**6. Applications in drug delivery**

MOFs are unique inorganic–organic hybrid materials possessing ultrahigh surface area and porosity. They are crystalline, have flexible and rigid frameworks, and also exhibit high chemical and thermal stability. MOFs have been continuously and thoroughly explored and reviewed for numerous applications. Several applications related to MOFs have been reported such as for gas storage and separation, [40–42] catalysis, [43, 44] sensing, [45] magnetism, [46] and energy [47]. In addition, various biomedical applications have also been reported, including biological sensing, [48] molecular imaging, targeted drug delivery, [21, 49] among others [11].

A large number of side effects are associated with uncontrolled and non-specific drug delivery by direct administration of a free drug inside the body. Great efforts have been made by researchers for the development of methods for targeted, systemic, and controlled drug administration. Nanocarriers have provided a simple and effective solution to this problem. Both organic (such as dendrimers, liposomes, etc.) and inorganic (such as noble metal and metal oxide nanoparticles, quantum dots, silica nanoparticles, etc.) nanocarriers have been reported as potential drug delivery vehicles. Organic nanocarriers such as liposomes are less stable and easily captured by the reticuloendothelial system (RES) once inside the body [50]. Inorganic nanocarriers such as gold, silver, and silica nanoparticles have been reported to be cytotoxic [51]. Inorganic– organic hybrid nanocarriers, such as porous NMOFs, offer many advantages over their pure organic and inorganic counterparts and have established themselves as optimal drug delivery vehicles. In the following subsections, the applications of NMOFs for the delivery and as carriers of therapeutic drugs, biomolecules such as proteins, enzymes, carbohydrates, nucleic acids, and other active agents, shall be discussed briefly.
