Core-shell structured quantum dot (QD)-silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core-shell structured CdTe/CdS@SiO₂ @CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red-emissive CdTe/CdS QDs (630 nm) were used as a built-in reference signal and green-emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S²^- , owing to a S²^- -induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S²^- -quenched CdTe/CdS@SiO₂ @CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric 'off-on' fluorescent probe for the detection of Cd²⁺ . The sensing mechanism was suggested to be: the newly formed CdS-like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.

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