## 2 Metal-Organic Frameworks  
The development of MOFs was initiated in the early 1990s. The founder of the MOF is considered to be [PERSON], who, in 1995, together with his team, introduced the term \"Metal-Organic Framework\" for the first time, for the created complex compound of copper and 4,4-bipyridyl, which was then still showing unstable porosity [13]. The first permanently porous MOF was obtained in 1997; it was a complex compound of cobalt and bipyridyl. For the first time, it was possible to carry out the reversible adsorption of small gas molecules, CH\({}_{4}\), O\({}_{2}\), and N\({}_{2}\)[14]. The beginning of extensive research on MOFs is  
Figure 3: A graph showing the number of publications containing the keywords “Metal-Organic Framework” and the selected technique for preparing liquid samples for analysis. SCOPUS database, [own elaboration]. considered to be [PERSON]'s work, in which the MOF-5 compound was described, characterized by exceptional durability and high porosity at that time, where the specific area of the material was \(\sim\)2900 m\({}^{2}\)/g [15]. Currently, the specific surfaces of some MOF materials can reach \(\sim\)7000 m\({}^{2}\)/g (several times more than the specific surfaces of, for example, activated carbon) and a pore volume reaching up to 2 cm\({}^{3}\)/g [16]. The characteristic features of MOF are crystallinity, stable porosity, strong metal-organic ligand bonds, and a three-dimensional structure. Due to the number of possible combinations of metal clusters and organic ligands capable of forming MOFs and the possibility of modifying existing structures, it is theoretically possible to adjust their structure and properties depending on the need [17]. From this perspective, they are more interesting than, for example, zeolites. However, they are characterized by lower thermal stability than zeolites, and lower resistance to water [7]. An MOF consists of two principal parts: an ion or cluster of metal, a \"node\", and a selected organic compound having at least two functional groups or two atoms capable of donating an electron pair, \"connectors\" or \"ligands\". 'Nodes' and 'ligands' are connected by one or more coordination bonds. An MOF is characterized by a rigid crystalline structure with an ordered pore arrangement. The most common metal ions included in MOFs include Zr\({}^{A+}\), F\({}^{3+}\), Cr\({}^{3+}\), Al\({}^{3+}\), Cu\({}^{2+}\), Zn\({}^{2+}\), Ni\({}^{2+}\), Mn\({}^{2+}\), and Ag\({}^{+}\). The type of element used, its valence, as well as the coordination number (which also depends on the ligand's structure) affect the final shape of the MOF network. Metal ions forming MOFs very often do not occur alone but in the form of clusters. A \"cluster\" is a compound containing groupings of atoms of a given element connected directly to each other. They can be formed by elements from the p, d, and f blocks. Metallic clusters are chemical forms containing a certain number of metal atoms that, in a \"significant percentage\", have an M-M (metal-to-metal) bond. M-L (metal-ligand) bonds can also be found in clusters. The basic feature of cluster compounds is a closed form, formed from M-M and M-L-M bonds [18]. In short, a metal cluster is a group of particles that form a shared network of connections, forming inert or charged molecules. Metals can combine in them, for example, with O, OH, or CO [19]. In relation to MOFs, they are often referred to as the \"Secondary Building Unit\" (SBU), whose spatial shape and properties can theoretically be selected to synthesize networks with specific properties [12]. An organic molecule, which is a ligand in the MOF, should contain at least two functional groups, or atoms, capable of donating an electron pair to form a coordination bond (e.g., N, O, OH, COOH). Aromatic compounds, one or more cyclic with at least two functional groups, are often used. An important parameter of ligands is their spatial structure, length, and the way of arrangement of their substituents.