![]() In most applications, it is the ultrahigh porosity ( >90%) and high specific surface area ( >7,000 m 2 /g) that is made use of, for sensing, separation, removal or delivery of specific species ( Figure 1C). Given the virtually infinite chemical space, a careful selection of MOF building blocks can undergo temperature induced self-assembly that can be modulated to facilitate a fine control over phase purity, porosity, particle size, morphology and surface chemistry ( Figure 1B) ( Forgan, 2020). Owing to the strength of the bonds and the use of relatively long organic molecules, the material formed has significant void space, making them permanently porous ( Furukawa et al., 2013). At the simplest level, these crystalline materials are composed of metallic centers (known as secondary building units) connected via organic linkers using relatively strong bonds ( Figure 1B). While the initial focus was on inorganic NMs, a hybrid inorganic-organic class of NMs known as metal-organic frameworks (or porous coordination polymers) have witnessed explosive growth owing to the modularity of their constituents and the ability to modify their composition and structure with relative ease. ![]() The engineering of materials at the nanoscale leverages the emergence of these unusual properties across consumer products, electronics and more recently, in medicine ( Nel et al., 2006 Valsami-Jones and Lynch, 2015). Since high free energies are unfavourable, these materials tend to be highly reactive and either bind to other species or undergo specific interactions in order to stabilise themselves ( Fruk and Kerbs, 2021). These surface atoms have fewer neighbors than their bulk counterparts, leading to a higher free energy ( Figure 1A). ![]() Nanomaterials have a significantly higher proportion of surface atoms than bulk materials. These unusual properties can be ascribed to the size of nanomaterials, where they tend to behave more like molecules and less as bulk materials (nanomaterials act as a bridge between the bulk and atomic scale) ( Nel et al., 2006). ![]() The observation of these phenomena at these length-scales has not only advanced fundamental understanding of science, but has also served as the basis for emergent applications with far-reaching consequences. Humanity’s quest towards pushing the boundaries of science has led to the nanoscale, where exciting physical and chemical phenomena have been unlocked owing solely to the size scales that can be accessed (generally <100 nm, however these definitions are arbitrary and often flexible) ( Silva, 2004). We briefly highlight potential mechanisms of toxicity and conclude with describing the need to transition towards data-intensive computational approaches such as machine learning to establish nMOFs as credible materials for their envisioned applications. The review also discusses the factors affecting toxicity of nMOFs, such as their size, shape, morphology, and composition. We first describe why nMOFs are of considerable interest to the scientific community followed by a discussion on routes through which they can be exposed to the environment and living organisms, particularly shedding light on their transformation mechanisms. In this mini review, we aim to initiate a discussion on the safety and toxicity of nMOFs, drawing parallels with the existing guidelines and literature on the safety of inorganic NMs. However, one aspect that is often ignored in these discussions is their safety at a nanoscale. These properties have prompted the investigation of these materials for applications in biological and environmental contexts. More recently, nanoscale metal-organic frameworks (nMOFs) have witnessed explosive growth due to the modularity of their chemical constituents, the ability to modify their composition and structure, and exceptional properties such as permanent porosity and high surface areas. These properties are leveraged by nanomaterials (NMs) across a plethora of applications. Owing to the size scales that can be accessed, the nanoscale has opened doors to new physical and chemical properties, not seen in the bulk.
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