Pyranine doped hydrogel based pH-strips for low pH regions

The fluorescence spectra of the hydrophilic pyrene derivative 8-hydroxy-1,3,6-pyrenetri-sulfonate (pyranine) is firmly connected to the degree of ionization of the 8-hydroxyl group and hence to the pH of media, in the range of 0<pH<5. Once entrapped in a polyacrylamide (PAAm) hydrogel, pyranine does not readily leak out of the gel. The fluorescence properties of entrapped pyranine are similar to those of bulk pyranine solution from the point of pH sensitivity. Therefore, pyranine can be used as a reliable reporter of aqueous pH changes within hydrogels. In this study, pyranine in PAAm hydrogel was used to produce a sensitive pH sensor working for aqueous acidic media. The ratio of the maximum fluorescence emission intensities at 445 nm and 512 nm i.e., l445/l512, changes with the pH of the aqueous solutions. This ratio is unaffected by the choice of the excitation wavelength. l445/15122 versus pH curves were evaluated for different gel compositions and thicknesses. The response time of the sensor was found to be about 50 seconds for the gel of thickness around 1.5-2.5 mm. It turns out that this period may be shortened even further by changing the thickness of the slab gel. Also the pH-strips can be used reversibly for many times.