A highly selective chemosensor for copper ion based on ICT fluorescence

Article abstract of DOI:10.1016/j.cclet.2012.03.007

Simple structural compounds 1 to 3 were synthesized. The presence of Cu²⁺ resulted in the fluorescence and absorption spectra change of 1 and 2, which indicated that 1 and 2 showed a highly selective response to Cu²⁺ over other metal ions. However, 3 showed no selectivity for metal ions, which means that the compound could bind with several metal ions, such as, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Fe³⁺, Mg²⁺, Ca²⁺, and Co ²⁺, except Cu²⁺ and Ag⁺. The different spectral responses were attributed to the difference in binding sites for 1 and 3.

Full text of DOI:10.1016/j.cclet.2012.03.007

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 607–610
www.elsevier.com/locate/cclet
A highly selective chemosensor for copper ion based on
ICT fluorescence
*
Fang Ying Wu , Sheng Gen Cao, Cai Xia Xie
Department of Chemistry and Center of Analysis and Testing, Nanchang University, Nanchang 330031, China
Received 23 December 2011
Available online 18 April 2012
Abstract
Simple structural compounds 1 to 3 were synthesized. The presence of Cu2+ resulted in the fluorescence and absorption spectra
change of 1 and 2, which indicated that 1 and 2 showed a highly selective response to Cu2+ over other metal ions. However, 3 showed
no selectivity for metal ions, which means that the compound could bind with several metal ions, such as, Ni2+, Zn2+, Cd2+, Hg2+
,
Pb2+, Fe3+, Mg2+, Ca2+, and Co2+, except Cu2+ and Ag+. The different spectral responses were attributed to the difference in binding
sites for 1 and 3.
# 2012 Fang Ying Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 4-Dimethylaminobenzoylamino derivatives; ICT fluorophore; Cu2+; Dual spectral response
Copper ion plays an important role in the fundamental physiological processes of organisms ranging from bacteria
to mammals [1]. The selective sensing of Cu2+ is a very important topic for the detection and treatment of various
chemical systems, including living systems [2]. However, Cu2+ can be toxic if its level exceeds cellular requirements.
Thus, designing sensors for Cu2+ has recently drawn worldwide attention. Among fluorescent sensors, the binding of
Cu2+ with an ionophore resulted in fluorescence enhancement [3] or fluorescence quenching [4] via photo-induced
electron transfer. Ratiometric assay has also been utilized recently to increase selectivity and sensitivity. Ratiometric
fluorescent probes allow signal rationing, and thus increase the dynamic range and provide built-in correction for
environmental effects [5]. Therefore, a few Cu2+ ratiometric sensors have been reported [6]. Sensors containing
intramolecular charge transfer (ICT) fluorophore [6d,6f] or excimer-forming pyrene moieties [3g,7] usually emit dual
fluorescence. p-Dimethylaminobenzoylamino is an ICT fluorophore and the N atom of amino could coordinate with
metal ions [6f,8], whereas amino acetic acid moiety serves as a good ligand for metal ions [3f]. The combination of
these two moieties could thus form a promising fluorescent probe for metal ions. In this paper, simple structural
compounds 1–3 that could emit dual fluorescence were synthesized. Compared with 3, 1 and 2 selectively recognize
Cu2+ over other metal ions because of the difference in binding sites between 1 or 2 and 3.
* Corresponding author.
E-mail address: fywu@ncu.edu.cn (F.Y. Wu).
1001-8417/$ – see front matter # 2012 Fang Ying Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2012.03.007

608
F.Y. Wu et al. / Chinese Chemical Letters 23 (2012) 607–610
O
O
OC H
2
5
N
HN CH
2
H N
2
N
N
COOC H
2
1
5
triethylamine,
in CH Cl , 0~5¡æ
C O Cl ,
2
2
2
N
2
2
N
in CH Cl
CH OH, rt
3
NaOH,
2
2
refuxing.
CO C H
2
2
5
HN
HO
O
Cl
O
CO C H
2
2
5
HO
O
O
N
C H O
O
O
N
2
5
O
O
OH
3
2
OC H
2
5
Scheme 1. Structure of fluorescent compounds 1–3.
1. Experimental
All fluorescence measurements were conducted on an F-4600 spectrofluorimeter (Hitachi, Japan) equipped with a
xenon lamp source and a 1.0 cm quartz cell. Absorption spectra were recorded on a Shimadzu-2501 UV–vis
spectrophotometer (Japan) with a 1.0 cm quartz cell. 1H NMR spectra were obtained on a Bruker Avance 600 MHz
NMR spectrometer with TMS as the internal standard. ESI mass spectra were recorded using a Waters ZQ4000/2695
LC–MS spectrometer. Infrared spectra were obtained as KBr pellets on a Nicolet 5700 FTIR spectrometer.
The structures of compounds 1–3 and the synthesis methods are depicted in Scheme 1. These structures were
characterized by 1H NMR, ESI mass, and IR data, which were consistent with the proposed formulation. 1: 1H NMR
(600 Hz, CDCl3): d 1.29–1.33 (t, 3H, CH3), 3.03 (s, 6H, CH3), 4.22–4.28 (m, 4H, CH2), 6.68 (d, 2H, J = 8.87 Hz, ArH),
7.72 (d, 2H, J = 8.89 Hz, ArH); ESI mass: m/z calcd. for C13H16N2O3 [M+H+] 249.12, found [M+H+] 249.37. 2: 1H
NMR (600 Hz, CDCl3): d 1.23–1.31 (t, 6H, CH3), 2.99 (s, 6H, CH3), 4.20–4.25, (m, 8H, CH2), 6.65 (d, 2H,
J = 8.78 Hz, ArH), 7.38 (d, 2H, J = 8.78 Hz, ArH); ESI mass: m/z calcd. for C17H24N2O5 [M+H+] 337.17, [M+Na+]
359.16; found [M+H+] 337.22, [M+Na+] 359.22. 3: 1H NMR (600 Hz, D2O): d 3.11 (s, 6H, CH3), 3.93 (s, 2H, CH2),
4.13 (s, 2H, CH2), 7.32 (d, 2H, J = 8.51 Hz, ArH), 7.52 (d, 2H, J = 8.50 Hz, ArH); ESI mass: m/z calcd. for
C13H16N2O5 [M+H+] 281.11; found [M+H+] 281.03.
2. Results and discussion
Compounds 1–3 emitted dual fluorescence in acetonitrile. The spectral parameters are presented in Table 1. The
shorter wavelength emission originated from the local excited state (LE), and the longer wavelength emission came
from the charge transfer state (CT). The intensity ratio of CTemission to LE emission for 1 or 2 was larger than that for
3, which implied that the ester group was favored for the charge transfer state in p-dimethylaminobenzoylamino
derivatives. Meanwhile, both emission wavelengths red shifted when the ether group was replaced by carboxyl.
However, the total quantum yield was higher for 3 than for 1 or 2.
The fluorescence and absorption titrations of 1 against Cu2+ were performed and presented in Fig. 1. As seen in
Fig. 1a, 1 emitted dual fluorescence in acetonitrile, which peaked at 365 and 480 nm. Upon addition of Cu2+, both
intensities decreased while the peak position was kept constant. Similar quenching was observed when the excitation
Table 1
Spectral parameters of receptor 1À3 in acetonitrile.
e (molÀ1 L cmÀ1
)
lex (nm)
lLE (nm)
lCT (nm)
ICT/ILE
a
FF
Receptor
lmax (nm)
1
2
3
298
288
281
2.41 Â 104
1.74 Â 104
1.10 Â 104
299
287
283
365
372
360
480
491
467
2.34
1.41
0.40
0.035
0.042
0.053
a
The total quantum yield including LE and CT emission.

F.Y. Wu et al. / Chinese Chemical Letters 23 (2012) 607–610
609
Fig. 1. (a) Fluorescence spectral change of 1 (3.5 Â 10À6 mol LÀ1) in the presence and absence of Cu2+ in acetonitrile. The excitation wavelength
was set at 310 nm. (b) Absorption spectral change of 1 (4.0 Â 10À5 mol LÀ1) with the addition of Cu2+ in acetonitrile.
was set at 264 nm, an isosbestic point. The intensity ratio between 480 nm and 365 nm showed good linear
relationship with the concentration of Cu2+ ranging from 0 to 2.5 Â 10À6 mol LÀ1 with a linear correlation coefficient
(R) of 0.9914. The absorption spectrum of 1 changed with the addition of Cu2+ as presented in Fig. 1b. The peak at
298 nm derived from 1 slightly blue shifted along with the decreased absorbance when increasing amounts of Cu2+
were added. Clear isosbestic points at 264 and 329 nm were observed to demonstrate the existence of a well-defined
stoichiometric complex. Absorbance at 298 nm leveled off (as seen in the inset plot of Fig. 1b) after introduction of 1
equivalent of Cu2+. This finding indicates the formation of a 1:1 complex. The binding constant (Kb) of copper
complex was calculated as 5.1 Â 107 molÀ1 L by non-linear fitting [8b]. The paramagnetic effect is the principal cause
of the fluorescence quenching by the d9 Cu(II) ion [9]. Thus, 1-Cu complex formation was assumed to result in
fluorescence quenching. Evidence of 1-Cu formation via ESI-mass was also provided. However, such evidence failed
because of complex decomposition at a high temperature.
Under identical condition the effects of other metal ions such as Ni2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+, Fe3+, Mg2+,
Ca2+ and Co2+ on the spectrum of 1 were also investigated. Compared with 1, only the presence of Pb2+ and Hg2+ made
the emission of 1 slight quench when the same amount of metal ion was added. The addition of other metal ions
resulted in little spectral change as seen in Fig. 2a. The results imply that 1 possessed a high selectivity toward Cu2+
over other metal ions.
The spectral responses of control compounds 2 and 3 to metal ions were also studied to elucidate high selectivity.
Compound2hadsimilarabsorptionandfluorescencespectralchangesinthepresenceofmetalions. Thebindingconstant
of 2-Cu was estimated as 8.5 Â 106 molÀ1 L, which was smaller than that of 1-Cu. However, the moiety of
CON(CH2COOC2H5)2 in compound 2 could provide more coordinating atoms compared with 1. Apparently, the moiety
of CON(CH2COOC2H5) 2 was not the binding site for Cu2+. Cu2+ was assumed to bind with the nitrogen atom of p-
dimethylamino moiety, which was similar to a report in the literature [8b]. As the ester group is an electron-attracting
Fig. 2. (a) Fluorescence intensity change ((I0 À I/I0) Â 100) of 1 (1.0 Â 10À5 mol LÀ1) in the presence of various metal ions. Various ions have the
same concentration like 1. (b) Fluorescence intensity changes ((I0 À I/I0) Â 100) of 3 (5.0 Â 10À6 mol LÀ1) in the presence of various metal ions.
Various ions have the same concentration like 3.

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F.Y. Wu et al. / Chinese Chemical Letters 23 (2012) 607–610
group, thereis oneestergroupin1 and twoestergroups in2. Thus, the electronclouddensity of N in thep-dimethylamino
moiety in 1 is larger than that in 2. The affinity of 1 toward Cu2+ is stronger than that of 2. Further, the fluorescence
quenching of 1 with the addition of Cu2+ was assumed to undergo isc by excitation from S1 to T1 state of the fluorophore,
that is deactivated by bimolecular non-radiative processes.
The fluorescence intensity change of 3 with the addition of metal ions is also studied and presented in Fig. 2b. For
most metal ions such as Hg2+, Fe3+, Pb2+, Cd2+, Ca2+, Mg2+, Co2+, except Cu2+ and Ag+, the presence of the above ions
initiates a decrease in the LE and CT fluorescence intensity of 3. The carboxyl groups in 3 possessed strong affinities
toward metal ions compared with nitrogen atom. Thus, 3 could bind with many metal ions. Obviously, the sensitivities
of 3 to Hg2+, Cd2+, and Co2+ were higher than that of Cu2+.
3. Conclusions
A simple compound, 4-dimethylaminobenzoylaminoacetic acid ethyl ester, was obtained. The total fluorescence
intensity, intensity ratio, and absorbance changes could act as indexes for the determination of Cu2+. The binding site
was assumed to play a key role in the selectivity of 1 to Cu2+.
Acknowledgments
This work is supported by Natural Science Foundation of China (No. 20965006).
References
[1] R. Uauy, M. Olivares, M. Gonzalez, Am. J. Clin. Nutr. 67 (1998) 952S.
[2] A. Torrado, G.K. Walkup, B. Imperiali, J. Am. Chem. Soc. 120 (1998) 609.
[3] (a) A.K. Mahapatra, G. Hazra, N.K. Das, et al. Sens. Actuators B 156 (2011) 456;
´
(b) E. Ballesteros, D. Moreno, T. Gomez, et al. Org. Lett. 11 (2009) 1269;
(c) M. Yu, M. Shi, Z. Chen, et al. Chem. Eur. J. 14 (2008) 6892;
(d) N. Shao, J.Y. Jin, H. Wang, et al. Anal. Chem. 80 (2008) 3466;
(e) J. Liu, Y. Lu, J. Am. Chem. Soc. 129 (2007) 9838;
(f) X. Zhang, Y. Shirashi, T. Hirai, Org. Lett. 9 (2007) 5039;
´
(g) R. Martınez, F. Zapata, A. Caballero, et al. Org. Lett. 8 (2006) 3235;
´
ˇ
(h) J. Kovacs, T. Rodler, A. Mokhir, Angew. Chem. Int. Ed. 45 (2006) 7815;
(i) P. Li, X. Duan, Z. Chen, et al. Chem. Commun. 47 (2011) 7755.
[4] (a) Y. Li, X. Zhang, B. Zhu, et al. Analyst 136 (2011) 1124;
(b) Q. Zeng, P. Cai, Z. Li, et al. Chem. Commun. 9 (2008) 1094;
(c) X. Meng, Q.G. He, H.M. Cao, et al. Chin. Chem. Lett. 22 (2011) 725;
(d) S.M. Park, M.H. Kim, J.I. Choe, et al. J. Org. Chem. 72 (2007) 3550;
(e) Z. Guo, W. Zhu, L. Shen, et al. Angew. Chem. Int. Ed. 46 (2007) 5549.
[5] (a) Y. Kubo, M. Yamamoto, M. Ikeda, et al. Angew. Chem. Int. Ed. 42 (2003) 2036;
(b) J. Raker, T.E. Glass, J. Org. Chem. 67 (2002) 6113;
(c) H. Fu, B.H. Loo, D. Xiao, et al. Angew. Chem. Int. Ed. 41 (2002) 962.
[6] (a) M. Royzen, Z. Dai, J.W. Canary, J. Am. Chem. Soc. 127 (2005) 1612;
(b) Z. Xu, X. Qian, J. Cui, J. Org. Lett. 7 (2005) 3029;
(c) H. Yang, Z.Q. Liu, Z.G. Zhou, et al. Tetrahedron Lett. 47 (2006) 2911;
(d) Z. Xu, Y. Xiao, X. Qian, et al. Org. Lett. 7 (2005) 889;
(e) V. Bhalla, R. Kumar, M. Kumar, et al. Tetrahedron 63 (2007) 11153;
(f) Z.C. Wen, R. Yang, H. He, et al. Chem. Commun. 1 (2006) 106.
´
´
[7] (a) R. Martınez, A. Espinosa, A. Tarrage, et al. Org. Lett. 7 (2005) 5869;
(b) D. Sahoo, V. Narayanaswami, C.M. Kay, et al. Biochemistry 39 (2000) 6594.
[8] (a) D. Braun, W. Rettig, S. Delmond, et al. J. Phys. Chem. A 101 (1997) 6836;
(b) Y.F. Cheng, D.T. Zhao, M. Zhang, et al. Tetrahedron Lett. 47 (2006) 6413.
[9] M. Formica, V. Fusi, L. Giorgi, et al. Coord. Chem. Rev. 256 (2012) 170.