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Natural Latex-Capped Silver Nanoparticles for Two-way Natural Latex-Capped Silver Nanoparticles for Two-way

Electrochemical Displacement Sensing of Eriochrome Black T Electrochemical Displacement Sensing of Eriochrome Black T

Ziad Khalifa ziad.khalifa@bue.edu.eg

Moustafa Zahran Dr Menoufia Company for Water and Wastewater, Holding Company for Water and Wastewater, Menoufia 32514, Egypt, moustafazahran76@yahoo.com

Magdy Zahran Prof Department of Chemistry, Faculty of Science, El-Menoufia University, Shibin El-Kom, magdyzahran@gmail.com

Magdi Abdel Azzem Prof Department of Chemistry, Faculty of Science, El-Menoufia University, Shibin El-Kom, magdiabdelazzem@gmail.com

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Recommended Citation Recommended Citation Khalifa, Ziad; Zahran, Moustafa Dr; Zahran, Magdy Prof; and Abdel Azzem, Magdi Prof, "Natural Latex-Capped Silver Nanoparticles for Two-way Electrochemical Displacement Sensing of Eriochrome Black T" (2020). Chemical Engineering. 117. https://buescholar.bue.edu.eg/chem_eng/117

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Electrochimica Acta 356 (2020) 136825

Contents lists available at ScienceDirect

Electrochimica Acta

journal homepage: www.elsevier.com/locate/electacta

Note

Natural latex-capped silver nanoparticles for two-way electrochemical

displacement sensing of Eriochrome black T

Moustafa Zahran

a , b , ∗, Ziad Khalifa

c , Magdy A.-H. Zahran

a , Magdi Abdel Azzem

a

a Department of Chemistry, Faculty of Science, El-Menoufia University, Shibin El-Kom 32512, Egypt b Menoufia Company for Water and Wastewater, Holding Company for Water and Wastewater, Menoufia 32514, Egypt c Chemical Engineering Deparetment, Faculty of Engineering, The British University in Egypt, El Sherouk City 11837, Egypt

a r t i c l e i n f o

Article history:

Received 8 June 2020

Revised 17 July 2020

Accepted 22 July 2020

Available online 25 July 2020

Keywords:

Electrochemical sensors

Eriochrome black T

Natural latex

Silver nanoparticles

Displacement

a b s t r a c t

A novel electrochemical displacement sensor has been reported for the first time standing on the oxi-

dation behavior of the modifier and displacer. The electrochemical displacement sensor based on glassy

carbon electrode (GC) modified with natural latex-capped silver nanoparticles (NL-AgNPs) has been con-

ducted for the Eriochrome Black T (EBT) assessment. NL-AgNPs have been synthesized by direct chem-

ical reduction under different reaction conditions such as NL and AgNO 3 concentration, temperature

and pH for optimizing the size, and stability of nanoparticles before modifying the GC electrode. Un-

der the optimized conditions, NL-AgNPs have been characterized with UV–VIS spectroscopy, fluorescence

emission spectroscopy (FL), transmission electron microscope (TEM), zeta potential analyzer and cyclic

voltammetry (CV). Under optimized electrolytic and voltammetric conditions, EBT has been detected at

NL-AgNPs/GC based on modifier (NL-AgNPs) and displacer (EBT) oxidation. Both displacer and modifier-

based sensor (two-way sensor) have shown a linear range of 2–96 μg/L and detection limits of 2.92 and

7.01 μg/L, respectively. A high selectivity for EBT detection in river water with 110 and 107.5 recovery

values has been shown for both modifier- and displacer-based sensor, respectively.

© 2020 Elsevier Ltd. All rights reserved.

1. Introduction

Colorants including dyes and pigments have been utilized ex-

tensively in this day and age in industries such as textile, leathers,

paper, paints and plastics. Dyes are soluble colored organic com-

pounds that could be classified according to their application

methods as reactive basic, acids, vat, direct and reactive [1] . Ac-

cording to their chemical structure, they have been classified into

azo, anthraquinone, phthalocyanine, indigo, nitro nitroso and sul-

fur dyes [2] . Azo dyes have been regarded as the most signifi-

cant dyes applied in pharmaceuticals, textiles and food industries

representing more than half of the world annual production com-

pared to other types of dyes [3 , 4] . The ability of azo dyes to im-

part color into the materials is attributed to the presence of azo

group as a chromophore in their chemical structures [5] . Approx-

imately, 30 0,0 0 0 tons of dyes have been released to the environ-

ment imposing highly colored wastewater challenges [6] . The con-

tamination of natural water with these dyes has resulted in various

hazardous effects on living organisms. The dye color prevents the

∗ Corresponding author.

E-mail addresses: moustafazahran76@yahoo.com ,

moustafazahran@science.menofia.edu.eg (M. Zahran).

light penetration through the water body, which reduces the pho-

tosynthesis processes rate and the dissolved oxygen level which

hardly affects the aquatic organisms [7] . The azo bond cleavage

may result in releasing numerous harmful products such as aro-

matic amines [8] . Previously, toxicity, mutagenicity and carcino-

genicity have been reported for such dyes [6] . The high solubil-

ity of dye in aqueous systems make its detection and removal us-

ing traditional methods a burdensome procedure [7] . Consequently,

there is a pressing need to develop detection and treatment pro-

tocols to ensure the sustainability of water and its safety for con-

sumers.

Chromatographic techniques such as gas chromatography/mass

spectrometry, high performance liquid chromatography, as well as,

optical and electrochemical techniques were used for the azo dyes

determination [8–12] . However chromatographic techniques have

shown low detection limits compared to other techniques. They

require many highly sophisticated tools, usage of organic solvents,

and onerous sample preparations [12] . Electrochemical techniques

have been regarded as a suitable replacement for the current tradi-

tional techniques due to their simplicity, high selectivity, optimized

sensitivity and high efficiency [13–15] . Recently, the application of

the voltammetric techniques on the identification and characteri-

zation of azo dyes has drawn a great attention due to the peculiar

https://doi.org/10.1016/j.electacta.2020.136825

0013-4686/© 2020 Elsevier Ltd. All rights reserved.

2 M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825

electron transfer mediator characteristic of the dye structure itself

[13 , 16] . Currently, the metallic nanoparticles and their composites

represent an excellent electrochemical probe for azo dyes determi-

nation [17 , 18] .

Recently, silver nanoparticles (AgNPs) have gained a great at-

tention due to their distinguishing physical and chemical proper-

ties [19–21] . Their potential roles in diverse biomedical and envi-

ronmental applications have been reported [22 , 23] . Various tech-

niques such as photochemical, electrochemical and direct chemical

reduction have been used in the fabrication of AgNPs [24] . The di-

rect chemical reduction using natural resources has represented an

elegant technique for the AgNPs synthesis returning to their sim-

plicity, efficiency and eco-friendliness [25] . The effect of metallic

nanoparticles on the degradation of organic compounds through

redox potential and catalysis have been studied [26] . Reportedly,

AgNPs have been used as a brilliant catalyst for the degradation of

azo dyes such as methyl orange and congo red [22 , 27] .

Eriochrome Black T (EBT) is one of the most significant azo

dyes that has been used extensively in industrial applications such

as textile, printing, cosmetics and printing [11] . Additionally, it is

used in analytical chemistry for sensing metal ions such as cal-

cium and magnesium in water systems [28] . The carcinogenic ef-

fect of EBT and its degradation products make its determination in

water a vital mission to ensure the sustainability of water safety.

However, few studies reported the determination of EBT in envi-

ronmental systems [11] . In the present study, we report a novel

electrochemical displacement method for EBT assessment in natu-

ral water based on a glassy carbon electrode (GC) modified with

AgNPs that were fabricated by Ficus sycomorus Latex (NL).

2. Experimental

2.1. Chemicals

Silver Nitrate (Cambrian chemicals, 99.8% purity), Eriochrome

Black T (PanreacAppliChem), Muroxide (Sigma-Aldrich), Methyl

Orange (PanreacAppliChem), Phenolphthalein (Sigma-Aldrich),

Sodium hydroxide (Sigma-Aldrich) and Hydrochloric acid (Schar-

lau) have been used as received. Phosphate Buffer Solution (PBS)

has been prepared in the laboratory using Sodium Hydrogen

Phosphate (Na 2 HPO 4 .H 2 O, Sigma-Aldrich), Sodium Dihydrogen

Phosphate (NaH 2 PO 4 .7H 2 O, Sigma-Aldrich). The Britton–Robinson

Buffer has been prepared from Boric Acid (Sigma-Aldrich), Phos-

phoric Acid (Sigma-Aldrich), Acetic Acid (Sigma-Aldrich) and

Sodium Hydroxide. Additionally, Acetate Buffer Solution has been

prepared using Acetic Acid and Sodium Acetate (CH 3 COONa.3H 2 O,

Sigma-Aldrich).

2.2. Instruments

The average size of natural latex (NL)-AgNPs formed under

different conditions has been measured using Particle Size An-

alyzer (Brookhaven, Holtsville, NY, USA). The size and morphol-

ogy of NL-AgNPs have been studied using Transmission electron

microscope (TEM) [JEM-2100 (JEOL), Japan]. In addition, stabil-

ity of NL-AgNPs has been determined using Zeta Potential An-

alyzer (Brookhaven, Holtsville, NY, USA). Optical features of NL-

AgNPs have been elaborated using both UV–VIS spectrophotome-

ter (UV-2450, Shimadzu, Japan) and spectrofluorophotometer (RF-

5301 PC, Shimadzu, Japan). All electrochemical measurements have

been performed using BAS Epsilon-EC (West Lafayette, IN 47,906,

USA) based on three-electrode setup, where glassy carbon elec-

trode (GC) (3 mm diameter), platinum wire and silver/silver chlo-

ride electrode act as working, counter and reference electrodes, re-

spectively.

2.3. Collection of Ficus sycomorous latex

Ficus sycomorous fruits have been sourced from Al-Shohdaa city,

Menoufia governorate, Egypt. It has been identified by the Botany

Department, Faculty of Science, Menoufia University, Egypt. The

white milky latex has been obtained through cutting the fruit ends

carefully then storing it in sterile glass containers at 4 °C for fur-

ther studies.

2.4. Natural latex- Capped Silver nanoparticles synthesis

Different concentrations of NL (1, 5, 10, 25, 50, 75 and 100%)

have been obtained by dissolving NL in double distilled water with

a gentle stirring. Next, 10 mL of this solution was added to 90 mL

of different concentrations of AgNO 3 (0.1, 0.5, 1, 1.5, 2, 2.5 and

3 mM). Each mixture has been heated at various temperatures (40,

45, 50, 55, 60, 65 and 70 °C). Also, the effect of pH on AgNPs syn-

thesis has been studied. Sodium hydroxide and hydrochloric acid

have been used to adjust the pH.

2.5. Glassy carbon electrode modification with NL-AgNPs

Firstly, GC electrode polishing has been carried out via alu-

mina slurry mounted on micro cloth pads until obtaining a mirror

shiny surface then followed up by electrolysis by applying 0.6 V for

30 min in a phosphate buffer solution. Modification of the GC elec-

trode with NL-AgNPs has been accomplished using transfer stick-

ing technique [25] . GC bare electrode has been immersed in NL-

AgNPs stock solution that has been obtained under optimized con-

ditions for the duration of 40 min, then immediately it has been

transferred to the buffer solution for electrochemical studies.

2.6. Square wave voltammetry of EBT at Nl-AgNPs/GC composite

The electrochemical behavior of EBT and NL-AgNPs were stud-

ied using Square wave voltammetry (SWV) on the potential range

−0.8 V to 0.8 V. Scan rate, electrolytic buffer and pH influences

on both EBT and NL-AgNPs current signals have been examined.

In addition, the sticking time effect on NL-AgNPs signal was stud-

ied extensively. Under optimized conditions, NL-AgNPs/GC sensor

has been used for the EBT determination based on electrochemical

displacement method. EBT and NL-AgNPs current signal peaks val-

ues were calculated from the straight lines connecting the minima

before and after the maxima of each peak. All obtained data was

analysed by Linear Least-square Regression in OriginPro 8.0 (Origin

Lab Corporation, USA). The limit of detection (LOD) and limit of

quantification (LOQ) of the calibration curves were calculated ac-

cording to Eqs. (1) and (2) . Furthermore, all measurements were

tested six times to ensure accuracy and quality of gained data.

LOD = 3s / b (1)

LOQ = 10s / b (2)

Where (s) is the standard deviation of y-intercept and (b) is the

slope of the calibration curve [29] . The slope of the calibration

curve represents the sensitivity of the sensor.

2.7. Interferences study

The selectivity and sensitivity of our novel sensor have been

evaluated by studying the effect of various inorganic interference

such as Na + , K

+ , Ca 2 + , Mg 2 + and Cu

2 + , as well as, diverse organic

interference such as methyl orange, muroxide and phenolphthalein

dyes.

M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825 3

Fig. 1. Synthesis of AgNPs from natural latex.

2.8. Real water samples analysis

The applicability of our novel sensor for the determination of

EBT in natural water samples was studied extensively. Water sam-

ples were collected from the raw water of Al-Shohdaa surface wa-

ter treatment plant, Al-Shohdaa city, Menoufia governorate, Egypt.

Thereafter, 5 mL of filtrated raw water was diluted to 10 mL us-

ing a phosphate buffer solution, then spiked with 40 μg/L EBT. The

current peak signal of spiked EBT was determined using SWV tech-

nique. Recovery percent values have been calculated by dividing

the gained concentration over the added one, then multiplied by

hundred. Additionally, a standard addition technique has been em-

ployed to examine the selectivity of the sensor under investigation.

3. Results and discussion

3.1. Synthesis of NL-AgNPs

AgNPs were fabricated effectively by the direct chemical reduc-

tion of Ag + using NL extracted from F. sycomorus [24] . To the best

of our knowledge, there is only one study about AgNPs synthe-

sis using F. sycomorus [30] . The capacity of NL for the formation

of metallic nanoparticles complex, chelate, has returned to its high

phenolic and flavonoids contents [31] . AgNPs synthesis has been

indicated visually ( Fig. 1 ) through color change from the NL white

color to the AgNPs brown color [32] . NL-AgNPs have been syn-

thesized via several reaction conditions to optimize the formation

conditions for the formations of metallic nanoparticle with the ad-

equate size and stability for the electrochemical studies. The con-

ditions that have been investigated include; NL concentration, re-

action temperature, AgNO 3 concentrations and pH. Decreasing the

concentration of NL from 100% to 1% has led to the formation of

nanoparticles with much larger size ( Fig. 2 A). This could be at-

tributed to the weak capping layer that has resulted from the in-

sufficient NL phenolic compounds. The weak capping layer does

not has had the ability to overcome the Wander Waals forces be-

tween the metallic nanoparticles that has led to form some aggre-

gations. On the other hand, increasing the concentration of AgNO 3

solution from 0.1 to 3.0 mM has induced a slight change in NL-

AgNPs particle size. However, 2.0 mM AgNO 3 solution has been

selected as the most optimum concentration that has led to forma-

tion of a concentrated AgNPs stock with an accepted particle size

( Fig. 2 B). Besides, AgNPs size was reduced gradually by increasing

the temperature from 40 to 60 °C ( Fig. 2 C). Increasing the temper-

ature above 60 °C led to aggregation of nanoparticles. Therefore,

AgNPs were synthesized at 60 °C for further studies. Furthermore,

the pH of AgNPs suspension is one of the most important factors

that affects the particle size and stability of the metallic nanopar-

ticles. A pH range of 4.0–9 has been studied to determine the ap-

propriate pH value for AgNPs synthesis ( Fig. 2 D). The pH 8.5 has

resulted in the synthesis of AgNPs with the smallest particle size

that has made it the best pH condition.

3.2. Characterization of NL-AgNPs

NL-AgNPs fabricated under optimized conditions has been char-

acterized using UV–VIS spectroscopy, fluorescence emission spec-

troscopy (FL), TEM, Zeta Potential Analyzer and cyclic voltamme-

try. UV–VIS and FL have been utilized to determine the optical

features of NL-AgNPs, however, TEM and Zeta Potential Analyzer

have been employed to examine its shape/size and stability respec-

tively. UV–VIS and FL spectra have indicated that the absorption

and emission bands of NL-AgNPs appeared at 443 and 469 nm, re-

spectively ( Fig. 3 A-B). The absorption peak is a result of the surface

plasmon resonance of AgNPs that represents the collective oscilla-

tion of the surface electrons that has interacted with the incident

light at the interface between silver nanoparticles and the dielec-

tric medium [33] . The red shifted emission peak has referred to

electron–phonon and hole–phonon that has been scattered with

the subsequent low energy release [21] . According to TEM im-

ages, AgNPs are spherical particles with an average size of 43 nm

( Fig. 3 C). The Zeta potential has been measured as −41 mV which

has proved the high stability of NL-AgNPs which has resulted from

the high electron repulsion between AgNPs capped by NL ( Fig. 3 D).

According to all these properties, NL-AgNPs represented a suitable

electrochemical modifier for constructing an electrochemical sen-

sor for EBT dye assessment. The electrochemical properties of GC

and NL-AgNPs/GC electrodes have been studied in 5.0 mM potas-

sium ferrocyanide K 4 Fe(CN) 6 •3H 2 O and 1.0 M KCl solution using

cyclic voltammetry (CV) ( Fig. 4 ). NL-AgNPs/GC has shown the re-

dox peaks with high current compared to bare GC electrode due

to the presence of NL-AgNPs. It indicated that the modification of

bare GC electrode with NL-AgNPs has developed its electrochem-

ical performance. The values of the electro active surface area of

both bare and modified electrodes have been calculated according

to Randles-Sevcik equation ( Eq. (3) ).

i p =

(2 . 69 × 1 0

5 )n

3 / 2 AC D

1 / 2 v 1 / 2 (3)

Where i p is the current (A), n is the number of transferred elec-

trons in the redox reaction ( n = 1), A is the electrode active surface

area (cm

2 ), C is the concentration of K 4 Fe(CN) 6 (mol/cm

3 ), D is the

K 4 Fe(CN) 6 diffusion coefficient (7.60 × 10 −6 cm

2 /s) and v is the

scan rate (V/s). Active surface area has been estimated for bare GC

and NL-AgNPs/GC electrodes as 0.024 and 0.13 cm

2 , respectively.

The surface area of NL-AgNPs/GC has been increased five-fold by

modifying the bare GC electrode with NL-AgNPs.

3.3. Electrochemical oxidation of EBT at NL-AgNPs/GC

The electrochemical behavior of NL-AgNPs/GC has been tested

in PBS buffer solution (pH 5.6) using SWV technique in absence of

EBT. The obtained voltammogram has shown one oxidation peak at

0.17 V with an anodic peak current of 7.85 μA ( Fig. 5 ) correspond-

ing to the oxidation of AgNPs. EBT oxidation at NL-AgNPs/GC has

shown two peaks, at −0.01 V and 0.23 V, corresponding to peaks

I and II, respectively. Peak I could be attributed to the oxidation of

EBT while peak II has been appeared at approximately the same

oxidation potential of AgNPs. The direct oxidation of 40 μg/L EBT

on bare GC electrode has resulted in an oxidation current signal of

1.86 μA. This demonstrated that the modification of bare GC elec-

trode with NL-AgNPs has enhanced the EBT oxidation by increasing

its peak current to 5.43 μA as shown in Fig. 5 . In all, this perfor-

mance could be attributed to the catalytic effect of NL-AgNPs and

the increase in the electrode surface area. Also, it is obvious that

the NL-AgNPs current signal has been decreased from 7.85 μA to

4 M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825

Fig. 2. Effect of NL concentration (A), AgNO 3 concentration (B), Temperature (C) and pH (D) on NL-AgNPs nanoparticles size.

Fig. 3. Characterization of NL-AgNPs using UV–VIS (A), FL (B), TEM (C) and zeta potential analyzer (D).

M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825 5

Fig. 4. CV of bare and NL-AgNPs/GC in 5.0 mM K 4 Fe(CN) 6 •3H 2 O and 1.0 M KCl,

scan rate 0.05 V/s.

Fig. 5. SWV of NL-AgNPs/GC in absence (black) and presence of 40 μg/L EBT (blue)

at PBS buffer of pH 5.6. SWV parameters: scan rate, 0.2 V/s; step potential, 0.004 V;

frequency, 50 Hz; amplitude, 0.05 V. (For interpretation of the references to colour

in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Chemical structure of EBT.

5.89 μA after EBT addition due to the replacement of some NL-

AgNPs with EBT.

In accordance with GC electrode modification with NL-AgNPs,

EBT has been oxidized and polymerized at the fabricated modified

electrode. The electrochemical oxidation of EBT at GC has been af-

fected by the presence of hydroxyl group in EBT structure it self

[34 , 35] . EBT has represented an azo dye with a hydroxyl group

in the ortho position with respect to the azo bridge ( Fig. 6 ). EBT

oxidation at NL-AgNPs/GC to give benzoquinone diimine structure

is shown in Fig. 7 . After oxidation, EBT could be polymerized ef-

ficiently at the electrode surface that has led to modify its sur-

face. EBT has been used as an efficient electrode modifier espe-

cially with metallic nanoparticles for the construction of various

sensors to determine organic molecules. For example, Poly(EBT)/GC

were used for dopamine, ascorbic acid and uric acid assessment

[35] . Also, Au nanoparticles/poly(EBT)/GC has been used for the

determination of L-cysteine and L-tyrosine [36] . Many techniques

have been used for the electrode modification such as drop cast-

ing, sticking and transfer-sticking. Transfer-sticking represented an

efficient technique for the modification of GC with AgNPs without

inducing agglomeration compared to other techniques [25] . The

proposed mechanism of GC modification with NL-AgNPs through

transfer-sticking and the subsequent EBT oxidation and polymer-

ization is shown in Fig. 7 .

3.4. Electrochemical displacement sensor

The chemical displacement phenomenon has been studied ex-

tensively. However, there are few reports that investigated the

electrochemical displacement at the electrode surface [37–39] .

Wang et al. (2008) developed an electrochemical displacement

method for studying the binding interaction of small polycyclic or-

ganic molecules with DNA [38] . In the present study, we have re-

ported a novel sensor based on the electrochemical displacement

at GC surface for the first time, to the best of our knowledge. The

sensor could be used for the determination of various inorganic

and organic compounds. In our study, the electrochemical displace-

ment sensor has been applied for the EBT assessment. The pro-

posed sensor has been constructed as follows: (1) Modification of

GC electrode with NL-AgNPs; (2) Different concentrations of EBT

solution has been added to the PBS buffer of pH 5.6; (3) followed

by oxidation using SWV. Adding EBT resulted in an increase in the

EBT oxidation peak signal (peak I) with simultaneous decreasing of

NL-AgNPs oxidation peak current signal (peak II) that have shown

a two-way sensor ( Fig. 8 ). This could be attributed to the high

affinity of EBT for binding to the GC electrode surface compared

to NL-AgNPs competitor. The high affinity of EBT has resulted in

the displacement of some NL-AgNPs with the added EBT molecules

at the GC surface. The selection of the metallic nanoparticles size

is critical for the proposed sensor. The oxidation behavior of NL-

AgNPs with different average sizes, 21, 27 and 43 nm, that have

been fabricated by varying the conditions of the synthesis process,

has been particularly studied ( Fig. 9 A-B). There is an obvious posi-

tive shift in the oxidation potential and lowering in current inten-

sity by the increase of NL-AgNPs size due to the diminution of sur-

face area to volume ratio [25] . The nanoparticles with size range of

20-30 nm are not idealistic to prevent the overlapping of EBT peak

with AgNPs one. Besides, AgNPs of particle size lower than 20 nm

should be avoided to exclude the aggregation effect that may re-

sult in the formation of nanoparticles with larger nanoparticles at

EBT defined potential. Reportedly, aggregation effect on the pos-

itive shift of metallic nanoparticles oxidation potential was stud-

ied [40 , 41] . Recently, we have reported a novel aggregation sen-

sor based on metallic AgNPs for determination of atrazine in wa-

ter [25] . NL-AgNPs with an average size of 43 nm (confirmed by

TEM), have been selected as a suitable modifier of GC for construc-

tion of the electrochemical displacement sensor. This size dimin-

ishes not only the effect of NL-AgNPs oxidation peak overlapping

with EBT oxidation peak, but also has made the possible induced-

aggregation peak away from EBT one.

3.5. Optimization of voltammetric and electrolytic conditions

Different factors such as sticking time, scan rate, supporting

electrolyte and pH have been studied to enhance the sensing pro-

cess. NL-AgNPs oxidation current signal under different sticking

time intervals has been determined. Sticking time of 40 min has

shown the highest oxidation current signal making it the preferred

6 M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825

Fig. 7. Schematic representation of EBT oxidation at NL-AgNPs/GC surface.

Fig. 8. Schematic representation of the electrochemical displacement technique.

time for NL-AgNPs sticking ( Fig. 10 A). Different scan rate ranges:

0.050, 0.075, 0.100, 0.150 and 0.200 V/s were studied for opti-

mizing the oxidation of both EBT and NL-AgNPs ( Fig. 10 B). EBT

has shown its maximum current signal at scan rate of 0.100 V/s,

while NL-AgNPs has exhibited its maximum signal at 0.200 V/s.

Scan rate of 0.200 V/s was selected as the optimized condition

for our novel sensor. The electrolytic conditions have been opti-

mized by selecting the appropriate supporting electrolyte and its

pH range. Supporting electrolyte and its pH effect not only in NL-

AgNPs oxidation but also in EBT oxidation, which has made the

selection of the suitable electrolyte and its pH a more complicated

procedure. Three supporting electrolytes; acetate, phosphate and

Britton-Robinson buffers, of pH 5.6 have been studied ( Fig. 10 C).

Acetate buffer has exhibited the lowest current signal for both NL-

AgNPs and EBT oxidation. However, Britton-Robinson buffer has

shown the highest current peak signal for NL-AgNPs, but it has ex-

hibited a lower current signal for EBT oxidation. Phosphate buffer

led to the highest current signal for EBT oxidation with an ac-

ceptable NL-AgNPs current signal making it the most appropriate

buffer for the electrochemical displacement sensor. Also, EBT and

NL-AgNPs peak current signals were studied under the pH range of

2-10 ( Fig. 10 D). Contrary to NL-AgNPs, EBT current signal has been

decreased with increasing the pH from 2 to 10. Phosphate buffer

Fig. 9. Different sizes of NL-AgNPs (A) and their oxidation behavior (B).

of pH 5.6 was selected as optimum electrolytic condition for EBT

and NL-AgNPs oxidations.

3.6. NL-AgNPs/GC electrochemical displacement sensor for EBT

determination

Employing previously optimized conditions, different concen-

trations of EBT (2–96 μg/L) were added to the phosphate buffer

pH 5.6 then SWV and the corresponding calibration curves of EBT

oxidation at NL-AgNPs/GC surface were determined ( Fig. 11 ). There

is an obvious increase in EBT peak current signal and reduction

of NL-AgNPs one with increasing EBT concentration. This two-way

electrochemical displacement sensor exhibited a linear range of 2–

96 μg/L. LOD for EBT (displacer) and NL-AgNPs (modifier) are 2.92

M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825 7

Fig. 10. Factors affecting AgNPs and EBT oxidation, sticking time (A), scan rate (B), electrolyte type (C) and pH (D).

Table 1

EBT detection parameters using electrochemical displacement method.

Sensor Linear range (μg/L) R 2 LOD (μg/L) LOQ (μg/L) Sensi. RSD (%)

Displacer 2–92 0.99 2.92 9.76 0.07 2.35

Modifier 0.97 7.01 23.39 0.12 2.84

Table 2

Reported methods for EBT determination.

Method Technique Linear range (μg/L) LOD (μg/L) Ref

Optical PL 46.1–3460.4 3183.5 [11]

Electrochemical reduction CV — — [42]

Electrochemical oxidation AdSLV 46.1–4613 — [43]

Electrochemical displacement SWV 2–92 2.92 (Displacer)

7.01 (Modifier)

This work

and 7.01 μg/L respectively, while their LOQs are 9.76 and 23.39 μg/L

respectively ( Table 1 ). Displacer and modifier regression equations

are I EBT = 0.071C EBT + 2.7 and I EBT = 0.125C EBT + 11.2 respectively.

Both displacer and modifier-based sensor has showed a good sensi-

tivity with closed values, 0.07 and 0.12, respectively. Relative stan-

dard deviation (RSD) of current signals has been obtained from

six replicates and has been measured as 2.35% and 2.84% respec-

tively for displacer and modifier-based sensor representing a good

repeatability.

3.7. Comparison between previously reported methods for EBT

detection

Few studies reported the EBT determination in environmental

systems in spite of its negative impact on human health. Table 2

represents the electrochemical and non-electrochemical methods

used in EBT determination. Recently, a non-electrochemical sensor

based on AgNPs/graphene oxide composite has been used as pho-

toluminescence sensor (PL) for sensitive determination of EBT [11] .

Regarding the electrochemical methods, two previous studies re-

ported the EBT azo dye determination based on its electrochemical

oxidation and reduction [42 , 43] . EBT was reduced at carbon paste

electrode in two-step one-electron reduction of the azo group [42] .

The oxidation of the EBT has occurred at multi-walled carbon nan-

otube modified GC using adsorptive stripping linear sweep voltam-

metry [43] . Our proposed electrochemical displacement sensor has

represented a more sensitive method for EBT determination with

the lowest LOD.

3.8. Interferences effect on EBT sensing

Interferences have represented a crucial effect in the validation

of the sensor for its applicability in real water samples. Table 3

exhibits the maximum concentration of inorganic metals includ-

8 M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825

Fig. 11. SWV and corresponding calibration curves of EBT oxidation at NL-AgNPs/GC (pH 5.6). SWV parameters: scan rate, 0.2 V/s; step potential, 0.004 V; frequency, 50 Hz;

amplitude, 0.05 V.

Table 3

Accepted concentration of interfering metal

ions for EBT sensing.

Metal

ion

Maximum concentration (mg/L) a

Modifier Displacer

Na + 320 ±5.4 510 ±4.2

K + 360 ±3.6 475 ±5.7

Ca 2 + 198 ±4.7 194 ±1.2

Mg 2 + 210 ±2.9 180 ±1.9

Cu 2 + 290 ±2.8 240 ±2.1

a 95% confidence interval calculated

( n = 6),.

ing Na + , K

+ , Ca 2 + , Mg 2 + and Cu

2 + that have an acceptable EBT sig-

nal change ( < 5%). In spite of the known binding interaction be-

tween our displacer (EBT) with Ca 2 + and Mg 2 + , there is a slight

effect of these metal ions on the displacer electrochemical re-

sponse. This could be explained by EBT-Ca 2+ and EBT-Mg 2 + inter-

actions that have taken place at higher pH values (8–12) compar-

atively to our sensor at pH (5.6) [44] . Maintaining moderate con-

Table 4

Effect of interfering dyes on EBT sensing.

Dye

Signal change (%) a

Modifier Displacer

Methyl orange −4.7 ± 0.14 4.9 ± 0.33

Muroxide −0.8 ± 0.11 3.7 ± 0.42

Phenolpthalein 1.1 ± 0.12 2.8 ± 0.14

a 95% confidence interval calculated ( n = 6).

centrations of metal ions have been recommended for the valid-

ity of the electrochemical displacement sensor. However, dialysis

could be a solution for samples with higher metal ions concen-

trations. The effects of organic interferences including methyl or-

ange, muroxide and phenolphthalein dyes were studied. The sig-

nal change after addition of 40 μg/L of each dye was calculated

( Table 4 ). Significantly, the signal change is accepted if it doesn’t

exceed 5%. Muroxide and Phenol pthalein have exhibited a lower

signal change compared to methyl orange. Methyl orange is a non–

hydroxyl-substituted azo dye which could be oxidized and poly-

M. Zahran, Z. Khalifa and M.A.-H. Zahran et al. / Electrochimica Acta 356 (2020) 136825 9

Table 5

EBT assessment in river water samples.

Sensor

Added EBT

(μg/L)

Found EBT

(μg/L) a Recovery

(%)

Modifier-based

sensor

40 44 ± 1.2 110

Displacer-based

sensor

43 ± 1.3 107.5

a 95% confidence interval calculated ( n = 6).

merized at GC surface in a different manner compared to EBT with

hydroxyl substitution [45] . However, it has slightly affected both

modifier and displacer of our electrochemical displacement sensor.

3.9. Real sample analysis

EBT azo dye has been detected in river water samples by utiliz-

ing our electrochemical displacement sensor based on the modi-

fier and displacer responses. Accepted recovery with 110 and 107.5

values has been reported for modifier and displacer, respectively

( Table 5 ). Both values have confirmed that there was no regarded

influence of the river water interferences. Additionally, the selec-

tivity of our two-way sensor has been determined using stan-

dard addition technique. Regression equations of the displacer- and

modifier-based sensors for EBT determination in river water were

calculated as I EBT = 0.074C EBT + 2.7 and I EBT = 0.132C EBT + 11.5 re-

spectively. By comparing the slopes of calibration and standard ad-

dition curves, there is no regarded difference between them, which

has made the proposed sensor efficient for the determination of

EBT in river water.

4. Conclusion

In summary, NL-capped AgNPs have been fabricated by

direct chemical reduction and characterized using Particle Size An-

alyzer, UV–VIS, FL, TEM, Zeta Potential Analyzer and Cyclic Voltam-

metry. The conditions of AgNPs synthesis such as NL concen-

tration, AgNO 3 concentration, reaction temperature and pH were

optimized for the formation of NL-AgNPs with an appropriate par-

ticle size and stability for our electrochemical sensor construction.

GC has been modified with NL-AgNPs using transfer sticking tech-

nique under optimized electrolytic and voltammetric conditions.

NL-AgNPs/GC has been used for the determination of EBT based

on an electrochemical displacement technique. Furthermore, the

sensing process has depended on the electrochemical responses of

NL-AgNPs (modifier) and EBT (displacer). The validity of the sensor

was examined by determining linear range, LOD, LOQ, repeatabil-

ity and sensitivity. Finally, the selectivity of our two-way sensor

has been verified for sensing EBT in river water samples.

Declaration of Competing Interest

The authors declare that they have no known competing finan-

cial interests or personal relationships that could have appeared to

influence the work reported in this paper.

CRediT authorship contribution statement

Moustafa Zahran: Investigation, Methodology, Writing - origi-

nal draft. Ziad Khalifa: Resources, Validation. Magdy A.-H. Zahran:

Supervision, Visualization. Magdi Abdel Azzem: Conceptualization,

Writing - review & editing.

Acknowledgment

The authors acknowledge the scientific donation of Alexander

von Humboldt Foundation, 53173 Bonn, Germany.

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