| Summary: | Dual-emission nanosensor for Hg<sup>2+</sup> detection was prepared by coupling CA-AEAPMS on the surface of RBS-doped modified silica microspheres. The CA-AEAPMS was synthesized by using <i>N</i>-(β-aminoethyl)-γ-aminopropyl methyldimethoxysilane (AEAPMS) and citric acid as the main raw material. The obtained nanosensor showed characteristic fluorescence emissions of Rhodamine B (red) and CA-AEAPMS (blue) under a single excitation wavelength (360 nm). Upon binding to Hg<sup>2+</sup>, only the fluorescence of CA-AEAPMS was quenched, resulting in the ratiometric fluorescence response of the dual-emission silica microspheres. This ratiometric nanosensor exhibited good selectivity to Hg<sup>2+</sup> over other metal ions, because of the amide groups on the surface of CA-AEAPMS serving as the Hg<sup>2+</sup> recognition sites. The ratio of F<sub>450</sub>/F<sub>580</sub> linearly decreased with the increasing of Hg<sup>2+</sup> concentration in the range of 0 to 3 × 10<sup>−6</sup> M, and a detection limit was as low as 97 nM was achieved. Then, the addition of three thiol-containing amino acids (Cys, Hcy, GSH) to the quenched fluorescence solution with Hg<sup>2+</sup> can restore the fluorescence, and the detection limits of the three biothiols (Cys, Hcy, GSH) are 0.133 μM, 0.086 μM, and 0.123 μM, respectively.
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