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 Cu 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/SiO nanoparticles. The red fluorescence of the AuNCs can be quenched by Cu owing to coordination between Cu and 11-MUA; however, the blue emission of the CDs was insensitive to Cu owing to the protective silica shell. The quenching of the AuNCs' fluorescence returned when PPi was added because of the higher affinity between Cu and PPi than that between Cu and 11-MUA. In the presence of ALP, PPi was catalytically hydrolyzed into phosphate (Pi), which showed a much weaker affinity for Cu. Thus, Cu ions were released, and the fluorescence of the AuNCs was quenched once more. Based on this principle, Cu and ALP could be simultaneously detected. The developed ratiometric fluorescent sensor could detect Cu over a range from 0.025 to 4 μM with a detection limit of 0.013 μM and ALP over a range from 0.12 to 15 U/L with a detection limit of 0.05 U/L. The present method was successfully applied for the detection of Cu 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 Cu and ALP sensing in environmental monitoring and medical diagnosis.