Detection of low concentrations of volatile amines in aqueous solutions using pH-dependent fluorophores

Abstract

The fluorescence of 2-haphthole (NL) and 1-naphthylamine (NA) has been studied depending on the concentration of amines: dimethylamine (DMA), trimethylamine (TMA), and pyridine (Pyr). The fluorescence spectrum of NL has two well-resolved bands with the maxima at 355 and 415 nm, corresponding to the protonated and deprotonated forms of 2-naphthole, the relative intensity of which depends on the concentration of aliphatic amines, but is essentially independent of the Pyr concentration. The parallel measurements of the pH of the medium showed that there is an unequivocal correlation between pH, ratio of intensities of two bands of the NL fluorescence, and concentration of aliphatic amines. It has been shown that NA has only one fluorescence band (447 nm); however, its intensity (quantum yield) is high and it is sensitive to the concentration of all three amines and pH. The pH-Metric titration of NA makes it possible to determine pK of protonation of this sensor and the pH value corresponding to the maximum slope of the titration curve, or to the maximum slope of the dependence of fluorescence intensity on the concentration of amine. This point (pH ≈ 4.2) is used as the initial one in the determination of low concentrations of amines. The effect of β-cyclodextrin (β-CD), crown ether 18-crown-6, and NA concentration on the NA sensitivity to the concentration of amines has been studied. The sensitivity of NA to Pyr is lower than to DMA, but higher than for TMA. In the presence of 2 mM of β-CD in the solution, the sensitivity of NA in relation to three amines is approximately similar.