Cyanine dye-embedded fluorescent film for ratiometric pH measurement.

The precise monitoring of pH is critical in various applications, particularly in biology-related areas. In this work, we report the synthesis and characterization of a novel cyanine-based fluorescent pH sensor with a pK_a around 6. This pH-sensitive dye features a cyanine chromophore coupled to a piperazine moiety, which modulates the protonation equilibrium and thus the optical response. DTF calculation corroborates the changes in the photophysical properties upon protonation. Upon immobilization within a hydrophilic polymer matrix, the resulting fluorescent sensor is exhibited a pK_a around 4.5, facilitating ratiometric pH measurements for acidic media (for instance in food-examples given in milk and fresh yogurt). Importantly, this probe design allowed a less than 5 min response time to pH changes while avoiding cytotoxic effects towards E. coli. The development of such a fluorescent pH indicator is motivated by the growing need for optical monitoring tools capable of providing real-time, non-invasive readouts of pH dynamics in complex media and microfluidic systems.