The green synthesis of blue fluorescent carbon nanodots (CNDs) from apricot kernel shells via a hydrothermal method was successfully executed. The interaction of the synthesized CNDs with various cations was systematically investigated using fluorescence spectroscopy. Fluorescence measurements were performed to evaluate the interaction of CNDs with 36 different cations, including Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mo⁶⁺, W⁶⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, B³⁺, Al³⁺, Tl⁺, As⁵⁺, Se²+, NH₄⁺, Au³⁺, Sb³⁺, Sn⁴⁺, Bi³⁺, Hg²⁺, Pd²⁺, and Pb²⁺. Among these, the CNDs exhibited exceptional selectivity and sensitivity as a fluorescent probe for the detection of Hg²⁺ ions. The working range for Hg²⁺ detection was established as 35–95 µM, with a detection limit of 14.0 µM and a quantification limit of 41.4 µM. The method was validated and successfully applied to tap water and river water, demonstrating the practical utility of CNDs derived from apricot seed shells for environmental monitoring and analytical applications.