Achieving High Quantum Efficiency in Mn5+ Activated Phosphors for NIR-II Deep Bioimaging Application

Abstract

Mn⁵⁺ emission is a promising candidate for imaging deep tissue structures (e.g., vessels, tumors) in the second near-infrared (NIR-II, 1000–1350 nm) region. However, its practical application is impeded by the limited quantum efficiency of the available phosphors due to the unstable valence state of Mn⁵⁺. Herein, a novel strategy involving site competition is proposed to stabilize the Mn⁵⁺ state by the introduction of valence-unstable Bi^2+/3+. The results demonstrate that Bi³⁺ ions tend to occupy two different Ca²⁺ ion sites within the Ca₆Ba(PO₄)₄O lattice. The incorporation of a small amount of Bi³⁺ effectively suppresses the amount of Mn²⁺ in Ca²⁺ sites. This is also confirmed by spectroscopic experiments and density function theory calculations. Notably, an ultra-high internal quantum efficiency of 82.3% is achieved under excitation at 653 nm, surpassing more than twofold the previously reported value of 37.5% in Ca₆Ba(PO₄)₄O: Mn⁵⁺. As a proof of concept, deep tissue imaging with a penetration depth of ≈2.8 cm is achieved using a self-produced NIR-II light-emitting diodes device embedded with Ca₆Ba(PO₄)₄O: 0.003Mn⁵⁺/0.003Bi³⁺ powder. These findings provide valuable insights into improving the luminescent properties associated with Mn⁵⁺ ions and pave the way for deep tissue imaging with high spatiotemporal resolution.