A "naked eye" and ratiometric chemosensor for cobalt(II) based on coumarin platform in aqueous solution

Article abstract of DOI:10.1016/j.inoche.2015.10.017

A novelcolorimetric cobalt(II) chemosensor ratiometric and (HL) based on coumarin platform has been synthesized. It displayed a remarkable (44 nm) red-shift in the absorption spectra upon reaction with Co²⁺, which is easily observed from yellow to red by the naked-eye. Co²⁺ could be quantitatively detected in the range of 0-10 μM by both normal and ratiometric absorption spectrometry methods. The detection limit of HL for Co²⁺ detection was as low as 0.31 μM.

Full text of DOI:10.1016/j.inoche.2015.10.017

Inorganic Chemistry Communications 62 (2015) 19–23
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A “naked eye” and ratiometric chemosensor for cobalt(II) based on
coumarin platform in aqueous solution
a,b,
a
a
a,
a
a
a
Zhaodi Liu ⁎, Wenlong Wang , Huajie Xu , Liangquan Sheng ⁎, Shuisheng Chen , Deqian Huang , Fei Sun
a
Department of Chemistry and Materials Engineering, Fuyang Normal College, Fuyang 236037, China
Anhui Provincial Key Laboratory for Degradation and Monitoring of the Pollution of the Environment, Fuyang 236037, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel ratiometric and colorimetric cobalt(II) chemosensor (HL) based on coumarin platform has been
Received 11 September 2015
Received in revised form 11 October 2015
Accepted 13 October 2015
Available online xxxx
2+
synthesized. It displayed a remarkable (44 nm) red-shift in the absorption spectra upon reaction with Co
,
2+
which is easily observed from yellow to red by the naked-eye. Co
could be quantitatively detected in the
range of 0–10 μM by both normal and ratiometric absorption spectrometry methods. The detection limit of HL
2
+
for Co detection was as low as 0.31 μM.
©
2015 Elsevier B.V. All rights reserved.
Keywords:
Cobalt(II)
Colorimetric
Ratiometric chemosensor
Coumarin
Cobalt is a vital trace element that plays an important role in various
biological systems as a key component of vitamin B12. Thus cobalt is
essential in small amounts for human life, and if consumed in large
doses and even long-term exposure at low level it could be harmful,
cause asthma, decreased cardiac output, cardiac enlargement, heart
disease, lung disease, dermatitis and vasodilation [1–4]. Therefore, it is
essential to monitor its concentration in biological and environmental
samples. Various analytical techniques are available for the detection
of Co , such as atomic absorption spectroscopy [5], inductively
coupled plasma atomic emission spectrometry [6], fabricate
nanomaterial [7–9], electrochemical methods [10,11] and phosphores-
cence/fluorescence techniques [12–21]. However many of these
methods are not simple and suitable for quick and onsite analysis. In
contrast, colorimetric methods can conveniently and easily monitor tar-
get ions without any sophisticated equipment. Colorimetric methods
have therefore attracted considerable attention regarding the detection
of toxic metal ions [22–25].
ligand [33], and metal complex [34], which always use single absorption
peak to detect Co2 rather than ratiometric measurement.
+
As we known, ratiometric chemosensors have a few advantages. The
ratio between the two absorption/emission intensities can be used to
evaluate the analyte concentration and provide a built-in correction
for environmental effects, such as photobleaching, sensor molecule con-
centration, the environment around the sensor molecule (pH, polarity,
temperature, etc.), and stability under illumination [35–39]. Thus, the
2
+
2+
design of colorimetric sensors for Co
is suitable for obtaining
quantitative information via internal calibration of dual absorption
and the altered color. Meanwhile, because coumarins possess desirable
photophysical properties, they are ideal platforms for development of
colorimetric chemosensors for specific heavy and transition metal
ions. The imine and keto/thioketo groups can act as chelating sites
of the metal cations, in particular, transition- and post-transition-
metal cations [29]. With these considerations in our mind, we
designed and synthesized a new coumarin derivative, (E)-benzyl-
2-((7-(diethylamino)-2-oxo-2H-chromen-3-
However, colorimetric chemosensors for cobalt(II) ions are still very
2
+
rare. Several strategies for detecting Co have been developed based
on metal nanoparticle [26,27], molecular microcrystal [28], coumarin
groups [29], julolidine-quinoline groups [30], 8-aminoquinoline-
salicylaldehyde [31], phthalazine–pyridine groups, quinolone–
pyridylaminophenol [25], thiazole based ligands [32], tripodal amide
yl)methylene)hydrazine-
carbodithioate (HL), which displays a selective and unique
ratiometric absorbance changes with Co2 in aqueous buffer solution.
As shown in Scheme 1, 7-(N,N-Diethylamino)coumarin (1) and 7-
(N,N-diethylamino) coumarin-3-aldehyde (2) were synthesized
according to the literature method [40]. HL was synthesized by one
step condensation of 7-(N,N-diethylamino) coumarin-3-aldehyde (2)
with S-benzyldithiocarbazate in ethanol solution. Target compound HL
was obtained as a red powder in a high yield. The structure of HL was
+
⁎
Correspondence to: Z. Liu, Department of Chemistry and Materials Engineering,
Fuyang Normal College, Fuyang 236037, China.
1
13
confirmed by H NMR C NMR, MALDI-TOF MS and X-ray crystallogra-
⁎
Corresponding author.
E-mail addresses: zhaodi_liu@163.com (Z. Liu), shenglq@fync.edu.cn (L. Sheng).
phy. Single crystals of HL were obtained by the slow evaporation of its
http://dx.doi.org/10.1016/j.inoche.2015.10.017
387-7003/© 2015 Elsevier B.V. All rights reserved.
1

20
Z. Liu et al. / Inorganic Chemistry Communications 62 (2015) 19–23
Scheme 1. The synthesis of HL.
saturated acetonitrile solution. The crystal structure of HL together with
the atomic numbering scheme is shown in Fig. 1a. In the molecule, the
C_N bond adopt trans configuration. The coumarin ring and the part
interfering metal ions could not develop any apparent change in the
UV–vis spectrum of HL, indicating that coexistence with competitive
metal ions cannot make substantial interferences in Co2 sensing.
+
N
2
–N
3
–C15–S
1
–S
2
–C16 of thiosemicarbazone backbone display an al-
3
The UV–vis absorption of HL in CH CN–water (1/1, v/v, 10 mM
most co-planar configuration with the dihedral angle of 5.5°. Compared
with the similar compound, (E)-2-[[7-(Diethylamino)-2-oxo-2H-chro-
men-3-yl]methylene]hydrazinecarbothioamide [41], the C_O
HEPES buffer, pH = 7.4) solution was investigated by spectrophotomet-
ric titration. As shown in Fig. 4, in the presence of Co2 (1.0 equiv), a
new absorption band appears at approximately ~517 nm. The absorp-
tion peak at 517 nm increased gradually upon gradual addition of
Co to the solution of HL, while the absorbance at 473 nm decreased
concomitantly with four isosbestic points at 306, 343, 386 and
+
(
1.210(2) Å), C_N (1.276(3) Å) and N–N (1.370(2) Å) bonds, are sim-
2
+
ilar, but the C_S (1.649(2) Å) bond is shorter than C_S (1.693(2) Å) in
that compound. In the crystal structure, molecules related by a centre of
symmetry are linked to form dimers via intermolecular hydrogen bonds
501 nm, implying a new coordination process between the coumarin
A
2+
(
N –H …O
3 1 2
, A = −x, −y + 1, −z + 1) (Fig. 1b). The crystal data and
platform and Co
.
structural refinement details are listed in ESI: Table 1.
As shown in the insets of Fig. 5, the absorbance at 517 nm first
increased quickly and then the increase slowed down. Its absorbance
ratio at 517 and 473 nm (A517 nm/A473 nm) reaches a saturation state
after total addition of about 0.5 equiv. of Co , and there it remained.
These imply the formation of a complex with 2:1 stoichiometry of HL
As the sensor contain both chromophoric group and binding site
i.e., favorable for metal ion binding so expected to work as ionophore
in cation sensing mechanism. Fig. 2 illustrates the absorption spectral
changes for the sensor HL (40 μM) upon addition 1.0 equiv. of tested
2
+
and Co2 , and the association constant (log K
+
) was found to be
(Fig. S1). Moreover, a Job's plot exhibits a maximum at
3
cations in CH CN–water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solu-
a
−
2
tion. The absorption spectrum of HL exhibits an obvious band at 473 nm.
6.91 M
2
+
2+
In the presence of Co , the band at 473 nm was weakened while a new
band appeared at longer wavelength (517 nm). Such a large red shift in
absorption spectrum of receptor is an easy assay for the colorimetric de-
tection of the receptor upon guest binding. The significant change to
wavelength resulted in a color change from yellow to red, indicating
0.34 M fraction of Co
2:1 complex is formed.
(Fig. S2), which further indicates that only a
In addition, the formation of 2:1 complex between HL and Co2+ is
further confirmed by the MALDI-TOF mass spectra. As shown in
Fig. S3, the mass spectrum exhibited a strong peak at m/z 906.983,
2
+
that there was chemical reaction between HL and Co . Addition of
2
which were assigned to the species [CoL ].
+
+
+
+
2+
other representative metal ions such as Li , Na , K , Ag , Cu
,
To further explore the binding structure, IR spectrometry was used
Hg , Pb , Ba , Mn , Ca , Cd , Ni , Mg , Zn , Fe , Cr3+,
and Al3 , did not give rise to significant color and wavelength changes.
These results suggest that HL could be served as a potential colorimetric
2
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
to investigate the free ligand HL and its Co -complex. As shown in
2+
+
Fig. S4, the shifts of υ(C = O), υ(C = N), υ(C = S) bands of HL at
−
1
−1
−1
1707 cm , 1616 cm , 1319 cm
to lower frequency of CoL
2
2
+
complex, 1686 cm−1, 1596 cm−1, and 1300 cm , respectively, indicate
−1
sensor selective for Co
.
Furthermore, to validate the high selectivity of HL toward Co2+, the
competitive experiments of other various metal ions were also investi-
gated. The receptor HL was treated with 1.0 equiv. of Co2 in the pres-
ence of other metal ions (5.0 equiv). As shown in Fig. 3, their absorption
intensities at 517 nm were recorded, respectively. The presence of other
the suggestion of coordination through the N, O, and S of the imine, keto,
−
1
thioketo groups, respectively. Moreover, a new band at 1617 cm is
observed for a new-C_N stretching vibrations formed. It is attributed
to HL has a proton adjacent to the thione group, as a rule, in the mono-
meric form and tends to remains as the thione tautomer. In the presence
+
Fig. 1. (a) The molecular structure of HL. All H atoms are omitted for clarity; and (b) a view of the hydrogen-bonded dimer of HL. Dashed lines indicate hydrogen bonds. [Symmetry code:
A = −x, −y + 1, −z + 1].

Z. Liu et al. / Inorganic Chemistry Communications 62 (2015) 19–23
21
3
Fig. 2. (a) UV–vis absorption spectra of HL (40 μM) upon addition of various metal ions (1.0 equiv) in CH CN–water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solution. (b) Color changes of
HL (40 μM) in CH CN–water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solution in the presence of 1.0 equiv. of various metal ions.
3
of metal ions, HL is rapidly converted into the thiol tautomeric forming
metal complexations with the deprotonated ligand L [42,43], as shown
We further investigated the sensitivity of the probe to quantify Co2+
in water buffer solution. As shown in Figs. 6 and S5, the absorbance at
in Scheme 2. Thus, the IR studies clearly proved the imine, keto, thioketo
473 nm, 517 nm and the values of A517 nm/A473 nm show a linear depen-
groups acting as chelating sites to bind with Co2
+
.
dence on Co
2+
concentration in the range of 0–10 μM. These linear
2
+
3
Fig. 3. (a) The ratio A517 nm/A473 nm of HL and its complexation with Co in the presence of various metal ions in CH CN–water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solution. Black
2+ 2+ 3+ 3+ 3+
+
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
bars: HL(40 μM) and HL with 5.0 equiv. of Li , Na , K , Ag , Cu , Hg , Pb , Ba , Mn , Ca , Cd , Ni , Mg , Zn , Fe , Cr , and Al stated. Red bars: 40 μM of HL and 1.0
2
+
2+
3
equiv. of Co with 5.0 equiv. of metal ions stated. (b) Photograph of the L–Co mixtures with various metal ions in CH CN–water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solution.

22
Z. Liu et al. / Inorganic Chemistry Communications 62 (2015) 19–23
Scheme 2. Proposed binding pattern between receptor HL and Co2+
.
medium (pH b 5.5) causes the dissociation of the complex and hydroly-
sis of the HL, therefore the decrease in the absorption intensity. At
−
2+
higher pH values (pH N 10.5), OH ions compete with the HL for Co
2
+
Fig. 4. UV–vis spectral changes of HL (40 μM) upon addition of Co (1.0 equiv) in CH
water (1/1, v/v, 10 mM HEPES buffer, pH = 7.4) solution.
3
CN–
ion, increasing the formation of the metal ion precipitation, accordingly
lowering the absorption intensity. Thus, HL exhibits satisfactory Co2
sensing abilities in the pH range of 6.0–9.0.
In summary, we have successfully developed a “naked eye” and
ratiometric chemosensor HL for Co2 ion based on coumarin platform.
HL exhibits selective ratiometric detection for Co2 ion in the presence
of other competing cations, and shows good colorimetric sensing ability
+
relationships allowed the detection of Co2+ by both normal and
ratiometric absorption spectrometry methods. The absorbance ratios
(A517 nm/A473 nm) of HL can be fitted to the equation A517 nm/
473 nm = 0.42357 + 0.03916C (Co , μM) over the range from 0 to
0 μM, R = 0.99354. The limit of detection (3σ) for Co is calculated
+
+
2
+
A
1
2
2+
2+
to Co ion in a large range pH (pH 5.5–10.5). It is likely therefore that
as 0.31 μM, which is much lower than the level (1.7 μM) of groundwater
quality criterion defined by the Department of Environmental Protec-
tion of the New Jersey [19] and comparable to previously reported for
experimental results will provide a basis for a new strategy for the
design of various coumarin platform chemosensors.
2
+
Co -binding colorimetric chemosensors.
Acknowledgments
Long response time in many chemosensors is undesired. Therefore,
the effect of the reaction time on the binding process of Co2 ion to
HL was studied as shown in Fig. S6. Following the addition of 1.0
+
This project is supported by the International sea area resources
survey and development of the 12th Five-year Plan of China
(DY125-15-E-01), the National Natural Science Foundation of China
(21171040), the Educational Commission of Anhui Province of
China (2014KJ016, 2014KJ024, 2014KJ012), the Natural Science
Foundation of Fuyang Normal College (2015FSKJ02ZD, 2014FSKJ06,
FS201402001B) and the Innovation Training Program for the College
students (AH201410371061).
2
+
equiv. Co
ion to the HL (40 μM), the absorption intensity of HL at
5
17 nm was turn on rapidly, and can reach a plateau within 1 min.
For practical applications, it is very important to ensure that the
sensor is suitable for measuring a specific cation under a physiological
pH range. The effect of pH in the range of pH 4.0–11.0 on absorption in-
tensity of HL at 517 nm was examined both in the absence and in the
2
+
presence of Co
ion. The results are shown in Fig. S7. Surprisingly,
the absorption intensity of HL at 517 nm was not at all affected by the
variation of pH. Intensity of HL in the absence of metal ion was almost
same in pH range of 4.0–11.0. However, the absorption intensity was
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the
online version. Crystallographic date reported in this paper has been
deposited with the Cambridge Crystallographic Date Center, CCDC
2
+
seen to change depending on pH values in the presence of Co
ion,
and reached to the maximum a pH range of 5.5–10.5. The acidic
1
419885 for HL. Copies of these information may be obtained free of
Fig. 5. Ratiometric calibration curve A517 nm/A473 nm as a function of Co2+ concentration.
Inset: absorbance plot of HL against increasing of Co at 473 and 517 nm.
2
+
Fig. 6. The relationship between A517 nm/A473 nm and Co2+ concentration.

Z. Liu et al. / Inorganic Chemistry Communications 62 (2015) 19–23
23
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