Multi-shell structured nanomaterials with strong red upconversion emission for trimodal biomedical imaging

Upconversion nanomaterials show promising applications in multimodal imaging due to their advantages such as sharp emission bands and non-autofluorescence in tissue. Obtaining intense red upconversion emission is crucial for fluorescence imaging. Herein, we designed a multi-shell structured NaErF4:0.5%Tm3+@NaYF4:20%Yb3+ @NaLuF4 upconversion nanomaterial exhibiting strong red emission and high red-to-green ratio under 980 nm irradiation. The main mechanism of the strong emission is attributed to the inert NaLuF4 shell which serves to block the energy transfer from Yb3+ to surface defects. In addition, a NaGdF4 shell is further cladded considering the property of Gd3+ of facilitating longitudinal relaxation rate of water. The properties of X-ray attenuation, longitudinal relaxation rate, and the upconversion emission of the measured sample suggest that our designed nanomaterial possesses potential for X-ray computed tomography (CT), magnetic resonance imaging (MRI), and fluorescence trimodal imaging.