Detection of copper ions is important for environmental protection and human healthy. In this study, a ratiometric fluorescent probe was developed by using silica nanoparticles capped carbon dots (CDs) as the core and red quantum dots (CdTe) as the shell linking onto the surface of silica. Cu(II) was found to quench the fluorescence of red-emitting CdTe on the shell, while not changing the fluorescence of the core blue-emitting CDs embedding silica nanoparticles. Based on multiple spectroscopic studies, we propose that selective binding of Cu(II) to CdTe of CDs@SiO2-CdTe via a photoinduced electron transfer (PET) process. The as-proposed sensor allows selective determination of Cu(II) in the range from 0.82 µM to 25 µM, with a detection limit of 0.053 µM.