A total internal reflectance fluorescence nanoscale probe of interfacial potential and ion screening in polyethylene oxide layers adsorbed onto silica

This work investigated the effect of the ionic environment on the fluorescence signal from layers of fluorescein-tagged polyethylene oxide (FITC-PEO) adsorbed onto silica from aqueous solution, The fluorescence intensity from an adsorbed layer in a total internal reflectance fluorescence (TIRF) study was found to be highly sensitive to the fluid contacting the layer: Deionized water gave no signal at all, while the signal was strong for phosphate-buffered saline, The bulk solution fluorescence of FITC-PEO, however, was comparatively less sensitive to the presence of buffer as opposed to deionized water, This suggested that the fluorescent sensitivity to ionic conditions was an interfacial effect, possibly stemming from the pH sensitivity of fluorescein. Titrations of the fluorescence signal from the adsorbed layer as a function of salt concentration and as a function of phosphate concentration at nearly constant ionic strength revealed an increase in fluorescence with increasing concentration of the ionic species, with a sharp sigmoidal transition, This transition occurred at 0.0001 M for the phosphate and near 1 M for NaCl, demonstrating the phosphate-buffering species to be more effective in producing fluorescence compared with an unreactive salt, These experimental observations were best explained by the weak acid-base reaction of the fluorescein with the buffer and by the influence of the ionic strength on the debye length near the acidic silica surface. In the presence of buffer, the transition point was shown to be sensitive to the local potential, and the extent of the quench at low ionic strengths to the local pH. (C) 1996 Academic Press, Inc.