Application of Chromogenic and Fluorogenic Reactands in the Optical Sensing of Dissolved Aliphatic Amines

• Published in: Analytical chemistry (Washington) 1998-09, Vol.70 (18), p.3868-3873
• Main Authors: Mohr, Gerhard J, Demuth, Caspar, Spichiger-Keller, Ursula E
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Dyes; Exact sciences and technology; Fluorescence; General, instrumentation
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac980279q

A new class of dyes has been investigated that reversibly interacts with aliphatic amines, resulting in changes in absorbance or fluorescence. These dyes, which combine the properties of both a chemical reagent and a ligand, are termed “reactands”. When embedded in plasticized PVC membranes, the reactand 4-(N,N-dioctylamino)-4‘-trifluoroacetylazobenzene (ETHT 4001) shows a significant signal change on exposure to aqueous amine solutions, with a decrease in absorbance around 490 nm and an increase in absorbance around 420 nm wavelength. This change in absorbance is caused by a conversion of the trifluoroacetyl group of the reactand into a hemiaminal or a zwitterion. Upon interaction with 1-butylamine, the sensor layer exhibits a dynamic range from 1 to 100 mM 1-butylamine, with maximum relative signal changes of 90%. The limit of detection with this approach is 0.3 mM.The forward response time (t 95) for a decade change in activity is 10 min, and the reverse response time is 5 min. The selectivity of ETHT 4001 toward amines correlates with the lipophilicity of the amines in plasticized PVC. Sterical factors additionally affect selectivity in that the reactions of secondary, tertiary, and bulky primary amines with the trifluoroacetyl group are hindered. A similar response is observed for the fluorescent reactand 4-(N,N-dioctylamino)-4‘-trifluoroacetylstilbene (ETHT 4004). When this reactand is exposed to 1-butylamine, a decrease in fluorescence around 580 nm is observed, which can be as high as 90% of the total fluorescence intensity. To prevent interferences from ions and ambient light, the sensor layer is coated with a protective layer of microporous white PTFE.