Fabrication of mesoporous CdTe/ZnO @SiO ₂ core/shell nanostructures with tunable dual emission and ultrasensitive fluorescence response to metal ions

In this paper, we demonstrated the encapsulation of CdTe quantum dots (QDs) and ZnO nanorods (NRs) with a layer of mesoporous SiO ₂ shell (pore size: 4.1 nm) for the purpose of integrating dual-emission property into one common nanostructure. Within the core-shell CdTe/ZnO@SiO ₂ nanocom-posítes, CdTe QDs and ZnO NRs provide visible emission and UV emission, respectively. The fluorescence intensity ratio of the dual emission can be tuned by altering the hydrolysis time of tetraethyl orthosilicate (TEOS). The core-shell CdTe/ZnO@SiO2 nanocomposites exhibit very interesting photoluminescent behaviors after interactions with heavy-metal ions such as Hg ²⁺, Pb ²⁺, and Cu ²⁺. The visible emission contributed by CdTe QDs was abnormally enhanced in a range of 20-90% at the concentration of Hg ²⁺ < 10 ^-8 M, Pb ²⁺ < 10 ^-5 M, and Cu ²⁺ < 10 ^-6 M, whereas the intensity of UV emission by ZnO NRs was kept constant in all cases. Consequently, the UV emission may serve as a reference. The mechanism of the fluorescence enhancement is presumed to be the adsorption of the metal ions by the trap sites and the resulting surface passivation of the QDs. The mesoporous silica shell of the nanocomposites also plays a key role in the process of fluorescence enhancement, which helps to protect the surface characteristics of QDs, prevent the flocculation of the particles, and promote the adsorption of Cu ²⁺, Hg ²⁺, and Pb ²⁺ ions. The variable visible emission in combination with the constant UV emission indicates that the as-prepared CdTe/ZnO @SiO ₂ core/shell nanostructures may serve as novel biluminescent materials as well as reliable and sensitive fluorescence probes.