Today, nanotechnology is expanding in different fields and increasing the potential benefits of application in the world . The dimensional range of nanoparticles is between 1-100 nm, and the small size and large surface area opens up new approaches for various sciences . In recent years, the study of nanoparticles with antimicrobial activity has increased due to the increased incidence of nosocomial infections and food poisoning caused by food pathogens . Bacterial infections are one of the global public health concerns and place a heavy economic burden on a country's health care, and nanomaterials have emerged as potential antimicrobial agents . The antimicrobial mechanism of nanoparticles is generally described by three models involving the induction of oxidative stress, the release of metal ion, and non-oxidative . Human infections are the result of gram-negative and gram-positive bacteria, and bacterial-susceptible strains are used to accurately determine the anti-bacterial activity of NPs . Among these, the nanoparticles of zinc oxide are one of the NPs that has high anti-bacterial potential and are important concerning gram-positive and gram-negative bacteria .
The metal organic frameworks as a class of porous hybrids, has the desired size and shape of pores. MOFs are synthesized through self-assembly metal centers (including metal ions or metal clusters) and linkers (including bridging ligands (. Metal organic frameworks have promising usage in many fields, such as protein immobilization , catalysis [10, 11], photocatalytic , sorbent , gas storage and separation , luminescence , sensing , ion exchange , biomedicine , magnetism , drug delivery [20, 21], preparation of nanoparticles [22, 23], and anti-bacterial . The release of metal ions from MOF structure makes them attractive antimicrobials for adjustable antibiotic applications . The metal organic frameworks prepare by different methods such as solution , hydrothermal , solvothermal , diffusion , microwave , sonochemical , electrochemical , mechanochemical , ion thermal , microfluidic , and laser ablation [35, 36]. MOF components can be connected with several types of connections. These types of bonds involve metal coordination, hydrogen bonding, electrostatic interactions, and π-π stacking that can show adjustable structures, compounds, and properties . The antimicrobial activity of MOFs is attributed to the presence of metal ions, such as Ag  and Zn . Zinc as a non-toxic metal is widely used in many drugs and cosmetics. Also, it has properties such as a cicatrizing agent and skin moisturizer with anti-dandruff, astringent, anti-inflammatory and anti-bacterial . Among the various MOFs, Zn2(BDC)2(DABCO) (BDC = 1,4- benzenedicarboxylic acid, DABCO = 1,4-diazabicyclo [2.2.2] octane) is common Zn-MOF with large surface area [8, 26]. In [Zn2(BDC)2(DABCO)].4DMF.1/2H2O MOF, di-nuclear Zn2 units are bridged by BDC to form a distorted 2D square grid [Zn2(BDC)], which is pillared by DABCO ligand to form a -Po type 3D network . In this study, Zn2(BDC)2(DABCO) was prepared by the methods of solution and thermal solution. Developing of the anti-bacterial properties of Zn2(BDC)2(DABCO) MOF is the main purpose of this project.
MATERIALS AND METHODS
All materials were purchased without purification from Merck (Darmshtadt, Germany). Zinc acetate dihydrate (Zn(OAc)2·2H2O) as metal ion, 1,4 benzenedicarboxylic acid (bdc) and 1,4-diazabicyclo[2.2.2] octane (dabco) as organic linkers, dimethylformamide (DMF) as solvents.
Synthesis of Zn2(BDC)2(DABCO) MOF
In a normal reaction, 0.132 g Zn (OAc)2.2H2O (2 mmol), 0.1 g BDC (2 mmol) and 0.035 g DABCO (1 mmol) were mixed to 25 ml DMF . The optimal ratios of metal, chelating ligand and bridging ligand were 2:2:1. The sample was mixed for 30 minutes at 25 °C and at 90 °C. After filtration of the reaction mixture and washing the white crystals by solvent to remove any metal and ligand remaining, DMF was removed from white crystals by a furnace at 120 °C for 5 h .
The prepared samples were analyzed by XRD, FTIR, UV blocking, DLS, and SEM. FTIR was obtained by utilizing a Spectrum Two (PerkinElmer-USA) using KBr disks for investigation functional groups. The diffraction of X-ray was performed for the estimation of samples in terms of crystalline structure by utilizing Cu Kα X-ray radiation with a voltage of 40 kV and a current of 30 mA by X'pert pro diffractometer (Panalytical). DLS was used for the samples size and size distribution (Zetasizer, England). The investigation of zeta potential of samples was performed by Zetasizer model Malvern company (England). Field emission scanning electron microscopy was utilized to observe the shape and size of the MOFs (Sigma VP, ZEISS). For the investigation of UV protective and energy gap (UV2550, Shimadzu), the spectroscopy of diffuse reflection was used. The anti-bacterial properties of samples were estimated by the method of disk diffusion against Escherichia coli (ATCC 1399, Islamic Azad University) and Staphylococcus aureus (ATCC 25923, Islamic Azad University).
RESULTS AND DISCUSSION
The IR spectra of samples in the range of 400–4000 cm-1 were noted. FTIR spectra of Zn2(BDC)2(DABCO) shows at 25 °C (Fig. 1a) and 90 °C (Fig. 1b). The stretch vibrations of O-H was observed at 3445 cm-1. The high intensity peak of C=O at 1385 cm-1 are assigned to carboxylic groups. The aromatic bands of C–H are displayed at 3400 cm-1 however the band of hydroxyl functional group is presented at 2960 cm-1. The CO2 peak was appear at 2357 cm-1 which exist in environment. C=O stretch band is assigned at 1600 cm-1. The carboxylic groups are assigned at 1387 cm-1 with the high intensity peak. As a result, Zn2(BDC)2(DABCO) MOF results confirm previous reports [8, 26]. Hence, the FTIR results confirm the quality of the MOF synthesis.
X-ray diffraction of Zn2(BDC)2(DABCO) was investigated from 5 to 80 (2°), which demonstrated the structure of cubic crystal (Fig. 2). According to the XRD analysis, MOFs are synthesized acceptably and the structure of obtained crystal is near to the previous report [8, 26]. The XRD results confirm the structure of crystal for synthetic MOF.
The particle size of samples and distribution diagram were estimated by DLS. The results indicated a single-peak at 643 nm with slight distribution at 25 °C (Fig. 3a), and single peak at 683 nm with narrow distribution at 90 °C (Fig. 3b). These results were first reported for Zn2(BDC)2(DABCO) MOF. The DLS results confirm the narrow size distribution of the synthetic MOF.
According to the results, higher synthesis temperature is caused to the growth increase of the MOF. The zeta potential was -14.1 (Fig. 4a) and -7.96 mV (Fig. 4b) at 25 ̊C and 90 ̊C respectively. Based on the results, the MOF is more stable at room temperature and the particle size is smaller. Zeta potential results were first reported for Zn2(BDC)2(DABCO) MOF.
The FESEM images of Zn2(BDC)2(DABCO) at 25 ̊C and 90 ̊C are shown in Fig. 5. The images revealed that the structures are cubic shaped with smooth surface with nanometer in diameter [8, 26]. The obtained results showed that the size of MOF particle is smaller at room temperature and confirms the DLS results. Consequently, higher temperatures can cause particle growth and size to increase.
The spectra absorption of DRS was used to evaluation of the optical properties of Zn2(BDC)2(DABCO) (Fig. 6). The MOF absorption of spectra was noted in the wavelength range of 200–800 nm in diffuse reflectance mode. The peak of absorption in the range of 200-400 nm was observed, which exhibited the protective properties of Zn2(BDC)2(DABCO) samples. DRS results were first reported for Zn2(BDC)2(DABCO) MOF.
The energy difference (in electron volts) between the highest capacitance band and the lowest conductance band is called the forbidden band energy or energy gap. Due to the band gap energies by the Tauc method as the transfer spectra of samples, MOF nanostructures were extracted. The band gap energies of MOF are approximately 3.6 and 3.55 eV at 25 ̊C and 90 ̊C respectively (Fig. 7). The obtained results showed that reducing the size increases the energy gap and is consistent with the previous report .
The method of agar diffusion with the certain concentration (0.01 g/ml) was used to the investigation of Zn2(BDC)2(DABCO) anti-bacterial activity against E. coli and S. aureus. The zone of inhibition was measured 9 mm at room temperature and 8 mm at 90 for Escherichia Coli and 8 mm at room temperature and 7 mm at 90 for Staphylococcus aureus. Many parameters such as concentration, morphology, particle size, and surface modifications affect the anti-bacterial activity. The results are based on previous reports [22, 42]. The principal mechanisms of MOF for anti-bacterial activity involve: the contact of bacterial through cell walls and cell destruction of it, the production of reactive oxygen species (ROS), and the release of Zn+2 ions as an antimicrobial ion . Anti-bacterial activity can affected be the release of COO- and Zn+2 ions, metallic Zn, and amine group. According to the results, it has more anti-bacterial activity against the MOF at ambient temperature due to its smaller size and gram-negative bacteria because of more absorption of opposite charge. Anti-bacterial activity results were first reported for Zn2(BDC)2(DABCO) MOF.
The Zn2(BDC)2(DABCO) MOF homogeneous samples. The purity of crystalline structure of MOF was confirmed by XRD spectrum. The applications of anti-bacterial activity and UV blocking of MOF, successfully were observed in this research. Based on the results, the solution method produces particles with smaller size, more band gap energy and higher anti-bacterial activity. The solution method as a simple synthesis can be used in various fields of biological application such as cosmetic, pharmaceutical, food, etc., and become an important research field in the future. Despite the material cost for industrial applications, the synthesis results in a very short time are very encouraging synthesized by solution and thermal solution methods. The DLS and SEM results showed.
CONFLICTS OF INTEREST
The authors declare that there are no conflicts of interest.