them are not sensitive enough to determine low concentration of
HSO3
−.
Fluorescence spectroscopy has become a powerful tool for
sensing and imaging trace amounts of samples because of its simplicity
and sensitivity [11]. Some fluorescent probes have been
exploited for detecting HSO3
− by their specific reactions with
an aldehyde [12] or levulinated group [13] in vitro assay. However,
the aldehyde-based probes can only be operated in acidic
conditions; and the labile ester linkage in levulinate-type probes
may induce a high background signal in biological imaging, as it
can potentially be cleaved by proteases and esterases to produce
active fluorophores. Guo has developed a ratiometric fluorescent
probe based on diethylaminocoumarin-hemicyanine dye for
sensing HSO3
− both in vitro and vivo [14]. Unfortunately, all
the probes that mention above displayed a response time more
than 5 min. Thereby, searching for quick reactions that are sensitive
and selective toward HSO3
− has always been attractive and
challenging.
Herein, we report a new fluorescent turn-on probe 1, which was
connected by a benzo[e]indolium fluorophore and a benzene moiety
through an ethylene group (Scheme 1). The probe has shown
a selective response to HSO3
− over other anions in PBS buffer
(pH = 7.4, 10 mM), and a distinguished fluorescent color change
from colorless to cyan could be observed. HSO3
− is expected to be
undergone 1,4-addition reactions with the C-4 atom in the ethylene
group, which is activated by the electron-withdrawing feature of
the positively-charged benzo[e]indolium fragment. The bulkiness
of HSO3
− prevented it to attack toward C-2 atom, although C-2
atom was an effective target for HS− [15] and CN−