Second near-infrared upconversion and downconversion luminescence in a core–multishell nanophotoswitch

Abstract

The development of a NIR-II (near-infrared-II: 1000–1700 nm) nanophotoswitch is urgently needed, due to its deeper tissue penetration and higher resolution imaging. In this work, a new type of NIR-II upconversion (UC) and downconversion (DC) nanophotoswitch (NaErF₄:Ho@NaYF₄@NaYbF₄:Tm@NaYF₄ core–multishell nanoparticles) is presented. When the switch is open, ultraviolet (UV)–blue emissions and intense red UC (Er³⁺:654 nm) and NIR-II DC (Er³⁺:1520 nm) emissions are simultaneously obtained under 980 nm NIR-I (near-infrared-I: 700–1000 nm) excitation, which shows potential application for real-time visible- and NIR-II-imaging in the photodynamic therapy (PDT) process. When the switch is closed, UV–blue emissions disappear while red UC (Er³⁺:654 nm) and NIR-II UC (Ho³⁺:1154 nm) emissions remain under 1532 nm NIR-II excitation, which may be used for real-time visible- and NIR-II-imaging before conducting PDT. This novel NIR-II nanophotoswitch displays special advantages in the biomedical field.