Highly sensitive sensing of Fe(III) harnessing Schiff based ionophore: An electrochemical approach supported with spectroscopic and DFT studies

Article abstract of DOI:10.1016/j.molliq.2021.115954

The vital role of iron in biological systems and health implications posed to humans due to associated toxicity by the consumption of iron contaminated food or drinking water and exposure to other environmental sectors demand sensitive methods for its det

Full text of DOI:10.1016/j.molliq.2021.115954

Journal of Molecular Liquids 333 (2021) 115954
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Highly sensitive sensing of Fe(III) harnessing Schiff based ionophore: An
electrochemical approach supported with spectroscopic and DFT studies
⇑
Sarbjeet Kaur, Bilal Ahmad Shiekh, Damanjit Kaur, Inderpreet Kaur
Department of Chemistry, Centre for Advanced Studies, Guru Nanak Dev University, Amritsar, Punjab 143005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The vital role of iron in biological systems and health implications posed to humans due to associated
toxicity by the consumption of iron contaminated food or drinking water and exposure to other environ-
mental sectors demand sensitive methods for its determination in various medicinal, biological and envi-
Received 2 December 2020
Revised 15 March 2021
Accepted 19 March 2021
Available online 25 March 2021
0
ronmental samples. In the current work, a potent Schiff-base ionophore 1,1 -((ethane-1,2-diylbis(oxy))bis
(2,1-phenylene))bis(N-p-tolylmethanimine) (SB) was synthesized and analysed for its redox behaviour
using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) followed by its application for
Keywords:
Fe detection
Schiff base
Voltammetric sensor
Carbon paste electrode
Potentiometric sensor
DFT studies
3+
3+
development of Fe sensing methods. The voltammetric sensor exhibited sensitivity towards Fe in
3+
-8
the linear concentration range of 1–19 mM with a detection limit of 5.57 ꢀ 10 M. The carbon paste elec-
3
+
trode incorporating SB (CPE-SB) showed sensitive potentiometric response for Fe with a Nernstian
-7
-2
slope of 18.6 ± 0.2 mV/decade over a wide concentration range of 1.0 ꢀ 10 -1.0 ꢀ 10 M with a detection
-8
limit of 5.0 ꢀ 10 M. Furthermore, UV–Vis and fluorescence studies confirmed the binding of SB with
3
+
Fe and found to exhibit the detection limits of 0.56 mM and 0.98 mM, respectively. Large binding constant
4
ꢁ1
ꢁ1
(
1.01 ꢀ 10
M ) and negative free energy (–22.84 kJmol ) calculated using Benesi-Hildebrand (BH)
3
+
equation confirmed 1:1 complexation between SB and Fe which was also validated by Job’s plot and
mass spectrometry. DFT studies also confirmed the structure of SB-Fe3+ complex with reduction in
HOMO-LUMO gap from 4.11 eV to 1.08 eV. CPE-SB electrode was successfully applied for the estimation
of Fe in real samples such as ground water, surface water and pharmaceutical samples.
Ó 2021 Elsevier B.V. All rights reserved.
3
+
1
. Introduction
and soil) and contribute to the disturbance in ecological balance
10]. Thus, in the present epoch, qualitative and quantitative deter-
[
Iron is well recognised for its vital role in physiological pro-
mination of iron content in different environmental and biological
samples gained extraordinary attention all over the world.
cesses for living beings such as oxygen transport, cellular metabo-
lism, transcriptional regulation and enzymatic reactions [1,2]. It is
In last two decades, various organic compounds like crown
ethers [11], cryptands [12], porphyrins [13], calixarenes [14],
macrocyclic assembies [15], Schiff bases [16], modified conducting
polymers [17] etc. have been explored for their applications in the
development of sensing methods for determination of various
potentially toxic elements. Among various ionophores, Schiff bases
are versatile ligands as they offer advantages such as thermal sta-
bility, flexible synthesis, capacity for complex formation with
metal ions; medicinal and biological properties [18]. Schiff bases
2
+
3+
present as Fe and Fe in the dissolved state and transformation
between the two oxidation states commonly occurs. It is necessary
to maintain the iron content up to appropriate level in human body
as its deficiency causes immunodeficiency disorders such as anae-
mia which include fatigue, pale skin, shortness of breath, drowsi-
ness, weakness and low blood pressure, whereas iron overload
leads to medical disorders such as cardiac arrest, cell damage, Alz-
heimer’s disease, Hepatitis, Parkinson’s disease, Hemochromatosis,
cancer, diabetes, etc [3-8]. According to World Health Organization
favour the interaction with transition metal ions through
p elec-
(
2
WHO), maximum permissible limit of iron in drinking water is
mg/L [9]. Besides, iron is widely used in industrial sectors includ-
trons of the C = N group. Furthermore, the presence of donor atoms
like O and N in Schiff bases also allows the formation of stable
complexes via strong coordination bonding between ligand and
metal ions. The chelation with transition metal ions may lead to
enhancement of ICT (intramolecular charge transfer) transition or
LMCT (ligand-to-metal charge-transfer) which are being utilized
for the sensing of transition metal ions [19,20]. Further the diverse
ing water pipe, plastic, steel, paint industries and medical applica-
tion which leads to its permeation into the environment (water
⇑
Corresponding author.
E-mail address: inderpreet11@yahoo.co.in (I. Kaur).
https://doi.org/10.1016/j.molliq.2021.115954
167-7322/Ó 2021 Elsevier B.V. All rights reserved.
0

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
nature of the Schiff bases can be revealed from their significant
applications in different areas of electrochemistry, optical sensing
and biological analysis [21-24].
in ethanol. The reaction mixture was refluxed for 6 h and on com-
pletion of reaction, the solid product was filtered, washed with
cold ethanol and subjected to vacuum drying. The ionophore (SB)
1
13
Several reports in literature addressed the application of Schiff
based ionophores for determination of Fe3 using various tech-
niques such as voltammetry [25,26], potentiometry utilizing poly-
mer membrane electrodes [27,28], spectrophotometry [29,30] and
fluorescence spectroscopy [31,32] but the reports describing
voltammetric detection of Fe3 using Schiff base ionophore are
quite limited. Thus, to design simple, fast, cheap and reliable meth-
ods for detection of metal ions in biological, environmental and
chemical fields using Schiff base ligand is coveted. Moreover, elec-
trochemical methods such as voltammetry and potentiometry are
associated with many advantages such as portability, accuracy,
easy operation, low cost, no requirement of sample pre-
treatment and direct quantification of analyte in real samples
which makes them superior and promising techniques for trace
analysis [33].
was characterized using H NMR, C NMR, FT-IR and Mass spec-
+
1
troscopy (S1-S4). Yield = 79%. H NMR (400 MHz, CDCl
2.34 (6H, s, –CH ), 4.45 (4H, br, –CH -), 6.98–7.13 (12H, m, ArH),
7.37–7.41 (2H, m, ArH), 8.13–8.15 (2H, dd, ArH), 8.86 (2H, s,).
NMR (100 MHz, CDCl ) d/ppm: 21.09, 67.29, 112.49, 121.01,
121.63, 125.40, 127.78, 129.76, 132.59, 135.61, 150.09, 155.45,
158.52. HRMS: m/z calculated for 448.2151
3
) d/ppm:
3
2
13
C
3
+
C
30
H
28
ꢁ1
N
:
2
O
m
2
:
+
Found: 449.2020 (M +1). IR (KBR)
m
max/cm
Ar,C-H = 3041,
m
Ar, C=C
=
1595,
m
C-H
=
2944,
C-O C=N
m = 1244, m = 1684,
mC-H (bend) = 1356.
2.3. Voltammetric measurements
The cyclic voltammetric (CV) and differential pulse voltammet-
ric (DPV) measurements were performed on Autolab PGSTAT302N
Metrohm workstation, using glass cell with three-electrode assem-
bly, i.e. platinum electrode (diameter 2 mm) as working electrode,
Ag/AgCl as reference electrode and platinum wire as counter elec-
trode. NOVA 2.0 software was used to record and collect the data.
Before measurements, the surface of platinum working electrode
was cleaned by polishing with 0.05-mm alumina followed by ultra-
sonication in acetonitrile. The CV and DPV studies were carried out
in the potential range from 0.5 V to 2.0 V using 0.01 molL-1 TBAP
Keeping in view the properties of Schiff bases and advantages
associated with electrochemical techniques, the present work
was planned with aim to synthesize Schiff base 1,1 -((ethane-1,2-
0
diylbis(oxy))bis(2,1-phenylene))bis(N-p-tolylmethanimine) (SB)
followed by its application for sensitive and selective detection of
3
+
Fe ions using electroanalytical techniques: Differential pulse
voltammetry (DPV) and Potentiometry. Further, spectroscopic
studies using UV–Visible and fluorescence were employed to sup-
3+
port the binding behaviour of SB with Fe . In order to explore the
sensing capability of SB for onsite monitoring of Fe3 content in
+
and acetonitrile (CH CN) as a supporting electrolyte and solvent,
3
respectively. Nitrogen gas (99.99% high purity) was used for purg-
various samples, SB based carbon paste electrode was fabricated
and subjected to potentiometric sensing of Fe3+ ions in real sam-
ples after simple pre-treatment.
ing the contents in electrochemical cell for 10 min prior to each
electrochemical measurement to create an inert environment.
The cell assembly was placed in a Faraday cage to avoid any kind
of external interference. All the electrochemical measurements
were performed at an ambient temperature of 25.0 ± 1 °C.
2
. Experimental
2.1. Materials
2.4. Fabrication of SB based carbon paste electrode (CPE)
Salicylaldehyde and 1,2-dibromoethane and p-toluidine were
purchased from Spectrochem, India. Potassium carbonate and high
viscosity paraffin oil (density = 0.840 g/cm ) were obtained from
For the preparation of carbon paste electrode (CPE), SB was
3
used as a modifier. For this, graphite powder (64 mg), ionophore
SB (10 mg) and paraffin oil (26 mg) were mixed thoroughly using
mortar pestle. The paste so obtained was then packed firmly into
the blank electrode (Metrohm, diameter 2 mm) and allowed to
dry at the room temperature. The modified CPE (CPE-SB) was pol-
ished on polishing paper to obtain a shiny surface by removing
extra composite material attached. The surface of prepared CPE-
Fischer Scientific and Merck, respectively. Graphite powder (GP),
tetrabutylammonium perchlorate (TBAP) and metal perchlorates
were procured from Sigma Aldrich and used without any further
purification. All other reagents and solvents used were of analytical
grade. Double distilled water (DDW) was used for preparing the
standard stock solutions and carrying out further dilutions wher-
ever required.
-2
3+
SB was preconditioned in 1.0 ꢀ 10 M Fe solution for 24 h.
The electrode was removed from the preconditioning solution,
rinsed with double distilled water and used as working electrode
for potentiometric measurements.
0
2
.2. Synthesis and characterization of 1,1 -((ethane-1,2-diylbis(oxy))
bis(2,1-phenylene))bis(N-p-tolylmethanimine) (SB)
SB was synthesized following two steps procedure as described
in Scheme 1. In the first step, 2,2 -(ethane-1,2-diylbis(oxy))diben
2.5. Potentiometric studies
0
zaldehyde was synthesized as described by Ilhan et al. [34]. In brief,
The potentiometric measurements were performed on Autolab
the solution of salicylaldehyde (0.1 mol) and potassium carbonate
PGSTAT302N Metrohm workstation using CPE-SB as a working
electrode and Ag/AgCl as a reference electrode. The performance
of the electrode was examined by measuring the electromotive
(
(
0.05 mol) in DMF was stirred, into which 1,2-dibromoethane
0.05 mol in DMF) was added drop wise. The reaction mixture
was refluxed at 150 °C for 10 h and then refluxing was continued
for 5 h at room temperature. The progress of reaction was moni-
tored using TLC and after completion of the reaction, double dis-
tilled water was added and the reaction mixture was kept in the
3+
force (EMF) of Fe solution in the concentration range from
-10
-2
1
.0 ꢀ 10 M to 1.0 ꢀ 10 M using the following electrochemical
cell assembly:
0
refrigerator for 1 h. The precipitates of 2,2 -(ethane-1,2-diylbis(ox
CPE ꢁ SBj Sample solution ðFe3þsolutionÞj Ag=AgClðsÞ;KCl ð3MÞ
y))dibenzaldehyde so obtained were filtered, washed with water
and then dried in the vacuum. In second step, the Schiff base
All measurements were performed at 25.0 ± 1 °C. The pH of the
solutions was monitored using Contech CPH-103 pH meter with a
conventional glass pH electrode.
0
(SB) was prepared by condensation reaction of 2,2 -(ethane-1,2-d
iylbis(oxy))dibenzaldehyde (0.05 mol) and p-toluidine (0.1 mol)
2

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
0
Scheme 1. Synthetic pathway for 1,1 -((ethane-1,2-diylbis(oxy))bis(2,1-phenylene))bis(N-p-tolylmethanimine) (SB).
2
.6. Spectroscopic studies
jected to the determination of Fe3+ content using CPE-SB. For the
sake of comparison, same samples were subjected to estimation
of Fe content using well established technique i.e. Microwave
3
+
The UV–Visible spectra were recorded using Shimadzu 1601
spectrophotometer in the range of 200–600 nm using a quartz cuv-
ette (path length of 1 cm). Fluorescence spectra were recorded on
Hitachi F-4600 spectrofluorimeter in the range of 335–450 nm at
an excitation wavelength of 320 nm using an excitation-emission
slit width of 5 nm. All UV–Vis and fluorescence titrations were per-
Plasma Atomic Emission Spectroscopy (MPAES).
3
+
2.8.2. Determination of Fe in pharmaceutical samples
In order to explore application of CPE-SB in pharmaceutical
analysis, the synthetic samples were prepared using pharmaceuti-
cal tablets of known compositions. For the preparation of samples,
tablets were first ground into powder and then 1 g of the powdered
1
13
formed using 5
l
M solution of SB in CH
3
CN. H and C NMR spec-
as a solvent. FTIR
tra were recorded on JEOL 400 MHz using CDCl
3
spectra were obtained using Cary 630 FT-IR (Agilent Technologies)
while high-resolution mass spectra were recorded using BRUKER
DALTONIK micrOTOF-Q11 spectrometer.
3
tablet was dissolved in minimum amount of concentrated HNO .
2
+
3+
Further, to oxidize Fe to Fe , 5 mL of 1% H O was added to
2
2
3
the sample solution followed by pH adjustment to 2 using HNO .
The samples so prepared were filtered and diluted to 100 mL using
double distilled water followed by determination of Fe3+ content
using CPE-SB and MPAES.
2
.7. Computational details
In order to establish the sensitivity of SB towards Fe3+, TD-DFT
calculations were carried out employing B3LYP-D3 density func-
tional theory in conjunction with 6-31G(d,p) basis set for all atoms
except for Fe for which MDF10 ECPs basis set and pseudo potential
were used. All the geometry optimizations of complexes without
any truncations have been performed at above mentioned level
of theory followed by Harmonic frequency calculations to confirm
that the optimized complexes are true minima on potential energy
surface. Solvent effects have been introduced using Integral Equa-
tion Formalism Polarizable Continuum Model IEFPCM/SMD solva-
tion model in Acetonitrile at normal temperature. All the above
mentioned calculations have been performed using Gaussian 09
suite of Programmes [35]
3. Results and discussion
0
SB was synthesized via condensation of 2,2 -(ethane-1,2-diylbis
(oxy))dibenzaldehyde (1) and p-toluidine. The structural feature of
SB consists of pseudo-cavity that can accommodate metal ions via
coordination through N and O donor atoms present in it which is
prerequisite for sensing. In order to explore its electrochemical
sensing potential, SB was subjected to voltammetric and potentio-
metric studies and results so obtained are discussed in following
sections:
3.1. Voltammetric measurements
2
2
.8. Sample preparation for real application
3.1.1. Electrochemical behaviour of SB
The voltammetric studies were aimed to investigate the redox
3
+
-4
.8.1. Determination of Fe in water samples
performance of SB (5 ꢀ 10 M) in CH
3
CN containing 0.01 M TBAP
For the determination of Fe3 content in real samples, ground
+
as supporting electrolyte using cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV). The CV of SB exhibits three irre-
versible anodic peaks at 1.58 V, 1.13 V and 0.89 V in the
potential range from 0.5 V to 2.0 V vs. Ag/AgCl at the scan rate of
50 mV/s as shown in Fig. 1(a). The peak at 1.58 V may be attributed
to O atoms while the peaks at 1.13 V and 0.89 V may be assigned to
N atoms of imine groups in SB molecule [36]. Similarly, three
and surface water samples were collected from various locations
of Amritsar, Punjab. The samples so collected were filtered through
Whatman filter paper no. 42 to remove the solid impurities. For the
conversion of Fe present in the real sample into Fe , the samples
were subjected to oxidation using 1% H and concentrated HNO
with pH maintained below 2 [25]. The samples were then sub-
2
+
3+
2
O
2
3
3

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
-
4
Fig. 1. (a) Cyclic Voltammogram (CV) of pure receptor SB (5 ꢀ 10 M in CH
3
CN). Supporting electrolyte: TBAP (0.01 M), Scan rate: 50 mV/sec; (b) Differential Pulse
-4
Voltammogram (DPV) of pure receptor SB (5 ꢀ 10 M in CH
3
CN); Supporting electrolyte: TBAP (0.01 M), Pulse amplitude: 0.05 V, Pulse width: 0.05 s, Pulse period: 0.5 s.
distinct anodic peaks at 1.50 V, 1.02 V and 0.83 V were also
observed when SB was subjected to DPV measurements (Fig. 1
enhancement in peak current values under similar conditions with
a remarkable shift of the peak at 1.02 V towards more positive
potential as shown in Fig. 2(a). Similar quenching of peak at
1.58 V and enhancement in peak current at 0.89 V were observed
in CV studies as expected, while the peak at 1.13 V showed anodic
shift along with the increase in current parameter indicating the
(
b)). The results obtained from CV and DPV measurements in pre-
liminary investigation confirmed the redox behaviour of SB which
could allow one to conclude that it could serve the purpose of elec-
trochemical sensing.
3
+
formation of SB-Fe complex (Fig. 2(b)).
The favourable behaviour of SB for complexation with only Fe3+
in comparison to other metal ions under investigation can be
explained on the basis of the fact that SB contains C = N and –O–
moieties that are associated with hard properties and prefer to
3.1.2. Interaction of SB with metal ions
To study the complexation behaviour of SB towards diverse
metal ions, differential pulse voltammetry (DPV) was performed
as it offers an advantage of well-defined and sharper current peaks
than CV under identical conditions. Upon addition of different
2
+
bind hard metal ions. Among the various metal ions tested, Ni
,
2
+
2+
2+
2+
2+
2+
Zn , Co , Pb , Cu , Fe are borderline acids and Cd is soft acid,
+
+
metal ions solutions (25 mM) such as alkali (Na , Li ), alkaline
so these ions are not expected to bind with SB so effectively.
2
+
2+
2+
2+
2+
2+
2+
+
+
2+
3+
3+
(
Ca ) and transition metal ions (Ni , Zn , Cd , Co , Pb , Cu
,
Although Na , Li , Ca , Cr and Al are hard but their charge den-
2
+
3+
3+
3+
-4
Fe , Fe , Al and Cr ) into SB solution (5 ꢀ 10 M), no major
sity values (qNa
þ
¼ 0:26; qLi
þ
= 0.59; qCa2þ ¼ 0:49; qCr3þ ¼ 2:18) are
3
+
3+
shift in positions and heights of various peaks was observed except
less than that of Fe
(q
3þ ¼ 4:30) and Al
(q
3þ ¼ 4:68). Charge
Fe
Al
3
+
3+
Fe (Fig. 2(a)). With Fe , there is complete quenching of peak cur-
density is an important parameter to interpret the electrophilicity
of the metal ions. Higher charge density favours stronger complex-
rent at 1.50 V while other two peaks at 1.02 V and 0.83 V showed
-
4
Fig. 2. (a) Differential Pulse Voltammograms (DPV) of receptor SB (5 ꢀ 10 M in CH
3
CN) in the absence and presence of various metal ions. Supporting electrolyte: TBAP
-4
(
0.01 M), Pulse amplitude: 0.05 V, Pulse width: 0.05 s, Pulse period: 0.5 s; (b) Cyclic Voltammogram (CV) of receptor SB (5 ꢀ 10 M in CH
3
CN) before and after addition of
2
5 mM addition of Fe3+. Supporting electrolyte: TBAP (0.01 M), Scan rate: 50 mV/sec.
4

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
ation with electron rich ligands. This fact rules out the hard metal
ions like Na , Li , Ca , and Cr for favourable complexation with
firmed 1:1 complexation between SB and Fe3+ (Fig. S5). The K
a
+
+
2+
3+
and free energy change (
D
G = -RTlnK
a
) were calculated to be
3
+
3+
3+
4
ꢁ1
ꢁ1
SB. Further, out of Fe and Al , Fe exhibit higher standard elec-
1.01 ꢀ 10 M and –22.84 kJmol , respectively. The larger value
of binding constant and negative Gibbs free energy indicated the
feasibility of the complex formation between two participants.
3+
3+
3+
trode potential in comparison to Al (Fe =0.77 V; Al = ꢁ1.66 V
3
+
and facilitate Fe to bind strongly with electron rich ligand SB.
Similar strong and selective complexation of polymer poly(1-ami
no-5-chloroanthraquinone) with Fe3 has been reported by Huang
et.al [37].
+
3.1.5. Interference study
In order to check the influence of foreign ions present along
3
+
with Fe in the same solution which normally occur in real sam-
3
+
3
.1.3. Quantitative application for Fe sensing
3+
ples, DPVs of SB containing 25 mM of Fe ions were recorded in
In order to confirm the electrochemical recognition sensitivity
the presence of equal amount (25 mM) of different metal ions such
of SB towards Fe3 ions, the voltammetric titration of SB was car-
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
as Na , Li , Ca , Ni , Zn , Cd , Co , Pb , Cu , Fe , Al and Cr
3
+
ried out with Fe solution in the potential range from 0.5 to
and it was observed that the anodic potentials and peak currents in
DPV plot were not or marginally influenced by the presence of
+
2
.0 V vs. Ag/AgCl. The solution of Fe3 with concentration ranged
from 0 to 25 mM was sequentially added into a solution of SB
3+
other metal ions showing no interference in the detection of Fe
-
4
3+
(
5 ꢀ 10 M). The increasing concentration of Fe ions in solution
(
shown in Fig. S6). Therefore, receptor SB could serve as a promis-
significantly influenced the position (potential) and height (peak
currents) of peaks observed in differential pulse voltammogram
of SB as shown in Fig. 3. The anodic peak at 1.50 V got vanished
on the successive addition of Fe3 ions up to 6 mM. While the peaks
at 1.02 V and 0.83 V gradually reaches to the maximum at 25 mM
concentration of Fe . Also, the peak at 1.02 V showed a shift
towards the more positive potential on the gradual addition of
Fe demonstrating the complexation of SB with Fe . The plot of
ing voltammetric sensor with excellent selectivity for detection of
3+
Fe in the presence of diverse metal ions.
+
3.2. Potentiometric measurements
3
+
After confirmation of favourable SB-Fe3+ complex formation
and keeping in view the requirement of on-site monitoring of
3
+
3+
3+
Fe without involving heavy instrumentation, CPE incorporating
the magnitude of peak current at 0.83 V against the concentration
SB as modifier was constructed and applied for potentiometric
3
+
of Fe is shown in the inset of Fig. 3. The peak current was found to
3+
sensing of Fe . In addition, using CPE-SB potentiometric sensing
3+
increase with successive addition of Fe and exhibit a linear rela-
tionship in the concentration range of 1 mM-19 mM with a correla-
tion coefficient of 0.99. The detection limit of the voltammetric
of various alkali, alkaline earth, and transition metal ions such as
+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
Na , Li , Ca , Ni , Zn , Cd , Co , Pb , Cu , Fe , Al and Cr
were also examined and the electrode characteristics such as linear
range and Nernstian slope so obtained are summarized in Table S1.
Results indicated that CPE-SB exhibited very inferior potentiomet-
-8
sensor was found to be 5.57 ꢀ 10 M.
3.1.4. Binding Stoichiometry, stability constant and free energy change
3+
ric response for all metal ions tested except Fe . The sensor
For determining the binding constant (K ) for the complexation
a
3+
responded to metal ions other than Fe with slope much below
3
+
between SB and Fe , Benesi-Hildebrand (BH) equation (1) was
the Nernstian slope values (in mV/decade) i.e. 19.72, 29.58 and
used [38]:
5
9.16 for trivalent, divalent and monovalent metal ions, respec-
3+
1
1
1
tively. As CPE-SB has shown high sensitivity toward Fe only,
hence it was decided to study systematically its electrode charac-
teristics such as response time, linear concentration range, Nerns-
tian slope, effect of pH and selectivity.
¼
þ
ð1Þ
n
D
I
p
D
I
0
Ka
D
I0½FeðIIIÞꢂ
where,
p
DI = Imax – I, Imax and I represent the current of SB
-
4
3+
(
[
5 ꢀ 10 M) in the presence and absence of Fe , respectively;
3+
Fe(III)] represents the molar concentration of Fe and n denotes
3
.2.1. Electrode characteristics
the stoichiometric coefficient. The double reciprocal plot of 1/DI
p
3
+
3
+
The potentiometric response of CPE-SB for Fe in the concen-
vs. 1/[Fe ] using data obtained from DPV measurements was
-
10
-2
2
tration range from 1.0 ꢀ 10
M to 1.0 ꢀ 10 M was examined
drawn which demonstrated a good linearity (R = 0.99) and con-
to obtain working concentration range and other electrode charac-
teristics. It was found that CPE-SB exhibited a near Nernstian slope
3
+
of 18.6 ± 0.2 mV/decade over a wide concentration range of Fe
-7
-2
from 1.0 ꢀ 10 to 1.0 ꢀ 10 M with a detection limit of
-8
5
.0 ꢀ 10 M. The variation in potentiometric response of CPE-SB
3+
with concentrations of Fe and corresponding calibration graph
are shown in Fig. 4. It can be seen that the potential increased with
3
+
increase in the concentration of Fe in solution due to the occu-
pancy of Fe in the pseudo cavity of SB via interaction with donor
O and N atoms.
3
+
3.2.2. Effect of pH and response time
The pH dependence of CPE-SB was analyzed over a wide pH
-
3
3+
range of 2.0–10.0 for 1.0 ꢀ 10 M Fe ions. 1 N HNO
3
and 1 N
3
+
NaOH solutions were used to adjust the pH of Fe solution. Fig.
S7 shows the variation in potentiometric response as a function
of pH. The potential was found to show marginal change in the
pH range of 2.0–7.0. The sharp change in potential at higher pH
values may be ascribed to the hydrolysis of Fe resulting in the
formation of hydroxides [39]. Similar results were obtained when
3
+
-4
Fig. 3. Differential Pulse Voltammograms (DPV) of receptor SB (5 ꢀ 10 M in
CN) in presence of Fe3+ ions (0–25 mM). Supporting electrolyte: TBAP (0.01 M),
CH
3
-2
3+
1
.0 ꢀ 10 M Fe solution was subjected to pH variation in the
Pulse amplitude: 0.05 V, Pulse width: 0.05 s, Pulse period: 0.5 s. Inset: Calibration
plot between [Fe3 ] (=1 mM- 19 mM) and corresponding peak currents.
+
range 2.0–10.0 followed by potentiometric measurements. There-
5

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
Fig. 4. (a) Schematic representation of potentiometric assembly used for potentiometric sensing using CPE-SB as indicator electrode. (b) Calibration curve of potentiometric
3
+
3+
response of CPE-SB with variation in [Fe ]. Inset: Potentiometric response of CPE-SB with various concentrations of Fe in solution.
fore, pH range 2.0–7.0 can be considered as the working pH range
of the CPE-SB
transition of C = N group, while the band at 327 nm corresponds
to n- * transition and intermolecular charge transfer (ICT) in the
molecule due to conjugation [42]. Upon addition of Fe , the
p
3+
The response time is an average time taken by an electrode to
achieve stable potential when it is dipped in successive analyte
solutions [40]. For the determination of response time, CPE-SB
absorption intensity of band at 327 nm was significantly increased
+
+
(Fig. S8). However, on addition of other metal ions such as Na , Li ,
3
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
electrode was subjected to potentiometric measurements in Fe
-
Ca , Ni , Zn , Cd , Co , Pb , Cu , Fe , Al and Cr , no signif-
icant change was observed in absorption intensity of signal at
327 nm. On the other hand, a shoulder appeared at 375 nm in case
solutions in concentration range studied and the corresponding
change in potential was observed with respect to time. It was
noted that the electrode achieved stable response within 15 s
and after which no fluctuations in potential was observed even
up to 5 min. Hence, the CPE-SB electrode showed an equilibrium
response within 15 s over whole Fe3 concentration range studied.
2
+
2+
2+
2+
3+
3+
3+
of Pb , Cu , Ca , Fe , Al , Cr including Fe . These changes in
absorption spectra i.e. significant enhancement in absorption band
at 327 nm confirmed the sensitivity of receptor SB towards Fe3+
.
+
Further, the titration experiment was carried out by the succes-
3
+
sive addition of Fe from 0 to 1.85 equiv. in the solution of recep-
3
+
tor SB (5 mM in CH
3
CN). Upon addition of Fe from 0 to 0.15 equiv.
-electron
3.2.3. Selectivity coefficient
a shoulder at 375 nm appeared which might be due to
p
The selectivity of CPE-SB was expressed in terms of potentio-
delocalization (Fig. 5(b)). This shoulder vanished on further addi-
tion, while the absorbance band corresponding to 327 nm gradu-
metric selectivity coefficients (KFe3þ ;B) which represents the
3+
response of electrode to the primary ion (Fe ) in the presence of
other ions in the same solution. KFe3þ ;B were evaluated by using
fixed interference method (FIM) [41] using a fixed concentration
-2
of secondary ions (1.0 ꢀ 10 M) in the background and varying
3
+
-10
the concentration of primary ion Fe ranged from 1.0 ꢀ 10
M
-
1
to 1.0 ꢀ 10 M as summarized in Table S2. It was revealed that
-3
the selectivity coefficients were in the order of 10 or lower for
+
almost all the diverse ions tested except univalent ions Na and
Li which may be attributed to their charge and these ions may
not cause any significant interference in the estimation of Fe ions
+
3
+
using CPE-SB unless present in large amounts. It can be concluded
3
+
that CPE-SB could be used for the selective determination of Fe
by direct potentiometric method even in the presence of foreign
-2
ions at higher concentration of 1.0 ꢀ 10 M.
3
+
3
3
.3. Spectroscopic studies supporting SB-Fe complex formation
.3.1. UV–Visible Spectroscopy
In order to confirm the complexation potential of SB with Fe3+
using UV–Vis spectroscopy, the absorption spectra of SB (5 mM in
CH CN) was recorded which exhibits distinct bands at 264 and
27 nm (Fig. S8). The band at 264 nm may be attributed to
Fig. 5. (a) Absorption behaviour of SB (5 mM in CH
3
CN) upon successive addition of
3+
3
Fe ; (b) enlarged scale showing appearance of new band; (c) Calibration plot of
3+
3
p-
p*
absorbance vs [Fe ].
6

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
Fig. 6. (a) Emission behaviour of SB (5 mM in CH
3
CN) upon successive addition of Fe3+ ; (b) corresponding change in emission intensity; (c) Calibration plot.
ally increased with successive additions of Fe3+ throughout the
titration with a red shift from 327 nm to 341 nm (Fig. 5(a)) which
may be attributed to ligand to metal charge transfer (LMCT) in SB-
of proton of HC = N (H
doublet which ensures the participation of C = N as binding site in
complexation of SB with Fe via imine nitrogens. Furthermore,
a
) along with the splitting of the singlet into
3
+
3
+
Fe complex. Moreover, there was change in colour from transpar-
downfield shift was also observed in signal of proton of methylene
3+
ent to brown on addition of 1.85 equiv. of Fe . The UV–Vis sensor
was found to exhibit good linearity in the concentration range of 0.
group (H
change in electronic environment upon binding of Fe with O
atoms adjacent to H protons. These observations confirm the par-
ticipation of imine N and O atoms of SB in the complex formation
b
) (d 4.47 to 4.51 ppm) which might be attributed to
3+
0
16–9.26 mM and detection limit calculated from the calibration
b
curve (Fig. 5(c)) was found to be 0.56 mM which is far below the
recommended WHO limit for iron content in drinking water i.e.
3
+
with Fe
.
2
mg/L [9].
Further, to gather more information and confirm the binding
stoichiometry, mass spectra of SB before and after complexation
3
+
3+
3
.3.2. Fluorescence spectroscopy
The emission behaviour of SB (5 mM in CH
with Fe was recorded and a peak at m/z = 543.1106 [SB + Fe
+
+
3
CN) was also inves-
+ K ] was observed which corresponds to expected mass of SB-
Fe complex (calculated 504.1500) . This analysis confirmed the
formation of 1:1 SB-Fe complex (Fig. S12).
It can be seen that the inferences obtained from spectroscopic
measurements such as UV–Visible spectroscopy, fluorescence,
NMR and HR-MS provides convincing evidences in support of the
major findings achieved from electrochemical techniques i.e. cyclic
voltammetry (CV), differential pulse voltammetry (DPV) and
potentiometry.
3
+
tigated using fluorescence measurements at an excitation wave-
length of 320 nm. The receptor SB displayed an emission band
centred at 354 nm which showed a gradual decrease in emission
intensity on successive addition of Fe3 up to 1.85 equiv. after
which the plateau was achieved (Fig. 6(a)). The plot of fluorescence
intensity versus [Fe ] was shown in Fig. 6(b). The sensor was
found to show response in the linear concentration range of
0
0
alkaline earth and transition metal ions such as Na , Li , Ca , Ni
Zn , Cd , Co , Pb , Cu , Fe , Al and Cr , no apparent alter-
ations in fluorescence spectra were observed (Fig S9).The decrease
in emission intensity in the presence of Fe may be attributed to
the paramagnetic nature of Fe3 which may induce electron/energy
transfer processes with fluorophores which may further results in
non-radioactive deactivation pathway [43].
Further, 1:1 stoichiometry was confirmed from Job’s plot which
showed maximum at mole fraction of 0.5 (Fig. S10). Thus, UV–Vis-
ible and Fluorescence studies confirmed the discriminating poten-
3
+
+
3
+
.83–5.83 mM and the lowest limit of detection was found to be
.98 mM (3 /slope). On the other hand, on addition of other alkali,
r
+
+
2+
2+
,
3.4. DFT studies
2
+
2+
2+
2+
2+
2+
3+
3+
The molecular orbital analysis was used to explain the sensing
behaviour of SB towards Fe in acetonitrile. The HOMO-LUMO
gap represents a key parameter to reflect chemical stability of
the molecule before and after binding with Fe . As shown in
Fig. 7, the HOMO-LUMO gap of SB in acetonitrile is 4.11 eV. How-
ever, after encapsulating Fe , the same energy gap reduces to
1.08 eV. Thus, it can be concluded that SB-Fe complex is easily
formed due to readily accessible energy states owing to the smaller
energy gap.
3+
3+
+
3
+
3
+
3
+
3
+
tial of SB to detect Fe in comparison to other metal ions under
investigation.
To establish the sensing behaviour of SB, TDDFT electronic
structure calculations at Frank Codon point were carried out at
the optimized geometry of Fe encapsulated SB. The brightest ver-
tical transitions are between molecular orbitals 115a to 119a and
116a to 119a with 0.0212 oscillatory strength and 3.71 eV excita-
tion energy. The 113b to 120b/121b, 116b to 119b, and 117b to
120b molecular orbital excitations are the second best vertical
1
3+
3
.3.3. H NMR and HR-MS studies
To understand the binding behaviour of SB with Fe3+, 1H NMR
titrations were also carried out in CH
3 3
CN (d ) (Fig. S11). All the sig-
nals have broadened owing to strong paramagnetic nature of SB-
3
+
Fe complex. Moreover, there was a slight downfield shift in signal
7

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
Fig.7. Pictorial representation of HOMO-LUMO gap of SB before (A) and after (B) encapsulating Fe3+. The HOMO-LUMO values are in atomic units.
transitions with 0.0161 oscillatory strength and 4.90 eV excitation
energy. The molecular orbital transition 115a to 119a corresponds
to HOMO-3 to LUMO transition whereas the transition 116a to
may be attributed to occupancy of the pseudocavity in SB via
coordination with enhancement in conduction behaviour of the
system (Scheme 2). Similarly, from potentiometric measurements,
3
+
1
19a is HOMO-2 to LUMO. According to molecular orbital analysis,
HOMO-3 and HOMO-2 are in nature while as LUMO is located
over the aromatic ring as portrayed in Fig. S13. Thus, the above said
excitations are mainly * in nature. Thus, a good agreement was
the potential values increased with Fe concentration which jus-
tifies the inferences drawn from DPV investigations. Furthermore,
p
1
3+
it was observed from H- NMR and mass spectra of the SB-Fe
3
+
p-
p
complex that Fe possibly interacts with O and C = N units of
the molecule and results in the formation of 1:1 complex
between SB and Fe . These investigations lead to the conclusion
seen between the TDDFT computational studies and inferences
obtained from electrochemical and spectroscopic measurements
described in previous sections.
3
+
3
+
that Fe occupies the pseudo cavity possibly via co-ordination
with the donor units in SB.
3.5. Proposed mechanism
3.6. Analytical application
Receptor SB is a Schiff base that consists of pseudo cavity
which can accommodate metal ions i.e. Fe3 in the present case.
It comprises donor units i.e. C = N and O atoms which have the
capability to coordinate with metal ion. DPV investigations indi-
cated the increase in peak currents corresponding to peak poten-
+
3.6.1. Potentiometric titration of Fe with EDTA
3+
The CPE-SB was successfully applied as an indicator electrode to
determine Fe content from the endpoint in potentiometric titra-
3
+
3
+
-3
tion of Fe with EDTA as a titrant. A 25 mL solution of 1.0 ꢀ 10
M
3
+
3+
-2
tials of imine moieties on increasing concentration of Fe which
Fe was titrated against 1.0 ꢀ 10 M EDTA solution at pH 6.0. The
Scheme 2. Possible mechanism of coordination responsible for encapsulation of Fe3+ in SB.
8

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
Table 1
Estimation of Fe3 content in real sample matrices using CPE-SB.
+
Real Samples
Concentration of Fe3+ (M)
CPE-SB sensor
% Compatibility
MPAES
-4
-4
-4
-4
-4
-4
-4
Surface water
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
8.90 ꢀ 10
9.26 ꢀ 10
9.37 ꢀ 10
9.67 ꢀ 10
5.82 ꢀ 10
5.65 ꢀ 10
M
M
M
M
M
M
8.65 ꢀ 10
9.39 ꢀ 10
9.58 ꢀ 10
9.74 ꢀ 10
5.99 ꢀ 10
5.83 ꢀ 10
M
M
M
M
M
M
102.9%
98.6%
97.8%
99.2%
97.2%
96.9%
-4
-4
-4
-4
-4
Ground water
Pharmaceutical tablets
*
All measurements were performed in triplicates.
potential data was plotted against the volume of EDTA added in
potentiometric titration and the curve so obtained is shown in
Fig. S14. The addition of EDTA causes a decrease in potential which
may be attributed to decreased concentration of free Fe3 ions in
solution due to their complexation with EDTA and after the end-
point, the potential remains constant due to non-availability or
3
+
very low concentration of free Fe in the solution. The sharp
3
+
breakpoint corresponds to the completion of Fe –EDTA complex
+
formation. Hence, the electrode CPE-SB can be used to determine
3
+
Fe content potentiometrically.
Table 2
Comparison of the proposed electrochemical sensors with previously reported Fe3+sensors based on Schiff base.
Voltammetric sensors
S.No.
Ionophore
Electrode
Linear range
Detection limit
Reference
[25]
-5
-5
-8
1
Glassy Carbon
1.6 ꢀ 10 to 4.4 ꢀ 10
M
5.2 ꢀ 10
M
2
3
SPE
0.625
l
M to 7.5
lM
0.93
l
M
[26]
-8
Platinum
1–19 mM
5.57 ꢀ 10
M
Present work
Potentiometric sensors
S.No.
Ionophore
CPE/PVC
PVC
Slope
mV/decade)
Linear range
(M)
Detection limit
(M)
Reference
[27]
(
-6
-2
-2
1.2x10-6
1
19.5
3.0 ꢀ 10 to 1.2 ꢀ 10
-7
7.4x10-8
2
3
PVC
PVC
19.3 ± 0.6
19.8 ± 0.3
1.0 ꢀ 10 to 1.0 ꢀ 10
[28]
[44]
1.0x10-2 to 1.0x10-8
8.3 ꢀ 10ꢁ9
3.5 ꢀ 10ꢁ to 4.0 ꢀ 10ꢁ2
6
2.5 (±0.1) ꢀ 10ꢁ6
[45]
4
PVC
28.5 (±0.5)
-
6
-1
5.0 ꢀ 10ꢁ6
5
6
PVC
20.0
6.3 ꢀ 10 to 1.0 ꢀ 10
[46]
-
7
-2
-8
CPE
18.6 ± 0.2
1.0 ꢀ 10 - 1.0 ꢀ 10
5.0 ꢀ 10
Present work
9

S. Kaur, Bilal Ahmad Shiekh, D. Kaur et al.
Journal of Molecular Liquids 333 (2021) 115954
3
.6.2. Real sample analysis
In order to explore the real applicability of the proposed CPE-SB,
measurements. The coordination mechanism was validated by
DFT, H NMR and mass spectroscopy. The characteristic perfor-
1
it was applied for determination of Fe3 content in real sample
matrices i.e. ground water, surface water and pharmaceutical sam-
ples and the results so obtained are presented in Table1. It was
found that the proposed sensors could determine Fe3 content in
real samples accurately. Moreover, the results were also compared
with a well-established technique Microwave Plasma Atomic
Emission Spectroscopy (MPAES) and were found to be in good
agreement (Table 1). Thus, the proposed method proved its candi-
dature for the estimation of Fe3 content in different environmen-
tal and biological samples after suitable pre-treatment if required.
Moreover, for analysis of environmental samples such as ground-
water and surface water, it offers additional advantages such as
portability, no sample pre-treatment and on-site monitoring.
+
mance of the proposed SB based sensor is found to be comparable
or even superior as compared to previously reported Fe3+ sensors.
CPE-SB was subjected to real analytical application and success-
+
3+
fully applied as an indicator electrode for determination of Fe
in real sample matrices such as surface water, ground water and
pharmaceutical tablets with great precision and accuracy. The
application of the proposed sensors could be extended for determi-
3
+
nation of Fe content in food crops, biological (blood, urine) and
environmental (water, soil) samples.
+
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
3
.7. Comparison with literature
In recent years, there have been an extensive reports utilizing
Acknowledgements
Schiff based ionophores for spectroscopic and electrochemical
3
+
detection for Fe . Attempts have been made to compare the per-
The authors acknowledge the financial support from the
Science & Engineering Research Board, Department of Science
and Technology (SERB-DST), New Delhi and Ministry of Human
Resources Development, Government of India under RUSA 2.0 Pro-
gramme. The first author (SK) is thankful to Department of Science
and Technology under PURSE (69919/Estt./A-11) and CSIR-SRF
(09/254(0306)/2020-EMR-I) for the fellowship.
formance of proposed SB based voltammetric and potentiometric
3
+
3+
Fe sensors with previously reported Fe ion-selective sensors
Table 2). There have been only two reports for voltammetric
(
3
+
detection of Fe using schiff based ionophores which showed
-8
comparable or inferior detection limits i.e. 5.2 ꢀ 10 M [25] and
0
.93 mM [26] in comparison to that of SB based voltammetric sen-
-
8
3+
sor i.e. 5.57 ꢀ 10 M. As far as potentiometric sensors for Fe are
concerned, many reports of Schiff based ion-selective electrodes
are available in literature but most of them involve PVC membrane
as matrix for supporting the ionophore and are associated with
their own limitations but no report on Fe3 -CPE incorporating
Schiff base is found. To the best of our knowledge, present work
is the first report of Schiff base incorporated in CPE for potentio-
metric determination of Fe3 . Moreover, potentiometric sensor
based on SB was found to exhibit better or comparable detection
limit than other ion-selective electrodes utilizing Schiff based iono-
phore [25,27,28,45,46] except the potentiometric sensor reported
by Zamani et. al [44] .In addition, CPEs offers additional advantages
of renewability, low ohmic resistance, easy modification, good
response and no need of any internal reference solution in compar-
ison to previously reported PVC membrane electrodes for detection
Appendix A. Supplementary data
+
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.115954.
+
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