Revisiting the SrAl₂O₄:Eu²⁺,Nd³⁺ persistent phosphor

SrAl₂O₄:Eu²⁺,Nd³⁺ is a well-known persistent phosphor due to its outstanding green persistent luminescence performance. In SrAl₂O₄:Eu²⁺,Nd³⁺, the Nd³⁺ ions are commonly considered as an auxiliary activator, even though they themselves are an excellent near-infrared luminescence center at 900-1400 nm. Moreover, though the SrAl₂O₄:Eu²⁺,Nd³⁺ phosphor was known to be effectively charged by ultraviolet and white light, the charging effectiveness of different wavelengths was never studied. Here, we revisit the SrAl₂O₄:Eu²⁺,Nd³⁺ persistent phosphor and report some new spectroscopic phenomena. Besides the long-lasting green persistent luminescence of Eu²⁺ (>20 h), the SrAl₂O₄:Eu²⁺,Nd³⁺ persistent phosphor also simultaneously emits long near-infrared persistent luminescence of Nd³⁺ (>4 h), due to efficient persistent energy transfer from Eu²⁺ to Nd³⁺. We acquire the first-ever persistent luminescence excitation spectrum on SrAl₂O₄:Eu²⁺,Nd³⁺ and surprisingly find that 365 nm ultraviolet light, which is often used to charge the SrAl₂O₄:Eu²⁺,Nd³⁺ phosphor, is actually least effective within the 250-470 nm wavelength range in producing persistent luminescence. Blue light at around 440 nm is the most suitable light to charge the phosphor. The charging mechanisms of different excitation wavelengths are studied through thermoluminescence measurements. In vitro bioimaging experiments suggest that the SrAl₂O₄:Eu²⁺,Nd³⁺ persistent phosphor is a promising near-infrared imaging probe for high-contrast deep-tissue bioimaging.