Selective detection of mercury ions using benzothiazole based colorimetric chemosensor

Article abstract of DOI:10.1016/j.inoche.2016.10.017

Three new receptors L1, L2 and L3 were designed and synthesized in a single pot synthesis. The synthesized receptors were characterized by ¹H NMR, FT-IR and Mass spectrometry. The cation binding affinity was studied using colorimetric and UV–Vi

Full text of DOI:10.1016/j.inoche.2016.10.017

Selective detection of mercury ions using benzothiazole based colorimetric
chemosensor
Bharath Kumar Momidi, Venkatadri Tekuri, Darshak R. Trivedi
PII:
DOI:
Reference:
S1387-7003(16)30409-9
doi:10.1016/j.inoche.2016.10.017
INOCHE 6454
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
27 July 2016
4 October 2016
7 October 2016
Please cite this article as:
Bharath Kumar Momidi, Venkatadri Tekuri,
Darshak R. Trivedi, Selective detection of mercury ions using benzothia-
zole based colorimetric chemosensor, Inorganic Chemistry Communications (2016),
doi:10.1016/j.inoche.2016.10.017
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Title: 1 SELECTIVE DETECTION OF MERCURY IONS USING BENZOTHIAZOLE
BASED COLORIMETRIC CHEMOSENSOR
Bharath Kumar Momidia, Venkatadri Tekuria and Dr. Darshak R. Trivedia*
a Supramolecular Chemistry Laboratory, Department of Chemistry,
National Institute of Technology Karnataka (NITK)-Surathkal, Mangalore-575 025. INDIA.
*Email: darshak_rtrivedi@yahoo.co.in
Abstract: Three new receptors L1, L2 and L3 were designed and synthesized in a single pot
1
synthesis. The synthesized receptors were characterized by H NMR, FT-IR and Mass
spectrometry. The cation binding affinity was studied using colorimetric and UV-Vis spectral
studies for the three receptors. Receptor L1 showed high selectivity with a significant color
change from yellow to colorless (turn-off response) in the presence of Hg2+ ions over the
other cations such as, Ni2+, Cr3+, Pb2+, Cu2+, Cd2+ , Co2+, Zn2+and Fe2+ and the receptor L2, L3
did not show any color change or change in the absorption in UV-Vis spectral studies with
Hg2+ ions and other tested metal ions, this may be due to lack of –OH group in receptor L2
and L3. The B-H Plot shown 1:1 complex between L1and Hg2+ ions.
Key Words: Chemosensors, Naked-eye detection and B-H Plot
Introduction: The development of new colorimetric receptors for selective recognition of
heavy metal ions [1-3] has received considerable attention in past few decades. This may be
because of the fact that certain metals when exceed permissible limit, become extremely toxic
and hazardous to mankind. Among various heavy metal ions mercury, lead and cadmium ions
are well known for their high toxicity towards environment and human health. Among them
mercury is one of the most poisonous metallic contaminants and can exist in ionic (Hg2+),
elemental (Hg0) and organic form methyl mercury (CH3Hg+) this powerful neurotoxin enters
the food chain and finally accumulates in the human body[4].the so called “ Minamata
Disease” is a tough consequence of methyl mercury. Mercury is the most lethal for living
organisms because it can easily enter through the skin and respiratory cell membranes,
leading to DNA damage, myocardial infarction, some kind of autism, damage of the brain,
kidneys and damage to the sensory parts of the nervous system. Knowing the seriousness of
problem, significant research efforts have been dedicated to improve heavy metal ion
detection [5]. Current industrial approaches count on costly, sophisticated instruments,
complicated procedures or low sensitivity and selectivity approaches like atomic
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absorption/emission spectroscopy or inductively coupled plasma mass spectroscopy. For these
reasons, chromogenic chemosensors are especially attractive because they permit naked eye
detection of color change without any use of spectroscopic instruments [6]. In addition these
colorimetric sensors ideally have the benefits of low cost, easy synthesis and storage, and
more tolerance towards different experimental conditions. Therefore, many researchers have
focused on “colorimetric,” and /or fluorometric chemosensors in this research area, for the
detection of heavy metals.
Recent time’s Ye won Choi group designed imidazole moiety as a metal ion receptor [7]
thiazole moiety was often chosen as a metal ion receptor [8-11], and 2 hydroxy
salicylaldehyde is well-known chromophore and has been frequently used for the colorimetric
chemosensors [11-14]. Therefore, we planned to incorporate the Benzothiazole moiety into 5-
nitro-2-hydroxy salicylaldehyde moiety to develop a novel chemosensor (L1), 2-(Benzothiazol-
2-yliminomethyl)-4-nitro-phenol.
Herein we report a simple and reliable colorimetric chemosensor (L1) for the detection of
Hg2+ ions in aqueous media. This work was motivated by the fact that most organic molecules
reported as Hg2+chemosensors perform the sensing process in a medium different than water,
which limits their function in environmental and life sciences. This new sensor is based on a
Schiff base derivative, this have been described as important tools for a long time, but their
usage as a signaling subunit in chemosensors is limited due to their low solubility in aqueous
media. Mercury is a soft metal and therefore favors sulfur-rich binding sites. It shows an
intense color change after coordination with metals and offers a wide variety of design
possibilities. Sensing characteristics of (L1) were investigated using UV-Vis absorption
spectroscopy in aqueous solutions in the presence of biologically and environmentally
relevant metals, i.e., Ni2+, Hg2+, Pb2+, Cu2+, Co2+, Zn2+and Fe2+ [15-23]. The results
demonstrated that L1 can be employed for sensing Hg2+ in water with a limit of 0.15 μM even
under the presence of other interfering ions. This is one of the lowest detection limit reported
for mercury in aqueous media.
Experimental:
Materials and methods:
All chemicals were procured from Sigma-Aldrich, Alfa Aesar or from Spectrochem
and used without further purification. All the solvents were procured from
Spectrochem, SD Fine, India of HPLC grade and used without further distillation. The
1H NMR spectra were recorded on a Bruker, (400MHz) instrument using TMS as
internal reference and DMSO-d6 as solvent. Resonance multiplicities are labelled as s
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(singlet), d (doublet), t (triplet) and m (multiplet). Melting points were measured on a
Stuart- SMP3 melting-point apparatus in open capillaries. Infrared spectra were
recorded on a Perkin-Elmer FT-IR spectrometer; signal designations: s (strong), m
(medium) and w (weak). UV-vis spectroscopy was carried out with Analytikjena
Specord S600 Spectrometer in standard 3.5 mL quartz cells (2 optical windows) with
10 mm path length.
Synthesis of receptors L1-L3:
Receptor L1-L3 was designed and synthesized as shown in Scheme 1, all receptors
were synthesized in a single step, through a condensation reaction of 2-Amino
benzothiazole (0.83 mM, 0.125 g) with 5-nitrosalicyaldehyde (0.83 mM, 0.138 g),
Benzaldehyde (0.83 mM, 0.088 g) and Biphenyl-4-carboxaldehyde (0.83 mM, 0.152 g)
in ethanol under reflux condition at 60°C for 3 hrs. The reaction was catalysed by a
drop of acetic acid. After cooling, the solid was filtered and washed with ethanol to
1
obtain the solid designed compounds. The obtained products were characterized by H
NMR, FT-IR spectroscopy, and Mass spectrometry (See the supporting information
Fig. 2a, 2b, 2c, 3a, 3b, 3c, 4a, 4b and 4c).
S
O2N
CHO
OH
O2N
EtOH / H+
600C 3 Hrs
N
N
OH
L1
N
CHO
S
N
EtOH / H+
600C 3 Hrs
S
N
NH2
L2
OHC
S
N
EtOH / H+
600C 3 Hrs
N
L3
Scheme-1: Synthesis of receptor L1, L2 and L3
Characterization data:
2-(Benzothiazol-2-yliminomethyl)-4-nitro-phenol, (L1):
Melting range: 247-250˚C; IR (KBr, frequency in cm-1): (C=C) 1463 cm-1, 1526 cm-1, (C=N)
1
1608 cm-1, (C-H aromatic str) 3075 cm-1, (OH) 3377 cm-1. H-NMR (500 Mzdmso-d6, δppm):
3

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7.01-7.25 (m,1H),7.38(m,4H), 7.2 ( m, 1H), 7.1(m,1 H), 8.0(s HC=N, 1 H), 9.0 (s,-OH
1H).13C NMR (CDCl3, 125 MHz) 51.71, 58.42, 102.05, 110.22, 118.61, 140.79, 154.21,
167.02. MS (ESI, m/z) calculated 299.30, observed (M+1) 300. Elemental analysis calculated C-
56.18, H- 3.03, N-14.04 Observed C- 56.32, H-3.08 and N- 13.85.
Benzothiazol-2-yl-benzylidene-amine (L2):
Melting range: 219-224˚C; IR (KBr, frequency in cm-1): (C=C) 1543 cm-1, (C=N) 1615 cm-1,
(C-H aromatic str), 3067 cm-1. 1H NMR (300Mzdmso-d6, δppm): 7.62 (m, 1H), 7.5 (m 4H), 7.0
(m, 1H),7.2 (m,1H) 6.89 (m, 1H), 8.95 (s,1H HC=N). 13C NMR (CDCl3, 100 MHz) 76.61,
119.44, 122.00, 126.03, 128.77, 137.43. MS (ESI, m/z) calculated 238.31, observed (M+1)
239.00. Elemental analysis calculated C-70.56, H- 4.23, N-11.76 Observed C- 70.42, H-4.28
and N- 11.87.
Benzothiazol-2-yl-biphenyl-4-ylmethylene-amine (L3):
Melting range: 220-226˚C; IR (KBr, frequency in cm-1): (C=C) 1543 cm-1, (C=N) 1615 cm-1,
1
(C-H aromatic str) 3062cm-1. H NMR (400Mz, dmso-d6, δppm): 8.29 (m, 2H), 7.82 (m, 2H)
7.67 (m, 3H), 7.59 (m, 3H), 6.79 (s,1H), 6.6(s, 2H) 5.89 (s, 1H). 13C NMR (CDCl3, 100 MHz)
39.70, 117.50, 125.53, 139.79, 166.57, 189.04. MS (ESI, m/z), calculated 314.40, observed
(M+1) 314.00, Elemental analysis calculated C-76.40, H- 4.49, N-8.91 Observed C- 76.32, H-
4.41, N- 8.78.
Results and discussion: An individual colorimetric studies were conducted for the
synthesized receptor L1-L3 in DMSO solvent, (2.5 x10-5 M) for the detection of different
cations such as Ni2+, Hg2+, Pb2+, Cu2+, Co2+, Cr3+, Zn2+ and Fe2+ in an aqueous solution (1.0 x
10-3 M). The receptor L1 showed a significant color change from yellow color to color less in
the presence of Hg2+ ions which can be easily identified by the naked eye and the L1 did not
show any color change in the presences of 3.4 equiv. of other tested cations. It indicates that
the receptor L1 has selective and sensitive towards Hg2+ ions over the other tested metal ions.
In the similar way the receptor L2 and L3 were investigated for colorimetric detection of
cations and they did not show any color change in the presence 3.4 equiv. of Hg2+ and other
tested cations.
The absorption spectra of receptor L1 in DMSO show major band at 445 nm. The
recoganization ability of receptor L1 towards different metal ions investigated by UV-Vis
measurement as shown in Fig.2a, receptor L1 did not show any absorption spectra in the
presence of 3.4 equiv. of an aqueous solution of different cations such as Ni2+, Hg2+, Pb2+,
Cu2+, Co2+, Cr3+, Zn2+ and Fe2+. In the presence 3.4 equiv. of Hg2+ ions the receptor L1 show
4

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a new absorption band at 345 nm and decreased absorption intensity at 445 nm in the UV-Vis
absorption spectra (Fig. 2a) and the solution of receptor L1-Hg2+ showed a significant color
change from yellow to colorless, which could be easily detected by naked-eye as shown Fig. 1
This signifies the interaction between receptor L1 and Hg2+ ions and the receptor L1 sense
Hg2+ in aqueous medium because of its thiophillic nature and having -OH group.
Fig. 1. The colorimetric changes observed by naked-eye of receptor L1 upon addition of 3.4
equiv. of various cations in an aqueous solution
0.9
1.4
1.2
L1 + Hg2+
0.8
1.0
0.8
0.6
0.4
0.2
0.0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
L1
Cu2+
Co2+
Fe2+
Cr3+
Zn2+
Ni2+
Pb2+
Hg2+
300
350
400
450
500
550
Zn2+
Ni2+
Pb2+ Hg2+
Co2+ Cr3+
Cu2+
Fe2+
L1
Wavelength (nm)
Fig. 2a. UV-Vis spectral change of receptor Fig. 2b. A bar chart representation of
L1 (2.5×10-5 M in DMSO solvent) in the absorption change of L1 in the presence of 3.4
presence of 3.4 equiv. various metal ions. equiv. of various metal ions at wavelength 345
Insert showing the absorbance of receptor nm
L1 and L1+Hg2+ ions
To explore the sensing properties of receptor L1 and Hg2+, UV-Vis spectral titration
experiment was performed between receptor L1 and Hg2+ ions as shown in Fig.3a. The
incremental addition from 0 to 3.4 equiv. of Hg2+ ions (1.0 x 10-3 M in an aqueous solution) to
a solution of receptor L1 in (2.5 x 10-5 M in DMSO solution), the absorption intensity band at
445 nm corresponds to -OH group in the receptor L1 gradually decreases and a new band at
345 nm gradually increases. This observation is due to the formation of the complexation of
Hg2+ ions with receptor L1, the electron rich hydroxyl group functioned as a donor group and
Hg2+ accepts electrons [16-18]. The binding modes of receptor L1 and Hg2+ ions were
supported by solid FT-IR studies. The free receptor L1 displayed -OH str. frequency at 3377
cm-1 and the -OH str. frequency of complex (L1 - Hg2+) at 3247 cm-1 (See supporting
5

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information Fig.12), this decrimental -OH str. frequency results suggests that Hg2+ ions might
bind to the phenolic O, S thiazole and Nimine
.
The binding stoichiometry between receptor L1 and Hg2+ ions was determined by the Benesi-
Hildebrand (B-H) method using UV-Vis spectrometric titration data wavelength at 345 nm.
The linearity of the graph confirms the formation of stable 1:1 complexation of receptor L1
with Hg2+ ions as shown in Fig. 3b. The association constant (K) found to be 6.8 × 103 M-1
using following equation
1
1
1
=
+
+ n
( A- Ao)
{K (Amax - Ao) . [Mx ] }
[ Amax - Ao]
Where, A0, A, Amax are the absorption considered in the absence of Hg2+, at an intermediate,
+
and at a concentration of saturation, K is binding constant, [Mx ] is concentration of Hg2+ ions
and n is the stoichiometric ratio.
1.4
60.0
0.9
y = 1E-05x - 0.4237
0.8
1.2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
R² = 0.9948
50.0
40.0
30.0
20.0
10.0
0.0
1.0
0.8
0.6
0.4
0.2
0.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
No. of equiv. of Hg2+
300
350
400
450
500
550
0.0E+00 1.0E+06 2.0E+06 3.0E+06 4.0E+06 5.0E+06
1 / [Hg2+
]
Wavelength (nm)
Fig. 3a: UV–vis spectra of L1 (2.5×10-5 M, Fig. 3b: Benesi–Hildebrand plot of receptor
DMSO) with increasing concentration of L1assuming 1:1 binding stoichiometry with
Hg2+ ions (0 – 3.4 equiv.) in an aqueous Hg2+ ions, wavelength at 345 nm
medium. Inset graph shows binding isotherm
at selective wavelength (345 nm)
The Detection limit (DL) and Quantization limit (QL) for Hg2+ ion was determined using the
a calibration plot between Hg2+ metal ion concentration (range from 2.5 x 10-7 M to 6.0 x 10-6
M) and corresponding absorbance of receptor L1-Hg2+complex measured wavelength at 345
nm.
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The DL and QL were determined based on the standard deviation of the response and the
slope using the following equation mentioned by ICH quality guideline Q2R1 [24]
Where, = standard deviation of blank measurement, m = slope of the calibration curve,
= constant (for DL 3.0 and QL 10.0).
The calibration plot was drawn between the concentration of metal ion and absorbance using
LINEST MS-excel as shown in the Fig. 4.
The calculated DL was found to be 0.15 µM and QL was found to be 0.51 µM. (See the
supporting information Fig. 10).
1.4
0.80
y = 112107x + 0.0358
R² = 0.9928
6.0 x 10-6 M of Hg2+
0.70
0.60
1.2
0.50
0.40
0.30
1.0
0.20
0.10
0.8
0.00
0.E+00
2.5 x 10-7 M of Hg2+
1.E-06
2.E-06
3.E-06
4.E-06
5.E-06
6.E-06
2+
[Hg ] M
0.6
0.4
0.2
0.0
300
350
400
450
500
550
Wavelength (nm)
Fig. 4: Linearity of UV-Vis spectra of complex (L1-Hg2+). Inset graph shows
calibration curve plot of absorbance of complex (L1-Hg2+) vs concentration of Hg2+
ions in M
Based on the UV-Vis titration studies experiment, solid FT-IR and1H NMR titrations
were carried out to further identify the binding mechanism. The figure (ESI-16) shows
the spectra of receptor L1 (5×10-4 mol /L) up on addition of 0-2 equiv. of Hg2+. When
treated with 1.0 equvi of Hg2+, the peak of phenol OH upfield shifted from 9.0 to8.8,
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while it completely disappeared after adding 2.0 equiv. of Hg2+ ion at the same time
the aromatic proton signals were also affected which underwent a concomitant upfield
shift. These observations clearly supported the –OH group involved in the binding
mechanism. The binding mechanism of the Hg2+ ions to the receptor L1 can be
proposed as showed in Scheme 2. The solid state FT-IR spectrum were recorded for
the free receptor L1 and L1 - Hg2+ complex individuality, the observation reveals that
the -OH Str. frequency of receptor L1 decreases in the presence 3.4 equiv. of Hg2+
ions, from 3377 cm-1 to 3247 cm-1 as shown in Fig. 5. From the UV-Vis titration and
FT-IR studies results reveals that -OH group present in receptor L1 responsible for
binding of Hg2+ ions. (25)
L1
L1 + Hg2+
70
65
60
55
50
3377 cm-1
45
40
35
3247 cm-1
3000
30
4000
3500
2500 2000
Wavenumber cm-1
150
Fig. 5: Solid state FT-IR Spectra of free receptor L1 and L1 - Hg2+ complex
N
N
S
NO2
Hg2+
O
H
Scheme 2: Proposed binding mechanism of Hg2+ ions with receptor L1
Conclusion:
To summarize, herein we report, the design and synthesis of receptor L1 for the colorimetric
detection of Hg2+ ions over other tested cations. The receptors L1 with phenolic-OH
functional group as a binding site showed a significant color change from yellow color to
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colorless, selectively in the presence of Hg2+ ions which can be easily identified by the naked-
eye, where as other tested cations failed to show any color change with receptor L1. The color
change was due to the formation of complex between Hg2+ ions and receptor L1, which was
confirmed by UV-Visible spectral titrations and solid state FT-IR studies. The receptor L1
showed a reasonable detection limit of 0.15 μM and a quantization limit of 0.51 μM for Hg2+
ions.
Acknowledgements: The authors acknowledge the Director and the HOD (Department of
chemistry), NITK for providing the research infrastructure. Authors are thankful to
Department of science and technology (DST, government of India, New Delhi,) for the
Financial support (DST-SERB Fast track scheme No. SB/FT/CS-137/2012) authors thank
NITK and CSMCRI Bhavnagar for the spectral analysis.
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 Detection of Hg2+ ion gained significance because of its key role in biological sciences as
well as in environmental sciences.
 On adding Hg2+ ions to the receptors L1 displayed instantaneous color change from
yellow to colorless (Turn-Off Mechanism).
 The receptor L1 showed a reasonable detection limit of 0.15 μM and a quantization limit
of 0.51 μM for Hg2+ ions.
 4. The receptor L1 shown good linear range from 2.5 x 10-7 M to 7.9 x10-6 M of Hg2+
ions.
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Graphical abstract
13