Abstract
1. Introduction
Nanotechnology is one of the developing fields of research in smart material sciences. It involves efficient manipulation, fabrication and stabilization of matter at an atomic and molecular level that ranges between 1 to 100 nm[
ZnO NPs exhibit antibacterial activity against a variety of pathogenic bacteria. The activity includes various process such as the generation of reactive oxygen species (ROS), cell membrane integrity disruption, enzyme inhibition[
Among various noble metals, Ag has great potential due to its unique properties such as good oxygen adsorption behavior, relatively non-toxic, cheaper, high thermal conductivity, high solubility, good electrical conductivity etc. Ag-decorated materials are used to treat infections caused by pathogens[
Recently, many researchers have been investigated microstructural effect (nanoparticles, micro/nanoflowers, hybrid nanostructures, hierarchically structures) of Ag-doped ZnO on their antimicrobial performance[
For the synthesis of NMOS, hydrothermal is preferred because it produces high crystalline structures. The hydrothermal method is also cost effective, eco-friendly, easy to handle and known as a low-temperature synthesis method[
In this paper, we have been synthesized ZnO nanostructures by using a facile single step hydrothermal method. Antimicrobial properties of pure ZnO and Ag-doped ZnO nanostructures have been investigated.
2. Preparation of nanostructures
ZnO NPs were synthesized by the hydrothermal method using zinc chloride (ZnCl2) (Sigma Aldrich) and sodium hydroxide (NaOH) (Sigma aldrich) as precursors. Different concentration of ZnCl2 and 1 M NaOH solution was prepared in 100 mL distilled water under stirring for 2 h. Furthermore, the ingot solution was retained into the autoclave (Teflon-lined, sealed) and heated at 200 °C for 7 h under autogenously pressure. Thereafter, it was cooled at room temperature. After completion of the process, white precipitate was washed with deionized water and further dried in an oven at 150 °C for 2 h. Finally, the fine powder of pure ZnO NPs was achieved by grinding. Silver nitrate (AgNO3) (Sigma Aldrich) and ZnCl2 were used as precursors to obtain the Ag-doped ZnO nanostructure. Zn1–xAgxO (where x = 0.5, 0.75 and 1) fine white powder was obtained via the same process as obtained pure ZnO NPs. ZnO and Ag-doped ZnO nanostructured samples were annealed at 800 °C for 2 h in a muffle furnace. A schematic diagram of hydrothermal process used to synthesis pure and Ag-doped ZnO nanostructures is shown in Fig. 1. The chemical Eqs. (1) and (2) used for synthesizing pure and Ag-doped ZnO nanostructures are as follows:
Figure 1.(Color online) The schematic diagram of hydrothermal method.
3. Characterization
The crystal structure, lattice plane and crystallite size of prepared ZnO nanostructures were determined by an X-ray diffractometer (Bruker AXS, D8 Advance). The XRD pattern was recorded by CuKα radiation with about 1.54060 Å (2θ range from 20° to 80°). The surface morphology of ZnO nanostructures was examined by FESEM (FEI, Quanta 200F) and their composition was determined by energy dispersive X-ray analysis (EDX). Photoluminescence (PL) (FLS 980, Edinburgh Instrument) was used to study the optical properties of the prepared ZnO nanostructures.
4. Antibacterial activity of ZnO NPs
The agar well diffusion method[
5. Results and discussion
5.1. Structural study of pure ZnO and Ag-doped ZnO nanostructures
XRD pattern of pure ZnO and Ag-doped ZnO nanostructures with different Ag doping concentrations are shown in Fig. 2. The diffraction peaks are observed at 2θ values of 31.76°, 34.41°, 36.24°, 47.62°, 56.54°, 62.88°, 67.93° and 69.09°, for the reflection from lattice planes (100), (002), (101), (110), (103), (112), (201) and (200), respectively and all are good matches with pure ZnO structure [JCPDS card number 36-1451]. The XRD patterns of Ag-doped ZnO nanostructures show the reflections at 38.04°, 44.30° and 77.35° corresponding to Ag along the lattice planes (111), (200) and (311), respectively [JCPDS card number 04-0783]. The crystallite sizes of ZnO nanostructures are observed to be decreased with increase of Ag doping concentration (Table 1). This crystallite size modification could be due to the fact that ionic radii of Ag (0.126 nm) are far greater than that of Zn (0.074nm)[
Figure 2.(Color online) XRD patterns of pure and Ag-doped ZnO nanostructures.
Table Infomation Is Not EnableThe crystallite size of pure ZnO and Ag-doped ZnO nanostructures were calculated by Scherrer's formula[
where d and (hkl) are inter-plane spacing and miller indices, respectively. Lattice parameters (a and c) are defined as:
where λ and θ are X-ray wavelength and Bragg's angle, respectively. It has been found that lattice parameters of Ag-doped ZnO nanostructures are increased with the increase of Ag doping content in ZnO (Table 1). This is due to the tiny ionic radius of Zn in compare to ionic radius of Ag[
Williamson's and Hall's Eq. (4)[
where β, λ, D, θ and ε is full width at half of the maximum (FWHM), wavelength of X-ray used, crystalline size, Bragg's angle, and lattice strain, respectively. The values of lattice strain (ε) are increased due to prompted lattice defects originated by the insertion of Ag in ZnO nanostructures (Table 1). W–H analysis reveals that all ZnO particles have positive (tensile) lattice strain (Fig. 3) and stretched may be the reason of this strain present in the prepared nanostructures[
Figure 3.(Color online) W–H analysis for pure and Ag-doped ZnO nanostructures.
5.2. XPS analysis
Core level spectra is illustrated in the Figs. 4(a)–4(c), corresponding to pure Ag 3d and Zn 2p and Ag-doped ZnO nanostructures probed by X-ray photoelectron spectroscopy (XPS). It is clearly realized that the (1–3 wt %) Ag-doped ZnO nanostructure shows three components: Zn, Ag, and O. The features attributed to the Ag peak is slightly weak due to the incorporation of Ag ions in ZnO due to the small scattering cross section. From the Fig. 4(a), the binding energies 367.5 and 373.6 eV are observed for the peaks of Ag 3d5/2 and Ag 3d3/2, respectively. It confirms that the Ag is successfully incorporated within the crystal lattice of ZnO. In Fig. 4(b), two peaks are observed at 1021.4 and 1044.5 eV in the high-resolution spectrum and these are attributed to the Zn 2p3/2 and Zn 2p1/2, respectively. It approves the Zn2+ oxidation state. Furthermore, a peak is found at 531.2 eV correspond to the O 1s and it is related to the surface hydroxyl oxygen, as shown in Fig. 4(c). The surface hydroxyl oxygen can inhibit photo-generated electron–hole recombination. The elemental analysis (XPS) of the Ag-doped ZnO nanostructure reveals the successful inclusion of Ag atoms into ZnO host matrix[
Figure 4.(Color online) XPS analysis for (a) Ag 3d, (b) Zn 2p and (c) O 1s.
5.3. Surface morphology study of pure ZnO and Ag-doped ZnO nanostructures
The prepared ZnO and Ag-doped ZnO nanostructures were investigated using FESEM in order to examine the surface morphology. The surface morphology and elemental composition of pure ZnO and Ag-doped ZnO nanostructures are shown in Fig. 5. FESEM images of pure ZnO particles demonstrate a hexagonal-like structure. The FESEM results are found to be consistent with XRD results. Ag-doped ZnO nanostructures clearly depicts the needle-shaped nanorods structure with very small diameter [Figs. 5(e) and 5(g)]. The rods have a broad hexagonal base with circular tips. The magnified image shows its hexagonal morphology with pointed tips (Fig. 5(g)). The morphological change from hexagonal (ZnO nanostructures) to needle-shaped nanorods (Ag-doped ZnO nanostructures) could be due to the replacement of Ag2+ to Zn2+ in the ZnO lattice[
Figure 5.FESEM with EDAX image of (a, b) pure ZnO, (c, d) 1 wt% Ag-doped ZnO, (e, f) 2 wt% Ag-doped ZnO and (g, h) 3 wt% Ag-doped ZnO.
5.4. Photoluminescence (PL) analysis
The photoluminescence (PL) spectra of pure ZnO and Ag-doped ZnO nanostructures were recorded at room temperature for the wavelength (350–800 nm) (Fig. 6). Xenon lamp with excitation wavelength of 329 nm was used to record PL spectra of all samples. The PL spectra depict two emission bands at wavelength 389 and 527 nm with different intensities for pure ZnO sample. The PL spectra peak analysis reveals that peak at 389 nm is ascribed to the optical excitation and broad peak at 527 nm is attributed to oxygen vacancies presenting in ZnO lattice. The peaks at 527 nm for green emission may be due to the radiating defects followed by the interface traps holding on the grain boundaries[
Figure 6.(Color online) PL emission spectra of pure ZnO and Ag-doped ZnO nanostructures.
The green emission is the result of recombination of the electrons in single ionized oxygen vacancies and the recombination of a photo-initiated hole with a single ionized charge state of the point defects (oxygen vacancies and Zn interstitial)[
The intensity at 389 nm initially decreases for 1 wt% Ag doping, and then increases for higher (>1 wt%) Ag doping concentrations. This can be attributed to Ag+ ions incorporation into the ZnO nanostructures by two different mechanisms: (i) substitution of Zn+2 ions fashioning more ionized oxygen vacancies and (ii) including as interstitials Ag. Whereas for low concentration of Ag doping most of the Ag+ ions might have been inserted into the ZnO lattice substitutionally. In the case of higher Ag doping concentration, the more quantity of Ag+ ions included into the ZnO nanostructure interstitially. Thus, higher doping concentration of Ag produces more amounts of lattice defects in ZnO nanostructures[
While the intensity of the emission peak at 638 nm is decreased with decrease in Ag doping amount (Fig. 6). The presence of Ag nanostructures on the surface of ZnO nanorods may be the cause of this quenching. Further, the peaks at 638 nm for the red emission show a shallow level emission (DLE) associate with the localized levels[
5.5. Antibacterial activity
The antimicrobial activity of synthesized pure ZnO nanostructures and Ag-doped ZnO NPs were assessed and their antibacterial activities against S. aureus were clearly observed in the agar well diffusion method. The difference in the zone of inhibition of pure ZnO NPs and Ag-doped ZnO nanostructures in comparison to the Zn+2 and Ag+ ions (100 µL of 0.7 M of ZnCl2 and AgNO3 solution) were evident and recorded. The zones of inhibition of 18 mm for the pure ZnO nanostructures and 19, 20 and 22 mm for Ag-doped ZnO NPs for concentration of 1, 2 and 3 wt%, respectively are achieved (Figs. 7 and 8 and Table 2). The results also show that S. aureus exhibits the resistivity against the ampicillin antibiotic at 25 mg/mL concentration similar to earlier reports[
Figure 7.Zone of inhibition of (a) pure ZnO, (b) 1 wt% Ag-doped ZnO, (c) 2 wt% Ag-doped ZnO, and (d) 3 wt% Ag-doped ZnO.
Figure 8.(Color online) Antibacterial activity of pure ZnO and Ag-doped ZnO nanostructures against Staphylococcus aureus.
Table Infomation Is Not EnableTwo possible mechanisms have been defined the interaction between bacteria and ZnO nanostructures: (i) decomposition of ZnO results in formation of ROS (hydroxyl radicals, Zn2+ ions, singlet oxygen and H2O2), which leads to destructive interaction with bacteria and grounds their death and (ii) ZnO nanostructures can gather on the surfaces of bacteria and leads disorganization of cellular function and interruption of cellular membranes. The antibacterial activity of the synthesized ZnO nanostructures might be due to the above defined mechanisms independently or cumulatively[
It has been reported that the antibacterial activity of ZnO nanostructures improved with the reducing of particle size. High surface-to-volume ratio of the prepared needle-shaped ZnO NPs is one of the reasons of such enhanced antibacterial activity. Ag doping into ZnO matrix is described to rise the Zn2+ ions releasing in water and participating efficiently in the antibacterial activity. An enhancement in Zn ions is probable on the interstitial sites by the doping of Ag ions on the sites of Zn ions in host matrix. These Zn ions may also be easily free from interstitial sites than from their native sites. In addition, Ag+ ions of Ag-doped ZnO nanostructures may also be released and this leads to the enhancement of the antibacterial activity[
6. Conclusions
We have demonstrated the present fast and controlled one-step method of ZnO nanomaterials preparation by low temperature hydrothermal method. XRD results evaluated the quality of the prepared nanostructures. Ag incorporation in ZnO alters the structural, surface, optical and antibacterial properties. FESEM images depict the change of morphology from hexagonal shape nanostructure (ZnO) to needle-shaped nanorods (Ag-doped ZnO). The synthesized pure ZnO and Ag-doped ZnO nanostructures have shown the antibacterial activity against gram positive human pathogenic bacteria S. aureus. A remarkable enhancement in antimicrobial activities is also observed through the doping of Ag in a pure ZnO nanostructure. Hence, the prepared nanostructures possess the profound applications in the pharmaceuticals, cosmeceuticals and agricultural industries.
Acknowledgements
The authors would show their gratitude to Prof. Ramesh Chandra, Institute Instrumentation Centre, Indian Institute of Technology, Roorkee, India for providing FESEM & XRD facilities and Dr. Preetam Singh, NPL, Delhi for his support in PL measurement. The authors would also be grateful to DST, Govt. of India for providing FIST grant for establishing the research facilities in the Department of Physics, Ch. Charan Singh University, Meerut, Uttar Pradesh, India. This work was supported by the UGC, Govt. of India [No. F.30–303/2016(BSR), F.D.Dy. No. 11299]
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