Bipyridine-mediated fluorescence charge transfer process based on copper ion grafted upconversion nanoparticle platform for ciprofloxacin sensing in aquatic products

• A label-free nanosensor for the determination of ciprofloxacin was developed. • The detection was realized through an “off-on” strategy of fluorescent signals. • Bipyridine as a bridging ligand enabled ultra-sensitive detection of ciprofloxacin. • The proposed nanosensor has good sensitivity with a LOD of 0.13 ng/mL. In this work, an ultrasensitive ciprofloxacin (CIP) detection strategy has been established based on copper (Cu 2+ ) ion-induced strong charge transfer in poly acrylic acid (PAA) functionalized upconversion nanoparticles (UCNPs)/2,2-bipyridine (bipy) system. The positively charged Cu 2+ ions electrostatically adhere to the surface of the PAA-UCNPs and deactivate the fluorescence via a charge transfer process. The bipy in this hybrid system controls the aggregation by chelating in proximity to the Cu 2+ center. Due to the strong affinity between pyridone oxygen and carboxy oxygen, CIP coordinates in high stoichiometry with the bipy-Cu complex as compared to the PAA-UCNPs, causing the trapped fluorescence to be released in an amount equivalent to the target concentration. Under the optimum assay conditions, a good calibration plot (0-1000 ng/mL) was acquired with a detection limit of 0.13 ng/mL. The satisfactory recoveries (85.93-96.87%) for real prawn and fish samples were further validated by enzyme-linked immunoassays (P>0.05).