Sensitive and smartphone-assisted visual detection of oxytetracycline by a ratiometric fluorescence sensor based on nitrogen-doped carbon quantum dots from banana peel cooperating with europium

Oxytetracycline (OTC) residues have serious adverse effects on human health. Thus, selective and sensitive field detection methods are needed. Here, we successfully synthesized nitrogen-doped carbon quantum dots (N-CQDs) with good water-solubility, prominent fluorescence (FL), and favorable stability via a one-step hydrothermal method using banana peel as a carbon source and diethylenetriamine as a nitrogen source. We used the N-CQDs to create a ratiometric FL probe by self-assembling them with europium ions (Eu3+), enabling OTC detection. The FL intensity of N-CQDs at 445 nm was quenched due to the inner filter effect, while the FL intensity of Eu3+ at 621 nm markedly increased via the antenna effect upon OTC introduction. The ratiometric FL sensor demonstrated two linear relationships with OTC concentrations, ranging from 0.01 to 5 μg mL−1 and 5 to 25 μg mL−1, with a limit-of-detection of 4.9 ng mL−1 for OTC. Additionally, this platform depicted an apparent color change from blue to purple under a 365 nm ultraviolet–visible lamp, and a visual semi-quantitative analysis of OTC was achieved using a mobile phone application. We successfully implemented this method to detect OTC in pork muscle and swine urine, achieving remarkable recovery results (96.89–103.20%). Therefore, our findings demonstrate the potential of this approach for real-time and semi-quantitative OTC detection.