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

In this work, an ultrasensitive ciprofloxacin (CIP) detection strategy has been established based on copper (Cu2+) ions-induced strong charge transfer in poly acrylic acid (PAA) functionalized upconversion nanoparticles (UCNPs)/2,2-bipyridine (bipy) system. The positively charged Cu2+ 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 Cu2+ 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 (0.05-1000 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).