Exploring and Exploiting Dynamic Noncovalent Chemistry for Effective Surface Modification of Nanoscale Metal–Organic Frameworks

Surface properties determine, to a great extent, the biologically relevant functions of various kinds of nanosized materials. Although the modification of the surface of traditional inorganic or polymeric nanoparticles can be routinely achieved through covalent or noncovalent manner or both, the surface modification of nanoscale metal–organic frameworks (nano-MOFs) is extremely challenging because of their rapid degradation in aqueous environments. In this work, we systematically studied the synergistic and dynamic noncovalent interactions between fluorescent probes and iron­(III) carboxylate nano-MOFs (i.e., MIL-101-NH₂ (Fe), one of the most prevalent MOFs used in drug delivery and imaging). We further examined the interplay between the surface binding of fluorescent probes and the degradation of MIL-101-NH₂ (Fe) in aqueous medium. It was demonstrated that the surface binding of probes is not only of high affinity but also dynamic and nonsheddable, even during the degradation, a feature that is essentially different from the covalent conjugation. Subsequently, we developed a unique and straightforward strategy for the surface modification of MIL-101-NH₂ (Fe) with polymer by exploiting the synergy of noncovalent interactions between functionalized copolymers and MIL-101-NH₂ (Fe). We demonstrated that the binding of polymers onto MIL-101-NH₂ (Fe) surface was very effective in aqueous solution and surprisingly nonsheddable during the process of degradation. Surface polymers can creep on the surface of MIL-101-NH₂ (Fe), in a dynamic and real-time manner, to the new sites formed immediately after the degradation. In addition, the stability of MIL-101-NH₂ (Fe) particles in aqueous environments can be improved to some extent by the surface polymer coating. The results presented herein constitute an important innovation for surface engineering of nano-MOFs, which would benefit the application of nano-MOFs as delivery systems in aqueous systems.