Electrocatalytic determination of morphine at the surface of a carbon paste electrode spiked with a hydroquinone derivative and carbon nanotubes

Journal of Electroanalytical Chemistry 665 (2012) 45–51

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Electrocatalytic determination of morphine at the surface of a carbon paste electrode spiked with a hydroquinone derivative and carbon nanotubes M. Reza Shishehbore a, Hamid R. Zare b,⇑, Davood Nematollahi c a

Department of Chemistry, Islamic Azad University , Science and Research Branch, Tehran, Iran Department of Chemistry, Yazd University, P.O. Box 89195-741, Yazd, Iran c Department of Chemistry, Faculty of Science, University of Bou-Ali-Sina, Hamedan, Iran b

a r t i c l e

i n f o

Article history: Received 11 May 2011 Received in revised form 22 October 2011 Accepted 13 November 2011 Available online 22 November 2011 Keywords: Morphine Acetaminophen Simultaneous determination Multi-wall carbon nanotubes 4-Hydroxy-2-(triphenylphosphonio) phenolate

a b s t r a c t This study aims at the electrochemical characterization of a novel sensor for the electrocatalytic determination of morphine (MO). The sensor is based on a carbon paste electrode spiked with 4-hydroxy-2-(triphenylphosphonio)phenolate (HTP) and multi-wall carbon nanotubes (HTP-MWCNT-CPE). The cyclic voltammetric responses of MO oxidation at the modified electrode surface at different potential scan rates show a characteristic shape typical of an EC catalytic mechanism. Cyclic voltammetry, chronoamperometry, and differential pulse voltammetry (DPV) were used to probe the characteristics of the modified electrode. The catalytic peak current obtained by DPV was linearly dependent on the MO concentration over the range 1.0–950.0 lM in two linear segments with a detection limit of 0.066 lM. The precision of DPV was found to be 3% for 15 replicate determinations of 4.0 lM of MO. For a binary mixture containing MO and acetaminophen (AC), two well-distinguished differential pulse voltammograms were obtained in the physiological pH (pH 7.0). The sensitivities of the modified electrode toward MO in the absence and presence of AC were found to be virtually the same, which refers to the fact that the electrocatalytic oxidation processes of MO are independent of AC. The modified electrode was successfully applied for the determination of MO and AC in a urine sample and pharmaceutical formulations. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction Morphine, MO, (5a,6a didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol) is a phenolic compound and an alkaloid which can cause disruption in the central nervous system. It is frequently used to relieve severe pains in patients, especially those undergoing a surgical operation. It is recommended by the World Health Organization (WHO) for the relief of moderate cancer-related pains [1]. MO usage in the dose of 120 mg can be fatal. It has been reported that almost 10% of the excreted morphine remains unmetabolized and 90% of orally administrated morphine is excreted in urine within 24 h [2]. Therefore to prevent overdose-induced toxicity, it is important for clinical medicine to determine morphine concentration in urine sample. MO is the main constituent of opium that is the most important drug in the opiates group. Also, heroin is hydrolyzed in the organism into morphine and 6monoacetylmorphine that cannot be normally detected in biological fluids for its short half-life of approximately 30 min [3]. Therefore, the determination of the MO content of biological samples is useful for clinical and forensic purposes to prevent drug abuse [4,5]. In forensic cases, the analytical strategy for drug abuse ⇑ Corresponding author. Tel.: +98 351 8122669; fax: +98 351 8210991. E-mail address: [email protected] (H.R. Zare).

testing in human urine involves a two-stage procedure, initial (chromatographic or immunoassays) and confirmation (chromatography–mass spectrometry) tests [6]. Thin-layer chromatography is simple, rapid, and inexpensive, but it suffers from a lack of sensitivity and specificity. Although immunological assay methods, as another alternative technique for the initial test of morphine in urine sample, are simple and inexpensive, it suffers from a lack of sensitivity and specificity too due to specific (e.g., for drugs of abuse screening, false-positive interference may occur from medications or their metabolites that have similar chemical structures) [7] and non-specific (e.g., sample-induced changes in pH and ionic strength of the reaction mixture) interferences [8]. Since, MO is presented in the urine sample of patients and addicts at trace levels, a sensitive assay method is desirable. In order to improve the sensitivity of detection, high efficiency detectors such as mass spectrometry (MS) [9–14] and fluorescence [15–17] were applied. However, MS is an expensive technique, and fluorescence always needs critical derivatization reactions. Spectrophotometric detection systems, which are simple and inexpensive methods, can improve sensitivity but suffer from a limited linear range and/or a high limit of detection [18,19]. Preconcentration by solid-phase extraction (SPE) is another procedure to improve sensitivity in the analyses of complex matrix samples such as biological fluids in order to reduce the influence of interferences. This procedure,

1572-6657/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.11.018

M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51

2. Experimental

The buffer solutions were prepared from ortho-phosphoric acid and its salts. The MO injection solution as morphine sulfate ampoule (10 mg mL1) was obtained from Darou Pakhsh Co (Iran). The AC tablets (325 mg) were purchased from Kimidaru Co (Tehran, Iran). Voltammetric measurements were performed using a computerized potentiostat/galvanostat Autolab (model PGSTAT 30, Eco Chemie B.V.A) equipped with General Purpose Electrochemical System (GPES) 4.9 software. A three-electrode assembly was employed to the experiment in a 50 mL glass cell containing 4-hydroxy-2-(triphenylphosphonio)phenolate, HTP, multi-wall carbon nanotubes modified carbon paste electrode (HTP-MWCNT-CPE) as the working electrode, an saturated calomel electrode (SCE) as the reference electrode, and a graphite counter electrode. All of the potentials were measured and reported versus SCE as the reference electrode. A Metrohm 781 pH/ion meter was also used for pH measurements. 2.2. Electrode preparation 0.5 mg of HTP was hand mixed with two times its weight of MWCNT and 200 times its weight of graphite powder in a mortar with a pestle. Paraffin was added to the mixture using a 5 mL syringe and mixed for 20 min until a uniformly wetted paste was obtained. The HTP-MWCNT-CP electrode (HTP-MWCNT-CPE) was fabricated by packing the paste into the end of a Teflon rod (ca. 2 mm i.d. and 10 cm long). Then electrical contact was made by inserting a copper wire into the Teflon rod at the end of the mixture. When necessary, a new surface was obtained by pushing an excess of paste out of the tube and polishing it on a white paper. The multi-wall carbon nanotubes modified carbon paste electrode (MWCNT-CPE) and HTP modified CPE (HTP-CPE) were prepared in the same way without adding HTP and MWCNT to the mixture respectively. Moreover, unmodified carbon paste electrode (CPE) was prepared by mixing graphite powder and paraffin to obtain a wetted paste and fabricated as explained. 2.3. Sample preparation 2.3.1. Urine sample preparation Fresh human urine samples were obtained form different volunteers who had not taken MO and AC. Each sample was diluted 50 times by a 0.15 M phosphate buffer solution at pH 7.0 after filtering by Whatman filter paper (No. 1). The electrochemical

0.95

0.07 (h) (g)

I / µA

that has disadvantages such as being time-consuming and need of organic solvents, was used in different reported studies for morphine determination [9,12,13,15,19]. Electrochemical analysis, which is safe, rapid, simple, and low-cost, has become of growing importance in medicine and biotechnology, environmental monitoring, and different applications in industrial process control. In the last decade, a few reports can be found to have used electrochemical methods for morphine determination [20–30]. There is also an access to a review manuscript that reported analytical methodologies for the determination of morphine and its metabolites [31]. Acetaminophen (AC, N-acetyl-p-aminophenol or paracetamol), one of the drugs known as aniline analgesics and the most extensively employed drug in the world, is an antipyretic and analgesic drug. AC commonly used against mild fevers or moderate pains, is a major ingredient in numerous cold and flu remedies. Also, it is a non-carcinogenic drug and an effective alternate for aspirin for the patients who are sensitive to aspirin. AC metabolites, mainly generated in liver, are toxic. Therefore, accumulation of toxic metabolites resulting from an overdose ingestion of AC may cause severe nephrotoxicity and fatal hepatotoxicity. Since AC is widely used as a drug, the development of accurate and rapid methods for its measurements in biological fluids and pharmaceutical formulations is necessary [32]. So far, numerous methods have been reported for the analysis of AC in pharmaceuticals or in biological fluids. The methods include titrimetry [33] UV–vis spectrophotometry [34–36], quantitative thin-layer chromatography [37], high-performance liquid chromatography [38–40], and electrochemical methods [41–44]. In a recent research, the effects of AC on morphine side-effects were investigated. The results show that AC administration is not associated with a decrease in the incidence of MO-related adverse effects or an increase in patient satisfaction. Adding AC to patientcontrolled analgesia was associated with a MO-sparing effect of 20% over the first postoperative 24 h. On the other hand, AC combined with MO induced a significant MO sparing-effect [45]. Therefore, simultaneous determination of AC and MO is important. To the best of our knowledge, there is no report on the simultaneous determination of MO and AC by using a modified carbon paste electrode. To continue our studies on the fabrication of 4-hydroxy-2-(triphenylphosphonio)phenolate (HTP)/multi-wall carbon nanotubes modified carbon paste electrode (HTP-MWCNT-CPE) and its applications for the determination of different analytes [46–48], we have reported in this paper the application of HTP-MWCNT-CPE for the electrocatalytic oxidation of MO and the simultaneous determination of MO and AC. The results show that the modified electrode has important advantages such as wide linear concentration range, low detection limit, and good repeatability for MO determination. Also, HTP-MWCNT-CPE exhibits a noticeable ability for the simultaneous determination of MO and AC in pharmaceutical preparations and urine samples.

I / µA

46

(b) -0.01

0.45 (a) (d) (c)

2.1. Chemicals and apparatus All of the solutions were freshly prepared using double-distilled water. Morphine, MO, (Sigma) and acetaminophen, AC, (Merck) with analytical grades were used as received. 4-Hydroxy-2(triphenylphosphonio)phenolate (HTP) was synthesized as reported previously [49]. Graphite fine powder (Fluka) and paraffin oil (DC 350, Merck, density = 0.88 g cm3) were used as binding agents for the graphite pastes. The multi-wall carbon nanotubes (MWCNT) with a diameter of 10–20 nm, length of 5–20 mM, and purity of >95% were purchased from Nanolab Inc. (Brighton, MA).

0.03

0

0.6

E vs SCE / V

1.2 (f) (e)

(h) (g)

-0.05 0

0.35

0.7

1.05

E vs SCE / V Fig. 1. Cyclic voltammograms of (a) HTP-MWCNT-CPE, (c) HTP-CPE, (e) MWCNTCPE and (g) CPE in a 0.15 M phosphate buffer solution (pH 7.0). (b) as (a), (d) as (c), (f) as (e) and (h) as (g) in presence of 0.05 mM of MO. Scan rate: 25 mV s1.

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M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51

determination of MO using DPV technique was done just after spiking MO to urine sample.

1.2 (v / V s –1 )1/2 0

0.024

3. Results and discussion

1

0.8

I p / µA

2.3.2. Pharmaceutical sample preparation Three AC tablets (in dose of 325 mg) were powdered and mixed thoroughly. 1.50 mg of the powdered AC was transferred to a 1 L volumetric flask and dissolved with a phosphate buffer solution at pH 7.0. The MO injection solution was diluted 103 times with a 0.15 M phosphate buffer solution at pH 7.0.

Ip v –1/2 / µA V –1/2 s1/2

15 A

0.048 1.06

y = 4.3048x + 0.8605 R2 = 0.9959

9

0.96 b

a 3 0

Ip / µA

8

0.86 0.042

0.021

v / V s -1

Cyclic voltammetry was used to study the efficiency of MWCNT and HTP as a modifier for the electrocatalytic oxidation of MO. For this purpose, the cyclic voltammograms of unmodified CPE (CPE), MWCNT modified CPE (MWCNT-CPE), HTP modified CPE (HTP-CPE), and HTP-MWCNT-CPE were recorded in the absence and presence of 0.05 mM of MO. The voltammograms of g and h of Fig. 1 (or inset of Fig. 1) show the cyclic voltammograms of CPE in the absence and presence of MO. Similar voltammograms at MWCNT-CPE are shown in Fig. 1, curves e and f. A comparison of the cyclic voltammograms of CPE (Fig. 1, curves g and h) and MWCNT-CPE (Fig. 1 curves e and f) in a 0.15 M phosphate buffer (pH 7.0) at the scan rate of 25 mV s1 in the absence (curves e and g) and the presence of MO (curves f and h) demonstrates the efficiency of MWCNT for MO oxidation. As it can be seen, MO oxidation at the CPE surface and the MWCNTCPE surface was performed at potentials about 700 mV (curve h) and 520 mV (curve f) respectively. The results show that the oxidation peak potential of MO at MWCNT-CPE (curve f) shifts about 180 mV to less positive potentials as compared to that at CPE. Also, the sensitivity of MO determination increased due to using MWCNT. Curves (c) and (d) of Fig. 1 show the cyclic voltamograms of HTP-CPE in a MO-free electrolyte, 0.15 M of a phosphate buffer solution at pH 7.0 (curve c), and 0.05 mM of MO (curve d) at the scan rate of 25 mV s1. It is apparent that MO oxidizes at about 220 mV on HTP-CPE, while the anodic peak current for the oxidation of MO at MWCNT-CPE is about 520 mV. Also, the peak current dramatically increased. Therefore, as expected for a modifier, HTP caused to decrease the overpotential, 300 mV, and to increase the sensitivity. Under identical experimental conditions, the cyclic voltammograms of HTP-MWCNTCPE in the absence (curve a) and the presence of 0.05 mM of MO (curve b) were recorded. A comparison of the electrocatalytic oxidation current of MO at HTP-CPE (curve d), and HTP-MWCNTCPE (curve b) shows that the current response of MO oxidation significantly increases when MWCNT is used in the structure of the modified electrode. The above results confirm that a combination of MWCNT and HTP definitely improves the sensitivity and the electrocatalytic effect of HTP-MWCNT-CPE with regard to MO oxidation. In order to achieve the maximum sensitivity, the effect of MWCNT percent in the composition of carbon paste was investigated in the range 0.8–1.2%. The best sensitivity for MO determination was obtained when the carbon past was spiked with about 1% of MWCNT. Thus, this percent was used for fabrication of the modified electrode in all subsequent studies. Fig. 2 shows the cyclic voltammograms of HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) containing 0.08 mM of MO at different potential scan rates. As it can be seen in Fig 2, plot (a) of inset A, the anodic peak currents of MO oxidation were linearly proportional to the square root of the scan rate. This indicates that, at a sufficiently positive potential, the reaction is controlled by the diffusion of

E vs SCE / V

0.14

3.1. Electrocatalytic characteristic of morphine at HTP-MWCNT-CPE

0.4

B 0.13

y = 0.0759x + 0.2193 R2 = 0.9986 0.12 -1.32

-1.17

-1.02

Log I (I / µA)

0 0

0.32

0.64

0.96

1.28

E vs SCE / V Fig. 2. Cyclic voltamograms of HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) containing 0.08 mM of MO at different scan rates. Numbers of 1–8 correspond to potential scan rates of 5, 10, 15, 20, 25, 30, 35 and 40 mV s1. Insets: (A, curve a) variation of the electrocatalytic peak currents versus the square root of scan rate and (A, curve b) variation of the scan rate normalized peak current (Ip v1/2) versus scan rate. (B) Tafel plot derived from the current potential curve recorded at scan rate 5 mV s1.

MO. The number of electrons in the overall reaction can be obtained using the slope of IP versus v1/2 plot. Based on the following equation for totally irreversible diffusion controlled processes [50]:

Ip ¼ 3:01  105 n½ð1  aÞna 1=2 AC b v 1=2 D1=2

ð1Þ

and considering (1  a)na = 0.78 (as calculated below), D = 1.16  105 (that was obtained by chronoamperometry), and A = 0.0314 cm2, the total number of electrons (n) corresponding to MO oxidation is calculated as 1.9 (n = 1.9 ffi 2). The calculated value of n = 2 for the electrocatalytic oxidation of MO is in agreement with the values reported in the literature [20,28,30,51–54]. Also, the electrocatalytic oxidation mechanism of MO was investigated by plotting the scan rate normalized current (Ip v1/2) versus the scan rate (inset A of Fig. 2, curve b). As it can be seen, this plot exhibits a characteristic shape typical of an EC catalytic (EC0 ) mechanism. These results show that the overall electrochemical oxidation of MO at HTP-MWCNT-CPE might be controlled by the cross-exchange process operating between the redox sites of HTPMWCNT-CPE and the diffusion of MO. Andrieux and Saveant [55] developed a theoretical model for EC catalytic mechanism and derived a relationship between the peak current and the concentration of substrate for a case of a slow scan rate, v, and a large catalytic rate constant, k0 :

Icat ¼ 0:496nFAC s D1=2 v 1=2 ðRT=nFÞ1=2

ð2Þ

where Cs is the bulk concentration (mol cm3) of the substrate (in this case; MO) and other symbols have their conventional meanings. According to the approach of Andrieux and Saveant and using Fig. 1 in their theoretical paper, the average value of k0 was calculated to be (9.7 ± 2.9)  104 cm s1. To evaluate kinetic parameters, Tafel plot (Fig. 2, inset B) was drawn from the data of the rising part of the current–voltage curve recorded at a scan rate of 5 mV s1. This part of voltammogram, known as Tafel region, is

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M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51

affected by the electron transfer kinetics between the substrate (MO) and HTP-MWCNT-CPE assuming the deprotonation of the substrate as a sufficiently fast step [56]. In this condition, the number of electrons involved in the rate-determining step can be calculated from the slope of Tafel plot [56]. The obtained numerical value of 0.0759 V decade1 for Tafel slope indicate a one-electron transfer for a rate-limiting step, assuming a charge transfer coefficient, a, is 0.22. Moreover, the value of the exchange current density (j0), which was calculated from the intercept of Tafel plot [56], is equal to 0.041 lA cm2. Based on the above results, the electrocatalytic oxidation mechanism of MO at HTP-MWCNT-CPE surface can be proposed as the following equations:

HTP ! HTPox þ 2Hþ þ 2e ðEÞ 0

k

HTPox þ MO ! HTP þ MOox

ðC0 Þ

ð3Þ

3.3. DPV technique for quantification of morphine and acetaminophen

ð4Þ

The main objective of this study was to develop a modified electrode capable of electrocatalytic oxidation of MO and simultaneous determination of MO and AC. Differential pulse voltammetry (DPV), which has lower charging current contribution to the background current and a much higher current sensitivity than cyclic voltammetry [56] was used to estimate the linear range of MO and the lower limit of detection. The effect of increasing the MO concentration on the voltammetric response is presented in Fig. 4 (in range of 1.0 to 95.0 lM) and its inset A (in range of 95.0–195. 0 lM). Insets B and C of Fig. 4 clearly show that the plot of the corrected peak current (Icorr., considered as the difference between the peak current of the analyte solution and the background current) versus the MO concentration is constructed from two linear segments of 1.0–95.0 lM and 95.0–195.0 lM with slopes of 0.0452 lA lM1 and 0.0056 lA lM1 respectively. Comparison of the sensitivities of the two linear segments indicates a decrease of the sensitivity in the second linear range of the calibration plot. It is well known that with increase of an analyte concentration in the solution, the thickness of diffusion layer and thus mass transfer limitation are reduced [56]. Thus, it is logical to conclude that under these conditions the electron transfer kinetic between the analyte and the electrodeposited modifier at the electrode surface has main role for the current limitation. In

Here MO and MOox stand for morphine and its two-electron oxidized form respectively. 3.2. Chronoamperometric studies The chronoamperometric measurements of MO at the HTPMWCNT-CPE surface were done to estimate the apparent diffusion coefficient, Dapp, of MO [56] under worked experimental conditions. Fig. 3 shows the current-time profiles obtained by setting the working electrode potential at 220 mV for different concentra-

4.5 0.64

13

I / µA

A

tions of MO. At long enough experimental times (t > 0.1 s), where the electron transfer reaction rate of MO is more than its diffusion rate toward the working electrode surface, the current is diffusioncontrolled. Fig. 3, inset A, shows the experimental plots of I versus t1/2 with the best fit for different concentrations of MO employed. The slopes of the resulting straight lines were then plotted versus the MO concentration (Fig. 3, inset B). Based on Cottrell equation [56], the slope of this plot (Fig. 3 inset B) can be used to estimate of the apparent diffusion coefficient, Dapp, of MO. From the slope of this plot (11.644 lA s1/2 mM1), the value of Dapp was found to be 1.16  105 cm2 s1 for MO.

0.32

1 0 0.16

0.21

0.26

t–1/2/ s –1/2

1.6 5.2

2.4

y = 0.0452x + 0.0025 R2 = 0.9995

Icorr. / µ A

21

3 B

1.2 I / µA

0

y = 11.644x + 0.1273

I / µA

Slope / µA s 1/2

I / µA 1.5

6 A

36

B

R2 = 0.9996 0 0

0.1

0

50

100

[MO] / µM

2.6

0.8

10 y = 0.0056x + 3.7916 R2 = 0.9987

22

0.2

[MO] / mM 1

Icorr. / µ A

3

7 C

0 0.02

4 0.17

0.32

50

E / V vs SCE

1

0 0

33

66

99

t/s Fig. 3. Chronoamperometric current responses at HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0), at a step potential of 220 mV, for different concentrations of MO. Numbers of 1–13 correspond to 0.01, 0.02, 0.04, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.15 and 0.17 mM of MO. Insets: (A) plots of I versus t1/2 obtained from chronoamperograms and (B) plot of the slopes of the straight lines against the MO concentrations.

0 -0.3

550

1050

[MO] / µM

13

0

0.3

0.6

E / V vs SCE Fig. 4. Differential pulse voltammograms (DPVs) of HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) containing different concentrations of MO. Numbers of 1–21 correspond to 0.0–95.0 lM of MO. Inset A shows the similar DPVs for concentration range 95.0–950.0 lM of MO. Insets (B) and (C) show the plots of electrocatalytic peak current as a function of MO concentration in the ranges of 1.0– 95.0 lM and 95.0–950.0 lM respectively.

49

M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51

other words, decrease of the sensitivity (slope) of calibration plot in the high concentrations of the analyte (Fig. 4, inset C) is likely due to the electron transfer kinetic limitation in the chemical reaction of between the analyte, MO, and the redox form of the modifier, HTPox. This chemical reaction was shown in Eq. (4). The lower detection limit was obtained according to the equation Cm = 3sb/m [57], where sb is the standard deviation of the blank response (lA) and m is the slope of the calibration plot (0.0452 lA lM1). Through the data analysis, the value of MO lower detection limit proved to be 0.066 lM. The average voltammetric peak current and the precision estimated in terms of the coefficient of variation for 15 replicates (n = 15) of 4.0 lM of MO at HTP-MWCNT-CPE were 0.68 ± 0.02 lA and 3.0% respectively. The coefficient of variation value indicates that the modified electrode is stable and does not undergo surface fouling during voltammetric measurements. This also demonstrates the fact that the results obtained at the sensor are reproducible in analytical applications. In Table 1, some of the analytical characteristics of the developed method were compared with those previously reported by others [20–30]. As it can be seen, the capabilities of the proposed sensor are superior in most cases than the others. Another attempt in this study was simultaneous detection of MO and AC with HTP-MWCNT-CPE. Fig. 5 shows the differential pulse voltammograms which were obtained by varying either the MO (Fig. 5A) or AC (Fig. 5B) concentration while the concentration of the other compound was kept constant. As indicated in Fig. 5A, the electrochemical response of MO in the presence of a constant concentration of 200.0 lM of AC increased linearly with the increase of the MO concentration in the range of 8.0–1050.0 lM, while the response of AC remaining almost constant. In the same way, the differential pulse voltammograms were recorded by

increasing the AC concentrations in the presence of 100.0 lM of MO. As Fig. 5B suggests, there comes about an increase in the peak current of AC when AC concentration increases in the range of 10.0–400.0 lM, while the voltammetric response of MO is almost unchanged during the oxidation of AC. The utility of the modified electrode was investigated for the simultaneous determination of MO and AC. For this purpose, the concentrations of MO and AC were changed simultaneously. The voltammetric responses show that the simultaneous determination of MO and AC with two anodic peaks at potentials of 50 and 160 mV, corresponding to the oxidation of MO and AC, is possible at HTP-MWCNT-CPE (Fig. 6A and its inset). Fig. 6B shows that the plot of peak current versus MO concentration is linear in the worked concentration range of 1.3–82.5 lM. Also, Fig. 6C demonstrates the linear correlation between peak current and AC concentration in the worked concentration range 5.0–152.0 lM while MWCNT-CPE could not separate the voltammogram of the mixed solution of 100.0 lM MO and 300.0 lM AC, and a broad signal in 180 mV was observed (Fig. 6D). Moreover, the sameness of sensitivities of HTP-MWCNT-CPE to MO in the absence (Fig. 4, inset B; 0.0452 lA lM1) and the presence (Fig. 6B; 0.0463 lA lM1) of AC indicates that the voltammetric responses of both analytes at the modified electrode surface are independent of each other and, therefore, individual or simultaneous determination of AC and MO are possible without any interference; if the MO signal was affected by AC, the sensitivities would be different. 3.4. Interference study Selectivity and applicability of the proposed method was also evaluated by investigating the effect of some common species that

Table 1 Comparison of the efficiency of different modified electrodes for the MO determination. Electrode name

Detection method

Linear range

Detection limit

Sensitivity

Oxidation peak potential/mV (worked pH)

Sample

Refs.

MCM-GCEa

Cyclic voltammetry Cyclic voltammetry

0.1–2.0 lM

0.01 lM

1.74 lA lM1

500 (pH 7.0)

Urine

[20]

4.0–18.0 lM

0.02 lM

0.0715 lA lM1

430 (pH 7.4)

Urine

[21]

500 (pH 7.0) 1200 (pH 6.0)

Urine –

[22] [23]

PGCEb

GNP-MITOc Palladized aluminum HMDEd HMDE Au-ME

Amperometry Amperomety DPVe DPV

f

P-MWCNT-GCEg MIP-MEh PB-MITOi CHF-GCEj HTP-MWCNT-CPEk

Cyclic voltammetry Amperometry Amperometry Amperometry Amperoetry DPV

18.0–100.0 lM 0.8–16.0 lM 2.0–30.0 lM 0.01– 3.1 lg mL1 0.01– 3.1 lg mL1 285– 305 pg mL1 0.5–150 lM 0.1–5.0 mM 0.09–1.0 lM 1.0–500.0 lM 1.0–95.0 lM 95.0–950.0 lM

a b c d e f g h i j k

1

0.21 lM 0.8 lM

0.0353 lA lM 0.22 lA lM1 –

3.0 ng mL1

372.70 nA L mg1

250 (pH 10.5)

Plasma

[24]

1

1

250 9 (pH 10)

Plasma

[25]

–

–

plasma

[26]

10 nA lM1 91.86 lA cm2 16.8 lA cm2 – 0.0452 lA lM1,

300 750 500 600 210

– – – Rat brain Urine and MO injection solution

[27] [28] [29] [30] This work

3.0 ng mL

95.5 pg mL 0.2 lM 0.2 mM 0.1 mM 0.5 lM 0.066 lM

1

372.70 nA L mg

(pH (pH (pH (pH (pH

7.0) 5.3) 5.0) 4.5) 7.0)

0.0056 lA lM1

MCM-GCE: Mespoporous carbon modified glassy carbon electrode. PGCE: Pretreated glassy carbon electrode. GNP-MITO: Gold nanoparticle modified indium tin oxide. HMDE: Hanging mercury dropping electrode. DPV: Differential pulse voltammetry. Au-ME: Au microelectrode. P-MWCNT-GCE: Preheated multi wall carbon nanotubes glassy carbon electrode. MIP-ME: Molecularly imprinted polymer-modified electrode. PB-MITO: Prussian blue modified indium tin oxide. CHF-MGE: Cobalt hexacyanoferrat modified glassy carbon electrode. HTP-MWCNT-CPE: 4-hydroxy-2-(triphenylphosphonio)phenolate multi-wall carbon nanotubes paste electrode.

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M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51

A

Table 2 Interference studies some of species for the determination of 50.0 lM of morphine under optimized conditions at HTP-MWCNT-CPE.

12

MO

I / µA

33

AC 6

Foreign species

Molar ratio (foreign species/morphine)

Na+, K+, NH4+, Cl NO2 Urea Uric acid Folic acid Ascorbic acid

1000 850 540 25 10 1

1 0 -0.08

0.16

Table 3 Differential pulse voltammetric determination of MO and AC in a human urine sample.

0.4

E vs SCE / V

B

Sample

8

AC 40 Human urine

I / µA

MO

a

4

0.1

0.3

E vs SCE / V Fig. 5. (A) Differential pulse voltammograms (DPVs) of the HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) containing 200.0 lM of AC and different concentrations of 8.0–1050.0 lM of MO. (B) DPVs of HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) containing 100.0 lM of MO and different concentrations of 10.0–400.0 lM of AC.

often accompany MO in real samples such as urine. This study was done for a solution of 50.0 lM of MO and the tolerance limit is 1.6

AC

MO

AC

MO

AC

MO

AC

– 10.00 30.00

– 20.00 30.00

 9.92 30.25

 20.25 29.91

– 99.2 100.8

– 101.2 99.7

– 2.1 1.9

– 2.0 2.1

RSD (%)

Average of three replicate measurements.

3.5. Application of HTP-MWCNT-CPE for determination of MO and AC in urine and pharmaceutical samples The applicability and reliability of the proposed modified electrode in real samples with different matrixes were tested to confirm the usefulness of HTP-MWCNT-CPE. A human urine sample prepared as discussed in Section 2.3.1 was spiked with different values of MO and AC. The measurements were performed using

B

5

AC

Recovery (%)

MO

MO

MO

6 y = 0.0463x + 0.5304

Ip / µA

A

Founda/lM

defined as the molar ratio of the foreign species to MO that causing 5% relative error for MO determination. The results, given in Table 2, show that ascorbic acid and folic acid and uric acid have seriously interfering effect on the determination of MO.

1 0 -0.1

Added/lM

AC

R 2 = 0.9991

3 0

1.2

0

45

90

2.5

C Ip / µA

I / µA

I/ µA

[MO] / µM

0.8

4 y = 0.0166x + 0.6464 R 2 = 0.9984

2 0 0

80

160

[AC] / µM 0.4

0 -0.1

0.15

0.4

D 1.7 I / µA

E vs SCE / V

0 -0.1

0.3

E vs SCE / V

0.7

1 0.3 -0.07

0.13

0.33

E vs SCE / V

Fig. 6. (A) Differential pulse voltammograms of HTP-MWCNT-CPE in a 0.15 M phosphate buffer solution (pH 7.0) in the mixed solutions of (from inner to outer): 1.3–18.0 lM of MO and 5.0–52.0 lM of AC. Inset shows the similar DVPs in the mixed solutions of (from inner to outer): 19.5–82.5 lM of MO and 56.0–152.0 lM of AC. Plots of the peak current as a function of (B) MO concentration in the worked concentration range of 1.3–82.5 lM and (C) AC concentration in the worked concentration range of 5.0–152.0 lM. (D) Shows the response of a mixed solution of 100.0 lM of MO and 300.0 lM of AC at the MWCNT-CPE surface.

51

M. Reza Shishehbore et al. / Journal of Electroanalytical Chemistry 665 (2012) 45–51 Table 4 Differential pulse voltammetric determination of MO and AC in the pharmaceutical samples. Sample

MO injection solution

Tablet of AC

a b

Added/lM

Founda/lM

Recovery (%)

RSD (%)

Total valueb

MO

AC

MO

AC

MO

AC

MO

AC

MO (mg mL

– 5.0 10.0 – – –

– – – – 5.0 10.0

10.38 15.12 20.58 – – –

– – –

– 98.3 101.0 – – –

– – – – 99.0 100.4

2.3 2.0 1.9 – – –

– – – 2.2 2.3 1.8

9.96 – – – – –

9.96 14.81 20.03

Declared value 1

)

1

AC (mg)

MO (mg mL

– – – 326.3 – –

10.0 – – – – –

)

RSD (%) AC (mg)

MO

AC

– – – 325.0 – –

2.9 – – – – –

– – – 3.1 – –

Average of three replicate measurements. The total values were calculated by multiplying the measured values by the appropriate dilution factor (n = 3).

the calibration curves which are shown in Fig. 6B and C, and the results listed in Table 3. The relative standard deviations (RSD%) and the recovery rates of the spiked samples were acceptable and confirmed the applicability of the modified electrode for AC and MO measurement in urine sample matrixes. Also, AC tablets and MO injection solution were used as pharmaceutical samples. Sample preparation was done as previously discussed in Section 2.3.2, and DPVs were recorded to estimate the MO and AC concentrations using the calibration curves (Fig. 6B and C). The results are shown in Table 4. As it can be seen, the total values obtained for both analytes are in agreement with those registered in the label of the pharmaceutical inhalation products. Also, the recovery of the spiked analytes and their RSD% are acceptable. Thus, HTP-MWCNT-CPE can be efficiently used for the simultaneous determination of AC and MO in real and pharmaceutical samples.

4. Conclusions This study has demonstrated that HTP-MWCNT-CPE exhibits excellent electrocatalytic activity for the electrocatalytic oxidation of MO at pH 7.0. Kinetic parameters such as the transfer coefficient, a, the catalytic electron transfer rate constant, k0 , and the overall number of electrons involved in the electrocatalytic oxidation of MO at the modified electrode surface have been determined using a cyclic voltammetry method. Chronoamperometry is used for the determination of the diffusion coefficient. Differential pulse voltammetric (DPV) measurements exhibit two linear dynamic ranges of 1.0–95.0 and 95.0–950.0 lM and a detection limit of 0.066 lM for MO. In DPV, HTP-MWCNT-CPE can separate the oxidation peak potentials of MO and AC present in a mixed solution though, at an unmodified CPE, the peak potentials are indistinguishable. Finally, the proposed modified electrode has been successfully applied for the determination of MO and AC in urine and pharmaceutical samples. Simplicity of preparation, good reproducibility and low cost of the modified electrode as well as wide linear concentration range, low detection limit and good repeatability for MO determination are the important advantages of HTP-MWCNT-CPE. References [1] World Health Organization, Cancer Pain Relief, 2nd, World Health Organization, Geneva, 1986, p. 21. [2] R.W. Milne, R.L. Nation, A.A. Somogyi, Drug Metab. Rev. 28 (1996) 345–472. [3] S.Y. Yeh, C.W. Gorodetzky, J. Pharm. Sci. 66 (1977) 1288–1293. [4] D.C. Fuller, J. Forensic Sci. 42 (1997) 685–689. [5] J. Sawynok, Can. J. Physiol. Pharmacol. 64 (1986) 1–6. [6] F. Moriya, K. Chan, Y. Hashimoto, Legal Med. 1 (1999) 140–144. [7] A. Alnajjar, B.M. Cord, J. Pharm. Biomed. Anal. 33 (2003) 463–473. [8] A.M. Idris, A.O. Alnajjar, Talanta 77 (2008) 522–526. [9] C. Meadway, S. George, R. Braithwaite, Forensic Sci. Int. 127 (2002) 136–141. [10] D. Whittington, E.D. Kharasch, J. Chromatogr. B 796 (2003) 95–103. [11] M. Mabuchi, S. Takatsuka, M. Matsuoka, K. Tagawa, J. Pharm. Biomed. Anal. 35 (2004) 563–573. [12] S.R. Edwards, M.T. Smith, J. Chromatogr. B 814 (2005) 241–249.

[13] K. Kudo, T. Ishida, N. Nishida, N. Yoshioka, H. Inoue, A. Tsuji, N. Ikeda, J. Chromatogr. B 830 (2006) 359–363. [14] S. Li, C. He, F. Gao, D. Li, Z. Chen, H. Liu, K. Li, F. Liu, Talanta 71 (2007) 784–789. [15] X.X. Zhang, J. Li, J. Gao, L. Sun, W.B. Chang, J. Chromatogr. A 895 (2000) 1–7. [16] S. Hara, S. Mochinaga, M. Fukuzawa, N. Ono, T. Kuroda, Anal. Chim. Acta 387 (1999) 121–126. [17] J. Beike, H. Kohler, B. Brinkmann, G. Blaschke, J. Chromatogr. B 726 (1999) 111– 119. [18] M. Sarwar, T. Aman, Microchem. J. 30 (1984) 304–309. [19] A. Sheibani, M.R. shishehbore, E. Mirparizi, Spectrochim. Acta A 77 (2010) 535–538. [20] F. Li, J. Song, C. Shan, D. Gao, X. Xu, L. Niu, Biosens. Bioelectron. 25 (2010) 1408–1413. [21] F. Li, J. Song, D. Gao, Q. Zhang, Do. Han, L. Niu, Talanta 79 (2009) 845–850. [22] Y.R. Zhao, Y. Wu, Y. Zhang, Z.G. Chen, X. Cao, J.W. Di, J.P. Yang, Electroanalysis 21 (2009) 939–943. [23] M.H. Pournaghi-Azar, A. Saadatirad, J. Electroanal. Chem. 624 (2008) 293–298. [24] A. Niazi, A. Yazdanipour, Chin. Chem. Lett. 19 (2008) 465–468. [25] A. Niazi, J. Ghasemi, M. Zendehdel, Talanta 74 (2007) 247–254. [26] P. Norouzi, M.R. Ganjali, A.A. Moosavi-movahedi, B. Larijani, Talanta 73 (2007) 54–61. [27] A. Salimi, R. Hallaj, G.R. Khayatian, Electroanalysis 17 (2005) 873–879. [28] W.M. Yeh, K.C. Ho, Anal. Chim. Acta 542 (2005) 76–82. [29] K.C. Ho, C.Y. Chen, H.C. Hsu, L.C. Chen, S.C. Shiesh, X.Z. Lin, Biosens. Bioelectron. 20 (2004) 3–8. [30] F. Xu, M. Gao, L. Wang, T. Zhou, L. Jin, J. Jin, Talanta 58 (2002) 427–432. [31] M.E. Bosch, A.R. Sanchez, F.S. Rojas, C.B. Ojeda, J. Pharm. Biomed. Anal. 43 (2007) 799–815. [32] F.F. Daly, J.S. Fountain, L. Murray, A. Graudins, N.A. Buckley, Med. J. Aust. 188 (2008) 296–301. [33] K.G. Kumar, R. Letha, J. Pharm. Biomed. Anal. 15 (1997) 1725–1728. [34] Z. Bouhsain, S. Garrigues, A.M. Rubio, M. Guardia, Anal. Chim. Acta 330 (1996) 59–69. [35] M.I. Toral, P. Richter, E. Araya, S. Fuentes, Anal. Lett. 29 (1996) 2679–2689. [36] N. Erk, F. Onur, Anal. Lett. 30 (1997) 1201–1210. [37] P. Kahela, E. Laine, M. Anttila, Drug Dev. Ind. Pharm. 13 (1987) 213–224. [38] M.L. Qi, P. Wang, X. Leng, J.L. Gu, R.N. Fu, Chromatographia 56 (2002) 295–298. [39] H. Fujino, H. Yoshida, H. Nohta, M. Yamaguchi, Anal. Sci. 21 (2005) 1121–1124. [40] B.M. Reddy, B.R. Mahdavi, N.K. DurgaDevi, P.S. Parveen, B.S. Maradula, T.N. Rani, Int. J. Chem. Anal. Sci. 1 (2010) 158–160. [41] F. Ghorbani-Bidkorbeh, S. Shahrokhian, A. Mohammadi, R. Dinarvand, Electrochim. Acta 55 (2010) 212–217. [42] Z.A. Alothman, N. Bukhari, S.M. Wabaidur, S. Haider, Sens. Actuators, B 146 (2010) 314–320. [43] M. Mazloum-Ardakani, H. Beitollahi, M.A. Sheikh Mohseni, A. Benvidi, H. Naeimi, M. Nejati-Barzoki, N. Taghavinia, Colloids Surf., B 76 (2010) 82–87. [44] M. Mazloum-Ardakani, H. Beitollahi, M.K. Amini, F. Mirkhalaf, M. AbdollahiAlibeika, Sen. Actuators, B 151 (2010) 243–249. [45] C. Remy, E. Marret, F. Bonnet, Brit. J. Anaesth. 94 (2005) 505–513. [46] H.R. Zare, M.R. Shishehbore, D. Nematollahi, M. Saber-Tehrani, Sens. Actuators, B 151 (2010) 153–161. [47] M.R. Shishehbore, H.R. Zare, D. Nematollahi, M. Saber-Tehrani, Anal. Methods 3 (2011) 306–313. [48] H.R. Zare, M.R. Shishehbore, D. Nematollahi, Electrochim. Acta 58 (2011) 654– 661. [49] D. Nematollahi, E. Tammari, R. Esmaili, J. Electroanal. Chem. 621 (2008) 113–116. [50] S. Antoniadou, A.D. Jannakoudakis, E. Theodoridou, Synth. Met. 30 (1989) 295– 304. [51] B. Proksa, L. Molnar, Anal. Chim. Acta 97 (1978) 149–153. [52] P.H. Jordan, J.P. Hart, Analyst 116 (1991) 991–996. [53] J.M.P.J. Garrido, C. Delerue-Matos, F. Borges, T.R.A. Macedo, A.M. Oliveira-Brett, Electroanalysis 16 (2004) 1419–1426. [54] Q.L. Zhang, J.J. Xu, X.Y. Li, H.Z. Lian, H.Y. Chen, J. Pharm. Biomed. Anal. 43 (2007) 237–242. [55] C.P. Andrieux, J.M. Saveant, J. Electroanal. Chem. 93 (1978) 163–168. [56] A.J. Bard, L.R. Falkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 2001. [57] D.A. Skoog, F.J. Holler, T.A. Nieman, Principles of Instrumental Analysis, fiveth ed., Harcourt Brace, Philadelphia, 1998.