A NBD-based highly sensitive and selective colorimetric chemosensor for Ni2?+ and Cu2?+

Article abstract of DOI:10.1016/j.inoche.2017.01.024

A new colorimetric chemosensor 1, containing the 7-nitrobenzo-2-oxo-1,3-diazolyl (NBD) moiety and the phenol one connected through Schiff-base linkage, has been synthesized. Sensor 1 showed remarkable color changes from pink to orange and pale brown, respectively, upon selective binding to Ni^2?+ and Cu^2?+ that can be identified by the naked-eye. The binding modes of sensor 1 to Ni^2?+ and Cu^2?+ were determined to be 1:1 stoichiometries using a Job plot and ESI-mass analysis. The sensor 1 showed high sensitivity toward Ni^2?+ and Cu^2?+ with the detection limits of 0.48?μM and 0.26?μM, respectively. The recognition properties of the sensor 1 toward Ni^2?+ and Cu^2?+ were explained by using photophysical experiments and theoretical calculations. Practically, sensor 1 functioned as a visible test strip for Ni^2?+ and Cu^2?+.

Full text of DOI:10.1016/j.inoche.2017.01.024

Inorganic Chemistry Communications 77 (2017) 6–10
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
A NBD-based highly sensitive and selective colorimetric chemosensor for
Ni2+ and Cu2+
⁎
Seong Youl Lee, Cheal Kim
Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new colorimetric chemosensor 1, containing the 7-nitrobenzo-2-oxo-1,3-diazolyl (NBD) moiety and the phe-
nol one connected through Schiff-base linkage, has been synthesized. Sensor 1 showed remarkable color changes
from pink to orange and pale brown, respectively, upon selective binding to Ni2+ and Cu2+ that can be identified
by the naked-eye. The binding modes of sensor 1 to Ni2+ and Cu2+ were determined to be 1:1 stoichiometries
using a Job plot and ESI-mass analysis. The sensor 1 showed high sensitivity toward Ni2+ and Cu2+ with the de-
tection limits of 0.48 μM and 0.26 μM, respectively. The recognition properties of the sensor 1 toward Ni2+ and
Cu2+ were explained by using photophysical experiments and theoretical calculations. Practically, sensor 1 func-
Received 14 December 2016
Received in revised form 12 January 2017
Accepted 17 January 2017
Available online 18 January 2017
Keywords:
Nickel
Copper
tioned as a visible test strip for Ni2+ and Cu2+
.
© 2017 Elsevier B.V. All rights reserved.
Colorimetric
Test strip
Theoretical calculations
Nickel is an essential trace element for supporting life, such as respi-
ration, metabolism and biosynthesis [1]. Moreover, metallic nickel and
its compounds are widely used in modern industries such as
electroplating, rods for arc welding, pigments for paints, ceramics, surgi-
cal and dental prostheses, catalysts for hydrogenation and magnetic
tapes of computers [2]. However, an oversupply of nickel in such indus-
tries can cause a variety of pathological effects in the human body as in a
form of lung cancer, prostate cancer, larynx cancer, lung embolism,
asthma and chronic bronchitis [3]. US EPA has classified nickel as one
of 13 priority metal pollutants for its widespread use [4]. On the other
hand, copper is the third most abundant essential trace element in the
human body and it plays an important role in many fundamental phys-
iological processes in organisms. However, unregulated overloading of
copper can cause extremely negative health effects such as gastrointes-
tinal disturbance and liver or kidney damage [5]. Therefore, the devel-
opment of a novel chemosensor for the rapid and convenient
detection of nickel and copper is of great importance [6–15]
sensing mechanisms of Ni2+ and Cu2+ were explained by theoretical
calculations.
The compound 2 was synthesized by the substitution reaction of
NBD chloride and hydrazine according to the literature method (see
Supporting Information), and the sensor 1 was obtained by the con-
densing of 2 and salicylaldehyde with 52% yield in EtOH (Scheme 1).
Both compounds 1 and 2 were characterized by 1H NMR and 13C
NMR, ESI-mass spectroscopy, and elemental analysis. To explore the
sensing behavior of sensor 1 toward metal ions, the absorption spectral
changes to various metal ions including Al3+, Ga3+, In3+, Zn2+, Cd2+
Cu2+, Fe2+, Fe3+, Mg2+, Cr3+, Hg2+, Ag+, Co2+, Ni2+, Na+, K+, Ca2+
,
,
Mn2+ and Pb2+ were investigated in CH3CN solution (Fig. 1). The re-
markable spectroscopic and visual responses of 1 were observed upon
the addition of Ni2+ and Cu2+, which were corresponded with color
changes from pink to orange and pale brown, respectively. In contrast,
no significant spectral or visible change was observed in the presence
of other metal ions under the identical conditions. Importantly, this is
the second example that an organo-chemosensor can operate for color-
imetric detection of both Ni2+ and Cu2+, to the best of our knowledge
[16]. The binding properties of 1 with Ni2+ were studied by UV–vis ti-
tration (Fig. 2). Upon gradual increase of Ni2+ concentration, the band
at 515 nm gradually shifted to 498 nm. At the same time, the absorption
peak at 400 nm gradually decreased while the absorption intensities at
460 nm and 625 nm increased until it reached a limiting value (1.6
equiv). Moreover, the presence of a clearly defined isosbestic point at
425 nm implied that sensor 1 reacted with Ni2+ to form a stable
complex.
Herein, we report a simple dual target chemosensor 1 based on the
NBD moiety and phenol one connected via a Schiff-base linkage. Sensor
1 is highly responsive to Ni2+ and Cu2+ with remarkable color changes
from pink to orange and pale brown, respectively, and it showed an ex-
cellent selectivity in the presence of other competing metal ions. Practi-
cally, 1 functioned as a colorimetric test strip for Ni2+ and Cu2+. The
⁎
Corresponding author.
E-mail address: chealkim@seoultech.ac.kr (C. Kim).
http://dx.doi.org/10.1016/j.inoche.2017.01.024
1387-7003/© 2017 Elsevier B.V. All rights reserved.

S.Y. Lee, C. Kim / Inorganic Chemistry Communications 77 (2017) 6–10
7
Scheme 1. Synthesis of 1.
To determine the stoichiometric ratio of 1 and Ni2+, Job plot analysis
[17] was carried out using absorption titration experiments in the pres-
ence of various molar fractions of Ni2+ (Fig. S1). When the molar frac-
tion of Ni2+ was 0.5, the absorbance at 550 nm reached an extreme
value, suggesting that the complex formation between 1 and Ni2+ has
a stoichiometric ratio of 1:1. The ESI-mass analysis of a mixture of 1
and Ni2+ also revealed the formation of a 1:1 complex through the
metal coordination interaction, with the peak at m/z = 432.12
(Fig. S2). It corresponded to the coordination structure of
[1 + Ni2+ − H+ + CH3CN + 2H2O]+ (calcd: 432.04). On the basis of
the 1:1 stoichiometry and UV–vis titration data, the binding constant
of 1-Ni2+ complex was determined to be 5.7 × 104 from Benesi-
Hildebrand equation (Fig. S3) [18]. The detection limit (3σ/K) [14] of
sensor 1 as a colorimetric sensor for the analysis of Ni2+ was found to
be 0.48 μM (Fig. S4). Thus, it is useful to detect the submicromolar con-
centration of Ni2+, which belongs to the range found in many chemical
and environmental systems.
UV–vis titrations of 1 with Cu2+ were also conducted (Fig. 3). Upon
gradual increase of the Cu2+ concentration, the absorption band at
500 nm gradually decreased and the band at 400 nm increased. A new
One of the important criterion for a selective sensor is the ability to
detect a specific ion in the presence of other competing ions. The absor-
bance (at 625 nm) of 1-Ni2+ complex remained unperturbed in the
presence of different cations such as Al3+, Ga3+, In3+, Zn2+, Cd2+
,
Fe2+, Fe3+, Mg2+, Cr3+, Ag+, Hg2+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+
and Pb2+ (Fig. S5), while Cu2+ did show some interference. The results
indicated that the presence of most background ions exerted no inter-
Fig. 2. Absorption spectral changes of 1 (20 μM) in the presence of different concentrations
of Ni2+ ions.
ference to the detection of Ni2+
.
Fig. 1. (a) Absorption spectral changes of 1 (20 μM) in the presence of 1.6 equiv. of different metal ions. (b) The color changes of 1 (20 μM) upon addition of various metal ions (1.6 equiv).
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

8
S.Y. Lee, C. Kim / Inorganic Chemistry Communications 77 (2017) 6–10
band at 600 nm appeared with an isosbestic point at 434 nm, which in-
dicated that the only one complex was formed between 1 and Cu2+. The
stoichiometry of the 1-Cu2+ complex was determined by Job plot [17]
and ESI-mass spectrometry analysis. The Job plot for the binding of 1
and Cu2+ exhibited a 1:1 stoichiometry (Fig. S6). The positive-ion
mass spectrum confirmed the formation of [1
+ −
Cu2+
H+ + CH3OH]+ based on the presence of a peak at m/z = 393.20
(calcd: 393.01) (Fig. S7). The association constant was calculated to be
1.3 × 105 M−1 from a Benesi-Hildebrand plot (Fig. S8) [18]. This value
is within the range of those (102–109) reported for Cu2+-binding sen-
sors. The detection limit [19] of the 1 for Cu2+ was determined to be
0.26 μM (Fig. S9).
To utilize 1 as an ion-selective colorimetric chemosensor for Cu2+
,
the effect of competing metal ions was carried out (Fig. 4). Upon addi-
tion of 1.0 equiv. of Cu2+ in the presence of other metal ions (1.0
equiv) such as Al3+, Ga3+, In3+, Zn2+, Cd2+, Fe2+, Fe3+, Cr3+, Mg2+
,
Hg2+, Ag+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+, coexistent
metal ions exerted no interference to the detection of Cu2+
.
Fig. 3. Absorption spectral changes of 1 (20 μM) in the presence of different concentrations
of Cu2+ ions.
Fig. 4. (a) Competitive selectivity of 1 (20 μM) toward Cu2+ (1.0 equiv) in the presence of other metal ions (1.0 equiv). (b) The color changes of 1 (20 μM) in the presence of Cu2+ (1.0
equiv) and other metal ions (1.0 equiv).
Fig. 5. Photographs of the filter papers coated with 1 for the detection of Ni2+ and Cu2+. Sensor-1 test strips (100 μM) immersed in various metal ions (20 μM).

S.Y. Lee, C. Kim / Inorganic Chemistry Communications 77 (2017) 6–10
9
Motivated by the favorable features of sensor 1 in solution, colori-
metric test strips were prepared (Fig. 5). When the test strips coated
with 1 were immersed in various metal-ion solutions, only Ni2+ and
Cu2+ showed the distinct visible color changes from pink to orange
and brown, respectively. Therefore, 1 could be used for “in the field”
measurements of Ni2+ and Cu2+ that did not require any additional
equipment.
In order to understand the sensing mechanisms of 1 toward Ni2+
and Cu2+, density functional theory (DFT) and time dependent-
density functional theory (TD-DFT) calculations were conducted. All op-
timizations were carried out using the B3LYP/6-31G(d, p) method basis
set on the Gaussian 03 program. The calculated energy-minimized
structures of 1, 1-Ni2+ and 1-Cu2+ species are shown in Fig. 6. The
energy-minimized structure of 1 showed a planar structure with the di-
hedral angle of 1C, 2C, 3C, 4C = −0.044o (Fig. 6a). 1-Ni2+ complex ex-
hibited a tetra-coordinated structure with the dihedral angle of 1C, 2C,
3C, 4C = 161.682°. Three coordination sites were satisfied by the 5N,
6N and 7O donor of 1, and a NO−3 occupied one site (Fig. 6b). For 1-
Cu2+ complex, Cu2+ was coordinated to 5N, 6N, 7O of 1 and an oxygen
atom of NO−3 with the dihedral angle of 1C, 2C, 3C, 4C = 152.962o
(Fig. 6c). To further interpret the electronic and absorption properties
of 1, 1-Ni2+ and 1-Cu2+ complexes, TD-SCF calculations were conduct-
ed from GEN basis set. In free sensor 1, the main molecular orbital (MO)
contribution of the 1st lowest excited state was determined for the
HOMO → LUMO transition (496.69 nm, Fig. S10). The HOMO spread
out the whole molecule of 1, and LUMO was on the NBD group, which
indicated an intra-molecular charge transfer (ICT). In 1-Ni2+ complex,
the 5th excited state was found to be related to the color change
(497.49 nm, Fig. S11). The MO of the HOMO – 2 was mainly located at
the metal-centered orbital while the LUMO was localized on the NBD
moiety (Fig. S12), which corresponds to metal-to-ligand charge-
transfer (MLCT) and causes the color change from pink to orange. In
1-Cu2+ complex, the 13th excited state was found to be relevant to
the color change from pink to pale brown (437.69 nm, Fig. S13). The
MOs of the HOMO (α) and HOMO – 1 (β) mainly lied in the phenol moi-
ety while the LUMO +1 (α) and LUMO +1 (β) spread around the NBD
moiety (Fig. S14). Their transitions were assigned to ICT transition. The
changes in absorption spectra due to complex formation are also
reflected in the theoretically calculated spectra of 1, 1-Ni2+ and 1-
Cu2+ complexes (Fig. S10, S11 and S13). With the integration of infor-
mation obtained from Job plot, ESI-mass spectroscopy analysis and the-
oretical calculations, the binding modes of 1-Ni2+ and 1-Cu2+
complexes are depicted in Scheme 2.
In conclusion, we have synthesized a new chemosensor 1, based on
the NBD and the phenol group connected via a Schiff-base linkage. Sen-
sor 1 could be used for the simultaneous detection of Ni2+ and Cu2+
through its visible color changes from pink to orange and pale brown,
respectively. The 1:1 stoichiometries of 1 toward Ni2+ and Cu2+ were
confirmed by the Job plot and ESI-mass analysis. The detection limits
were calculated to be 0.48 μM for Ni2+ and 0.26 μM for Cu2+, respec-
tively. The sensing mechanisms of 1 toward Ni2+ and Cu2+ were ex-
plained by DFT calculations. Based on the colorimetric response of 1 to
Ni2+ and Cu2+, test strips containing 1 were fabricated, which exhibited
the obvious color changes from pink to orange and brown, respectively.
Therefore, sensor 1 will offer an important guidance to the development
of single sensors for recognizing both Ni2+ and Cu2+
.
Acknowledgements
Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (NRF-2014R1A2A1A11051794 and NRF-
2015R1A2A2A09001301) are gratefully acknowledged. We thank the
Nano-Inorganic Laboratory, Department of Nano & Bio Chemistry,
Kookmin University for the access of the Gaussian 03 program packages.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2017.01.024.
Fig. 6. Energy minimized structures of (a) 1, (b) 1-Ni2+ complex and (c) 1-Cu2+ complex.

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S.Y. Lee, C. Kim / Inorganic Chemistry Communications 77 (2017) 6–10
Scheme 2. Proposed binding modes of 1-Ni2+ and 1-Cu2+ complexes.
[10] T.G. Jo, Y.J. Na, J.J. Lee, M.M. Lee, S.Y. Lee, C. Kim, Sensors Actuators B Chem. 211
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