Tailoring the sensitivity and spectral properties of a chromoreactand for the detection of amines and alcohols

The chromoreactand N,N-dioctylamino-4ʹ-trifluoroacetyl-2ʹ-nitroazobenzene (CR-546) is sensitive to amines and alcohols when dissolved in plasticised PVC. The chromoreactand provides an improvement over related reactands in that a nitro group meta to the trifluoroacetyl group enhances the chemical reactivity and, consequently, the sensitivity by a factor of 20 compared to the derivative without nitro group. Furthermore, the absorbance is shifted by around 40 nm to longer wavelengths, making the sensor membranes compatible with the green LED as a light source for a miniaturised sensor device. The sensor membrane responds to aqueous 1-propylamine in the 0.5–50 mM concentration range and exhibits a detection limit of 0.1 mM. Response times are in the range of 5–15 min. A fluorescent ethanol-sensitive membrane is obtained by dissolving the inert fluorophore N,Nʹ-bis(1-hexylheptyl)perylene-3,4:9,10-bis(dicarboximide) together with CR-546 and the catalyst tridodecylmethylammonium chloride in the polymer matrix. The trifluoroacetyl form of CR-546 with its absorbance at around 560 nm overlaps the emission of the fluorophore. Increasing ethanol concentrations cause decreasing absorbance at 560 nm and consequently, increasing luminescence of the fluorophore. The resulting layer has been evaluated for its detection of ethanol via changes in luminescence intensity. The calibration is linear in the range from 1 to 15% (v/v) ethanol with a limit of detection of 0.1% (v/v). Response times are in the range from 20 to 40 min for both forward and reverse reaction.