The pH sensitive fluorescent dyes, fluorescein-5-(and-6)-sulfonate (FS), 8-hydroxy-1, 3, 6-pyrenetrisulfonate (HPTS), 6,8-dihydroxy-1, 3-pyrenedisulfonate (DHPDS) and 3,6-dihydroxy-2, 7-naphthalenedisulfonate (DHNDS) were immobilized in thin films of polyethylene glycol (PEG) hydrogel. FS and HPTS were physically immobilized in the PEG hydrogel through ion-pairing with the common reagent, cetyltrimethylammonium bromide (CTAB). DHPDS and DHNDS were covalently bound within hydrogel films by forming their respective methacryloyl-analogs, MA-HPDS and MA-HNDS. The presence of the methacrylate moiety allowed MA-HPDS and MA-HNDS to participate as co-monomers in the polymerization reactions. Fluorescence excitation spectra of hydrogel-immobilized dyes in buffer solutions were recorded with emission wavelength at 540 nm (FS), 520 nm (HPTS), 510 nm (DHPDS) and 460 nm (DHNDS). Solution pH was determined from excitation ratio of protonated and deprotonated dyes. No shifts of the acid dissociation constants (pKa) were observed in the hydrogel films compared to the dyes in solutions. The fluorescent lifetimes of dyes in both hydrogel films and buffer solutions were measured. For physically trapped fluorescent dyes, FS and HPTS, the fluorescent lifetimes of deprotonated excited states (4.0 ns and 5.3 ns) were unchanged compared to those in buffer solutions. For covalently bound fluorescent dyes, DHPDS and DHNDS, the fluorescent lifetimes of deprotonated excited states (5.0 ns and 12 ns, respectively) were shorter compared to those in buffer solutions (6.3 ns and 17 ns, respectively). As potential optical pH sensors, the equilibrium times for the fluorescent dyes in the hydrogel films were affected by both the film thickness and the ionic strength of the buffer solutions. The fastest equilibrium time, less than 5 minutes, was obtained with the thinnest film and at high ionic strength (0.1 M). The ionic strength effect on the apparent pKa of the immobilized dyes was small and consistent. The pH sensitive range for these four fluorescent dyes, FS, HPTS, DHPDS, and DHNDS in hydrogel films were 5-8, 7-10, 7-10, and 9-11.5 respectively. Minimal leaching of fluorescent dyes from the hydrogel films was observed.

pH response of fluorescent dyes immobilized in hydrogel films

244th National Meeting (2012)

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