Carbon dots as multifunctional fluorescent probe for Fe³⁺ sensing in ubiquitous water environments and living cells as well as lysine detection via "on-off-on" mechanism

Iron and amino acids are essential nutrients for living organisms, and their deficiency or excess can cause a range of diseases. Therefore, there is considerable interest in developing sensing assays capable of detecting these nutrients with sensitivity, selectivity, and multifunctionality even in complex environments. In this report, hydrothermally synthesized blue fluorescent carbon dots (C-dots) from zinc gluconate were utilized for the detection of Fe3+ and lysine via "on-off" and "on-off-on" mechanisms, respectively. Specifically, the Fe3+ sensing assay achieved a broad linear range of 0-200 mu M and a low limit of detection (LOD) of 1.9 mu M. It is worth mentioning that the assay was also well adapted to natural aqueous environments (e.g., lake water), and its linear detection range could be extended to 0-1000 mu M with a LOD of 3.3 mu M. Furthermore, the assay was also effective for intracellular Fe3+ tracking. Most importantly, the assay could also be applied for the quantitative detection of lysine with a linear range of 0-1200 mu M and LOD of 8.6 mu M. Systematic mechanistic studies revealed that Fe3+ sensing was based on a static quenching process between C-dots and Fe3+, whereas a stronger complexation might have formed between Fe3+ and Lys, leading to the release of C-dots and thus the recovery of fluorescence.