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.