Selective colorimetric and "turn-on" fluorimetric detection of cyanide using an acylhydrazone sensor in aqueous media

Article abstract of DOI:10.1039/c5nj00089k

A simple, selective colorimetric and fluorimetric cyanide chemosensor L was designed and synthesized, which showed both colorimetric and fluorescence turn-on responses for cyanide ions in aqueous solution with specific selectivity and high sensitivity. The probe shows an immediate visible change in color from colorless to yellow only after the addition of CN^- in aqueous solution; these color changes can be readily observed visually. Moreover, the detection limit on fluorescence response of the sensor to CN^- is down to 1.20 × 10^-9 M, which is far lower than the maximum level of 1.9 × 10^-6 M for cyanide in drinking water according to WHO guidelines. Test strips based on L were fabricated, which could be used as a convenient and efficient CN^- test kit to detect CN^- in aqueous solution for "in-the-field" measurement.

Full text of DOI:10.1039/c5nj00089k

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Selective colorimetric and ‘‘turn-on’’ fluorimetric
detection of cyanide using an acylhydrazone
sensor in aqueous media†
Cite this:DOI: 10.1039/c5nj00089k
JingHan Hu,* JianBin Li, Jing Qi and You Sun
A simple, selective colorimetric and fluorimetric cyanide chemosensor L was designed and synthesized,
which showed both colorimetric and fluorescence turn-on responses for cyanide ions in aqueous
solution with specific selectivity and high sensitivity. The probe shows an immediate visible change in
color from colorless to yellow only after the addition of CNꢀ in aqueous solution; these color changes
can be readily observed visually. Moreover, the detection limit on fluorescence response of the sensor
to CNꢀ is down to 1.20 ꢁ 10ꢀ9 M, which is far lower than the maximum level of 1.9 ꢁ 10ꢀ6 M for
cyanide in drinking water according to WHO guidelines. Test strips based on L were fabricated, which
could be used as a convenient and efficient CNꢀ test kit to detect CNꢀ in aqueous solution for ‘‘in-the-
field’’ measurement.
Received (in Montpellier, France)
13th January 2015,
Accepted 9th March 2015
DOI: 10.1039/c5nj00089k
www.rsc.org/njc
Several organic molecules and transition metal complexes
able to detect the presence of cyanide based on supramolecular
1. Introduction
The cyanide anion (CNꢀ) is known to be an extremely toxic approaches using hydrogen-bonding motifs with dramatic
anion and can directly lead to death of human beings in several changes in their absorption and emission properties have
minutes because it strongly binds to cytochrome-c, thereby already been identified.8–14 Some of these chemosensors can
disrupting the mitochondrial electron-transport chain and causing even detect micromolar amounts of cyanide.15–19 However,
a decreased oxidative metabolism and oxygen utilization.1,2 The some of them suffer severe interference from coexisting anions
cyanide ion also detrimentally affects vascular, visual, central such as Fꢀ,11 AcOꢀ,13 and H2PO4ꢀ 15 In addition, many of them
.
nervous, cardiac, endocrine, and metabolic functions. However, are reported to work only in organic media,20–22 but relatively
despite cyanide toxicity, large quantities of cyanide salts are still few of them exhibit spectral changes in both absorption and
widely used in industrial applications such as metallurgy emission spectra. Therefore, it is still worthwhile to exploit new
(1.5 million tons per year), electroplating, and the synthesis of colorimetric and ‘‘Turn-on’’ fluorimetric sensors for selective
fine chemicals.3 As a result, in recent years, the purposeful recognition of toxic anions (CNꢀ) in the organo-aqueous media and
design and synthesis of efficient chemosensors to selectively then in pure water solution for applications in environmental or
detect CNꢀ ions in the environmental and biological fields have biological systems.
attracted much attention. Although a wide variety of chemical
Our research group has a longstanding interest in molecular
and physical sensors for the detection of CNꢀ have been recognition.23 Herein, we have designed and synthesized a
reported,4,5 most of the physical methods require expensive compound (L), L as a probe for cyanide comprising 2-hydroxy-
and sophisticated equipment or involve time-consuming and 1-napthaldehyde and isoniazid functionalities. Receptor L possesses
laborious procedures that can be carried out only by well-trained a hydroxyl group and it is well established that the OH group
professionals, which are serious stumbling blocks to the practical due to strong acid is readily deprotonated when basic ions
application of these CNꢀ sensors.6 For purposes of simplicity, appear. The naphthalene group is introduced as a fluorophore
convenience, easy preparation and high sensitivity,7 colorimetric to achieve fluorescence recognition. Sensor L showed fluores-
and fluorimetric chemodosimeters for CNꢀ have become particularly cence and UV-vis selectivity for CNꢀ in DMSO/H2O (6 : 4, v/v)
attractive.
binary solution over other anions. The detection limit on
fluorescence response of the sensor to CNꢀ is down to 1.22 ꢁ
10ꢀ9 M, which indicates that this sensor could potentially be
used as a probe for monitoring CNꢀ levels. The mechanism of
this process has been investigated by 1H NMR, UV and ESI-
College of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: hujinghan62@163.com
† Electronic supplementary information (ESI) available: Complete experimental
procedures and some of the spectroscopic techniques. See DOI: 10.1039/c5nj00089k mass spectrometry.
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2. Experimental section
2.1. Materials and physical methods
For 1H NMR titrations, the sensor of stock solutions was
prepared in DMSO-d6, and the cyanide anion was prepared in
distilled water. Aliquots of the two solutions were mixed directly
in NMR tubes.
Test strips were prepared by immersing filter papers into a
DMSO/H2O (6 : 4, v/v) binary solution of L (0.01 M) followed by
exposure to air until complete drying. The test strips containing
sensor L were utilized to detect CNꢀ and other cations.
Fresh double distilled water was used throughout the experiment.
The tetrabutylammonium salts and N-2-hydroxyethylpiperazine-N0-
2-ethanesulfonic acid (HEPES) were purchased from Alfa Aesar
Chemical Reagent Co. (Tianjin, China). All reagents and solvents
were commercially available and were of analytical grade and were
1
used without further purification. H NMR and 13C NMR spectra
were recorded on a Mercury-400BB spectrometer at 400 MHz and
100 MHz, respectively. Chemical shifts are reported in ppm
downfield from tetramethylsilane (TMS, d scale with solvent
resonances as internal standards) UV-vis spectra were recorded
on a Shimadzu UV-2550 spectrometer. Photoluminescence spec-
tra were recorded on a Shimadzu RF-5301 fluorescence spectro-
photometer. Melting points were measured on an X-4 digital
melting-point apparatus purchased from Beijing Tech Instrument
Co. (uncorrected). Infrared spectra were recorded on a Digilab FTS-
3000 FT-IR spectrophotometer.
3. Results and discussion
The receptor was found to have limited solubility in water, and
this compelled us to use this sensor in a mixed solvent, such as
H2O/DMSO (4 : 6, v/v), for recognition studies of L. UV-vis
absorption spectral response of chemosensor L was tested with
aqueous solutions of the tetrabutylammonium salts of all
common anionic analytes such as Fꢀ, CIꢀ, Brꢀ, Iꢀ, AcOꢀ,
H2PO4ꢀ, HSO4ꢀ, ClO4ꢀ and SCNꢀ as well as CNꢀ. The solution
of L resulted in an immediate change in color from colorless to
yellow (Fig. 1). In the corresponding UV-vis spectrum, two
strong absorption bands at 328 nm and 368 nm disappeared,
while a new band appeared at 420 nm after the addition of CNꢀ
(Fig. 2). All examined anions including the more basic and
nucleophilic ones such as Fꢀ and SCNꢀ did not cause any
obvious color and spectral changes. These results suggested
that sensor L shows excellent selectivity for CNꢀ over all other
anions tested.
2.2. Synthesis of sensor L
The synthesis route of receptor molecule L is demonstrated in
Scheme 1. To an ethanol solution (25 mL) of 2-hydroxy-1-
naphthaldehyde (0.86 g, 5 mmol), isoniazid (0.54 g, 5 mmol)
was added. Then the reaction mixture solution was stirred and
refluxed for 6 h. After cooling to room temperature, quietly
placed, a large amount of yellow precipitate was formed, which
was filtered and washed with distilled ethanol three times, then
recrystallized with absolute ethanol to get yellow crystals of L in
89% yield (m.p. 273–275 1C), 1H NMR (400 MHz, DMSO) d:
12.54 (s, 1H), 12.42 (s, 1H), 9.49 (s, 1H), 8.85 (d, J = 4.3 Hz, 2H),
8.33 (d, J = 8.5 Hz, 1H), 8.08–7.74 (m, 4H), 7.64 (t, J = 7.6 Hz,
1H), 7.43 (t, J = 7.3 Hz, 1H), 7.26 (d, J = 8.9 Hz, 1H). 13C NMR
(101 MHz, DMSO) d: 160.96, 158.12, 150.39, 147.94, 139.74,
133.03, 131.56, 128.90, 127.80, 123.53, 121.35, 120.83, 118.75,
108.46. IR (KBr) n: 3447.66 cmꢀ1 (–OH), 3218.90 cmꢀ1 (–NH),
3038.39 cmꢀ1, 1576.92 cmꢀ1 (CQN–H), 1680.77 cmꢀ1 (–CQO).
ESI-MS calcd for C17H13N3O2 + H 292.10, found 292.02.
Fig. 1 Color changes observed upon the addition of various anions
(50 equiv.) to solutions of sensor L (2 ꢁ 10ꢀ5 M) in DMSO/H2O (6 : 4, v/v).
2.3. General procedure
Both UV-vis spectroscopy and fluorescence spectroscopy were
carried out just after the addition of tetrabutylammonium anion
salt (0.01 M) in DMSO/H2O (6 : 4, v/v) solution, while keeping
the ligand concentration constant (2.0 ꢁ 10ꢀ5 M) on a Shimadzu
UV-2550 spectrometer and a Shimadzu RF-5301 spectrometer,
respectively. The anion solutions were prepared from the tetrꢀa-
butylammonium salts of Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4
,
ClO4ꢀ and SCNꢀ, but CNꢀ was prepared in NaCN. The excitation
wavelength was 368 nm.
Fig. 2 UV-vis spectra of L (2 ꢁ 10ꢀ5 M) in the presence of 50 equiv. of various
Scheme 1 Synthetic procedures for receptor L.
anions in H2O/DMSO (4 : 6, v/v) binary solution at room temperature.
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Fig. 5 Absorbance spectra of L (2 ꢁ 10ꢀ5 M) in the presence of various
Fig. 3 Fluorescence spectra of L (2 ꢁ 10ꢀ5 M) in the presence of 50 equiv.
of various anions in H2O/DMSO (4 : 6, v/v) binary solution at room
temperature.
anions (50 equiv.) in DMSO/H2O (6 : 4, v/v) in response to CNꢀ (50 equiv.).
Fig. 4 Color changes observed upon the addition of various anions
(50 equiv.) to solutions of sensor L (2 ꢁ 10ꢀ5 M) in DMSO/H2O (6: 4, v/v),
under a UV-lamp (365 nm).
Compound L alone displays a weak, single fluorescence
emission band at 494 nm when excited at 368 nm in aqueous
media H2O/DMSO (4 : 6, v/v), which exhibits a low quantum
yield (FR = 0.14). Changes in the spectral pattern were observed
only in the presence of 50 equivalents of CNꢀ; chemosensor L
produced a strong fluorescence response band shifted toward
530 nm, with an increase in quantum yield (FR = 0.42), and
Fig. 6 Fluorescence spectra of L (2 ꢁ 10ꢀ5 M) in the presence of various
anions (50 equiv.) in DMSO/H2O (6 : 4, v/v) in response to CNꢀ (50 equiv.).
The detection limit is one of the most important parameters
responded with a dramatic color change, from pale colorless in ion sensing. For many practical purposes, it is very important
to yellow. No change in the spectral pattern for receptor L to detect the analytes at low concentrations. The fluorimetric
in the presence of other anions suggests either a very weak detection limits of sensor L for CNꢀ were also determined. The
or no interaction between these anions and the compound detection limit of the fluorescence measurements is calculated
(Fig. 3 and 4).
on the basis of 3SB/S (ref. 24) (where SB is the standard
To validate the selectivity of sensor L, various anions includ- deviation of the blank solution and S is the slope of the
ing Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4 and SCNꢀ calibration curve); Fig. 7 shows the detection limit of 1.20 ꢁ
ꢀ
contacting with L were investigated by UV-vis absorption and 10ꢀ9 M for CNꢀ, which is far lower than the maximum level of
fluorescence spectroscopy. Among various anions, only cyanide 1.9 ꢁ 10ꢀ6 M for cyanide in drinking water according to WHO
displayed noticeable color changes from colorless to yellow. guidelines.
Fluorescence spectra showed that L had a high selectivity for
The signaling mechanism of L (0.01 M) in the presence of
the cyanide ion; other anions almost caused no change in cyanide (0.5 M) was based on the deprotonation of hydroxyl and
fluorescence. This highly selective detection of cyanide could amino groups. NMR spectra gave a strong evidence to support
be demonstrated even in the presence of other anions (Fig. 5 this assumption (Fig. 8). According to 1H NMR spectra of L and
and 6). The selectivity of cyanide over other anions, especially CNꢀ, the proton signals of –OH and –NH at 12.54 (s, 1H) and
Fꢀ, AcOꢀ, and H2PO4ꢀ, is important because many reported 12.42 (s, 1H) ppm completely disappeared, which clearly shows
cyanide sensors suffer from deleterious interference of these that –OH and –NH undergo deprotonation and the other
anions.
aromatic proton chemical upfield shifts from 9.49–7.26 to
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Scheme 2 Possible sensing mechanism.
could not occur in the case of a fluoride ion due to its high
hydration enthalpy and low basicity (DH0hyd = ꢀ504 kJ molꢀ1
,
pKa = 3.18) in an aqueous system.25 Furthermore, the recogni-
tion properties of receptor L for CNꢀ were determined by UV-vis
and fluorometric titrations. The absorption spectral changes in
L with the addition of cyanide (0.01 M) in DMSO/H2O (6 : 4, v/v)
solutions are shown in Fig. 9. The absorption band at 368 nm
decreased, while a new band appeared at 420 nm. A clear
isosbestic point of 395 nm indicated an interconversion into
single discrete chemical species during the titration process.
Fig. 10 shows the emission spectrum for compound L with the
addition of CNꢀ (0.1 M). When excited at 352 nm, the emission
intensity at 533 nm remarkably increased upon changing the
concentration of CNꢀ from 0 to 3.8 mM.
Fig. 7 Fluorescence detection limit spectra of L (2 ꢁ 10ꢀ5 M) in (H2O/
DMSO, 4 : 6, v/v) solution upon adding CNꢀ (1 ꢁ 10ꢀ4 M).
To further investigate the interaction between sensor L and
CNꢀ, the infrared spectra were recorded and are displayed in
Fig. 11. The stretching vibration absorption peaks at 3447.66 cmꢀ1
(–OH), 3218.90 cmꢀ1 (–NH), 3038.29 cmꢀ1, 1576.92 cmꢀ1 (CQN–H),
1680.77 cmꢀ1 (–CQO) of sensor L compared with the N–H peak at
3218.90 cmꢀ1 of the L + CNꢀ compound disappeared; at the same
time, the peaks of (–CQO) and (CQN–H) at 1680.77 cmꢀ1 and
1576.92 cmꢀ1 moved to 1605.52 cmꢀ1 and 1536.29 cmꢀ1, respec-
tively, which demonstrates that receptor L reacted with CNꢀ. More-
over, the mass spectrum obtained and confirmed that the ion peak
of sensor L was detected at m/z 292.02, which corresponds to [L + H]+
(Fig. S2, ESI†). The probe with NaCN confirmed the revival of m/z
Fig. 8 Partial 1H NMR spectra of L (0.01 M) and in the presence of varying
amounts of CNꢀ (0.5 M).
9.02–6.79 ppm. This indicates that the aromatic protons were
shielded by the negative charge of the deprotonated
hydoxyl group.
This deprotonation caused a high charge separation between
acceptor and donor units in L, and consequently caused a
excellent electron delocalization to the isoniazid unit. The
excited states in the complexes were stabilized upon the binding
of CNꢀ, resulting in a bathochromic shift in the absorption band
with Dl = 81 nm for L. Moreover, a new emission band at 530 nm
for L and CNꢀ was observed. It was possibly caused by a large
charge separation between the isoniazid unit and the naphtha-
lene rings resulting in a strong intramolecular charge transfer
(ICT). Consequently, the observed fluorescence enhancement is
most likely caused by the ICT. A schematic illustration is shown
in Scheme 2. Therefore, it can be clearly seen that chemosensor
L selectively detects cyanide over other anions such as Fꢀ, Clꢀ,
ꢀ
Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ, ClO4 and SCNꢀ in aqueous
media (H2O/DMSO, 4 : 6, v/v).
The selective detecting properties of L for CNꢀ can be
described by the basicity of anions. The charge transfer process
Fig. 9 UV-vis spectra of L (2 ꢁ 10ꢀ5 M) in DMSO/H2O (6 : 4, v/v) upon
adding an increasing concentration of CNꢀ (0.01 M).
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Fig. 10 Fluorescence titration spectra (lex = 368 nm) of L (2 ꢁ 10ꢀ5 M) in
(H2O/DMSO, 4 : 6, v/v) solution upon adding an increasing concentration
of CNꢀ (0.1 M). The inset shows a fluorescence change of L observed upon
excitation at 365 nm after the addition of CNꢀ anions (0–3.8 mM).
Fig. 12 Photographs of L (0.01 M) on test strips (a) only L, (b) after
immersion into water solutions with CNꢀ, (c) after immersion into water
solutions with other anions, (d) after immersion into water solutions with
CNꢀ and other anions at room temperature and irradiation under UV light
at 365 nm, respectively.
(Fig. 12). Therefore, chemosensor L exhibits excellent UV visual and
fluorescence sensing performance, which will be very useful for the
fabrication of sensing devices with fast and convenient detection of
cyanide ions.
4. Conclusions
In conclusion, we have presented a facile, rapid and efficient
chemosensor L, which showed special selectivity and high
sensitivity UV-vis absorption and fluorescence recognition for
CNꢀ in DMSO/H2O (6 : 4, v/v) solutions. Moreover, the sensor
demonstrates that the detection limit on fluorescence response
of the sensor to CNꢀ is down to 1.20 ꢁ 10ꢀ9 M, which is far
lower than the maximum level of 1.9 ꢁ 10ꢀ6 M for cyanide in
drinking water according to WHO guidelines. In addition, test
strips based on L were fabricated, which could serve as a
practical colorimetric and fluorimetric sensor to detect CNꢀ
in field measurements or in test kits. We believe that these
characteristics of L make it attractive for further molecular
modifications and underlying applications as a colorimetric
and fluorimetric sensor for CNꢀ.
Fig. 11 Infrared spectra of L (black line) and its complex L + CNꢀ (red line).
313.97 and also demonstrated the presence of compound [L – 2H +
Na+ + H]+ (Fig. S3, ESI†). In conclusion, the IR and mass spectro-
scopy experiments suggested the sensing mode of chemosensor L
and cyanide.
The pH dependence of sensor L in the HEPES buffer system
was also checked by UV-vis and fluorescence spectroscopy.
Cyanide ions were added to the buffer solution of L at different
pH values. No apparent changes in the spectra were observed,
indicating that the binding of L with the CNꢀ cannot work well
in the pH range of 1.0–12.0 (Fig. S5, ESI†).
The realization of quick response to cyanide is quite meaningful
for the sensor for its practical application in portable sensing
devices. To facilitate the use of L for the detection of cyanide, test
strips were prepared by immersing filter papers into a DMSO/H2O
(6 : 4, v/v) binary solution of L (0.01 M) followed by its exposure to
air for drying. Intriguingly, the fluorescence color changed imme-
diately from blue to mazarine once the test papers were immersed
into an aqueous solution (5 mM) of cyanide under UV irradiation
Acknowledgements
‘‘This work was supported by the Nature Science Foundation of
China (No. 21467012), the Science and Technology Bureau of
Lanzhou, Gansu Province of China (No. 2013-4-63).’’
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