Smartphone-assisted dual-mode fluorescent/ colorimetric biosensors based on Fe₃O₄-CDs nanozyme for self-calibrated...

Exploring multifunctional nanozyme-based analytical platforms is essential to broaden sensing modalities and enhance detection accuracy. In this study, we develop a composite Fe₃O₄-carbon dot (Fe₃O₄-CD) nanozyme that combines fluorescent emission, peroxidase-mimicking catalysis, and magnetic separation capabilities. Leveraging these features of Fe₃O₄-CD, a ratiometric fluorescence - colorimetric dual-mode biosensor was developed for the rapid, on-site, and visual analysis of oxytetracycline (OTC). Upon the addition of OTC, the red fluorescence of europium ions was significantly enhanced due to the antenna effect (AE), whereas the blue fluorescence of Fe₃O₄-CDs was diminished by the internal filter effect (IFE). These dual opposite fluorescence signal change enabling a highly sensitive ratiometric fluorescence detection (F₆₁₇/F₄₄₁). Concurrently, the peroxidase-like activity of Fe₃O₄-CDs was also enhanced by OTC, promoting the catalytic oxidation of TMB and generating a distinct colorimetric signal. Under optimal conditions, the limits of detection were as low as 1.8 nM for the colorimetric mode and 7.9 nM for the fluorescence mode. Moreover, this dual-mode sensing strategy can be seamlessly integrated with a smartphone-based platform, enabling portable, user-friendly, and instrument-free quantification, thereby demonstrating great potential for on-site testing applications. Moreover, satisfactory recovery results were obtained using the developed dual-mode sensing probe in spiked honey and water samples, demonstrating its practical applicability. This multifunctional nanozyme probe provides an alternative strategy for the rapid, sensitive, and accurate screening of oxytetracycline in environmental monitoring and food safety.