Metal-Organic Frameworks are coordinated compounds consisting of metal ions correlated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. These metals are known to offer flexible, co-ordination environment with different geometries, e.g., tetrahedral, trigonal bipyramidal square and pyramidal octahedral. In addition, due to the usual liability of metal complexes, the formation of coordination bonds between the metal ions and the organic linkers can be reversible. This property in fact enables the rearrangement of metal ions and organic linkers during the process of polymerization to render (provide) the highly ordered framework structures.
MOFs are usually prepared under solvothermal or hydrothermal conditions in pure N, N-diethylformamide or N, N-dimethylformamide used as solvents, which slowly decompose upon heating in oven and generate bases of organic linker molecules which later on react with metal salts and produce 3D metal-organic networks.
Role of metals and organic linkers
Metal ions consist of vacant orbital’s which defines their coordination number which indeed it the size and shape of pores by dictating how many ligands can bind to the metal. It acts as secondary building units to create open crystalline frameworks with permanent porosity.
The organic units are di or tri organic carboxylates, amylates (and other similar negatively charged molecules), which, when linked to metal-containing units, yield architecturally robust (construct) crystalline MOF structures with a typical porosity, greater than 50% of the MOF crystal volume. Longer organic linkers provide larger storage space and a greater number of adsorption sites within a given material. It increases the adsorption sites which in turn increases the surface area.
Properties of Metal Organic Frameworks:
The unique properties of metal organic frameworks, like internal surface areas, uniform channels, low density, (sub) nanometer sized cavities, thermal stability (above 300°C), adjustable chemical functionalities or chemical tailor ability, offer an ideal platform for the development of the sensitive layer. They can be considered as crystalline materials with tunable porosity (Pore volume: 1.59cm2/g), large internal surface area and organic functionality. The topology structure or surface area of MOFs can be controlled through choosing different organic linkers. They have structural diversity and specific adsorption affinity for the application in gas storage.
In MOFs, paramagnetic metal ions are linked with diamagnetic organic ligands which can be used efficiently in transmitting magnetic exchange. Therefore the magnetic properties in MOFs depend on the properties of both metal and ligands as well as the structural organization of the resulting network.
Applications of Metal Organic Frameworks:
• Metal organic frameworks in drug delivery
Drug delivery has become increasingly important mainly due to the awareness of the difficulties associated with a variety of old and new drugs. Off the drug delivery systems, metal organic frameworks have been widely used as drug delivery system for various applications because of their pore size and density.
Metal-organic frameworks are a recently identified class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands, and are a new development on the interface between molecular coordination chemistry and materials science. A range of novel structures has been prepared which feature amongst the largest pores known for crystalline compounds, very high absorption capacities and complex absorption behavior. Among the 10,000 of known MOFs, MIL (Fe-MOFs) family shows a unique candidate for storage and controlled release of biologically important molecules. In order for MOFs to be useful as efficient delivery vehicles for drugs and imaging contrast agents, the material composition must be biocompatible and the particle sizes must be carefully controlled to be uniform and below several hundred nanometers. The imaging and drug components can be directly incorporated into the MOFs either as metal-connecting points or as bridging ligands during the MOF synthesis.
• Medical Imaging
Medical imaging such as MRI relies on large doses of administered contrast agents to differentiate between normal and diseased tissues. MOFs are intrinsically biodegradable, and their high porosity makes them ideal for targeted delivery of entrapped agents.
The manipulation of nanoporosity and ultrahigh surface area of MOFs make them ideal candidates for recognizing analytes in sensing applications. MOFs have the potential to overcome many of the challenges of selectivity that plague other sensor materials and form the basis of robust, highly-sensitive and compact sensing devices.
• Biomedical Applications of MOFs:
The applications of MOFs in the field of biomedical science have recently been explored. The initial studies of MOFs in this field show a promising role for biomedical applications. Stability and the toxicology of the material are the main issues that should be concerned when MOFs are used in this field. Since a large number of MOFs have been synthesized to date, it is hard to make a general comment on the stability of the MOFs. For example Fe-MIL-100 are stable in biological solutions for extended periods whereas MOF-5 is only partially stable in humid environments.