A new large - Scale synthesis of magnesium oxide nanowires: Structural and antibacterial properties.

Large-scale one-dimensional magnesium oxide (MgO) nanowires with diameters of 6 nm and lengths of 10 μm have been successfully synthesized by a new facile and simple reaction. This production was performed via a microwave hydrothermal approach at low temperature growth of 180 °C for 30 min. The structure of as synthesized MgO nanowires were investigated by means of X-ray diffraction (X-ray), Fourier Transformation Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) and Energy Dispersive X-ray (EDS). The antibacterial behavior of MgO nanowires concentration in solid media against Gram negative and Gram positive for different bacteria has been tested in details. The results show that the MgO nanowires have bacteriostatic activity against Escherichia coli and Bacillus sp. The antibacterial activity increases with increasing MgO nanowires concentration. Furthermore, the presence of one-dimensional MgO nanowires has high antibacterial efficacy and damages the membrane wall of bacteria. Finally, this study offered the prospect of developing ultrafine nanoscale devices utilizing MgO nanowires and implementing their useful potential in biological control.

Introduction

Recently, fabrication of one-dimensional (1D) materials at the nanometric scale such as, nanowires, nanorods, nanotubes, nanosheets, nanoribbons, nanorings, and nanobelts were able to create new and enhanced structural and physicochemical properties [1-5]. Furthermore, these materials have great potential serving as building components for future nanotechnology devices compared with their bulk parent counterparts [1,2]. The desirable properties of materials can be tailored by the control of dimension, size, microstructure, and composition of material, often achieved using synthesis methods [3-6]. Every day, human beings, animals and plants fall under the threat of microbes in their living environment [7,8]. Currently, multiple drug resistance is developed by pathogenic bacteria due to the indiscriminate use of commercial antimicrobial drugs, commonly used in the treatment of infectious diseases [9,10]. In addition to this problem, antibiotics are sometimes associated with adverse effects on the host including hypersensitivity, immune-suppression and allergic reactions [2,11-13]. With increasing antibiotic resistance, a motivation for looking at other sources of antibacterial active nanomaterials was developed. Antibacterial agents organic and/or inorganic are of relevance to a number of industrial sectors including environmental, food, synthetic textiles, packaging, healthcare, medical care, as well as construction and decoration [9,13]. A severe limitation to the use of organic antibacterial material is their extreme oxidation sensitivity and instability, which frequently becomes a barrier to the use of organic materials as antibacterial agents [14,15]. In fact, inorganic materials, such as metals and metal oxides, have attracted lots of attention over the past decade due to their ability to withstand harsh process conditions [6,7]. Applying metal oxides, such as TiO2, ZnO, Fe2O3, and MgO are of particular interest as they are not only stable under harsh conditions but also generally regarded as safe materials to human beings, animals and plants [8,9,16]. Nano-structured MgO is an essential minerals for human health and is an exceptionally important materials that has a unique ability to destructively adsorb different gases [17,18], including chemical warfare agents, surrogates, catalysis, additive in refractory, toxic waste remediation, paint, flame retardants, polymer reinforcement agents, and superconductor product and antibacterial materials [9,10,19]. The traditional method for preparing MgO is the thermal decomposition of either magnesium salts or magnesium hydroxides, which results in an inhomogeneous morphology and crystallite size as well as a low surface area [15,20]. Many synthetic strategies have used different physical and chemical transformation techniques to synthesis one-dimentional MgO with high surface area and enriched surface chemistry. These techniques include aerogel [21], thermal evaporation [22], chemical vapor deposition [23], laser ablation [18], vapor–solid process [16], sol–gel [3], hydrothermal method [7], epitaxial growth [9], nonepitaxial growth [6], direct chemical transformation [4], solid-state interfacial diffusion reaction [2], and so on.

To the best of our knowledge, no reports on the synthesis of MgO nanowires by the use of magnesium acetate with urea in microwave hydrothermal process are available in literature. In this regards, we first report the synthesis of one-dimensional MgO nanowires by a microwave hydrothermal method using magnesium acetate with urea as precursors. The antibacterial properties of MgO nanowires are examined in details. It is suggested that the one-dimensional MgO nanowires may find potential application in the design of nanotechnology devices and antibacterial agents.

Experimental section

Synthesis of MgO nanowires

All chemical reagents used in the experiments were analytical grade without further purification. The method of synthesizing large-scale one-dimensional magnesium oxide nanowires by directly reacting magnesium acetate and urea in microwave hydrothermal technique is as follows: 6.44 gm of magnesium acetate was dissolved in 75 ml distilled water and magnetic stirred for 30 min at room temperature. Simultaneously, a 1.2 gm urea in 25 ml water was added; drop wise into this aqueous magnesium acetate solution under vigorous magnetic stirring for 5 min. Further, the above solution was loaded into a 100 mL Teflon-lined autoclave. Finally, the autoclave was sealed and maintained at 180 °C for 15 min in microwave furnace with a power of 1000 w. The autoclave was then allowed to cool down to room temperature naturally when the reaction time was finished. After completion of the reaction, the products were collected by centrifugation at 8000 rpm for 5 min, filtered with distilled water followed by ethanol to reduce the agglomeration, and later dried at 60 °C for 24 h. Finally, the white colored material that resulted was calcined at 600 °C for 1 h.

Microstructure tests

The X-ray measurements were performed using Shimadzu XRD-6000 X-ray powder diffractometer using CuK radiation (λ = 0.15406 nm) at 15 kV and 30 mA for the X-ray tube. The scan speed was 5°/min to analyze the phase type and content. For each measurement, a complete 2θ scan was made between 10° and 80°. The surface morphology of the magnesium oxide nanowires was investigated by field emission scanning electron microscope (FESEM) with a microscope (JEOL JSM-6500F) combined with an energy dispersive X-ray (EDS) probe, using electron beam energy of 15 keV and a beam current of 40 mA. Prior to investigation, the MgO powders were coated with a uniform carbon layer for good electrical conductivity. EDS analysis was performed to identify the elemental composition at specific locations on the surface of the as synthesized MgO oxide. Transmission Electron Microscopy (TEM) was carried out using a JEOL 2011 electron microscope at an acceleration voltage of 100 kV. The MgO/H2O solution was dropped onto carbon-coated copper grids and allowed to dry under ambient conditions. Fourier Transform Infrared (FTIR) spectroscopy was performed using a Nicolet DTGS TEC detector spectrophotometer from 4000 to 400 cm−1 by the KBr pellet method.

Antibacterial activities in solid agar medium

This test was carried out according to the method described in references [17,18], which was performed in sterile Petri-dishes with 90 mm diameter containing sterile Nutrient agar medium (15 ml). After the renewal of cultures bacterial for 24 h, the freshly prepared bacterial inoculums was swabbed over the entire surface of the medium three times, rotating the plate 60° after each application by using sterile cotton swab, to ensure the spread of bacteria on the surface of the plates. One well of 6 mm diameter was pored in the medium for each plates with the help of sterile cork-borer and was filled with 50 μl of the bacterial suspension of the tested material using micropipette. Ampicilin (5 μg/ml) was used as positive control and water was used as negative control. Plates were left for 45 min at room temperature to allow proper diffusion of the extract to occur in the medium. All plates were incubated at 37 °C for 24 h, followed by the measurement of the diameters of inhibition zones. Inhibition of bacterial growth was measured as zone diameters (mm) at 3-equidistant points taken from the center of the inhibition zone, and the average value was taken. All experiments were carried out in triplicate and the reported data represents average values.

Antibacterial activities in liquid medium

Different concentrations of the tested MgO nanowires were prepared and added to 48 ml of nutrient broth medium in 250 ml Erlenmeyer flasks to give 100–900 μg/ml. Each flask was inoculated with 2 ml of the tested bacterium, containing 5 × 105/ml, and the flasks were incubated at 37 °C. The bacterial growth was determined by measuring the absorbance at 595 nm. Control flasks without MgO nanomaterial were prepared as control.

Mode of action of the tested nanomaterials

After incubation of the tested bacterial in nutrient broth medium, containing different concentration of the MgO ranging from 100–900 μg/ml, bacterial cells were collected by centrifugation at 3000 rpm and were washed several times with sterile distilled water. The collected cells were re-suspended in sterile distilled water (OD. at 550 = 0.65). Cell respiration (quantity of O2 consumed/min) was determined using Oxigraphe. The quantity of K+ flowed from the treated and untreated cells were determined using atomic adsorption [18,19].

Results and discussion

X-ray studies

The crystal phases and crystallinity of the as synthesized MgO nanowires were investigated by X-ray powder diffraction. Typical X-ray diffraction pattern for the synthesized MgO nanowires is demonstrated in Fig. 1. Miller indices are indicated on top of each diffraction peak. As seen in Fig. 1, all the obtained Bragg reflection peaks in the X-ray pattern of the synthesized MgO nanowires have been assigned to a pure cubic phase of MgO, which is consistent with bulk MgO crystal (JCPDS Card No. 77–2364, MgO), [1-5]. Five reflection peaks characterizing MgO were identified, these were indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) diffraction planes. No obvious peaks, other than that corresponding to MgO, were observed in the X-ray pattern, up to the detection limit of X-ray. These results confirm the high purity of the synthesized MgO nanowires. It is interesting to note that the obtained X-ray pattern exhibited sharp reflections which could be correlated with good crystallinity of the synthesized MgO nanowires. The X-ray pattern revealed that the MgO nanowires are cubic face centered with lattice constants of a = 0.42 nm and the interplanar spacing of the (2 0 0) plane was approximately 0.21 nm. These values are in good agreement with the known values of bulk MgO crystal [1,2]. No peaks of other phases were found in the X-ray spectrum, indicating that the MgO nanowire obtained via this synthesis method possesses an ultra-pure crystalline phase, without any impurity.

Fig. 1

X-ray powder diffraction pattern of the synthesized MgO nanowires by microwave hydrothermal technique.

From X-ray data, the crystallite size (D) of the synthesized MgO was calculated from the full width at half maximum (FWHM) of all the peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) using the Debye–Scherrer formula [10]:where β is the breadth of the observed diffraction line at its half-intensity maximum, k is the shape coefficient for the reciprocal lattice point, which usually takes a value of about 0.9 [11], and λ is the wavelength of X-ray source used in X-ray spectra.

The crystallite size, as obtained from the Scherrers’ formula, was 6 nm and matches well with the size obtained from FESM and TEM images in this article.

The specific surface area (SA) is one of the important parameters used to characterize powder samples and it depends on particle size, shape, and density of the sample and is given by [16]:where ρ is the density of MgO (3.59 g/cm3).

The computed value of specific surface area is approximately 461 m2/g.

Fourier Transformation Spectroscopy (FTIR) analysis

To characterize the structure in further detail, the FTIR of the as synthesized MgO nanowire was measured. Fig. 2 shows the FTIR spectrum of synthesized one-dimensional MgO nanowires. Various well-defined absorption bands have been observed in the FTIR spectrum.

Fig. 2

FTIR spectrum of the synthesized cubic-shaped MgO nanowires synthesized by facile aqueous solution based microwave hydrothermal process.

The presence of two distinct bands at 412 and 703 cm−1 were assigned to the Mg–O stretching vibration [6,7]. Bands at 815 and 1008 cm−1 were attributed to the υ1 and υ2 stretching vibrations of metal–oxygen bonds, which confirm the formation of MgO [8,9]. A high absorption band, which appeared at 1628 cm−1, is also observed in the MgO spectrum. This band is related to bending vibration of absorbed water and surface hydroxyl (-OH) [14,15]. It is interesting to note that the 3700 cm−1 band on the FTIR spectrum of the synthesized MgO sample completely disappeared. This is a consequence of the complete decomposition of Mg(OH)2 to MgO [10,2]. Finally, FTIR provided compelling evidence for the formation of MgO nanowires which agreed well with the X-ray spectra.

Field Effect Scanning Electron Microscopy (FESEM) studies

The general morphology and particle size of synthesized one-dimensional MgO nanowires with different magnifications were examined by observed Field Emission Scanning Electron Microscopy (FESEM) images as depicted in Fig. 3a–c. It can be seen that the nanowires are continuous and arranged roughly parallel to each other, and growth of the nanowires are very high; as most of the nanowires are attached to each other through one of their surfaces and exhibit sharp edges with smooth and clean surfaces. In addition to that, it is clear that the synthesized MgO are nanowires which are formed in large-quantity. The MgO nanowires exhibits diameters in the range of 6–8 nm and lengths of about 10–15 μm, and is consistent with the X-ray result above.

Fig. 3

(a–c) Typical FE-SEM image of the synthesized cubic-shaped MgO nanowires synthesized by facile aqueous solution based microwave hydrothermal process, (d) EDS spectra of as synthesized MgO nanowires, which indicates the existence of oxygen, Mg. Note: the carbon (C) peak is due to the carbon tape used as the sample supporter.

Results further confirming the composition and formation of the synthesized MgO nanowires were conducted and observed by Energy Dispersive Spectroscopy (EDS), depicted in Fig. 3d. As can be seen from the EDS spectrum, the presence of magnesium (Mg) and oxygen (O2) are shown with the proper percentage. Since the sample was supported on a piece of double-sided adhesive conductive carbon tape, a signal of carbon (C) is also shown.

Transmission Electron Microscopy (TEM) studies

For more detailed structural and compositional characterizations, the synthesized MgO nanowire was examined by Transmission Electron Microscopy (TEM) and depicted in Fig. 4a. Fig. 4a exhibits TEM image of the synthesized MgO which shows full consistency with the observed FESEM results.

Fig. 4

(a) TEM image of MgO nanowire, (b) The selected area electron diffraction (SAED) pattern is the inset, and (c) is lattice-fringe image of MgO (2 0 0).

The Selected Area Electron Diffraction (SAED) pattern confirms the structural nature of the synthesized MgO nanowire and show reflections corresponding to (1 1 1), (2 0 0), ((2 2 0), (2 0 0), (2 2 0) and (2 2 2) planes, supporting the presence of MgO in cubic structure, inset is shown in Fig. 4b.

Fig. 4c demonstrates the typical high-resolution TEM image which exhibits clear and well-defined lattice fringes of the synthesized MgO nanowire. The measured spacing between two lattice fringes is 0.21 nm, which corresponds to the (2 2 0) plane of cubic MgO [2,11,12].

Antibacterial properties

The dependence of the bacterial growth in liquid medium of Escherichia coli and Bacillus sp. against different concentration of MgO nanowires is illustrated in Fig. 5. It is seen that the bacterial growth decreases with increasing MgO nanowires concentration into bacterial species of E. coli and Bacillus. In Fig. 5, it is worthily to note that the antibacterial activity of E. coli is higher than that of Bacillus sp., with increasing MgO concentration. The following reasons suggest a possible explanation for the decrease of the bacterial growth with increasing MgO concentration. First, the MgO is easily hydrated forming Mg(OH)2 which covers the surface of MgO [17,18]. As the surface area of the MgO nanowires increases, so does the OH− concentration on their surface. This in turn leads to an increase in the O−2 concentration in the solution and results in a more effective destruction of the cell wall of the spores [17-21]. Second, the formation of superoxide anions on the MgO surface [22-24]. It can be suggested that the bactericidal action of MgO results from attack of these superoxide ions on the Mg+ of the carbonyl group in the peptide linkages leading to degradation of the proteins [23,24]. High concentrations of highly active superoxide ions are generated on the surface of the MgO nanowires, which can react with the peptide linkages in the cell wall of bacteria or spores and thus destroy them [18-21].

Fig. 5

Bacterial growth in liquid medium of E. coli and Bacillus sp. after treatment with different concentration of MgO nanwires.

In support of these facts of the antibacterial efficacy of MgO nanowire, we performed the SEM of E. Coli bacterial sample before and after inhibition test. Fig. 6a and b represent the SEM images of E. Coli before and after treatment for 24 h with 0.7 g/l MgO nanowires, respectively. It is clear that treatment of the E. Coli bacteria with MgO nanowires has led to considerable damage to E. Coli which caused the breakdown of the bacterial cell wall [20]. In addition, because particle size of MgO is about 6 nm, the aggregation effect becomes very significant due to the very high surface area of the synthesized MgO nanowires which is about 461 m2/g. Therefore, the MgO nanowires activate the bacterial grovel and spores so that bactericidal efficiency becomes higher. Furthermore, the inhibition of bacterial growth for cell wall is attributed to production of active oxygen species due to the presence of MgO, electrostatic interaction between MgO nanoparticles and cell wall, penetration of individual MgO nanoparticles into the cell and reformation of MgO in the entire cell [22-24].

Fig. 6

(a) and (b) SEM images of E. Coli before and after treatment for 24 h with 0.7 g/l MgO nanowires, respectively.

The antibacterial power of MgO nanowires may be associated with some characteristics of bacterial species [18,19]. The antimicrobial activity of the MgO nanowires compared to ampicilin, as a positive control for different bacterial species, is recorded in Table 1. It is clear that, Gram-positive bacteria (such as Bacillus sp., Micrococcus sp., Staphylococcus aureus, Staphylococcus epiderimdis, Streptococcus pneumonia) are less susceptible to Mg ions than Gram-negative bacteria (such as Acinetobacter sp., E. coli, Klebsiella pneumonia, Proteus mirabilis, Pseudomonas aeruginosa, Salmonella sp.) due to differences in their membrane structure [1,2]. The Gram-positive bacteria have more peptidoglycan than Gram-negative bacteria because of their thicker cell wall, and because peptidoglycan are negatively charged and MgO are positively charged [18]. Hence, more Mg ions may get trapped by peptidoglycan in Gram-positive bacteria than in Gram-negative bacteria [19,20].

Table 1

The antimicrobial activity (Diameter of inhibition zone, mm) of the MgO nanowires compared to ampicilin as a positive control.

Positive control (Ampicilin)	MgO	Gram reaction	Tested bacteria
15 ± 1.0	10.3 ± 0. 60	−ve	Acinetobacter sp.
30 ± 1.5	12.2 ± 0.50	+ve	Bacillus sp.
35 ± 4.5	13.8 ± 0.34	−ve	Escherichia coli
34 ± 0.5	8.8 ± 0.28	−ve	Klebsiella pneumonia
30 ± 2.5	10 ± 0.86	+ve	Micrococcus sp.
16 ± 1.5	11.2 ± 1.56	−ve	Proteus mirabilis
19 ± 2.50	9.8 ± 0.50	−ve	Pseudomonas aeruginosa
20 ± 0.56	10.8 ± 4.56	−ve	Salmonella sp.
32 ± 0.36	9.0 ± 2.56	+ve	Staphylococcus aureus
34 ± 0.34	11.7 ± 1.56	+ve	Staphylococcus epiderimdis
30 ± 0.54	ND	+ve	Streptococcus pneumonia
28.7			Antibacterial index

ND: no detected inhibition zone, +ve: Gram positive, −ve: Gram negative.

The dependence of MgO nanowires on the respiration of oxygen consumed and the flow of potassium for E. coli and Bacillus sp. bacteria is recorded in Table 2. By taking a closer look at the data in Table 2, it is seen that the E. coli have higher respiration of oxygen consumed and flow of potassium compared with Bacillus sp. This demonstrates that E. coli are more sensitive to MgO nanowires than Bacillus sp. One explanation for the higher respiration of oxygen consumed and flow of potassium in E. coli compared to Bacillus sp. is ascribed to differences in the polarity of their cell membrane [23,24]. This reflects that the ability of MgO nanowires to inhibit growth by generation of radical oxygen species is well suggested.

Table 2

Effect of nano particles of MgO on both respiration and flow of potassium from the plasma membranes of E. coli and Bacillus sp.

Control		MgO		Bacteria
Flow of potassium (mg/g cells)	Respiration Quantity of O2 consumed (Hr. mg cells)	Flow of potassium (mg/g cells)	Respiration Quantity of O2 consumed (Hr. mg cells)
14 × 10−7	29.5	12.64 × 10−7	22.6	E. coli
14 × 10−7	22.8	11.94 × 10−7	18.9	Bacillus sp.

Conclusion

In this work, and for the first time using microwave hydrothermal process, the synthesis of one-dimensional MgO nanowires and its antibacterial properties were examined. Based on the results obtained, the following conclusions can be drawn:

A new facile and simple microwave hydrothermal process was used to synthesize MgO one dimentional nanowires at low temperature of 180 °C for 15 min; using magnesium acetate and urea as precursors.

X-ray and FESEM analysis coincidentally indicate that the MgO nanowire is crystalline. Most of the crystallites have nanowire morphology with a length of 10 μm and a diameter of a 6 nm.

The bactericidal growth decreased with increasing MgO nanowires concentration. A mechanism is proposed on the basis of the results obtained. The surface of MgO can produce high concentrations of O2− which is highly active and can react with the peptide linkages in the cell wall of bacteria or spores and thus destroy them.

This finding is expected to be useful for fabrication of one-dimensional devices with potentially useful in nanoelectronics and antibacterial properties.