Enhancement of near-infrared emission through Gd3+ mediated energy transfer in fluoride nanoparticles under X-ray excitation

High energy X-rays have been widely employed in biological and medical fields because of its unlimited penetration depth in biological tissues and near-infrared (NIR) emission is considered as a preferred signal in bioimaging and biosensing. However, it remains a big challenge for nanoparticles to emit high intensity NIR light upon X-ray radiation due to the presence of defects and material's intrinsic low X-ray absorption ability. In this work, we prepare NaLuF4: Gd, Tm nanoparticles with different doping levels of Gd3+ and Tm3+ ions and systematically study the NIR emission properties under X-ray irradiation. NaLuF4: 20Gd, 1 Tm nanoparticles exhibits ∼3.5 times stronger radioluminescence at 800 nm as compared to solely Tm3+ doped nanoparticles. When X-ray induced electrons and holes recombine, Gd3+ ions serve as energy absorber and transfer the optical energy to Tm3+ ions. Excessive amount of Gd3+ ions will result in efficient energy flow to defects and apparent decreased NIR emission which can be confirmed by the lifetime curves of 3H4 states of Tm3+ ion. We further examine the X-ray-induced afterglow intensity of nanoparticles at 800 nm and demonstrate conceptually bioimaging with a deep penetration depth using the developed nanoparticles. The clear picture taken by a NIR camera indicates the NaLuF4: 20Gd, 1 Tm nanoparticles can be used a promising probe for X-ray luminescence optical imaging. •NaLuF4: Gd, Tm nanoparticles containing different amount of Gd3+ and Tm3+ ions were prepared.•Mechanism of enhanced near-infrared emission of Gd3+-doped fluoride nanoparticles upon X-ray excitation was clarified.•A clear deep penetration bioimaging was achieved by utilizing the X-ray-induced afterglow at 800 nm.