Transferrin-terbium complexes as luminescent pH sensing devices.

Three kinds of protein-metal complexes were developed as pH sensing devices in the aqueous solutions. Biological transferrin and lactoferrin proteins formed stable Tb3+ complexes, which exhibited long-lived and green luminescence signals. They showed pH-dependent luminescence changes around neutral pH, coupled with their Tb3+ complexation/decomplexation. Since fluorescein-labeled transferrin-Fe3+ complex exhibited fluorescence changes at lower pH, these three protein complexes covered wide pH-responsive ranges. The immobilized biotinylated transferrin-Tb3+ complexes on the micrometer-scaled bead also operated as a pH-responsive luminescent indicator. Since the transferrin ligand had rapid Tb3+ complexation/decomplexation process, its Tb3+ complex exhibited pH-responsive luminescence signals even in the presence of Fe3+ cation. A new series of tunable luminescent pH sensing devices were developed by combination of biological protein and luminescent lanthanide center.