A highly selective colorimetric chemosensor for cyanide and sulfide in aqueous solution: Experimental and theoretical studies

Article abstract of DOI:10.1039/c6nj01832g

A new colorimetric chemosensor 1 was developed by a combination of 4-(p-tolyl)thiazol-2-amine and 2-hydroxy-1-naphthaldehyde. Sensor 1 showed selective colorimetric responses to CN^- and S^2- in aqueous solution. A clear color change o

Full text of DOI:10.1039/c6nj01832g

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A highly selective colorimetric chemosensor for cyanide and sulfide in
aqueous solution: experimental and theoretical studies
Soo Min Kim,a Minjae Kang,a Isaac Choi,a Jae Jun Lee,b* Cheal Kima,b*
aNowon Institute of Education for the Gifted at Seoultech, Seoul National University of
Science and Technology, Seoul 139-743, Korea
bDepartment of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul
National University of Science and Technology, Seoul 139-743, Korea. Fax: +82-2-973-9149;
Tel: +82-2-970-6693; E-mail: idtwar@gmail.com and chealkim@seoultech.ac.kr
Abstract
A new colorimetric chemosensor 1 was developed by combination of 4-(p-
tolyl)thiazol-2-amine and 2-hydroxy-1-naphthaldehyde. The sensor 1 showed selective
colorimetric responses to CN- and S2- in aqueous solution. A clear color change of 1 from
yellow to colorless was observed with CN- through a nucleophilic addition mechanism, while
S2- was detected through deprotonating mechanism with a distinct color change from yellow to
orange. The binding stoichiometries of 1 with CN- and S2- were proposed to be 1:1, based on
1
Job plot, H HMR titration and ESI-mass spectrometry analysis. Additionally, the sensing
mechanisms of 1 with CN- and S2- were theoretically explained by DFT and TD-DFT
calculations. Moreover, 1 could be used to quantify CN- in water samples.
Keywords: Cyanide, Sulfide, Colorimetric, Theoretical calculations
1

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Introduction
There is a considerable interest for detecting anions, due to their important roles in
biological, industrial and environmental processes.1-4 Among the various anions, cyanide is
extensively researched due to its wide use in many fields such as gold mining, electroplating,
metallurgy, synthetic fibers and resins industry. Therefore, the wide use of cyanide is inevitable,
and many industries produce nearly 140,000 tons of cyanide per year worldwide.5-9 On the
other hand, cyanide is known to be an extremely toxic anion in biology and environment. Its
toxicity results from its propensity to bind to the iron in cytochrome c oxidase, interfering with
electron transport and resulting hypoxia.10-13 Thus, there is a need for an efficient sensing
system to detect cyanide from biology and environment.
Hydrogen sulfide (H2S) has been considered to be the third gasotransmitter, together
with carbon monoxide (CO) and nitric oxide (NO). Many studies have proved that H2S takes
part in many physiological processes, such as angiogenesis, vasodilation, apoptosis, regulation
of inflammation and neuromodulation.14-16 On the other hand, H2S is known to be a widespread
environmental contaminant released from leather industries, petroleum refineries and
delignification of pulp. Moreover, H2S was known as a poisonous gas in nervous system of
human being, which could cause serious diseases such as hyperglycemia, Down’s syndrome
and Alzheimer’s disease.17-19 Therefore, it is important to develop an efficient method for the
detection of H2S in both biological and environmental systems.
There are various kinds of sensing methods for analytes, such as mass spectroscopy,
electrochemical methods and atomic absorption.20-23 These methods usually have the use of
time-consuming procedures, involving the use of sophisticated and expensive instrumentation,
and the complicated pre-processing of the samples. However, colorimetric approach on
detecting analytes allows a simple naked-eye detection of the color change without the use of
expensive instruments.24-27 Therefore, the colorimetric approach is recently popular due to its
capability to detect interesting analytes with fast response, convenient procedures, and high
sensitivity.28-33 Compared with the many known colorimetric organic molecule sensors for
anions,34-47 there is only one colorimetric chemosensor capable of simultaneously detecting
CN- and S2- in aqueous solution.39 Therefore, there is a great need for the development of new
colorimetric chemosensors that can simultaneously detect both CN- and S2- in aqueous
solutions.
2

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Herein, we report a new thiazol-based chemosensor 1, which was synthesized by
condensation reaction of 4-(p-tolyl)thiazol-2-amine and 2-hydroxy-1-naphthaldehyde (Scheme
1). Chemosensor 1 can detect CN- and S2- by the color change from yellow to colorless and to
orange, respectively. The sensing mechanisms were proposed for CN- with the nucleophilic
addition toward carbon of imine moiety and for S2- with the deprotonation of phenol moiety.
Moreover, the sensing mechanisms were explained by theoretical calculations.
Experimental section
Materials and Instrumentation
All the solvents and reagents (analytical grade and spectroscopic grade) were obtained
1
from Sigma-Aldrich and used as received. H NMR and 13C NMR measurements were
performed on a Varian 400 MHz and 100 MHz spectrometer, respectively, and chemical shifts
were recorded in ppm. Electrospray ionization mass spectra (ESI-MS) were collected on a
Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadruple ion trap
instrument. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a
Flash EA 1112 elemental analyzer (thermo) in Organic Chemistry Research Center of Sogang
University, Korea. UV-vis spectra were recorded at room temperature using a Perkin Elmer
model Lambda 25 UV/Vis spectrometer. All electronic figures were created by Origin 8.0.
Synthesis of receptor 1
4-(p-Tolyl)thiazol-2-amine (0.29 g, 1.5 mmol) was dissolved in 25 mL of ethanol and
2-hydroxy-1-naphthaldehyde (0.26 g, 1.5 mmol) was added into the solution. The reaction
mixture was stirred for 2 h and then, three drops of HCl were added into the reaction solution.
After 2 d, the yellow precipitate appeared. It was filtered and washed with ethanol. The yield:
0.51 g (89 %). 1H NMR (400 MHz, DMSO-d6) δ 13.86 (s, 1H), 10.03 (s, 1H), 8.72 (d, J = 9
Hz, 1H), 8.12 (d, J = 9.0 Hz, 1H), 8.01 (s, 1H), 7.94 (d, J = 8 Hz, 3H), 7.67 (t, J = 8 Hz, 1H),
13
7.47 (t, J = 8 Hz, 1H), 7.28 (m, 3H), 2.36 (s, 3H). C NMR (100 MHz, DMSO-d6) δ 163.73
(1C), 161.01 (1C), 152.59 (1C), 137.60 (1C), 137.15 (1C), 132.32 (1C), 131.23 (1C), 129.30
(2C), 129.11 (1C), 128.66 (1C), 125.91 (2C), 124.19 (1C), 121.48 (1C), 119.14 (1C), 112.40
(1C), 109.50 (1C), 20.80 (3C). LRMS (ESI): m/z calcd for C21H16N2OS-H+ ([M-H+]), 343.09;
3

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found, 343.32 Anal. calcd for C21H16N2OS (344.43): C, 73.23; H, 4.68; N, 8.13%; Found: C,
73.52; H, 4.57; N, 8.01%.
UV-vis measurements of receptor 1 with CN- and S2-
For CN-; receptor 1 (1.03 mg, 0.003 mmol) was dissolved in dimethyl sulfoxide
(DMSO, 1 mL) and 15 μL of the receptor 1 (3 mM) were diluted to 2.985 mL bis-tris
buffer/DMSO solution (v/v=1:1, pH 7.0) to make the final concentration of 15 μM.
Tetraethylammonium cyanide (TEACN, 49.3 mg, 0.3 mmol) was dissolved in DMSO (1 mL).
0.3-7.5 μL of the CN- solution (300 mM) were transferred to each receptor solution (15 μM)
prepared above. After mixing them for a few seconds, UV-vis absorption spectra were taken at
room temperature.
For S2-; receptor 1 (1.03 mg, 0.003 mmol) was dissolved in DMSO (1 mL) and 15 μL
of the receptor 1 (3 mM) were diluted to 2.985 mL bis-tris buffer/DMSO solution (v/v=1:1, pH
7.0) to make the final concentration of 15 μM. Sodium sulfide nonahydrate (Na2S·9H2O, 72.0
mg, 0.3 mmol) was dissolved in bis-tris buffer (1 mL). 0.3-7.5 μL of the S2- solution (300 mM)
were transferred to each receptor solution (15 μM) prepared above. After mixing them for a
few seconds, UV-vis absorption spectra were taken at room temperature.
Job plot measurements
For CN-; receptor 1 (3.4 mg, 0.01 mmol) was dissolved in DMSO (1 mL). 3, 2.7, 2.4,
2.1, 1.8, 1.5, 1.2, 0.9, 0.6, 0.3 and 0 µL of the 1 solution were taken and transferred to vials.
Each vial was diluted with bis-tris buffer/DMSO solution (10 mM, pH 7.0) to make a total
volume of 2.997 mL. TEACN (1.6 mg, 0.3 mmol) was dissolved in DMSO (1 mL). 0, 0.3, 0.6,
0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, and 3 µL of the TEACN solution were added to each diluted 1
solution. Each vial had a total volume of 3 mL. After reacting them for a few seconds, UV-vis
absorption spectra were taken at room temperature.
For S2-; receptor 1 (3.4 mg, 0.01 mmol) was dissolved in DMSO (1 mL). 3, 2.7, 2.4,
2.1, 1.8, 1.5, 1.2, 0.9, 0.6, 0.3 and 0 µL of the 1 solution were taken and transferred to vials.
Each vial was diluted with bis-tris buffer/DMSO solution (10 mM, pH 7.0) to make a total
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volume of 2.997 mL. Na2S·9H2O (72.0 mg, 0.3 mmol) was dissolved in bis-tris buffer (1 mL).
0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, and 3 µL of the S2- solution were added to each
diluted 1 solution. Each vial had a total volume of 3 mL. After reacting them for a few seconds,
UV-vis absorption spectra were taken at room temperature.
Competitive experiments
For CN-; receptor 1 (1.03 mg, 0.003 mmol) was dissolved in DMSO (1 mL) and 15 μL
of the receptor 1 (3 mM) were diluted to 2.985 mL bis-tris buffer/DMSO solution (v/v=1:1, pH
7.0) to make the final concentration of 15 μM. TEA salts of Cl-, Br-, and I-, and
-
-
-
tetrabuthylammonium (TBA) salts of OAc-, H2PO4 , N3 , SCN- and BzO-, and Na salts of NO3
and S2- (0.3 mmol) were separately dissolved in DMSO (1 mL). 6.8 µL of each anion solution
(300 mM) were taken and added to 2.993 mL of the solution of receptor 1 (15 µM) to give 45
equiv of anions. TEACN (49.3 mg, 0.3 mmol) was dissolved in DMSO (1 mL). Then, 6.8 µL
of the TEACN solution (300 mM) were added into the mixed solution of each anion and 1 to
make 45 equiv. After mixing them for a few seconds, UV-vis spectra were taken at room
temperature.
For S2-; receptor 1 (1.03 mg, 0.003 mmol) was dissolved in DMSO (1 mL) and 15 μL
of the receptor 1 (3 mM) were diluted to 2.985 mL bis-tris buffer/DMSO solution (v/v=1:1, pH
7.0) to make the final concentration of 15 μM. TEA salts of CN-, Cl-, Br-, and I-, and TBA salts
-
-
of OAc-, H2PO4 , N3 , SCN- and BzO-, and NaNO2 (0.3 mmol) were separately dissolved in
DMSO (1 mL). 6.8 µL of each anion solution (300 mM) were taken and added to 2.993 mL of
the solution of receptor 1 (15 µM) to give 45 equiv of anions. Na2S·9H2O (72.0 mg, 0.3 mmol)
was dissolved in bis-tris buffer (1 mL). Then, 6.8 µL of the S2- solution (300 mM) were added
into the mixed solution of each anion and 1 to make 45 equiv. After mixing them for a few
seconds, UV-vis spectra were taken at room temperature.
1H NMR titrations
For CN-; three NMR tubes of receptor 1 (3.4 mg, 0.01 mmol) dissolved in DMSO-
d6/CD3OD solution (v/v=6:4) were prepared and then three different concentrations (0, 0.01
and 0.02 mmol) of TEACN dissolved in DMSO-d6/CD3OD solution (v/v=6:4) were added to
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1
each solution of receptor 1. After shaking them for a minute, H NMR spectra were taken at
room temperature.
For S2-; four NMR tubes of receptor 1 (3.4 mg, 0.01 mmol) dissolved in DMSO-d6/
CD3OD solution (v/v=1:1) were prepared and then four different concentrations (0, 0.005, 0.01
and 0.02 mmol) of Na2S·9H2O dissolved in D2O were added to each solution of receptor 1.
After shaking them for a minute, 1H NMR spectra were taken at room temperature.
Determination of CN- in water samples
UV-vis spectral measurement of water samples containing CN- was carried by adding
15 μL of 3 mmol/L stock solution of 1 and 0.60 mL of 50 mmol/L bis-tris buffer stock solution
to 2.385 mL sample solutions. After well mixed, the solutions were allowed to stand at 25 ◦C
for 2 min before the test.
Theoretical calculation methods
All theoretical calculations were performed by DFT/TD-DFT method with the hybrid
exchange-correlation functional B3LYP48,49 applying the 6-31G**50,51 basis set without any
symmetry restrictions in the gas phase. The energy-minimized structure of 1 was obtained in
various geometric forms. On the basis of the optimized ground (S0) structure of 1, the optimized
structures of 1-CN- and 1-S2- were also obtained. In vibrational frequency calculations, there
was no imaginary frequency for the optimized geometries of 1, 1-CN- and 1-S2-, suggesting
that these geometries represented local minima. For all calculations, the solvent effect of water
was considered by using the Cossi and Barone’s CPCM (conductor-like polarizable continuum
model).52,53 In order to investigate the transition energies for the optimized structures of 1, 1-
CN- and 1-S2-, we calculated the lowest 20 singlet-singlet transitions using their ground state
geometries (S0) with TD-DFT (B3LYP) method. The GaussSum 2.1 was used to calculate the
contribution of molecular orbital in electronic transitions.54 All the calculations were performed
with Gaussian 03 suite.55
Results and discussion
Synthesis of 1
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The receptor 1 was synthesized by coupling 4-(p-tolyl)thiazol-2-amine and 2-hydroxy-
1-naphthaldehyde with 89 % yield in ethanol (Scheme 1) and analysed by 1H NMR, 13C NMR,
ESI-mass spectrometry and elemental analysis.
Scheme 1. Synthetic procedure of chemosensor 1.
Colorimetric detections of CN- and S2-
To examine the colorimetric sensing properties of 1, UV-vis absorption was measured
-
-
with various anions (TEA salts: CN-, F-, Cl-, Br-, and I-; TBA salts: OAc-, H2PO4 , N3 , SCN-,
-
-
and BzO-; Na salts: NO2 and S2 ) in bis-tris buffer/DMSO solution (v/v=1:1, pH 7.0). As shown
in Fig. 1a, 1 showed a yellow color with absorbance at 423 nm (ε = 2.0 x 104 L mol-1 cm-1),
suggesting intramolecular charge transfer (ICT) band with a π-conjugate push-pull system from
a donor of p-tolyl to an acceptor of imine.56-58 When 45 equiv of various anions such as OAc-,
-
-
-
F-, Cl-, Br-, I-, H2PO4 , BzO-, N3 , SCN- and NO2 were added into receptor 1, there were no
obvious changes of the absorbance compared with 1. In contrast, the addition of CN- showed
the complete decrease of the absorbance at 423 nm and that of S2- did a new absorbance at 470
nm. In addition, obvious color changes of 1 were observed with CN- and S2-, from yellow to
colorless and orange, respectively (Fig, 1b). These results indicated that 1 could be used as a
colorimetric chemosensor for CN- and S2-. In particular, the sensor 1 is the second colorimetric
chemosensor for the simultaneous detection of CN- and S2- in aqueous solution, to the best of
our knowledge.
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Fig. 1 (a) Absorption spectral changes of 1 (15 in the presence of 45 equiv of different
anions in bis-tris buffer/DMSO solution (v/v=1:1, pH 7.0). (b) The color changes of 1 (15
upon addition of various anions (45 equiv) in bis-tris buffer/DMSO solution (v/v=1:1, pH
7.0).
To further investigate the binding property of 1 with CN-, the UV-vis titration
experiments were performed (Fig. 2). The absorption spectrum of 1 indicated a broad band in
a range of 375 to 500 nm, which might be attributed to the transition of ICT band as proposed
above. On the treatment with CN- to solution of 1, the absorption band at 423 nm gradually
decreased and reached a minimum at 45 equiv of CN-. Two isosbestic points were observed at
290 and 316 nm. A noticeable decrease of the absorption band at 423 nm led us to propose that
the transition of ICT might be interrupted by the nucleophilic addition reaction of CN- to 1
(Scheme 2).28,29,59 Moreover, the time-dependent change in the absorption spectra was
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examined (Fig. S1(a)). The reaction time was determined to be about 30 min withDtOhI:e10s.1e0c3o9/nCd6N-J01832G
order rate constant of 1.77 s-1M-1 (Fig. S1(b)).
Fig. 2 Absorption spectral changes of 1 (15 μM) in the presence of different concentrations of
CN- (from 0 to 50 equiv) at room temperature. Inset: Plot of the absorbance at 423 nm as a
function of CN- concentration.
Scheme 2. The proposed colorimetric sensing mechanism of 1 for CN-.
The Job plot60 referred to a 1:1 stoichiometry between 1 and CN- (Fig. S2), which was
further confirmed by ESI-mass spectrometry analysis (Fig. 3). The negative-ion mass spectrum
at m/z 369.93 was assigned to 1-CN- species [calculated: m/z 370.10 for 1 + CN-]. Based on the
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UV-vis titration, the binding constant for 1 with CN- was estimated to be 2.2 x 103 by using
non-linear fitting analysis (Fig. S3).61,62 This value is within the range of those (1.0 ~ 1.0 x 105)
reported for CN- chemosensor.
Fig. 3 Negative-ion electrospray ionization mass spectrum of 1 (15 μM) upon addition of
TEACN (45 equiv).
To further check the practical applicability of 1 as CN- sensor, the competition
experiments were performed (Fig. 4). When 1 was treated with 45 equiv of CN- in the presence
of the same concentration of other anions, there was no interference for the detection of CN- in
-
-
-
the presence of S2-, OAc-, F-, Cl-, Br-, I-, H2PO4 , BzO-, N3 , SCN- and NO2 , while only S2-
interfered with the detection of CN-. These results suggested that 1 could be a selective
colorimetric sensor for CN- in the presence of various competing anions.
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Fig. 4 (a) Absorption spectral changes of 1 (15 μM) upon addition of CN- (45 equiv) in the
absence and presence of 45 equiv of various anions in bis-tris buffer/DMSO solution (v/v=1:1,
pH 7.0). (b) The color changes of 1 (15 μM) upon addition of CN- (45 equiv) in the absence
and presence of 45 equiv of various anions in bis-tris buffer/DMSO solution (v/v=1:1, pH 7.0).
In order to further examine the nucleophilic addition of CN- to 1, 1H NMR titrations
were performed (Fig. 5). Upon addition of 2.0 equiv of CN- to 1, the signal of the H5 proton of
the imine group at 9.4 ppm gradually disappeared and a new H5’ at 6.1 ppm started to appear.
This result strongly suggests that the nucleophilic addition of CN- occurs at the carbon atom of
imine group of 1. Most aromatic protons shifted to upfield, which indicated that the negative
charge developed from the nucleophilic addition of CN- toward 1 might be delocalized through
the whole receptor.28,29,63-65 Based on the UV-vis titration, Job plot, ESI-mass spectrometry
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analysis and 1H NMR titrations, we proposed the sensing mechanism of 1 for CN- as shown in
scheme 2.
Fig. 5 1H NMR titration of 1 with CN-.
The effect of pH on the UV-vis response of 1 to CN- was investigated in a series of
solutions with various pH values ranging 2 to 12 (Fig. 6). Its stable absorbance and colorless
solution were retained in the range of pH 5 to 11. Thus, these results indicated that CN- could
be clearly detected by the naked eye or UV-vis measurements using 1 over the wide pH range
of 5 to 11.
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Fig. 6 (a) Absorbance (at 423 nm) of 1 (15 μM) with CN- (45 equiv) at different pH (2-12). (b)
The color changes of 1 (15 μM) upon addition of CN- (45 equiv) at different pH (2-12).
We constructed a calibration curve for the determination of CN- by 1 (Fig. S4). A good
linear relationship was observed between the absorbance of 1 and CN- concentration (0.00-
30.00 μM) with a correlation coefficient of R2 = 0.9944 (n = 3). This result indicates that 1
could be a suitable candidate for quantitative detection of CN-. The detection limit was
calculated as 19.4 μM based on the definition by IUPAC (CDL =3σ/k). Under the condition of
the detection limit, CN- reacted with 1 by 40% for 1 h (Fig. S5). To confirm the practical
applicability of 1 with CN-, 1 was applied for the determination of CN- in both tap water and
drinking water samples (Table 1). Suitable recovery and R.S.D. values from water samples
were obtained. These results suggest that the receptor 1 could be useful for the measurement
of CN- in chemical and environmental applications.
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Table 1. Determination of CN- in water samples.
Sample
CN- added
CN- found
Recovery
(%)
R.S.D. (n = 3)
(μmol/L)
(μmol/L)
(%)
Tap water
0.00
20.00 a
0.00
0.00
21.23
0.00
106.15
2.72
Drinking
water
30.00 b
31.06
103.53
3.97
Conditions: [1] = 15 μmol/L in 10 mM bis-tris buffer-DMSO solution (1:1, pH 7.0). a. 20.00 μmol/L of CN- ion was artificially
added into the tap water. b. 30.00 μmol/L of CN- ion was artificially added into the drinking water.
Next, UV-vis titration was carried out to investigate the binding property of 1 with S2-
(Fig. 7). Upon the addition of S2- to 1, the absorption band at 423 nm decreased and a new
absorption band at 470 nm steadily increased. This bathochromic shift led us to propose the
change of ICT band through the deprotonation of 1 by S2-.29,66 If one looks carefully into the
UV-vis titration in Fig. 7a, there is the two-step change of the UV-vis spectra. The first step up
to 16 equiv of S2- showed that the absorption band at 423 nm gradually decreased with two
isosbestic points at 324 and 465 nm (Fig. 7b). The second step up to 50 equiv of S2- showed
that a new absorption band at 470 nm increased, and one isosbestic point was observed at 347
nm (Fig. 7c). Based on the two-step process, we proposed that the first step is the formation of
the hydrogen bond between the protonated form HS- of S2- and the hydroxyl proton of phenol
(Scheme 3). The second step is the deprotonating process of the hydroxyl proton in the presence
of an excess of S2-. Moreover, the time-dependent change in the absorption spectra was
examined (Fig. S6(a)). The reaction time was determined to be 2 min with the second-order
rate constant of 26.67 s-1M-1 (Fig. S6(b)).
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Fig. 7 (a) Absorption spectral changes of 1 (15 μM) in the presence of different concentrations
of S2- (from 0 to 50 equiv) at room temperature. (b) Absorption spectral changes of 1 (15 μM),
extracted from the range of 0-16 equiv of S2- in (a). (c) Absorption spectral changes of 1 (15
μM), extracted from the range of 18-50 equiv of S2- in (a).
Scheme 3. The proposed colorimetric sensing mechanism of 1 for S2-.
The Job plot60 referred to a 1:1 stoichiometry between 1 and S2- (Fig. S7), which was
further confirmed by ESI-mass spectrometry analysis (Fig. S8). The negative-ion mass
spectrum at m/z 343.36 was assigned to 1- species [calculated: m/z 343.09 for 1 - H-]. Based on
the UV-vis titration, the binding constant for 1 with S2- was estimated to be 5.7 x 102 by using
Benesi-Hildebrand equation (Fig. S9).67 The detection limit (3σ/K) of receptor 1 as a
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colorimetric sensor for the analysis of S2- ions was found to be 2.2 mM (Fig. S10). This value
is about three times lower than a Secondary Maximum Contaminant Level (7.8 mM) by odor
in drinking water, established by the EPA.68 Thus, 1 could be used as a good detector for
monitoring S2- ion in drinking water.
To investigate the practical applicability of 1 as S2- sensor, the competition
experiments were performed (Fig. 8). When 1 was treated with 45 equiv of S2- in the presence
of the same concentration of other anions, there was no interference for the detection of S2- in
-
-
-
the presence of CN-, OAc-, F-, Cl-, Br-, I-, H2PO4 , BzO-, N3 and SCN-, while only NO2
interfered somewhat with the detection of S2-. These results suggested that 1 could be an
excellent colorimetric sensor for S2- in the presence of various competing anions.
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Fig. 8 (a) Absorption spectral changes of 1 (15 μM) upon addition of S2- (45 eDqOuI:iv10).10i3n9/tCh6eNJ01832G
absence and presence of 45 equiv of various anions in bis-tris buffer/DMSO solution (v/v=1:1,
pH 7.0). (b) The color changes of 1 (15 μM) upon addition of S2- (45 equiv) in the absence and
presence of 45 equiv of various anions in bis-tris buffer/DMSO solution (v/v=1:1, pH 7.0).
1
To further check the sensing mechanism of 1 with S2-, H NMR titrations were
performed (Fig. 9). Upon the addition of 2.0 equiv to 1, most aromatic protons shifted to upfield,
while H8 showed a down-field shift. There was no shift in the position of proton on the further
addition of S2- (>2.0 equiv).These results indicated that the negative charge generated from
deprotonation of the naphtholic proton by S2- might be delocalized through the 1. Base on the
UV-vis titration, Job plot, ESI-mass spectrometry analysis and1H NMR titrations, we proposed
the sensing mechanism of 1 for S2- as shown in scheme 3.
Fig. 9 1H NMR titration of 1 with S2-.
For practical application, the effect of pH on the UV-vis response of 1 to S2- was
investigated in a series of solutions with various pH values ranging 2 to 12 (Fig. S11). Its stable
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absorbance and orange color were retained in the range of pH 7 to 11. Thus, these results
indicated that S2- could be clearly detected by the naked eye or UV-vis measurements using 1
over the wide pH range of 7 to 11.
Theoretical calculations for sensing mechanisms of CN- and S2-
To further get understanding on the electronic structures of 1, 1-HS- and 1-CN-, we
optimized energy-minimized structures of chemosensor 1, 1-HS- and 1-CN- at DFT/B3LYP/6-
31G** level. In Fig. 10 are shown their energy-minimized structures with bond lengths and
bond angles. The energy-minimized structure of 1 indicated a square planar structure with the
sp2 hybridized imine group (bond angle = 118.82° (H3, C2, N4)) (Fig. 10a). 1-CN- showed a
tilted conformation with sp2 hybridized imine group (bond angle = 108.62° (H3, C2, N4)) (Fig.
10b). This conformational difference caused a significant change in π-conjugation between 1
and 1-CN-, expecting that no ICT was observed in the 1-CN- adduct. In case of 1-HS-, it showed
almost planar conformation (dihedral angle = 173.69° (C1, C2, N4, C5)) and two hydrogen bonds
were observed in O7 - H9, and N6 - H10 (Fig. 10c).
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Fig. 10 Energy-minimized structures of (a) 1, (b) 1-CN- and (c) 1-S2- from B3LYP level.
To gain an insight into colorimetric sensing mechanism for 1-CN- and 1-HS-, time-
dependent density functional theory (TD-DFT) calculations were performed at the optimized
geometries (S0). The main transition property of 1 showed that the HOMO to LUMO (435.33
nm, 99 % contribution) transition predominantly contributes to the electronic configurations
(Fig. S12). The HOMO-LUMO transition of 1 was assigned as the ICT band from p-tolyl to
imine moiety, which is consistent with the experimental absorption wavelength of 1 (423 nm,
ε = 2.0 x 104 L mol-1 cm-1). For 1-CN-, the TD-DFT calculations indicated the major oscillator
strength at 363.19 nm, which showed HOMO→LUMO+1 (82 % contribution) and
HOMO→LUMO (14 % contribution) (Fig. S13). These transitions were assigned as the π→π*
transition of the naphthol group and the ICT transition from the naphthol to 4-(p-tolyl)thiazol
group with the minor oscillator strength. Thus, the sensing mechanism of CN- could be
explained by blocking of HOMO to LUMO transition of 1 by the nucleophilic addition of CN-
at imine carbon, resulting in the formation of the colorless leuconitrile species. The TD-DFT
calculations for 1-HS- indicated the obvious red shift of π→π* transition (446.40 nm, HOMO→
LUMO, Fig. S14). Thus, deprotonating process of 1 caused the decrease of HOMO to LUMO
energy gap, which substantially results in the bathochromic shift and color change of 1 from
yellow to orange.
It is worthwhile to mention why sensor 1 interacts CN- and S2- by two different ways
(nucleophilic addition vs deprotonating process), although both of them behave as a
nucleophile. S2- is more basic than CN-. Therefore, we assume that the strong base S2- could
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undergo the acid-base reaction with the weak base hydroxyl group of the phenol moiety of 1,
while the weak base CN- might attack the carbon center of the imine moiety of 1 instead of the
acid-base reaction with the hydroxyl group.
Conclusion
We have synthesized a thiazol-based chemosensor 1 for detection of CN- and S2-
through color changes. Chemosensor 1 showed highly selective colorimetric responses to
cyanide based on a nucleophilic addition and to sulfide based on a deprotonating process. Based
1
on the Job plot, H NMR titration and ESI-mass spectrometry analysis, it has been proposed
that the receptor 1 bound CN- and S2- in a 1:1 stoichiometric manner, respectively. Moreover,
DFT and TD-DFT studies supported the experimental data and proposed sensing mechanisms.
Additionally, 1 could be used to quantify CN- in water samples. In particular, the sensor 1 is
the second colorimetric chemosensor for the simultaneous detection of CN- and S2- in aqueous
solution.
Acknowledgements
Financial support from Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology
(NRF-2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301) are gratefully
acknowledged. We thank Nano-Inorganic Laboratory, Department of Nano & Bio chemistry,
Kookmin University to access the Gaussian 03 program packages.
Supplementary Information
Supplementary data (experimental procedures and additional experimental data) associated
with this article can be found. This material is available free of charge via the Internet at http://
~.
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Graphical abstract
A selective chemosensor was developed for the colorimetric detection of CN- and S2- in
aqueous solution.