## 3 Results and Discussion
### Adsorption and Release of Diclefonac Sodium  
All synthesized under different conditions, MIL-88A materials were applied as diclofenac sodium carriers. To evaluate the efficacy of the drug adsorption process on the sample surfaces, the FT-IR spectra were recorded (Figure 5A-D). Regarding the pristine MIL-88A samples, the absorption band in the wavenumber range of 3200-3600 cm\({}^{-1}\) is associated with hydroxyl groups in the structure or adsorbed moisture. The bands at -1604 and -1400 cm\({}^{-1}\) could be attributed to the symmetric and asymmetric vibrations of the fumarate ligand C=O groups [60,61]. In turn, at around 1215 and 983 cm\({}^{-1}\), the absorption bands originating from the vibration mode of C-O in carboxylic groups and C-H were detected. The band at 640 cm\({}^{-1}\) could be associated with the presence of the carbonyl group. In addition, the absorption band at -556 cm\({}^{-1}\) can be ascribed to the Fe-O stretching vibrations [61, 62, 63]. The presence of the mentioned absorption bands signifies the successful coordination of fumaric acid and iron ions in the MIL-88A structure [64, 65, 66]. The FT-IR spectrum of diclofenac sodium contains distinct bands at 3390 cm\({}^{-1}\), 1574 cm\({}^{-1}\), 1289 cm\({}^{-1}\), and 746 cm\({}^{-1}\) related to the N-H groups of the secondary amine, C=O from carboxyl ions, and C-CO-C, as well as C-Cl vibrations, respectively (Figure 5E) [67]. The drug-loaded MIL-88A samples spectra revealed the band at approximately 3320 cm\({}^{-1}\) which could be associated with the N-H stretching vibrations of the secondary amine from diclofenac sodium. Moreover, the bands at wavenumbers around 1580 cm\({}^{-1}\) and 1450 cm\({}^{-1}\), attributed in the literature to C=O and C=C stretching vibrations, are noted [67, 68, 69]. The materials show a new band at 1290 cm\({}^{-1}\) from the C-CO-C bond, confirming the presence of the drug on the sample surfaces [70]. The band at 766 cm\({}^{-1}\) can be related to the stretching vibrations of  
Figure 4: TG and DTG curves of (**A**)—MIL-88A-1; (**B**)—MIL-88A-2; (**C**)—MIL-88A-3; (**D**)—MIL-88A-4. C-Cl. The existence of these absorption bands validated the successful adsorption of the drug on the surface of all MIL-88A materials. Figure 6 depicts the equilibrium adsorption isotherms of diclofenac sodium onto metal-organic frameworks. It was noted that the quantity of adsorbed drug significantly  
Figure 5: FT-IR spectra of (**A**)—MIL-88A-1; (**B**)—MIL-88A-2; (**C**)—MIL-88A-3; (**D**)—MIL-88A-4 samples before and after drug adsorption, and (**E**)—diclofenac sodium. increased with increasing initial concentration of its solution until reaching the saturation point of adsorption. At low initial drug concentrations, the active sites of samples were highly accessible for diclofenac sodium, and its adsorption was random. At the higher drug solution concentrations, the sorption capacity was constant because all available active sites were occupied by either drug or water molecules [71]. The lowest sorption capacity toward diclofenac sodium exhibits samples synthesized in DMF: MIL-88A-3 and MIL-88A-4 (833 mg/g and 1244 mg/g, respectively). The most effective adsorbents of the drug are MIL-88A-1 (2021 mg/g) and MIL-88A-2 (1500 mg/g) prepared in distilled water, although they have a smaller specific surface area and pore volume than samples synthesized in DMF. This phenomenon could be associated with the structural flexibility of MIL-88A affected by the adsorption of polar solvent molecules [72]. Materials synthesized in distilled water are more flexible owing to the presence of hydrogen bonds between water (guest molecules) and the framework [73; 74; 75]. The structural flexibility of these MOFs allows them to accommodate drug molecules of different sizes [76]. Based on the aforementioned results, the drug adsorption process engaged structural parameters and interactions between the samples and drug molecules. One of the primary factors influencing adsorption is the pore size of porous adsorbents.