Controlling the Adsorption and Release of Ocular Drugs in Metal-Organic Frameworks: Effect of Polar Functional Groups

A series of UiO-66 materials with different functional groups (-H, -NH₂, and -NO₂) have been evaluated for the adsorption and release of a common ocular drug such as brimonidine tartrate. UiO-66 samples were synthesized under solvothermal conditions and activated by solvent exchange with ethanol. Experimental results suggest that the incorporation of surface functionalities gives rise to the development of structural defects (missing linker defects) but without altering the basic topology of the UiO-66 framework. These defects improve the adsorption performance of the parent metal-organic framework (MOF), while the bulkier functionalities infer slower release kinetics, with the associated benefits for prolonged delivery of brimonidine. Among the evaluated MOFs, defective UiO-66-NO₂ can be proposed as the most promising candidate due to the combination of a larger brimonidine volumetric uptake (680 mg/cm³), a prolonged delivery (period of up to 25 days), a small particle size, and a larger instability. Contrariwise, at high concentrations UiO-66-NO₂ has higher toxicity toward human retinal pigment epithelium cells (ARPE-19) compared to the pure and NH₂-functionalized UiO-66.