Selective Filter Effect Induced by Cu²⁺ Adsorption on the Fluorescence of a GdVO₄:Eu Nanoprobe

Human blood contains substantial amounts of metal ions such as Mg²⁺, Ca²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Al³⁺. Most biomedical applications of nanoparticles require understanding the influence of these metal ions because adsorbed metal ions can affect the function of nanoparticles to limit their sensitivity, performance, stability, and/or resolution in applications. In the present work, the adsorption of various metal ions at the surface of GdVO₄:Eu nanoparticles was studied to assess their spectral filter effect on the fluorescence of GdVO₄:Eu. Due to the negative surface potential, the electrostatic attraction caused an intensive adsorption reaction of GdVO₄:Eu nanoparticles with metal cations. Compared to the adsorption of other common metal ions in human blood, the distinct fluorescence quenching of GdVO₄:Eu was induced in the presence of Cu²⁺ ions. On the basis of the UV-vis absorption spectrum of an aqueous CuCl₂ solution and reflectance spectrum of Cu(OH)₂, in which the surroundings of Cu²⁺ ions are supposedly similar to the hydroxylated surface of GdVO₄:Eu nanoparticles, it is proposed that the complementary overlap of the emission band of GdVO₄:Eu with the absorption band of Cu²⁺ results in the effective filter effect to quench the red emission. Because GdVO₄:Eu nanoparticles are attractive candidates for applications as magnetic/fluorescent multimodal nanoprobes, it is important to recognize that the average amount of Cu²⁺ ion in human blood is sufficient to interfere with or limit the fluorescence probe function of GdVO₄:Eu nanoparticles.