Sensitizer-rich core–shell–shell upconversion nanoparticles for enhancing luminescence by spatial separation†

Lanthanide-doped upconversion nanoparticles (UCNPs) play an important role in sensing. However, the application of UCNPs is hampered due to their generally low luminescence intensity. In this paper, the NaYbF₄@NaYF₄:2%Er,20%Gd@NaYF₄:20%Gd (CSS) was designed to enhance the luminescence intensity, and the luminescence intensity was about 2.5 times that of traditional NaYF₄:20%Yb,2%Er,20%Gd@NaYF₄:20%Gd (CS). The enhancement of luminescence intensity was mainly attributed to three factors: (1) a larger absorption cross-section provided by a sensitizer-rich core (100% Yb³⁺ doped), which was conducive to harvesting more near infrared light; (2) the spatial separation of the sensitizer and activator reduced harmful cross relaxation (CR); (3) the compact inert shell inhibited surface quenching. To further demonstrate the reasons for enhanced luminescence, the impact of different structures, the doping concentration of Yb³⁺ and Er³⁺, and the thickness of the inert shell on optical properties were studied. Besides, the CSS can still maintain strong UCL after being treated with NOBF₄ and dispersed in DMF. Based on this, a CSS-doped fibrous membrane (CDFM) with a promising hydrophobicity was fabricated and applied to detect single drop rhodamine B (RhB) taking advantage of the inner filter effect (IFE) with a low limit of 0.382 μM.