The synthesis of water-soluble and low cytotoxicity quantum dots (QDs) in aqueous solution has received much attention recently. A one-step, rapid, cost-efficient and convenient method has been developed for synthesis of water-soluble CdTe QDs using N-acetyl-L-cysteine (NAC) as the stabilizer via hydrothermal route. NAC not only acts as a stabilizer but also undergoes gradual thermal decomposition to release free S-2(-), forming a CdS shell on the CdTe core, and resulting in low cytotoxicity. The optical properties of QDs are characterized by absorption and fluorescence (FL) spectra. The structure of QDs was verified by TEM, XRD, XPS, EDS, and FTIR. The formation mechanism of core/shell QDs is also discussed in detail. Furthermore, the result of microcalorimetric technique showed our as-prepared CdTe QDs exhibit lower cytotoxicity than previous synthesized QDs (e.g., MPA-capped CdTe QDs). (C) 2013 Elsevier B.V. All rights reserved.