<p>Chitosan with different molecular weights (i.e., CS₂₀₀ and CS₂) as the ligand was employed to stabilize copper nanoclusters&#xa0;(CuNCs), developing a fluorescence “turn-on” probe to recognize fluoroquinolones. Chitosan chains not only can prevent the CuNCs from oxidation and overgrowth, but also provide the regulable luminescence and aggregation-induced emission (AIE) behaviors. The experimental results revealed that the CS₂₀₀@CuNCs probe displayed better sensitivity, selectivity and anti-interference towards ciprofloxacin and several other specific fluoroquinolones than CS₂@CuNCs. When recognizing the analytes, a more obvious AIE phenomenon occurred in CS₂₀₀@CuNCs. The ciprofloxacin concentration for linear response was 1.0 ~ 180 µM, with a detection limit of 0.067 µM. Moreover, the CS₂₀₀@CuNCs probe could be practically used to determine the ciprofloxacin residue from real samples with good recoveries, implying the perspective in various scenarios. These advantages are attributed to the flexile chain conformation for chitosan in CS₂₀₀@CuNCs, which can generate the reasonable spatial-constraint. This structural feature significantly promotes the recognition and capture of target molecules. This work paves a distinctive way for designing novel biosensors by regulation of the ligand chain length, demonstrating high potential for rapid antibiotic screening.</p> Graphical Abstract <p></p>