Smartphone-integrated ratiometric fluorescence sensor using Al³⁺/BSA-stabilized CuNCs for on-site visual detection of ciprofloxacin in aquatic foods

The residual presence of ciprofloxacin (CIP) in aquatic products raises, increasing concerns regarding bioaccumulation and antimicrobial resistance, necessitating the development of highly sensitive and selective detection methods. Here, we present a novel ratiometric fluorescence sensor based on bovine serum albumin-stabilized copper nanoclusters (BSA-CuNCs) and Al³⁺ for CIP detection. CIP complexes with Al³⁺ to simultaneously quench the orange-red emission of BSA-CuNCs at 600 nm and enhance its intrinsic blue emission at 490 nm, thereby producing a self-calibrated ratiometric signal (F₄₉₀/F₆₀₀) with a clear concentration-dependent response and an observable color transitions from orange-red to blue. Benefiting from macromolecular surface modification of CuNCs and the ratiometric sensing strategy, the sensor displays good stability, high selectivity, and strong anti-interference capacity. It achieves a wide linear dynamic range of 0.025-50.0 mg L^-1 (R² = 0.9935), a low limit of detection (0.01 mg L^-1), and practical potential in complex matrices. Validation in diverse aquatic food samples has yielded satisfactory recoveries of 87.0%-108.8% with RSD < 2.4%. To further expand its practical applicability, a smartphone-assisted sensing platform has been developed, enabling accurate RGB-based quantification (R² = 0.9929). Statistical comparison with HPLC using paired t-test showed no significant difference between the two methods (p > 0.05), indicating high analytical concordance. This work establishes a reliable and versatile approach for accurate quantification and intuitive visual screening of CIP residues, and also suggests the potential for future extension toward simultaneous detection of Al³⁺ and CIP in real-world applications.