
the amount of nucleation clusters during sono-chemical synthesis,
which is currently not addressed in the field.
Metal-organic framework (MOF) colloids hold great potential for bioanalytical and biomedical applications due to their unique features. These include responsive luminescent properties and exceptionally high loading capacities for small molecular drugs. However, currently the lack of a surface functionality independent method for biomolecule conjugation is strongly limiting the advancement of colloidal MOFs in bioanalytical or biomedical applications. Bioanalytical methods, especially for environmental analysis, would benefit dramatically when responsive luminescent properties of MOFs could be coupled with a specific antibody interaction. Targeted drug delivery in biomedical applications often requires specificity towards tissues of interest in addition to a high drug loading capacity. Thus, both envisioned applications require biomolecules and in particular antibodies to be conjugated to colloidal MOFs.
Here, we propose a robust and easy to handle method that is suitable for a wide range of MOF templates and that allows reliable conjugation of biomolecules.
Colloidal ZIF-8 MOFs with particle size between 300nm and 5µm were used as templates to adsorb polymeric multilayer through a Layer-by-Layer self-assembly process. Multilayer build up was confirmed by the change in zeta-potential of particles upon polyelectrolyte adsorption as well as dynamic light scattering experiments and SEM/TEM microscopy. Subsequent biomolecule conjugation to functional groups of the polymeric multilayer on the surface of colloidal MOFs was achieved by established carbodiimide/succinimide conjugation chemistry. Successful and stable conjugation was confirmed by employing fluorescent labeled biomolecules.
Subsequently, colloidal MOF-antibody (IgG) conjugates were utilized in a solid-phase immuno assay using antibodies against carbamazepine and lithocholic acid, two interesting markers in environmental analysis.
In brief, mouse anti-CBZ (Carbamazepine) and rabbit anti-lithocholic acid were separately and covalently immobilized on 2D polystyrene surfaces to form a small micro spot array. Two separate populations of colloidal MOF particles were conjugated with goat anti mouse IgG and goat anti rabbit IgG, respectively. The different fluorescent labels of the two MOF colloid populations allowed the determination of binding selectivity and strength of the colloidal MOF-antibody conjugates to the immobilized binding partners.
The ability to coat MOFs in solution independent of their surface charge with polymers and subsequently to conjugate biomolecules to their surface makes this method a powerful tool to foster bioanalytical, biomedical as well as other applications.