Sensitivity and selectivity of p-(N-dimethylamino cinnamylidene) malononitrile, 6-propionyl-2-(dimethylamino) naphthalene (PRODAN) and fluorescein

Abstract

The ability of p-(N-dimethylamino cinnamylidene) malononitrile (CMN), 6-propionyl-2-(dimethylamino) naphthalene (PRODAN) and fluorescein to monitor the microviscosity ( η), dielectric constant ( ε) and pH of the medium, respectively, was studied with particular emphasis given to sensitivity and selectivity. Each fluorescent probe was added to various solutions of known microviscosity, dielectric constant and pH. The fluorescence intensity of CMN and fluorescein was monitored at their wavelength of maximum emission, whereas for PRODAN, the maximum emission wavelength was monitored. Emission intensity of CMN was sensitive to microviscosity (3.2–160 cps) and insensitive to pH (1.5–8.0), dielectric constant (30–68) and ionic strength (0.12–1.20). The maximum emission wavelength of PRODAN showed a red shift with increasing dielectric constant (28–78) and was insensitive to pH (2.75–8.0), microviscosity (0.9–8.7 cps) and ionic strength (0.12–1.20). The emission intensity of fluorescein was sensitive to pH (5.00–8.00) and unperturbed by ionic strength (0.12–1.20). However, the fluorescence intensity of fluorescein was strongly influenced by the dielectric constant of the medium (28–78). A correction for this effect using PRODAN fluorescence can be found in a companion paper.