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In this study, a facile formulation of near-infrared (NIR)-emitting CdHgTe/CdS/CdZnS quantum dots (QDs) was fabricated and coated with three different thiol ligands, N-acetyl-L-cysteine (NAC), 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA). The optical properties, morphology, hydrodynamic diameters, structure and surface chemistries of the developed QDs were comprehensively assessed. As a therapeutic drug, NAC provides excellent biocompatibility and anti-oxidant activity both in vitro and in vivo. Given their core–shell–shell structure, the QDs induced no toxicity in living bodies even at a concentration of 20 mg kg−1. Compared with NAC-capped QDs, TGA- and MPA-capped QDs demonstrated poor tumor-targeting capabilities because of their larger hydrodynamic diameters and inferior biocompatibility. Furthermore, the use of different stabilizing ligands resulted in various major changes in organ biodistribution and clearance from the body. In terms of rapid metabolization through renal clearance, the NAC-capped core–shell–shell QDs will be an alternative probe in cellular metabolism studies and image-guided surgery for tumor targeting and removal.

In this article, highly photoluminescent near-infrared (NIR)-emitting quantum dots (QDs) were directly synthesized in water by a fast, inexpensive and facile method. Unlike the conventional technique, biocompatible N-acetyl-L-cysteine (NAC) capped CdHgTe/CdS core/shell QDs were successfully synthesized through a one-pot sequential growth of the core and shell in a controlled manner. An additional ZnS shell was deposited around the outer layer of CdHgTe/CdS QDs to render the core/shell/shell QDs less sensitive to environmental changes and less toxic to living bodies. The experimental results from high resolution transmission electron microscopy and X-ray powder diffraction indicated the as-prepared QDs have a very small diameter and good monodispersity. Furthermore, the CdHgTe/CdS/ZnS core/shell/shell QDs were successfully applied for in vitro imaging, and the results demonstrated that the biocompatible NIR QDs exhibited highly spectrally and spatially resolved imaging. Moreover, the NAC capped QDs were directly used for in vivo tumor targeting of nude mice by passive targeting. The tumor was identified distinctly by the ultra-small QDs within only 2 h of blood circulation. Through the leakiness of the tumor vasculature, the QDs showed excellent accumulation in the tumor region for as long as 4 h. The results of acute toxicity tests showed that the biocompatible NIR QDs caused negligible harmful effects to the nude mice at experimental dosages, indicating that the high fluorescence probes can be highly effective in long-term diagnostics and therapy in in vivo observation.