As
traditional pH meters cannot work well for minute regions (such
as subcellular organelles) and in harsh media, molecular pH-sensitive
devices for monitoring pH changes in diverse local heterogeneous environments
are urgently needed. Here, we report a new dual-modal colorimetric/fluorescence
merocyanine-based molecular probe (CPH) for ratiometric sensing of
pH. Compared with previously reported pH probes, CPH bearing the benzyl
group at the nitrogen position of the indolium group and the phenol,
which is used as the acceptor for proton, could respond to pH changes
immediately through both the ratiometric fluorescence signal readout
and naked-eye colorimetric observation. The sensing process was highly
stable and reversible. Most importantly, the suitable pKa value (6.44) allows CPH to presumably accumulate in
lysosomes and become a lysosome-target fluorescent probe. By using
CPH, the intralysosomal pH fluctuation stimulated by antimalaria drug
chloroquine was successfully tracked in live cells through the ratiometric
fluorescence images. Additionally, CPH could be immobilized on test
papers, which exhibited a rapid and reversible colorimetric response
to acid/base vapor through the naked-eye colorimetric analysis. This
proof-of-concept study presents the potential application of CPH as
a molecular tool for monitoring intralysosomal pH fluctuation in live
cells, as well as paves the way for developing the economic, reusable,
and fast-response optical pH meters for colorimetric sensing acid/base
vapor with direct naked-eye observation.