Membrane and nucleus targeting for highly sensitive cancer cell detection using pyrophosphate and alkaline phosphatase activity-mediated fluorescence switching of functionalized carbon dots

A specific membrane and nucleus targeted fluorescence OFF-ON-OFF system, using the dodecane/sulfobetaine group of functionalized carbon dots (CD) with a copper ion (Cu2+-CD) based on the presence of pyrophosphate (PPi) molecules and alkaline phosphatase (ALP) activity, for cancer cell detection was designed. The biosensor could be effectively transported from the cytosol to the nucleus in MDAMB cells, but not in MDCK cells due to the response to a change in pH by CD functionalized with zwitterionic groups. The biosensor also showed a membrane-selective regulated route for fusion of long alkyl chain grafted-CD on cell membranes. As a potential sensor, the fluorescence of the prepared Cu2+-CD was significantly quenched due to aggregation. In human cancer MDAMB cells, a nearly complete restoration of the fluorescence intensity of the Cu2+-CD was observed because of the high levels of intracellular PPi, which preferentially bound to Cu2+. After 10 min, in the MDAMB cells, re-quenching of the CD fluorescence occurred because of the high level of intracellular ALP, which can hydrolyze PPi and release the Cu2+ to re-aggregate the CD. In contrast to MDAMB cells, MDCK cells did not show an obvious response to the specific intracellular biomolecules, thus, enabling the biosensor to be used to distinguish between cancer and normal cells. In conclusion, this biosensor has the potential to be a simple and sensitive cancer diagnostic tool that can differentiate normal cells from cancer cells on coated surfaces and in aqueous states.