Schiff base derived from salicylaldehyde-based azo dye as chromogenic anionic sensor and specific turn-on emission sensor for cyanide ion

Article abstract of DOI:10.1002/jhet.3693

A novel Schiff base has been derived from condensing 4-aminoantipyrine with diazotized salicylaldehyde. The derived compound acted as a colorimetric sensor for hazardous aqueous anions like CN^?, F^?, and CH₃COO^?

Full text of DOI:10.1002/jhet.3693

Received: 24 May 2019
DOI: 10.1002/jhet.3693
Revised: 23 July 2019
Accepted: 11 August 2019
A R T I C L E
Schiff base derived from salicylaldehyde‐based azo dye as
chromogenic anionic sensor and specific turn‐on emission
sensor for cyanide ion
Nilanjan Chakraborty1 | Arijit Chakraborty2 | Suman Das1
1 Biophysical Chemistry Laboratory,
Abstract
Department of Chemistry, Jadavpur
A novel Schiff base has been derived from condensing 4‐aminoantipyrine with
University, Kolkata, India
2 Department of Chemistry, Acharya
Brojendra Nath Seal College, Cooch
Behar, India
diazotized salicylaldehyde. The derived compound acted as a colorimetric sen-
sor for hazardous aqueous anions like CN−, F−, and CH3COO− among a list of
anions. The colorimetric changes were further verified through absorption
titrations. The detection limits were of the order of 10−10 M, which makes
the sensor significant. The interaction of the anions with the sensor was stoi-
chiometrically 1:1 with good binding constants. The sensor turns out to be a
specific turn‐on emission sensor for CN− even in competitive environments.
The F− ion sensing ability was extended to the determination of F− in a com-
mercial toothpaste with good results.
Correspondence
Suman Das, Biophysical Chemistry
Laboratory, Department of Chemistry,
Jadavpur University, Kolkata, West
Bengal 700032, India.
Email: sumandas10@yahoo.com
Funding information
University Grants Commission; Ministry
of Human Resource Development, Grant/
Award Number: R‐11/101/19; Science and
Engineering Research Board, Grant/
Award Number: SR/FT/CS‐116/2010
1 | INTRODUCTION
preventing tooth decay and in osteoporosis treatment.[24]
In surplus amount, fluoride ion exposure can lead to del-
Anion sensing and detection have been a progressing
arena in the field of host‐guest interaction studies in the
last two decades.[1–16] This approach has overwhelming
applications in the medical, biological, environmental,
and industrial areas. The importance of colorimetric and
fluorescent sensors is increasing day by day owing to
nonexpensive instrumentation and naked‐eye signaling
of the event.
Cyanide, being the most toxic among anions, demands
its detection in lower concentrations[17]; 0.2 to 0.5 mg/kg
body weight of cyanide ion is extremely fatal to human
beings.[18] The cellular respiration in mammalian cells is
inhibited owing to interaction of cyanide with active site
of cytochrome a3.[19] We are exposed to cyanide ion via
industrial pollution, as cyanide is involved in various
chemical processes like electroplating, tanning, and
extraction of metals like gold and silver.[8,20–23] Fluoride
ion is benign in nature owing to its beneficial effect in
eterious outcomes like fluorosis, weakening of bones and
ligaments, hypocalcaemia, and variation in thyroid hor-
mone status.[25] Fluoride ion is also released from hydro-
lysis of chemical warfare agent sarin, and it is also
present in many psychiatric drugs, and hypnotic and
anesthetic agents.[26] Acetate ion plays a pivotal role in
living systems as acetyl coenzyme.[27] Acetate ion in form
of (11)C‐acetate acts as a specific and sensitive radiotracer
in positron emission tomography (PET) imaging of hepa-
tocellular carcinoma (HCC) and evaluation of other liver
masses[28] and in protein‐ion binding measurements.[29]
Thus, these ions require efficient detectors.
Earlier in our works, we have shown an azo dye derived
from fluorophore 4‐hydroxy coumarin to be a sensing ele-
ment for F− and CH3COO− ions[11] and a Schiff base from
4‐aminoantipyrine with pyrene‐1‐carboxaldehyde as Cu2+
sensors.[30] From our previous knowledge, we have diazo-
tized salicylaldehyde with 4‐nitroaniline and derived a
J Heterocyclic Chem. 2019;1–7.
wileyonlinelibrary.com/journal/jhet
© 2019 Wiley Periodicals, Inc.
1

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CHAKRABORTY ET AL.
Schiff base with 4‐aminoantipyrine in our present piece of
work. The resulting moiety was investigated for anion
sensing where it was responsive towards CN−, F−, and
CH3COO− ions colorimetrically. The application of F−
ion sensing was extended for the determination of F− in
toothpaste samples.
concentration of complete saturation, respectively. K rep-
resents the binding constant of the complexation between
compounds 2 and anions, and [C] represents concentra-
tion of the anions. The plot was created by plotting
ΔAmax/ΔA as a function of 1/[C]. The binding constants
were determined from the obtained slope of the plot.
2 | EXPERIMENTAL SECTION
2.1 | Materials
2.2.4 | Determination of binding
stoichiometry
From the Job's plot[32] (method of continuous variation),
stoichiometry of complexation of anions (CN−, F−, and
CH3COO−) with the analyte was determined. The absor-
bance differences (ΔA) between the free ligand and after
anionic interaction were plotted against the increasing
mole fraction of anions. Total concentration of ligand
and anion was kept at 50 μM while varying their mole
fraction from 0 to 1. The absorbances at λmax of 364 nm
were recorded to obtain Jobs plot. The mole fraction of
complexation of the ligand with anions was obtained
from the break point in the plot.
The solvents are of analytical grade and were obtained
from Merck, India; and reagents were purchased from
Spectrochem, India, and used without further purification.
2.2 | Methods
2.2.1 | Physical measurements
Bruker 300 MHz (Bruker AVANCE 300) spectrometer
was used to record 1H nuclear magnetic resonance
(NMR) and 13C NMR at an ambient temperature. The
melting points were determined on a LabX, India, digital
melting point apparatus and are uncorrected. Mettler
Toledo Analytical Balance Band (model MS 2045) was
used for all weightings.
2.2.5 | Limit of detection
The limit of detection (LOD) was obtained using calibra-
tion curve obtained from the titration profile of absorp-
tion studies. The equation mentioned below was utilized
to serve the purpose:
2.2.2 | UV‐vis and spectrofluorimetric
studies
3σ
S
LOD ¼
;
(2)
All UV‐vis spectra of receptor were recorded in high‐
performance liquid chromatography (HPLC)‐grade aceto-
nitrile on a PerkinElmer UV/VIS spectrometer (model
Lambda 25) in matched quartz cells of 1‐cm path length.
PerkinElmer fluorescence spectrometer (model LS 55)
was used for steady‐state fluorescence spectroscopy.
Fluorescence‐free quartz cell (path length 1 cm) was
used. All the measurements were done while keeping
excitation wavelength at 363 nm and excitation and emis-
sion band passes at 5 and 15 nm, respectively.
where σ is the standard deviation of the blank solution
(relative absorbance ratio at shifted maximum to ligand
maximum) and m is the slope of the plot of relative absor-
bance versus anionic concentration in the lowest concen-
tration range.
2.2.6 | Anion sensing by NMR
spectroscopy
1H NMR spectra of sensor 2 was procured in DMSO‐d6
both in the presence and absence of various equivalents
of F− ion (tetrabutylammonium fluoride) to know the
nature of interaction between them using an NMR spec-
trometer (300 MHz) at room temperature.
2.2.3 | Determination of binding constant
A Benisi‐Hildebrand plot[31] was constructed from the
absorption titration data for binding constant determina-
tion using the following equation:
ΔAmax
ΔA
1
K½Cꢀ
2.2.7 | Solvent effects on the spectral
properties
¼ 1 þ
;
(1)
ΔA = |Ax − A0| and ΔAmax = |A∞ − A0|. A0, Ax, and A∞ are
the absorbances of sensor 2 in the absence of anions, at an
intermediate concentration of anions, and at an anionic
To study solvatochromism in the absorption spectra,
25 μM of compound 2 in various solvents was utilized.

CHAKRABORTY ET AL.
3
Solvents of varying dielectric constant, viz, ethanol,
ethanol, and dried over water bath. For purification, the
compound was crystallized from acetonitrile. Yield
79.23%; mp 230°C to 235°C. 1H NMR (300 MHz,
DMSO‐d6): δ 13.65 (s, 1H), 9.79 (s, 1H), 8.37 (d,
J = 8.58, 2H), 8.18 (s, 1H), 8.01 (d, J = 8.37, 2H), 7.94
(d, J = 7.35, 1H), 7.53 (d, J = 7.05, 2H), 7.39 (s, 3H),
7.11 (d, J = 8.58, 1H), 3.23 (s, 3H), 2.48 (s, 4H), 2.43 (s,
3H), 2.07 (s, 1H) (Fig. S1). 13C NMR (75 MHz,
DMSO‐d6): δ 163.97, 159.42, 155.80, 150.77, 148.44,
145.59, 134.55, 129.93, 127.93, 126.38, 125.59, 125.53,
123.65, 121.43, 118.29, 114.17, 35.48, 10.34 (Figure S2).
High‐resolution mass spectrometry (HRMS) (m/z).
water,
hexane,
tetrahydrofuran,
toluene,
N,N‐
dimethylformamide (DMF), dimethylsulfoxide (DMSO),
chloroform, and acetonitrile were engaged to figure out
the effect of solvent polarities in the absorption spectrum.
2.2.8 | pH dependence of receptor 1 in
absorption spectra
pH dependence of sensor 2 was investigated, where
50 μM of sensor 2 in buffer solutions of various pH values
was examined. The changes in wavelength of the absorp-
tion were noted. The buffers were as follows: pH 1 to 2,
HCl/KCl; pH 3 to 7, citric acid/Na2HPO4; pH 8,
NaH2PO4/NaHPO4; pH 9, Na2B4O7/H3BO3; and pH 10,
Na2CO3/NaHCO3.
+
[M + H] : calculated 457.16, found 457.11 (Figure S3).
3 | RESULT AND DISCUSSION
3.1 | Synthesis of the colorimetric anion
sensor
2.2.9 | Practical application of fluoride
sensing
We have diazotized salicylaldehyde with 4‐nitroaniline
following a well‐established procedure,[33] where diazo-
nium coupling occurs at the para position. The resulting
azo dye of salicylaldehyde was further condensed with
4‐aminoantipyrine (Scheme 1). The bright red–colored
product obtained was purified by crystallization from ace-
tonitrile, and purity was checked by thin‐layer chroma-
tography. The sensor obtained was in good yields.
The practical suitability of sensor 2 was tested to detect
concentration of F− in a toothpaste sample. Commer-
cially available toothpaste was weighed and extracted
with various amounts of water and centrifuged to obtain
a supernatant clear solution. The supernatant solution
was further filtered and centrifuged to get clear extracts.
The estimation was done in DMSO medium where a
known volume of toothpaste extract was added to sensor
solution 2 (25 μM) in DMSO and changes in the absorp-
tion spectrum were noted. The estimated fluoride ion
concentration was calculated according to the F− amount
in the ingredient list of the toothpaste and the dilutions.
The found F− ion concentrations in the toothpastes were
obtained according to absorbance ratio response and the
calibration curve. The calibration curve was plotted from
absorption titration profile of sensor 2 (25 μM) with aque-
ous solution of F− in DMSO medium (Figure S10).
3.2 | Naked‐eye sensing
The chromogenic recognition ability of sensor 2 were
checked by addition of aqueous solutions of
tetrabutylammonium salts of F−, Br−, and I− and sodium
salts of CN−, CH3COO−, H2PO4−, HPO42−, and NO3− into
sensor solution in acetonitrile. The colorimetric response
was obtained in case of CN−,F− and CH3COO− where
yellow color of the receptor solution was changed to
crimson red in case of CN− and light red in case of F−
and CH3COO− (Fig. 1). No such characteristic color
2.2.10 | Synthesis of 4‐((E)‐(2‐hydroxy‐5‐
((E)‐(4‐nitrophenyl)diazenyl)benzylidene)
amino)‐1,5‐dimethyl‐2‐phenyl‐1H‐pyrazol‐
3(2H)‐one (2)
4‐((E)‐(2‐Hydroxy‐5‐((E)‐(4‐nitrophenyl)diazenyl)benzyli-
dene)amino)‐1,5‐dimethyl‐2‐phenyl‐1H‐pyrazol‐3(2H)‐
one was prepared by condensation of diazotized
salicylaldehyde (1) with 4‐aminoantipyrine. The
salicylaldehyde was diazotized with 4‐nitroaniline using
reported method.[29] Diazotized salicylaldehyde (59 mg,
0.22 mmol) in 5 mL of ethanol was refluxed with 4‐
aminoantipyrine (54 mg, 0.27 mmol) for 8 hours. Cata-
lytic amount of acetic acid was added for better yield.
The resulting solid was filtered, washed with chilled
SCHEME 1 Synthesis of sensor 2.

4
CHAKRABORTY ET AL.
changes were observed in the presence of other men-
tioned anions. The observations acknowledge ligand 2
as sensor for highly basic anions like CN−, F−, and
CH3COO− and not for the mild ones.
3.3 | Absorption studies on anionic
sensing
To further ratify the naked‐eye observations, the whole
sensing process was monitored through absorption spec-
trum. Sensor 2 exhibited an absorption maximum at
364 nm in acetonitrile medium. Upon addition of various
anions (50 μM), a bathochromic shift was observed in the
presence of overly basic anions such as CN−, F−, and
CH3COO− (Fig. 2). No significant changes in the spec-
trum were noticed in case of other anions that were in
the list in our study. Titrations were carried out with
25 μM of ligand 2 in acetonitrile throughout absorption
studies. In case of the most basic CN− ion, the 364‐nm
peak of the sensor exhibited decrease in intensity with a
redshift to 578 nm with increase in intensity (Fig. 3).
The changes continued until saturation at 39 μM of ana-
lyte concentration. Explicit isosbestic points were absent
in the spectrum. While titrating with F− and CH3COO−
ions, the ratiometric changes were perceived with an
enhanced redshift to 589 nm (Figs 4 and S4). Distinct
isosbestic points were noted at 290, 311, 409, and
438 nm in titration profile with F− while 313, 407,
and 445 nm were isosbestic for titration with CH3COO−.
The observed redshift in the presence of CN−, F−, and
CH3COO− articulates the fact of deprotonation of the
phenolic proton at the azo dye part owing to high basicity
of anions. Exhibition of sharp isosbestic points stipulates
at the equilibrium between phenol and phenolate anion.
The deprotonation is also justified as anions that respond
are of more basic and least hydrogen bonding character
that favors deprotonation over hydrogen‐bond formation.
FIGURE 2 Changes on absorption spectrum of sensor 2 (25 μM)
on addition of various anions (50 μM) in polar aprotic acetonitrile.
[Color figure can be viewed at wileyonlinelibrary.com]
The NMR titration was only carried out with
tetrabutylammonium fluoride where a complete experi-
ment was performed in DMSO‐d6 (Fig. 5). The antipyrine
coupled with azo dye manifests an NMR peak at
13.6 ppm for phenolic OH and at 9.9 ppm for imine bond,
and the rest are aromatic protons and protons of the anti-
pyrine part. Upon addition of 1 equivalent of F−, the peak
at 13.6 ppm disappears and there is a slight upfielded shift
of aromatic protons, and on further F− addition, more
upfield shift was noted for aromatic protons. The result
of NMR titration suggests deprotonation of phenolic
hydrogen and formation of phenolate ion, which
increases the electron density of the aromatic ring. The
1:1 stoichiometry of interaction can also be predicted
from NMR studies.
3.5 | Determination of binding constant,
binding stoichiometry, and detection limit
3.4 | 1H NMR studies
The binding constant was determined from aforemen-
tioned Benisi‐Hildebrand plot (Figure S5) using data of
the absorption titration profile. The binding constants
To understand the mechanism of interaction of anions,
the study was extended to 1H NMR experiments.
were determined to be 3.66 × 104 M−1, 6.02 × 104 M−1
,
FIGURE 1 Colorimetric changes on addition of various anions to 25 μM of sensor solution in polar aprotic acetonitrile. [Color figure can
be viewed at wileyonlinelibrary.com]

CHAKRABORTY ET AL.
5
FIGURE 3 Absorption titration of 25 μM of sensor 2 (curve 1)
with aqueous CN− up to 39 μM (curve 10) in polar aprotic
acetonitrile. Inset: Variation in absorbance at 364 and 578 nm with
changes in CN− concentration. [Color figure can be viewed at
wileyonlinelibrary.com]
FIGURE 5 1H NMR titration of sensor 2 with F− in highly polar
aprotic DMSO‐d6. [Color figure can be viewed at wileyonlinelibrary.
com]
3.6 | Emission studies on sensing
FIGURE 4 Absorption titration of 25 μM of sensor 2 (curve 1)
with aqueous F− up to 45 μM (curve 10) in polar aprotic
acetonitrile. Inset: Ratiometric changes in absorbance at 364 and
589 nm with changes in F− concentration. [Color figure can be
viewed at wileyonlinelibrary.com]
The absence of a good fluorophore in the azo dye of
salicylaldehyde makes it very poorly emissive. Herein,
the condensed product of the dye with 4‐aminoantipyrine
induces some rigidity in the structure through the imine
bond, which imparts a weak emissive character. Ligand
2 emits at 417.5 nm in acetonitrile medium. The aqueous
solutions of anions up to 120 μM were added to 50 μM of
ligand solution in acetonitrile. Except in CN−, no distinct
changes were observed for other anions (Fig. 6). Sensor 2
turned out to be turn‐on sensor for CN− ion (Fig. 7). The
acetonitrile solution of sensor 2 was titrated with increas-
ing CN− ion concentration till saturation at 118 μM.
Turn‐on CN− sensing was examined in competitive
environments. The competitive anions were firstly added
to acetonitrile solution of sensor 2, an equal amount of
and 3.40 × 104 M−1 for CN−, F−, and CH3COO−, respec-
tively. The forenamed Job's method of continuous varia-
tion was employed to obtain the stoichiometry of
interaction. The plot (Figure S6) suggests 1:1 stoichiometry
for all three ions, viz, CN−, F−, and CH3COO−. The result
acquired complemented the result of the 1H NMR titration.
The important parameter that characterizes a sensor is its
detection limit. The detection limit was obtained in the
way mentioned in Section 2 from the calibration curve
(Figure S7). Sensor 2 has detection limits of 0.604, 0.380,
and 0.347 nM for CN−, F−, and CH3COO−, respectively.

6
CHAKRABORTY ET AL.
FIGURE 8 Turn‐on emission of sensor 2 in the presence of
aqueous CN− in competitive environment of other anions
(solvent: polar aprotic acetonitrile). [Color figure can be viewed at
wileyonlinelibrary.com]
FIGURE 6 Changes on emission spectrum of sensor 2 (50 μM)
on addition of various anions (120 μM) in polar aprotic
acetonitrile. [Color figure can be viewed at wileyonlinelibrary.com]
CN− ion was added, and the emission spectrum was run.
The enhancement in emission was well maintained in
the presence of other anions (Fig. 8), which signifies the
turn‐on CN− sensing to be a selective and specific one.
significant changes in wavelength with solvent polarity
except highly polar electron pair–donating solvents like
DMF and DMSO (Figure S8). In DMF and DMSO, the
absorption band exhibited a redshifted hump at 600 and
617 nm, respectively. When the absorption spectrum of
sensor 2 was taken on various pH values, no noteworthy
change in spectrum was noticed (Figure S9). Thus, it can
be further exploited in a wide range of pH for future use.
3.7 | Effect of pH and solvent on the
sensor
To further extend the utility of the designed sensor, the
effect of pH and other solvents must be known. The
absorption spectrum of the compound 2 was viewed on
various solvents, viz, ethanol, water, hexane tetrahydrofu-
ran, toluene, DMF, DMSO, chloroform, and acetonitrile.
In the absorption spectrum of the sensor, there were no
3.8 | Practical application of fluoride
sensing
The method to test practical utility of sensor 2 to detect
concentration of fluoride in toothpaste has been per-
formed as discussed in Section 2. The working procedures
have been already discussed in Section 2. The found
fluoride ion concentration in the toothpastes was deter-
mined from ratiometric absorbance response and the
calibration curve. The calibration curve was obtained
from spectrophotometric titration data of sensor 2 with
aqueous solution of F− in DMSO medium (Figure S10).
It is noted that the determined fluoride concentrations
were in agreement with the estimated data as per the
ingredient list (Table 1).
TABLE 1 Results of determination of F− in toothpaste samples.
Estimated [F−], μM
Found [F−], μM
Recovery, %
6.48
9.72
12.94
7.00
108
130
107
FIGURE 7 Emission titration of 50 μM of sensor 2 (curve 1) with
aqueous CN− up to 118 μM (curve 13) in polar aprotic acetonitrile.
[Color figure can be viewed at wileyonlinelibrary.com]
12.73
13.88

CHAKRABORTY ET AL.
7
[11] Chakraborty N, Bhuiya S, Chakraborty A, Das S. Indian J
Chem. 2018;57A:59.
4 | CONCLUSION
[12] Chakraborty N, Bhuiya S, Chakraborty A, Mandal D, Das S. J
Photochem Photobiol A. 2018;359:53.
We have designed an azo dye‐based Schiff base as a selec-
tive chromogenic sensor for aqueous anions such as CN−,
F−, and CH3COO− via proton abstraction in acetonitrile
medium. The stoichiometry of anionic interaction was
concluded to be 1:1. Good detection limits of approxi-
mately 10−10 M were reported for the anions detected.
The detection limits are much bellow the maximum
allowed limit for CN− and F− in drinking water as recom-
mended by the World Health Organization (WHO). The
sensor developed has been inferred to be a specific turn‐
on emission sensor only for CN−. The utility of chromo-
phore 2 has been extended to determine F− in an oral
care product with good estimation. The compound has
also been concluded to be stable in wide range of pH
and solvent polarity. Thus, the antipyrine‐coupled azo
dye of salicylaldehyde is a new attribute to the world of
anion sensing, which has a good future prospect.
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ACKNOWLEDGMENTS
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[23] Vallejos S, Estevez P, Garcia FC, Serna F, Jose L, Garcia JM.
AC gratefully acknowledges financial supports from Sci-
ence and Engineering Research Board, DST, Government
of India (SERB no. SR/FT/CS‐116/2010), and SD grate-
fully acknowledges the research support under RUSA
2.0 Grant to Jadavpur University, Kolkata, provided by
Ministry of Human Resource Development, Government
of India (reference no. R‐11/101/19 dated 15(19)‐02‐
2019). NC thanks University Grants Commission, Gov-
ernment of India, for Senior Research Fellowship. We
also thank the Department of Chemistry, Maulana Azad
College (Kolkata, West Bengal, India) and Department
of Chemistry, Jadavpur University (Kolkata, India).
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ORCID
Suman Das https://orcid.org/0000-0002-4148-1002
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
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How to cite this article: Chakraborty N,
Chakraborty A, Das S. Schiff base derived from
salicylaldehyde‐based azo dye as chromogenic
anionic sensor and specific turn‐on emission sensor
for cyanide ion. J Heterocyclic Chem. 2019;1–7.
https://doi.org/10.1111/jhet.3693
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