Influences of Substituents and pH on Fluorescence Characteristics of Water-Soluble Polycyclic Aromatic Hydrocarbons

In the range of pH at 3 similar to 11, the fluorescence spectra of water-soluble (10 mu g . L-1) pyrene and substituted polycyclic aromatic hydrocarbons (S-PAHs) including 1-hydroxypyrene, 1-aminopyrene, 1-methylpyrene, 1-pyrenecarboxylic acid, 1-bromopyrene and 1-nitropyrene were measured by three-dimensional fluorescence spectroscopy. The influences of various substituents and solution pH on the fluorescence properties of seven kinds of PAHs were explored. The results showed that the fluorescence peak and fluorescence intensity of the pyrene ring were dependent on the substituent structure. The substituents of -OH, -NH2, -CH3, -COOH, and -Br can cause varying degrees of red shift in the excitation wavelength and emission wavelength of the pyrene ring. Meanwhile, the fluorescence intensities of 1-hydroxypyrene and 1-aminopyrene were higher than those of pyrene, which is mainly because the electron-donating group -OH, -CH3 can increase the conjugated 7 electron cloud density on the pyrene ring, and then reduce the ground state excitation energy and increase the fluorescence efficiency of the fluorescence molecule. The electron-withdrawing group -NO2 and heavy atom substituent -Br have a strong inductive effect, leading to decrease the electron cloud density on the pyrene ring conjugation system and reduce the fluorescence efficiency of the fluorescence molecule. The electron-donating group -NH, has no obvious effect on the fluorescence intensity of the pyrene ring, while the electron-withdrawing group -COOH could sluggishly increase the fluorescence efficiency of the pyrene ring. Under this experimental condition, there were no significant effects of pH on the fluorescence peak position and fluorescence intensity of pyrene, 1-methylyrene and 1-bromopyrene. Compared with pH <= 9 , the maximum fluorescence peak of 1-hydroxypyrene was red-shifted at pH-11, and the fluorescence intensity decreases significantly. In addition, the effects of pH on the fluorescence properties of 1-aminopyrene and 1-pyrenecarboxylic acid were also observed clearly. The fluorescence peaks of 1-aminopyrene and 1-pyrenecarboxylic acid were blueshifted and redshifted respectively, owing to the protonation effect at pH= 3. The fluorescence intensity of 1-aminopyrene at pH=3 was slightly greater than that of pH >= 5, while the fluorescence intensity of 1-pyrenecarboxylic acid at pH =3 was slightly lower than that of pH >= 5. These studies are of great significance to the establishment of S-PAHs fluorescence analysis method and in situ determination of S-PAHs in the water environment.