Although the above intensity-based probes have been useful
for visualizing the localization and redistribution of Cu+ in cells,
variations in probe concentrations between cells, heterogeneous
distribution within cells, and issues associated with cell
thickness and movement complicate their use in detecting
quantifiable changes in copper concentrations. These issues can
be addressed by ratiometric imaging. The Chang group
developed a small-molecule ratiometric reporter for Cu+,
based on an asymmetric BODIPY platform and referred to as
ratiometric CS1 (RCS1).170 Upon binding Cu+, RCS1
undergoes an impressive 20-fold fluorescence ratio change
with excitation and emission in the visible regime. Treatment of
HEK 293T cells with RCS1 enabled monitoring transient