A chromone based Schiff base: An efficient colorimetric sensor for specific detection of Cu (II) ion in real water samples

Article abstract of DOI:10.1016/j.molstruc.2020.129549

A new chromone based Schiff base ligand L was synthesized by the condensation of 3-formyl chromone and pyrazine-2-carbohydrazide as a colorimetric probe to detect Cu (II) ions selectively. An instant visual colour change from colourless to yellow was obtained on addition of Cu²⁺ ions to the probe L solution, while other metal ions found ineffective. The ligand L was characterized by ¹H NMR, FTIR and HRMS spectral techniques. UV-Visible spectroscopic technique was used to study the sensing ability of probe L for copper ions above other metal ions. The Job's plot obtained from absorption studies and HRMS data confirmed that the Cu²⁺ ions bind with ligand L in 1:1 stoichiometric ratio. DFT computations were also supported the binding framework between L and Cu (II) ions. The LOD value and the association constant were obtained 3.9 × 10^?7 M and 2.3 × 10⁵ M^?1 respectively, via Benesi-Hildebrand equation. Selectivity of L towards Cu²⁺ ions was also studied and it was found that the probe L worked specifically for copper ions without any considerable influence of other intruding metal ions. In addition, in real water samples, the ligand L was fully implemented for identification and quantification of Cu²⁺ ions.

Full text of DOI:10.1016/j.molstruc.2020.129549

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Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
A chromone based Schiff base: An efficient colorimetric sensor for
specific detection of Cu (II) ion in real water samples
Nisha Tomer a, Apurva Goela, Vikas D. Ghuleb, Rajesh Malhotraa,∗
a Department of Chemistry, Guru Jambheshwar University of Science and Technology, Hisar -125001, India
b Department of Chemistry, National Institute of Technology, Kurukshetra, 136119, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chromone based Schiff base ligand L was synthesized by the condensation of 3-formyl chromone
and pyrazine-2-carbohydrazide as a colorimetric probe to detect Cu (II) ions selectively. An instant visual
colour change from colourless to yellow was obtained on addition of Cu2+ ions to the probe L solution,
while other metal ions found ineffective. The ligand L was characterized by 1H NMR, FTIR and HRMS
spectral techniques. UV-Visible spectroscopic technique was used to study the sensing ability of probe L
for copper ions above other metal ions. The Job’s plot obtained from absorption studies and HRMS data
confirmed that the Cu2+ ions bind with ligand L in 1:1 stoichiometric ratio. DFT computations were also
supported the binding framework between L and Cu (II) ions. The LOD value and the association constant
were obtained 3.9 × 10−7 M and 2.3 × 105 M−1 respectively, via Benesi-Hildebrand equation. Selectivity
of L towards Cu2+ ions was also studied and it was found that the probe L worked specifically for copper
ions without any considerable influence of other intruding metal ions. In addition, in real water samples,
the ligand L was fully implemented for identification and quantification of Cu2+ ions.
Received 2 October 2020
Revised 29 October 2020
Accepted 29 October 2020
Available online xxx
Keywords:
Schiff base
Colorimetric probe
Cu2+ ions
HRMS
LOD
Binding constant
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
sensing [14], atomic absorption spectrometry [15], fluorescence
sensing [16], inductively coupled plasma atomic emission spec-
Copper is an essential transition metal which plays vital roles in
various physiological processes [1,2]. It acts as a helper molecule
that assists in several biochemical transformations as cuproen-
zymes [3,4]. Excessive utilization of copper may cause severe side
effects to human beings like heptolenticular degeneration, growth
retardation, liver damage, melena, Wilson’s disease, gastrointesti-
nal disorders etc., in addition less intake of this trace metal can
cause diseases like Menkes, connective tissues, low white blood
cell count, fatigue, brittle bones etc. in human beings [5-9]. Be-
sides toxicity, neurological disorders and other health problems
[10], because of its wide usage in industrial sector, it turns out
to be an environment pollutant in combined form [11]. Accord-
ing to a report of the US-EPA (Environmental Protection Agency),
maximum acceptable concentration of Cu2+ ions in potable water
should not exceed 1.3 mg/litre [12]. Consequently, a simple, effec-
tual and facile approach is needed for monitoring Cu2+ ions in en-
vironmental samples.
trometry [17], cold vapour atomic absorption spectroscopy [18], in-
ductively coupled plasma mass spectroscopy [19] mainly include,
but their expensive instrument, high maintenance cost and diffi-
cult sampling methods constraints their usage. Spectrophotomet-
ric technique is usually recommended for metal ion assessments
in aqueous phase, due to its simplicity, cost effectiveness and easy
availability [20]. Apart from that colorimetric detection is modest,
fast and easy method for metal ion sensing; owing to its simplicity
it drags researcher’s attraction towards itself. There are many col-
orimetric sensors introduced to detect copper metal ions but their
difficult synthetic route, weak LOD values and solvent pH sensi-
tivity put restrictions to their use. Therefore an effective probe is
required which is extremely sensitive and selective for metal ion
detection.
Schiff bases are well known class of compounds for their large
number of applications in various fields. In addition to their sev-
eral antimicrobial and cytotoxicity evaluation [21-23], Schiff bases
act as potent corrosion inhibitors and used as catalysts in vari-
ous chemical reactions [24]. These Schiff bases act as coordinat-
ing ligands and binds with different metal ions through their ac-
tive -C = N- group which is greatly responsible for their nu-
merous applications [25-26]. Many Schiff bases were introduced
as colorimetric probes [27-31] but a few chromone based Schiff
Innumerable techniques were developed for the metal ion sens-
ing namely, anodic stripping voltammetry [13], electrochemical
∗
Corresponding author.
E-mail address: malhotra_ksrk@yahoo.co.in (R. Malhotra).
https://doi.org/10.1016/j.molstruc.2020.129549
0022-2860/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: N. Tomer, A. Goel, V.D. Ghule et al., A chromone based Schiff base: An efficient colorimetric sensor for specific
detection of Cu (II) ion in real water samples, Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2020.129549

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Scheme 1. Synthetic route for Schiff base L.
bases were familiarised. Chromone derivatives have been widely
useful due to their tremendous pharmacological properties and
sensing applications [32,33]. Moreover, the term chromone derives
from the Greek word Chroma means “colour”, and can display
a wide range of colours [34]. Considering this, we have synthe-
sized a new chromone based Schiff base Pyrazine-2-carboxylic acid
(4-oxo-4H-chromen-3-ylmethylene)-hydrazide from the condensa-
tion of pyrazine −2-carbohydrazide and 3-formyl chromone (L) as
shown in Scheme 1. The prepared Schiff base was characterized
by means of different analytical techniques like 1H NMR, FT-IR
and HRMS. Ligand L worked as an extremely sensitive colorimet-
ric probe for Cu (II) ions detection in methanol and the colour
response can be observed through naked eyes. The practical use-
fulness of sensor L was successfully tested in real water samples
at room temperature. Importantly, this probe L can be utilized for
the sensing of copper ions at ppm concentration over a wide pH
range.
2.2. Synthesis
The formation of Schiff Base ligand L involved two steps; first
step was the synthesis of pyrazine-2-carbohydrazide while sec-
ond step was the condensation of prepared hydrazide with 3-
formylchromone as given below;
2.2.1. Preparation of pyrazine-2-carbohydrazide
Pyrazine-2-carboxylic acid (0.1 g) was dissolved in 20 ml
ethanol in a round bottom flask followed by the addition of few
drops of conc. sulphuric acid and refluxed for 22 hrs. Then hy-
drazine hydrate 100% (3 ml) was added to the solution and again
refluxed for a period of 7 hr. The product was obtained by filtration
and washed with cold water, further recrystallized with ethanol.
Yield: 84%.
2.2.2. Preparation of ligand [L]
3-formylchromone (0.25 g) and pyrazine-2-carbohydrazide
(0.2 g) were dissolved in 30 ml ethanol in separate beakers
and mixed with constant stirring in a round bottom flask. The
solution mixture was stirred for 30 min at room temperature.
TLC plates were used to monitor progress of reaction. The re-
action mixture was then filtered, crude precipitates were ob-
tained and recrystallization of precipitates was done using ethanol.
The recrystallized product was then washed with distilled wa-
ter and ethanol (Scheme 1). Yield: 83.4%. m.p:205 °C. 1H NMR
(400 MHz, DMSO–d6), δ=12.51 ppm (s,1H), δ=9.27 ppm (s,1H),
δ=8.94 ppm (d,1H), δ= 8.87 ppm (s,1H), δ=8.81 ppm (d,1H),
δ=8.80 ppm (s,1H), δ=8.15 ppm (dd,1H), δ=7.88 ppm (t,1H),
δ=7.75 ppm (dd,1H), δ=7.58 ppm (t,1H). FT-IR (KBr pellet, cm1):
2. Experiment
2.1. Materials and apparatus
3-formylchromone, pyrazine-2-carboxylic acid and hydrazine
hydrate were obtained from Sigma Aldrich. Concentrated sul-
phuric acid, glacial acetic acid, absolute ethanol, dimethyl sul-
foxide (DMSO), ethyl acetate, hexane, methanol, hydrated nitrate
salts of Cd2+, Hg2+, Cr3+, Mn2+, Ca2+, Mg2+, Fe3+, Co2+, Ni2+
,
Cu2+, Zn2+, Al3+ and anhydrous nitrate salts of Ag+, Zr2+, Na+,
K+ and Pb2+were purchased from CDH and used as procured with-
out further refinement. 1H NMR (TMS as internal standard) was
recorded from Avance III 400 MHz Bruker instrument using DMSO-
d6 as solvent. FTIR spectrum was obtained using Perkin Elmer
Spectrum, BX II FT-IR Spectrophotometer over a range from 4000
to 400 cm−1 with KBR pellets. The UV-VIS studies were per-
formed in 200 to 800 nm region by using Varian Cary-5000 UV–
VIS-NIR Spectrophotometer with slit width 2.0 nm. The HRMS
data of ligand was obtained from SCIX-QTOF Mass Spectrome-
ter in methanol solvent. Systronics Digital pH Meter 335 was
employed to check pH sensitivity of the compound synthesized.
Throughout all the experiments distilled water was used wherever
needed.
υN= = 3282cm−1, υC=O = 1681cm−1, υC=N = 1644cm−1. HRMS
H
peak at m/z = 295.08 corresponds to [L + 1]+ [calculated mass,
294.28].
2.3. Titration measurements
Primarily, the stock solutions of ligand L (10 mM) and metal
salts (10 mM) were prepared taking methanol as a solvent and
then desired concentration for further titrations was obtained by
the dilution. All the solutions were kept at room temperature. The
solutions of varying pH to check the pH sensitivity was prepared
employing acid (HCl) and base (NaOH) solutions.
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Scheme 2. The expected binding framework between Cu2+ ions and receptor L.
Fig. 1. Colorimetric response of ligand L (17 μM) after the addition of various metal ions (10−5 M).
Fig. 2. (a) The UV-Visible absorption spectra of prepared ligand L. (b) Absorption changes when ligand L (17 μM) solution was titrated with different metal ions solution.
2.4. Copper ion detection in water samples
whatman filter paper, the collected water samples were filtered
to remove insoluble impurities; afterwards samples were spiked
with different Cu2+ ions concentration (5 ppm & 10 ppm). The
absorbance spectra of the spiked samples were obtained and the
calculations were made by using Beer–Lambert’s Law.
Spectrophotometric titrations for the detection of Cu2+ions were
performed using ligand L (17 μM) solution in various water sam-
ples (tap water, ground water and canal water). Initially, using
3

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Fig. 3. (a) Absorption spectra of L with incremental addition of Cu2+ ions from 0.0 to 1.0 equivalent. (b) Absorbance versus mole fraction plot for L-Cu2+ at wavelengths
428 nm and 311 nm.
Fig. 4. (a) Job’s plot corresponding to wavelength 428 nm of L-Cu2+complex with varying concentration of copper ions from 0.0 to 1.0 equivalents, using the UV–Vis absorp-
tion spectra. (b) The Benesi-Hildebrand plot to find out association constant (Ka) among L and Cu2+ ions.
Fig. 5. (a) The HRMS analysis for Schiff base L. (b) Mass spectrum of L in the presence of Cu2+ions in methanol solvent.
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Fig. 6. Bar graph showing the selectivity of ligand L (17 μM) for Cu2+ions over other competing ions.
the addition of hydrazine hydrate solution. The prepared hydrazide
was then allowed to condensed with 3-formylchromone to give the
desired Schiff base Pyrazine-2-carboxylic acid (4-oxo-4H-chromen-
3-ylmethylene)-hydrazide (Scheme 1). The prepared ligand L was
characterized by spectral techniques such as; 1H NMR, FTIR and
HRMS spectrometer. The FT-IR data [Figure S1] confirms the struc-
ture of ligand by showing the characteristic band of functional
groups like C = N at 1644 cm−1, C = O at1681 cm−1 and N H at
–
3282 cm−1. Also, the structure is supported by 1H NMR spectrum
–
that shows peaks at δ=12.51 ppm corresponds to N H proton and
δ=8.87 ppm for imine proton [Figure S2]. The HRMS data shows
molecular ion peak [M + 1]+ at 295.08 also in close agreement
with the calculated mass 294.28 of the ligand L [Fig. 5(a)].
3.2. Colorimetric analysis
Naked eye or colorimetric detection of ions using a chemosen-
sor is the modest and benign method. When methanolic solution
of prepared ligand L (17 μM) was added individually with differ-
Fig. 7. The influence of pH changes on the absorbance of L-Cu (II) solution at wave-
length 428 nm.
ent metal ions i.e. Na+, Mg2+, Ca2+, K+, Pb2+, Al3+, Cr3+, Mn2+
,
2.5. DFT- Calculations
Fe3+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+ and Zr2+, an in-
stant colour response was observed for Cu2+ ions above various
other metal ions. Upon addition of Cu2+ ions, the colourless liquid
L turned into yellow which was visible through naked eyes, while
no noticeable change was observed for other metal ions as shown
in Fig. 1. The results indicated that ligand L might serve as a colori-
metric probe for selective recognition of Cu2+ ions through naked
eyes.
Quantum mechanical calculations were performed using the
B3LYP density functional theory (DFT) employed in Gaussian 09
program package [35]. Geometry optimizations of the probe L and
metal attached to L were characterized to the minima without im-
posing any constrains on the potential energy surfaces. The com-
putations were carried out with 6–311 G (d, p) basis set for ligands
L, and LANL2DZ basis set for its Cu (II) complex.
3.3. Absorption studies
3. Results and discussions
The UV-Visible absorption studies are well recognized for un-
derstanding the interaction of metal ions with the Schiff base lig-
ands [36]. Taking the colorimetric results under consideration, fur-
ther metal ions interaction with L was examined through UV-
Visible titrations using methanol as a solvent. Absorption spectra
of ligand L showed two strong absorption bands one at 258 nm
and the other one at 311 nm.
The band at 258 nm may be due to ꢀ- ꢀ∗ transition and at
311 nm could be due to n- ꢀ∗ transition as shown in Fig. 2(a).
On addition of distinct metal ions no considerable changes in the
3.1. Design and synthesis of L
Schiff bases are well known chelating ligands. The azo methine
(-C = N-) group of Schiff bases act as an active binding site for
transition and post transition metal ions and therefore selected as
chromophores for the metal ions detection. Taking this under con-
sideration, a chromone based Schiff base L has been synthesized.
Pyrazine-2-carbohydrazide was prepared by refluxing pyrazine-2-
carboxylic acid with conc. sulphuric acid in ethanol followed by
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˚
Fig. 8. Geometrical optimization of ligand L (a) and l-Cu (II) complex (b) at B3LYP/ LAN L 2DZ level of theory. Selective bond lengths (in A) are shown.
absorption bands were obtained apart from Cu (II) ions as shown
in Fig. 2(b). In case of Cu2+ ions, a new band was appeared at
428 nm with the complete disappearance of 311 nm absorption
band which indicated a bathochromic shift in the absorption spec-
trum. This new band must be due to the complexation amongst
the Cu2+ions and ligand L [37]. The intramolecular charge transfer
(ICT) process was evident during the complex formation [38, 39].
Upon incremental addition of Cu2+ ions from 0.0 to 1.0 equivalent
in terms of molar ratio to the ligand L solution, band at 311 nm
decreased gradually and a new band at 428 nm appeared progres-
sively. This gave a clear isosbestic point at 363 nm and proposed
that a stable metal complex between Cu (II) ions and ligand L is
formed as shown in Fig. 3(a). The Fig. 3(b) shows graph between
the absorbance changes in bands at 428 nm and 311 nm versus the
mole fraction of L-Cu2+, indicating that with the hike in concentra-
tion of copper ions in the solution, the band at 428 nm increases
and at 311 nm decreases gradually.
Cu (II) ions and L is 1:1. The High Resolution Mass Spectrometry
(HRMS) data also validate the 1:1 stoichiometry between L and
Cu2+ ion as shown in Fig. 5(b) and the pe+ak at m / z 355.9 in the
mass spectrum corresponding to [L+Cu2+
]
clarified the metal co-
ordination with ligand in 1:1 molar ratio. With the help of Benesi-
Hildebrand equation, association constant (Ka) for L was computed
2.3 × 105 M−1 using:
1
1
1
=
+
]
(1)
A − Ao
Ka Amax − Ao Cu2+
Amax − Ao
[
][
where A0 is absorbance value of ligand L in absence of copper
ions, A is absorbance recorded in the presence of copper ions at
particular concentration, Amaxis maximum absorbance recorded in
the presence of copper ions and Ka stands for association con-
stant. The limit of detection (LOD) of L for Cu2+ ions was com-
puted 3.9 × 10−7 M using expression;
LOD = 3σ/K
(2)
where σ stands for the standard deviation and Ka stands for the
association constant obtained from equation-1 with the help cali-
bration curve shown in Fig. 4(b).
3.4. Job’s plot measurements
To understand the mechanism of binding among Schiff base
ligand L and the Cu2+ ions, the job’s plot measurement was im-
plemented. In order to obtain the plot, solutions of ligand L and
copper metal salt of equal concentration (17 μM) were prepared
in methanol. UV-Visible titrations of L were carried out with in-
creasing extents of Cu2+ions from 0.1 to 1.0 equivalents and overall
concentration of solution was maintained constant at 17 μM. The
Fig. 4(a) shows a Job’s plot of absorbance at wavelength 428 nm
with increase in mole fraction of Cu2+ ions. It was observed that
absorbance of ligand L reached its maximum value at ~0.5 mole
fraction suggesting that the stoichiometric binding ratio among
3.5. Selectivity of l for Cu2+ ions
To check the selectivity of L for Cu (II) ions over Na+, Mg2+
,
Ca2+, K+, Pb2+, Al3+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+,
Zn2+, Cd2+, Hg2+, and Zr2+ ions, competitive titrations were per-
formed using 17 μM solution of L. When 1:1 equivalent solutions
of L and various other metal ions were treated with copper ions
individually, no considerable changes were observed in the absorp-
tion spectra, even after using 2.0 equivalents of the other metal
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Table 1
Some reported Schiff base colorimetric probe with their properties.
Building Block
Colour Change
Detection limit
Reference
Solvent
–
Bisthiophene
Napthaldehyde
Cinnamaldehyde
Thiosemicarbazide
Amide
Yellow to colourless
Yellow to claybank
Yellow to pink
6.21 × 10−6
4.65 × 10−7
2.85 × 10−6
M
M
M
[40]
CH3OH H2O
DMF-H2O
[41]
–
[42]
CH3CN H2O
Colourless to yellow
Colourless to yellow
Colourless to yellow
Colourless to yellow
1.7 × 10−6
1.8 × 10−6
M
[43]
CH3OH H2O
–
M
[44]
CH3OH-tris buffer
CH3OH-tris buffer
CH3OH
Triazole
12 × 10−7M
[45]
Chromone
3.9 × 10−7
M
Present work
ions no significant interference was observed. The Fig. 6 displaying
the bar graph of absorbance changes at wavelength 428 nm for
L + Cu and L + Cu + competitive ions, which gave clear indication
that the Schiff base L might work as a selective colorimetric sensor
in the presence and in absence of various other analytes.
ligand L was tested for different water samples using spectropho-
tometric method and the relative standard deviation (RSD) values
of all the taken samples are shown in Table 2, RSD values were
not found more than 2.8% in case of entire sample range, which
was quite reliable for analysis at ppm level. These studies revealed
the potency of the synthesized ligand L as an effective chemosen-
sor for Cu2+ ions detection in environment samples.
3.6. pH effects
Many of the chemosensors involves the detection of metal ions
through the proton transfer phenomenon and this process is af-
fected by pH of the solution. To check the acceptable pH range for
metal ion detection in biological and environmental samples, effect
of pH changes on the liability and selectivity of ligand L towards
Cu2+ ions was too tested. For this intent, 17 μM ligand L solution
was prepared using methanol and the pH of solution was sustained
from 2 to 11 in separate vessels using pH meter with the help of
acid (HCl) and base (NaOH) solutions. Solutions of ligand L of dif-
ferent pH were then treated with Cu2+ ions solution and further
these solutions were subjected for UV-Visible absorbance studies.
The Fig. 7 shows the changes in absorbance at 428 nm with re-
spect to the change in pH from 2 to 11. It was observed that ligand
L work effectively in neutral and basic medium (pH 6–11) while no
satisfactory results were obtained at very low pH values (2–3). The
results justified the working pH which is 7.9; also it was revealed
that the sensor can work over a broad pH range. Apart from that,
it was noted that as the basicity increases the colorimetric changes
were observed more effectively.
3.8. DFT-Calculations
Full geometrical optimization of ligand L and its Cu (II) complex
was carried out at the B3LYP level in their ground state shown
in Figs. 8(a) and 8(b). The bond distances of coordination bonds
formed among Cu (II) and donor atoms of ligand L are Cu-N:
˚
˚
˚
2.1258 A; Cu-O: 1.9411 A and Cu-O: 1.9763 A. The frontier or-
bitals of highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of ligand L and L-Cu2+ com-
plex are also given in Figs. 9(a) and 9(b). It was indicated that
electron density of HOMO in ligand L is mainly located on 3-
(iminomethyl)−4H-1-benzopyran-4-one site and anticipated as a
possible binding site for the Cu2+ions, while LUMO for L was lo-
calized on pyrazine-2-carboxamide site. Similar fashion of MOs lo-
calization was observed in Cu (II) complex of ligand L.
The energy gap between HOMO-LUMO of ligand L and L-Cu (II)
complex was measured to be 3.77 eV and 0.98 eV, correspondingly.
The Cu (II) complex of ligand L exhibited decrease in energy gap,
which gave clear indication of the easy electron transfer mecha-
nism and intensive colour of the complex. These results supported
the colorimetric changes in L with Cu (II) ions and the formation
of stable L-Cu2+ complex.
To compare the efficiency of the ensuing probe, comparison of
probe L in terms of selectivity and detection limit for Cu2+ ions
with other formerly reported colorimetric probes is given below
in Table 1 with their visual colour change and detection limit, con-
firming that the developed chromone based Schiff base L is compa-
rable and has even better performance in terms of detection limit.
From the results obtained by Job’s plot, HRMS analysis and DFT-
calculations, it was concluded that the expected binding frame-
work of Cu2+ions with the receptor L would be as depicted in
Scheme 2.
3.7. Sensor’s stability and reproducibility
The stability of the prepared compound L was tested after 15
days and no specific change was observed in characteristic ab-
sorbance band at 428 nm for Cu2+ ions. The ligand L showed the
identical results as were obtained earlier. Moreover, L showed an
immediate colour response as soon as the Cu2+ ions solution was
added to the L solution and a fast response time ~40 s for the lig-
and L was recorded. In order to find the reproducibility of probe L
for the detection of Cu (II) ions, the recommended procedure was
repeated three times under optimal conditions. The reliability of
4. Practical utility
So as to further examine the feasibility and suitability of
chemosensor L in environmental samples, a calibration curve was
constructed as shown in Fig. 10(a). Probe L displayed a straight line
graph for varying concentration of Cu2+ions from 0 to 10 ppm with
a correlation coefficient R2 = 0.9865, which signifies that L could
be suitable for Cu2+ions quantitative measurement. The real sam-
ple analysis was accomplished with canal, ground and tap water
Table 2
Percentage RSD and Recovery values for different water samples.
Water Samples
Canal Water
Cu Spiked (ppm)
Cu Found (ppm)
% Recovery
% RSD (n = 3)
5
5.16
9.87
5.19
10.15
5.44
10.40
103%
98%
2.1
10
5
1.04
2.07
1.32
2.81
1.14
Ground Water
Tap Water
104%
101.5%
108%
104%
10
5
10
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Fig. 9. (a) Calculated frontier molecular orbitals of ligand L and (b) the corresponding L-Cu2+binding.
Fig. 10. (a) Calibration curve for absorbance of L-Cu2+against varying concentration of Cu ions from 0 to 10 ppm. (b) Spectrophotometric analysis of Cu (II) ions at varied
concentrations in different water samples.
samples and the UV-Visible titration response of L against these
water samples is as shown in Fig. 10(b).
5. Conclusion
The standard procedure of addition was followed for the real
time analysis. Acquired to the calibration curve, relevant recovery
values of the water samples were obtained using Beer-Lambert’s
Law as depicts in Table 2. Based on the obtained results, we could
say that sensor L might be applicable for the recognition and quan-
tification of Cu (II) ions in biological and environmental samples at
a very low concentration level.
In present work, a chromone based Schiff base L was being
synthesized and characterized by means of different spectral tech-
niques like FTIR, 1H NMR and HRMS. The ligand L showed selective
recognition of Cu2+ ions over other ions such as, Na+, Mg2+, Ca2+
K+, Pb2+, Al3+, Cr3+, Mn2+, Fe3+, Co2+, Ag+, Ni2+, Zn2+, Cd2+
,
,
Hg2+ and Zr2+through colorimetric response from colourless to yel-
low. UV-Visible titrations also assisted the selectivity of L towards
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Cu2+ ions. HRMS and the Job’s plot data authenticated the 1:1 stoi-
chiometry among L and Cu2+ ions. The LOD value for the detection
of Cu2+ ions was determined to be 3.9 × 10−7 M and association
constant (Ka) was computed to be 2.3 × 105 M−1 using Benesi-
Hildebrand equation. Density functional Theory (DFT) computation
data also certify the binding stoichiometry among L and Cu2+ ions.
The pH tests performed to check the sensitivity of chemosensor L
revealed that it can operate over large pH values. Sensor L was ef-
fectively implemented for quantified sensing of Cu2+ions in differ-
ent water samples at different concentrations. From overall results,
it has been concluded that L could work as an excellent colorimet-
ric and selective probe to sense Cu2+ ions for environmental and
biological monitoring.
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Nisha Tomer and Apurva Goel contributed equally to this work.
Rajesh Malhotra contributed as being the supervisor of this re-
search work.
Vikas D. Ghule contributed to DFT calculations.
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.
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Acknowledgements
Nisha Tomer is thankful to UGC-JRF vide letter no. F.No.16–
9(June 2018)/2019(NET/CSIR) and Ref. No.:264/ (CSIR-UGC NET
JUNE 2018) from UGC, New Delhi, for providing financial support
to her in the form of JRF fellowship. The authors are extremely
gratified for providing instrumental assistance to the Central In-
strumentation Lab (CIL), Guru Jambheshwar University of Science
and Technology, Hisar.
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