CeO₂-CePO₄ and Ag@CeO₂-CePO₄ nanocomposites from Penaeus semisulcatus for heavy metals sensing, UV shielding and cytotoxic applications

CeO₂-CePO₄ and Ag@CeO₂-CePO₄ nanocomposites (NCs) were prepared using Penaeus semisulcatus and characterized by various analytical techniques. The NCs exhibited spherical morphology, with average particle sizes measuring 2.36 nm, 3.30 nm and 29.64 nm for the samples air dried, annealed at 550 °C and 800 °C respectively, and the Selected Area Electron Diffraction patterns from HR-TEM revealed their crystalline nature. In 3d X-ray Photoelectron Spectra (XPS) of NCs, the highest binding energies were observed at 916.46 eV (3d_3/2) and 898.16 eV (3d_5/2) with spin–orbit splitting of 18.3 eV. The X-Ray Diffraction (XRD) patterns of the NCs showed peaks typical of both CeO₂ and CePO₄ phases. The Fourier Transform-Infra Red (FT-IR) spectrum contained bands due to Ce-O stretching at 560 cm⁻¹ and PO₄^3- groups at 1008 and 711 cm⁻¹, which was further substantiated by Raman spectrum, which had an intense peak at 468 cm⁻¹ corresponding to F_2g band of CeO₂ and another peak at 976 cm⁻¹ due to PO₄^3- tetrahedron. In the Ultraviolet-Visible Diffuse Reflectance Spectra (UV-Vis DRS), the broad absorption peak at 352 nm and minor peaks around 300 nm confirmed the formation of CeO₂-CePO₄ NCs. The green synthesized NCs were excellent fluorescence sensors for Pb²⁺, Cd²⁺ and Hg²⁺ in a wide range of concentrations, from 5.0x10^-9 M to 4.0x10^-5 M. The concentration Vs. emission fitting plot had three distinctive linear ranges. At a low concentration range (5x10^-9 to 3.5x10^-7 M), the limit of detection for Pb²⁺, Cd²⁺ and Hg²⁺ was found to be 2.41x10^-8, 2.56x10^-8 and 3.11x10^-8 M. The sensitive detection of a mixture of heavy metal ions in ppb levels by biocompatible NCs gained insight in treating aqueous industrial wastes. Further, NCs showed cytotoxicity and UV shielding properties.