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@SiO 2 ) via carbodiimide chemistry. The Q-dots form kind of “satellites” on the surface of the C-dot@SiO 2 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@SiO 2 @Q-dots hybrid spheres are shown to represent a ratiometric fluorescent probe for Cu 2+ in that the emission of the Q-dots is quenched by Cu 2+ , while the emission of the C-dots (which are coated with SiO 2 ) is not quenched. The ratio of the fluorescence intensities at 441 and 655 nm is related to the concentration of Cu 2+ in the range from 0.1 to 1.0 μM, with a 0.096 μM lower detection limit. The ratiometric probe was successfully applied to the determination of Cu 2+ in (spiked) vegetable and fruit samples by the standard addition method, and recoveries ranged from 96.7 to 100.8 %. Carbon dots were first coated with silica and then with CdTe quantum dots to give nanoparticles with an architecture of the type C-dot@SiO 2 @Q-dots. These are shown to be a viable ratiometric fluorescent probe for the detection of Cu(II).