Physics of Ce³⁺↔Ce⁴⁺ electronic transition in phytosynthesized CeO₂/CePO₄ nanocomposites and its antibacterial activities

Highlights

• Stabilization of Ce³⁺ as CePO₄ phase on the surface of CeO₂/CePO₄ nanocomposites via green synthesis.

• Excellent antibacterial efficacy observed against both gram positive and gram negative bacteria.

• Insignificant cytotoxicity observed on two mammalian cell lines up to moderately high concentration of nanocomposites.

Abstract

The interplay between physics and chemistry of nanoparticles dictate many useful properties for their practical applications. In this context, we synthesized well controlled CeO₂/CePO₄ nanocomposites using Artocarpus heterophyllus aqueous leaf extract as reducing agent. The as-synthesized nanocomposites were annealed at elevated temperatures (500–900 °C) for 3 h under air atmosphere and their characterizations were performed using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Differential Scanning Calorimetry (DSC) –Thermo Gravimetric (TG) analysis, Raman, Fourier Transform Infrared (FTIR) and UV–Visible spectroscopy analysis. The formation of the CeO₂/CePO₄ nanocomposites could be realized by the presence of phosphate ions in aqueous leaf extract which has been confirmed by Gas Chromatography – Mass Spectrometry (GC-MS) analysis. Such stabilization of Ce³⁺ ions as CePO₄ phase on the surface of nanoceria induced the reduction of grain growth and lowering of the bandgap of the nanocomposites. The antibacterial efficacy against both gram positive (S. aureus and B. cereus) and gram negative (S. typhimurium and E. coli) bacteria is attributed to the redox cycling between Ce³⁺ and Ce⁴⁺ ions at the oxide-phosphate interface of CeO₂/CePO₄ nanocomposites. The cytotoxicity analysis observed on two mammalian cell lines (HeLa and Vero) shows that the functional nanocomposites were non–toxic up to higher concentration (3 g/L). Our findings have implication that the phyto-synthesized CeO₂/CePO₄ nanocomposites could provide novel insights for mimicking multienzymes’ activities and safe for antibacterial applications in terms of in vitro cytotoxicity.

Introduction

Nanoceria (CeO₂) has drawn immense attention because of its multifunctional properties suitable for versatile applications in solid oxide fuel cells, catalysis, bio-imaging, sensors, UV resistant coatings and nanomedicine to mention but a few [[1], [2], [3], [4], [5], [6], [7]]. CeO₂ nanoparticle has cubic fluorite structure with a mixed valence state of Ce⁴⁺/Ce³⁺ that makes it highly efficient for destroying bacterial cell membrane and cells via generation of Reactive Oxygen Species (ROS) [8]. Indeed, the excellent antibacterial activity of nanoceria against both gram positive and gram negative bacteria has raised enormous enthusiasm among scientists over last few years for its potential in antibacterial applications [[9], [10], [11]]. However, the antibacterial efficacy of nanoceria against gram positive bacteria is still considerably lower than its efficacy against gram negative bacteria [[12], [13], [14]]. Recently, green synthesized nanoceria showed promising antibacterial activities against both gram positive and gram negative bacteria in contrast with the physically and chemically synthesized nanoceria which could be attributed to their reduced grain growth upon annealing at elevated temperatures and high Ce³⁺/Ce⁴⁺ ratio on surface of the nanoceria [[14], [15], [16], [17]]. Besides, the phyto-constituents assisted green synthesis of metal and/or metal oxide nanoparticles is useful because of the cost effective, environmentally benign and capping effect of this process [[18], [19], [20], [21], [22]]. Interestingly, phosphate ions in plant reductant can stabilize the Ce³⁺ as CePO₄ phase on the surface of nanoceria which could be beneficial to enhance the Ce³⁺/Ce⁴⁺ ratio by modulating the reversible Ce³⁺↔Ce⁴⁺ electronic transition at oxide-phosphate interface [[23], [24], [25], [26]]. The immobilization of Ce³⁺ by phosphate ions as CePO₄ on the surface of CeO₂ leads to a phosphate deficient environment which in turn can kill E. coli by generating adequate oxidative stress [27]. Although, it is evident from previous literatures that CeO₂/CePO₄ nanocomposites exhibit excellent catalytic activities, an investigation of its antibacterial efficacy against both gram positive and gram negative bacteria has not been reported yet to the best knowledge of authors [[28], [29], [30], [31], [32]].

In the present work, CeO₂/CePO₄ nanocomposites were synthesized by using Artocarpus heterophyllus aqueous leaf extract as bio-reducing and surface modifying agent. A. heterophyllus is widely known as jackfruit plant and grows as evergreen throughout the tropical regions of the world. Green synthesized metal and metal oxide nanoparticles by using A. heterophyllus aqueous leaf extract showed promising antibacterial activities in many previous studies [[33], [34], [35]]. Here, the structural, thermal and optical properties of CeO₂/CePO₄ nanocomposites were analyzed and correlated by XRD, Raman spectroscopy, EDX, FT-IR, DSC-TGA and UV–Vis spectroscopy analysis while the estimation of average particle size of the nanocomposites was realized by FESEM images. Finally, the antibacterial efficacy of the nanocomposites against both gram positive (S. aureus and B. cereus) and gram negative bacteria (S. typhimurium and E. coli) were investigated by following disc diffusion assay. Moreover, the cytotoxicity of the nanocomposites was observed on two mammalian cell lines (HeLa and Vero).