Synthesis of ZnO nanoparticles using surfactant free in-air and microwave method
Highlights
► Synthesis of ZnO nanostructures of different morphologies via surfactant free methods. ► Effective antimicrobial activity of synthesized ZnO nanostructures. ► Photodegradation of methylene blue dye in the presence of ZnO nanoparticles. ► Evaluation of optical properties.
Introduction
Zinc oxide is a unique and key inorganic material that has attracted extensive research due to its characteristic features and novel applications in wide areas of science and technology. It has multiple properties like semiconducting, piezoelectric, pyroelectric, catalysis and optoelectronics [1], [2]. The lack of centre of symmetry in the wurtuzite structure of ZnO finds its use as mechanical actuators and piezoelectric sensors [3]. ZnO is a wide band gap n-type semiconductor (3.37 eV) with large exciton binding energy of 60 meV at room temperature. This property has made ZnO as one of the most attractive photocatalyst in the treatment of wastes and pollutants present in air and water by photodegradation mechanism. As the photocatalyst reaches the nanoscale, the photogenerated electron–hole pair combination decreases due to the fast photocatalytic reaction at the surface of the catalyst [4], [5], [6], [7], [8]. In addition, the optical properties of ZnO nanoparticles play an important role in optoelectronic, catalytic and photochemical properties.
In the recent years, workers all over the world have used different preparative techniques like chemical vapour deposition (CVD), electrodeposition (ED), hydrothermal route, sol–gel process, vapour–liquid–solid process, pulsed layer deposition, layer-by-layer method and thermal decomposition for the preparation of zinc oxide nanoparticles with varied morphologies [9], [10], [11], [12], [13], [14], [15], [16].
ZnO nanoparticles have been considered to possess potential biological applications as efficient antimicrobial agents, drug carriers, bioimaging probes and possessing cytotoxic behaviour for the treatment of cancer [17], [18]. Being a semiconducting material, the band gap between conduction and valance electrons plays a vital role in the generation of reactive oxygen species (ROS), which bring about conformational changes/oxidant injury to the surface of the membrane of the microorganism [19]. Moreover, for the production of ROS and rupture of the microbial cell membrane, the surface contact of nanoparticle with the biological membrane is necessary, which assists in the damage of microbial enzyme bodies, thereby killing the pathogenic microbes. Though the application of microwave radiation in synthetic chemistry started in 1980s but in the recent years, the use of microwave radiations has provided a new, efficient and environmentally benign methodology for the synthesis of various metal oxide nanoparticles of diverse morphologies and size. These radiations have unique properties like homogenous volumetric heating, which causes heating directly inside the sample, high reaction rate, selectivity and increased product yield. Moreover, it is an energy saving process [20], [21], [22]. A detailed study of kinetic and thermodynamic aspects in the microwave assisted synthesis of ZnO nanoparticles has been reported in the literature [23]. As compared to conventional heating, microwave heating causes the uniform distribution of temperature between the surface and the bulk material thereby leading to the fast formation of ZnO nanoparticles [24], [25]. In our previous work [19] ZnO nanoparticles were synthesized under different reaction conditions (under microwave, under pressure, under vacuum and at room temperature) using surfactant, which was isolated from natural plant source. Then the antifungal and antibacterial activity was evaluated by using a particular concentration (10−1 M) of ZnO nanoparticles. In the present study, ZnO nanoparticles have been synthesized under three different reaction conditions viz in-air (IA) drop-by-drop mixing, in-air instant mixing and under the influence of microwave radiations without the use of any surfactant. The synthesized ZnO nanostructures were further used to study the antimicrobial activity, photodegradation of methylene blue (MB) dye and investigations of optical properties. As compared to previous work, here, antimicrobial activity has been evaluated at different concentrations of ZnO nanoparticles and reduction in the growth of pathogenic bacteria and fungi at high concentrations of synthesized ZnO nanoparticles has been observed.
Section snippets
Materials and method
ZnSO4·7 H2O (SdFine) and NaOH (SdFine) were used as received.
For antimicrobial activity, Horizontal Laminar Air Flow, petri-dishes, agar medium, pathogenic bacteria (E. coli and S. aureus), fungi (Pythium debarynum and Sclerotium rolfsii) and ZnO nanoparticles were used. Fungal and bacterial strains were acquired from the Department of Basic Sciences, Solan. All the glassware and materials to be used for antimicrobial activity were sterilized in an autoclave for 2 h before use to prevent
Results and discussion
The synthesis of ZnO nanoparticles under different reaction conditions has a marked effect on the morphology and size. Moreover, synthesis under the influence of microwave radiations resulted with the product of smaller particle size.
Conclusions
In this work, ZnO nanoparticles have been prepared by using different methods, drop-by-drop method, instant method and microwave method. ZnO nanoparticles synthesized via microwave method are smaller in size as compared to nanoparticles obtained from other two reported methods. Microwave irradiations caused the complete decomposition of Zn(OH)2 into ZnO nanoparticles. The ZnO nanoparticles exhibited concentration dependent antimicrobial activity against pathogenic bacteria and fungi. The
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