A novel dual-channel chemosensor for CN? using asymmetric double-azine derivatives in aqueous media and its application in bitter almond

Article abstract of DOI:10.1016/j.saa.2017.03.052

In this paper, we have designed and synthesized a novel sensor L1 based on asymmetric double-azine derivatives, which showed both “naked eye” recognition and fluorescence responses for CN^? in DMSO/H₂O (v/v?=?4:1, pH?=?7.20) solution. This simple sensor L1 could distinguish CN^? from coexisting anions via the way of deprotonation and sensing mechanism of intramolecular charge transfer (ICT), and the minimum detection limit on fluorescence response of the sensor L1 towards CN^? was down to 9.47?×?10^??7?M. Moreover, we have successfully utilized the sensor L1 to detect CN^? in bitter almond. Test strips containing L1 were also prepared, which could act as a practical colorimetric tool to detect CN^? in aqueous media.

Full text of DOI:10.1016/j.saa.2017.03.052

Accepted Manuscript
A novel dual-channel chemosensor for CN− using asymmetric
double-azine derivatives in aqueous media and its application in
bitter almond
Peng-Xiang Pei, Jing-Han Hu, Ying Chen, You Sun, Jing Qi
PII:
DOI:
Reference:
S1386-1425(17)30238-X
doi: 10.1016/j.saa.2017.03.052
SAA 15031
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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11 January 2017
15 March 2017
19 March 2017
Please cite this article as: Peng-Xiang Pei, Jing-Han Hu, Ying Chen, You Sun, Jing Qi ,
A novel dual-channel chemosensor for CN− using asymmetric double-azine derivatives
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ACCEPTED MANUSCRIPT
A novel dual-channel chemosensor for CN− using
asymmetric double-azine derivatives in aqueous media and
its application in bitter almond
Peng-Xiang Pei, Jing-Han Hu*, Ying Chen, You Sun, Jing Qi
-------------------------------------------
E-mail: hujinghan62@163.com
-------------------------------------------
College of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, gansu, 730070, P.R.China
Abstract:
In this paper, we have designed and synthesized a novel sensor L1 based on
asymmetric double-azine derivatives, which showed both “naked eye” recognition
and fluorescence responses for CN− in DMSO/H2O (v/v=4:1, pH=7.20) solution. This
simple sensor L1 could distinguish CN− from coexisting anions via the way of
deprotonation and sensing mechanism of intramolecular charge transfer (ICT), and the
minimum detection limit on fluorescence response of the sensor L1 towards CN− was
down to 9.47×10-7 M. Moreover, we have successfully utilized the sensor L1 to detect
CN− in bitter almond. Test strips containing L1 were also prepared, which could act as
a practical colorimetric tool to detect CN− in aqueous media.
Key words: Anion Sensor, Fluorescence, Cyanide, Test Strip, DFT
1. Introduction

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Currently, the development of anions probes such as cyanide has been a subject
of research hotspot, because anions play vital roles in environment pollutant and
biological systems [1-6]. It is well known that cyanide as one of the most toxic anions
can damage many biological functions including endocrine, vascular, central nervous
and metabolic systems in addition to serious lead to eventually death [7-12]. However,
cyanide is widely used in industry like gold extraction, synthetic fibers, electroplating
and etc [13-18]. Thus, there is a need for developing an efficient sensing system to
monitor cyanide concentration from contaminant sources.
In the past few decades, with the development of the detection techniques, many
methods have been used to detect CN−, such as titrimetric, potentiometric,
electrochemical methods and ion chromatography [19-22] and etc. However, these
methods usually suffer from a series of problems such as long response time and
selectivity shortage. Hence, it is of great importance to design and develop efficient
methods to solve above mentioned problems.
Up to now, a great deal of effort has been invested in developing fluorimetric and
colorimetric sensors due to simplicity, low detection limit and practical application
[23-27]. Fortunately, fluorimetric and colorimetric sensors based on various
mechanisms (intramolecular charge transfer (ICT) [28-29], excited state
intramolecular proton transfer (ESIPT) [30], photo-induced electron transfer (PET)
[31], and metal-ligand charge transfer (MLCT) [32]) have been partly developed.
Among the different intelligent strategies in designing sensors, the deprotonation
approach is often used to detect CN− due to easy design and comprehension. In many

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cases, sensors usually containing groups such as amide, thiourea, pyrrole and
hydroxyl could detect CN− [33-34].
Our research group has a longstanding interest in molecular recognition [35-39].
Herein, we have synthesized a new chemosensor L1 based on asymmetric double-
azine derivatives, which could recognize CN− in DMSO/H2O (v/v=4:1, pH=7.20)
solution. To the best of our knowledge, the double azine-based chemosensors reported
so far are very few in the field of supramolecular chemistry. Here, the function of
sensor L1 for recognizing CN− is based on the appended salicylaldehyde hydrazone
that act as binding sites as well as fluorescent signal group. Moreover, the sensor L1
could successfully detect CN− in the bitter almond, and the detection limit was down
to 9.47 ×10–7 M from fluorescence spectrum changes. The mechanism of this
recognition was investigated by mass spectrometry and 1H-NMR.
2. Materials and Instruments
1H-NMR measurements were obtained from a Varian Mercury Plus-400MHz
spectrometer and chemical shifts were recorded in ppm (DMSO-d6 as solvent).
Melting points were performed on an X-4 digital melting point apparatus and were
uncorrected. 13C-NMR measurements were measured using a Mercury-400BB
spectrometer at 100 MHz. UV-Vis absorption spectra was measured on a Shimadzu
UV-2550 spectrometer. ESI-MS was measured on an Agilent 1100 LC-MSD-Trap-VL
system. The photoluminescence spectra were performed on a Shimadzu RF-5301
fluorescence spectrophotometer.
3. Experimental

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3.1 Synthesis
The chemosensor L1 was synthesized in good yields (72.3％) by one-step
condensation of salicylaldehyde hydrazone (0.01 mol, 1.36 g), 1,4-phthalaldehyde
(0.01 mol, 1.34 g) in DMF (20 mL). The reaction was refluxed for 6 h with 0.2
mL acetic acid as catalyst, and then cooling room temperature, the mixture was
filtrated. Finally, the crude product was purified by recrystallisation with
DMF/H2O, obtaining the target compound L1 (Scheme 1). Colour: yellow
1
solid, mp: 247~249℃. H-NMR (DMSO-d6, 400 MHz, ppm) δ: 11.26 (s, 2H), 9.00
(s, 2H), 8.88 (s, 2H), 8.00-7.99 (m, 4H), 7.72 (d, J=7.93 Hz 2H), 7.40 (m, 2H),
6.98-7.00 (t, J=8.32 Hz, 4H). 13C-NMR (DMSO-d6, 100 MHz, ppm) δ: 163.88,
162.04, 159.22, 136.67, 133.72, 131.58, 129.36, 120.03, 118.64, 116.96. MS/ EI: m/z:
371.17 (M+H+).
Scheme 1 The synthetic procedure for sensor L1.
3.2 General procedure of spectroscopy
All UV-Vis spectroscopy and fluorescence spectroscopy were performed after
−
−
adding tetrabutylammonium salt of anions (F−, Cl−, Br−, I−, AcO−, H2PO4 , HSO4 ,
−
−
ClO4 , but N3 , PO43−, B2O72−, SCN−and CN− were added in the form of sodium salts)
in DMSO solution, while keeping the host concentration constant (2.0×10-5 M).
1
For H-NMR titrations, L1 and sodium cyanide were respectively prepared in
DMSO-d6, and the DMSO-d6 solution of sodium cyanide was added to L1 according

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to different quantities.
4. Results and Discussions
The recognition of sensor L1 towards anions (F−, Cl−, Br−, I−, N3 , PO4
−
3−
,
−
−
−
B2O72−, AcO−, H2PO4 , HSO4 , ClO4 , SCN− and CN−) in DMSO/H2O (v/v=4:1,
pH=7.20) solution was investigated by UV-Vis spectroscopy and fluorescence
spectroscopy. When adding 50.0 equiv. of various anions respectively to the solution
of L1, we found that only CN− could induce an obvious color change from faint
yellow to red, which could be easily distinguished under visible light. In the
corresponding UV-Vis spectrum, an obvious red shift band from 412 nm to 500 nm
was observed. Moreover, F− also gave rise to a red shift from 412 nm to 434 nm in the
absorption spectrum, but it had no influence on detecting CN−. In contrast, other
examined anions didn't induce any dramatic color and spectra changes. These results
suggested that sensor L1 showed a specific selectivity for CN− in aqueous media
(Figure 1).
Figure 1 (a) UV-Vis spectra of sensor L1 (20 μM) with various anions (50 equiv.) in
DMSO/H2O (v/v=4:1, pH=7.20) solution (b) Color changes of L1 (20 μM) with
various anions (50 equiv.) in DMSO solution.

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The fluorescence measurements for sensor L1 were performed in DMSO/H2O
(v/v=4:1, pH=7.20) solution. The fluorescence spectrum of free L1, which was
excited at 412 nm and showed a strong emission band at 554 nm in aqueous solution,
when 50.0 equiv. of CN− were added to L1, a dramatic fluorescence quenching was
observed. However, other anions could not induce any similar response. It showed
that L1 could be able to instantly and selectively detect CN− in DMSO/H2O (v/v=4:1,
pH=7.20) solution. (Figure 2)
Figure 2 (a) Fluorescence spectra of L1 (20 μM) with various anions (50 equiv.) in
DMSO/H2O (v/v=4:1, pH=7.20). (b) Color changes observed for L1 (20 μM) with
various anions (50 equiv.) in DMSO/H2O (v/v =4:1, pH=7.20) solution under the UV
lamp.
In order to investigate the sensing behavior between L1 and CN−, we have
worked absorption spectra titration. As shown in Figure 3, with an increasing amount
of CN− from 0 equiv. to 55 equiv., the absorption band at 412 nm was gradually
decreasing while the band at 500 nm was increasing little by little before obtaining the
maximum value, which showed one red shift in the absorption band with ∆λ=88 nm.
Moreover, an isosbestic point at 450 nm clearly observed, which indicated that an

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interconversion into single discrete chemical species during the titration process. In
addition, we also have performed anti-interference experiments, results obviously
showed that other coexisting anions had no or little influence on detecting CN− in
aqueous solution. (Figure. 4)
Figure 3 Absorption spectra titration of sensor L1 (20 μM) in presence of different
concentration of CN− (0-55 equiv.) in DMSO/H2O (v/v =4:1, pH=7.20) solution. Inset:
a plot of absorbance at 412 nm and 500nm depending on the concentration of CN− in
the range from 0 equiv. to 55 equiv.
Figure 4 Absorption spectra change for a 1:20 mixture of L1-CN− upon addition of
20 equiv. of other examined anions in DMSO/H2O (v/v =4:1, pH=7.20) solution.
Similarly, the variation in fluorescence emission spectral of sensor L1 was
measured by titrations methods. As shown in Figure 5, the fluorescence intensity at

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554 nm of L1 decreased steadily on changing the concentration of CN− from 0 equiv.
to 76 equiv, and it was quenched almost completely after the addition of 76 equiv.
CN−. Furthermore, the fluorescence intensities at 554nm were plotted to obtain a
calibration graph, which clearly showed an excellent linear relationship between
fluorescence intensity and CN− concentration. Moreover, we have also validated the
selectivity of sensor L1 towards CN− in the context of coexisting anions (F−, Cl−, Br−,
−
−
−
−
I−, AcO−, H2PO4 , N3 , PO43−, B2O72−, HSO4 , ClO4 and SCN−), which clearly
showed that other examined anions had little impact on CN− detection. (Figure 6)
Figure 5 Fluorescence titration of sensor L1 (20 μM) with different concentration of
CN− (0-76 equiv.) in DMSO/H2O (v/v=4:1, pH=7.20) solution. Inset: the change of
fluorescence 554 nm depending on the concentrations of CN−
Since the charge distribution and inherent fluorescence properties of a sensor are
affected by pH value. Thus, the effect of pH to L1-CN− was also studied by emission
spectra (Fig. S1). Results suggested that single L1 had no fluorescence response when
pH was between 2.0 and 8.0, but the L1-CN− systems showed a significant
fluorescence response when pH was between 7.0 and 8.0, which indicated that L1

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showed an excellent fluorescence response for CN− within the basic pH range
(pH=7.0–8.0).
Figure 6 Fluorescence spectra of L1 (20 μM) in presence of 20 equiv. of CN− and 20
equiv. of various interference anions.
The reversibility of a chemosensor is one of the essential aspects for its
applications. Therefore, the reversibility of sensor L1 was performed by respectively
addition of H+ and CN−. To our delight, this switching process was repeated more
than six times in the absence of fluorescence loss, which indicated that L1 showed an
excellent reversibility to CN− (Figure 7).
\
Figure 7 Switching cycles of L1 controlled by alternating addition of H+ and CN−.
Moreover, the fluorimetric detection limit on sensor L1 for CN− was also tested.

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As shown in Figure 8, the fluorescence detection limits on sensor L1 for CN− was
down to 9.47×10–7 M according to the basis of 3σ/s (where σ and s represent
respectively the standard deviation of blank solution and the slope of calibration
curve), indicating that the sensor L1 could detect very low concentration of CN−.
In addition, a lot of experiments have been done to investigate the stoichiometry
between L1 and CN−. As shown in Figure 9, as expected, a job plot was implemented,
demonstrating a stoichiometry for L1-CN− was 1:2.
Figure 8 Fluorescence detection limit of L1 (20 μM) towards CN−in DMSO/H2O
(v/v= 4:1, pH=7.20) solution.
Figure 9 Job plots of L1 and CN–.

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1
The recognition mechanism was finally investigated by ESI-MS, H-NMR
titration methods and Job plot of L1-CN−. As shown in Figure 10, the –OH peak at
11.25 disappeared when adding 0.5 equiv. of CN– to the DMSO-d6 solution of L1,
which clearly showed that –OH undergo deprotonation. With the increasing amount of
CN−, the proton chemical of aromatic gradually showed upfield shift that indicated
the aromatic of protons were shielded by the increase of electron density through
charge delocalization in the conjugated system.
1
Figure 10 H-NMR spectra of free L1 (DMSO-d6) and in the presence of different
amounts of CN−
Moreover, ESI-MS further gave a strong support to above result. As shown in
Fig. S2, L1 ion peak was detected at m/z 371.17 (ESI. L1+H+), and it appeared at m/z
413.2716 (ESI. L1-2H++2Na+-H+) when the addition of CN− to the solution of L1
(Fig. S3), simultaneously, according to the stoichiometry of L1-CN−, which clearly
suggested that L1 did undergo deprotonation and lost two protons.
This deprotonation caused a high charge separation between acceptor and donor
units in L1, which led to a bathochromic shift in the absorption band with ∆λ=88 nm
nm for L1 upon the addition of CN−. Moreover, we observed that the fluorescence

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intensity at 554nm was quenched, which was possibly caused by a large charge
separation resulting in a strong intramolecular charge transfer (ICT) in L1.
Furthermore, according to the change of mass spectrum after the addition of CN− and
the stoichiometry of L1-CN− 1:2, we proposed that a possible interaction mechanism
between L1 and CN−. (Scheme 2)
Scheme 2 A possible mechanism of L1 and CN−.
In order to realize quick response of sensor L1 for cyanide, test strips were
fabricated by immersing filter papers into the DMSO solution of L1 (2×10−3 M) and
then drying in air, as shown in Figure 11, the color of test strip changed from faint
yellow to red under visible light; and the fluorescence color also changed from yellow
to black under UV lamp. Herein, the test strips could be used to conveniently detect
CN− in solutions.
Figure 11 Photographs of test strips containing L1 under different conditions; (A)
only L1 (B) L1 with CN− by naked eyes, (C) only L1 under UV lamp, (D) L1 with
CN− under UV lamp.

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Finally, we have investigated its applicability of L1 in our lives. 30 g of crushed
bitter almond were put into a flask by adding 100 mL of water and 0.5 g of NaOH.
And the mixture solution was filtered after stirring 20 min. Then we made the pH=9
which diluted the solution with fresh double water. When addition of the solution into
L1, we found that the fluorescence of L1 quenched. This result indicated that could be
applied successfully for detecting CN− in bitter almond.
Figure 12 Fluorescence spectral response of L1 (20 µM) in diluted bitter almond.
Comparison with reported CN− receptors, L1 could detect CN− in solution with
high water content, as shown in Table S1, so L1 could act as a preferable
fluorescence sensor for CN−.
4.1. Theoretical Calculations
Figure 13 The DFT computed HOMO and LUMO diagram of L1 and L1-CN−

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complex.
The mechanism of sensor L1 with CN− was further confirmed by DFT (Density
Functional Theory) calculations. As shown in Figure 13, the HOMO–LUMO energy
band gap of L1 and L1-CN− complex were 337.59 kcal/mol and 283.34 kcal/mol
respectively, it is obviously that the former was higher than the latter, which led to the
red shift of L1 in absorption spectra.
5. Conclusion
In summary, we have designed and synthesized a colorimetric and fluorescence
sensor L1, which could selectively sense CN− in DMSO/H2O (v/v=4:1) solution
within the basic pH range (pH=7.0–8.0), and the detection limit of sensor L1 to CN−
is down to 9.47 ×10-7 M from fluorescence spectrum changes. The CN− induced
fluorescence process could be reversed by adding H+ and the switching process could
be repeated more than six times without a large fluorescence loss. Moreover, test trips
based on sensor L1 could be utilized to detect CN− in aqueous solution. Above all, the
probe was successfully applied to the detection of cyanide in bitter almond. On
account of those advantages, we believe that L1 as a CN−sensor makes it more
conspicuous for its potential applications.
Acknowledgment
We gratefully acknowledge the support of the Nature Science Foundation of
China (No. 21467012).
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A novel dual-channel chemosensor for CN− using
asymmetric double-azine derivatives in aqueous media and
its application in bitter almond
Peng-Xiang Pei, Jing-Han Hu*, Ying Chen, You Sun, Jing Qi
Graphical abstract: A novel cyanide specifically selective and highly sensitive
fluorescence enhanced chemosensor L1 based on asymmetric double-azine
derivatives had been designed and synthesized.

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Highlights
1 Chemosensor L1 is based on asymmetric double-azine derivatives.
2 Sensor L1 demonstrates specifically selective “naked eye” recognition and
fluorescence response for CN−
3 Receptor L1 shows highly selective colorimtric and fluorimetric response for CN−.
4 The probe L1 responds with CN− in aqueous solution system.
5 The probe L1has been detected successfully CN− in bitter almond.
6 Test strips of L1 was prepared and convenient and rapidly detected CN−.