Highly selective hydrazone based reversible colorimetric chemosensors for expeditious detection of CN? in aqueous media

Article abstract of DOI:10.1016/j.ica.2018.01.013

Herein we have described the design and syntheses of two novel hydrazone based N, O donor Schiff-base colorimetric sensors L₁ and L₂ for selective sensing of cyanide ions in 2:1 CH₃CN-H₂O mixture. L₁

Full text of DOI:10.1016/j.ica.2018.01.013

Inorganica Chimica Acta 474 (2018) 22–29
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Highly selective hydrazone based reversible colorimetric chemosensors
ꢀ
for expeditious detection of CN in aqueous media
⇑
Jahangir Mondal, Amit Kumar Manna, Goutam K. Patra
Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G.), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we have described the design and syntheses of two novel hydrazone based N, O donor Schiff-base
Received 30 October 2017
Received in revised form 12 January 2018
Accepted 15 January 2018
1 2 3 2 1 2
colorimetric sensors L and L for selective sensing of cyanide ions in 2:1 CH CN-H O mixture. L and L
1
have been characterized by H NMR, IR spectroscopy, HRMS spectrometry and elemental analyses.
ꢀ
Interactions of L
to pink (for L
1
and L
2
with CN provide remarkable color change from yellow to red (for L
1
) and yellow
ꢀ
2
) along with change in the absorption maxima, enabling naked-eye sensing of CN ion,
1
without use of any expensive equipment. From the job’s plot analyses, HRMS spectral studies and
NMR analyses, 1:1 binding stoichiometry of chemosensor L and 1:2 binding stoichiometry of chemosen-
sor L towards cyanide ion have been confirmed. The detection limits reach up to 1.3 mM for L and 1.0
mM for L which are lower than the maximum permissible level of CN in drinking water set by WHO. The
other competitive anions (OAc , F , Cl , Br , I , H PO , NO , NO , HSO , N , CO , PO , S , and BO3
H
Keywords:
Schiff base
Hydrazones
1
2
1
ꢀ
Colorimetric sensor
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
3ꢀ 2ꢀ
4
3ꢀ
ꢀ
)
CN sensor
2
4
2
3
4
3
3
Reversible sensor
showed very negligible interference for detection of cyanide at that concentration level. The sensing
mechanism has further been confirmed by DFT studies. Both the chemosensors L
successfully applied for the determination of CN ions in real water samples and simulated urine
1
and L
2
have been
ꢀ
samples.
Ó 2018 Elsevier B.V. All rights reserved.
1
. Introduction
The development of chemosensors for anions has become a sub-
focuses on novel colorimetric cyanide sensors, which allow naked-
eye detection of the color change without resorting to the use of
expensive instruments [18–21]. Colorimetric materials have
advantages such as low cost, a rapid response rate, easy detection
and high selectivity [22–26]. Therefore, colorimetric sensors that
are capable of recognizing cyanide in an aqueous environment
are of current interest.
ject of intense research interest, because anions play important
roles in a wide range of environmental, clinical, chemical, and bio-
logical applications [1–5]. Among the various anions, cyanide is
one of the maximum concerned, because it is one of the most
rapidly acting and powerful poisons. Its toxicity results from its
propensity to bind to the iron in cytochrome c oxidase, interfering
with electron transport and resulting in hypoxia [6–11]. Cyanide
could be absorbed through the lungs, gastrointestinal tract and
skin, leading to vomiting, convulsion, loss of consciousness, and
eventual death [12–14]. In spite of its toxicity, its application in
various areas as raw material for synthetic fibers, resins, herbi-
cides, and the gold extraction process is unavoidable [15–17],
which releases cyanide into the environment as a toxic contami-
nant. Thus, there is a need for an effective sensing system to mon-
itor cyanide concentration from pollutant sources. Among various
approaches such as fluorescence techniques and electrochemical
methods for the detection of cyanide, the most attractive approach
Hydrazone derivatives form an important class of most widely
used organic compounds and have versatile applications in various
advanced fields including the ink jet printing [27], pigments [28],
photography [29], molecular recognition [30], multidentate
ligands [31] and fluorescent materials [32]. These are also found
to be significant due to their extensive applications in pharmaceu-
tical and biological activities like anticancer, anti-inflammatory,
antibacterial, antifungal, antimalarial, antitubercular activities
[33], and potential inhibitor for various enzymes [34]. Hydrazones
containing chromophoric group like nitro are having hydrogen
donor capability and are reported as good anion receptors
[35,36]. The success of hydrazone derivatives is also due to the
simplicity of their synthesis, to the many possibilities presented
by variation of the diazo compounds and coupling components,
the generally high molar extinction coefficient as well as to the
good light and wet fastness properties [37]. Although many optical
⇑
Corresponding author.
E-mail address: patra29in@yahoo.co.in (G.K. Patra).
https://doi.org/10.1016/j.ica.2018.01.013
020-1693/Ó 2018 Elsevier B.V. All rights reserved.
0

J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
23
sensors for cyanide detection have been reported based on H-
bonding interaction [38], nucleophilic addition reaction [39], metal
complex displacement approach [40], most of them suffer from
various limitations such as lengthy synthetic protocol, severely
interference from F and AcO . In these connections and in contin-
uation to our earlier research [41], we have reported here two
5.5–6.5) by addition of diluted HCl. The precipitate was finally re-
crystallized in MeOH/H O to obtain the pure product.
2
To 25 mL of an anhydrous methanol solution of benzildihydra-
zone (0.238 g, 1 mmol), the azo dye, 2-(4-nitrophenyl)diazenyl-2-
hydroxybenzaldehyde (1, 0.542 g, 2 mmol) was added. The result-
ing yellow mixture was refluxed for 6 h, under dry condition. Then
it was slowly cooled to room temperature. Yellow crystalline solid
separated out, which was filtered off and dried in air. Yield, 0.543 g
ꢀ
ꢀ
novel reversible chromogenic chemosensors L
1
and L
2
containing
ꢀ
hydrogen donor for the detection of CN in 2:1 CH
3
CN-H O mix-
2
ꢀ
ture. Interactions of L
1
and L
2
with CN provide remarkable color
28 10 6
(73%); mp > 200 °C Anal. found (calc. for C40H N O ): C, 64.50
change from yellow to red (for L
1
2
) and yellow to pink (for L ).
(64.52%); H, 3.75 (3.79%); N, 18.85 (18.81%). HRMS: 744.22 (M,
ꢀ1
1
00%) (Fig. S5). FTIR/cm (KBr): 3444(br), 3059(w,br), 1614(vs)
(
6
8
(
C@N), 1516 (vs), 1317(vs), 1344(m), 1279(m), 1116(m), 860(m),
2
. Experimental
.1. General information
Melting points were determined on an X-4 digital melting-point
1
3
89(w) (Fig. S6). H NMR (200 MHz, CDCl , TMS): d 11.58 (s, 2H)
.91 (s, 2H), 8.35 (m, 5H), 7.94–8.00 (m, 12H), 7.49 (m, 5H), 6.99
2
3
ꢀ1
ꢀ1
3
s, 2H) (Fig. S7). UV–VIS: kmax/nm(e/dm mol cm )(CH CN):
3
14 (37 800); 374 (44 450).
apparatus and not corrected. UV–Visible spectra were recorded on
a Shimadzu UV 1800 spectrophotometer using a 10 mm path
2
.4. UV–Vis titrations
length quartz cuvette. 1H NMR spectra of ligand L
and L
at room temper-
ature and NMR titrations were carried out dissolving L and L in
O. The chemical shifts are reported
1
2
were
The chemosensors L
.01 mmol) were dissolved in acetonitrile-water solvent mixture
L of them were diluted to 3 mL with
the solvent mixture to make a final concentration of 10 M.
Sodium cyanide (0.1 mmol) was dissolved in 10 mL of triple dis-
tilled water and 1.5–90 L of the anions solution (10 mM) were
transferred to the solutions of L and L (10 M) prepared above.
1 2
(3.90 mg, 0.01 mmol) and L (7.44 mg,
recorded on a Bruker Ultra shield 200 using CDCl
3
0
1
2
6
(2:1, v/v, 10 mL) and 30 l
DMSO d and cyanide salts in D
2
l
in d values (ppm) relative to TMS. High resolution mass (HRMS)
spectra were recorded on a Waters mass spectrometer using mixed
solvent HPLC methanol and triple distilled water. All the chemicals
and metal salts were purchased from Merck and the counter ion
was used as nitrate for metal and sodium for anions.
l
1
2
l
After mixing them for a few seconds, UV–Vis spectra were obtained
at room temperature.
2.2. Synthesis and characterization of the probe L
1
2.5. Colorimetric test kit
To 25 mL of an anhydrous methanol solution of benzil (0.420 g,
mmol), 2,4-(dinitrophenyl)hydrazine (0.396 g, 2 mmol) was
added. The resulting pale yellow mixture was refluxed for 6 h,
under dry condition. Then it was slowly cooled to room tempera-
ture. Yellow crystalline solid separated out, which was filtered off
and dried in air. Yield, 0.546 g (70%); mp 178 °C Anal. found (calc.
Chemosensors L (3.90 mg, 0.01 mmol) and L (7.44 mg, 0.01
1
2
2
mmol) were dissolved in acetonitrile-water solvent mixture (2:1,
v/v) (10 mL) to get 1 mM solution. Test kits were prepared by
immersing filter-papers into these solutions (1 mM), and then
ꢀ
dried in air to get rid of the solvent. CN and different anions
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ ꢀ ꢀ ꢀ 2ꢀ
3ꢀ
4
(
S
OAc , F , Cl , Br , I , H
2
PO
4 2 3 4 3 3
, NO , NO , HSO , N , CO , PO ,
14 4 5
for C20H N O ): C, 61.54 (61.53%); H, 3.62 (3.61%); N, 14.35
2
ꢀ
3ꢀ
+
ꢀ1
, and BO
3
; 0.001 mmol) were dissolved in acetonitrile-water
(
(
1
14.33%). EI-MS: m/z 390.10 (M , 100%) (Fig. S1). FTIR/cm
solvent mixture (2:1, v/v, 10 mL) to prepare 0.1 mM solution. The
test kits prepared above were dipped into above-mentioned ace-
tonitrile-water mixture (2:1, v/v) of cyanide ion and other anions
and then dried at room temperature.
KBr): 3441(br), 3269(m), 3112 (w), 2926(w), 1657(w) (˜C@O),
621(vs) (C@N), 1592(s), 1506(s), 1413(m), 1342(s), 1306(s),
1
1
227(m), 1012(m), 940(w), 840(w), 747(w), 589(w) (Fig. S2). H
3
NMR (200 MHz, CDCl , TMS): d 11.91 (s, 1H), 9.08 (s,1H), 8.41 (d,
1
H), 8.21 (d, 1H), 7.94 (d, 2H), 7.69 (m, 3H), 7.42–7.51 (m, 2H)
1
3
(
1
1
Fig. S3). C NMR (200 MHz, CDCl
3
, TMS): d 194.27, 151.42,
44.62, 139.07, 135.53, 134.57, 130.81, 129.86, 129.43, 129.02,
27.21, 123.75, 123.21, 116.57, 115.25 (Fig. S4). UV–VIS kmax/nm
2.6. Computational details
The GAUSSIAN-09 Revision C.01 program package was used for
all calculations [43]. The gas phase geometries of the compound
was fully optimized without any symmetry restrictions in singlet
ground state with the gradient-corrected DFT level coupled with
the hybrid exchange-correlation functional that uses Coulomb-
attenuating method B3LYP [44]. Basis set 6-31++G was found to
be suitable for the whole molecule.
3
ꢀ1
ꢀ1
3
(e/dm mol cm )(CH CN): 374 (45,600).
2
.3. Synthesis and characterization of the probe L
2
For the synthesis of the probe L , the azo dye 2-(4-nitrophenyl)-
2
diazenyl-2-hydroxybenzaldehyde (1) was prepared first by the fol-
lowing way.
The diazonium salts from aniline derivatives were prepared in
good yield according to the previously described methods [42]
with slight modification. After completion of diazotization reac-
tion, in a second flask, salicylaldehyde was dissolved in water con-
taining sodium hydroxide and sodium carbonate and cooled to 0 °C
in an ice bath. The diazonium solution was slowly added to the
phenolate solution in basic medium by adjusting the pH at 7.5–
3. Results and discussion
1 2
3.1. Synthesis and structures of L and L
Receptor L
sation reaction of benzil and 2,4-(dinitrophenyl) hydrazone in
anhydrous methanol, whereas the receptor L was prepared by
1
was obtained in good yield (70%) by the 1:1 conden-
2
8
.5 over 30–45 min. The resulting solution was stirred for 1–2 h
the 1:2 condensation reaction of the azo dye 2-(4-nitrophenyl)di-
azenyl-2-hydroxybenzaldehyde and benzildihydrazone. Both the
probes were characterized by H NMR, IR and HRMS spectrometry
in an ice bath and then allowed to reach room temperature to
obtain a precipitate. The precipitate was collected and was washed
several times with cool water after acidified of the solution (pH =
1
and elemental analyses. Optimized geometries and surface plots of

2
4
J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
the chromophores L
1
and L
2
and their deprotonated species L
1
-H+
metal ions did not show any significant color change. These indi-
cate that the probes L and L can serve as ‘naked-eye’ cyanide
indicators. The spectral responses were also investigated upon
+
and L
2
-2H are shown in Figs. 1 and 2 respectively.
1
2
ꢀ5
3.2. Colorimetric and UV–vis spectral studies
addition of 10 equiv. of various anions to L
solution in acetonitrile-water (2:1, v/v) mixture (Fig. 4). Absorption
spectra of free receptors L and L in acetonitrile water (2:1, v/v)
mixture exhibited same absorption band 374 nm. A large batho-
1
and L
2
(2 ꢁ 10 M)
In order to determine the solvatometric behavior of the synthe-
sized chemosensors, we have recorded the absorption spectra L
and L in different solvents (Figs. S8 and S9) with different polarity
Table 1) as the absorption spectra of the colorant materials in
1
2
1
chromic shift (
spectra for L
D
k
max) of 136 nm was observed in the absorption
2
ꢀ
(
1
in the presence of CN ions whereas for (
D
kmax) of
solution depend strongly on the solvent nature and solvent–solute
interactions. With increase in solvent polarity, maximum absorp-
tion band (kmax) of the chemosensors L
129 nm was observed in the absorption spectra for L
ence of CN ions. In Fig. S10 of Supporting Information, bar graphs
2
in the pres-
ꢀ
and L
2
showed bathochro-
1 2
showing the relative absorption intensities of L and L upon treat-
1
mic shifts (positive solvate chromism). The new absorption bands
at higher wavelengths are due to the interaction at the H-atoms of
ment with various anions have been given.
The large bathochromic shifts were attributed due to the depro-
amine moiety in L
relatively basic solvent DMSO. These results suggest that in polar
solvents, the ground states of the tested chemosensors L and L
are better stabilized relative to the excited state. For this reason,
moderate polar solvent acetonitrile was selected as the best sol-
1
and of hydroxyl group of the chemosensor L
2
, in
tonation of the –NH proton and –OH proton (from L
1
and L
2
respec-
ꢀ
tively) by the CN ion and as a result of which the electron density
on nitrogen and oxygen atoms are increased and the electrons are
rearranged in the whole molecule leading to the more stable delo-
calized structures. Moreover due to the presence of chromophoric
1
2
vent for the colorimetric sensing of chemosensors L
1
and L
2
.
azo group (N@N) and the electron-deficient –NO
2
group,
In order to investigate the selectivity of the probes L
1
and L
2
col-
intramolecular charge transfer (ICT) processes become more effi-
cient. The other anions cannot show any significant absorption
band shift as well as color change indicating no such de-
protonation.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
orimetrically towards various anions such as OAc , F , Cl , Br , I ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
3ꢀ
2ꢀ
3ꢀ
H
2
PO
detection was carried out in 2:1 (v/v) CH
is observed by the naked eye, L and L showed a remarkable color
and yellow to pink for L upon the
4
, NO
2
, NO
3
, HSO
4
, N
3
, CO
3
, PO
4
, S , and BO
3
, visual
3
CN-H O mixture. As it
2
In order to determine the sensitivities of L
1
and L
2
towards
1
2
ꢀ
change from yellow to red for L
addition of 10 equiv. of CN ions (Fig. 3). In contrast, the other
1
2
CN ion, we have carried out UV–vis spectrophotometric titrations
ꢀ
ꢀ
by adding a standard solution of CN as their sodium salts to the
1 2
Fig. 1. Geometry optimized diagrams of the molecules (a) L and (b) L .
1 1 2 2
Fig. 2. Surface plots of HOMO and LUMO of (a) L and L -H ; (b) L and L
-2H+ with their energy differences.
+

J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
25
ꢀ
Table 1
cating single product was formed after the addition of CN in each
case.
1 2
Absorption properties of L and L in various solvents.
Solvent
k
max(nm) for L
1
k
max(nm) for L
2
1 2
From the above titration experiments of L and L , ratiometric
calibration curves have been determined, as it is essential to
increase dynamical behaviors of the chemosensors towards cya-
nide detection. The curves show linear dependence of the ratio of
Methanol
Chloroform
Acetonitrile
DMSO
378
372
374
393, 525
355
369
378
374
571
–
the absorbances at 510 nm and 374 nm (A510/A374) for L
1
and
n-Hexane
ꢀ
5
05 nm and 374 nm (A505/A374) for L
2
as function of CN concen-
tration (Fig. S11, Supporting Information), which enables ratiomet-
ꢀ
ric quantification of CN . Good linearity was observed in the range
ꢀ
5
CH
3
CN-H
2
O (2:1, v/v) solution of L
1
and L
2
(2.25 ꢁ 10 M) (Fig. 5).
of 0–1.2 mM. From these calibration curves detection limit can be
calculated based on the formula 3SD/m where SD is the standard
deviation of the blank solution and ‘m’ is the slope of the calibra-
ꢀ
On the treatment of CN ions, the intensity of absorption band at
74 nm decreased significantly for both the ligand L and L , and
1 2
3
new absorption bands at 510 and 503 nm appeared respectively
tion curve. The calculated values of detection limit are 1.3
L1 and 1.0 M for L . These values are lower than the permissible
limit (1.9
lM for
ꢀ
for L
1
and L
2
, on the addition of 10 equiv. of CN ion. Clear isos-
l
2
bestic points were observed at 435 and 408 nm for L
1
and L
2
indi-
lM) in drinking water set by the WHO.
Fig. 3. Color changes of (a) L
1
and (b) L
2
3 2
after addition of 10 equiv. of different anions in CH CN-H O (2:1, v/v) mixture.
(2.25 ꢁ 10ꢀ M) before and after addition of 10 equiv. of different anions in CH
5
Fig. 4. UV–vis spectral responses of (a) L
1
and (b) L
2
3 2
CN-H O (2:1, v/v) mixture.

2
6
J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
3 2
Fig. 5. UV–vis titration of L with cyanide ion (0–10 equiv.) in CH CN-H O (2:1, v/v) solution.
ꢀ
To determine the binding selectivity of CN ion towards L
1
and
ger interaction is present between host and guest, which shifted
the maximum absorption band towards higher wavelength.
L
2
in presence of other interfering ions the UV–visible spectra was
studied in the presence of various competing anions such as OAc ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ 3ꢀ
2ꢀ
F , Cl , Br , I , H
2
PO
4
, NO
2
, NO
3
, HSO
4
, N
3
, CO
were treated with
3 4
, PO , S , and
3.3. pH and time dependence
3
ꢀ
BO
5
3
. For competition tests, receptors L
1
and L
2
ꢀ
equiv. of CN in the presence of 6 equiv. of other anions. No inter-
The pH dependence of L
troscopy using 100 mM HEPES buffer. No significant change
occurred in pH less than 6 for both L and L probably due to
1 2
and L were checked by UV–vis spec-
ꢀ
ference was observed for the detection of CN in the presence of
other anions (Fig. S12, Supporting Information). This result indi-
1
2
cates that the chemosensors L
1
and L
2
have excellent selectivity
towards CN in presence of competing ions even OAc , F and
hydrolysis of imine linkage. As the pH raises (pH = 7–8), the inten-
sity of the absorption band at ꢃ500 nm increases; this is due to an
increase in the hydroxyl ions concentration which interacts with
the –NH and –OH protons of the chemosensors L and L . It was
1 2
also observed that, the interaction of chemosensors with the amine
or hydroxyl proton occurred effectively at pH values higher than 8
ꢀ
ꢀ
ꢀ
ꢀ
H
2
PO
4
ions.
In order to determine the binding stereochemistry of L
towards cyanide ion Job’s continuous variation method was used.
For the probe L absorption band of wavelength 510 nm reaches
maximum intensity when mole fraction of cyanide ion within the
range of 0.45–0.55, indicating that the receptor L
: CN ] ratio (Fig. S13, Supporting Information).
, absorption band of wavelength 505 nm
reaches maximum intensity when mole fraction of cyanide ion at
1 2
and L
1
(Fig. S16, Supporting Information). This effect has been attributed
ꢀ
1
binds to CN
to the de-protonating of amine and phenolic group of the probes
in highly basic conditions. These findings indicated that the syn-
thesized chemosensors were sensitive to the pH values, and can
be used any pH range from 6 to 12.
ꢀ
ions in a 1:1 [L
1
But in the case of L
2
ꢀ
around 0.38, indicating that the receptor binds to CN ions in a
The time evolution of the receptors L
1
and L
2
in the presence of
ꢀ
1
:2 [L
2
: CN ] ratio (Fig. S14, Supporting Information).
From UV–Vis spectral change, the binding constants for the for-
mation of the respective complexes were evaluated using the Ben-
esi-Hildebrand (B-H) plot, Eq. (1).
ꢀ
5
3 2
equiv. of CN ion in CH CN–H O (2:1 v/v) were also investigated.
The recognition interactions get almost completed just after the
ꢀ
addition of 5 equiv. of CN ion and the absorbance intensity
remains almost the same up to 15 min. This ensures that the recep-
n
1 2
tor L and L to be a sensitive sensor.
1
=ðA ꢀ A
o
Þ ¼ 1=fKðAmax ꢀ A
o
Þ½Cꢂ g þ 1=ðAmax ꢀ A
o
Þ
ð1Þ
where A
o
is the absorbance of free receptor L
1
/L
2
at particular wave-
3
.4. Reversibility test
length, and A is the observed absorbance at that wavelength in any
intermediate metal ion concentration (C). Amax, is the absorbance
intensity value that was obtained in presence of anion at saturation.
Assuming 1:1 stoichiometry for L
stant (K) have been determined from the intercept of the linear
plot (Fig. S15a, Supporting Information). Considering 1:2 stoi-
chiometry for L
determined from the intercept of the linear plot (Fig. S15b, Sup-
porting Information). The free energy changes of complex forma-
tion reactions have been determined using the relation,
To examine the reversibility of the receptors L
1
and L
2
towards
ꢀ
CN ion we have added hydrochloric acid (HCl, 5 equiv.) to the
guest adduct L-CN (Fig. 6). The red and pink color solution of
ꢀ
-CN adduct, binding con-
ꢀ
1
ꢀ
ꢀ
L
1
-CN and L
2
-CN adducts reappear to original host yellow color
after addition of HCl solution along with the disappearance of red
shifted band at 510 and 505 nm for L
excess CN into the solution again, the color and the absorbance
were recovered. The color changes were almost reversible even
after several cycles with the sequentially alternative addition of
CN and HCl. These results indicate that receptors L
be recyclable than any irreversible base catalysed reaction and fur-
ther confirm that both the probes L
the deprotonation mechanism.
ꢀ
2
-CN adduct, binding constant (K) has also been
and L
. Upon addition of
1
2
ꢀ
DG =
ꢀRTlnK (Table 2). Higher the value of negative free energy, stron-
ꢀ
1
and L
2
could
ꢀ
1
and L
2
would detect CN by
Table 2
Calculated thermodynamic parameters for L-CN (where L = L
ꢀ
1
and L
2
).
In order to be confirmed whether the color and spectral changes
initiated from the ICT mechanism through the de-protonation of –
Chemosensor
Binding const. (K) (Mꢀ1)
Free energy (ꢀ
DG) (kJ/mole)
5
4
L
L
1
1.71 ꢁ 10
2.91 ꢁ 10
NH and –OH moiety and the interactions between chemosensors L
1
1
1
4
2
2.77 ꢁ 10
6.52 ꢁ 10
ꢀ
and L
2
and OH were also conducted. The color and UV–vis

J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
27
ꢀ
1 2
Fig. 6. Change of absorbance spectra of (a) L and (b) L on alternate addition of CN and HCl.
ꢀ
ꢀ
spectral change of the probes L
1
and L
2
upon addition of OH were
L
1
completely disappeared after the addition of 1.5 equiv of CN
ꢀ
nearly identical to those of addition of CN except slightly less
ion to the probe L
1
. The other sharp IR peaks become broad due
intensity, which support the deprotonation mechanism of L by
to delocalization effect of whole molecule which demonstrates
ꢀ
ꢀ
CN ion, which have further been supported by HRMS studies. In
that chemosensor L
1
reacted with CN ion. For L
2
, addition of 2.5
+
ꢀ
HRMS, the peak around 389 was assignable to L
1
–H (Fig. S17 of
equiv. of CN ion to the probe the stretching vibration peaks at
ꢀ
1
Supporting Information) and the peak around 742 was assignable
3444 cm (–OH) of chemosenor L
2
also completely disappeared
–2H+ (Fig. S18, Supporting Information). The de-protonation
(Fig. S20b, Supporting Information). Therefore, these results of H
1
to L
2
+
mechanism has also been verified by the DFT study of the L
1
–H
NMR experiments and IR studies validate the de-protonation of
amine group and hydroxyl group during the anion recognition pro-
cess as shown in Scheme 1.
+
and L
eV) and L
2
–2H+ species. The HOMO-LUMO energy in L
–2H+ (3.471 eV) are much lower than that in free L
3.087 eV) and L (6.031 eV) (shown in Fig. 2).
1
–H (1.815
2
1
(
2
The chemosensors L
withdrawing –NO groups, which increase the acidity as well as
the H-bond donating ability of the H-atom of the –NH/–OH moiety
towards suitable anions. In the aqueous solution of anions, H
1 2
and L contain azo groups and electron-
2
3.5. Sensing mechanism
2
O
itself compete with the anions for the binding sites of receptor
and therefore, could disturb hydrogen bonding interactions
between CN and the receptor. Additionally there is a red shifted
band above 500 nm of the receptors L and L in basic solvent
1 2
DMSO which has less capacity to participate in H-bond formation.
Thus deprotonation predominant over H-bonding interaction;
In order to study the nature of the interactions between the
with CN ion, 1H NMR experiments have
ꢀ
chemosensors L
1
and L
2
ꢀ
been conducted. Fig. S19a of Supporting Information shows the
1
H NMR spectra of L
of CN ion (in DMSO d
1
in the absence and presence of 1.5 equiv.
ꢀ
1
6
and D
2
O mixture). It is clear from the
H
ꢀ
NMR spectra that the addition of 1.5 equiv. of CN results in the
disappearance of the proton signal at d 11.91 (–NH proton). From
Fig. S19b it can also be seen that on the addition of 2.5 equiv. of
which is also confirmed by NMR and IR studies. The pK
a
value sup-
ꢀ
ꢀ
ꢀ
ports the selectivity of CN ion over competing ion F , OAc and
ꢀ
H
2
PO
4
ions in aqueous solution. The pKa value of the two ligands
have been obtained by the pH-dependent UV–vis spectra
ꢀ
CN the proton signal at d 11.58 (–OH proton) disappeared. More-
L
1
and L
2
over, the other aromatic protons are shifted slightly towards up
field for both the cases (Fig. S19, Supporting Information).
using the following Eq. (2)
To further investigate the interactions between the probes L
1
pKa ¼ pH ꢀ log½ðAacidic ꢀ AintÞ=ðAint ꢀ AbasicÞꢂ
ð2Þ
ꢀ
ꢀ
2
and L and CN ions, the infrared spectra of L and L-CN adduct
were compared. As shown in Fig. 20a of Supporting Information,
the stretching vibration peaks at 3441 cm (–NH) of chemosenor
where, Aacidic is the absorbance in acidic condition, Aint is the inter-
mediate absorbance (absorbance at pH at 7) and Abasic is the
ꢀ1
1 2
Scheme 1. Proposed sensing mechanism of (a) L and (b) L .

2
8
J. Mondal et al. / Inorganica Chimica Acta 474 (2018) 22–29
Table 3
L
towards various anions were tested visually and with UV–vis
absorption spectrometry. The probes L and L are having two
strong electron withdrawing groups (–NO ) and showed remark-
) and yellow to pink
2
1
Determination of cyanide in different water samples using L .
1
2
Samples
Added
mM)
Found
(mM)
Recovery
(%)
RSD*
(%)
2
(
able color change from yellow to red (for L
(for L
solution. The detection limits reach up to 1.3 mM for L
mM for L , far lower than the WHO permissible limit. The solva-
tochromic behaviors of the chemosensors L and L revealed that
1
ꢀ
Tap water
Rain water
10
10.9
19.7
109
98
2.1
1.5
2
) upon addition of CN ion in acetonitrile-water (2:1, v/v)
20
1
and 1.0
10
11.7
21.2
117
106
1.8
2.7
2
20
1
2
Simulated urine
sample
10
20
9.3
19.1
93
95
2.2
1.3
there are large bathochromic shifts upon increasing solvent polar-
ity and pH values. The sensing mechanisms have been confirmed
by H NMR experiments, HRMS and IR analysis as well as DFT stud-
1
*
Three independent measurements.
ies. L
1
and L
2
can be successfully applied for the determination of
ꢀ
CN ion in real samples.
absorbance in basic condition. We considered Aacidic, in the absor-
bance at pH = 6 (because of the fact that at lower pH the imine
Acknowledgements
bonds of the Schiff bases L
at pH at 10. Calculated pKa values of the ligands L
and 8.18 respectively. The pK value of HCN in water (9.30) is much
higher than that of other acids (pK = 4.75 and 3.17 for HOAc and HF
in water respectively) and those of the chemosensors L and L . Sol-
1
and L
2
may break) and Abasic, absorbance
G.K.P would like to thank the Department of Science and Tech-
nology and Department of Biotechnology, Government of India,
New Delhi for financial support.
1
and L are 8.30
2
a
a
1
2
ꢀ
ꢀ
Appendix A. Supplementary data
vation of F and OAc in water is high for small size and small
radius. As a result their basicity is decreased; they cannot show
ꢀ
ꢀ
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.ica.2018.01.013.
any competition with CN ions. CN ion has less solvation in water,
ꢀ
ꢀ
so the weaker H-bonding ability in comparison with F and AcO ,
1 2
which also favours the deprotonation in the probes L and L and
therefore result in high selectivity for cyanide ion.
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