COMMUNICATION 1703296 (1 of 7) www.small-journal.com small NANO MICRO © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Ultrasmall Pb:Ag 2 S Quantum Dots with Uniform Particle Size and Bright Tunable Fluorescence in the NIR-II Window He He, Yi Lin, Zhi-Quan Tian, Dong-Liang Zhu, Zhi-Ling Zhang, and Dai-Wen Pang* Dr. H. He, Dr. Y. Lin, Dr. Z.-Q. Tian, D.-L. Zhu, Prof. Z.-L. Zhang, Prof. D.-W. Pang Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education) College of Chemistry and Molecular Sciences State Key Laboratory of Virology The Institute for Advanced Studies Wuhan Institute of Biotechnology Wuhan University Wuhan 430072, P. R. China E-mail: dwpang@whu.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201703296. DOI: 10.1002/smll.201703296 is tuning the emission wavelength in a controllable way, which is critically important to multicolor imaging. There- fore, preparing NIR-II Ag 2 S QDs with high photoluminescence quantum yields (PLQYs) and expediently tuning their emission is urgent to develop advanced in vivo imaging methods and better serve both fundamental research and clinical practices. Although various synthetic approaches had been applied to prepare Ag 2 S QDs, including tuning particle size, engineering surface ligands, coating shells etc., none of these Ag 2 S QDs had a PLQY that could compete with traditional Cd-, Pb-based QDs. [2,8–15] The high Ag + ions’ mobility in Ag 2 S crystal lattice, [16,17] which causes abundant cation vacancies and crystal defects, is detrimental to photolumines- cence effciency of QDs [18] and hard to be solved by routine synthetic methods. Seeking a technique to effectively reduce this intrinsic drawback of Ag 2 S is essential to produce high-quality Ag 2 S nanocrystals and improve their optical performance. Based on quantum size effect, emission bandgap of intrinsic QDs has a “size-dependent” property, [19] and tuning emission wavelength of QDs is usually fulflled by controlling their size. [20] Hence, NIR-II QDs usually have a relatively large par- ticle size, which is undesired in biocirculations. [21] Moreover, particle size of QDs frequently runs out of control in synthesis when highly reactive precursors are utilized to facilitate the nucleation and crystallization of nanocrystals. This dilemma is common in the synthesis of I–VI group QDs. For example, the Ag 2 Se QDs with tunable emission from 1080 to 1330 nm reported by Zhu et al. had very low PLQYs; [22] in their later work, they replaced the Se-precursor TOPSe with a higher reac- tive bis(trimethylsilyl)selenide [(TMS) 2 Se], and obtained the Ag 2 Se QDs with a markedly improved PLQY (10.2%), while the emission wavelength of this Ag 2 Se QDs was nontunable. [23] Therefore, to develop a “non-size-dependent” method to tuning the emission bandgap is critical for preparing ultrasmall and emission tunable NIR-II Ag 2 S QDs. Intentionally introducing impurities into host materials (doping) is a fundamental approach to modify properties of host materials and has been intensively applied in prepara- tion of quantum dots to achieve enhanced or novel properties, including electronic, optic, magnetic, photovoltaic, and Ag 2 S quantum dots (QDs) are well-known near-infrared fluorophores and have attracted great interest in biomedical labeling and imaging in the past years. However, their photoluminescence efficiency is hard to compete with Cd-, Pb-based QDs. The high Ag + mobility in Ag 2 S crystal, which causes plenty of cation deficiency and crystal defects, may be responsible mainly for the low photoluminescence quantum yield (PLQY) of Ag 2 S QDs. Herein, a cation- doping strategy is presented via introducing a certain dosage of transition metal Pb 2+ ions into Ag 2 S nanocrystals to mitigate this intrinsic shortcoming. The Pb-doped Ag 2 S QDs (designated as Pb:Ag 2 S QDs) present a renovated crystal structure and significantly enhanced optical performance. Moreover, by simply adjusting the levels of Pb doping in the doped nanocrystals, Pb:Ag 2 S QDs with bright emission (PLQY up to 30.2%) from 975 to 1242 nm can be prepared without altering the ultrasmall particle size (≈2.7–2.8 nm). Evidently, this cation-doping strategy facilitates both the renovation of crystal structure of Ag 2 S QDs and modulation of their optical properties. In Vivo Imaging Narrow bandgap semiconductor Ag 2 S quantum dots (QDs) have attracted arising interest in in vivo fuorescence imaging due to their decent biocompatibility and the potential to emit in the second near-infrared window (NIR-II, 1000–1700 nm), [1–4] where body autofuorescence and photon scattering are mini- mized, leading to a greatly improved imaging resolution (micrometer-scale) and light penetration depth (up to several millimeters). [5–7] However, NIR-II fuorescence imaging is a new feld of research, and lacking of high-quality fuorescence probes severely restricts its development. Although Ag 2 S QDs have multiple advantages in in vivo imaging, their low photo- luminescence effciency is still a big obstacle on their way to various applications. Another challenge in the synthesis Small 2018, 1703296