Green Synthesis and the Study of Some Physical Properties of MgO Nanoparticles and Their Antibacterial Activity

Magnesium oxide nanoparticles (MgO NPs) were synthesized by a green method using the peels of Persimmon extract as the reducing agent , magnesium nitrate, and NaOH. This method is eco-friendly and non-toxic. In this study, an ultrasound device was used to reduce the particle size, with the impact on the energy gap was set at the beginning at 5.39 eV and then turned to 4.10 eV. The morphological analysis using atomic force microscopy (AFM) showed that the grain size for MgO NPs was 67.70 nm which became 42.33 nm after the use of the ultrasound. The shape of the particles was almost spherical and became cylindrical. In addition the Field-Emission Scanning Electron Microscopy (FESEM) analysis showed that the average particle size was reduced and the spherical shape was changed into cylindrical flakes. The antibacterial activity of MgO Nps was measured against both gram positive and negative bacteria (Staphylococcus aureus and Escherichia coli, respectively) for both the synthesized and the scaled-down particles by the ultrasonic. MgO NPs showed an efficacy at a minimum inhibitory concentration (MIC) of 500 μg/ml, with the better effect being observed after the ultrasonic treatment of the MgO NPs.


Introduction
Modern studies show that the metal oxide and metal nanomaterials demonstrate noticeable positive results in many fields [1][2][3][4]. Metal oxide nanoparticles attracted attention due to their increased use in various fields such as cosmetics, electronics, material sciences, catalysis, environment, energy and Biomedicine [5,6]. MgO nanoparticles have interesting applications in microelectronics, diagnostics, and biomolecular detection. Green synthesis of MgO nanoparticles was carried out using the peels of Persimmon extract for the predominantly ecofriendly processing with remarkable novel technologies [7]. MgO is considered as a safe material for humans and animals. There are several methods for the synthesis of nano-sized MgO particles, including laser deposition, sol-gel , hydrothermal synthesis , aerosol synthesis, and chemical gas deposition [8,9]. The toxicity of metal oxide nanoparticals and their antimicrobial activity are attracting very much research. Among such oxides is the MgO with its low cost and eco-friendly properties, while its toxicity is conferred by the production of reactive oxygen species (ROS) [10]. MgO NPs were shown to have antimicrobial activities against different microbial species, especially bacteria [11], which needs to be further investigated on different levels. Persimmon is also named kaki, which has been cultivated in Japan for several centuries. It is believed to have originated in southern China. The fruits of kaki are astringent until fully ripe. Persimmon contains many biologically active compounds, such as tannins, carotenoids, flavonoids, steroids, naphthoquinones, terpenoids, sugars, minerals, amino acids, and lipids [12]. In general, the peel was seen as a waste matter, although recent studies have shown that peel contains biologically and nutritionally beneficial compounds [13]. Ultrasound is a very active processing method in the application and generation of nano-size materials. In general, ultrasonic cavitation in liquids may cause fast and complete degassing; it accelerates chemical reactions by facilitating the mixing of reactants; it enhances polymerization and depolymerization reactions by temporarily dispersing aggregates or by permanently breaking chemical bonds in polymeric chains [14]. Ultrasound is applied in a vast range of physical, biological and chemical processes. Emulsifying and dispersing are examples of physical processes. Most of the uses of high-intensity ultrasound are based on the cavitation effects [15]. The nanoparticles and mini nanoparticles of MgO synthesized using ultrasonic processor were analyzed as related to their antibacterial activities.

Materials and methods Green method of synthesis of MgO NPs Preparation of hot aqueous extracts of the Persimmon peels
Fresh fruits of Iranian origin were bought from the markets, washed with water and distilled water several times, peeled and dried in an 40 ° C oven. They were grinded with an electric grinder and a soft powder of kaki peels was obtained. The powder (15 gm) was mixed completely with 200 ml of boiled distilled water and then homogenized on a magnetic stirrer. The final product solution was extracted by centrifugation at 4000 rpm for 25 min, and thereafter kept at 4 untill use [16] .

Synthesis of MgO NPs by hot aqueous extract of the Persimmon peels
An aliquot of 40 ml aqueous extracts from the peels of Persimmon (P.P) was heated at 60 ° C on a magnetic stirrer, then 0.1 M (40 ml) of magnesium Nitrate Mg(NO3)2.6H2O was added and kept on stirring condition at 1500 rpm. After reaching the required temperature, 2M NaOH solution was added drop-wise into the reaction mixture to achieve a pH between 10-12. A solution of Mg (OH) 2 was formed after adding the alkaline solution and left for 2hr until its color changed to a brown color, as shown in Figure-1. The solution was then centrifuged at 4000 rpm for 15min. The precipitate was placed in the oven at 400° C for 3hr. A white powder was obtained and collected carefully, as shown in Figure  The synthesized MgO NPs (0.1mg) was mixed with 100 ml of distilled water and processed in an ultrasonic processor device at 750 Watt and 20 kHz .The device operated for 5 days with 1and a half hours a day (10: 5 seconds of working: resting cycles).
The synthesized MgO nanoparticles and the reduced grain size of MgO NPS were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and energy dispersive x-ray analysis (EDX).

Antibacterial activity of MgO nanoparticles Preparation of bacterial culture solution
Two previously-identified bacterial strains (Staphylococcus aureus and Escherichia coli) were obtained from Biology department -College of Science /University of Baghdad. Each strain was subculturted in nutrient broth and incubated for 18 hr. at 37 °C, and then used as a bacterial stock solution for further experiments.

Minimal Inhibitory Concentration (MIC)
Antibacterial activity of MgO NPs was examined against pathogenic bacteria S. aureus (Grampositive) and E. coli (Gram negative) using MIC examination in a microtiter polysteren plate with 96 flat bottomed wells. The freshly bacterial subcultures were prepared before starting the experiment by inoculating the bacteria in a 10 ml nutrient broth test tube and incubated for 18 hr. at 3 iffe en concentration of MgO NPs (250, 500, 1000, and 2000 μg/ml) we e used and 100 µl of each MgO NPs concentration was mixed with 100 µl bacterial growth (after adjusting its concentration to 1.5*10 8 cell/ml, 0.5 MacFerland turbidity) in each well of the microtiter plate. The lowest concentration of MgO NPs that showed no detectable growth was determined as MIC. A volume of 100 µl was transferred from the bacterial culture at the MIC as well as the previous higher concentration and spread on sterile nutrient agar plates, which were then incubated at 37 fo 24 h he lowes concentration of MgO NPs that killed 100% of the bacteria was determined as the Minimal Bactericidal Concentration (MBC) [17].

Resazurin microplate assay
The test was performed in a microtiter polysterene 96 well plate as described by Coban (2012) [18], using the broth microdilution method with modifications. Resazurin powder was dissolved in phosphate-buffered saline (PBS), pH 7.5, to a final concentration of 0.1 mg/ml. Vortex mixer was used to ensure that the powder was well-dissolved. Fifty microliters of nutrient broth was dispensed into the 1st to the 6th well in two rows, while 50 µl of each MgO NPs concentration was pipeted into the 1st to the 3rd well in two raws for the ultrasonic-treated MgO NPs and into the 4th to the 6th well in two raws for the ultrasonic-untreated MgO NPs. Five microliters of bacterial suspention (at 0.5 MacFerland turbidity standard with aproximately 1.5*10 8 cell/ml bacterial concentration) was inoculated into each previous well (1st raw with E coli bacterial growth and 2nd raw with S aureus bacterial growth). The plate was then covered loosely to avoid dehydration of the well suspention and incubated aerobically at 35 fo 1 h f e incu a ion of he o h mic odilu ion pla e, 15 μl of he dye was added to each well and mixed using an electronic multichannel dispenser. The plates were incubated for an additional 75 min a 3 ( Figure-10). The change in blue color into pink indicated live bacterial cells while the maintanence of the blue or purple color indicate thed inhibited or killed bacterial cells.

Results and discussion X-ray Diffraction
The cubic crystal system of the synthesized MgO was confirmed by the XRD (Figure-3). he 2θ peak positions were well identified with the JCPDS NO: 00-045-0946. Also, the crystalline size was evaluated by the following e ye Sche e 'sfo mula D= 0.94λ/β cos θ The crystalline size values and structural parameters of MgO NP s synthesized by the green method are shown in Table- [19,20]. The calculated crystallite size of the MgO NPs shows that the nanoparticles were prepared in the quantum confinement system, as shown in Table-1. There were no impurities, while the extra peaks in the synthesized material and the sharp peaks specified that the material is purely crystalline in nature.  The optical absorption analysis UV-VIS is the optic absorption spectrum [21]. The absorption spectra of the MgO were recorded within the wavelengths of 200-800. From Figure- [5,22]. The energy band gaps of MgO NPs were calculed by the following formula E= hc/λ It was observed that the shift in the increasing wavelength leads to a decrease in the energy gap from 7.8 eV in the bulk MgO to 5.16 eV in the MgO NPs synthesized by the green method. This is possibly due to the increase in the aggregation of MgO NPs. This occurs because of defects that accompany the preparation of MgO NPs [9,22].

Atomic Force Microscopy (AFM)
Atomic force microscopic examination allows identifying the plot topographies representing the surface elevation and the structure of the surface. This technique refers to digital images that allow quantitative measurements of surface features, such as root mean square roughness (Rq) , average roughness (Ra) and the analysis of images from different perspectives, including 3D simulation [23]. Figure-5 illustrates the three dimensional AFM images and granularity distribution of the MgO NPs synthesized by the green methods. It is important to note that the mean values were obtained and showed a statistical variance, depending on the location of the measurements performed on the samples. The average grain size for the MgO NPs was 67.73 nm, as show in Table-2, and It can be seen that the nanoparticles were approximately spherical.    Table-3 . It can be seen that the nanoparticles were approximately cylindrical. This result corresponds with that of the UV-Vis analysis.  Figure 6-AFM images after treatment with ultrasonic processor (a) three dimensions, and (b) histogram of the distribution of grain size. Figure-7 shows the surface morphology of MgO nanoparticles that were obtained using the FE-SEM image. By analyzing the image, it shows that the synthesized MgO NP s were approximately spherical with an average diameter of 36.74 nm. In addition, they were aggregated and formed predominantly dense and irregular sharped flakes. The FESEM analysis for MgO NP s with ultrasound effects was obtained as shown in Figure-8, with an average size of 23.88nm. There have been a noticeable change in the shape after using the ultrasonic mixer; the spherical shape was changed into cylindrical flakes with a homogeneous and almost uniform distribution for magnesium oxide nanoparticles .  [16]. A strong peak was obtained at 1.1 keV to 1.3 keV for magnesium , while an oxygen peak was obtained around 0.2-0.4 keV. The observed strong peaks can be correlated to the formation mechanism to confirm the purity of the synthesized materials.

Antibacterial activity of MgO nanoparticles Antibacterial activity by MIC assay
The anibacterial activity of MgO NPs was detected against both S.aureus and E.coli pathogenic bacteria . The MIC value of MgO NPs against E.coli and S.aureus was 500 μg/ml wi h ul asonic treatment. Whease the MgO NPs without ultrasonic treatment showed no effect against E.coli, although a highe concen a ion was used (2000 μg/ml)which showed the same MIC value against S.aureus a 500 μg/ml he MgO NPs in general were previously shown to have antibacterial activities [24,25]. Green synthesis of nanoparticles from this oxid metal is superior to chemical syntheses in different ways [26]. In this study, we used the peels of Persimmon extract in the green synthesis of MgO NPs, which showed antibacterial activities especially against Gram positive bacteria ( S. aureus). This may be explained by the differences in the cell wall of Gram positive and Gram negative bacteria [27]. The presence of active oxygen, such as superoxide, on the surface of the synthesized MgO NPs may be one of the reasons behind the significant antibacterial activity by causing cell death related to the cell membrane damage [28]. The results also showed the positive effects of using ulrasonic treatment for the produced MgO NPs on inhibiting the growth of bacteria. This can be attributed to the ability of the ultrasonic treatment to maintain the nanoparticles flouting and dispresed in the suspension rather then being aggregated as a sediment with no effect. In other words, ultrasound disruption shows more energy and reach higher levels of powder fragmentation [29], which enhance the effects of the MgO NPs against the pathogenic bacteria that grow in the broth suspention.

Antibacterial activity by resazurin microdilution assay
Antibacterial activity by resazurin was used as a confirmatory assay (growth indicator ) of the MIC assay In he 250 μg/ml ea ed wells, via le ac e ial cells we e de ec ed (pink colo ) fo both E .coli and S.aureus after 75 min of incubation ( Figure-10) f e incu a ion wi h 500 μg/ml, nanoflakes the non-viable cells started to be detected (purple color) (Figure-10), whereas totally non-viable cells were seen ( blue color) at 2000 and 1000 μg/ml-treated samples. Growth was highly inhibited when the bacterial cells were incubated with MgO NPs treated with ultrasonic processor at a concnetrattion of 1000 μg/ml nanopa icles. From this assay, it was observed that the population of dead cells was increased with the increase of concentration. Furthermore, very low viability/growth of S. aureus was seen af e incu a ion wi h 500, 1000 and 2000μg/ml of MgO NPs compared to control and compared to E.coli where. It was also observed that the population of dead cells increased in the concentrations of 2000 and 1000 of MgO NPs treated with ultrasonic mixer. The data obtained is similar to MIC assay , which gives a good indication for the useful and simple use of resazurin in the detection of the antibacterial activity of different nanopartical materials since this assay was used in different other materials such as plant extracts and essential oils.

Conclusion
This research is the result of biological synthesis and reduced particle size by ultrasonic treatment of MgO NPs and their ability to be applied as antibacterial agents. XRD confirms the molecular structure of MgO nanoparticles with a crystal size of 25.62 nm. Analyses of FESEM and AFM show the spherical nature of the MgO NPs prior to ultrasound processing. The effect of ultrasound reduced particle size and changed their shape from spherical to cylindrical. MgO Nps showed a minimum inhi i o y concen a ion (MI ) a 500 μg / ml wi h a e e effec fo he ultrasound-treated sample.