Quinoline-based colorimetric chemosensor for Cu2+: Cu 2+-induced deprotonation leading to color change

Article abstract of DOI:10.1016/j.cclet.2013.08.002

A quinoline-based colorimetric chemosensor (QDB) for Cu²⁺ was synthesized by coupling quinoline-2-carbaldehyde with 4-(dimethylamino) benzohydrazide. Although most transition metal cations can cause redshifts in the UV-vis spectrum of QDB, the response of the chemosensor for Cu²⁺ can be easily distinguished because it exhibits the largest redshift together with a color change from colorless to red in response to Cu²⁺. Other metal ions have no effect on the specific response of QDB to Cu²⁺. The significant redshift and color change were attributed to Cu ²⁺-induced deprotonation of NH in the sensor.

Full text of DOI:10.1016/j.cclet.2013.08.002

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Chinese Chemical Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Quinoline-based colorimetric chemosensor for Cu2+: Cu2+-induced
deprotonation leading to color change
*
Xiao-Bo Li, Zhi-Gang Niu, Ling-Ling Chang, Men-Xia Chen, En-Ju Wang
School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, China
A R T I C L E I N F O
A B S T R A C T
A quinoline-based colorimetric chemosensor (QDB) for Cu2+ was synthesized by coupling quinoline-2-
carbaldehyde with 4-(dimethylamino)benzohydrazide. Although most transition metal cations can
cause redshifts in the UV–vis spectrum of QDB, the response of the chemosensor for Cu2+ can be easily
distinguished because it exhibits the largest redshift together with a color change from colorless to red in
response to Cu2+. Other metal ions have no effect on the specific response of QDB to Cu2+. The significant
redshift and color change were attributed to Cu2+-induced deprotonation of NH in the sensor.
ß 2013 En-Ju Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Article history:
Received 17 June 2013
Received in revised form 17 July 2013
Accepted 23 August 2013
Available online xxx
Keywords:
Quinoline-based sensor
Colorimetric sensor
Copper
Deprotonation
1. Introduction
carbaldehyde with 4-(dimethylamino)benzohydrazide. QDB exhibits
a color change response from colorless to red only toward Cu2+
.
Copper is an essential trace element and plays important
physiological roles in many biological systems. But at the same
time, copper is a significant metal pollutant due to its widespread
use. Both deficiency and excess of copper can lead to health
problems. Therefore the development of colorimetric or fluores-
cent sensors for the selective and sensitive detection of Cu2+ in
biological and environmental systems has attracted considerable
attention [1–10].
Some metal ions can induce deprotonation of active NH, such as
the NH groups conjugated to aromatic or carbonyl groups. This
deprotonation process caused by complexation can be used for metal
ions recognition and sensing. For the chemosensors based on
intramolecular charge transfer, the deprotonations of coordination
atoms in electron-donating groups can greatly enhance the electron-
donating ability of electron donors and result in a redshift in emission
and absorption spectra. Great achievement in the field of deprotona-
tion-based chemosenors for metal ions has been made [6–12]. Guo
reported twoZn2+ fluorescent sensors based on the metal ion-induced
deprotonation of an imide group, which show red-shifts of both
2. Experimental
Unless otherwise noted, all chemicals were obtained from
commercial suppliers and used without further purification. 4-
(Dimethylamino)benzohydrazide was prepared according to the
procedure reported in the literature [13]. The corresponding metal
nitrates were used as the metal cation sources. 1H NMR and 13C
NMR spectra were recorded on a Bruker-400 spectrometer with
TMS as the internal standard. ESI-MS spectra were performed on a
Bruker esquire HCT-Agilent 1200 spectrometer. UV–vis absorption
spectra were recorded on TU-1901 spectrophotometer. X-ray
diffraction data collection was performed on Gemini A Ultra
diffractometer.
2.1. Synthesis of N0-(quinolin-2-ylmethylene)-4-
(dimethylamino)benzohydrazide (QDB)
Quinoline-2-carbaldehyde (0.30 g, 1.91 mmol) and 4-(dimethy-
lamino)benzohydrazide (0.35 g, 1.95 mmol) were stirred at 72 8C
in methanol (30 mL) under nitrogen atmosphere until complete
consumption of quinoline-2-carbaldehyde, monitored by TLC
(about 10 h). The reaction mixture was cooled to room tempera-
ture to give a precipitate. The precipitate was collected by filtration
and washed three times with cooled methanol to afford QDB as a
excitation and emission bands due to an extensive p-system resulted
from deprotonation of imide groups [11]. Inspired by the idea, we
synthesized a new chemosensor (QDB) by coupling quinoline-2-
pale yellow solid (0.38 g, 63%). 1H NMR (DMSO-d6):
8.60 (s, 1H), 8.42 (d, 1H, J = 8.8 Hz), 8.12 (d, 1H, J = 8.8 Hz), 8.05 (d,
d 11.92 (s, 1H),
*
Corresponding author.
E-mail address: enjuwang@163.com (E.-J. Wang).
1001-8417/$ – see front matter ß 2013 En-Ju Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.08.002
Please cite this article in press as: X.-B. Li, et al., Quinoline-based colorimetric chemosensor for Cu2+: Cu2+-induced deprotonation
leading to color change, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.08.002

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1H, J = 8.4 Hz), 8.02 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J = 8.8 Hz), 7.77–
7.83 (m, 1H), 7.61–7.67 (m, 1H), 6.80 (d, 2H, J = 8.8 Hz), 3.03 (s, 6H).
13C NMR (DMSO-d6):
d 163.2, 154.2, 152.7, 147.4, 146.2, 136.6,
130.0, 129.5, 128.8, 128.0, 127.8, 127.1, 119.1, 117.4, 110.8, 39.7;
ESI-MS m/z: 319.4 [M+H]+, 341.4 [M+Na]+.
2.2. Preparation of QDB–Cu(II) complex
Needle-like crystals suitable for X-ray analysis were obtained
by evaporating a solution of QDB/Cu(NO3)2 (1:1 molar ratio) in
ethanol.
3. Results and discussion
Free QDB shows absorption maximum at 356 nm in ethanol.
Upon the gradual addition of Cu2+, the intensity of the absorption
at 356 nm decreases and a new absorption band at 474 nm appears
(Fig. 1), resulting in a color change from colorless to red. Other
transition metal cations, such as Zn2+, Co2+ and Ni2+, also cause a
decline of the absorbance at 356 nm and induce new absorption
bands at longer wavelength than 356 nm. Nevertheless the
induced new absorptions are lower in intensity and shorter at
wavelength than that induced by Cu2+ (Fig. 2a), and Cu2+ is the only
cation that causes an observable color change from colorless to red
(Fig. 2b).
Fig. 2. (a) UV/vis responses of QDB (5.0 ꢀ 10ꢁ5 mol/L) to various metal ions in
ethanol. (b) The color changes of QDB (5.0 ꢀ 10ꢁ5 mol/L) upon addition of various
metal ions in ethanol. (Amount of the metal ions: 10 equiv. for Na+, K+, Ca2+, Mg2+
and 1.0 equiv. for other metal ions).
The selectivity of QDB for Cu2+ was examined upon treatment
with binary mixtures of Cu2+ with various metal cations in ethanol.
The intensity of the Cu2+-induced absorption at 474 nm is hardly
affected by any other metal cations (Fig. 3). The sensor displays an
extremely high selectivity for Cu2+ than other metal ions. That was
attributed to the far higher binding affinity of QDB to Cu2+ than that
to other metal cations.
In aqueous ethanol, QDB exhibits similar absorption spectral
behavior as that in ethanol, which is significant for detection of
Cu2+, because metal ion-containing samples usually are aqueous.
The UV/vis responses of QDB to various metal cations and the
selective responses of QDB to Cu2+ in 50% aqueous ethanol are
shown in Fig. 4a and b. The Ni2+-induced change in the UV/vis
spectrum of QDB is very similar to that of Cu2+-induced, which
perhaps interferes with distinguishing between Cu2+ and Ni2+. But
fortunately, Ni2+ leads to a color change from colorless to yellow,
while Cu2+ induces a change from colorless to red. In the presence
of mixed Cu2+ and Ni2+
, the UV/vis spectrum QBD shows
absorption maximum at 451 nm (454 nm for Cu2+, 440 nm for
Ni2+) and the color change from colorless to red occurs (Fig. 4c). We
can deduce that Cu2+ is more competitive in complexation with
QDB than Ni2+. Therefore QDB can selectively recognize and signal
Cu2+ and other metal cations have no effect on the specific
response of QDB to Cu2+ in 50% aqueous ethanol.
With the stepwise addition of Cu2+ (0–1.6 equiv.) to QDB, the
absorbance increases approximately linearly. When the molar
ratio of Cu2+ to QDB was over 1, the absorption intensity remained
unchanged (Fig. 1 inset). This implies the formation of a 1:1
complex between QDB and Cu2+. ESI-MS spectra of QDB in the
presence of Cu2+ exhibits a peak at m/z 380.3, which is assigned to
[Cu(QDB)-1]+ (calcd. 380.1). This confirms the 1:1 stoichiometry
QDB+Mn+
QDB+Cu2++Mn+
2.0
1.8
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
20
40
60
Cu /10 mol/L
80
2+
-6
0.0
2+
Cu Na+ K+ Ca2+ Mg2+ Mn2+ Ag+ Fe2+ Co2+ Ni2+ Zn2+ Cd2+ Hg2+
None
200
300
400
500
600
Wavelength (nm)
Fig. 3. Absorbance of QDB in the presence of various cations (10 equiv. for Na+, K+,
Ca2+, Mg2+ and 1.0 equiv. for other metal ions) at 474 nm (black bars) and the
selective responses of QDB to Cu2+ (1.0 equiv.) in the presence of other cations
((10 equiv. for Na+, K+, Ca2+, Mg2+ and 1.0 equiv. for other metal ions) at 474 nm
(gray bars) in ethanol.
Fig. 1. UV/vis spectra of QDB (5.0 ꢀ 10ꢁ5 mol/L) upon the titration of Cu2+ (0–
1.6 equiv.) in ethanol. Inset: Absorbance at 474 nm as
concentrations.
a
function of Cu2+
Please cite this article in press as: X.-B. Li, et al., Quinoline-based colorimetric chemosensor for Cu2+: Cu2+-induced deprotonation
leading to color change, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.08.002

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X.-B. Li et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
3
QDB+Mn+
QDB+Cu2++Mn+
Fig. 4. (a) UV/vis responses of QDB (5.0 ꢀ 10ꢁ5 mol/L) to various metal cations (10 equiv. for Na+, K+, Ca2+, Mg2+ and 1.0 equiv. for other metal ions) in aqueous ethanol (50%, v/
v). (b) Absorption responses of QDB in the presence of various metal cations (10 equiv. for Na+, K+, Ca2+, Mg2+ and 1.0 equiv. for other metal ions) (black bars) and in the
presence of the mixtures of Cu2+ (1.0 equiv.) and various metal cations (10 equiv. for Na+, K+, Ca2+, Mg2+ and 1.0 equiv. for other metal ions) (gray bars) in aqueous ethanol
(50% v/v). (c) The color changes of QDB (2.0 ꢀ 10ꢁ5 mol/L) in the presence of Cu2+ (1.0 equiv.), Ni2+ (1.0 equiv.) or their mixture (1.0 equiv. for each ion).
Acknowledgments
This work is supported by the National Natural Science
Foundation of China (No. 21162010) and the College Students’
Innovation Training Project of Hainan Normal University (No.
cxcyxj2013005).
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4. Conclusion
In summary, we have developed a quinine-based colorimetric
chemosensor for Cu2+ detection. The sensor displays an apparent
response to Cu2+ by a significantly red-shifted absorption band and a
color changefromcolorlesstored. Othermetalionshavenoeffect on
its specific response to Cu2+. The 1:1 binding mode was proposed
basedonUV–vistitrations, ESI-MSandX-rayanalysis. Whenbinding
with Cu2+, the deprotonation process of NH in the sensor would
occur, which causes the large redshift and the color change.
Please cite this article in press as: X.-B. Li, et al., Quinoline-based colorimetric chemosensor for Cu2+: Cu2+-induced deprotonation
leading to color change, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.08.002