There has been a continuous threat to health, food packaging, cosmetics, and many more industries due to microbes and these industries highly depend on various antimicrobial agents (Ananpattarachai et al., 2009). Contaminated surfaces, colonization, subsequent biofilm formation, improper cleaning of equipments are found to be the primary carriers of microorganisms leading to several foodborne and other outbreaks (Swaminathan and Smidt, 2007; Yemmireddy and Hung, 2017). Nanotechnology has its impacts on all fields of science related to nanomedicine, biomedical, biosensor, development of smart cities, energy, environment, etc. (Kumar et al., 2019a; Bhawna et al., 2020; Gupta et al., 2020a,b).

Nanoparticles have been long known for their antimicrobial behavior against gram-positive and gram-negative bacteria, pathogens, and other microbes (Azam et al., 2012; Vargas-Reus et al., 2012). Metal oxide nanoparticles serve as antimicrobial agents owing to their large surface area (Raghunath and Perumal, 2017). Out of several metal oxide nanomaterials, scientists have more interest in SnO₂ nanoparticles because of their novel properties such as high chemical stability, high transparency, and low electrical sheet resistance, etc. (Jarzebski and Marton, 1976a,b; Jarzebski and Morton, 1976). The modified SnO₂ also has great technical and scientific interests because of its diverse applications, e.g., transparent conducting electrodes, gas sensors, as electrodes in lithium-ion batteries, electronic devices, dye-based solar cells, H₂ generation, etc. (Jiang et al., 2017, 2018; Park et al., 2017; Xie et al., 2017; Wang et al., 2018; Bhawna et al., 2020). Other than these applications, SnO₂ has been seeking attention as an antimicrobial agent and has played an essential role against the growth of various bacterial strains like Staphylococcus aureus, E. coli (Kumari and Philip, 2015; Vidhu and Philip, 2015; Phukan et al., 2017). Green synthesis of SnO₂ nanoparticles using Aloe barbadensis miller showed antibacterial and antifungal activities (Ayeshamariam et al., 2013). Apart from antimicrobial activities, other important biological properties like anticancer, antitumor, and antioxidant activities have also been reported using green synthesized SnO₂ nanoparticles (Kamaraj et al., 2014; Khan et al., 2018). However, when doped with transition metal ions, SnO₂ disinfects microbes with good efficiency, i.e., Co-doped SnO₂ and Ag-doped SnO₂ nanoparticles have shown potent antibacterial activities (Chandran et al., 2015; Nasir et al., 2017; Qamar et al., 2017; Ali et al., 2018). Only a few reports are available on antimicrobial activities of Ce doped metal oxide NPs. Ce-doped ZnO NPs showed antimicrobial activity against both gram-negative and gram-positive bacteria (Rooshde et al., 2020). Similarly, Ce doped CuO NPs completely eradicated the E. coli and S. aureus bacteria (Jan et al., 2014) and Ce-doped ZrO₂ NPs showed high antibacterial property against gram-positive bacteria than gram-negative bacteria (Mekala et al., 2018).

Water is a crucial factor for the existence of life on earth and clean water is the necessity of the hour. Consumption of water by a rapid increasing population is leading to the depletion of major aquifers. On the other hand, organic manufacturing industries have been the target for disposing of their chemical wastes into water bodies. According to research, the world's dye production of about 0.7 million tons (>11%) is annually released as industrial wastewater (Samadi et al., 2019). Among various known dyes, Malachite green dye has its extensive uses worldwide. Besides its use as a dye in silk, jute, leather, wool and paper industries; it is also used as a food additive, coloring agent and as a disinfectant. However, due to its carcinogenic effects on human health and aquatic life, it has now become a controversial compound and has been banned in many countries. Continuous efforts are being made to recycle contaminated water containing bacteria, toxic chemicals, dyes, heavy metals, etc. to make it safe for drinking and other purposes. Some conventional methods are- photocatalysis, ozonation, Fenton's reagent, electrochemical routes, membrane filtration, coagulation, adsorption, ion-exchange, irradiation, anaerobic and aerobic degradation, etc. (Gusain et al., 2019). Though, metal oxides such as TiO₂, SnO₂, ZnO have been found as better photocatalysts for the degradation of organic dyes in aquoues solution. SnO₂ as an n-type semiconductor has also been reported for the degradation of various azo dyes. Besides antimicrobial activities, doped SnO₂ finds improved results in photocatalytic activities. Ce doping has been known for bandgap tailoring as well as lattice distortion in SnO₂. There are various methods reported in literature for the synthesis of Ce doped SnO₂ such as- sol-gel (Shide et al., 2010; Ahmed et al., 2019), hydrothermal (Lian et al., 2017), co-precipitation (Bharathi et al., 2017; Kumar et al., 2018), wet-chemical (Kumar et al., 2019b), flame spray method (Kotchasak et al., 2018), etc. Kumar et al. showed degradation of dyes such as methylene blue and methyl orange using Ce doped SnO₂ nanoparticles (Kumar et al., 2019b) whereas Wu et al. degraded methyl orange dye using Ce doped SnO₂ (Shide et al., 2010).

To the best of our knowledge, until now, no work has been reported on antimicrobial behavior using Ce doped SnO₂. This work involves the facile synthesis of Ce doped SnO₂ nanoparticles and reports its antimicrobial behavior against microbes. It also represents photocatalytic degradation of malachite green dye using Ce doped SnO₂.

Ce doped SnO₂ nanoparticles were synthesized by a wet-chemical method using hydrogen peroxide, as mentioned in our previous report (Kumar et al., 2019b). Solutions of SnCl₂.2H₂O (Merck, 18 ml of 0.5 M) and CeCl₃ (Merck, 6 mL of 40 mM) were mixed and 30 mL hydrogen peroxide was added into the mixture. Then, the mixture was refluxed at 100°C for 14 h. The white suspension was cooled to room temperature, centrifuged, and was dried after washing several times to remove dissolved impurities.

The Antimicrobial activity of Ce doped SnO₂ nanoparticles was carried out using Diffusion Susceptibility Test method (Bauer et al., 1966). The bacterial strain, E. coli was inoculated in 5 ml LB Media (Luria-Bertani; HiMedia Laboratories) and was kept at 37°C and 180 rpm for overnight incubation. The overnight incubated bacterial culture was diluted in 1:100 ratios with fresh LB media. A zone of inhibition experiment was analyzed using an LB Agar plate well-diffusion method. Then, the sterilized well cutter was used for boring the LB Agar plate. The diluted overnight bacterial culture of E. coli was spread on LB agar plate. Thereafter, seven concentrations of Ce: SnO₂ NPs, namely, 0.25, 0.50, 1, 2, 3, 4, and 5 mg were poured into LB Agar wells. The well in the center of LB agar plate did not contain Ce doped SnO₂ NPs and was used as a control. Thereafter, the LB agar plate with Ce doped SnO₂ NP was incubated at 37°C for 16 h.

The Powder X-ray diffraction pattern of the synthesized Ce doped SnO₂ nanoparticles has been shown in the Figure 1. It shows a rutile structure with tetragonal symmetry space group P4₂/mnm [a = 4.680 (4) Å and c = 3.167 (4) Å] and shows clear reflection at (110), (101), (200), and (211) crystallographic planes corresponding to JCPDS file no. 41-1445 (Kumar et al., 2019b). The absence of any other characteristic peaks rule out possibilities of impurities or other species within the lattice represents high phase purity. The broadness of the diffracted peaks depicts a small size of crystals and the average crystallite size determined using the Scherrer formula was found to be ~6 nm (Scherrer, 1912).

The morphology and elemental mapping of Ce doped SnO₂ nanoparticles was investigated through FESEM (Figure 2). Irregularly shaped particles distributed unevenly over the lattice surface has been shown through SEM imaging. Elemental mapping shows the spatial distribution of elements within the lattice and provides the evidence that Ce (yellow), Sn (purple), and O (green) were homogeneously distributed within the crystal lattice.

After elucidation of the phase formation and morphology of the formed nanoparticles, the concentration of Ce ions was found to be 5% as determined through X-ray photoelectron spectroscopy in our previous report (Kumar et al., 2019b). Also, the presence of Ce³⁺ and Ce⁴⁺ ions was confirmed into SnO₂ lattice, which caused charge imbalance and hence disorderness in the lattice (Kumar et al., 2019b).

Facile and economical synthesis of Ce doped SnO₂ NPs showed potent antimicrobial properties so far. Also, nanomaterials were able to degrade toxic organic pollutants like malachite green. These nanomaterials could be used against bacterial infection as well as for multidrug-resistant bacteria along with wastewater treatment purposes.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.