Description
Nowadays, fluorescent chemosensors play an important role in analytical and environmental chemistry especially for detection of toxic metal ions. In this work, three new 8-hydroxyquinoline (8HQ) derivatives Q1, Q2 and Q3 are synthesized and were used as fluorescence turn-on sensors for metal ions. For Q1, 8HQ was O-substitued with acetate ester of diethylene glycol. The O-substitution of 8HQ with diethylene glycol followed by the extension of the pi-conjugation at 5-position with 4-ethynyl N,N-dimethyl aniline moiety gives Q2 and the acetylation of Q2 gives Q3. In acetonitrile, the absorption spectra of 8HQ and Q1 were similar showing two absorption maxima around 240 and 300 nm, while those of Q2 and Q3 were around 300 and 370 nm. The emission maximum of 8HQ and Q1 was observed at 400 nm whereas that of Q2 and Q3 was red-shifted to 560 nm. In polar aprotic solvent, the fluorescence of Q2 and Q3 are visible to naked-eye under black light that becomes invisible in protic solvents such as $CH_{3}OH$ and $H_{2}O$. Therefore, 8HQ and its derivatives are investigated as turn-on fluorescent sensors for metal ions in mixed solvents. In $CH_{3}CN$/$H_{2}O$ (90/10 v/v), only Q1 shows selective turn-on fluorescence with trivalent ions such as $Al^{3+}$, $Cr^{3+}$ and $Fe^{3+}$, while 8HQ, Q2 and Q3 show non-selective and low fluorescence response to metal ions. In $CH_{3}OH$/$H_{2}O$ (30/70 v/v), 8HQ shows known green fluorescence enhancement with $Al^{3+}$ while Q3 shows interestingly strong green fluorescence (510 nm) enhancement selectively with $Hg^{2+}$. The detection limit of $Hg^{2+}$ by Q3 is 64 nM or 13 ppb. The Tyndall effect observed along with the increase of fluorescence intensity of Q3 upon the addition of $Hg^{2+}$ suggests that the fluorescence enhancement of Q3 with $Hg^{2+}$ is due to the aggregation induced emission (AIE). The AIE of Q3 is also observed without $Hg^{2+}$ at higher fraction of $H_{2}O$ (80-90%).
