Positively charged carbon nanoparticulate/sodium dodecyl sulphate bilayer electrode for extraction and voltammetric determination of ciprofloxacin in real samples

Abstract

In this study, ciprofloxacin, a second-generation fluoroquinolone antibiotic, has been analyzed in human serum and a pharmaceutical preparation by a modified carbon paste electrode. The modified electrode was prepared using a layer-by-layer method with functionalized carbon nanoparticles and sodium dodecyl sulphate. Carbon nanoparticles (Emperor 2000™) were functionalized with ethylene diamine using a chemical method. The functionalized carbon nanoparticles were studied with scanning electron microscopy and FTIR spectroscopy. The modified electrode was characterized with electrochemical impedance spectroscopy and cyclic voltammetry. The ability of modified electrode to adsorb ciprofloxacin was investigated. The extraction parameters such as time, pH and agitation rate were optimized. In optimum conditions, ciprofloxacin was adsorbed at the surface of electrode and its electro-oxidation was studied. Differential pulse voltammetry was applied for quantitative determinations. Two linear ranges 5.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol L⁻¹ and 3.0 × 10⁻⁸ to 5.0 × 10⁻⁷ mol L⁻¹ with a limit of detection of 5.0 × 10⁻⁹ mol L⁻¹ were obtained. The prepared electrode exhibited the successful ability for the determination of ciprofloxacin in real samples such as human serum and commercial tablets with recoveries 99.85% and 101.5%, respectively.

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Introduction

Ciprofloxacin (CPFX) [1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7(piperazinyl) quinolone-3-carboxylic acid], a second-generation fluoroquinolone antibiotic, has been approved for exclusive use in humans.¹ Adverse effects of the drug include swelling or tearing of a tendon, especially the Achilles' tendon of the heel, toxic epidermal necrolysis, Stevens–Johnson syndrome, and aggravation of muscle weakness in patients with neurological disorder myasthenia gravis leading to breathing problems, agranulocytosis, thrombocytopenia and myelosuppression.^2–5 An overdose of ciprofloxacin results in severe hepatotoxicity.⁶ Thus, it is essential to quantify CPFX in human plasma/serum/urine to monitor drug accumulation. CPFX analysis also plays a critical role in drug quality control. Hence, there is a need to develop a reliable, sensitive, simple and rapid method for the determination of CPFX in pharmaceutical preparations.⁷

Over the previous years, a wide range of techniques such as spectrophotometry,^8–10 spectrofluorometry,^11–13 flow injection chemiluminescence,¹⁴ capillary electrophoresis,¹⁵ high-performance liquid chromatography,^16,17 micro-emulsion electrokinetic chromatography has been used for the simultaneous separation of seven fluoroquinolones and to measure ciprofloxacin and lomefloxacin concentrations in urine samples.¹⁸ In addition, combinations of capillary electrophoresis and photodiode array detection,^19,20 mass spectrometry,²¹ laser induced fluorescence²² and electrochemical analysis^23–26 have been used for analysis of CPFX. Among all the above, electrochemical techniques may be the most interesting owing to its advantages of low cost, quick response, simple instruction, high sensitivity, facile miniaturization and low power requirement.

Recently, new types of carbon nano-materials such as nanotubes,²⁷ graphenes,²⁸ and nanofibers²⁹ have attracted considerable interests and have become a wide range of research owing to their unique physical and chemical properties which can provide an important and feasible platform for electroanalysis particularly in the design of modified electrodes for electrochemical sensing.

In contrast to structurally more pure forms of nanocarbons such as nanotubes³⁰ or graphene,³¹ carbon nanoparticles are already used widely in industry and available at low cost and with an approved safety record. Carbon nanoparticles (CNPs), for example those derived from carbon blacks,³² provide a versatile platform for the development of electroanalytical tools³³ with high surface area and a high degree of functionalisation.³⁴ In recent study, we demonstrated attaching the cysteine on the surface of CNPs.³⁵ Frank Marken and et al. have been reported functionalizing CNPs with redox enzymes³⁶ and the use of dioctylamine functionalized CNPs as a novel substrate for lipids and membrane components.³⁷

Chemically modified carbon-paste electrodes (CMCPEs) have attracted large interest due to their potential applications in various analyses. These electrodes have been widely used in electroanalysis because of their ability to catalyze the redox processes of some molecules of interest. CMCPEs are inexpensive and possess many advantages such as low background current, wide range of potential windows (in both cathodic and anodic region), easy fabrication, rapid surface renewal,³⁸ and modification compatibility with various modifiers such as metal complexes,³⁹ metal nanoparticles,⁴⁰ carbon nanostructures,⁴¹ and enzymes.⁴²

Surfactant with a long hydrophobic C–H chain and a hydrophilic head group can adsorb at hydrophobic electrode surface (such as the bare or modified CPE electrode surface) via hydrophobic interaction. Therefore, surfactant will alter the properties of electrode/solution interface, and heavily influences the electrochemical process of electroactive species. To date, surfactant was widely used in electroanalytical chemistry to improve the sensitivity and selectivity.⁴³

In this approach, carbon nanoparticles (Emperor 2000™) have been functionalized with ethylene diamine. Carbon paste electrode has been modified using layer-by-layer method with functionalized CNPs and sodium dodecyl sulphate (SDS). The bilayer has shown good adsorption ability toward CPFX. The modified electrode exhibited the electrocatalytic effect through electro-oxidation of CPFX. The effects of adsorption parameters on CPFX extraction were studied. Finally, the modified electrode has been used for determination of CPFX in real samples, successfully.

Experimental

Materials

Carbon nanoparticles were obtained from Cabot (ca. 9 to 18 nm diameters, Emperor 2000™, Cabot Corporation). All other chemicals (thionyl chloride), dichloromethane. Acetonitrile, ethylene diamine (>99%) and SDS (>95%) were analytical reagent grade from Merck. All aqueous solutions were prepared with doubly distilled deionized water. Ciprofloxacin was obtained from TEMAD Co., Ltd (>99%). The CPFX tablets were purchased from a local pharmacy, Arya (Tehran, Iran).

Apparatus

Voltammetric experiments were performed using a Metrohm Computrace Voltammetric Analyzer model 797 VA. A conventional three-electrode system was used with a carbon paste electrode (3 mm diameter CPE), a KCl-saturated calomel reference electrode (SCE), and a Pt wire as the counter electrode. A digital pH/mV/Ion meter (Metrohm) was applied for the preparing of the buffer solutions, which were used as the supporting electrolyte in voltammetric experiments. Electrochemical impedance spectroscopic measurements were performed using a Galvanostat/Potentiostat microAutolab3. The surface morphology of the films was studied by a scanning electron microscope (SEM) images were obtained using LEO 1430VP. Fourier transform spectroscopy (FTIR) spectra was recorded by FTIR Spectroscopy RXI, Perkin Elmer.

Procedures (I): preparation of positively charged CNPs

Positively charged CNPs have been prepared by using literature method.⁴⁴ For step (A) (see Fig. 1): typically 1 g of carbon nanoparticles (Emperor 2000) were sonicated in dry dichloromethane in a round bottom flask for 30 min. The flask was degassed with nitrogen gas at 0 °C and 10 cm³ of thionyl chloride was added dropwise under continuous stirring. The flask was then allowed to warm to room temperature whilst stirring for 2–3 h. Excess thionyl chloride and solvent were removed by rotary evaporation.

Fig. 1 (A) Conversion of sulfonate to sulfonylchloride with thionylchloride at 0 °C. (B) Formation of sulfonamide by reacting sulfonylchloride with ethylene diamine at 0 °C in dichloromethane.

For step (B) (see Fig. 1): 10 cm³ of ethylene diamine was added into a 250 cm³ round bottom flask with 30 cm³ of dry dichloromethane and the temperature cooled to 0 °C. The sulfonyl chloride functionalized carbon nanoparticles were added in small portions, and then the reaction was allowed to warm to room temperature whilst stirring for 2 h. Excess amine and dichloromethane were removed by rotary evaporation. Aqueous 1 M HCl was then added and a black solid was collected by Buchner filtration (CNPs–NH₃⁺).

Procedure (II): preparation of modified electrode

The unmodified carbon paste electrode (CPE) was prepared by mixing graphite powder with appropriate amount of mineral oil (Nujol) and thorough hand mixing in a mortar and pestle (∼75 : 25, w/w),⁴⁵ and a portion of the composite mixture was packed into the end of a Teflon tube (about 3.0 mm i.d.). Electrical contact was made by forcing a copper pin down into the Teflon and into the back of the composite. Both modified and unmodified electrodes were polished on a paper in order to obtain a smooth and shiny surface.

For preparation of modified electrode, a stable suspension of CNP–NH₃⁺ containing 3 mg/10 mL in acetonitrile was dispersed using ultrasonic agitation for 60 min. The suspension is very stable during two months. 10 μL of this suspension was casted on the CPE surface and dried in the air to evaporate solvent. Bilayer of CNP–NH₃⁺/SDS has been prepared by soaking the CNP–NH₃⁺ modified electrode in SDS solution (8.0 mM) for 10 s immersion time. The CNP–NH₃⁺/SDS electrode was completely rinsed with ultrapure water to remove the unadsorbed SDS. The surface of electrode should be prepared before every experiment by casting of CNP–NH₃⁺ at the surface of CPE. The obtained modified electrode (CPE/CNP–NH₃⁺/SDS) was characterized by using electrochemical impedance spectroscopy (ESI) and cyclic voltammetry techniques (CV).

Procedure (III): extraction of CPFX at the surface of CPE/CNP–NH₃⁺/SDS⁻ and cyclic voltammetric studies

The stock standard solution of 1 mM CPFX was prepared by dissolving 9.64 mg of CPFX in phosphate buffer (pH 3.0) and diluting with the buffer for preparation of various concentrations. The extraction process was performed onto the modified electrode within 5 min under stirring. The electrode was taken out of the adsorption cell and was then transferred into voltammetric cell. Voltammogram was recorded by applying potential from −0.50 to +1.40 V.

Procedure (IV): preparation real samples

Serum samples obtained from healthy individuals (after obtaining their written consent) were stored frozen until assay. Methanol were tried as a serum precipitating agents. 5 mL of serum treated with 5 mL of methanol as serum denaturating and precipitating agent. The tubes were vortexed for 10 min and then centrifuged for 40 min at 5000 rpm for removing of protein residues. The supernatant was taken carefully, then the volume was completed to 25 mL with the buffer solution of pH 3.0. The standard addition method was applied for calculation of recoveries in spiking of CPFX to human serum.

A total of three tablets of CPFX (with labelled values of 500 mg CPFX per a tablet) accurately weighed and powdered. This powder accurately transferred into 50 mL calibrated flask and the flask was filled to volume with 0.1 M phosphate buffer with pH 3.0, then the flask was sonicated to 15 min at room temperature. The resulting solutions were filtered through Whatman filter paper no 42 and suitably diluted into 100 mL to get final concentration of 0.038 mol L⁻¹. Standard addition method was used for determination of CPFX in pharmaceutical tablet.

Results and discussion

Scanning electron microscopic results

To study the morphology of functionalized CNPs (CNP–NH₃⁺) and the surface of electrodes, scanning electron microscopy (SEM) has been utilized. Scanning electron image of the CNP–NH₃⁺ exhibits the carbon nanoparticles with a particle size in the order of 20–40 nm radiuses (see Fig. 2a). Scanning electron micrographs of the surface of CPE and CPE/CNP–NH₃⁺/SDS⁻ have been shown in Fig. 2b and c, respectively. It seems the CNP–NH₃⁺ aggregate during the functionalization and casting procedures. Due to the porous nature of film, water and electrolyte can readily access the film whereas electrical contact and conductivity via carbon nanoparticles is maintained.

Fig. 2 SEM image of (a) CNP–NH₃⁺ powder (b) CPE surface (c) CPE/CNP–NH₃⁺/SDS⁻ surface.

FTIR spectroscopy results

The formation of sulfonamide-functionalized carbon nanoparticles is clearly indicated by FT-IR (ESI data 1†). In the spectrum of sulfonamide-functionalized carbon nanoparticles the absorption band appeared at 1588 cm⁻¹ can be attributed to bending vibrations of N–H bond. The stretching vibrations related to S=O and C–N bonds are appeared at 1180 cm⁻¹ and 1036 cm⁻¹, respectively. The broad adsorption band centred at 3418 cm⁻¹ is related to N–H stretching vibration of amino group.

Voltammetric studies of CPE/CNP–NH₃⁺/SDS⁻

Capacitive currents. The surface characteristics and microscopic area of CPE, CPE/CNP–NH₃⁺, CPE/CNP–NH₃⁺/SDS⁻ were obtained by comparison of the CV responses in 0.1 M phosphate buffer pH 2.0 (Fig. 3). Capacitance values of ca. 9.36 μF, 897 μF and 780 μF for a CPE, CPE/CNP–NH₃⁺, CPE/CNP–NH₃⁺/SDS⁻ (ca. 3 μg carbon) were estimated, respectively. A significant increase in capacitive current suggests a great increasing active electrode surface area (with ca. 260 F g⁻¹ CNPs). This increase in surface area can be further controlled by changing the amount of film deposited at the electrode surface.

Fig. 3 Capacitive currents for CPE, CPE/CNP–NH₃⁺/SDS⁻​ ​electrodes.

Cyclic voltammetric results and microscopic area calculation. The microscopic areas of the modified and the unmodified carbon paste electrode were obtained by the cyclic voltammetry using K₄Fe(CN)₆ as a probe at different potential scan rates. For a reversible process, the following Randles–Sevcik formula is used:⁴⁰

i_p,a = 2.69 × 10⁵n^3/2AC_oν^1/21/2D_o^1/2 (1)

here, i_p,a refers to the anodic peak current, n the electron transfer number, A the microscopic surface area of the electrode (cm²), D_o the diffusion coefficient (cm² s⁻¹), C_o the bulk concentration of K₄Fe(CN)₆ (mol cm⁻³) and ν is the scan rate (V s⁻¹). The microscopic areas can be calculated from the slope of the plot of i_p,a vs. ν^1/2 (ESI data 2†). For 1 mM K₄Fe(CN)₆ in 0.1 M KCl electrolyte (n = 1 and D_o = 6.7 × 10⁻⁶ cm² s⁻¹), the electrode surface area of the modified electrode was 0.288 cm², and for the unmodified electrode it was 0.086 cm². This shows that the microscopic area of the modified electrode increased significantly. It is more than 3 times larger than the microscopic area of the unmodified electrode.

Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) was employed to investigate the impedance changes of the electrode surface due to the modification procedure. Fig. 4 shows the Nyquist plots of K₃Fe(CN)₆/K₄Fe(CN)₆ at the surface of CPE and CPE/CNP–NH₃⁺/SDS⁻ In these studies, high frequency zone, which appears as a nearly semicircle plot, can be ascribed to the kinetic limitations (R_ct) of the electrochemical reaction. On the other hand, the linear behavior of Z_Im versus Z_Re in a low frequency region is characteristic of a diffusion-controlled electrode process. As it can be seen in Fig. 4, a semicircle with a very large diameter is observed at CPE. However, the diameter of the semicircle is significantly reduced with CPE/CNP–NH₃⁺/SDS⁻, which suggests that the surface of the modified electrode exhibits lower electron transfer resistance and greatly increases the electron transfer rate. It should be mentioned the diameter of the semicircle for CPE/CNP–NH₃⁺ is slightly smaller than CPE/CNP–NH₃⁺/SDS⁻ (see inset of Fig. 4).

Fig. 4 Nyquist plots (Z′′ versus Z′) for the EIS measurements of (●) CPE, (○) CPE/CNP–NH₃⁺/SDS⁻ (inset) (a) CPE/CNP–NH₃⁺ (b) CPE/CNP–NH₃⁺/SDS⁻ in 5 mM K₄Fe(CN)₆/K₃Fe(CN)₆ + 0.1 M KCl at the formal potential: 0.2 V, frequency range: 0.01–1 000 000 Hz.

Voltammetric studies of adsorbed CPFX and CPFX solution at the surface of CPE/CNP–NH₃⁺/SDS⁻

The affinity of CPFX to adsorb CNP–NH₃⁺/SDS⁻ film at the carbon paste electrode is been studied (i) in aqueous solution of CPFX and (ii) in pure electrolyte after pre-immersion of the electrode into CPFX. Fig. 5 shows a typical set of voltammograms for the oxidation of 1.0 × 10⁻⁵ M solution of CPFX in buffer solution pH 3.0 at CPE/CNP–NH₃⁺/SDS⁻ electrode (solid line), CPE/CNP–NH₃⁺ electrode (dashed line), CPE/SDS (dashed-dotted line) and CPE (dotted line) electrode. For pre-adsorption measurements the modified electrode is immersed for 5 min in CPFX solution and then rinsed and transfer into clean phosphate buffer solution pH 3.0 for cyclic voltammetry analysis.

Fig. 5 (a) DPV of 1.0 × 10⁻⁵ M solution of CPFX in buffer solution pH 3.0 at CPE/CNP–NH₃⁺/SDS⁻ electrode (solid line), at CPE/CNP–NH₃⁺ (dashed line), at CPE/CNP (dotted line) and at CPE/SDS (dashed-dotted line). (b) Comparison of 1.0 × 10⁻⁵ M solution of CPFX with adsorption and without adsorption. For pre-adsorption measurements the modified electrode is immersed for 5 min in CPFX solution (phosphate buffer, pH 3.0) and then rinsed and transfer into clean phosphate buffer solution pH 3.0 for voltammetry analysis. Scan rate was 100 mV s⁻¹.

As can be seen, a considerable enhancement in the peak current using the CNP–NH₃⁺/SDS⁻ modified electrode obtained. The comparison of the charge under the voltammetric peak and peak current of CPFX in solution (Fig. 5b dashed line) and adsorbed (Fig. 5b solid line) CPFX confirms that the affinity of it to adsorb to CNP–NH₃⁺SDS⁻ is clearly evident.

The optimization of CPFX extraction at the surface of CPE/CNP–NH₃⁺/SDS⁻

To obtain the optimum conditions of CPFX extraction, the effect of several parameters has been investigated on oxidation peak current of CPFX. The time, pH, SDS concentration and agitation rate in extraction process were optimized.

Various times 2, 5, 10, 20 and 30 min were studied and the maximum peak current has been observed for 5 min (see Fig. 6a). By increasing time, extraction process was progressing and after 5 min all binding site have been filled by CPFX. An extraction time of 5 min was used for further study. Since the extraction is a mass transfer process, agitation may have a certain effect. Agitation rates 250, 375, 500 and 750 rpm were examined, the best signal was observed for 375 rpm (see Fig. 6b). Fig. 6c exhibits the role of pH in extraction step. The well-defined anodic peak currents were obtained for pHs 3 and 5. Because the shape of peak for pH 3 was sharper especially in lower concentrations, the pH 3 was chosen for further studies. Fig. 6d exhibits the role of SDS concentration in extraction step. SDS concentration 1.0, 4.0 and 8.0 mM were examined, the best signal was observed for 8.0 mM. Because the critical micelle concentration (CMC) of SDS is immediately after 8.0 mM, a further increase in concentration was not studied.

Fig. 6 (a) The plot of I_pa versus extraction time (b) the plot of I_pa versus agitation rate (c) the plot of I_pa versus pH for solutions of CPFX during extraction (d) the plot of I_pa versus SDS concentration. For extraction measurements the modified electrode is immersed for appropriate time in CPFX solution and then rinsed and transfer into clean phosphate buffer solution pH 3.0 for cyclic voltammetry analysis. Scan rate was 100 mV s⁻¹.

The effect of pH and scan rate on voltammetric response

The cyclic voltammetric studies for adsorbed CPFX in optimized conditions were performed at the surface of the CPE/CNP–NH₃⁺/SDS⁻ in a buffered solution of pH 2.0 at different potential sweep rates (see Fig. 7a). Experiments at various scan rates indicate fast electron conduction within the CNP–NH₃⁺/SDS⁻ film. The peak current is approximately linearly related to the scan rate by following equations, suggesting that confirms the CPFX oxidation follows a surface controlled mechanism (see Fig. 7b).

I_pa (μA) = 1047.9ν (V s⁻¹) − 4.8521, R² = 0.9974 (2)

Fig. 7 (a) Cyclic voltammograms of pre-adsorbed CPFX in various scan rates (b) plot of the anodic peak current versus the scan rate consistent with an pre-adsorbed CPFX onto the modified electrode is immersed for 5 min in CPFX solution (phosphate buffer, pH 3.0) and then rinsed and transfer into clean phosphate buffer solution pH 2.0.

Fig. 8a exhibits the cyclic voltammograms of adsorbed CPFX at the surface of CPE/CNP–NH₃⁺/SDS⁻ with various pHs of voltammetric buffer solution. Following the peak potentials of oxidation of CPFX shows a negative shift by increasing of the pH of the buffer solution (Fig. 8b). This confirms that H⁺ participates in oxidation of CPFX correspond to Scheme 1. With considering the resulted slope for peak potentials, the process follows a two electron–two proton mechanism (eqn (3)).⁴⁶

E_p = 0.0719 pH + 1.372, R² = 0.9959 (3)

Fig. 8 (a) Cyclic voltammograms of pre-adsorbed CPFX in various voltammetric cell pHs (b) plot of the anodic peak potential versus pH consistent with an pre-adsorbed CPFX onto the modified electrode is immersed for 5 min in CPFX solution (phosphate buffer, pH 3.0) and then rinsed and transfer into buffered solution with various pHs. Scan rate was 100 mV s⁻¹.

Scheme 1 Electro-oxidation mechanism of ciprofloxacin.

Analytical results

Calibration curves. The effect of CPFX concentration during the extraction process is investigated. Fig. 9a and b shows a typical set of CPFX oxidation responses as a function of CPFX concentration. The CPFX oxidation is observed at nanomolar level and the signal increases systematically with concentration, after filling the binding sites the signal become constant. Under the optimum conditions, the calibration curves of CPFX were obtained (insets of Fig. 9a and b).

Fig. 9 (a) DPV of various concentrations of CPFX down to up: 3 × 10⁻⁸, 5 × 10⁻⁸, 8 × 10⁻⁸, 1 × 10⁻⁷, 3 × 10⁻⁷, 5 × 10⁻⁷ M (b) 5 × 10⁻⁷, 8 × 10⁻⁷, 1 × 10⁻⁶, 3 × 10⁻⁶, 5 × 10⁻⁶, 8 × 10⁻⁶, 1 × 10⁻⁵ M. For pre-adsorption measurements the modified electrode is immersed for 5 min various concentration of CPFX solution (phosphate buffer, pH 3.0) then rinsed and transferred into clean phosphate buffer solution pH 2.0 for cyclic voltammetry analysis at electrode. (50 mV pulse amplitude, 5 mV step potential, 0.05 s step time).

The oxidation peak current was linearly proportional to the concentration in the ranges 5.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol L⁻¹ and 3.0 × 10⁻⁸ to 5.0 × 10⁻⁷ mol L⁻¹, respectively (insets of Fig. 9a and b). The linear equations are I_pa (μA) = 0.7622C (μM) − 4.567 (R² = 0.9922), I_pa (μA) = 6.908C (μM) + 0.5238 (R² = 0.9947) and the limit of detection was estimated 5.0 × 10⁻⁹ mol L⁻¹ respectively. Table 1 compares some electrochemical methods which exist in literature for determination of Hg. The presented modified electrode exhibits the better analytical results and also it is less expensive than most of the electrode used previously.

Table 1 Comparison of some electrochemical methods which previously used for the determination of CPFX

Electrode	Method	LOD (μmol L^−1)	Linear range (μmol L^−1)	Reference
a Poly(alizarin red)/electrodeposited graphene/glassy carbon electrode.b Multi-wall carbon nanotubes film-modified glassy carbon electrode.c Electrochemically activated glassy carbon paste.d Hanging mercury drop electrode.
PAR/EGR/GCE^a	DPV	0.01	0.04–10.0	46
			10.0–12.0
MWCNT/GCE^b	Chronoamperometry	6.00	40.0–1000.0	47
Nano Ba0.5Co0.5Fe2O4/GCE	LSV	0.006	0.01–1500.0	48
GCPE^c	SWASV	0.033	0.27–20.0	49
CPE-CTBA	DPV	0.05	0.10–20	50
HMDE^d	SWV	0.007	0.30–2.0	51
CPE/CNP–NH3^+/SDS^−	DPV	0.005	0.03–0.50	This work
			0.50–10.0

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Real samples. For calculating the applicability of the proposed route in the real sample analysis, it was used to determine CPFX in human serum. The standard addition method was applied for calculation of recoveries in spiking of CPFX to human serum. The slope of the calibration curve, which was obtained with the spiked standard solution of CPFX in the range of 3.0 × 10⁻⁸ to 5.0 × 10⁻⁷ mol L⁻¹, was 6.898 μA μM⁻¹ with a correlation coefficient of (R²) 0.9973. Compared with the standard curve, 6.908 μA μM⁻¹ a recovery of 99.85% was obtained with the new method, revealing that the method is appropriate for accurate determination of CPFX in real and complex human serum samples.

CPFX was determined in pharmaceutical tablet sample containing 500 mg CPFX by using standard addition method. Three tablet samples of CPFX (with labelled values of 500 mg CPFX per a tablet) were powdered and solution was prepared in 0.1 M phosphate buffer with pH 3.0. The slope of the calibration curve, which was obtained with the spiked standard solution of CPFX in the range of 3.0 × 10⁻⁸ to 5.0 × 10⁻⁷ mol L⁻¹, was 7.017 μA μM⁻¹ with a correlation coefficient of (R²) 0.9965. Compared with the standard curve, 6.908 μA μM⁻¹ a recovery of 101.5% was obtained with the new method.

Reproducibility of sensor

The reproducibility of the modified electrode was investigated in two concentration levels of CPFX (5.0 × 10⁻⁶ M and 1.0 × 10⁻⁷ M) in buffer solution pH 3.0 and potential scan rate 0.05 V s⁻¹ by using voltammetric measurements for ten repeated measurements. The relative standard deviations for CPFX determination, based on the ten replicates of analysis were 0.61%, 1.56% for 5.0 × 10⁻⁶ M and 1.0 × 10⁻⁷ M, respectively.

Conclusions

The voltammetric studies of CPFX at the surface of the CPE/CNP–NH₃⁺/SDS⁻ modified electrode have shown adsorption-like behaviour. Differential pulse voltammetry is applied successfully for the determination of trace amounts of CPFX in the clinical and pharmaceutical solutions. Fast electron transfer, excellent sensitivity, reproducibility and easy preparation, suggested that the CPE/CNP–NH₃⁺/SDS⁻ electrodes may provide a new strategy for CPFX concentration determination in physiological solutions.

Acknowledgements

The authors gratefully acknowledge the support of this work by University of Mohaghegh Ardabili research council, Ardabil, Iran.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03170f

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