Colorimetric fluorescent cyanide chemodosimeter based on triphenylimidazole derivative

Article abstract of DOI:10.1016/j.saa.2013.12.098

In this paper, we demonstrated a highly selective colorimetric chemodosimeter for cyanide anion detection. This chemodosimeter having a triphenylimidazole group as a fluorescent signal unit and a dicyano-vinyl group as a reaction unit was synthesized by t

Full text of DOI:10.1016/j.saa.2013.12.098

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Colorimetric fluorescent cyanide chemodosimeter based
on triphenylimidazole derivative
a
a,b,
Wei Zheng a, Xiangzhu He a, Hongbiao Chen a, , Yong Gao , Huaming Li
⇑
⇑
a College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China
b Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province,
and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
A triarylimidazole-based colorimetric
chemodosimeter for CNÀ was
demonstrated.
The chemodosimeter can recognize
CNÀ with obvious color and
fluorescence change.
The chemodosimeter exhibits a very
low limit of detection (0.11
lM) for
CNÀ.
The chemodosimeter allows naked
eye detection of CNÀ.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we demonstrated a highly selective colorimetric chemodosimeter for cyanide anion
detection. This chemodosimeter having a triphenylimidazole group as a fluorescent signal unit and a
dicyano-vinyl group as a reaction unit was synthesized by the Knoevenagel condensation of 4-(4,5-diphe-
nyl-1H-imidazol-2-yl)benzaldehyde with malononitrile in a reasonable yield. The probe exhibited an
intramolecular charge transfer (ICT) absorption band at 420 nm and emission band at 620 nm, respec-
tively. Upon the addition of cyanide anion, the probe displayed a blue-shifted spectrum and loss in color
due to the disruption of conjugation. With the aid of the fluorescence spectrometer, the chemodosimeter
Received 28 September 2013
Received in revised form 6 December 2013
Accepted 22 December 2013
Available online 4 January 2014
Keywords:
Cyanide anion
Chemodosimeter
Colorimetry
Intramolecular charge transfer
exhibited a detection limit of 0.11
lM (S/N = 3). Interferences from other common anions associated with
cyanide anion analysis were effectively inhibited.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
chrome can prevent transport of electrons from cytochrome c
oxidase to oxygen, causing cytotoxic hypoxia in the presence of
normal hemoglobin oxygenation [2–4]. Consequently, tissues that
depend highly on aerobic respiration, such as the central nervous
system and the heart, are particularly affected. Nowadays, the con-
cern over the effect of cyanide on humans was heightened by the
fact that the use of cyanide salts remained widespread, particularly
in gold and silver mining, electroplating, plastics manufacturing,
and metallurgy [5–8]. Unfortunately, accidental releases of cyanide
into the environment do occur although safeguards and increasing
levels of monitoring and control. In addition, certain plastics, espe-
cially those derived from acrylonitrile, release hydrogen cyanide
Cyanide is considered the most toxic of all anions and can be
absorbed through inhalation, ingestion, or skin contact [1]. Even
a trace amount of cyanide intake will lead to lethal damage to
the human body, since cyanide is a powerful inhibitor of the activ-
ity of cytochrome c oxidase. The binding of cyanide to this cyto-
⇑
Corresponding authors. Address: College of Chemistry, Xiangtan University,
Xiangtan 411105, Hunan Province, PR China. Tel.: +86 731 58298572; fax: +86 731
58293264 (H. Li).
E-mail addresses: chhb606@163.com (H. Chen), lihuaming@xtu.edu.cn (H. Li).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.12.098

98
W. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
when heated or burnt [9]. Therefore, the development of novel
methods for the determination of cyanide at trace concentrations
has become one of the most attractive subjects of investigation
in analytical chemistry because of the practical applications.
Traditionally, cyanide has been determined by using ion
chromatography, electrochemical analysis, and spectroscopic tech-
niques [10–13]. However, most of these strategies require either
multiple experimental steps with tedious sample pretreatments
or sophisticated instrumentation. In addition, these methods often
suffer from interference by other anions, such as NOÀ3 , AcOÀ,
H2POÀ, HSOÀ, ClOÀ and halides. Specifically, the discrimination of
cyanide from halides is rather problematic [14,15]. In this regard,
analytical methods based on colorimetric chemosensors present
many advantages, including high sensitivity, easy detection, inex-
pensive, and rapid in real-time monitoring. The sensing process
is often accompanied by changes in absorption or fluorescence
spectra that can be precisely monitored and sometimes detected
by the naked eye [16–21].
spectroscopic evaluation of the new colorimetric fluorescent
cyanide chemodosimeter in detail.
Experimental
Reagents and apparatus
All reagents and solvents were purchased from commercial
source and used without further purification unless otherwise
noted. Triethylamine (Et3N) was distilled and kept over potassium
hydroxide. NMR spectra were recorded with a Bruker AV-400 NMR
spectrometer. MALDI-TOF mass spectra were recorded on a Bruker
BIFLEXeIII mass spectrometer using a nitrogen laser (337 nm) and
an accelerating potential of 20 kV. UV–vis spectra were recorded
with a Perkin–Elmer Lamda-25 UV–vis spectrometer. Photolumi-
nescence emission spectra were recorded with a Perkin–Elmer
LS-50b luminescence spectrometer.
4
4
4
Compared to the relatively well-developed cyanide chemosen-
sors, colorimetric chemodosimeters based on the special nucleo-
Synthesis of 4-(4,5-diphenyl-1H-imidazol-2-yl)benzaldehyde (TPI-0)
philicity of cyanide have emerged as
a research area of
Under an atmosphere of dry argon, benzil (2.10 g, 10 mmol),
terephthalaldehyde (1.34 g 10 mmol) and ammonium acetate
(6.16 g, 80 mmol) in 50 mL acetic acid were refluxing for 6 h. After
cooling to room temperature, the reaction mixture was poured into
water, filtered, and dried in vacuo. The crude product was purified
by silica gel chromatography using ethyl acetate/dichloromethane
(1/10, v/v) as an eluent to isolate pure compound TPI-0 (2.60 g,
80%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.99 (s, 1H, NH),
10.01 (s, 1H, CHO), 8.29-8.27 (d, 2H, ArH), 8.01-7.99 (d, 2H, ArH),
7.55-7.21 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d (ppm):
192.8, 144.7, 138.6, 135.9, 131.2, 130.5, 129.1, 128.7, 127.6,
125.9. MALDI-TOF MS (C22H16N2O) m/z: calcd for 324.12, found:
325.20 [M + H]+.
significant importance. Generally, colorimetric chemodosimeters
are used to detect an analyte through a highly selective and irre-
versible chemical reaction between the dosimeter molecule and
the target analyte, leading to signal changes in both the absorption
wavelength and color that has an accumulative effect and hence is
directly related to the concentration of the analyte. Taking advan-
tage of the unique nucleophilicity of cyanide, various colorimetric
cyanide chemodosimeters have been demonstrated, in which the
chemodosimetric molecule contains a
p-conjugated chromogenic
unit and reactive subunit. Hitherto, dicyano-vinyl group
a
[22–26], diketone group in benzyl [27], salicylaldehyde group
[28–31], benzamide group [32–34], trifluoroacetyl group [19,35],
N-acyl triazenes [17,36], and [1,3]oxazine ring [37,38] have been
adopted as the reactive subunit. Among these, dicyano-vinyl group
is a popular reaction counterpart for cyanide nucleophilic addition
reactions owing to its high efficiency. Although the design of color-
imetric cyanide chemodosimeters with dicyano-vinyl groups as
the recognition site has currently attracted attention, there are
only a few examples of fluorescent chemodosimeters for cyanide
[22–26]. Therefore, it is still a challenge to fabricate new colorimet-
ric cyanide chemodosimeters adopting dicyano-vinyl group as the
reactive subunit.
Synthesis of 2-(4-(4,5-diphenyl-1H-imidazol-2-
yl)benzylidene)malononitrile (TPI-1)
Under an atmosphere of dry argon, compound TPI-0 (325 mg,
1.0 mmol) and malononitrile (132 mg, 2.0 mmol) in 20 mL abso-
lute ethanol were refluxing overnight with trace Et3N as catalyst.
After cooling to room temperature, the reaction mixture was
poured into water, filtered, and dried in vacuo. The crude product
was purified by silica gel chromatography using ethyl acetate/
dichloromethane (1/20, v/v) as an eluent to isolate pure compound
TPI-1 (240 mg, 65%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 13.05
(s, 1H, NH), 8.51 (s, 1H, CH@C), 8.27-8.25 (d, 2H, ArH), 8.06-8.04 (d,
2H, ArH), 7.5-7.2 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d
(ppm): 160.8, 144.4, 135.7, 131.7, 130.5, 128.9, 126.1, 114.9,
114.0, 80.8. MALDI-TOF MS (C25H16N4) m/z: calcd for 372.13,
found: 373.21 [M + H]+.
Herein, we demonstrated a highly sensitive colorimetric fluo-
rescent chemodosimeter for the detection of cyanide by covalent
linking triphenylimidazole and dicyano-vinyl units. As is well
known, arylimidazole derivatives have attracted considerable
attention in recent years because of their unique properties and di-
verse applications such as photographic materials, luminescent
materials, optical materials, and therapeutic agents [39–41]. In
particularly, 2,4,5-triphenylimidazole (TPI) is a typical fluorophore,
which shows a maximum absorption wavelength at 308 nm
= 2.62 Â 104 MÀ1 cmÀ1), fluorescence emission wavelength at
General spectroscopic procedures
(e
385 nm, and fluorescence quantum yield of 0.10 in CH3CN solution,
anthracene in cyclohexane used as a standard (UF = 0.31; excita-
tion wavelength 366 nm) [49]. (Figs. S1 and S2, see Supporting
Information, SI). Additionally, TPI contains an imidazole ring with
dicoordinate nitrogen atom, which can potentially build complexes
assembled by hydrogen bonds with a molecule containing hydro-
gen-bond donor, such as a carboxylic acid. The chemical flexibility
of this class of compounds allows the preparation of a large variety
of related structures and, consequently, the tailoring of their
optical properties. Therefore, one would expect that novel chemod-
osimeter based on directly linked arylimidazole-dicyano-vinyl
units might result in new potential applications in cyanide
detection. In the present work, we describe the synthesis and the
A solution of TPI-1 (10 lM and 20 lM) was prepared in CH3CN
solution. Titration experiments were carried out in 10-mm quartz
cell at room temperature. Anions (as the tetrabutylammonium
salt) in CH3CN were added to the host solution and used for the
titration experiments.
Binding stoichiometry
The binding stoichiometry of TPI-1 with cyanide ion was inves-
tigated through the Job’s plot. For the Job’s plot analyses, a series of
solutions with varying mole fraction of cyanide ion were prepared
by keeping the total concentration of TPI-1 and cyanide ion

W. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
99
constant (10
l
M). Fluorescence spectra of these solutions were
hypsochromic shift of the absorption band at 420 nm together with
the bathochromic shift of the absorbance at 298 nm. Absorption
spectra changes with different concentrations of cyanide anion fur-
ther displayed the shift in the absorption bands, and showed an
isosbestic point at about 358 nm, indicating the formation of new
species (Figs. S7, SI). Meanwhile, the fluorescence emission of TPI-
measured for each sample at an excitation wavelength of
420 nm. The fluorescence intensity at 620 nm was monitored for
each spectrum.
Results and discussion
2 in CH3CN (10 lM) was completely quenched upon excitation at
420 nm (Fig. 1b), making it possible to construct a colorimetric
chemodosimeter based on the platform of TPI-1.
The synthetic procedure for TPI-1 is shown in Scheme 1.
Initially, the precursor TPI-0 was synthesized by a one-step reac-
tion from benzil, terephthalaldehyde, and ammonium acetate in
acetic acid with a yield of 81%. Subsequently, TPI-0 was further
condensed with malononitrile by using a Et3N-mediated Knoeve-
nagel condensation reaction [23,42] to afford the TPI-1 in 65%
yield. The whole synthetic route was simple and the purification
was easy. Both TPI-0 and TPI-1 were carefully purified and charac-
terized by spectroscopic methods, from which satisfactory analysis
data corresponding to their molecular structures were obtained
(see Experimental section and SI, Figs. S3–S6).
Considering that the optical signal changes depended on the
rate of nucleophilic addition reaction between compound TPI-1
and cyanide anion, the influence of the reaction time on the prob-
ing results was thus investigated by monitoring the changes in
fluorescence intensity at 620 nm (I620). As can be seen in Fig. 2,
there are gradual changes in the fluorescence intensity from 0 to
15 min when the concentration of cyanide anion is lower than
8.0
reaction time to 10 min. With an increase in the concentration of
cyanide anion to 15 M, less than 4 min are needed to achieve a
lM, however, the changes begin to level off after prolonging
l
Similar to compound TPI-0, TPI-1 was soluble in common
organic solvents such as CH3CN, THF, EtOAc, DMF, acetone, etc.
The UV–vis absorption spectra of TPI-0 and TPI-1 are shown in
Fig. 1a. As can be seen, compound TPI-0 showed an intense absorp-
plateau. That is to say, the nucleophilic addition reaction almost
completes within 4 min in high cyanide concentrations, whereas
the reaction time is longer in low concentrations.
To obtain a highly sensitive probe for cyanide anion, the sensing
behavior of TPI-1 toward cyanide anion was investigated by fluo-
tion band centering at 357 nm (
colorless solution in CH3CN (20
e
= 2.55 Â 104 MÀ1 cmÀ1), giving a
l
M). Upon condensation with mal-
rescence titration in CH3CN solvent (10 lM) at an excitation wave-
ononitrile, the resultant compound TPI-1 displayed two main
absorption bands centering at 295 nm (
e
= 2.09 Â 104 MÀ1 cmÀ1
)
length of 420 nm. It is important to note that the globular shape of
TPI-1 and the spherical hindrance cause this compound to solubi-
lize with different aggregates of the solute in some solvents [43]. In
the current case, we choose CH3CN as the solvent for cyanide anion
determination since a typical absorption spectrum of monomeric
TPI-1 is observed in this solvent as evidenced by UV–vis measure-
ment. Fig. 3 displays the fluorescence titration results at room tem-
perature. As can be seen, upon addition of cyanide anions, the
fluorescence emission intensity of the probe at 620 nm was de-
creased gradually and was saturated at 1.5 equiv of cyanide anion
and 420 nm (
e
= 2.35 Â 10À4 MÀ1 cmÀ1), which can be assigned to
the p–
pà transition and the intramolecular charge transfer (ICT)
bands, respectively, giving a brilliant yellow solution in CH3CN
(20 M). Fig. 1b presents the fluorescence emission spectra of
TPI-0 and TPI-1 in their CH3CN solution (10 M). As demonstrated
l
l
in Fig. 1b, both of TPI-0 and TPI-1 exhibited strong luminescence in
diluted solutions with the maximum emission wavelength at 490
and 620 nm, respectively. Due to the fact that the electronic
property of triarylimidazole derivatives changed before and after
the condensation reaction with malononitrile, these compounds
thus exhibited different absorption behaviors and obvious color
changes.
(Fig. 3a). In the range of 0.5–10
plot of the fluorescence intensity at 620 nm as a function of the
cyanide concentrations showed good linear relationship
lM cyanide concentration, the
a
(R = 0.998, Fig. 3b), indicating that TPI-1 could be used to detect
cyanide concentration quantitatively. The detection limit of
TPI-1 was determined to be 0.11 lM at a ratio of signal to noise
of 3 [28,44], which was lower than those reported previously
[22,23,26,28,45,46]. Moreover, this detection limit was also much
lower than the maximum contaminant level (MCL) for cyanide
As mentioned previously, nucleophilic attack by cyanide anion
was expected to occur toward the a-position of the dicyano-vinyl
group in compound TPI-1 as shown in Scheme 1. Upon addition
of equivalent cyanide anion, the electron-withdrawing dicyano-
vinyl group was expected to be transformed into an anionic elec-
tron-pushing group due to the destruction of the dicyano-vinyl
group. Therefore, the dicyano-vinyl acceptor in TPI-1 became donor
in TPI-2 after cyanide addition. As a result, the extent of effective
conjugation in the TPI-1 molecule was broken, which could affect
the ICT efficiency and optical properties of the sensing system. In-
deed, the solution of stabilized anionic specie TPI-2 in CH3CN
anion in drinking water (2.7
Organization (WHO) [47].
lM) set by the World Health
Considering the fact that cyanide anion is usually found in
water or biocompatible solutions, we further studied the influence
of trace water on the sensing process. The absorption of TPI-1
(10
lM, 10 mL) changed at a very limited degree after 15 lL of
(20
lM) was colorless and exhibited a new absorption band center-
H2O was introduced (Fig. S10, SI), indicating that the water in the
ing at 310 nm (Fig. 1a), which was originated by the decrease and
Scheme 1. Structure of TPI-1 and the sensing mechanism.

100
W. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
Fig. 1. (a) Absorption spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (20
lM); (b) fluorescence spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (10 lM). Inset a: the fluorescence
images of TPI-1 in CH3CN (100 M) in the absence and presence of cyanide under a UV lamp at 365 nm.
l
Fig. 2. Reaction time profile of probe TPI-1 (10 lM, in CH3CN) in the presence of
different concentrations of cyanide anion.
cyanide solutions did not affect the results of the titration experi-
ment. Indeed, the plot of fluorescence intensity at 620 nm as a
function of cyanide concentration showed a good linear relation-
ship (R = 0.9958, Fig. S11, SI) when the cyanide aqueous solution
was used instead of cyanide/CH3CN solution, demonstrating that
TPI-1 could be used to detect real cyanide samples.
To evaluate the selectivity of probe TPI-1 for cyanide anion detec-
tion, fluorescence spectral changes upon addition 3 equiv of various
anions including NOÀ3 , AcOÀ, H2POÀ4 , HSO4À, ClOÀ4 , FÀ, ClÀ, BrÀ, and IÀ
were studied. Each spectrum was obtained after addition of various
analytes at room temperature for 25 min. As shown in Fig. 4 and
Fig. S12 (see SI), the selectivity observed by fluorescence monitoring
was matched when TPI-1 was used as a chemodosimeter for cyanide
anion. That is to say, the reaction of TPI-1 with cyanide anion gave a
dramatic decrease of the fluorescence intensity at 620 nm. In con-
trast, no significant fluorescence spectral changes were promoted
by addition of other anions (Fig. 4). Moreover, the significant color
changes of TPI-1 solution could be applied for the detection of cya-
nide anion upon the addition of cyanide anion as shown in Fig. S13
(see SI). In contrast, other anions did not induce any significant color
changes. In addition, the color changes of TPI-1 solution with differ-
ent concentrations of cyanide anion were recorded in Figs. S8 and S9
(see SI). These results further confirm that TPI-1 can act as a cyanide-
specific colorimetric fluorescent sensor.
Fig. 3. (a) Fluorescence spectra of TPI-1 (10
different concentrations of cyanide anion upon excitation at 420 nm. (b) Fluores-
cence intensity at 620 nm of TPI-1 (10 M, in CH3CN) versus the concentration of
cyanide anion at low concentration range of 0.5–10 M.
lM, in CH3CN) in the presence of
l
l
Fig. 5, the signals at 8.51 ppm and 13.05 ppm can be ascribed to
the vinylic proton (Ha) and imidazole proton (Hc), respectively,
for the free TPI-1. After the addition of excess cyanide anion, a
new signal at 4.62 ppm corresponding to the
a-proton (Hb) was
appeared, and the vinylic proton (Ha) at 8.51 ppm vanished, sug-
gesting that the dicyano-vinyl moiety of TPI-1 had reacted with
the cyanide anion through breaking of the C@C double bond and
The mechanism of TPI-1 in sensing of cyanide anion was
monitored the 1H NMR spectral changes induced via the addition
of cyanide anion in DMSO-d6 at room temperature. As shown in

W. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
101
Hunan Province (2013WK3036), Open Project of Hunan Provincial
University Innovation Platform (12K050), and the Construct
Program of the Key Discipline in Hunan Province is greatly
acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.098.
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In summary, we have designed and synthesized a novel fluores-
cent triarylimidazole-based colorimetric chemodosimeter TPI-1 for
cyanide anion. Based on the nucleophilic addition reaction
between the dicyano-vinyl group and cyanide anion, TPI-1 showed
high selectivity for cyanide detection over other common anions
and could recognize cyanide by naked eye. With the aid of the
fluorescence spectrometer, the TPI-1 in ethanenitrile (10
exhibited linearity range from 0.5 M to 10 M cyanide anion with
a detection limit of 0.11 M (S/N = 3), which was among the best
lM)
l
l
l
results for cyanide anion sensing by the fluorescence dicyano-vi-
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Financial support from Program for NSFC (51273170), RFDP
(20124301110006), International S&T Cooperation Program of