Peptide-based fluorescent probe with large Stokes shift for specific sequential detection of copper (II) ions and histidine: Smartphone device, real samples, test strips and bioimaging applications

A novel and facile fluorescent probe, DNH, was developed based on a dipeptide backbone (Asn-His-NH₂) labelled with a dansyl fluorophore. DNH exhibited significant advantages in detecting Cu²⁺ based on the paramagnetic quenching mechanism, including satisfactory water solubility (100 % aqueous medium), a large Stokes shift (230 nm), excellent anti-interference capability (no interference), a low detection limit (19.6 nM), rapid response time (within 30 s) and a wide pH range (7–12). Employing a displacement approach, the DNH-Cu²⁺ ensemble showed strong fluorescence enhancement upon recognition of _L-His with outstanding selectivity, and the detection limit for the DNH-Cu²⁺ ensemble with respect to _L-His was calculated at 22.8 nM. Notably, DNH demonstrated favourable reversibility for at least seven cycles with minor changes in fluorescence intensity when Cu²⁺ and _L-His were added alternately. Moreover, DNH exhibited excellent low cytotoxicity and good biocompatibility, and was utilised as an outstanding probe for the sequential determination of Cu²⁺ and _L-His in living cells and zebrafish. DNH was successfully used to determine Cu²⁺ and _L-His in three actual water samples with good recovery and accuracy. In addition, DNH was applied to filter paper to develop visual test strips for the rapid and high-efficiency analysis of Cu²⁺ and _L-His. Furthermore, the semi-quantitative visual monitoring of Cu²⁺ and _L-His was achieved by developing a smartphone colour recogniser into a portable analytical device, achieving low detection limits of 0.49 μM and 0.63 μM, respectively. Given these attributes, DNH presents itself as a promising peptide-based fluorescent probe, showcasing considerable potential for real-time visual monitoring of Cu²⁺ and _L-His in various environmental and biological settings.