Accepted Manuscript Title: Simultaneous determination of codeine and caffeine using single-walled carbon nanotubes modified carbon-ceramic electrode Author: Biuck Habibi Mehri Abazari Mohammad Hossein Pournaghi-Azar PII: DOI: Reference:

S0927-7765(13)00594-8 http://dx.doi.org/doi:10.1016/j.colsurfb.2013.09.026 COLSUB 6027

To appear in:

Colloids and Surfaces B: Biointerfaces

Received date: Revised date: Accepted date:

15-2-2013 24-8-2013 13-9-2013

Please cite this article as: B. Habibi, M. Abazari, M.H. Pournaghi-Azar, Simultaneous determination of codeine and caffeine using single-walled carbon nanotubes modified carbon-ceramic electrode, Colloids and Surfaces B: Biointerfaces (2013), http://dx.doi.org/10.1016/j.colsurfb.2013.09.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1 Simultaneous determination of codeine and caffeine using single-walled carbon nanotubes

Biuck Habibi*, Mehri Abazari and Mohammad Hossein Pournaghi-Azar

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modified carbon-ceramic electrode

Azarbaijan Shahid Madani University, Tabriz 53714-161, Iran *

Tel and Fax: +98 412 4327541; E-mail:

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Corresponding author (Biuck Habibi).

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Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences,

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[email protected]

Abstract

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In the present paper, the simultaneous determination of codeine (CO) and caffeine (CF) is

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described by the use of single-walled carbon nanotubes modified carbon-ceramic electrode

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(SWCNT/CCE); prepared via a simple and rapid method. The results show that the SWCNT/CCE exhibits excellent electrochemical catalytic activity towards the oxidation of these

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compounds with respect to the bare CCE and offers two anodic peaks at 1.05 and 1.38 V vs. saturated calomel electrode for oxidation of CO and CF, respectively. Differential pulse voltammetry was used for simultaneous determination of CO and CF at micromolar concentration level. In the optimum conditions, it is found that the calibration graphs for CO and CF are linear in the concentration ranges 0.2-230 and 0.4-300 μM with detection limits of 0.11 and 0.25 µM for CO and CF, respectively. The SWCNT/CCE presents good stability, reproducibility, and repeatability and the proposed method has been successfully applied for determination of CO and CF in some pharmaceutical, drinking and biological samples with high recovery rate.

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2 Keywords: Codeine; Caffeine; Differential pulse voltammetry; Carbon nanotubes; Carbon-

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ceramic

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3 1. Introduction Codeine (C18H21NO3) or 3-methylmorphine (a natural isomer of methylated morphine) is an opiate used for its analgesic, antitussive, and antidiarrheal properties [1, 2]. However, codeine

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(CO) is used to treat mild to moderate pain and also to relieve cough. CO is marketed as both a single-ingredient and in combination preparations drugs. These combinations always provide

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greater pain relief than the agent alone. These marketed combination products containing CO

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with other pain killers or muscle relaxers, as well as CO with phenacetin, naproxen,

paracetamol + CO ± caffeine ± antihistamines.

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indomethacin, diclofenac and others and even more complex mixtures including aspirin +

Caffeine (C8H10N4O2) is the common name for the 1, 3, 7-trimethylxanthine (3, 7-

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dihydro-1, 3, 7-trimethyl-1H-purine-2, 6-dione). It is widely distributed in plant products and

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beverages such as tea, coffee bean, cocoa nuts, cola nuts, and coca-cola [3, 4]. It is also one of

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the most widely used drugs in the world and has many important pharmacological effects such as the stimulant of central nervous system, diuresis and positive effect on cardiovascular system [5,

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6]. Caffeine (CF) is also used therapeutically in combination with ergotamine in the treatment of migraine or in combination with nonsteroidal anti- inflammatory drugs in analgesic formulations [7].

An overdose of these drugs may induce nausea, vomiting, diarrhea, abdominal pain, sweating, seizures, confusion or an irregular heartbeat. On the other hand, in pharmaceutical preparations, these compounds are used as the sedative, analgesic and antitussive agents. Therefore, determination of CO and CF at trace quantities in the pharmaceutical preparations and biological samples as in individual or simultaneous form is of great importance and it is necessary to develop the simple, rapid, precise, and reliable methods. Some papers on individual

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4 or simultaneous determination of these analytes have been published [8-23]: including spectrophotometry [8], thin layer chromatography [9-11], capillary electrophoresis [12], gas chromatography [13], and liquid chromatography [14-23]. Generally, these methods are

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expensive, time-consuming, complicated, laborious and requiring pretreatment of the samples. In contrast, the electrochemical methods have better features, such as simplicity, high sensitivity,

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and ease of use. A survey in the literature indicates that there is only one electrochemical method

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for simultaneous determination of some alkaloids (such as CO and CF) and other compounds in pharmaceuticals and urine [24].

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In this work, we report for the first time, the application of single-walled carbon nanotubes modified carbon-ceramic electrode (SWCNT/CCE) for simultaneous determination of

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CO and CF. Based on the different electrocatalytic activities of the SWCNT/CCE towards these

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two species, a sensitive and selective method for simultaneous determination of these drugs was

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set up via differential pulse voltammetry. The method possesses wide linearity and sufficiently low detection limits. The practical analytical utility of the procedure has been illustrated by the

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selective measurements of CO and CF in several real samples such as pharmaceutical, drinking and biological samples, without any preliminary treatment.

2. Experimental 2.1. Chemicals

SWCNT has been manufactured by CNI company (USA). Methyltrimethoxysilane, codeine, caffeine, H2SO4, and other chemicals were purchased from Merck or Fluka and used without any further purification. Throughout the whole experiment doubly distilled water was used. Supporting electrolyte solutions (0.01 M H2SO4) pH 1.7, and other pHs were prepared from stock solution of 0.1M H2SO4. pH of the solutions was adjusted by 0.1M NaOH.

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5 2.2. Apparatus The electrochemical experiments were carried out using an AUTOLAB PGSTAT-100 (potentiostat/galvanostat) equipped with a USB electrochemical interface and driven by a GPES

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4.9 software package (Eco Chemie, The Netherlands) in conjunction with a three-electrode system and a personal computer for data storage and processing. A three-electrode cell system

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composed of a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as

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the auxiliary electrode, and the SWCNT modified CCE (geometric surface area of 0.119 cm2) as the working electrode was employed for the electrochemical studies. The preparation of the bare

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procedure described in our previous paper [25].

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carbon-ceramic electrode and its modification by SWCNT was achieved according to the

2.3. Preparation of real samples

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Ten tablets of pharmaceutical samples were weighed and then completely grinded and

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homogenized. Powdered tablets were dissolved in 100 mL distilled water and the as-prepared

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suspension was stirred and centrifuged at 6000 rpm for 10 min. Then, 10 mL supernatant was diluted 50-times with 0.01 M H2SO4 solution. A portion of the resultant solution was then taken out and subjected as the sample for the detection of drugs concentration. The drinking samples were appropriately diluted (1:10) with supporting electrolyte to bring them into the working concentration range. In order to determine CO and CF in tea sample, a known amount of tea (15.0 g) was weighted and dissolved in 500 mL of double distilled water and boiled for 1 h on a hot plate with stirring. After allowing the residue to settle down, the hot solution was filtered and used for further experiments. A known amount of filtered solution was added to 10 mL of supporting electrolyte solution of pH 1.7. Finally, the urine sample of a healthy person was diluted ten times by 0.01 M H2SO4 solution. The diluted urine samples were spiked with a

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6 certain amount of CO and CF and the drugs were determined by DPV method via standard addition method. The quantitative determination of CO and CF was achieved by measuring the

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oxidation peak current after background subtraction using DPV method.

3. Results and Discussion

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3.1. Electrooxidation of CO and CF on the SWCNT/CCE

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The morphological and electrochemical characterizations of the SWCNT/CCE have been studied and reported in our previous papers [25-27]. The obtained results showed that the

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SWCNT/CCE has some advantageous such as its simple and fast preparation, the low cost, the high reproducibility, the high stability in different pH, and the high electrocatalytic properties.

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The results also indicate that the SWCNT/CCE offers a broad potential window in different pHs making it particularly suitable for electrochemical studies of analytes with a high oxidation

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potential. Therefore, here the SWCNT/CCE was used for the electrochemical study and

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simultaneous determination of CO and CF. Fig. 1 shows the cyclic voltammetric response of the

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SWCNT/CCE in the absence (curves 1) and presence (curves 2) of 3.0 mM CO (A) and CF (B) in individual solution of 0.01 M H2SO4 medium (pH 1.7) at a scan rate of 50 mV s-1, respectively. As a comparison experiment, the oxidation of CO and CF at the bare CCE was also investigated (not shown here). The obtained results at the bare CCE reveal that the oxidation of CO occurs as a weak oxidation peak at about 1.26 V and CF oxidation also shows an anodic wave at 1.48 V. Compared to the bare CCE, at the SWCNT/CCE the oxidation peak current of CO increased significantly and the oxidation peak potential shifted negatively to 1.05 V. On the other hand, as is seen in Fig. 1 (B), the anodic peak potential for oxidation of CF at the SWCNT/CCE is about 1.38 V vs SCE. The observed 210 and 100 mV less-positive potential shifts for CO and CF at the SWCNT/CCE clearly indicate that the SWCNT decreases the

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7 thermodynamic overpotential of CO and CF electrooxidation. Enhancement the oxidation peak current and decreasing (less positive potential) the anodic peak potential in electrooxidation of CO and CF at the SWCNT modified electrode are attributed to the increase of effective surface

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area of the immobilized SWCNT modified electrode, compared to the bare CCE. To check out this feature, we have measured the effective surface area of the bare CCE and SWCNT/CCE via

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cyclic voltammetry method using 5 mM K3[Fe(CN)6] ( as a reference compound) in 0.1 M KCl

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[27]. The real surface area was calculated according to the Randles-Sevcik equation and the calculated effective surface area of the SWCNT/CCE was 0.330 ± 0.012 cm2, while, the real

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surface area for the bare CCE was found to be 0.162 ±0.018 cm2. The significant increase in effective surface area suggests that the SWCNT/CCE is capable for electrocatalysis and

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determination of various electroactive analytes.

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On the other hand, the absence of any reduction peak in the reverse sweep for both

electrode reactions [28-34].

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compounds in Fig. 1 A and B on the SWCNT/CCE clearly indicates the irreversibility of the

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In order to further investigate the electrochemical behavior of CO and CF on the SWCNT/CCE, the CVs were recorded for various potential scan rates at the range 10-650 mVs-1 in 0.01 M H2SO4 solution containing 3.0 mM of these drugs. The results showed that the anodic peak currents vary linearly with the square rote of scan rate (υ1/2) for both drugs, which confirm the diffusion-controlled process for electrooxidation of CO [28] and CF [31, 32] on the surface of SWCNT/CCE in the studied range of potential scan rates. The regression equations are as follows:

CO: Ipa (μA) = 61.84 υ1/2 - 210 (υ: mV s-1, R2 = 0.998).

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CF: Ipa (μA) = 80.33 υ1/2 -190 (υ: mV s-1, R2 = 0.997).

(2)

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8 A brief review of the literature shows that no information on the oxidation process of CO on the SWCNT exists. For obtaining further information on the rate of determining steps of the CO on the SWCNT/CCE, a Tafel plot was created using the data derived from the raising part of

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the current-voltage curve of 3.0 mM of CO, monitored at slow scan rate of 10 mV s-1. The slope of the Tafel plot is equal to nα(1-α)F/2.3RT which comes up to 9.18 V decade-1. We obtained

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nα(1-α) as 0.54. Assuming nα = 1, gives α = 0.46. In addition, the value of αnα (nα is the number

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SWCNT/CCE at pH 1.7 using the following equation [35]:

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of electrons involved in the rate determining step) was calculated for the oxidation of CO on the

|Ep - Ep/2| = 1.857RT/αnαF = 0.0477/αnα → αnα= 0.0477/|Ep - Ep/2|

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(3)

where Ep/2 is the potential corresponding to Ip/2. The value for αnα was found to be 0.41

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(assuming nα = 1, hance α = 0.41) on the surfaces of SWCNT/CCE. Comparison of the obtained

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α-values from two ways shows that there is a good agreement between the obtained results. On

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the other hand, the value for αnα for CO oxidation on the unmodified CCE was found to be 0.11. These values (αnα on the SWCNT/CCE and CCE) show that the over-potential of CO oxidation is reduced at the surface of SWCNT/CCE, and that the rate of electron transfer process is greatly enhanced. This is another indication of the SWCNT effect on the improvement of the electrooxidation of CO. We also evaluated the number of electrons (n) involved in the thorough oxidation process of CO. For a totally irreversible diffusion-controlled electrode process, the following equation can be used for evaluation of n [35]:

Ipa = 2.99 × 105 n [(1- α) nα ]1/2ACCODCO1/2 υ1/2

(4)

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9 where nα, α, and A are the number of electrons involved in the rate-determining step, transfer coefficient, and surface area of the electrode, respectively. According to Eq. 4, the plot of Ipa versus υ1/2 should be linear with a slope of 2.99 × 105 n [(1 - α) nα]1/2ACCODCO1/2 allowing for

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calculation of n. For this propose, the values of (1- α) nα and DCO must be already determined. The value of (1- α) nα was determined via Tafel polt and DCO was evaluated by

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chronoamperometry experiment. The current-time profiles were obtained by setting the working

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electrode potential at 1.1 V, in the absence and presence of 3.0 mM CO (not shown here). The plot of I versus t-1/2 gives a straight line (Cottrell equation), and slope of such a line can be used

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for the estimation of the DCO. From the slope of this plot, the value of DCO is found to be 3.6×10cm2 s-1. Based on Eq. 4, the plot of Ipa versus υ1/2 is linear with slope of 0.0618 V. Using the

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value of the slope and other quantities, it is found that the number of electrons involved in

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thorough anodic oxidation of CO is 2.18 (about 2 per mole).

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In order to investigate the effect of pH on the electrochemical behavior of CO and also CF at the SWCNT/CCE, the effect of pH on the oxidation peak potential and peak current was

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investigated by cyclic voltammetry in the solution containing 3.0 mM of these drugs. The values of peak potential (E'p), depending on the pH value of the supporting solution, show that the electrode reaction of these drugs includes some proton transfer in the oxidation processes. According to the Nernst equation, the slope of about 60 mV/pH reveals that the proportion of the electron and proton involved in the reactions is 1:1. In this work, within the pH range 0.5 to 3.5, the oxidation peak potential of CO shifts negatively with rising pH, according to the linear equation Ep(V)=1.152 - 0.059 pH [R2=0.98]. The value of dEp/dpH 59 mV/pH for CO indicates the involvement of equal number of protons and electrons in the electrooxidation reaction of CO [28, 29]. The relationship between the peak potential and the pH for CF oxidation is also linear,

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10 with the regression equation being: Ep(V)=1.191 - 0.058 pH (R2=0.993) [3, 26, 36]. On the other hand, within the studied pH ranges (0.5-3.5), the anodic peak current of CO oxidation increases from pH 0.5 to pH 3.5, while, the peak current of the CF oxidation being pH dependent in

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different way [26]. The anodic peak current value of CF oxidation is constant starting from pH 0.5 to pH

Simultaneous determination of codeine and caffeine using single-walled carbon nanotubes modified carbon-ceramic electrode.

In the present paper, the simultaneous determination of codeine (CO) and caffeine (CF) is described by the use of single-walled carbon nanotubes modif...
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