Bipyridine- and Copper-Functionalized N-doped Carbon Dots for Fluorescence Turn Off–On Detection of Ciprofloxacin

Herein, a fluorescence turn off-on nanosensor has been successfully developed using functionalized N-doped carbon dots (N-CDs) as the label-free sensing probe for the ultrasensitive detection of Cu²⁺ ions first and then ciprofloxacin (CIP), one of the most commonly used antibiotics for disease control, in the presence of bipyridine. The homogeneous and narrowly distributed N-CDs with a mean size of 5.7 nm and a high quantum yield of 84% are fabricated via the hydrothermal method in the presence of citric acid and ethylenediamine as the carbon and nitrogen sources, respectively. The Cu²⁺ ions serve as both analyte and fluorescence quenchers in the sensing platform of N-CDs, and a good linear response to Cu²⁺ in the range of 0.01-0.35 μM with a limit of detection (LOD) of 0.076 nM is observed. Then, 0.35 μM Cu²⁺ is used as the fluorescence quencher of N-CDs to build up the fluorescence turn off-on sensing probe for the detection of CIP using bipyridine (bipy) as the linker for CIP and Cu²⁺ ions. The addition of CIP to the bipy-Cu@N-CD composites triggers the formation of CIP-bipy-Cu conjugate as well as the release of N-CDs, resulting in the recovery of fluorescence intensity after 6 min of incubation. The sensing probe exhibits a two-phase linear response to CIP in the concentration range of 0.05-1 and 1-50 μM with a LOD of 0.4 nM. In addition, the bipy-Cu@N-CD probe shows high sensitivity toward CIP over the 19 other interferences. Good recovery of 96-110% is also observed when 0.1-0.9 μM CIP is spiked into the real samples. Results obtained in this study clearly demonstrate a newly developed sensing platform with rapid detection of metal ions and antibiotics, which can open an avenue to develop highly efficient and robust sensing probes for the detection of metal ions, organic metabolites, and biomarkers in biological applications.