Iron oxide (Fe2O3) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation of paracetamol and dopamine

Iron oxide (Fe2O3) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation of paracetamol and dopamine.. M.M.[r]

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Iron oxide (Fe2O3) nanoparticles modified carbon paste electrode as an advanced

material for electrochemical investigation of paracetamol and dopamine

M.M Vinay, Y Arthoba Nayaka

PII: S2468-2179(19)30208-4

DOI: https://doi.org/10.1016/j.jsamd.2019.07.006

Reference: JSAMD 239

To appear in: Journal of Science: Advanced Materials and Devices

Received Date: 10 April 2019

Revised Date: 22 July 2019

Accepted Date: 28 July 2019

Please cite this article as: M.M Vinay, Y Arthoba Nayaka, Iron oxide (Fe2O3) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation of paracetamol

and dopamine, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/

j.jsamd.2019.07.006

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Iron oxide (Fe 2 O 3 ) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation of paracetamol and dopamine

Vinay M Ma, Y Arthoba Nayakaa*

a

Department of Chemistry, School of Chemical Science, Kuvempu University, Shankaraghatta -

577451, Karnataka, India, tel.: +919448855078; fax: +9108282 256255

*Corresponding author E-mail address: drarthoba@yahoo.co.in (Y Arthoba Nayaka)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 -50

-40 -30 -20 -10 0 10 20

b c a

NH2

Dopamine-o-quinone

O H OH

2e -2H+

To, Date 04.07.2019

The Editor,

JSAMD

Subject: Submission of Revised Manuscript

With reference to the above subject, herewith I am submitting manuscript of the research paper

your journal, which is entitled your journal, which is entitled “Iron oxide (Fe 2 O 3 ) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation

of paracetamol and dopamine” The article is ORIGINAL and unpublished and is not being

considered for publication elsewhere Kindly consider and do the needful

Thanking You,

With Regards

(Prof Y Arthoba Nayaka)

Iron oxide (Fe 2 O 3 ) nanoparticles modified carbon paste electrode as an advanced material for electrochemical investigation of paracetamol and dopamine

Vinay M Ma, Y Arthoba Nayakaa*

Abstract

Electrochemical behaviors of paracetamol (PA) and dopamine (DA) have been investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and square wave voltammetry (SWV) using a selective and sensitive iron oxide (Fe2O3) nanoparticle modified carbon paste electrode (IOCPE) The PA and DA showed anodic peak potential at 0.458 V and 0.247 V and cathodic peak potential at 0.088 V and 0.11 V (vs Ag/AgCl), respectively In DPV

mode PA and DA gave linear response over the concentration range of 2 to 150 µM (R2 = 0.998)

and 2 to 170 µM (R2 = 0.989), respectively The limit of detection (LOD = 3s/m) for PA and DA

were found to be 1.16 and 0.79 µM, respectively IOCPE possess excellent electrocatalytic activity towards the determination of PA and DA The proposed method could be successfully validated for the simultaneous and individual determination of PA and DA present in pharmaceutical and real samples

Keywords: Carbon paste electrode; Differential pulse voltammetry; Electrochemical impedance

spectroscopy; Iron oxide nanoparticles

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 -50

-40 -30 -20 -10 0 10 20

b c a

NH2

Dopamine-o-quinone

O H OH

2e -2H+

1 Introduction

Paracetamol (PA) (N-acetyl-p-aminophenol or acetaminophen) is one of the most widely

used analgesic and antipyretic drug [1] PA release the pain associated with headache, backache, arthritis and postoperative effect, and it is commonly used for reducing the fever [2] PA having

pKa value 9.5, rapidly gets distributed after oral administration and is easily excreted in urine PA does not exhibit any harmful side effects, but in few cases, it leads to the formation of liver damage and nephrotoxic metabolites [3] An overdose of PA leads to hepatic toxicity, liver and kidney damage and in some cases cause death [4] Dopamine (DA) another most leading catecholamine-based neurotransmitter drug and it was monitoring the central nervous system (CNS) and thus by arbitrating the multiple CNS functions such as memory, learning, neuroendocrine secretion, cognition and control of locomotion The difference in the concentration of DA level leads to severe causes such as Parkinson’s disease, depression, schizophrenia and Huntington’s disease, HIV infection, epilepsy and senile dementia [5] So, it is necessary to determine the drugs present in pharmaceutical samples and human biological fluids (urine and blood) Therefore, the development of simple, rapid, sensitive, and precise analytical procedures for the detection and quantification of these drugs is of great importance [6]

Numerous methods are available for the determination of PA and DA such us spectrophotometry [7], TLC [8], HPLC [9-10], LC-MS [11], flow-injection analysis [12], UV-Vis spectrometry [13], electrochemical methods [14], fluorimetry methods [15] and chemiluminescence methods [16] The electrochemical techniques have received a remarkable consideration due to their unique qualities such as simple pretreatment procedure, good sensitivity, better selectivity, less time-consumption and low cost [17] In recent years, the modification of electrode surface fascinated significant attention because of their extremely

enhanced sensitivities Literature survey reveals that PA and DA undergoes electrochemical reactions at different electrodes, such as graphite electrodes (GE) [18], polyurethane modified

GE [19], glassy carbon electrodes (GCE) [20], modified GCE [21], screen-printed electrodes [22], carbon paste electrodes (CPE) [23], modified CPE [24], carbon fiber microelectrodes [25], carbon ionic liquid electrodes [26], platinum electrodes [27], gold electrodes [28] and boron doped diamond electrode (BDDE) [29]

Currently, metal nanoparticles, nanodiamonds, fullerenes, carbon nanotubes, etc., were being used in biomedical and sensor applications, since nanomaterials possess fundamental features like smaller size, good optical, magnetic and mechanical properties [30] Metal nanoparticles have much more considerations during the last few decades due to their unique properties as well as a large surface-to-volume ratio as compared to their bulk counter parts [31] The various electrochemical methods are available for the determination of drugs and have been reported in the literature The modification of carbon paste electrode with different metal nanoparticles can improve the performance in terms of sensitivity and selectivity, and hence fastens the rate of electron transfer between the electroactive species and the electrode surface Several metal nanoparticles are reported in literature, such as platinum [32], gold [33], silver [34], copper [35] and as well as metal oxide nanoparticles MnO2 [36], NiO [37] CuO [38], and ZnO [39] these were used for the development of electrochemical sensors The Fe2O3

nanoparticles revealed sole features which powerfully differ from those of massiveness phases The electronic, magnetic and optical properties of Fe2O3 nanoparticles have a wide importance in many industrial applications including the extended new electronic, optical devices, information storage, magnetocaloric refrigeration, color imaging, bioprocessing, ferrofluid technology or the manufacture of magnetic recording media [40]

The present work reports the determination of PA and DA simultaneously, using electrochemical techniques such as CV, DPV, SWV and EIS The analytical applications of IOCPE have been tested via the redox reaction of PA and DA present in biological (urine and serum) and pharmaceutical samples The proposed modified electrode IOCPE has got low limit

of detection, good sensitivity, better selectivity and so it could be used for the estimation of PA and DA present in pharmaceutical and real samples

2 Experimental

2.1 Apparatus

All the voltammetric experiments were carried out using Electrochemical Workstation (Model number: CH Instrument 660D, USA) The electrochemical experiments were performed using conventional three-electrode system The IOCPE, platinum wire and silver/silver chloride were used as working, auxiliary and reference electrode, respectively The Equiptronics EQ-611

pH meter was used for the pH measurements All the electrochemical studies were carried under lab temperature

2.2 Chemicals and reagents

The standard drugs Paracetamol (Acetaminophen) (≥99%) and Dopamine hydrochloride (≥99%) have been procured from Sigma Aldrich (USA and Germany, respectively), di-Potassium hydrogen phosphate anhydrous (≥98%), Potassium hydrogen phosphate (≥98%) and Potassium chloride purified (≥99%) were procured from Merck (Mumbai, India) Silicon oil, Potassium ferricyanide (III) (99%, A.R.) obtained from Himedia (Mumbai, India) Paracetamol tablets and Dopamine injection tubes have been purchased from local market (Shivamogga, India) PBS of 0.1 M was prepared (lab temperature at 26±2 oC) and the pH was adjusted using NaOH and H3PO4 All the solutions were prepared using doubly distilled water

2.3 Procedure

2.3.1 Preparation of iron oxide nanoparticles (ION’s)

Iron oxide nanoparticles have been synthesized by simple precipitation method under laboratory condition using FeCl3.6H2O and liquid ammonia The stoichiometric ratio of FeCl3.6H2O powder has been dissolved in doubly distilled water and stirred well To this constant stirring solution, 14 % of NH4OH solution has been added drop-wise (0.2 ml min-1) with maintaining the pH 8.0 The obtained brown coloured iron oxide was filtered The excess of base was washed with water and dried in an oven for about 12 h, and finally subjected to calcination at 500 oC for 5 h The so obtained Fe2O3 sample has been characterized by scanning electron microscopy (SEM), energy dispersive X-Ray analysis (EDAX) and X-ray diffraction

(XRD) techniques [41,42]

2.3.2 Preparation of bare carbon paste electrode (BCPE) and IOCPE

The working electrode (IOCPE) was developed by mixing graphite powder, silicon oil

and ION’s in the ratio 76:20:4 (w/w) and the resultant mixture were thoroughly homogenized

using mortar and pestle The obtained paste was tightly packed into the glass tube (60 mm height, 3 mm diameter) without any air gap The electrical contact has been made at one end by inserting a copper wire through the center of the paste packed glass tube without any crack The exposed end of the electrode was mechanically polished and renewed using butter sheet to get reproducible smooth and shiny working surface This operation has been repeated before start of each experiment The BCPE has been prepared in the same way without addition of modifier

3 Result and Discussion

3.1 Characterization of prepared ION’s and IOCPE

The prepared ION’s sample has been characterized by powder X-ray diffractometer

(Cu-Kα, radiation at λ = 1.5406 Å) to confirm the physical properties like structure, crystallinity,

lattice planes etc Fig 1(a) depicts reflection planes (220), (311), (400), (422), (511), (440)

corresponding to 2 angles 30.26, 35.65, 43.32, 53.77, 57.32, 62.98, respectively These values

were well matches with standard JCPDS file no.: 39-1346 (volume-582.5, lattice-primitive, system-cubic, space group-P4 1 32, cell parameter a = 8.351) The sharp peak with small FWHM (full width at half maximum) showed that sample has got good crystallinity Debye-Scherrer

formula has been used for the calculation of crystal size and was found to be 27 nm Fig 1(b)

represents the SEM image of synthesized iron oxide nanoparticles, which shows the prepared nanoparticles were agglomerated The EDAX analysis has shown the energy peaks of iron oxide nanoparticle in the range 0.5 to 6.5 keV corresponds to iron and oxygen atoms of prepared iron oxide nanoparticles (figure not shown) and it has been revealed that iron oxide has got crystalline

nature [43] Fig 1(c) represents the SEM (analysis of surface morphology) image of prepared IOCPE

Fig 1 Characterization of prepared ION’s: (a) X-ray diffraction spectra; (b) SEM image; (c)

SEM image of prepared IOCPE

The electrochemical behavior of IOCPE has been monitored through the redox process of [Fe(CN)6]3−/4−couple Fig 2 shows the cyclic voltammograms obtained a) absence of analyte at

IOCPE; b) at BCPE and c) at IOCPE in 1 mM K3[Fe(CN)6] containing 0.1 M KCl solution In the forward scan (positive scan) Fe[(CN)6]3- was electrochemically oxidized from Fe[(CN)6]4- (anodic process) and in the reverse scan (negative scan) this Fe[(CN)6]3- has been reduced back

to Fe[(CN)6]4- (cathodic process) The obtained voltammogram of [Fe(CN)6]3−/4−couple shows a

peak to peak separation potential (∆Ep) of 309 mV, 136 mV, at BCPE and IOCPE, respectively

The value of ∆Ep has been decreased at IOCPE compared to BCPE and it clearly indicated the more-reversible charge-transfer process at IOCPE than that of BCPE Also the peak currents for [Fe(CN)6]3−/4− couple have been enhanced at IOCPE compared to BCPE The decrease in ∆Epvalue with an increase in peak current suggested that the IOCPE has got increased surface area than BCPE this may be the presence of iron oxide nanoparticles, enhances the electron transfer rate The surface area of BCPE and IOCPE have been calculated by Randles-Sevcik equation using 1 mM K3[Fe(CN)6] in 0.1 M KCl solution at different scan rate [44]

(c)

where Ip and ν refers the peak current (µA) and scan rate (V s-1), A is the active surface area of

electrode (cm2), Do and C0 * represents the diffusion co-efficient (cm2 s-1) and bulk concentration

of K3[Fe(CN)6] (mol cm-3) The diffusion co-efficient for 1 mM K3[Fe(CN)6] in 0.1 M KCl can

be obtained by plotting Ipa vs ν1/2 (n = 1, Do = 7.6 x 10-6 cm2 s-1) [9] On substituting the above values, effective surface area of BCPE and IOCPE were found to be 0.116 cm2 and 0.1786 cm2, respectively

Fig 2 Cyclic voltammograms of: (a) blank at IOCPE; (b) BCPE; (c) IOCPE in 1 mM

K3[Fe(CN)6] containing 0.1 M KCl (at scan rate 100 mV s-1)

3.3 Effect of modifier (ION’s) concentration

The effect of modifier concentration was studied by cyclic voltammetry using the 1 mM PA in

PBS (pH 7.0) and plotting the graph of current vs potential (Fig 3 –supplementary) The 4-wt%

of ION’s in carbon paste electrode demonstrates good redox behavior of PA compared to other compositions The resistive electrochemical signal was observed for more than 4-wt% of the modifier this may be saturation point of iron oxide nanoparticles have been attained and hence this electrode composition has been selected for further electrochemical studies

-60 -40 -20 0 20 40 60

3.4 Electrochemical characterization of IOCPE

Electrochemical impedance spectroscopy (EIS) is an efficient method to analyze the interfacial properties of surface modified electrodes and it provides valuable information regarding the impedance change at the electrode surface EIS uses the frequency sweep of very minute AC voltage modulated over DC bias to extract the equivalent circuit The circuit has been obtained

by plotting the real and imaginary part of impedance (Nyquist diagram) [45] The electron

charge transfer resistance (Rct) has been measured from the semicircle diameter of the Nyquist

plots Fig 4 explains the Nyquist diagram of 5 mM K3[Fe(CN)6] containing 0.1 M KCl at BCPE and IOCPE in the frequency range from 0.1 to 103 kHz and equivalent circuit has been preferred

to fit the obtained impedance data and result showed that Rct value of BCPE was found to be 181.3 kΩ (open circuit potential (OCP) = 0.2891 V and error = 0.421) and for IOCPE it was 34.06 kΩ (OCP = 0.2931 V and error = 0.3857) Decrease in the diameter of semicircle was observed for IOCPE due to the fast-interfacial electron transfer process and low resistance value Low resistance value of IOCPE indicates the presence of ION’s in carbon paste accelerates the electron transfer process and good electrical conductivity The smaller semicircle diameter was viewed due to synergistic effect of ION’s The synergistic consequence could significantly develop the interfacial electron transfer capability and redox reaction of probe on the electrode surface This indicated the presence of mediator ION’s on CPE played a significant role in increasing the charge transfer capability [46-48]

0 1 2 3 0.0

-0.4 -0.8 -1.2

-1.6

b-BCPE

(frequency range from 0.1 to 103 kHz, inset: Randle’s equivalent circuit)

3.5 Voltammetric behaviors of PA and DA at BCPE and IOCPE

The BCPE and IOCPE electrodes were used for the investigation of PA signal by cyclic

voltammetry The curve a depicts the cyclic voltammogram (CV) of blank solution at IOCPE and curve b and c shows the cyclic voltammograms (CVs’) in presence of 1 mM PA in PBS (pH

7.0) at BCPE and IOCPE, respectively at a scan rate 100 mV s-1 (Fig 5) At IOCPE, PA exhibits

the quasi-reversible redox behavior with anodic (Epa) and cathodic peak potential (Epc) of 45 and

8.8 mV, respectively The significant enhancement of anodic (Ipa) and cathodic peak currents

(Ipc) provides a clear evidence for the catalytic effect of IOCPE towards the redox behavior of

PA [44,49] After 120 days, the same procedure has been performed for the determination of PA

and DA using the same IOCPE The curves e and f show CVs’ for blank solution and 1 mM DA

at BCPE, respectively and curve d and g represents the redox behavior of 1 mM PA and 1 mM

DA at IOCPE, respectively in PBS (pH 7.0) at scan rate 100 mV s-1 (Fig 5) The results have

0.0 0.3 0.6 0.9 -100

-50 0

Fig 5 CV’s of: (a) and (e) buffer at IOCPE; 1 mM PA at (b) BCPE ; (c) and (d) IOCPE; and 1

mM DA at (f) BCPE; (g) IOCPE at scan rate of 100 mV s-1( (d), (e), (f) and (g) obtained at IOCPE after 120 days)

3.6 Effect of scan rate and pH

The influence of scan rate on peak currents (Ip) of PA at IOCPE was investigated by

cyclic voltammetry Fig 6(a) shows the voltammetric response of 1 mM PA at IOCPE at

different scan rate of 50 to 350 mV s-1 The redox peak current increases linearly with increasing

in the scan rate and the reduction peak shifted towards more negative potential where as the oxidation peak shifted towards more positive potential which indicates the diffusion-controlled kinetics of electron transfer reaction at the electrode surface The linear regression equations

were obtained from the graph Ipa and Ipc vs ν1/2 (square root of scan rate) as follows; Ipa = -11.11

× 10-6 + 6.892 ν1/2, (R2 = 0.999 ; N=12); Ipc = 22.85 × 10-6 - 5.516 ν1/2, (R2 = 0.9977 ; N=12) for

anodic and cathodic process, respectively which indicates the reaction of PA at IOCPE was diffusion controlled [49] The linear relationship between log Ipa vs log ν can be obtained as follows; log Ipa = -5.383 + 0.572 log ν(mV s-1), (R2 = 0.9993 ; N=12) The slope value 0.57 is very close to the theoretical value 0.5 and confirmed the reaction at the electrode surface was diffusion controlled [50,51] For comparison, the scan rate from 50 to 250 mV s-1 was performed for PA and DA individually using long stored working electrode (IOCPE) The redox peak currents of both PA and DA were obtained and found to be increased linearly with increase in

scan rate (Fig 6(b) and 6(c)) The linear correlation equations have been given by: Ipa = -28.07 ×

10-6 + (-4.87 × 10-6) v1/2, (R2 = 0.9965 ; N=9); Ipc = -27.85 × 10-6 + 3.503 × 10-6 v1/2, (R2 = 0.9993

; N=9) for PA and Ipa = -5.8 × 10-6 + (-3.21 × 10-6) v1/2, (R2 = 0.9937 ; N=9); Ipc = -14.49 × 10-6 + 2.56 × 10-6 v1/2, (R2 = 0.9984 ; N=9) for DA The plots of log Ipa vs log ν for PA and DA as follows; log Ipa = -4.744 + 0.417 log ν(mV s-1), (R2 = 0.9917 ; N=9) and log Ipa = -5.237 + 0.41 log ν(mV s-1), (R2 = 0.9903 ; N=9), respectively The slope values for PA and DA have been found to be 0.417 and 0.41, respectively and these confirmed the redox process of PA and DA at IOCPE were diffusion controlled [42,50,51] The CVs’ were performed in the scan rate from 50

to 225 mV s-1 for the determination of number of electrons transferred and electron transfer

coefficients in PA and DA The linear regression equations (Epa and Epc vs log ν) for PA and DA

are as follows: Epa (PA) = 0.3438 + 0.08 log ν (R2 = 0.9948); Epc (PA) = 0.2512 – 0.09 log v (R2

= 0.9927) and Epa (DA) = 0.254 + 0.061 log v (R2 = 0.9969); Epc (DA) = 0.2431 – 0.078 log v (R2

= 0.9927) According to Laviron’s equation slopes of line obtained from the above linear

regression equations have been found to be 2.303RT/(1-α)nF and -2.303RT/αnF, where R is gas

constant (8.314 J K-1 mol-1), T is temperature (298 K), F is Faraday’s constant (96,485 C), n and

α are the transfer number and transfer coefficient, respectively The

electron 0.2 0.0 0.2 0.4 0.6 0.8 -120

-100 -80 -60 -40 -20 0 20 40

1.6 1.8 2.0 2.2 2.4 -4.48

-4.40 -4.32 -4.24 -4.16 -4.08 -4.00

R 2 = 0.9903

-100 -50 0 50 100

l a

-100 -50 0 50

(inset: linearity plot of current vs scan rate and log current vs log scan rate)

(a)

Fig 6(b) and 6(c) CV’s of individual scan rates of 1 mM PA and 1 mM DA, respectively at

different scan rate from 50 to 250 mV s-1 (a to i) in 0.1 M PBS (pH 7.0) at long stored IOCPE (inset: linearity plot of PA and DA from log peak current vs log scan rate)

The square wave voltammetric method (frequency 15 Hz; amplitude 25 mV) has been employed to investigate the effect of pH on electrochemical behavior of PA (20 µM) and DA (50

µM) in 0.1 M PBS Fig 7 shows the variation of peak currents at different pH and inset plot

shows the variations of peak potentials with pH The shift in peak potentials towards negative side with increase in pH indicates the involvement of protons in the reaction The increase in peak current with increase in pH (from 4.0 to 6.0) in case of PA is attributed to the formation of p-benzoquinone The maximum peak current obtained at pH 7.0 is due to the formation of N-acetyl-p-quinone-imine which exists in unprotonated stable form [52,53] At higher pH (above 8.0) the peak current found to be decrease and is due to the dimerization of N-acetyl-p-benzoquinone-imine [54] The above results clearly indicated that at pH 7.0 there is a good electrostatic interaction exists between unprotonated PA and IOCPE which in turn responsible for the fast electron transfer reaction Hence PBS of pH 7.0 has been selected as an appropriate standard electrolytic medium for further studies [55] The square wave peak potential for PA has

a linear relationship with pH (from 4.0 to 8.0) and corresponding regression equation has been

given by the following equation: Ep (V) = 0.739 - 0.050 pH (R2 = 0.9993) Further from pH 4.0 to 6.0, the square wave peak potential of DA found to be shifted towards negative side and at pH 7.0 a rapid increase in peak current was observed Above pH 8.0 the peak potential again shifted towards negative side with decrease in peak current may be self-polymerization of DA into polydopamine in alkaline medium [56] The corresponding linear regression equation for DA is

as follows: Ep (V) = 0.562 – 0.0558 pH (R2 = 0.9993) The slope value for PA and DA were

found to be 50 and 55 mV pH-1 respectively and this was nearly equal to theoretical value of 59

mV pH-1 which is in good agreement with the Nernst equation These results indicated that the electrochemical redox process of PA and DA at IOCPE involves two protons and two electrons transfer reactions [52,57] The probable redox reaction mechanisms of PA and DA have shown

in (Scheme 1 Supplementary)

2 4 6 8 10

0.1 0.2 0.3 0.4 0.5

amplitude 25 mV; inset: linearity plot of pH vs potential)

3.7 Effect of concentration

The DPV method has been employed for the electrochemical detection of PA at IOCPE

Fig 8 shows the differential pulse voltammograms (DPVs’) for different concentration of PA (1

µM to 14 µM) in 0.1 M PBS at pH 7.0 The oxidation peak current was directly proportional to the concentration of PA and has been increased linearly with increase in the concentration The linear regression equation for PA is as follows: Ip(µA) = -7.65×10-7 - 2.19×10-2 C(µM) and

correlation coefficient R2 = 0.9994 The limit of detection (LOD) can be detected by using the

formula LOD = 3s/m where s indicates the standard deviation of first five runs and m indicates

slope obtained from linearity graph (concentration vs current) and LOD has been found to be 0.904 The IOCPE has been shown enhanced performance for electrochemical determination of

PA After 120 days the above experimental procedure has been repeated for the analysis of PA

and DA using the same IOCPE Fig 9(a) shows DPVs’ of PA keeping the concentration of DA

at constant and Fig 9(b) represents DPVs’ of DA keeping the concentration of PA at constant

Fig 9(c) shows DPVs’ for the simultaneous estimation of PA and DA The concentration of PA

has been varied from 2 to 150 µM keeping DA concentration at 15 µM and the regression

equation is given by: Ipa (PA) = 7.74 × 10-7 + 1.7 × 10-2 C (µM) with correlation coefficient R2 = 0.9982 Similarly, the DA concentration has been varied from 2 to 170 µM keeping PA at 14 µM

and the corresponding regression equation is as follows: Ipa (DA) = 1.45 × 10-6 + 2.49 × 10-2

C (µM) with correlation coefficient R2 = 0.989 From the regression equations, the LOD has been

calculated as 1.16 and 0.79 µM (S/N=3) for PA and DA respectively It was observed that the

change in concentration of one species does not alter the peak potential and current of other species Further, the concentrations of both PA and DA have been varied from 1 to 170 µM and they exhibited the good separation peaks with the following regression equations: 9.35 × 10-7 + 1.367 × 10-2 C (µM) (R2 = 0.9924) for PA and 1.44 × 10-6 + 1.81 × 10-2 C (µM) (R2 = 0.9786) for

DA From the above equations the LODs were found to be 1.44 and 1.09 µM and these values are in good agreement with the individual determination of PA and DA The above results shows that the modified electrode IOCPE exhibited good and acceptable LOD and LR values compared

to other modified electrodes reported in literature (Table 1 -Supplementary)

-0.2 0.0 0.2 0.4 0.6 0.8 -0.35

-0.30 -0.25 -0.20 -0.15 -0.10 -0.05

0.5 1.0 1.5 2.0 2.5 3.0 3.5

R 2 = 0.9982

Fig 8 DPV plots at IOCPE of different concentration from 1 to 14 µM (a to l) obtained at

IOCPE in 0.1 M PBS at pH 7.0; (inset: linearity graph of current vs concentration)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 -1.3

-1.2 -1.1 -1.0 -0.9 -0.8 -0.7

2 4 6 8 10 12 14 -0.80

-0.85 -0.90 -0.95 -1.00 -1.05 -1.10