A simple, rapid, and specific method for Hg(II) detection has been proposed based on the fluorescence change of N-acetyl-L-cysteine-capped CdTe quantum dots (QDs). The presence of Hg(II) ions could quench the fluorescence of QDs at 565？nm and meanwhile produce new peak in 700–860？nm wavelength range. The linear response range is 20–430？nM with the detection limit at 8.0？nM Hg(II). It was found that the position of the new peak was irrelevant to the size of QDs. Furthermore, the mechanism of the quenching of QDs fluorescence by Hg(II) and the appearance of new peak in near-infrared area were also discussed and deduced through ultraviolet absorption spectrum, fluorescence spectrum, and X-ray photoelectron spectrum. 1. Introduction As a new class of potential fluorescence probes, quantum dots (QDs) have attracted great interests of the researchers because of their unique and excellent properties over traditional fluorescent dyes and fluorescent proteins [1–3]. Compared to conventional organic fluorescent dyes, QDs possess higher photoluminescence (PL), excellent quantum yield (QY), size-dependent tunable luminescence wavelength, wide continuous absorption, narrow fluorescence band, and better photostability. Over the past two decades, great efforts have been focused on the development of sensors [4–8] based on QDs, and the detection of metal ions is the active field. Some researchers have realized the specific detection of metal ions through modification of QDs with different surface-attached ligands [9–13], such as the detection of Cu2+ ions through thioglycerol-capped CdS QDs [9] and mercaptopropionic acid-coated core/shell CdTe/CdSe QDs [10], the detection of Zn2+ ions through L-cysteine-capped CdS QDs [9], the detection of Ag+ ions through thioglycolic acid-coated CdSe QDs [11], the detection of Cu2+ and Ag+ ions through peptide-coated CdS QDs [12], and the detection of Pb2+ ions through glutathione-capped ZnCdSe and CdTe QDs [13]. As one of the most toxic heavy metals and persistent contaminants which cannot be biodegraded in ecosystem [14, 15], mercuric ion (Hg2+) requires new and efficient detection methods. The major challenges in developing QDs-based Hg probe are the preparation of water-soluble QDs with high luminescence quantum yield and the selectivity of the system [16–19]. Herein, through hydrothermal route, a series of high-quality N-acetyl-L-cysteine- (NAC-) capped QDs with excellent water solubility, stability, and high QY (the average QY is 50%) have been synthesized [20–22]. Based on the prepared NAC-capped CdTe QDs as the fluorescence
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