A novel electrochemical strategy for simple and facile synthesis of semicarbazide functionalized nitrogen-doped graphene quantum dots (N-GQDs) was reported, based on direct exfoliation and oxidation from graphite rods. The average diameter of the as-synthesized N-GQDs is about 20 nm, and their dispersion is bright yellow due to the rich nitrogen and oxygen functional groups on their surface. The N-GQD dispersion was further applied in the selective detection of ferric ions (Fe³⁺) based on the photoluminescence (PL) quenching of N-GQDs after adding Fe³⁺. The fluorescent sensor has a wide linear range of 0–200 μM and a detection limit of 0.87 μM, which is much lower than the maximum level (0.3 mg L⁻¹, equivalent to 5.4 μM) of Fe³⁺ permitted in drinking water by the U.S. Environmental Protection Agency (EPA). Moreover, these novel N-GQDs exhibit much wider emission bands, which extend into the entire visible region, and emit three primary color fluorescence independently. This distinctive behavior of the as-prepared GQDs not only breaks the limitation that traditional reported GQDs only exhibit blue emission in the short-wavelength region, but may also provide a new research platform for further applications of GQDs in real environmental detection and biological imaging systems.