Tailoring the upconversion emission and magnetic properties of NaGdF<inf>4</inf>:Yb, Er by Mg²⁺ or Fe³⁺ doping and optical trapping of individual magnetic nanoparticle at NIR 980 nm

This paper presents the upconversion emission and magnetic properties of NaGdF4:Yb, Er nanoparticles separately doped with Fe and Mg in the 10–40% concentration. The XRD analysis validates the structural properties of NaGdF4:Yb, Er nanoparticles with different concentrations of the dopants. The XPS spectra of Fe-doped NaGdF4:Yb, Er and Mg-doped NaGdF4:Yb, Er nanoparticles reveal the presence of the dopants and their valency. The 30%Mg doping resulted in an intense red upconversion emission at 650 nm compared to the Fe-doped NaGdF4:Yb, Er due to the high lifetime of 254 μs for the 4F9/2 → 4I15/2 (660 nm) energy level transitions of Er3+ ion. The Mg-doped nanoparticles showed a 2.7 red-to-green emission intensity ratio, 5.7 for Fe doping at the same 30% concentration and above this doping level, upconversion emission quenching resulted. The room temperature VSM study demonstrated paramagnetic property in the Mg-doped NaGdF4:Yb, Er nanoparticles characterized by negligible coercivity and retentivity with low saturation magnetization. Similar behaviour is also observed for the Fe-doped nanoparticles up to 20% concentration, and it became weak ferromagnetic at a high 30 and 40% dopant. In addition, the present work demonstrated the optical trapping of individual magnetic nanoparticles of 30%Fe-doped NaGdF4:Yb, Er by using a single beam optical tweezer technique under the tightly focused 980 nm diode laser interaction and the estimated trap stiffnesses is 1.85 pN μm−1 W−1. The multifunctional upconversion red emission, magnetic and optical trapping properties obtained in NaGdF4:Yb,Er,30%Fe nanoparticles can be used as a single particle probe in the theragnostic application.