808 nm Excited Multicolor Upconversion Tuning through Energy Migration in Core–Shell–Shell Nanoarchitecture

NaGdF4: A (A = Eu, Tb)@NaGdF4: Yb, Tm@NaGdF4: Yb, Nd core–shell–shell is designed to achieve 808 nm excited upconversion emission tuning. On the basis of the above core–shell–shell nanostructure, intense upconversion emission has been realized for activators without long-lived intermediate states (Eu3+, Tb3+) through Gd3+-mediated energy migration under 808 nm irradiation, enriching the emission colors. The spatial separation, where sensitizer (Nd3+), accumulator (Tm3+), and activator (Eu3+, Tb3+) are doped into separated layers, effectively suppresses nonradiative decays so that the doping concentration of Nd3+ can reach to 40%, vastly enhancing the luminescence intensity. Notably, when Gd3+ ions are replaced by Nd3+ or inert Y3+ in NaGdF4: Yb, Nd outer shell, without Gd3+-mediated energy migration, the deleterious energy transfer from Tm3+ in the interlayer to surface quenchers is suppressed, and thus, more active energy is trapped by activators, which induces the further change of upconversion emission color. Furthermore, the multicolor upconversion tuning can also be realized via Tb3+-mediated energy migration. 808 nm excited multicolor upconversion tuning, overcoming low tissue penetration and overheating effect under 980 nm excitation, improves the feasibility of upconversion nanoparticles in multicolor imaging and multiplexed detection areas.