much-depressed photon absorption and scattering by tissues, which offers deeper tissue penetration and higher spatial and temporal resolution compared with the first NIR window (NIR-I) with wavelengths from 650 to 900 nm. [1][2][3][4][5][6][7][8] To achieve the optimum performance of in vivo imaging, NIR-II fluorophores with high quantum yield (QY) and excellent biocompatibility are necessary. Up to now, various NIR-II nanoprobes have been widely studied, including organic dyes, [1,[9][10][11][12][13][14][15][16][17] single-wall carbon nanotubes, [18][19][20] rare earth luminescent materials [21] and inorganic semiconductor quantum dots (QDs). [2,3,[22][23][24] Among these nanoprobes, QDs have stimulated extensive researches due to their small size, high QY, and facile surface functionalization. Therefore, NIR-II QDs has been a prominent fluorescent probe for bioimaging and has stimulated great attention. To date, some NIR-II QDs, such as Ag 2 S and Ag 2 Se, different from CdHgTe, [25,26] PbS, [27][28][29] and PbSe, [30,31] were reported for their good optical properties and excellent biocompatibility. [22,32] Silver telluride (Ag 2 Te) has a narrower direct bandgap of 0.06 eV [33] than that of Ag 2 S or Ag 2 Se and does not contain highly toxic heavy metal ions, promising it as an ideal nanoprobes to have a broader emissive spectrum in the NIR-II window for bioimaging. However, the poor fluorescence brightness and stability prevent its potential applications. Consequently, it is of great significance to develop Ag 2 Te-based nanoprobes with high QYs and stability in NIR-II window along with high biocompatibility.The dangling bond on the QDs surface is one of the crucial factors responsible for its low QY. An effective strategy to improve QY is to passivate the QDs surface by overgrowing a shell of a second semiconductor to construct a core-shell structure. [34] In this manner, the fluorescence intensity and stability of the QDs will gain vast enhancement. [35,36] Considering the crystalline structure, lattice parameters and the bandgap, as well as the biocompatibility of the shell materials, herein, Ag 2 S is selected as the shell to passivate Ag 2 Te QDs. The as-obtained Ag 2 Te@Ag 2 S QDs show superior photoluminescence properties in comparison with Ag 2 Te QDs itself.Ag 2 Te@Ag 2 S QDs were synthesized via a facile two-step method as shown in Figure 1a. First, Ag 2 Te QDs was prepared Fluorescence in the second near-infrared window (NIR-II, 900-1700 nm) has drawn great interest for bioimaging, owing to its high tissue penetration depth and high spatiotemporal resolution. NIR-II fluorophores with high photoluminescence quantum yield (PLQY) and stability along with high biocompatibility are urgently pursued. In this work, a Ag-rich Ag 2 Te quantum dots (QDs) surface with sulfur source is successfully engineered to prepare a larger bandgap of Ag 2 S shell to passivate the Ag 2 Te core via a facile colloidal route, which greatly enhances the PLQY of Ag 2 Te QDs and significantly improves the stability of Ag 2 Te ...

The water-soluble PEG-PATU Ag2S QDs have been successfully prepared by one-pot method, which display the favorable NIR-II fluorescence property for in vivo bioimaging.

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