Water-soluble MoS₂ quantum dots for facile and sensitive fluorescence sensing of alkaline phosphatase activity in serum and live cells based on the inner filter effect

Abstract

We report a facile and sensitive method for the detection of alkaline phosphatase (ALP) activity in serum and live cells using molybdenum disulfide quantum dots (MoS₂ QDs) based on the Inner Filter Effect (IFE). In the present work, water soluble MoS₂ QDs with bright green fluorescence were synthesized through direct ultrasonic exfoliation of MoS₂ powder in 85 vol% aqueous ethanol solution. p-Nitrophenylphosphate (PNPP) was employed to act as an ALP substrate, and its enzyme catalytic product (p-nitrophenol (PNP)) functioned as a powerful absorber in the IFE to influence the excitation of MoS₂ QDs. PNPP was transformed into PNP in the presence of ALP, leading to the transition of the absorption peak from 310 nm to 405 nm and therefore resulted in a complementary overlap between the absorption of PNP and the excitation of MoS₂ QDs. The fluorescence of MoS₂ QDs was quenched due to the significant weakening of the excitation of MoS₂ QDs by competitive absorption between QDs and PNP. A good linear relationship was obtained from 0 to 5 U L^-1 (R² = 0.9919) using the present IFE based sensing strategy with the lowest detection activity of 0.1 U L^-1. The proposed sensing approach was successfully applied to ALP sensing in serum samples and ALP inhibitor investigation, as well as in ALP cell imaging.