Open system synthesis of narrow-bandwidth red-fluorescent carbon quantum dots with a function of multi-metal ion sensing

Carbon quantum dots (CQDs) are environmentally friendly phosphors. However, there have been few reports on red-emitting CQDs with narrow-bandwidths, all of which were synthesized via solvothermal methods using autoclaves. Here, red-emitting CQDs were synthesized from 4,6-diaminoresorcinol dihydrochloride (DAR), which is an aromatic compound with electron-donating hydroxy and amino groups. The synthesis involved heating at 180 °C for 6 h in a high-boiling-point 1,2-pentanediol solvent in an open system without using autoclaves and catalysts. The DAR-derived-CQDs in ethanol exhibited red photoluminescence with a 0.14 eV full width at half-maximum that was comparable to that of narrow-bandwidth blue-fluorescent phloroglucinol (PG)-derived-CQDs. The red emission was attributed to a weakened quantum-size effect and electron-donating NH₂ groups. The hydroxy groups of PG-derived-CQDs interacted with Al³⁺ and Fe³⁺ ions, resulting in fluorescence quenching. DAR-derived-CQDs have hydroxy and amino groups, which facilitate the fluorescence quenching for Al³⁺, Fe³⁺, Cu²⁺, and Pb²⁺ ions.