## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Stir-Bar Sorptive Extraction (SBSE)  
The first mention of SBSE dates back to 1999, and an article was published by [PERSON] et al [135]. The authors of the article developed a new, for their time, technique using the PDMS sorbent, which is still popular to this day. SBSE was a response to the inadequacies of the then-used open tubular trapping (OTT) technique and a competitor to the more recently developed SPME. The initial approach to the technique was to remove a layer of Teflon from the magnetic core of a magnetic stirrer, then place it in a glass tube and cover it with a layer of PDMS (polydimethylsiloxane) sorbent. The device prepared in this way was placed in a liquid sample and subjected to stirring for a specified time. The size of the magnetic stirrer and the amount of PDMS sorbent used can be adjusted depending on the amount of sample. In the first article, the authors used thermal desorption of analytes, although desorption using organic solvents and liquid desorption is equally possible [135]. The SBSE technique appears to be an extremely interesting technique for several reasons, including the ubiquity of magnetic stirrers, which can be easily found in almost any laboratory, which significantly reduces the cost and complexity of using the technique in practice, the relatively large amount of sorbent material used, which can increase the sensitivity of the analyses conducted, and the ease of controlling the length of extraction time, which, due to the direct contact of the sorbent with the sample, can be very efficient. Since the publication of the first historical article, the SBSE technique, like all others, has undergone significant development. However, to this day, still the most popular sorbent used in the SBSE technique is polydimethylsiloxane. It is excellent for extracting non-polar compounds from aqueous matrices and is mainly used for this purpose. The most typical applications of SBSE include the extraction of PAHs from environmental matrices, foodstuffs, and biological samples. In addition to it, commercially, you can only get two other types of sorption coatings, polyethylene glycol (PEG) and polyacrylate (PA), which are better for the extraction of more polar compounds. However, the extraction of highly polar and hydrophilic analytes from these matrices remains a challenging issue. A review article published by [PERSON] et al. [136] describes these problems and focuses on a comparison of different application methods (adhesion, sol-gel, MIP, monolithic) of modern sorption materials, such as nano- and micro-carbon-based materials, functional polymers, and inorganic nanoparticles, to change the selectivity of the stir bar to more polar compounds. Although MOF is mentioned, it is very insignificant [136]. Another review article published by [PERSON] et al. [137] does not focus on changing the selectivity of the stir bar but on the use of various modern nanomaterials, including MOFs, in the most typical application of this technique, namely the extraction of PAHs from environmental samples. Here, too, the most common MOFs are MIL-100(Cr), MIL-100(Fe), and ZIF-7(Zn) [137]. In relation to the latter, a lot of interesting information can be found in the article by [PERSON] et al. [138], which focuses exclusively on using ZIF-type materials, omitting the whole range of the rest of the MOFs. An interesting approach to using MOFs was taken by [PERSON] et al. [139] with in-situ growth of ZIFs on the surface of layered double hydroxides (LDHs) for the preparation of a nanosorbent used in stir-bar sorptive extraction. The designed nanosorbent was used to extract and detect benzylpenicillin in biological and food samples. Under optimized conditions, the limits of detection and quantification were 0.05 and 0.15 \(\upmu\)g/L, respectively [139]. UiO-66, one of the most known and durable MOFs, has been used as a component for a sorbent by [PERSON] et al.