Ultra-bright near-infrared-emitting HgS/ZnS core/shell nanocrystals for in vitro and in vivo imaging

Near-infrared (NIR)-emitting nanocrystals have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Mercury chalcogenide NCs are one of the attractive NCs with NIR emission; however, the potential toxicity of Hg restricts their diverse applications. Herein, we synthesized low-toxic, highly luminescent and stable GSH-capped HgS/ZnS core/shell NCs by an aqueous route for the first time. The core/shell structure was characterized by using TEM, XRD and XPS, which provide evidence for the shell growth. After the successful growth of an appropriate ZnS shell around HgS NCs, poorly luminescent HgS NCs converted into ultra-bright HgS/ZnS NCs, substantially increasing photoluminescence quantum yield up to 43.8% at room temperature. The fluorescence peak of HgS/ZnS NCs was successfully tuned in a wide NIR window ranging from 785 nm to 1060 nm with high emission efficiency by controlling the synthetic pH values. Significantly, an in vitro cytotoxicity study clearly demonstrated that the HgS/ZnS NCs exhibited good biocompatibility as evidenced by the cell viability retained above 80% at a dose of HgS/ZnS NCs up to 150 mg mL(-1). More importantly, the low-toxic NIR-emitting HgS/ZnS NCs have proved to be an effective fluorescent label in in vitro and in vivo imaging. The penetration depth reached 2 cm in a nude mouse with distinct separation of autofluorescence and NCs' fluorescence, giving excellent contrast at all depths. The novel highly-luminescent NIR-emitting HgS/ZnS NCs open up new possibilities for highly-sensitive, highly spectrally resolved and multicolor imaging in biomedical applications.