Ratiometric fluorescent sensor for visual determination of copper ions and alkaline phosphatase based on carbon quantum dots and gold nanoclusters

In this work, a novel ratiometric fluorescent sensor, based on carbon dots (CDs) and gold nanoclusters (AuNCs), is developed for highly sensitive and selective visual colorimetric detection of Cu2+ and alkaline phosphatase (ALP). The ratiometric fluorescent sensor was synthesized by covalently linking 11-mercaptoundecanoic acid (11-MUA)-stabilized AuNCs to the surface of amino-functionalized CD/SiO2 nanoparticles. The red fluorescence of the AuNCs can be quenched by Cu2+ owing to coordination between Cu2+ and 11-MUA; however, the blue emission of the CDs was insensitive to Cu2+ owing to the protective silica shell. The quenching of the AuNCs' fluorescence returned when PPi was added because of the higher affinity between Cu2+ and PPi than that between Cu2+ and 11-MUA. In the presence of ALP, PPi was catalytically hydrolyzed into phosphate (Pi), which showed a much weaker affinity for Cu2+. Thus, Cu2+ ions were released, and the fluorescence of the AuNCs was quenched once more. Based on this principle, Cu2+ and ALP could be simultaneously detected. The developed ratiometric fluorescent sensor could detect Cu2+ over a range from 0.025 to 4M with a detection limit of 0.013M and ALP over a range from 0.12 to 15U/L with a detection limit of 0.05U/L. The present method was successfully applied for the detection of Cu2+ and ALP in real water samples and in human serum samples, respectively. This ratiometric fluorescent approach may provide a highly sensitive and accurate platform for visual Cu2+ and ALP sensing in environmental monitoring and medical diagnosis.