Wide range luminescence lifetime-based pH sensing with covalently immobilized multi-protonatable Ru(II) complexes

To overcome the limitations of optical pH sensors, the response of which is just ca. one pH unit around the pK_a value of the indicator dye and capitalize on the advantages of red-luminescent Ru(II) complexes for optical sensing, multi-pK_a members of this family are proposed. Bipyridine ligands bearing protonatable groups (amines and carboxylates) are the basis to design and prepare the indicator dyes. Their photophysical properties and response to pH changes were investigated in the pH 3.5−9.0 range. The complex [Ru(DCB)₂DEAMB], where DCB and DEAMB stand for 2,2’-bipyridine-4,4’-dicarboxylate and 4,4’-(N,N-diethylaminomethyl)-2,2’-bipyridine, respectively, shows a reversible monotonic change of its emission wavelength maximum (648 – 634 nm), luminescence intensity (1.40x) and lifetime (335 – 429 ns) in solution with increasing pH. This complex and two similar ones were covalently tethered to commercial TentaGel® M Br polymer microbeads for luminescence phase shift fiberoptic pH measurements. The immobilized [Ru(DCB)₂DEAMB] displays the widest pH sensitivity over pH 3.5 to pH 8.5. The stability of this sensor was satisfactory when cycling between pH 3.5 and 8.5 for five days as well as under a three-day cycle between each pH unit. Changes of the buffer type, buffer concentration and osmolarity did not significantly influence the sensor response; however, large variations of dissolved oxygen and, naturally, temperature would require correction by the corresponding sensors. The novel, robust luminescent pH sensor has been tested in phase-sensitive mode for cell cultures monitoring in commercial bioreactors, but its response would also be suitable for in situ monitoring of natural waters.