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Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
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Scheme 1. The synthetic route of L.
2.2. Synthesis
The synthetic route of (E)-N′-((2-hydroxynaphthalen-1-
yl)methylene)benzohydrazide (L) is shown in Scheme 1. A solution of
2-hydroxy-1-naphthaldehyde (0.34 g, 2 mmol) and benzoyl hydrazine
(0.27 g, 2 mmol) in 30 mL ethanol was stirred at 60 °C for 2 h. After com-
pletion of the reaction, the obtained yellow precipitate was filtered and
washed several times with cold ethanol. After drying under reduced
pressure, the reaction afforded 0.52 g (89%) as a yellow solid. 1H NMR
(400 MHz, CD3CN) δ 12.91 (s, 1H), 10.54 (s, 1H), 9.40 (s, 1H), 8.10 (d,
J = 8.6 Hz, 1H), 7.98 (d, J = 7.5 Hz, 2H), 7.95–7.86 (m, 2H), 7.70–7.54
(m, 4H), 7.44 (t, J = 7.5 Hz, 1H), 7.25 (d, J = 9.0 Hz, 1H). 13C NMR
(100 MHz, DMSO) δ 162.50, 157.98, 146.82, 132.74, 132.60, 132.10,
131.58, 128.97, 128.65, 127.78, 127.56, 123.54, 120.58, 118.89, 108.48.
MALDI-TOF MS: 290.79.
Fig. 2. Absorbance at 359 nm and 402 nm of L as a function of Al3+ concentration.
absorbance value approached a maximum, which demonstrated the
formation of a 1:1 complex between sensor Al3+ and L.
UV–vis spectroscopy analysis of L toward various metal ions such as
3. Results and discussion
Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+
,
Ca2+, Cd2+, Ni2+, Mg2+, Zn2+, and Al3+ was conducted (Fig. 4).
Upon the addition of 5 equiv. of Al3+ to the L solution, a remarkable
change was observed by visual inspection and UV–vis absorption spec-
troscopy. The significant change to wavelength resulted in a color
change from colorless to blue, which can be easily observed by the
“naked-eye”. The addition of other representative metal ions (5.0
3.1. Absorption spectra studies
UV–vis experiments were carried out to obtain the detailed absorp-
tion properties. The UV–vis titration of the Al3+ was conducted by using
50 μM of L in CH3CN–H2O (1:1, v/v) solution, as shown in Fig. 1. Upon
binding with Al3+, a new absorption peak appears at 402 nm and its ab-
sorption intensity gradually increases with the addition of Al3+ ion with
the color changing from colorless to blue, while the absorbance of L at
359 nm gradually decreases. At the same time, an isosbestic point ap-
pears at 377 nm between them. Since Al3+ concentration increased
up to 50 μM, the absorbance at 359 nm and 402 nm changed slowly at
even higher Al3+ concentration, which implied 1:1 binding stoichiome-
try between L and Al3+ (Fig. 2).
equiv.) such as Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+
,
Sr2+, Mn2+, Co2+, Ca2+, Cd2+, Ni2+, Mg2+, and Zn2+ did not give
rise to significant color and UV–vis absorption spectroscopy changes.
These results suggest that L could be served as a potential colorimetric
sensor selective for Al3+
.
3.2. Fluorescence study
To gain insight into the stoichiometry of the sensor L–Al3+, the
method of continuous variations (Job's plot) was used, as shown in
Fig. 3. As expected, when the molar fraction of sensor L was 0.5, the
The fluorescence titration was carried out using 50 μM L in the
presence of different concentrations of Al3+ from 0 to 2 equiv. in
CH3CN–H2O (1:1, v/v) solution. As shown in Fig. 5, Al3+ ion was
Fig. 1. UV–vis absorption spectra of L (50 μM) in CH3CN–H2O(1:1, v/v) solution with Al3+
ions (0–100 μM). Inset: color change of the solution upon Al3+ addition. (For interpreta-
tion of the references to color in this figure legend, the reader is referred to the web version
of this article.)
Fig. 3. Job's plots according to the method for continuous variations. The total concentra-
tion of L and Al3+ is 100 μM.