High quantum yield carbon quantum dots as selective fluorescent turn-off probes for dual detection of Fe2+/Fe3+ ions

Abstract

Carbon quantum dots (CQDs) are fluorescent nanoparticles that have great potential for use as nontoxic fluorescent probes. However, synthesizing CQDs with high photoluminescence quantum yield (QY) in a rapid and facile way from commonly available and low-cost reagents is still challenging. In this work, Tween®80 was used as a surface passivation agent along with citric acid/urea in a one-step microwave procedure to make extremely blue luminous nitrogen doped CQDs. When excited at 350 nm, these CQDs had a fluorescent quantum yield of 75.5 %. The significant high quantum yield and stability of these CQDs are attributed to the high aromatic sp2 domain size within the CQD core and improved carbon and oxygen functional groups at the CQDs surface. We employed the CQDs as a fluorescent probe to detect Fe2+ and Fe3+ ions and could establish a linear relationship between photoluminescence quenching efficiency (F0-F/F0) and ion concentration with a detection limit of 6.5 µM and 2.5 µM for Fe2+ and Fe3+, respectively. Our studies suggest that the highly selective detection of iron ions is a result of partial transfers of excited electrons from the CQDs to the d orbital of the iron ions, instead of contributing to radiative relaxation that induces photoluminescence quenching of the CQDs. Other reported carbon dots for this application have a substantially lower quantum yield and only detect Fe3+ ions. Therefore, our CQDs have considerable benefits over similar probes. Subsequent research may adapt them to other areas due to the high quantum yield and stability.