## 3 Results and Discussion
### Physicochemical Characterization of MIL-88A Carriers
Figure 1: XRD patterns of materials obtained in the high-angle range. Scanning electron microscopy (SEM) images demonstrate that the MIL-88A materials with different morphologies were successfully prepared using the hydro-/solvothermal method. The particle size is strongly influenced by the value of the dipole moment of solvents due to the changes in linker solubility, solvent-nanoparticle interface, and superficial tension [50]. It is observed that after synthesis in distilled water at 70 \({}^{\circ}\)C, the MIL-88A-1 sample presents a hexagonal rod-like morphology with an average length of 7.5 \(\upmu\)m (Figure 3A) [51]. Upon increasing the synthesis temperature, the mean particle size of MIL-88A-2 decreases to approximately 5.5 \(\upmu\)m, but also the surface roughness is visible (Figure 3B). The morphology of MIL-88A-3 prepared in DMF at 70 \({}^{\circ}\)C changed to a spherical (Figure 3C). DMF retards the crystallization rate of MOF by effectively solvating Fe(III) ions in the organic medium, which could result in lower particle agglomeration and stop rod-like crystallite formation [45; 52]. The particle size of this sample is much smaller compared to the materials prepared in distilled water: about 500 nm. These size differences are associated with higher dipole moment of DMF than water (3.86 D and 1.856 D, respectively) and greater linker solubility in DMF than in water [53]. It could result in faster nucleation rates, leading to smaller crystal sizes [54]. The converse phenomenon is evident in the case of the MIL-88A-4 sample (Figure 3D). Implementation of the same solvent (DMF) but a higher temperature (120 \({}^{\circ}\)C) caused the formation of crystals with two different shapes--rods and diamonds, the most common for materials synthesized in DMF. It was observed that the size of diamond-like particles was about 13 \(\upmu\)m and rod-like--2 \(\upmu\)m in length. The thermal stability of the MIL-88A samples was evaluated via TG analysis (Figure 4). The initial weight loss, occurring below 100 \({}^{\circ}\)C, is associated with the evaporation of moisture [55]. For MIL-88A-1 and MIL-88A-2, the further mass loss observed at 300 and 298 \({}^{\circ}\)C, respectively, is attributed to fumaric acid degradation, which proves that it is inherent in the MIL-88A structure [56]. The mass loss noted in the temperature range of 360-440 \({}^{\circ}\)C is related to the collapse of the metal-organic skeleton [57; 58]. The third stage visible on the TG curve of MIL-88A-1 at 646 \({}^{\circ}\)C can be associated with the iron oxide conformation or the conversion between iron oxides [59]. Weight loss of MIL-88A-1  
Figure 2: N\({}_{2}\) adsorption/desorption isotherms of MIL-88A materials. and MIL-88A-2 was 69 and 55%, respectively. In turn, for MIL-88A-3 and MIL-88A-4, weight loss associated with linker decomposition occurred to a greater extent at lower temperatures: 283 and 287 \({}^{\circ}\)C. After this step, the weight of MIL-88A-3 and MIL-88A-4 samples is almost unchanged until 1000 \({}^{\circ}\)C. The samples synthesized in distilled water (MIL-88A-1 and MIL-88A-2) are slightly more thermally stable than materials synthesized in DMF. Figure 3: SEM images of: (**A**) MIL-88A-1, (**B**) MIL-88A-2, (**C**) MIL-88A-3, (**D**) MIL-88A-4 carriers.