Integrating Antibiotic Sensing and Anticounterfeiting: A Dual‐Mode Platform Based on S,N‐Co‐Doped Carbon Nanodots

ABSTRACT The urgent need to detect hazardous tetracycline (TC) antibiotics in the environment and to combat product counterfeiting has driven the development of a novel dual‐purpose carbon dot platform for sensitive monitoring and covert security printing. In this study, we report sulfur‐ and nitrogen‐co‐doped carbon dots (S, N‐CDs) as a dual‐mode fluorescent platform for both sensitive TC detection and covert security printing. The S, N‐CDs exhibit bright blue fluorescence that is efficiently quenched by TC, enabling a rapid on‐off fluorescence response. This sensing strategy provides a low limit of detection (0.14 µM) and a wide linear range (0.5–38 µM) for TC, with minimal interference from other substances. Furthermore, this system has been successfully applied for the detection of TC in water samples and pharmaceutical analysis, demonstrating its potential for practical applications. Smartphone‐assisted visual mode was also designed and showed 2 µM as LOD and a wide linear range (10‐150 µM). The fluorescence intensity remains stable after 90 days of storage, demonstrating the high stability of the S, N‐CDs probes. In addition to sensing, the S, N‐CDs are colorless under ambient light and highly photostable; under UV illumination, they emit strong fluorescence, making them ideal as an invisible ink for anti‐counterfeiting. Patterns drawn with the S, N‐CDs ink remain undetectable to the naked eye under normal lighting but become brightly visible under UV light. This multifunctional S, N‐CDs platform thus integrates effective antibiotic monitoring with advanced security features, offering a promising approach for environmental safety and product authentication.