Triple-Mode Detection of Doxycycline Using Highly Emissive Red Carbon Dots: A Versatile Platform for Environmental and Bioanalytical Applications.

Doxycycline (DOX), widely used in human medicine and animal feed, poses health and environmental risks due to residue accumulation, emphasizing the need for reliable detection methods. Unlike conventional methods relying on single- or dual-detection strategies, our approach enables the selective detection of DOX among tetracycline antibiotics through a triple-mode system (fluorescence, electrochemical, and colorimetric), achieving an exceptionally low detection limit. To expand their utility, red-emitting carbon dots (R-CDs) were embedded in PMMA to form solid films, where restricted intramolecular motion and surface structure conferred stable fluorescence for effective colorimetric sensing. The low limits of detection for DOX were found to be 15 nM for visual detection, 31.44 nM for fluorescence detection, and 17.9 nM for electrochemical detection. For supporting antimicrobial resistance control, detection of DOX residues was also done in tap water, milk, and egg samples. Owing to their excellent biocompatibility and minimal toxicity, the R-CD system was effectively employed as a "turn-off" fluorescent probe for intracellular imaging in HeLa cells. This work highlights the ability of the synthesized single probe to enable the detection of DOX via three distinct sensing modalities, each of which provides complementary evidence for the presence of DOX.