## 3 Results and Discussion
### In Vitro Release Studies  
The release profile of MTX (Figure 7) from UiO-66-MTX and UiO-66-NH\({}_{2}\)-MTX, in water and in PBS (pH 7.4) +10% FBS at 37 \({}^{\circ}\)C, mimicking the culture conditions surrounding cancer cell lines, was investigated. The stability of Zr-based MOFs, particularly UiO-66 and UiO-66-NH\({}_{2}\), in PBS remains a concern. Previous investigations have indicated potential instability of Zr-MOFs in PBS, prompting the exploration of alternative conditions for drug release studies [1; 59; 60; 61; 62]. In this study, water and PBS supplemented with 10% FBS was used for drug release experiments. The addition of 10% FBS was deliberate and served a dual purpose: it helped mitigate the observed instability in PBS and introduced biologically relevant components that may influence drug release dynamics. The protein content in FBS-supplemented PBS also simulates the conditions closer to physiological environments, enhancing the clinical relevance of drug release assessments. The results indicate that UiO-66-MTX showed a gradual release of approximately 25% of the drug after 96 h in water at 120 h in PBS +10% FBS, demonstrating the stability of UiO-66-MTX in both water and PBS +10% FBS. UiO-66-NH\({}_{2}\)-MTX showed a slower and more consistent release of MTX with a release of 62 \(\pm\) 4.9% in water and 47 \(\pm\) 3.6% in PBS +10% FBS after 168 h. The percentage release of MTX after 96 h from UiO-66-MTX was lower compared to UiO-66-NH\({}_{2}\)-MTX in water. In PBS +10% FBS, a release of 20 \(\pm\) 2% by UiO-66-MTX and 19 \(\pm\) 4% by UiO-66-NH\({}_{2}\)-MTX was reported, which simulates the culture conditions used for a cytotoxicity assay in different cell lines and agreed with the results obtained after 96 h. The slow release of MTX from both MOFs in PBS at pH 7.4 is consistent with prior findings [63]. The UiO-66-MTX material demonstrated a decrease in release percentage after 96 h in water and after 120 h in PBS +10% FBS. In contrast, the UiO-66-NH\({}_{2}\)-MTX material showed an increase in release percentage until 168 h in both water and PBS +10% FBS. These results demonstrate that, despite UiO-66-NH\({}_{2}\) having a lower loading capacity than UiO-66, it still showed a slow and consistent MTX release for a prolonged duration. The observed slower release profile in the presence of FBS necessitates further discussion to consider its implications on drug delivery. A comparison with the existing literature reveals a common trend where nanoparticles, including liposomes and polymers, often exhibit faster release profiles in biological media due to destabilisation [64; 65; 66]. However,  
Figure 6: SEM images showing surface morphology of (**A**) UiO-66, (**B**) UiO-66-MTX, (**C**) UiO-66-NH\({}_{2}\), and (**D**) UiO-66-NH\({}_{2}\)-MTX. The scale bar represents 200 nm. All the images are in same magnification (20 K).  
our results with MOFs deviate from this norm. This divergence prompts an exploration of the possible formation of a protein corona around the MOFs, influencing their release kinetics. For instance, the encapsulation of doxorubicin within a BSA-coated ZIF-8 MOF exhibited enhanced biocompatibility, leading to increased uptake by MCF7 breast cancer cells. This phenomenon can be attributed to the positive charge present on the outer surface, facilitating improved interaction with the cellular environment [67]. Importantly, this distinctive behaviour to proteins could present an advantageous feature in MOF-based drug delivery systems [44,68] compared to other nanoparticle formulations.