A simple and green route was developed for the first time to produce fluorescent graphitic carbon nitride (F-g-C₃N₄) by hydrothermal treatment of bulk g-C₃N₄. The produced F-g-C₃N₄ dots have blue emission and a high quantum yield, and were applied as a very effective fluorescent probe for label-free selective and sensitive detection of Cu(2+) and Fe(3+) ions; the limits of detection were as low as 0.5 nM and 1.0 nM, respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe(3+), Cu(2+) was exclusively detected in the presence of Fe(3+) ions. Cu(2+) and Fe(3+) ions in real water samples were also detected successfully. This exceptional fluorescent performance makes the probes based on F-g-C₃N₄ dots attractive for highly sensitive detection of Cu(2+) and Fe(3+) ions in real water.