Abstract: Water-soluble luminescent nitrogen-doped CDs (N-CDs) having a high QY of 29.83% were prepared by a one-step microwave-assisted pyrolysis method using ammonium citrate and triethylenetetramine as the precursors. The microstructure, optical properties and biocompatibility of the N-CDs were investigated. Results indicate that they exhibit a strong fluorescence, favorable biocompatibility and excellent optical stability. Their photoluminescence is quenched by Cu²⁺ ions through the formation of a complex between the N-CDs and Cu²⁺, which can be used for Cu²⁺ detection. Compared with other CD-based nanomaterials, the N-CDs show outstanding optical properties, and excellent selectivity and sensitivity for Cu²⁺ detection with a linear range of 0.01-11 μM and a detection limit of 4.5 nM. The selectivity for Cu²⁺ detection is so good that the N-CDs have been successfully used in Cu²⁺ detection for real water samples. They have also been used to draw fluorescent patterns and image living cells.