A novel Iron(III)-based MOF: Synthesis, characterization, biological, and antimicrobial activity study

Recently infectious diseases and increasing microbial drug-resistant have caused many humorless health problems. To fill knowledge gaps and guide strategies at all levels for antimicrobials which represent a challenge and an urgent need. For these reasons, our target is developing a new effective antimicrobial drug with extended action time, multi-antimicrobial agents, low toxicity, and safe strategies. Metal–organic frameworks are promising materials for antimicrobial agents. Herein, a novel affordable Fe(III)-MOF was simply prepared via a reflux method. FE-SEM images showed an octahedral structure with sharp edges with high crystallinity and purity of Fe(III)-MOF. Under optimum conditions, the Fe(III)-MOF showed excellent antimicrobial efficiency against ± bacteria, fungus, and yeast with an inhibition zone ranging between 40–46 and 22–24 mm at a concentration of 50 and 25 μg/mL Fe(III)-MOF, respectively. As well, the mechanism of interaction is also well studied. The results open the door for the use of prepared materials as an effective and efficient antimicrobial agent.


Introduction
Infectious diseases and human infections like pathogenic bacterial infection as an example, remain a major driver of morbidity/mortality and pose an immense threat to public health worldwide annually [1][2][3]. Moreover, to tackle this threat, antibiotics are widely used in large doses to treat bacterial infection conventionally [4,5]. However, the use of administered antibiotics is often less effective or even ineffective in many cases, which can be attributed to increased drug resistance [1,5,6] Consequently, increasing the numbers of multidrug-resistant bacteria is considered one of the vital challenging issues that face global public health [3,5,7]. A report published by the "World Health Organization" (WHO) in 2017 included a list of many types of drug-resistant bacteria [2]. Therefore, traditional antimicrobial agents/drugs became not as efficient as demanded. At the same time, the antimicrobial agent's development becomes have considerable attention and attracted many scientists around the world due to their important role in different fields such as environmental, textiles, food industry/storage, healthcare sector, medical area, treatment of infectious disease, and many technical applications [2,[8][9][10]. In this regard, a lot of reports handled several transition metals, metal oxides, metal complexes, and metallic-nanoparticles as antimicrobial agents [11][12][13][14]. Although the above materials showed efficient high antimicrobial activities, unfortunately, they sometimes have high cytotoxicity in human normal cells/tissues [3,7]. Due to this reason, the application of some of the above materials as antimicrobial agent's significant drawbacks.
On the other hand, MOFs and nanocomposites-based MOFs systems have demonstrated significant long period antimicrobial activity [2,3,7,[15][16][17]. MOFs are highly well-ordered skeletons and structures, constituting organic linker and metal center molecules [18][19][20][21][22][23][24]. The possibility of a combination of multiplefunctionalities within the framework; makes these polymeric materials an ideal system for the designing of many periodic arrangements with multi-functional molecular subunits. Besides the distinct properties of these materials, they have been used in wide areas of applications [25][26][27][28]. The advantages of this category of materials as antimicrobial agents, they act as a metal ions reservoir and release these ions progressively in a prolonged period due to their synergistic action [29]. In addition, they are chemically stable and active, tunable skeletons and structure and can be controlled in their interaction with the microorganisms;

Fe(III)-MOF characterization
The Fe(.III)-MOF was prepared via a simple reaction of 1.0 mmol of NL reported by Sheta et al., [24] with 2.0 mmol ferric sulfate according to the reaction scheme (Fig. S1)

FE-SEM/EDX
The FE-SEM images at different magnifications [ Fig. 1(a)-(c)] of Fe(III)-MOF morphology appear to be an octahedral shape with dimensions in few micro-size. The results are similar to the previously reported [46]. Whereas, the EDX-analysis of Fe(III)-MOF [ Fig. 1

Mass spectrum
The Fe(III)-MOF mass spectrum (Inside proposed fragmentation Scheme) was presented in Fig. S2. Fig. S2 shows that the ion peak is 1085.13 m/z (theoretically calculated 1085.24 g/mol) as well m/z ions peaks were fully agreed with the proposed structure molecular weight and empirical formula C 44 H 43 Fe 4 N 7 O 12 which was obtained from elemental analysis. Moreover, as shown in Fig. S2 the subsequent fragmentations were in good harmony with the mass spectrum peaks.

H/ 13 C NMR spectra
The 1 H/ 13 C-NMR spectra of the Fe(III)-MOF were examined at room temperature in DMSO-d 6 and represented in Figs. S3 and S4, respectively. The 1 H-NMR spectrum (Fig. S3) ethanol molecules show three peaks; a triplet peak at 1.026, 1.040, and 1.055 ppm for (CH 3 ), the quartet peaks for (CH 2 ) at 3.404, 3.418, 3.431, and 3.446 ppm, besides the signal observed at 4.372 ppm assigned to NH 2 protons [48,49]. The phenyl protons peaks appeared between 6.923 and 7.946 ppm. Furthermore, the 13 C-NMR spectrum (Fig. S4), of the Fe(III)-MOF confirmed the 1 H-NMR spectrum that showed two peaks at 18.563 and 56.020 ppm due to CH 3 and CH 2 of ethanol. The signal at 30.715 ppm is due to the carbon of amine groups. The signals between 117.380 and 149.324 ppm are assigned to the carbon of phenyl rings. Finally, the peak appeared at 167.199 due to the amide carbon.

XPS analysis
The XPS survey scans of the Fe(III)-MOF sample revealed in Fig. 3(a) and the data summarized in Table S2. The obtained data confirm the existence of (Fe, O, N, and C), also these data were in excellent harmonization that obtained from EDX analysis as well as, calculated theoretically. Moreover, The Fe 2p XPS spectrum [ Fig. 3(b)] showed two satellite peaks at 715.09 for Fe(III) 2p3/2 and 728.12 eV for Fe(III) 2p1/2. As well, the spectrum showed two doublets' peaks appeared at 710.69 eV for Fe 2p3/2 and 725.26 eV for Fe 2p1/2. These main Fe 2p peaks confirmed the oxidation state of Fe 3+ in the MOF sample [38,56]. Figure S5 shows the O 1s peaks region located at 531.14, 531.93, and 533.06 eV [57][58][59]. Fig.  S6 shows the N 1s peaks region and displays two signals at 398.79, and 399.81 eV corresponding to C 6 H 5 -N-H 2 and N-C=O [60,61]. Finally, Fig. 7 displays the C 1s peaks region, which is located at 284.09, 285.34, and 288.57 eV attributed to C-N, C=O, and C=C respectively [21,25].

BET-surface area analysis
The N 2 adsorption\desorption isotherm of Fe(III)-MOF is represented in Fig. S8. The isotherm of prepared Fe(III)-MOF is of type III, according to the IUPAC classification. The BET surface area of the Fe(III)-MOF is 101.55 m 2 g −1 higher than the published reports in the literature with values about 17.79 and 30.09 m 2 g −1 higher than the published reports in the literatures [38,62], respectively. Whereas, the calculated pore volume for the Fe(III)-MOF is about 0.62 cm 3 g −1 . Additionally, the pore size distribution curve is represented in Fig. S9. From this Figure, it can be noticed that the Fe(III)-MOF pore size was about 1.34 nm, which indicated the presence of the micropores cages in the sample (zeolitic structure).

Thermal analysis
The thermalgravimetric behavior (TGA/DSC) of the Fe(III)-MOF is presented in Fig. 3(c). The thermalgravimetric plot suggested the breakdowns of the Fe(III)-MOF through three steps. The first weight loss was about 16.97% due to the loss of four molecules of C 2 H 5 OH at 73.05 °C (theoretically calculated weight loss: 16.98%). Then the Fe(III)-MOF obeyed to decomposition in two subsequent steps of weight losses about 26 0.46, and 35.48% due to the exclusion of organic skeleton. Finally, the remaining iron residue is about 21.09% (theoretically calculated 20.58%). The thermalgravimetric behavior confirmed the other obtained data from mass, 1 H and 13 C NMR spectra as well obtained from EDX.
Given the physical and spectral results discussed above, it can be assumed the monomeric unit 3D structure of the Fe(III)-MOF as presented in Fig. 3(d).

Magnetic behavior and photoluminescence property of the Fe(III)-MOF
The Fe(III)-MOF magnetization curve (Fig. S10) was demonstrated the superparamagnetic behavior of the prepared Fe(III)-MOF sample, that the values of magnetization (Ms), coercivity, and remanence were 18.656 emu/g, 12.349 G, and 0.198 emu/g, respectively. Moreover, The PL-spectrum of the Fe(III)-MOF in DMSO was performed. As represented in Fig. S11, the Fe(III)-MOF exhibits a strong photoluminescence emission band (ƛ max ) at 522 nm when excited at 384 nm.

Biological and antimicrobial activity studies
The antimicrobial activity of Fe(III)-MOF in vitro was investigated against gram-positive/negative bacteria and fungus/yeast. According to the experimental part, the enriched blood nutrient-agar medium was prepared and poured down to a Petridishes. Afterward, the fungus or bacteria were sub-cultured in the prepared media to grow microorganisms. Subsequently, the growth microorganisms were distributed over nutrient agar Petri-dishes using a sterile loop. Different families of standard antimicrobial/antifungal drugs were investigated in parallel with Fe(III)-MOF (Fig. 4). The activities of the Fe(III)-MOF were evaluated by calculating the inhibition zone diameter. The effectiveness-sensitive material is directly proportional to the diameter inhibition zone. The results of the biological activity of the Fe(III)-MOF, and the comparison of the gained data with the antifungal and antimicrobial agents used in the current study were presented in Table 1. The activities histograms for each type of microorganisms were displayed in Fig. 4. Figure 4

Mechanism of interaction and biological activity
Generally, the high antibacterial activity of the investigated MOF could be attributed to a variety of properties, such as the high surface area, unique shape, texture, structure, enormous porosity, the metallic plates of Fe(III) MOFs, their diffusion framework on the surface of microorganisms and the effect on the surrounding environment of the bacterial cell [63].
In the present case, as represented in Fig. 5 the suggested high antibacterial activity of Fe(III)-MOFs is due to the presence of Fe +3 on the MOF surface which led to the interaction potential between Fe +3 ions and the cell walls of microorganisms [64][65][66]. The exclusive antibacterial activity also results from the active surface of the Fe(III)-MOFs metal sites and is most probably a result of having a lot of free ions in the solution, which can promote the cooperative interaction between metal ions and bond [65]. In addition, Fe +3 ions can also bind to donor bond atoms, such as N − , O − , and S − [65,67,68]. These reactions depend on the coordination chemistry and are the bonding forces of -OH, -C=O, -COO-, -NH 2 and -SH groups in bacterial cell walls which formed and then destroy bacterial cell walls, causing bacterial death. This activity can also be explained due to the release of Fe +3 ions from MOFs which bear the opposite charge of the metal-negative bacteria partly with the donor atoms present in the bonds (the decreasing due to the feasible  delocalization of the π-electrons within chelate-ring beside the relatively partially sharing of metal-ion positive charges with the donor-site) and there is electron discrimination on the whole chelating band. This in turn increases the lipophilic-character of the Fe +3 chelate and favors its permeation across the lipid layers of bacterial membranes [15,[69][70][71][72]. Probably, the most common reason for the biotoxicity of Fe-MOF is the release of the Fe-metal from the bulk of the tire, as a cation or as small segments of the Fe-framework. An advanced topic published by Berchel et al. [73], in which they postulated that the antibacterial activity depends on the ease release of cations due to the high rigidity of the MOFs structures. Based on the Pearson theory (Pearson acid-base concept) of hard-soft Lewis-acids/bases. It suggested that the releasing of cations easily depends on the relatively stiffness of the cations "Lewis acids", and the organic bonds "Lewis bases" of the metal framework. When soft-acid is attached to hard-base, making it more susceptible to hydrolysis and releasing the cations. In addition, the resulting electrostatic attraction is one of the reasons why killing bacteria and yeast is effective and easier. Furthermore, the effectiveness of biological activity increases with an increase in the dose (concentration) of Fe. Moreover, MOF components and their biodegradability make metal ions and sterilization ligands an auxiliary important mechanism [65].
Additionally, the ligand in MOF acts as a reservoir for metal-ions when they come into contact with the bacterial cell walls. The Fe(III)-MOF mode-of-action (a mechanism) involves hydrogen-bond formation between the NH 2 -groups of Fe(III)-MOF and constituents of microorganism cell-active sites. Due to the formation of these bonds a disturbance within the cell wall is generated and successively causes to damage the cytoplasmicmembrane and increases the cell-permeability which causes cell fatality. Furthermore, the functional groups of organic bonds in MOFs can react with cations in the cell, which leads to the generation of oxygen reactive species in the cytoplasm, which leads to fragmentation and modification of the DNA [15,[74][75][76].
Finally, the obtained results confirm that the Fe(III)-MOF showed higher biological/antimicrobial activity against a different class of pathogens contrasted with standard antimicrobial/ antifungal agents. In addition, the results suggested that Fe(III)-MOF could be used effectively as an antimicrobial/antifungal agent and can address future medical concerns.

Conclusion
This work presents a novel affordable Fe(III)-MOF that was prepared and well-characterized. Octahedral structure with high crystallinity, purity, and sharp edges of Fe(III)-MOF was obtained. The prepared Fe(III)-MOF was evaluated as an antimicrobial agent against different types of pathogens. The Fe(III)-MOF showed antimicrobial activity against different tested bacteria such as gram-positive like (S. Aureus) and gram-negative like (E. Coli) with an inhibition zone ranging between 40-46 and 22-24 mm at a concentration of 50 and 25 μg/mL Fe(III)-MOF, respectively. The promising results of the antimicrobial study revealed that it can be one of the promising affordable future antimicrobial agents and may be used as antimicrobial for drug-resistant pathogen and simple outstanding solution for one of a vital environmental and public human health issue. Meanwhile, the vital globally reasons to prevent the advance emerging pathogens and decrease the deaths rate.

Instruments and characterization
The characterization of the prepared Fe(III)-MOF was carried out using the following analytical techniques: The structure morphology was examined by using a combination of "field-emission scanning electron microscope (FE-SEM)/ element mapping via spatially resolved energy-dispersive X-ray spectroscopy (EDX)" (JEOL JSM-6510LV, Japan). The Fe(III)-MOF Fourier-transform-infrared (FT-IR) and UV-Vis spectra were performed by (JASCO-FT/IR-460 spectrophotometer, and V-770-UV-vis, USA). The bandgap energy was estimated with Optbandgap-204B soft wear. Elemental analysis was done using (Costech ECS-4010-analyzer, Italy). The mass spectrum of solid Fe(III)-MOF was analyzed using a "Thermo Scientific-IQS single quadrupole spectrometer" (Thermo Scientific, USA). The 1 H/ 13 C-NMR spectra of Fe(III)-MOF in DMSO-D 6 were performed with (JEOL-ECA 500II-500 MHz NMR spectrometer, Japan). The recognition of the relative crystallinity phase, and crystal size of the Fe(III)-MOF was carried out using "X-ray diffraction" (XRD) (Bruker-X-ray D8-AVANCE-diffractometer, Germany). The species and oxidation states in the Fe(III)-MOF were analyzed by an "X-ray photoelectron spectrometer" (XPS) (ThermoScien-tific™ K-α™, USA). The surface area of the prepared Fe(III)-MOF was measured using Quantachrome, TouchWin™ v1.2 instrument as a results of nitrogen isotherms at 77.35 K using the "Brunauer-Emmett-Teller (BET) theory". The pore volume/size was calculated according to many methods and fitted according to the "density functional theory (DFT)" and applied to the adsorption curves. The thermal performance of the Fe(III)-MOF was investigated using "differential-scanning calorimetry/thermogravimetric analysis" (DSC/TGA) by (Universal V4.5-TA, USA). The magnetic possessions of the sample were assessed using a "Vibrating-sample magnetometer" (VSM-7400-1, USA). The photoluminescence (PL) study was performed using a (Shimadzu RF-5301PC spectro-fluorophotometer). The sample was analyzed at different excitation wavelengths and recorded the maximum emission wavelength in quartz-cuvette (1.0 cm path length). ChemBioDraw Ultra12 and Origin-8 programs were used for data analysis and drawn the structure and schemes.

Fe(III)-MOF synthesis
The Fe(III)-MOF was prepared according to the reaction scheme represented in Fig. S1. In 20 mL distilled-water (DW), Fe 2 (SO 4 ) 3. 5H 2 O (2.0 mmol, 0.9799 g) was dissolved and then drowsily added to the organic linker previously reported by Sheta et al. [24] with stirring and then refluxed at 80 °C for 48 h. The solution color (yellowish orange) was changed to a dark red-brown precipitate which filtered off, washed, and finally dried well.

Biological and antimicrobial activity
The biological and antimicrobial activities of the Fe(III)-MOF were performed in the microbiology lab., Ministry-of-Health, Egypt, via diffusion method. Scheme 1, represented a schematic representation for the biological activity evaluation. Different types of microorganisms (bacteria: gram-positive and negative), fungus, and yeast were used in the current study; gram-positive was (S. Aureus), gram-negative was (E. coli), whereas the fungus/yeast was (Candida spp. and A. Niger).
In the nutrient-agar medium, the microorganisms were subcultured. Nutrient-agar media was prepared according to kit-procedure: "Suspend nutrient-agar (28.0 g) in 1.0 Liter DW, then heat to boiling then sterilizing for 15 min in an autoclave at 121 °C. Left the prepared media to cool to reach 45-50 °C then poured into Petri-dishes (3 and 9 cm diameter). The media can be enhanced with 5-10% blood (calling blood-agar media), then after cooling to room temperature, the Petri-dishes were stored at 4 °C till used". The microorganism's culture is prepared by spreading them over each Petri dish prepared before using a sterile loop rod and incubated at 37 °C. Subsequently, after subculturing different types of bacteria, fungus, and yeast, the following step was the sensitivity evaluations of the prepared material (Fe(III)-MOF) against different families of common standard antimicrobial/antifungal agents. The microorganisms were transferred to Petri-dishes including the media, then an about 100 µL of dissolved synthesized Fe(III)-MOF (25 and 50 μmol) was added to the Petri-dishes and examined under comparable conditions with "Amikacin 500 mg, Ceftriaxone 500 mg, Ciprofloxacin 500 mg, Unasyn 375 mg, Durosiff 1 g, Flagyl 500 mg". After incubation for 48 h at 37 °C, the inhibition zones were evaluated and measured in millimeters cautiously.

Acknowledgments
This work was supported by the financial support of the Science and Technology Development Fund (STDF) Foundation of the Project No. (37068).

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability
All data generated or analysed during this study are included in this published article [and its supplementary information files].

Conflict of interest
The authors declare that they have no conflict of interest.

Open Access
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