Regulating the color output and simultaneously enhancing the intensity of upconversion nanoparticles via a dye sensitization strategy

Abstract

Color-modulated upconversion nanoparticles (UCNPs) have garnered widespread optical applications owing to their superior properties, including low auto-fluorescence background noise, large anti-Stokes shift, and distinguishable sharp emission bands. Recent success in multicolor alteration of UCNPs has mostly relied on the cross-relaxation between activators and other elements, which allows them to perform excellently in optical encoding and bioimaging. However, the color modulation is subject to the cross-relaxation strategy, which heavily weakens the original emission intensity of the activators. Herein, we report a dye-sensitization strategy of conjugating strong near-infrared absorptive dyes on the surface of UCNPs. It was proven that dye sensitization can manipulate the red to green (R/G) emission ratio of NaYF₄:Yb/Er@NaYF₄:Nd UCNPs and simultaneously enhance their luminescence intensity. The R/G emission ratio can be tuned to increase or decrease, benefiting from the dye sensitization strategy. Proof of concept was demonstrated for anti-counterfeiting application under orthogonal excitation of 808 nm and 980 nm.