Azo dye-based colorimetric chemodosimeter for the rapid and selective sensing of cyanide in aqueous solvent

Article abstract of DOI:10.1016/j.tetlet.2014.12.083

We developed an azo dye-based chemodosimeter possessing oxime functionality as a reaction unit with cyanides in the aqueous environment. The azo oxime chemical probe rapidly reacted with cyanides through the nucleophilic addition and subsequent proton transfer of resonance-assisted hydrogen bonding OH, which displayed a dramatic color change from colorless to dark violet in DMSO/HEPES buffer.

Full text of DOI:10.1016/j.tetlet.2014.12.083

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Azo dye-based colorimetric chemodosimeter for the rapid
and selective sensing of cyanide in aqueous solvent
⇑
Sang-Yun Na, Hae-Jo Kim
Department of Chemistry, Hankuk University of Foreign Studies, Yongin 449-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We developed an azo dye-based chemodosimeter possessing oxime functionality as a reaction unit with
cyanides in the aqueous environment. The azo oxime chemical probe rapidly reacted with cyanides
through the nucleophilic addition and subsequent proton transfer of resonance-assisted hydrogen bond-
ing OH, which displayed a dramatic color change from colorless to dark violet in DMSO/HEPES buffer.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 27 October 2014
Revised 2 December 2014
Accepted 14 December 2014
Available online xxxx
Keywords:
Azo dye
Colorimetric chemodosimeter
Cyanide
Oxime
Probe
In spite of its toxicity,1 cyanide is widely used in various chem-
ical industries including metallurgy, which has resulted in severe
pollution of water supplies.2 Cyanides do harm to human beings
by absorption through the lungs, gastrointestinal tract, and skin,
leading to vomiting, convulsion, loss of consciousness, and even-
tual death. Therefore, highly selective and sensitive probes for
the cyanide anion are of considerable interest due to their applica-
bility to environmental cyanide detection and to the pathological
imaging of the anions.3 Despite considerable effort, however, in
the development of efficient probes for cyanide, major challenges
remain in the development of visual cyanide probes that are sus-
tainable in the aqueous environment.
Herein, we report an azo dye-based colorimetric probe (1),
which has an oxime functional group as a recognition unit of cya-
nides. For the visual and rapid detection of cyanides, an azo scaf-
fold was introduced as the chromogenic unit and an oxime
functional group as the reaction unit of cyanide in the aqueous
environment.
replacing the aldehyde with an oxime group, where the oxime
nitrogen was expected to reinforce the hydrogen bonding with
the phenolic OH group through the strong resonance-assisted
H-bond.5 Therefore, probe 1 was expected to be applicable to the
detection of cyanides in the aqueous environment.6 For the aim,
the azo oxime probe (1) was prepared from p-nitroaniline via
Azo according to the modified literature procedure.4 The oxime
functional group of 1 was easily introduced by treatment of Azo
with hydroxylamine (Scheme 1), of which structure was confirmed
by NMR and mass spectral analyses (See the Supplementary infor-
mation [SI]).
The colorimetric oxime probe (1) was very stable in aqueous
solvent but rapidly reacted with cyanides in DMSO/HEPES buffer
(8:2, 0.10 M, pH 7.4), where the reaction with cyanides was com-
pleted within a few minutes. On addition of cyanides to 1, the ini-
tial maximum of 1 appeared at k 388 nm, which was dramatically
Previously we prepared an azo compound with a salicylalde-
hyde functional group (Azo) and applied it for the sensing of cya-
nides in an organic solvent such as dimethyl sulfoxide (DMSO).4a
The azo salicylaldehyde, however, was not operable in aqueous
solvent due to the insoluble property as well as rapid deprotona-
tion of the phenolic OH of Azo in an aqueous buffer. To overcome
the solvent limitation, a new azo scaffold (1) was designed by
H
N
HO
H
H
O
O
N
N
O
N
H
NH2OH
N
NO2
NO2
Azo
1
⇑
Corresponding author. Tel./fax: +82 31 330 4703.
E-mail address: haejkim@hufs.ac.kr (H.-J. Kim).
Scheme 1. Synthesis of 1.
http://dx.doi.org/10.1016/j.tetlet.2014.12.083
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Na, S.-Y.; Kim, H.-J. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.12.083

2
S.-Y. Na, H.-J. Kim / Tetrahedron Letters xxx (2014) xxx–xxx
shifted to k 555 nm with a pseudo-isosbestic point at k 446 nm
(Fig. 1).
of probe (1) turned from colorless to dark violet (Fig. 3). As
observed in the selectivity experiment, acetate also induced a light
violet color of 1.
Based on the significant bathochromic shift of 1 in the presence
of cyanides, we investigated the substrate selectivity of 1 for vari-
ous anions in DMSO/HEPES buffer (8:2, 0.10 M, pH 7.4) and
observed that probe 1 exhibited a highly selective response to cya-
nide over other anions (Fig. 2). It was noticeable that acetate anion
(AcOÀ) also induced a significant red shift in the UV–vis spectra
possibly due to its strong nucleophilicity.
To elucidate the reaction mechanism, 1H NMR spectra of 1 were
monitored in the absence and presence of cyanides. On addition of
1.0 equiv of CNÀ to 1, a new set of 1H NMR spectra was observed,
which was very stable even in excess cyanides (Fig. 4). The imine
proton (Hc) of 1 was slightly shifted to an upfield region (
Dd
<À0.2 ppm) upon addition of cyanide anions, whereas the aromatic
A competitive assay also demonstrated the consistent selectiv-
ity of probe 1 toward cyanide (Fig. 2B). When cyanide was added to
proton (Hd) at the ortho position of phenolic OH of 1 was dramat-
ically shifted into a high field region (
D
d = À1.0 ppm) due to the
the mixtures of 1 (20
lM) and other anions (20 mM) in DMSO/
possible phenolate formation. To rationalize the observed UV–vis
and 1H NMR spectral changes, we initially speculated on the nucle-
ophilic addition of cyanide anions to the activated imine carbon
and a subsequent proton transfer from phenol to amine (path a).
Later, we found that these phenomena were also observable by
the acid–base reaction of 1 with cyanide followed by internal pro-
ton transfer of the phenolic proton and nucleophilic addition of
cyanide (path b). From the careful analysis of 1H NMR spectra,7
we knew that the colorimetric change could take place partly
through path b. We reasoned that the basic cyanide anions (pKa
<9.4)8 are expected to readily abstract the acidic proton of oxime
OH (Ha, pKa <4) and then to induce an intramolecular proton trans-
fer of the RAHB phenolic OH (Hb) of 1 to afford iminium salt
(1+CN). These two different modes of action induced a significant
electronic change at the para-substituted azo group to display a
dramatic color change of 1+CN (Scheme 2).
HEPES buffer (8:2, 0.10 M, pH 7.4), all the 1+anion mixtures dis-
played almost the same bathochromic shifts as that of 1+CN except
bisulfate (HSOÀ4 ), for which the acidity of bisulfate plausibly inter-
fered with cyanide.
The significant UV–vis change of 1 could be applied for the
detection of cyanide by the naked eye. Upon addition of cyanides
to 1 (20
l
M) in DMSO/HEPES buffer (8:2, 0.10 M, pH 7.4), the color
0.9
0.9
0.6
388 nm
0.3
555 nm
0
0.6
0
4
8
12
time/sec
The stoichiometry between 1 and CNÀ was determined by Job’s
plot, which showed one-to-one binding between 1 and CNÀ
(Fig. 5).
0.3
0
The limit of detection (LOD) of CNÀ was measured in DMSO/
HEPES buffer (8:2, 0.10 M, pH 7.4) through the UV–vis titration
experiment. The standard deviation of the UV–vis absorbance of
250
350
450
/nm
550
650
1 without CNÀ was obtained as
r
= 0.0068 (n = 3). The absorbance
Figure 1. Time-dependent UV–vis spectra of
1
(20
lM) with cyanide anions
at 555 nm was measured by the incremental addition of CNÀ to 1
(20 mM) in DMSO/HEPES buffer (8:2, 0.10 M, pH 7.4).
(20
l
M), the slope of which gave m = 0.0002. The UV–vis titration
0.9
A
CN -
0.6
AcO-
-
-
-
-
-
CN-
F-
Cl-
Br-
I- H2PO4 HSO4 AcO-
N3
ClO4
NO3
1
1 & others
0.3
0
Figure 3. Naked eye detection of cyanide ions by 1 (20
(8:2, 0.10 M, pH 7.4).
lM) in DMSO/HEPES buffer
300
400
500
600
700
/nm
C
B
A
0.9
0.6
0.3
0
Anion
B
Anion+CN
OHa
OHb
Hc
Hd
1
CN-
F-
Cl-
Br-
I- H2PO4-HSO4- AcO- N3- ClO4- NO3-
10.0
5.0
Figure 2. (A) UV–vis spectra of 1 (20
their competitive UV–vis assay with cyanides (20 mM) in DMSO/HEPES buffer (8:2,
0.10 M, pH 7.4).
l
M) with various anions (20 mM) and (B)
Figure 4. 1H NMR spectra of 1 (20 mM) in the absence (A) and presence of 1.0 equiv
(B), and 2.0 equiv (C) of NaCN in DMSO-d6/D2O (50:1, v/v).
Please cite this article in press as: Na, S.-Y.; Kim, H.-J. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.12.083

S.-Y. Na, H.-J. Kim / Tetrahedron Letters xxx (2014) xxx–xxx
3
HO
H
H
carbon or acid-base reaction of acidic oxime OH. The subsequent
proton transfer from the RAHB phenolic OH to imine displayed a
dramatic color change of 1. The color of chemodosimeter 1 turned
from colorless to dark violet in the aqueous environment, which is
easily detectable by the naked eye and further applicable to the
environmentally toxic anion analysis.
HO
H
NC
H+ transfer
H
N
O
N
N
O
N
NC
nucleophilic
addition
N
N
path a
(
)
H
aO
Hb
N
O
Hd
Hc
NO2
NO2
CN
N
N
1+CN
Acknowledgment
O
H
H
O
H
N
O
N
N
O
This work was supported by the National Research Foundation
of Korea (NRF 2014R1A2A2A01002430).
H
NO2
H+ transfer
CN
1
1+CN
N
acid-base
reaction
N
N
Supplementary data
(path b)
NO2
NO2
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.12.
083.
Scheme 2. A proposed response mechanism of 1 to cyanide.
References and notes
0.9
0.6
1. (a) World Health Organization. Concise International Chemical Assessment
Document 61. Hydrogen cyanide and cyanides: human health aspects; Geneva,
2004;
4
(http://www.who.int/ipcs/publications/cicad/en/cicad61.pdf).; (b)
Taylor, J.; Roney, N.; Harper, C.; Fransen, M.; Swarts, S. In Toxicological Profile
for Cyanide; U.S. Department of Health and Human Services: Atlanta, GA, 2006;
6,
2. The United States Environmental Protection Agency has set the maximum
contaminant level for CNÀ in drinking water at 0.2 ppm: Kulig, K. W. In Cyanide
Toxicity; U.S. Department of Health and Human Services: Atlanta, GA, 1991.
3. (a) Wang, F.; Wang, L.; Chen, X.; Yoon, J. Chem. Soc. Rev. 2014, 43, 4312; (b) Xu,
Z.; Chen, X.; Kim, H. N.; Yoon, J. Chem. Soc. Rev. 2010, 39, 127; (c) Baird, C.; Cann,
M. Environmental Chemistry; Freeman: New York, 2005; (d) Ryall, B.; Davies, J.
C.; Wilson, R.; Shoemark, A.; Williams, H. D. Eur. Respir. J. 2008, 32, 740; (e)
Enderby, B.; Smith, D.; Carroll, W.; Lenney, W. Pediatr. Pulmonol. 2009, 44, 142;
(f) Nam, S.-W.; Chen, X.; Lim, J.; Kim, S. H.; Kim, S.-T.; Cho, Y.-H.; Yoon, J.; Park, S.
PLoS ONE 2011, 6, e21387.
0.3
0
0
0.2
0.4
0.6
0.8
[CN]/([1]+[CN])
4. (a) Hong, K.-H.; Kim, H.-J. Supramolecular Chem. 2013, 25, 24; (b) Park, S.; Hong,
K.-H.; Hong, J.-I.; Kim, H.-J. Sens. Actuators, B 2012, 174, 140; (c) Lee, K.-S.; Lee, J.
T.; Hong, J.-I.; Kim, H.-J. Chem. Lett. 2007, 36, 816.
Figure 5. Job’s plot between 1 and cyanide in DMSO/ HEPES buffer (8:2, 0.10 M, pH
7.4), where total concentration of 1 and cyanides was kept constant as 200 M.
l
5. (a) Lee, D.-N.; Kim, H.; Mui, L.; Myung, S.-W.; Chin, J.; Kim, H.-J. J. Org. Chem.
2009, 74, 3330; (b) Kim, H.; So, S. M.; Yen, C. P.-H.; Vinhato, E.; Lough, A. J.; Hong,
J.-I.; Kim, H.-J.; Chin, J. Angew. Chem., Int. Ed. 2008, 47, 8657; (c) Kim, H.-J.; Kim,
H.; Alhakimi, G.; Jeong, E. J.; Thavarajah, N.; Studnicki, L.; Koprianiuk, A.; Lough,
A. J.; Suh, J.; Chin, J. J. Am. Chem. Soc. 2005, 127, 16370; (d) Kim, H.-J.; Kim, W.;
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experiment of 1 revealed that the LOD of CNÀ was 10.3
lM at 3r/m
(Fig. S3 in SI).9
The colorimetric chemodosimeter (1) showed a stable pH pro-
file at pH 3 ꢀ 7.4, where 1 itself showed a negligible absorbance
at 555 nm but 1+CN displayed a strong absorbance at the long
wavelength (Fig. S4 in SI). The pH-dependent UV–vis absorbance
change of 1+CN was observable with the naked eye. We expect that
the characteristic color change of 1 could be utilized for the detec-
tion of cyanides in the neutral aqueous environment.
In summary, we developed an azo dye-based chemical probe (1)
which has an oxime functional group as the reaction site of cya-
nides in the aqueous environment. The dosimeter (1) readily
reacted with cyanides through the nucleophilic addition of imine
7. While oxime proton (Ha) rapidly disappeared upon the addition of 0.5 equiv of
cyanide, the RAHB phenol proton (Hb) was still observed at the same position as
1 with the significant peak broadening in 1H NMR titration experiment (Fig. S2
in SI).
8. Jencks, W. P.; Regenstein, J. Handbook of Biochemistry and Molecular Biology:
Ionization constants of acids and bases; CRC Press: Cleveland, 1976.
9. MacDougall, D.; Crummett, W. B. Anal. Chem. 1980, 52, 2242.
Please cite this article in press as: Na, S.-Y.; Kim, H.-J. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.12.083