Green synthesis of multifunctional nitrogen-doped carbon quantum dots (N-CQDs) derived from citrus lemon juice: applications on antioxidant and antimicrobial activities and highly selective detection for Fe3+ ion sensing

In this research work, a facile, eco-friendly, and scalable one-pot microwave-assisted synthesis of nitrogen-doped carbon quantum dots (N-CQDs) has been reported using citrus lemon as renewable carbon. The as-synthesized N-CQDs were characterized for their optical properties using UV–Vis, Fluorescence, TCSPC, FTIR, and Raman spectroscopy, and for structural properties using, HR-TEM, FE-SEM coupled with EDX, XRD, XPS, and TGA. The as-fabricated N-CQDs were confirmed to be monodispersed spherical particles (~ 8.7 nm) with an amorphous carbon phase, as evidenced by TEM and FE-SEM analyses, while FTIR spectroscopy revealed the presence of –OH/–NH, C=O, C=N, C=C, and C−O functional groups on their surface. The synthesized N-CQDs exhibited high thermal stability, excellent biocompatibility, and a high quantum yield (33%), along with outstanding optical and chemical properties, and promising biological activities. They demonstrated remarkable free radical scavenging activity (EC50: 83.67 μg/mL) and effective antimicrobial properties. Furthermore, the N-CQDs showed excellent selectivity and sensitivity for Fe³⁺ ion detection, with a limit of detection (LOD) of 0.98 μM, distinguishing them among various metal ions. The fluorescence quenching mechanism was identified as dynamic quenching via electron transfer to Fe³⁺ ions, confirmed by fluorescence lifetime studies. Besides, the fluorescence of N-CQDs was significantly restored upon the addition of EDTA to the Fe³⁺- containing N-CQDs solution. Owing to their high photoluminescence, low toxicity, and metal ion selectivity, the synthesized N-CQDs offer a multifunctional platform for environmental sensing, bio-imaging, optoelectronics, and drug delivery.

Graphical abstract