Synthesis and defect characterization of hybrid ceria nanostructures as a possible novel therapeutic material towards COVID-19 mitigation

Abstract

This study reports the synthesis of hybrid nanostructures composed of cerium dioxide and microcrystalline cellulose prepared by the microwave-assisted hydrothermal route under distinct temperature and pH values. Their structural, morphological and spectroscopic behaviors were investigated by X-Rays Diffraction, Field Emission Gun Scanning Electron Microscopy, High-Resolution Transmission Electron Microscopy, and Fourier-Transform Infrared, Ultraviolet-Visible, Raman and Positron Annihilation Lifetime spectroscopies to evaluate the presence of structural defects and their correlation with the underlying mechanism regarding the biocide activity of the studied material. The samples showed mean crystallite sizes around 10 nm, characterizing the formation of quantum dots unevenly distributed along the cellulose surface with a certain agglomeration degree. The samples presented the characteristic Ce-O vibration close to 450 cm^-1 and a second-order mode around 1050 cm^-1, which is indicative of distribution of localized energetic levels originated from defective species, essential in the scavenging of reactive oxygen species. Positron spectroscopic studies showed first and second lifetime components ranging between 202-223 ps and 360-373 ps, respectively, revealing the presence of two distinct defective oxygen species, in addition to an increment in the concentration of Ce³⁺-oxygen vacancy associates as a function of temperature. Therefore, we have successfully synthesized hybrid nanoceria structures with potential multifunctional therapeutic properties to be further evaluated against the COVID-19.

Figure 1

XRD patterns of the samples synthesized by microwave-assisted hydrothermal method at 100 and 120 °C with pH 10, 12 and 14.

Figure 2

SEM micrographs of the (a) raw microcrystalline cellulose and hybrid structures prepared with the MAH route at (b) 100 °C, pH 10, (c) 100 °C, pH 12, (d) 100 °C, pH 14, (e) 120 °C, pH 10, (f) 120 °C, pH12 and (g) 120 °C, pH 14.

Figure 2

SEM micrographs of the (a) raw microcrystalline cellulose and hybrid structures prepared with the MAH route at (b) 100 °C, pH 10, (c) 100 °C, pH 12, (d) 100 °C, pH 14, (e) 120 °C, pH 10, (f) 120 °C, pH12 and (g) 120 °C, pH 14.

Figure 3

TEM micrographs of samples prepared at 100 °C and 120 °C with pH 10 and 14, respectively.

Figure 3

TEM micrographs of samples prepared at 100 °C and 120 °C with pH 10 and 14, respectively.

Figure 4

UV–Vis spectra of the microcrystalline cellulose (upper) and the hybrid nanostructures composed of CeO₂@MCC (lower).

Figure 5

FT-IR spectra of the as-synthesized particles and the raw microcrystalline cellulose.

Figure 6

Raman spectra of the hybrid structures and the MCC.

Figure 7

Schematic representation of the τ₁ evolution as a function of reaction pH for the different synthesis temperatures. The corresponding lifetimes for the free-defect CeO₂ and the Ce³⁺-oxygen vacancy associates are also presented.