A selective fluorescence probe based on benzothiazole for the detection of Cr3+

Article abstract of DOI:10.1515/hc-2017-0120

A novel benzothiazole-functionalized Schiff-base derivative L was prepared and its metal-ion sensing properties were investigated. Sensor L displays selective naked-eye color change from yellow to green in the presence of Cr³⁺ in aqueous solution at pH 7.2, while other cations do not interfere with the recognition of Cr³⁺. The proposed mechanism is supported by Job's plot evaluation, IR and ESI-MS studies. The association constant and detection limit of sensor L to Cr³⁺ are 5.73×10⁴ m^-1 and 2.1×10^-8 m, respectively. A B3LYP/6-31G (d,p) basis set was employed for optimization of L and L-Cr³⁺ complex.

Full text of DOI:10.1515/hc-2017-0120

Heterocycl. Commun. 2017; aop
RAens-GeanlgeLcv,tSihvu-eWefnlCuheon*reansd cYaen nGaco*e probe based
on benzothiazole for the detection of Cr3+
https://doi.org/10.1515/hc-2017-0120
probes, metal ions have been sensed in organic media,
mixed organic solvents and aqueous-organic media [11–
16]. There are reports in the literature for specific sensing
of Cr3+ in aqueous solution using spectrophotometer,
spectrofluorimeter and confocal microscope [17–23]. The
potent toxicity of Cr3+ drives the need of simple, conveni-
ent and naked-eye visualization method for the detection
in aqueous solution.
Recently, benzothiazole and Schiff base derivatives
have become research hotspot in recognition of various
analytes [24–29]. Here, we report a novel colorimetric
chemosensor L based on a benzothiazole-functionalized
Schiff-base derivative for the quantification of Cr3+ in
aqueous solution. Ligand L specifically binds to Cr3+ in the
Received June 8, 2017; accepted July 10, 2017
Abstract: A novel benzothiazole-functionalized Schiff-
base derivative L was prepared and its metal-ion sensing
properties were investigated. Sensor L displays selective
naked-eye color change from yellow to green in the pres-
ence of Cr3+ in aqueous solution at pH 7.2, while other
cations do not interfere with the recognition of Cr3+. The
proposed mechanism is supported by Job’s plot evalu-
ation, IR and ESI-MS studies. The association constant
and detection limit of sensor L to Cr3+ are 5.73ꢀ×ꢀ104 m−1 and
2.1ꢀ×ꢀ10−8 m, respectively. A B3LYP/6-31G (d,p) basis set was
employed for optimization of L and L-Cr3+ complex.
Keywords: aqueous solution; benzothiazole; Cr3+; naked- presence of other competing cations with a substantial
eye; recognition.
color change in HEPES buffer. This chemosensor provides
a convenient and practical way to detect both Cr3+ in the
environment.
Introduction
Design and synthesis of fluorescent chemosensors capable
of selective recognition of metal ions is an active field of
research in supramolecular chemistry [1–4]. Among the
metal ions, Cr3+ ion performs a crucial role in a wide range
of biochemical processes, which is essential for good
health in moderate intake; however, it is toxic at high
concentration [5, 6]. Several biological functions depend
directly or indirectly on the proper concentration and oxi-
dation states of chromium to maintain the homoeostatic
mechanism. Deficiency or overdose of chromium can lead
to serious diseases, including Alzheimer’s, Huntington’s
and Parkinson’s disease [7, 8]. Ion recognition, particu-
larly of toxic heavy metal ions is of intense interest as it
has implications in the fields of environment, medicine
and biology [9, 10]. Chemosensors for on-site selective and
sensitive detection of such metal ions in aqueous phase
is always endeavored. With the aid of suitable molecular
Results and discussion
Compound 2 was synthesized by condensation of 2-for-
mylbenzothiazole (1) and thiosemicarbazide in the reflux-
ing acetic acid for 8 h. The unique ligand L was then
obtained by condensation of 2 and 2-bromoacetophenone
(Scheme 1). The molecular structure of 2 and L were con-
firmed by NMR, mass spectra and elemental analysis (see
Figures S1, S2, S5–S8 in the online supplement).
Host-guest recognition experiments in aqueous
DMSO, HEPES buffered solution, pH 7.2, were carried out.
The coordination of L with Cr3+ was first investigated using
UV-vis absorption spectroscopy. As shown in Figure 1, the
addition of Cr3+ (10 μm) to L (5 μm) results in the spectral
shift of the absorption peak from 378 nm to 589 nm along
with color changes from yellow to green. Meanwhile, only
slight absorption changes are induced by adding Cu2+. In
an analogous way, no significant adsorption and color
changes occur in the presence of other metal ions includ-
ing Fe3+, Ag , Hg2+, Co2+, Ni2+, Cd2+, Pb2+, Ba2+, Mg2+, Al3+, Ca2+
*Corresponding authors: Shu-Wen Chen and Yan Gao, School
of Chemical Engineering, University of Science and Technology
LiaoNing, Anshan 114051, China, e-mail: wangweisit@163.com
(S.-W. Chen); gys20080901@163.com (Y. Gao)
Ren-Gang Lv: School of Chemical Engineering, University of Science
and Technology LiaoNing, Anshan 114051, China
+
and Zn2+. This result indicates that L can serve as a ‘naked-
eye’ Cr3+ indicator in aqueous media.
Additional studies of the Cr3+ – L complexation in
HEPES buffer is shown in Figure 2. The initial absorption
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ꢀꢀꢀꢁ
2ꢀ
ꢀR.-G. Lv et al.: Fluorescence probe for the detection of Cr
Ph
N
S
S
O
S
NH2
C
Br
N
NH
N NH
N
SeO 2
N
S
H2N
NHNH2
N
S
N
S
Ph
CH3
CHO
S
AcOH
EtOH
1
2
L
Scheme 1ꢀ
decreases upon further addition of Cr3+ ion and a new peak
at 589 nm gradually increases, implicating the formation
of L-Cr3+ ensembles.
1.0
0.8
0.6
0.4
0.2
0.0
The fluorescence spectra (λexꢀ=ꢀ358 nm) of L in the
absence and presence of various metal cations are shown
in Figure 3. The sensor L shows a negligible fluorescence
emission around 500 nm in the absence and presence of
L + other metal ions
Cu2+
+
Fe3+, Ag , Cu2+, Hg2+, Co2+, Ni2+, Cd2+, Pb2+, Ba2+, Mg2+, Al3+,
Cr3+
Ca2+ and Zn2+. By contrast, the addition of Cr3+ creates a
remarkable fluorescence enhancement at 536 nm. These
results demonstrate that probe L shows an excellent selec-
tivity toward Cr3+.
Fluorescence titration experiments were also per-
formed. As shown in Figure 4, upon incremental addition
of Cr3+ (0–2.0 equiv.) to L solution, the fluorescence emis-
sion gradually increases and reaches the saturation state
with 1.0 equiv. of Cr3+. The nonlinear fit of the data reveals
that the binding of L to Cr3+ is most probably of 1:1 stoichi-
ometry. This binding mode is also supported by the data
of Job’s plots evaluated from the fluorescence spectra of L
and Cr3+ with a total concentration of 20 μm (Figure S9).
According to the linear Benesi-Hildebrand expres-
sion, the measured fluorescence intensity [1/(Fꢀ−ꢀF0)] at
515 nm changes as a function of 1/[Cr3+] in a linear rela-
tionship (R2ꢀ=ꢀ0.9962). The association constant of L with
Cr3+ in HEPES buffer was calculated to be 5.73ꢀ×ꢀ104 m−1
300
400
500
Wavelength (nm)
600
700
Figure 1ꢀThe absorption spectra of L (5 μm) in DMSO/H2O solution
(3:7, v/v; HEPES buffered, pH 7.2) upon addition of 2.0 equiv. of
various metal ions.
Investigated metal ions are Cr3+, Fe3+, Ag , Cu2+, Hg2+, Co2+, Ni2+, Cd2+
Pb2+, Ba2+, Mg2+, Al3+, Ca2+ and Zn2+. Inset shows colors of L solution
in the absence and presence of Cr3+ ion.
,
+
1.0
0.8
L
0.6
0.4
0.2
0.0
120
120
L + Cr3+
100
100
80
60
80
L
40
20
0
60
40
20
0
300
400
500
600
700
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 2ꢀUV-vis absorption spectra of L (5 μm) in the presence of
Cr3+ (0–15 μm) in DMSO/H2O (3:7, v/v; HEPES buffered, pH 7.2).
L Cr Fe Ag Cu Hg Co Ni Cd Pb Ba Mg Al Ca Zn
maximum of L at 378 nm decreases upon gradual addi-
Figure 3ꢀFluorescence spectra of L (10 μm, λexꢁ=ꢁ358 nm) in the pres-
tion of Cr3+ ion with the concomitant appearance of a new
+
ence of different metal ions: Cr3+, Fe3+, Ag , Cu2+, Hg2+, Co2+, Ni2+, Cd2+
,
absorption band at 426 nm. This new band also gradually Pb2+, Ba2+, Mg2+, Al3+, Ca2+ and Zn2+ (20 μm).
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R.-G. Lv et al.: Fluorescence probe for the detection of Cr3+
ꢁꢀꢀꢀ
ꢂ ꢂ3
7
6
5
4
3
2
1
0
experiments (Figure 6). For the competition tests, recep-
120
100
80
60
40
20
0
tor L was treated with 2 equiv. of Cr3+ and 2 equiv. of other
+
metal ions such as Fe3+, Ag , Cu2+, Hg2+, Co2+, Ni2+, Cd2+,
Pb2+, Ba2+, Mg2+, Al3+, Ca2+ and Zn2+. No interference was
observed for the detection of Cr3+ by L in aqueous solu-
tion under these conditions. These results indicate that
L could be an excellent chromogenic sensor for Cr3+ over
competing relevant metal ions in aqueous solution.
The green bars represent the emission changes of
L in the presence of cations of interest (all are 20 μm).
The red bars represent the changes of the emission that
occurs upon the subsequent addition of 20 μm of Cr3+ to
the solution.
Cr
0.0
0.5
1.0
1.5
2.0
[Cr3+]/[L]
400
500
600
700
800
Wavelength (nm)
Figure 4ꢀFluorescence spectra of L (10 μm, pH 7.2) in buffered
aqueous phase upon addition of increasing concentrations of Cr3+
(0–20 μm, λexꢁ=ꢁ358 nm).
The reversibility of the recognition process of L was
analyzed by adding a bonding agent, Na2EDTA. As shown
in Figure 7, the addition of Na2EDTA to a mixture of L and
(Figure S10). The detection limit for Cr3+ was estimated to
be 2.1ꢀ×ꢀ10−8 m based on a 3σ/slope analysis under these
experimental conditions (Figure S11). Furthermore, the
fast fluorescence responses of L towards Cr3+ was also con-
firmed by the addition of Cr3+ (20 μm) to the solution of
L (10 μm) in HEPES buffer. As shown in Figure S12, upon
the addition of Cr3+, a significant fluorescence response is
observed within 5 min, indicating that the probe L could
be used for the real-time detection of Cr3+.
For the biological application of L, the sensing should
operate in the physiological range of pH. As shown in
Figure 5, the fluorescence intensity of L is stable in the pH
range of 5–11. This result indicates that the probe L can
be used as a chemosensor for Cr3+ detection under physi-
ological conditions.
L + Mn+
L + Mn+ + Cr3+
120
100
80
60
40
20
0
L
Cr Fe Ag Cu Hg Co Ni Cd Pb Ba Mg Al Ca Zn
To check further the practical applicability of recep-
tor L as a selective receptor, we carried out competition
Figure 6ꢀRelative fluorescence intensity of L (10 μm) in the presence
of various cations in DMSO/H2O solution (3:7, v/v; HEPES buffered,
pH 7.2, λexꢁ=ꢁ358 nm).
120
100
120
Cr3+
100
L
80
L + Cr3+
L
80
60
40
20
60
40
20
0
EDTA
5
6
7
8
9
10
11
0
2
4
6
8
10
pH
Number of addition
Figure 5ꢀThe effect of pH (5.0–11.0) on the fluorescence intensity of
L (10 μm) with 2.0 equiv. Cr3+ in DMSO/H2O solution (3:7, v/v; HEPES
buffered, pH 7.2).
Figure 7ꢀFluorescence responses of L (10 μm) after the sequential
addition of Cr3+ and EDTA.
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ꢀꢀꢀꢁ
4ꢀ ꢀR.-G. Lv et al.: Fluorescence probe for the detection of Cr
Cr3+ results in the decrease of the fluorescence intensity, are consistent with the proposed chelation as shown in
which indicates the regeneration of the free sensor L. It Scheme 2. The L-Cr3+ interactions may involve the imine
means that the receptor L could be used as a selective fluo- nitrogen and the nitrogen of the thiazole group to forms
rescent sensor for detection and recognition of Cr3+ in such a 1:1 complex (Scheme 2).
fields of environmental analysis.
To better understand the nature of the coordination
To verify the potential application of the fluores- of Cr3+ with L, energy-optimized structures of L and its
cent sensor L, the Cr3+ determination in tap and bottled corresponding complexes with Cr3 were obtained using
+
water samples was conducted using the fluorescence density functional theory (DFT) calculations at the
spectroscopy. The results were calculated by linear B3LYP/6-31G(d) level using Lanl2dz basis set for simple
equation and are listed in Table 1. All measurements receptor L and L-Cr3+ complex. The spatial distributions
were repeated three times. As can be seen, the recovery and orbital energies of the highest occupied molecular
of Cr3+ in all determined water samples ranges from 96.7 orbital (HOMO) and the lowest unoccupied molecu-
to 104.8% and the relative standard deviation (RSD) is lar orbital (LUMO) of L and the corresponding metal
under 1.83%. These data demonstrate that the present complexes were also calculated. As shown in Figure 8,
probe can be used for the practical detection of Cr3+ in the HOMO is distributed on the whole molecule in L,
aqueous samples in the presence of potentially compet- whereas LUMO is distributed on the benzothiazole ring
ing metal ion species.
in L. Meanwhile, the π electrons of HOMO of L-Cr3+ is
The recognition mechanism of the sensor L with mainly distributed on the chromium and the benzothia-
Cr3+ was investigated by using mass and infrared zole. The LUMO is distributed on the chromium and the
spectra. The mass spectrum obtained in the presence corresponding coordination site in L-Cr3+. The energy
+
+
of sodium ion shows a molecular-ion peak [Lꢀ+ꢀNa ] at gaps between the HOMO and LUMO in the L and L-Cr3+
m/z 359.03931. When Cr3+ ion is added into the solution were calculated to be 3.50210 eV and 2.34834 eV, respec-
of L, the resultant peak at m/z 458.95294 is assignable tively, indicating that the binding of Cr3+ to L decreases
+
to [Lꢀ+ꢀCr3+ꢀ+ꢀ2Cl−] species (Figure S3). We studied the IR the HOMO-LUMO energy gap of the complex and stabi-
spectra of free ligand L and the complex L-Cr3+ again and lizes the system. Thus, these results are consistent with
the results are shown in Figure S4. Upon addition of Cr3+ a favorable complexation according to the proposed
to L, the IR spectrum of L undergoes changes with the coordination.
vibrational frequency of CH=N undergoing a red shift
from 1636 cm−1 to 1653 cm−1. This result suggests that the
nitrogen atom of CH=N may participate in binding with
Cr3+. The IR frequency of N-H undergoes a blue shift from
S
N
3182 cm−1 to 3169 cm−1, which can be attributed to the
change of chemical environment near the N-H moiety.
In addition, the slight change of aromatic frequency
can be attributed to the coordination of the nitrogen of
thiazole with Cr3+. Therefore, the observed differences in
the IR spectra of L in the absence and presence of Cr3+,
coupled with ESI-MS analysis and Job’s plot analysis,
N
Cr3+
Cr3+
NH
S
N
NH
N
N
S
N
S
L-Cr3+
L
λ
ex = 358 nm
λ
ex = 358 nm
Scheme 2ꢀProposed mechanism for Cr3+ chelation by L.
Table 1ꢀAnalytical results of Cr3+ in water samples (nꢁ=ꢁ3).
–1.97990
–2.60059
L-Cr3+
LUMO
Samples
ꢁ
Added (μm)ꢁ Founda (μm)ꢁ Recovery (%)ꢁ RSDb (%)
L
LUMO
∆E = 2.34834
Bottled water ꢃ
10.0ꢃ
50.0ꢃ
100.0ꢃ
10.0ꢃ
50.0ꢃ
100.0ꢃ
9.87ꢃ
48.36ꢃ
98.74ꢃ
10.48ꢃ
50.93ꢃ
99.55ꢃ
98.7ꢃ
96.7ꢃ
0.29
0.36
0.45
0.52
0.84
1.67
∆E = 3.50210
ꢃ
ꢃ
98.7ꢃ
–4.94893
–5.48200
Tap water
ꢃ
ꢃ
ꢃ
104.8ꢃ
101.9ꢃ
99.6ꢃ
L-Cr3+
HOMO
L
HOMO
aMean value from three determinations. bRelative standard deviation Figure 8ꢀEnergy diagram of HOMO and LUMO orbital L and L-Cr3+
from three determinations. complex.
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R.-G. Lv et al.: Fluorescence probe for the detection of Cr3+
ꢁꢀꢀꢀ
ꢂ ꢂ5
orange solid: yield 0.99 g (59%); IR: ν 3182, 3062, 1636, 1587, 1480,
Conclusion
−1
1
1457, 783 cm ; H NMR: δ 12.90 (s, 1H, NH), 8.30 (s, 1H, CH=N), 8.11
(d, Jꢀ=ꢀ7.9 Hz, 1H, Ar), 8.00 (d, Jꢀ=ꢀ8.1 Hz, 1H, Ar), 7.88 (d, Jꢀ=ꢀ7.6 Hz, 2H,
Ar), 7.61–7.39 (m, 5H, Ar), 7.33 (t, Jꢀ=ꢀ7.2 Hz, 1H, Ar); 13C NMR: δ 167.3,
165.3, 153.8, 151.3, 135.6, 134.2, 129.2, 128.3, 127.0, 126.6, 126.0, 123.4,
122.8, 105.6. ESI-MS. Calcd for C17H12N4S2: m/z 336.4335. Found: m/z
A new benzothiazole-based fluorescent chemosensor L
for Cr3+ detection in mixed aqueous medium (DMSO/H2O
solution, 3:7, HEPES buffered, pHꢀ=ꢀ7.2) was developed.
The experimental results clearly indicate that compound
L is a highly sensitive and selective chemosensor for Cr3+.
The receptor selectivity and sensitivity are not affected in
the presence of other metal ions. According to Job’s plot,
a 1:1 stoichiometry complex between L and Cr3+ is formed.
This excellent selectivity suggests a potential application
of the chemosensor in the biological monitoring and track-
ing of Cr3+. A B3LYP/6-31G (d,p) basis set was employed for
optimization of L and L-Cr3+ complex.
+
359.03931 (Mꢀ+ꢀNa ). Anal. Calcd for: C, 60.69; H, 3.59; N, 16.65. Found:
C, 60.74; H, 3.57; N, 16.61.
Recognition studies
The metal binding studies were performed in 10-mL volumetric flasks
with fixed concentration of metal ion (0.2 mm) along with compound
L (0.1 mm) in HEPES buffered solution (50 mm, pH 7.2). The titration
experiments were conducted manually by stepwise addition of Cr3+
to the buffered solution of compound L (10 mL). To guarantee the
uniformity of solution, enough time was given before recording any
spectrum. The association constant was calculated according to the
equation of Benesi-Hildebrand [31]. The stoichiometry of compound
L and Cr3+ was determined by Job’s method from the fluorescence
data [32]. For fluorescence intensity measurements, the excitation
wavelength was fixed at 358 nm. The slit widths were 10 nm/10 nm.
Experimental
2-Formylbenzothiazole [30] was prepared as reported previously. All
chemicals were commercially available, and the organic solvents were
dried over appropriate drying agents and distilled prior to use. Dou-
ble-distilled water was used. Salts of metal ions Ca2+, Mg2+, Ni2+, Co2+,
Zn2+, Cu2+, Hg2+, Pb2+, Fe3+, Ba2+, Ag , Al3+, Cr3+ and Cd2+ were used to
+
Association constant calculation
evaluate the probe selectivity. 1H NMR spectra (500 MHz) and 13C NMR
spectra (125 MHz) were recorded on a Bruker AV 500 spectrometer
using DMSO-d6 as solvent. ESI mass spectra were recorded on a Bruker
Solarix 70 high-resolution instrument. Elemental analyses were per-
formed on a Perkin-Elmer 240 analyzer. The UV-vis and fluorescence
experiments were performed on a Lambda-900 spectrometer and a
Perkin-Elmer LS-55 fluorescence spectrophotometer, respectively.
For the formation of a 1:1 complex between the chemosensor L and
cation Cr3+, the following Benesi-Hildebrand equation was used:
1
1
1
=
+
Ka (Fmax −F )[Cr3+
]
F −F
Fmax −F
0
0
0
where F and F0 represent the fluorescence emission of L in the pres-
ence and absence of Cr3+, respectively, Fmax is the saturated emission
of L in the presence of excess amount of Cr3+; [Cr3+] is the concentra-
tion of Cr3+ ion added, and Ka is the binding constant.
Synthesis of 2
A mixture of 2-formylbenzothiazole (5 mmol), thiosemicarbazide
(5 mmol), glacial acetic acid (3 mL) and ethanol (20 mL) was mag-
netically stirred at reflux until the reaction was completed, as
monitored by TLC. The resulting precipitate of 2 was collected and
purified by silica gel column chromatography eluting with petro-
Acknowledgments: This work was supported by the finan-
cial support of the Key Laboratory Project of Liaoning
Province (2008S127).
1
leum ether/ethyl acetate, 9:1 to afford 0.87 g (74%); H NMR: δ 11.96
(s, 1H, NH), 8.61 (s, 1H, CH=N), 8.37 (s, 1H, NH), 8.12 (d, Jꢀ=ꢀ7.9 Hz,
1H, Ar), 8.06–7.95 (m, 2H, NH, Ar), 7.54 (t, Jꢀ=ꢀ7.6 Hz, 1H, Ar), 7.49 (t,
Jꢀ=ꢀ7.6 Hz, 1H, Ar); 13C NMR: δ 179.0, 164.9, 153.7, 137.2, 134.6, 127.0,
126.8, 123.5, 122.8. ESI-MS. Calcd for C9H8N4S2: m/z 236.3163. Found:
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