Ultrasensitive and highly selective detection of Cu2 + ions based on a new carbazole-Schiff

Article abstract of DOI:10.1016/j.saa.2017.08.057

A new chemosensor for Cu^{2 +} based on Schiff base with high sensitivity and selectivity was designed and synthesized. The fluorescence intensity of the chemosensor in CH₃CN solution was enhanced 160-fold after the addition of 10 equiv. Cu^{2 +} over other metal ions. In addition, it also facilitates colorimetric detection for Cu^{2 +} in CH₃CN solution. The chemosensor displayed low detection limit and fast response time to Cu^{2 +}.

Full text of DOI:10.1016/j.saa.2017.08.057

Accepted Manuscript
Ultrasensitive and highly selective detection of Cu2+ ions based
on a new carbazole-Schiff
Jun Yin, Qijing Bing, Lin Wang, Guang Wang
PII:
S1386-1425(17)30688-1
doi: 10.1016/j.saa.2017.08.057
SAA 15414
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
25 June 2017
6 August 2017
19 August 2017
Please cite this article as: Jun Yin, Qijing Bing, Lin Wang, Guang Wang , Ultrasensitive
and highly selective detection of Cu2+ ions based on a new carbazole-Schiff,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi:
1
0.1016/j.saa.2017.08.057
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ACCEPTED MANUSCRIPT
2
+
Ultrasensitive and highly selective detection of Cu ions based on a
new carbazole-Schiff
Jun Yin, Qijing Bing, Lin Wang, Guang Wang*
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Corresponding author: Guang Wang
Tel: 86+431+85099371;
Fax: 86+431+85099371.
E-mail address:
wangg923@nenu.edu.cn (G. Wang)

ACCEPTED MANUSCRIPT
Abstract: A new chemosensor for Cu2+ based on Schiff base with high sensitivity and selectivity was
designed and synthesized. The fluorescence intensity of the chemosensor in CH
enhanced 160-fold after the addition of 10 equiv. Cu2+ over other metal ions. In addition, it also
facilitates colorimetric detection for Cu2+ in CH
3
CN solution was
3
CN solution. The chemosensor displayed low
2
+
detection limit and fast response time to Cu .
Keywords: Fluorescence; Chemosensor; Copper ion; Schiff base; Carbazole.
1. Introduction
Heavy metal ions are hazardous to human health and environment, of which, Cu2+ ions perform an
important role in physiology of human beings [1-3]. Cu2+ ion is one kind of the essential trace metal ion
to sustain normal human health and sever as a cofactor for a wide variety of enzymes in all living
organisms. For example, Cu2+ ions play a cofactor role in various redox processes, enzyme functions and
pigments [4]. Although Cu2+ are the third most abundant essential trace element after iron and zinc in the
human body, they are harmful to humans and organism at high concentration [5]. Excessive loading
copper ions can cause extremely negative health effects such as prion disease, several neurodegenerative
diseases including Menkes, Wilson, Alzheimer and Parkinson [6-8] and could turn into a biologically
hazard because of its ability to generate reactive oxygen species, which causes irritation of nose and
throat resulting in nausea, vomiting, and diarrhea [9]. In our daily lives, Cu2+ ion is generated and
accumulated in food chain and the environment with the development of various industries such as
electroplating, painting, wood and paper industries. The limit of copper in drinking water is 1.3 ppm (~20
μM) set by US Environmental Protection Agency (EPA) [10]. The traditional detection methods for Cu2+
such as inductively coupled plasma atomic emission spectrometry (ICP-AES) [11-14], atomic absorption
spectroscopy (AAS) [15-16], fluorescence techniques [17-19] and electrochemical methods (EM)
[
[
20-22] require sophisticated equipment, tedious sample preparation procedures and trained operators
23].
Fluorescent chemosensor is a highly sensitive and selective approach for detection of metal ions
with speed, precise accurateness and low-cost [24-25]. Up to now, a variety of fluorescent and
colorimetric Cu2+ chemosensor based on small organic molecules, conjugated polymers, nanoparticles

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and biomolecules were reported [2, 26-31]. Even so, exploring of fluorescence emission probes for Cu2+
sensing with high selectivity and sensitivity is still necessary.
Schiff bases, condensation of reactive aldehydes and amines, are important compounds with
various potential applications. Schiff bases attract much interest both from a synthetic and biological
point of view. Some of Schiff base derivatives had been used as coordination unit in fluorescent
chemosensors [32-35]. The bridged C=N structure of Schiff bases easily isomerize in the excited state
and tend to exhibit very weak fluorescence. But when they coordinate with some special metal ions, the
C=N isomerization is inhibited, and then Schiff bases show strong fluorescence [36-37].
Carbazole is a conjugated unit with interesting optical and electronic properties. At present, the
most carbazole derivatives are usually used as photoelectric functional materials. There are a few reports
on carbazole-based fluorescent chemosensor for selective detection of metal ions [10, 38-39].
Diaminomaleonitrile has special steric structure and containing four nitrogen atoms and C=C double
bond, it can provide high selectivity in coordination process with metal ions.
In this paper, a novel Schiff base (Scheme 1) composed of carbazole and diaminomaleonitrile has
been developed to behave as a highly selective and sensitive fluorescent chemsensor for Cu2+ in CH
CN
3
solution. Moreover, this Schiff base presented the application potential as fluorescent for the detection of
metal ions.
Scheme 1. The synthetic route of sensor 1
2
. Materials and Methods
2.1. Instruments and materials
Unless otherwise stated, solvents and reagents were of analytical grade from commercial
suppliers and were used without further purification. All chromatographic examinations were
1
performed using silica gel, and TLC was performed on silica plates (made in China). H NMR

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13
spectra and C NMR spectra were recorded on a Varian Unity Inova Spectrometer (500 MHz) at
room temperature using d-chloroform as solvent. The IR spectra were recorded on a Nicolet 6700
Fourier Transform Infrared Spectrometer (FT-IR). The absorbance spectra were examined using a
Varian Cary 500 UV-vis spectrophotometer. Deionized water was used throughout all experiments.
The salts used in stock solutions of metal ions are Cu(NO
Al(NO ·9H O, Zn(NO ·6H O, Cr(NO ·9H O, Cd(NO
Co(NO ·6H O, Mg(NO ·6H O, KNO , AgNO , BaCl ·2H O, MnCl
SrSO and FeSO ·7H O.
3
)
2
·3H
O,
, HgCl
2
O, Ni(NO
3
)
2
·6H
·9H
2
O,
3
)
3
2
3
)
2
2
3
)
3
2
3
)
2`4H
2
Fe(NO
3
)
3
2
O,
3
)
2
2
3
)
2
2
3
3
2
2
2
2
, PbCl
2
, CaCl ,
2
4
4
2
2.2. Synthesis
2.2.1. Synthesis of N-hexyl carbazole 3
After the mixture of 85.0 ml DMF and KOH (13.0 g, 232.0 mmol) in a 250 ml flask was stirred
for 20 min at room temperature, Carbazole (6.20 g, 37.0 mmol) was added into the mixture and
continued to be stirred. After stirring for 40 min, bromohexane (5.27 ml, 37.0 mmol) was added
dropwise into the mixture. Then, the mixture was stirred for another 9 h. The mixture was poured
into cool water and the white precipitate was obtained. Then the filtered residue was
chromatographed over silica gel using petroleum ether/dichloromethane (3/1) as an eluent to give
1
N-hexyl carbazole, 9.10g. Yield: 98.0%. H NMR (500 MHz, CDCl
3
, ppm) : δ 8.10 (d, J = 4.0 Hz,
2
1
H), 7.48-7.45 (m, 2H), 7.40 (d, J = 4.0 Hz, 2H), 7.25-7.21 (m, 2H), 4.30 (t, J = 7.5 Hz, 2H),
.90-1.84 (m, 2H), 1.40 (t, J = 6.25 Hz, 2H), 1.38-1.25 (m, 4H), 0.86 (t, J = 7.0 Hz, 3H).
2.2.2. Synthesis of 3-formyl-N-hexyl carbazole 2
After POCl (9.1 ml, 100.0 mmol) was added dropwise into DMF (81.0 ml, 100.0 mmol)
3
under stirring in ice-water bath, the reaction mixture was stirred for another 40 min and brought to
room temperature under stirring for 1 h. N-hexyl carbazole 3 (2.51 g, 10.0 mmol) in 1,
2
-dichloroethane (50.0 ml) was added dropwise and then the reaction mixture was refluxed for 24
h. After cooling to room temperature, the mixture was poured into ice water. The mixture was
neutralized with 50% NaOH aqueous solution and then extracted with CH Cl (4×50 ml). The
2
2

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combined organic phase was washed with saturated sodium bicarbonate solution and saturated
sodium chloride solution, dried with anhydrous Na
2
SO . The solvent was evaporated under
4
reduced pressure and the crude product was purified by silica chromatography using petroleum
ether: ethyl acetate = 3:1 as eluent to give white powder, 4.17 g, yield, 75.0%. 1H NMR (500
3
MHz, CDCl , ppm) δ: 10.10 (s, 1H), 8.61(d, J = 0.5 Hz, 1H), 8.16 (d, J = 3.75 Hz, 1H), 8.02-8.00
(m, 1H), 7.55-7.52 (m, 1H), 7.47 (t, J = 8.25 Hz, 2H), 7.34-7.31 (m, 1H), 4.33(t, J = 7.25 Hz, 2H),
1.92-1.86 (m, 2H), 1.41-1.35 (m, 2H), 1.34-1.26 (m, 4H), 0.86 (t, J = 7.25 Hz, 3H).
2.2.3. Synthesis of 2-amino-3-((Z)-((9-hexyl-9H-carbazol-3-yl) methylene)amino)maleonitrile
(sensor 1)
A mixture of 30 ml ethanol, 2, 3 - diaminomaleonitrile (0.648 g, 6 mmol) and
-hexyl-9H-carbazole-3-carbaldehyde (0.837 g, 3 mmol) in a 100 ml flask was stirred at 70°C for 6
9
h. The solvent was distilled off and residue was chromatographed over silica gel using petroleum
1
ether/ethyl acetate (3:1) to afford sensor 1 with a yield of 68.8%. H NMR (500MHz, CDCl
3
, ppm)
δ : 8.58 (s, 1H), 8.53 (d, J = 0.5 Hz, 1H), 8.14 (d, J = 4.0Hz, 1H), 7.97-7.95 (m, 1H), 7.54-7.51
m,1H), 7.44 (t, J = 7.5 Hz, 1H), 7.31 (t, J = 7.25 Hz, 2H), 7.26 (s, 1H), 5.01 (s, 2H), 4.32 (t, J = 7.25
(
1
3
Hz, 2H), 1.90-1.86 (m, 2H), 1.41-1.34 (m, 2H), 1.33-1.26 (m, 4H), 0.865 (t, J = 7.0 Hz, 3H). C
NMR (125 MHz, CDCl , ppm): δ = 160.07, 143.09, 141.02, 126.86, 126.61, 126.03, 123.36,
22.75, 120.66, 120.09, 114.00, 112.68, 109.48, 109.34, 109.21, 43.39, 31.49, 28.91, 26.91, 22.51,
3.98 ppm. IR (KBr): 3435.30, 3330.26, 2923.87, 2857.53, 2228.29, 2193.72, 1684.55, 1599.90,
3
1
1
1
−
1
467.60, 1368.92, 1237.25, 1126.51, 962.91, 895.09, 805.00, 746.44, 603.42, 563.54, 465.21 cm .
, requires [M + Na] 392.18.
HRMS: m/z: found [M + Na]. 392.45, molecular formula C23
H
23
N
5
2
2
.3. Detection of sensing properties of sensor 1 for copper ions
.3.1. Selective experiments
Both absorbance and fluorescence spectra were used to investigated the selective response of
sensor 1 to metal ions. Sensor 1 (184.5 mg, 0.5 mmol) was dissolved in acetonitrile (CH CN, 100
ml) to get the 5×10 M stock solution. And then, 0.2 ml stock solution were taken with microliter
3
-3

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pipette and added into 100 ml volumetric flask. After the CH
3
CN was added to the volumetric scale
mark, the 1×10-5 M solution of sensor 1 was obtained. 0.25 mmol of Cu(NO
)
·3H
O,
O,
, HgCl
were dissolved into deionized water (50 ml) to make 5×10-3
M metal ion solutions, respectively. For selective test, the each test solutions were prepared by
3
2
2
Ni(NO
Fe(NO
3
)
)
2
·6H
·9H
2
O,
O, Co(NO
O, CaCl
Al(NO
·6H
and SrSO
3
)
3
·9H
2
O,
Zn(NO
3
)
2
·6H
2
O,
Cr(NO
, AgNO
3
)
3
·9H
2
O,
·2H
Cd(NO
3
)
2`4H
2
3
3
2
3
)
2
2
O, Mg(NO
)
3 2
·6H
2
O, KNO
3
3
, BaCl
2
2
O, MnCl
2
2
,
PbCl
2
, FeSO
4
·7H
2
2
4
-3
-5
placing 0.05 ml of the each metal solution (5×10 M) into 2.5 ml sensor 1 solution (1×10 M),
respectively. In the mixture solutions, the metal ion is 10 equiv. of sensor 1. After mixing them for
three minutes, UV–vis absorbance and fluorescence spectra were taken at room temperature.
2.3.2. Absorbance and fluorescence titration
For each titration solution with different molar ratio of Cu2+ to sensor 1 was prepared by mixing
different volumes [µL] of Cu2+ and sensor 1 stock solutions, and then the mixing solution was
diluted using CH
CN to 2.5 mL. The concentration of sensor 1 in every solution was kept at 1×10-5
3
M. The absorbance and fluorescence spectra were taken at room temperature after finishing the
solution preparation for three minutes.
2.3.3. Job’ plot measurements
The stoichiometry of complexes can be obtained using Job’ plot method via the
measurement of absorbance and fluorescence spectra. Job’ plot was drawn based on the
measurement of a series of solutions in which the molar concentrations of ligand and metal ion vary
but their sum remains constant. The absorbance or fluorescence of each solution is measured and
plotted the maximum value of absorbance or emission against the mole fraction of ligand or metal
ion. The maximum or the inflection point on the Job’ plot appear at the mole ratio corresponding to
the combining ratio of the complex.
3. Results and Discussion
3.1. Synthesis and structural characterization

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The general synthetic procedure is given in Scheme 1. The target sensor 1 was easily synthesized
by the reaction of 2, 3-diaminomaleonitrile and 9-hexyl-9H-carbazole-3-carbaldehyde. The
1
13
structure of sensor 1 was well characterized by IR, H NMR, C NMR, and HRMS analyses. The
alkyl connected on the N atom of carbazole unit is aimed to increase the solubility in solvents.
3
.2 Fluorescent recognition of Cu2+
There are five nitrogen atoms with lone pair electrons in sensor 1, it is anticipated that sensor 1
can coordinate with metal ions and further change its fluorescent property. The preliminary
fluorescence spectra studies revealed that sensor 1 exhibited a better selectivity to Cu2+ in CH
3
CN
solution. Fig. 1 presents the fluorescence spectra of sensor 1, compound 2 (aldehyde carbazole)
and their mixed solutions with Cu2+ in CH
CN solutions, the concentration of sensor 1 and
3
-5
2+
compound 2 are kept at same as 1×10 M. The results obviously indicates that Cu endows the
large enhancement effect on the fluorescence of sensor 1 with the 70 nm red-shift compared with
the maximum fluorescence peak of sensor 1. At the excitation wavelength of 298 nm, sensor 1
presented two main emission peaks at 335 and 381nm, compound 2 also showed two emission
peaks at 334 and 430 nm. Their emissions are weak under measurement condition and relative
intensity are just larger than 100, which is probably due to the C=N isomerization [40]. In the
2
+
same concentration and measurement condition, after addition of 10 equiv. Cu , the fluorescence
spectrum of compound 2 had not obvious change and its intensity did not increase or decrease,
2
+
which indirectly indicated that compound 2 did not have obvious interaction with Cu . However,
upon the addition of Cu2+ into the sensor 1 solution, a new emission peak at 450 nm appeared and
its intensity was dramatic large to be 4800, it should be ascribed to the complex of sensor 1 with
2
+
2+
Cu . Before addition of Cu , the relative fluorescence intensity at 450 nm of sensor 1 was just
2
+
3
0, but it increased to 4800 after addition of 10 equiv. Cu . The fluorescence enhancement was
up to 160-fold, which also confirmed the complex of sensor 1 with Cu2+ has large fluorescence
-5
efficiency. The solution of sensor 1 possessed a quantum yield of 0.01 (1×10 M) in CH CN, after
3
2+
addition of 10 equiv. Cu , the quantum yield increased to 0.22. Meanwhile, the difference of
fluorescence spectra of sensor 1 with compound 2 upon the addition of Cu2+ also showed that the

ACCEPTED MANUSCRIPT
coordination effect of sensor 1 with Cu2+ comes from diaminomaleonitrile moiety but not
carbazole and aldehyde units. The fluorescence images of sensor 1 before and after addition of
2
+
2+
Cu in Fig. 1 also illustrated the large fluorescence enhancement after addition of Cu .
-
5
2+
Fig 1. Fluorescence spectra comparison of sensor 1, compound 2 (1×10 M) and their mixed solutions with Cu
ions (10 equiv.) in CH
CN solutions and fluorescence images of sensor 1 before and after addition of Cu2+ ions.
3
3.3 Selectivity
High selectivity toward target analyte over the other potentially competitive species is a very
important parameter to evaluate the performance of a chemosensor. Therefore, the fluorescence
-5
response of sensor 1 solution (1×10 M) in CH CN solutions toward various metal ions (10.0
3
equiv.) was conducted and the results were shown in Fig. 2. Free sensor 1 showed a very weak
emission with two peaks at 335 and 381 nm under the excitation of 298 nm light, upon addition of
1
0.0 equiv. of Cu2+ into the sensor 1 solution, a new emission band centered at 450 nm appeared
+
+
and displayed a dramatic fluorescence enhancement (Fig. 2a). Other metal ions such as K , Ag ,
2
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
3+
Mn , Co , Ni , Zn , Cd , Pb , Mg , Ca , Sr , Ba , Hg , Fe , Fe , Al and Cr (10.0
equiv. of each) did not induce significant fluorescence emission changes under the identical
conditions (Fig. 2b). Some of these potentially competitive metal ions also induced a new
emission peak at 450 nm but the intensities were very weak, which may be due to their very weak
interaction with sensor 1. These observations indicate that sensor 1 has an excellent selectivity to
2
+
Cu ion in CH CN solutions.
3

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-
5
3
Fig 2. Fluorescence response of sensor 1 (1×10 M) in CH CN towards to various metal ions (10.0 equiv.).
This fluorescence enhancement could be ascribed to the complex formation between sensor 1
2
+
and Cu . The fluorescence of sensor 1 is weak in CH CN solution due to C=N isomerization
3
2
+
which is the predominant decay process of the excited state. Upon addition of Cu , the
coordination of sensor 1 with the metal ion inhibits the C=N isomerization. On the contrary, other
ions fail to form such framework and did not increase the fluorescence intensity, which may be
due to the unsuitable coordination geometry conformation of the Schiff-base unit of sensor 1 and
the inappropriate ion radius and insufficient binding energy of these metal ions.
The anti-jamming ability is also important for one sensor to be used in practical applicability,
+
+
2+
2+
2+
2+
2+
2+
the possible interferences by metal ions including K , Ag , Mn , Co , Ni , Zn , Cd , Pb ,
Mg , Ca , Sr , Ba , Hg , Fe , Fe , Al and Cr3+ were measured through competitive
experiments. The competition experiment was carried out by monitoring the change of
fluorescence intensity at 450 nm upon addition of 10 equiv. Cu2+ and 10 equiv. other metal ions to
a solution of sensor 1, the results are shown in Fig. 3. Although exists of a certain degree of
perturbation caused by the above competitive metal ion, the same concentration of Cu2+ still
enhanced the fluorescence intensity of sensor 1 near to 100-fold of its original value. This result
indicated that the exists of the competitive metal ion could not interfere detection of sensor 1 for
2
+
2+
2+
2+
2+
2+
3+
3+
2
+
2+
Cu , in other words, the sensing of sensor 1 toward Cu has the high selectivity and sensitivity.

ACCEPTED MANUSCRIPT
Therefore, it is safe to say that sensor 1 is capable of fluorescent recognition of Cu2+ with high
selectivity from different interference metal ions.
Fig 3. (a) The fluorescence intensity changes of sensor 1 (1×10-5 M) to various metal ions. The black bars
represent the fluorescence intensity of sensor 1 in the presence of miscellaneous metal ions (10 equiv.), the red
bars represent the fluorescence intensity of the above solution upon further addition of 10 equiv. of Cu2+ (λem
=450 nm). (b) Fluorescence images of sensor 1 after addition of different metal ions. (For interpretation of the
reference to color in this figure legend, the reader is referred to the web version of this article.)
3.4 Fluorescence titration
2
+
To evaluate the sensing property of sensor 1 toward Cu , fluorescence titration experiment
-5
2+
of sensor 1 (1×10 M) with Cu (0~20 equiv) in CH CN solution was carried out. As shown in
3
2
+
Fig. 4, upon gradual increasement of Cu , the fluorescence intensity of the solution at 450 nm
increased. The fluorescence intensity reached to the highest value and the enhancement was
almost 160-fold when the 6 equiv. Cu2+ was added, see inset in Fig. 4. When more Cu2+ was
titrated, the fluorescence intensity did not further increase.
Fig 4. Fluorescence response of sensor 1 in CH
ex = 298 nm, λem = 450 nm. The inset is the plot of fluorescence intensity at 450 nm of sensor 1 as a function
of Cu2+ concentration.
CN solution (1×10-5 M) upon addition of Cu2+ (0~20 equiv.).
3
λ
Based on the titration data, the binding constant (K
) of sensor 1 with Cu2+ can be calculated
a

ACCEPTED MANUSCRIPT
using the Benesi-Hildebrand plot (Fig. 5a) [41]. Linear fitting of the experiment plot based on the
1
:1 binding stoichiometry of sensor 1 and Cu2+ was examined and the fitted curve are almost
superimposed over the experimental plot with a correlation coefficient over 0.9785, which
2
+
strongly supports the 1:1 binding stoichiometry of sensor 1 and Cu . The binding constant
between sensor 1 and Cu2+ was evaluated to be 1.26×10 M . The Job’s plot further confirmed
-6
-1
the 1:1 binding stoichiometry (Fig. 5b).
-
5
CN solution in the presence of Cu2+ (2.2~4.0
CN
Fig 5. (a) Benesi-Hildebrand plot of sensor 1 (1×10 ) in CH
equiv.) (R=0.9973). (b) Job’s plot from the fluorescence emission spectra of sensor 1 and Cu2+ in CH
3
3
-
5
solution with the total concentration of 5×10 M. λex = 298 nm, λem = 450 nm.
In addition, the fluorescent detection limit of sensor 1 for Cu2+ was also evaluated via the
calibration curve (Fig. S1) and the standard deviation of the blank solutions. The calibration curve
2
+
of fluorescence intensity at 450 nm versus [Cu ] was made and it displayed a linear graph, the
-8
linear relationship R is 0.9766. The detection limit CLOD was calculated to be 2.74×10 M from
the following equation: CLOD = K × Sb1/S. Here, K = 3; Sb1 is the standard deviation of the blank
solution; S is the slope of the calibration curve [42]. The detection limit is much lower than the
limit of copper in drinking water (~20 µM) permitted by US Environment Agency ( EPA ) [43]
and typical concentration of blood copper (1.57×10-5 ~ 2.36 ×10-5 M ) in normal individuals
defined by the U.S. Environmental Protection Agency [44]. This result indicates that sensor 1 is
2+
sensitive enough to monitor Cu concentration in solution.
The effect of the reaction time on the fluorescence emission of the system was examined and
the results are shown in Fig. S2. It can be seen that there is almost no time-dependent effects in the

ACCEPTED MANUSCRIPT
fluorescence sensing process, the Cu2+ solution was added into to sensor 1 solution under hand
shake, the emission intensity reaches to its saturation value in just 15 s due to the rapid complex
reaction. The solution is uniformly mixed ，the emission intensity reaches the saturation
immediately. Based on its very fast response, sensor 1, as a fluorescent chemosensor, can provide
2+
a very rapid detection of Cu .
The reversibility of fluorescence response process of sensor 1 for Cu2+ was also investigated
with ethylenediamine tetraaetic acid disodium salt (EDTANa
2
). After addition of excess EDTA
2+
2+
(
20 equiv. of Cu ) to sensor 1 and Cu solution, the fluorescence spectrum of the resulted
solution restored to the original spectrum of sensor 1 (Fig. S3), which indicates that the Cu2+
recognition is complexation process and reversible.
3
.5 Colormetric sensoring of sensor 1 for Cu2+
2
+
The selective response of sensor 1 for Cu was also confirmed by the changes of both color
and absorbance spectra of sensor 1 solution upon additions of different metal ions. Sensor 1 in
CH CN solution displayed two absorption bands centered at 290 nm and 398 nm corresponding to
3
the π–π transition of the C=N group and an intermolecular charge transfer (ICT) of the entire
conjugated molecule, respectively [40].
+
+
2+
2+
2+
2+
After addition of a variety of metal ions (10 equiv.) such as K , Ag , Mn , Co , Ni , Zn ,
Cd , Pb , Mg , Ca , Sr , Ba , Hg , Fe , Fe , Al and Cr3+ in deionized water (5×10 M ),
2
+
2+
2+
2+
2+
2+
2+
2+
3+
3+
-3
the absorption intensity did not show obvious change except for Fe , Pb and Cu2+ as shown in
3
+
2+
3+
2+
Fig.6a. The addition of Fe and Pb just caused the slight absorption increase. As the same as
fluorescence response, the absorbance spectra of sensor 1 presented significant change after
addition of 10 equiv. Cu2+ ions, the absorption band at 398 nm disappeared completely.
Meanwhile, the color of the solution became colorless completely from yellowish (Fig.6b), which
indicates that sensor 1 can serve as a colorimetric chemosensor for Cu2+ ion with high selectivity

ACCEPTED MANUSCRIPT
and sensitivity.
-
5
Fig 6. (a) Absorption spectral changes of sensor 1 (1×10 M) upon the addition of 10 equiv of various metal
-
4
ions in CH
ions.
3
CN. (b) The color changes of sensor 1 (1×10 M) upon the addition of 10 equiv of various metal
Spectrometric titration experiments can help us to disclose the interaction mechanism of
2+
CN with Cu2+ was subsequently
sensor 1 with Cu . Therefore, titration of sensor 1 in CH
3
2
+
-5
performed. Upon incremental addition of Cu (0～20 equiv.) to the sensor 1 solution (1×10 M)
results in a stepwise decrease and increase in absorbance at 398 nm and 290 nm, respectively.
Meanwhile, a new absorption peak at 320 nm appeared and increased gradually with the addition
2
+
-4
-1
-1
of Cu (Fig.7). The mole extinction coefficient (ɛ = 1.1×10 L·g ·cm )) of sensor 1 at 398 nm
decreased near to 0. There also existed an isobestic point at 340 nm in the titration spectra. From
these results, we could estimate that the complex between sensor 1 and Cu2+ formed and the
binding effect of sensor 1 with Cu2+ was the direct reason for inducing the fluorescence
2
+
enhancement. In other word, the complexes of sensor 1 with Cu have high fluorescent quantum
yield.
Fig 7. Changes in the absorption spectra of sensor 1 (1×10-5 M) upon addition of Cu2+ (0～20 equiv.) in CH
CN
3
solution and the color change of the solution.

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The binding constant was also evaluated using the Benesi-Hildebrand plot via fitting of the
2
+
absorption data obtained by titration of sensor 1 with Cu (Fig. S4) [45]. The absorbance at 398
2
+
nm gradually decreased with the successive addition of Cu . The Benesi-Hildebrand plot showed
a linear line with the correlation coefficient 0.9976 and the binding constant was calculated to be
-6
2+
1
.98×10 . The linear plot also illustrates that sensor 1 form 1:1 stoichiometric complex with Cu ,
and 1:1 binding mode was further confirmed by Job’s plot (Fig. S5). On the Job’s plot, the
2
+
2+
inflection point appeared at 0.5 of the mole ratio of [Cu ]/[sensor 1] +[Cu ]. These results are
consistent with those of fluorescence measurement.
3.6 Proposed binding mode
The response of the fluorescence and absorbance of sensor 1 to Cu2+ provided an obvious
symbol of interaction and complex formation between sensor 1 and Cu2+ with the 1:1
stoichiometry. Comparison of absorbance spectra of carbazole (compound 2 in Scheme 1), sensor
2+
1
and sensor 1-Cu in CH CN solution can help us to interpret the sensing mechanism of sensor 1
3
for Cu2+ (Fig. S6). Carbazole presents two main absorption peaks at 262 and 293 nm, sensor 1
displays one more absorption peak at 398 nm, which is ascribed to the intermolecular charge
transfer (ICT) band of the entire conjugated molecule. So, it can be stated that the absorption peak
at 398 nm of sensor 1 is caused by combining of carbazole with diaminomaleonitrile. The
absorption peak of sensor 1 at 398 nm disappeared completely but absorption peaks of carbazole
2
+
did not change obviously after coordination with Cu , which state that the coordination unit of
sensor 1 with Cu2+ is diaminomaleonitrile unit but not carbazole moiety. In addition, the
fluorescence spectra of compound 2 also did not change obviously after addition of Cu2+ in
CH
3
CN solution (Fig. 1), which indicated that both aldehyde group and carbazole could not take
2+ 1
part in coordination with Cu . H NMR studies also provide further evidence for the interaction
2+
2+
between sensor 1 and Cu . Upon the addition of 10 equiv. Cu , protons peaks on the sensor 1

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broadened and shifted downfield as shown in Fig. S7. Based on the above results, we proposed the
binding mode as in scheme 2.
Scheme 2. Proposed mechanism of fluorescence enhancement of Sensor 1
4. Conclusions
In conclusion, we have developed a new simple fluorescent chemosensor based on Schiff base
for Cu2+ with high sensitivity and selectivity in CH
3
CN solution. The fluorescence intensity of
2
+
chemosensor was significantly enhanced about 160 fold with the addition of 6 equiv of Cu . In
2
+
addition, it also displayed a greater colorimetric sensing ability for Cu via the changes of color and
absorbance spectra. Most importantly, both the color and fluorescence changes of the chemosensor
solution are significant for Cu2+ in the presence of other heavy and transition metal ions (even in
2
+
high concentration). The low detection limit and fast response time of chemosensor to Cu meet the
selective detection requirements for biomedical and environmental monitoring application. We
expect that this chemosensor provides a potential tool for the detection and quantification of Cu2+
ions in environmental samples.
Acknowledgments: This work was supported by the National Natural Science Foundation of China
(
no. 50873019) and Research Fund for the Doctoral Program of Higher Education of China
20120043110007).
(
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Figure Caption:
Scheme 1. The synthetic route of sensor 1.
Scheme 2. Proposed mechanism of fluorescece enhancement of Sensor 1.
-
5
2+
Fig 1. Fluorescence spectra comparison of sensor 1, compound 2 (1×10 M) and their mixed solutions with Cu
ions (10 equiv.) in CH
CN solutions and fluorescence images of sensor 1 before and after addition of Cu2+ ions.
3
-
5
3
Fig 2. Fluorescence response of sensor 1 (1×10 M) in CH CN towards to various metal ions (10.0 equiv.).
Fig 3. (a) The fluorescence intensity changes of sensor 1 (1×10-5 M) to various metal ions. The black bars
represent the fluorescence intensity of sensor 1 in the presence of miscellaneous metal ions (10 equiv.), the red
bars represent the fluorescence intensity of the above solution upon further addition of 10 equiv. of Cu2+ (λem
=450 nm). (b) Fluorescence images of sensor 1 after addition of different metal ions. (For interpretation of the
reference to color in this figure legend, the reader is referred to the web version of this article.)
-
5
2+
Fig 4. Fluorescence response of sensor 1 in CH
298 nm, λem = 450 nm. The inset is the plot of fluorescence intensity at 450 nm of sensor 1 as a function of
Cu2+ concentration.
3
CN solution (1×10 M) upon addition of Cu (0~20 equiv.). λex
=
-
5
CN solution in the presence of Cu2+ (2.2~4.0
CN
Fig 5. (a) Benesi-Hildebrand plot of sensor 1 (1×10 ) in CH
equiv.) (R=0.9973). (b) Job’s plot from the fluorescence emission spectra of sensor 1 and Cu2+ in CH
3
3
-
5
solution with the total concentration of 5×10 M. λex = 298 nm, λem = 450 nm.
-
5
Fig 6. (a) Absorption spectral changes of sensor 1 (1×10 M) upon the addition of 10 equiv of various metal
-
4
ions in CH
ions.
3
CN. (b) The color changes of sensor 1 (1×10 M) upon the addition of 10 equiv of various metal
Fig 7. Changes in the absorption spectra of sensor 1 (1×10-5 M) upon addition of Cu2+ (0～20 equiv.) in CH
CN
3
solution and the color change of the solution.

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Ultrasensitive and highly selective detection of Cu2+ ions based on a new
carbazole-Schiff
Jun Yin, Qijing Bing, Lin Wang, Guang Wang*
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Graphical abstract

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2+
Ultrasensitive and highly selective detection of Cu ions based on a
new carbazole-Schiff
Jun Yin, Qijing Bing, Lin Wang, Guang Wang*
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Highlights
►
Carbazole-based Schiff as fluorescent chemosensor (sensor 1) was successfully
synthesized.
Sensor 1 displayed high selective and sensitive fluorescence sensing for Cu .
Sensor 1 showed high anti-interference ability in the co-existence of other metal
ions.
Sensor 1 also showed the ability of colorimetric sensing for Cu ions.
2
+
►
►
2
+
►