Engineering sensitive ratiometric probe based on Tb@BNCDs and thiolated-protected AuNCs for melamine sensing via competitive coordination mechanism

A novel ratiometric fluorescent probe was developed by combining terbium-doped boron/nitrogen co-doped carbon dots (Tb@BNCDs) with thiolated-protected gold nanoclusters (Th@Au NCs) for efficient melamine detection based on a competitive coordination mechanism. The designed system exploits the synergistic effects of aggregation-induced emission (AIE) and Förster resonance energy transfer (FRET) processes. The Tb@BNCDs, formed via coordination of Tb³⁺ ions with the amino and carboxyl groups on BNCDs, serve as both a stable fluorescence source and a coordination site for melamine interaction. Simultaneously, the weakly emissive Th@Au NCs were activated through Tb³⁺-induced aggregation, yielding a hybrid nanostructure exhibiting dual emission peaks at 470 nm and 630 nm. Upon the introduction of melamine, competitive binding to Tb³⁺ ions occurs via melamine's nitrogen-rich triazine structure, disrupting the original coordination complexes. This displacement interferes with AIE and FRET pathways, resulting in quantifiable changes in fluorescence emission intensity ratios (F₄₇₀/F₆₃₀), and enabling ratiometric detection. The probe demonstrates a broad linear detection range (0.01–300 µM), an ultra-low limit of detection (LOD) of 2.6 nM, and rapid response time (~ 1.5 min), outperforming many existing single-signal fluorescent systems. Stability assessments confirmed robust optical performance under varying pH, ionic strength, and UV exposure conditions, though long-term storage stability remains to be evaluated. The probe's high selectivity for melamine was validated against common food matrix interferents. Furthermore, practical applicability was demonstrated by analyzing spiked milk and infant formula samples, yielding satisfactory recoveries and precision, comparable to standard HPLC/UV methods. Overall, this ratiometric fluorescence platform offers a sensitive, rapid, and user-friendly approach for melamine monitoring, with potential for adaptation to other food safety applications through surface modification or doping strategies.