Asparagine modified downconversion NaGdF4:Dy3+/Tb3+ nanophosphor for selective and sensitive detection of Cu(ii) ion

Abstract

Pure NaGdF₄, NaGdF₄:Dy³⁺, and Dy³⁺/Tb³⁺ co-doped NaGdF₄ nanoparticles with different concentrations of Tb³⁺ (ranging from 3 to 20 mol%) were prepared via a hydrothermal method. The influence of the Tb³⁺ ion content on the phase, crystallite size, morphology, and spectroscopic properties of the NaGdF₄:Dy³⁺/Tb³⁺ nanophosphors was explored with the help of PXRD, FE-SEM, TEM, HR-TEM, EDS, and photoluminescence studies. Moreover, the synthesized NaGdF₄:Dy³⁺/Tb³⁺ (3% Tb³⁺) nanophosphor was functionalized with the asparagine amino acid to render water dispersibility and to sense the heavy metal ions such as Cu²⁺, Mn²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺. Asparagine on the surface of nanomaterial not only prevents aggregation but also increases the stability and solubility of the nanophosphor in aqueous media. The asparagine-functionalized NaGdF₄:Dy³⁺/Tb³⁺ nanophosphor (Asp-NP) shows sharp emission peaks at 493, 548, 588, and 625 nm. The presence of –NH₂ and –COO⁻ groups on the surface of the nanophosphor material helps to detect heavy metal ions via electrostatic interactions. Significantly, the peak at 548 nm displayed a notable quenching effect upon the addition of metal ion solutions to an aqueous solution of the prepared Asp-NP. Particularly, Cu²⁺ ion showed the lowest limit of detection (1.362 ppm) and highest quenching constant (2.578 × 10⁵ M⁻¹) compared to the other hazardous metal ions, which suggests the high sensitivity and selectivity of the synthesized nanophosphor toward Cu²⁺ ion.