Effects of the polymer matrix on an optical nitrate sensor based on a polarity-sensitive dye

Abstract

A membrane responsive to nitrate has been developed; it is composed of a polymer matrix, an anion exchanger (tridodecylmethyl-ammonium chloride), and a polarity-sensitive dye (rhodamine B octadecyl ester perchlorate). On exposure to nitrate, its fluorescence intensity increases, while the wavelength of the excitation and emission maxima remain unchanged. The effect of the polymer matrix on the relative signal change, sensitivity and selectivity has been investigated. It is shown that the sensitivity and selectivity for nitrate over other anions strongly depends on the lipophilicity and polarity of the polymer: increasing lipophilicity results in both higher sensitivity and selectivity. The limit of detection (LOD) in the case of lipophilic matrices is of the order of 0.1 ppm nitrate, and highest signal changes are observed between 1 and 100 ppm. Hydrophilic matrices have an LOD of 1 ppm and are most sensitive to nitrate at levels between 10 and 1000 ppm. The selectivity factor of lipophilic matrices for nitrate over chloride is 200, whereas it is around 10 only in hydrophilic matrices. The optical signal of the sensor material is pH dependent and this dependence again is affected by the lipophilicity of the matrix. in general, lower lipophilicity increases the cross-sensitivity of the nitrate sensor to pH. As a result, the response of the sensor may be fine-tuned by the appropriate choice of the polymer material.