Plasmonic‐Enhanced NIR‐II Downconversion Fluorescence beyond 1500 nm from Core–Shell–Shell Lanthanide Nanoparticles

Abstract This paper reports on the light amplification of NaGdF₄:Yb,Er,Ce@NaGdF₄:Yb,Nd@NaGdF₄ core–shell–shell downconversion nanoparticles (CSS‐DCNPs) in the near‐infrared second biological window (NIR‐II: 1000–1700 nm) by plasmonic nanostructures. Through a precisely controlled plasmonic metallic nanostructure, fluorescence from Yb³⁺ induced 1000 nm emission, Nd³⁺ induced 1060 nm emission, and Er³⁺ induced 1527 nm emission are enhanced 1.6‐fold, 1.7‐fold, and 2.2‐fold, respectively, under an 808 nm laser excitation for the CSS‐DCNPs coupled with a gold hole‐cap nanoarray (Au‐HCNA), while the Er³⁺ induced 1527 nm emission under a 980 nm laser excitation is enhanced up to 6‐fold. To gain insight into the enhancement mechanism, the plasmonic modulation of Er³⁺ induced NIR‐II emission at 1550 nm under 980 nm excitation is studied by FDTD simulation and lifetime measurements, showing the observed fluorescence enhancement can be attributed to a combination of enhanced excitation and an increased radiative decay rate.