Tannic Acid-Derived Carbon Dots with Antibacterial Activity for Detection of 5-Nitroimidazole Antibiotics

The detection of antibiotics has emerged as a critical area of research due to growing global concerns over antimicrobial resistance, environmental safety, and public health. In this study, we report the successful synthesis of carbon dots (TA:OPDA CDs) from tannic acid (TA) and o-phenylenediamine (OPDA) for the selective detection of commonly used 5-nitroimidazole (5-NI) class of antibiotics, such as metronidazole (MDN), ornidazole (ODN), secnidazole (SDN), and tinidazole (TDN). The synthesized CDs were thoroughly characterized by using ultraviolet–visible (UV–vis), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) analysis. The TA:OPDA CDs exhibited selective fluorescence quenching in response to the 5-NI antibiotics. Notably, the TA:OPDA (1:2) CDs, as a representative example, demonstrated excellent sensitivity, with limits of detection (LoD) in the nanomolar range (4.15–5.01 nM) via a static quenching mechanism. A simple paper strip-based detection method was also developed, enabling practical application. Furthermore, CDs successfully detected 5-NI antibiotics in environmental samples and animal products, demonstrating their real-world applicability. Excellent recovery rates (95.5 - 103.3%) confirmed the reliability of the method. Additionally, TA:OPDA (1:2) CDs exhibited no cytotoxicity even at 1000 μg/mL and successfully detected 5-NI antibiotics in live 3T3-L1 mouse fibroblast cells, highlighting their potential for cellular imaging applications. Beyond antibiotics detection, the CDs also demonstrated promising antibacterial activity against both Bacillus subtilis and Escherichia coli with minimum inhibitory concentrations of 0.2 and 0.5 mg/mL, respectively. Generation of reactive oxygen species (ROS) was found to be the plausible antibacterial mechanism. Overall, TA:OPDA CDs represent a versatile and multifunctional platform with integrated applications in antibiotic sensing, antibacterial activity, and cellular imaging.