Highly-sensitive ratiometric determination of tetracyclines and smartphone-assisted visual assay by carrot-derived nitrogen-doped carbon dots-europium ion dual-emission fluorescence probe

• Carrot-derived CDs were prepared using water as solvent (W-CDs). • W-CDs/Eu 3+ enables ratiometric detection of TC and OTC with detection limits down to 2.96 nM and 2.85 nM, respectively. • W-CDs/Eu 3+ exhibits the lowest LOD among biomass-derived Eu 3+ -CDs system for monitoring TCs. • This method demonstrates a wider linear range of nearly four orders of magnitude for monitoring TC/OTC. • The smartphone-assisted sensing platform enables both visual and quantitative detection of TC . The persistent residues of tetracyclines (TCs), particularly tetracycline (TC) and oxytetracycline (OTC) in aquaculture have attracted widespread attention due to their significant threat to the environment and human health. Therefore, there is an urgent need to develop facile and highly sensitive methods to detect TC and OTC . In this study, using biomass carrot as the carbon source, polyethyleneimine as the nitrogen source, nitrogen-doped carbon dots (W-CDs) were prepared by one-pot hydrothermal method. Subsequently, a ratiometric fluorescent probe was constructed by combining W-CDs with europium ions (Eu 3+ ), enabling the specific recognition of TC and OTC. Based on the opposite response signals, W-CDs/Eu 3+ enables the ratiometric detection of TC in a wide linear range of 0.0067–25 μM and OTC from 0.0067 to 21 μM, with a sensitive detection limit down to 2.96 nM and 2.85 nM, respectively. Moreover, under 365 nm ultraviolet (UV) lamp, the color change from blue to red is clearly observed. Furthermore, the smartphone-assisted sensing platform enables both visual and quantitative detection of TC. The method was successfully applied to detect TC and OTC in actual water samples, demonstrating the promising potential of this probe.