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[Display omitted] • Highly fluorescent N, B-doped CNDs discerning interactions with Fe(II) and Fe(III). • The "urn-Off" optical probe relies on a concentration-dependent fluorescence decrease. • The N, B-doped CNDs are selective towards Fe(II) ions in the presence of other metals. • The CNDs can monitor Fe(II) presence in living cells by fluorescent imaging. The interactions between metal ions and fluorescent probes are fundamentals for discerning the metal ions. Co-doped carbon nanodots (CNDs) have demonstrated the capacity to optically detect a wide variety of targets, including metal ions. In this work, the quenching of highly fluorescent, nitrogen (N) and boron (B) co-doped CNDs by Fe(II) ions was discovered and employed to differentiate Fe2+ in water samples and live cells. The fluorescence of the N,B-CNDs was quenched in a concentration-dependent manner with a need of very short reaction time (30 s), and the CNDs show good selectivity towards Fe(II) ions relative to other metal ions including Fe(III). The limit of detection (LOD) was calculated as 1.8 nM and the linear response range was 7 nM to 1 μM. The recovery percentage was determined to be 97%, 101%, and 103% for spiked Fe(II) concentrations of 31, 125, and 1000 nM, respectively, in tap water. The N,B-CNDs were able to monitor the presence of Fe(II) in living cells using the optical imaging of Fe(II) ions, and the quenching in cells was completed within one hour. Regarding the quenching mechanism, the photoinduced electron transfer (PET) procedure is proposed with support of cyclic voltammetry study on the CNDs upon addition of Fe(II). Overall, the N,B-CNDs promise as optical probes for the detection of Fe2+ in water and living cells, and demonstrated a satisfactory level of sensitivity and selectivity. [ABSTRACT FROM AUTHOR]