Enzyme cascade reaction-based ratiometric fluorescence probe for visual monitoring the activity of alkaline phosphatase

Inspired by the intrinsic catalytic properties of alkaline phosphatase (ALP) and tyrosinase (TYR) as well as the fluorogenic reaction between levodopa and resorcinol, a ratiometric fluorescent probe based on the CdTe/CdS/ZnS/SiO 2 QDs for visual monitoring the activity of ALP was developed. Phosphotyrosine was catalyzed into tyrosine by ALP, and then tyrosine was catalyzed into levodopa by TYR. Carboxyazamonardine, the reactive product of levodopa and resorcinol, gave rise to a strong blue fluorescence at 470 nm and quenched the red fluorescence of CdTe/CdS/ZnS/SiO 2 QDs at 620 nm due to their inner filter effect. Thus, the resulting ratiometric fluorescence acted as the signal output for quantitative ALP detection, accompanying with the fluorescence color changing from red to purple and finally blue for semiquantitative ALP detection. A good linear relationship was obtained ranging from 0.08 to 500 U L −1 with the detection limit of 0.02 U L −1 . Moreover, the probe was successfully applied to monitoring ALP activity in human serum samples without any pretreatment, and also used to screen ALP inhibitors. These results suggested that the developed method offered a highly sensitive and promising analytical platform for ALP in clinical diagnostics and paved a new way for monitoring enzymes activity visually for point-of-care diagnosis.