Hydrogen-Bonded Molecular Clusters Transform into Surface-Passivated Fluorophores in Carbon Nanodots: Mechanistic Insight and Sensing Application Toward Bilirubin and Cu²⁺ Ions

The present study aims to explore the origin of fluorescence in nitrogen-doped carbon dots (NCDs) and the impact of their distinct molecular domain characteristics on sensing applications where bilirubin (BR) and Cu²⁺ have been chosen as model analytes. The transformation of H-bonded molecular clusters of fluorophores [citrazinic acid and 4-hydroxy-1H-pyrrolo[3,4-c] pyridine-1,3,6(2H, 5H)-trione (HPPT)] in dual emissive (blue/green) NCD1 into amorphous single emissive (green, HPPT dominated) carbon nanodots, NCD2, synthesized by thermal pyrolysis of citric acid and urea at the two optimized temperatures of 140°C and 240°C, respectively, was observed. Depending on the synthesis temperature, the nature and population of molecular constituents of NCDs can be altered, revealed through time-resolved and steady-state optical spectroscopy, HRTEM, XPS and Raman analysis. Furthermore, these NCDs were used as fluorescent probes for the detection of BR and Cu²⁺ ions in aqueous solution. Cu²⁺ detection, monitored through fluorescence quenching of the NCDs, occurred via electron transfer between the NCDs (donor) and Cu²⁺ (acceptor), with limit of detection differences attributed to the distinct molecular compositions of NCD1 and NCD2. On the contrary, BR sensing was found to occur via ground-state complex formation for NCD1 (evidenced by isosbestic points) and through the inner filter effect for NCD2.