One-pot Fabrication and Characterization of Bioactive CeO_2-x Nanocrystals with Enhanced Radical Scavenging Potential

Ceria nanoparticles (CeO₂ NPs) have seen rapid advancements due to both fundamental and technological interest in view of its potential applications in the energy, agriculture, environment and biomedicine sectors. The co-existence of the redox couple consisting of Ce (IV) and Ce (III) ions results in the generation of oxygen vacancies (OVs), the effect of which becomes more pronounced at the nanoregime and nurtures it with a distinctive role as an oxygen reservoir. The electrons that result from the inception of these OVs can rapidly switch between the +3 and +4 states of CeO₂ NPs and confers them with a lucrative characteristic of scavenging the reactive oxygen species (ROS). It thereby prevents the generation of intracellular oxidative stress and thus has immense prospects in biomedicine.<br/>In this current study, a systematic investigation is carried out to understand the structural, physiochemical, and biological properties of the novel oxygen vacancy-rich CeO_2-x NCs synthesized by a first-of-its-kind batch reactor assembly. The synthesis protocol and the reactor assembly were rationalized and designed so that the nucleation-growth kinetics of CeO_2-x NCs do not influence the self-regenerative cycle of Ce³⁺ and Ce⁴⁺. A remarkable improvement in the colloidal stability (-30.3±7.2 mV) was observed from the zeta potential measurement which was achieved through accurately regulated ambient conditions where the reaction took place in a limited oxygen zone to engineer more OVs into the fluorite lattice. Estimation of the hydrodynamic size from dynamic light scattering (DLS) revealed a size of 9.26 nm indicating no agglomeration which is in accordance with the Derjaguin-Landau-Verwey-Overbeek (DLVO) model. The structural characterizations were performed with X-ray diffraction (XRD) that exhibited a cubic fluorite phase of CeO_2-x NCs with high phase purity, and an average crystallite size of 7.41 nm was estimated from the Debye-Scherrer’s equation. This was in good agreement with the observed broadening of XRD peaks because of the small size of the NCs. A high intrinsic lattice strain of 0.0017 was revealed by the Williamson-Hall plot while the determination of the lattice parameter of the NCs gave a value of 0.5425 nm which was higher when compared to 0.5411 nm of the bulk CeO₂, affirming a size-dependent change. This was ascribed to the lattice expansion induced by increased surface defect concentration at low particle dimensions. An in-depth understanding of the surface chemical composition by Fourier transform infrared spectroscopy (FTIR) corroborated the presence of Ce-O vibrations at 650 cm^-1in the fingerprint region. Additionally, a broad peak in the range of 3200-3700 cm⁻¹ was typically due to O-H stretching and bending modes of hydroxyl groups attached to the surface via chemisorption during the synthesis. Further, CeO_2-x NCs displayed significantly enhanced biological properties in contrast to their metal oxide counterparts ZnO and TiO₂ NPs, especially their superior antioxidant nature as observed by the capability to scavenge H₂O₂. This was evaluated mechanistically from the fluorescence quenching studies and the photoluminescence quantum yield (PLQY) was calculated with quinine sulphate in 0.1N H₂SO₄ as standard. Drop in the corresponding PLQY from 0.109 to 0.042 with the addition of increasing concentration of H₂O₂ assay to the test samples with time was accredited to the efficient scavenging by a large number of OVs. Redshift (λ_shift:5-100 nm) in the UV-vis absorption spectra at a constant optical density (0.1) was quantitatively estimated to substantiate the notable radical scavenging ability of CeO_2-x NCs for varying concentrations of H₂O_2.Thus, these stable colloidal solutions of CeO_2-x NCs with enriched antioxidant nature can be employed as potential bioactive ingredients for theranostics of various ailments mediated by the formation of ROS.