Dual-emission nitrogen-doped carbon dots for ratiometric and smartphone-based detection of levofloxacin

Levofloxacin (LEV) is a third-generation fluoroquinolone commonly applied in treatment against infections in both humans and animals. However, its overuse has led to increasing concerns regarding environmental contamination, health risks, and the emergence of antibiotic-resistant bacteria. In this study, we present a novel dual-emission ratiometric fluorescence probe for the detection of LEV, utilizing the intrinsic green fluorescence of LEV as the sensing signal and blue-emissive nitrogen-doped carbon dots (N-CDs) as an internal reference. N-CDs were synthesized via a one-pot hydrothermal method using 4-aminoantipyrine (4AA) and hydrazine hydrate (HH) as precursors. Elemental analysis revealed a high nitrogen content (22.1%), which enhanced the photoluminescence properties, achieving a quantum yield of 35.6%. Surface functional groups (-OH, -NH₂, and -COOH) contributed to excellent water dispersibility. Comprehensive characterization (FTIR, ¹H-NMR, ¹³C-NMR, and Raman spectroscopy, XPS, XRD, and TEM) was conducted, and a plausible formation mechanism of N-CDs was proposed. Upon interaction with LEV, a clear fluorescence shift from blue to green enabled ratiometric detection, which was further implemented using a smartphone-based sensing platform. The probe exhibited a detection limit of 43 μmol L^-1, a linear range of 0.4-20 μmol L^-1, and a rapid response time of 3 min. Successful application to pharmaceutical samples demonstrated the probe's high accuracy and precision.