Core-shell structured CdTe/CdS@SiO2 @CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection.

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.