Metal-organic frameworks (MOFs) are an interesting class of hybrid materials that are built from metal ion connectors and polydentate bridging ligands. They have shown potential in a number of applications, such as nonlinear optics, [1] gas adsorption, [2] catalysis, [3] and even controlled drug release. [4] MOFs on the nanometer scale can offer an interesting approach to designing functional nanomaterials for biological and biomedical applications, because their compositions can be systematically tuned by judicious choice of building blocks. We have recently developed a room-temperature reversephase microemulsion procedure for the synthesis of nanoscale metal-organic frameworks (NMOFs). Although such a synthetic procedure afforded NMOFs of several metal/ ligand combinations, it led to gel-like amorphous materials in many other cases, presumably as a result of rapid and irreversible metal-ligand coordination bond formation at room temperature. Alternative synthetic methods are thus needed before we can fully take advantage of the intrinsic tunability of NMOFs in designing functional NMOFs for ultimate applications in imaging, biosensing, and drug delivery. Herein we report the surfactant-assisted synthesis of two novel gadolinium NMOFs at elevated temperatures and demonstrate the potential utility of NMOFs as magnetic resonance imaging (MRI) and optical contrast agents.
We chose Gd III ions as the metal connectors for their highly paramagnetic nature and the benzenehexacarboxylate moiety (bhc; the conjugate acid is also called mellitic acid) as the bridging ligand for its ability to form stable Gd NMOFs and to carry a high payload of Gd III ions. Numerous attempts at synthesizing Gd bhc NMOFs using the room-temperature reverse-phase microemulsion procedure only produced amorphous materials with ill-defined morphologies and wide size distributions. We reasoned that the bhc ligand might have too high a tendency to bridge Gd III ions, thus leading to amorphous materials by rapid irreversible crosslinking at room temperature. On the other hand, hydrothermal reactions have shown to be an excellent method for the synthesis of a variety of nanomaterials. Presumably, the elevated [*] K.