<p>Heavy metal ions (HMIs) pollution poses severe threats to ecological integrity and human health, necessitating the development of sensitive, cost-effective, and matrix-adaptable detection technologies. Carbon quantum dots (CQDs) and their composites have emerged as promising candidates for HMI detection, owing to their exceptional optical properties, high stability, and excellent biocompatibility. In this study, we report a facile, cost-effective, and green synthesis route for the fabrication of red-emitting (R-CQDs) and green-emitting (G-CQDs) carbon quantum dots. These materials were systematically evaluated for the sensitive and selective detection of Hg²⁺, Ag⁺, and Cu²⁺ ions in both aqueous and saliva samples, thereby addressing critical needs in environmental monitoring and non-invasive health assessment. The detection performance was quantified using both ultraviolet–visible (UV–Vis) absorption spectroscopy and fluorescence (FL) spectroscopy. UV–Vis spectroscopy measurements revealed a limit of detection (LOD) of 50&#xa0;nmol/L for Hg²⁺ at 235&#xa0;nm (using G-CQDs), 100&#xa0;nmol/L for Ag⁺ at 226&#xa0;nm (using G-CQDs), and 5&#xa0;nmol/L for Cu²⁺ at 215&#xa0;nm (using R-CQDs). Fluorescence spectroscopy further demonstrated LODs of 250&#xa0;nmol/L for Hg²⁺ and 5&#xa0;µmol/L for Ag⁺ (both at an excitation wavelength of 480&#xa0;nm using G-CQDs), and 0.5&#xa0;µmol/L for Cu²⁺ at an excitation wavelength of 427&#xa0;nm (using R-CQDs). Collectively, our findings demonstrate that the as-synthesized R-CQDs and G-CQDs exhibit high sensitivity and selectivity toward target HMIs. Their successful application in real water and saliva samples underscores their significant potential for practical environmental monitoring and personal health screening.</p>