Colorimetric and fluorescent dual-mode sensing of alkaline phosphatase activity in L-02 cells and its application in living cell imaging based on in-situ growth of silver nanoparticles on graphene quantum dots

Herein we report a new colorimetric and fluorescent dual-mode sensor for alkaline phosphatase (ALP) activity based on the specific enzyme amplification of ALP and the unique optical properties of graphene quantum dots (GQDs)/silver nanoparticles (AgNPs) hybrid. In this strategy, ALP enabled the removal of phosphate group from ascorbic acid 2-phosphate to yield ascorbic acid. Silver ions could be attached on the surfaces of GQDs via electrostatic interaction and reduced by ascorbic acid to produce AgNPs, which in-situ grew on the surfaces of GQDs, accompanied by a substantial increase in the SPR band of AgNPs and an evident fluorescence quenching of GQDs simultaneously. AgNPs acted as a nanoquencher to decrease the fluorescence of GQDs by fluorescence resonance energy transfer from GQDs (donor) to AgNPs (acceptor). The mechanism of ALP sensor was examined by transmission electron microscope (TEM), atomic force microscope (AFM), energy dispersive spectrometer (EDS) and elemental mapping. Under optimal conditions, the detection limits of ALP activity are as low as 0.1 U/L and 0.02 U/L by colorimetric and fluorometric method, respectively. The dual-mode sensor could discriminatively detect ALP in L-02 cell lysates with the recoveries ranging from 93.9% to 106.5%. The probe can be employed to monitor the ALP levels in L-02 cells related to different extents of alcoholic fatty liver injury. The images of confocal laser scanning microscopy reflect that the GQD-based sensor was successfully applied to intracellular imaging of ALP activity in L-02 cells due to its favorable biocompatibility and outstanding fluorescent property. (c) 2017 Elsevier B.V. All rights reserved.