Silver-filled MWCNT nanocomposite as a sensing element for voltammetric determination of sulfamethoxazole

Accepted Manuscript Silver-filled MWCNT nanocomposite as a sensing element for voltammetric determination of sulfamethoxazole Abdollah Yari, Azim Shams PII:

S0003-2670(18)30925-5

DOI:

10.1016/j.aca.2018.07.061

Reference:

ACA 236162

To appear in:

Analytica Chimica Acta

Received Date: 30 April 2018 Revised Date:

18 July 2018

Accepted Date: 26 July 2018

Please cite this article as: A. Yari, A. Shams, Silver-filled MWCNT nanocomposite as a sensing element for voltammetric determination of sulfamethoxazole, Analytica Chimica Acta (2018), doi: 10.1016/ j.aca.2018.07.061. 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.

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Silver-filled MWCNT nanocomposite a sensing element for voltammetric determination of sulfamethoxazole

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Silver-filled MWCNT nanocomposite as a sensing element for voltammetric determination of sulfamethoxazole Abdollah Yari*, Azim Shams

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Department of analytical chemistry, Lorestan University, 68137-17133, Khorramabad–Iran

Abstract

Here, we introduce a new electrode based on Silver-filled multi-walled carbon nanotube

voltammetric

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(Ag-MWCNT) and methyltrioctyl ammonium chloride (MTOAC) for highly sensitive measurement of Sulfamethoxazole (SMX).

The

electrode showed

an

electrocatalytic activity as it led to the diminution of the overpotential and an increase in peak

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current for SMX oxidation in a phosphate buffer solution (pH 6.0).

Analysis of surface topography and electrochemical properties of the modified electrode was examined by TEM, EDX and EIS, respectively. Electrochemical performance of the modified electrode was investigated with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques for determination of SMX in aqueous solution. In addition, the oxidation

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process was found to be dependent on the pH of the buffer solution. Under optimal conditions, a linear relationship between the oxidation current and SMX concentration was found in a range 0.05-70 µM (R2= 0.997) with a detection limit of 0.01 µM after 2 min of accumulating time. The electrode was successfully used to quantify SMX in pharmaceutical formulations and human

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urine by DPV.

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Keywords: Sulfamethoxazole; Methyltrioctyl ammonium chloride; Silver-filled MWCNT; Electrochemical sensor; Nanocomposite

*

To whom correspondence should be addressed. Tel/Fax +98 66 33120612, E-mail: [email protected].

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1. Introduction Sulfamethoxazole, 4-amino-N-(5-methylisoxazole-3-yl)-benzene sulfonamide, (SMX) is a member of the antibacterial sulfonamides family. SMX is widely used for the treatment of

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infectious diseases due to its antibacterial properties via preventing the formation of dihydrofolic acid [1]. SMX is most commonly used in combination with other drugs to increase its power because of the importance of this drug in fighting bacterial infection and HIV-infected patients [2, 3]. The interest in the investigation of the SMX as well as in the analytical determination of

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its content in pharmaceutical formulations and biological fluids is always increasing.

Numerous methods such as solid-phase extraction [4], HPLC [5-7], spectrophotometry [8, 9], capillary electrophoresis [6,10-12], spectrofluorimetry [13,14], potentiometry [15] and

formulations or biological samples.

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voltammetry [2,16-19] have been reported for determination of SMX in pharmaceutical

Electrochemical methods are powerful analytical techniques in the pharmaceutical and drug analysis because they have many advantages including low-cost instrumentation, simple and quick to use, high sensitivity and selectivity, large linear dynamic range and the ability to

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determine kinetic and mechanistic parameters [20]. Carbon-based electrodes have been widely investigated in the electrochemical studies. Carbon nanotubes are suitable for electrode preparation due to high electrocatalytic activity, high chemical stability and low fouling. Moreover, multiwall carbon nanotubes (MWCNTs) are used extensively as the appropriate

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modifiers in preparation of some electrodes because of their ability to improve the facility of electron transfer between electro-active species and electrodes [21,22]. Studies show that the introduction of a metal into an MWCNT may alter some of the MWCNT properties or function

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of the metal, such as conducting, electronic and mechanical properties. Therefore, these filled nanotubes may be used as nanowires, novel catalysts and prevent magnetic nanoparticles of Fe, Co or Ni from oxidation [23-25]. Mesoporous carbon materials can act as electrode materials for energy storage and supports for well-dispersed active nanoparticles [26]. Furthermore, silver filled nanotubes have been used as spectroscopic enhancers [27]. A carbon paste electrode (CPE) modified with Ag-filled MWCNT as a potentiometric sensor was introduced for determination of free cyanide ion in aqueous solutions [28]. There have been many reports of the use of nanotubes-modified glassy carbon electrode (GCE) in electrochemical analysis such as 2

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abrasive immobilization of MWCNTs [29], the coating amount of nanocomposite suspension [30, 31] and electrodeposition of suitable nanostructures in the MWCNTs modified GC surface [31, 32]. In addition, aliquot 336 or methyltrioctyl ammonium chloride (MTOAC) is a quaternary ammonium salt have been used to modify an electrode surface in the electrochemical

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analysis [33, 34].

In this paper, we report an electrochemical sensor for determination of SMX in pharmaceutical formulations and urine samples with square wave voltammetry (SWV) using an Ag-MWCNT/GCE. The determination of the SMX in commercial formulations and urine

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samples was also carried out and the results were compared with those obtained with the HPLC. 2. Experimental

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2. 1. Instrumentation

The voltammetric measurements were obtained using an Autolab Potentiostat/Galvanostat model PGSTAT 204 (Eco Chemie, Netherlands) computer-controlled with NOVA 2.1 software. A three-electrode cell containing a modified GCE, SCE and a platinum wire counter electrode was used as the working, reference and counter electrode, respectively. frequency

response

detector

(EG&G

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A

model

1025)

controlled

with

a

Potentiostat/Galvanostat (model 263A) was used to measure the electrochemical impedance spectroscopy (EIS) of the electrode surface. A pH-meter (Metrohm model 713, Herisau, Switzerland) was used to measure the pH of the solutions. The IR spectra were recorded on an

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FTIR-8400s (Shimadzu, Japan). A 203H ultrasound bath (Rocker, 50 Hz, Taiwan) was used for the ultrasonic agitation of the nanoparticles.

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2. 2. Chemicals and solutions

All chemicals and reagents were of analytical grade and used without further purification. SMX, MTOAC, and MWCNT were obtained from Sigma-Aldrich. Phosphoric acid, sodium phosphate, AgNO3 and other chemicals were purchased from Merck. All needed solutions were prepared with double distilled water. Phosphate buffer solution was prepared from 0.1 M NaH2PO4 and Na2HPO4 as supporting electrolyte. Because of low aqueous solubility, a stock solution of SMX (0.01 M) was prepared by dissolving an appropriate amount in 0.1 M of HCl

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solution and the desired SMX concentrations were obtained by successive dilutions of the stock solution with the phosphate buffer solution. 2. 3. Preparation of the Ag-filled MWCNT/GCE

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The Ag-filled MWCNT was synthesized according to the reported procedures in the literature [36, 37]. Therefore, a certain amount of MWCNT was mixed with concentrated H2SO4:HNO3 (3:1) and refluxed at 100 ̊C for 15 hours. The oxidized MWCNT was filtered, washed, and dried to obtain a purified product. For the synthesis of Ag-MWCNT, a proper

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quantity of the product was dispersed in a solution of AgNO3 (4:1 v/v of water:ethanol), then stirred for 72 h at 25 °C, filtered, washed (several times) and dried. Energy dispersive X-ray spectroscopy (EDX) was used for chemical characterization and elemental analysis of the silver-

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filled MWCNT (Fig. 1). The presence of C, Ag and O elements in the produced nanocomposite was confirmed. Further characterization of the nanocomposite has been reported in the previous works [36, 37]. The TEM image of the product is given as an inset in Fig. 1 for more clarity. The formation of encapsulated Ag nanoparticles in the internal channels of CNTs can be seen. (Figure 1)

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A GCE was used as the core conducting rod and carefully polished with 0.3-µm alumina slurry on a micro-cloth pad and sonicated in 1:1 ethanol and distilled water to remove adsorbed particles. Then, 20 mg of Ag-MWCNT powder was added to a 90 ml 5% (v/v) MTOAC in hexane and then sonicated for 10 min to obtain a homogenous mixture. Afterward, 5 µl of the

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prepared mixture was transferred on the cleaned GC and dried until a thin layer of the modifier was observed on the GCE surface. The electrode was rinsed with distilled water and scanned 5

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times in the potential range of 0.6-1.2 to eliminate the electrochemical effect of the matrix. 2. 4. Preparation of the real sample The pretreatment procedures for the determination of SMX in commercially available tablets and urine are as follows: Ten Co-trimoxazole tablets (that combined SMX and trimethoprim) were individually weighted and powdered homogeneously in a mortar. Then, an accurately weighed portion of the powder transferred into a 100-ml volumetric flask, dissolved in sufficient solvent and diluted to volume with ethanol-water (70:30 v/v). After sonication and stirring for 30 min, the solution centrifuged and filtered to remove any insoluble materials. Aliquots of the 4

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solution were transferred into the electrochemical cell in sufficient phosphate buffer (0.1 M pH 6). Then, the real samples were spiked with known and different amounts of SMX for analysis using the standard addition method. On the other hand, five urine sample were collected from 5 healthy men, mixed and stored in a refrigerator for a day, then centrifuged at 6000 rpm for 10

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min. An appropriate amount of the urine sample was diluted by the phosphate solution to decrease the matrix effect. Aliquots of the urine samples were transferred into the electrochemical cell and standard addition method was used for SMX determination by DPV technique in the range of calibration graph.

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3. Results and discussion

3. 1. Electrochemical characterization of the Ag-filled MWCNT/GCE

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We used EIS to study the electrochemical properties of the electrode surface. EIS is a powerful technique to investigate the kinetics of electron transfer at the electrode/solution interface [38]. The EIS and CV voltammograms of 5.0 mM [Fe(CN)6]3-/4- were recorded against different electrodes. Fig. 2 shows the EIS spectra (A) and CV voltammograms (B) of a bare GCE (a),

MWCNT/GCE

(b),

MTOAC/GCE

(c),

Ag-MWCNT/GCE 3-/4-

and

the

Ag-

and 0.12 M KCl solution in

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MWCNT/MTOAC/GCE (e) in a mixture of 5.0 mM [Fe(CN)6]

(d)

the 0.1 Hz to10 KHz frequency range. From Fig. 2B, there are two well-defined peaks due to [Fe(CN)6]3-/4- redox couple. These peaks were shifted towards potential that is more negative by applying the Ag-MWCNT/MTOAC electrode. The Ag-MWCNT nanocomposite can act as the

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probe attracter. On the other hand, the other new content MTOAC, an ionic liquid, acts as a phase transfer catalyst that can facilitate the transfer of Fe(CN)64-/Fe(CN)63- anions from the aqueous phase to the thin film at the electrode surface where the electrochemical reactions can

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occur. This reveals that the redox reaction of Fe(CN)64-/Fe(CN)63- is a fast electron transfer process at the presence of MTOAC component and the Ag-MWCNT nanocomposite at the surface of the electrodes. Both these factors coherently reduce the needed potentials towards negative values.

The Nyquist impedance spectra often consist of a semicircular and linear part correspond to electron transfer and the mass transfer processes, respectively. The surface electron transfer resistance (Rct) could be evaluated from the semicircular diameter of the EIS spectra in the

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Nyquist plot. The obtained results showed that Ag-MWCNT/MTOAC/GCE is electrically more conductive than the other electrodes (Fig. 2A e). (Figure 2)

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The electrochemical behavior of SMX at the proposed electrode was investigated by DPV technique in the buffer (pH 6.0) within a potential range of 0.65 to 1.1 V vs. SCE. The DPV voltammograms of 30 µM SMX were taken at the Ag-MWCNTs/MTOAC/GCE in the buffer solution only (a white solution) (a) and at a bare GCE (b), the MWCNT/GCE (c), the

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MTOAC/GCE (d), Ag-MWCNT/GCE (e) the Ag-MWCNTs/MTOAC/GCE (f) in the buffer solution. The results are demonstrated in Fig. 3 for five replicate measurements after 2 min as accumulation time. Furthermore, as seen in Fig. 3, the Ag-MWCNT/MTOAC/GC electrode is

properties.

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more sensitive to oxidation of SMX than other electrodes because of its better electrochemical

(Figure 3)

3. 2. Effect of pH on the SMX oxidation

In order to investigate the electrochemical oxidation of SMX at different pHs, the CV

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voltammograms of 30.0 µM SMX were recorded over the pH range of 2.0 - 8.0 in solutions of phosphate buffers at the surface of the proposed sensor. The DPV voltammograms of SMX at different pHs (A), the anodic peak current Ip vs. pH (B) and the oxidation peak potential Ep vs.

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pH (C) are shown in Fig. 4.

(Figure 4)

As shown in Fig. 4B, the Ip increases with increase in pH up to pH 6.0 and then decreases.

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Therefore, a solution of phosphate buffer (0.1 M, pH 6.0) was chosen as the supporting electrolyte. As seen in Fig. 4C, the Ep decreases with increasing in pH and shifts toward less positive potentials an evidence that the electrode reaction is governed by a proton transfer process [13, 17]. Fig. 4C shows that there is a linear region for the plot of Ep versus pH with a Tafel slope of 0.052 V/pH in the pH range. The relationship between the Ep of SMX and the pH of the solution could be evaluated from Equation 1: Ep(V) = 1.198 - 0.052pH, r2= 0.997

(1)

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SMX contains amino groups that the can be oxidized via a pH-dependent reaction [39, 40]. Thus, the slope of 0.052 V/pH for SMX oxidation indicated that the conjugate base of the amino group was oxidized with an equal number of electrons and protons participating in the reaction. The slope of 0.052 value is close to the theoretically expected of 0.059 V/pH (m/n, where m and

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n denote the number of protons and electrons transferred in the electrochemical process, respectively) reveals the number of protons and electrons transferred were same in electrooxidation of SMX [41].

In order to find the exact number of the transferred electrons, the coulometric method was

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used as an appropriate technique to determine the electron transfer number in electro-oxidation of SMX at Ag-MWCNT/MTOAC/GCE. A 20 ml of phosphate solution (pH= 6) containing 0.3

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mmol of SMX was prepared and used until SMX completely oxidized at the modified electrode. The electricity consumed for oxidation of 0.3 mmol SMX was about 55 coulomb. Therefore, according to the Faraday’s law, the number of electrons transferred, n is 2. Therefore, a possible mechanism for electro oxidation of SMX at the mentioned modified electrode can be shown as in Scheme 1 [42].

3. 3. Scan rate effect

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(Scheme 1)

As shown in Fig. 5, to investigate the effect of scan rate on the oxidation of SMX at the surface of the optimized electrode, the CV voltammograms of 60 µM SMX were recorded at

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different scan rates in the range 25 to 175 mV/s (Fig. 5A, a to g). (Figure 5)

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From Fig. 5B, Ip increased linearly with increasing the scan rate (ν) in the range of 25 - 175 mV/s after a 2 min accumulation time. Equation 2 could evaluate the relationship between Ip and ν.

Ip(µA) = 0.147 + 0.025ν,

r2=0.995

(2)

The logI - logν graph showed a slope of 0.927 that is very close to the theoretical value of 1 (Fig. 5C). Therefore, the rate of the electrode reaction is limited under an adsorption process. As shown in Fig. 5D, the Ep values shifted toward more positive potentials with increasing the scan rate indicating the electrochemical reactions are irreversible and an adsorption-controlled process 7

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is going on for the electrochemical oxidation of SMX in this work [17, 41]. The relationship between the Ep and ν can be expressed by Equation 3. Ep(V) = 0.784 + 0.064 log ν, r2= 0.997

(3)

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3. 4. Analytical characteristics The optimized DPV parameters for the determination of SMX at the surface of the AgMWCNT/MTOAC/GC electrode are listed in Table 1. (Table 1)

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Furthermore, DPV was used to quantify the different concentrations of SMX in the buffer solution (Fig. 6). Fig. 6A depicts the DPV responses of the proposed sensor (Ag-

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MWCNT/MTOAC/GCE) toward various SMX concentrations. (Fig. 6)

The analytical curve in the linear range of 0.05 - 70 µM (Fig. 6B) is described as the following linear regression, Equation 4: I(µA) = 0.415 + 0.244C (µM),

r2 = 0.997

(4)

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From Eq. 4, the results obtained showed a reasonable linear range and a low limit of detection (LOD) for electrochemical determination of SMX at the proposed electrode, for which the results were compared with those obtained with some other reported electrodes (Table 2). (Table 2)

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The LOD was found to be 0.01 µM according to 3Sb/m ratio, where Sb is the standard deviation of the 5 repeated measurements of blank sample and m is the slope of the linear part of

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the calibration curve that interprets the sensitivity of system [43]. At optimal conditions, the repeatability, reproducibility of the proposed electrode were investigated by DPV in the buffer solution containing 30 µM SMX. Repeatability of the sensor was tested by performing seven replicate measurements of the electrode signal and the relative standard deviation (RSD) of ±3.4% was obtained, representing the high precision of the sensor. To determine the reproducibility of the sensor, seven similar electrodes were made from the optimized modification components and their responses were measured individually against a 30 µM SMX. The resulting RSD of ±3.8% for the responses indicating a high reproducibility of the 8

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proposed sensor for DPV determination of SMX. Verifying the stability of the modified electrode, 40 consecutive CV voltammograms were performed by the electrode for 30 µM SMX under the optimal conditions for them, no significant differences were observed (< ±5%). Therefore, the proposed sensor is suitable and reliable for determination of SMX in aqueous

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solutions. 3. 5. Effect of interferes

The influences of some foreign species on the voltammetric determination of SMX were

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investigated by immersing the electrode into the buffer solution (pH 6.0) containing 30 µM SMX and varying amounts of the foreign substances using DPV (Table 3).

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(Table 3)

The Trimethoprim (TMP) interfering effect, as one of the most popular materials that combined with SMX in tablets, on the oxidation peak current of SMX was investigated. It was found that there was negligible interference at tenfold excess of TMP on electrochemical oxidation of SMX.

Because of no significant changes in the peak current for SMX oxidation (< ±5%) in the

are listed in Table 3. 3. 6. Real sample analysis

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presence of foreign species, the proposed sensor acts selectively with respect SMX. The results

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In order to study the analytical application of the modified electrode, Co-trimoxazole tablet and human urine were investigated to determine the concentration of SMX content as real

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sample by DPV technique. The tablet formulations and urine sample solutions were prepared according to section 2.4. To analysis of SMX formulations, two different Co-trimoxazole tablet were analyzed using the standard addition method. For this purpose, five aliquots of Cotrimoxazole solutions with different concentrations of SMX were prepared in the phosphate buffer solution (pH 6.0). Table 4 gives the corresponding DPV data obtained for measurement of the tablet formulations. The data revealed a good agreement between the declared SMX values and the proposed method. (Table 4)

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An HPLC method was used to measure the SMX contents as a standard method. As shown in Table 4, the obtained results of the new sensor were compared with those of the HPLC method by student t-test and F-test [17, 44]. At a 95% confidence level, the calculated t-value and F-

the results given by the proposed sensor and HPLC method.

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value were less than the statistical critical values so there were no significant differences between

To determine the SMX concentration in the human urine sample by DPV, each sample was diluted 3 times with the phosphate buffer and spiked with standard concentrations of SMX,

104%. (Table 5)

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before. Table 5 shows the evaluated recoveries obtained. The recoveries were reasonable, 97 –

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According to the results of this sensor, we can use the sensor to determine SMX in real samples such as pharmaceutical formulations and urine as real samples. 4. Conclusions

We developed a novel electrochemical sensor by introducing a new nanocomposite, silverfilled MWCNT/MTOAC, coated on a glassy carbon rod for voltammetric determination of

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Sulfamethoxazole drug in pharmaceutical and real samples as the sensing element in the proposed sensor. The electrode shows a good sensitivity due to electrocatalytic properties of the sensing element. High repeatability, good reproducibility and top sensitivity for the determination of Sulfamethoxazole due to robust and durable modification components are the

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outstanding performances of the new electrochemical sensor.

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[35] S. Tsang, Y. Chen, P. Harris, M. Green, A simple chemical method of opening and filling carbon nanotubes, Nature 372 (1994) 159-162. [36] R. Sepahvand, M. Adeli, B. Astinchap, R. Kabiri, New nanocomposites containing metal nanoparticles, carbon nanotube and polymer, J. Nanopart. Res. 10 (2008) 1309-1318. [37] M. Adeli, R. Sepahvand, A. Bahari, B. Astinchap, Carbon nanotube-graft-block copolymers containing silver nanoparticles, Int. J. Nanosci. 8 (2009) 533-541. [38] L. V. Protsailo, W. R. Fawcett, Studies of electron transfer through self-assembled monolayers using impedance spectroscopy, Electrochim. Acta 45 (2000) 3497-3505.

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[39] T. N. Rao, B. Sarada, D. Tryk, A. Fujishima, Electroanalytical study of sulfa drugs at diamond electrodes and their determination by HPLC with amperometric detection, J. Electroanal. Chem. 491 (2000) 175-181. [40] S. M. Sabry, Polarographic and voltammetric assays of sulfonamides as α‐oxo‐γ‐butyrolactone

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arylhydrazones, Anal. lett. 40 (2007) 233-256. [41] H. Chasta, R. N. Goyal, A Simple and sensitive poly‐1, 5‐diaminonaphthalene modified sensor for the determination of sulfamethoxazole in biological samples, Electroanalysis 27 (2015) 1229-1237.

[42] R. Joseph, K. G. Kumar, Differential pulse voltammetric determination and catalytic oxidation of sulfamethoxazole using [5,10,15,20‐tetrakis (3‐methoxy‐4‐hydroxy phenyl) porphyrinato] Cu(II)

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modified carbon paste sensor. Drug Test. Anal. 2 (2010) 278-283.

[43] K. Hasebe, J. Osteryoung, Differential pulse polarographic determination of some carcinogenic

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nitrosamines, Anal. Chem. 47 (1975) 2412-2418.

[44] N. Zheng, Y. Z. Li, M. J. Wen, Sulfamethoxazole-imprinted polymer for selective determination of sulfamethoxazole in tablets, J. Chromatogr. A 1033 (2004) 179-182.

[45] C. D. Souza, O. C. Braga, I. C. Vieira, A. Spinelli, Electroanalytical determination of sulfadiazine and sulfamethoxazole in pharmaceuticals using a boron-doped diamond electrode, Sens. Actuat. B 135 (2008) 66-73.

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[46] I. Cesarino, V. Cesarino, M. R. Lanza, Carbon nanotubes modified with antimony nanoparticles in a paraffin composite electrode: Simultaneous determination of sulfamethoxazole and trimethoprim, Sens. Actuat. B 188 (2013) 1293-1299.

[47] M. Meshki, M. Behpour, S. Masoum, Application of Fe doped ZnO nanorods-based modified sensor

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for determination of sulfamethoxazole and sulfamethizole using chemometric methods in voltammetric

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studies, J. Electroanal. Chem. 740 (2015) 1-7.

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Table 1. The optimized DPV parameters for determination of SMX at surface of the AgMWCNT/MTOAC/GCE. Variable parameter

Function range Optimum value 5.0 - 100

50

Frequency (Hz)

5.0 - 40

20

Scan interval (mV)

2.0 - 20

Accumulation time (s)

10 - 500

120

Potential (V) vs. SCE

0.0 - 1.3

0.6 - 1.2

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Pulse amplitude (mV)

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5

Table 2. Comparison of the analytical performance obtained with the constructed AgMWCNT/MTOAC/GCE and other electrodes for determination of SMX (µM). Type of electrode

Technique

Linear range

LOD

Ref.

1.4 - 118.6

3.95 × 10-1

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DPV

NiO/GO/1M3BIB/CPE

SWV

0.08 - 550

4.0 × 10-2

19

BDDE

SWV

6.1 - 60.1

1.15

45

SWV

0.5 - 150

0.05 × 10-3

41

DPV

0.1 - 0.7

2.4 × 10-2

46

DPV

2.0 - 160

3.0 × 10-2

47

DPV

0.05 - 70

1.0 × 10-2

This work

p-DAN/GCE

FeZnO/CPE

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P/MWCNT-SbNPs/CE

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MWCNTs/PE

Ag-MWCNT/MTOAC/GCE

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MWCNT/PE: MWCNT paste electrode; NiO/GO/1M3BIB/CPE: NiO/graphene oxide nanocomposite-ionic liquids (1-methyl-3-butylimidazolium bromide) modified carbon paste electrode; BDDE: Born doped diamond electrode; pDAN/GCE: poly-1,5-diaminonaphthalene modified glassy carbon electrode; P/MWCNT-SbNPs/CE: paraffin/MWCNT-SbNPs composite electrode.

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Table 3. Effects of some foreign species on the oxidation of 5.0 µM SMX at AgMWCNT/MTOAC/GCE.

Glucose

400

Lactose

400

Ascorbic acid

400

Uric acid

600

Glycine

600

Citric acid

600

Na+

1000

NH4+

1000

Cl a

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Trimethoprim

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Tolerance (µM)a

Foreign species

-

1000

The level with maximum concentration of foreign species for deviation less than ±5%

Table 4. Determination of SMX in Co-trimoxazole samples (mg/tablet) by DPV at the AgMWCNT/MTOAC/GCE and HPLC. Doses (mg)

Co-trimoxazole

400

Cotrim pediatric

200

a

Founda DPV

HPLC

Recovery (%)

412 ± 1.9

409 ± 0.9

103

2.47

4.46

2.78

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195 ± 1.4

197.1 ± 0.7

97.5

2.32

4.00

2.78

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Sample

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Average of 3 replicate measurements per sample. Critical t-value for (4) degrees of freedom at P (0.05). c Critical F-value for (2, 2) degrees of freedom at P (0.05).

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b

Table 5. Determination of SMX (µM) in urine samples. Sample

Human urine

a

Added

Founda

Recovery(%)a

0.0


10.0

9.7

97.0 ± 0.5

20.0

20.9

104.5 ± 0.5

30.0

31.1

103.7 ± 0.5

40.0

39.1

97.8 ± 0.5

---

Average for five measurements (n= 5). 16

t-test

F-test

tb

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Legend of figures: Fig. 1. EDX spectra of the Ag-filled MWCNT. The inset shows the TEM image that confirms the insertion of Ag nanoparticles into the MWCNT channel [36, 37].

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Fig. 2. EIS characteristics (A) and CV voltammograms (B) of 5.0 mM [Fe(CN)6]3−/4− solution containing 0.1 M KCl at a bare GCE (a), MTOAC/GCE (b), Ag-MWCNT/GCE (c) and the Ag-MWCNT/MTOAC/GCE (d).

Fig. 3. DPV voltammograms Ag-MWCNT/MTOAC/GCE in the buffer solution only (a) of 30.0

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µM SMX at a bare GCE (b), MWCNT/GCE (c), Ag-MWCNT/GCE (d) MTOAC/GCE (e), Ag-MWCNT/MTOAC/GCE (f) with pulse amplitude 50 mV, frequency 20 Hz, accumulation time of 2 min.

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Fig. 4. DPV voltammograms of 30.0 µM SMX on the surface of the Ag-MWCNT/MTOAC/GC electrode at various pHs from 2.0 to 8.0 (A), the relationship between Ip and the solution pH (B) and the peak potential (Ep) vs. pH values (C).

Fig. 5. The electrochemical oxidation of 60.0 µM SMX on the Ag-MWCNT/MTOAC/GCE at different scan rates 25, 50, 75, 100, 125, 150 and 175 mV/s. CV voltammograms (A), the

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dependence of Ip on the scan rate (B), the logI - logѵ plot (C) and the relationship between the Ep and logѵ (D).

Fig. 6. DPV voltammograms of different concentration of SMX (A), the calibration curve for the DPV voltammograms (B), under the optimal measurement conditions listed in Table 1 at

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Ag-MWCNT/MTOAC/GCE. Scheme 1. The possible oxidation mechanism of SMX at the surface of the modified Ag-

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MWCNT/MTOAC/GCE electrode.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Scheme 1.

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Highlights Introducing a new nanocomposite as the sensing element of the developed sensor.

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The sensing element with high electrical conductivity and low charge transfer resistance. A novel voltammetric sensor for sulfamethoxazole determination.

Outstanding performances due to robust and durable modification components.

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Selective and highly sensitive twoard sulfamethoxazole.