Ratiometric Fluorescence and Smartphone‐Assisted Visual Assay of Tetracycline and Levofloxacin Based on Dual‐Emission Carbon Dots

The growing antibiotic resistance poses a severe threat to global public health, and timely identification of bacterial resistance can help humans treat diseases better. Herein, dual-emission fluorescent carbon dots (CDs) exhibiting emission peaks at 440 and 520 nm were fabricated using citric acid and riboflavin as precursors. The prepared CDs demonstrated excellent fluorescence stability and a quantum yield of up to 37.6%. The fluorescence of CDs was substantially quenched through an inner filter effect induced by tetracycline (TC). Subsequent addition of levofloxacin (LEV) led to a significant recovery of the quenched fluorescence signal. As a result, the developed CDs served as a highly sensitive and accurate ratiometric fluorescent probe for the detection of TC and LEV, with a limit of detection (LOD) of 0.084 and 0.03 μM, respectively. Notably, upon LEV addition, the fluorescence color of the system shifted from green to bright blue. By coupling smartphone imaging with RGB analysis, this platform enabled rapid and convenient visual quantification of LEV, with an LOD of 0.013 μM. Furthermore, the practical utility of the CDs was validated through the quantitative determination of TC and LEV in water, milk, and fruit juice, yielding satisfactory recoveries and precision.