Green synthesis of strontium-doped tin dioxide ( SrSnO 2 ) nanoparticles using the Mahonia bealei leaf extract and evaluation of their anticancer and antimicrobial activities

In this study, a simple green method was employed to produce strontium (Sr)-doped-tin-dioxide (SnO2) nanoparticles (SrSnO2 NPs) using the Mahonia bealei leaf extract. The synthesized NPs were characterized with XRD, FE-SEM, FTIR, and PL spectroscopy measurements. SrSnO2 NPs were analysed for antimicrobial and anticancer activities. The XRD analysis revealed that the synthesized samples exhibited a tetragonal rutile crystal structure type of tin oxide. The EDX spectrum conforms to the chemical composition and elemental mapping of SrSnO2 NP synthesis. At 632 cm , the O–Sn–O band was observed and chemical bonding was confirmed using an FTIR spectrum. The PL spectrum identified surface defects and oxygen vacancies. The SrSnO2 NPs were tested against both Gram-positive and Gram-negative human pathogens. The synthesized nanoparticles exhibited effective antibacterial properties. The anticancer effects of SrSnO2 nanoparticles were also assessed against MCF-7 cells, and growth was decreased with increasing concentrations of the nanoparticles. Dual staining revealed high apoptosis in SrSnO2 NP-treated MCF-7 cells, proving its apoptotic potential. To conclude, we synthesized and characterized potential SrSnO2 nanoparticles using a green approach from the Mahonia bealei leaf extract. Further, green SrSnO2 nanoparticles showed significant antibacterial and anticancer properties against breast cancer cells (MCF-7) through apoptosis, which suggests a healthcare application for these nanoparticles.


Introduction
Tin oxide nanoparticles (SnO 2 NPs) have recently received increasing interest due to their extraordinary chemical, physical, and biological properties, most notably their excellent antibacterial activity. It is an N-type semiconductor with a wide bandgap and the most promising material owing to its unique physicochemical properties. On the other hand, those methods require more energy and are more expensive. Additionally, both environmental and human health hazardous chemicals have been employed for tin oxide nanoparticle synthesis. As a result, there is a necessity to create safer eco-friendly tin oxide synthesis methods that avoid unsafe solvents and chemicals while maximizing the use of natural biodegradable materials [1].
Mahonia bealei is a significant member of the Mahonia genus and is widely employed in traditional Chinese medicine. Many bioactive compounds have been extracted from this plant and have beneficial properties. It is an ethnomedicinal plant [2] and therefore utilized in a variety of nations, including India, Malaysia, Mauritius, Bangladesh, Nepal, Mozambique, and Oman for abortion, asthma, constipation, diarrhoea, earache, epilepsy, fever, ganglion, gum, and teeth disease, insect bites, headaches, decreased sugar levels, a mouth ulcer, acne, inflammatory bowel disease, syphilis, and healing of wounds, as well as an anti-helminthic, anti-parasite, aphrodisiac, dermatology ailment, emmenagogue, expectorant, and laxative [3][4][5][6][7][8][9][10][11]. The World Health Organization (WHO, 2022) has acknowledged that a reliable source of medicinal plants for traditional medicinal practices is based on plant sources for therapeutic applications [12].
The photocatalytic properties of tin oxide nanomaterials are highly promising candidates for microbial and cancer cell inactivation. Nanomaterials generate reactive oxygen species (ROS) and are strongly linked to antibacterial and anticancer activity [13]. This is because of their large surface areas, increased oxygen vacancies, reactant molecule diffusion, and active ion release. When tin oxide nanomaterials come into contact with light, they induce oxidative stress in cells, which leads to bacterial and cancer cell death. Tin oxide has, therefore, been employed to synthesize nanomaterials from both organic and inorganic substances due to these factors [14].
These ethnomedicinal plants can grow along the side of the road, in outdoor play areas, and in house compounds throughout settlements. In addition, they can be found in the wild or cultivated for personal use by some people. Moreover, the leaves contain valuable phytochemicals including alkaloids, anthraquinone, catechol, flavonoids, phenols, saponins, steroids, triterpenoids, and tannins [15]. The alkaline-earth metal was doped with tin oxide to enhance its physical, chemical, and biological properties. Technological applications can be expanded by doping with foreign metal ions. According to a literature report, charge compensation and ionic radius differences may arise through the doping of alkaline earth metals (Mg 2+ , Ba 2+ , Sr 2+ ), which can further boost the antibacterial activity [16].
In this study, the synthesis of alkaline-earth metal strontium-doped tin oxide NPs utilizing a simple green method has been carried out, which could increase optical efficiency. These nanoparticles open up additional opportunities for incorporating oxide nanoparticles into tin oxide. This will support a variety of fundamental and advanced concepts in a variety of applications. Though some research has been performed on the optical, antibacterial, and anticancer activity of SrSnO 2 NPs, there has been only limited research on the structural, optical, and antibacterial behaviours of SrSnO 2 . As a result, we attempted to verify the impact of variations caused by the amalgamation of Sr ions into the tin oxide lattice site. Tin oxide NPs also play an important role in antibacterial and anticancer activities [17,18]. Previous studies reported that green synthesized cerium-doped tin oxide (Ce-SnO 2 ) nanoparticles exhibit talented anticancer effects against several tumours [19].
To synthesize and evaluate the properties of Sr ions on the structural, morphological, and elemental properties of SrSnO 2 nanoparticles, a Sr ion on tin oxide nanoparticles was hypothesized. Molecular characterization of SrSnO 2 NPs was performed using several characterization techniques. The antimicrobial properties of SrSnO 2 NPs were investigated. Additionally, their ability to inhibit the proliferation of cancer cells by inducing cytotoxicity has been investigated. The synthesized nanoparticles were further tested for their efficacy in inducing cytotoxicity via promoting apoptosis on MCF-7 cells.

Materials
Chemicals such as tin chloride and strontium nitrate were acquired from Sigma Aldrich, USA. All the other required chemicals and reagents attained were of the analytical category.

Synthesis of green SrSnO 2 nanoparticles
Tin chloride and strontium nitrate were utilized as precursors without additional purification. Approximately, 10 g of fresh M. bealei leaves was boiled with deionized water (100 mL) for 10 min at 50-60°C.

Characterization of SrSnO 2 nanoparticles
An X'PERT PRO PANalytical XRD was employed to characterize the SrSnO 2 NPs. Diffraction patterns between 25°2 and 80°were recorded with a monochromatic wavelength of 1.54. A NanoPlus DLS Nano Particle Sizer was utilized for the particle size comparison of SrSnO 2 NPs. FE-SEM (Carl Zeiss Ultra-55 FESEM) with EDAX and mapping analysis was employed to investigate the samples (model: Inca). A Perkin-Elmer spectrometer was utilized to obtain FT-IR spectra in the range of 400-4,000 cm −1 . The Cary Eclipse spectrometer was used to measure photoluminescence (PL) spectra [21].

Antibacterial activity of SrSnO 2 NPs
The well diffusion technique was used to determine the antibacterial properties of SrSnO 2 NPs against Gram-positive (Staphylococcus aureus, Streptococcus pneumonia, and Bacillus subtilis) and Gram-negative (Klebsiella pneumonia, Escherichia coli, and Vibrio cholerae) strains. To ensure homogeneous distribution of the inoculums, the bacteria were streaked onto the media plates (NA) twice, with each streak being rotated at 60°. After inoculation, sterile forceps were used to place wells containing 1, 1.5, and 2 mg·mL −1 SrSnO 2 NPs on the bacteria-inoculated plates. After that, the plates were incubated at 37℃ for 24 h. Around the discs, the inhibition zone was noted and recorded. The positive control was amoxicillin (Hi-Media), which was tested against pathogens to compare the efficacy of the test samples.

Culturing of MCF-7 cells
The MCF-7 cells were cultured in Eagle's minimum essential medium supplemented with 1% antibiotics and 10% FBS. The culture medium with cells was incubated at 37°C by supplying 5% CO 2 . A trypsin-EDTA solution containing 0.25% trypsin was used to perform experiments with sub-cultured cells at 80% confluency.

Statistical analysis
GraphPad Prism version 6.02 software was employed to examine the values after the experiments were conducted. We analysed the data with a one-way ANOVA and subsequently the post hoc Duncan's test; significance we fixed at P < 0.05.

Characterization of SrSnO 2 nanoparticles
The ultraviolet-visible absorbance spectrum of SrSnO 2 NPs is shown in Figure 1a. The absorption band edge values of the synthesized SrSnO 2 NPs were observed at 394 nm [24]. With the help of Planck's formula, where E g is the band-gap energy  [27]. SrSnO 2 NPs were reduced and then stabilized by these phytocomponents (Figure 1b).
The PL spectrum of SrSnO 2 NPs with an excitation wavelength of 380 nm is shown in Figure 1c. The peaks in the PL emission spectrum of the SrSnO 2 sample were found at 413, 442, 465, 482, 499, and 522 nm. The violet emission, found at 413 nm, is accredited to the electron transition from the natural zinc interstitials' shallow donor level to the valence band's top level [28]. The blue emission peaks observed at 442, 465, 482, and 499 nm are allocated to single ionized Zn vacancies [29]. Finally, the green emission peak is located at 522 nm, and this emission corresponds to the single ionized oxygen vacancy [30]. Figure 2a represents the XRD pattern of the tin oxide and SrSnO 2 NPs that were synthesized using the green approach. Tin oxide NPs have XRD peaks that appear at where λ is the X-ray wavelength (= 1.54060), β is the angular peak width at half maximum (in rad), and θ is Bragg's diffraction angle. The average crystallite sizes of tin oxide and SrSnO 2 NPs were calculated to be 35.2 and 27.3 nm, respectively. The hydrodynamic diameter of SrSnO 2 NPs was determined using dynamic light scattering to obtain the particle size ( Figure 2b). The size of SrSnO 2 NPs was 143 nm; as the water medium surrounded the NPs, the DLS particle size was higher than XRD and TEM findings and is called hydrodynamic size.
The morphology, chemical composition, and elemental mapping of the synthesized SrSnO 2 NPs are shown in Figure 3a-c. As shown in the FESEM images, SrSnO 2 NPs exhibit a nanorod structure (Figure 3a). The average particle size was 200 nm. The chemical composition of SrSnO 2 NPs was demonstrated using the EDAX spectra and is shown in Figure 3b. In SrSnO 2 nanocomposites, the atomic percentages were as follows: 17.88% Sn, 9.88% Sr, and 72.24% O. The elemental mapping analysis revealed the presence of Sn, Sr, and O (Figure 3c). The NPs of SrSnO 2 exhibited uniform distribution of Sn, Sr, and O atoms throughout the structure.

Antimicrobial activity of SrSnO 2 NPs
The tin oxide and SrSnO 2 NPs were tested against S. aureus, S. pneumonia, B. subtilis, K. pneumonia, E. coli, and V. cholerae pathogens via the agar well diffusion method, as shown in Figure 4a. Figure 4a and b illustrates the zone of inhibition of tin oxide, SrSnO 2 , and conventional antibiotics such as amoxicillin when treated with the bacterial strains. The SrSnO 2 NPs displayed higher antibacterial activity than tin oxide NPs. The bacterial inhibitory mechanism depends on the adsorptiondesorption (A-D) and chemical-physical (C-P) activity between the SrSnO 2 NPs and various human pathogens. The interaction between the SrSnO 2 NPs and the human pathogens leads to distinct antibacterial activity. Each cytoplasmic lipid cell membrane produced A-D and C-P activities when SrSnO 2 NPs accumulated on the surface of each microbe. Nanomaterials synthesized through the green approach cause different disturbances on each cellular membrane, leading to cell death. The other antibacterial effects could be because varied amounts of active free radicals (ROSs) were formed on the cell walls during the contact, contributing to varying oxidative stress levels in the cells [31].
In previous studies, SnCl 2 H 2 O was synthesized from leaf extracts of Aloe barbadensis miller as a precursor for SnO 2 nanoparticles. These nanoparticles were found to be spherical and ranged in size from 50 to 100 nm, and they revealed substantial antibacterial properties against E. coli and S. aureus [32]. Green synthesis of SnO 2 NPs can also be done efficiently and cheaply using the P. amboinicus leaf extract and SnCl 2 H 2 O [33]. A previous study indicated that the Nyctanthes arbortristis (Parijataka) flower extract could reduce and stabilize SnO 2 nanoparticles and antimicrobial activity [34]. Previous studies demonstrated the antimicrobial activity of undoped and co-doped SnO 2 NPs against B. subtilis, A. flavus, E. coli, A. niger, and C. albicans [35]. In the past, similar studies reported that the formulation of SnO 2 NPs using the P. pinnata leaf extract revealed that the flower-like shape of NPs renders antibacterial properties against K. pneumoniae, E. coli, S. aureus, and S. pyogenes, which is comparable and even higher than tin oxide nanoparticles reported in previous studies [36]. Previously, similar studies reported that Ce-doped SnO 2 nanoparticles exhibited antimicrobial activity against E. coli and inhibited bacterial growth [37].
The particle size of SrSnO 2 NPs was 27.3 nm in this study. The XRD and PL results show several interstitial oxygen vacancies (O i ) at 522 nm for the sample's oxygen vacancies (O v ). The effects of the high number of ROS generated in SrSnO 2 NPs are responsible for the change. These defects allow more electron-hole pairs to migrate onto the nanomaterial matrix surface of SrSnO 2 NPs. While the electrons and holes can react with the superoxide anion (⋅ − O 2 ), hydroxyl radicals (OH°), hydrogen peroxide (H 2 O 2 ), and singlet oxygen ( 1 O 2 ) present in the aqueous environment of SrSnO 2 NPs may have contributed to ROS production. When free radicals contact the cellular environment, they can oxidize and reduce macromolecules such as nucleic acids, proteins, and lipids, leading to oxidative stress. Consequently, the cells are in a state of imbalance between ROS accumulation and the capacity of the biological system to rapidly eliminate reactive radicals or repair injury to tumour cells. Tin oxide NPs were tested for cytotoxicity against liver cancer HepG2 cells using an aqueous extract of A. squamosa, and cells treated with tin oxide nanoparticles were inhibited in a dose-and time-dependent manner due to chromatin breakdown in the nucleus [38]. Tin oxide nanoparticles synthesized from the Piper nigrum seed extract demonstrated increased cytotoxicity against lung (A549) and colorectal (HCT116) cancer cell lines [39]. Tin oxide nanoparticles made from the Pruni spinosae flos aqueous extract exhibited substantial cytotoxicity against lung cancer A549 and CCD-39Lu cells in a concentration-and time-dependent manner, and the authors reported that the cytotoxicity was related to the accumulation ROS and increased oxidative stress [40]. In previous reports, in vitro cytotoxicity of green-synthesized undoped tin oxide and codoped tin oxide NPs revealed a substantial mortality rate using the MTT assay, which further showed significant cytotoxicity and apoptosis in MCF-7 cells when compared with normal cell lines such as human lung fibroblast (WI38) and human amnion (WISH). The co-doped tin oxide NPs demonstrated more ROS accumulation than the undoped/doped tin oxide NPs [41].

Acridine orange/ethidium bromide staining of SrSnO 2 NPs
Apoptotic changes and nuclear condensation are involved in SrSnO 2 NP cytotoxicity. Fluorescent DNA-binding AO/EB dyes were utilized to detect and quantify apoptosis induction and necrosis formation in MCF-7 cells before and after treatment with SrSnO 2 NPs. AO dye was absorbed by both living cells, as indicated by green fluorescence. On the other hand, EB dye only absorbs dead cells (red fluorescence). The bright green nuclei and orange cytoplasm were evenly distributed throughout viable cells. In the early stages of apoptosis, apoptosis-inducing cells appear to have a green-coloured nucleus because their membranes are still intact but their DNA breaks down, resulting in a green-coloured nucleus. A bright orange-coloured nucleus with dense chromatin reveals late apoptotic and necrotic cells (Figure 6a and b). The tin and oxygen vacancies in SrSnO 2 NPs are believed to be responsible for anticancer activity, leading to higher ROS production. Because of the composition of SrSnO 2 NPs, they can produce significant ROS spontaneously. Reeves et al. [42] reported a similar result, where a nanogel containing curcumin appeared more effective against MDA-MB 231 cells than curcumin alone. HT-29 cells treated with curcumin-containing chitosan NPs (CUR-CS-NPs) and free curcumin at 75 µM also displayed fragmentation of nuclei and irregular edges, clearly indicative of apoptosis [43]. MTT assays of SrSnO 2 nanoparticles were performed on MCF-7 cells and the nanoparticles showed potential anticancer activity. Nanoparticles also caused cancer cell death through apoptosis when stained with AO/EtBr. Overall, the nanoparticles demonstrated greater anticancer activity than tin oxide alone.

Conclusion
An eco-friendly green approach was employed to synthesize SrSnO 2 NPs, in which the Mahonia bealei leaf extract was used as a reducing and capping agent. The phase analysis results showed a tetragonal rutile-type structure and an average crystallite size of 27.3 nm. When compared to pure tin oxide crystals, SrSnO 2 NPs were reduced in size. UV absorption band edge values were observed at 394 nm. Moreover, the PL spectrum showed that a lot of oxygen vacancies induced ROS production and cell death. SrSnO 2 NPs displayed higher antibacterial activity than pure tin oxide nanoparticles. In addition, SrSnO 2 nanoparticles exhibited potent anticancer effects against breast cancer MCF-7 cells. In the near future, we believe that SrSnO 2 nanoparticles could serve as potential anticancer agents through further research.