Near-infrared fluorescence probe for the determination of alkaline phosphatase

• Water-soluble CuInS2 QDs were functionalized with tryptophan molecules.
• Functionalized CuInS2 QDs had a fluorescence emission peak around 689 nm.
• Cu2+-quenched functionalized CuInS2 QDs could be restored by pyrophosphate.
• The QDs–Cu2+–pyrophosphate system was utilized to detect alkaline phosphatase.

Water-soluble CuInS2 quantum dots (QDs) were directly synthesized in an aqueous solution with mercaptopropionic acid (MPA) as stablizers, and were functionalized using tryptophan molecules to form tryptophan-functionalized CuInS2 QDs (W-CuInS2 QDs) that had a strong fluorescence emission around 689 nm. The fluorescence of W-CuInS2 QDs could be quenched by Cu2+ and then the addition of pyrophosphate (PPi) could effectively turn on the quenched fluorescence due to the strong interaction between Cu2+ and PPi. The alkaline phosphatase (ALP) could catalyze the hydrolysis of PPi that would disassemble the complex of PPi–Cu2+–PPi. Therefore, the recovered fluorescence could be quenched again by the addition of ALP. In this paper, we developed a novel near-infrared fluorescence probe for the simple and convenient assay of ALP.