A highly selective and sensitive fluorescent sensor for copper(ii) ion characterized by one dichlorofluorescein moiety and two azathia-crown ether

Article abstract of DOI:10.14233/ajchem.2013.14722

A new fluorescent sensor 1, which functionalized with one dichlorofluorescein moiety as fluorogenic signaling subunit and two azathiacrown ether macrocyclic as binding site was designed, synthesized and characterized. It was found that sensor 1 can selectively recognize Cu²⁺ by a remarkable emission quenching in DMSO-H₂O (1:1, v/v), which was attributed to the 1:2 complex formation between 1 and Cu²⁺, while other ion including K⁺, Na⁺, Mg²⁺, Zn ²⁺, Ba²⁺, Co²⁺, Ca²⁺, Fe ²⁺, Fe³⁺, Pb²⁺, Pb²⁺, Ni ²⁺ and Ag⁺ induced no or much small fluorescence changes. In addition, the sensor 1 exihibited a rapid, stable and linear response to Cu²⁺ in the concentration range from 5 × 10^-7 to 1 × 10^-5 mol L^-1 with a detection limit as low as 8.7 × 10^-8 mol L^-1. Furthermore, the sensor 1 was applied to practical determination of Cu²⁺ in different water samples with satisfactory results.

Full text of DOI:10.14233/ajchem.2013.14722

Asian Journal of Chemistry; Vol. 25, No. 15 (2013), 8292-8296
http://dx.doi.org/10.14233/ajchem.2013.14722
A Highly Selective and Sensitive Fluorescent Sensor for Copper(II) Ion Characterized
by One Dichlorofluorescein Moiety and Two Azathia-Crown Ether
1
2
1
1
1
1,*
YUZHU JIANG , ZHONG HUANG , HUAJUN DAI , LIHUA WANG , LIU YING and XINGMING KOU
1College of Chemistry, Sichuan University, Chengdu 610064, P.R. China
2Academy of Engineering Physics, Mianyang, Sichuan 621900, P.R. China
*Corresponding author: Fax: +8685412291; Tel: +13980995879; E-mail: kouxm@scu.edu.cn
(Received: 20 October 2012;
Accepted: 19 August 2013)
AJC-13936
A new fluorescent sensor 1, which functionalized with one dichlorofluorescein moiety as fluorogenic signaling subunit and two azathia-
crown ether macrocyclic as binding site was designed, synthesized and characterized. It was found that sensor 1 can selectively recognize
Cu2+ by a remarkable emission quenching in DMSO-H2O (1:1, v/v), which was attributed to the 1:2 complex formation between 1 and
Cu2+, while other ion including K+, Na+, Mg2+, Zn2+, Ba2+, Co2+, Ca2+, Fe2+, Fe3+, Pb2+, Pb2+, Ni2+ and Ag+ induced no or much small
fluorescence changes. In addition, the sensor 1 exihibited a rapid, stable and linear response to Cu2+ in the concentration range from 5 ×
10-7 to 1 × 10-5 mol L-1 with a detection limit as low as 8.7 × 10-8 mol L-1. Furthermore, the sensor 1 was applied to practical determination
of Cu2+ in different water samples with satisfactory results.
Key Words: Fuorescent probe, Azathia-crown ether, Dichlorofluorescein, Copper ion, Fluorescence quenching.
which are extensively used in the field of heavy metal ion
recognition and detection because of their sensitivity, simplicity
and practicability15. So far, many effective fluorescent sensors
for heavy metal ion including Cu2+ have been successfully
developed16-18. However, there is still plenty of room for impro-
vements in terms of fluorescence intensity quenching, selec-
tivity, aqueous solubility, suitable fluorophores with a high
fluorescence quantum yield and visible excitation and emission
wavelengths, as well as availability without resorting to instru-
ments.
INTRODUCTION
Copper ions are essential for normal physiological
processes of living things1,2. In humans, copper is the third
most abundant transition metal ion3-6, acting in several biolo-
gical systems and also a cofactor in diverse biochemical
processes of enzymes syntheses.Additionally, excessive amounts
or defects in intake of copper ions cause several possible
alterations to physiological processes7,8, which result in
systemic diseases and triggers, such as Menkes syndrome,
amyotrophic lateral sclerosis and Wilson's disease9.At the same
time, copper is an economically important element that is found
in only trace quantities in the Earth's crust and main copper is
often connected to effluents from septic tanks and municipal
wastewaters, discharges from power plants as well as leaching
from antifouling paints and pressure-treated docks pilings. It
is well known that the free cupric ion is highly toxic for marine
organisms10,11. Therefore, an efficient and reliable analytical
systems for the trace levels of copper detection is urgent nec-
essary. Thus far, various efficient and reproducible methods,
such as atomic absorption spectrometry12, inductively coupled
plasma mass spectroscopy (ICP-MS)13 and inductively coupled
plasma atomic emission spectrometry (ICP-AES)14 were
established. However, most of these methods are usually
complicated, time-consuming and costly. To the best of our
knowledge, fluorescent probes are a class of superior sensors,
Moving along these lines, we have developed a series of
chemosensors for Hg2+ detection19,20. In this paper, we designed
and synthesized a novel fluorescent chemosensor 1 (Scheme-
I) for Cu2+ based on intramolecular charge transfer mechanism
(ICT)21,22, which is an effective signaling mechanism employed
in the design of fluorescent sensors.As usual, the chemosensor
is composed of two sections, namely a binding site and a
signaling subunit connected through a covalent bond. The
binding site is responsible for the interaction with the host
and is designed bearing in mind supramolecular chemistry
principles in order to achieve a high degree of complementarity
between both components. The interaction between the binding
site and the host is transformed in an easy-to observe output
by the signalling subunit. Due to its superior properties to
coordinate with heavy and transition metal ions23-26, azathia-
crown ether was chosen as receptors of sensor 1. Similarly,

Vol. 25, No. 15 (2013)
A Highly Selective and Sensitive Fluorescent Sensor for Copper(II) 8293
dichlorofluorescein moiety, which is widely used as a
fluorophore with high fluorescence quantum yield and visible
light excitation, was selected as signaling subunit in sensor
127. Sensor 1 shows a selective response for Cu2+ in the pres-
ence of other related heavy and transition metal ions in a wide
pH range from 3 to 6 and a remarkable fluorescence quenching,
which was ascribed to the suppressing of intramolecular charge
transfer in sensor 1. Keeping this in mind, a highly selective,
sensitive and rapid-response fluorescent chemosensor 1
containing one signaling subunit and two binding sites was
developed.
40 ºC and allowed to stirred for 6 h. The excess carbon disulfide
was removed by distillation at normal pressure. Then the
procreant turkey red liquid was diluted with 75 mL of water,
giving a transparent solution. Then 40 mL of carbon disulfide
(0.3 mol) was added dropwise to the aqueous sodium
trithiocarbonate obtained above at 20 ºC. Then the reaction
mixture was allowed to stir for 5 h at 65 ºC. The resulting
basic solution was extracted three times with ether after cooling
to remove unreacted material and other nonacid impurity. Then
sulfuric acid (3 mol L-1) was added to the aqueous layer to
adjust pH to 2 and the acidic aqueous medium was extracted
four times with ether. The combined extract was washed with
water until it became neutral and dried over anhydrous magne-
sium sulfate. Distillation at reduced pressure (15 mm Hg) after
removing solvent gave β,β-dimercaptodi-ethyl ether as a
colourless but quite odoriferous liquid in 65 % yield. Bp.: 110
Na2CS3
Cl
O
Cl
H2SO4
HS
O
SH
Cl
Cl
OH
OH
SOCl2
HN
Cl H2N
1
ºC (15 mm Hg). H NMR (ppm, CDCl3, 400 MHz): δ 1.56-
1.52 (t, 2H, SH), 2.62-2.57 (m, 4H, SCH2), 3.51-3.48 (t, 4H,
OCH2). MS (m/z): 137 [M-1].
Bis-(2-chloroethyl)amine hydrochloride28 diethanol-
amine (21.9 g, 0.21 mol) was dissolved in chloroform (30
mL) in a 250 mL three necked round-bottomed flask keeping
at 0 ºC. A solution of thionyl chloride (99.3 g, 0.84 mol) in
CHCl3
S
LiOH
THF
Cl
Cl
NH
O
Cl H2N
HS
O
SH
S
O
O
S
HCHO
CH3CN
O
HN
Cl
HO
Cl
OH
O
O
Cl
Cl
OH
S
O
HO
S
O
chloroform (30 mL) was added dropwise to the above solution
and stirred at 4-6 ºC for 2 h. Then the reaction mixture was
placed at room temperature to react for 3 h and was heated to
60 ºC to react for 4 h. The reactant was then filtered and washed
with chloroform to get a white solid bis-(2-chloroethyl)amine
hydrochloride in 62 % yield. m.p.: 212-214 ºC. 1H NMR (ppm,
D2O, 400 MHz): δ 3.49-3.46 (t, 4H, ClCH2), 3.86-3.83 (t, 4H,
NCH2). IR (KBr, νmax, cm-1): 3423 (NH), 2952 (CH2).
N
O
N
O
S
S
S
1
Scheme-I: Synthetic route of 1
EXPERIMENTAL
Water used was twice-distilled through out all experi-
1-Oxa-4,7-dithia-10-azacyclododecane29 a mixture of
β,β'-dimercaptodiethyl ether (2.76 g, 120 mmol), bis-(2-
chloroethyl)amine hydrochloride (3.58 g, 20 mmol) and
lithium hydroxide (2.88 g,120 mmol) in absolute tetrahydro-
furan (1500 mL) was refluxed under a nitrogen atmosphere
for 5 days. The reaction mixture was filtered and then concen-
trated at reduced pressure. The residue was extracted three
times with chloroform and the extract was washed with water.
The solvent was removed after drying over anhydrous sodium
sulfate. The crude product was recrystallized from hexane to
give a white solid 1-oxa-4,7- dithia-10- azacyclododecane in
37 % yield. m.p.:62-63 º. 1H NMR (ppm, CDCl3, 400 MHz): δ
2.81-2.72 (m,13H, CH2SCH2CH2NH), 3.58-3.56 (t, 4H,
OCH2). IR (KBr, νmax, cm-1): 3274, 3249 (NH), 2916, 2860,
2815 (CH2), 1116 (OCH2). MS (m/z): 208 (M+1)+.
ments. All the chemicals were of analytic grade and used as
received. Dimethyl sulfoxide, sodium sulfide, carbon disulfide,
concentrated sulfuric acid (98 %), thionyl chloride, concen-
trated hydrochloric acid (37 %), triethylamine, dichloro-
methane, chloroform, sodium carbonate, acetonitrile, tetrahy-
drofuran, diethanol amine, diethyl ether, ethanol, lithium
hydroxide, paraformaldehyde were obtained from Chengdu
KeLong Chemical Reagents Factory, dichlorofluorescein and
dichlorodiethyl sulfide were purchased from Bailingwei
Chemical Company.All the reagents used as received without
further purification. Both acetonitrile and triethylamine were
prepared by refluxing with calcium hydride and distilled at
atmospheric pressure. Lithium hydroxide was dried at 120 ºC
for about 10 h in vacuum oven.
Fluorescence spectrometric data were obtained on a
Hitachi F-4500 fluorescence spectrophotometer with a 1-cm
1
quartz cell. H NMR spectra were determined on a Varian
The sensor 130 dichlorofluorescein (0.1368 g, 0.66 mmol)
and paraformaldehyde (0.17 g, 5.66 mmol) were dissolved in
acetonitrile (15 mL) in a 100 mL of three necked round-
bottomed flask, then the mixture was refluxed under a nitrogen
atmosphere for 0.5 h. With stirring, a solution of dichloro-
fluorescein (0.1245 g, 0.33 mmol) in 30 mL of CH3CN-H2O
(1:1, v/v) was added to the above solution and then the mixture
was refluxed for 24 h. The reaction mixture was first concen-
trated at reduced pressure to give a crude product. It was puri-
fied by silica gel chromatography using CH2Cl2-CH3OH (10:1,
v/v) as eluent, finally, a red solid was obtained by 45 % yield.
UNITY INVOA-400 MHz spectrometer. Mass spectra (MS)
were measured on an API-3000LC/MS/MS spectrometer.
Melting points were measured with a XRC-I melting point
apparatus. The IR spectrometric data were obtained on a Perkin
Elmer 16PC FT-IR spectrometer. All pH measurements were
obtained on a pH-25 pH meter.
Synthesis: β,β-dimercaptodiethyl ether20. To a solution
of sodium sulfide (48 g, 200 mmol) dissolved in 30 mL water,
25 mL of carbon disulfide (600 mmol) was added dropwise at
room temperature. Then the reaction mixture was warmed to
1
The synthetic route of sensor 1 was shown in Scheme-I. H

8294 Jiang et al.
Asian J. Chem.
NMR (ppm, CDCl3, 400 MHz): δ 2.83-2.67 (m, 24H, CH2),
3.72-3.51 (m, 12H, CH2), 6.79 (s, 2H,ArH), 7.31 (d, 1H,ArH),
7.66 (m, 2H, ArH), 8.20-8.16 (d, 1H, ArH); IR (KBr, νmax, cm-1):
3443 (OH), 2925 (CH2), 1630.
RESULTS AND DISCUSSION
Scheme-II: Proposed binding mode of 1 with Cu2+ and sensing mechanis
Fluorescence spectral characteristic of 1: The fluore-
scence behaviour of sensor 1 was investigated in DMSO-water
(1:1, v/v) solvent systems. The excitation spectrum of sensor
1 was characterized by a maximum excitation at 500 nm. As
shown in Fig. 1, the maximum emission wavelength was
located at 546 nm. When Cu2+ was added to the solution of 1,
the fluorescence of 1 was quenched. In addition, the intensity
of emission peak at 546 nm was found to be decreased gradu-
ally with the addition of Cu2+ to the solution of 1 and the maxi-
mum emission wavelength did not change, which constituted
the basis for the recognition of Cu2+ with sensor 1 proposed in
this work. At the same time, the coordination of sensor 1 and
Cu2+ was found to be reversible by the changes of fluore-
scence. When 25 equiv. of Cu2+ was added to the solution of 1,
the fluorescence disappeared. While 25 times of concentration
of EDTA is added subsequently, the fluorescence recovered
at once, finally, fluorescence was quenched again upon the
addition of Cu2+ (equiv). This phenomenon could be explained
by intramolecular charge transfer mechanism (ICT), which is
generally thought to be a typical signaling mechanism. Just as
shown in Scheme-II, sensor 1 displayed intense fluorescence
based on the charge transfer of electron-rich group (azathia-
crown ether) to the electron-withdrawing group (dichloro-
fluorescein). After addition of Cu2+, owning to the interaction
between the fluorescent sensor 1 and Cu2+, the above-mentioned
ICT was suppressed. As a result, a fluorescence quenching
was observed. Furthermore, the complexation stoichiometry
of Cu2+ and 1 was studied and found to be 2:1 by the Job's
plot, which was shown in Fig. 2.
500
400
300
200
100
0.0
0.2
0.4
0.6
0.8
1.0
Mole fraction of 2 (X2)
Fig. 2. Job's plot of 1-Cu2+ system in DMSO-water (1:1), [1]+[ Cu2+] = 1.0
× 10-5
ions. It is found that Cu2+ has caused the most obvious
fluorescence quenching after 5 equiv of K+, Na+, Mg2+, Ca2+,
Ba2+, Zn2+, Fe3+, Fe2+, Co2+, Ni2+, Cd2+, Pb2+, Ag+, Hg2+ and
Cu2+ were added respectively. Except forAg+, other ions induce
a little fluorescence quenching under identical conditions as
depicted in Fig. 3. This indicated that sensor 1 owns high
selectivity to Cu2+, thoughAg+ may induce some interferences.
To evaluate quantitatively the selectivity of sensor 1, we have
investigated the selectivity coefficients31 of Cu2+ via the formula
SCu2+/S0, SCu2+ is the fluorescent change caused by Cu2+, S0 is
the fluorescent change induced by other ions. The selectivity
coefficients of Cu2+ against other metal ions were listed in
Selectivity: Generally, high selectivity plays a vital role
to determine whether a chemosensor is an excellent one or
not, so the selectivity of sensor 1 has been studied in the
presence of different alkali, alkaline earth and transition metal
1400
1200
1400
1200
1000
800
600
400
200
0
1, K+, Na+, Mg2+, Zn2+, Ba2+, Co2+
Ca2+, Fe2+, Fe3+, Pb2+, Ni2+, Hg2+
,
1000
800
600
400
200
0
Ag+
Cu2+
520
540
560
580
600
620
640
520
540
560
580
600
620
640
Wavelength (nm)
Wavelength (nm)
Fig. 1. Fluorescence titration of 1 (10-5 mol L-1) with increasing concen-
tration of Cu2+ (0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 ×
10-5 mol L-1) in water-DMSO (1:1,v/v)
Fig. 3. Fluorescence spectra (excitation at 500 nm) of 1 in water-DMSO
(1:1,v/v) and in the presence of 5 equiv of K+, Na+, Mg2+, Zn2+,
Ba2+, Co2+, Ca2+, Fe2+, Fe3+, Pb2+, Pb2+, Ni2+, Ag+, Cu2+

Vol. 25, No. 15 (2013)
A Highly Selective and Sensitive Fluorescent Sensor for Copper(II) 8295
Table-1. Apparently, 1 can recognize Cu2+ specifically in the
presence of other metal ions.Actually, the azathia-crown ether
moiety of sensor 1 played an important role in Cu2+ recognition.
According to the theory of soft and hard acid-base, sulfur is a
soft base, nitrogen is a middle-intensity base and Cu2+ is a soft
acid, therefore, Cu2+ has a strong ability to coordinate with
sulfur and nitrogen atoms to form a stable coordination comp-
ound, which gave 1 a high selectivity.
1 in the absence of Cu2+ showed a trend of slow decrease.
When pH is higher than 6, the fluorescence intensity decreased
greatly with the increasing pH, which might be induced by
the isomerization of dichlorofluorescein32. Upon addition of
Cu2+, the fluorescence quenching varied barely with the pH in
the range of 5-7. Nevertheless, with the pH varied form 3 to 5,
fluorescence quenched much more intently, which may due
to the protonation of the nitrogens of 1. In order to avoid the
interference caused by pH, the pH was controlled during 5-7
used phosphate buffer.
TABLE-1
SELECTIVITY COEFFICIENTS OF Cu2+ OVER
OTHER SELECTED METAL IONS
1400
1200
1000
800
600
400
200
0
Metal
ion
Ca2+
Co2+
Fe3+
Ba2+
Mg2+
Pb2+
Hg2+
Selectivity
coefficient
Metal
ion
Selectivity
coefficient
40.0
42.7
42.5
44.5
46.5
25.2
47.0
Cd2+
Zn2+
Ni2+
Na+
K+
40.1
43.3
41.2
56.3
56.0
47.4
2.43
Fe2+
Ag+
For further investigation the interference from other metal
ions on the fluorescence determination of Cu2+, quenching
ratio29 (I0/I) change of 1 was studied, where I0 is fluorescence
intensity of 1, I is the fluorescence intensity in the presence of
5 equiv. of Cu2+, coexisting with 50 equiv. of other background
metal ions, including K+, Na+, Mg2+, Ca2+, Ba2+, Zn2+, Fe3+,
Fe2+, Co2+, Ni2+, Hg2+, Cd2+, Pb2+, Ag+ respectively. The results
was shown in Fig. 4. Clearly, the coexisted ions cause a little
interference except Ag+. All of these results indicated that the
sensor 1 was of a good selectivity.
3
4
5
6
7
pH
Fig. 5. Effect of pH on the fluorescence intensity of 1 [in the absence (a)
and presence (b) of 5 equiv. of Cu2+]
Detection limit and linear range: Under the optimal
experiment condition, quenching ratio (I0-I)/I0 of 1 was linearly
proportional to the concentration of Cu2+ in the range of 5.0 ×
10-7-1.0 × 10-5 mol L-1 with the correlation coefficient of 0.9947
and the linear regression equation was proposed to be (I0-I)/I0
= 0.1003 × 106c + 0.0323, where I0 is the fluorescence intensity
of 1, I is the fluorescence intensity in the presence of Cu2+, c is
the concentration of Cu2+. The detection limit was as low as
8.7 × 10-8 mol L-1, which was measured by following equation.
10
8
6
3σbi
Detection limit =
m
4
where m is the slope of standard curve based on the fluore-
scence quenching ratio (I0-I); I0 of 1 vs. Cu2+ concentration.
σbi Is the standard deviation of blank measurements (n = 11).
Effect of reaction time: For an excellent chemosensor,
fast response is a matter of necessity. Consequently, the effect
of reaction time on the binding process of Cu2+ to 1 was investi-
gated. The results are depicted in Fig. 6. As shown in Fig. 6,
the fluorescence intensity reached the plateau region after 2
to 3 s and kept stable until to 1200 s. This indicated that the
response time of 1 to Cu2+ is very short, which was owed to
the fast and complete reaction between 1 and Cu2+. Conse-
quently, we can obtain the fluorescence emission spectrum of
1 immediately upon addition of Cu2+.
2
0
Fig. 4. Quenching ratio (I0/I) of fluorescence intensity of 1 (1.0 × 10-5 mol
L-1) at 546 nm in the presence of 5 equiv. of Cu2+ upon addition of
50 equiv. of other metal ions respectively
Effect of pH: The intense fluorescence of the sensor 1
originates from dichlorofluorescein moiety, which was affected
by pH in some extent. Therefore, the effect of pH on fluore-
scence intensity of the sensor 1 in the absence and presence of
Cu2+ was studied. pH was controlled by buffer and Cu2+ concen-
tration was fixed at 1 × 10-5 mol L-1 in the experiments. The
results were shown in Fig. 5, it can be seen from the Fig. 5 that
during the pH range from 3 to 6, the fluorescence intensity of
Stability constant: As stated above, 1 shows high selec-
tivity to Cu2+, which can be attributed to its excellent coordi-
nation with Cu2+. So stability constant of coordination com-
pound was measured to investigate its stability. Stability
constant was obtained by following formula:

8296 Jiang et al.
Asian J. Chem.
1.0
Conclusion
In summary, the investigation described above has resulted
in the development of a highly selective and sensitive chemo-
sensor 1 for Cu2+ based on ICT mechanism. The system, which
using a novel strategy by designing a molecule containing one
fluorophore and two ionophores, has displayed a considerable
selectivity for Cu2+. In addition, sensor 1 showed fairly low
detection limit for Cu2+, which was as low as 8.7 × 10-8 mol L-1,
this was attributed to the specific affinity of Cu2+ for 1. In
addition, the proposed sensor 1 was successfully applied to
the determination of Cu2+ in water samples with satisfatory
results.
0.8
0.6
0.4
0.2
0
ACKNOWLEDGEMENTS
The authors gratefully acknowledged Analytical and
Testing Center, Sichuan University.
0
200
400
600
800
1000
1200
Time (s)
Fig. 6. Effect of reaction time on the fluorescence intensity of 1 in the
presence of Cu2+
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
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







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(mol L-1)
Cu2+ recovered
(mol L-1)
Recovery
(%)
Drinking water
1
0
Not detected
(1.19 0.02) × 10-6
(3.61 0.01) × 10-6
-
2
1.20 × 10-6
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Tap water
1
0
Not detected
(1.18 0.02) × 10-6
(3.53 0.03) × 10-6
-
2
1.20 × 10-6
3.60 × 10-6
98.3
98.1
27. H. Yin, Y.F. Xu and X.H. Qian, Bioorg. Med. Chem., 15, 56 (2007).
28. Y. Habata and F. Osaka, Dalton Trans., 18, 36 (2006).
29. E.M. Zahran, V. Gavalas, M. Valiente and L.G. Bachas, Anal. Chem.,
82, 3622 (2010).
3
River water
1
2
3
0
Not detected
(1.17 0.02) × 10-6
(3.45 0.04) × 10-6
-
30. Y.L. Zhang, P.T. Jing and Q.H. Zeng, J. Phys. Chem., 113, 1886 (2009).
31. D. Yang, H.-L. Wang and Z.-N. Sun, Am. Chem. Soc., 18, 6004 (2006).
32. S. Goswami and R. Chakrabarty, Tetrahedron Lett., 43, 5910 (2009).
1.20 × 10-6
3.60 × 10-6
97.5
95.8