Green and Cost-Effective Synthesis of Tin Oxide Nanoparticles: A Review on the Synthesis Methodologies, Mechanism of Formation, and Their Potential Applications

Nanotechnology has become the most promising area of research with its momentous application in all fields of science. In recent years, tin oxide has received tremendous attention due to its fascinating properties, which have been improved with the synthesis of this material in the nanometer range. Numerous physical and chemical methods are being used these days to produce tin oxide nanoparticles. However, these methods are expensive, require high energy, and also utilize various toxic chemicals during the synthesis. The increased concerns related to human health and environmental impact have led to the development of a cost-effective and environmentally benign process for its production. Recently, tin oxide nanoparticles have been successfully synthesized by green methods using different biological entities such as plant extract, bacteria, and natural biomolecules. However, industrial-scale production using green synthesis approaches remains a challenge due to the complexity of the biological substrates that poses a difficulty to the elucidations of the reactions and mechanism of formations that occur during the synthesis. Hence, the present review summarizes the different sources of biological entities and methodologies used for the green synthesis of tin oxide nanoparticles and the impact on their properties. This work also describes the advances in the understanding of the mechanism of formation reported in the literature and the different analytical techniques used for characterizing these nanoparticles.


Introduction
In the last few decades, nanotechnology has emerged as a new area of research dealing with the synthesis, characterization, modification and utilization of nanomaterials for their tremendous application in areas like pharmaceutics, food industry, cosmetics, textile industry, medicine, optics, electronics, energy science and electrochemical applications [1][2][3][4]. Nanomaterials are materials having one dimension in a size range of 1-100 nm. The very large surface area-to-volume ratio and extremely small size of these materials may result in a completely new or enhanced electrical, optical, magnetic, catalytic, and antimicrobial activities as compared to their bulk materials [5][6][7]. Because of these unique properties, nanoparticles find application in various fields of modern science and engineering, such as nanomedicine, photocatalysis, biosensors, cleaning agents, and textile industry [1,8]. Among the nanoparticles, tin(IV) oxide (SnO 2 ) in particular has gained immense attention due to their versatile applications such as optoelectronic devices [9], solid-state gas sensors [10], electrodes for lithium-ion batteries [11], field emission displays [12], light-emitting diodes [13], catalysis [14], dye-based solar cells [15], medicine [16], photo-sensors and antistatic coatings [17].
In material science, SnO 2 is considered as an oxygendeficient n-type semiconductor, which crystallizes as the tetragonal rutile structure with lattice constants a = b = 4.7374 Å and c = 3.1864 Å. The unit cell consists of two sixfold coordinated tin and four three-fold coordinated oxygen atoms [18,19]. A wide energy gap (3.6 to 3.8 eV), strong thermal stability (up to 500 °C), a high degree of transparency in the visible spectrum, strong chemical and physical interactions with the adsorbed species make SnO 2 a promising candidate for a potential application in the lithium-ion batteries, sensors, catalysis, energy storage, glass coatings, medicine and environmental remediation [20][21][22][23]. SnO 2 is used as a sensor to improve the response time and sensitivity owing to its high specific area, high chemical stability, low electrical resistance, and low density [24]. From the past few years, SnO 2 was thoroughly explored for its applications in the solar cells [25] and gas sensors to detect the combustible gases such as CO, NO, NO 2 , H 2 S, and C 2 H 5 OH [26][27][28][29]. Due to the unique physicochemical properties and potential applications of nanoparticles (NPs), the scientific community has been developing several methods for producing nanoparticles. However, the chemical and physical methods used for the synthesis of metal and metal oxide nanoparticles are quite expensive and use toxic substances that are hazardous to the environment and human health [30]. In recent years, most researchers have changed their research interest toward the green synthesis of NPs because it has many advantages such as cost-effective, simple manufacturing procedure, reproducibility in production, and often results in more stable nanoparticles [31]. During the last decade, several studies on the green synthesis of SnO 2 NPs were reported. However, no single review article is available in the literature that demonstrates the methodology of synthesis and mechanism of formation. Hence, this paper describes the green synthesis, mechanism of formation, characterization techniques, and potential applications of SnO 2 NPs. SnO 2 NPs are synthesized by various physical, chemical, and green methods. The chemical methods include sol-gel, hydrothermal, precipitation, mechanochemical method, microemulsion, and so forth [31][32][33][34][35][36][37]. Among the chemical methods, the most widely used technique is the sol-gel synthesis, which utilizes tin precursor salt and chemical reagents that regulate the formation of the tin-containing gel. After that, the gel is exposed to heat treatment under temperatures up to 800 °C to obtain SnO 2 NPs [32,38]. Chemical stabilizers and capping agents, such as oxalic acid or ethylene glycols, can be added during the synthesis of SnO 2 NPs to control the size and forbid agglomeration of the nanoparticles [32,39]. A solution pH, the concentration of chemicals, reaction time, and calcination temperature can also influence the size and morphology of nanoparticles [31,[34][35][36][37]. The aforesaid methods of synthesizing SnO 2 NPs utilize various perilous chemical reagents, solvents, and surfactants, which create a serious threat to the environment and human health [4,30].

Green Synthesis of Tin Oxide Nanoparticles
SnO 2 NPs can also be synthesized by physical techniques such as spray pyrolysis, thermal oxidation, chemical vapor deposition, laser ablation, and ultrasonication [40][41][42][43][44]. Among these methods, laser ablation is considered a cost-effective and simple method for synthesizing metal and metal oxide nanoparticles in liquid [44,45]. In contrast to the other conventional methods, this method does not require capping/reducing agents, high temperature, or high pressure, and allows us to produce nanoparticles of high purity [44,45]. The variation in the parameters of the pulses applied from the laser beam and the ablation time are important parameters that define the particle size, morphology, and surface chemistry of the nanoparticles [44]. However, most of the physical methods require complex equipment, high energy, and skilled man power [46]. Therefore, it is highly important to develop synthesis methods that are eco-friendly, cheap, efficient, and that works at ambient conditions. One such solution is a green synthesis, and many researchers have developed a green chemistry approach for synthesizing tin oxide nanoparticles. In the green synthetic strategy, biological entities like plant extract, microorganisms or other green sources could be used as an alternative to the conventional physical and chemical methods [47]. These days, the biologically inspired methods of synthesis are also known as green synthesis because they go in agreement with the twelve principles of green chemistry [48]. Some of the distinct advantages that biological synthesis has over physical and chemical methods are (a) clean and environmentally friendly method, as nontoxic chemicals are used, (b) the use of renewable sources, (c) the active biological components like enzyme itself as well as phytochemicals acts as reducing and capping agent, thereby minimizing the overall cost of the synthesis process, (d) external experimental conditions like high pressure and temperature are not required, causing significant energy savings [49,50].
In the last decade, the interest in synthesizing SnO 2 NPs via the biological method has increased considerably, because the process is more reliable, eco-friendly, cost-effective, low input-high yield, and simple procedures without causing any adverse effect on the environment. A variety of biological substrates such as plant extracts, bacteria, and natural biomolecules have been successfully employed for green synthesis of SnO 2 NPs. The phytochemicals of various plants and enzymes from bacteria are primarily responsible for the green synthesis. The active compounds present in the green sources also play as reducing, capping, and stabilizing agent during the synthesis. The desired NPs are often obtained after calcination or annealing at specified temperatures [51][52][53][54].

Plant-Mediated Synthesis of Tin Oxide Nanoparticles
Plant-mediated synthesis has become the best platform of synthesis over conventional physicochemical methods, because it is free from toxic chemicals and provides natural capping as well as reducing agents. Moreover, it is easy and eco-friendly and gives quantity-enriched product free of impurities. In this method, there is no need to use high temperature, high pressure, and costly equipment. Additionally, plant-mediated synthesis leads to large-scale production of more stable nanoparticles with varying shapes and sizes [55,56]. Extracts of a large number of parts of different plant species have been utilized for the green synthesis of SnO 2 NPs. In general, the plant-mediated synthesis of SnO 2 NPs is a very simple process in which a thin salt is added to the extract previously prepared. After the reaction, the solution is subjected to centrifugation. Finally, the pellets are then submitted to thermal treatment followed by characterization using various analytical techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), energy-dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyzer (PSA), and dynamic light scattering (DLS). UV-visible spectroscopy (UV-Vis) is used to monitor the formation of nanoparticles. The detailed protocol for the plant-mediated synthesis of SnO 2 NPs is schematically illustrated in Fig. 1. Diallo et al. reported the green synthesis of SnO 2 NPs by using Aspalathus linearis and tin chloride pentahydrate (SnCl 4 ·5H 2 O) as a precursor [51]. The salt was dissolved in the plant extract, and the formation of a white deposit was observed after 10 min. The white deposit was collected after the centrifugation process and was dried at about 80 °C. The powder was annealed at various temperatures for about 4 h and subjected to various analytical techniques like high-resolution transmission electron microscopy (HR-TEM), EDS, XRD, and X-ray photoemission spectroscopy (XPS). The NPs were quasispherical in shape with an average size in the range of 2.5-11.40 nm. It was observed that the particle size and crystalline nature of the NPs increase with increasing the annealing temperature. Moreover, the bioactive components like aspalathin, nothofagin, and aspalalinin present in the plant extract act as both chelating and reducing agents. Camellia sinensis leaf extract was also used to synthesize SnO 2 NPs [53]. The polyphenols present in the leaf extract act both as stabilizing and as capping agents. High-resolution scanning electron microscopy (HR-SEM) and XRD analysis revealed spherical shape SnO 2 NPs with size in the range of 5-30 nm. Furthermore, the bandgap of the NPs was found to decrease with increasing annealing temperature. The Catunaregam spinosamediated green synthesized SnO 2 NPs showed excellent photocatalytic activity against Congo red dye [57]. The biosynthesized NPs are spherical in shape and an average size of 47 nm.
The leaf extract of Aloe barbadensis miller has been used to synthesize SnO 2 NPs from SnCl 2 ·2H 2 O as a precursor [58]. The resultant NPs was found to be spherical with an average size varying from 50 to 100 nm. Moreover, the synthesized nanoparticles exhibited excellent antibacterial activity against S. aureus and E. coli.
In another study, the green synthesis of SnO 2 NPs was done by an efficient and cheap method using Plectranthus amboinicus leaf extract and SnCl 2 ·2H 2 O as starting material [59]. The plant extract acts as a reducing and stabilizing agent. The resulting NPs were characterized by SEM, EDX, XRD, and PSA. The NPs were tetragonal in shape with an average particle size of 63 nm. Furthermore, the authors revealed that the biosynthesized NPs exhibited greater photocatalytic activity against Rhodamine B as compared to the commercial SnO 2 . Green synthesis of SnO 2 NPs was reported using Nyctanthes arbor-tristis (Parijataka) flower extract [60]. As it is evident from SEM and PSA analysis, the synthesized nanoparticles exhibited the morphology of fine granules with tiny agglomeration stage and average grain size of 2-8 nm. The study also explored the hydrolysis and capping potential of the plant extract.
The green synthesis of SnO 2 NPs was carried out by a low-cost and eco-friendly process using Psidium guajava leaf extract [61]. The UV-Vis result revealed a surface plasmon resonance peak at 314 nm, confirming the formation of SnO 2 NPs. Moreover, the NPs were spherical with size ranging from 8 to 10 nm. The study also showed the NPs exhibit 90% degradation of reactive yellow 186 within 180 min under sunlight irradiation. Bhosale et al. [62] identified the leaf extract of Calotropis gigantea as a natural source for the synthesis of SnO 2 NPs. The secondary metabolites of the extract act as both stabilizing and capping agent in the conversion of tin chloride to SnO 2 NPs. The authors suggested that the NPs were spherical and an average size between 30 and 40 nm. The synthesized NPs degrade the methyl orange dye up to 80%, within 120 min. Singh et al. [63] reported the biosynthesis of SnO 2 NPs using Piper betle leaf extract. SEM and TEM analysis revealed the formation of spherical NPs with an average size of 8.4 nm. The NPs degrade the reactive yellow 186 in a pseudo-first order with an efficiency of 92.17%. Additionally, the NPs showed excellent selectivity toward the removal of reactive yellow 186 as compared to reactive red 120 and reactive green 119. In the literature, different plants and plant extracts have been used for the preparation of SnO 2 (Table 1). From the table, it can be seen that the nature of plant species and the reaction conditions affect the size and shape of SnO 2 NPs.

Bacteria-Mediated Synthesis of Tin Oxide Nanoparticles
Microbes are important nano-factories that gain immense potential as eco-friendly and cost-effective proteins available at the surface reduce the metal ions and are responsible for providing stabilization to the NPs [73]. However, the extracellular synthesis is more advantageous compared to the intracellular pathway because it can be used to fabricate large quantities of NPs and eliminates various steps of synthesis required for the recovery of NPs [74]. Biological synthesis utilizing bacteria is green and costeffective approach compared to chemical synthesis. However, this method has several drawbacks: (a) screening of the microbes is a time-consuming process, (b) requires careful monitoring of the culture broth and the entire process, and (c) is difficult to control the size and morphology of the NPs. Not all bacteria can synthesize NPs due to their intrinsic metabolic process and enzyme activities. Therefore, careful selection of an appropriate bacteria is necessary to produce NPs with well-defined size and morphology [74]. For instance, a study conducted by Srivastava and Mukhopadhyay [54] reported a low-cost, green, and simplest procedure for the synthesis of SnO 2 NPs using fresh and clean Erwinia herbicola bacterial cells and an aqueous tin(II) chloride solution. The synthesized SnO 2 NPs were mostly spherical with size in the range of 10 to 42 nm. It was reported that the bacterial protein and other biomolecules serve as a reducing and stabilizing agent during the synthesis of SnO 2 NPs. These biomolecules also helped in controlling the size and aggregation of SnO 2 NPs.

Biomolecules and Other Green Source-Mediated Syntheses of Tin Oxide Nanoparticles
Apart from the plant-and bacterial-mediated synthesis of SnO 2 NPs, researchers have developed a nontoxic, environmentally benign, and green chemistry approach by utilizing other biomolecules such as amino acids, vitamins, enzymes, and sugars (Table 2). Yang et al. [75] synthesized SnO 2 NPs using a low-cost and environmentally friendly method using vitamin C (ascorbic acid), a naturally available biomolecule. TEM analysis showed the formation of spherical NPs with an average size around 30 nm. Vitamin C acts as both a capping and a reducing agent during the synthesis. The study suggested the vitamin C capped on the surface of SnO 2 NPs decreased the oxidative stress caused by SnO 2 NPs on the cells, leading to a reduced weight loss in neonatal mice. Spherical SnO 2 NPs within the average particle size of 13 nm was prepared using carbohydrate (starch) [76]. It was suggested that the carbohydrate acts as a template, which could bind several metal cations through their functional groups, and hence, a uniform dispersion of the cations was observed. In another study, remnant water collected from soaked Bengal gram beans (Cicer arietnum L.) was employed for synthesizing un-doped SnO 2 , and Ni, Fe, and Au-doped SnO 2 NPs [77][78][79]. The authors suggested that the pectin in the extract was responsible for the synthesis of the SnO 2 NPs. The average crystallite size of the spherically shaped un-doped SnO 2 , Ni-doped SnO 2 , and Au-doped SnO 2 NPs was found to be 11 nm, 6 nm, and 25 nm, respectively. Another study has shown a green approach to synthesize SnO 2 NPs using an eggshell membrane (ESM) [80], a natural bio-waste from the chicken eggshell. It was  16:97 suggested that the ESM constituents biomolecules such as uronic acid and saccharides containing the aldehyde moieties, which act as a reducing agent during the synthesis. Morphological analysis showed the formation of rod, hexagonal, and spherical shape of SnO 2 NPs with a particle size in the range 13-40 nm. Different amino acids like glycine, arginine, aspartic acid, lysine, and tyrosine [81][82][83][84][85] have been used for the green synthesis of SnO 2 NPs due to their good capping or complexing agents. Amino acid-mediated synthesis eliminates the use of toxic chemicals during the synthesis. Spherical SnO 2 NPs were synthesized using arginine [81]. The morphological studies suggest that the synthesized SnO 2 NPs were spherical with an average size in the range of 4-5 nm. Bhattacharjee et al. [83] used glycine to produce SnO 2 NPs from stannous chloride. It was suggested that the NPs formed at 200 °C, 400 °C, and 600 °C are spherical, polycrystalline, and monodispersed in nature with an average size of 6, 16, and 33 nm, respectively. Further, the nanoparticle obtained at 400 °C was luminescent. Similarly, Begum et al. [84] demonstrated the synthesis of tetragonal rutile structure SnO 2 NPs using L-lysine with size in the range of 4-17 nm.

Mechanism of Formation of Tin Oxide Nanoparticles Through Green Synthesis
In the last few years, different plant extracts, microorganisms, and other biological derivatives have been used in the green synthesis of metal and metal oxide NPs. Studies suggested that the secondary metabolites such as phenols, flavonoids, tannins, saponins, terpenoids, and carbohydrates present in the plant extract play a significant role by acting as both reducing and stabilizing agents in the synthesis of metal or metal oxide NPs [87]. Additionally, microbes-mediated synthesis can be attained either intracellularly or extracellularly according to the location where the NPs are formed. In extracellular synthesis, bioreduction occurs on the surface of the microbial cells in the presence of the enzymes available at the surface. Furthermore, in intracellular biosynthesis, metal ions are transported into the microbial cell, and the NPs easily form there in the presence of enzymes inside the cell [73]. Selvakumari et al. [53] utilized Camellia sinensis extract for the biosynthesis of SnO 2 NPs. They demonstrated from their studies that the polyphenolic compounds (epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate) present in the extract acts as both capping and stabilizing agents. The SnO 2 NPs formed by this method consists of some major steps, including (i) reduction of Sn 2+ to Sn 0 , (ii) the reducing effect of phenolic compounds (-OH) of extract to form Sn species, and (iii) thermal transformation of the Sn species into the SnO 2 NPs. In the SnO 2 NPs synthesized using Calotropis gigantean leaves extract, polyphenolic compounds are responsible for the biochemical transformations of tin ions [62]. Spherical SnO 2 NPs of size in the range 3.62-6.34 nm were synthesized using Cauliflower extract [52]. It was suggested that the polyphenols and the flavonoids of the extract coordinate with the metal ions, resulting in the formation of Sn(OH) 2 intermediates, which upon calcination leads to the formation of SnO 2 NPs. In another study, Srivastava and Mukhopadhyay [54] reported both intracellular and extracellular synthesis of spherical SnO 2 NPs by the enzymes secreted by bacterium E. herbicola. In this method, the Sn 2+ ions are trapped by the extracellular enzymes secreted by the bacterial cell or by membrane-associated proteins on the cell surface, and the reduction was initiated by dehydrogenase enzymes. The Sn 2+ ions are reduced by gaining two electrons, and a molecule of NAD + is oxidized to form NADH, which finally results in the production of extracellular Sn NPs. The biosynthesized Sn nanoparticles are then oxidized by the oxygen present in the solution, which leads to the formation of SnO 2 NPs. FTIR analysis revealed the presence of protein-like molecules on the surface, which provide natural support and stability of the SnO 2 NPs. Furthermore, amino acids play as capping or complexing agents, thereby minimizing the use of toxic chemicals during the synthesis. The possible mechanism for the green synthesis of SnO 2 NPs is shown in Fig. 2:

Characterization of Tin Oxide Nanoparticles
Characterization of NPs is very crucial to know and control the synthesis procedure and application. The surface morphology and conformational details about size, shape, crystallinity, and surface area of the synthesized NPs are studied by utilizing various techniques. Some of the techniques used for characterizing green synthesized SnO 2 NPs are UV-Vis, FTIR, XRD, EDS, SEM, and TEM. UV-Vis spectroscopy is used to monitor nanoparticle formation by studying their optical properties. UV-Vis spectra of the SnO 2 NPs synthesized using Catunaregam spinosa extract exhibited the highest absorption peak of 223 nm due to its surface Plasmon resonance [57]. XRD is a powerful technique used for studying the crystal structure of materials. Bhattacharjee et al. [81] characterized SnO 2 NPs with XRD peaks at diffraction angles (2θ) of 26.7, 34.  (Fig. 3). The standard diffraction peaks show the tetragonal rutile structure of SnO 2 NPs with an average crystallite size 4.6 nm calculated using the Debye-Scherer equation [81].
FTIR spectroscopy is used to investigate the surface chemistry and identify the functional groups present  [70].
Microscopy-based techniques such as SEM and TEM have been widely employed to determine the morphological properties of the nanoparticles. However, TEM gives better resolution and information on the internal structure, such as crystal structure and morphology, as compared to SEM. The more accurate results of the surface properties can be acquired by using the FE-SEM. These techniques are also useful in estimating the average size of the synthesized nanoparticles [89]. FE-SEM and TEM analysis revealed the formation of slightly agglomerated spherical-shaped SnO 2 NPs with an average size of 30-40 nm (Fig. 4) [62]. Furthermore, selective area electron diffraction (SAED) analysis showed the particles are nanocrystalline in nature (Fig. 4) [62]. The formation of spherical shape SnO 2 NPs synthesized using Piper betle aqueous extract was ascertained by FE-SEM [63]. From TEM and XRD analysis, the average size of the NPs was found to be 8.4 nm. EDS is an analytical technique used to analyze the elemental composition of a sample. The EDS spectra of SnO 2 NPs synthesized using Psidium Guajava leaf extract revealed the presence of Sn and O peaks, which confirms the formation of pure SnO 2 NPs [61].

Biological Application of Green Synthesized Tin Oxide Nanoparticles
Green synthesized SnO 2 NPs showed enhanced photocatalytic, antimicrobial, antioxidant, and anticancer activity as compared to their bulk form. In this section, we have discussed the applications of green synthesized SnO 2 NPs in various fields as guidance to new researchers for future prospects.

Antimicrobial Activity
Many researchers have observed antimicrobial activity SnO 2 NPs. For instance, the antibacterial activity of SnO 2 NPs synthesized using aloe vera plant extract was studied using E. coli and S. aureus. It was suggested that the NPs were more active against S. aureus than E. coli [58]. This may be because the cell wall of E. coli is more complex than S. aureus. S. aureus has a membrane composed of thick peptidoglycan. However, the cell wall of E. coli has peptidoglycan layer plus an outer membrane composed of lipopolysaccharide. The outer membrane of E. coli acts as a barrier that lowers the penetration level of ROS into the cell [90][91][92]. Another reason could be the difference in the polarity of their cell membrane. The membrane of S aureus has a more positive charge than E. coli, which result in greater penetration level of negatively charged free radicals causing more cell damage and death in S. aureus than in E. coli [93,94]. The antibacterial activity of SnO 2 NPs synthesized using Punica granatum seed extract has been tested against E. coli. The bactericidal effect increased with increasing the concentration of the nanoparticles [95]. SnO 2 NPs synthesized using Trigonella foenumgraecum seed extract also showed similar antibacterial activity against E. coli [72]. In a recent study, Clerodendrum inerme leave extract was used for the synthesis of un-doped and Co-doped SnO 2 NPs [96]. The green synthesized un-doped and Co-doped SnO 2 NPs were subjected toward antimicrobial activity against five disease-causing pathogens such as E. coli, B. subtillis, A. niger, A. flavus, and C. Albicans,, and their zone of inhibition diameters (ZOIs), minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) were calculated. The Co-doped SnO 2 NPs showed substantial antibacterial activity against E. coli and B. subtillis in a concentration-dependent manner compared to the un-doped SnO 2 NPs, plant extract, and standard drugs in terms of their MIC (22 ± 0.7, 18 ± 0.8 mg/mL) and MBC (31 ± 0.9, 21 ± 0.6 mg/ mL) as well as ZOIs (30 ± 0.08, 26 ± 0.06 nm), respectively. The authors suggested that the broad-spectrum antibacterial activity of Co-doped SnO 2 NPs is due to cobalt doping, which leads to increased grain size and surface area of the NPs as compared to un-doped SnO 2 NPs. As more is the surface area with smaller particle sizes, the greater will be the antimicrobial activity. The associations of biomolecules like flavonoids and phenolic compounds with Co-doped SnO 2 NPs are also responsible for the enhancement of their antimicrobial activity. Furthermore, the green synthesized Co-doped SnO 2 NPs showed extraordinary antifungal activity with maximum ZOIs of 17 ± 0.04, 23 ± 0.08, and 26 ± 0.06 nm against A. niger, A. flavus, and C. Albicans, respectively, in comparison with plant extract, un-doped SnO 2 NPs, and standard drugs [96].
The actual mechanism of action of SnO 2 NPs against microbial strains is still unknown. However, several mechanisms of action against bacteria have been suggested for metal oxide nanoparticles, such as the decomposition of nanoparticles, electrostatic interaction of nanoparticles with the cell wall of microorganisms, and formation of reactive oxygen species (ROS) by the effect of light radiation [97][98][99]. One possible cause for the antibacterial effect of SnO 2 NPs may be the accumulation of the NPs on the surface of the bacterial cell membrane. The ROS generated due to the presence of SnO 2 NPs interacts with the cell membrane and disturbs the membrane permeability and respiration system of the bacteria, which leads to cell death [72,95,100]. For instance, Khan et al. suggested that the release of Sn 4+ and Co 2+ is Page 10 of 16 Gebreslassie and Gebretnsae Nanoscale Res Lett (2021) 16:97 responsible for the damage of bacterial DNA and mitochondria, which inactivates the bacterial enzyme and finally leads to cell death [96].

Antioxidant Activity
Many researchers examined the antioxidant activity of NPs by monitoring the ability in quenching of stable DPPH (2,2-diphenyl-1-picrylhydrazyl) radical into nonradical form (DPPH-H). Kamaraj et al. [101] reported the antioxidant activity of SnO 2 NPs biosynthesized using Cleistanthus Collinus leaves extract. The antioxidant activity of SnO 2 NPs increased with increasing concentration of SnO 2 NPs and reaction time. In another study [95], the free radical scavenging activity of green synthesized SnO 2 NPs increased in a dose-dependent manner. However, the annealed sample exhibited a lower scavenging activity as compared to the as-prepared sample. The decrease in scavenging activity with increasing annealing temperature may be due to a decrease in surface areato-volume ratio of the NPs. Moreover, the antioxidant efficacy of SnO 2 NPs against DPPH is due to the transfer of electron density between the NPs and the free radical located at nitrogen in DPPH. A similar result was found for SnO 2 NPs synthesized using Trigonelle foenum-graecum aqueous extract [72]. Hong [96].

Cytotoxic Activity
SnO 2 NPs synthesized using an aqueous extract of agricultural waste of dried peel of Annona squamosa were evaluated for cytotoxicity test against the hepatocellular carcinoma cell line (HepG2) [64]. TEM results revealed the loss of cell volume, considerable swelling of the cells, and nuclear condensation. The nuclear condensation seen in SnO 2 NP-treated HepG2 cells might be due to the breakdown of chromatin in the nucleus. SnO 2 NPs inhibited cell proliferation in a dose-and time-dependent manner with an IC 50 value of 148 µg/ mL. SnO 2 NPs synthesized using piper nigrum seed extract exhibited higher cytotoxic activity against colorectal (HCT116) and lung (A549) cancer cell lines [69].  Figure 5 shows the probable cytotoxicity mechanism of green synthesized SnO 2 NPs.

Photocatalytic Activity
The uncontrolled release of toxic chemicals, hazardous textile dyes, and pesticides from various industries into the running water has led to severe environmental problems. These superfluous water contaminants cause longterm adverse effects and pose a real threat to aquatic and human life. Besides, some organic dyes are carcinogenic and toxic. Hence, treating water that contains poisonous chemicals before disposal to the environment is very crucial to reduce environmental pollution. Recent studies have shown that nanostructured semiconductor metal oxides act as an excellent photocatalyst for the removal of various water pollutants [102][103][104][105]. Among the semiconductor metal oxides, SnO 2 is extensively used in the removal of common textile dyes and organic compounds owing to its beneficial characteristics, which include physical and chemical stability, high surface reactivity, high photocatalytic efficiency, low cost, and low toxicity [68,106]. Manjula et al. [107] synthesized SnO 2 nanoparticles using glucose. The NPs were effectively used as a catalyst in degrading methyl orange (MO) dye. The effect of calcination temperature (150-500 °C) on the photocatalytic activity of the NPs was investigated, and the results revealed that the as-synthesized SnO 2 NPs calcinated at 150 °C is the best photocatalyst for the reaction under study among the studied materials. Moreover, the as-prepared SnO 2 nanoparticles degraded methyl orange completely in 30 min, and also, the nanomaterials may be recycled with enhanced efficiency a minimum of five times. In another study [108], the photocatalytic activity of the SnO 2 QDs (quantum dots) synthesized by using serine was evaluated by monitoring the optical absorption spectra of eosin Y solution under direct sunlight. It was observed that the rate of degradation of eosin Y using SnO 2 QDs (98%) is higher than that using commercial SnO 2 (96%) and P25 (88%). Begum et al. [84] reported the synthesis of SnO 2 NPs using an amino acid, L-lysine monohydrate. The synthesized nanoparticles were evaluated for their photocatalytic behavior toward toxic organic dyes, namely malachite green oxalate (MGO) and Victoria blue B (VBB) under direct sunlight. The absorption peak of these dyes has begun to reduce, which shows that the chromophore structure has been demolished (Fig. 6). Furthermore, the as-prepared SnO 2 NPs exhibited an outstanding photocatalytic degradation of MGO (97.3%) and VBB (98%) dye within 120 min. Literature reports on the photocatalytic activity of green synthesized SnO 2 NPs are summarized in Table 3.

Gas-Sensing Property
Many metal oxide-based gas sensors are widely used for gas-sensing applications. However, SnO 2 NPs has gained tremendous attention in gas sensing under atmospheric conditions because of its beneficial properties, which include high sensitivity, high selectivity, easy reversibility, and cheap manufacturing costs. Green synthesized porous SnO 2 nanospheres demonstrated excellent gas sensing capabilities [107]. It was observed that the assimilation of 0.5 wt% Pd into the SnO 2 matrix improved the sensitivity and made it highly selective for low-temperature hydrogen detection. Moreover, the fabric was able to respond to even 50 ppm H 2 in N 2 at room temperature with an interval of 10 s. These sensing properties are due to the synergetic effect of both the porous structure of SnO 2 nanospheres and also the catalytic property of Pd nanoparticles. Gattu et al. [77] reported the gas-sensing behavior of biosynthesized and chemically synthesized Ni-doped SnO 2 NPs thin films. The biosynthesized Ni-doped SnO 2 NPs thin film showed higher NO 2 gas-sensing response as compared to the chemically synthesized ones. The sensor response was found to be increased with Ni doping for both biosynthesized and chemically synthesized Ni-doped SnO 2 NPs. This may be due to the reduction of particle size with Ni-doping, which results in increased surface area for adsorption of NO 2 gas. Furthermore, the Ni-doped SnO 2 thin film exhibited excellent selectivity toward NO 2 gas when put next to other gases like NH 3 , LPG and H 2 S. In another study [78], the gas-sensing properties of un-doped and Fe-doped SnO 2 NPs synthesized using Cicer arietnum L. extract were reported. The gas response within the presence of 100 ppm NH 3 gas at 200 °C operating temperature was found to be 28% for un-doped SnO 2 and 46% for Fe-doped SnO 2 thin films. Moreover, the Fe-doped SnO 2 -based sensor was found to be more selective for NH 3 gas as compared to the un-doped SnO 2 sensor. The biosynthesized Au-doped SnO 2 NPs were found to be highly sensitive to NO 2 gas at 200 °C operating temperature [79]. Gas sensor supported Au-doped SnO 2 NPs showed the gas response of ~ 30% for 100 ppm of NO 2 gas. Additionally, the gas sensor of Au-doped SnO 2 NPs showed excellent selectivity toward NO 2 gas when put next to other gases like H 2 S, LPG, and NH 3 . The improved gas response and selectivity toward NO 2 gas are because of the lattice distortion induced by Au-doping and also the oxygen vacancies generation within the SnO 2 lattice.

Conclusions
The use of green methods for the production of NPs has been the area of focused research because it is an ecofriendly, inexpensive, nontoxic, and sustainable method. Numerous studies report the possibility of producing SnO 2 NPs via a green protocol using a range of plant materials, bacteria, and natural biomolecules. The literature survey shows that the green substrates act as reducing and stabilizing agents or capping agents regardless of their source. Among the various green methods of SnO 2 synthesis, plant-mediated synthesis is cost-effective, easy to process, and less hazardous than microorganisms. However, the plant extracts consist of a large number of active compounds in a different composition, which makes it difficult to know the exact amount of the molecules responsible for the reduction of metal ions. Due to this complexity, it is difficult to evaluate the synthesis of nanoparticles. Therefore, further study on the mechanism of formation of SnO 2 NPs is required to understand the chemical reactions that occur during the synthesis.
With the knowledge of the actual reaction mechanism, it will be possible to monitor and optimize the biosynthesis process, which is essential for the large-scale production of SnO 2 NPs. Hence, understanding of the rapidly growing method of synthesis discussed herein will help