A coumarin-connected carboxylic indolinium sensor for cyanide detection in absolute aqueous medium and its application in biological cell imaging

Article abstract of DOI:10.1016/j.saa.2019.117710

Fluorescent sensor has been noticed in detecting system due to its sensitive, selective, operational simplicity and low cost. We designed a coumarin-connected carboxylic indolium sensor molecule that is water-soluble and cytomembrane-permeable. This infrared (IR) emitter is selectively sensitive towards cyanide detection in aqueous media according to CN^? nucleophilic attack on the indole C=N function. Upon the addition of CN^? anion, the color of sensor in water varied from blue to colorless by naked eyes and fluorescence quenching was observed by spectroscopic method. This was because the intramolecular charge transfer (ICT) effect occurred when the fluorescent sensor was added with CN^?. The minimum detection limit of the sensor's fluorescence response to CN^? is 4.44 × 10^?7 mol/L. Furthermore, the cytotoxicity test shows the sensor has lower cytotoxicity, and indicates that this sensor can be utilized for practical detection of trace cyanide in wastewater.

Full text of DOI:10.1016/j.saa.2019.117710

Journal Pre-proof
A coumarin-connected carboxylic indolinium sensor for cyanide detection in absolute
aqueous medium and its application in biological cell imaging
Siyao Cheng, Xihao Pan, Mingyang Shi, Ting Su, Cheng Zhang, Wei Zhao, Wei Dong
PII:
S1386-1425(19)31100-X
https://doi.org/10.1016/j.saa.2019.117710
SAA 117710
DOI:
Reference:
To appear in: Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
Received Date: 30 May 2019
Revised Date: 22 October 2019
Accepted Date: 24 October 2019
Please cite this article as: S. Cheng, X. Pan, M. Shi, T. Su, C. Zhang, W. Zhao, W. Dong, A coumarin-
connected carboxylic indolinium sensor for cyanide detection in absolute aqueous medium and
its application in biological cell imaging, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117710.
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Graphical Abstract
A coumarin-connected carboxylic indolinium sensor for
cyanide detection in absolute aqueous medium and its
application in biological cell imaging
Siyao Cheng, Xihao Pan, Mingyang Shi, Ting Su, Cheng Zhang, Wei Zhao,
*
Wei Dong
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, 210094, China.
*
E-mail address: weidong@njust.edu.cn (W. Dong)
1

A coumarin-connected carboxylic indolinium sensor for
cyanide detection in absolute aqueous medium and its
application in biological cell imaging
Siyao Cheng, Xihao Pan, Mingyang Shi, Ting Su, Cheng Zhang, Wei Zhao,
*
Wei Dong
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, 210094, China.
Abstract: Fluorescent sensor has been noticed in detecting system due to its
sensitive, selective, operational simplicity and low cost. We designed a
coumarin-connected carboxylic indolium sensor molecule that is water-soluble and
cytomembrane-permeable. This infrared (IR) emitter is selectively sensitive towards
-
cyanide detection in aqueous media according to CN nucleophilic attack on the indole
-
C=N function. Upon the addition of CN anion, the color of sensor in water varied
from blue to colorless by naked eyes and fluorescence quenching was observed by
spectroscopic method. This was because the intramolecular charge transfer (ICT)
-
effect occurred when the fluorescent sensor was added with CN . The minimum
-
-7
detection limit of the sensor's fluorescence response to CN is 4.44× 10 mol/L.
Furthermore, the cytotoxicity test shows the sensor has lower cytotoxicity, and
indicates that this sensor can be utilized for practical detection of trace cyanide in in
wastewater.
Keywords: Fluorescent sensor; water-soluble; cyanide; nucleophilic attack; cell
imaging
1
. Introduction
With the rapid development of industry, all kind of industrial pollution has been
produced. Various contaminants were released directly into rivers and lakes without
any further treatment, which caused dystrophication and soil contamination.
Furthermore, long-term accumulation of hazardous substances could lead to serious
health problems [1-5]. As we all know, cyanide is terribly hazardous material to
human tissue at very low concentration that destroys vascular, visual and metabolic
*
E-mail address: weidong@njust.edu.cn (W. Dong)
1

systems. Nevertheless, cyanides are widespread in various important industrial
applications including gold mining, electroplating, tanning and metallurgy [6-8]. It is
very urgent to get rid of the poisonous cyanide in waste water, and determination of
cyanide in wastewater samples is a key factor. Therefore, it is great of important to
develop effective techniques for the determination of cyanide in drinking water [9].
-
Up to now, some options are used for detecting CN such as titrimetric,
electrochemistry and spectroscopy [10-13]. They are sensitive and selective, but
tedious operation, insufficient detection limit and not real time. Recently, fluorescent
-
sensor has been noticed for CN ions detection, because it’s specificity, sensitivity,
operational simplicity and low cost [14-25]. Its detection mechanism contains
coordination mechanism, hydrogen bonding interaction, supramolecular self-assembly,
nucleophilic addition reaction. However, the majority of fluorescent sensors have
some drawbacks, for example, poor solubility in water, low selectivity under from
coexisting anions interference, poor application in the “naked eye” detection [26-35].
As well as lots of fluorescent probe’s emission signals can be interfered by the cell
autofluorescence, so they cannot be used in cell imaging well. Therefore, it is of vital
importance to design one that the fluorescent sensor’s emission signals discrete from
the autofluorescence of biomolecules (195-600 nm) in biological window [36-40]. It
has been confirmed that the coumarin molecule can offer enough charge transfer in
the coumarin-connected molecule to promote those sensors’ absorption and emission
in the red region so as to effectively eliminate cell autofluorescence [40-42].
In this work, we have designed and synthesized a fluorescent coumarin - indole
probe CID (as shown in Scheme 1) as a colorimetric and turn-off fluorescent probe
-
for detecting CN in absolute aqueous [43-49]. The Carboxylate group was tethered to
indole moiety for improving water solubility. Carboxylated indole unit as an electron
withdrawing group and 7-(Diethylamino) coumarin-3-carbaldehyde as an electron
donating group were covalently linked through a double bond (D-π-A). The color of
sensor CID is observed to change from blue to colorless by naked eye due to
quenching of the fluorescence because ICT mechanism is disrupted in presence of
-
CN . The spectroscopic experiments have shown that the probe CID had a fast
2

-7
kinetics, high sensitivity, good selectivity and low detection limit (4.44× 10 mol/L)
-
for selective recognition of CN in 100% aqueous solution. Also, the synthesized IR
emitting sensor (CID) is found to possess lower cytotoxicity, cytomemebrane
permeability and may be useful for detection of cyanide in biological cells by
fluorescence imaging in the spectral region 580 nm to 630 nm, where
autofluorescence of living cells is low. The binding mechanism of CID-CN was
further confirmed by the 1H NMR, MS analyses and DFT optimized structures of
CID and CID-CN adduct.
2
. Experimental
2
.1. Chemicals and Apparatus
All reagents and solvents were purchased from commercial sources without
further purification. The deionized water was used throughout the experiment. The
1
13
HNMR and CNMR spectra were taken on 500 MHz and 126 MHz respectively,
using DMSO as solvent and tetramethylsilane as an internal standard. UV-vis spectras
were detected on a Shimadzu UV-2550 spectrometer. Fluorescent spectras were
recorded at a Shimadzu RF-5301 fluorescence spectrophotometer. pH values were
achieved by a Sartorius (PB-10) digital pH meter (LeiCi, Shanghai, China).
2
.2. Preparation and characterization of sensor CID
Scheme 1. Synthesis of probe CID.
The synthesis route of CID is shown in Scheme 1. The compound (1) was
3

prepared according to literature [50]. Then compound (1) (0.232g, 1ꢀmmol) and (2)
(0.245ꢀg, 1ꢀmmol) were dissolved in ethyl alcohol (20ꢀmL), stirred and refluxed at 76
o
C for 12ꢀh. when the reaction is finished, we evaporated the solvent and purified by
recrystallization from Hexane-EtOH to get CID as a dark green powder.
1
CID: Yield 73%, H NMR (500 MHz, DMSO) δ 13.30 (s, 0H), 8.88 (s, 1H), 8.39 (d,
J = 12.3 Hz, 1H), 8.15 (d, J = 8.2 Hz, 1H), 7.90 (dd, J = 51.3, 11.9 Hz, 1H), 7.60 (d, J
=
9.0 Hz, 1H), 6.94 (d, J = 8.9 Hz, 1H), 6.74 (s, 1H), 4.48 (d, J = 6.9 Hz, 2H), 3.59 (d,
1
3
J = 6.6 Hz, 2H), 1.81 (s, 3H), 1.46 (t, J = 6.8 Hz, 2H), 1.19 (t, J = 6.6 Hz, 3H). C
NMR (126 MHz, DMSO) δ 166.95, 159.95, 158.38, 154.89, 144.26, 143.79, 133.15,
1
31.06, 130.89, 124.37, 114.66, 112.73, 112.14, 110.32, 109.43, 97.18, 51.88, 45.47,
0.50, 40.34, 40.17, 40.00, 39.84, 39.67, 39.50, 26.54, 13.31, 13.00.
4
+
+
MS m/z [M] Calcd for C H N O 459.2299, found 459.2278.
28
31
2
4
2
.3. General procedures for UV–vis and fluorescence spectrum experiments
All spectrum experiments were conducted in 100% aqueous solution. Most of the
-
-
-
-
-
-
-
-
anions (F , Cl , Br , I , AcO , H PO , HSO and ClO ) were prepared from their
2
4
4
4
-
-
-
tetrabutylammonium salt, CN , N and SCN were from their sodium salts. Any
3
change in the spectroscopy of the sensor CID with different anions will be recorded
and the concentration of the bulk solution will be maintained at 10 µM in all
experiments.
2
.4. Fluorescence quantum yield
The fluorescence quantum yield of CID was calculated by the following
equation.
Φ =Φ (D /D ) (A /A ) (η /η )
2
x
2
x
st
x
st
st
x
st
Where ‘st’ subscript represents the reference and the Φ as the quantum yield of
st
the standard. ‘x’ stands for the sample. A is the absorbance at the excitation
wavelength, η is the refractive index of the solvent used, D is the area under the
emission optical spectrum. Rohidamine B (Φ =0.89 in water) at pH 7.4 as the
criterion.
4

2
.5. Cell culture and imaging
B16-F10 cells were cultured in RPMI-1640 supplemented with 10% FBS (Fetal
Bovine Serum), 100 µg/mL penicillin and 100 µg/mL streptomycin at 37 ℃ in an
atmosphere of 5% CO and 95% air for 24 h. Before the experiment, the cells well
2
placed in 6-well plate, followed by adding the sensor CID (10 µM). The cells were
incubated at 37 ℃ in environment of 5% CO and 95% air for 3 h, and then washed
2
-
three of the holes with PBS for three times. Subsequently, 20 µM CN were injected
into the three holes and incubated for 20 min to enter recipient cell. Subsequently, the
-
cells contained sensor and CN were washed by PBS buffer for three times to
eliminate floating colors. Then the detection results were verified by taking
fluorescence images with the red channel of the fluorescence microscope (λem = 570–
6
20 nm).
3
. Results and discussions
In the design process, we are committed to develop a sensor with water-soluble,
-
cytomembrane-permeable, IR emitting, and sensitive detection of CN in order to
-
specifically bind CN- in 100% aqueous solution and detect the content of CN in
biological and environmental systems. The synthesis of CID by aldol condensation
mechanism were outlined in Scheme 1. In order to detect the sensitivity and
-
selectivity of CID toward CN , a series of spectral experiments about CID were
-
-
-
−
-
-
-
carried out in the present of different anions (F , Cl , Br , I , AcO , H PO , HSO ,
2
4
4
-
-
-
-
-
ClO , CN , N and SCN ). With the addition of CN , spectral experiments showed that
4
3
-
CID could measure the concentration of CN . We also explored the performance of
-
the sensor CID in detecting CN in biological cells.
3
.1. Anion sensing properties in absolutely aqueous solution
In order to study the ability of specific recognition of sensors CID, we conducted
experiments with UV-vis spectrophotometry and fluorescence spectrometer. The
maximum absorption bond of sensor CID was at 584 nm in H O (0.01 M Tris-HCl
2
5

buffer, pH 7.2). From Fig. 2a, it showed that a significant absorbance decrease was
-
-
-
-
-
-
observed in presence of CN and the other anions, including F , Cl , Br , I , AcO ,
−
−
-
-
-
-
HSO , H PO , ClO , N and SCN (50 equiv). With the addition of CN to CID
4
2
4
4
3
solution, the absorption peak at 584 nm dramatically decreased and a new peak
suddenly appeared at 425 nm. In the fluorescence spectrum, the sensor CID had
-
strongly fluorescence in red region (λem max= 642 nm). Only addition of CN , the
fluorescence emission was at 642 nm turn-off in Fig. 1b. From Fig. 1a insert and Fig.
1
b insert, it observed that the color of solution varied from blue to colorless with
naked eye and the color of solution varied from red to yellow under 365 nm UV lamp
-
irradiation after upon addition of CN . Under the same conditions, other anions had no
significant effect on UV–vis spectra and fluorescence spectra so that the sensor CID
-
had a promising application for selectively detecting CN .
−
5
Fig. 1. (a) UV–vis absorption spectra of CID (1.0×10 M) with various analytes in H O [0.01 M
2
-
-
-
-
-
-
2-
3-
-
Tris-HCl buffer, pH 7.2] in the presence of CN , F , Cl , Br , I , Ac , HSO , H PO , ClO ,
4
2
4
4
-
-
−5
N and SCN . Inset: Color change. (b) Fluorescence spectra of CID (1.0×10 M) with various
3
analytes in H O. (λ = 500 nm, slit: 5 nm/10 nm). Inset: Color change.
2
ex
-
3
.2. The spectral response of sensor CID to CN -titration
6

-
The UV–vis titration and fluorophotometric titration of sensor CID toward CN (0.1M)
-
were carried out in aqueous solution. When the concentration of CN increased in the
bulk solution contained the CID (10 µM), an obvious reduction of the UV–vis
absorption peak at 584 nm and a significant increasing of absorbance at 425 nm
occurred (Fig. 2a). Moreover, the calculated limit of detection for the UV–vis for
-
−7
sensing CN according to the equation 3σB/S was estimated to be 3.35 × 10 mol/L,
which was far below the World Health Organization (WHO) permissive level for
-
cyanide concentration 1.9 μmol/L in drinking water. After 1.0 eq CN was added, the
fluorescent emission intensity a t 642 nm rapidly weakened until the fluorescence of
sensor CID was quenched. From Fig. 2c-d, it showed that the absorbance and
-
fluorescence intensity both appeared huge mutation at a function of [CN ] / [µL]. In
-
Fig. 2c insert, the absorbance of CID displayed good linearity with CN -titration with
the linear regression coefficient of 0.97597. Besides, Fig. 2d insert indicated that the
-
fluorescence intensity of CID and the function [CN ] / [µL] was a perfect linear
2
relation (R =0.98177). The fluorescence quantum yield of CID was calculated (Φ
x
=0.151). Therefore, the results indicated that CID was a promising fluorescent sensor
-
for quantitative determination of CN in absolutely aqueous solution.
7

−
5
Fig. 2. (a) UV–vis spectra of CID (1.0ꢀ×ꢀ10 M) in H O [0.01 M Tris-HCl buffer, pH 7.2] upon
2
-
adding an increasing concentration of CN . Inset: Color change. (b) Fluorescence spectra of CID
-
upon adding an increasing concentration of CN . Inset: Color change. (c) Changes of absorbance
-
-
intensity at 584 nm with the addition of CN (0.1 M). Inset: the good linearity in titration CN 0 - 8
2
-
μLꢀ(R = 0.97597). (d) Changes of fluorescence intensity at 642 nm with the addition of CN (0.1
-
2
M); insert: the good linearity in titration CN 0 - 8 μLꢀ(R = 0.98177).
3
.3. Selectivity studies
In order to prove the selectivity response of CID, but no response to other anions
-
-
-
-
-
-
-
-
-
-
-
including F , Cl , Br , I , AcO , H PO , HSO , ClO , CN , N and SCN . The contrast
2
4
4
4
3
-
experiments were performed with 50 equiv. of CN and 50 equiv. of other anions in
1
00% water solution [51, 52]. From Fig. 3a, 3b, their absorbance and fluorescence
-
intensity were informed that the anti-interference ability of CID to CN in the
presence of other commonly competitive anions was outstanding and the CID could
-
apply in complicated waste water for detecting CN .
8

-
Fig. 3. (a) UV–vis spectra of sensor CID at 584 nm with addition of CN in the presence of 50
-
equiv. of other anions in H O. (b) Fluorescence of CID at 642 nm with addition of CN in the
2
presence of 50 equiv. of other anions in H O.
2
1
-
Fig. 4. HNMR spectra of sensor CID in DMSO-d6 before and after the addition of CN .
9

Fig. 5. The main contribution of the frontier molecular orbitals of CID and CID-CN.
3
.4. The stoichiometric sensing mechanism
To get insight into the plausible binding mechanism between CID and CN by
-
1
HNMR-titration experiments and MS methods. Sensor CID and
tetrabutylammonium cyanide were dissolved in DMSO-d respectively, and then
6
tetrabutylammonium cyanide solution was added into CID solution with dropwise. As
-
shown in Fig. 4, all 1H NMR signals were redshifted upon addition of CN and the
corresponding OH signal disappeared at 13.30. The vinylic protons (H and H )
a
b
presented up-field shift from 7.96 ppm, 7.61 ppm to 7.68ppm, 7.48 ppm due to
breaking of the conjugation. In addition, the chemical shifts of the N-methyl protons
(
H ) shifted from 4.49 ppm to 4.20 ppm. The binding mechanism of CID-CN was
c
further confirmed by the MS analyses, where a peak at 486.23 m/z was occurred.
These observations exposed the formation of stable anionic species CID-CN in this
process.
-
In order to complete the mechanism of CID upon binding with CN , we used the
Gaussian 09 program, B3LYP / 6-31G(d) method. The density functional theory (DFT)
optimized structures of CID and CID-CN have been displayed in Fig. 5. The HOMO
orbitals of CID were homogeneous diffused on coumarin unit and indolium vinylene
unit due to integrated π-conjugation and active ICT processes. After the addition of
-
CN , the π-conjugate state of CID is broken due to the transfer of electron density
1
0

from anion to the receptor CID and ICT process is blocked, showing that the electron
-
clouds of CID-CN- are localization on coumarin moiety and the band gap of CID-CN
(3.4797 eV) obviously surpassed the band gap of CID (2.4704 eV). The theoretical
results well explained the red shift phenomenon in UV–vis. Thus, we can conclude
-
that the CID detected CN by a nucleophilic addition reaction.
-
3
.5. Detection of CN in biological cells
To explore the potential biological application of sensor CID, we tested the
biocompatibility of the CID against B16-F10 cells in vitro. MTT assay is an
important evaluation criterion for biological systems. As shown in Fig. 7, the cell
viability was reached above 85%, even though the concentration of sensor CID was
-
5
0 µM. Therefore, the sensor CID had a possibility for applying in detecting CN in
biological cells. It can be seen in Fig. 7 that sensor CID could enter cells and widely
spread in the whole cytoplasmic matrix. When cells were incubated for 30 min at 37
◦
C in the presence of CID (10 µM), and then it showed vividness red fluorescence. In
contrast, there only presented a large black area when CID has been pre-processed by
-
-
CN . These results demonstrate that sensor CID could be used for CN detection in
biological cells.
Fig. 6. (a) Cell cytotoxicity effect of CID on B10-F10 cells. 1, contrast; 2, 0.01 µM; 3, 0.1 µM; 4,
1
µM; 5, 10 µM; 6, 50 µM. Data are expressed as mean values ± standard error of the mean of
three independent experiments, each performed in three triplicates.
11

Fig. 7. Fluorescence microscopy images of B16-F10 cells with CID (a, b, c); Fluorescence
-
microscopy images of B16-F10 cells with CID and CN (d, e, f). a, d shows the corresponding
brightfield image. (red channel, λ =488 nm, λ =580–630 nm).
ex
em
4
. Conclusions
In conclusion, we have successfully developed
a
water-soluble,
cytomembrane-permeable, NIR emitting Indole-Based coumarin fluorescent sensor
CID based on coumarin-connected carboxylic indolium. The sensor shows
-
noteworthy color change from blue to colorless with the addition of CN . The
-
-7
desirable LOD of sensor on fluorescence response towards CN is 4.44× 10 mol/L,
-
and the sensor has good anti-interference ability to CN over other anions in water.
Furthermore, the cytotoxicity test shows the CID has lower cytotoxicity. Due to the
sensor CID outstanding performances, the sensor CID has a great application
-
prospect in detecting CN in biological systems and environment systems.
Acknowledgement
We acknowledge the financial support by the National Natural Science
Foundation of China (NSFC) (grant number 51573078).
1
2

Conflict of interest
The authors have declared no conflict of interest.
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Highlights
1
. A simple coumarin-connected carboxylic indolinium sensor has been prepared.
. The sensor could specifically detect cyanide in absolute aqueous.
2
3
. An ICT mechanism plays a key role in the sensing properties.
. The sensor could respond to cyanide in biological cells.
4

We declare that we do not have any commercial or associative interest that
represents a conflict of interest in connection with the work submitted.