A novel phenylacetylene-indolium fluorophore for detection of cyanide by the naked eye

Article abstract of DOI:10.1039/c5ra10273a

A novel fluorescent turn-on cyanide sensor containing an indolium salt as the selective CN^- receptor was synthesized in 2 steps via Sonogashira coupling and Knoevenagel reaction. Compound 2 was developed as a new fluorometric and colorimetric sensor for cyanide with a sensing mechanism through cyanide ion addition to the indolium group. Importantly, the inclusion complexes between compound 2 and α-cyclodrextin, exhibited fluorescent turn-on detection of CN^- in 100% aqueous media with the detection limit of 1.3 μM. The turn-on signal is the result of the conversion of the conjugated hemicyanine moiety to a cyano-hemicyanine group via the addition of the cyanide ion. Using paper sensor strips, naked eye detection of cyanide is possible down to 4.59 ng mm^-2. Test strips based on compound 2 can act as a convenient and efficient CN^- testing kit.

Full text of DOI:10.1039/c5ra10273a

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Grapical abstract
Novel phenylacetylene-indolium fluorophore for rapid cyanide detection
The compound 2 having phenylene-ethynylene as fluorophore and indolium salt tips
exhibits the highest CN- recognition sensitivity in aqueous solution with the detection limit
of 1.3 μM. Using paper sensor strips, a naked eye detection of cyanide is possible down to
4.59 ng.mm-2. Test strips based on compound 2 can act as a convenient and efficient CN-
test kit.

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Novel phenylacetylene-indolium fluorophore for naked eye
detection of cyanide
Nakorn Niamnont,*a Apiwat Promchat,b Chutima Siangma,a Chuenjai Pramaulpornsatit,a and
Mongkol Sukwattanasinittb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A novel fluorescent turn-on cyanide sensor containing indolium salt as the selective CN- receptor was synthesized 2 steps
via Sonogashira coupling and Knoevenagel reaction. The compound 2 was developed as a new fluorometric and
colorimetric sensor for cyanide with a sensing mechanism through cyanide ion addition to the indolium group.
Importantly, the inclusion complexes between compound 2 and α-cyclodrextin, exhibited fluorescent turn-on detection of
CN- in 100 % aqueous media with the detection limit of 1.3 μM. The turn-on signal is the result of the conversion of the
conjugated hemicyanine moiety to cyano-hemicyanine group via the addition of cyanide ion. Using paper sensor strips, a
naked eye detection of cyanide is possible down to 4.59 ng.mm-2. Test strips based on compound 2 can act as a convenient
and efficient CN- test kit.
www.rsc.org/
containing significant amount of organic solvents. As common
cyanide salts are soluble in water and real samples are more
common in aqueous media, it is thus of interest to have a
Introduction
Cyanide anion (CN−) is one of the most poisonous chemicals to
human and animals health.1 The World Health Organization
(WHO) designates the maximum level of cyanide permissible in
drinking water as at 1.9 µM.2 On the other hand, cyanide has
been used in many industry processes such as plastics
industry, metal mining, and electroplating processes.3
Additionally, cyanide is produced in nature by certain algae
and cassava plant.4 Cyanide contamination in wastewater,
ground water, river water, and drinking water has been
seriously recognized.5
cyanide sensor operate well in medium containing mostly
water. Paper based indicator for naked eye detection of
cyanide ion can also be very handy for on-site analyses.
Due to their strong fluorescence and good stability, we
have recently utilized arylene-ethynylene as fluorogenic
building blocks for metal ion, proteins and DNA sensors.12 We
also found that water solubility of the fluorophore containing
rod-shaped phenylene-ethynylene moieties could be
significantly enhanced via the formation of inclusion
complexes with cyclodextrins (CyD's).13 In this work, a novel
To detect cyanide anions, fluorescent and/or colorimetric
sensors are highly attractive for their rapid detection and ease
of use for on-site analyses where instrument-free methods are
usually desirable.6 For organic compounds, a few numbers of
functional groups such as boronic acid,7 aldehyde,8 and
dicyanovinyl9 have been reported as selective CN- receptors.
However, the sensors constructed from these functional
groups have cross reactivity with other anions such as fluoride,
acetate and phosphate ions and usually require some long
response time to give stable signals.10
cyanide fluorescent turn-on sensing compound
based on phenylene-ethynylene fluorogenic rod attached to
indolium group serving as
CN- receptor (Scheme. 1)
Compound should also be able to form an inclusion complex
2 was designed
a
2
with CyD's that lead to its application for cyanide detection in
aqueous media.
O
Br
O
H
H
N
H
N
PdCl2(PPh3)2, PPh3,
CuI, DIPA, Toluene,
70 oC, 12 h
1
Recently, indolium has emerged as a very promising probe
for CN- because it provided fast response and dual modes of
detection in both fluorescence and color change.11 However,
most of these cyanide sensors have been operated in solutions
67 %
N
I
Al2O3, CH2Cl2,
reflux, 3 h
N
I
N
2
70%
Scheme. 1 The synthetic routes of the compound 2.
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purified by column chromatography on silica (from haxene to
DCM/MeOH; 100:1 v/v) to give fluorophore as dark violet
Experimental
2
Reagents and apparatus
solid (145 mg, 70 %). mp: decomposed > 250 °C; 1H-NMR (500
MHz, Acetone-d6): δ (ppm) 8.40 (d, J = 20 Hz, 1 H), 8.04 (d, J =
10 Hz, 2H), 7.86-7.80 (m, 1H), 7.79-7.75 (m, 1H), (m, 2H), 7.67-
7.60 (m, 5H), 7.40 (d, J = 10 Hz, 2H), 6.74 (d, J = 10 Hz, 2H),
4.19 (s, 3H), 3.01 (s, 6H), 1.86 (s, 6H). 13C-NMR (500 MHz,
Acetone-d6): δ (ppm) 183.0, 153.7, 151.5, 144.1, 142.4, 133.3,
132.0, 130.7, 130.3, 129.7, 123.6, 123.1, 115.3(2), 112.2, 96.9,
87.8, 53.2, 39.5, 25.5, 21.7. HR-ESI-MS m/z for C29H29N2 calcd.
405.2325; found 405.2322.
Benzyltrimethylammonium chloride, potassium carbonate, and
were purchased from Fluka. Triphenylamine, iodine
monochloride, Diisopropylamine, copper (I) iodide, and,
bis(triphenylphosphine)palladium(II) dichloride, indolium salts,
N,N-dimethylaniline, potassiumhydroxide, triphenylphosphine,
trimethylsilylethyne, and sodium thiosulfate were purchased
from Aldrich. All other reagents were non-selectively
purchased from Sigma-Aldrich, Fluka or Merck and used
without further purification. Generally, solvents such as
dichloromethane and acetonitrile were reagent grade stored
over 4A molecular sieves. However, for anhydrous reactions,
solvents such as THF and toluene were dried and distilled
before use according to the standard procedures. All column
chromatography was operated using Merck silica gel 60 (70-
230 mesh). Thin layer chromatography (TLC) was performed
on silica gel plates (Merck F245). Solvents used for extraction
and chromatography such as dichloromethane, hexane, ethyl
acetate and methanol were commercial grade and distilled
before use while diethyl ether and chloroform were reagent
grade. All reactions were carried out under positive pressure of
N2 filled in rubber balloons. Milli-Q water was used in all
aqueous experiments unless specified otherwise.
Photophysical property study
The stock solution of 500 μM compound
2 in 10 mM HEPES
buffer pH 7.4 was prepared. The UV-vis absorption spectra of
the solutions were recorded in a 1 cm quartz cell from 300 nm
to 600 nm at ambient temperature. The fluorescence spectra
were recorded from 380 nm to 700 nm at ambient
temperature using an excitation wavelength at 370 nm.
Fluorescence quantum yields of the fluorophores were
determined from the slope of the plot between the integral of
emission intensity against the absorbance (always kept below
0.1) in relation to quinine sulphate (ΦF = 0.54) in 0.1 M
H2SO4.16 The spectra were acquired from 1-5 μM solutions,
diluted from the stock solution with their respective solvents,
in a 1.4 mL fluorescence quartz cell with 1 cm path length.
Analytical instruments
Nuclear magnetic resonance (NMR) spectra were recorded in
DMSO-d6, CDCl3, Acetone-d6 and CD3CN on a Varian 400 MHz
or Bruker 400 MHz or Bruker 500 MHz spectrometers.
Electrospray mass spectra were acquired from a Micromass
Platform quadrupole mass analyser with an electrospray ion
source using acetonitrile as a solvent. Absorption spectra were
measured by Perkin Elmer 35/FIAS300 and Varian Cary 50 UV-
vis spectrophotometer. Fluorescence spectroscopy was
performed on Agilent Cary Eclipse and Hitachi F-2500
fluorescence spectrometers.
UV-vis absorption measurements
All the UV-vis absorption spectra were acquired from aqueous
solution with 10 mM HEPES buffer, pH 7.4. The UV-vis spectral
changes of compound
2 (50 µM) were recorded upon the
addition of sodium salts of the anion while keeping the
concentration constant at 2 mM. The stock solution of α-
cyclodextrin (2 mM) was prepared in HEPES buffer 7.4. The
mixture between α-cyclodrextrin (400 µM), compound 2 (50
µM) and sodium salt of the anions tested (Br-, Cl-, CO32-, F-, I-,
-
NO2 , S2-, and SO42- (1 mM) and CN- (100 µM) were used for the
UV-vis experiments.
Synthesis of Compound 1
A mixture of 4-ethynyl-N,N-dimethylaniline14 (146.0 mg, 1.0
mmol), 4-Bromobenzaldehyde (190 mg, 1.1 mmol), PdCl2-
(PPh3)2 (35.0 mg, 0.05 mmol), CuI (9.5 mg, 0.05 mmol) in THF
(5 mL) was added with isopropylamine (1 mL) and the mixture
was stirred at 70 oC for 15 h. The reaction mixture was then
evaporated. The residue was eluted through a silica gel column
by gradient from pure hexane to hexane/CH2Cl2 1:2 (v/v) as an
Fluorescence sensing study
The 150 mM stock solutions of the anions were prepared by
dissolving their sodium salts in Milli-Q water. The anion stock
solutions (10 μL) were individually mixed with the stock
solution of compound
2 (500 μM, 10 μL) in HEPES buffer 7.4.
The volumes of the mixtures were adjusted to 1 mL by HEPES
buffer 7.4 to afford the final concentration of 50 μM for the
fluorophores and 1.0 mM for the anions. In the experiments
involving α-CD (400 µM), the stock solution of α-CD (2 mM)
was mixed with the fluorophore stock solution (500 μM, 100
μL) prior to the addition of the anion salts. The emission
spectra were recorded from 380 to 650 nm with the excitation
wavelengths of 360 nm after 6 minutes of mixing under 25 oC.
eluent to afford fluorophore
1 as a yellow solid. (166.3 mg,
67% yield). mp: 150-152°C; 1H-NMR (CDCl3, 500 MHz): δ (ppm)
10.02 (s, 1H,), 7.85 (d, 2H, J = 8.2 Hz), 7.63 (d, 2H, J = 8.2 Hz),
7.45 (d, 2H, J = 8.4 Hz), 6.75 (d, 2H), 3.03 (s, 6H).15
Synthesis of Compound 2
Compound
1 (99.8 mg, 0.39 mmol) and indolium salt (120.5
mg, 0.40 mmol) were dissolved in 20 mL of CH2Cl2 and then
AlCl3 (50 mg, 0.38 mmol) was added. The reaction mixture was
stirred for 3 h at room temperature and the solvent was
removed under reduced pressure. The resulting residue was
Paper-based sensor
The solution of compound
2 (1.0 mM) in ethanol was pipetted
at 2.0 µL onto the surface of marked filter paper and allowed
for air dry to generate fluorescent spots under an ordinary
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decreased with an increase of a new band at 360 nm (Fig. 1).
The color of the solution also changed from orange to colorless
which was easily observed by naked eye. A large hypsochromic
shift of 90 nm in the electronic absorption peak is associated
with the nucleophilic attack of CN- to the iminium carbon of
the indolium group.11a,b This addition reaction disrupts the
electron accepting ability of the indolium group as well as
electron delocalization within its π-conjugated system. Job’s
plot analysis of the fluorescence responses revealed that the 2
formed 1:1 complexes with CN-. The analysis mechanism of the
method was formed stoichiometrically (Fig. 2).
20 W black light lamp. Water samples, i.e. drinking water and
mineral water, spiked with sodium cyanide at various
concentrations were filtered and pipetted at 2 µL on top of the
spots of 2. After air drying, the images of the filter paper under
the black light were photographically recorded.
Results and discussion
The synthesis of the target compound
Sonogashira coupling and Knoevenagal condensation (Scheme.
1). Starting from 4-ethynyl- -dimethylaniline, the
Sonogashira coupling with 4-bromobenzaldehyde gave
compound in satisfactory yield. The Knoevenagal
condensation of with indolium salts afforded the target
product in good yield. The reaction set up was simple and
the purification was easy. The structure of
2 was done in two steps,
N,N
1
1
2
2
was confirmed by
1H-NMR, 13C-NMR, and HRMS data (see electronic
supplementary material (ESM); Fig. S1-3).
Compound
2 exhibited good solubility in common polar
organic solvents, such as acetone, DMF, DMSO, CH3CN, THF,
and EtOH but it was slightly soluble in 100% water media (Fig.
S4). The α, β, and γ-CD have been used to increase water
solubility of 2 13
The ESM; Fig. S4 showed that 2 can
.
completely solute in HEPES buffer at pH 7.4. Upon the addition Fig. 2 Job’s plot of fluorescence responses of
2 (50 µM) mixed
cyclodextrins (400 μM). To various mole faction of CN- in
HEPES buffer pH 7.4.
of CN- (100 µM), the I/I0 of
exhibited the highest ratio about 10 times as shown in ESM;
Fig. S5. A solution of mixed with α-CD (400 µM) in HEPES
2 (50 µM) with α–CD (400 µM)
2
Compound
maximum at 595 nm. Upon the addition of sodium cyanide,
the fluorescence of dramatically and rapidly increased with
2 showed weak emission (ΦF = 0.025) with a
buffer at pH 7.4 was light reddish orange with two distinct
absorption bands around 360 and 450 nm (Fig. 1). The
emission maxima appeared around 450 and 495 nm with the
fluorescence quantum efficiency of 0.025 (Quinine sulfate =
0.54 was used as reference).16 The low quantum efficiency is
probably due to the intramolecular charge transfer (ICT) from
the strong electron donor amino group to the strong electron
acceptor indolium group via its π-conjugated system resulting
in excited ICT state with much smaller band gap.17 Additionally,
intermolecular H-type π–π interaction may also contribute to
the self-quenching effect.18
2
the blue-green emission and maximum emission wavelength
o
at 450 nm (Fig. 3) at 6 minutes under 25 C (ESM; Fig. S6). The
quantum efficiency of the cyanide adduct improved to 0.34
(ESM; Table S1). The fluorescence turn-on response ratio (I/I0)
linearly increased with the cyanide concentration in the range
of 0–80 µM (ESM; Fig S7). The detection limit of cyanide
determined at S/N =3 was 1.3 µM below the World Health
Organization (WHO) limit of 1.9 µM allowed in drinkable
water.2,19
Fig. 1 UV−vis spectra of 2 (50 μM) and cyclodextrin (400 μM) in
10 mM HEPES buffer pH 7.4 in the presence of different
concentrations of CN-.
Fig. 3 Emission spectra of
2 (50 µM) and cyclodextrin (400 μM)
in response to addition of sodium cyanide at various
concentrations in 10 mM HEPES buffer pH 7.4 (λex = 360 nm).
Upon the addition of sodium cyanide to the solution of 2 in
10 mM HEPES buffer pH 7.4, the absorption band at 450 nm
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1H-NMR titration was used to verify that nucleophilic The results suggested that a selective naked eye detection of
addition of CN- to the imminium carbon of the indolium group cyanide with
(Fig. 4) really participates in the cyanide sensing mechanism of
2 was possible.
2. The spectra in Fig. 5 show that the signal at 6.42 ppm
corresponding to the gˊ proton grows in expense of the signal
of g vinylic proton at 8.40 ppm upon the addition of sodium
cyanide. Meanwhile, the signals of the aromatic protons also
displayed large magnetically shielded shifts compared with
those of
electron withdrawing character of the indolium group.11
Additionally, the formation of cyanide with was confirmed by
2 in good agreement with the loss of conjugated
2
high resolution mass HRMS where a peak at m/z 432.2400
corresponding to the cyanation adduct (Fig. S8).
Fig. 6 Photographs of
2 (100 µM) and cyclodextrin (800 μM) in
HEPES buffer (10 mM) pH 7.4 in the presence of sodium salt of
anion (1.0 mM) taken under (a) room light and (b) black light.
To better evaluate the interference test of
2 mixed α-CD (400
µM), ratiometric absorption (A360/A450) and fluorescence (I/I0)
changes caused by the addition of other anions. Including with
the interfering anion ions at 10 times concentration did not
show any significant ratiometric absorption of all anion ions
Fig.4 Proposed structure of inclusion complexes between
2
tested on 2 (Fig. 7a). The fluorescence enhancement ratio (I/I0)
also showed high selectivity toward CN- and low interference
from other types of anions including S2- (Fig. 7b).
with α-CD and cyanide sensing mechanism.
Fig. 5 1H-NMR spectra (500 MHz, CD3OD) of
2 only (0.9 mM) in
the absence and presence of cyanide.
Fig. 7 Bar charts of (a) ratiometric absorption (A360/A450) and
(b) fluorescence intensity ratio (I/I0) of
2/α-CD (50/400 μM)
To evaluate the selectivity of
2 toward various anions i.e. Orange bars represent the addition of a single analyte, red
Br-, Cl-, CO32-, F-, I-, NO2 , S2- and SO42- were examined in parallel bars exhibit the present of CN- (100 μM) in the mixture of 10
to CN- using their sodium salts. As shown in Fig. 6, only sodium mM HEPES buffer pH 7.4) and other interfering anions (1 mM).
cyanide induced the color disappearance and blue emission of
-
the original orange color of
2
. Therefore,
2
has much higher
Compound 2 was prepared as a solid state sensor by drop
selectivity for CN- than other anions. Notably, S2- also induced casting on a strip of filter paper from its ethanol solution (100
some color fading, probably the similar addition mechanism of µM, 2 µL). The test strip was dropped with sodium cyanide
S2- to 2 11c
but without apparent fluorescence emission. solution (2 µL) with various concentrations. Visualization of the
,
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dry test strip under black light (20 W) showed intense blue Table 2 Comparison of CN- detection limit of
Fluorescence when the concentration of sodium cyanide optical materials.
solution was 40 µM or higher (Fig. 8). The results can be
2 with other
translated to the detection ability of 5 nanograms of cyanide
per square millimeters of the testing area.
Detection
limit
Sensing
Signal detected
Solvent
Ref.
materials
Turn-on emission
Turn-on emission
Turn-on emission
CH3CN
30 µM
10 µM
14 µM
1.5 µM
1.1 µM
1.9 µM
1.3 μM
20
8a
21
22
23
34
Boradiazaindacene
40% DMSO in HEPES
buffer pH 7.4
Cumarin
Salicylaldehyde
Dicyano-vinyl
polymer
DMF
Turn-on emission 95 % CH3CN/ 5% H2O
Benzofuranzan
Azobenzylmalono-
nitrile
Fig. 8 Photographic images of 2 paper-based detection of CN-
Absorption
Absorption
CH3CN
EtOH
Quinolinium
bromide
under a 20 W black light (Scale bar = 7 mm).
Compound 2
Turn-on emission 100 % HEPES buffer,
(visual
The present method was applied to determine cyanide ion
in the real water samples i.e. drinking water, mineral water
and brine (DI water with 10% NaCl). The water samples were
buffered with 10 mM HEPES buffer, pH 7.4 and spiked with
known amount of sodium cyanide (10 and 20 µM). The analysis
of each sample was carried out in three replicates. Table 1
demonstrates that cyanide ion was determined in with a
satisfactory analytical accuracy (<10% error) and precision
(RSD ≤ 5%) although the sample high concentration of brine
gave a little higher positive error values. The results confirmed
the accuracy and reproducibility of this sensing system. The
method is also relatively simple and economical as it has high
tolerance to other inferring ions.
pH 7.4
paper
This
observation)
4.59 ng.mm-1 work
Conclusions
In summary, the study on fluorophores
2 containing
phenylene-ethynylene fluorescent units with indolium
termininal as the most sensitive and selective sensor for CN-
detection is highly reactive to CN-. The detection limit was
estimated to be 1.3 µM in aqueous media. The ratio
colorimetric and fluorometric sensing behaviour of
was from the reduced π-conjugation and block ICT process
induced by CN- upon the addition of
. Effectively, the probe
could be applied to detect CN- in drinking water with
satisfactory results. The test strip containing also exhibits a
good sensitivity to CN- ion at 4.59 ng.mm-1 level. The probe
2
for CN-
2
2
Table 1 The % recovery data for the detection of cyanide ions
2
in spiked water samples (n=3).
2
was constructed by taking advantages of the cyanide sensor. In
addition, the probe could serve as practical colorimetric
sensors for “in-the-field” measurement, which did not require
any additional equipment.
Samples
Spiked
(µM)
Found
(µM)
Recovery
(%)
RSD
(%)
Drinking water
10.0
20.0
10.0
20.0
10.0
20.0
9.78 ± 0.21 97.84 ± 2.85
20.45 ± 0.27 102.72 ± 2.50
9.92 ± 0.13 99.25 ± 0.89
19.56 ± 0.41 97.83 ± 4.92
10.97 ± 0.52 109.71 ± 5.45
21.15 ± 0.29 105.75 ± 3.12
2.16
1.78
0.85
2.42
3.64
2.18
Acknowledgements
Minerals water
10 % NaCl (aq)
The authors gratefully thank the Thailand Research Fund
(MRG5680031) and KMUTT Research Fund. M.S. would also
like to acknowledge the financial supports from the Thailand
Nanotechnology Center (NANOTEC, NSTDA), the IIAC
Chulalongkorn University Centenary Academic Development
Project, the National Research University Project of Thailand,
Office of the Higher Education Commission (AM1006A), the
Project for Establishment of Comprehensive Centre for
Innovative Food, Health Products and Agriculture supported by
the Thai government stimulus package 2 (TKK2555, SP2), the
Ratchadaphiseksomphot Endowment Fund (90th Anniversary
CU Fund).
aMean ± SD
In comparison with other sensing materials used for CN-
detection in aqueous solution, the CN- detection limit of
is
2
one of the lowest as shown in Table 2. To the best of our
knowledge, this work is also one of the most sensitive and
convenient techniques for the naked eye detection of CN-
(Table 2).
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