Date Published: January 18, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Xiaochen Qiu, Xingjun Zhu, Xianlong Su, Ming Xu, Wei Yuan, Qingyun Liu, Meng Xue, Yawei Liu, Wei Feng, Fuyou Li.
Recently, upconversion luminescence (UCL) has been widely applied in bioimaging due to its low autofluorescence and high contrast. However, a relatively high power density is still needed in conventional UCL bioimaging. In the present study, an ultralow power density light, as low as 0.06 mW cm−2, is applied as an excitation source for UCL bioimaging with PbS/CdS/ZnS quantum dots (UCL‐QDs) as probes. The speculated UCL mechanism is a phonon‐assisted single‐photon process, and the relative quantum yield is up to 4.6%. As determined by continuous irradiation with a 980 nm laser, the UCL‐QDs show excellent photostability. Furthermore, UCL‐QDs‐based probe is applied in tumor, blood vessel, and lymph node bioimaging excited with an eye‐safe low‐power light‐emitting diode light in a nude mouse with few heat effects.
Upconversion luminescence (UCL) can convert low‐energy photons, usually in the near‐infrared (NIR) region, into higher ones.1, 2 In the past decade, UCL has gained considerable attention and has been widely applied in bioimaging,3, 4, 5, 6 biodetection,7, 8 molecular diagnostics,9 and clinical therapeutics.10, 11, 12 In bioimaging, UCL shows great promise owing to its various merits such as low autofluorescence, deep tissue penetration, and little photodamage to living organisms. During the past decades, rare earth–doped nanoparticles and triplet–triplet annihilation‐based upconversion systems have been widely examined by many research groups for bioapplication.13 However, for rare earth–doped nanoparticles, due to the narrow band absorption cross section of the Yb3+ sensitizer14 and low UCL quantum yield,15 a relatively high power density excitation is still necessary, to ensure that a strong enough emission signal can be achieved for bioapplication. Triplet–triplet annihilation‐based UCL has a low power density excitation16; however, an organic solution system is required,17, 18, 19 and in aqueous solution, it is still challenging to fabricate a low power density light excited small‐size nanoprobe,20 which has limited its bioapplication in vivo.
In conclusion, we fabricated an ultralow power density 980 nm light excited UCL based on UCL‐QDs, which can even be excited by a power density (0.06 mW cm−2) lower than sunlight. Our results demonstrate that UCL emission may be achieved through a phonon‐assisted single‐photon process. It should be noted that PEG‐modified UCL‐QDs show little biotoxicity and excellent biocompatibility. Importantly, the quantum yield of photostable UCL‐QDs is approximately 4.6%, and is unrelated to the excitation power density. Moreover, tumor, blood vessel, and lymph node UCL imaging in nude mouse was obtained following excitation with a low power density eye‐safe 980 nm LED light (≈20 mW cm−12), which has only a slight heating effect. We believe that the development of these UCL‐QDs will open a new door for UCL bioimaging in the future.
Materials and Characterization: Lead(II) oxide, cadmium oxide, zinc oxide, and sulfur were obtained from Shanghai Macklin Biochemical Co., Ltd. Octadecene (ODE; technical grade, 90%), oleic acid (OA; technical grade, 90%), and bis(trimethylsilyl)sulfide ((TMS)2S) (synthesis grade) were purchased from Sigma Aldrich. Ethanol anhydrous (99.5%) was bought from Adamas‐beta. Chloroform, toluene, and acetone were brought from Sinopharm Chemical Reagent Co., China. DSPE‐PEG‐2000 was obtained from Shanghai Ponsure Biological Technology Co., Ltd. All chemicals were used without further purification.
The authors declare no conflict of interest.