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Graphical abstract Highlights • The dual-emission carbon dots were directly prepared by carbonizing glutathione in the water/formamide mixture. • The carbon dots exhibited dual emission peaks at 470 nm and 655 nm with excitation of 400 nm. • Fe3+ can be detected based on the fluorescence quenching at 655 nm of DCDs in acidic environments. • Zn2+ can be detected based on the fluorescence quenching at 470 nm of DCDs in alkaline environments. Abstract Herein, we developed a facile route for the preparation of dual-emission carbon dots (DCDs) by carbonizing glutathione in the water/formamide mixture. The synthesized DCDs displayed dual emission band respectively at 470 nm and 655 nm under the single-wavelength excitation at 400 nm, which can be used for the detection iron ions (Fe3+) and zinc ions (Zn2+) in different pH environments. An obvious fluorescence quenching around 655 nm was detected with the addition of Fe3+ in acidic environments, and a linear range of 2.5–30 μmol·L−1 and a detection limit of 0.8 μmol L−1 for Fe3+ were achieved. Moreover, another fluorescence quenching at 470 nm was also observed with the addition of Zn2+ in alkaline environments, and a linear range of 2.5–50 μmol L−1 and a detection limit of 1.2 μmol L−1 for Zn2+ were achieved. The prepared DCDs was used as a ratiometric fluorescent probe for determination of Fe3+ and Zn2+ ions in serum samples. [ABSTRACT FROM AUTHOR]