## 4 MOFs as Smart Carriers and Delivery Systems of Sensitive Volatile Compounds
### \(\gamma\)-CD-MOFs  
Among the diverse families of MOFs those based on cyclodextrin (CD) are at the forefront of bio-related applications. That is due to their unique physicochemical and structural characteristics combined with their biocompatible nature. This unique family of materials combines the properties of MOFs and those of CDs (Figure 4) resulting to a smart versatile platform for bio-related applications including the smart release of molecules with biological interest, such as drugs, agrochemicals and active compounds with increased chemical sensitivity as scents and essential oils [161; 162]. CD-MOFs are easily synthesized by combining CDs (Figure 4), serving as the organic ligand and potassium or sodium salts. CD-MOFs present high specific surface area values in combination with low density, due to the chemical nature of their building blocks, along with tunable pore size depending on the type of CD which is used. The increased available space that they provide in their framework allows the encapsulation of a high volume of active compounds, while the tunable pore sizes allow the encapsulation of diverse molecules of biological interest, ranging from small volatile molecules as terpenes and terpenoids to larger biomolecules. In detail, CDs are cyclic saccharides with a conical hollow barrel structure, featuring a hydrophilic outer structure and a hydrophobic inner cavity. This leads to a unique framework architecture allowing the encapsulation of hydrophobic guest molecules e.g., EOs and their components (Figure 3), through van der Walls interactions or hydrogen bonding. At the same time, the hydrophilic outer space of these porous structures allows them to act as a vessel, transferring those hydrophobic molecules to hydrophilic environments which otherwise would not be approachable to them. Furthermore, the encapsulation of such active molecules, allows also their protection from degradation. Keeping in mind the chemical sensitivity of volatiles, their encapsulation in a porous framework helps them to be shielded against light, heat, or moisture improving their stability and therefore their mode of action. For example, CD-MOFs have been found effective towards the protection of curcumin, catechins and sucrose [161] but also, they have been employed for the safe and smart delivery for a plethora of volatiles as eugenol, cinnamaldehyde, oregano EO, thymol, carvacrol and clove EO just to name a few [139; 140; 148; 162]. More specifically, recent studies highlight the versatility and effectiveness of CD-MOFs, particularly \(\gamma\)-CD and \(\beta\)-CD variants, in enhancing the preservation and functionality of various EOs. Below, we discuss important examples of CD-MOF-based carriers employed in agrifood sector, with a particular emphasis on food packaging examples. A notable example that has been reported recently, is the in-situ growth of \(\gamma\)-CD-MOF on a chitosan-cellulose film aiming at the encapsulation of carvacrol EO. This hybrid material has been employed as an advanced packaging medium in order to extend the shelf-life  
Figure 4: \(\alpha\)-CD (6 glucose units and cavity size, 4.7–5.3 Å), \(\beta\)-CD (7 glucose units and cavity size, 6.0–6.5 Å), \(\gamma\)-CD (8 glucose units and cavity size, 7.5–8.3 Å) (CD refers to cyclodextrin) Reprinted with permission from Ref. [163]. © Elsevier 2022.  
of sensitive fruits such as strawberries [148]. Through this approach, the physicochemical and structural characteristics of \(\gamma\)-CD-MOF are employed (namely, low toxicity, biocompatibility, and appropriate pore size) in order to prepare humidity-responsive active packaging. This work also highlights the challenges arising from employing non-toxic components for MOFs' synthesis while it also emphasizes on the benefits of employing in situ growth of MOFs on films in order to overcome the common issue of nanoparticle aggregation in such cases [148].