N-Doped carbon dots (CDs) were directly synthesized with a high yield of 21.85% by one-pot pyrolysis of a gel composed of ethanolamine and 1-carboxyethyl-3-methylimidazolium chloride at 220 °C for 2 h. The as-synthesized CDs with a mean particle size of 7.8 nm were uniform, amorphous, and abundant in nitrogen content (23.15 wt%) and surface groups such as amide and hydroxyl. Thus, the CDs exhibited good water-solubility, bright blue excitation- and pH-dependent fluorescence with a quantum yield of 17.93%, and high ionic strength tolerance. In addition, a CD-based fluorescent sensor towards Cr(vi) ions with favorable sensitivity and selectivity was constructed based on the inner filter effect. Two good linear relationships between the concentration of Cr(vi) ions and the PL quenching efficiency were obtained in the ranges from 0.2 to 2 (R2 = 0.9965) and 2-40 μM (R2 = 0.9918), and the limits of detection (LOD = 3σ/S) were calculated as 0.018 and 0.25 μM, respectively. Importantly, this sensor was solid and capable of rapidly detecting Cr(vi) ions in tap water with detection ranges of 0.2-2 (R2 = 0.9817) and 2-60 μM (R2 = 0.9902), LODs of 0.048 and 0.40 μM, and recoveries of 102.1-106%.