Carbon nanodots (CNDs) were synthesized from hazelnut shells using the pyrolysis method in an oven at temperatures ranging from 250 to 400 °C and reaction times between 1 and 3 h. The structural and optical properties of the CNDs, which exhibited strong blue fluorescence under a UV lamp (365 nm), were characterized using UV-vis absorption spectroscopy, fluorescence spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The quantum yield of the CNDs was determined to be between 2.2% and 7.8%. The effect of various cations on the fluorescence spectra of the CNDs was investigated using fluorescence spectrometry. Among the synthesized CNDs, those prepared via pyrolysis for 3 h at 300 °C, 315 °C, and 350 °C (designated as HS300-3, HS315-3, and HS350-3, respectively) exhibited selective fluorescence quenching in the presence of Fe³⁺, Sn²⁺, and Pd²⁺. These CNDs were applied for the determination of Fe³⁺ in tap water and soil samples, while HS315-3 was also employed for the detection of Sn²⁺ and Pd²⁺ in tap water. The limits of detection (LOD) were 4.5 μg l⁻¹ for Fe³⁺ and 15.8 μg l⁻¹ for both Sn²⁺ and Pd²⁺. The accuracy of the methods was validated through spiked recovery experiments with tap water samples and by analyzing a certified reference material (CRM-SA-C Sandy Soil C).