Synthesis, Characterization And Antibacterial Activity of Modified Zeolitic Imidazole Framework-8 (Zif-8)

MOFs are a class of crystalline microporous materials that consist of endless lattices composed of metal ions or clusters and organic ligands connected via strong coordinate bonds. ZIF-8 is an example of a metal-organic framework comprising inorganic units of Zinc metal centers coordinated by four organic linkages (2-methyl imidazolate), forming a zeolitic sodalite topology. ZIF-8 possesses unique physical and chemical features, including high porosity, large pores, high surface area (BET: 700-1300 m2/g), exceptional thermal and chemical stability (unlike other MOFs), and biocompatibility, making it a promising candidate for drug delivery applications. ZIF-8 combines the advantages of both organic (biocompatibility and biodegradability) and inorganic (controllable size and shape, easy functionalization, and compatibility with different types of molecules) drug delivery systems. ZIF-8 is also utilized in wound healing due to its antibacterial activity. It can accelerate the wound healing process through a synergistic antibacterial effect, collagen deposition, reduced inflammation, and fibroblast proliferation. Copper possesses antibacterial, angiogenic, and antioxidant properties. It binds and interacts with several growth factors involved in vessel formation. Copper-dependent stimulation of vessel formation is primarily attributed to its regulation of vascular endothelial growth factor (VEGF) and, to a lesser extent, angiogenin. VEGF is considered the most effective mediator of angiogenic activity during the proliferative phase of wound healing. Therefore, copper availability, uptake, and delivery can play a role in regulating the overall process. Given the properties of ZIF-8 and copper ions, it can be expected that the wound-healing process will be accelerated. Cu/ZIF-8 was synthesized using the following process: 1.8 mmol Cu(NO3)2·3H2O and 2.2 mmol Zn(NO3)2·6H2O were dissolved in 40 ml methanol (solution 1). 32 mmol 2-methylimidazole was dissolved in 40 ml methanol (solution 2). Solution 1 was stirred for approximately 1.5 hours. Then, solution 1 was added dropwise to solution 2, and the final solution was stirred for 12 hours. Cu/ZIF-8 was collected via centrifugation (8000 rpm) and washed with methanol three times. The powder sample was obtained by drying in an incubator. The XRD pattern of ZIF-8 nanoparticles exhibited diffraction peaks at 2θ values of 7.15, 10.34, 12.52, 14.54, 16.24, 17.75, 22.02, 24.51, 26.52, 29.29, and 30.33, corresponding to ZIF-8 nanoparticles. According to XRD characterization, the presence of Cu2+ during the growth of ZIF-8 crystals did not cause any structural alteration, and ZIF-8 materials retained their crystalline integrity. However, a decrease in crystallinity was observed because Copper ions may interfere with the formation of well-defined crystal structures, leading to a decrease in crystallinity. The FT-IR spectra of ZIF-8 and Cu-ZIF-8 exhibited remarkable similarity. However, notable differences were observed in the Cu-ZIF-8 spectrum, particularly in the 3100 to 3600 cm-1 range, where stronger peaks corresponding to –OH and –NH stretching were observed. This suggests that the introduction of copper (Cu) into ZIF-8 resulted in modifications to its pore structure. Consequently, an increased amount of water molecules was found to condense within the pores of Cu-ZIF-8. Additionally, the characteristic 800 - 1600 cm-1 was attributed to the 2-methylimidazole ligand, indicating that the Cu-doped modification did not eliminate the organic ligand. These findings from the FT-IR analysis support the XRD results, which indicated that the structure of ZIF-8 remained unaffected by the Cu-doped modification. FESEM analysis revealed that copper doping increased the size of nanocrystals. The size distribution of ZIF-8 nanoparticles was approximately 75-125 nm, whereas Cu@ZIF-8 exhibited a size distribution of 125-325 nm. This confirms that copper ions may act as nucleation sites, promoting the formation of larger crystals during synthesis. The presence of copper ions can slow down the crystal growth process, allowing larger particles to form. To verify the antibacterial activity of Cu@ZIF-8, samples were co-cultured with E. coli and S. aureus. The number of bacterial colonies in the Cu@ZIF-8 groups was lower than that in the control group. The antimicrobial activity mechanism of ZIF-8 includes electrostatic interactions between bacterial cell walls and Zn2+ ion particles, internalization of Zn2+ ions into the cell, as well as the hydrophobic chains of methyl imidazole ligands. Copper ions can participate in redox reactions within bacterial cells, leading to the generation of reactive oxygen species (ROS). These ROS can cause oxidative stress, damaging cellular components and interfering with bacterial metabolism.