Fluorescent pH indicators in polymer research: What can fluorescence spectroscopy tell us about counterion distribution around polyelectrolyte chains?

Abstract: The behavior of pH indicators adsorbed at the charged interface has been studied by many authors after Hartley and Roe first pointed in 1940 that such indicators can be used for probing the local concentration of counterions at the interface. The observed shift in the titration curve (that is, absorbance or fluorescence emission intensity vs. pH) of the adsorbed indicator compared to the free indicator molecule have been interpreted either as the "local pH" at the interface or as the "effective pK" of the indicator at the interface. In this communication, we report on fluorescence studies of (i) poly(methacrylic acid) (PMAA) with a pH-sensitive fluorescent label (umbelliferone) at the end of the PMAA chain and (ii) polystyrene-block-polymethacrylic acid micelles with a pH-sensitive fluorescent probe (N-(dodecanoyl)aminofluorescein) sorbed in the PMAA corona. We compare fluorescent data with the results of MD simulations of counterion distributions for the PMAA homopolymer, showing that both "local pH" and "effective pK" approaches are equivalent.  

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