Sigma-2 receptor-targeted functionalized carbon dots enable selective Fe ³⁺ detection and cellular imaging

Photoluminescent nitrogen doped carbon dots bearing a sigma-2 receptor pharmacophore (N-CD_ISQ) were synthesized and validated for <i>in vitro</i> biosensing of Fe³⁺. N-CD_ISQ exhibited intense blue photoluminescence at <i>λ</i> = 450 nm when excited at <i>λ</i> = 350 nm. N-CD_ISQ also exhibited selective fluorescence quenching by Fe³⁺ ions in the presence of anions and biomolecules. Quantitative analysis of photoluminescence quenching followed Stern-Volmer kinetics with a linear response for Fe³⁺ (5-90 μM). The detection limit was estimated to be 4.13 μM. Mechanistic studies confirmed static quenching and the involvement of interactions between Fe³⁺ and isoquinoline in the carbon dots. N-CD_ISQ demonstrated good biocompatibility, with cell viability above 80% after a 48-h treatment with concentrations up to 100 μM. Negligible erythrocyte rupture was observed in the haemolysis assay. Cellular imaging experiments demonstrated the bio-sensing capability of N-CD_ISQ to visualize Fe³⁺ ions within cells. The synthesized N-CD_ISQ represents a novel class of nitrogen-doped carbon dot-based fluorescent probes with dual functionality for quantitative Fe³⁺ analysis and cellular imaging.