Epitaxially grown core–shell NaGdF4:Tm,Yb@NaGdF4:Ce,Tb nanoparticles exhibiting down- and up-conversion luminescence with multifunctional properties

Abstract

Core@shell nanoparticles, NaGdF₄:Tm–Yb@NaGdF₄:Ce–Tb, were synthesized via a thermolysis method, enabling epitaxial shell growth over the core as a single entity. These nanoparticles exhibit both upconversion and downshifting luminescence. Under 980 nm excitation, emissions from 474 and 800 nm corresponding to Tm³⁺ ions show significantly enhanced intensity in core@shell structures compared to core only nanoparticles, particularly at higher laser powers. This enhancement arises from reduced dipole–dipole interactions among Tm³⁺ ions due to dilution and suppression of surface defects and quenchers. In the down-conversion process, Tm³⁺ emission is quenched under UV excitation (258, 273, and 361 nm), whereas strong Tb³⁺ emission is observed due to efficient energy transfer from Ce³⁺/Gd³⁺ to Tb³⁺ ions. The dual-mode excitation and emission tunability make these nanoparticles promising candidates for security ink applications. Additionally, folic acid–functionalized nanoparticles demonstrate potential for targeted cancer therapy.