A colorimetric and fluorescent cyanide chemosensor based on dicyanovinyl derivatives: Utilization of the mechanism of intramolecular charge transfer blocking

Article abstract of DOI:10.1016/j.saa.2014.09.128

Chemosensor (CS1) was designed and synthesized by simple green chemistry procedure. CS1 exhibited both colorimetric and fluorescence turn-off responses for cyanide (CN^-) ion in aqueous solution. The probe showed an immediate visible color chang

Full text of DOI:10.1016/j.saa.2014.09.128

Accepted Manuscript
A Colorimetric and Fluorescent Cyanide Chemosensor Based on Dicyanovinyl
Derivatives: Utilization of the Mechanism of Intramolecular Charge Transfer
Blocking
Qiao Li, Yi Cai, Hong Yao, Qi Lin, Yuan-Rong Zhu, Hui Li, You-Ming Zhang,
Tai-Bao Wei
PII:
S1386-1425(14)01486-3
http://dx.doi.org/10.1016/j.saa.2014.09.128
SAA 12804
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 June 2014
27 September 2014
29 September 2014
Please cite this article as: Q. Li, Y. Cai, H. Yao, Q. Lin, Y-R. Zhu, H. Li, Y-M. Zhang, T-B. Wei, A Colorimetric
and Fluorescent Cyanide Chemosensor Based on Dicyanovinyl Derivatives: Utilization of the Mechanism of
Intramolecular Charge Transfer Blocking, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.09.128
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A Colorimetric and Fluorescent Cyanide Chemosensor Based on
Dicyanovinyl Derivatives: Utilization of the Mechanism of
Intramolecular Charge Transfer Blocking
Qiao Li, Yi Cai, Hong Yao, Qi Lin, Yuan-Rong Zhu, Hui Li, You-Ming Zhang and 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, Gansu, 730070ˈP. R. China
Abstract˖Chemosensor (CS1) was designed and synthesized by simple green chemistry procedure. CS1
í
exhibited both colorimetric and fluorescence turn-off responses for cyanide (CN ) ion in aqueous solution.
The probe showed an immediate visible color changes from yellow to colorless and green fluorescence
í
í
disappearance when CN was added. The mechanism of chemosensor reaction with CN was studied using
1H NMR and C NMR spectroscopies and mass spectrometry. Moreover, test strips based on the sensor
13
í
were fabricated, which served as convenient and efficient CN test kits.
Keywords: Cyanide (CN-); green synthesize; colorimetric; Fluorometric; ICT blocking; highly selectivity
²²²
*
Corresponding author. E-mail: weitaibao@126.com (Prof. Wei T. B.)
Tel:+086 931 7973120;
1

1. Introduction
In past decades, substantial research has focused on the development of molecular probes for anions
such as cyanide because these substances have important functions in biological systems; however, these
probes are also known as pollutants in our environment[1-3]. Cyanide (CN-) is a highly toxic anion that is
deadly to humans because it can affect many biological functions, such as vascular, visual, central nervous,
cardiac, endocrine and metabolic systems[4]. About 0.5 mg to 3.5 mg of cyanide per kilogram of body
weight is fatal to humans [5]. Nevertheless, cyanide is widely used in numerous chemical processes, such
as electroplating, plastic manufacturing, gold and silver extraction, tanning and metallurgy[6-10].
Therefore, sensitive, selective, simple, low-cost, and easy-to-prepare CN- colorimetric or fluorometric
chemosensors[11-13]have become good choices for various applications. Furthermore, in biological and
environmental systems, cyanide±sensor interactions commonly occur in aqueous solutions; therefore,
much attention has been directed toward developing CN- optical chemosensors that work in aqueous
solution [14-20].
Currently, several chemosensor systems for CN- detection are based on the mechanism of
coordination [21-23], hydrogen bonding interaction[24], supramolecular self-assembly [25-26],
nucleophilic addition reaction [27-31], and so on. Among these CN- sensors, reaction-based sensors
display both specific selectivity and high sensitivity to the CN-. However, most reaction-based CN- sensors
often employ sophisticated structures that require complicated synthetic steps, high temperature, or long
reaction time for CN- detection. In addition, these structures can only be operated in pure or mixed organic
solvents. Therefore, developing a simple and efficient optical chemosensor for CN- in aqueous solution at
room temperature is essential.
With these considerations and our interest in ion recognition [24, 32, 33], we report on an efficient
optical chemosensor (Scheme 1) that can sense CN- with specific selectivity and high sensitivity in
aqueous solutions. In addition, this sensor was synthesized via a simple, efficient, and environmentally
friendly route in pure water without using any catalyst. And the design focuses on utilizing the
2

nucleophilic addition reaction of CN- with a dicyano-vinyl group as the strategy for chemosensor design in
tracking CN- levels. Considering the dicyano-vinyl acceptor group as electron-withdrawing group in CS1,
we introduced an electron-donating group, such as dimethylaminia, onto the 4-carbon position, thereby
changing the initial charge distribution of the chemosensor. This technique can effectively change the
intramolecular charge configurations and construct an acceptor±donor±acceptor (A-D-A) molecule that
improves intramolecular charge transfer [34-36].
The electron-withdrawing nature of dicyano-vinyl group can be modulated by CN-, which interrupts
ʌ-conjugation through nucleophilic addition reaction. When the reaction starts, this group would rapidly
respond to the spectrum change, resulting in very high sensitivity and selectivity. To further confirm the
effect of intramolecular charger transfer on recognition, CS2 without an electron donating group was
synthesized and compared with CS1.
2. Experimental
2.1. Materials and Instruments
Fresh, double-distilled water was used throughout the experiments. All other reagents and solvents
were commercially available at analytical grade and were used without further purification. 1H NMR and
13C NMR spectra were recorded using a Mercury-400BB spectrometer at 400 and 100 MHz, respectively.
Chemical shifts were reported in ppm downfield from tHWUDPHWK\OVLODQHꢀ ꢁ706ꢂꢀ įꢀ VFDOHꢀ ZLWKꢀ VROYHQWꢀ
resonances as internal standards). UV±vis spectra were recorded using a Shimadzu UV-2550 spectrometer.
Photoluminescence spectra were obtained using a Shimadzu RF-5301 fluorescence spectrophotometer.
Melting points were measured on an uncorrected X-4 digital melting-point apparatus. Infrared spectra were
obtained using a Digilab FTS-3000 FT-IR spectrophotometer.
2.2. Chemosensor synthesis
General procedures for the synthesis of CS1: p-Dimethylaminobenzaldehyde (0.74 g, 5 mmol) and
malononitrile (0.33 g, 5 mmol) were combined in 30 mL pure water. The solution was stirred at 90 °C for
2 h. After cooling to room temperature, the yellow precipitate was filtered, washed with water thrice, and
3

recrystallized in ethanol to obtain a yellow powdery product CS1 (0.84 g, 4.3 mmol) at 85% yield. CS2
was prepared following the same procedure.
Scheme 1
CS1: (m.p. 185 °C±186 °C), IR: (KBr, cm-1) v: 3066 (Ar±H), 2212 (CŁN), 1616 (C=C), 1580 (C=C),
810 (Ar±H). 1H NMR (CDCl3, 400 MHz): 7.82 (d, 2H, ArH), 7.47 (s, 1H, NH), 6.69 (d, 2H, ArH ), 3.14 (d,
6H, ArH); 13C NMR (CDCl3,100 MHz): d 158.00, 154.19, 152.99, 133.74, 119.19, 115.96, 114.89, 111.54,
171.66, 40.04, 40.02; Anal. calcd for C11H12N3: C 73.07, H 5.62, N 21.30; found C, 73.23; H, 5.43; N,
21.03.
CS2: (m.p. 87 °C±88 °C), IR: (KBr, cm-1) v: 3032 (Ar±H), 2933 (C±H), 2212 (CŁN), 1590 (C=C),
1564 (C=C), 1485 (C=C), 775 (Ar±H). 1H NMR (CDCl3, 400 MHz): 7.92 (d, 2H, ArH), 7.79 (s, 1H, CH),
7.64 (t, 1H, ArH ) 7.55 (t, 2H, ArH ); 13C NMR (CDCl3,100 MHz): d 160.16, 134.85, 130.94, 129.85,
113.92, 112.76; Anal. calcd for C10H6N2: C 77.91, H 3.92, N 14.62; found C, 77.98; H, 3.84; N, 14.68.
2.3. General procedures for UV±vis spectrum experiments
All UV±vis experiments were performed in aqueous solution [0.01 M 4-(2-hydroxyethyl)-1-
piperazineethanesulfonic acid (HEPES) buffer, pH 7.26, 20% dimethyl sulfoxide (DMSO)] on a Shimadzu
UV-2550 spectrometer. Any changes in the UV±vis spectra of the synthesized compound were recorded
upon the addition of tetrabutylammonium salts while keeping the ligand concentration constant (2.0×10-
-
-
5 M) in all experiments. Tetrabutylammonium salt of anions (i.e., F-, Cl-, Br-, I-, AcO-, H2PO4 , HSO4 ,
-
ClO4 , CN- and SCN-) was used for the UV±vis experiments.
2.4.General procedures for fluorescence spectrum experiments
Fluorescence spectroscopy was performed in aqueous solution (0.01 M HEPES buffer, pH 7.26, 20%
DMSO) on a Shimadzu RF-5301spectrometer. Any changes in the fluorescence spectra of the synthesized
compound were recorded upon the addition of tetrabutylammonium salts while keeping the ligand
concentration constant (2.0×10-5 M) in all experiments. Tetrabutylammonium salt of anions (F-, Cl-, Br-, I-,
-
-
-
AcO-, H2PO4 , HSO4 , ClO4 , CN- , and SCN-) was used for the fluorescence experiments.
4

For 1H NMR and 13C NMR titrations, the CS1 solution was prepared in DMSO-d6 and the appropriate
concentrated solution of the guest was prepared in double-distilled water. Aliquots of the two solutions
were mixed directly in NMR tubes. The NMR probe temperature was kept constant at 298 K.
3. Results and discussion
To investigate the CN--recognition abilities of sensors CS1 and CS2 in aqueous solution, we
performed a series of host±guest recognition experiments. The colorimetric and fluorimetric sensing
abilities were primarily investigated by adding pure water solution of various anions to the DMSO/H2O
(2/8, v/v; pH 7.26) solutions of sensor CS1. As shown in Fig. 1a, the sensor immediately responded with
obvious color changes (from yellow to colorless) when CN- water solution was added to the CS1 solution
at room temperature. In the corresponding UV±vis spectra, the absorption peak at 440 nm decreased.
However, as shown in Fig. 1b, a green fluorescence with one emission band centered at 492 nm appeared
when the solution of sensor CS1 was excited at 440 nm. Upon the addition of CN- water solution, the
fluorescence emission band decreased remarkably and shifted to 536 nm. However, when water solutions
-
-
-
of other anions (such as F-, Cl-, Br-, I-, AcO-, H2PO4 , HSO4 , ClO4 , and SCN-) were added to the sensor
CS1 solution, neither significant color nor fluorescence change was observed. CS1 was confirmed to be
capable of selectively and instantly (dual-channel) detecting CN- in DMSO/H2O binary solution.
Fig. 1a Fig. 1b
The same tests were applied to CS2. In this case, when the CN- water solution was added to the
DMSO/H2O solution of CS2, no change in the color of the solution was observed. In the corresponding
UV±vis spectra, the absorption peak at 308 nm decreased; however, this peak was not in the visible area
(Fig. S1). By contrast, other anions cannot cause such color and UV±vis and fluorescence spectra changes.
Therefore, CS2 cannot colorimetrically detect CN- selectively. This phenomenon illustrates that an A±D±A
molecule CS1 has higher sensitivity than CS2.
An important feature of the sensor is its specific selectivity toward the analyte over other competitive
species. The variations of UV±vis absorbance and the fluorescence of CS1 in DMSO/H2O binary solutions
5

-
-
-
caused by the anions (F-, Cl-, Br-, I-, AcO-, H2PO4 , HSO4 , ClO4 , and SCN-) were recorded in Fig. 2 and
S2 The miscellaneous competitive anions did not lead to any significant interference. In the presence of
these ions, CN- still produced similar color and optical spectral changes. The results showed that sensor
CS1 selectivity toward CN- was not affected by the presence of other anions, especially F-, AcO-, and
-
H2PO4 [37-38].
Fig. 2
We also plotted UV±vis absorbance as CN- concentration (Fig. S3). The change of absorbance
intensity ratio was linear with increasing CN- concentration. The detection limit was 1.4×10-7 M (0.14 ȝ0ꢃꢀ
on the basis of 3 ıꢄPꢅꢀıꢀLVꢀWKHꢀVWDQGDUGꢀGHYLDWLRQꢀRIꢀWKH blank measurements and m is the slope of the
intensity ratio versus the sample concentration plot.
The pH dependence of the chemosensor in HEPES buffer system was checked using UV±vis
spectroscopy. CN- was added to the buffered solution of CS1 at different pH values. The results indicated
that CS1 binding with CN- occurred effectively in the range of pH= 2 to 13 (Fig. 3).
Fig. 3
To evaluate the practical applicability of chemosensor CS1, test strips were prepared by immersing
filter papers into a DMSO solution of CS1 (0.1 M). The strips were subsequently air dried. The test strips
that contained CS1 were utilized to sense different anions. As shown in Fig. 4, obvious color changes were
observed only in CN- water solution when different anion solutions were added to the test kits. Therefore,
the test strips can be conveniently detected in CN- water solutions with satisfactory selectivity.
Fig. 4
Further CS1±CN- reaction mechanism was observed from a series of experiments. To obtain an
insight into CS1±CN- addition reaction stoichiometry, the method of UV±vis titration was used. As shown
in Fig. 5, free CS1 has an absorption band centered at 440 nm ꢁİꢀ ꢀ8.6 × 104 M-1cm-1), which can be
assigned to the ICT transition bands. When 1.8 equiv. of CN- was added, distinct reduction of the bands
and an isosbestic point at 321 nm were observed. Moreover, fluorescence titration with CN- in varying
6

concentrations was also carried. As shown in Fig. 6, the addition of increasing concentrations of CN-, the
emission peak at 490 nm gradually diminished intensity, and the fluorescence of CS was essentially
quenched by 1.5 equiv. of CN- ion. Meanwhile, the fluorescence quantum yields (ĭꢃ increase from 0.023
to 0.061 (see supporting information). These results indicate that CS1 reacts with CN- to form a stable
complex. From the ESI±MS spectrum, we can also observe an obvious peak at 269.0 m/z that can be
assigned to [CS1+CN-+2Na]+ (m/zcaled = 269. 1) (Fig. S4).
Fig. 5
Fig. 6
1H NMR titration displayed the changes of chemical shift upon the addition of CN-. As shown in Fig.
7, the vinylic proton (Ha) at 8.04 ppm completely disappeared upon the addition of 1 equiv. of CN-,
whereas, a new signal emerged at 4.32 ppm (Ha'). Meanwhile, protons Hb and Hc at CS1 showed a large
upfield shift from 7.82 and 6.84 ppm to 7.13 and 6.67 ppm, respectively.
Fig. 7 Fig. 8
13C NMR titration can further explain the recognition mechanism. After the addition of 1.0 equiv. of
CN-, a new signal contemporarily appeared at 126.82 ppm (Fig. 8). Such change suggests the formation of
the new C±C single bond between CN- and C1 of dicyano-vinyl during nucleophilic addition reaction. In
13
addition, another C NMR upfield shift from 158.84 ppm (C1) to 34.53 ppm (C1') and from 154.29 ppm
(C2) to 16.08 ppm (C2') was observed, which may have resulted from the breaking of the C=C double bond.
These observations indicate that the CN- was added to the vinyl group.
Scheme 2
4. Conclusion
In summary, based on the intramolecular charge transfer mechanism, an easy-to-make CN- sensor
CS1 was designed and synthesized via a green chemistry method. The sensor achieved dual-channel
instant detection CN- in water solution at room temperature. Investigation of the recognition mechanism
indicated that the sensor CS1 recognized CN- by a nucleophilic addition reaction. The coexistence of other
anions did not interfere with the CN- recognition process. The detection limit of the sensor CS1 toward
7

CN- was 2.7×10-7 M, which was satisfactory for monitoring CN- levels in physiological and environmental
systems. In addition, the home-made, CS1-based test strips exhibited good selectivity to CN-. Furthermore,
the strips can be used as convenient and efficient CN- test kits.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 21064006᧷
21161018; 21262032), the Natural Science Foundation of Gansu Province (1010RJZA018) and the
Program for Changjiang Scholars and innovative Research Team in University of Ministry of Education of
China (IRT1177).
8

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Figures and schemes
Fig. 1a UV±vis spectra of CS1(2.0×10-5 M) with various anions in DMSO/H2O solution (2:8, v/v,
containing 0.01 M HEPES, pH=7.26) in response to CN- (50 equiv.). Inset: color changes of CS1 with
various anions.
Fig. 1b Fluorescence responses of CS1(2.0×10-5 M) with various anions in DMSO/H2O solution (2:8, v/v,
containing 0.01 M HEPES, pH=7.26 ȜH[ꢀ ꢀꢆꢆꢇ nm) in response to CN- (50 equiv.). Inset: color changes of
í
CS1 with various anions. Inset: fluorescent photograph of CS1 and CS1 + CN .
11

Fig. 2 Absorbance spectra of CS1 ((2.0×10-5 M)) in the presence of various anions (50 equiv.) in
DMSO/H2O (2:8, v/v, containing 0.01 M HEPES, pH=7.26) in response to CN- (50 equiv.).
12

Fig. 3 Effect of pH on the absorbance spectra of CS1 ((2.0×10-5 M)) in response to CN- (50 equiv.) from 1
to 13 in DMSO/H2O (2:8, v/v, containing 0.01 M HEPES, pH=7.26) solution.
13

Fig. 4 Photographs of test strips (A: CS1; B: CS1+CN-).
14

Fig. 5 UV±vis spectra of CS1 ((2.0×10-5 M) in DMSO/H2O (2:8, v/v, containing 0.01 M HEPES, pH=7.26)
upon adding an increasing concentration of CN-.
15

300
200
100
0
0 eq CN-
1.5 eq CN-
500
600
Wavelength/nm
Fig. 6 Fluorescence spectra of CS1 ((2.0×10-5 M) in DMSO/H2O (2:8, v/v, containing 0.01 M HEPES,
pH=7.26) upon adding an increasing concentration of CN-.
16

CS1+CN-(1.0 equiv.)
CS1+CN-(0.5 equiv.)
Ha'
Ha'
Hc'
Hb'
Ha
Hc'
Hc
Hb
Hb'
CS1+CN-(0.1 equiv.)
Ha
Hc
Hc' Hb
Ha'
Hb'
Ha
Hc
Hb
free CS1
Fig. 7 Partial 1H NMR spectra of CS1 (0.1M, DMSO-d6) and in the presence of varying amounts of CN-.
17

Fig. 8 Partial 13C NMR spectra of CS1 (DMSO-d6) and in the presence of CN-.
18

Scheme 1 Synthetic procedures for sensors CS1 and CS2.
Hb'
Hb Ha
Ha'
CN
CN
Hc'
C1' C2'
CN
Hc
C1 C2
CN
CN-
CN
N
Hb'
N
Hb
Hc'
Hc
Scheme 2 Proposed sensing mechanism of sensor CS1 for CN- detection.
19

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Highlights
1. We presented a new colorimetric and fluorescent cyanide chemosensor according
to the mechanism of intramolecular charge transfer blocking and explored the
1
13
interaction of the sensor and CN- by H NMR and C NMR spectroscopies and
mass spectrometry.
2. The fluorescence revealed that receptor CS1 is an excellent sensor in the
recognition to CN- selectively in aqueous solution.
3. The test strips were containing CS1 were fabricated, which also shows a highly
sensitively to CN-.

ICT
ICT
×
CN-
N
N
CN
CN
CN
CN
CS1
CN
Graphical Abstract᧶Chemosensor (CS1) was designed and synthesized via a
simple green chemistry procedure. CS1 exhibited both colorimetric and fluorescence
turn-off responses for cyanide ion in aqueous solution.