Nanomolar detection of lead using electrochemical methods based on a novel phthalocyanine

Article abstract of DOI:10.1016/j.ica.2020.119564

4-4-[(Bromo-phenylimino)-methyl]-2-methoxy-phenoxy-phthalocyanine (CoTBrIMPPc) complex was synthesized and characterized by different spectroscopic as well as analytical techniques for the first time. The iminephthalocyanine derivative displayed redox pea

Full text of DOI:10.1016/j.ica.2020.119564

InorganicaChimicaActa506(2020)119564
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Nanomolar detection of lead using electrochemical methods based on a
novel phthalocyanine
Veeresh A. Sajjan, Shambhulinga Aralekallu, Manjunatha Nemakal, Manjunatha Palanna,
C.P. Keshavananda Prabhu, Lokesh Koodlur Sannegowda⁎
Department of Studies in Chemistry/Industrial Chemistry, Vijayanagara Sri Krishnadevaraya University, Ballari, Karnataka 583105, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cobalt phthalocyanine
Lead sensor
Heavy metals
Cyclic voltammetry (CV)
Amperometry
4-4-[(Bromo-phenylimino)-methyl]-2-methoxy-phenoxy-phthalocyanine (CoTBrIMPPc) complex was synthe-
sized and characterized by different spectroscopic as well as analytical techniques for the first time. The imi-
nephthalocyanine derivative displayed redox peaks ascribed to cobalt (Co+2/Co+1) metal and phthalocyanine
ligand (Pc−2/Pc−3). The (CoTBrIMPPc) complex was immobilized by drop coating method on the glassy carbon
surface and the modified electrode was utilised for detecting lead (Pb) at nanomolar concentration level by
voltammetry and amperometric methods. The cyclic voltammetric method showed linearity in the
100–1000 nmol L−1 range with the limit of detection (LOD) 37 nmol L−1. The addition of lead species did not
show the considerable amperometric current response under normal amperometric conditions and hence ex-
periments were performed by the modified methodology. Amperometry technique showed linearity in the 500 to
3000 nmol L−1 range, LOD value of 180 nmol L−1. The current response vs. Lead (Pb) concentration in graph by
amperometric method exhibited correlation coefficient of 0.9991 and sensitivity of 0.0035 µA nM−1. The fab-
ricated iminephthalocyanine based sensor is stable, reproducible and free of interference from other metal ion
impurities in the electrolyte. The designed sensor has been employed for practical application.
1. Introduction
blood results in abdominal pain, constipation, tiredness, headache, ir-
ritation, and memory loss behaviour, tingling in the body of hands or
Lead is a highly poisonous metal among the various heavy metals,
both to humans and the environment [1,2]. A common source for lead
is mining; available in mineral galena (PbS) and trace quantity of lead is
also associated with zinc, silver and copper ores. Besides that, Pb finds
potential application in lead ion batteries, projectiles, solder, plumbing,
gasoline fuels as an antiknock agent, anti-corrosion agent, paint (black
printing ink), coloring agent in ceramic glazes, radiation shielding, and
electrode in the process of electrolysis, etc.
Lead is considered as a source of neurotoxin and has adverse affects
on health. In particular, it affect the normal growth of the nervous
system and brain causing permanent illness in adults with higher risk of
kidney damage and varied blood pressure [3,4]. Lead is known to be a
toxicant that harms multiple body organs including gastrointestinal,
hematological, cardiovascular, neurological and renal organs. Lead
stores in the teeth and bones, where it accumulates and concentrates
with time and the stored lead in bone is slowly released into the blood
system during the period of pregnancy in woman and responsible for
the source of lead contact to the growing baby in womb. The lead in
feet, tiredness, etc [5]. The lead exposure accounts for 1,43,000 deaths
on average every year and it accounts for 0.6% of the global diseases
[6]. The allowed limit of consumption of lead is 10 μg/dL as per WHO
standards. Further, the Environmental Protection Agency (EPA) has
estimated around 20% lead exposure incidents are due to the usage of
lead contaminated drinking water [4].
Hence, it has become a priority to detect and monitor the amount of
lead in environmental samples. A simple, sensitive and selective
method for the detection of lead at nanomolar concentration is essential
at the moment for environmental monitoring of lead. The different
analytical techniques like chromatography, spectrophotometry,
fluorogenic probing, chemiluminescence, electrochemical sensing are
being used for the sensing of lead. Among these, electrochemical
methods show great promise due to their advantages such as easy op-
eration, cheaper cost, quick response and high sensitivity. But, it is very
essential to find suitable catalyst material which is stable, decreases the
overpotential and increases the sensitivity for lead detection [7]. In the
last few years, high-performance electrochemical sensors have been
⁎ Corresponding author.
E-mail address: kslokesh@vskub.ac.in (L.K. Sannegowda).
https://doi.org/10.1016/j.ica.2020.119564
Received 13 December 2019; Received in revised form 29 February 2020; Accepted 1 March 2020
Availableonline02March2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
developed based on cyclic voltammetry and amperometry as they are
known to be simple, easy, reliable, reproducible and needs low cost. In
recent years modified electrodes have been widely used owing to their
favorable signal to noise ratio. Various modifiers including polymers,
carbon nanotubes (CNTs), Graphene Oxide (GO), metal phthalocya-
nines and biological molecules such as DNA and enzymes have been
employed to modify the electrode surface for the detection of heavy
metals [8–11]. Tebello Nyokong et al. reports the simultaneous detec-
tion of mercury (II), Lead (II), copper (II) and cadmium (II) ions with
low symmetric metallophthalocyanine via click chemistry [12]. 3-D
ZnO@graphene nanocomposite was used to detect Pb by mixing with Bi
at the surface of the modified electrode for improved sensitivity and
generation of intense peaks [13]. Further, an electrochemical sensor
fabricated by GO modified with N-doped quantum dots (QDs) has been
used for adsorption of heavy metal ions due to availability of active
sites in the sensor surface [14]. Bi based modified electrodes have been
frequently used for the detection of lead ions [15]. However, in order to
use Bi based modified electrodes, Bi (III) needs to be preconcentrated at
the electrode to form an alloy with the targeted analyte for which an in
situ electro-deposition or an electric discharge method or exfoliation
method must be carried out [15]. Hence, there is a dire need of a more
easy method and material for the direct detection of lead ions.
N4 macrocyclic molecules are known to be very good electro-
chemical catalysts for sensing and these molecules represent in-
expensive alternatives to platinum and precious metals. More im-
portantly Cobalt phthalocyanines (CoPcs) have received significant
attention because of inherent redox behavior of cobalt and substituted
functional groups. It is well known that the properties of the Pcs are
dependent on metal center and functional groups at the peripheral sites.
In turn, CoPcs are highly stable, conjugated with enhanced electron
delocalization. Moreover, CoPcs have been shown to act as effective
electrocatalysts towards a wide range of redox systems. Electrode
modified with this compound has shown great promise for the elec-
trocatalytic determination of many important compounds like catalytic
oxygen reduction reaction and to decrease the effective oxidation or
reduction potentials of various reactions. The redox transformation of
CoII/CoIII and CoIII/CoII is very facile compared to other metals in the
macrocyclic system and as such cobalt metal is very good electro-active
metal [16]. Further, the imine group represents a class of biologically
reactive intermediate and has been the focus of intensive toxicological
research. The bis-imines are biologically important due to the presence
of > C]N moiety. Compound containing an azomethine group (–CH]
N–) are known as schiff bases and these Schiff bases form complexes
with various metal ions which have shown significant biological ac-
tivities [17]. In addition, imine groups enhances the electrocatalytic
activity. Hence, we expected that the synthesis of cobalt phthalocyanine
with imine peripheral group will lead to the biocompatible molecule
which can be incorporated in the biosensor for the lead detection.
colored product. It was recrystallized from ethanol to form 4-[(4-
bromo-phenylimino)-methyl]-2-methoxy-phenol (i).
Mol. Wt.: 306. Melting point: 140 °C. Yield: 70%, Anal. For:
C
14H12BrNO2: Calc. C, 54.92; H, 3.95; N, 4.58; O, 10.45; Br, 26.10.
Found: C, 55.23; H, 4.08; N, 4.22. FTIR (cm−1): 1626 (–C]C str.),
2932, 2866 (CeH str.) and 3383 (–OH str). 1H NMR (400 MHz,
DMSO‑d6): δ 1.780 (s, 1H), 2.73 (s, 1H), 3.37 (d, 1H), 3.824 (s, 1H),
7.237 (m, 4H), 7.79 (d, 2H), 7.57 (d, 2H), 7.60 (t, 1H), 7.954 (d, 2H),
7.39 (s, 1H), 7.341 (s, 1H), 8.086 (s, 1H), 8.66 (s, 1H). Mass
Spectroscopy: M: 306, M+1: 307.
2.2.2. Preparation of 4-{4-[-bromo-phenylimino)-methyl]-2-methoxy-
phenoxy}-phthalonitrile) ligand (ii)
The precursor
i (1.0 g, 0.00326 mol) and 4-nitrophthalonitrile
(0.5657 g, 0.00326 mol) were taken in 10 mL DMF solvent and slowly
added potassium carbonate (2.24 g, 0.01304 mol) with stirring in ni-
trogen atmosphere. Further, the mixture was stirred for 72 h to yield
compound ii [18]. The solid product was washed with water and dried
in vacuum. The ligand ii was recrystallized with ethanol to obtain pure
imine phthalonitrile ligand (ii).
Mol. Wt.: 433. Melting point: 122 °C. Yield: 74%. Anal. For
C
22H14N3O2Br: Calc. C, 61.13; H, 3.26; N, 9.72; O, 7.40; Br, 18.48,
Found: C, 61.58; H, 3.54; N, 9.36.FTIR (cm−1): 1622 (C]C str.), 2928,
2856 (CeH str.), and 2233 (–CN str.). 1H NMR:(400 MHz, DMSO‑d6): δ
1.49 (s, 2H), 3.358 (s, 1H), 7.197 (d, J = 4.00 Hz, 1H), 7.218 (d, 1H),
7.239 (d, 1H), 7.26 (dd, H), 7.34 (d, 2H), 7.535 (d, 2H), 7.54 (d, 2H),
7.924 (d, 2H), 8.174 (dd,1H), 8.675 (s, 2H). Mass Spectroscopy: M+2
435.
:
2.2.3. Preparation of 4-{4-[-bromo-phenylimino)-methyl]-2-methoxy-
phenoxy}-phthalocyanine (CoTBrIMPPc) (iii)
Compound ii (1.0 g, 0.00231 mol) and CoCl2 (0.137 g, 0.00057 mol)
were taken in 10 mL of 1-pentanol and added small amount of DBU as
catalyst. The reaction was progressed by refluxing the mixture for 24 h
at 140 °C to form dark green colored product which is then washed with
ethanol and hexane to yield pure compound iii.
Mol. Wt.: 1414. Yield: 78%. Anal. For C88H56N12O8Br4Co: Calc. C,
59.11; H, 3.16; N, 9.40; O, 7.16; Br, 17.88; Co, 3.30. Found: C, 59.55; H,
3.42; N, 9.21; Co, 3.66. FTIR (cm−1):715, 844, 981, 1106, 1119 (ske-
letal vibrations of phthalocyanine) and 2860–2920 (Ar-CH stretching
and aliphatic –CH stretching). UV–Visible (nm): B- band (325 nm), Q-
band (667 nm).
2.3. Modification of GCE
The GCE was cleaned by polishing to mirror-like surface utilizing
alumina (0.05 µ, Baikolox, Japan) over the polishing pad prior to
modification. Then the electrode was sonicated sequentially in
Millipore water and ethanol 2 times in each solvent to wash-off the
trapped alumina particles on the surface and then dried. Suspension of
phthalocyanine was prepared by dispersing 0.1 mM CoTBrIMPPc in
1 mL of isopropyl alcohol, sonicated and 5 µL of suspension was drop-
coated on a clean bare GCE and dried under vacuum to obtain GCE/
CoTBrIMPPc electrode.
2. Materials and methods
2.1. Materials
The chemicals for the CoTBrIMPPc synthesis as well as electro-
chemical characterization were obtained from Sigma Aldrich or SD Fine
chemicals, India, and all the analytical solutions were diluted and
prepared from distilled water.
3. Characterization
The elemental analyzer Vario EL III CHNS, Germany was employed
for the analysis of different elements in the complex. The cobalt content
was determined by gravimetric procedure [19]. The Perkin-Elmer
spectrophotometer model lambda 35 was used to record the UV–Vis
spectrum in the 280–950 nm range with 0.1 mM of CoTBrIMPPc in
DMSO. FTIR spectral analysis was carried out on Perkin-Elmer FT-IR
spectrometer by employing KBr sampling technique. The thermogram
of the CoTBrIMPPc was recorded over a STA6000 instrument in the
30–800 °C temperature with 10 °C min−1 heating rate in air
2.2. Preparation of 4-{4-[-bromo-phenylimino)-methyl]-2-methoxy-
phenoxy}-phthalocyanine (CoTBrIMPPc)
2.2.1. Preparation
of
4-[(4-bromo-phenylimino)-methyl]-2-methoxy-
phenol (i)
A mixture of 4-bromoaniline (2.0 g, 0.0116 mol) and vaniline
(1.76 g, 0.0116 mol) with 10 mL methanol as solvent and triethylamine
(4–5 drops) as catalyst and refluxed for about 8 h to yield light yellow
2

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
H3CO
O
Br
N
CN
CN
N
O
Br
OH
OH
OCH3
O
OCH3
O2N
N
N
N
CN
CN
H3CO
OCH3
Br
N Co
N
O
OCH3
CoCl2
CHO
N
N
N
N
MeOH Reflux 8 h
DMF, K2CO3
N
N
NH2
O
Br
N
OCH3
Br
Br
Br
i
ii
iii
Scheme 1. Schematic route for the preparation of cobalt phthalocyanine macrocyclic complex (CoTBrIMPPc).
atmosphere (30 mL min−1). XRD profile of the complex was recorded
on D8 Advance X-ray diffraction machine from Bruker. The 1H NMR
spectra (D6-DMSO as solvent) of the ligands were performed using
Bruker AMX-400. The electrochemical analyser (CHI6005E) with cyclic
voltammogram and chronoamperometry techniques in a three-elec-
trode setup with GCE as working, Pt foil counter and Ag/AgCl reference
was employed for electrochemical investigation. The measurements
were carried out in inert atmosphere of nitrogen by purging and stirring
the electrochemical cell with N2 gas for 20 min prior to the experiment.
The voltammetric analysis of the complex was carried out with 0.1 mM
CoTBrIMPPc in DMSO and 0.1 M TBAP. Further, 0.1 mM K4[Fe(CN)6] in
buffer (phosphate buffer) was used for studying the charge transfer
characteristics of the GCE/ CoTBrIMPPc.
in Q-band region and is ascribed to π-π* transition [20]. This absorp-
tion peak in the visible region is responsible for characteristic blue color
of the phthalocyanine complex. The peak observed at 325 nm is due to
the absorption in B-band region and can be ascribed to n-π* and π-π*
transitions. The small shoulder-like peak appeared around 610 nm in-
dicates the vibronic behavior of the complex and the presence of di-
meric and oligomeric forms of phthalocyanine in solution [21].
4.1.2. IR spectroscopy
Fourier transform infrared spectra for the synthesized precursor,
ligand and complex were performed in 4000 to 500 cm−1 region and
presented as shown in Fig. 1. The IR spectra of precursor (i), ligand (ii)
and phthalocyanine complex iii exhibited a band at 1620 cm−1 corre-
sponding to the aromatic C]C stretching and the peak at
2850–2930 cm−1 is attributed to the –CH stretching of aromatic rings.
The compound i exhibited broad peak at 3500 cm−1 for –OH group.
Whereas the phthalonitrile ligand ii showed a new peak at 2233 cm−1
with the disappearance of –OH peak at 3500 cm−1. New peak observed
at 2233 cm−1 may be ascribed to the –CN group and this confirms the
successful preparation of ligand ii from i. The spectrum of phthalo-
cyanine complex iii, displayed characteristic bands for phthalocyanine
macrocycle skeletal vibrations at 715, 844, 980, 1105, 1119 cm−1 [22].
The–CN peak disappearance in CoTBrIMPPc and the appearance of
phthalocyanine skeletal vibration peaks confirm the formation of
phthalocyanine complex from ligand ii [23].
4. Results and discussion
The synthetic scheme for the preparation of imine linkage cobalt
phthalocyanine (CoTBrIMPPc) is presented in Scheme 1. The electro-
philic carbon of aldehyde and ketone are the target for nucleophilic
attack from the amine group. This reaction yields a product in which
the C]O is replaced by C]N as shown in Scheme 1. This kind of
compound formed is well-known by the name imine or Schiff base.
The mechanism for imine complex formation involves two steps.
Firstly, the nitrogen in amine behaves like a nucleophile and attacks the
carbonyl carbon. This is closely related to hemiacetal and hemiketal
formation as shown in Scheme ES. 1a. According to the mechanism
involved in the formation of acetal and ketal, the next step of the re-
action includes the attack from a second amine group to yield a product
with carbon binding to the both amine groups i.e. nitrogen version of
the ketal. The nitrogen atom gets deprotonated, and electrons of the
NeH bond slowly pushes the oxygen atom from the carbon forming
imine, C]N bond (an imine) and water molecule is removed as shown
in Scheme ES. 1b.
4.1.3. NMR spectra
The NMR spectrum of precursor (i) in DMSO‑d6 was recorded and is
presented in Fig. S2a. The position of the peaks obtained for (i) are
The synthesized Pc complex is obtained with good yield and possess
bluish colour. The CoTBrIMPPc complex is amorphous powder-like
material and dissolves only in DMF, DMSO, DMA and concentrated
sulphuric acid without any degradation. The experimental elemental
analytical data of CoTBrIMPPc displayed good agreement with theo-
retical data referring that the compounds synthesized are highly pure.
4.1. Characterization of the ligands and complex
4.1.1. Absorption studies
The synthesized cobalt phthalocyanine complex UV–Vis absorption
spectrum was measured in DMSO solvent in 300–800 nm wavelength
range. The CoTBrIMPPc solution was uniform, clear and the electronic
absorption spectrum of CoTBrIMPPc exhibited characteristic absorption
peaks for phthalocyanine as shown in Fig. S1. The absorption spectrum
of the complex exhibited a strong and sharp absorption peak at 667 nm
Fig. 1. FTIR spectra of i) precursor, ii) phthalonitrile ligand and iii) phthalo-
cyanine complex.
3

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
presented in synthesis sub-section. Peak at 8.66 ppm is due to proton of
imine linkage and this confirms the imine bond formation. Further, the
peak for the proton of phenolic –OH was noticed at 3.824 ppm.
Further, the 1H NMR spectrum for 4-{4-[-bromo-phenylimino)-me-
thyl]-2-methoxy-phenoxy}-phthalonitrile) ligand was recorded using
DMSO‑d6, Fig. S2b. The observed 1HNMR peaks, splitting and their
assignment to different protons of ligand are presented in the synthetic
section. Further, the spectrum did not display the peak for –OH at
3.824 ppm and the –OH peak disappearance confirms the formation of
phthalonitrile ligand from the precursor. The aromatic protons were
observed in the chemical shift range 7 to 8.5 ppm.
with the redox peaks for CoTBrIMPPc in solution. The stability of the
GCE/CoTBrIMPPc electrode was evaluated in the experimental condi-
tion by repeated potential cycling for 50 cycles in 1.0 to −1.0 V and
Fig. 2b shows no-leaching effect of the Pc layer from the GCE.
4.2.2. Charge transfer behavior
The ferri/ferrocyanide redox couple provides the information re-
garding the charge transfer behavior of the CoTBrIMPPc complex which
is on the GCE. Voltammograms for the 0.1 mM [Fe(CN)4]3−/4− in PBS
(phosphate buffer solution) pH 7.0 on the pristine and GCE/
CoTBrIMPPc is as in Fig. S6. The peak currents observed for the [Fe
(CN)4]3−/4 decreased on the GCE/CoTBrIMPPc in comparison to the
bare electrode. The GCE/CoTBrIMPPc film displayed slight hindrance
for the transfer of charge compared to pristine GCE. The GCE/CoT-
BrIMPPc film is slightly resistive in nature for the charge transfer that
occurs between the electrode and electrolyte. The minimizing of peak
current and blocking behavior of the coated film on GCE may be ac-
counted for the thick CoTBrIMPPc film on the electrode which restricts
the facile charge transfer [26].
4.1.4. Mass spectral characterization
The mass spectrum of 4-[(4-bromo-phenylimino)-methyl]-2-
methoxy-phenol precursor is shown in Fig. S3a. The theoretical mole-
cular mass of the precursor compound is 306 and the peak at m/
z = 306, 307 in the mass spectrum correspond to the molecular ion M
and M+1. The mass spectrum confirms the formation of (i) successfully.
The phthalonitrile ligand (ii) mass spectrum is presented in Fig. S3b.
Theoretical molecular mass of the compound is 433 and the peak at m/
z = 435 appeared in the spectrum due to molecular ion, M+2. Hence,
the formation of phthalonitrile ligand (ii) was confirmed by mass
spectrum.
4.2.3. Electrochemical impedance studies
The electrochemical impedance spectroscopy was employed to
analyze the modified electrode to obtain information about charge
transfer characteristics and interfacial resistance. The Nyquist plot ob-
served for bare and GCE/CoTBrIMPPc electrode in phosphate buffer at
amplitude of 5 mV is as shown in Fig. S7. The Nyquist plot has been
fitted theoretically with an appropriate equivalent circuit as R(Q(RW))
and is shown in the inset of Fig. S7. R1 is electrolytic resistance, R2 is
charge transfer resistance (Ret), Q is the constant phase element and W
is Warburg element. The equivalent circuit parameters of the equivalent
circuit are presented in Table S1. The impedance spectra demonstrated
that the Ret of the modified electrode varied slightly compared to bare
electrode. The change in Ret value for the GCE/CoTBrIMPPc electrode
in comparison to bare GCE confirms the coating of phthalocyanine on
GCE [27].
4.1.5. TGA analysis
TGA data was collected to study the thermal behaviour and de-
composition nature of imine linkage cobalt 4-{4-[-bromo-phenylimino)-
methyl]-2-methoxy-phenoxy}-phthalocyanine (CoTBrIMPPc) complex
shown in Fig. S4. The small weight loss in the weight of CoTBrIMPPc,
noticed below 120 °C is due to the removal of moisture and volatile
components in the complex. The thermogram reveals that the complex
formed is stable upto 380 °C and the degradation starts at 385 °C in the
air atmosphere. The gradual weight loss noticed in 380–500 °C tem-
perature range is attributed to thermal degradation of the phthalocya-
nine inner structure in the elevated oxidizing atmosphere. The stable
degradation product formed from the phthalocyanine complex was
equivalent to the theoretical mass of CoO. The TGA data along as well
as elemental composition data confirms the pure nature of the CoT-
BrIMPPc [20].
4.2.4. Electrocatalytic sensing of lead by cyclic voltammetry
The cyclic voltammograms in Fig. 3a represents the electrocatalytic
detection of lead at pristine GCE and GCE/CoTBrIMPPc electrode in
PBS with 200 nM of lead. The catalytic peak appeared at −0.6 V for
lead is due to the electrocatalytic reduction of lead at the modified
electrode. The literature reports indicate that the reduction of lead
occurs at −0.55 V in 1 M HCl at the substituted cobalt phthalocyanine
[12] and −0.66 V in pH = 4.5 buffer solution at the substituted cobalt
phthalocyanine [28]. The observed reduction potential values are in
agreement with the literature values. The peak current for the reduction
of lead at bare GCE was very minimal and reduction peak appeared at
higher negative potential compared to that of GCE/CoTBrIMPPc. The
redox peak noticed at −0.6 V for the GCE/CoTBrIMPPc may be as-
cribed for the electrocatalytic reduction of lead at −0.6 V. The ap-
pearance of the lead reduction peak at less overpotential with increased
peak current on the GCE/CoTBrIMPPc electrode reveals that the GCE/
CoTBrIMPPc electrode could be a potential sensor for the detection and
sensing of lead. Hence, voltammetric curves were carried out for
varying concentrations of lead at GCE/CoTBrIMPPc and the peak cur-
rent (ip) for the Pb reduction increased as the Pb concentration in-
creased as in Fig. 3b. Inset of figure Fig. 3b, depicts ip vs. Pb con-
centration plot at the GCE/CoTBrIMPPc electrode displayed a linear
behaviour in the 100–1000 nM concentration with LOD value of 30 nM
(S/N = 3) in 0.1 M PBS at of 50 mV/s. The reduction ip vs. concentra-
tion of Pb exhibited linear characteristics, Y = 0.0011x + 1.7829 with
R2 = 0.9996 and sensitivity of 0.0011 µA nM−1. Regression value (R2)
reveals a good linear dependency between the dependent variable (ip)
and independent variable (concentration of lead).
4.1.6. Powder X-ray diffraction
Powder X-ray diffraction pattern for the synthesized novel
CoTBrIMPPc complex was recorded in 2θ angle range 10-70°. The imine
linkage phthalocyanine showed highly diffused and noisy pattern in-
dicating that the compound is highly amorphous in nature and the
diffraction pattern is as shown in the Fig. S5.
4.2. Electrochemical studies
4.2.1. Electrochemical behaviour of CoTBrIMPPc
Cyclic voltammograms (CVs) were performed in DMSO solution
with 0.1 mM CoTBrIMPPc and 0.1 M TBAP as supporting electrolyte.
The CV was recorded in the window of −1.1 to 0.9 V in nitrogen at-
mosphere at 50 mV/s. The voltammetric cycle showed that the
CoTBrIMPPc complex exhibits two pairs of redox peaks revealing that
the phthalocyanine molecule is electro-active [24]. The redox peaks
that were observed at ∼0.35 and 0.45 V with reduction and oxidation
peaks difference (ΔE) of 0.1 V corresponds to the metal (Co2+/Co+1
)
and another redox peak appeared at −0.5 and −0.3 V with a separa-
tion of the reduction and oxidation peaks (ΔE) of −0.2 V is due to the
phthalocyanine macrocycle (Pc−2/Pc−1) [9,25] as illustrated in the
Fig. 2a.
The CoTBrIMPPc modified GCE was analyzed by cyclic voltammetry
in dimethyl formamide (DMF) with 0.1 M tetrabutyl ammonium per-
chlorate (TBAP) in nitrogen (N2) atmosphere. The CV of the modified
electrode displayed all the redox peaks which are in well agreement
4

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
Fig. 2. Cyclic voltammograms for a) 0.1 mM CoTBrIMPPc and b) i) bare and ii) GCE/CoTBrIMPPc modified electrode in 10 mL DMSO containing 0.1 M TBAP
supporting electrolyte in N2 atmosphere.
4.2.5. Effect of scan rate
Fig. S8 presents voltammograms for 200 nM lead in 0.1 M PBS at
different scan rates of 10–100 mV/s on GCE/CoTBrIMPPc electrode.
The voltammograms clearly demonstrated an increasing peak current
(ip) when the scan rate (ν was increased and the inset in Fig. S8 presents
½
the ip vs. ν plot. The peak current exhibits a linear dependence on the
ν
1/2. This linear behavior infers that the CoTBrIMPPc modified elec-
trode involves diffusion controlled mass transfer phenomena between
the Pc thin film modified electrode/electrolyte interface.
The diffusion coefficient for the catalytic reaction of lead at the
GCE/CoTBrIMPPc was calculated using the following equation (Eq. (1))
[29].
Ip = 2.69 × 105n3/2C D1/2
(1)
where, peak current response (Ip), number of electrons (n) take part in
the redox process of lead having value n = 2, bulk concentration (C) of
lead (200 nM). Diffusion coefficient (Do) was 3.9 × 10−5 cm2 s−1
.
Fig. 4. Amperometric i-t response for different concentration of lead on GCE/
CoTBrIMPPc modified electrode in 0.1 M phosphate buffer pH 7.0 under stir-
ring condition. Inset shows the plot of stabilised current response vs. con-
centration of lead.
Larger value of Do infers the rapid diffusion of the Pb species at the
electrode-electrolyte interface. The value of Do obtained for the lead
reduction is appreciably higher when compared to the results docu-
mented for other electrodes [30].
monitored separately for the amperometric current response.
Amperometric curve was recorded at an applied voltage of −0.6 V in
phosphate buffer solution with continuous stirring to maintain the
homogeneity in the solution. The Fig. 4, presents the step-wise current
response on the successive incremental addition of 500 nM lead to
0.1 M PBS. On each addition of 500 nM lead to the electrolyte solution,
the amperometric current increased linearly. The step-wise
4.2.6. Electrocatalytic sensing of lead by amperometry
The voltammetric measurements encouraged us to develop sensitive
amperometric sensor for lead using GCE/CoTBrIMPPc electrode.
Normal amperometric procedure could not be employed for the de-
tection of lead as it did not show appreciable amperometric current
response for successive addition of lead. Hence, a modified metho-
dology has been employed where each addition of lead has been
Fig. 3. CV profile in phosphate buffer pH 7.0 for a) i) bare GCE without analyte, ii) bare GCE with analyte (200 nM lead), iii) GCE/CoTBrIMPPc without analyte and
iv) GCE/CoTBrIMPPc with analyte (200 nM lead). b) electrocatalytic reduction of lead with different concentration (100–1000 nM) on CoTBrIMPPc electrode. Inset
b: Plot of peak current at −0.6 V vs. lead concentration.
5

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
Table 1
real sample measurements and the results are reported in the Table 2.
Different concentration of solder sample solution was added to 10 mL of
PBS and lead was detected on GCE/CoTBrIMPPc electrode at −0.6 V by
amperometry. The amount of lead detected by amperometry was
agreeing with that of the theoretical value as given in Table 2. Further,
the standard addition method was employed to understand the inter-
ference from the matrix substances in the real sample. To the 1000 nM
solder solution in electrolyte, a standard lead solution was added. The
amperometric response was used to find the recovery of lead and the
results are documented in Table 2. The % recovery values of the lead
are satisfactory at the GCE/CoTBrIMPPc and indicates that the impurity
or matrix of the solder do not interfere in the detection of lead at GCE/
CoTBrIMPPc. Further, the Pb content in blood and urine samples was
assessed at the GCE/CoTBrIMPPc. The blood sample was collected from
the person working in soldering industry from a nearby hospital and
was stored in a clean vial at 2 °C before use. 1 mL of blood sample was
mixed with 10 mL PBS. The blood sample did not show any lead content
by amperometric method in the detectable range of the GCE/CoT-
BrIMPPc sensor. Therefore, the standard addition method was em-
ployed to detect lead in blood sample and known amount of lead was
spiked into the blood sample. The amperometric method was used to
detect lead and the recovery values were between 100 and 102% for
different amounts of lead added to blood sample. In the same manner,
the urine sample was also analysed for lead content by standard addi-
tion method and the recovery values varied between 100 and 101%,
Table 2. The results illustrated that the GCE/CoTBrIMPPc electrode
presents the feasibility and practical applicability of lead detection in
biological samples which are comparable with the literature data [35].
Comparison of the analytical parameters observed for the detection of lead with
the literature reports.
Electrode material Method Linear
range (μM)
LOD (μM) Sensitivity
Reference
(µA nM−1
)
aGCE/HDME
CV
CV
CV
CV
20–1000
30–3000
10–1000
0.1–10000
0.1–1
3.55
2.7
0.06
0.049
0.03
13.01
—
—
50
0.0035
[31]
[32]
[33]
[34]
bGCE/Pt wire
cGCE/Ch-N-Gr
dGCE/GSH-AgNPs
eGCE/CoTBrImPPc CV
This work
a
HDME-Hanging dropping mercury electrode.
GCE/Pt wire – Platinum wire electrode.
GE/Ch-N-Gr-Glassy electrode N graphene.
GCE/GSH-AgNPs-glutathione stabilized silver nanoparticles.
CoTBrImPPc-4-4-[(Bromo-phenylimino)-methyl]-2-methoxy-phenoxy-
b
c
d
e
phthalocyanine.
enhancement in the amperometric current for the successive and re-
petitive addition of lead demonstrates that the lead undergoes sensitive
and quantitative reduction at GCE/CoTBrIMPPc electrode.
The inset in Fig. 4 exhibits the current response vs. Pb concentration
plot at the GCE/CoTBrIMPPc. The straight line response was obtained
in 500 to 3000 nM concentration range of lead with the LOD 180 nM
and sensitivity of 0.0035 µA nM−1. The analytical parameters for de-
tection of lead at GCE/CoTBrIMPPc was compared with other elec-
trodes in literature and some of the values are better for some important
parameters achieved by GCE/CoTBrIMPPc and the comparison data of
our work with the literature values are shown in Table 1 [31–34].
In order to evaluate the fabricated sensor for selectivity and speci-
ficity, the interference from co-existing molecules was studied with
various metal ions. The interference study involved amperometric
measurement of lead in presence of interfering common ions such as
3 µM nickel sulphate, 3 µM zinc sulphate, 3 µM sodium chloride, 3 µM
potassium chloride, along with 500 nM lead. Fig. S9 shows the am-
perometric response for lead and other interfering ions. The experiment
was performed by recording the amperometric current with the suc-
cessive incremental injection of lead and 3000 nM of different inter-
fering molecules at a regular interval of 200 s with an applied voltage of
−0.6 V at GCE/CoTBrIMPPc under stirring condition. The plot clearly
demonstrates an increasing of the current on addition of lead but, the
addition of co-existing species shows negligible or non-measurable
current response. Despite the presence of excess interfering species, the
GCE/CoTBrIMPPc demonstrated very good selectivity for the detection
of lead. Hence, GCE/CoTBrIMPPc electrode provides highly selective
and sensitive signal towards the lead detection even when the inter-
fering molecules are present in higher concentration.
4.2.8. Reproducibility, repeatability and stability of GCE/CoTBrIMPPc
The GCE/CoTBrIMPPc electrode was prepared in 4 sets to evaluate
the reproducibility. The results from the fabricated four sets of sensor
showed RSD value of less than 4% which indicates the acceptable re-
producibility. The sensor repeatability parameter was investigated by
taking five repeated amperometric measurements for 500 nM lead using
the CoTBrIMPPc electrode. The designed sensor exhibited over-
whelming repeatability with RSD of 2% for the detection of lead. The
amperometric response was collected for freshly prepared and electrode
stored in the desiccator for 10 days to monitor the stability of the
modified electrode. The current response for the stored electrode was in
well agreement and satisfactorily matching with the freshly prepared
electrode response.
Further, to understand the type of interaction between lead and MPc
during the lead detection, absorption experiments were carried out and
shown in Fig. S10. It was noticed that the addition of an aqueous lead
solution to CoTBrIMPPc in DMSO displayed small shift in both B and Q
band towards lower wavelength with splitting of B-band which may be
due to formation of adduct of CoPc with lead or aggregation [16].
4.2.7. Real sample analysis at GCE/CoTBrIMPPc modified electrode
The commercial 1.0 g solder sample [1.0 g of lead solder (Recorn
Classic, Delhi) contains 0.4 g of lead] was dissolved in 10 mL of 1 M HCl
and diluted to obtain the applicable concentration range of the mod-
ified electrode. This commercial solder sample solution was used for the
5. Conclusions
N4 macrocycle containing imine linkage at the periphery was
Table 2
Analytical data obtained for the commercial solder sample.
Sample
Theoretical value (nM)
Detected
Spiking (nM)
Detected RSD§ (%)
Recovery (%)
Solder Sample
500
1000
1500
–
–
–
–
–
–
503
1004
1505
–
–
–
–
–
–
100
200
400
500
1000
1500
500
1000
1500
606
0.4157
0.6403
0.3404
0.5024
0.1054
0.2046
0.1804
0.4404
0.2098
103
101
100.25
102
101.5
101.2
100.8
101.2
100.26
1206
1906
510
1015
1518
504
Blood Sample
Urine Sample
1012
1504
6

V.A. Sajjan, et al.
InorganicaChimicaActa506(2020)119564
synthesised first time with good yield. Various analytical and physico-
chemical characterization like elemental composition, UV–Vis, IR
spectra, XRD, TGA, NMR and mass spectroscopy have been performed
to confirm the purity. The cyclic voltammetry reveals the redox-active
nature of iminephthalocyanine. The synthesized CoTBrIMPPc was
coated on GCE and investigated for the Pb detection by voltammetry
and amperometry. The fabricated electrode showed improved analy-
tical parameters with respect to low LOD, high sensitivity, stability and
reproducibility. The GCE/CoTBrIMPPc electrode was highly selective
for Pb detection and did not respond to the co-existing molecules.
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