A green synthesis of a simple chemosensor that could instantly detect cyanide with high selectivity in aqueous solution

Article abstract of DOI:10.1016/j.cclet.2013.05.005

A novel and simple cyanide chemosensor 2-(naphthalen-1-ylmethylene) malononitrile (L) was designed and synthesized via a green chemistry method in water without using any catalyst. The chemosensor showed an excellent sensitivity and selectivity for CN^- in aqueous solution. The detection limit could be as low as 1.6 × 10^-7 mol/L (0.16 μmol/L), which is far lower than the WHO guideline of 1.9 μmol/L cyanide for drink water.

Full text of DOI:10.1016/j.cclet.2013.05.005

Chinese Chemical Letters 24 (2013) 699–702
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
A green synthesis of a simple chemosensor that could instantly detect cyanide
with high selectivity in aqueous solution
*
Qi Lin, Pei Chen, Yong-Peng Fu, You-Ming Zhang, Bing-Bing Shi, Peng Zhang, Tai-Bao Wei
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel and simple cyanide chemosensor 2-(naphthalen-1-ylmethylene)malononitrile (L) was designed
and synthesized via a green chemistry method in water without using any catalyst. The chemosensor
showed an excellent sensitivity and selectivity for CNꢀ in aqueous solution. The detection limit could be
Received 2 March 2013
Received in revised form 16 April 2013
Accepted 19 April 2013
Available online 12 June 2013
as low as 1.6 ꢁ 10ꢀ7 mol/L (0.16
mmol/L), which is far lower than the WHO guideline of 1.9 mmol/L
cyanide for drink water.
ß 2013 Tai-Bao Wei. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords:
Green synthesis
Cyanide
Chemosensor
Selectively detect
1. Introduction
solutions. Therefore, much attention has been paid to developing
CNꢀ optical chemosensors that work in aqueous solutions [13–18].
Cyanide anion (CNꢀ) is one of the most toxic and dangerous
anions [1–3]. Owing to the extreme toxicity, the maximum
Several chemosensor systems for cyanide anion detection
reported to date are based on the mechanism of coordination
[16], hydrogen-bonding interactions [17,18], nucleophilic addition
reactions [13,15,19,20] and so on. Among these cyanide sensors,
reaction-based sensors display both high selectivity and sensitivity
for the cyanide anion. However, most the reaction-based cyanide
anion sensors employ sophisticated structures, require complicated
synthetic sequences, can only operate in the pure or mixed organic
solvent and require high temperature or long reaction time for
detection CNꢀ. Therefore, simple and efficient CNꢀ optical
chemosensors that could instantly detect CNꢀ in the aqueous
solution at room temperature are essential. On the other hand, the
synthetic procedures of most artificial sensors usuallyinvolve rather
harsh reaction conditions and often employ hazardous materials
suchasstartingmaterials, solutionsand catalysts, whichposeahuge
risk to the environment. To minimize the generation and application
of hazardous substances, a green synthetic procedure should be
encouraged in the synthesis of CNꢀ chemosensors.
permissive level of cyanide in drinking water is set at 1.9
mmol/L
by the World Health Organization (WHO) [4]. On the other hand, a
large quantity of cyanide anions are widely used in industry for the
synthesis of fine chemicals, electroplating and precious metal
mining and so on [5]. Therefore, finding methods to detect cyanide
at the environmental level has attracted much attention [6–10].
Although previous work has involved the development of a wide
variety of chemical and physical sensors for the detection of CNꢀ
[6–10], so far, improving the detection selectivity and sensitivity in
the context of interference from coexisting anions such as Fꢀ,
AcOꢀ, and H2PO4 in the aqueous solution has been challenging.
ꢀ
Moreover, most physical methods require expensive equipment,
involve time-consuming and laborious procedures that can be
carried out only by well-trained professionals and seriously
restrict the practical application of these CNꢀ sensors
[6,7,11,12]. For the purposes of simplicity, convenience and low
cost, easily-prepared CNꢀ optical chemosensors [8–10] become an
excellent choice. Furthermore, in biological and environmental
systems, cyanide–sensor interactions commonly occur in aqueous
With these considerations and our interest in ion recognition in
mind [21–23], we here report an efficient optical chemosensor
(Scheme 1) that could sense CNꢀ with high selectivity and
sensitivity in aqueous solutions. In addition, the sensor was
synthesized via a simple, efficient and environmentally friendly
route in pure water without using any catalyst.
The strategies for the design of the sensor are as follows. Firstly,
in order to achieve the instantaneous detection of cyanide, a
* Corresponding author.
E-mail address: weitaibao@126.com (T.-B. Wei).
1001-8417/$ – see front matter ß 2013 Tai-Bao Wei. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.05.005

[
Q. Lin et al. / Chinese Chemical Letters 24 (2013) 699–702
2.4. Synthesis of sensor L
Naphthaldehyde (0.468 g, 3 mmol) and malononitrile (0.234 g,
3 mmol) were combined in 20 mL of pure water. The solution was
stirred at 90 8C for 2 h. After cooling to room temperature, the
yellow precipitates were collected by filtration, washed with 75%
ethanol three times, and then recrystallized in ethanol to obtain
the yellow powdery product L. Yield: 82%, mp 173–174 8C, 1H-
NMR (400 MHz, DMSO-d6,):
8.20 (t, 1H, J = 7.2 Hz, Ar–H), 8.09 (t, 1H, J = 7.2 Hz, Ar–H), 7.70 (m,
3H, Ar–H). IR (KBr, cmꢀ1):
2224 (CBB N), 1562 (C55C), 1506 (C55C),
d 9.38 (s, 1H, C–H), 8.27 (m, 2H, Ar–H),
n
1456 (C55C). Anal. Calcd. for C14H8N2: C 82.33, H 3.95, N 13.72;
found: C 82.41, H 4.02, N 13.64. APCI-MS: Calcd. for C14H8N2:
204.1, found: 204.0.
3. Results and discussion
Scheme 1. The green synthesis of L and the CNꢀ sensing mechanism.
Sensor L was synthesized by the Kno¨evenagel reaction as
depicted in Scheme 1. Usually, the Kno¨evenagel reaction is carried
out in polar aprotic solvents such as DMF or CH3CN and catalyzed
by such bases as piperidine, or sodium hydroxide [19]. We,
however, attempted to synthesize the sensor L in pure water under
the catalyst free conditions to avoid the use of organic solvents and
catalyst and prevent environmental contaminations. It is exciting
that the naphthaldehyde could undergo the Kno¨evenagel reaction
with malononitrile in pure water without using any catalyst to give
2-(naphthalen-1-ylmethylene)malononitrile (L) in high yields. The
proposed reaction mechanism involves the ionization of mal-
ononitrile, the nucleophilic addition of dicyanomethyl carbanion
to naphthaldehyde followed by a dehydration condensation
process (Scheme 2) [24]. This is an excellent green chemistry
method for the preparation of these kinds of fine chemicals.
The sensing ability of L toward various anions, such as Fꢀ, Clꢀ,
Brꢀ, Iꢀ, Acꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4ꢀ, SCNꢀ, N3ꢀ, SO42ꢀ, NO3ꢀ, S2ꢀ and
CNꢀ was investigated by UV–vis spectroscopy. With the aim of
excluding the possible influence of pH fluctuation, we carried out
experiments in DMSO/H2O (8/2, v/v), HEPES (4-(2-hydroxyethyl)-
1-piperazineethanesulfonic acid) (10 mmol/L) buffered solutions
at pH 7.04. Upon adding 50 equiv. of CNꢀ to the solution, the
absorption at 369 nm disappeared immediately. To validate the
selectivity of L, the same tests were applied to other anions and no
such changes were observed (Fig. 1). It was confirmed that L could
selectively and instantly detect CNꢀ in DMSO/H2O binary solution.
To further explore the utility of sensor L as an ion-selective
chemosensor for CNꢀ, competitive experiments were carried out in
the presence of 50 equiv. of CNꢀ and 50 equiv. of various other
anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, Acꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4ꢀ, SCNꢀ,
dicyanovinyl group was introduced as the binding site. According
to the literatures [19,20], vinyl-substituted derivatives display
both selective and sensitive responses to various concentrations of
the cyanide anion. Additionally, the dicyano substitution on the
vinyl group could significantly enhance the sensitivity of the
nucleophilic addition reaction between the vinyl group and CNꢀ.
Secondly, we introduced a naphthyl group as the signal groups.
Finally, the sensors were designed to be easily synthesized via a
green chemistry method.
2. Experimental
2.1. Materials and apparatus
All reagents for synthesis were of analytical grade, commer-
cially purchased and were used without further purification.
Tetrabutylammonium (TBA) salts of anions were purchased from
Sigma–Aldrich Chemical and stored in a vacuum desiccator.
Melting points were measured on an X-4 digital melting point
apparatus and were uncorrected. UV–vis spectra were recorded on
a Shimadzu UV-2550 spectrometer. 1H NMR spectra were recorded
on a Varian Mercury Plus-400 MHz spectrometer with DMSO-d6 as
solvent and tetramethylsilane (TMS) as an internal reference. The
infrared spectra were performed on a Digilab FTS-3000 FT-IR
spectrophotometer. Elemental analyses were performed by
Thermo Scientific Flash 2000 organic elemental analyzer. Electro-
spray ionization mass spectra (ESI-MS) were measured on a Bruker
Daltonics Esquire6000 ESI-ION TRAP system.
[(cShem_)2DT$FI]G
2.2. General procedure for the UV–vis experiments
nucleophilic addition
A solution of sensor L (2.0 ꢁ 10ꢀ4 mol/L) in DMSO was prepared
and stored in dry atmosphere. The solution was used for all
spectroscopic studies after appropriate dilutions. Solutions of
1.0 ꢁ 10ꢀ2 mol/L TBA salts of the respective anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ,
O
NC
NC
NC
NC
H+
+ H+
CH2
CH
AcOꢀ, H2PO4ꢀ, HSO4 and ClO4ꢀ) were prepared in H2O, and the
ꢀ
sodium salts of CNꢀ, NO3ꢀ, S2ꢀ and SCNꢀ were prepared in H2O.
Any changes in the UV–vis spectra of sensor L were recorded upon
the addition of salts while keeping the concentration of sensor L
(2.0 ꢁ 10ꢀ5 mol/L) constant in all experiments.
CN
H
CN
2.3. General procedure for 1H NMR experiments
H2O
For 1H NMR titrations, sensor L was prepared in DMSO-d6, NaCN
was prepared in D2O. Sensor L in DMSO-d6 was added into an NMR
tube, then 0.2, 0.5, 1.0 and 1.5 equiv. of CNꢀ anion was added
sequentially. All solutions were mixed directly in the NMR tube.
Scheme 2. The proposed reaction mechanism for the condensation between
malononitrile and naphthaldehyde.

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Q. Lin et al. / Chinese Chemical L
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etters 24 (2013) 699–702
701
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.2
0.1
0.0
0.2
0.1
0.0
L and L +other anions
L + CN
0.0
0.5
1.0
1.5
[CN ]/[L]
300
400
500
Wavelength/nm
600
300
400
500
600
700
Wavelength/nm
Fig. 1. UV–vis absorption of sensor L (2 ꢁ 10ꢀ5 mol/L) with various anion in DMSO/
Fig. 3. UV–vis titration of sensor L (2 ꢁ 10ꢀ5 mol/L) with CNꢀ (0–1.8 equiv.) in
H2O (8/2, v/v), HEPES (10 mmol) at room temperature, other anions are Fꢀ, Clꢀ, Brꢀ,
DMSO solution.
Iꢀ, Acꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4ꢀ, SCNꢀ, N3ꢀ, SO42ꢀ, NO3ꢀ, S2ꢀ
.
[(Fig._2)TD$FIG]
absorbance at 369 nm as a function of [CNꢀ]/[L]. The plot of the
decrease in absorbance at 369 nm reached the saturation point at
the ratio of [CNꢀ]/[L] of 1.2, indicating a 1:1 stoichiometry between
L and CNꢀ ions. Furthermore, the detection limit of the UV–vis
0.3
0.2
0.1
0.0
L
changes calculated on the basis of 3
s
B/S [25] is 1.6 ꢁ 10ꢀ7 mol/L
L + other anions
for CNꢀ, which is far lower than the WHO guideline of 1.9
cyanide.
mmol/L
L +CN + other anions
L +CN
To further elucidate the binding mode, 1H NMR-titration
experiments of sensor
L were conducted in DMSO-d6/D2O
(Fig. 4). Sensor L was dissolved in DMSO-d6 and NaCN was
dissolved in D2O. Before adding CNꢀ, the 1H NMR chemical shift of
the vinylic proton (Ha) on sensor L was at 9.38 ppm. After the
addition of 0.2 equiv. CNꢀ, it was obvious that the intensity of the
vinylic proton (Ha) at 9.38 ppm decreased and a new signal grew at
300
400
Wavelength/nm
500
600
a0
5.26 ppm corresponding to the
a
-proton (H ). With the addition of
1.0 equiv. of CNꢀ, the peak at 9.38 ppm disappeared completely.
Meanwhile, the proton signals on naphthyl displayed an upfield
shift due to the loss of the conjugation between vinyl and naphthyl
groups. These observations clearly indicated that the cyanide anion
was added to the vinyl group and suggested 1:1 reaction between L
and CNꢀ ions (as showed in Scheme 1).
Fig. 2. UV–vis absorption of sensor L (2 ꢁ 10ꢀ5 mol/L) with miscellaneous anions
exist in DMSO/H2O (8/2, v/v), HEPES (10 mmol/L) at room temperature, other
anions are Fꢀ, Clꢀ, Brꢀ, Iꢀ, Acꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4ꢀ, SCNꢀ, N3ꢀ, SO42ꢀ, NO3ꢀ, S2ꢀ
.
N3ꢀ, SO42ꢀ, NO3ꢀ, S2ꢀ) in a HEPES-buffered solution of sensor L. The
optical absorbance of sensor L with CNꢀ (Fig. 2) was not influenced
by the subsequent addition of competing anions, which indicated
that L had shown specific selectivity for CNꢀ.
The results of APCI-MS experiments also support this hypothe-
sis. In the APCI-MS spectra, the peak of L appeared at 204.0 (Calcd.
for C14H8N2: 204.1). However, when 1 equiv. CNꢀ was added to the
solution of L, the [L+CN]ꢀ peak appeared at 230.5, coinciding well
with that for the species [L+CN] ꢀ (calcd. for C15H8N3ꢀ: 230.1) and
indicating the formation of the stabilized anionic species L-CN.
The UV–vis titration of L with CNꢀ is carry out in DMSO
solutions (Fig. 3). The inset shows a good linear dependence of the
[(Fig._4)TD$FIG]
Fig. 4. CNꢀ 1HNMR titrationofsensorL (10.0 mmol/L)in DMSO-d6: (a) L only, (b) L + 0.2 equiv. ofCNꢀ, (c) L + 0.5 equiv. ofCNꢀ, (d) L + 1.0 equiv. ofCNꢀ and (e) L + 1.5 equiv. ofCNꢀ.

702
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In conclusion, an easy-to-make chemosensor L was designed
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instantly detect CNꢀ in water solution at room temperature with
high selectivity and sensitivity. The investigation of the recognition
mechanism indicated that the sensor L recognized CNꢀ by forming
a nucleophilic addition adduct. The coexistence of other anions did
not interfere with the CNꢀ recognition process. Moreover, the
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This work was supported by the National Natural Science
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