Bimetallic-doped carbon dot nanozyme for orthogonal dual-mode fluorescence-colorimetric detection of doxycycline

Doxycycline (DOX) is a widely used tetracycline antibiotic that frequently persists as a contaminant in food and environmental waters, necessitating reliable methods for routine monitoring. We report an orthogonal dual-mode fluorescence–colorimetric sensing platform based on Mn, Ce-doped carbon dots (Mn, Ce@CDs), a one-pot hydrothermally synthesized nanozyme with bright blue fluorescence and intrinsic oxidase-like activity capable of catalyzing 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to produce a colorimetric signal. DOX modulates both signal channels via two independent mechanisms: it coordinates with surface metal centers to suppress oxidase-like activity and quenches fluorescence through a combination of inner-filter effect (IFE) and collisional dynamic quenching. The orthogonal responses enable two complementary detection modes. The fluorescence channel offers high sensitivity with a linear range of 0.1–100 µM and a limit of detection (LOD) of 43 nM, while the colorimetric channel provides a linear range of 1–15 µM and a LOD of 0.347 µM. The orthogonal dual-mode design mitigates false signals common to single-mode methods, as the colorimetric channel remains unaffected by interferents that compromise fluorescence detection. Practical applicability is validated by recoveries in spiked wastewater (99.6-103.1%) and milk samples (99.1-100.8%) with relative standard deviations (RSDs) below 3.9%. This work presents a self-validating analytical approach for DOX monitoring in complex matrices, leveraging orthogonal signal outputs to enhance detection reliability. Graphical abstract The alternative text for this image may have been generated using AI.