Silica-coated carbon dots conjugated to CdTe quantum dots: a ratiometric fluorescent probe for copper(II)

The surface of CdTe quantum dots (Q-dots) was modified with thioglycolic acid (TGA) and these Q-dots were then covalently conjugated to amino-functionalized silica-coated carbon dots (C-dot@SiO2) via carbodiimide chemistry. The Q-dots form kind of "satellites" on the surface of the C-dot@SiO2 nanoparticles. The nanoparticle conjugates display dual emission (with peaks at 441 nm and 605 nm) under UV excitation and were further characterized by transmission electron microscopy, UV-vis absorption and FTIR spectroscopy. The C-dot@SiO2@Q-dots hybrid spheres are shown to represent a ratiometric fluorescent probe for Cu2+ in that the emission of the Q-dots is quenched by Cu2+, while the emission of the C-dots (which are coated with SiO2) is not quenched. The ratio of the fluorescence intensities at 441 and 655 nm is related to the concentration of Cu2+ in the range from 0.1 to 1.0 mu M, with a 0.096 mu M lower detection limit. The ratiometric probe was successfully applied to the determination of Cu2+ in (spiked) vegetable and fruit samples by the standard addition method, and recoveries ranged from 96.7 to 100.8 %.