Photoluminescent semiconductor quantum dots (QDs) are a new kind of contrast agent employed in bio-imaging due to their outstanding optical properties. Copper indium sulfide QDs (CIS QDs) are particularly attractive as biolabels since they have tunable emission from the visible to near-infrared spectral range. In particular, their near-infrared emission has drawn much attention because this wavelength range penetrates deeper in tissues than visible light ^[1]. Additionally, CIS QDs do not contain heavy metal elements such as Pb and Cd, and are therefore potentially less toxic. Unfortunately, the surface of CIS QDs contain many defects which trap photoexcited carriers leading to low (＜5%) photoluminescence quantum yields (PLQY). Moreover, the stability of CIS QDs under ambient conditions is unsatisfactory. Therefore, surface modification of the bare CIS QDs is necessary to allow their application as bioimaging labels.

Here, we developed a synthesis protocol for water-soluble CIS/ZnS QDs. The emission wavelength of the CIS QDs that were used as cores can be tuned from 630nm to 825nm by controlling their size. After overcoating with ZnS, the PLQY and the stability of the QDs are significantly enhanced. This clearly shows that the ZnS shell effectively passivates non-radiative recombination centers, while confining the exciton in the QD core, thereby preventing quenching at surface defects and photochemical degradation. A number of different zinc compounds (zinc stearate, zinc diethyldithiocarbamate, zinc iodide, zinc acetate) were used as Zn precursors for the ZnS overcoating ^{[2, 3]}. Our results clearly show that the nature of the ZnS precursors have a dramatic impact on the optical properties and quality of final products. After overcoating, the PLQY of CIS/ZnS QDs were increased from blew 10% to 50% with a certain extent photoluminescence blue shift. This can be attributed to the different chemical reactivities of the Zn precursors and their impact on the degree of Zn diffusion into the core and shell growth. Finally, optimized CIS/ZnS QDs with near-infrared (NIR) emission are transferred into the water phase by Jeffamine modified amphiphilic polymer (Jeffamine@M-1000 polyetheramine) ^[4]. These NIR-emitting water-soluble CIS/ZnS QDs have high stability over a wide range of pHs, making them promising materials for bioimaging.