[en] Highlights: • Core-shell colloidal GdVO₄:Eu³⁺@SiO₂ nanocrystals were fabricated. • Intense red emission from these nanocrystals could be selectively quenched by Cu²⁺. • The limit of detection for Cu²⁺ is as low as 80 nM in aqueous solution. • The detection ability is highly reversible by the addition of EDTA. • The luminescence quenching and recovery processes can be repeated. Nowadays, in view of health and safety demands, the controlled design of selective and sensitive sensors for Cu²⁺ detection is of considerable importance. Therefore, we construct herein core-shell colloidal GdVO₄:Eu³⁺@SiO₂ nanocrystals (NCs) as optical sensor for the detection of Cu²⁺, which were synthesized by a facile hydrothermal reaction and encapsulated with a uniform layer of ultrathin silica through a sol-gel strategy. The NCs present strong red emission due to energy transfer from VO₄³⁻ groups to Eu³⁺ when exciting with ultraviolet (UV) light. This intense red emission from Eu³⁺ could be selectively quenched in the presence of Cu²⁺ in comparison to other metal ions and the limit of detection is as low as 80 nM in aqueous solution. It is revealed that the spectral overlap between the emission band of NCs and the absorption of Cu²⁺ accounts for this intriguing luminescence behavior. The detection ability is highly reversible by the addition of ethylenediaminetetraacetic acid (EDTA) with the recovery of almost 100% of the original luminescence. The luminescence quenching and recovery processes can be performed repeatedly with good sensing ability. These remarkable performances allow the colloidal GdVO₄:Eu³⁺@SiO₂ NCs a promising fluorescence chemosensor for detecting Cu²⁺ ions in aqueous solution.