Electrochemical detection and degradation of ibuprofen from water on multi-walled carbon nanotubes-epoxy composite electrode

JOURNAL OFENVIRONMENTALSCIENCES

ISSN 1001-0742

CN 11-2629/X

www.jesc.ac.cn

Available online at www.sciencedirect.com

Journal of Environmental Sciences 2013, 25(4) 838–847

Electrochemical detection and degradation of ibuprofen from water onmulti-walled carbon nanotubes-epoxy composite electrode

Sorina Motoc1, Adriana Remes1, Aniela Pop1, Florica Manea1,∗, Joop Schoonman2

1. Politehnica University of Timisoara, P-ta Victoriei No.2, 300006 Timisoara, Romania. E-mail: [email protected]. Materials for Energy Conversion and Storage, Department of Chemical Engineering, Delft University of Technology,

Julianalaan 136, 2626 BL, Delft, The Netherlands

Received 30 June 2012; revised 05 November 2012; accepted 06 November, 2012

AbstractThis work describes the electrochemical behaviour of ibuprofen on two types of multi-walled carbon nanotubes based composite

electrodes, i.e., multi-walled carbon nanotubes-epoxy (MWCNT) and silver-modified zeolite-multi-walled carbon nanotubes-epoxy

(AgZMWCNT) composites electrodes. The composite electrodes were obtained using two-roll mill procedure. SEM images of surfaces

of the composites revealed a homogeneous distribution of the composite components within the epoxy matrix. AgZMWCNT composite

electrode exhibited the better electrical conductivity and larger electroactive surface area. The electrochemical determination of

ibuprofen (IBP) was achieved using AgZMWCNT by cyclic voltammetry, differential-pulsed voltammetry, square-wave voltammetry

and chronoamperometry. The IBP degradation occurred on both composite electrodes under controlled electrolysis at 1.2 and 1.75 V vs.

Ag/AgCl, and IBP concentration was determined comparatively by differential-pulsed voltammetry, under optimized conditions using

AgZMWCNT electrode and UV-Vis spectrophotometry methods to determine the IBP degradation performance for each electrode.

AgZMWCNT electrode exhibited a dual character allowing a double application in IBP degradation process and its control.

Key words: multi-walled carbon nanotubes composite electrodes; electrochemical oxidation; electrochemical detection; ibuprofen

DOI: 10.1016/S1001-0742(12)60068-0

Introduction

In the recent years, there has been increasing concern

about the presence of pharmaceuticals in water. These

compounds are included within the class of emerging

pollutants. Their presence in water can be explained in

terms of both their use in medicine and the inefficiency

of water treatment systems (Jones et al., 2005; Fent et al.,

2006; Miege et al., 2009; Santos et al., 2010).

Ibuprofen (IBP) is the third most popular drug in the

world, non-prescription, non-steroidal drug used as an

anti-inflammatory analgesic and antipyretic in the human

treatment of fever, migraine, muscle aches, rheumatoid

arthritis, tooth aches, and osteoarthritis (Jones et al., 2005;

Fent et al., 2006). IBP presence in water requires viable

methods for its determination and removal and several

determination methods have been reported in the litera-

ture, e.g., ratio spectra (Issa et al., 2011), capillary zone

electrophoresis (Hamoudova and Pospısilova, 2006), spec-

trophotometry (Hassan, 2008; Khoshayand et al., 2008),

HPLC (Mendez -Arriaga et al., 2008), and ultrasonic

* Corresponding author. E-mail: [email protected]

methods (Whelan et al., 2002).

Recent progress of nanotechnology has created huge po-

tential to build highly sensitive, low cost, portable sensors

with low power consumption. Carbon nanotubes (CNTs)

represent very interesting nanomaterial, due to the various

novel properties that make them useful in the field of

nanotechnology and pharmaceuticals (Dresselhaus et al.,

2004; Merkoci, 2006; Shanov et al., 2006; Uslu and Ozkan,

2007; Endo et al., 2008). CNTs are the best conductor of

electricity on a nanoscale level, similar to copper, but with

the ability to carry much higher currents (Hamada et al.,

2005; Kim et al., 2006). Analytical chemistry seems to be

one of the sciences interested in several advantages which

are brought by CNTs for various potential applications,

e.g., chromatography (Cao et al., 2004; Cruz-Vera et al.,

2008), battery electrode materials (Morris et al., 2004),

sensors and biosensors (Dai et al., 2002; Gooding, 2005;

Sinha et al., 2006; Ahammad et al., 2009; Hirlekar et al.,

2009).

There are two main kinds of carbon nanotubes, single-

walled carbon nanotubes (SWCNTs) that are individual

cylinders of 1–2 nm in diameter, and multi-walled carbon

No. 4 Electrochemical detection and degradation of ibuprofen from water on multi-walled carbon nanotubes-epoxy composite electrode 839

nanotubes (MWCNTs) with diameters in the range of 2

and 100 nm, which are a collection of several concentric

graphene cylinders, where weak van der Waals forces bind

the tubes together. Since carbon-carbon covalent bonds are

among the strongest bonds in the nature, a structure based

on a perfect arrangement of these bonds oriented along the

axis of the nanotubes produces a very strong material with

an extremely high strength-to-weight ratio (Barisci et al.,

2000; Hamada et al., 2005; Kim et al., 2006).

The interesting properties of carbon nanotubes have

led to an explosion of research efforts worldwide. For

sensing applications, CNTs exhibit many advantages, such

as: small size with larger surface area, excellent electron

transfer promoting ability in electrochemical reactions,

and CNTs play an important role in the performance

of electrochemical sensors. Therefore, a large number of

relevant publications can be found in the literature, especial

for the analytical applications (Wang and Musameh, 2004;

Merkoci et al., 2005; Pumera et al., 2006; Ye and Sheu,

2006; Varghese et al., 2010; Yanez-Sedeno et al., 2010).

Zeolite-modified electrodes have been largely devel-

oped during the past two decades because they combine

the essential properties of the zeolites (mainly their

ion-exchange capacity and their size selectivity at the

molecular level) with electron-transfer reactions. Zeolite-

modified carbon based electrodes have been largely and

successfully applied for electrooxidation and detection

purposes (Manea et al., 2008; Pop et al., 2008; Arvand et

al., 2009; Manea et al., 2010).

In the last decades, modern computer-based voltammet-

ric techniques have been used to realize the determination

of organic chemicals in various types of samples, espe-

cially pharmaceutical field (Ye et al., 2004; Ly, 2006;

Abbaspour and Mirzajani, 2007; Yogeswaran et al., 2007;

Manisankar et al., 2009; Shahrokhiana et al., 2009; Ensafi

et al., 2011). The advance in experimental electrochemical

techniques in the field of drug analysis is due to their

simplicity, low cost, and relatively short analysis times,

no need for derivatizations or time-consuming extraction

steps.

Moreover, the electrochemical methods possess the

dual character in relation with both pollutants detection

and destruction. Several electrochemical methods for the

destruction of pharmaceuticals have been reported (Gar-

rido et al., 2007; Guinea et al., 2008; Murugananthan

et al., 2008). These methods use the electron as the

main reagent, but also require the presence of support-

ing electrolytes. In general, the supporting electrolytes

exist in the wastewaters to be treated, but not always in

sufficient concentrations. These processes can operate at

ambient temperature without a need of temperature control

(Koparal et al., 2007). The applications of electrochemical

technologies for wastewater treatment are benefiting taking

into account their advantages, e.g., versatility, environmen-

tal compatibility and potential cost effectiveness among

others (Bensalah et al., 2009). However, the electrode

material is the most important parameter in the elec-

trochemical process and represents the key of process

performance.

The aim of this article is to explore the dual character of

the electrochemical methods for ibuprofen (IBP) detection

and degradation on two types of multi-walled carbon

nanotubes based composite electrodes, i.e., multi-walled

carbon nanotube-epoxy (MWCNT) and silver-modified-

zeolite-multi-walled carbon nanotube-epoxy (AgZMWC-

NT) composites electrodes. Cyclic voltammetry technique

was used to characterize the electrochemical behaviour

of IBP on both composite electrodes. Cyclic voltammetry

(CV), differential-pulsed voltammetry (DPV), square-

wave voltammetry (SWV) and chronoamperometry (CA)

were applied for the electrochemical determination of IBP

at AgZMWCNT electrode. The degradation studies were

performed on both composite under controlled electrolysis

at 1.2 and 1.75 V vs. Ag/AgCl, and the control of the

IBP degradation process was performed comparatively by

optimized SWV at AgZMWCNT electrode, UV-Vis spec-

trophotometry and total organic carbon (TOC) parameter.

1 Experimental

1.1 Preparation of electrodes

The dispersion of MWCNTs in tetrahydrofuran, 99.9%

(THF, Sigma Aldrich) was achieved by ultrasonication

using a Cole-Parmer� 750-Watt Ultrasonic Processor for

about 10 min prior to mixing with the polymer resin. After

the sonication process, the solution of MWCNTs/THF

was sonicated again with epoxy resin to obtain a more

homogeneous mixture. An effective method based on the

two roll mill (TRM) of achieving high levels of dispersion

and distribution was used to prepare the both electrodes.

The ratio between the components was chosen to reach 20

wt.% content of MWCNT and respective 20 wt.% content

of silver-modified zeolite (AgZ). During processing the

temperature was kept constant at 70°C, the mixing speed

was maintained at 10 and 20 r/min for about 40 min, after

then the curing agent (weight ratio of epoxy resin:curing

agent was 100:38) was added to MWCNT- resin mixture,

and mixing was continued for an additional 20 min to

ensure a uniform dispersion within the sample. The same

procedure was applied for AgZMWCNT electrode. The

mixture was then poured into PVC tubes and cured in a

vacuum oven at 80°C for 24 hr, after which it was left to

cool down at room temperature, and the composite elec-

trode with disc surface area of 19.63 mm2 was obtained.

The electrical contacts of the electrodes were assured using

copper wire.

1.2 Instrumental and measurement

The electrical conductivity of each composite material

was determined by a four-point resistance measurement

840 Journal of Environmental Sciences 2013, 25(4) 838–847 / Sorina Motoc et al. Vol. 25

(Mironov et al., 2007). A Scanning Electron Microscope

(Philips CM30T) was used to examine morphologically

the working electrode surface. BET specific surface area

analysis of the composite electrodes was carried out by

N2 adsorption at 77 K. Prior to the adsorption runs, the

samples were subjected to outgassing under vacuum at the

temperature of 333 K. The adsorption measurements were

performed on a Quantachrome Nova 1200e analzyer.

The electrochemical behaviours of AgZMWCNT and

MWCNT electrodes with the disc geometry in the presence

of IBP were studied by cyclic voltammetry.

The electrochemical detection experiments were per-

formed using differential-pulsed voltammetry (DPV),

square-wave voltammetry (SWV), and chronoamperome-

try (CA).

The electrochemical degradation of IBP was carried out

under potentiostatic conditions by CA at potential values

of +1.2 V and +1.75 V vs. Ag/AgCl.

Prior to use, the working electrodes were gradually

cleaned, first polished with abrasive paper and then on

a felt-polishing pad by using 0.5 μm alumina powder

(Metrohm, Switzerland) in distilled water for 5 minutes

and rinsing with distilled water. In addition, an electro-

chemical pre-treatment by three repetitive cycling between

–0.5 V to 1.5 V vs. Ag/AgCl in 0.1 mol/L Na2SO4

supporting electrolytes was performed.

The electrochemical detection method validation was

achieved by comparison with UV-Vis spectrophotometric

determination.

The assessment of the IBP electrochemical oxidation

was performed by IBP concentration monitoring using the

proposed electrochemical method. Also, the total organic

carbon (TOC) parameter was monitored to evaluate the

mineralization degree of IBP. The performance of the

IBP electrooxidation process was assessed in terms of

degradation degree ( ηIBP) and electrochemical efficiency

(EIBP, mg C/cm2) using Eqs. (1) and (2).

ηIBP =(CIBP0 −CIBP)

CIBP0

× 100 (1)

EIBP =(CIBP0 −CIBP)

Q × S× V (2)

where, CIBP0 – CIBP represents the change in the

IBP concentration determined by SWV voltammetry

at AgZMWCNT composite electrode, verified also by

UV-Vis spectrophotometric method. Q represents charge

consumption corresponding to applied electrolysis time, V(700 cm3) is the sample volume and S (cm2) is the area of

the electrode surface.

Also, the mineralization degree and electrochemical

efficiency was determined. The degree and electrochemical

efficiency for IBP mineralization defined as an overall

efficiency for complete oxidation to CO2 was determined

based on Eqs. (1) and (2) taking into consideration the

change in TOC measurements during experiments, which

means using TOC0–TOC instead of CIBP0–CIBP.

UV-Vis spectrophotometric and TOC measurements

were performed using Varian Cary 100 UV-Vis spec-

trophotometer and respective, Shimadzu TOC analyzer.

All electrochemical measurements were carried out us-

ing an Autolab potentiostat/galvanostat PGSTAT 302 (Eco

Chemie, The Netherlands) controlled with GPES 4.9 soft-

ware and a three-electrode cell, with a Ag/AgCl reference

electrode, a platinum counter electrode and AgZMWCNT

and MWCNT as working electrodes.

Standard solution of 1 g/L IBP was prepared from ana-

lytical reagent from BASF SE (Ludwigshafen, Germany)

using distilled water and 0.1 mol/L NaOH solution. The

0.1 mol/L Na2SO4 solutions supporting electrolytes for

the characterization and application of electrodes materials

were freshly prepared from Na2SO4 and NaOH of analyti-

cal purity (Merck) with distilled water.

2 Results and discussion

2.1 Morphological and electrical behaviours of thecomposite electrodes

Figure 1 shows the molecular structure of IBP. Figure2 shows the comparative scanning electron microscopy

(SEM) images of the MWCNT and AgZMWCNT com-

posites with 20 wt.% of MWCNTs and respective, 20

wt.% of AgZ, proving a uniform distributions of both

components within the epoxy matrix. A more porous

structure is noticed for AgZMWCNT composite, which

was expected because of the presence of zeolite. A slight

increase of specific surface area was noticed also, for

AgZMWCNT (30.9 m2/g) in comparison with MWCNT

(26 m2/g). The Ag-modified zeolite affected slightly the

electrical conductivity of the composite, the value of about

1.177 S/cm was determined for AgZMWCNT compos-

ite, which is two times higher than MWCNT composite

electrical conductivity (0.596 S/cm). Even if the zeolite is

an insulating material, the enhancement of the electrical

conductivity due to its presence should be attributed to the

silver content within silver-modified zeolite.

2.2 Electrochemical behaviour of ibuprofen

Prior to the studies regarding the electrochemical be-

haviour of IBP on both MWCNT and AgZMWCNT com-

posites electrodes, their electroactive surface areas were

determined by classical potassium ferricyanide method.

CH3

CH3

H3C

COOH

Fig. 1 Molecular structure of ibuprofen.


Acc.V Spot Magn Det WD Exp

15.0 kV 4.0 2500x SE 18.2 110 μm

Acc.V Spot Magn Det WD Exp

15.0 kV 4.0 2500x SE 5.5 110 μm

a b

Fig. 2 SEM imaging of the cross-section of electrode surface: MWCNT (a); AgZMWCNT (b).

This method was achieved by cyclic voltammetry in 1

mol/L KNO3 supporting electrolyte recorded at different

scan rates (the CVs results are not shown here). Based on

Randles-Sevcik equation (Konopka and McDuffle, 1970):

Ip = 2.69 × 105AD1/2n3/2v1/2C (3)

where, A (cm2) represents the area of the electrode, nis the number of electrons participating in the reaction

and is equal to 1, D is the diffusion coefficient of the

molecule in solution, C is the concentration of the probe

molecule in the solution (4 mmol/L), and v (V/sec) is the

scan rate, the electroactive surface area of 0.443 cm2 was

calculated for MWCNT composite electrode and 0.473

cm2 for AgZMWCNT composite electrode.

Figure 3 presents the cyclic voltammograms (CVs)

recorded at AgZMWCNT and MWCNT composites elec-

trodes in 0.1 mol/L Na2SO4 supporting electrolyte and

in the presence of various IBP concentrations. It must be

noticed the occurrence of the oxidation and the reduction

processes at the potential value of about +0.315 V/sec and

respective, +0.065 V/sec of multi-walled carbon nanotubes

in 0.1 mol/L Na2SO4 supporting electrolyte (curve 1 from

Fig. 3). The presence of silver-modified zeolite into the

electrode composition led to higher capacitive current and

the lower potential value corresponding to the oxygen

evolution, which are common features for the electrode

materials that exhibit the electrocatalytic activity.

CVs recorded on MWCNT electrode in the presence

of IBP (Fig. 3) showed that the anodic current increased

non-linearly with IBP concentration without appearance of

peak corresponding to IBP oxidation. Taking into account

the chemical structure of IBP that is benzene derivate,

it is well-known that a complex oxidation process of

benzene occurred on carbon-based electrodes, which is

in direct relation with the potential value of the oxygen

evolution (Kim et al., 2000). The oxidation process of the

aromatic organic compounds involves both the adsorption

of reactant/intermediate or IBP oxidation products and

the formation of passive layer by electropolymerization.

This aspect was noticed during repetitive scanning (the

results are not shown here), when anodic current decreased

starting with the second scanning.

CVs recorded on AgZMWCNT electrode under the

-0.5 0.0 0.5 1.0 1.5

-0.1

0.0

0.1

0.2

0.3

0.4

E (V) vs. Ag/AgCl

I (m

A)

I (m

A)

1

62

-0.5 0.0 0.5 1.0 1.5

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10

0.00

0.02

0.04

0.06

0.08

0.10

0.12

y = 0.0031 + 0.01088x

R2 = 0.997

∆I

(mA

)

IBP concentration (mg/L)

E (V) vs. Ag/AgCl

1

6

MWCNT AgZMWCNT

Fig. 3 Cyclic voltammograms recorded in 0.1 mol/L Na2SO4 supporting electrolyte (curve 1) and in the presence of various IBP concentrations (5–50

mg/L for MWCNT and from 2–10 mg/L for AgZMWCNT) at the electrodes. Conditions: potential scan rate of 0.05 V/sec; potential range: –0.5 to +1.5

V vs. Ag/AgCl; inset: calibration plots of the current vs. IBP concentration of the CVs recorded AgZMWCNT electrode at: E = +1.2 V vs. Ag/AgCl.


similar conditions (Fig. 3) show that the anodic currents

recorded at the potential value of +1.2 V vs. Ag/AgCl in-

creased linearly with IBP concentrations, which informed

about a possible diffusion controlled-electrooxidation of

IBP on AgZMWCNT composite electrode.

Based on the comparative results relating the elec-

trochemical behaviours of IBP on both electrodes,

AgZMWCNT composite electrode exhibited better perfor-

mance regarding IBP electrooxidation, and it was selected

for further electrochemical studies.

In order to investigate more detailed the mechanistic

aspects of IBP oxidation on AgZMWCNT electrode, the

effect of scan rate on the cyclic voltammogram shapes was

studied. Figure 4 shows the CVs recorded on AgZMWC-

NT electrode at the scan rate ranged from 0.01 to 0.3

V/sec, and it can be noticed that the anodic peak currents

increased linearly with the square root of the scan rates.

The slope of the curve obtained for the potential value of

+1.2 V is about 1. This result indicated that the electro-

catalytic oxidation of IBP on AgZMWCNT electrode is a

diffusion-controlled oxidation process, and the adsorption

phenomena can not be neglected (no zero intercept).

2.3 Detection measurements

The electroanalytical procedures were developed using

cyclic voltammetry (CV), differential-pulsed voltammetry

(DPV), square-wave voltammetry (SWV), and chronoam-

perometry (CA).

Based on the CV results (see Fig. 3 for AgZMWCNT,

the previous section), the linear dependence of the oxida-

tion currents versus IBP concentration was achieved at the

potential value of +1.2 V vs. Ag/AgCl with a correlation

coefficient better than 0.99. The electroanalytical parame-

ters achieved by this technique are gathered in Table 1.

Differential-pulsed voltammetry (DPV) has been em-

ployed as a very highly sensitive and fast electrochemical

method for the evaluation of the performance of the

AgZMWCNT electrode for the IBP assessment. The ef-

fects of DPV working parameters on the response of the

AgZMWCNT electrode have been studied under different

modulation amplitude, step potential and corresponding

scan rate, scanning the potential from +0.5 to +1.2 V

(vs. Ag/AgCl) (Fig. 5) to achieve the best performance of

the electrode. The linear dependences of the peak current

recorded at +1.1 V vs. Ag/AgCl with IBP concentrations

were achieved for the IBP concentrations ranged from

1 to 10 mg/L. The best electrode sensitivity for IBP

determination using the optimum DPV condition (0.0511

mA/(mg·L)) was obtained for modulation amplitude of 0.2

V, with an effective scan rate of 0.025 V/sec and step

potential of 0.025 V. The sensitivities, the lowest limit

of detection, the limit of quantification and the relative

standard deviation determined using DPV technique for

three replicates are presented in Table 1.

Square-wave voltammetry was also applied in order to

improve the electroanalytical parameters for IBP detection.

Based on the optimum conditions determined for DPV

in relation with the modulation amplitude and the scan

rate, the effect of the frequency was studied (the results

are not shown here) and 10 Hz was found as the best

frequency, which was used for IBP detection. The anodic

peak current observed at +1.1 V vs. Ag/AgCl increased

linearly with the IBP concentration in the range of 1–5

mg/L (Fig. 6a). The sensitivity for IBP detection under

these working conditions was 0.0239 mA/(mg·L) with

a correlation coefficient better than 0.96 (Fig. 6b). All

electroanalytical parameters obtained by SWV are shown

in Table 1.

Based on the voltammetric results, the chronoamperom-

etry was applied as the easiest electrochemical technique

used for IBP detection at the detection potential value

of +1.2 V vs. Ag/AgCl, which was applied continuously

using the standard addition method and the IBP concentra-

tion step of 2 mg/L (Fig. 7). The detection potential value

-0.5 0.0 0.5 1.0 1.5

-0.5

0.0

0.5

1.0

1.5

E (V) vs. Ag/AgCl

I (m

A)

1

8

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

0.6

y = -0.010 + 1.068x

R2 = 0.99

I (m

A)

v1/2 (V/sec)1/2

Fig. 4 (a) Cyclic voltammograms recorded at AgZMWCNT electrode in 0.1 mol/L Na2SO4 supporting electrolyte in the presence of 10 mg/L IBP at

different scan rates: 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.3 V/sec (curves 1–8); (b) anodic peak current vs. square root of the scan rate.


0.00.2

0.4 0.60.8

1.01.2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.01

0.02

0.03

0.040.050.06

∆I

(mA

)

Ste

p p

ote

nti

al (

V)

E (V)

1

2

3

4

Fig. 5 Differential-pulsed voltammograms recorded in 0.1 mol/L

Na2SO4 supporting electrolyte and in the presence of 10 mg/L IBP

concentration under different conditions. Line 1: MA = 0.05 V, SP = 0.01

V; line 2: MA = 0.1 V, SP = 0.01 V; line 3: MA = 0.1 V, SP = 0.025

V; line 4: MA=0.2 V, SP = 0.025 V, on AgZMWCNT electrode. MA:

modulation amplitude; SP: step potential.

was chosen based on the consideration to be in line with the

cyclic voltammetry, but slight more positive to avoid the

electrode fouling. It must be noticed that the sensitivity was

lower in comparison with voltammetric techniques (0.0025

vs. 0.0112 mA/(mg·L) for CV) (Inset of Fig. 7) probably

due to the electrode fouling occurred.

The lowest limit of detection (LOD) and the lowest limit

of quantification (LQ) were evaluated based on 3SB/b, and

respective 10SB/b, where SB is the standard deviation of

the mean value of three voltammograms/amperograms of

the blank and b is the slope of the straight line in the ana-

lytical curve obtained by each voltammetric/amperometric

technique (Codognoto et al., 2004). The reproducibility of

the electrode using the above-mentioned techniques was

evaluated for three replicates measurements of IBP detec-

tion for each technique. The relative standard deviation

(RSD) of maximum 3.29% showed the good reproducibil-

ity of the electrode.

The best electroanalytical parameters for IBP determi-

nation was achieved for differential-pulsed volttammetry

technique under optimized conditions of modulation am-

plitude of 0.2 V, with an effective scan rate of 0.025 V/sec

and step potential of 0.025 V, the electrochemical method

selected for the validation.

A recovery test was also performed by analyzing three

parallel tap water samples, which contain 2 mg/L IBP. This

test was run in 0.1 mol/L Na2SO4 supporting electrolyte

and a recovery of 98.5% was found with a RSD of 2.5%

using DPV under optimized conditions. Finally, the results

obtained by this method were compared with those ob-

tained by means of a UV-Vis spectrophotometric method,

and can be concluded that the results obtained by the two

methods are very closely and the accuracy of the proposed

voltammetric method is excellent.

2.4 IBP degradation measurements

To investigate the dual character potential of the

AgZMWCNT electrode applying in both IBP detection

and degradation, the comparative electrochemical oxida-

tion of IBP on both MWCNT and AgMWCNT composite

electrodes was studied by controlled potential electrolysis

at room temperature, and the process performance was

assessed based on IBP concentration after electrolysis,

determined by DPV at the AgZMWCNT electrode under

previous-established optimized conditions.

The experiments were conducted at two different ap-

plied potentials in selected regions of the voltammogram,

at +1.25 V (vs. Ag/AgCl) just after the IBP second

voltammetric peak corresponding to the direct oxidation

and at +1.75 V under oxygen evolution range. Figure 8

0.6 0.8 1.0 1.2 1.41.75

1.80

1.85

1.90

1.95

2.00

2.05

2.10

2.15

2.20

2.25

2.30

2.35

2.40

I (m

A)

E (V) vs. Ag/AgCl

1

2

6

1 2 3 4 5 60.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

∆I

(mA

)


y = 0.039 + 0.0239x

R2 = 0.96

a b

Fig. 6 (a) SWV voltammograms recorded in 0.1 mol/L Na2SO4 supporting electrolyte (curve 1) and in the presence of various IBP concentrations

(1–5 mg/L) recorded at AgZMWCNT electrode (curve 2–6). Conditions: potential scan rate of 0.05 V/sec; potential range: –0.5 to +1.5 V vs. Ag/AgCl;

frequency = 10 Hz; amplitude = 0.1 V; step potential = 0.05 V, (b) calibration plots of the current vs. IBP concentration of the SWVs recorded on

AgZMWCNT electrode at: E = +1.1 V vs. Ag/AgCl.


Table 1 Electroanalytical performance of the AgZMWCNT electrode for the detection of ibuprofen

Used Peak Electrode Correlation LOD LQ RSD

technique potential sensitivity coefficient

(V) (mA/(mg·L)) (R2) (mg/L) (mg/L) (%)

CV +1.2 0.0112 0.97 0.55 1.84 1.76

DPV +1.1 0.0511 0.99 0.082 0.28 2.51

SWV +1.1 0.0239 0.96 0.31 1.05 0.13

CA +1.2 0.0025 0.97 0.84 2.8 1.64

LOD: lowest limit detection; LQ: lowest limit of quantification, RSD: relative standard deviation.

0 100 200 300 400 5000.00

0.05

0.10

0.15

0.20

0.25

0 2 4 6 8 10 12 14 16 180.00

0.01

0.02

0.03

0.04y = -0.0045 + 0.00252x

R2 = 0.97

∆I

(mA

)


I (m

A)

Time (sec)

2 mg/L IBP

Fig. 7 Chronoampero metric response of AgZMWCNT electrode in 0.1

mol/L Na2SO4 supporting electrolyte and in the presence of different IBP

concentrations (2–16 mg/L) recorded at E = +1.25 V vs. Ag/AgCl. Inset:

The calibration plots of the currents vs. IBP concentration.

shows the chronoamperometric responses recorded at both

composite electrodes at +1.25 and +1.75 V vs. Ag/AgCl. It

can be seen that the currents measured at the beginning of

the experiment were much higher for AgZMWCNT than

for MWCNT composite electrode and the current decay

with time was much steeper.

The dual character of AgMWCNT composite electrode

for IBP degradation and control was checked using DPV

analysis of the solution after the electrolysis at both

potential values for both composite electrodes, under pre-

viously discussed optimized conditions. The degradation

degree and electrochemical efficiency for IBP degradation

were assessed based on the IBP concentration determined

by DPV and compared with UV-Vis spectrophotomet-

ric method based on Relations (1) and (2). Also, the

mineralization degree and electrochemical efficiency were

determined by TOC parameter monitoring after elec-

trolysis. The results of the IBP electrooxidation process

0 1000 2000 3000 4000 5000 6000 70000.00

0.05

0.10

0.8

1.0

1.2

1.4

1.6

1.8

2.0

I (m

A)

Time (sec)

1

2

3

4

Fig. 8 Chronoamperometric responses recorded in 0.1 mol/L Na2SO4

supporting electrolyte and in the presence of 10 mg/L IBP concentration

at E = +1. 25 V vs. Ag/AgCl on MWCNT (curve 1) and AgZMWCNT

(curve 2) and at E = +1.75 V vs. Ag/AgCl on MWCNT (curve 3) and

AgZMWCNT (curve 4).

efficiencies are gathered in Table 2.

It must be noticed that AgZMWCNT composite elec-

trode exhibited the better electrooxidation and quite

mineralization performances for IBP degradation from

aqueous solution in comparison with MWCNT. The differ-

ence between IBP concentration and TOC evolutions with

the electrolysis time for both applied potentials (+1.25 V

and respective, +1.75 V) proved the existence of the two

oxidation stages. The slightly improved results in relation

with degradation and mineralization degrees were obtained

by the application of a higher electrolysis potential (+1.75

V vs. Ag/AgCl) for the same electrolysis time, but it

has to take into consideration that for lower potential

applied, lower specific electrical charge passed in the

electrochemical oxidation. This means that even if the

higher degradation and mineralization degree is reached by

electrolysis at higher potential value, the electrochemical

Table 2 Electrooxidation performance for the IBP degradation using chronoamperometry

Electrode Working ηdegradation Edegradation ηTOC ETOC

material conditions (%) (g/(C·cm2)) (%) (g/(C·cm2))

AgZMWCNT Eox = 1.25 V 78 2.30 60.84 2.58

AgZMWCNT Eox = 1.75 V 82 0.068 67.24 0.087

MWCNT Eox = 1.25 V 57 1.62 0 32.49 2.09

MWCNT Eox= 1.75 V 61 0.10 37.21 0.085


efficiency is lower, and the potential value must be chosen

based on certain practical requirements in relation with the

technical and economic criteria. Based on these presented

studies, the AgZMWCNT composite electrode was used

successfully for IBP degradation process control proving

dual character of this electrode for IBP degradation and

detection.

3 Conclusions

Two types of the composite electrodes, i.e., multiwalled-

carbon nanotubes – epoxy (MWCNT) and silver-modified

zeolite – multiwalled-carbon nanotubes (AgZMWCNT)

composites electrodes were successfully obtained using

two-roll mill procedure. SEM images of surfaces of the

composites revealed a homogeneous distribution of both

multiwalled carbon nanotubes and silver-modified zeolite

within the epoxy matrix. A better electrical conductivity

was determined using the standard four point probe for

AgZMWCNT composite, which exhibited also, a slight

higher specific surface area.

In comparison with the MWCNT, AgZMWCNT com-

posite electrode exhibited the electrocatalytic effect to-

wards the electrochemical oxidation of ibuprofen, envis-

aging both its degradation and determination.

The best electroanalytical parameters for IBP determi-

nation was achieved for differential-pulsed voltammetry

technique under optimized conditions of modulation am-

plitude of 0.2 V, with an effective scan rate of 0.025

V/sec and step potential of 0.025 V. The validation of

this electrochemical method by comparison with UV-

Vis spectrophotometric method informed about a good

accuracy of this method.

AgZMWCNT composite electrode exhibited the better

electrooxidation and quite mineralization performances of

IBP from aqueous solution in comparison with MWCNT

composite electrode.

AgZMWCNT electrode exhibited a dual character al-

lowing a double application in IBP degradation process

and its control by IBP monitoring using electrochemical

method.

Acknowledgments

This work was partially supported by the strategic grant

POSDRU/88/1.5/S/50783; POSDRU/21/1.5/G/13798;

POSDRU/89/1.5/S/57649 co-financed by the European

Social Fund – Investing in People, within the Sectoral

Operational Programme Human Resources Development

2007-2013 and partially by the PN II-RU-PD129/2010

and PN II Ideas 165/2011.

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Documents

Electrochemical detection and degradation of ibuprofen from water on multi-walled carbon nanotubes-epoxy composite electrode