A novel colorimetric and fluorescent probe based on indolium salt for detection of cyanide in 100% aqueous solution

Article abstract of DOI:10.1016/j.dyepig.2019.04.059

We reported a novel indolium salt -based colorimetric and fluorescent probe N1 for cyanide. The probe N1 showed a bleaching solution and fluorescence quenching toward CN^? in 100% aqueous solution over other anions. In UV-vis and fluorescent spectrometry method, a large blue shift (261 nm), color bleaching and fluorescence quenching was observed. The minimum detection limit on fluorescence response of the probe N1 towards CN^? was 3.34 × 10^?7 mol/L. This probe N1 operates via intramolecular charge transfer (ICT) mechanism, because the nucleophilic attacking of CN^? to the indol C[dbnd]N, the ICT progress was blocked with color changed and fluorescence quenched, which is further confirmed by ¹H NMR, MS and DFT studies. Therefore, the synthesized probe would be a promising device for the detection of cyanide.

Full text of DOI:10.1016/j.dyepig.2019.04.059

Accepted Manuscript
A novel colorimetric and fluorescent probe based on indolium salt for detection of
cyanide in 100% aqueous solution
Junjian Li, Zengyan Chang, Xihao Pan, Wei Dong, Ai-Qun Jia
PII:
S0143-7208(19)30553-4
https://doi.org/10.1016/j.dyepig.2019.04.059
DYPI 7514
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 13 March 2019
Revised Date: 24 April 2019
Accepted Date: 24 April 2019
Please cite this article as: Li J, Chang Z, Pan X, Dong W, Jia A-Q, A novel colorimetric and fluorescent
probe based on indolium salt for detection of cyanide in 100% aqueous solution, Dyes and Pigments
(2019), doi: https://doi.org/10.1016/j.dyepig.2019.04.059.
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Graphical Abstract
A novel colorimetric and fluorescent probe based on indolium
salt for detection of cyanide in 100% aqueous solution
Junjian Li a, b, Xihao Pan c, Wei Dong c , Ai-Qun Jia a,b
a State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan
University, Haikou 570228, China
b Key Laboratory of Tropical Biological Resources of Ministry Education, Hainan
University, Haikou 570228, China
c School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
# the two authors contributed this work equally.
E-mail address: weidong@njust.edu.cn
E-mail address: jiaaiqun@gmail.com

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A novel colorimetric and fluorescent probe based on indolium salt for
detection of cyanide in 100% aqueous solution
Junjian Li a, b, Zengyan Chang a, b, Xihao Pan c, Wei Dong c , Ai-Qun Jia a,b
a State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University,
Haikou 570228, China
b Key Laboratory of Tropical Biological Resources of Ministry Education, Hainan University,
Haikou 570228, China
c School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing
210094, China
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Abstract：We reported a novel indolium salt -based colorimetric and fluorescent
probe N1 for cyanide. The probe N1 showed a bleaching solution and fluorescence
quenching toward CN- in 100% aqueous solution over other anions. In UV-vis and
fluorescent spectrometry method, a large blue shift (261 nm), color bleaching and
fluorescence quenching was observed. The minimum detection limit on fluorescence
response of the probe N1 towards CN- was 3.34× 10-7 mol/L. This probe N1 operates
via intramolecular charge transfer (ICT) mechanism, because the nucleophilic
attacking of CN- to the indol C=N, the ICT progress was blocked with color changed
1
and fluorescence quenched, which is further confirmed by H NMR, MS and DFT
studies. Therefore, the synthesized probe would be a promising device for the
detection of cyanide.
# the two authors contributed this work equally.
E-mail address: weidong@njust.edu.cn
E-mail address: jiaaiqun@gmail.com

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Keywords: cyanide; water soluble; intramolecular charge transfer; nucleophilic
addition
1. Introduction
24 Cyanide plays a critical role in several industries such as organic synthesis,
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metallurgical engineering, electroplating technology and other fields due to its special
characteristics. However, it is still extremely damaging to human beings even at very
low concentrations for 0.5 mg per kilogram of body weight, which leads to cellular
asphyxiation and death [1-5]. Hence, in human terms, it’s very urge to develop
reliable and efficient approaches to detect cyanide in environmental systems [6, 7].
Up to now, several traditional detection methods, such as titrimetric, electrochemistry
and spectroscopy [8-11], have been devised for the detection of CN-. With the
development of analytical instruments, the devised and prepared probes for
colorimetric and fluorescent detection of cyanide have gained increasing attention
because of their outstanding properties [12, 13]. Compared with various traditional
approaches, colorimetric and fluorescent probes are regarded as the powerful tools for
detection method owing to their simplicity, high sensitivity and naked eye detection
[14-17]. And lots of fluorescent probes have been reported for CN- detection so far.
The above probes had been designed on the basis of various chemical reaction
mechanisms, nucleophilic addition reaction [18-23], supramolecular self-assembly
[24], hydrogen bonding motifs [25-28], cyanide complexation addition[9], electron
deficient alkenes [14, 29, 30], and so on. However, the majority of these probes are
inclined to poor solubility in water [31-35]. Thereby, there is a significant challenge

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to develop the CN- probes in 100% aqueous solution in biological and environmental
systems.
Herein, we synthesized a novel colorimetric and fluorescent dual-channel probes N1
with carboxylated indole as an electron-withdrawing group and dimethylamine as an
electron-donating group (Scheme 1), which showed highly sensitive detection for CN-
in 100% H2O solution (0.01 M Tris-HCl buffer, pH 7.2). The mechanism of detection
for CN- was mainly based on the nucleophilic addition reaction, which collapsed the
π-conjugation and interrupted the ICT process to form a sensitive colorimetric and
fluorescent turn-on response. The possible mechanism of the probe N1 sensing CN-
1
was confirmed by the measurements of H NMR, MS and DFT. Therefore, the
synthesized probe would be a promising device for the detection of cyanide.
Scheme 1
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2. Materials and methods
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2.1 Chemicals and Apparatus
All reagents and solvents were analytical grade and obtained commercially without
further purification. The ultrapure water was used throughout the experiment.
Tetrabutylammonium salt of anions (CN-, F-, Cl-, Br-, I-, Ac-, SO4 , H2PO4 and ClO4 )
2-
-
-
were purchased from Sigma-Aldrich. Sodium salts (S2-, SCN- and CO3 ) was
2-
obtained from Energy Chemical (Shanghai, China). 1H NMR and 13C NMR spectra
were carried out on a 500 MHz, and TMS was used as an internal standard. A
Shimadzu UV-2550 spectrometer was performed to get UV-vis spectra. Fluorescent

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spectra was achieved by use of a Shimadzu RF-5301 fluorescence spectrophotometer.
2.2 Synthesis of the probe N1
The schematic illustration of synthesis methods for probe N1 is shown in Scheme 1.
5-carboxy-1-ethyl-2,3,3-trimethyl-3H-indol-1-ium iodide (1) was prepared by
references [36]. Compound 5-carboxy-1-ethyl-2,3,3-trimethyl-3H-indol-1-ium iodide
(1) (0.72g, 2ꢀmmol), p-dimethylaminobenzaldehyde (2) (0.33ꢀg, 2.2ꢀmmol) and
piperidine (3 drop) were dissolved in ethyl alcohol (20ꢀmL), stirred and refluxed for
12ꢀh. After the above reaction cooled to ambient temperature, we removed the solvent
and refined the left residue with column chromatography to get N1 as a black red
powder.
1: Yield 75%, 1H NMR (500 MHz, DMSO) δ 13.20 (s, 1H), 8.39 (s, 1H), 8.14 (d, J =
13
32.0 Hz, 2H), 4.53 (s, 2H), 2.90 (s, 3H), 1.58 (s, 6H), 1.45 (s, 3H). C NMR (126
MHz, DMSO) δ 196.38, 171.47, 149.00, 141.11, 140.05, 125.85, 120.25, 114.23,
53.72, 42.67, 21.27, 20.54, 13.37, 12.09.
1
N1: Yield 65%, H NMR (500 MHz, DMSO) δ 13.21 (s, 1H), 8.42 (d, J = 15.3 Hz,
1H), 8.31 (s, 1H), 8.25 – 7.94 (m, 3H), 7.79 (d, J = 8.3 Hz, 1H), 7.27 (d, J = 15.4 Hz,
1H), 6.92 (d, J = 8.5 Hz, 2H), 4.54 (d, J = 6.5 Hz, 2H), 3.21 (s, 6H), 1.79 (s, 6H), 1.38
13
(t, J = 7.0 Hz, 3H). C NMR (126 MHz, DMSO) δ 180.28, 167.18, 156.38, 155.58,
144.78, 143.30, 131.00, 129.74, 124.21, 123.07, 113.54, 112.96, 104.51, 51.14, 41.00,
40.50, 40.33, 40.16, 40.00, 39.83, 39.67, 39.50, 38.93, 26.88, 13.50. MS m/z [M]+
+
C23H27N2O2 363.2073; C23H26N2O2Na+ 385.1892. FT-IR: (KBr, cm−1) νꢀ=ꢀ3465.12
(-COOH), 3049.25 (C–H), 1700.41 (C=O), 1613.18 (C=C)

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2.3 Theoretical approach of the probe N1 and N1-CN
The Gaussian 09 program was performed. The optimization of the molecular
configurations of N1 and N1-CN in the ground state and excited state were
implemented using the density functional theory (DFT) with the b3lyp methodology
containing 6-31G(d) basis sets.
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3. Results and discussion
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Compound N1 was synthesized by the aldol condensation reaction of
5-carboxy-1-ethyl-2,3,3-trimethyl-3H-indol-1-ium
iodide
(1)
and
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p-dimethylaminobenzaldehyde (2) in ethanol (Scheme 1). The N1 was prepared to
contain a strong electron withdrawing carboxylated indole derivatives and carboxyl
and an electron donating dimethylamine, which were linked via an ethylene. Carboxyl
group was selected because it is an excellent fluorophore with good
electron-withdrawing property and a good water-soluble group. Therefore, N1 can be
detected the CN- in water.
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3.1. UV–vis and Fluorescence spectroscopic studies of N1
In order to obtain the optical properties of N1 for detection of anions, the UV-Vis and
fluorescence titration experimentations of N1 were performed. The absorbance of N1
in H2O (0.01 M Tris-HCl buffer, pH 7.2) exhibited one absorption bands at 569. In the
UV-Vis spectrum, the significant changes were discovered of N1 in the presence of
CN- compared to the other anions, including F-, Cl-, Br-, I-, Ac-, SO4 , H2PO4 , ClO4 ,
2-
-
-
S2-, SCN-, CO3 , Cys and GSH. Upon the addition of 50 equiv.CN- into the H2O solution
2-

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of N1 (20µM), the absorption band at 569ꢀnm disappeared and a new band started to
grow at 308ꢀnm (Fig. 1a). In the fluorescence spectrum, the probe N1 displays a
strongly fluorescence peak at 603 nm. By addition of CN-, the fluorescence band at
603ꢀnm disappeared (Fig. 1b). However, the addition 50 equiv. of other anions, the
absorption band and fluorescence did not remarkable change, which show the fact that
probe N1 possesses a reasonable selectively in respect of cyanide anion.
(Fig. 1)
In order to obtain more information of the N1 with CN-, we carried out UV-Vis and
fluorescence titration experimentations. By gradual addition of CN-, the UV-Vis
absorption peak at 569 nm declined, while the absorption gradually increased at 308
nm. It is noteworthy that the blue shift of absorption wavelength consistent with a
pretty evident color change from amaranth to colorless demonstrates that N1 can
achieve the goal of detection of CN- with visual observation. Meanwhile, the
fluorescence titration of N1 with CN- was carried out, the emission intensity at 603
nm weakens gradually while enhancing CN- concentration, along with an obvious
fluorescence quenching. The LOD of the N1 was calculated as 3.34× 10-7 mol/L using
the formula 3σ/S. Which is far below the WHO standard of 1.9 × 10-6 mol/L cyanide.
(Fig. 2)
To study the selectivity of N1, competition experiments were carried out in the
presence of other anions. The addition of 50 equiv. of CN- and other anions, the
UV-Vis absorption band and fluorescence intensity is not lead to observable changes
in absorption at 569ꢀnm and the intensity of emission at 603ꢀnm by the presence of

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other anions (Fig. 3). These results indicated that the interference of other anions is
negligible for the selective detection of CN-.
(Fig. 3)
3.2. Plausible mechanism
1
To examine the possible mechanism of interaction between N1 and CN-, H NMR
titrations and MS methods have been implemented. 1H NMR titrations were dissolved
in DMSO-d6. The incremental addition of cyanide to the N1 caused the vanishing of
the signal corresponding to the carboxyl OH at 13.21 and all 1H NMR signals shifted
upfield. The peaks belonging to the vinylic protons H exhibited an up-field shift from
7.79 and 7.27 ppm to 6.85 and 6.57 ppm upon adding 1 equiv. CN-. Additionally, the
signals in accordance with –N–CH2–, i.e. Hc protons presented an up-field shift from
4.54 to 3.18 ppm. By mass spectral analyses, the peak of N1 was at 363.34 m/z. The
CN- was dropped into the solution of N1, and then the N1-CN peak arised at 390.37
m/z. Therefore, we thus proposed that the suggested mechanism of CN- as a
proverbial nucleophile is the strong nucleophilic attack of CN- to the -C=N group of
N1.
(Fig. 4)
3.3. Theoretical studies
The geometry of N1 was optimized using Gaussian 09 program, B3LYP methodology,
6-31G(d) basis sets. As shown in Fig. 5(a), the optimized ground-state (S0) structures,
the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular
orbitals (LUMO) of N1 and N1-CN have been showed. The HOMO of probe N1 was

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mainly distributed on the entire indol and benzene ring, demonstrating the well
delocalization of the electron cloud density and the active ICT processes. However,
when the N1 was converted to N1-CN, the conjugation is interrupted and the ICT
process is blocked. The HOMO-LUMO energy gap of N1 was calculated to be 2.69
eV, which is much lower than that of N1-CN (4.04 eV) (Fig. 5b). This building up
energy gap is responsible for the blue shift of the UV–vis of N1. The experiment
phenomenon was well in line with the theoretical results. Hence, we can summarily
conclusion that the N1 detected CN- by a nucleophilic addition reaction.
(Fig. 5)
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4. Conclusions
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In this study, we have successfully designed a simple colorimetric and fluorescent
probe N1 based on indolium-derived for detection of cyanide in 100% H2O (0.01 M
Tris-HCl buffer, pH 7.2) solution. The probe performs good anti-interference ability
and high selectivity toward CN- over other anions. The potential or possible
1
mechanism is characterized as a nucleophilic addition reaction by conducting H
NMR, MS and DFT measurements. The probe N1 exhibits high selectivity for CN-
even in the presence other anions and a desirable low LOD of 3.34× 10-7 mol/L. These
unique properties of prepared probe N1 renders its promising application for detecting
cyanide in environmental systems.
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Acknowledgement
This study was supported by the scientific research foundation of hainan university

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(KYQD(ZR)1917) and the National Natural Science Foundation of China
(41766006).
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Figure & Scheme captions
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Scheme 1 Synthetic procedures for probe N1. (a) AcOH, KOH. (b), CH3CH2I, acetonitrile,
refluxed,75%. (c) piperidine, EtOH, refluxed. 65%.
Fig. 1. (a) UV–vis absorption spectra of N1 (2.0×10−5 M) with various analytes in H2O [0.01 M
2-
-
-
-
Tris-HCl buffer, pH 7.2] in the presence of CN-, F-, Cl-, Br-, I-, Ac-, SO4 , H2PO4 , ClO4 , N3 S2-,
,
SCN-, CO3 , Cys and GSH. (b) Fluorescence spectra of N1 (2.0×10−5 M) with various analytes in
2-
H2O.
Fig. 2. (a) UV–vis spectra of N1 (2.0ꢀ×ꢀ10−5ꢀM) in H2O [0.01 M Tris-HCl buffer, pH 7.2] upon
adding an increasing concentration of CN−. Inset: Color change. (b) Fluorescence spectra of N1
upon adding an increasing concentration of CN−. Inset: Color change.
Fig. 3. (a)UV–vis spectra of N1 at 569 nm with addition of CN- in the presence of 50 equiv. of
2-
-
-
-
other anions (0 L1; 1 CN-; 2 F-; 3 Cl-; 4 Br-; 5 I-; 6 Ac-; 7 SO4 ; 8 H2PO4 ; 9 ClO4 ; 10 N3 ; 11 S2-;
2-
12 SCN-; 13 CO3 ; 14 Cys; 15 GSH.) inH2O (b). Fluorescence of N1 at 603 nm with addition of
CN- in the presence of 50 equiv. of other anions in H2O.
1
Fig. 4. Suggested mechanism for the N1 to CN−. Partial H NMR spectra of N1 and in the
presence of different equiv. of CN−.
Fig. 5. (a) The optimized ground-state structures of N1 and N1-CN (b) Frontier molecular orbitals
of N1 and N1-CN obtained from the DFT calculations using Gaussian 09 program.
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302

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Figures and schemes
Scheme 1 Synthetic procedures for probe N1. (a) AcOH, KOH. (b), CH3CH2I, acetonitrile,
refluxed,75%. (c) piperidine, EtOH, refluxed. 65%.
Fig. 1. (a) UV–vis absorption spectra of N1 (2.0×10−5 M) with various analytes in H2O [0.01 M
2-
-
-
-
Tris-HCl buffer, pH 7.2] in the presence of CN-, F-, Cl-, Br-, I-, Ac-, SO4 , H2PO4 , ClO4 , N3 S2-,
,
2-
SCN-, CO3 , Cys and GSH. (b) Fluorescence spectra of N1 (2.0×10−5 M) with various analytes in
H2O.

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Fig. 2. (a) UV–vis spectra of N1 (2.0ꢀ×ꢀ10−5ꢀM) in H2O [0.01 M Tris-HCl buffer, pH 7.2] upon
adding an increasing concentration of CN−. Inset: Color change. (b) Fluorescence spectra of N1
upon adding an increasing concentration of CN−. Inset: Color change.
Fig. 3. (a)UV–vis spectra of N1 at 569 nm with addition of CN- in the presence of 50 equiv. of
2-
-
-
-
other anions (0 L1; 1 CN-; 2 F-; 3 Cl-; 4 Br-; 5 I-; 6 Ac-; 7 SO4 ; 8 H2PO4 ; 9 ClO4 ; 10 N3 ; 11 S2-;
2-
12 SCN-; 13 CO3 ; 14 Cys; 15 GSH.) inH2O (b). Fluorescence of N1 at 603 nm with addition of
CN- in the presence of 50 equiv. of other anions in H2O.

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1
Fig. 4. Suggested mechanism for the N1 to CN−. Partial H NMR spectra of N1 and in the
presence of different equiv. of CN−
Fig. 5. (a) The optimized ground-state structures of N1 and N1-CN (b) Frontier molecular orbitals
of N1 and N1-CN obtained from the DFT calculations using Gaussian 09 program.

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Highlights
1. A cyanide probe based on a conjugated indolium system.
2. The probe recognizes CN- selectively in 100% aqueous solution
3. The detection of cyanide with probe can be performed clearly by color changes