Electron transfer quenching by nitroxide radicals of the fluorescence of carbon dots

Blue fluorescent carbon dots (CDs) were synthesized by a microwave assisted one-step procedure without surface passivation. The intermolecular interactions between the CDs and nitroxide radicals in aqueous and aprotic solution were investigated. The fluorescence of the CDs was found to be efficiently quenched by the paramagnetic nitroxide radical. A SOMO (singly occupied molecular orbital) facilitated electron transfer was proposed to account for the quenching of the fluorescence of the CDs in the CD@TEMPO conjugate, formed via electrostatic interactions between the negatively charged CDs and the cationic 4-amino-2,2,6,6-tetramethylpiperidine-N-oxide free radical (4-AT). Covalently spin-labeled CDs by the paramagnetic nitroxide radical yield a weakly fluorescent spin-labeled conjugate (CD–TEMPO), which exhibits sensitive fluorescence and electron spin resonance (ESR) bimodal response towards ascorbic acid (AA) at the μM level. The fluorescence intensity of the spin-labeled CDs was also found to be sensitive to paramagnetic factors and thus establishes its promising potential for constructing a bimodal responsive sensor with off–on fluorescence and on–off ESR signaling for the detection of antioxidants and carbon-centered radicals.