Synthesis, physical properties and antibacterial activity of metal oxides nanostructures

Materials Science in Semiconductor Processing 21 (2014) 154–160

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Synthesis, physical properties and antibacterial activity of metal oxides nanostructures Tariq Jan a, Javed Iqbal a,n, Muhammad Ismail b, Noor Badshah c, Qaisar Mansoor b, Aqsa Arshad a, Qazi M. Ahkam a a b c

Laboratory of Nanoscience and Technology, Department of Physics, International Islamic University, Islamabad, Pakistan Institute of Biomedical and Genetic Engineering (IBGE), Islamabad, Pakistan Department of Basic Science, University of Engineering and Technology, Peshawar, Pakistan

a r t i c l e in f o

abstract

Available online 19 February 2014

Metal oxides (MOs) nanostructures represent a new class of materials which have been explored for the health related applications. Highly ionic MOs nanostrucrures are important for their unique physicochemical properties as well as antibacterial activity. In this work, MOs nanostructures (ZnO, CuO, SnO2 and CeO2) have been synthesized by chemical co-precipitation technique and characterized by XRD, SEM, EDS, FTIR and UV–visible spectroscopy analysis. XRD results reveal the single-phase formation of all metal oxides. Spherical nanoparticles are observed in case of ZnO, SnO2 and CeO2 samples, while hierarchal nanostructures are observed in case of CuO sample. Antibacterial activity of four different MOs nanostructures against E. coli bacterium has been assessed by agar disc method. The antibacterial activity results have shown that the ZnO nanostructures exhibit maximum sensitivity (10 mm ZOI) towards E. coli bacterium. The order of antibacterial activity for different MOs nanostructures is found to be the following: ZnO 4 SnO2 4CeO2 4 CuO. Our findings suggest that the particle size, morphology and type of MOs nanostructures play vital role in their antibacterial activity. It is concluded from the present findings that ZnO nanostructures can be used as an efficient antibacterial agent. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Metal oxides Nanostructures Co-precipitation Antibacterial activity E. coli

1. Introduction MOs play very critical role in many fields of science and technology such as physics, chemistry, material science, engineering and medicine. Metal elements are capable to make chemical bonds with oxygen to form oxidic compounds. These oxidic compounds can take up large number of structural geometries with an electronic structure that can demonstrate metallic, semiconductor, or insulator distinctiveness. These days, MOs have been used widely in the fabrication of solar cells, sensors, fuel cells,

n

Corresponding author. Tel.: þ92 51 901 9713; fax: þ 92 51 925 7954. E-mail address: [email protected] (J. Iqbal).

1369-8001/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2014.01.006

microelectronic circuits, catalysis process and in piezoelectric devices [1–6]. Nanostructures of MOs can demonstrate unique physical and chemical properties because of their nano-scale size, high aspect ratio (i.e. surface to volume ratio of atoms) and edge effect [7,8]. Due to these unique properties, MOs nanostructures have attracted wide variety of applications such as lithium ion batteries, fuel cells, field effect transistor, light emitting diodes, solar cells, magnetic storage devices, bio-sensors, cancer cell treatment and antibacterial agents [8–14]. However for successful applications of MOs nanostructures, the strict control over their size and morphology is essential. Also the development of synthesis techniques, which are versatile, easy to handle, cost effective and reproducible is of the core importance. Several routes have been reported for

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the synthesis of MOs nanostructures such as sol–gel method, hydrothermal method, combustion route, ball milling and co-precipitation method [15–18]. Among these, the chemical co-precipitation technique is the most useful and easiest way for the synthesis of MOs nanostructures. Antibacterial agents are of great interest in several industries, such as hospital implants, medicine, food packaging and preservation, textile fabrics and water disinfection [19,20]. Traditionally, the organic compounds are used for disinfection but are limited due to various disadvantages such as toxicity towards healthy cells and low chemical and physical stability especially at high temperatures and pressure [3]. MOs nanostructures are considered as most promising candidates for this purpose because of their non toxic nature, high thermal stability, photocatalytic and antibacterial properties [21,22]. Here, we present the synthesis of various MOs nanostructures such as ZnO, CuO, SnO2 and CeO2 by a simple, versatile and easily reproducible chemical co-precipitation technique. MOs nanostructures synthesized by co-precipitation technique have been characterized for the study of several physical properties such as structural, morphological and optical properties. Furthermore, the antibacterial activity of these prepared MOs nanostructures have been investigated by agar disc method. 2. Materials and methods 2.1. Synthesis of MOs nanostructures The chemicals used for synthesis of MOs nanostructures were zinc chloride (ZnCl2), copper chloride (CuCl2  5H2O), stannous chloride (SnCl4  5H2O), cerium nitrate (CeNO3), sodium hydroxide (NaOH) and acetic acid (CH3COOH). All MOs samples were prepared by chemical co-precipitation method using distilled water as a solvent. For synthesis of MOs nanostructures, 0.1 M solution of all metal precursors were prepared in distilled water and stirred until the complete dissolution. CH3COOH was added as a surfactant to control the size of MOs particles. Then 1 M NaOH solution in distilled water was added to the above solution drop wise and pH value was adjusted to approximately 8. After adjusting pH value, the solution was further stirred for one h. The precipitates were collected from the solution by centrifugation and washed with distilled water. The cleaned precipitates were dried in an oven for 12 h at 80 1C and then grind to acquire the powder. For the decomposition of organic matter, the grind samples were again heated in oven for 2 h at 180 1C. Finally, the prepared MOs samples were annealed at 300 1C for 2 h to enhance their crystallinity. The structure, morphology, chemical composition, vibrational modes and optical properties were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared (FTIR) spectroscopy and UV–visible spectroscopy, respectively. 2.2. Determination of antibacterial activity Antibacterial activities of the prepared MOs nanostructures were examined against clinically isolated; Gram

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negative E. coli bacterium. E. coli strain was grown aerobically at 37 1C under normal ambient light conditions. The in vitro antibacterial activities of the prepared MOs nanostructures were investigated by agar disc method. Single colony of E. coli was cultured in agar medium by lawn formation. The colloidal suspensions of MOs nanostructures (2 mg/ml) were applied to agar Petri plates by disc method. These agar plates were incubated at 37 1C for 24 h and the zone of inhibition (ZOI) was measured in millimeters (mm). 3. Results and discussion The crystal structures and phase purity of the prepared MOs samples have been investigated by XRD. Fig. 1 depicts the typical XRD patterns of ZnO, CuO, SnO2 and CeO2 nanostructures. All peaks in XRD patterns can be well indexed to the typical wurtzite, monoclinic, rutile type tetragonal and cubic fluorite structures of ZnO, CuO, SnO2 and CeO2, respectively [17,18,23]. Furthermore, no impurity peaks are detected, which confirms the phase purity of all the prepared MOs nanostructures. The crystallite sizes for all prepared samples are calculated using Scherrer formula; D ¼ 0:9λ=β cos θ where λ is the wavelength of X-ray, β is full width at half maximum of the peak at diffracting angle θ. The calculated crystallite sizes are found to be 18 nm, 12 nm, 8 nm and 17 nm for ZnO, CuO, SnO2 and CeO2, respectively. The morphology and chemical composition of the prepared samples have been investigated by SEM coupled with EDS. SEM images of the synthesized MOs samples are shown in Fig. 2. Spherical nanoparticles are found in case of ZnO, SnO2 and CeO2 samples, while the hierarchal nanostructures are found in case of CuO sample. The particle sizes obtained from SEM images are 25 nm, 28 nm and 30 nm for ZnO, SnO2, and CeO2 nanostructures, respectively. Fig. 3 depicts the EDS spectra of prepared MOs nanostructures. All the spectra show only the presence of respective metals and oxygen, which demonstrates the purity of the all prepared MOs nanostructures. Nanostructures have very high aspect ratio as compared to their bulk counterpart. This property makes them more chemically reactive because more atoms are accommodated on the surface. Therefore, the study of surface chemistry of prepared MOs nanostructures is of great interest. To investigate the presence or absence of various vibration modes on the surfaces of samples, FTIR spectra have been recorded in the range of 400–4000 cm  1. FTIR spectra of ZnO, CuO, SnO2 and CeO2 nanostructures are shown in Fig. 4. The FTIR spectra of all MOs nanostructures demonstrate series of vibrational modes from 400 to 4000 cm  1. The presence of vibrational modes in the region of 400–600 cm  1, for all samples, shows the existance of M–O bonding (M ¼Zn, Cu, Sn, and Ce) [17]. Thus, FTIR spectroscopy results corroborate very well with the XRD results. After determining the structure, morphology and purity, the optical properties of MOs nanostructures have been investigated at room temperature. UV–visible

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Fig. 1. XRD patterns of MOs nanostructures.

ZnO

CuO

SnO2

CeO2

Fig. 2. SEM images of MOs nanostructures.

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Fig. 3. EDS spectra of (a) ZnO, (b) CuO, (c) SnO2 and (d) CeO2 nanostructures.

Fig. 4. FTIR spectra of MOs nanostructures.

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Fig. 5. UV–visible absorption spectra of MOs nanostructures.

absorption spectra of the MOs nanostructures dispersed in distilled water are shown in Fig. 5. ZnO nanoparticles exhibit typical exciton band gap absorption at 376 nm which is blue shifted as compared to its bulk counterpart [24]. CuO nanostructures show maximum absorption peak at 380 nm, which is attributed to the surface plasmon absorption [25]. SnO2 and CeO2 exhibit band edge absorption at 326 nm and 322 nm, respectively. The obtained results are matched well with the reported ones [24–27]. E. coli is a Gram-negative bacterium having complex cell wall. Its cell wall is composed of a thick PG layer as well as outer membrane which covers the surface membrane. Due to this complex cell wall, E. coli shows resistance to mostly available drugs [28]. Additionally, the E. coli bacterium protects itself by creating bio-films. Biofilms cover the bacterial cells community and protect it from antibiotics [29]. Hence MOs nanostructures can possibly be used to break these protection layers of bacteria. At nanoscale, MOs possess high surface area which leads to higher chemical and biological reactivity of MOs nanostructures. Due to these characteristics, the MOs nanostructures react efficiently with the cell membranes and inactivate the bacteria [30]. Several mechanisms of action against bacteria have been reported for MOs nanostructures such as decomposition of MOs and formation of reactive oxygen species (ROS), electrostatic interaction of nanostructures with cell wall and their photocatalytical light activation [31]. Nanostructures react through these mechanisms with bacterial cells and lead to zone of inhibition (ZOI) around nanomaterials. The size of ZOI depends on the bactericidal potency of nanomaterials. Disc diffusion method has been adopted to investigate the in vitro antibacterial activity of MOs nanostructures against multi-drug resistant E. coli bacterium. Antibacterial activity results reveal that the ZnO nanoparticles exhibit

highest level of bactericidal potency against E. coli. ZnO, SnO2 and CeO2 nanoparticles produce 10 mm, 6 mm, and 3 mm of ZOI, while CuO nanostructures could not produce any ZOI (Fig. 6). The ZOIs formed around the SnO2 and CeO2 nanoparticles (white spot) are not much clear, which demonstrate that some E. coli are still proliferated within the ZOI. This shows that the bactericidal effectiveness of SnO2 and CeO2 nanoparticles is poor. While,the ZOI around ZnO nanoparticles (white spot) is very much clear, which reveals that ZnO nanoparticles act as an excellent antibacterial agent as compared to other MOs nanostructures tested. Particle sizes and morphology play an important role in the antibacterial activity of MOs nanostructures [17,19,32]. Tam et al. have reported the highest antibacterial activity for small size spherical nanoparticles as compared to nanopowders and nanorods [32]. The SEM results demonstrate that ZnO has the smallest particle size (25 nm) among all the MOs samples. It can also be seen from the SEM results that CuO has a hierarchal morphology. Thus, the higher antibacterial activity of ZnO nanoparticles may be attributed to its smaller particle sizes. The resistance of E. coli towards CuO may be attributed to its hierarchal morphology. The solubility of MOs increases with the decrease in particle sizes. The highsolubility of MOs nanostructures can increase the concentration of soluble metal ions as a result there is enhancement in the antibacterial activity of MOs nanostructures [33]. Furthermore, the solubility of MOs nanostructures also depends on the type of MOs. Recently, it is reported that ZnO nanoparticles have higher solubility than CuO, Sb2O3 and NiO nanoparticles [34]. Antibacterial activity also depends on the physicochemical properties of MOs nanostructures [29]. Hence, the difference in antibacterial activity of different MOs nanostructures may be attributed to their particle sizes, different morphology, solubility and physicochemical properties.

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Fig. 6. ZOI produced by different MOs nanostructures (a) ZnO, (b) CuO, (c) SnO2 and (d) CeO2.

4. Conclusions

References

In this work, the various MOs (ZnO, CuO, SnO2 and CeO2) nanostructures have been successfully synthesized by a simple, versatile and low temperature chemical coprecipitation method. The structural characterizations have revealed the single-phase formation of metal oxides. Spherical nanoparticles are found in case of ZnO, SnO2, and CeO2 samples, while hierarchal nanostructures in case of CuO sample. The particle sizes obtained from SEM images are 25 nm, 28 nm and 30 nm for ZnO, SnO2 and CeO2 nanostructures, respectively. The antibacterial activity of four different MOs nanostructures against E. coli bacterium has been assessed by agar disc method. ZnO nanoparticles exhibit maximum sensitivity (10 mm ZOI), while the CuO nanostructures have shown the least sensitivity against E. coli. The higher antibacterial activity of ZnO nanoparticles can be attributed to smaller particle size. It is concluded from the present preliminary findings that ZnO nanoparticles can be used as an efficient antibacterial agent.

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Acknowledgments This work was funded by the HEC Project IPFP (Grant no. PM-IPFP/HRD/HEC/2011/3386) and funding for HEC Ph. D. Scholars (Tariq Jan and Aqsa Arshad).

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