Assessment of Structural, Optical, and Antibacterial Properties of Green Sn(Fe : Ni)O2 Nanoparticles Synthesized Using Azadirachta indica Leaf Extract

Metal oxide nanoparticles have attained notable recognition due to their interesting physicochemical properties. Although these nanoparticles can be synthesized using a variety of approaches, the biological method involving plant extracts is preferred since it provides a simple, uncomplicated, ecologically friendly, efficient, rapid, and economical way for synthesis. In this study, the Azadirachta indica leaf extract was used as a reducing agent, and a green process was used to synthesize tin(ferrous: nickel)dioxide (Sn(Fe : Ni)O2) nanoparticles. The synthesized nanoparticles were subjected to characterization by using X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy analysis, field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), and photoluminescence (PL) measurement. Furthermore, Sn(Fe : Ni)O2 nanoparticles were analyzed for their antimicrobial activity against Gram-positive and Gram-negative organisms including Staphylococcus aureus, Streptococcus pneumoniae, Bacillus subtilis, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa bacterial strains. XRD patterns revealed that Sn(Fe : Ni)O2 nanoparticles exhibited a tetragonal structure. The hydrodynamic diameter of the nanoparticles was 143 nm, as confirmed by the DLS spectrum. The FESEM image showed the spherical form of the synthesized nanoparticles. Chemical composites and mapping analyses were performed through the EDAX spectrum. The Sn–O–Sn and Sn–O stretching bands were 615 cm−1 and 550 cm−1 in the FTIR spectrum, respectively. Various surface defects of the synthesized Sn(Fe : Ni)O2 nanoparticles were identified by photoluminescence spectra. Compared to traditional antibiotics like amoxicillin, the inhibition zone revealed that Sn(Fe : Ni)O2 nanoparticles displayed remarkable antibacterial activity against all tested organisms, indicating the valuable potential of nanoparticles in the healthcare industry.


Introduction
Metal oxide nanoparticles (NPs) difer from bulk materials in terms of their optical, thermal, magnetic, and electrochemical properties [1]. Owing to their small size and distinct features from bulk materials, these are particularly efective in applications such as pharmaceuticals, energy, chemicals, communications, agricultural machinery, manufacturing, industries, and consumer goods [2]. Metal oxide NP characteristics are known to be sensitive to the environment in which they are produced. To synthesize nanoparticles, researchers have employed the hydrothermal approach [3], combustion [4], coprecipitation [5], the sol-gel method [6], and the green method [7]. Due to the utilization of hazardous and harmful compounds, some NP synthesis pathways [8,9] using ionic liquids, pulsed laser [10], thermal decomposition [11], irradiation through microwaves [12], and so on, are not suitable for the safe fabrication of nanoparticles. As a result, in the feld of nanoscience, a green method for synthesizing nanoparticles that are environmentally acceptable, harmless, and inexpensive is required.
Among the various routes of NP synthesis, the green method has some advantages over physical-chemical methods, such as the usage of safe compounds, the ability to synthesize nanoparticles without producing harmful byproducts, the lack of toxic reagents, and the fact that it is an environmentally friendly, safe, and low-cost method. Plant extracts serve as both reducing and capping agents [13][14][15], and the extracts' phytochemicals help decrease and stabilize nanoparticles.
Te biological activity of inorganic nanoparticles is infuenced by various factors, including their size, morphology, surface charge, surface chemistry, capping agents, and other properties. With regard to the synthesis of NPs, the capping agent is one of the most signifcant elements. As a result, selecting suitable capping components is critical for stabilizing colloidal solutions as well as their absorption into living cells and the environment. After capping with biocompatible surfactants, the surface chemistry and particle size of nanoparticles are changed. Capping agents should have the ability to decompose and be well scattered, soluble, biocompatible, and nontoxic.
In this regard, for the synthesis of nanoparticles, the Azadirachta indica leaf extract was utilized. Azadirachta indica (family Meliaceae) is a plant that can be found in abundance across the tropics of the world. It has been shown that A. indica leaves possess anti-infammatory, antipyretic, antimicrobial, antidiabetic, and diverse pharmacological properties [16] by increasing insulin secretion and lowering blood glucose levels. In Asia, the leaves of the plant (A. indica) have long been utilized for medicinal purposes. A. indica can also be employed as a capping and reducing agent in the manufacturing process of nanoparticles [17].
Semiconductor nanostructures are currently attracting a lot of attention due to their unique physicochemical properties. Te N-type semiconductor, SnO 2 (nanostructured tin dioxide), with a bandgap width of nearly 3.6 eV, has a wide range of applications. SnO 2 also possesses excellent optical and electrical properties, making it suitable for photocatalysis, solar cells, gas sensors, transistors, and transparent electrodes, and displays high antibacterial activity [18][19][20][21][22][23].
Te doping of SnO 2 nanoparticles with transition metals or nonmetals has been observed to enhance their physical, chemical, and biological properties. Iron (Fe) is the most popular metal because of its half-flled electronic arrangement, which is expected to aid in narrowing the bandgap by forming new intermediate band levels and trapping electrons to reduce the recombination rate of pairs by catching photogenerated electron/hole pairs [24]. Terefore, a decrease in the recombination consequences of charge carriers is an explanation for how bandgap energy enhances the physical and biological activities of SnO 2 [25].
Te material's structure, surface morphology, composition, optical properties, photocatalytic dye degradation, and antibacterial properties are all thoroughly examined [26]. Previous studies reported that increasing the concentration of Fe-doped ions improved photocatalytic degradation efciency and antibacterial activity [27]. Particularly, metal and metal oxide NPs are thought to have antibacterial activity due to the generation of reactive oxygen species such as H 2 O 2 , superoxide, and hydroxyl radicals. Reactive oxygen species penetrate the bacterial cell membrane, causing DNA and protein damage and inhibiting bacterial growth [28]. According to XRD and FESEM analysis, the increased antibacterial activity of Fe-SnO 2 NPs can be attributed to their small particle size, which causes bacterial cells to leak intracellular components and die as a result of reactive oxygen species generated on their surfaces [29].
In the present work, the synthesis of Sn(Fe : Ni)O 2 nanoparticles was carried out by the green process using the Azadirachta indica leaf extract. Structural, morphological, optical, and antibacterial studies were carried out in an attempt to assess their potential to be employed in further biomedical applications.

Preparation of Green Sn(Fe : Ni)O 2 Nanoparticles.
Te synthesis of Sn(Fe : Ni)O 2 nanoparticles by the green method and antimicrobial activity of the entire study are schematically represented in Figure 1.
Te freshly collected Azadirachta indica leaves were washed multiple times with deionized water to remove adhering foreign impurities. Te aqueous leaf extract was prepared by boiling 10 g of fresh leaves at 80°C in 100 ml of deionized water for 15 min. Furthermore, the leaf extract was fltered using flter paper.
First, 0.002 M of ferrous nitrate solution and 0.002 M of nickel nitrate solution were added to an aqueous tin chloride solution (0.096 M). Ten, the obtained metal ion solution was mixed with 100 mL of the Azadirachta indica leaf extract and magnetically stirred at room temperature for 20 min to achieve a green-colored homogeneous solution. Next, the resultant solution was irradiated by using a microwave at 800 W for 10 min in polypropylene-capped autoclave bottles. Later, the obtained precipitate was cooled to room temperature and washed several times with deionized water and ethanol. At 120°C, the obtained residue was dried, and a light white powder was obtained. Finally, Sn(Fe : Ni)O 2 nanoparticles were annealed at 800°C for 5 h and then utilized for further analysis.

Characterization of Sn(Fe : Ni)O 2
Nanoparticles. An Xray difractometer (PANalytical X'Pert Pro) was used to characterize the synthesized SnO 2 and Sn(Fe : Ni)O 2 nanoparticles. Teir morphology and chemical composition were examined by using Carl Zeiss Ultra 55 FESEM with Inca : EDAX). Te particle size was used to measure dynamic light scattering (DLS) using NanoPlus instruments. Te Fourier transform infrared spectra were measured in the range between 400 and 4000 cm −1 by using a Perkin-Elmer spectrometer. Te photoluminescence spectra were measured using a JASCO FP-8200 spectrofuorometer.

Antibacterial Activity of Sn(Fe : Ni)O 2 Nanoparticles.
A culture collection from ATCC was used in this study, and using Mueller-Hinton agar (MHA), we tested the antibacterial activity of Sn(Fe : Ni)O 2 nanoparticles against Grampositive and Gram-negative bacteria including Staphylococcus aureus, Streptococcus pneumoniae, Bacillus subtilis, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa based on the Clinical and Laboratory Standards Institute methodology. Te nanoparticles were tested at a concentration of 1, 1.5, and 2 mg/ml dispersed in dimethyl sulphoxide (DMSO). Te zone of inhibition was measured after incubating the plates at 37°C for 24 h. Te antibiotic (amoxicillin) (10 µg disc) was used as a positive control.

Statistical Analysis.
Te mean and standard deviation of each result were calculated using descriptive statistics. Te signifcant diferences between control and treated groups were determined by using Student's t-test. p value of less than 0.05 was considered signifcant. SPSS statistical software version 11 (SPSS Inc., Chicago, USA) was employed to perform all statistical analyses.  [30]. In addition, no secondary phase was observed in the XRD difraction peaks of Sn(Fe : Ni)O 2 nanoparticles. Tis implies that Fe and Ni ions can ft into the lattice sites of SnO 2 instead of the interstitial space. Figure 2(b) shows information about the difraction angle shift in the (1 1 0) hkl plane, which is a shift towards the higher angle side with the substitution of Fe and Ni atoms in the SnO2 surface matrix. Tese efects are accompanied by changes in lattice parameter values. Debye-Scherrer's equation [31] was used to calculate the average crystallite size of SnO 2 and Sn(Fe : Ni)O 2 nanoparticles as follows:

Results and Discussion
where λ is equal to 1.54060Å (the wavelength of X-ray used), β is the angular peak width at half maximum in radians, and θ is Bragg' s difraction angle. Te average crystallite size is calculated at 65 nm and 52 nm for SnO 2 and Sn(Fe : Ni)O 2 nanoparticles, respectively.

Energy-Dispersive X-Ray (EDX) Spectroscopy Analysis.
Te energy-dispersive X-ray (EDX) spectroscopy spectrum shows the chemical composition of Sn(Fe : Ni)O 2 nanoparticles, as depicted in Figure 3. In the present investigation, the Sn    [32][33][34]. Te FTIR spectra of the synthesized nanoparticles are shown in Figure 4. Te O-H stretching and O-H bending were observed at 3441 and 1627 cm −1 ;respectively, asymmetric and symmetric C-H stretching peaks were at 2917 and 2849 cm −1 , respectively. Te antisymmetric Sn-O-Sn and Sn-O stretching peaks appeared at 615 cm −1 and 550 cm −1 , respectively. Tis result confrmed that the A. indica leaf extract's -OH group attaches to metal ions (ferric, nickel, and tin) and forms a coordination compound.

FESEM and TEM Analysis of Sn(Fe : Ni)O 2 Nanoparticles.
Te FESEM images of Sn(Fe : Ni)O 2 nanoparticles are shown in Figures 5(a) and 5(b). Te FESEM images with lower and higher (i.e., ×10,000 and ×50,000, respectively) magnifcations were captured at the same operating voltage of 5 kV. Te prepared Sn(Fe : Ni)O 2 nanoparticles were well crystallized and formed spherical and agglomerated shapes. Tese agglomerated shapes may be due to the strong interaction between hydrogen bonds in the precipitate during the green synthesis [35]. Te average size of the nanoparticles in the Sn(Fe : Ni)O 2 sample was estimated to be between 30 nm. Te TEM pattern of Sn(Fe : Ni)O 2 nanoparticles is shown in Figures 5(c) and 5(d). Te diferent sizes of Sn(Fe : Ni)O 2 nanoparticles were detected by TEM analysis and plotted as a bar chart based on the count versus particle size, as shown in Figure 5

Photoluminescence (PL) Spectrum of Sn(Fe : Ni)O 2
Nanoparticles. Figure 8 shows the PL spectrum of the synthesized Sn(Fe : Ni)O 2 nanoparticles. Te exciting wavelength was found to be 325 nm. Te spectrum of PL emission was observed in UV emission due to a recombination of electron-hole pairs and in visible emission due to various intrinsic defects in SnO 2 nanoparticles including V Sn , V O , O Sn , O i , and Sn i corresponding to tin vacancies, oxygen vacancies, oxygen antisites, oxygen interstitials, and tin interstitials, respectively [36].     In general, Sn(Fe : Ni)O 2 NPs have uneven microsurfaces with the surface containing active molecules, which readily adhere to the bacterial wall and cause damage to the cell membrane, resulting in cellular organelle extrusion and bacterial death. Te FESEM images of synthesized Sn(Fe : Ni)O 2 nanoparticles also displayed uneven ridges at their outer surface, which could have led to the potential antibacterial activity in the current study.
In this study, we determined that Sn(Fe : Ni)O 2 NPs have antibacterial activity against both Gram-positive (B. subtilis, S. aureus, and S. pneumoniae) and Gram-negative (E. coli, P. aeruginosa, and K. pneumoniae) bacteria. A wide variety of metal-or metal ion-based NPs engineered from various nanomaterials have been synthesized. Te majority of nanomaterials described in recent studies have antibacterial activity that can be attributed to one or more of the following mechanisms: Cell wall/membrane synthesis is inhibited, energy transduction is disrupted, toxic ROS are produced, photocatalysis is inhibited, enzymes are inhibited, and DNA production is reduced [37]. Te Sn(Fe : Ni)O 2 sample's MIC and MBC for inhibiting bacterial growth are 1.2 and 1.5 mg ml −1 , respectively, ( Table 1).
As shown in Figure 10, Sn(Fe : Ni)O 2 NPs inhibited only B subtilis, then E coli, P aeruginosa, and S. aureus. Te generation of reactive oxygen species (ROS) within the microbial cell membrane is a major reason for the increased antimicrobial efect of Sn(Fe : Ni)O 2 NPs. ROS generate three types of free radicals to increase antimicrobial properties: hydrogen peroxide (H 2 O 2 ), superoxide free radicals (O 2 %), and hydroxyl free radicals (OH%) [38]. As a result of ROS production, NPs in hydrogen peroxide penetrate the cell membrane, causing DNA damage and cell death [39]. In pure Sn (Fe : Ni)O 2 NPs, higher inhibition zones were observed against B. subtilis and reduced activities were observed against S. aureus, E. coli, and P. aeruginosa. Sn (Fe : Ni)O 2 NPs have much higher activity against E. coli than against S. aureus, and Gram-negative bacteria deactivate more efciently than Gram-positive bacteria.

Conclusion
In conclusion, an eco-friendly method to synthesize Sn(Fe : Ni)O 2 nanoparticles using the Azadirachta indica leaf extract as a reducing agent has been demonstrated in the current study. Te XRD patterns of Sn(Fe : Ni)O 2 nanoparticles exhibited a tetragonal structure. From the FESEM image, the spherical structure of the synthesized nanoparticles was noticed. Chemical composite and mapping analyses were performed through the EDAX spectrum. Various functional groups were identifed using the FTIR spectrum. Te antibacterial activity of Sn(Fe : Ni)O 2 nanoparticles was found to be greater than that of conventional antibiotics such as amoxicillin in this study.

Data Availability
All the data incorporated in the manuscript can be obtained from the corresponding author upon request.

Conflicts of Interest
Te author declares that there are no conficts of interest.