A novel synthesis of new antibacterial nanostructures based on Zn-MOF compound: design, characterization and a high performance application.

In this study, the novel zinc metal-organic frameworks (MOF) nanostructure has been employed, which was developed using an affordable, environmental friendly, efficient and fast method of ultrasound-assisted reverse micelle (UARM). These nanostructures were identified with various techniques such as FT-IR, XRD, BET, SEM, TG-DSC, TEM and EDS. It was found that the Zn-MOF samples have favorable physicochemical properties. The impact of experimental parameters of the UARM method is effective on the resulting properties, such as high surface area of the products that increases the interactions between the Zn-MOF nanostructure and bacteria.Their antibacterial activities were investigated using diffusion methods in agar and also with dilutions of Zn-MOF samples. Antibiotics (tetracycline and ampicillin) and their anti-biofilm effects were evaluated using microplate method. Obtained results revealed that the Zn-MOF nanostructures have high antibacterial properties which, could be due to the nature of the applied Zn-MOF as well as the optimization process. The Zn- MOF nanostructures could be a novel antibacterial material as biocatalyst processes.

Introduction

The use of antimicrobial compounds has become widespread due to their importance. The application ranges of these compounds included in various fields such as biomedical equipment, food packaging, reduced food corruption, and improved water purification equipment [1, 2]. However, the problem with these materials is that they show little resistance to the microbial cells that stick to surfaces permanently. The growth of biofilms on food processing equipment and medical devices causes the microbial contamination, and increases the prevalence of food-borne diseases and hospital infections [3]. For example, the bacteria such as Staphylococcus aureus, Escherichia coli, Salmonella enterica, Klebsiella pneumonia, Bacillus subtilis and Acinetobacter baumannii have a strong tendency to biofilm formation, and are found to be highly resistant to disinfectants [4, 5].

Given the importance of antibacterial materials, the introduction of nanotechnology-based antimicrobial structures has been expanded in the last few decades. Selecting compounds that not only have the side effects of conventional antimicrobials, but also having the specific surface and required structural capabilities that affect antimicrobial properties is highly important [6, 7, 8].

MOFs have some unique advantages compared to conventional porous materials such as zeolites and activated carbon including high-specific surface area, adjustable pores and adjustable porous surfaces. It is a made through coordination bonds between organic ligands and metal ions and found to be an important class in material science due to their diversity in structure and function [9, 10, 11]. MOFs have some unique advantages compared to conventional porous materials such as zeolites and activated carbon including high-specific surface area, adjustable pores and adjustable porous surfaces [12, 13].

The high surface area, structural porosity and dynamics, high mechanical and physiological properties of MOFs lead to the potential applications in gas storage and separation [14, 15], chemical sensor [16], heterogeneous catalyst [17] and also in antimicrobial fields [18, 19].The formation of these nanomaterials that affect the properties of these compounds is highly important. In recent years, MOFs have been made by fast, affordable and effective ultrasonic assisted reverse micelle method (UARM). If these methods are combined, an increase in the physicochemical properties of these products affecting the antimicrobial properties is expected [20].

Selecting the type of nanostructure that not only has an antibacterial nature, but also affects the properties of the product is very important. In recent years, the extensive studies have been conducted in this area. Liu and coworkers have developed to prepare an Ag-based MOF, which have showed antibacterial activities against gram-negative and gram-positive bacteria [21]. Zhuang et al. have synthesized Co-MOF compound as a disinfectant against E. coli. In both aforementioned study, the ligand used in preparation of MOF has not been commercially available and has been synthesized in laboratory during almost four separate phases [22].

ZnO nanoparticles are non-toxic, photooxidant and photocatalyst, and have skin-friendly properties. Besides, due to the antimicrobial properties of materials in nanostructure nature, therefore, in case of MOF production, they will find not only desirable properties, but also considerable physicochemical properties, which can be used in various fields [23, 24].Zhang and coworkers have synthesized two dimensional pillar-layered Zn MOF by solvothermally which can selectively adsorb carbon di oxide over nitrogen and methane [25].

Zhou and coworkers incorporated porphyrin systems into a surface-anchored MOF converted into a metal-free and extremely water-resistant antimicrobial coating [26].

In ionic liquid microemulsions stabilized by the surfactant TX-100, Zn-metal – organic frameworks (Zn-MOFs) are prepared using Zn2 + and the aromatic ligand 1,3,5-benzenetricarboxylic acid (BTC) [27] Restrepo and coworkers have developed Zn-MOF with hyderazine benzoate linker having reasonably good antibacterial activities against Gram-positive bacterium Staphylococcus aureus due to the controlled release of the ligand [28]. Moreover, nano Zn-MOFs have been examined alone and mixed with antibiotics such as amppicilin and kanamycin against bacteria like Escherichia coli, Staphylococcus aureus, Staphylococcus lentus, and Listeria monocytogenes antibiotics [29].

In this study, the Zn- MOF are made by effective ultrasonic assisted reverse micelle method for the first time and their physicochemical properties has identified with various techniques. Furthermore, the antibacterial effects of the Zn-MOF compound with antibiotics has also investigated. Also, its biofilm effects has been evaluated on six strains of bacteria such as Staphylococcus aureus, Escherichia coli, Salmonella enterica, Klebsiella pneumoniae, Bacillus subtilis, and Acinetobacter baumannii.

Materials and methods

material characterization

Zn (NO3)2.6H2O (Merck, 98%) and 2, 6 pyridine dicarboxilic acid (Merck, 99%), and sodium dodecyl sulfate (Sigma, 99%) were prepared using analytical-grade chemicals without any purification. Crystallinity data of the synthesized Zn-MOF samples was investigated by X′- PERT PRO (Philips) X-ray diffractometer device. Brunauer–Emmett–Teller (BET) surface area of the Zn-MOF products was determined using N2 adsorption/desorption isotherms (ASAP 2460, Micromeritics). The FTIR spectra of the Zn-MOF samples have been recorded between 4000 and 500 cm_1 as KBr pellets on SHIMADZU FT 8400 Spectrometer. EDS analysis was carry out by XL30. The particle size distribution and morphology of the samples were examined by scanning electron microscopy (SEM, Hitachi S-4800). Thermogravimetric analysis (TGA, 851e-Mettler Toledo) was studied from room temperature to 800 °C with a heating rate of 10 °C min−1 under an Argon atmosphere. TEM images were obtained by using a HITACHI H-7650 electron microscope with an accelerating voltage of 100 kV.

Ultrasonic assisted reverse micelle method

A mixture of zinc nitrate hexahydrate (0,120 mmol) and 2,6-pyridine dicarboxylic acid (0,120 mmol) is dissolved in 21 mL of double distilled water during the preparation of the specimens using the UARM process. The resulting solution was added to a mixture of 0.077 mmol of sodium dodecyl sulfate and 8 mL of hexane. Then, the resulting mixture was stirred at temperature of 85 °C for 1 h. The resulting solution was put into the ultrasonic device and was placed under ultrasonic irradiation in optimum conditions including ultrasonic duration of 21 min, power of 175 W and ultrasonic temperature of 40 °C. After 30 min, the Zn-MOF crystals are formed that are separated by centrifugation and washed by DMF.

Antibactrial behavior

In order to investigate the antibacterial behaviour of the Zn-MOF samples, the direct effects of powder are used. Initially, the plates are filled by the relevant culture medium of each bacteria, which was Mueller-Hinton agar. Then, from the desired six strains, including E. coli bacteria, after a 24-hour cultivation, the McFarland 0.5 (1.5 × 108 cfu/ml) is prepared. Hereafter, the lawn cultivation is made in the completely sterile conditions. In the following, about 2 mg of Zn-MOF powder is directly placed in the center of already inoculated plate. After 24 h of incubation at 37 °C, the diameter of non-growth halos is measured per mm. Moreover, the plates containing non-inoculated sterile culture medium are also incubated, so the possible contamination is examined. The procedure was repeated three times for each of the strains of the bacteria and the mean diameter for non-growth halos was obtained.

Determining minimum inhibitory concentration

The serial dilution method was used for testing. First, 11 sterilized tubes were placed in the tube container and numbered from 1 to 11. To all tubes, 0.5 mL of Mueller-Hinton broth agar was added. Then, 0.5 mm of Zn-MOF solution that was diluted was added to the tubes. The concentrations were (0.1, 0.05, 0.025, 0.0125, 0.006, 0.003), respectively. Afterwards, 100 μl of the bacteria containing McFarland 0.5 (1.5 × 108 cfu/ml) were prepared and entered the tubes (1–11). Then, the tubes were incubated at 37 °C for 24 h, where the tube No. 10 is Zn-MOF control and tube No.11 is the control culture medium, and after 24 h of testing, MBC was examined at 37 °C.

Biofilm formation measurement

The standard microtiter plate was used for quantitative evaluation of biofilm in bacterial strains. 100 μl of TSB medium and 100 μl of 0.5 McFarland suspension (1.5 × 108 cfu/ml) were added to each well of the microtiter plate. After 24 h of incubation at 37 °C, the culture medium within wells were removed and the wells were re-rinsed. Hereafter, 250 μl of 0.1% crystal violet and safranin were added to the wells for 20 min. In order to remove extra stains, the wells were washed 3 to 4 times with deionized water. The microtiter plate was dried in open air for 30 min. In order to dissolve the crystal violet and safranin, 250 μl of glacial acetic acid (33%) was added to the wells for 20 min. In the next step, 250 μl was transferred from a well to a new well. The absorbance of wells was measured by ELISA at the wavelength of 492 nm. In this method, the non-bacterial culture medium was used for negative control, and the strains were evaluated based on absorption values of OD. The staining was repeated for three times. Given the various states of biofilm formation, the non-bacteria OD was considered as control OD, and the crystal violet staining was appropriate for this study (Figure 1).

Figure 1

Crystal violet and safranin staining for different conditions.

Results and discussion

Physicochemical properties of Zn-MOF

The FT-IR spectrum of the Zn-MOF is depicted in Figure 2. The broad absorption bands appear at 3426.1–2851.7 cm−1 is attributed to stretching vibrations of H2O molecules. The absorption bands at 1645.6 and 1471.5 cm−1 which can be attributed to the stretching vibrations and asymmetric stretching vibrations of the carboxylate groups in the PDC (Pyridine dicarboxylate) ligand, respectively [30]. The vibrations of the C–H groups in the pyridine ring give adsorption bands from 830–725 cm−1. The characteristic peaks of protonated carboxyl groups (1730–1700 cm−1) are not observed in the FT-IR spectra, indicating that the PDC has been completely deprotonated in the reaction and acts as the bridging ligand to form Zn-MOF.

Figure 2

FT-IR spectrum of Zn-MOF.

X-ray diffraction pattern of Zn-MOF is depicted in Figure 3A. Based on Xpert High score software, the diffraction patterns are well indexed with the reference code (JCPD card no. 234578). In the present study, the average particle size has been calculated using the Scherer formula [31] given by where λ is the wavelength of the X-ray source, β the full width at half maximum, and the diffraction angle, the average crystalline size of MnO2 nanoparticle is found as 20 nm.

Figure 3

X-ray diffraction (A), N2 adsorption/desorption isotherms (B), SEM image (C) and TG/DSC analysis (D) of Zn-MOF synthesized under optimum conditions of UARM.

the size of crystals is about 41 nm, which approves the nanostructure nature of the Zn-MOF sample [27]. The small size of crystals can affect the performance of antibacterial properties of the samples. The observed peak patterns are properly compatible with other references, which is the reason for the correct synthesis of the structure with high crystallinity [32, 33, 34, 35, 36, 37, 38, 39, 40].

The specific surface area of the Zn-MOF was characterized by the N2 adsorption–desorption method depicted in Figure 3B. Based on classical isotherms, the sample has a similar behavior with the fourth type of isotherms. This type of isotherm represents the distribution of mesoporous with a mean size of 3.4 nm for the samples. The mesoporous size distribution increases the surface area of the samples, which considerably affects the antibacterial properties of the sample. Moreover, based on the BET technique, the sample has a surface area of 800 m2/g, which approves the interaction required for the antibacterial performance of the Zn-MOF sample [41].

Figure 3Cdepicts the SEM image of Zn-MOF having mean particle size of 97 nm. It could also be suggested that Zn MOF has multi-dimensional morphology, which is attributed to proper bonding in between the metal and linker. There is also no indication of agglomeration of the particles and this feature optimizes the antibacterial properties of the samples.

The thermal stability of Zn-MOF was determined with TGA and DSC thermograms depected in Figure 3D. It has shown that thermal decomposition was proceeded with two distinct weight losses between 40 and 700 °C. The first weight loss was attributed to the departure of all the water molecules in the temperature range 90–150 °C, without further weight loss up to 350 °C. The second weight loss corresponded to the release of the organic ligands from 400 to 550 °C, resulting in ZnO and inoorganic residue. The results showed that the Zn-MOFs had been prepared. Based on these results, the stability temperature of the sample is about 218 °C. The DSC results also supports TGA data based on weight loss of the sample, approves the range of temperature stability of the sample [42].

The X-ray diffraction spectroscopy was used to evaluate the chemical composition of Zn-MOF. This analysis was clearly showed the identification strong peaks with weight percentage of Zn (24.59), C (46.12) and O (29.31) elements and no other impurities are present in spectrum depicted in Figure 4A. TEM (Transmission electron microscopy) image of prepared Zn-MOF compound is depicted in Figure 4B, which gives a useful information about size and crystallographic parameters. It can be seen, the that Zn-MOF have pore size less than 200 nm which is good agreement to results obtained from BET, SEM and XRD.

Figure 4

EDS (A) and TEM (B) of prepared Zn-MOF.

Antibacterial effect of Zn-MOF

The findings of the analysis of the powder effect of Zn-MOF on the screening of bacterial strains are obtained by calculating the diameter of the non-growth halos. This method has been carried out for each strain of bacteria with three replicates in compliance with Table 1. In contrast, the mean diameter of non-growing halos for a range of bacteria is shown in Figure 5. Based on the results, the Zn-MOF samples have reasonable good antibacterial properties. The nanostructure design of the material, the physicochemical properties of the Zn-MOF compounds and the synthesis process seem to have a significant impact on the performance of the materials.

Table 1

Diameter of the halos in six type of testing bacteria.

No	Bactria	REP 1	REP 2	REP 3	Average diameter (mm)
1	Staphyloccus aureus	18mm	16mm	17mm	17mm
2	Bacillus subtilis	16mm	16mm	17mm	16mm
3	Acinetobacter baumannii	12mm	11mm	10mm	11mm
4	Kelbsiella pneumoniae	10mm	10mm	9mm	9.7mm
5	Salmonella entrica	10mm	9mm	10mm	9.7 mm
6	Escherichia coli	9mm	9mm	8mm	8.6 mm

Figure 5

Average Diameter of the halos in different Bacteria.

MIC results for the Zn-MOF sample

The results for the series of desired tubes for the inhibitory effect of synthesized Zn-MOF samples on several bacterial strains in different concentrations are presented in Table 2. As can be seen, in some concentrations, the tubes are clear and no opacity is observed, and in subculture of tubes in Mueller-Hinton agar and incubation at 37 °C for 24 h, no obvious growth is seen. Investigating the effect of the compound of the synthesized Zn-MOF and tetracycline antibiotic showed that the material is at a dilution of 0.006 g/100 cc for Zn-MOF samples and dilution of 0.8 g/100cc for tetracycline MIC.

Table 2

Growth status of various types of bacteria at different concentrations of Zn-MOF.

Bacteria	Concentration	Growth
Staphylococcus.aureus	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	non-growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	Growth
Bacillus subtilis	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	non-growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	Growth
Escherichia coli	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	Growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	non-growth
Acinetobacter.baumannii	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	Growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	Growth
Klebsiella pneumoniae	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	Growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	Growth
Salmonella enterica	0.2	non-growth
	0.1	non-growth
	0.05	non-growth
	0.025	Growth
	0.0125	Growth
	0.006	Growth
	0.003	Growth
	0.001	Growth
	0.0005	Growth

The results showed that from 45 tubes containing different bacteria at various concentrations of Zn-MOF, the bacterial growth was seen in 8 test tubes (17.8%) and 37 test tubes (82.2%) have no bacterial growth. This result shows that the frequency of test tubes lacks the bacterial growth more than 4 times of the frequency of test tubes with bacterial growth (Table 3). Based on Table 4, the results of Chi-square test also showed that with an increase in concentration of Zn-MOF from 0.006 to 0.1, there is a significant difference in the frequency of bacterial growth and lack of bacterial growth in different concentrations (p < 0.01) (it is synergistic).

Table 3

Results of growth and non-growth of all bacteria in different concentrations of combined tetracycline antibiotic and synthesized Zn-MOF samples.

Tetracyclin g/100cc	(Zn-MOF) g/100cc
	0.1	0.05	0.025	0. 0125	0.006	Tetracycline
12.8	non-growth	non-growth	non-growth	non-growth	non-growth	non-growth
6.4	non-growth	non-growth	non-growth	non-growth	non-growth	non-growth
3.2	non-growth	non-growth	non-growth	non-growth	non-growth	non-growth
1.6	non-growth	non-growth	non-growth	non-growth	non-growth	non-growth
0.8	non-growth	non-growth	non-growth	non-growth	non-growth	growth
0.4	non-growth	non-growth	non-growth	non-growth	growth	growth
0.2	non-growth	non-growth	non-growth	growth	growth	growth
0.1	non-growth	non-growth	non-growth	growth	growth	growth
0.05	non-growth	non-growth	non-growth	growth	growth	growth
Zn-MOF	non-growth	non-growth	non-growth	growth	growth	growth

Table 4

Comparing the observed frequency of growth or non-growth of all bacteria in various concentrations of synthesized Zn-MOF with tetracycline.

P value	sum	12.8	6.4	3.2	1.6	0.8	0.4		0.3	0.1	0.05	Tetracyclil
		0.1		0.05		0.025		0.0125		0.006		Zn-MOF
0.006	(82.2%) 37	(20%) 9		(20%) 9		(20%) 9		(11.1%) 5		(11.1%) 5		non-growth
	(17.8%) 8	(0%)0		(0%)0		(0%) 0		(8.9%) 4		(8.9%) 4		growth
	(100%) 45	(20%) 9		(20%) 9		(20%) 9		(20%) 9		(20%) 9		sum

Biofilm testing

The results of the biofilm testing that are presented in Figure 6 indicate that from 6 strains of testing bacteria during 48 h in crystal violet staining, the bacteria such as Acinetobacter baumannii and Salmonella enterica formed the medium biofilm, and the bacteria such as Escherichia coli, Bacillus subtilis, Staphylococcus aureus and Klebsiella pneumonia formed the weak biofilm. The results of the biofilm testing with the Zn-MOF showed that from 6 types of bacteria that formed medium and weak biofilm, at concentrations of (0.1, 0.05, 0.025, 0.0125, 0.006, 0.003) μg/mL, the synthesized Zn-MOF sample has the highest inhibitory that is shown in Figure 7, and also in accordance to the formula and statistical calculations, the diagram of -3% biofilm decrease during 48h.

Figure 6

Results of biofilm measurement in the microtiter plate with crystal violet stain (A) and results of biofilm measurement with the synthesized Zn-MOF on six strains of bacteria with crystal violet stain (B).

Figure 7

Biofilm testing with the synthesized Zn-MOF on six strains of bacteria.

Conclusion

In the present work, Zn-MOF has been synthesized using ultrasound-assisted reverse micelle and characterized by various physicochemical techniques such as FTIR, SEM, TEM, XRD, BET, EDS, TG-DSC and also antibacterial properties. The dilatation method was used to obtain its antibacterial activity in order to perform tests to determine the minimum growth inhibitor concentration. In the antibacterial behavior of such substances, the particle size, the surface area, the Zn-MOF concentration play an important role. The results of the examination of the minimum growth inhibitory concentration of antibiotics alone and in combination with the sample indicate the antimicrobial effects in both cases and therefore, given the synergistic effects of antibiotics and nanostructures, the effects of combined nanostructure toxicity can reduce the consumption of Zn-MOF. The improvement in the antibacterial properties of the samples may be attributed to the type of nanostructure, the process used and the optimization operations.

Declarations

Author contribution statement

Fatemeh Akbarzadeh, Mehdi Motaghi: Conceived and designed the experiments; Performed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Narendra Pal Singh Chauhan: Analyzed and interpreted the data; Wrote the paper.

Ghasem Sargazi: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.