A fluorescent turn-on probe based on benzo [E] indolium for cyanide ion in water with high selectivity

Article abstract of DOI:10.1007/s10895-013-1258-y

A highly selective fluorescent turn-on probe for CN^- bearing a benzo [e] indolium group as a fluorophore and binding site was reported. The detection of CN^- was performed via the nucleophilic attack of CN ^- toward the benzo [e] indolium group of the probe, resulting in a prominent fluorescence enhancement and an obvious color change. The probe shown a highly selectivity in water and could be performed in a range of pH value between 6 and 9. The detection limit was 4.6 × 10^-8 M, which could be detected CN^- in drinking water. A simple paper test strip system for the rapid monitoring of CN^- was developed.

Full text of DOI:10.1007/s10895-013-1258-y

J Fluoresc
DOI 10.1007/s10895-013-1258-y
ORIGINAL PAPER
A Fluorescent Turn-On Probe Based on Benzo [E] Indolium
for Cyanide Ion in Water With High Selectivity
Yue Sun & Shanwei Fan & Dong Zhao & Lian Duan &
Ruifeng Li
Received: 20 May 2013 /Accepted: 25 June 2013
#
Springer Science+Business Media New York 2013
2
+
Abstract A highly selective fluorescent turn-on probe for
Co -salen [6–8], displacement [9–13], as well as bond-
–
–
CN bearing a benzo [e] indolium group as a fluorophore
forming reaction between the CN and either an electrophilic
–
and binding site was reported. The detection of CN was
carbon [14–32] or a boron center [33–35]. However, only a
few probes can perform in water for the poor water-solubility
of most probes [29, 36, 37].
–
performed via the nucleophilic attack of CN toward the
benzo [e] indolium group of the probe, resulting in a promi-
nent fluorescence enhancement and an obvious color change.
The probe shown a highly selectivity in water and could be
performed in a range of pH value between 6 and 9. The
Probes based on the chemical species induced changes in
fluorescence appear to be particularly attractive due to the
highly sensitive, quick, simple and real time monitoring of
the fluorescence. Many fluorescent and colorimetric probes
bearing an indoline moiety have been employed to detect
−8
detection limit was 4.6×10 M, which could be detected
–
CN in drinking water. A simple paper test strip system for
–
–
the rapid monitoring of CN was developed.
CN for its highly reactive binding site and high selectivity
[
18, 24, 28, 30, 37–41]. However, the fluorescence of
.
.
Keywords benzo [e] indolium Cyanide-sensing
indoline is rather weak, and another fluorophore is essential
to ensure the probe could be detected on a fluorescence
spectrophotometer. Undoubtedly, the long synthetic path-
ways make the probes difficult to prepare, and also make
the water-solubility of the probes worse. Here, we introduced
a new fluorescence turn-on probe 1 bearing a positively-
.
Chemosensor Fluorescent turn-on
Introduction
–
–
Cyanide (CN ) is extremely toxic to mammals with even a
small amount, and it can be absorbed through the lungs,
gastrointestinal track and skin, leading to vomiting, convul-
sion, loss of consciousness, and eventual death. The highest
charged benzo [e] indolium fragments for detecting CN in
water with high selectivity and sensitivity. CN is expected
–
to be detectable by nucleophilic attack toward the carbon
atom of the C=N group, which is activated by the strong
electron-withdrawing feature of the positively-charged benzo
[e] indolium fragments. We were attracted to benzo [e]
indolium because (1) the benzo [e] indoline possesses an
obvious fluorescence emission caused by a large π-
–
allowable level of CN in drinking water is only 1.9 μM
according to the World Health Organization (WHO) [1–3].
Nevertheless, cyanide has been produced in large quantities
and used in various industrial processes, such as synthetic
fibers, resins, herbicide and the gold-extraction [4], which
has led to environmental contamination as well. Therefore,
there is an emergency requirement to develop efficient and
–
conjugation after the addition of CN ; (2) any other
fluorophore is unnecessary, the probe is easy-to-make, and
(3) the positively-charged benzo [e] indolium fragments make
the probe water-soluble.
–
simple sensing systems to monitor CN from the contaminant
sources. Multiple probes for CN have been developed over
–
past 10 years through coordination ability and the nucleophilic
–
2+
2+
reactivity of CN [5], such as Zn -porphyrin, Ru -pyridine,
Experimental Section
Apparatus and Materials
:
:
:
:
Y. Sun S. Fan D. Zhao L. Duan (*) R. Li
School of Chemistry and Chemical Engineering, Taiyuan
University of Technology, Taiyuan 030024, China
e-mail: duanlian@tyut.edu.cn
Absorption spectra were taken on a Shimadzu UV-1800 spec-
trophotometer. Fluorescence spectra were taken on a Hitachi F-

J Fluoresc
CH3I
p-CH3OC6H4CHO
O
CH3CN
I-
EtOH
N
I-
N
N
2
1
Scheme 1 Synthesis route of probe 1
1
13
2
700 fluorescence spectrometer. H NMR and C NMR mea-
Results and Discussion
surements were recorded at 600 and 150 MHz on a Brucker
Avance 600-MHz spectrometer, respectively. Dimethyl sulf-
Cyanide Binding Studies of 1
oxide (DMSO-d ) was solvent, and tetramethylsilane (TMS)
6
1
was used as internal standard. The following abbreviations
were used to explain the multiplicities: s = singlet; d = doublet;
m = multiplet. ESIMS were taken on a Fourier transform ion
cyclotron resonance mass spectrometry (Varian 7.0 T).
All reagents and solvent were purchased from commercial
source and used without further purification, if not otherwise
stated. All reactions were carried out on the magnetic stirrers
and their reaction process was monitored on thin layer chro-
matography (TLC).
H NMR analysis was firstly carried out to demonstrate the
proposed addition mechanism. The solution of probe 1
-3
(3×10 M) in DMSO-d (450 μL) and D O (50 μL) was
6
2
placed in the NMR tube, and potassium cyanide powder
(1 equiv. and 2equiv) was added. All the potassium cyanide
powder was soluble. As anticipate, after addition of potassi-
um cyanide powder (1 equiv.) into the solution of 1, the
–
nucleophilic attack of CN toward the positively-charged
benzo [e] indolium group weakened its electron-
1
withdrawing character and lead all the H NMR signals up-
a
Synthesis
field shifted (Fig. 1). It is clearly that the proton signal (H , at
+
δ 4.23) of methyl group connected with N was dramatically
Preparation of 2
2
, 3, 3-Trimethylbenz [e] indole (10.0 g, 47.78 mmol) was
dissolved in acetonitrile (50 mL), then methyl iodide (3.57 mL,
7.34 mmol) was added and the reaction mixture was heated at
Hb
H3C
Hb
CH3
Hb'' Hb'
H3C
CH3
O
O
CN-
5
I-
N
CN
N
reflux for 1 h with vigorous stir. The mixture was cooled to
room temperature and the precipitate was collected by filtra-
tion, washed with diethyl ether, and dried in vacuo. Product 2
CH3
CH3
a
Ha'
H
1
was obtained as a white solid in 86 % yield (14.41 g). H NMR
(
600 MHz, DMSO-d ): δ 8.36 (d, 1H, J=8.2), 8.29 (d, 1H,
6
J=8.8), 8.22 (d, 1H. J=8.2), 8.10 (d, 1H, J=8.8), 7.78 (m, 1H),
.72 (m, 1H), 4.09 (s, 3H), 2.87 (s, 3H), 1.75 (s, 6H).
7
a
b
Preparation of Probe 1
(1.05 g, 3 mmol) was dissolved in ethanol (5 mL), then 4-
methoxybenzaldehyde (0.36 g, 3 mmol) was added and the
reaction mixture was heated at reflux for 12 h. When the
mixture was cooled to room temperature and the precipitate
was collected by filtration, washed with cold ethanol, and
2
dried in vacuo. Probe 1 was obtained as an orange solid in
1
5
2
7
6
1
1
2
6 % yield (0.8 g). H NMR (600 MHz, DMSO-d ): δ 8.47 (m,
6
H), 8.28 (m, 3H), 8.23 (d, 1H), 8.11 (d, 1H), 7.77 (m, 2H),
.61 (d, 1H), 7.19 (m, 2H), 4.27 (s, 3H), 3.92 (s, 3H), 2.02 (s,
H). C NMR (150 MHz, DMSO-d ): δ 185.7, 167.1, 155.4,
42.8, 141.0, 136.4, 136.3, 134.2, 133.9, 133.4, 131.7, 130.8,
30.4, 130.0, 126.5, 118.3, 116.6, 113.5, 59.2, 56.9, 38.3, 28.7,
4.7. HRMS (ESI) calcd. for [1] 342.1852, found 342.1855
1
3
6
Fig. 1 1H NMR spectral change of 1 (3×10−3 M) in the absent (a)
and present of 1 equiv. of potassium cyanide (b) in DMSO-
+
Scheme 1.
6 2
d :D O=9:1 (v/v).

J Fluoresc
b
shifted up-field to δ 2.83. Moreover, the proton signal (H , at
δ 2.00) of two methyl groups was also shifted up-field and
7
6
00
50
b’
b”
divided into two single signals (H , at δ 1.78 and H , at δ
.24), which become non-equivalent after formation of 1-
1
CN. The unvaried integral intensity ratio between the
methoxyphenyl and benzo [e] indolium moiety and the
methyl signals indicates that the formation of 1-CN keeps
600
1
them intact. The H NMR signals remained essentially
0
50
100
150
200
250
unchanged when more than 1 equiv. cyanide was added into
the solution, suggesting the 1: 1 binding stoichiometry be-
tween 1 and CN . In addition, the formation of the 1-CN was
Time/s
–
Fig. 3 Response time of reaction between 1 (10 μM) and CN (3 equiv.).
lex=345 nm. Slits: 5 nm/ 5 nm
–
further confirmed by mass spectrometry analysis, in which
the peak at m/z 407.1520 (calcd. = 407.1512.) corresponding
to [1 + CN + K] was clearly observed (Fig. 2).
+
–
ensure complete reaction between 1 and CN at room tem-
perature (Fig. 3).
Spectral Properties
Figure 4 illustrates the absorption (A) and fluorescence
B) spectral changes for probe 1 upon addition of CN in
–
(
Fluorescence and UV–vis absorption studies were
performed using 10 μM and 20 μM solution of 1 in deion-
ized water with appropriate amounts of anions (as their Na
water. The absorption spectrum of 1 (20 μM) exhibits an
obvious broad band at 445 nm, which is ascribed to the typical
intramolecular charge transfer (ICT) band from the methoxy
group to the positively-charged benzo [e] indolium fragments.
+
salt, 10 mM and 100 mM). The excitation wavelength was
–
3
45 nm. The excitation and emission slit width were 5 nm
Upon addition of CN , the band at 445 nm was gradually
and 5 nm, respectively. The detection was delayed 4 min to
attenuated until saturation after 7 equiv., suggesting that the
Fig. 2 The ESIMS spectral of 1 and 1-CN

J Fluoresc
a
b 1000
1000
8
6
4
2
00
00
00
00
0
0
0
0
0
.6
.4
.2
.0
8
6
4
2
00
00
00
00
0
0
20 40 60 80 100 120 140 160
Concentration/µM
2
50
300
350
400
450
500
550
400
450
500
550
Wavelength/nm
Wavelength/nm
c 2000
d
80
1
1
500
000
70
60
50
40
5
00
0
0
.0
0.2
0.4
0.6
0.8
1.0
0.000000
0.000005
0.000010
_
_
[
CN ]:[CN ]+[1]
Concentration/M
–
Fig. 4 a Absorption spectra of 1 (20 μM) upon the addition of CN in
the following order: 0, 5, 10, 15, 20, 30, 40, 50, 70, 90, 110, 130,
–
water. CN concentrations were varied in the following order: 0, 10, 20,
150 μM. (C) The titration probe evaluated from fluorescent intensity at
–
3
0, 40, 50, 60, 70, 80, 90, 100,120, 140, 160, 180, 200 μM. Inset:
photos of 1 (20 μM) in the absence (left) or presence (right) of CN
100 μM) under the day light. b Fluorescence spectra of 1 (10 μM)
425 nm. Job’ plot for determining the stoichiometry of 1 and CN ([1]-
–
–
–
+[CN ]=100 μM). (D) The linear relation for concentration of CN in
the range of 0–10 μM. lex=345 nm. Slits: 5 nm/ 5 nm
(
–
–
upon the addition of CN in water. CN concentrations were varied in
–
ICT process is blocked due to the nucleophilic attack of CN ;
shown in Fig. 5. At pH values lower than 5, no obvious
meanwhile, an obvious color change from yellow to colorless
was also observed (inset of Fig. 4a). Moreover, a clear
isosbestic point was also noted at 343 nm, indicating the
characteristic fluorescent of 1 could be observed regardless
–
of the presence and absence of CN , indicating that no
–
reaction occurs between 1 and CN . This could be attributed
–
–
formation of the 1-CN adduct. Only 7 equiv. of CN used in
to the protonation of CN decreases the actual concentration
–
–
water is impressive compared to many reported CN probes
of CN in the sample solution. Between pH 6 and 9, upon
–
–
–
require high equivalents of CN to reach a maximal spectral
addition of CN , 1 responded stably to CN without any
signal. Figure 1b shows the fluorescence spectra of 1 (10 μM)
after addition of different amounts of CN in water. The
interference by protons. At pH higher than 9, the fluorescent
–
aqueous solution of 1 was almost non-emissive in the absence
of CN . However, the fluorescence increases gradually after
addition of cyanide ions. The fluorescence at 425 nm
1200
1
–
-
1+CN
9
00
belonged to benzo [e] indoline increases by more than 90 -
fold when the concentration of CN reaches 7 equiv. in the
–
600
solution of 1. Job’ plot analysis of the fluorescence titrations
3
00
0
revealed a maximum at about 0.5 mol fraction suggesting 1: 1
–
binding between 1 and CN (Fig. 4c). The titration profile at
–
0
2
4
6
8
10
12
4
25 nm versus concentration of CN is shown in the inset of
pH
–
Fig. 4b, and the association constant for CN was estimated to
3
2
Fig. 5 Changes in fluorescence spectra of 1 (10 μM) in the absence and
present of CN (10 equiv.) in water as a function of pH. pH value were
adjusted by the dilute solutions of NaOH and HCl, respectively.
lex=345 nm. Slits: 5 nm/ 5 nm
be 5.2×10 (R =0.9914).
–
The fluorescent spectral response of 1 in the absence and
–
presence of CN at different pH values was also evaluated as

J Fluoresc
intensity at 425 nm of 1 increased dramatically. This could
–
be ascribed to the nucleophilic attack by OH to 1. The
–
results indicate that 1 can successfully react with CN and
–
allows CN detection at a range of pH values from 6 to 9.
Detection Limit Studies
Fig. 7 Color changes of the filter paper containing 1 treated with
The detection limit for 1 was calculated based on the fluo-
–
–
–
–
–
various anions: a (1), b (CN ), c (F ), d (Cl ), e (Br ), f (I ), g
rescence titration. To determine the S/N ratio, the emission
–
–
–
–
–
2–
(
(
3 4 3 4
AcO ), h (SCN ), i (N ), j (ClO ), k (NO ), l (SO ), m
–
–
intensity of 1 without CN was measured by 10 times and the
2–
3–
2–
4 4 4 2 4
HSO ), n (PO ), o (HPO ) and p (H PO )
standard deviation of blank measurements was determined.
Under the present conditions, a good linear relationship
–
between the fluorescence intensity and the CN concentra-
enhancement was observed at 465 nm which was attributed
to the ICT process from HS to the naphthalene moiety. Thus,
−5
tion could be obtained in the range of 0–1×10
R =0.9876), as shown in the inset of Fig. 4d. The detection
limit was then calculated with the equation: detection
M
2
–
(
probe 1 shows a high selectivity toward to CN . Moreover, in
–
the presence of the competing anions, the CN still resulted
limit=3σ /m, where σ is the standard deviation of blank
measurements, m is the slope between intensity versus sam-
in similar intensity changes (Fig. 6b).
Filter Paper Test
bi
bi
ple concentration. The detection limit was measured to be
8
4
.6×10− M. This means that our proposed methods based
–
on probe 1 is sensitive enough to monitor CN concentra-
A preliminary paper test strip system was constructed as
shown in Fig. 7. The neutral filter papers were dipped into
the stock solution of 1 (10 mM) and dried. Then, the pre-
pared test papers were dipped into the various solutions of
tions in water.
Selectivity Experiment
–
anions (1 mM). Only CN made the color fade from dark
–
The fluorescent intensity of 1 (10 μM) caused by CN (5
yellow to colorless. This phenomenon demonstrated that
equiv.) and other miscellaneous competing species (100
probe 1 could be used as a simple paper test for the rapid
detecting CN .
–
–
–
–
–
–
–
–
–
equiv.) including F , Cl , Br , I , AcO , SCN , N , ClO ,
3
4
–
2–
2–
3–
2–
–
–
NO , SO , HSO4 , PO4 , HPO4 , H PO and HS in
3
4
2
4
water were demonstrated in Fig. 6. It can be seen that these
–
–
2–
3–
competitive species, just like F , AcO , HSO , PO4
,
Conclusion
4
2
–
–
HPO4 and H PO which often show strong interference
to CN detection, did not disturb the fluorescence emission
2
4
–
In summary, probe 1 bearing a benzo [e] indolium group as
the binding site and fluorophore was synthesized, and its
absorption and fluorescence spectral properties in the presence
–
of 1. HS could attack to the same binding site as its nucle-
–
ophilicity is comparable to CN [42], and an intensity
a 900
b 900
600
300
0
6
3
00
00
0
Fig. 6 a Fluorescence intensity at 425 nm of 1 (10 μM) upon addition of various anions (100 equiv.). b Intensity changes in the presence of the
–
competing anions (100 equiv. respectively) followed by CN (5 equiv.). lex=345 nm. Slits: 5 nm/ 5 nm

J Fluoresc
1
1
6. Jo J, Lee D, Am J (2009) Turn-on fluorescence detection of cyanide
in water: Activation of latent fluorophores through remote hydrogen
bonds that mimic peptide β-turn motif. Chem Soc 131:16283–16291
7. Kim S, Hong S, Yoo J, Kim S, Sessler JL, Lee CH (2009) Strapped
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change upon the addition of CN was observed in water
–
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−8
selectivity in water at a very low concentration (4.6×10 M),
which were below the WHO detection level. Moreover, 1
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3
629
1
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–
the rapid monitoring of CN .
Acknowledgments This work was supported by Postgraduate Inno-
vation Foundation of Taiyuan University of Technology and Youth
Foundation of Taiyuan University of Technology (No. 2012 L018 and
No. 2012 L062).
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