Optical sensing of anions via polarity-sensitive dyes: A bulk sensor membrane for nitrate

The first application of a lipophilic polarity-sensitive dye (PSD) to optically sense an anion is reported. A membrane responsive to nitrate has been developed which is composed of plasticized poly(vinyl chloride), an anion “carrier” (tridodecylmethylammonium chloride), and a lipophilic cationic PSD (rhodamine B octadecyl ester perchlorate). On exposure to nitrate, the fluorescence intensity increases, while the wavelengths of the excitation and emission maxima remain unchanged. The sensor membrane exhibits its highest sensitivity to nitrate in the 0.6 to 60 μg ml−1 range, and the detection limit is 0.1 μg ml−1. The signal change on exposure to 100 mM nitrate is as high as +95%. The effect of pH is significant: in going from pH 5.0 to pH 9.0 and in the absence of nitrate, the fluorescence intensity changes almost linearly by around −6% per pH unit. The results show that the PSD-based ion sensing scheme — which so far has been applied to cations only — is applicable to anions as well and in this case results in much larger signal changes.