## 4 MOFs as Smart Carriers and Delivery Systems of Sensitive Volatile Compounds
### Zr-MOFs  
Zirconium-based metal-organic frameworks (Zr-MOFs), particularly UiO analogues, have emerged as highly promising materials for the encapsulation and controlled release EOs, as well as smaller active compounds of EOs, such as monoterenes, addressing the challenges of volatility, stability, and water solubility that limit their wider practical applications. These Zr-MOFs are characterized by their exceptional chemical and thermal stability, structural and functional tunability, and high loading efficiency, offering versatile platforms for enhancing the efficacy of bioactive compounds in agrifood applications. For example, UiO-66-(COOH)\({}_{2}\), has been utilized for the encapsulation of thymol and limonene. This example demonstrated the ability of this MOF platform to accommodate both polar and non-polar molecules. More specifically, the presence of the free carboxyl functional groups in UiO-66-(COOH)\({}_{2}\) enhances its framework interaction with polar molecules like thymol, improving its encapsulation efficiency. The sustained release of thymol from UiO-66-(COOH)\({}_{2}\), was observed over 11 days at 25 \({}^{\circ}\)C. This formulation has been tested for its prolonged antifungal activity against _Colletotrichum musae_. This gradual diffusion of thymol from the pores of the MOF allows the continuous exposure of fungal cells to the antifungal compound, disrupting their cellular processes [160]. Regarding the exploration of the possibilities of the UiO family in the encapsulation of volatiles, their structural characteristics have been meticulously studied, emphasizing their cavities size and functionalization. In detail, the pore size effect of Zr-MOF is a critical factor in the encapsulation and release of volatiles as monoterenes, which are actually the primary components of EOs. UiO-66, UiO-67, and UiO-68 are a family of isostructural MOFs, built from Zr\({}_{6}\)-octahedra connected by dicarboxylate linkers of different length, forming three-dimensional structures with high stability that makes them ideal platforms for controlled release systems. Through the careful selection of the pore size of the MOFs, it is possible to achieve a high loading and at the next step a sustained release of encapsulated volatiles with antimicrobial and antioxidant activity. These MOFs have been studied for their behavior regarding the smart release of monoterepenes providing us with key information in order to further expand and address key challenges as the relationship between pore size and guest molecule diffusion [147]. NH\({}_{2}\)-UiO-66, with a pore size of 4.4 nm, has been successfully employed for the encapsulation of CEO, achieving a high loading capacity of 654.4 mg/g. The release kinetics of CEO from starch matrix packaging films incorporating NH\({}_{2}\)-UiO-66 follow the Avrami model, indicating a slow and sustained release. This controlled release is attributed to the mesoporous structure and large specific surface area of NH\({}_{2}\)-UiO-66, which effectively encapsulates CEO and limits its volatility and hydrophobicity. The resulting starch-based packaging films exhibit increased mechanical strength, enhanced oxidation resistance, and improved vapor and oxygen barrier properties [164]. The presence of eugenol, eugenyl acetate, and \(\beta\)-caryophyllene in CEO enhance its antimicrobial and antioxidant activity, as they can disrupt bacterial cell structures. The prepared films demonstrate excellent antibacterial activity against _E.coli_ and _Staphylococcus aureus_. This highlights the potential of NH\({}_{2}\)-UiO-66 as a carrier for antimicrobial EOs in food packaging applications [166]. The slow and sustained release of CEO from this MOF, as described by the Avrami model, ensures prolonged antibacterial effects, contributing to the extended shelf life of packaged products like blueberries [159]. In conclusion, Zr-MOFs, particularly UiO analogues, provide versatile platforms for the encapsulation and controlled release of EOs and monoterepenes.