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In the above equations, ? (nm) is the wavelength of the maximum absorption and A is the absorbency. Also, the units of parameters ? and l are nm and cm, respectively. To characterize the chemical structure of [email protected] QDs, the FT-IR spectra were recorded in the range 600-4000 cm-1. As presented in Fig. 2C, the obvious peaks at about 1583 cm-1 and 1389 cm-1 are attributed to asymmetric stretching vibration of COO- group in the as-prepared QDs. The strong vibrational modes at 2969-2850 cm-1 corresponds to the CH2 of the TGA indicating the presence of TGA molecules that are coated surface of the QDs. Meanwhile, disappearance of the S-H stretching vibration peak at 2565 cm-1 was the result of the covalent bonding between S and Cd atom on the QDs surfaces.
For the further characterization of [email protected] QDs, the UV–visible absorption spectroscopy and fluorescence spectroscopy were recorded. As presented in Fig. 2A, the [email protected] QDs shows a narrow PL emission spectrum centered at 547 nm (?ex = 352 nm). Also, the UV/vis spectrum shows that the QDs exhibit a considerable UV/vis absorption band at approximately 505 nm.
3.2. Investigation of the sensing mechanism of Hg2+
A schematic of the turn on-off fluorescent sensor for detecting of Hg2+ based on the was shown in Scheme 1, 2. As displayed in scheme 1, the [email protected] QDs nanostructure had an obvious fluorescence emission (curve a) in pH 6.00 PBS. In the following, in the presence of Hg2+ the fluorescence signal decreased gradually, due to the efficient quenching effect of Hg2+ (curve b). This fact was owing to that Hg2+ formed a stable complex with DDT ligand 26, that leading to facilitating nonradiative electron/hole recombination annihilation through an effective electron or energy transfer process (scheme 2).

Hg2+ + 2 RS ?

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