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Water-soluble and reductive carbon quantum dots (CQDs) were fabricated by the hydrothermal carbonization of chitosan. Acting as a reducing agent and stabilizer, the as-prepared CQDs were further used to synthesize gold nanoparticles (AuNPs). This synthetic process was carried out in aqueous solution, which was absolutely “green„. Furthermore, the CQDs/AuNPs composite was used to detect iodine ions by the colorimetric method. A color change from pink to colorless was observed with the constant addition of I^− ions, accompanied by a decrease in the absorbance of the CQDs/AuNPs composite. According to the absorbance change, a favorable linear relationship was obtained between ΔA and I^− concentration in the range of 20⁻140 μM and 140⁻400 μM. The detection limit of iodide ions, depending on the 3δ/slope, was estimated to be 2.3 μM, indicating high sensitivity to the determination of iodide. More importantly, it also showed good selectivity toward I^− over other anion ions, and was used for the analysis of salt samples. Moreover, TEM results indicated that I^− ions induced the aggregation of CQDs/AuNPs, resulting in changes in color and absorbance.