Abstract:
The fluorescence resonance energy transfer (FRET)
mechanism has been established between carbon dots (CDs) and
naphthalimide to monitor the activity of thioredoxin reductase (TrxR),
which is often overexpressed in many cancer cells. The naphthalimide
moiety was covalently attached to the surface of CDs through a
disulfide linkage. In normal cell conditions (when devoid of high
concentrations of TrxR), the CDs act as an energy donor and
naphthalimide acts as an acceptor, which establishes the FRET pair as
interpreted from the emission at λem = 565 nm, when excited at λex =
360 nm. However, contrary to this, the elevated levels of TrxR cause
the breakage of disulfide bonds and consequently abolishes the FRET
pair through the release of the naphthalimide moiety from the surface
of CDs. This process was studied by monitoring of fluorescence
intensity at λem = 565 and 440 nm, when excited at the same
wavelength (λex = 360 nm). The TrxR based ratiometric quenching and enhancement of fluorescence intensity offers an
interesting opportunity to monitor the enzyme activities and has many advantages over conventional monitoring of fluorescence
intensity at a single wavelength to avoid interference of external factors. Fluorescence images of cancer cells in response to the
nanosensor were visualized under a confocal microscope. Cytotoxicity study of nanosensor retards the growth of HeLa and
MCF-7 cell lines in the presence of visible light. Therefore, the nanosensor also acts as a theranostic agent to diagnose as well as
killing of cancer cells
