Colorimetric chemosensor for multiple targets, Cu2+, CN- and S2-

Article abstract of DOI:10.1039/c5ra27553a

A simple and easily synthesized colorimetric chemosensor 1, (E)-2-(((2-((2,4-dinitrophenyl)amino)phenyl)imino)methyl)phenol, was designed and synthesized for the detection of Cu²⁺, CN^- and S^2-. Receptor 1 showed a highly selective colorimetric response to copper(ii) ions by changing its color from pale yellow to orange immediately without any interference from other metal ions. Also, the sensitivity of UV-vis based assay (8.77 μM) for Cu²⁺ is below the World Health Organization (WHO) guidelines for drinking water (31.5 μM). In addition, chemosensor 1 could be used to quantify Cu²⁺ in water samples and the sensing mechanism of the 1-Cu²⁺ complex was explained by theoretical calculations. The 1-Cu²⁺ complex could be reversible simply through treatment with an appropriate reagent such as EDTA. Moreover, with the "naked eye" 1 could sense cyanide and sulfide by color changes from pale yellow to pale pink and deep pink, respectively, in aqueous solution. Furthermore, sensor 1 can detect both CN^- (0.28 μM) and S^2- (10.7 μM) below the given guidelines (WHO: 1.9 μM for CN^- and Environmental Protection Agency: 7.8 mM for S^2-).

Full text of DOI:10.1039/c5ra27553a

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Page 1 of 41
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DOI: 10.1039/C5RA27553A
Colorimetric chemosensor for multiple targets, Cu2+, CN- and S2-
Ka Young Ryu, Jae Jun Lee, Jin Ah Kim, Dae Yul Park, Cheal Kim*
Department 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: chealkim@seoultech.ac.kr
Abstract
A simple and easily synthesized colorimetric chemosensor 1, (E)-2-(((2-((2,4-
dinitrophenyl)amino)phenyl)imino)methyl)phenol was designed and synthesized for the
detection of Cu2+, CN- and S2-. Receptor 1 showed a highly selective colorimetric response to
copper (II) ion by changing its color from pale yellow to orange immediately without any
interference from other metal ions. Also, the sensitivity of UV-vis based assay (8.77 μM) for
Cu2+ is below the World Health Organization (WHO) guideline for drinking water (31.5 μM).
In addition, the chemosensor 1 could be used to quantify Cu2+ in water samples and the sensing
mechanism of 1-Cu2+ complex was explained by theoretical calculations. 1-Cu2+ complex
could be reversible simply through treatment with a proper reagent such as EDTA. Moreover,
1 with “naked eye” could sense cyanide and sulfide by color changes from pale yellow to pale
pink and deep pink, respectively, in aqueous solution. Furthermore, this sensor 1 can detect
both CN- (0.28 μM) and S2- (10.7 μM) below the given guidelines (WHO: 1.9 μM for CN- and
Environmental Protection Agency: 7.8 mM for S2-).
Keywords: copper, cyanide, sulfide, colorimetric chemosensor, naked-eye
1

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1. Introduction
The development of selective and sensitive chemosensors for the detection of metal ions
and anions has received considerable attention because of their important roles in medicine,
living systems and the ecosystems.1-3 Among the different types of chemosensors, the sensors
based on colorimetric determination of anions and metal ions have many advantages because
of the convenience, low cost, and rapid tracking of analytes.4-5 Therefore, colorimetric sensors
are recently getting popular due to their capability to detect analyte by the naked eye without
utilizing to any expensive instruments.6-11
Among the various metal ions, copper serves as an indispensable cofactor by composing
the active part in a large variety of enzymes, including cytochrome c oxidase, superoxide
dismutase and tyrosinase.12-15 Thus, steady ingestion of copper is necessary for our good
health.16 However, unregulated overloading of copper can induce several diseases including
Parkinson’s, Alzheimer’s and prion diseases.17-21 On the other hand, Cu2+ is a major
environmental pollutant due to its widespread use. Therefore, World Health Organization
(WHO) has prescribed the safe limit of copper in drinking water at 2 ppm (31.5 μM).22 Thus,
the development of chemosensors for the detection and monitoring of Cu2+, with high
sensitivity, low detection limit and quick response, is of great importance.23
The development of molecular sensors for anions has been a subject of great research
interest, because anions play important roles in a wide range of environmental, clinical,
chemical, and biological applications.24-32 Among the various anions, cyanide with a wide use
in our life is most concerned, because it is one of the most rapidly acting and powerful poisons.
Its toxicity results from its disposition to bind to the iron in cytochrome c oxidase, interfering
with electron transport and resulting in hypoxia.33-45 Cyanide could be absorbed through the
lungs, gastrointestinal tract and skin, leading to vomiting, convulsion, loss of consciousness,
and eventual death.46-48 Despite its toxicity, its application in various fields as raw material for
synthetic fibers, resins, herbicides, and the gold extraction process is inevitable,49-51 which
releases cyanide into the environment as a toxic contaminant. Thus, there is a need for an
effective sensing system to monitor cyanide concentration from contaminant sources.
Sulfide is an extensive environmental pollutant generally released from the paper,
petrochemical and leather industries.52 In consequence, sulfide as a toxic traditional pollutant
can be widely found in water not only due to industrial processes but also thanks to microbial
2

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reduction of sulfate by anaerobic bacteria or from sulfur-containing amino acids in meat
proteins. The protonated forms, HS- or H2S, are even more toxic than sulfide itself.53 It can
damage the human nerve and respiratory systems, causing people to lose consciousness or even
die at very low concentrations (ppm levels).54 However, in biological systems, H2S is usually
afforded from L-cysteine in reactions catalyzed by enzymes such as cystathionine β-synthase
(CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfur transferase (3MST), as
well as from nonenzymatic processes.55 Following nitric oxide (NO) and carbon monoxide
(CO), H2S has been regarded to be the third gasotransmitter to regulate cardiovascular, neuroma,
immune, endocrine, and gastro-intestinal systems.56-58 Several studies have shown that H2S
participates in many physiological processes, such as angiogenesis, vasodilation, regulation of
inflammation, neuromodulation and apoptosis.59-62 In addition, it has also proved that abnormal
H2S production is linked to human diseases such as Alzheimer's disease, Down's syndrome,
hypertension, and liver cirrhosis.63-66 Therefore, the quantitative detection of H2S is of great
significance for both environmental and biological systems.67
Herein, we report on a new Schiff base chemosensor 1, based on the combination of a
dinitrobenzene and a phenol group (Scheme 1). Chemosensor 1 detected Cu2+ by color change
from pale yellow to orange, CN- from pale yellow to pale pink, and S2- from pale yellow to
deep pink in aqueous solution. The detection mechanisms were proposed for Cu2+ with intra-
molecular charge transfer (ICT) and for S2- and CN- with the deprotonation process.
2. Experimental
2.1. Materials and Instrumentation
All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from
Sigma-Aldrich and used as received. 1H NMR and 13C NMR spectra were recorded on a Varian
400 and 100 MHz spectrometer. Chemical shifts (δ) were reported in ppm, relative to
tetramethylsilane Si(CH3)4. Absorption spectra were recorded at room temperature using a
Perkin Elmer model Lambda 25 UV/Vis spectrometer. Electrospray ionization mass spectra
(ESI-mass) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage
MAX quadrupole ion trap instrument.
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2.2. Synthesis of N1-(2,4-dinitrophenyl)benzene-1,2-diamine
1-Fluoro-2,4-dinitrobenzene (0.17 g, 1 mmol) and benzene-1,2-diamine (0.11 g, 1 mmol)
were dissolved in 5 mL of acetonitrile. Then, this solution was stirred for 4 h at room
temperature. The red powder produced was filtered, washed using methanol and diethyl ether,
1
and air-dried. Yield: 90 %. H NMR (400MHz, DMSO-d6) δ 9.74 (s, 1H), 8.91 (d, 1H), 8.23
(d, 1H), 7.12 (m, 2H), 6.83 (d, 1H), 6.64 (t, 2H), 5.28 (s, 2H). 13C NMR (100MHz, DMSO-d6)
δ 148.28, 145.30, 136.26, 131.46, 130.09, 129.23, 128.72, 123.54, 121.53, 117.15, 116.76,
116.15 ppm.
2.3. Synthesis of 1
N1-(2,4-dinitrophenyl)benzene-1,2-diamine (0.27 g, 1 mmol) and salicylaldehyde (0.11
mL, 1.1 mmol) were dissolved in 5 mL of methanol. Then, three drops of phosphoric acid were
added into the reaction mixture, which was stirred for 1 h at room temperature. The pale yellow
1
powder produced was filtered, washed using iced-methanol, and air dried. Yield: 80%. H
NMR (400 MHz, DMSO-d6) δ 12.33 (s, 1H), 10.26 (s, 1H), 8.99 (s, 1H), 8.91 (s, 1H), 8.20 (d,
1H), 7.65 (d, 2H), 7.53 (d, 2H), 7.48 (d, 1H), 7.37 (t, 1H), 6.96 (m, 2H), 6.86(d, 1H). 13C NMR
(100 MHz, DMSO-d6) δ 163.74, 159.96, 146.43, 144.21, 136.28, 133.53, 132.15, 131.54,
130.86, 129.69, 128.48, 127.80, 127.09, 123.13, 119.56, 119.33, 119.04, 116.97, 116.48 ppm.
ESI-MS m/z [M-H+]-: calcd, 377.09; found, 377.33. Anal. calcd for C19H14N4O5 (378.10): C,
62.01; H, 4.19; N, 14.06 %. Found: C, 62.13; H, 3.84; N, 14.07 %.
2.4. UV-vis titration measurement of 1 with Cu2+
Receptor 1 (3.78 mg, 0.01 mmol) was dissolved in dimethyl sulfoxide (DMSO, 1 mL) and
6 μL of the receptor 1 (10 mM) was diluted to 2.994 mL of bis-tris buffer/DMSO (1/1, v/v) to
make the final concentration of 20 μM. Cu(NO3)2·2.5H2O (2.4 mg, 0.01 mmol) was dissolved
in DMSO (1mL). 0-180 μL of the Cu(NO3)2 solution (10 mM) was transferred to the receptor
1 solution (20 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were
taken at room temperature.
2.5. Job plot measurement of 1 with Cu2+
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Receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 μL of the
receptor 1 (10 mM) was diluted to 29.7 mL of bis-tris buffer/DMSO (1:1, v/v) to make the final
concentration of 100 μM. Cu(NO3)2·2.5H2O (2.4 mg, 0.01 mmol) was dissolved in DMSO (1
mL) and 300 μL of the copper solution (10 mM) was diluted to 29.7 mL of bis-tris
buffer/DMSO (1:1, v/v) to make the final concentration of 100 μM. 4.5, 4.0, 3.5, 3.0, 2.5, 2.0,
1.5, 1.0, and 0.5 mL of the 1 solution were taken and transferred to vials. 0.5, 1.0, 1.5, 2.0, 2.5,
3.0, 3.5, 4.0, and 4.5 mL of the copper solution were added to solutions of 1 prepared above.
Each vial had a total volume of 5 mL. After shaking the vials for a few seconds, UV-vis spectra
were taken at room temperature.
2.6. Competition of 1 toward various metal ions
Receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 6 μL of this solution
(10 mM) was diluted with 2.994 mL of bis-tris buffer/DMSO (1:1, v/v) to make the final
concentration of 20 μM. MNO3 (M = Na, K; 0.01 mmol), M(NO3)2 (M = Mn, Co, Ni, Cu, Zn,
Cd, Mg, Ca, Pb; 0.01 mmol), M(ClO4)2 (M = Fe; 0.01 mmol) and M(NO3)3 (M = Al, Fe, Cr,
Ga, In; 0.01 mmol) were separately dissolved in bis-tris buffer (1 mL). 48 μL of each metal
solution (10 mM) was taken and added into 3 mL of each receptor 1 (20 μM) prepared above
to make 8 equiv. Then, 48 μL of the Cu(NO3)2 solution (10 mM) was added into the mixed
solution of each metal ion and receptor 1 to make 8 equiv. After mixing them for a few seconds,
UV-vis spectra were taken at room temperature.
2.7. UV-vis titration measurements of 1 with anions
For CN-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 3 μL of the
receptor 1 (10 mM) was diluted to 2.997 mL of bis-tris buffer/DMSO (1/9, v/v) to make the
final concentration of 10 μM. Tetraethylammonium cyanide (16.56 mg, 0.1 mmol) was
dissolved in bis-tris buffer (1mL). 0-84 μL of the CN- solution (100 mM) was transferred to
the receptor 1 solution (10 μM) prepared above. After mixing them for a few seconds, UV-vis
spectra were taken at room temperature.
For S2-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 3 μL of the
receptor 1 (10 mM) was diluted to 2.997 mL of bis-tris buffer/DMSO (1/9, v/v) to make the
final concentration of 10 μM. Sodium sulfide nonahydrate (24.01 mg, 0.1 mmol) was dissolved
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in bis-tris buffer (1 mL). 0-27 μL of the S2- solution (100 mM) was transferred to the receptor
1 solution (10 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were
taken at room temperature.
2.8. Job plot measurements of 1 with anions
For CN-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 μL of
the receptor 1 (10 mM) was diluted to 29.7 mL of bis-tris buffer/DMSO (1:9, v/v) to make the
final concentration of 100 μM. Tetraethylammonium cyanide (16.56 mg, 0.1 mmol) was
dissolved in bis-tris buffer (1 mL) and 30 μL of the tetraethylammonium cyanide solution (100
mM) was diluted to 29.97 mL of bis-tris buffer/DMSO (1:9, v/v) to make the final
concentration of 100 μM. 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 mL of the 1 solution were
taken and transferred to vials. 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 mL of the
tetraethylammonium cyanide solution were added to solutions of 1 prepared above. Each vial
had a total volume of 5 mL. After shaking the vials for a few seconds, UV-vis spectra were
taken at room temperature.
For S2-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 300 μL of
the receptor 1 (10 mM) was diluted to 29.7 mL of bis-tris buffer/DMSO (1:9, v/v) to make the
final concentration of 100 μM. Sodium sulfide nonahydrate (24.01 mg, 0.1 mmol) was
dissolved in bis-tris buffer (1 mL) and 30 μL of the sodium sulfide nonahydrate solution (100
mM) was diluted to 29.97 mL of bis-tris buffer/DMSO (1:9, v/v) to make the final
concentration of 100 μM. 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 mL of the 1 solution were
taken and transferred to vials. 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 mL of the sodium
sulfide nonahydrate solution were added to solutions of 1 prepared above. Each vial had a total
volume of 5 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room
temperature.
2.9. Competition of 1 toward various anions
For CN-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 3 μL of this
solution (10 mM) was diluted with 2.997 mL of bis-tris buffer/DMSO (1:9, v/v) to make the
final concentration of 10 μM. Tetraethylammonium salts of F-, Cl-, Br- and I-, and
-
tetrabutylammonium salts of OAc-, PO43-, N3 , SCN- and BzO- (0.1 mmol), and sodium salts
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-
of SCN-, N3 , SO42- and S2- (0.1 mmol) were separately dissolved in bis-tris buffer (1 mL). 78
μL of each anion solution (100 mM) was taken and added into 3 mL of each receptor 1 (10 μM)
prepared above to make 260 equiv. Then, 78 μL of the tetraethylammonium cyanide solution
(100 mM) was added into the mixed solution of each anion and receptor 1 to make 260 equiv.
After mixing them for a few seconds, UV-vis spectra were taken at room temperature.
For S2-; receptor 1 (3.78 mg, 0.01 mmol) was dissolved in DMSO (1 mL) and 3 μL of this
solution (10 mM) was diluted with 2.997 mL of bis-tris buffer/DMSO (1:9, v/v) to make the
final concentration of 10 μM. Tetraethylammonium salts of F-, Cl-, Br- and I-, and
-
tetrabutylammonium salts of OAc-, PO43-, N3 , SCN- and BzO− (0.1 mmol), and sodium salts
-
2-
of SCN-, N3 , SO4 and S2- were separately dissolved in bis-tris buffer (1 mL). 26.4 μL of each
anion solution (100 mM) was taken and added into 3 mL of each receptor 1 (10 μM) prepared
above to make 88 equiv. Then, 26.4 μL of the sodium sulfide nonahydrate solution (100 mM)
were added into the mixed solution of each anion and receptor 1 to make 88 equiv. After mixing
them for a few seconds, UV-vis spectra were taken at room temperature.
2.10. 1H NMR titrations of 1 with anions
For CN-; five NMR tubes of 1 (3.78 mg, 0.01 mmol) dissolved in DMSO-d6 were prepared,
and then five different equiv (0, 0.1, 0.5 and 1 equiv) of tetraethylammonium cyanide dissolved
1
in DMSO-d6 were added to 1 solution separately. After shaking them for a minute, their H
NMR spectra were taken.
For S2-; four NMR tubes of 1 (3.78 mg, 0.01 mmol) dissolved in DMSO-d6 were prepared,
and then four different equiv (0, 0.5, 1, 5 and 10 equiv) of sodium sulfide nonahydrate dissolved
1
in D2O were added to 1 solution separately. After shaking them for a minute, their H NMR
spectra were taken.
2.11. Determination of Cu2+ in water samples
UV-vis spectral measurements of water samples containing Cu2+ were carried out by adding
6 μL of 1 mM stock solution of 1 and 0.60 mL of 50 mmol/L bis-tris buffer stock solution to
2.394 mL sample solutions. After well mixed, the solutions were allowed to stand at 25 ◦C for
2 min before the test.
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2.12. Theoretical calculation methods of 1 with Cu2+
All theoretical calculations were performed by DFT/TD-DFT method with the hybrid
exchange-correlation functional B3LYP68-69 applying the 6-31G**70-71 basis set for the main
group elements, whereas the Lanl2DZ effective core potential (ECP)72-74 was employed for Cu.
The energy-minimized structures of 1 and 1-Cu2+ complex were obtained in various geometric
forms. In vibrational frequency calculations, there was no imaginary frequency for the
optimized geometries of 1 and 1-Cu2+ complex, 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).75-76 In order to investigate
the transition energies for the optimized structures of 1 and 1-Cu2+ complex, we calculated the
lowest 25 singlet-singlet transitions using their ground state geometry (S0) with TD-DFT
(B3LYP) method. The GaussSum 2.1 was used to calculate the contribution of molecular
orbital in electronic transitions.77 All the calculations were performed with Gaussian 03 suite.78
3. Results and discussion
The receptor 1 was synthesized by coupling N1-(2,4-dinitrophenyl)benzene-1,2-diamine
with salicylaldehyde in methanol (Scheme 1). The precursor N1-(2,4-dinitrophenyl)benzene-
1,2-diamine was obtained by substitution reaction of 1-fluoro-2,4-dinitrobenzene and benzene-
1,2-diamine in acetonitrile. Receptor 1 and the precursor N1-(2,4-dinitrophenyl)benzene-1,2-
1
diamine were characterized by H NMR, 13C NMR, ESI-mass spectrometry and elemental
analysis.
3.1. Spectroscopic studies of 1 toward Cu2+
To examine the colorimetric properties of 1, the UV-vis absorption was measured with
various metal ions (Na+, K+, Mg2+, Ca2+, Al3+, Ga3+, In3+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+,
Cu2+, Zn2+ Cd2+, Hg2+, Ag+ and Pb2+) in bis-tris buffer/DMSO (1:1, v/v, 10 mM, pH 7.0). Upon
the addition of 8 equiv of each metal ion, only Cu2+ induced a distinct spectral change at 412
nm, while other metal ions showed either no or small changes in the absorption spectra relative
to the free receptor 1 (Fig. 1a). For example, Fe2+ and Fe3+ showed a small red-shift. In
consistency with the absorption spectrum, the solution color of 1 changed from pale yellow to
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orange with copper ion (Fig. 1b), indicating that the receptor 1 could serve as a potential
candidate of colorimetric chemosensor for Cu2+.
The binding properties of 1 with Cu2+ were studied by UV-vis titration experiments. On
treatment with Cu2+ to solution of 1, the absorption band at 348 nm significantly decreased,
and a new band at 412 nm gradually reached a maximum at 8 equiv of Cu2+ (Fig. 2). An
isosbestic point was observed at 381 nm, indicating that only one product was generated from
1 upon binding to Cu2+. The Job plot analysis79 showed a 1:1 stoichiometry of Cu2+ to 1 (Fig.
S1). To further examine the binding mode between 1 and Cu2+, a positive-ion ESI-mass
experiment was carried out. The positive-ion mass spectrum of 1 with 1 equiv of Cu2+ showed
the formation of the [12- + Cu2+ + DMSO + H+] complex [m/z 517.93, calcd m/z 518.03] (Fig.
S2).
The sensing mechanism of Cu2+ by 1 could be explained by intra-molecular charge
transfer (ICT). Kaur et al. suggested that the ICT mechanism referred to the push-pull effect of
the electron-donating and electron-withdrawing groups.80 That is, the red shift indicates that
the energy gap of ICT band decreases, upon binding Cu2+ ion to the electron-withdrawing
moieties. Likewise, we assume that the red shift of 1-Cu2+ complex might be induced by
binding of Cu2+ ion to the electron withdrawing groups such as the imine and the aniline with
the nitro moiety. Therefore, the change of ICT band might be responsible for the color change
from pale yellow to orange.
Based on the UV-vis titration of 1 with Cu2+, the association constant (K) of 1 with Cu2+
was calculated by using Benesi–Hildebrand equation (Fig. S3).81 The K value was determined
to be 2.7 x 103 M−1, which is within the range of those (103 - 1012) reported for Cu2+ sensing
chemosensors.82-84 The detection limit85 of receptor 1 for Cu2+ ions on the basis of 3σ/K was
found to be 8.77 μM (Fig. S4), which is lower than the WHO guideline (31.5 μM) for Cu2+ in
drinking water.22 Therefore, receptor 1 can serve as a good indicator for monitoring Cu2+ ions
in aqueous solution.
To further check the practical applicability of receptor 1 as a Cu2+ selective sensor, the
competition experiments were conducted (Fig. 3). Upon treatment with 8 equiv of Cu2+ in the
presence of the same concentration of other metal ions, there were no obvious interferences for
the detection of Cu2+. In cases of Fe2+ and Fe3+, somewhat higher absorbance at 412 nm was
observed, due to their original absorbance (compare Fig. 3 with Fig. 1). Thus, 1 could be used
as a selective colorimetric sensor for Cu2+ in the presence of most competing metal ions.
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We investigated the effect of pH on the absorption response of receptor 1 to Cu2+ ion in a
series of solutions with pH values ranging from 2 to 12 (Fig. S5). The color of the 1-Cu2+
complex remained in the orange color between pH 6 and 10. These results indicated that Cu2+
could be clearly detected by the naked eye or UV-vis absorption measurements using 1 over
the pH range of 6-10.
To examine the reversibility of sensor 1 toward Cu2+, ethylenediaminetetraacetic acid
(EDTA, 1 equiv) was added to the complexed solution of sensor 1 and Cu2+. As shown in Fig.
4, the absorbance at 412 nm disappeared immediately. Upon addition of Cu2+ again, the
absorbance was recovered. The absorbance changes were almost reversible even after several
cycles with the sequentially alternative addition of Cu2+ and EDTA. These results indicated
that sensor 1 could be recyclable simply through treatment with a proper reagent such as EDTA.
Such reversibility and regeneration could be important for the fabrication of chemosensors to
detect Cu2+.
In order to examine the practical application of the chemosensor 1 in environmental
samples, the chemosensor 1 was applied for the determination of Cu2+ in aqueous environment
samples, using the calibration curve of 1 toward Cu2+ (Fig. S6). Drinking water samples were
chosen (Table 1) and showed a suitable recovery and R.S.D. values. These results suggested
that the chemosensor 1 was suitable for the determination of Cu2+ concentration.
To clearly demonstrate the sensing mechanisms of 1 toward Cu2+, theoretical calculations
were performed with the 1:1 stoichiometry (Cu2+ : receptor), based on Job plot and ESI-mass
spectrometry analysis. To get the energy-minimized structures of 1 and 1-Cu2+ complex, their
geometric optimizations were performed by DFT/B3LYP level (main group atom: 6-31G**
and Cu: Lanl2DZ/ECP). The significant structural properties of the energy-minimized
structures were shown in Fig. 5. The energy-minimized structure of 1 has a bent structure with
the dihedral angles of -65.983o (1C, 2N, 3C, 4C) and -28.185 (3C, 4C, 5N, 6C), and the
hydrogen bonds are observed in 3N-5H and 9O-10H (Fig. 5a). 1-Cu2+ complex exhibits a
square planar structure with the dihedral angles of 8.833o (9O, 2N, 5N, 6O), -28.185 (3C, 4C,
5N, 6C) and -150.565 (1C, 2N, 3C, 4C), and Cu2+ is coordinated with 9O, 2N, 5N and 7O
atoms of 1 (Fig. 5b). We also investigated the absorption to the singlet excited states of 1 and
1-Cu2+ complex via TDDFT calculations. In case of 1, the third lowest excited state was
considered to be responsible for the yellow color of 1, which was determined to HOMO - 1→
LUMO and HOMO → LUMO + 1 transitions (403.53 nm, Fig. S7). The major composition,
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HOMO - 1→ LUMO, indicated ICT transition from the three benzene rings to two nitro groups.
For 1-Cu2+ complex, the 14th lowest excited state was considered to be responsible for orange
color of 1-Cu2+ complex, which was determined to HOMO → LUMO + 1 (α) and HOMO →
LUMO + 2 (β) transitions (438.86 nm, Fig S8). The transitions were almost similar to the
HOMO - 1→ LUMO transition of 1 (Fig. S9). These results indicated that the chelation of Cu2+
with 1 stabilized the energy of ICT transition, which induced the different color change of 1 in
the presence of Cu2+.
Based on Job plot, ESI-mass spectrometry analysis and theoretical calculations, we
propose that the oxygen atom of the hydroxyl group, the nitrogen atom of the imine group,
the nitrogen atom of amine group, and the oxygen atom of the nitro group in 1 might bind to
Cu2+ (Scheme 2).
3.2. Spectroscopic studies of 1 toward CN- and S2-
The colorimetric sensing abilities of 1 toward various anions were also investigated in a
mixture of bis-tris buffer/DMSO (1:9, v/v) upon addition of tetraethylammonium (TEA) salts
-
of CN-, F-, Cl-, Br-, and I-, and tetrabutylammonium (TBA) salts of OAc-, H2PO4-, N3 , SCN-
2
-
-
-
and BzO-, and sodium salts of NO2 , HSO4 and S . Upon the addition of 260 equiv of each
anion, the 1 showed almost no change in absorption peak in the presence of F-, Cl-, Br-, I-, OAc-,
-
2-
-
-
-
-
H2PO4 , N3 , SCN-, BzO-, NO2 and HSO4 (Fig. 6a). By contrast, CN and S showed distinct
spectral changes. Consistent with the changes in their UV-vis spectra, CN- and S2- showed
color changes from yellow to pale pink and to deep pink, respectively, while other anions had
no change in color (Fig. 6b). Additionally, a cross-validation study of the 1-Cu2+ complex
toward cyanide and sulfide ion was conducted (Fig. S10). The addition of S2- to the complexed
solution of sensor 1 and Cu2+ changed its color from yellow to dark yellow, confirming
existence of both Cu2+ and S2-. However, the addition of CN- to 1-Cu2+ solution changed its
color from yellow to colorless. Therefore, the existence of both Cu2+ and CN- could not be
discernible with 1.
First of all, the binding properties of 1 with CN- were studied by UV-vis titration
experiments (Fig. 7a). Once one looks carefully into Fig. 7a, there is the two-step change of
the absorption bands. The first change is that the absorbance at 350 nm decreased gradually,
while a new absorbance at 500 nm increased with a clear isosbestic point at 398 nm (Fig. 7b).
The second change is that the absorbance at 350 nm continuously decreased, and the broad
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band in the range of 425-525 nm gradually increased with a well-defined isosbestic point at
418 nm (Fig. 7c). Based on this two-step UV-vis process, we proposed that, in the first step,
the NH and OH of 1 might form hydrogen bonding with CN-, and in the second step, the
hydroxyl group be completely deprotonated by CN- (Scheme 3). The negative charge of 1-
generated from the deprotonation would lead to the red shift, resulting in the pale pink color.
Therefore, the bathochromic shift of the absorption band led us to propose the transition of
intra-molecular charge transfer (ICT) band through the deprotonation of the chemosensor 1 by
the cyanide, based on Bhattacharya’s proposal.87-89
To further prove the sensing mechanism between 1 and CN-, the interaction between 1 and
OH- was also investigated (Fig. S11). UV-vis spectral change of 1 upon addition of 10 equiv
of OH- was similar to that of 1 obtained from the addition of CN-, which indicated the
deprotonation between 1 and CN-. The formation of 1- generated from the deprotonation was
further supported by ESI-mass spectrometry analysis (Fig. S12). The positive-ion mass
spectrum of 1 upon addition of CN- showed the formation of the 1- + 2·TEA [m/z; 636.93,
calcd; 637.41]. The Job plot74 showed a 1:1 stoichiometry between 1 and CN- (Fig. S13).
Based on the UV-vis titration of 1 with CN-, the association constant (K) of 1 with CN- was
calculated by using non-linear fitting analysis (Fig. S14). The K value was determined to be
4.2 × 103. This value is within the range of those (1.0 - 1.0 x 105) reported for CN-
chemosensors.90-93 The detection limit (3σ/k) of receptor 1 for the analysis of CN- was
calculated to be 0.28 μM (Fig. S15).80 The value is 6 times lower than the WHO guideline (1.9
μM) for CN- in drinking water.81 Hence, receptor 1 could be used as a good detector for
monitoring CN- ion in drinking water.
The preferential selectivity of 1 as a naked eye chemosensor for the detection of CN- was
studied in the presence of various competing anions. For competition studies, receptor 1 was
treated with 260 equiv of CN- in the presence of 260 equiv of other anions (Fig. 8). There was
3
-
-
no interference in the detection of CN- in the presence F-, OAc-, Cl-, Br-, I-, PO4 , N3 , SCN-,
-
2-
BzO-, NO2 and SO4 , except S2-. Thus, 1 could be used as a selective colorimetric sensor for
CN- in the presence of the competing anions.
To further understand the nature of interaction between sensor 1 and the cyanide, 1H NMR
study was initiated (Fig. S16). Upon addition of 0.2 equiv of TEA(CN) as the cyanide source,
two singlets at 12.39 ppm (-OH1) and 10.31 ppm (-NH11) almost disappeared. This result might
be attributed to possible double hydrogen bonding of the Y-shaped ion causing stronger
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binding.94 On excess addition of CN- to 1 solution, the two singlets at 12.39 ppm (-OH1) and
10.31 ppm (-NH11) completely disappeared. At the same time, all aromatic protons shifted to
upfield, which suggests that the negative charge generated from the deprotonation of 1 by CN-
was delocalized over the whole receptor molecule. Therefore, these results could support the
proposal that the sensing mechanism of 1 for cyanide occurs by the deprotonation pathway.
1
Based on UV-vis titration, Job plot, ESI-mass spectrometry analysis and H NMR study, we
propose that receptor 1 senses CN- by the deprotonation process, as shown in Scheme 3.
The effect of pH on the absorption response of receptor 1 to CN- was investigated in a series
of solutions with pH values ranging from 2 to 12 (Fig. S17). The color of the 1-CN- species
remained in the pale pink color between pH 7 and 11. These results indicated that CN-could be
clearly detected by the naked eye or UV-vis absorption measurements using 1 over the pH
range of 7.0-11.0.
Next, the binding properties of 1 with S2- were also studied by UV-vis titration experiments
(Fig. S18). If one looks carefully into the UV-vis titration, there is the three-step change of the
absorption bands. The first change is up to 60 equiv of S2- that the absorbance at 348 nm
decreased gradually, while a new absorbance at 500 nm increased with a clear isosbestic point
at 385 nm (Fig. S18b). The second change is up to 75 equiv of S2- that the absorbance at 348
nm continuously deceased, and at the same time, a new band at 425 nm increased with a well-
define isosbestic point at 390 nm (Fig. S18c). The third change is up to 88 equiv of S2- that the
absorbance at 348 nm slightly deceased, and a new band at 412 nm showed a slight increase
with the isosbestic point at 373 nm (Fig. S18d). Based on this three-step UV-vis process, we
2
-
proposed that, in the first step, the NH and OH of 1 might form hydrogen bonding with S , and
2
-
in the second step, the hydroxyl group be completely deprotonated by S , and in the third step,
the amine group be completely deprotonated by S2- (see Scheme 4). The deep pink color of the
solution observed upon addition of sulfide to receptor 1 might be attributed to the
intramolecular charge transfer (ICT) transition from the deprotonation of the chemosensor 1
by the sulfide, based on Bhattacharya’s proposal.83-85 To further prove the sensing mechanism
between 1 and S2-, the interaction between 1 and OH- was also investigated (Fig. S19). UV-vis
spectral change of 1 upon addition of 14 equiv of OH- was nearly identical to that of 1 obtained
from the addition of S2-.
The Job plot74 indicated a 1:1 stoichiometric ratio between the S2- and 1 (Fig. S20).
Moreover, the negative-ion mass spectrum of 1 with 1 equiv of S2- showed the formation of the
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1- + NaCl + H2O complex [m/z 453.33, calcd. m/z 453.06] (Fig. S21).
1
To further explain the proposed sensing mechanism between the receptor 1 and S2-, H
NMR titrations of 1 were performed by the addition of S2- (Fig. 9). Upon addition of 0.2 equiv
of S2-, two singlets at 12.39 (-OH1) and 10.31 ppm (-NH11) disappeared. This result might be
due to possible double hydrogen bonding of the Y-shaped ion causing stronger binding.94 Upon
further addition of S2-, all aromatic protons shifted to upfield, which was much larger than that
observed with CN-. These results suggest that both protons of 1 in the presence of an excess of
S2- might undergo the deprotonation to generate 12- species. Therefore, the double negative
charges developed from deprotonation of 1 by excess S2- were delocalized through the whole
receptor molecule and then, all protons showed large upfield-shift. Based on the UV-vis
1
titration, Job plot, ESI-mass analysis and H NMR titrations, we proposed the sensing
mechanism of S2- by 1, as shown in Scheme 4.
Based on the UV-vis titration data, the binding constant for 1 with S2- was estimated to be
1.2 x 103 M−1 from Benesi–Hildebrand equation (Fig. S22).81 The detection limit85 (3σ/K) of
receptor 1 as a colorimetric sensor for the analysis of S2- ion was found to be 10.7 μM (Fig.
S23). The value is 80 times lower than a Secondary Maximum Contaminant Level (SMCL) set
for odor in drinking water as established by the EPA (7.8 mM).95 Hence, receptor 1 could be
used as a good detector for monitoring S2- ion in drinking water.
The preferential selectivity of 1 as a naked eye chemosensor for the detection of S2- was
studied in the presence of various competing anions. For competition studies, receptor 1 was
treated with 88 equiv of S2- in the presence of 88 equiv of other anions, as shown in Fig. 10.
3-
-
There was no interference in the detection of S2- in the presence F-, OAc-, Cl-, Br-, I-, PO4 , N3 ,
2-
-
SCN-, BzO-, SO4 , NO2 and CN-. Just in a case, if S2- and CN- species are present
simultaneously, only S2- could be detected by 1. That is, S2- species interacts with 1 much more
strongly than CN- one. Thus, 1 could be used as an excellent selective colorimetric sensor for
S2- in the presence of the competing anions.
The effect of pH on the absorption response of receptor 1 to S2- was studied in a series of
solutions with pH values ranging from 2 to 12 (Fig. S24). The color of the 1-S2- species
remained in the deep pink color between pH 7 and 11. These results indicated that S2- could be
clearly detected by the naked eye or UV-vis absorption measurements using 1 over the pH
range of 7.0-11.0.
To study the reversible behaviors of 1 toward CN- and S2-, Al3+ was added to the each
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solution of 1-CN- and 1-S2-. For CN-, the absorbance at 500 nm disappeared immediately (Fig.
S25). Upon addition of CN- again, the absorbance was recovered. The absorbance changes
were almost reversible with the sequentially alternative addition of CN- and Al3+. In case of S2-,
the absorbance at 427 nm also disappeared immediately (Fig. S26). Upon addition of S2- again,
the absorbance was also recovered, and the absorbance changes were reversible even for
several cycles.
4. Conclusion
We have developed an outstanding colorimetric chemosensor 1, based on the combination
of
(E)-2-(((2-((2,4-dinitrophenyl)amino)phenyl)imino)methyl)phenol
moiety
and
salycilaldehyde, capable of recognizing the multiple targets, Cu2+, CN-, and S2-, in aqueous
solution. The receptor 1 displayed a highly selective and sensitive colorimetric recognition
toward Cu2+ by a color change from pale yellow to orange, and enabled the analysis of Cu2+
ions with a sensitivity limit of 8.77 μM, which was below the WHO acceptable limit (31.5 μM)
in drinking water. In addition, the chemosensor 1 could be used to quantify Cu2+ in water
samples and the sensing mechanism of 1-Cu2+ complex was explained by theoretical
calculations. Moreover, 1 can detect CN- with a color change from pale yellow to pale pink.
The detection limit for CN- was found to be 0.28 μM, which was lower than the WHO
acceptable limit (1.9 μM) in drinking water. Moreover, 1 has the ability to detect S2- with a
color change form pale yellow to deep pink, and showed a very low sensitivity limit of 10.7
μM, much lower than the EPA acceptable limit (7.8 mM) in drinking water. On the basis of the
results, we believe that receptor 1 will offer an important guidance to the development of single
receptors for recognizing both cations and anions.
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.
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C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J.
Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D.
J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W.
Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian 03, Revision
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[79] P. Job, Ann. Chim., 1928, 9, 113-203.
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Reactivity, 4th ed., Harper Collins College Publishers, New York, 1993.
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Scheme 1. Synthetic procedure of 1.
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Scheme 2. Proposed structure of 1-Cu2+ complex.
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Scheme 3. Proposed sensing mechanism of 1 for cyanide.
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Scheme 4. Proposed sensing mechanism of 1 for sulfide.
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Table 1. Determination of Cu(II) in water samples
Sample
Cu(II) added
Cu(II) found
Recovery
(%)
R.S.D. (n = 3)
(μmol/L)
(μmol/L)
(%)
Drinking water 0.00
10.00
0.00
-
-
10.75
107.5
7.9
Conditions: [1] = 10 μmol/L in 10 mM bis-tris buffer/DMSO solution (1:1, pH 7.0).
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Figure captions
Figure 1. (a) Absorption spectral change of 1 (20 μM) in the presence of 8 equiv of various
metal ions in a mixture of bis-tris buffer/DMSO (1:1, v/v). (b) Color change of receptor 1 (20
μM) in the presence of 8 equiv of various metal ions in a mixture of bis-tris buffer/DMSO
(1:1, v/v).
Figure 2. Absorption spectral changes of 1 (20 μM) in the presence of different concentrations
of Cu2+ (from 0 to 9 equiv) at room temperature.
Figure 3. (a) Competitive selectivity of 1 (20 μM) towards Cu2+ (8 equiv) in the presence of
other metal ions (8 equiv) in a mixture of bis-tris buffer/DMSO (1:1, v/v). (b) Colorimetric
competitive experiment of 1 (20 μM) in the presence of Cu2+ (8 equiv) and other metal ions
(8 equiv) in a mixture of bis-tris buffer/DMSO (1:1, v/v).
Figure 4. (a) UV-vis spectral changes of 1 (20 mM) after the sequential addition of Cu2+ and
EDTA in bis-tris buffer/DMSO (v/v, 1:9). (b) Reversible changes in absorbance of 1 (20 mM)
at 412 nm after the sequential addition of Cu2+ and EDTA; 1 (1), 2 (1 + Cu2+), 3 (1 + Cu2+ +
EDTA), 4 (1 + Cu2+ + EDTA + Cu2+)
Figure 5. The energy-minimized structures of (a) 1 and (b) 1-Cu2+ complex.
Figure 6. (a) Absorption spectral change of 1 (10 μM) in the presence of 260 equiv of various
anions in a mixture of bis-tris buffer/DMSO (1:9, v/v). (b) Color change of receptor 1 (10 μM)
in the presence of 260 equiv of various anions in a mixture of bis-tris buffer/DMSO (1:9, v/v).
Figure 7. (a) Absorption spectral changes of 1 (10 μM) in the presence of different
concentrations of CN- (from 0 to 280 equiv) at room temperature. (b) Absorption spectral
changes of 1 (10 μM), extracted from the range of 0-100 equiv of CN- in (a). (c) Absorption
spectral changes of 1 (10 μM), extracted from the range of 110-280 equiv of CN- in (a).
Figure 8. (a) Competitive selectivity of 1 (10 μM) towards CN- (260 equiv) in the presence
of other anions (260 equiv) in a mixture of bis-tris buffer/DMSO (1:9, v/v). (b) Colorimetric
competitive experiment of 1 (10 μM) in the presenceof CN- (260 equiv) and other anions
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(260 equiv) in a mixture of bis-tris buffer/DMSO (1:9, v/v).
Figure 9. 1H NMR titration of receptor 1 with S2-.
Figure 10. (a) Competitive selectivity of 1 (10 μM) towards S2- (88 equiv) in the presence of
other anions (88 equiv) in a mixture of bis-tris buffer/DMSO (1:9, v/v). (b) Colorimetric
competitive experiment of 1 (10 μM) in the presence of S- (88 equiv) and other anions (88
equiv) in a mixture of bis-tris buffer/DMSO (1:9, v/v).
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