Zn2SnO4 an inverse structure of AB2O4 compound (space group Fd3m), has fascinated and unique properties rendering it suitable for a wide variety of applications such as chemical sensors, photoelectrical devices, transparent conducting electrodes, functional coatings and photocatalysts [1- 3]. And also, as an important transparent semiconductor with a wide bandgap of 3.6 eV, Zn2SnO4 is known to have high chemical sensitivity, high electrical conductivity, and low visible absorption . To realize the universal application of nanomaterials, the key point is to devise simple and efficient methods for their preparation on a large scale at a low cost. Various methods have been employed to produce Zn2SnO4 nanostructures such as mechanochemical synthesis, thermal evaporation method by heating metal or metal oxide powder at high temperatures, simple co-precipitation method, and hydrothermal synthesis [5-7].
Metal oxide nanoparticles (NPs) are the most widely used antimicrobial agent in the food industry applications . Zn2SnO4 NPs display biocidal activity against a broad range of Gram-positive and Gram-negative microorganisms . The antimicrobial activity of Zn2SnO4 NPs is mainly based on the following mechanisms: (a) release of Zn2+/Sn2+ ions which bind to electron donor groups in molecules containing sulfur, oxygen or nitrogen, (b) disruption of DNA replication and (c) oxidative stress through the catalysis of reactive oxygen species (ROS) formation . ROS contains the most reactive hydroxyl radical (OH), the less toxic superoxide anion radical (˙O2-), and hydrogen peroxide with a weaker oxidizer (H2O2). This can damage DNA, cell membranes, etc., leading to cell death .
Samreen Fatema et al.,  had investigated Saraca Asoka leave extracts used successfully for the synthesis of silver nanoparticles. Their results showed anti-bacterial activities against gram-positive bacteria. Leila Kafi-Ahmadi et al.,  had examined the influence of reaction parameters on crystal phase growth and optical properties of ultrasonic-assisted Hydro-and solvothermal synthesized sub-micrometer-sized CdS spheres. Navid Assi et al.,  had studied the synthesis of ZnO-nanoparticles by microwave-assisted sol-gel method and its role in photocatalytic degradation of food dye Tartrazine. Rejani et al.,  had reported that structural, optical, and dielectric studies in ZnO nanorods by the microwave-assisted method.
In the present work, Zn2SnO4 nanorods were prepared by a microwave-assisted method. The structural properties of the nanorods were characterized in detail. Besides, further development of a wide range of the optical behavior of Zn2SnO4 nanorods and the antibacterial activity was also investigated.
The subsequent high purity chemicals such as Zinc (II) nitrate, Tin (II) chloride dihydrate and ammonia solution were used as precursors without further purification.
Zn2SnO4 nanoparticle was prepared in different ratios of Zn and Sn (2 : 1, 1 : 1, and 1 : 2 named as P1, P2, and P3 respectively) by Microwave-assisted precipitation method. Zinc (II) nitrate and Tin (II) chloride aqueous solutions (50 mL) of the appropriate amount was prepared and stirred together for 1 h 20 min to get a homogeneous mixture. Then Ammonia solution was added to get a white precipitate and stirred at room temperature for 15 minutes. The product solution was transferred to a polypropylene capped autoclave bottle and the solution was irradiated by a microwave oven with a power of 600 W for 10 min. After irradiation, the solution was allowed to cool down naturally to room temperature. The precipitation was collected and washed numerous times with double distilled water and ethanol. Then the precipitate was dried at 120 °C or 12 h at the atmospheric condition and annealed at 800 °C for 5 hours to obtain Zn2SnO4 nanorods. The schematic diagram for the preparation of Zn2SnO4 nanorods is shown in Fig.1.
The antibacterial activity of microwave assisted Zn2SnO4 nanorods was tested against Streptococcus pneumoniae, Escherichia coli, Klebsiella pneumoniae and Shigella dysenteriae bacterial strain were carried out in agar by well diffusion method. These four strains were collected from Microbiology department of Alagappa University, Karaikudi, Tamilnadu. The antibacterial activity was tested at a concentration of 1 and 1.5 mg/ml of the Zn2SnO4nanorods dispersed in dimethylsulphoxide (DMSO). Zn2SnO4 nanorods -impregnated discs (30 μl) were placed on agar plates and incubated at 37°C for 24 hours. Pure DMSO (30 μl) was used as a negative control. The zone of inhibition levels (mm) was measured subsequently after 24 h at 37 °C. For positive control, standard antibiotic Amoxicillin (30 µg disc) was used.
The structural properties of Zn2SnO4 nanorod was investigated from the X-Ray diffraction patterns obtained using X’PERT PRO Panalytical Diffractometer. The morphology of Zn2SnO4 nanorods was examined by FESEM (Carl Zeiss Ultra 55) with EDAX (Inca). The functional groups were analysed from FT-IR spectra documented by Perkin-Elmer spectrometer in the range of 400-4000 cm-1. Photoluminescence spectra were taken using a spectrometer JASCO spectro flurometer FP-8200 to study the optical properties.
RESULTS AND DISCUSSION
X-ray diffraction patterns
The X-ray diffraction pattern was obtained in reflection mode with Cu Kα (λ=1.5406 Å) radiation, in the 2Ө range from 10º to 80º at room temperature. Fig. 2 shows the X-ray diffractions patterns of the synthesized of Zn2SnO4 nanoparticles. XRD patterns of synthesized Zn2SnO4 nanorods with diffraction planes (111), (220), (311), (222), (400), (331), (442), (511), (440), (531), (620), (533) and (622), exhibit spinel cubic structure and the values are in a good agreement with the reference JCPDS #74-2184. The lattice constant values a = 8.5814, 8.5871 and 8.5720 Å and volume V = 631.93, 633.19 and 629.86 Å3 for P1, P2 and P3 respectively. The crystallite size of the Zn2SnO4 nanorods is measured from Debye Scherrer’s relation(eq.1)and the crystallite sizes were found to be 42nm for all the prepared samples (P1, P2, and P3) and the micro-strain (μ) (eq.2) was 0.00082 . In a hydrothermal process, the alkaline concentration is a key factor that influences the crystallinity, morphology, and size of the as-synthesised sample . The XRD pattern indicates that the reaction is complete and the as-synthesised nanorods are close to the expected stoichiometric ratio.
The surface morphology of microwave assisted synthesis of all Zn2SnO4 nanorods was examined through FESEM analysis was shown in Fig. 3 (P1, P2 and P3). FESEM images clearly showed the synthesized Zn2SnO4 exhibits, rod like structure and average particle size in the nanoscale range. The formation of nanorods may be due to two reasons such as crystal nucleation and crystal growth direction. The growth of zinc stannate nanorods is initiated by the reaction between zinc ions and tin ions during hydrothermal synthesis. A part from the reaction time and temperature, the concentration of sodium hydroxide in the precursor solution plays an important role in determining the size and shape of zinc stannate nanoparticles. Zinc hydroxystannate is transformed into zinc stannate at a medium concentration of sodium hydroxide. The growth mechanism of Zn2SnO4 nanorods can be explained in terms of chemical reactions and crystal growth, as follows: From the crystallization point of view, the synthesis of an oxide during an aqueous solution reaction is expected to experience a hydrolysis-condensation process. Growth of Zn2SnO4 nanorods occurs according to the reaction.
2Zn(g) + Sn(g) + 2O2= Zn2SnO4(s).
Elemental compositions analysis
The elemental compositions of the Zn2SnO4 nanorods were represented in Fig. 4 (P1, P2 and P3). From the EDAX spectra, the several area positions of the sample were chosen and scanning, the same Zn, Sn and O content was present. In the present work, the Zn, Sn and O elements atomic percentage was given Table 1. The increasing concentration of Tin chloride during synthesis, the oxygen percentage increased and Zinc and tin percentage decreased, this may be a local lattice strain.
FTIR spectroscopic analysis
Fig. 5 shows the FTIR spectra of various concentration of Zn2SnO4 (2 : 1 (P1), 1 : 1(P2) and 1 : 2 (P3)) NPs. Zn2SnO4 samples various functional group are, O-H stretching at (3430, 3432 and 3416 cm-1) , C-H stretching at (2921 and 2924 cm-1) , C-H bands at (2361 and 2336 cm-1), this can be absorb atmospheric CO2. The asymmetric and symmetric stretching COO-group was found to be (1620, 1627 and 1632 cm-1) and (1469, 1416 and 1454 cm-1)  for P1, P2 and P3 samples. The Zn-Sn-O bands found to be 502, 475 and 460 cm-1 for all Zn2SnO4 NPs respectively, may be vibration of ZnO and SnO2 groups, and results formation of the Sn-O-Zn bonding in the Zn2SnO4 .
Photoluminescence spectroscopic studies
The photoluminescence spectra of microwave assisted Zn2SnO4 nanorods was shown in Fig. 6 (P1, P2 and P3).The Zn2SnO4 nanorods measured at the excitation wavelength of 460 nm. The two blue-green emission, four green emissions, and orange-yellow emission are located at (484, 499, 508, 519, 526, 541, 555, 566, and 584 nm), (484, 498, 508, 523, 540, 551, 566, and 582 nm) and (484, 499, 509, 519, 526, 541, 555, 566, and 579 nm) for P1, P2 and P3 respectively. The blue-green emission found to be (484-499 nm) for all Zn2SnO4 nanorods, which is attributed to oxygen vacancies [21, 22]. The green emission observed at (508-551 nm) for P1, P2 and P3 samples respectively, usually the oxygen vacancies existing in ZnSnO4 [23, 24]. The yellow-orange emission centered at (566 and 579 nm) for Zn2SnO4 nanorods respectively, due to the interaction between oxygen vacancies, tin interstitials and oxygen interstitials can be accountable for the yellow-orange emission ,and defects has formation of a huge amount of trapped states and sum of meta stable energy levels in the band gap of the as-synthesized Zn2SnO4 NPs. However for the P3 sample, green emission values (579 nm) increased as compared to the P2 (582 nm) and P1 (584 nm) for Zn2SnO4 nanorods samples respectively. From the optoelectronic application generally depends on decrease in defect level, which is mainly influenced via electron phonon coupling interaction. In the present work, the P3 samples emission decreased as compared to P1 and P2 samples, this results sustenance for the future development of optoelectronic application.
Fig. 7 (P1, P2 and P3) shows antibacterial activity of Microwave assisted Zn2SnO4 NPstested against (a) Streptococcus pneumoniae, (b) Escherichia coli,(c) Klebsiella pneumoniae and (d) Shigella dysenteriae bacterial strains to determine by the well diffusion method. The Zn2SnO4 NPs and Amoxicillin shows the antibacterial activity, which clearly shows the inhibition zone and specifies the biocidal action of Zn2SnO4 NPs. The maximum Zone of inhibition was observed E. coli as compared to the other S. pneumoniae, K. pneumoniae and S. dysenteriae bacterial strain (Fig. 8). In the present work, increasing the concentration (P3) of Zn2SnO4 NPs the inhibition zone also gets increased. The mechanism of action Zn2SnO4 nanorods is by disruption of bacterial cell membrane by electrostatic interaction leading to bacterial cell death .
The size of the inhibition zone increases significantly with the increasing concentration of NPs. Zn2SnO4 nanorods have the maximum antibacterial efficacy against E. coli with the highest zone of inhibition of 19 mm followed by a zone of inhibition of 18 mm against S. pneumoniae. Moderate antibacterial activity against K. pneumonia and S. dysenteriae with lower zone of inhibition of 12 mm is observed. The toxicity of Zn2SnO4 nanorods depends on their concentration and these nanorods are mildly toxic at low concentration. The mechanism of nanoparticle toxicity depends on composition, size, surface modification, intrinsic properties, and bacterial species. Nanorods attach to the bacterial cell membrane by electrostatic interaction and disrupt the integrity of bacterial cell, which in turn increases its permeability leading to cell death. The toxicity induced due to dissolved Zn ions and Sn ions from Zn2SnO4 nanorods is negligible and the toxicity strength of nanorods depends on intrinsic toxic properties of heavy metals .
The antibacterial activity generally depends on production of reactive oxygen species (ROS) [27-29].This ROS on the surface of these nanoparticles in light causes oxidative stress in microbial cells membrane, ultimately leading to the death of the cells.
The ROS production using UV light can be given in equation form as the following .
Zn2SnO4 + hυ e- + h+
h+ + H2O •OH + H+
e- + O2•O2-
•O2- + H+ HO2•
HO2• + H+ H2O2
The Zn2SnO4 nanorods through defects can be activated via both UV and visible light, electron-hole pairs (e- h+) can be created. The holes fragmented H2O molecules hooked on OH- and H+. Dissolved (O2) can be converted to (•O2-) radical anions. The (•O2-) superoxide radical anions in turn react with H+ to create (HO2•) radicals. The hydrogen ions (H+) react with (HO2•) to produce molecules of H2O2. The production of H2O2 be able to penetrate the cell membrane and finally bacteria are death .On other hand, Zn2+/Sn2+ions are released by Zn2SnO4 comes into contact with microbial cell membranes, the cell membranes with (-) charge and Zn2+/Sn2+ ions with (+) charge mutually attract. The Zn2+/Sn2+ metal ions are penetrates hooked on the cell membrane and react by sulfydryl groups inside the cell membrane. As a result, the damaged microbe synthetase activity and cells losing their ability of cell division, which leads to the cell death of the bacteria.
In summary, the Zn2SnO4 nanorods were prepared through facile hydrothermal method using microwave oven. From the XRD patterns showed that synthesized nanorods exhibits spinel cubic structure. Nanorod like morphology and chemical composition was observed through FESEM and EDAX spectra. In case of FT-IR spectra, the Zn-Sn-O stretching bands was observed at 502, 475 and 460 cm-1 for all Zn2SnO4 NPs. PL spectra, the Zn2SnO4(P3) samples emission decreased as compared to P1 and P2 samples, these results strong support for the potential development of wide-range of optical and electrical device application. The Zn2SnO4 nanorods showed the antibacterial activity, the inhibition zone indicates the biocidal action of Zn2SnO4 nanorods. This material was used as a bactericidal agent to prevent and control the spread and persistence of infectious diseases.
CONFLICTS OF INTEREST
The authors do not have any personal or financial conflicts of interest.