Development of tribenzamide functionalized electrochemical sensor for femtomolar level sensing of multiple inorganic water pollutants

Article abstract of DOI:10.1016/j.electacta.2020.136569

A novel tribenzamides derivative N-{4-[2-(1,3-Benzoxazolyl)]phenyl}-3,5-N,N′-bis(4-hexyloxybenzoyl)benzamide (BOC6), containing three amide groups, was successfully synthesized and characterized through multiple spectroscopy techniques. The ultrasensitive electrochemical platform was developed by fabrication of glassy carbon electrode (GCE) with synthesized multifunctional tribenzamide appended BOC6 for the removal of water pollutants. The performance of the engineered sensor was studied by its electrochemical characterization with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and square wave anodic stripping voltammetry (SWASV). The modifier played a facilitating role between the GCE surface and the target metal ions by bringing analytes closer to transducer surface resulting in appearance of intense electrochemical signals. Under pre-defined optimized conditions of SWASV, the BOC6 modified electrode was able to attain distinct oxidative current signals of thallium, arsenic, and mercuric ions simultaneously in one voltammogram scan. The sharp electrochemical signal and peak potential shift of thallium (Tl), arsenic (As) and mercury (Hg) oxidation at ?0.79 V, ?0.06 V and 0.22 V respectively, versus reference electrode Ag/AgCl evidence the electrocatalytic role of BOC6/GCE as compared to bare GCE at ?0.68 V, ?0.01 V and 0.3 V, respectively. Moreover, it was found to sense femtomolar concentration of aforementioned toxic ions, much lower than the world health organization (WHO) and environmental protection agency (EPA) guidelines for drinking water [1]. The limit of detections (LODs) obtained on for Tl(I), As(III), and Hg(II) detection were 2.19 fM, 1.97 fM, and 2.52 fM, respectively. Conclusively, the designed electrode was portable, cost effective, resistant to the interference species, highly stable and gives repeatable and reproducible voltammograms of analytes via strong interactions with target ions even in real water samples.

Full text of DOI:10.1016/j.electacta.2020.136569

Electrochimica Acta 353 (2020) 136569
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Development of tribenzamide functionalized electrochemical sensor
for femtomolar level sensing of multiple inorganic water pollutants
,
, b,
Tayyaba Kokab a, Alia Manzoor a ***, Afzal Shah a **, Humaira Masood Siddiqi a,
Jan Nisar c, Muhammad Naeem Ashiq d, Aamir Hassan Shah e
,
*
a Department of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan
b Department of Chemistry, College of Science, University of Bahrain, Sakhir, 32038, Bahrain
c National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan
d Institute of Chemical Sciences, Bahauddin Zakaryia University, Multan, 6100, Pakistan
e Department of Chemistry and Biochemistry, University of California, Los Angeles, CA, 90095, USA
a r t i c l e i n f o
a b s t r a c t
A novel tribenzamides derivative N-{4-[2-(1,3-Benzoxazolyl)]phenyl}-3,5-N,N0-bis(4-hexyloxybenzoyl)
benzamide (BOC6), containing three amide groups, was successfully synthesized and characterized
through multiple spectroscopy techniques. The ultrasensitive electrochemical platform was developed by
fabrication of glassy carbon electrode (GCE) with synthesized multifunctional tribenzamide appended
BOC6 for the removal of water pollutants. The performance of the engineered sensor was studied by its
electrochemical characterization with cyclic voltammetry (CV), electrochemical impedance spectroscopy
(EIS), and square wave anodic stripping voltammetry (SWASV). The modifier played a facilitating role
between the GCE surface and the target metal ions by bringing analytes closer to transducer surface
resulting in appearance of intense electrochemical signals. Under pre-defined optimized conditions of
SWASV, the BOC6 modified electrode was able to attain distinct oxidative current signals of thallium,
arsenic, and mercuric ions simultaneously in one voltammogram scan. The sharp electrochemical signal
and peak potential shift of thallium (Tl), arsenic (As) and mercury (Hg) oxidation at ꢀ0.79 V, ꢀ0.06 V and
0.22 V respectively, versus reference electrode Ag/AgCl evidence the electrocatalytic role of BOC6/GCE as
compared to bare GCE at ꢀ0.68 V, ꢀ0.01 V and 0.3 V, respectively. Moreover, it was found to sense
femtomolar concentration of aforementioned toxic ions, much lower than the world health organization
(WHO) and environmental protection agency (EPA) guidelines for drinking water [1]. The limit of de-
tections (LODs) obtained on for Tl(I), As(III), and Hg(II) detection were 2.19 fM, 1.97 fM, and 2.52 fM,
respectively. Conclusively, the designed electrode was portable, cost effective, resistant to the interfer-
ence species, highly stable and gives repeatable and reproducible voltammograms of analytes via strong
interactions with target ions even in real water samples.
Article history:
Received 23 April 2020
Received in revised form
1 June 2020
Accepted 2 June 2020
Available online 6 June 2020
Keywords:
Electrochemical sensor
Femtomolar detection
Electrochemical characterizations
Limit of detection
© 2020 Elsevier Ltd. All rights reserved.
1. Introduction
organisms via food chains [1]. Anthropogenic activities such as,
rapid industrialization, disposal of municipal and industrial
Heavy metals (HMs) like thallium, arsenic, mercury and their
amalgams are highly toxic, ubiquitous in nature, non-
biodegradable in environment, and can bio-accumulate in living
wastewater, mining, incineration, smelting, and agriculture are
main sources of these contaminants [1]. These HMs are consider-
ably hazardous to human health and water dwelling species even at
minuscular level as declared by several international organizations
such as, World Health Organization (WHO), Joint Food and Agri-
cultural Organization (FAO), Centre for Disease Control (CDC), and
Environmental Protection Agency (EPA) [2]. For instance, organic
and inorganic arsenide’s (As) toxicity lead to numerous diseases
including, skin itching, keratosis, vomiting, nervous, cardiovascular,
and respiratory system complications, bone marrow depression,
* Corresponding author.
** Corresponding author. Department of Chemistry, Quaid-i-Azam University,
Islamabad, 45320, Pakistan.
*** Corresponding author.
E-mail addresses: afzals_qau@yahoo.com (A. Shah), ah.shah@chem.ucla.edu
(A.H. Shah).
https://doi.org/10.1016/j.electacta.2020.136569
0013-4686/© 2020 Elsevier Ltd. All rights reserved.

2
T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
cutaneous arsenicosis, and muscle cramps [3,4]. Furthermore, they
can cause cancer of vital organs as identified by the International
Agency for Cancer Research, IACR [2e9]. Likewise, the mercury (Hg)
compounds are most frequently used in paintings, cosmetics, teeth
filling, thermometers, barometers, blood pressure measuring in-
struments, and chloralkali industry etc. The mercury exposure can
produce birth defects, respiratory system and gastrointestinal tract
damages, autism, kidney malfunction, vestibular dysfunction,
anxiety, speaking impairment, skin diseases, carcinogenic, muta-
genic and teratogenic activities after crossing the threshold limit
[7,10e14]. Similarly, thallium (Tl) is another heavy metal, which is
even more toxic than mercury due to its high water solubility, bio-
accumulation tendency, and susceptible entry to the body via po-
tassium uptake pathways [14]. Correspondingly, thallium
poisoning includes the alopecia, gastroenteritis, encephalopathy,
polyneuropathy, peripheral and central nervous systems effects,
tachycardia and degenerative changes of vital organs [14e18].
Moreover, amalgams of Tl forms eutectic alloys that have freezing
point of ꢀ60 ꢁC and have extensive applications in thermostatic
devices [19]. However, in spite of high toxicity problems of these
metal ions, their analysis and detection by conventional analytical
methods is not feasible due to their poor responsiveness and low
sensitivity at trace levels [20]. Additionally, these techniques have
high cost and complicated instrumentation, time taking sample
preparation and requirement of highly skilled operators [20].
Therefore, a simple ultrasensitive portable tool for their trace level
sensing is urgently required to adopt effective measures in water
reservoirs.
To develop new methods for better sensing and quantifying of
such water pollutants, the stripping techniques and particularly
square wave and differential pulse anodic stripping voltammetry
(SWASV & DPASV) ensured alternative and extensive explored
sensitive electrochemical methods for HMs ions detection [21].
Herein, the design of experiment (DOE) is based on primary vol-
tammetric analysis (SWASV, CV and EIS) for the HM ions detection
via cost-effective electrochemical computer-controlled worksta-
tion with advantages of simple electrode fabrication, portable
handling, low voltage consumption and fast analysis. Moreover,
anodic stripping voltammetry (ASV) is highly sensitive and selec-
tive due to electrochemical preconcentration/reduction of metal
ions onto the surface of electrode and afterward subsequent anodic
stripping steps [22,23]. Anodic stripping voltammetric (ASV) elec-
trochemical sensor’s performance is primarily required choice of
some electroactive recognition materials on their working elec-
trode that provide significant functionality to sensors along with
boosting up their conductivity, stability, selectivity, surface area,
and sensitivity [16,24]. Usually, electrocatalysts used for fabrication
of sensors are surfactants, nanomaterials, polymers, carbon-based
materials, organic ligands, and biomaterials [23,25,26]. However,
HMs better analysis in water reservoir samples can be done via
organic ligands that are robust complexing agents and are capable
to sequester these metal ions [13,19,27,28]. In recent advances of
organic ligands based electroactive materials, the organic ligands
having the amide functional moiety revealed strong and selective
complexing ability towards metal ions when used to fabricate
sensor [13,22,28,31] than other organic ligands [2,9,17,29,30]. For
instance, they are vital constructing units of proteins, act as donor
atoms in Ca2þ, Naþ, and Kþ ions channels, and selectively use in the
extraction and separation of precious metals and f-block elements
through complexation with metal ions via O and N-donor potential
binding sites of amide groups [25,32,33]. Considering their
worthwhile bioactivity, benzamides and their synthesized oxazole,
thiazole and imidazole derivatives having multi-functional electron
releasing groups, have been registered to interact and chelate with
metal ions to reduce them on sensor surface during accumulation
step of ASV [34,35].
In this manuscript, we synthesized tribenzamide derivatives, N-
{4-[2-(1,3-Benzimidazolyl)]phenyl}-3,5-N,N0-bis(4-
dodecyloxybenzoyl)benzamide
(BIC12)
and
N-{4-[2-(1,3-
Benzoxazolyl)]phenyl}-3,5-N,N0-bis(4-hexyloxybenzoyl)benza-
mide (BOC6), that are designated as electroactive modifiers for the
co-sensing of mercuric, arsenic, and thallium ions due to their
metal ions complexing propensity (Scheme 1). Their coordinating
ability and electrocatalytic role is credited to the electron donating
benzamide tri-ring having amide moieties along with oxazoles,
imidazoles, and ethers groups in its molecule [36e38]. These
organic ligands are insoluble in aqueous solutions and thus stable
on working electrode surface. Consequently, the fabricated sensors
have high stability, viability and accuracy. Moreover, they possess
electrode-anchoring hydrophobic groups and metal ion complex-
ing agents like amides, oxazoles, imidazoles, and ethers that pro-
duce synergic effect for the coordination with metal ions and act as
connecting bridge and facilitator between the sensor (host) and
guest (target analytes) in the analyte’s solution [35]. Likewise, they
have highly accessible adsorption sites and non-agglomeration
characteristics [34]. Furthermore, the BOC6 modified GCE
revealed the better electrochemical sensing behavior in CV and
SWASV for the co-sensing of mercury, arsenic, and thallium ions in
water samples than BIC12, because heterocyclic organic ligands
that have dissimilar donor atoms such as OeN type create more
stable metal ion complexes than organic ligands having similar
donor atoms like OeO, NeN types [35,39,40]. We have obtained
six-fold high SWASV response for target analytes on BOC6/GCE
than bare GCE on our DOE, which leads to high sensitivity of
fabricated sensor. Because during pre-concentration step of
SWASV, the amide group’s presence in the tribenzamide molecules
interact with metal ions and increase their absorptivity on elec-
trode surface. The metal ions get reduced on BOC6/GCE surface by
establish of coordinate covalent bonds with amide groups of tri-
ring benzamide molecules and produce transition metal com-
plexes. Thus, the increase of metal ions adsorptive sites and accu-
mulated amount of reduced metals, by BOC6 electron donor groups
Scheme 1. (A) BOC6 and (B) BIC12 molecular structures.

T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
3
leads to significant enhancement of sensitivity. Moreover, it was
found to sense femtomolar concentration of Tl(I) 2.19 fM, As(III)
1.97 fM, and Hg(II) 2.52 fM ions which is much lower than the WHO
and EPA guidelines for drinking water [1]. The designed electrode is
resistant to the interference species and gives repeatable and
reproducible voltammograms of analytes via strong interactions
with target ions even in real water samples. The results infer nov-
elty of this work based on superior figure of merits of
tribenzamide-based sensor and its excellent stability. Surprisingly,
no single report of such a cost-effective, simple, selective BOC6
based electrochemical device for the on-site simultaneous detec-
tion of these toxic ions at femtomolar level in aqueous medium is
available in literature.
wire were used as reference (RE) and counter electrode (CE),
respectively. WE and RE were set at nearby distance in the elec-
trochemical cell to avoid ohmic/IR drop. All electrochemical studies
were done at the room temperature (25
mosphere in order to evacuate the atmospheric oxygen by
continuous purging of N2 gas.
2
ꢁC) under inert at-
2.3. Synthesis and structural confirmations of tribenzamide
derivatives
Tribenzamide derivative BOC6 was synthesized by reacting
freshly prepared diamines N-(1,3-benzoxazol-2-ylphenyl)-3,5-
diaminobenzamide (BODA) and p-hexyloxybenzoyl chloride (p-
HOBzCl) according to the reported procedure [41]. The general
steps involved in the synthesis of BOC6 are presented in Scheme 2.
Firstly, p-hexyloxybenzoic acid (40 mmol) and thionyl chloride
(8 mL) were mixed and set at 6 h continuous reflux in an oil bath to
get the p-hexyloxybenzoyl chloride. Excess thionyl chloride was
recovered by vacuum distillation after completion of the reaction.
Secondly, 20 mmol solution of BODA in dimethylacetamide (DMAc)
was added dropwise in reaction flask at 0 ꢁC via dropping funnel.
This solution was mixed for 8 h to get the final product of BOC6. In
order to observe the reaction proceedings, TLC was performed at
25 ꢁC by using n-hexane and ethyl acetate in 1:4. After completion
of the reaction, the whole mixture was decanted into doubly
distilled water to filter the product. The product was purified with
excess water and then repeatedly recrystallized in alcohol, ethyl
acetate, and THF, respectively. Moreover, second tribenzamide de-
rivative BIC12 was formed by the same process using freshly pre-
2. Experimental
2.1. Chemical reagents and real water samples
Synthetic details of BOC6 and BIC12 electrocatalysts are given in
the succeeding section. All chemicals of analytical grade were
procured from sigma Aldrich and Merck Germany and used
without any purification steps. While, arsenic chloride, thallium
sulphate and mercuric chloride were used to prepare analyte so-
lutions. Strontium nitrate, zinc acetate, cesium chloride, cobalt
chloride, calcium chloride, lead nitrate, alanine, lysine, threonine,
glutamic acid, surfactants (CTAB, SDS), EDTA, glucose, citric acid, 2-
amino-4-nitrophenol, and 3-chloro-5-nitrophenol were used to
prepare interfering agents solutions. Acetic acid, sodium acetate,
HCl, NaOH, sodium phosphate monobasic monohydrate, KCl,
H2SO4, H3BO3, H3PO4, dimethyl sulphoxide (DMSO), and sodium
phosphate dibasic heptahydrate were used to prepare supporting
electrolytes solutions. The selection of the solvents was based on
the solubility of analyte and tribenzamide compounds, doubly
distilled water (ddw) was used for the preparation of metal ions,
buffers and interferents solutions while the DMSO of the analytical
grade was used for BOC6 and BIC12 solutions based on the insol-
ubility of tribenzamide compounds in aqueous media. Glass
apparatus was used after washing it with 6 M HNO3 and then
thoroughly rinsing it with ddw to prevent any metal contamina-
tions. Real-life samples like tap water, drinking water, spring water,
river water, soil wastewater, and industrial wastewater were ob-
tained from different areas in Islamabad, Pakistan, in order to check
the validity and applicability of our established sensor. These water
samples were filtered through the Whatman filter paper No. 40 to
remove soil particles or any other impurity before further use.
pared
diamine
N-(1,3-benzimidazol-2-ylphenyl)-3,5-
diaminobenzamide (BIDA) and p-dodecyloxybenzoyl chloride (p-
DOBzCl) reaction as shown in Scheme 2. Synthesized BOC6 and
BIC12 were obtained in powder form with 82% and 90% yields,
respectively.
BOC6: Rf ¼ 0.40, M.P. 183e185 ꢁC; FTIR (ymax, cmꢀ1): 3304 (NeH
amide stretch), 3008 (Cs2p-H stretch), 2920, 2850 (Cs3p-H stretch),
1671 (C]O stretch), 1607 (C]C aromatic bend), 1245 (CeO
stretch). 1H NMR (DMSO-d6
d ppm, 300 MHz): 0.85 (6H, t,
J ¼ 6.9 Hz, 15, 150), 1.28 (6H, m, 12, 120-14, 140), 1.74 (4H, quin,
J ¼ 6.6 Hz, 11, 110), 4.02 (4H, t, J ¼ 6.6 Hz, 10, 100), 6.96 (4H, d,
J ¼ 7.8 Hz, 9, 90), 7.06 (2H, d, J ¼ 8.6 Hz, 2, 20), 7.40 (2H, d, J ¼ 8.2 Hz,
3, 30), 7.75 (2H, d, J ¼ 8.5 Hz,1,10), 7.85 (4H, m, 4, 4’; 8, 80), 8.01(2H, s,
6, 60), 8.24 (1H, s, 7), 10.36 (3H, s, 5, 50, 5’’); (Figure. S2 and S3) 13C
NMR (DMSO-d6 d ppm, 300 MHz): 14.34 (C, 26), 22.50 (C, 25), 25.59
(C, 24), 28.93 (C, 23), 31.40 (C, 22), 68.22 (C, 21), 110.02 (C, 1), 111.23
(C, 14, 140), 114.53 (C, 19, 190, 19’’, 19000), 114.65 (C, 16), 120.03 (C, 4),
120.54 (C, 10, 100), 123.17 (C, 2), 124.03 (C, 3), 125.78 (C, 8, 17, 170),
128.40 (C, 9, 90), 130.16 (C, 13, 18, 180, 18’’), 131.81 (C, 11, 15, 150),
142.05 (C, 5),150.60 (C, 6),161.98 (C, 20, 200),162.76 (C, 7),167.53 (C,
12, 120, 12’’) (Figure. S4).
2.2. Instrumentation
Nuclear magnetic resonance (NMR) spectra were obtained from
Bruker 300 MHz digital NMR instruments, Germany. Fourier
transform infrared spectroscopy (FT-IR) spectra were obtained on
Thermo Scientific Nicolet 6700 FTIR, U.S. Melting point (M.P) of the
synthetic tribenzamide samples were obtained on m.p apparatus of
Mel-Temp, Mitamura Riken Kogyo, Inc. Tokyo Japan. The EDX
analysis along with SEM were carried out on ZEISS EVO 40 (Merlin,
Carl Zeiss) for morphological characterization of sensor. Design of
experiment (DOE) that is, voltammetric measurements (ASV, CV
and EIS) were carried out on electrochemical computer-controlled
workstation Metrohm Auto lab (PGSTAT 302N), Switzerland with
general three electrode scheme as shown in Figure S1. Moreover, an
INOLAB pH meter, Xylem Analytics Germany was used to set pH
values of the buffers and working solutions. Glassy carbon elec-
trode (GCE) of 0.071 cm2 active area was employed as working
electrode (WE), whereas Ag/AgCl (concentrated KCl) and Platinum
BIC12: Rf ¼ 0.70, M.P. 199e201 ꢁC; FTIR (ymax, cmꢀ1): 3343
(NeH amide stretch), 3050 (C2sp-H stretch), 2920, 2850 (Cs3p-H
stretch), 1680 (C]O stretch), 1608 (C]C aromatic bend), 1250
(CeO stretch). 1HNMR (DMSO-d6
d ppm, 300 MHz): 0.82 (6H, t,
J ¼ 6.9 Hz, 22, 220), 1.23 (18H, m, 13, 130-21, 210), 1.75 (4H, quin,
J ¼ 6.6 Hz, 12, 120), 4.01 (4H, t, J ¼ 6.6 Hz, 11, 110), 6.97 (4H, d,
J ¼ 8.8 Hz, 10, 100), 7.39 (2H, d, J ¼ 8.7 Hz, 2, 20), 7.42 (2H, d,
J ¼ 8.4 Hz, 4, 40), 7.76 (2H, d, J ¼ 8.6 Hz, 1, 10), 7.88 (4H, m, 5, 50, 9, 90),
8.04 (2H, s, 7, 70), 8.22 (1H, s, 8), 10.80 (3H, s, 6, 60, 6’’), 12.80 (1H, s,
3); (Figure. S5 and S6) 13CNMR (DMSO-d6 d ppm, 300 MHz): 14.41
(C, 29), 22.57, 25.89, 28.98, 29.19, 29.49, 31.77 (C, 19, 190), 68.19, (C,
18, 180), 111.23 (C, 11, 110), 114.62 (C, 16, 160, 16’’, 16000), 115.13 (C, 1, 10),
120.08 (C, 13), 121.09 (7, 70), 123.21 (C,2, 20), 125.28 (C, 5, 14, 140),
128.59 (C, 6, 60), 130.32 (C, 10, 15, 150, 15’’), 131.78 (C, 8, 12, 120),

4
T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
Scheme 2. Representation of synthetic route for the tribenzamide derivative BOC6 and BIC12.
142.15 (C, 3, 30), 150.63 (C, 4), 162.74 (C, 17, 170), 167.45 (C, 9, 90, 9’’)
section (Figure S8).
(Figure. S7).
2.5. SWASV of fabricated electrodes
2.4. Fabrication and electrochemical measurements of working
electrode
The bare, BOC6, and BIC12 sensors used as working electrodes
and was dipped in electrochemical cell containing 1
m ,
M of Tl1þ
0.75
m
M A3þ and 1.2 M Hg2þ ions in 50% BRB solution at pH value
m
For cleaning of GCE, firstly it was physically clean on a nylon
of 5 as a stripping electrolyte. The metal ions were electro-reduced
onto sensor surface at negative 1.2 V deposition potential for
accumulation time of 135 s under continuous magnetic stirring.
After that, the magnetic stir was stopped at equilibrium time of 5 s
and the SWASV scan run from ꢀ1.3 V to þ0.6 V at 20 mV pulse
amplitude, 20 Hz frequency, 100 mVsꢀ1 sweep rate, and 0.5 mV
potential step. Hence, oxidative stripping of the metals from the
sensor surface was occur in the quiescent solution and resultant
well-resolved current peaks of metal ions was observe on vol-
tammogram. For comparative purpose, SWASV electroplating and
stripping analysis steps were also done on bare GCE under same
experimental conditions. Moreover, for each reading a new recog-
nition layer was immobilized on electrode to confirm the repro-
ducibility of results.
polishing pad with m-alumina slurry to get shiny surface by up to
14% constant O/C ratio [42]. Then, it was ultrasonicated for 5 min in
ddw, alcohol and aq. HNO3 (1:1, v/v). In the second step, electro-
chemical cleaning of GCE was performed by dipping it in PBS and
taking several scans of cyclic voltammogram from ꢀ1.4 V to þ0.9 V
until reproducible cyclic voltammograms are obtained (Figure S8A).
Fabrication of WE was then performed at perfectly cleaned and
activated GCE having 0.07 cm2 active surface area, by casting 5
drop of the 10 M synthetic tribenzamide solution in DMSO fol-
mL
m
lowed by drying in vacuum oven to ensure proper adsorption of
modifier and to avoid non-uniformity in recognition layers.
Furthermore, loosely bound molecules were stripped off by careful
rinsing of the modified GCE with ddw.
At the GCE surface typically number of chemical functionalities
i.e. aldehydes, ketones, carboxylic acids, quinones, lactones, and
alcohols are found in configuration of fused aromatic rings which
cause electrons delocalization throughout the carbon network that
contribute to its O/C ratio [43]. The activation of GCE surface by
physiochemical process improve its surface O/C ratio which mean
the number of chemical functionalities increase on activated GCE
surface. The casted tribenzamides molecules interact with them
and get attached on the activated GCE surface as uniform recog-
nition layer. During the electrode fabrication process, strong van
der Waals forces result in tight binding of their hydrophobic
electrode-anchoring tails to the GCE surface. Consequently, their
metal ion complexing moieties become freely available for the
electron transfer and Lewis acid-base interactions with target cat-
ions. The superficial description and elemental analysis of fabri-
cated sensor on SEM and EDX provided in supporting information
3. Results and discussion
3.1. Tribenzamide modified GCE electrochemical characterizations
Cyclic voltammetry (CV) was carried out to characterize the
BOC6 and BIC12 tribenzamide modified GCE designed sensor by
probing current response of [Fe (CN)] 3-/4- redox couple compare to
the bare GCE, by immersing the electrodes in 5 mM solution of the
K3[Fe(CN)6] redox probe and a 0.1 M KCl stripping electrolyte.
Fig. 1A shows obvious increase and displacement in electro-
3-/4-
chemical reversible signal of [Fe (CN)]
redox couple at BOC6
and BIC12 immobilized GCE than bare GCE. The tribenzamides
molecules offer more availability, electrocatalysis and faster elec-
tron transduction of ions at the surface of electrode due to their

T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
5
Fig. 1. Comparative plots obtained from bare GCE and tribenzamide-modified GCEs in a medium containing 5 mM K3[Fe(CN)6] solution and a 0.1 M KCl electrolyte (A) Cyclic
voltammograms at scan rate of 100 mV/s from potential ꢀ1.0 Ve1.0 V (B) Nyquist plots using electrochemical impedance spectroscopic data with applied DC voltage of 0 V vs. EOC
and amplitude of 10 mV over applied frequency ranges varying from 100 kHz to 0.1 Hz. (Inset) Randles equivalent circuit model for the system under study showing resistors,
capacitor, and Warburg impedance elements.
excessive active sites for the redox probe accumulation on designed
sensor. However, to calculate the large surface area of working
electrode after modification with BOC6, the CV data was used in
modified GCE (Rct ¼ 2.2 k
U
), and the bare GCE (Rct ¼ 4.35 k
U) which
confirmed that the recognition layer of the BOC6 indeed improves
the sensing properties of the electrode by increasing the accessible
active sites and the providing faster charge transductions.
Randles-Sevcik equation of IP ¼ 2:69 ꢂ 105 n32AD12n12C. Where
Ip represents the oxidative peak current in Amperes, n the number
of electrons appearing in the half-reaction of probe, A the electro-
active surface area in cm2, D the analyte diffusion coefficient in cm2
The electrocatalytic role and sensing ability of the electroactive
modifier corresponds to a kinetic parameter, exchange current
density (Jo), which is inversely related to the Rct value of EIS data via
RT
ꢀ1, v the scan rate at which the potential is swept in Vsꢀ1 and C the
equation Jo
¼
. Higher Jo value associates to the remarkable
nFRct
s
concentration of the probe system in mol cmꢀ3 [44]. As cyclic
voltammograms of bare GCE and tribenzamide-modified GCEs
obtained in a medium containing 5 mM K3[Fe(CN)6] solution and a
0.1 M KCl electrolyte at scan rate of 100 mV/s that’s why
D ¼ 7.6 ꢂ 10ꢀ6 cm2 sꢀ1, C ¼ 5 mM and n ¼ 1 were put in equation.
The calculated areas of bare GCE, BIC12/GCE and BOC6/GCE were
0.02 cm2, 0.05 cm2 and 0.095 cm2, respectively. Thus, after modi-
fication BOC6 tribenzamides the electroactive surface area of the
modified GCE enhanced five times compared to bare electrode.
Similarly, the variation of designed sensor interfacial properties
like, charge transfer features and extent of modification were
analyzed via electrochemical impedance spectroscopy (EIS) in
same solution [45]. In the EIS Nyquist plots, a semicircle diameter in
the higher frequency region and a straight line in the lower fre-
quency regime were obtained on bare GCE, BOC6 and B1C12 tri-
benzamide derivatives modified GCE. The semicircle arc region
corresponds to charge transfer resistance (Rct) at electrode/analyte
interface and the linear line corresponds to the Warburg diffusion
region as shown in Fig. 1B. The EIS data was fitted by the Randles
equivalent circuit (inset of Fig. 1B) and attained parameters values
were sum up in Table 1. The diameter of the semicircle that cor-
responds to the charge transfer resistance, is significantly smaller in
case of BOC6/GCE and B1C6/GCE as compared to the bare GCE that
corresponds to the facilitated charge transfer kinetics with effect of
the tribenzamides fabrication. The Rct value calculated for the BOC6
electrocatalytic function of the recognition layer and its feasibility
towards the interfacial surface electrochemical reactions. Thus, the
higher exchange current density of the tribenzamide BOC6 modi-
fied GCE (127.5
m
A/cm2) as listed in Table 1, is attributed to its
enhanced heterogeneous electrocatalysis and feasible redox re-
actions at BOC6/GCE surface [46].
3.2. Voltammetric analysis of modifier-metal ions complexation
effect
The amide group is a bidentate ligand that forms a stable
chelating tri-ring in the BOC6 and BIC12 tribenzamides and has
strong affinity for the metal ions chelation. Hence, the BOC6 and
BIC12 tribenzamides complexation effect on the modified elec-
trodes performance for co-sensing of target triple ions was deter-
mined by CV in solutions of Tl1þ, As3þ and Hg2þ ions with a
BrittoneRobinson buffer (BRB) of pH 5 supporting electrolyte at
sweep rate of 100 mV/s. The current intensity variation of Tl1þ/Tlꢁ,
As3þ/Asꢁ and Hg2þ/Hgꢁ redox couples show noticeable current in-
crease and potential shift in CV of the tribenzamides immobilized
GCE compared to bare GCE as indicated in Fig. 2A. The enhance-
ment further confirms the EIS results and attributed to the tri-
benzamide molecules that provide effective active sites and an
increased surface area on the electrode surface for accumulation of
the reduced metal ions [32]. Furthermore, the CV profile revealed
reversibility of the design sensor and it can recognize the oxidized
and reduced forms of target triple ions clearly. These results
confirm the reversible interactions between host and guest i.e.
complexation and decomplexation process of metal ions at sensor
surface during run of an electrochemical scan. Moreover, the
further enhancement of the current signals in case of the BOC6/GCE
as compared to BIC12/GCE specify that the BOC6 molecules facili-
tate the redox probe approachability to the electrode surface better
than the BIC12 molecules. This facilitation can be related to the
difference of electroactive groups present in the BOC6 and BIC12
modified GCE (Rct ¼ 0.21 k
U) is quite low as compared to the BIC12
Table 1
Randles circuit parameters calculated for bare and tribenzamide modified GCEs
from electrochemical impedance spectroscopy (EIS).
Electrodes
R
ct (k
U
)
Re (U
)
CPE (
m
F)
Jo
(m
A/cm2)
GCE
BIC12/GCE
BOC6/GCE
4.35 0.04
2.2 0.01
0.21 0.05
198.2 1.58
142.2 1.2
139 1.3
3.63 0.12
2.38 0.10
1.52 0.23
5.9
11.7
127.5

6
T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
Fig. 2. (A) Cyclic voltammograms of Tl1þ, As3þ and Hg2þ ions at scan rate of 100 mV/s from working potential ꢀ1.3 Ve0.5 V (B) SWASVs obtained from a) BOC6 modified GC
electrode, b) BIC12 modified GC electrode, and c) bare GC electrode for the detection of 1
m
M of Tl1þ, 0.75
m
M A3þ and 1.2
m
M Hg2þ ions in BRB of pH ¼ 5 as striping solvent, keeping
a scan rate of 100 mV/s, a deposition potential of ꢀ1.2 V and a deposition time of 135 s d) The BOC6 designed sensor in aqueous solvent containing BRB (pH 5.0) under same
experimental conditions.
molecular structures and their molecular geometries. The BIC12
possesses bulky molecular structure as shown in Scheme 1, that
offers more structural hindrance for the analyte’s interaction on the
electrode surface as compared to the BOC6. Likewise, the tri-
benzamide BOC6 has more hetero-atomic electron donating groups
with more electronegative units and interact better with the metal
cations. Consequently, BOC6 is better electrochemical device for the
co-sensing of Tl1þ, As3þ and Hg2þ ions as compared to BIC12
[36e38].
3.3. Evaluation of analytical features
Under the pre-optimized SWASV conditions (supporting infor-
mation), the linear concentration ranges (LCRs), limit of detection
(LOD) and limit of quantification (LOQ) of the BOC6/GCE sensor
were investigated for the co-sensing of Tl1þ, As3þ and Hg2þ ions. A
robustness in analytical response of the sensor was observed in the
voltammograms obtain at investigated concentration ranges of
0.05 pMe0.15 mM, 0.025 pMe0.125 mM, and 0.075 pMe0.175 mM of
The SWASVs were recorded at the tribenzamide modified
the Tl(I), As(III), and Hg(II) respectively, as shown in Fig. 3A. The
linear calibration curves were obtained by plotting oxidative peak
current values of the Tl1þ, As3þ and Hg2þ ions from the SWASVs
versus their concentrations. The LCRs for Tl1þ, As3þ and Hg2þ ions
were determined in the range of 0.05 pMe37.5 nM, 0.025
pMe25 nM and 0.075 pMe50 nM with correlation coefficients R2 of
0.997, 0.995, and 0.997 respectively, as shown in Fig. 3B. The LODs
values for Tl1þ, As3þ and Hg2þ ions were assessed by their IUPAC
regulations and found 2.19 fM, 1.97 fM, and 2.52 fM respectively.
Likewise, the LOQs for Tl1þ, As3þ and Hg2þ were obtained as 7.3 fM,
6.56 fM, and 8.4 fM respectively. The lower LODs are related to the
better sensitivity and higher current response of the BOC6/GCE.
Comparatively LOD of 1.97 fM for arsenic ions at the BOC6/GCE
under pre-defined SWASV conditions was the lowest than thallium
(2.19 fM) and mercury (2.52 fM) ions. This enhance response of the
sensor for arsenic ions can be related to its smaller radius with high
charge density and thus greatest tendency for interaction and
complex formation with the modifier. Henceforth, it is concluded
that the BOC6 is more selective and sensitive for complexation with
arsenic than other target analytes, but at the same time, it possesses
better sensing ability and offers effective interactions for all metal
cations under investigation.
electrodes (BOC6/GCE & BIC12/GCE) for 1
m m
M of Tl1þ, 0.75 M A3þ
and 1.2
m
M Hg2þ ions to further assess their electrochemical per-
formance in terms of sensitivity and selectivity toward the target
analytes as shown in Fig. 2B. The SWASV oxidative signatures of
Tl1þ, As3þ and Hg2þ metal ions on the BOC6 & BIC12 modified
electrodes significantly shifted to the more negative potential than
the bare GCE, which indicate the electrocatalytic role of these tri-
benzamide derivatives via their electroactive sites. Moreover, in
SWASV a noticeable approximately six folds and two folds’ ampli-
fication of the oxidative signal intensity of Tl1þ, As3þ and Hg2þ ions,
was seen at the BOC6 and BIC12 immobilized GCEs, respectively
than the bare GCE. At fabricated GCE surface, the modifier molecule
configure itself in such a way that its chelating moieties are freely
available to interact with the aqueous target analytes [43]. There-
fore, the enhancement of Tl1þ, As3þ and Hg2þ ions peak current
intensity can be related to the contribution of the amide groups
present in the tri-ring of the modifiers and metal ions complexation
(that increase their electroplating efficacy in the accumulation step
of SWASV). Likewise, the ether, imidazole, and oxazole electron-
rich functionalities of the recognition layers also offer adsorption
sites for the metal cations to form M/MXþ couple and, enhanced
their current responses by serving as a venturing stone between the
host and guest for faster electron transfers [47]. These SWASV re-
sults are in good agreement with the electrochemical character-
izations mentioned in the outcomes of designed sensors and
3.4. Reproducibility and repeatability of the BOC6 sensor
The stability, durability, and repeatability of the BOC6/GCE was
probed by recording eight successive SWASVs at different intervals
confirm
their
electrocatalytic
sensing
mechanism via
of times for 0.15 m m m
M of Tl1þ, 0.125 M A3þ and 0.175 M Hg2þ ions at
complexation-decomplexation process. Based on these results, the
BOC6/GCE was selected as an electrochemical sensor of Tl1þ, As3þ
and Hg2þ ions detection in all onward electrochemical in-
vestigations. The optimization of the parameters like modifier
concentration, stripping electrolyte and its pH values, deposition
times, and accumulation potentials on the sensing sensitivity,
shape, and intensity of the metal ions current signals at BOC6/ GCE
are provided in Supporting information section.
a single BOC6/GC electrode under the constant pre-defined opti-
mum experimental conditions. After intra and inter day repetitive
measurements as illustrated in Fig. 4A, the obtained voltammetric
signals demonstrate the analogous responses with RSD of 2.1%,
1.84%, and 1.36% for Tl1þ, As3þ and Hg2þ ions, respectively. Thus, the
developed sensor shows outstanding stability over a long period of
storage time due to its insolubility in aqueous solutions.

T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
7
Fig. 3. (A) SWASV recorded at BOC6/GCE by varying concentrations of Tl1þ, As3þ and Hg2þ ions in BRB (pH ¼ 5), a scan rate of 100 mV/s, a deposition potential ꢀ1.2 V, and a
deposition time of 135 s. The investigated concentration ranges are mentioned above each peak. (B) The corresponding calibration plots with error bars, linearity ranges and linear
equations, obtained by SWASV data, showing linearity of Tl1þ, As3þ and Hg2þ ions concentration with Ip under optimized conditions used for sensitivity, applicability and validity of
BOC6/GCE for multiple ions detection.
Fig. 4. Validation of the applied methodology testified by monitoring the SWASV peak current responses for 0.15 m m m
M of Tl1þ, 0.125 M A3þ, and 0.175 M Hg2þ ions solutions under
optimized conditions; (A) showing repeatability of the designed BOC6/GCE electrode at intra and inter days scans obtained at electrode dipped time mentioned in figure (n ¼ 8), (B)
showing reproducibility of multiple fabricated BOC6/GCE electrodes (n ¼ 6).
Likewise, in an attempt to assess the reproducibility of the
devised sensor, six independently BOC6 fabricated electrodes were
used for the successive scans of SWASV for the electrodeposited Tl,
As, and Hg metals as shown in Fig. 4B. No obvious inconsistencies in
derivative is summarized in Table 2, suggesting the promising
properties of our tribenzamide based electroanalytical sensor.
Furthermore, these calculated FOMs such as LOD values are
much lower than WHO and USEPA recommended threshold limits,
and also from those sensors already reported in the literature for
detection of Tl1þ, As3þ and Hg2þ ions [1e7,10e14,16e18,30,35].
Briefly, in comparative Table 3, the LODs of previously reported
graphene oxides, carbon nanotubes, precious metals nanoparticles
and nanocomposites, silica, amines, and polymers fabricated sen-
sors for Tl1þ, As3þ and Hg2þ ions detection are significantly inferior
the oxidation peak signatures of 0.15
m m
M of Tl1þ, 0.125 M A3þ and
0.175
m
M Hg2þ ions were found with RSD of 1.50%, 1.53%, and 1.71%.
These outcomes elucidate noteworthy reproducibility and repeat-
ability of the developed electrochemical sensor and hence, render it
an ultrasensitive reliable platform for HM ions detection. The figure
of merits (FOM) of the designed sensor of the BOC6 tribenzamide
Table 2
Figures of merits of the BOC6 modified GCE for Tl1þ, As3þ and Hg2þ ions.
Figures of merits
Units
Tl1þ
As3þ
Hg2þ
Investigated range
Linearity range
LOD
m
M to pM
0.15e0.05
37.5e0.05
2.19
0.125e0.025
25e0.025
1.97
0.175e0.075
50e0.075
2.52
nM to pM
fM
LOQ
fM
7.3
6.56
8.4
% RSD (Reproducibility)
% RSD (Repeatability)
% RSD (Validity)
% RSD (Anti-interference ability)
Recovery (%)
n¼6
n¼8
n¼6
n¼15
n¼12
1.50
2.1
1.02e1.96
2.22
95e100
1.53
1.84
1.02e2.05
2.16
95e100
1.71
1.36
0.96e2.02
2.18
95e100

8
T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
Table 3
Comparison of some figures of merit related to the different reported modified electrodes for the sensing ability of Tl1þ, As3þ and Hg2þ ions.
Sensors
Measurement technique
LOD (nM)
Tl1þ
Ref.
As3þ
Hg2þ
MEA/Au electrodea
Au NPs/CNTs/GCE
Au NPs/GCE
Au/MPS-(PDDA-AuNPs)
Pt NPs/CNTs
layer-by-layer assembled DNA
[Ru(bpy)3]2þ-GO/GE
GCeIIPeMWCNTs
CueCoHCF/GCE with
IL/SBA 15-silica/CPE
PPy/Pct/GR/GCE
Bi nanopowder electrode
Gly/GCE
Bi film electrode
LB-p-allylcalix [4]arene/GCE
IL/Gr/L/CPE
4-MPY/Au NPs
ZE-2/GCE
BOC6/GCE
DPASV
SWV
ASV
DPASV
LSV
N.Mb
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
30
0.175
10.8
5.0
6.01
N.M
N.M
2.19x10¡6
20
100
24
480
1.6
0.67
2.3
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
N.M
1.97x10¡6
N.M
N.M
N.M
N.M
N.M
N.M
1.6
[2]
[3]
[4]
[5]
[6]
[6]
[7]
[10]
[11]
[12]
[13]
[13]
[14]
[16]
[17]
[18]
[30]
[35]
This work
LSV
DPV
DPASV
DPV
5.0
L-Cysteine
80.0
10.0
6 ꢂ 10ꢀ6
N.M
0.23
N.M
N.M
N.M
8
DPASV
DPASV
SWASV
SWASV
SWASV
DPASV
SWASV
SPR
SWASV
SWASV
1.28
2.52x10¡6
a
MEA/Au electrode
¼
mercaptoethylamine modified Au electrode; Au NPs/CNTs
¼
gold nanoparticles/carbon nanotubes (CNTs); Au/MPS-(PDDA-
AuNPs) ¼ poly(diallyldimethylammonium chloride) and citrate capped Au nanoparticles anchored on sodium 3-mercapto-1-propanesulfonate modified gold electrode;
[Ru(bpy)3]2þ-GO ¼ ruthenium(II) bipyridine complex-textured graphene oxide nanocomposite; IIPeMWCNTs ¼ ion imprinted polymeric nanobeads and multi-wall carbon
nanotubes; CueCoHCF ¼ copper-cobalt hexacyanoferrate; IL/SBA 15-silica/CPE ¼ ionic liquid-ordered mesoporous silica/carbon paste electrode; PPy/Pct/GR ¼ polypyrrole,
pectin and graphene nanocomposites; Gly ¼ glycine; LB-p-allylcalix [4]arene ¼ LangmuireBlodgett (LB) film of p -allylcalix [4]arene; 4-MPY/Au NPs ¼ mercaptopyridine-
functionalized gold nanoparticles; ZE-2 ¼ Co-based zeolitic imidazolate framework (ZIF-67) and ex-panded graphite (EG); BOC6 ¼ Tribenzamide derivative.
b
N.M ¼ not measured.
to our novel designed electroanalytical BOC6 fabricated sensor.
These results infer superior properties of our tribenzamide-based
sensor and its excellent stability.
small signal of lead ions, with no significant deviation in target
analytes signals was also observed. These results indicate that lead
ions can also be detected and quantified by designed sensor, but at
different experimental conditions. Hence, our devised electro-
chemical sensing platform possess signifying resistance to the in-
terferences and capable to recognize the target analytes with
absolute discrimination ability.
3.5. Interference study for validation of sensor
In real matrices of water samples, commonly competitive redox
species and complexing agents are present along with the target
analytes that can be reduced on sensor surface and influence the
sensing of focus HM ions in practical application of electrochemical
sensors [25]. However, multiple co-existed cations can display their
oxidation peaks on SWASVs at different stripping potentials under
different experimental conditions. Therefore, it is necessitated to
thoroughly investigate the various potential interferents effect on
the sensor performance and its validity for Tl1þ, As3þ and Hg2þ ions
3.6. Practical applicability of sensor by study of real samples
To ensure the precision and practical applicability of the pro-
posed method of the electrochemical sensing, the BOC6 modified
GCE was applied for monitoring of Tl1þ, As3þ and Hg2þ ions in real
sample matrixes under defined experimental conditions. For this
purpose, the SWASV was performed in the electrochemical cell
having a water sample with 50% of the BRB (pH ¼ 5) to determine
the initial concentration of Tl1þ, As3þ and Hg2þ ions in tap water,
drinking water, spring water, river water, soil and industrial waste
water samples. Afterward, by the conventional standard addition
method, the known amount of metal ions from their stock solution
was spiked in water sample and recovery tests were performed
accordingly at the predefined experimental conditions of SWASV.
The concentration of spiked ions was then evaluated from the
linear calibration plot of Ip versus concentration and the recovery
percentages extended from 95 to 100% with RSD of 0.96e2.05% are
tabulated in Table 4. These results suggest the practical applica-
bility, certain feasibility and precision of the suggested method for
multi-metal ions detection.
detection. In this regard, the SWASVs of 0.15
mM ,
of Tl1þ
0.125
mM
A3þ and 0.175 Hg2þ ions in potential range
mM
from ꢀ1.2 V to 0.6 V at deposition potential of ꢀ1.2 V, was inves-
tigated in the presence of 2 mM concentration of organic and
inorganic interfering species such as, 2-amino-4- nitrophenol, 3-
chloro-5-nitrophenol, Csþ, Ca2þ, Sr2þ, Co2þ, Pb2þ, Kþ, Zn2þ, EDTA,
glucose, SDS, CTAB, amino acid and citric acid as displayed in vol-
tammograms of Fig. 5A. Consequently, SWASVs electrochemical
signals revealed that the presence of multi-fold higher amount of
the interfering agents can slightly affect the metal ions oxidative
peak currents under optimized conditions with RSD of 2.22%, 2.16%,
and 2.18% for Tl1þ, As3þ and Hg2þ ions, respectively as shown in bar
grFigure 5Baph in. The multiple ions in the homogeneous BRB pH 5
electrolyte may have adsorption competition on the sensor surface
depending on their concentrations and interactions toward modi-
fier. Nevertheless, SWASVs results reveal no obvious discrepancy,
this phenomenon might be attributed to enrichment of target triple
ions on the designed sensor surface by qualifying the competition
for accumulation sites because of strong binding affinity and
complexation of the tribenzamide recognition layer. However, a
4. Conclusion
In summary, we designed a novel tribenzamide derivative
modified GCE and tested it for the recognizing of multiple toxic ions
i.e. Tl(I), As(III), and Hg(II) in water systems. At BOC6/GCE, low Rct
value in EIS, noticeable redox ions reversibility in CV, enhance

T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
9
Fig. 5. (A) Voltammograms of metal analytes performed with a BOC6 modified electrode in the presence of 2 mM of one of the interfering agents i.e., 2-amino-4- nitrophenol, 3-
chloro-5-nitrophenol, Csþ, Ca2þ, Sr2þ, Co2þ, Pb2þ, Kþ, Zn2þ, EDTA, glucose, SDS, CTAB, amino acid and citric acid in cell having 0.15 M of Tl1þ, 0.125 M A3þ and 0.175 M Hg2þ ions
m m m
in BRB of pH 5 under chosen optimized conditions. (B) Corresponding bar graph with error bar showing adsorptive stripping peak current Ip of Tl1þ, As3þ and Hg2þ ions SWASV
effected by 2 mM concentrations of various organic and inorganic interfering agents.
target ions peak current intensity and negative potential shift in
SWASVs, ensure that the fabricated working electrode offer faster
electron transduction, better electrocatalysis and enhanced sensi-
tivity. These improved characteristics of fabricated sensor are
associated with availability of excessive adsorption sites on sensor
surface that increase the effective surface area of electrode for
target ions sensing. Furthermore, under optimized conditions of
SWASV, significantly enhanced well-defined oxidation current
signals of the target metals were achieved at the fabricated elec-
trode as compared to the bare GCE. The recommended sensing
platform has remarkable characteristics of low cost, easy
fabrication, portable and the ecological benign. The SWASVs results
showed that the BOC6 modified GCE possess excellent repeatable,
reproducible, electrocatalytic, ultrasensitive and the selective
electrode surface. The wide LCRs for the target metal ions and LODs
of 2.19 fM, 1.97 fM, and 2.52 fM for Tl(I), As(III), and Hg(II) ions
respectively are far below than the limits set by USEPA and WHO
[1]. In addition, the developed analytical protocol has efficacy for
sequestering the environmental pollutant in real-life water
matrices with excellent anti-interference ability, reliability, validity
and stability.

10
T. Kokab et al. / Electrochimica Acta 353 (2020) 136569
Table 4
Results for Tl1þ, As3þ and Hg2þ ions determination in real water samples obtained under the optimum experimental conditions.
Metal ions
Sample
Initially found (nM)
Spiked amount (nM)
Found amount (nM)
RSD (%)
Recovery (%)
Tl1þ
Drinking water 1
Drinking water 2
Tap water 1
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.0001
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.0008
0.00
0.00
17.50
17.50
17.50
17.50
17.50
17.50
17.50
17.50
17.50
17.50
17.50
17.50
15.00
15.00
15.00
15.00
15.00
15.00
15.00
15.00
15.00
15.00
15.00
15.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00
17.45
17.51
17.47
17.44
17.52
17.49
16.87
17.33
17.22
16.98
16.70
17.43
14.75
14.50
14.90
14.60
14.25
15.09
14.60
14.93
14.90
14.63
14.37
14.93
19.9
1.15
1.02
1.05
1.29
1.36
1.22
1.96
1.87
1.82
1.69
1.01
1.62
2.05
1.02
1.25
1.19
1.76
1.12
1.16
1.37
1.42
1.39
1.51
1.32
1.05
1.12
1.50
1.90
1.96
2.02
0.96
1.73
1.12
1.90
1.42
1.51
99.71
100.05
99.83
99.65
100.11
99.99
96.4
99.0
98.4
97.0
95.4
Tap water 2
Spring water 1
Spring water 2
River water 1
River water 2
Industrial wastewater 1
Industrial wastewater 2
Soil 1
Soil 2
99.6
As3þ
Drinking water 1
Drinking water 2
Tap water 1
98.33
96.66
99.33
97.30
95.00
100.60
97.4
99.5
99.4
97.5
95.8
Tap water 2
Spring water 1
Spring water 2
River water 1
River water 2
Industrial wastewater 1
Industrial wastewater 2
Soil 1
Soil 2
99.5
Hg2þ
Drinking water 1
Drinking water 2
Tap water 1
99.50
97.50
95.00
97.00
96.50
99.75
99.4
19.5
19.0
19.4
19.3
Tap water 2
Spring water 1
Spring water 2
River water 1
River water 2
Industrial wastewater 1
Industrial wastewater 2
Soil 1
19.9
19.88
19.94
20.08
20.10
19.56
19.30
99.7
100.4
100.5
97.8
Soil 2
96.5
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Authors declare no conflict of interest.
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