## 2 Metal-Organic Frameworks  
MOFs are hybrid inorganic-organic open frameworks with potential high porosity. More specifically, they are built through the combination of organic linkers and inorganic building blocks, which can either be metal ions or polynuclear metal clusters, also described as secondary building units (SBUs) [24; 25; 26]. The combination of these building blocks towards the formation of a porous framework is governed by reticular chemistry principles [27; 28], which have been proven to be a powerful tool for the design and synthesis of materials with specific structural characteristics and installed functionality to enhance their interactions with specific chemical moieties and improve their performance in targeted applications. In detail, the organic building blocks of a MOF are usually organic carboxylate or azolate polytopic linkers of diverse geometry, which are pre-synthesized [26; 29]. This makes them a robust building block that can dictate the structure of the final framework to a great extent, in contrast to the inorganic building block, which is rarely pre-synthesized [30; 31], but usually forms in situ under specific reaction conditions. So, in this way the fact that the organic linker acts as an actual preformed building unit [26], not only allows us to a great extent to control and therefore target specific structural characteristics to the synthesized material but also to ensure that specific chemical groups are present to the backbone of the material [29; 32]. This is very important because in this way we can synthesize porous materials that not only present specific pore shapes and sizes towards the storage of targeted molecules, but also through the presence of specific chemical groups. Thus, the affinity of the framework with the targeted molecules can be increased, achieving in this way high storage percentages. Regarding the inorganic building blocks, also described as SBUs, they can be polynuclear oxo-metal clusters, one-dimensional metal-oxo chains, or single metal cations [28]. However, MOFs based on polynuclear metal oxo clusters have gained the attention of the scientific community in the last decade, and a plethora of groups have been dedicated to the exploratory synthesis of such novel frameworks. This is due to two main factors. Firstly, polynuclear metal clusters are known to adopt specific geometrical shapes, which is a crucial parameter as in this way the prediction of the final network can be more accurate by employing reticular chemistry principles [33; 34; 35; 36]. Also, the increased connectivity that these kinds of inorganic moieties allow with the organic linkers enhances the stability of the final network, in a plethora of environments, including water, which is a crucial prerequisite for various applications concerning the environment, human health, and agriculture. Recently, MOFs have been at the forefront of materials science due to a combination of key features, as their remarkable chemical stability, their unique tunability, their high surface area, and their modular synthesis [37; 38]. These properties have established MOFs as a unique family of tailor-made highly porous materials that can be designed and synthesized for targeted applications across diverse fields. Also, their highly crystalline nature allows their detailed structural characterization and better understanding of their structure-property interactions, paving the way for future innovations [39]. Therefore,MOFs have found widespread use in a broad range of applications as gas storage and separation [40], photoluminescence [41; 42], sensing [41; 43], magnetism [44] catalysis [45; 46], optoelectronics [47] drug delivery [48; 49; 50], bioimaging [51; 52], conductivity [53], energy storage [54; 55; 56], among others. A full profile characterization of MOFs includes a combination of structural, morphological, and spectroscopic characterization methods employing state-of-the-art methodology.