Static and time-resolved fluorescence of fluorescein-labeled dextran dissolved in aqueous solution or sequestered within a sol–gel-derived hydrogel

Abstract

Recently, fluorescein-labeled dextrans have been incorporated within sol–gel-derived thin films for pH sensing applications. The fluorescein-labeled dextran exhibited a resistance to leaching when compared with fluorescein-doped sol–gel-processed films and also some improved analytical features. In this work, we sought to determine the origin of this improved performance. Toward this end, this paper reports the effects of pH on the static and time-resolved fluorescence anisotropy and intensity decay kinetics of fluorescein-labeled dextran dissolved in aqueous solution and when sequestered within a porous sol–gel-derived hydrogel. The results reveal the following key points. (1) There is a 1–1.5 pH unit increase in the fluorescein anion to dianion pK_a when the fluorescein-labeled dextran is incorporated within the xerogel glass. (2) In aqueous solution, the molar volume associated with the largest detected fluorescein rotational motion is one quarter to one third of the value expected for the entire dextran polymer. (3) The local motion associated with the fluorescein probe itself, when attached to the dextran polymer, is completely unaffected by the solution pH. (4) The range over which the fluorescein moiety (attached to the dextran polymer) can reorient increases as the pH increases, regardless of the environment (solution versus hydrogel). (5) The primary differences between the fluorescein-labeled dextran dynamics when it is dissolved in solution or sequestered within the hydrogel are associated with the magnitude of local and ‘global’ rotational reorientation times for the fluorescein residue. However, the observed rotational motions differ only by a factor of two for the fluorescein-labeled dextran dissolved in solution versus the same species sequestered within a hydrogel. This demonstrates that the polymer-bound fluorescein residue is remarkably mobile within these hydrogels; however, the fluorescent reporter group cannot leach from the glass because it is attached to a larger polymer. This particular result is most likely the key to the favorable analytical performance of these types of dextran-doped sol–gel-derived materials as sensor platforms. (6) Even though the magnitude of the fluorescence intensity changes with pH are comparable between solution and the hydrogel, the sensitivity of the fluorescein-labeled dextran within a hydrogel is clearly greater near physiological pH. These results suggest the possibility of using tagged dextrans or other polymers in concert with sol–gel-derived materials to modulate the analytically useful range of chemical sensing.