Azo dye-based colorimetric chemodosimeter for cyanide in aqueous solution

Article abstract of DOI:10.1080/10610278.2012.712121

A latent colorimetric probe (1) was prepared through the Baylis-Hillman condensation reaction and applied to a Michael acceptor type of probe for cyanide in aqueous solution. The probe has shown a highly selective response to the cyanide anion over other various anions through 1,4-addition of cyanide to the α,β-unsaturated ketone unit of probe 1. When cyanides were added, dramatic colour change of 1 was observable by the naked eye in aqueous buffer.

Full text of DOI:10.1080/10610278.2012.712121

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Supramolecular Chemistry
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Azo dye-based colorimetric chemodosimeter for
cyanide in aqueous solution
Keum-Hee Hong a & Hae-Jo Kim a
a Department of Chemistry, Hankuk University of Foreign Studies, Yongin, 449-791, South
Korea
Published online: 13 Aug 2012.
To cite this article: Keum-Hee Hong & Hae-Jo Kim (2013): Azo dye-based colorimetric chemodosimeter for cyanide in aqueous
solution, Supramolecular Chemistry, 25:1, 24-27
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Supramolecular Chemistry
Vol. 25, No. 1, January 2013, 24–27
Azo dye-based colorimetric chemodosimeter for cyanide in aqueous solution
Keum-Hee Hong and Hae-Jo Kim*
Department of Chemistry, Hankuk University of Foreign Studies, Yongin 449-791, South Korea
(Received 11 June 2012; final version received 20 June 2012)
Dedicated to Professor A. P. de Silva on the occasion of his 60th birthday
A latent colorimetric probe (1) was prepared through the Baylis–Hillman condensation reaction and applied to a Michael
acceptor type of probe for cyanide in aqueous solution. The probe has shown a highly selective response to the cyanide anion
over other various anions through 1,4-addition of cyanide to the a,b-unsaturated ketone unit of probe 1. When cyanides were
added, dramatic colour change of 1 was observable by the naked eye in aqueous buffer.
Keywords: Baylis–Hillman; colorimetric; cyanide; Michael acceptor; probe
Introduction
Results and discussion
Cyanide ion is a highly toxic anion that does harm to
organisms by absorption through the lungs, gastrointesti-
nal tract and skin (1). The cyanide anion can strongly bind
to the haem unit at cytochrome a3, and can inhibit the
electron transport chain in enzyme cytochrome oxidase at
cytochrome a3 (2). After binding of cyanide to cytochrome
oxidase, the process of cellular oxygen metabolism is
inhibited by the cyanide ion (3). As a result, the electron
transport chain in mammals is disturbed, leading to death.
Cyanide ions are also widely distributed in the
environment and affect human beings. The cyanide ions
have been found in many foods and plants. In addition,
cyanide ions are used industrially in gold mining,
electroplating and metallurgy (4). Humans are exposed
to cyanides from food, industrial, environmental and other
sources. Therefore, highly selective probes for the cyanide
ion are of great interest. Despite much effort to develop
efficient and selective probes for cyanide, a few
chemosensors have been reported operating in aqueous
environments due to the high solvation of the anions in
water. To overcome the water desolvation energy during
the cyanide–probe complexation, it is recently employed
to take advantage of the nucleophilic nature of cyanide
anion and to produce a chemodosimeter (5). Herein, we
report an azo dye-based latent colorimetric chemodosi-
meter (1) possessing a masked phenol in the para position
of an azo dye group, which exhibits a highly selective
colorimetric response to cyanide over other anions in
aqueous solution.
Probe 1 was prepared through the Baylis–Hillman
condensation reaction with 2-cyclopentenone from sali-
cylaldehyde with an azo dye functional group (6). Probe
(1) was expected to play a role of a Michael acceptor type
of chemodosimeter for cyanide ions and to exhibit a
dramatic colour change by the resulting free phenol
group. As expected, probe 1 has shown selective response
towards cyanide over various anions through 1,4-addition
of cyanide to a,b-unsaturated ketone of the probe
(Scheme 1).
The chemosensing behaviour of 1 was investigated by
UV–vis spectroscopy. Time-dependent UV–vis spectra of
1 exhibited ratiometric changes when 1 (20 mM) was
treated with CN2 (20 mM) in aqueous dimethylformamide
(DMF) solution (DMF–HEPES 1:1, v/v, 0.10 M, pH 7.4).
Upon the addition of cyanides to 1, the absorbance of 1 at
380 nm decreased, whereas the absorbance at 500 nm
gradually increased to afford a clear isosbestic point at
425 nm (Figure 1). The transformation of 1 to 1-CN was
completed within 6 h with the second-order rate constant
k2 ¼ 3.6 £ 1022 M21 s21 at 258C.
In order to get insight into the reaction, we monitored
1H NMR spectrum after adding 0.5 equiv sodium
cyanide to 1 and compared it with that of the probe itself
(Figure 2). After the reaction was complete, its structure
was elucidated by 1H NMR spectral analysis. The
prominent upfield shifts of vinylic proton (Ha) from 8.15
to 5.10 ppm and aromatic proton (Hb) from 7.21 to
6.23 ppm indicated that Michael addition reaction took
place and significant electronic density was accumulated
*Corresponding author. Email: haejkim@hufs.ac.kr
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2013 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.712121
http://www.tandfonline.com

Supramolecular Chemistry
25
in the aromatic region of 1 owing to the free phenol
formation upon the addition of cyanide anions as observed
in a similar system (7).
O
O
O
O
N
O
N
O
N
NC
NC
The selective recognition of 1 towards CN2 could be
monitored by UV–vis spectroscopy. The relative absor-
bance ratio (A500/A380) of 1 was significantly increased
only by the cyanide ion, whereas other anions such as F2,
Cl2, Br2, I2, H2PO42, HSO42, AcO2 and N23 did not
induce any detectable changes in the UV–vis spectra
(Figure 3). Competitive anion assay was carried out by
adding CN2 (20 mM) to 1 (20 mM) in the presence of other
anions (20 mM) in 50% HEPES–DMF buffer (0.10 M, pH
7.4). It also showed a consistent result for the selectivity of
NaCN
N
N
N
NO2
NO2
NO2
1
1-CN
Red
Colorless
Scheme 1. The Michael reaction of 1 with cyanide.
0.4
0.3
0.2
0.4
0.3
0.2
0.1
0
380nm
500nm
0.1
0
0
2
4
6
8
10
12
Time / h
250
350
450
550
650
λ/nm
Figure 1. Time-dependent UV–vis spectra upon the addition of 1000 equiv of cyanides to 1 (20 mM, DMF–HEPES, 1:1, pH 7.4).
Inset: its kinetics.
Figure 2. Partial 1H NMR spectra of 1 (10 mM) upon the addition of NaCN (0.5 equiv) in DMSO-d6–D2O (v/v, 100:1). (A) 1,
(B) 1 þ NaCN.

26
K.-H. Hong and H.-J. Kim
Anion
Anion+CN–
1.2
0.8
0.4
0
CN–
F–
CI–
Br–
I– H2PO4 HSO4 AcO–
N3
–
–
–
1
Figure 3. Ratiometric UV–vis absorbance of 1 (20 mM) in the presence of 1000 equiv anions in DMF–HEPES (1:1. v/v, 0.1 M, pH 7.4).
Figure 4. Naked-eye images of 1 in DMF–HEPES (1:1. v/v, 0.1 M, pH 7.4) upon the addition of various anions (1000 equiv).
1 towards CN2. The addition of CN2 induced a significant
ratiometric response in the mixture of 1 and other anions as
much as that of 1-CN solution. However, some protic
anions such as H2PO24 and HSO24 interrupted the changes
by decreasing the nucleophilic character of cyanide anions
due to the possible hydrogen bonding between cyanide and
the protic anions.
The prominent bathochromic shift (.120 nm) was
observable by the naked eye. The colour of 1 turned
into red from colourless upon the addition of cyanide,
whereas other anions did not induce any colour changes
(Figure 4).
To investigate the effect of pH on the reaction of 1 to
cyanide ion, the absorbance changes in 1 in the presence
and absence of cyanide ions were measured at various pH
(Figure 5). In the absence of CN2, the colourless probe
(1) showed no detectable UV–vis changes in a wide
range of pH (pH 3–9). In the presence of CN2, however,
dramatic UV–vis changes were observed with concomi-
tant colour changes into red at pH 7–9 with the maximal
intensity observed at pH 9. This indicates that probe 1 can
be used to detect cyanide ion around the biological pH
condition.
0.8
1
1+CN–
0.6
0.4
0.2
0
Conclusion
3
5
7
9
11
13
We prepared a latent colorimetric probe (1) for cyanide.
Upon addition of cyanides, probe 1 underwent a ring-
opening reaction through the Michael addition of cyanide
to give rise to a free phenol group, the signal of which was
transduced into the azo dye unit to afford ratiometric UV–
vis changes. The chemodosimeter exhibited a highly
pH
Figure 5. UV–vis absorbance change at 500 nm of 1 (20 mM) in
the presence (square) and absence (diamond) of CN2
(1000 equiv) DMF–HEPES (1:1. v/v, 0.1 M, pH 7.4) as a
function of pH.

Supramolecular Chemistry
27
selective and colorimetric response to cyanide ions over
other anions in aqueous solution. The dramatic colour
changes of 1 allowed us to detect toxic cyanide anions
even by the naked eye in aqueous solvent.
HRMS (FABþ, m-NBA): m/z obsd 336.0981
([M þ H]þ, calcd 336.0984 for C18H14N3O4).
Acknowledgement
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Korea Government (MEST)
(No. 2011-0028456).
Experimental
2-Cyclopenten-1-one and imidazole were purchased from
Aldrich Chemical Co. and used without further purifi-
cation. All solvents used for the measurements of UV–vis
were purchased from Aldrich Chemical Co. as ‘spectro-
scopic grade’. NMR measurements were carried out using
200 MHz spectrometer. All peaks were given as d in ppm
and were related to the signals of residual non-deuterated
peaks. Mass spectra were recorded on a G6401A
MS-spectrometer. thin layer chromatography (TLC)
analyses were carried out on silica gel plates, and flash
chromatography was conducted by using silica gel column
packages purchased from Merck.
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