Carbon dots (CDs) with concentration-modulated luminescence were obtained by pyrolysis of citric acid (CA) and thiourea (TU). CDs exhibit multi-colored emission by varying their concentrations. As a contrast, Zr-doped CDs (ZrCDs) were prepared from CA, TU and ZrCl4 in a similar way. To our surprise, no concentration-dependent emission were observed in ZrCDs. Structural and optical characterizations prove that the concentration-tunable fluorescence is attributed to the aggregation between CDs. As the concentration increases, the aggregation of CDs results in a gradual decrease in the band gaps (from 2.18 eV to 1.56 eV), which ultimately causes the fluorescence to redshift continuously. However, ZrCDs are always in a highly dispersed state no matter how the concentration goes up, the zirconium complexes on the surface of ZrCDs prevents them from aggregation and maintains the band gap of ZrCDs at about 2.21 eV. Therefore, we proposed an aggregation-induced multicolor emission mechanism for CDs, which has a certain reference significance for exploiting the potential applications of CDs. (C) 2020 Elsevier Inc. All rights reserved.