Hg2+ induced hydrolysis of thiazole amine based Schiff base: Colorimetric and fluorogenic chemodosimeter for Hg2+ ions in an aqueous medium

Article abstract of DOI:10.1016/j.saa.2019.03.106

Simple pyrene-based chemosensors 1 to 3, were synthesized from pyrene-1-carboxaldehyde and they were characterized using various spectroscopic techniques like UV–Vis, FT-IR, Mass, ¹H NMR and ¹³C NMR. Among synthesized receptors, the receptor 1 shows high selectivity towards Hg²⁺ ions. Further, the high selectivity of receptor 1 towards Hg²⁺ ions in the presence of various other interfering metal ions like Ni²⁺, Zn²⁺, Mn²⁺, Co²⁺, Cu²⁺, Cr³⁺, Fe³⁺, Al³⁺, Ag⁺, Fe²⁺, Cd²⁺, Mg²⁺, Pb²⁺, Ca²⁺, Na⁺, K⁺ was confirmed by UV–Vis and fluorescence methods. The detection limit for Hg²⁺ ions was found to be 0.270 μM. The chemodosimetric irreversible hydrolysis of the receptor 1 in the presence of Hg²⁺ was studied by UV/Vis, fluorescence, FT-IR, LC-MS, ¹H NMR and theoretical DFT study. Further, the real life applications of receptor 1 for the determination of Hg²⁺ ions were demonstrated by UV–Vis method.

Full text of DOI:10.1016/j.saa.2019.03.106

Accepted Manuscript
Hg2+ induced hydrolysis of thiazole amine based Schiff base:
Colorimetric and fluorogenic chemodosimeter for Hg2+ ions in an
aqueous medium
Venkatadri Tekuri, Suban K. Sahoo, Darshak R. Trivedi
PII:
S1386-1425(19)30354-3
https://doi.org/10.1016/j.saa.2019.03.106
SAA 16996
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 22 November 2018
Revised date:
Accepted
date:
26 March 2019
2
8 March 2019
Please cite this article as: V. Tekuri, S.K. Sahoo and D.R. Trivedi, Hg2+ induced
hydrolysis of thiazole amine based Schiff base: Colorimetric and fluorogenic
chemodosimeter for Hg2+ ions in an aqueous medium, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.03.106
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ACCEPTED MANUSCRIPT
2
+
Hg induced hydrolysis of thiazole amine based Schiff base: Colorimetric and
2
+
fluorogenic chemodosimeter for Hg ions in an aqueous medium
a
b
a*
Venkatadri Tekuri , Suban K. Sahoo and Darshak R. Trivedi
a
Supramolecular Chemistry Laboratory, Department of Chemistry,
National Institute of Technology Karnataka (NITK), Surathkal, Mangalore-575025,
Karnataka, INDIA.
b
Department of Applied Chemistry, S.V. National Institute Technology Surat-395 007
Gujarat, India.
*
Corresponding Author: darshak_rtrivedi@yahoo.co.in
Tel.: +91 824 2473205 fax: +91 824 2474033
Abstract:
Simple pyrene-based chemosensors 1 to 3, were synthesized from pyrene-1-
carboxaldehyde and they were characterized using various spectroscopic techniques like UV-
1
13
Vis, FT-IR, Mass, H NMR and C NMR. Among synthesized receptors, the receptor 1
2
+
shows high selectivity towards Hg ions. Further, the high selectivity of receptor 1 towards
2
+
2+
2+
2+
2+
Hg ions in the presence of various other interfering metal ions like Ni , Zn , Mn , Co ,
2
+
3+
3+
3+
+
2+
2+
2+
2+
2+
+
+
Cu , Cr , Fe , Al , Ag , Fe , Cd , Mg , Pb , Ca , Na , K was confirmed by UV-Vis
2
+
and fluorescence methods. The detection limit for Hg ions was found to be 0.270µM. The
2
+
chemodosimetric irreversible hydrolysis of the receptor 1 in the presence of Hg was studied
1
by UV/Vis, fluorescence, FT-IR, LC-MS, H-NMR and theoretical DFT study. Further, the
2
+
real life applications of receptor 1 for the determination of Hg ions was demonstrated by
UV-Vis method.
Key Words: Chemosensor/ receptor; Chemodosimeter; DFT; Colorimetric.
1
.0 Introduction:
Research in the field of molecular detection and sensing has received the substantial
attention in last few decades. Several strategies can be followed in the design and production
of molecular sensors with optimal selectivity toward particular analyte and the recognition
process converted into a read-out signal, either in the form of spectroscopic (Optical /
Fluorescence / NMR) or electrochemical signal. “Cation Recognition” as a research area has
been actively pursued by many scientists including chemists, biologists, clinical biochemists
and environmentalists in past decades. Ecological contamination is one of the most important
concerns of modern research. Immense efforts have been put towards the production of
2
+
2+
2+
molecular receptor for the determination of poisonous elements, like Hg , Pb and Cd ,
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2
+
3+
As and Cr [1–3]. These heavy metals damages human health as well as the
environment[4–6]. Therefore, early detection of these metals is desirable.
The broad uses of mercury in chemical manufacturing industries and farming
processes has led to its increased concentration in the environment [4–7], thus quantitative
2
+
determination of the Hg necessary. Mercury has more adverse effect on human life's, when
exceeds its permeable limit can cause neurological syndrome. An excessive amount of
mercury can cause paralysis, coma and death in human beings [8,9].
Till date, numerous receptors have developed and reported using fluorescence "turn-
2
+
on" mechanism, attributed to quenching nature of the Hg ions [10–15]. Recently, numerous
2
+
‘
turn-on’ fluorescence receptors for the detection of Hg ion have reported in organic and
semi-aqueous media [16–21]. Few fluorescence and colorimetric receptors are available in
2
+
the literature for the detection of Hg ions in aqueous media[17,18] [3,22]. In present work,
2
+
we have reported a new receptor 1, for the qualitative and quantitative determination of Hg
ion through a dual channel (colorimetric and fluorometric) response in a water medium. The
more poisonous nature of heavy metal ions in drinking water, the international standards like
WHO [23] and US-EPA [24] have set a limit for their daily consumption. Currently, many
advanced and accurate techniques like, ICP-MS and Atomic Absorption Spectroscopy (AAS)
[
25,26], which offers the quantification of heavy metal ions. The above mentioned
sophisticated methods require high cost and maintenance making it unsuitable for field
monitoring. In this regard colorimetric chemosensors are being developed for the recognition
of soft and poisonous metals like mercury. The present developed receptor 1 is having N and
S as interacting sites in its molecular structure, which help in selective binging of mercury
ions.
From past decades, there are three different approaches being used in the area of
chemosensor for the recognition of metal ions such as, i). Binding site-signaling, ii).
Displacement, iii). Chemodosimeter approach. In the chemodosimeter approach, a specific
ion-induced chemical reaction occurs which results in an optical signal [27–30]. In the
present work, we have reported simple Schiff base chemosensors 1-3 (Scheme-1). In receptor
2
+
1
, thiazole moiety undergoes Hg induced hydrolysis of -C=N-(imine) bond in water media
2
+
whereas pyrenecarboxaldehyde molecules act like ‘Turn On’ chemodosimeter for Hg ions.
The receptor 1, is more susceptible and efficient to detect mercury ions in the attendance of
other tested competitive metals in an aqueous media.
2
2
.0 Experimental details
.1 Materials
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In the present study we have used all chromatography grade solvents, analytical grade
metal salts and chemicals were procured from commercial sources and utilized as such.
2
.2 General synthesis procedure for receptors 1-3
A good percetage yield of receptors 1-3 has synthesized through the condensation of
Pyrene-1-carbaldehyde (1.1 mmol) with Thiazol-2-ylamine (1.1 mmol) (receptor 1), 1H-
Benzoimidazol-2-ylamine (1.1 mmol) (receptor 2) and 2-Thiophene carboxylic acid
hydrazide (1.1 mmol ) (receptor 3) in ethanol as a solvent and a drop of acetic acid as catalyst
by refluxing at RT for 3 hrs as represented in scheme 1.
2
.3 General techniques used for characterizations of receptors 1-3
Melting point (M.P) have determined on Bio-cote (SMP10). Bruker Alpha (FT-IR)
instrument was used to record the IR spectra of synthesized receptors 1-3.
1
3
1
The C NMR (100 MHz) and H NMR (400 MHz) spectra were recorded using
Varian mercury plus and Brukar FT-NMR using TMS as internal standard and DMSO-d6
solvent. Represented the chemical shift in ppm and coupling constant (J) in Hz. The Peak
multiplicities are represented with s, d, t & m letters (s-singlet, d-doublet, t-triplet & m-
multiplet). Agilent LC-MS 6410 instrument was used to record the mass spectra of reported
receptors 1-3. Analytik Jena Specord S600 UV-Vis spectrophotometer was used for the
titration experiments with sample cell (quartz) having 3.0 mL capacity and path length of 10
mm. Jasco-FP-6200 fluorescence spectrophotometer instrument was used for the fluorescence
titration in 3.0 mL quartz cells having four transparent optical windows and path length of 10
mm. The characterization results are described below.
Receptor 1 (Pyren-1-ylmethylene-thiazol-2-yl-amine).
-1
Yield: 94.4%. Melting range: 206 - 208°C. FT-IR: (KBr, cm ) 3073.21(Aromatic C-H,
str), 1563.02 (imine C=N, str), 1478.01, 1384.08, 1316.86 (aromatic C=C, str), 1142.31 (C-N,
1
str), 878.24 (C-H, bent), 712.61 (C-S, Str). H-NMR (dmso-d δppm): 9.416 - 9.393 (d, 1 H,
6
J=9.2 Hz), 9.253 - 9.229 (d, 1H, J=9.6 Hz), 8.823 - 8.803 ( d, 1H, J=8.0 Hz), 8.607 - 8.586 (d,
1
7
1
1
H, J=8.4 Hz), 8.496 - 8.304 (m, 4H), 8.293 - 8.271 (m, 1H), 8209 - 8.147 (m, 1H), 7.821 -
13
.813 (d, 1H, J=3.2 Hz), 7.722 - 7.714 (d, 1H, J=3.2 Hz). C-NMR (dmso-d δppm): 173.160,
6
62.680, 142.046, 135.452, 131.453, 131.165, 130.990, 130.474, 130.232, 128.289, 127.894,
27.687, 127.401, 127.097, 125.716, 125.413, 124.472, 123.205, 123.060, 120.146. LC-MS
(
ESI) m/z: calculated for C H N S; 312.38, found, 313.10 (M+1).
20 12 2
Receptor 2 ((1H-Benzoimidazol-2-yl)-pyren-1-ylmethylene-amine)
-1
Yield: 92.2%. Melting range: 141 - 143°C, FT-IR: (KBr, cm ) 3242.21(N-H str), 3085.33
(aromatic, C-H str), 1685.71 (Imine C=N, str), 1507.32, 1478.64, 1411.35, 1317.51 (aromatic
Page 3 of 19

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1
C=C, str), 1220.16 (C-N, str), 844.06 (C-H, bent). H-NMR(dmso-d δppm): 9.420-9.397
6
(1H, d, J=9.2Hz), 8.6092 - 8.589 (1H, d, J=8.0Hz), 8.496-8.408(3H, m), 8.305-8.283(1H, d,
J=8.8Hz), 8.220-8.182 (1H, t, J=7.2), 7.125-7.085 (2H, m), 6.881 - 6.841 (2H, m), 6.219 (2H,
1
3
s). C-NMR(dmso-d δppm): 193.713, 172.249, 155.147, 130.910, 130.673, 130.635,
6
1
27.266, 127.236, 127.198, 126.930, 126.877, 124.883, 122.546, 119.040, 111.439. LC-MS
(
ESI) m/z: calculated for C H N 345.29, found, 346.20 (M+1).
24
15 3;
Receptor 3 (Thiophen-2-yl-acetic acid pyren-1-ylmethylene-hydrazide)
-1
Yield: 93.0%. Melting range: 268 - 270°C. FT-IR: (KBr, cm ): 3217.15 (N-H, Str),
3
1
070.17 (Aromatic C-H, Str), 1635.02 (amide C=O, Str), 1559.39 (imine C=N, str), 1418.57,
359.68, 1294.23 (aromatic C=C, Str), 1135.15 (C-N, str), 845.08 (C-H, bent), 717.42 (C-S,
1
Str). H-NMR (dmso-d δppm): 9.496 (1H, s), 8.846(1H, s), 8.846 - 8.742 (1H, d), 8.620 -
6
,
8
7
1
.573 (1H, m),8.393 - 8.373 (3H, m), 8.301- 8.321 (2H, q), 8.231- 8.120 (2H, m), 8.033 -
1
3
.936 (2H,m), 7.305 - 7.284 (1H,t, J=4.4Hz). C-NMR (dmso-d δppm): 156.217, 130.849,
6
,
30.123, 128.716, 128.420, 127.396, 126.625, 126.120, 125.784, 123.760. LC-MS (ESI)
+
m/z: calculated for C H N OS; 356.44, found, 355.10 (M-H) .
2
2
16
2
+
Ethanol /H , RT, 3 h
H2N
S
S
N
N
N
O
Receptor 1
H
N
H2N
N
HN
N
+
N
Ethanol /H , RT, 3 h
Receptor 2
Receptor 3
S
H
N
H2N
O
S
+
H
N
Ethanol /H , RT, 3 h
N
O
Scheme 1: Synthesis of receptor 1 - 3
2
.4 Colorimetric, UV–Vis and Fluorescence experiments
-3
Individual metal ion stock solutions of 1.0 × 10 M were prepared in de-ionized water in
-3
1
0 mL volumetric flasks. 1.0 × 10 M solution of receptors 1-3 were made in dimethyl
sulphoxide (DMSO) as stock solutions. Further, these stock solutions were diluted to 2.5 ×
-6
1
0 M using DMSO. Micropipette was used for additions of metal solutions to the receptor
Page 4 of 19

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solutions during all the instrumental analysis such as colorimetric, UV-Vis and fluorescence
studies. After each addition of metal solution to the receptor spectra's were recorded.
3
3
.0 Results and Discussion
.1. Colorimetric sensing of cations
-6
The receptors 1-3 (2.5 x 10 M in DMSO) were tested for various metal ions like,
2
+
3+
2+
3+
3+
2+
2+
+
2+
2+
2+
2+
2+
2+
2+
+
Ni , Cr , Co , Fe , Al , Cu , Zn , Ag , Hg , Fe , Cd , Mn , Mg , Pb , Ca , Na
+
and K in aqueous solution. Among the receptors 1-3, receptor 1 produced selectively color
2
+
change (yellow to colorless) with the addition of 2.0 equivalence of Hg ions. Receptor 2
2
+
and 3 did not show any response with the adding of other interfering metals and Hg ions.
Receptor 1 showed linear decrement of color intensity (yellow to colorless) with the
2
+
incremental addition of Hg ions from 0, 0.5, 1.0, 1.5 and 2.0 equivalence, as shown in Fig.
1
.
Fig. 1: Linear decreasing of color intensity of the receptor 1 with the linear incremental
2
+
addition of Hg ions (0.5, 1.0, 1.5, and 2.0 equivalence)
3
.2. UV-Vis and fluorescence recognization of cations
The qualitative and quantitative determination of mercury ions, binding constant,
interference of other metals and stoichiometric ratio between receptor-metal ions were carried
out using UV-Vis and fluorescence titration.
In the UV-Vis spectra of receptor 1, the absorption bands in the region at 290 nm and
4
17 nm, are assigned to the π-π* and n -π*. Upon adding 3.0 equiv. of mercury ions a
new band was observed at 355 nm where as the band at 417 nm was disappeared. This
behavior can be explained metal-receptor complex formation. Whereas, receptor 1 did not
produce any absorption or color changes in the attendance of 4.0 equiv. of interfering metals
as represented in Fig. 2-4 in UV-Vis study experiments.
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-6
Fig. 2: Color change of receptor 1 (2.5 x10 M in DMSO) in the presence of 4.0 equi. of
2
+
various cations and 3.0 equivalence of Hg ions
S2R1
2
+
2
1
1
1
1
0
0
0
0
.00
.75
.50
.25
.00
.75
.50
.25
.00
Mn
2
+
+
Fe
Fe
3
2
+
Co
2
Ni
+
2
+
Zn
3
+
2+
Al
Pb
Ag
1 + Hg
2
+
+
2
+
Cd
3
Cr
+
2+
+
Ca
Na
2+
Mg
+
K
2
+
Cu
Hg
2
+
-0.25
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3: UV-Vis spectra of receptor 1, with the addition of 4.0 equi. of other interfering ions
2
+
2+
and 3.0 equivalence of Hg ions. Insert shows free receptor 1 and Receptor 1-Hg complex.
0
0
0
0
0
.8
.6
.4
.2
.0
1
2+
2+
+
2+
+
2+
+
+
3
+
2+
2+
2+
3
2+
2
+
3
2
+
2+
+
r
Fe
Fe
Ni
Zn
Al
Pb
Ag
Cr
Na
Mg
K
Cu
Hg
pto
Mn
Co
Cd
Ca
Rece
Fig. 4: A Bar-chart depiction of UV-Vis absorption intensity changes in the attendance of
various cations, the measured wavelength at 417 nm
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2
+
Titration experiments (UV-Vis) were performed with the incremental adding of Hg
ions (0 - 3.0 equivalence). The UV-Vis maxima peak at 417 nm linearly diminishes and a
2
+
new absorption peak appears at 355 nm with gradual addition of Hg ions (Fig. 5a).
Similarly, fluorescence titration performed with the incremental addition of mercury ions (0.0
-6
-
3.0 equivalences) to the solution of receptor 1 (2.5 10 M.) with excitation wavelength
55 nm. The emission band at 417 nm increased progressively and the solution showed
3
strong fluorescent in the emission spectra. (Fig. 5b).
1
0
0
.0
.5
.0
1
100
000
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
1000
1
R1
R1+Hg2+
900
Receptor 1
800
700
600
500
800
600
400
200
0
400
00
200
3
0
1
2
3
2
+
0.0
0.5
1.0
1.5
2.0
2.5
3.0
No. of eque. of Hg
2
+
Number Of Equivalance of Hg
0
Eque.
2+
3.0 Equei. of Hg
3
.0 Eque.
500
2+
0.0 Equei. of Hg
3
00
350
400
450
550
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 5a: UV-Vis titration spectra of receptor, 1 Fig. 5b: Fluorescence titration spectra of
-6
2+
-6
(
2.5 10 M) with the linear addition of Hg
receptor, 1 (2.5 x10 M) with incremental
2
+
ion (0.0 to 3.0 equivalence). Insert showing addition of Hg ions (0.0 to 3.0 equi.). The
binding isotherm at the selected wavelength inset shows the number of equivalence of
(417 nm)
metal solution vs. Intensity at 417 nm (Exited
wavelength at 355 nm)
The binding interaction was examined through fluorescence and UV-Vis titration with
2
+
the gradual addition of Hg ions. The binding constant (k) was determined using B-H plot
Fig. 6 [31,32].
+
5
-2
+5
-2
The binding constant calculated to be 4.2 x 10 M , 5.8 x 10 M for UV-vis and
fluorescence, respectively.
From the B-H plot, it was confirmed that the linear absorption plot between
2
+
absorbance/fluorescence intensity and concentration of Hg ions (Fig. 6), which provides the
:1 stoichiometry ratio of receptor and metal ions. Further, it was confirmed with the Job's
2
plot experiment using UV-Vis method and the figure given in the Fig. S10 Supporting
information.
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1
/ [M+]2
0.04
0
20
40
60
80
y = 8E-15x + 0.0016
R² = 0.9908
0
5E+12
1E+13
1.5E+13
2E+13
0.035
-
-
-
-
0
.03
.025
.02
0
0
0
0
.015
0.01
.005
0
-
100
120
y = -6E-12x - 1.5897
R² = 0.9962
0
1.5E+12
3E+12
4.5E+12
-
1
/ [M+]2
Fig. 6a: Benesi-Hildebrand plot of receptor Fig. 6b: B-H Plot of receptor 1 binding with
2
+
2+
1
binding with Hg ions (2:1), associated Hg ions (2:1 ratio) associated with a change
with absorbance change at 417 nm of Intensity at 417 nm. (Exited wavelength at
55 nm)
.3. Determination of detection limit (DL) and quantization limit (QL)
According to ICH Q2R1guidelines [33], defining the DL and QL for any quantification
3
3
analytical methods is most important to understand the minimal quantification and detection
of an analyte present in the sample. The DL and QL were calculated with the help of linear
2
+
-6
calibration curve drown b/w concentration of Hg ions (range from 0.0 to 5 x 10 M) vs.
response of the complex at 417 nm wavelength (Fig. 7).
DL / QL = K * S / S
d
Where K = 3.0 for DL and 10.0 for QL [33].
Sd = standard deviation of the response of the measurement,
S = Slope.
UV-Vis
Fluorescence
DL = 3 * 11.954935 / 1.33 x 10
8
DL = 3 * 0.019681 / 179169.41
-7
-7
=
3.89 x 10 M ≈ 0.389 µM.
= 2.7 x 10 M ≈ 0.270 µM.
8
QL = 10 * 0.019681 / 179169.41
QL = 10 * 11.954935 / 1.33 x 10
-6
-7
=
1.30 x 10 M ≈ 1.297 µM.
= 8.99 x 10 M ≈ 0.899 µM.
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1
.2
1
y = 1E+08x + 266.82
R² = 0.9917
y = -174144x + 0.9882
R² = 0.9946
1000
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
0
0
0
0
.8
.6
.4
.2
0
0
.E+00
2.E-06
3.E-06
5.E-06
6.E-06
0.E+00
3.E-06
5.E-06
8.E-06
2
+
2+
[
Hg ] M
[Hg ] M
Fig. 7a: Linear calibration curve between the Fig. 7b: A linear calibration curve of
2
+
2+
concentration of Hg and absorbance of emission of receptor 1-Hg vs. concentration
2
+
2+
receptor 1-Hg
wavelength at 417 nm
.4. Density Functional Theory (DFT)
Considering the experimental evidence on the hydrolytic cleavage of imine linkage of
complex, monitored of Hg ions. (Exited wavelength at 355 nm)
3
2
+
receptor 1 in the presence of Hg leading to the selective optical responses, the quantum
mechanical calculations of receptor 1 and its starting reactant 1-pyrenecarboxaldehyde were
done by adopting the density functional theory (DFT) method to complement the sensing
mechanism. All calculations were done in the gas phase by applying the B3LYP exchange-
correlation functional and the 6-31G** basis set coded in the Gaussian 09W and Gaussview
computational program [34]. Further, the time-dependent density functional theory (TD-
DFT) calculations were done to complement the spectral changes at the same level of theory.
As shown in Fig. 8a, the optimized geometry of receptor 1 was found to be planar leading to
a highly conjugated system resulting in the yellow coloration in the solution. The calculated
UV-Vis spectrum of receptor 1 has three absorption bands at 426.74 nm (Oscillatory strength,
f = 0.9315), 371.36 nm (f = 0.0826) and 336.33 (f = 0.0291) in the gas phase whereas at
4
55.34 (f =1.1237), 375.13 (f =0.1128) and 338.88 (f =0.0594) in the DMSO medium. The
calculated band in the visible region complemented well with the experimental value (417
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nm), which was responsible for the yellow coloration. The molecular electrostatic potential
map (MEP) of receptor 1 indicates that the most negative sites shown in red color are
observed over the nitrogen atoms (Fig. 8b). Considering all the possible coordination sites of
2
+
receptor 1, we have tried to obtain the Hg complex structure, but the structures are found to
be energetically not favourable. Further, the reactant molecule 1-pyrenecarboxaldehyde was
optimized and its spectral data were obtained which showed two bands at 370.88 (f = 0.3327)
and 343.24 (f = 0.0445) in the gas phase whereas at 390.24 (f =0.4957) and 347.27 (f
=0.0482) in the DMSO medium. The calculated absorption bands of 1-pyrenecarboxaldehyde
are correlated well with the experimental evidence obtained on the UV-Vis spectral changes
2
+
arise in receptor 1 upon binding with Hg . Further, the blue spectral shift of receptor 1 in the
2
+
attendance of Hg was also complemented with the HOMO-LUMO energy levels, where the
band gap of receptor 1 was increased from 0.1114 eV to 0.1276 eV (Fig. 8c). Analyzing the
MEP as well as the electron density in receptor 1, the possible weak fluorescence from
receptor 1 may be due to the PET (photoinduced electron transfer) occurred from the nitrogen
2
+
atoms to the pyrene fluorophore. Upon interaction with Hg followed by the separation of 1-
pyrenecarboxaldehyde and 2-aminothiazole, the PET process is inhibited resulting in the
vertical enhancement in the fluorescence.
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Fig. 8. (a) The DFT computed structure of L and (b) its MEP map. (c) The DFT computed
HOMO-LUMO energy levels of receptor 1 and 1-pyrenecarboxaldehyde.
.5. Binding mechanism of receptor 1 with mercury metal ion
3
The response (A /A) of receptor 1 to different metal ions vs. ionic radii/electronegativity
0
is depicted in Fig. S5 (Supporting information) and it further substantiates that mercury ion
exhibits better response than other examined metals [32,35–37]. This specific binding and
recognition of mercury ions explained by different mechanisms like Hard and soft acid-base
(
(
[
HSAB) theory [38], electronegativity and ionic radii. No correlation is noticeable when the
A /A) of the receptor 1 in the presence of various metal ions are drawned against ionic radii
0
32,35–37] (Fig. S5 supporting information) and same results were observed when data
2
+
plotted against electronegativity. From the correlation study, it is observed that Hg has
more selectivity than that of other cations and further, the results can be supported by
Pearson’s HSAB theory [38]. The sulfur atom is a soft base, so it has a greater affinity to
bind with mercury ion which is soft acid, hence the better response was shown by receptor 1.
2
+
The fluorescence response of 1 significantly increased at 417 nm when the Hg ions
were introduced to a receptor 1 solution. Enhancement at 417 nm in the fluorescence spectra
was due to the chemodosimetric event of receptor 1 that produced a strong fluorescent moiety
2
+
(
1-Pyrenecarboxaldehyde). To prove the chemodosimetric reaction of receptor 1 with Hg ,
spectroscopic structural analysis of the reaction products were investigated by FT-IR, LC-MS
1
and H-NMR spectroscopy [39,40]. FT-IR spectrum was recorded for receptor 1 in the
2
+
attendance and absence of Hg ions (Fig. S6 Supporting information). When compared FT-
2
+
-1
IR spectra of receptor 1 with the receptor 1+ Hg , the imine (-C=N-) frequency at 1562 cm
-1
was disappeared and aldehyde (-C=O) characteristic frequency at 1668 cm was found after
2
+
the addition of Hg ions. From the FT-IR analysis, it was identified that the imine bond in
2
+
receptor 1 undergone hydrolysis with the addition of Hg . The mass peak at m/z 313.08,
2
13.08 and 101.05 are corresponding to receptor 1, aldehyde and thiazole amine,
2
+
respectively, were identified in the LC-MS spectra of receptor 1 in the attendance of Hg
ions. (Fig. S7 Supporting information). It indicates that the irreversible hydrolysis of the
2
+
imine bond (in receptor 1) in the attendance of Hg ions. Another direct evidence was
1
obtained from the H-NMR (Fig. S8 Supporting information), receptor 1 showed a proton
2+
peak at 10.9 ppm which corresponding to pyrene aldehyde proton in the attendance of Hg
ions. These observations show that irreversible hydrolysis of imine bond in the attendance of
2
+
Hg ions.
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The genesis of pyrenecarboxaldehyde as a result of Hg-promoted hydrolysis of receptor
1
is confirmed by the absorption, fluorescence spectra (Fig. S9 Supporting information). The
literature reports also support the proposed reaction mechanism [39,40]. The absorption
2
+
spectra of pyrenecarboxaldehyde and receptor 1+Hg are closely undifferentiated, represent
2
+
that hydrolysis of receptor 1 by Hg ions affords pyrenecarboxaldehyde within a short
reaction time.
1
The UV-Vis, FT-IR, LC-MS and H-NMR results corroborate the proposed mechanism
2
+
of Hg -promoted imine hydrolysis of receptor 1 as reprasented in the scheme -2.
2
+
Scheme -2: Proposed mechanism of Hg -promoted imine hydrolysis of receptor 1.
4
.0 Real life application:
2
+
Receptor 1 selectively binds with Hg ions, with this mind we tried to find the trace
2
+
quantity of Hg ions in drinking and tap water samples with quantitative determination.
2
+
Initially, linear calibration plots drown between absorbance of receptor 1-Hg vs. the amount
2
+
of Hg ions (0.0 to 7.0 μM) (Fig. 8). From the linear plot, it was identified the absence of
2
+
Hg ions in the collected samples. Spiking studies were carried out to investigate the
interference of the other competitive ions present in the collected water samples. Mercury
2
+
spiked sample was prepared by adding a known quantity of Hg ions (1.3 μM and 2.5 μM)
to the water samples. The result is tabulated in table-1, it shows good percentage recovery
between three replicate spiked samples during the analysis with good precession.
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1
0
0
0
0
0
.00
.80
.60
.40
.20
.00
y = -0.1193x + 0.8815
R² = 0.9926
0
.00
2.00
4.00
6.00
8.00
Concentration of Hg2+ in μM
2
+
2+
Fig. 8: A linear calibration plot of 1-Hg vs. amount of Hg ions
2
+
Table-1: Estimation of Hg ions amount in water samples collected from different sources
Type of
Water
sample
Amount (μM)
n=3)
Found (μM) &
% RSD (n=3)
% Recovery
(n=3)
Spiked (μM)
(
1
2
1
2
.30
.50
.30
.50
1.31 + 1.17
2.49 + 0.23
1.31 + 1.16
2.48 + 0.40
100.5
99.5
Drinking
Tap
Not detected
Not detected
101.0
99.2
RSD: Relative standard deviation (RSD)
From the spiking study experimental results, it suggests that the quantitative
2
+
estimation of Hg ions in the tested two different water samples. This method can be utilized
2
+
for regular analysis to measure the amount of Hg in samples.
.0 Conclusion
Among the reported chemosensors, receptor 1 showed selectivity and sensitivity
towards mercury ions by producing a noticeable color change whic is easily recognized by
5
2
+
naked-eye. Receptor 1 was utilized for colorimetric detection and estimation of Hg ions in
the attendance of competitive tested metal ions qualitative and quantitatively. Through optical
1
studies (UV-Vis/ Fluorescence), spectroscopy (FT-IR, Mass and H-NMR) and theoretical
2
+
DFT study results were confirmed the Hg -induced hydrolysis of receptor 1. The prepared
receptor 1 was accomplished for the detection of 0.389 μM (UV-Vis) and 0.270 μM
2
+
(
Fluorescence) of Hg ions in a water medium. The receptor 2 and 3 did not show any
2
+
activity towards other tested cations and Hg ions; this may be a lack of suitable binding site
in their molecular structure like receptor 1. Receptor 1 has more acidic nature compared to
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2
+
the receptor 2 and 3, may be due to this the receptor, 1 undergone hydrolysis with the Hg
ions. The real-time analysis results suggest that receptor 1 could produce accurate and linear
results for routine application.
Acknowledgments
The authors are thankful to the Director and HOD (Department of Chemistry), NITK
Surathkal for providing the research facilities and the infrastructure. VT thankful to NITK
Surathkal, for the research fellowship and thankful to Venkatesh Pilli for the characterization
support and S. Ramanjaneyulu for LC-Mass analysis.
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39] V. Bhalla, Roopa, M. Kumar, P.R. Sharma, T. Kaur, Hg induced hydrolysis of
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Graphical Abstract
1
0
0
.0
.5
.0
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
S2R1
Receptor 1
0
1
2
3
2+
No. of eque. of Hg
0
Eque.
3
.0 Eque.
500
3
00
350
400
450
550
Receptor 1
Wavelength (nm)
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Highlights
2
+




New colorimetric/fluorescent chemodosimeter for Hg ions was reported
2
+
Receptor 1 showed sensitive response towards Hg ions at μM detection limit
Receptor 1 has the advantage to be a dual channel chemosensor
2
+
The hydrolysis of imine by Hg ions was confirmed by various spectroscopic
methods

The receptor 1 could be used for real-life application
Page 19 of 19