Nanomolar detection of mercury(II) using electropolymerized phthalocyanine film

Article abstract of DOI:10.1016/j.electacta.2020.137519

The synthesis and characterization of novel cobalt(II) tetraamide benzimidazole phthalocyanine (CoTABImPc) has been reported for the first time and applied for the determination of mercury(II) at nanomolar concentration using electrochemical techniques. CoTABImPc was prepared by coupling 4-(1H-benzimidazol-2-yl)aniline with cobalt(II) tetracarboxylic acid phthalocyanine. CoTABImPc was characterized by elemental analysis, TGA, FT-IR, UV–visible, Mass, and NMR spectroscopic techniques. The CoTABImPc was electropolymerized on clean glassy carbon electrode (GCE) by continuous cycling of the potential (GCE/poly(CoTABImPc)). The polymeric film was employed for the detection of highly toxic Hg(II) by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometric (CA) methods. The designed GCE/poly(CoTABImPc) electrode exhibited good electrochemical response and high electrocatalytic activity for detecting Hg(II). The Pc polymeric film electrode displayed a sensitivity of 1.2178 μA nM^?1 cm^?2 for the detection of Hg(II) in the linear range 10–300 nM, and low detection limit of 4 nM using CV technique. Further, DPV exhibited a sensitivity of 1.4024 μA nM^?1 cm^?2 in the 10–500 nM range with LOD of 3.8 nM. The CA method delivered an excellent sensitivity of 3.7389 μA nM^?1 cm^?2 in the linear range 10–400 nM with LOD of 3 nM. The proposed method augments the selective electrochemical determination of Hg(II) in environment pollution analysis.

Full text of DOI:10.1016/j.electacta.2020.137519

Electrochimica Acta 367 (2021) 137519
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Nanomolar detection of mercury(II) using electropolymerized
phthalocyanine film
Manjunatha Palanna, Shambhulinga Aralekallu, CP Keshavananda Prabhu, Veeresh A Sajjan,
∗
Mounesh, Lokesh Koodlur Sannegowda
Department of Studies in Chemistry, Vijayanagara Sri Krishnadevaraya University, Ballari, 583105, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of novel cobalt(II) tetraamide benzimidazole phthalocyanine
Received 21 August 2020
Revised 24 October 2020
Accepted 19 November 2020
Available online 25 November 2020
(
CoTABImPc) has been reported for the first time and applied for the determination of mercury(II) at
nanomolar concentration using electrochemical techniques. CoTABImPc was prepared by coupling 4-
1H-benzimidazol-2-yl)aniline with cobalt(II) tetracarboxylic acid phthalocyanine. CoTABImPc was char-
(
acterized by elemental analysis, TGA, FT-IR, UV–visible, Mass, and NMR spectroscopic techniques. The
CoTABImPc was electropolymerized on clean glassy carbon electrode (GCE) by continuous cycling of the
potential (GCE/poly(CoTABImPc)). The polymeric film was employed for the detection of highly toxic
Hg(II) by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometric (CA)
methods. The designed GCE/poly(CoTABImPc) electrode exhibited good electrochemical response and high
electrocatalytic activity for detecting Hg(II). The Pc polymeric film electrode displayed a sensitivity of
Keywords:
Benzimidazole phthalocyanine
Electropolymerization
Mercury(II)
Cyclic voltammetry
Chronoamperometry
.2178 μA nM 1 cm for the detection of Hg(II) in the linear range 10–300 nM, and low detection limit of
−
−2
1
4
nM using CV technique. Further, DPV exhibited a sensitivity of 1.4024 μA nM 1 cm in the 10–500 nM
−
−2
−
1
−2
range with LOD of 3.8 nM. The CA method delivered an excellent sensitivity of 3.7389 μA nM cm
in the linear range 10–400 nM with LOD of 3 nM. The proposed method augments the selective electro-
chemical determination of Hg(II) in environment pollution analysis.
©
2020 Elsevier Ltd. All rights reserved.
1
. Introduction
gloves [5]. The excess Hg(II) levels in water and meat can lead
to death, mental retardation, dysarthria, blindness, neurological
deficits, hearing impairment, developmental defects, and abnor-
mal muscle tone [6]. Besides, it can result in disorders including
Hunter-Russell syndrome, pink disease, Minamata disease and se-
vere kidney damage. Hence the contamination caused by heavy
metals has aroused extensive public concern [4].
Heavy metal ions like As, Cd, Pb, Cr and Hg are highly harm-
ful to the environment and human health [1]. Heavy metal ions
from industrial wastewater are majorly responsible for water pol-
lution [2]. Heavy metal ions get enriched in the living organisms
through the food chain and due to their nondegradability and non-
biocompatibility results in serious damage and problems to ani-
mals, plants, and humans. Particularly, mercury (Hg(II)) is consid-
ered as the most toxic element that could easily accumulate in the
human body and environment [3]. The World Health Organization
In this regard, accurate, reliable detection, and monitoring of
trace level Hg(II) in water, as well as noxious waste samples, has
profound importance to secure the health and environment. Dif-
ferent analytical methods have been developed for the reliable de-
tection of Hg(II), including traditional methods such as inductively
coupled atomic absorption and emission spectroscopy [7], X-ray
fluorescence spectrometry [8], inductively coupled plasma mass
spectrometry [9] and capillary electrophoresis [10]. Also, different
sensing strategies have been explored for the detection of Hg(II)
using photoelectrochemical [11], surface Plasmon resonance [12],
surface-enhanced resonance Raman scattering [13], electrochemi-
cal [14], fluorescence and colorimetric methods [15, 16].
(
WHO) has set a permissible level of Hg(II) as 30 ppb in drink-
ing water, whilst the U.S Food and Drug Administration corpora-
tion has mentioned a maximum permissible level of Hg(II) in meat
as 1000 nM L 1 [4].
−
Among the different Hg compounds, most dangerous is
dimethylmercury ((CH3)2 Hg) which is toxic enough to cause death
even if few microliters is spilt on the skin, or even on latex
However, among the various analytical methods, electrochem-
ical methods are simple, reliable and straightforward when com-
pared with the accessible aforementioned conventional and ad-
∗
Corresponding author.
E-mail address: kslokesh@vskub.ac.in (L.K. Sannegowda).
https://doi.org/10.1016/j.electacta.2020.137519
0
013-4686/© 2020 Elsevier Ltd. All rights reserved.

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
vanced methods. But the electrochemical techniques require an ap-
propriate catalyst which can reduce the overpotential and increase
the catalytic activity.
crude product was purified using chloroform to get corresponding
4-(1H-benzimidazol-2-yl)aniline [35].
Yield: 73%. Melting point: 226–228 °C. Anal. for (i), Mol. Wt.
In recent years, metal phthalocyanines (Pcs) have clinched great
interest because of their excellent properties like environment-
friendliness, low-cost, stability, and semiconducting nature [17].
Moreover, Pcs are easy to synthesize and are widely studied as
sensors [18–20], supercapacitors and battery [21, 22] materials in
succession to their traditional applications as dyes and pigments
209.26. C13H11 N ; Calc. (%): C, 74.62; H, 5.30; N, 20.08. Found
3
(%): C, 74.43; H, 5.33; N, 20.32. IR absorption bands (KBr (pellet),
cm 1): 494, 691, 730, 897, 922, 1001, 1031, 1109, 1243, 1273, 1312,
−
1460, 1479, 1568, 1666, 1760, 1878, 1918, 2763, 3057, 3216, 3343
and 3432. 1H NMR (400 MHz, DMSO–d ): δ 5.58 (s, 2H), 7.47 (d,
6
J = 8.00 Hz, 1H), 7.56 (d, J = 9.00 Hz, 1H), 7.88 (d, J = 9.00 Hz,
+
3
[
23]. Metal phthalocyanines (MPcs) are redox-active molecules and
1H), 7.49 (d, J = 8 Hz, 1H), 11.38 (s, 1H). Mass: M (212.13).
are known to catalyze the reactions effectively [24–27].
Hence, MPcs find great potential as surface modifiers in elec-
trochemical sensors as they yield high surface area, good absorba-
bility, and high catalytic activity [28]. A variety of MPcs composed
of different metal ions and ligands have been reported as efficient
catalysts in electrochemical sensors [29, 30]. Among them, Co-
based MPcs are explored extensively, as they exhibit exceptional
chemical stability and flexibility for catalytic applications [31, 32].
However Pc-based materials have not yet been employed in the
electrochemical sensing of heavy metals and in particular Hg(II).
The benzimidazole substituent at the periphery of Pc ring is
electrochemically active and it forms an electropolymerized film
on potential cycling [33]. The polymeric film deposits on the elec-
trode surface and provides chemical interaction between the elec-
trode and Pc polymeric film. The Pc film uniformly distributes and
expected to have high conductivity due to the extension of conju-
gation and delocalization with higher surface area and also, pro-
vides higher stability and less interfacial resistance for the charge
transfer compared to physical adsorption process [34].
2.2.3. Preparation of cobalt(II) tetracarboxylic acid phthalocyanine
complex (CoTCAPc) (ii)
CoTCAPc was synthesized by utilizing the procedure as reported
in the literature [33, 34]. A ground mixture of trimellitic anhydride
(100 mM), urea (5 M), a catalytic amount of ammonium molyb-
date (0.2 mM) and CoCl2 (25 mM) was charged into a three-necked
flask and stirred at 170–180 °C for 5 h and then cooled to room
temperature. The obtained shady green colored product was pu-
rified by washing with alcohol and concurrently with hot dil.HCl
and 1 N NaOH solutions. At last, CoTCAPc was washed with a large
amount of distilled water to remove the acid. The product CoTCAPc
was dried over P O .
2
5
Yield: 78%. Anal. CoTCAPc, Mol. Wt. 747.49. C36H16 N O Co: Calc.
8
8
(%) C, 57.84; H, 2.16; N, 14.99; O, 17.12; Co, 7.88. Found: C, 57.52; H,
2.13; N, 15.24; Co, 7.75. Absorption spectra (DMSO, λmax(nm)): 330,
430, 600, 635. IR absorption bands (KBr (pellet), cm 1): 736, 884,
−
1091, 1157, 1283, 1323, 1403, 1469, 1520, 1606, 1718, 2224, 2851,
.
2927, and 3406. Mass: M (747).
In this study, CoTABImPc was prepared for the first time and
was electropolymerized and utilized as a highly sensitive and se-
lective catalyst for Hg(II) electrochemical sensor.
2.2.4. Synthesis of cobalt(II) tetraamide
benzimidazole phthalocyanine complex (iii)
The precursor compound 4-(1H-benzimidazol-2-yl)aniline
(1.01 g, 0.0040 mol) was mixed with CoTCAPc (0.241 g, 0.0010
mol) and, DCC (1.51 g, 0.0060 mol) in an RB flask containing
10 mL dimethylformamide. The mixture was refluxed at 90 °C
for about 6 h. The dim bluish-green colored product formed was
washed with alcohol, hexane and acetone. Then dried in an oven
at 60 °C for an overnight.
2
. Experimental section
2
.1. Materials
4
-aminobenzoic acid, anhydrous cobaltous acetate, anhy-
drous potassium carbonate, orthophenylenediamine and mer-
cury(II) chloride were procured from Alfa Aesar India. 5% Nafion
solution, N, N’-dicyclohexylcarbodiimide, dimethylsulphoxide, N,
N’-dimethylformamide, methanol, ethanol were purchased from
Merck, India. Electrodes were obtained from fuel cell store, India.
Yield: 75%. Anal. for CoTABImPc (deep blue), Mol. Wt. 1512.42
C88H52N20O Co; Calc. (%): C, 69.88; H, 3.47; N, 18.52; Co, 3.90; O,
4
4.23. Found (%): C, 69.49; H, 3.41; N, 18.79; Co, 3.61. Absorption
spectra (DMSO, λmax/ nm (log ε, mol 1 L cm )): 339 (3.69), 612
(
−
−1
4.03), 677 (4.21). IR absorption bands (KBr (pellet), cm 1): 642,
−
2
2
.2. Synthesis of phthalocyanine complex
722, 752, 806, 892, 1043, 1090, 1154, 1244, 1309, 1446, 1578, 1625,
123, 2856, 2930, 3217, 3349, and 3430. Mass: M 3 (1509.18).
−
2
.2.1. Synthesis of cobalt(II) tetraamide benzimidazole phthalocyanine
complex
The synthesis of CoTABImPc was performed as revealed in
2.3. Instrumentation and characterization
Scheme 1. The preparation of benzimidazole phthalocyanine com-
plex has two reaction steps. The first step involves the formation of
All the analytical techniques used for the characterization are
mentioned in the Supporting Information.
4
-aminobenzimidazole moiety and the second step is the coupling
of the precursor with carboxylic acid phthalocyanine to yield the
cobalt tetraamide benzimidazole phthalocyanine complex as the
concluding product.
2.4. Cleaning of glassy carbon electrode (GCE)
The GCE surface was polished with alumina powder slurry (0.3
and 0.05 μm, respectively) on a polishing pad. The polished elec-
trode was then sonicated in ethanol to remove the trapped parti-
cles. Then CV scans were performed in −1 to 1 V range in 1 M
H SO electrolyte to remove the adsorbed particles on the surface
2
.2.2. Preparation of 4-(1H-benzimidazol-2-yl)aniline (i)
The mixture of o-phenylenediamine (2.531 g, 0.0234 mol) and
-aminobenzoic acid (3.209 g, 0.0234 mol) was grounded well and
4
2
4
transferred into a clean dried RB flask containing ethanol (20 mL)
of the GCE.
as a solvent and added a catalytic amount of NH Cl to the solution
4
and refluxed at 65–70 °C for 4 h to get compound i. The reaction
progress was monitored using thin-layer chromatography (TLC). Af-
ter completion of the product formation, crude was charged into
deionized ice-cold water to get a brown precipitate. The product
obtained was filtered, washed with distilled water and finally, the
2.5. Modification of the electrode
The electropolymerization methodology was employed to de-
posit a thin uniform film of Pc on the electrode. Electropolymeriza-
tion process was carried out using cyclic voltammetry with 0.1 mM
2

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
Scheme 1. Synthetic approach for CoTABImPc.
CoTABImPc in DMSO containing 0.1 M TBAP at cleaned GCE. The
polymeric film electrode was represented as GCE/poly(CoTABImPc)
and employed for voltammetric and amperometric determination
of Hg(II).
2
.6. Preparation of mercury(II) solution
The 0.01 M mercury(II) chloride stock solution was prepared us-
ing doubly distilled water and was diluted to achieve a working
standard solution of 0.01 mM. Different aliquots of the standard
solution of Hg(II) was injected into 10 mL PBS electrolyte through-
out the analysis.
3. Results and discussion
The scheme for preparing cobalt tetraamide benzimidazole ph-
thalocyanine (CoTABImPc) is presented in Scheme 1. The pre-
cursor was prepared by reacting 4-aminobenzoic acid with
OPDA in the presence of ethanol with a catalytic amount of
i
Fig. 1. FTIR spectra for (i) 4-(1H-benzimidazol-2-yl)aniline, (ii) CoTCAPc and (iii)
NH Cl. The CoTABImPc formation involves the coupling of 4-
4
CoTABImPc.
aminobenzimidazole with CoTCAPc (Scheme 1).
The amide bond formation mechanism is presented in the Sup-
porting Information (Scheme. S1). The amide bond is formed by
the reaction of an acid and amine in the presence of a coupling
agent. The amide bond is stable because of the resonance struc-
ture of (-CO=NH–). The nitrogen in amide is having lone pair of
electron capable of delocalization with carbonyl carbon. This forms
the partial double bond between –N=C by pushing electrons from
C=O and results, in turn, the formation of oxygen anion [36].
The electrophilic carbonyl carbon (–COOH) of CoTCAPc reacts
−1
recorded in the range of 400–4000 cm . The compound 4-
−1
aminobenzimidazole displayed peaks at 3343, 3216 and 3432 cm
corresponding to –NH2 and –NH of benzimidazole group. The char-
acteristic peaks for Pc complex were observed around 735–750,
−1
8
75–890, 1050–1060, 1090, 1150–1160 cm
and assigned for
−1
phthalocyanine skeletal vibrations [34]. The peak at 1718 cm
in
the spectrum of CoTCAPc corresponds to –C=O and broad peak
−1
at 3406 cm
for –OH of –COOH present in the complex. The
with a nucleophilic amine group (–NH ) of 4-amino benzimidazole
−1
−1
2
disappearance of peaks at 3406 cm
and the appearance of a
indicate the conversion of
in the presence of coupling agent dicyclohexylcarbodiimide (DCC)
resulting in protonation/deprotonation. DCC acts as a dehydrating
agent and also enhances the synthetic efficiency of the forward re-
action forming the product. This coupling method is mild, efficient
and compatible for peptide bond formation and the driving force
of the reaction is the formation of urea by-product. The CoTABImPc
complex obtained was bluish-green amorphous powder. The com-
new peak at 3217 and 3349 cm
carboxylic acid Pc to benzimidazole cobalt phthalocyanine. The
−1
peak at 3430 cm
group.
can be assigned for –NH of benzimidazole
3
.2. Mass spectra
The mass spectra of CoTCAPc and CoTABImPc are character-
plex is soluble in DMF, DMSO, DMA, quinoline and Conc H SO .
2
4
3
.1. FT-IR studies
ized by many competitive and consecutive pathways, thus form-
ing many intense fragment ions. The mass spectrum was recorded
at various retention times and the mass spectrum of the precur-
sor compound 4-(1H-benzimidazol-2-yl)aniline is in Fig. S1. The
Fig. 1 depicts the FTIR spectra of precursor compound 4-
aminobenzimidazole, CoTCAPc, and CoTABImPc complexes
3

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
+
3
base peak for i appears at m/z = 212.13 which corresponds to M
molecular ion peak. The theoretical molecular weight of CoTCAPc is
7
47.49 and the mass spectrum demonstrated peaks at m/z = 743,
−4
−3
−2
,
744,745, 746 and 747.0 which can be accounted for M , M , M
−1
and M. molecular ion peaks, Fig. S2. Similarly, CoTABImPc
M
has a theoretical molecular mass of 1512.4 and the peak appeared
at m/z = 1509, 1527, 1528, 1529 and 1530 can be attributed to
−
3
−3
−2
−1
M
, (M+H O) , (M+H O) , (M+H O) , (M+H O) molecular
2
2
2
2
ion peaks as shown in Fig. S3.
.3. 1HNMR studies
The 1H NMR spectrum of the precursor was measured in
3
Fig. 2. CVs for (i) blank and (ii) 1 mM CoTABImPc in DMSO containing 0.1 M TBAP
−
1
DMSO–d6 at 400 MHz and chemical shift values were recorded
in ppm. Fig. S4 is the 1H NMR spectrum of 4-aminobenzimidazole
and the peaks were observed at δ 5.58 (s, 2H), 7.47 (d, J = 8.00 Hz,
on GCE at 50 mVs in N2 atmosphere.
II −2
II −1
II −2
I
−2
1
H), 7.56 (d, J = 9.00 Hz, 1H), 7.88 (d, J = 9.00 Hz, 1H), 7.49 (d,
[Co Pc ]/[Co Pc ] and [Co Pc ]/[Co Pc ] redox couples, re-
spectively according to the literature reports [29–31].
J = 8 Hz, 1H), 11.38 (s, 1H). The peak for NH of amine (in Fig.
2
S4) was singlet peak at δ 5.58 (s, 2H) and peak for NH proton ap-
peared at δ 11.38 (s, 1H). 1H NMR spectrum successfully confirmed
the purity and formation of 4-aminobenzimidazole compound (i).
3
.7. Electrochemical polymerization of CoTABImPc
The benzimidazole moiety is electrochemically active and it
3
.4. UV–visible studies
successfully forms polymerization product on continuous cycling
of the potential. The electropolymerization of CoTABImPc was car-
ried out on cleaned GCE with 0.1 M CoTABImPc in DMSO contain-
ing TBAP supporting electrolyte and scanning the potential amid
Two characteristic bands were noticed in the UV–vis spectrum
of phthalocyanine complexes i.e., B band or soret band in the UV
region and Q band in the visible region (Fig. S5). The spectrum
of CoTCAPc exhibited a peak at 330 nm in B band region and
−1
1
to −1 V at 20 mV s
for 20 cycles under the N2 atmosphere
as presented in Fig. S7. The gradual progress in the number of po-
tential cycles displayed a proportional change in the current which
is accounted for the sequential addition of repeating units to the
polymer on the electrode. Further, peak potential shift was no-
ticed for the consecutive CVs which might be due to the expo-
sure of new polymeric surface at the GCE. The dark sapphire film
was noticed after the deposition of polymer on the GCE surface.
Polymerization of CoTABImPc complex involves the stacking of the
supramolecular blocks with great structural flexibility. It is note-
worthy that the acidity of the -NH proton of benzimidazole moi-
ety increases in presence of electronegative atoms located near the
carbon atom. The interactions between electron-withdrawing and
donating atoms are believed to be electrostatic interactions be-
tween π-donors and π-acceptors, which have been efficiently uti-
lized in electrochemical polymerization [33, 34].
6
35 nm in the Q band region with a shoulder peak at 600 nm.
Whereas, CoTABImPc displayed a peak at 339 and 677 nm in B
and Q band regions with a shoulder peak at 612 nm. The shoul-
der peak can be ascribed to vibronic nature as well as a dimer,
oligomeric molecules present in the complex. The Q-band is re-
sponsible for the green color of the Pc complexes prepared. The
small shift in the absorption peak position towards longer wave-
length for CoTABImPc compared to CoTCAPc can be accounted for
augmenting in the conjugation and resonance stabilization in ben-
zimidazole phthalocyanine.
3
.5. TGA studies
Fig. S6 depicts the thermograms of CoTCAPc and CoTBImPc. The
little weight loss below 150 °C can be accounted to the loss of
moisture and volatile components in the sample. The complexes
were stable up to 450 °C and then the Pc macrocycle undergoes
gradual decomposition in 400–550 °C region. At this temperature,
the peripheral functional groups first get detached and then the
complex undergoes decomposition in air atmosphere. The weight
corresponding to the horizontal portion after 600 °C represents
the stable oxidized compound, CoO formed which is equivalent to
the theoretical weight of the CoO in the initial weight of Pc. The
CoTABImPc demonstrated enhanced thermal stability than the CoT-
CAPc as a result of higher conjugation and extended delocalization
in the CoTABImPc.
The polymeric film was analysed by CV to evaluate its electro-
chemical behavior and stability in DMSO containing 0.1 M TBAP
at 20 mVs 1 under N2 atmosphere. The first cycle in Fig S7, dis-
played peaks analogous to peaks in Fig. 2 which confirmed the ad-
hering of poly(CoTABImPc) film on the electrode surface. Besides,
Fig. S8 depicts a small or negligible change in the background and
peak current for the fresh CV and the 100th cycle which infers that
the poly(CoTABImPc) possess good electrochemical stability on the
electrode.
−
3.8. Charge transfer behavior
3
.6. Cyclic voltammetric studies
Redox probe, Ferro/ferricyanide system was employed to eval-
uate the electron transfer characteristics of GCE/poly(CoTABImPc).
Fig. S9 depicts the CVs of the redox probe at virgin GCE
and GCE/poly(CoTABImPc) film. The virgin GCE showed well-
The electrochemical activity and redox performance of the
CoTABImPc complex was investigated by cyclic voltammetry (CV).
Before the experiments, the DMSO solution was purged with a
constant flow of N2 gas to eliminate the dissolved oxygen for
defined characteristic redox peaks for the Fe2 /Fe
+
3+
redox probe
whereas, partial flattening of the peaks was noticed at the
GCE/poly(CoTABImPc) with a peak current decrease which infers
that the electron transfer is sluggish at the GCE/poly(CoTABImPc)
compared to pure GCE. The slight sluggish behavior experienced
at the poly(CoTABImPc)electrode may be accounted for the lesser
conductivity and thick film of Pc on the surface which slightly
2
0 min. Fig. 2 shows the first cyclic voltammogram for 1 mM
CoTABImPc in DMSO containing 0.1 M TBAP as supporting elec-
−1
trolyte at a scan rate of 50 mVs
on pristine GCE. The CV
of CoTABImPc demonstrated two pairs of redox peaks at −0.3
to −0.5 V and 0.25 to 0.45 V which can be assigned for the
4

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
Table 1
Intrinsic electrochemical parameters for the Hg(II) detection at
GCE/poly(CoTABImPc).
Tafel slope
113 mV/dec
7.41 × 10− A cm
5
−2
4
Exchange current density
Diffusion coefficient
(2.2.0 ± 0.2) × 10− cm2
−1
s
strated a well-defined and sharp peak for the detection of Hg(II)
at 0.18 V and successive increase in response was noticed for en-
hancement in the Hg(II) concentration. The plot of peak current
(
ip) versus the Hg(II) concentration displayed a linear characteristic
in the 10–300 nM concentration range (Fig. 4b). The linear equa-
tion for the straight-line relationship is; i/μA = 1.2769 + 0.0853 C
Fig. 3. Nyquist plot of
1
mM K4[Fe(CN)6] at (i) bare GCE and (iii)
2
(
nM), with the correlation coefficient (R ) of 0.9979. The sensitivity
GCE/poly(CoTABImPc) and the theoretical fit for (ii) bare GCE and (iv)
GCE/poly(CoTABImPc) in 0.1 M PBS pH 7.0. Inset: Equivalence electrical circuit used
to fit the EIS data.
of the GCE/poly(CoTABImPc) was calculated based on the slope of
−1
−2
the calibration plot and the sensitivity was 1.2178 μA nM
cm
with 10 nM as the lowest quantifiable concentration. The experi-
mental limit of detection (LOD) was 4 nM by the 3σ method. The
value of LOD for Hg(II) at GCE/poly(CoTABImPc) is much smaller
than the reported values in the literature [37–43].
blocks the facile transfer of electrons at the boundary of electrode
and electrolyte.
The sensitivity, concentration range and LOD of the fabricated
poly(CoTABImPc) electrode are correlated with various modified
electrodes in literature for Hg(II) detection and are summarized in
Table S2. The LOD offered at GCE/poly(CoTABImPc) is much lower
than other electrodes.
3
.9. Impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) is helpful to ex-
amine the interfacial charge transfer and charge storage process
of the polymeric electrode. Further EIS demonstrates the advan-
tage of separating interfacial processes at different frequency do-
mains. EIS also provides input about the mechanism of reaction
arises at the electrode and electrolyte system. EIS was measured
The mechanism of Hg(II) detection involves the adsorption of
Hg(II) onto the surface of GCE/poly(CoTABImPc) and the presence
of benzimidazole and amide groups in the Pc may enhance the ad-
sorption capacity for Hg(II).
in PBS pH 7.0 containing 1 mM K [Fe(CN) ] in the 0.001 Hz to
4
6
10,000 Hz frequency range with 5.0 mV amplitude. The Nyquist
3
.11. Scan rate studies
plot at virgin GCE and poly(CoTABImPc) are placed in Fig. 3. The
Randel’s equivalent circuit was applied to theoretically fit the
The scan rate dependence of the Hg(II) redox reaction at
Nyquist plot by using the model R (C(R W)) as shown in the in-
1
2
GCE/poly(CoTABImPc) was monitored by CV. The CV curves for
set of Fig. 3, where R , R , C and W symbolize the resistance of
1
2
6
0 nM Hg(II) at various scan rates (10–80 mVs 1) in PBS at
−
solution, charge transfer resistance, capacitance and the complex
Warburg impedance, respectively. The semicircle diameter in the
Nyquist plot represents the charge transfer resistance. As shown
in Fig. 3, GCE/poly(CoTABImPc) displayed larger semicircle diam-
GCE/Poly(CoTABImPc) are shown in Fig. 5. The ip for Hg(II) detec-
tion increased with the scan rate ν. Inset of Fig. 5 established that
ip of Hg(II) increased linearly with the square root of the ν from 10
to 80 mVs 1 and the linear regression equation for the straight-
line characteristics was i/(μA) =4.5934 ((mVs 1)1/2) + 0.9294x
with R2 as 0.9983. The linear variation in ip with the ν concludes
that the Hg(II) detection at the poly(CoTABImPc) electrode is driven
by the diffusion process.
−
eter than GCE indicating the hindrance of electron transfer from
−
Fe(CN) ]3
−/4
[
to electrode owing to the poor conductivity of Pc ma-
6
terial. Change in the Rct value of poly(CoTABImPc) compared to the
unmodified electrode reveals the modification of poly(CoTABImPc)
on the electrode surface. Table S1 summarizes the equivalent elec-
trical circuit parameters evaluated by fitting the impedance spec-
tra.
The diffusion coefficient,
D for the Hg(II) detection at
poly(CoTABImPc) is calculated by the Eq. (1),
◦
1/2
i = 0.4463 n FAC (nFvD/RT )
(1)
p
3
.10. Detection of Hg(II)
where n is the total number of electrons transferred in the over-
all Hg(II) detection reaction (n = 2), A is the GCE surface area
The GCE/poly(CoTABImPc) electrode was evaluated for the de-
2
−1
tection of highly toxic Hg(II) by CV in 0.1 M PBS (pH = 7). When
0 nM Hg(II) was added into PBS, a peak appeared at 0.18 V on
(0.0701 cm ), F is the Faraday constant (96,485.332 C mol ), C is
−
4
the bulk Hg(II) concentration (60 nM) in mol cm 3, ν is the scan
−
−1
−1
the GCE/poly(CoTABImPc), which is responsible for the detection
of Hg(II), but the bare GCE did not show an appreciable peak
for the Hg(II) addition (Fig.S11). The CV revealed that the electro-
catalytic current for Hg(II) detection at GCE/poly(CoTABImPc) was
much larger with lesser overpotential compared to GCE. The en-
hanced peak current response and decreased in the overpotential
established that the poly(CoTABImPc) film acts as an effective cat-
alyst to activate the detection of analyte.
rate in mVs 1, R is the gas constant (8.314 J K
mol ), T is the
temperature in K.
−4
cm2s
−1
The D value was found to be 2.202 × 10
for Hg(II)
detection at GCE/poly(CoTABImPc) (Table 1) which is comparable
with the reported electrodes for Hg(II) [44, 45]. The kinetics of
mercury nucleation from Hg 2 and Hg
+
2+
solutions on vitreous
2
carbon electrodes showed D value of (1.4 × 10 cm2s ) [44] and
−
5
−1
D value for cathodic and anodic deposition of mercury and silver
The augmented electrochemical signal for Hg(II) detection at
polymeric film electrode, motivated the concentration-dependent
studies at the proposed GCE/poly(CoTABImPc) electrode. Fig. 4a il-
lustrates the CV behavior towards different concentrations of 10 to
at boron-doped diamond electrodes is (~10 5 cm2s ) [45]. This
concludes that GCE/poly(CoTABImPc) has shown a superior D value
for Hg(II) compared to other electrodes in the literature. The D
value provides an understanding of the ability of Hg(II) to diffuse
through a unit area in 1 s in the applied potential gradient of one
−
−1
3
00 nM Hg(II) at GCE/poly(CoTABImPc) electrode. The CVs demon-
5

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
Fig. 4. (a) CV response for Hg(II) in 10 to 300 nM range at GCE/poly(CoTABImPc) in 0.1 M PBS pH 7.0. Inset; zoomed portion of the CV. (b) calibration plot of the peak
current versus Hg(II) concentration.
GCE/poly(CoTABImPc). The Tafel slope value and scan rate studies
for Hg(II) detection at GCE/poly(CoTABImPc) show an electrochem-
ically irreversible reaction for the detection of Hg(II). Obtained in-
trinsic values for the detection of Hg(II) at GCE/poly(CoTABImPc)
are shown in Table 1.
3
.13. Differential pulse voltammetry
The electrochemical response of the GCE/poly(CoTABImPc) elec-
trode towards Hg(II) detection was also investigated by DPV in PBS
(
pH 7.0), Fig. 7.
Different concentrations of Hg(II) were added and peak re-
sponse was monitored under the optimized working conditions.
As in Fig. 7a, the GCE/poly(CoTABImPc) demonstrated a gradual in-
crease in peak current with the enhancement of Hg(II) concentra-
tion. The relationship between Hg(II) concentration and peak cur-
rent showed linear range in 10 to 500 nM with straight-line equa-
tion i/μA = 2.6690 + 0.0983 C (nM) and a good linear correlation
Fig. 5. CVs for 60 nM Hg(II) at GCE/poly(CoTABImPc) in 0.1 M PBS pH 7.0 with
different scan rates. Inset: plot for ip versus square root of scan rate.
2
−1
−2
cm . Further,
of R = 0.9961 with sensitivity as 1.4024 μA nM
the LOD was calculated as 3.8 nM based on the 3σ method. The
LOD of this fabricated sensor is lower than that of the reported
LOD for electrochemical detection of Hg(II) (Table 2) [37–43].
Fig. 7 confirms that the polymeric Pc exhibits an excellent activ-
ity for Hg(II) detection with increased response and sensitivity at
GCE/poly(CoTABImPc) interface due to the presence of polymeric
phthalocyanine which has an effective interaction or complexing
ability with Hg(II).
3
.14. Amperometric detection of Hg(II)
Fig. 6. Tafel plot for the Hg(II) detection at GCE/poly(CoTABImPc).
The electroanalytical performance of the GCE/poly(CoTABImPc)
unit. The D value suggests a facile diffusion of the Hg(II) through
electrode towards Hg(II) detection was assessed using amperom-
etry. The i–t curve for Hg(II) detection was monitored using PBS
pH 7.0 at a fixed potential of 0.18 V and purging the N2 with con-
stant stirring, Fig 8. The gradual increase in Hg(II) concentration
displayed a proportional current increase and the current response
was almost five times higher than the CV and DPV methods in-
dicating the superiority of the amperometric method over CV and
DPV methods.
the electrolyte to electrode interface.
3
.12. Tafel slope
The reaction kinetics and mechanism of Hg(II) detection at
GCE/poly(CoTABImPc) can be studied by using Tafel plot, and the
slope value in Tafel plot is used to compare the catalytic activ-
ities of different electrocatalysts. The Tafel slope value measures
the sensitivity of response to an analyte at an appropriate voltage
and also explores the rate-determining step for understanding the
kinetics of the reaction. The log I vs. E (V) (Tafel map) is a linear
plot with 1/b slope. Tafel plot in Fig. 6 follows the typical Tafel per-
Amperometric
response
for
Hg(II)
detection
on
GCE/poly(CoTABImPc) was quick, and further, the stabilization
of the current response takes less than 2–3 s following the sub-
sequent Hg(II) addition. Quick response and faster stabilization
time for Hg(II) by the sensor are very important parameters
in the fabrication and development of sensor technology. The
developed amperometric sensor showed a linear response in 10
to 400 nM range and the linear behavior satisfies the straight-line
−1
formance. The Tafel slope is obtained as 113 mV dec
for Hg(II)
detection at the polymeric electrode. The exchange current den-
−
5
sity for the rate of the reaction was found to be 7.41 × 10
A
−2
2
cm
which infers that the intrinsic Hg(II) detection kinetics is
equation i/μA = 2.0116 + 0.2621 C (nM) with the R , 0.9988 and
−1
−2
cm . The LOD of Hg(II) was 3 nM
aided by the polymerized electrode for the Hg(II) detection at
sensitivity of 3.7389 μA nM
6

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
Fig. 7. (a) DPV response of the GCE/poly(CoTABImPc) electrode for the detection of Hg(II) over a concentration range of 10 nM to 500 nM in 0.1 M PBS pH 7.0. (b) calibration
plot of the peak current versus Hg(II) concentration.
Table 2
The analytical data of the proposed voltammetric, differential pulse voltammetric and amperometric sensor.
−
1
−2
Modifier
Method
Linear Range (nM)
Sensitivity μA nM cm
LOD (nM)
GCE/poly(CoTABImPc)
CV
10–300
10–500
10–400
1.2178
1.4024
3.7389
4
DPV
CA
3.8
3
Fig. 9. Selective response for Hg(II) (10 nM) in presence of different heavy metal in-
terfering species like 5 μM Pb(II), Cd(II), Zn(II), and K(I) at the GCE/poly(CoTABImPc)
in 0.1 M (pH 7.0) PBS.
Fig. 8. Amperometric i–t curve for Hg(II) of different concentration at
GCE/poly(CoTABImPc) in 0.1 PBS pH 7.0, applied potential 0.18 V. Inset:
M
Plot for current response versus Hg(II) concentration.
The repeatability of the GCE/poly(CoTABImPc) sensor was in-
vestigated using the same electrode for 10 successive measure-
ments. This yielded a relative standard deviation (RSD) of 3.31% for
by the 3σ method. The fabricated GCE/poly(CoTABImPc) sensor
is capable to detect the Hg(II) at lesser concentration levels than
the allowed maximum concentration prescribed by the WHO.
The analytical data obtained by voltammetric, differential pulse
voltammetric and amperometric techniques for the detection of
Hg(II) at GCE/poly(CoTABImPc) are tabulated in Table 2.
6
0 nM Hg(II). Furthermore, the electrode-to-electrode reproducibil-
ity was measured using 5 modified electrodes, which were fabri-
cated utilizing the same fabrication procedure and it displayed an
RSD value of 2.9% for the detection of 60 nM Hg(II).
3
.15. Interference, stability and reproducibility of the sensor
3
.16. Real sample analysis using GCE/poly(CoTABImPc)
The influence of interferents during the Hg(II) detection at
GCE/poly(CoTABImPc) was evaluated using various metal ions. The
selectivity of the designed GCE/poly(CoTABImPc) was assessed by
introducing 5 μM of different metal ions like Pb(II), Cd(II), Zn(II),
KCl and 10 nM of Hg(II) in 0.1 M PB solution of pH(7.0), Fig 9.
The current response curve (Fig. 9) exhibited a prominent response
for Hg(II), whereas even 5000 times larger concentration of other
interferent metal ions like Pb(II), Cd(II), Zn(II), and K(I) did not
show significant current at designed GCE/poly(CoTABImPc) elec-
trode. Compared to various interferents, the response for Hg(II)
on the GCE/poly(CoTABImPc) was much higher representing that
the developed electrode selectively responds for Hg(II) even in
the presence of higher concentration of interferents (Fig. 9).
This shows that the proposed sensor has an excellent anti-
interference capability, superior selectivity and recognition capac-
ity for the Hg(II) detection.
The developed GCE/poly(CoTABImPc) sensor was tested for the
Hg(II) determination in real samples. Water samples from indus-
trial effluents, pond and river were collected, filtered and used for
the Hg(II) analysis at GCE/poly(CoTABImPc) and no response was
noticed in the i–t curve indicating that either Hg(II) is absent in
water samples or it may contain Hg(II) lesser than the applica-
ble range of the GCE/poly(CoTABImPc) sensor. Standard addition
method was utilized to study the matrix effect by monitoring the
recovery values. A known quantity of Hg(II) was added to the wa-
ter sample and then i–t curve was recorded to find the recovery
value. The results are summarized in Table 3 and the recovery of
Hg(II) ranged between 98.3 and 102.6% in water samples. More-
over, the RSD (%) values are in between 1.7 and 2.7% demonstrat-
ing that the GCE/poly(CoTABImPc) could be practically applied for
the Hg(II) determination in real samples.
7

M. Palanna, S. Aralekallu, C. Keshavananda Prabhu et al.
Electrochimica Acta 367 (2021) 137519
Table 3
Practical application data obtained for the analysis of different water samples and fish sample.
−
−1
)
Sample
Hg(II) found
–
Spiked (nM L 1)
Detected (nM L
Recovery (%)
RSD (%) (n = 3)
River water
30
31
103.3
102
(± 2.5)
(± 2.3)
(± 1.9)
(± 2.8)
(± 2.1)
(± 2.0)
(± 2.4)
(± 2.0)
(± 1.6)
(± 2.2)
(± 2.1)
(± 1.7)
(± 2.8)
(± 2.6)
(± 2.3)
1
3
00
00
102
305
101.6
104.3
104
Industrial effluent water
Tap water
–
–
–
–
30
31.3
104
1
3
00
00
309
103
30
30.7
101.5
306
102.3
101.5
102
1
3
00
00
Drinking water
30
30.2
101.6
302.5
30.8
100.9
304.3
100.6
101.6
100.8
102.6
100.9
101.4
1
3
00
00
5
g Fish
30
1
3
00
00
RSD-Relative standard deviation for n = 3 measurements.
Further, to apply the polymerized sensor for practical applica-
tions, it was also tested for the Hg(II) determination in (angler)
fish. The fish sample was purchased from a market in Ballari which
was bought from the coastal part of Karnataka, India. 5 g angler
fish sample was digested with 20.0 mL concentrated nitric acid at
Acknowledgments
Authors acknowledge the financial support from DST-SERB,
Govt. of India (No. SERB/F/9388/2016-17), CSIR grant No.
01(2915)/17/EMR-II, DST-FIST support SR/FST/CSI-274/2016 and
VGST K-FIST support (GRD No. 555). Manjunatha P acknowledges
VSK University, Ballari for the SC/ST fellowship.
120 °C for 2 h and then cooled sample was digested in the mi-
crowave system for complete digestion. After cooling, the solution
was heated at 120 °C to remove the acid. The residue was diluted
by PB saline (pH 7.0) to 10 mL and mixed uniformly [46]. Mercury
content in the angler fish sample was tested by amperometry and
the sample did not yield a significant response inferring that the
fish sample contains either no or less Hg(II) than the detectable
range of the GCE/poly(CoTABImPc) sensor. The recovery study for
angler fish sample was performed and the values are listed in
Table 3. The Hg(II) recovery in the fish sample was from 101 to
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.electacta.2020.137519.
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within an acceptable range and these results reveal that developed
GCE/poly(CoTABImPc) sensor acts as an efficient, convenient and
consistent platform for monitoring Hg(II) content in real samples.
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voltammetric and mutual interference analysis of Cd2 , Pb
+
2+
and Hg with
2+
[
[
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