Emission enhancement through Nd³⁺-Yb³⁺ energy transfer in multifunctional NaGdF₄ nanocrystals

The growing need for biomedical contrast agents has led to the current development of multi-functional materials such as lanthanide-based nanoparticles (NPs). The optical and magnetic properties these nanoparticles (NPs) possess are important to enhance current biomedical imaging techniques. To increase the optical emissions of the nanoparticles, neodymium (Nd³⁺) and ytterbium (Yb³⁺) were introduced into a magnetic host of NaGdF₄. The energy transfer between Nd³⁺ and the Yb³⁺ was then investigated at multiple concentrations to determine the optimal dopant levels. The NaGdF₄:Nd³⁺,Yb³⁺ nanoparticles were synthesized through a modified solvothermal method, resulting in rectangular structures, with an average side length of 17.87 ± 4.38 nm. A double dopant concentration of 10% Nd³⁺ and 4% Yb³⁺ was found to be optimal, increasing the emission intensity by 71.5% when compared to the widely used Nd³⁺ single dopant. Decay measurements confirm energy transfer from Nd³⁺ to Yb³⁺, with a lifetime shortening from Nd³⁺ 1064 nm emission and a calculated lifetime of 12.72 ms with 98% efficiency. Despite NaGdF₄:Nd³⁺,Yb³⁺ NPs showing a slight decrease in their magnetic response at the expense of optimizing optical emission, as it is directly dependent on the Gd³⁺ concentration, a strong paramagnetic behavior was still observed. These results corroborate that NaGdF₄:Nd³⁺,Yb³⁺ NPs are viable candidates for multimodal imaging.