The effect of PEG length on the size and guest uptake of PEG-capped MIL-88A particles

The surface functionalization of MOF particles with poly(ethylene glycol) (PEG) is important for their use in biomedical applications. Here, the effect of the molecular weight of a monovalent PEG-carboxylate capping ligand (MWPEG) was investigated in a newly developed one-step, stoichiometric procedure that aims at functionalizing MIL-88A particles and achieving size control at the same time. The bulk of the MIL-88A particles is composed of iron(III) oxide metal clusters connected by fumaric acid as the organic ligand. The surface is functionalized with monovalent PEG-carboxylate capping ligands of different lengths. The size of the PEG-functionalized MIL-88A decreased with increasing MWPEG, and nanoMOFs were obtained for long (≥2 kDa) PEG chains. For lower MWPEG, higher concentrations of PEG were needed to obtain the maximum size effect, but the resulting sizes were still larger than for long PEGs. BET surface area, elemental analysis, zeta potential, and infrared spectroscopy measurements showed that the PEG chains were attached to the surface of the MOF particles and not in their interior. Moreover, it was demonstrated that longer chains occupy a larger surface area, and the PEG chains adopt the low-density brush conformation. Uptake and release experiments with sulforhodamine B dye (as a model drug) showed a higher and faster uptake and release for MIL-88A functionalized with PEG (20 kDa) than for native MIL-88A, which is attributed to a larger surface-to-volume ratio for the PEG-covered particles, and to the well-hydrated and accessible nature of the PEG layer in an aqueous medium. Complete release of the dye was achieved in phosphate buffered saline, the majority by counter ion exchange, and a smaller fraction in the salt form.