Turn-Off Fluorometric Detection of Co²⁺ by Polyethylenimine-Passivated Carbon Nanoprobes: Mechanistic Insights through ITC

The development of sensitive and selective probes for cobalt ion (Co²⁺) detection is critical, given the increasing environmental burden of heavy metal contamination from industrial discharge and its consequences on lifeforms. To this end, we present polyethylenimine-passivated carbon dots (PEI-MCDs), synthesized via a rapid microwave-assisted pyrolysis of <i>Moringa oleifera</i> leaves, as efficient fluorescent nanoprobes for Co²⁺ detection. Comprehensive spectroscopic and microscopic analyses confirmed that the PEI-MCDs are water-soluble and exhibit excitation-dependent emission. The nanoprobes exhibited a wide linear detection range (20-180 μM) and a low detection limit of 1.1 μM for Co²⁺ detection, with high accuracy in real water samples. Further investigation of the selective affinity of PEI-MCDs to Co²⁺ ions with isothermal titration calorimetry (ITC) revealed van der Waals force-driven strong and specific exothermic interactions at the interface. Complementary time-resolved fluorescence studies indicated that Förster resonance energy transfer (FRET) contributes to the observed fluorescence quenching upon Co²⁺ binding, offering further insight into the energy transfer dynamics at the nanoprobe interface. Together, these findings offer a comprehensive mechanistic understanding of the Co²⁺-nanoprobe interaction, establishing PEI-MCDs as a promising sustainable platform for reliable, real-time fluorometric detection of cobalt ions in complex water bodies.