Europium(III) Coordination-Mediated Self-Assembly of N-Doped Carbon Dot Nanoclusters for Ratiometric Detection of Tetracycline

Effective control of carbon dot (CD) self-assembly into larger nanoclusters in the liquid state for ultrasensitive detection is challenging and still in its infancy. For the first time, a europium(III) coordination-mediated strategy for self-assembly of N-doped carbon dots into large nanoclusters (N, Eu³⁺-CD nanoclusters) in aqueous solution was developed. It was found that the sizes and photoluminescence (PL) properties of nanoclusters could be effectively tuned by adjusting the Eu³⁺ content. The larger clusters with relatively longer fluorescence lifetimes were obtained by increasing the Eu³⁺ content. The simultaneous coordination of multi-CD particles with a Eu³⁺ and a dot particle with multiple Eu³⁺ was the key to inducing the self-assembly of CDs and regulating their optical properties. Furthermore, a dual-emitting N, Eu³⁺-CD nanocluster-based probe was constructed for the fluorescence ratiometric detection of tetracycline (TC). Once introducing TC, the blue emission of CDs was quenched through an IFE effect, but red emission of Eu³⁺ was significantly sensitized through an AE effect. Taking advantage of aggregation of CDs in aqueous solution, the N, Eu³⁺-CD nanocluster possessed a high sensitivity for detection of TC due to the spatial proximity of a large number of sensing units in the aggregated nanocluster assembly. The detection limit for TC was low to 35 nM. This work will provide a new insight into tuning the self-assembly of CDs and to regulate the photoluminescence and sensing performance of CDs.