## 3 Metal-Organic Frameworks
### MOFs by the Co-Precipitation Method
#### 3.1.1 PVP Co-Precipitating Agent  
In the process of encapsulating enzymes into MOFs, PVP is commonly used as a co-precipitant to facilitate the formation of complexes. PVP aids in maintaining the dispersion of enzymes during immobilization and stabilizes enzymes in the solution through electrostatic/hydrogen bonding interactions [22; 108]. Simultaneously, PVP promotes MOF nucleation through its weak coordination affinity for metal cations via the pyrrolone \begin{table}
\begin{tabular}{c c c c c c} \hline
**MOF** & **Metal Ions** & **Enzyme** & **Applications** & **Improved Performance** & **Ref.** \\ \hline ZIF-8 & Zn\({}^{2+}\) & Cytochrome C & Oxidation of Amplex red & Enhanced activity & [103] \\ \hline ZIF-8 & Zn\({}^{2+}\) & \begin{tabular}{c} Horseradish Peroxidase and \\ Glucose Oxidase \\ \end{tabular} & Selective glucose detection & \begin{tabular}{c} Enhanced activity and \\ selectivity \\ \end{tabular} & [104] \\ \hline ZIF-8 & Zn\({}^{2+}\) & Carbonic Anhydrase & \begin{tabular}{c} CO\({}_{2}\)/N\({}_{2}\) selectivity \\ composite membranes \\ \end{tabular} & Enhanced stability & [105] \\ \hline ZIF-8 & Zn\({}^{2+}\) & Horseradish Peroxidase & Proof of concept & \begin{tabular}{c} Enhanced thermal \\ stability \\ \end{tabular} & [106] \\ \hline ZIF-8 & Zn\({}^{2+}\) & Lipase QLM & \begin{tabular}{c} Kinetic resolution of (\(R\), \\ \(S\))-2-octanol \\ \end{tabular} & \begin{tabular}{c} Enhanced activity and \\ reusability \\ \end{tabular} & [107] \\ \hline ZIF-90 & Zn\({}^{2+}\) & Catalase & Biocatalysis & Enhanced activity & [108] \\ \hline MIL-53/ & Al\({}^{3+}\) & \(\beta\)-glucosidase/ & \begin{tabular}{c} Proof of concept \\ Lacase \\ \end{tabular} & \begin{tabular}{c} Enhanced organic solvent \\ stability \\ \end{tabular} & [109] \\ \hline MIL-88A & Fe\({}^{3+}\) & \begin{tabular}{c} Glucose Dehydrogenase/ \\ Horseradish Peroxidase \\ \(/\)Acetylcholinesteraseterase \\ \end{tabular} & Proof of concept & Enhanced reusability & [110] \\ \hline MIL-100 & Fe\({}^{3+}\) & Lipase PPL & \begin{tabular}{c} Synthesis of benzyl \\ cinnamate \\ \end{tabular} & \
\begin{tabular}{c} Enhanced thermal, pH, \\ and stability \\ \end{tabular} & [111] \\ \hline Fe-MOF & Fe\({}^{3+}\) &
\begin{tabular}{c} Alcohol dehydrogenase/ \\ Lipase/Glucose Oxidase \\ \end{tabular} & Biocatalysis & Enhanced reusability & [112] \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of the enzyme-MOF composite’s one-step synthesis. groups. [PERSON] and colleagues reported a straightforward method for the direct synthesis of biologically active protein-encapsulated MOFs (Figure 5). They achieved the direct embedding of cytochrome c (Cyt c) into zeolitic imidazolate framework-8 (ZIF-8) by adding a solution containing Cyt c and PVP to a methanol solution of zinc nitrate hexahydrate and 2-methylimidazole (mIM). They find that the embedded Cyt c shows a significantly decreased K\({}_{\text{m}}\) of H\({}_{2}\)O\({}_{2}\), which suggests the possibility that the immobilized Cyt c has a higher substrate affinity toward H\({}_{2}\)O\({}_{2}\) molecules. The Cyt c embedded in ZIF-8 displayed a tenfold increase in peroxidase activity compared to free Cyt c in solution [103]. [PERSON] and colleagues introduced zinc acetate solution into a water solution containing a mixture of \(\alpha\)-amylase, glucoamylase, PVP K-30, and mIM, preparing a dual-enzyme combination MOF. Its stability increased threefold, and after five consecutive reuse cycles, it retained as much as 52% of its residual activity [113]. In recent years, there has been increasing interest in synthesizing novel hybrid MOFs for enzyme immobilization by employing competitive ligands (amino acids, short peptide molecules, etc.) [114] along with traditional ligands. Competitive ligands inherently contain multiple coordination sites and various functional groups suitable for binding metal ions, enhancing the concentration of metal cations around enzymes to promote MOF nucleation.