Simple and sensitive colorimetric sensors for the selective detection of Cu2+ in aqueous buffer

Article abstract of DOI:10.1016/j.saa.2013.09.104

Simple chromogenic sensor for the selective detection of Cu²⁺ was described. With the addition of Cu²⁺, a bathochromic shift about 82 nm was observed in the UV-VIS spectra, with the color change from colorless to bright yellow. This suggested that the coordination between receptor and Cu²⁺ was formed, and the strong push-pull system occurred. The followed IR spectra indicated that Cu²⁺ coordinated to the two phenolic oxygen atoms and one of two azomethines in the receptor.

Full text of DOI:10.1016/j.saa.2013.09.104

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 789–792
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short Communication
Simple and sensitive colorimetric sensors for the selective detection
of Cu2+ in aqueous buffer
Jianzhong Huo a, Kai Liu a, , Xiaojun Zhao b, Xingxing Zhang b, Ying Wang b
⇑
a Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Ministry of Education, Tianjin Normal University, Tianjin 300387, China
b Tianjin Key Laboratory of Structure and Performance for Functional Molecule, College of Chemistry, Tianjin Normal University, Tianjin 300387, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
An easy-to-make Cu2+ colorimetric
sensor SH was designed and
synthesized.
SH displayed colorimetric specific
selectivity for Cu2+ in aqueous
solution.
Cu2+
N
N
N
N
Cu2+
OH
O
O
OH
The simple approach is useful for
routine analysis.
SH-Cu2+
SH
a r t i c l e i n f o
a b s t r a c t
Simple chromogenic sensor for the selective detection of Cu2+ was described. With the addition of Cu2+, a
bathochromic shift about 82 nm was observed in the UV–VIS spectra, with the color change from color-
less to bright yellow. This suggested that the coordination between receptor and Cu2+ was formed, and
the strong push–pull system occurred. The followed IR spectra indicated that Cu2+ coordinated to the
two phenolic oxygen atoms and one of two azomethines in the receptor.
Article history:
Received 3 August 2013
Received in revised form 12 September
2013
Accepted 26 September 2013
Available online 8 October 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Colorimetric sensor
Cu2+
Ratiometric detection
Naked-eye cation detection
Introduction
response attracted much attention [4,8,9] because of their simple
use and without sophisticated equipments than closely related
Recognition and sensing of heavy and transition metal (HTM)
ions attracted much attention in the supramolecular chemistry
[1–5] due to their significant importance in the chemistry, biology
and environment science. Among these species, Cu2+, as the third
in abundance in the human body, plays a crucial role in the funda-
mental physiological processes in organism [6]. However, the
superfluous Cu2+ in the body resulted in serous neurodegenerative
diseases [6]. Furthermore, Cu2+ is a significant environmental
pollutant due to its widespread use. Thus, the detection and
monitoring of Cu2+, with high sensitivity, high detection limit
and quick response, are in great demand [7]. Within these
detection approaches for Cu2+, those related with the colorimetric
approaches involving fluorescence and electrochemical response,
as well as the intrinsic paramagnetic nature of Cu2+ turning off
the fluorescence response of most of fluorescent sensors [10–12]
and rendering low signal outputs less advantageous for practical
analysis. Colorimetric sensors would have direct application in
the development of the disposable dip-stick rapid assays or
optodes based on the absorption changes. With the simplicity,
convenience and low cost in mind, the efficient Cu2+ colorimetric
sensors are necessary. Moreover, the interaction between the
sensors and Cu2+ commonly occur in aqueous solution in the
practical application. Therefore, the design and synthesis of cupric
sensors operating in the system containing water are desirable.
According to these requirements, various chemosensors with
soft heteroatoms (e.g. nitrogen and sulfur) for chelate of metal cat-
ions [13–15], have been used for the selective detection of Cu2+. In
⇑
Corresponding author. Tel.: +86 22 23766515; fax: +86 22 23766532.
E-mail address: hxxylk@mail.tjnu.edu.cn (K. Liu).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.09.104

790
J. Huo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 789–792
these chemosensors, receptor containing the Schiff base structure
was prevalent [14–16], because (a) it is easily prepared in high
yield by simple starting materials, (b) Schiff base can adjust the
coordination cavity to accommodate metal ions by configuration
change. Furthermore, Schiff bases and their metal complexes are
used as models for biological system for their antimicrobial and
anticancer activities [17,18]. Therefore, the biomimetic of the
interaction between Schiff base and metal is significant [17]. On
the other hand, up to now, only few investigations involved the
phenol groups as a key component to obtain the Cu2+ binding
[14,15,19], although the phenol commonly plays an important role
in the cation binding in the biological system. Another issue relat-
ing to the cupric recognition came from the serious interference by
other metal ions (e.g. Ag+, Hg2+) [20–23]. Based on the above-
mentioned background and as part of our research effort devoted
to ion recognition, herein we reported the cupric sensing and bind-
ing properties of SH based on 2-hydroxyl Schiff base as the binding
site by UV–VIS in dilute solution. Results showed that SH was
0.60
0.45
0.30
0.15
0.00
SH and metal ions (Pb2+ Ag+ Fe3+ Co2+ Zn2+ Hg2+
Cd2+ Cr3+ Ba2+ Mg2+ Ca2+ K+ Na+ Li+)
Cu2+
Ni2+
300
400
500
600
Wavelength (nm)
preferential selective colorimetric cation sensors for Cu2+
.
Fig. 1. Absorption spectra of SH (1 Â 10À5 M) recorded in DMSO/HEPES buffer
(10 mM, pH = 7.0, v/v, 4:1) after the addition of 2 equiv. of Cu2+ and 10 equiv of
other metal ions.
which can be assigned to p–
pà and n–pà [24] (namely, the intramo-
lecular charge transfer from C@N and OH to the phenyl group). In
the presence of 2.0 equiv. of Cu2+, the absorption band at 360 nm
disappeared, at the same time, the new bathochromic shift band
at 442 nm occurred with the concomitant color change from color-
less to yellow. This new band was attributed to the metal-induced
intramolecular charge transfer [25] from the ‘push’ SH to the ‘pull’
Cu2+ [26]. A similar but less remarkable spectral change was ob-
served upon addition of Ni2+, however, this perturbation did not af-
ford naked-eye detection. On the other hand, the other metal ions
(even at higher concentration) resulted in negligible changes in the
UV–VIS spectra under the same conditions. The significant cation-
induced spectroscopic differences between Cu2+ and other metal
ions were ascribed to the high thermodynamic affinity of Cu2+ for
typical N- and O-donor ligand and fast metal-to-ligand binding
kinetics [27,28].
Experimental
Reagents
All reagents for synthesis were of analytic grade without further
purification. In the titration experiments, Cu2+, Pb2+, Ag+, Fe3+, Co2+
,
Zn2+, Ni2+, Hg2+, Cd2+, Cr3+, Mg2+, Ca2+, Ba2+, Na+, K+, and Li+ were
added in the form of nitrate.
Synthesis
To a stirred ethanol solution of 1.22 g salicylaldehyde at room
temperature, 0.25 ml of hydrazine hydrate (99%) was added. The
mixture was stirred continually for 16 h. The yellow precipitated
product was rapidly filtered, washed several times with cold etha-
nol to afford pure SH. Yield: 89%. m.p. 219.5-220.6 °C. EI-MS: m/z
241.3 (M + H+), 1H NMR (DMSO-d6): d 11.122 (s, 2H, ArAOH),
9.001 (s, 2H, CH@N), 7.673-7.701 (m, 2H, ArACH), 7.374-7.416
(m, 2H, ArACH), 6.946-6.985 (m, 2H, ArACH).
In order to explore the effect of other metal ions on the binding
of Cu2+ with SH, the competitive experiments were carried out by
addition of Cu2+ to the solution of SH containing other metal ions.
As shown in Fig. 2, the visible absorbance of SH–Cu2+ complex at
442 nm and the corresponding color changes were largely uninflu-
enced by other metal ions (Pb2+, Ag+, Fe3+, Co2+, Zn2+, Ni2+, Hg2+
,
Cd2+, Cr3+, Ba2+, Mg2+, Ca2+, Li+, Na+ and K+) even at higher concen-
tration. These results indicated that the simple SH was preferential
0.24
0.18
0.12
0.06
0
General approaches for UV–VIS spectroscopy
Serial working solutions were prepared by adding the incre-
mental multiples of metal ions in the HEPES buffer (10 mM,
pH = 7.0) to the DMSO solution of the SH (2.5 Â 10À5 M), and
finally diluted with HEPES buffer to make the volume ratio of
DMSO to HEPES buffer constant (4:1), and were stored at room
temperature for 0.5 h before used in the experiment. UV–VIS
absorption spectra were measured on Shimadzu UV-2450PC in
the range of 250–600 nm with a slit width of 1.0 nm.
Results and discussion
The optical properties of SH were investigated by mixing it with
metal ions (Cu2+, Pb2+, Ag+, Fe3+, Co2+, Zn2+, Ni2+, Hg2+, Cd2+, Cr3+
,
Ba2+, Mg2+, Ca2+, Na+, K+ and Li+). The spectroscopic studies were
performed in DMSO/HEPES buffer (v/v, 4:1, pH = 7.0). As shown
in Fig. 1, in the absence of metal ions, the UV–VIS spectra of SH
exhibited two maximum absorption band at 360 nm and 300 nm,
Fig. 2. Optical absorbance of SH (1 Â 10À5M) in DMSO/HEPES buffer (v/v, 4:1,
pH = 7.0) in presence of 2 equiv. of Cu2+ and other metal ions (Pb2+, Ag+, Fe3+, Co2+
,
Zn2+, Ni2+, Hg2+, Cd2+, Cr3+, Ba2+, Mg2+, Ca2+, Na+, K+ and Li+). Detection wavelength:
442 nm.

J. Huo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 789–792
791
selective colorimetric sensors for Cu2+ compared with other metal
0.5
0.4
0.3
0.2
0.1
0.0
ions at the same conditions. This also represented a significant
improvement in the sense of Cu2+ for SH compared to the other re-
ported sensors, which was generally interfered by other metal ions
(e.g. Ag+, Hg2+) [20–23]. Moreover, this suggested the stronger
0.25
0.20
0.15
0.10
0.05
0.00
binding capacity of SH toward Cu2+
.
The significant optical response of probe SH exhibited a ratio-
metric response to Cu2+. The ratio of the long-wavelength at
442 nm to the short-wavelength at 360 nm increased with the
increasing Cu2+ concentration. These changes allowed the Cu2+
concentration to be determined ratiometrically. Other tested metal
ions did not greatly alter the ratiometric value relative to SH itself
(Supplementary data, Fig. S1). This indicated that the co-existence
of other metal ions had a negligible interfering effect on the reli-
able detection of Cu2+. Furthermore, the Cu2+-induced absorption
ratio A442/A360 displayed good linearity, which can be expressed
as A442/A360 = 18.87 [Cu2+]À0.104 (R = 0.996) in the Cu2+ concentra-
0.0 0.2
0.4
0.6
0.8 1.0
]
[SH]/([SH]+[Cu2+
300
400
500
600
tion range of 10.0–80.0 lM (Fig. 3). This indicated that the probe
Wavelength (nm)
SH could be used for the practical quantitative detection. The limit
of detection (LOD) was determined to be 6.5 Â 10À6 M. Then the
proposed Cu2+-selective approach was used for the determination
of Cu2+ in the real-life samples according to the literature [29].
Fig. 4. UV–VIS titration of SH (1 Â 10À5M) with Cu2+ (0–1.5 equiv.) in DMSO/HEPES
buffer (pH = 7.0, v/v, 4:1). Insert displays the Job plot of absorbance of SH at 442 nm
the complex formed by SH and Cu2+ at an invariant total concentration of 20
lM.
The interfering species, such as Cd2+ÀNi2+, Ca2+ÀMg2+, Co2+ÀZn2+
,
Ag+ÀHg2+ and various anions (AcOÀ, NOÀ3 and ClÀ), were added
to a standard solution containing Cu2+ (40
l
M), with the succedent
2+
addition of SH (2.5 Â 10À5 M). Subsequently, the samples were
quantitatively measured by UV–VIS analysis at 442 nm and
360 nm, respectively. The quantification data of Cu2+ was fitted
in the calibration (Fig. 3). This suggested that the sensors could
be applied to detect Cu2+ real sample.
2+
The absorption peak at 442 nm and 360 nm for SH–Cu2+ is clo-
sely related to the degree of deprotonation of the phenolic subunit,
which is dependent on the pH of the medium. So, the effect of pH
on the SH–Cu2+ was investigated. Result indicated that SH–Cu2+
showed an optimum absorption at pH = 7.0 (Supplementary data,
Fig. S2). Therefore, pH = 7.0 was selected for recognition. Further-
more, addition of EDTAÁNa2 to the solution of SH–Cu2+ resulted
in a reverse color change from bright yellow to colorless. This sug-
gested that Cu2+-induced color change is reversible.
Scheme 1. A proposed response mechanism of SH–Cu2+
.
push–pull SH displayed strong intramolecular charge transfer
[26]. Simultaneously, the well-defined isosbestic point at 385 nm
occurred, which indicated that SH forms a 1:1 molecular complex
with Cu2+ in DMSO/H2O. This was also confirmed by Job plot anal-
ysis (Fig. 4). The binding constant of complex SH–Cu2+ was esti-
mated at Ka = 1.3 Â 106 molÀ1 dm3 (R = 0.97) by the satisfactory
non-linear least-square analysis [30,31] of the absorbance changes
at the new band maximum (Supplementary data, Fig. S3).
The binding properties of SH with Cu2+ were studied in detail by
UV–VIS titration experiment. As shown in Fig. 4, with the gradual
increase amount of Cu2+, the absorption band at 360 nm decreased
slowly and the new red-shifted maximum absorption band at
442 nm formed and developed. As a result, the color of the solution
changed from colorless to bright yellow. These indicated that
To further explore the binding mode of SH and Cu2+, the impor-
tant IR spectra were performed which illustrated the characteristic
structure changes during the interaction of SH with Cu2+. The band
at 2624 cmÀ1, which was assigned to the OHÁ Á ÁN@C intramolecular
hydrogen bond of SH [32], disappeared upon the coordination of
Cu2+. This suggested that Cu2+ coordinated to the two deprotonated
phenolate groups in SH. Moreover, the C@N stretching vibration
1628 cmÀ1 of SH appeared in the SH:Cu2+ complex, and the new
lower frequency at 1620 cmÀ1 emerged which was ascribed to
the coordination of C@N and Cu2+ [32]. These results clearly indi-
cated that one of two azomethines in the ligand SH coordinated
to the Cu2+, as illustrated in Scheme 1. The occurrence of complex-
ation of two deprotonated phenolate groups and the single C@N to
Cu2+ allowed the charge transfer from ligand to metal, which re-
sulted in the new absorption band at 442 nm and the obvious color
change from colorless to bright yellow. Furthermore, the coordina-
tion also indicated that the planar SH distorted, and one of the aro-
matic rings rotated perpendicular to another around C@N
1.5
1.0
0.5
0.0
facilitating the binding of Cu2+
.
Conclusion
2
4
6
8
[Cu2+] (
×
10-5M)
The simple probe SH displayed colorimetric and ratiometric
response with a large red-shift of UV–VIS absorption that was
applied to the detection of cupric cation with high selectivity and
Fig. 3. Linear response of the absorbance ratio A442/A360 as a function of Cu2+
concentration.

792
J. Huo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 789–792
[7] R. Krämer, Angew. Chem. Int. Ed. 37 (1998) 772.
[8] Y.-F. Lee, T.-W. Deng, W.-J. Chiu, T.-Y. Wei, P. Roy, C.-C. Huang, Analyst 137
(2012) 1800.
[9] E. Oliveira, J.D. Nunes-Miranda, H.M. Santos, Inorg. Chim. Acta 380 (2012) 22.
[10] J. Huang, Y. Xu, X. Qian, Org. Biomol. Chem. 7 (2009) 1299.
[11] Y. Hu, Q. Li, H. Li, Q. Guo, Y. Lua, Z. Li, Dalton. Trans. 39 (2010) 11344.
[12] L. Fabbrizzi, M. Licchelli, P. Pallavicini, A. Perotti, D. Sacchi, Angew. Chem. Int.
Ed. Engl 33 (1994) 1975.
low LOD among HTM ions. The dramatic color change from color-
less to bright yellow was ascribed to the intramolecular charge
transfer from the phenolic oxygen atoms and nitrogen atom of
SH–Cu2+
.
Acknowledgements
[13] X.-J. Yuan, R.-Y. Wang, C.-B. Mao, L. Wu, C.-Q. Chu, R. Yao, Z.-Y. Gao, B.-L. Wu,
H.-Y. Zhang, Inorg. Chem. Commun. 15 (2012) 29.
[14] S.H. Mashraqui, M. Chandiramani, R. Betkar, S. Ghorpade, Sensors. Actutat. B:
Chem. 150 (2010) 574.
[15] Z. Yang, M. She, J. Zhang, X. Chen, Y. Huang, H. Zhu, P. Liu, J. Li, Z. Shi, Sensors.
Actutat. B: Chem. 176 (2013) 482.
[16] T. Li, Z. Yang, Y. Li, Z. Liu, G. Qi, B. Wang, Dyes. Pigments 88 (2011) 103.
[17] B. Duff, V.R. Thangella, B.S. Creaven, M. Walsh, D.A. Egan, Eur. J. Pharmacol. 689
(2012) 45.
This work was supported by the National Natural Science Foun-
dation of China (Nos. 21002069 and 21206127), Tianjin Science &
Technology Development Fund Planning Project for Colleges and
Universities (No. 20120507), National Science Foundation of Tian-
jin (No. 13JCQNJC06000), Lab Innovation Foundation of Tianjin
Normal University (No. A201209) and Innovation Project of the
Undergraduate (No. 2013100).
[18] A.A.A. Abou-Hussein, W. Linert, Spectrochim. Acta Part A: Mol. Biomol.
Spectrosc. 95 (2012) 596.
[19] X. Chen, M.J. Jou, H. Lee, S. Kou, J. Lim, S.-W. Nam, S. Park, K.-M. Kim, J. Yoon,
Sensors. Actutat. B: Chem. 137 (2009) 597.
[20] K.M. Gattás-Asfura, R.M. Leblanc, Chem. Commun. (2003) 2684.
[21] K. Rurack, M. Kollmannsberger, U. Resch-Genger, J. Daub, J. Am. Chem. Soc. 122
(2000) 968.
[22] Z.-T. Jiang, R.-R. Deng, L. Tang, P. Lu, Sensors. Actutat. B: Chem. 135 (2008) 128.
[23] S.H. Kim, J.S. Kim, S.M. Park, S.-K. Chang, Org. Lett. 8 (2006) 371.
[24] E.C. Okafor, Spectrochim. Acta. Part A: Mol. Biomol. Spectrosc. 36 (1980) 207.
[25] M.M. Abo Aly, B.A. El Sayed, A.M. Hassan, Spectrosc. Lett. 35 (2002) 337.
[26] R. Nishiyabu, P. Anzenbacher, Org. Lett. 8 (2006) 359.
[27] T. Gunnalaugsson, D.A.M. Donail, D. Parker, Chem. Commun. (2000) 93.
[28] Z. Liang, Z. Liu, L. Jiang, Y. Gao, Tetrahedron Lett. 48 (2007) 1629.
[29] S.A. El-Safty, A.A. Ismail, A. Shahat, Talanta 83 (2011) 1341.
[30] J. Bourson, J. Pouget, B. Valeur, J. Phys. Chem. 97 (1993) 4552.
[31] M.H. Litt, J. Welllnghoff, J. Phys. Chem. 81 (1977) 2644.
[32] M. Salavati-Niasari, A. Amiri, Appl. Catal. A: Gen. 290 (2005) 46.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.104.
References
[1] P.A. Vigato, V. Peruzzo, S. Tamburini, Coord. Chem. Rev. 256 (2012) 953.
[2] X. Wang, Z. Liu, F. Qian, W. He, Inorg. Chem. Commun. 15 (2012) 176.
[3] A. Basoglu, S. Parlayan, M. Ocak, H. Alp, H. Kantekin, M. Ozdemir, U. Ocak, J.
Fluoresc. 19 (2009) 655.
[4] Y. Jeong, J. Yoon, Inorg. Chim. Acta. 381 (2012) 2.
[5] J.F. Zhang, Y. Zhou, J. Yoon, J.S. Kim, Chem. Soc. Rev. 40 (2011) 3416.
[6] K.J. Barnham, C.L. Masters, A.I. Bush, Nat. Rev. Drug Discov. 3 (2004) 205.