Abstract
Metal-organic framework nanoparticles (MOF NPs) are promising guest-host materials with applications in separation, storage, catalysis, and drug delivery. However, on- and off-loading of guest molecules by porous MOF nanostructures are still poorly understood. Here we study uptake and release of fluorescein by two representative MOF NPs, MIL-100(Fe) and MIL-101(Cr). Suspensions of these MOF NPs exhibit well-defined size distributions and crystallinity, as verified by electron microscopy, dynamic light scattering, and X-ray diffraction. Using absorbance spectroscopy the equilibrium dissociation constants and maximum numbers of adsorbed fluorescein molecules per NP were determined. Time-resolved fluorescence studies reveal that rates of release and loading are pH dependent. The kinetics observed are compared to theoretical estimates that account for bulk diffusion into NPs, and retarded internal diffusion and adsorption rates. Our study shows that, rather than being simple volumetric carriers, MOF-NPs are dominated by internal surface properties. The findings will help to optimize payload levels and develop release strategies that exploit varying pH for drug delivery.
Highlights
The widespread use of porous materials in the field of separation, storage, and catalytic process technologies requires a thorough understanding of the adsorption and desorption of guest molecules within the porous structure
Several reports have pointed to the general applicability of metal-organic frameworks (MOFs) nanoparticles (MOF-NPs) for drug delivery, as they have high loading capacities and are functionalizable, and certain structures have been shown to be biocompatible (e.g., MIL-100(Fe); MIL stands for Materials
These findings demonstrate that physicochemical studies of MOF-NP loading enable rational, predictive design of release scenarios, with regard to varying pH conditions
Summary
The widespread use of porous materials in the field of separation, storage, and catalytic process technologies requires a thorough understanding of the adsorption and desorption of guest molecules within the porous structure. As a matter of fact, MOFs exhibit some of the highest porosities (1000 to 7000 m2 /g) of all known porous solids, with pore sizes in the range of 0.3 to 6 nm [5] Their high porosities and, in particular, the combination of high surface area with tunable pore size render MOFs ideal for applications in gas storage and separation [6,7], catalysis [8,9,10], sensing [11,12], electronics [13], drug delivery [14,15,16,17,18], and X-ray analysis of the structures of guest molecules within the MOF scaffold [4,19].
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