Spatiotemporal pH imaging using fluorescence lifetime
imaging microscopy
(FLIM) is an excellent technique for investigating dynamic (electro)chemical
processes. However, probes that are responsive at high pH values are
not available. Here, we describe the development and application of
dedicated pH probes based on the 1-methyl-7-amino-quinolinium fluorophore.
The high fluorescence lifetime and quantum yield, the high (photo)stability,
and the inherent water solubility make the quinolinium fluorophore
well suited for the development of FLIM probes. Due to the flexible
fluorophore-spacer–receptor architecture, probe lifetimes are
tunable in the pH range between 5.5 and 11. An additional fluorescence
lifetime response, at tunable pH values between 11 and 13, is achieved
by deprotonation of the aromatic amine at the quinolinium core. Probe
lifetimes are hardly affected by temperature and the presence of most
inorganic ions, thus making FLIM imaging highly reliable and convenient.
At 0.1 mM probe concentrations, imaging at rates of 3 images per second,
at a resolution of 4 μm, while measuring pH values up to 12
is achieved. This enables the pH imaging of dynamic electrochemical
processes involving chemical reactions and mass transport.