An ICT based highly selective and sensitive sulfur-free sensor for naked eye as well as fluorogenic detection of Hg2+ in mixed aqueous media

Article abstract of DOI:10.1016/j.tetlet.2013.06.035

A new internal charge transfer (ICT) based colorimetric as well as fluorogenic sulfur-free probe for highly selective and sensitive monitoring of Hg²⁺ has been developed and reported in this Letter. It gives a colorimetric and fluorometric resp

Full text of DOI:10.1016/j.tetlet.2013.06.035

Tetrahedron Letters 54 (2013) 4620–4623
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An ICT based highly selective and sensitive sulfur-free sensor
for naked eye as well as fluorogenic detection of Hg2+ in
mixed aqueous media
⇑
Shyamaprosad Goswami , Sangita Das, Krishnendu Aich
Department of Chemistry, Bengal Engg. and Science University, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new internal charge transfer (ICT) based colorimetric as well as fluorogenic sulfur-free probe for highly
selective and sensitive monitoring of Hg2+ has been developed and reported in this Letter. It gives a col-
orimetric and fluorometric response upon addition of Hg2+ in acetonitrile:water (1:1) solution at a phys-
iological pH, with no significant interference of other competitive metal ions. Receptor (TPM) shows a
distinct color change from light yellow to orange upon progressive addition of Hg2+, so that it can serve
as a naked-eye sensor for Hg2+ ion. It exhibits excellent reproducibility, reversibility, and selectivity with
the detection limit at micro-molar level.
Received 3 April 2013
Revised 6 June 2013
Accepted 10 June 2013
Available online 14 June 2013
Keywords:
Sensor
Ó 2013 Elsevier Ltd. All rights reserved.
Hg2+
Colorimetric
Fluorometric
ICT
In recent times, the development of chemosensors for the rec-
ognition and detection of physiologically and environmentally
important analytes has been receiving considerable attention.1
Mercury pollution is a global problem and remains a significant
threat to human health, yet global mercury emissions continue
to rise.2 Mercury is one of the most dangerous metal elements to
humans and the environment, and exists in nature mainly in three
forms, namely elemental, inorganic, and organic. Various forms of
mercury and its compounds have high chemical activity. They can
associate strongly with thiol, carboxyl, and phosphate in biological
bodies and cause great damage on human health.3 Due to the high
toxic effect of mercury in biological bodies and in environment the
recognition of mercury is a great area of research.4 Keeping in view
the roles played by mercury in day to day life, the development of
techniques for mercury hazard assessment and mercury pollution
management is in great demand.
Therefore, easy and affordable detection methods are in great
demand for various situations. In this regard, optical sensors for
mercury, in which a change in color is monitored, have been stud-
ied actively over the recent decades due to their simple, inexpen-
sive, and rapid implementation.5 Especially, colorimetric sensors
are promising because the color change can easily be observed
by the naked-eye, thus requiring less labor and no equipment.6
In the past few years, fluorescence chemosensors are widely re-
garded as one of the most effective ways for sensor design due to
the high sensitivity, specificity, simplicity of implementation, and
ability for real-time monitoring.7 Therefore, the development of
new fluorescence chemosensor for Hg2+ detection, especially those
that exhibit selective Hg2+ amplified absorption color change in
aqueous media, is still a challenging task.
Here, we report the synthesis and cation binding properties of a
new triphenyl amine based Schiff’s base derivative. Experimental
studies revealed that the present sulfur-free sensor exhibits
remarkable affinity for Hg2+ in aqueous media which is associated
with a color change from light yellow to orange due to the
enhancement of ICT8 (internal charge transfer) effect.
The receptor (TPM) is synthesized starting from triphenyl
amine as shown in Scheme 1. 4,40-diformyl triphenyl amine is syn-
thesized using the known literature procedure.9 The treatment of
diaminomaleonitrile with 4,40-diformyl triphenyl amine in ethanol
at refluxing condition affords the receptor as a yellow powder in
94% yield. The receptor is characterized by 1H NMR, 13C NMR,
and mass spectrometry analysis.
The photophysical properties of the sensor are investigated by
monitoring the absorption spectral behavior upon addition of sev-
eral metal ions such as Na+, Mg2+, Cr3+, Al3+, Mn2+, Fe3+, Cu2+, Zn2+
,
Cd2+, Hg2+, Pb2+, and Co2+ in aqueous medium [1:1, water/MeCN,
pH 7.1]. pH of the solution was maintained using HEPES [4-(2-
hydroxyethyl)-1-piperazineethanesulfonic acid] buffer (50 mM)
solution. Now from Figure 1, it is clear that the sensor (10
exhibits an absorption maximum centered at 447 nm. Upon
lM)
⇑
Corresponding author. Tel.: +91 33 2668 2961 3; fax: +91 33 2668 2916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.035

S. Goswami et al. / Tetrahedron Letters 54 (2013) 4620–4623
4621
CN
CN
N
OHC
CHO
NH2
N
N
N
(i)
(ii)
N
NH2
CN
NC
Scheme 1. Synthesis of the receptor (TPM). Reagents and conditions: (i) DMF/POCl3, 90 °C, 12 h; (ii) diamino maleonitrile, EtOH, reflux, 6 h.
3+
0.4
Receptor, Al
2+ 3+
,
1.0
0.8
0.6
0.4
0.2
0.0
2+
Cd , Cr , Co
3+ 2+
,
2+
Fe , Mn , Pb
,
2+
+
2+
0.3
0.2
0.1
0.0
Zn , Na , Mg
2+
Cu
2+
Hg
300
400
500
600
0
20
40
60
[Hg ] / μM
80
100
Wavelength (nm)
2+
a
function of Hg2+
Figure 1. UV–vis spectra of TPM (10 lM) in (CH3CN:H2O, 1:1, v/v, pH 7.1) in the
presence of different metal ions (5 equiv).
Figure 3. Change of absorbance of TPM at 508 nm as
concentration.
addition of Hg2+ a new band appears at 508 nm and the color of the
solution has changed from light yellow to orange.
This red shift (60 nm) of the absorption spectra is observed which
may be due to the enhancement of ICT effect after complexation of
the receptor with Hg2+
.
The other competing metal ions have no or insignificant effect
on the absorbance of the receptor. After addition of Cu2+ ion the
absorbance of the receptor decreases whereas no red shifted band
appeared. From this point of view, it is clear that the receptor is
The absorbance of the receptor at 508 nm increases linearly
with the incremental amount of Hg2+ in the range of 0–20
lM
(Fig. 3). The detection limit of the sensor for Hg2+ is determined
from the absorption spectral change, upon addition of Hg2+ to be
highly selective toward Hg2+
.
An absorption study of the receptor (TPM) upon gradual addi-
tion of increasing concentrations of Hg2+ (0–5.0 equiv) is shown
in Figure 2. The absorption band at 447 nm gradually decreases
and a new band appears at 508 nm and gradually increases with
4.15
l
M, using the equation DL = K ꢀ Sb1/S, where K = 3, Sb1 is
the standard deviation of the blank solution, and S is the slope of
the calibration curve10 (Supplementary data).
Job’s continuous variation method has been used to determine
the stoichiometry of the interaction between the receptor (TPM)
and Hg2+. Now Job’s plot obtained from absorbance spectral change
showed 1:2 stoichiometric complexation between TPM and Hg2+ in
[CH3CN:H2O, 1:1, v/v, pH 7.1] (Fig. 4). From the absorption spectral
data, we have evaluated the association constant of TPM using
the incremental amount of Hg2+
.
The intensity of the new band (at 508 nm) increases regularly as
the amount of Hg2+ is added progressively (up to 5 equiv) (Fig. 2)
resulting a color change of the solution from light yellow to orange.
0.9
0.6
0.3
0.0
0.5
0.4
0.3
0.2
0.1
0.0
300
400
500
600
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
X
g
Figure 2. UV–vis spectra of TPM (10
lM) in (CH3CN:H2O, 1:1, v/v, pH 7.1) in the
presence of increasing amounts of Hg2+ (0–5 equiv). Inset: photograph of TPM,
Figure 4. Job’s plot diagram of receptor for Hg2+ ion (where Xg is the mole fraction
of the guest and I indicates the change of absorbance at 447 nm).
before and after addition of Hg2+
.
D

4622
S. Goswami et al. / Tetrahedron Letters 54 (2013) 4620–4623
Hg2+
Hg2+
N
N
N
NC
NC
H2O OH2
Hg2+ 2+Hg
H2OOH2
NC
NC
N
N
CN
CN
N
CN
CN
N
NC
N
N
CN
Hg2+
N
H
N
H
N
H
N
H
NC NH2
H2N CN
S2-
Scheme 2. Probable binding mode of receptor with Hg2+
.
Benesi–Hildebrand equation11 and it is found to be 1.7 ꢀ 105 Mꢁ1
.
with 50 mM HEPES (pH 7.1). TPM shows a strong broad emission
band centered at about 575 nm in the absence of Hg2+. Now the
gradual addition of Hg2+ shows a distinct change in the emission
spectrum (Fig. 6). On continued addition of Hg2+ a gradual decrease
in the emission intensity at 575 nm is observed whereas a new
band at 475 nm appears. Essentially these changes in the fluores-
cence spectrum stop when the addition of Hg2+ reaches 5 equiv.
The difference between the two emission intensities is very large
(100 nm), which not only explains the accurate measurement of
the intensities of the two emission peaks but also holds a good
Assuming the (1:2) stoichiometry between TPM and Hg2+, we plot
1/[A ꢁ A0] versus 1/[Hg2+ 2
] and the data obtained from the graph
give a linear fit, which also support the (1:2) binding stoichiometry
of TPM and Hg2+ 12
. (Supplementary data)
The colorimetric sensor TPM is an electronically conjugated
form of two subunits, triphenyl amine (signaling unit) and diami-
nomaleonitrile (binding unit). The binding interaction between
TPM and Hg2+ induces the ICT (internal charge transfer) from the
electron rich nitrogen atom of the triphenylamine moiety to Hg2+
coordinating unit, which is responsible for the bathochromic shift
of the UV spectra. The plausible binding mode of TPM with Hg2+
in a 1:2 manner is shown in Scheme 2.
ratiometric value. The detection limit was found to be 5.2 lM,
which indicates the sensitivity of the receptor toward Hg2+ is in
micro molar scale (Supplementary data).
The selectivity and the interference are the two very important
parameters to evaluate the performance of a receptor. Now the
sensitivity and the selectivity of the TPM toward Hg2+ are exam-
Analogous experiments in fluorescence are carried out with
other series of cations such as Al3+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+
,
Mn2+, Mg2+, Na+, Pb2+, and Zn2+ using their perchlorate salts.
ined by employing different metal ions (Al3+, Cd2+, Co2+, Cr3+
,
In these cases small quenching of emission intensity at 575 nm
is observed whereas no blue shifted spectrum is found (Fig. 7).
Thus the sensor is exclusively selective for Hg2+, observed from
the fluorescence titration experiment.
Cu2+, Fe3+, Mn2+, Mg2+, Na+, Pb2+, and Zn2+) in aqueous acetonitrile
solution of TPM. TPM shows an extraordinary selective enhance-
ment of absorbance toward Hg2+ in CH3CN:H2O (1:1) solution at
508 nm, whereas, no notable change is observed upon addition of
other competing metal ions in the TPM solution at 508 nm. So
the selectivity of TPM toward Hg2+ is well proved in this section.
The interference of other competing metal ions is investigated
by the competition experiment which is demonstrated in Figure 5.
When TPM is treated with 2.0 equiv of Hg2+ in the presence of
other metal ions in the same concentration, the detection of Hg2+
in the presence of other metal ions is not hampered, that is, the
interference for the detection of the Hg2+ is not observed. So TPM
can be used as a selective and sensitive colorimetric sensor for
the Hg2+ ion.
Taking the advantage of thiophilic nature of Hg2+, in order to
show the reversibility of the receptor (TPM), absorption titration
experiment is performed using the TPM–Hg2+ complex with S2ꢁ
.
Now from the titration experimental data it is clear that the return
of the original TPM spectra is restored and disappearance of the or-
ange color is also well defined (Fig. 8). It indicates the decomplex-
ation of TPM–Hg2+ complex, as S2ꢁ strips away Hg2+ from the
binding side.
In order to examine the sensitivity of the receptor (TPM), the
fluorescence spectra of TPM (10
lM, kex = 390 nm) are explored
in aqueous acetonitrile (H2O:CH3CN, 1:1, v/v) solution buffered
125
100
575 nm
2
R =0.9834
75
50
25
100
75
50
25
0
2
R =0.9759
475 nm
0
10
20
30
40
2+
[Hg ]/μM
450 500 550 600 650 700 750
Wavelength (nm)
Figure 6. Change of emission spectra of TPM (10 l
M) upon gradual addition of Hg2+
Figure 5. Metal ion selectivity profile of the receptor (10
of absorbance intensity of receptor + 2 equiv Mn+; (brow bars) change of absorbance
intensity of receptor + 2 equiv Mn+, followed by 2 equiv Hg2+
l
M): (yellow bars) change
(0–6 equiv) in [H2O:CH3CN, 1:1, v/v, pH 7.1]. Inset: Plot of [Hg2+] versus fluores-
cence intensity at two different wavelengths (right). Photograph of TPM before and
after addition of Hg2+ (left).
.

S. Goswami et al. / Tetrahedron Letters 54 (2013) 4620–4623
4623
120
100
80
60
40
20
0
Supplementary data
3+ 2+
2+
Cr ,Cu ,Fe
Al ,Cd ,CO ,
3+ 2+ 3+
,
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
06.035.
2+
2+
+
Mn ,Mg ,Na ,
2+ 2+
Pb ,Zn
2+
Hg
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In summary, we have designed and synthesized a simple (inter-
nal charge transfer) ICT based probe TPM which exhibits a selec-
tive, sensitive, colorimetric, and ratiometric sensing toward the
Hg2+ ion in an aqueous acetonitrile solution. Thus from the UV–
vis and fluorescence data, it is clear that this sulfur-free receptor
is solely selective to the Hg2+ ion over other ions. Water solubility,
reversibility, and high sensitivity make this probe suitable to use as
an excellent chemosensor for Hg2+
.
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Acknowledgments
The authors thank CSIR and DST, Government of India for finan-
cial support. S.D. and K.A. acknowledge CSIR for providing them
with fellowships.