β-NaErF4:0.005Tm upconversion nanocrystals (UCNCs) with core–shell structure are quite popular due to efficient red upconversion luminescence. However, the traditional seed-mediated growth method is not continuous and quite time consuming, while the successive layer-by-layer strategy is unfriendly because of the usage of toxic sodium trifluoroacetate (Na-TFA). In this work, we utilized a novel one-pot strategy to synthesize β-NaErF4:0.005Tm with core–shell–shell structure. The morphology of the core could be significantly regulated. NaYbF4 and NaYF4 shells could be epitaxially grown on the core by simply adding fresh raw materials and heating to 300 °C, successively. Thanks to the suppression of surface quenching and energy back transfer from Yb3+ to Er3+, the population density of excited states of Er3+ (4I13/2, 4I11/2 and 4I9/2) was much improved, resulting in the tremendously enhanced upconversion intensity of β-NaErF4:0.005Tm@NaYbF4@NaYF4. This one-pot protocol could supply core–shell–shell red UCNCs with regular morphology, enhanced intensity and long-lasting decay lifetime, offering promising opportunities for applications such as bioprobing, anti-counterfeiting, photodynamic therapy, etc.
Jing Zuo,Langping Tu,Qiqing Li,Yansong Feng,Ivo Que,Youlin Zhang,Xiaomin Liu,Bin Xue,Luis J. Cruz,Yulei Chang,Hong Zhang,Xianggui Kong
Photoswitchable materials are important in broad applications. Recently appeared inorganic photoswitchable upconversion nanoparticles (PUCNPs) become a competitive candidate to surmount the widespread issue of the organic counterparts —photobleaching. However, current PUCNPs follow solely Yb3+/Nd3+ cosensitizing mode, which results in complex multilayer doping patterns and imperfectness of switching in UV–blue region. In this work, we have adopted a new strategy to construct Nd3+ free PUCNPs—NaErF4@NaYF4@NaYbF4:0.5%Tm@NaYF4. These PUCNPs demonstrate the superior property of photoswitching. A prominent UV–blue emission from Tm3+ is turned on upon 980 nm excitation, which can be completely turned off by 800 nm light. The quasi-monochromatic red upconversion emission upon 800 nm excitation—a distinct feature of undoping NaErF4 upconversion system—endows the PUCNPs with promising image-guided photoinduced “off–on” therapy in biomedicine. As a proof-of-concept we have demonstrated the imaging-guided photodynamic therapy (PDT) of cancer, where 800 nm excitation turns off the UV–blue emission and leaves the emission at 660 nm for imaging. Once the tumor site is targeted, excitation switching to 980 nm results in UV–blue emission and the red emission. The former is used to induce PDT, whereas the latter is to monitor the therapeutic process. Our study implies that this upconversion photoswitching material is suitable for real-time imaging and image-guided therapy under temporal and spatial control.
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