Highly sensitive voltammetric sensor based on immobilization of bisphosphoramidate-derivative and quantum dots onto multi-walled carbon nanotubes modified gold electrode for the electrocatalytic determination of olanzapine

Materials Science and Engineering C 60 (2016) 67–77

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Highly sensitive voltammetric sensor based on immobilization of bisphosphoramidate-derivative and quantum dots onto multi-walled carbon nanotubes modified gold electrode for the electrocatalytic determination of olanzapine Leila Mohammadi-Behzad a, Mohammad Bagher Gholivand a,⁎, Mojtaba Shamsipur a, Khodayar Gholivand b, Ali Barati a, Akram Gholami b a b

Department of Analytical Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran Department of Chemistry, Tarbiat Modares University, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 9 September 2014 Received in revised form 14 August 2015 Accepted 23 October 2015 Available online 27 October 2015 Keywords: Olanzapine Quantum dots Carbon nanotubes Bisphosphoramidate compounds Scanning electron microscopy

a b s t r a c t In the present paper, a new bisphosphoramidate derivative compound, 1, 4-bis(N-methyl)-benzene-bis(N-phenyl, N-benzoylphosphoramidate) (BMBPBP), was synthesized and used as a mediator for the electrocatalytic oxidation of olanzapine. The electro-oxidation of olanzapine at the surface of the BMBPBP/CdS-quantum dots/multi-walled carbon nanotubes (BMBPBP/CdS-QDs/MWCNTs) modified gold electrode was studied using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. This sensor showed an excellent electrocatalytic oxidation activity toward olanzapine at less positive potential, pronounced current response, and good sensitivity. The diffusion coefficient and kinetic parameters (such as electron transfer coefficient and the heterogeneous rate constant) were determined for olanzapine oxidation, using the electrochemical approaches. Surface morphology and electrochemical properties of the prepared modified electrode were investigated by scanning electron microscopy (SEM), cyclic voltammetry and electrochemical impedance spectroscopy techniques. The hydrodynamic amperometry at rotating modified electrode at constant potential versus reference electrode was used for detection of olanzapine. Under optimized conditions, the calibration plot was linear in the concentration range of 20 nM to 100 μM and detection limit was found to be 6 nM. The proposed method was successfully applied to the determination of olanzapine in pharmaceuticals and human serum samples. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Olanzapine (OLZ), a new antipsychotic agent, was approved by the U.S. Food and Drug Administration in 1996 for use in the treatment of schizophrenia and related disorders [1]. Schizophrenia is a neurological disorder that strikes about 1% of the world's population, or around 50 million people. Typical neuroleptic drugs treat the positive symptoms of psychosis (hallucinations, paranoia and delusions) but are largely ineffective in treating many of the negative symptoms (low levels of drug therapy. Interest, lack of motivation, social withdrawal and poverty of speech). Clinical studies indicate that olanzapine is as effective as haloperidol in reducing positive symptoms and superior in reducing negative symptoms [2]. Olanzapine is well absorbed after oral dosing Abbreviations: OLZ, olanzapine; MWCNTs, multi-walled carbon nanotubes; QDs, quantum dots; BMBPBP, 1, 4-bis(N-methyl)-benzene-bis(N-phenyl, Nbenzoylphosphoramidate); EIS, electrochemical impedance spectroscopic; NHS, Nhydroxysuccinimid; EDC, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride; TEM, transmission electron microscope; PBS, phosphate buffer solution. ⁎ Corresponding author. E-mail address: [email protected] (M.B. Gholivand).

http://dx.doi.org/10.1016/j.msec.2015.10.068 0928-4931/© 2015 Elsevier B.V. All rights reserved.

and extensively metabolized to many primary metabolites such as N-desmethyl, N-oxide, 2-hydroxymethyl, 4′-N-glucuronide and 10-Nglucuronide metabolites [3]. Several analytical methods for the determination of OLZ in pharmaceutical and biological matrices have been published. These include liquid chromatography coupled to mass [4], electrochemical detection [5–8] and ultraviolet (UV) [9–11] detection, high performance thin layer chromatography (HPTLC) [12,13] as well as spectrophotometry [14–18] and electroanalytical methods [19–21]. In addition to low sensitivity and selectivity in some cases, these methods are time consuming, high cost, and often needs to the pretreatment step, and thus they are unsuitable for a routine analysis. Electrochemical techniques have been shown to be excellent procedures for the sensitive determination of drugs and related compounds in pharmaceutical dosage forms and biological fluids [22]. The advance in electrochemical techniques in the field of drug analysis is due to their simplicity, high sensitivity, low cost, and relatively short analysis time compared to the other techniques. The selectivity of the voltammetric techniques can be reached by specific applied potential. Moreover, electrochemistry is most suitable for investigating the redox properties of drugs that can give insight into its metabolic fate. The data obtained

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from electrochemical techniques are often correlated with molecular structures and pharmacological activities of drugs. Electrochemistry of olanzapine has been slightly investigated. Mashhadizadeh et al. [19] studied the electrochemistry of olanzapine on a carbon paste electrode modified with ZnS Nanoparticles. Merli et al. [20] determined olanzapine in pharmaceutical using a gold electrode modified with oxidized single walled carbon nanotubes. Arvand et al. [21] employed amine-functionalized TiO2/multi-walled carbon nanotubes modified glassy carbon electrode for electrooxidation and determination of olanzapine. Thus, there is still a need to develop other electrode materials that can detect olanzapine in pure and pharmaceutical forms as well as analysis in biological fluids. Recently, nanoparticles have been used as modifier in chemically modified electrode for analysis of drug molecules. Nanostructured carbon materials, especially functionalized carbon nanotubes (CNTs) with unique properties, such as high chemical stability, good electrical conductivity, high surface-volume ratio and high adsorption capacity, and when they anchored with functional groups, are one of the most promising supporting materials for surface modification of electrodes [22–27]. Due to unique optical and electronic properties, semiconductor quantum dots (QDs) have found potential applications in several areas, including catalysis, coatings, textiles, data storage, biotechnology, health care, biomedical, pharmaceutical industries and most recently, in construction of sensors [28]. Compared to using QDs and or multi-walled carbon nanotubes (MWCNTs) alone, the junction of QDs and CNTs could render a better platform for fabrication of nanoscale modifiers as sensing materials [29–33]. Chemical binding is one of the practical strategies for assembling QDs onto carbon nanotube (CNT) surface through coavalent binding between functional groups of surface modifiers of QDs and MWCNTs. Cysteine is one of these capping agents that has been used for modification of QDs [34,35]. In the present work the cysteine capped CdS-QDs was attached to carboxylated MWCNTs which their combination exhibits synergistic effects toward target analysis. Bisphosphoramidates are some important instances of phosphorus derivatives containing P–N function have a wide range of biological activities [36,37]. The common feature of most bisphosphoramidate compounds arises from the presence of alkyl diamine in their structure, providing a quasi-irreversible electron transfer character for these compounds, and making them as suitable mediators. In the present work, we describe an electrochemical method based on a new bisphosphoramidate-derivative, 1, 4-bis(N-methyl)-benzene-bis(Nphenyl, N-benzoylphosphoramidate) (BMBPBP) film immobilized on the surface of CdS-QDs/MWCNTs Au electrode for the electrocatalytic determination of olanzapine. Finally, the catalytic ability of the modified electrode is tested through the electro-oxidation of olanzapine in the pharmaceuticals and blood serum samples.

2. Experimental 2.1. Chemicals Carboxylated multiwall carbon nanotubes with purity 95% (10 nm diameters) and 1–2 μm lengths were obtained from Dropsens (Llanera, Spain). N-hydroxysuccinimide (NHS), 1-ethyl3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and cysteine were purchased from Sigma-Aldrich (St. Louis, USA). Aniline, acetonitrile, dichloromethane, 1, 4-bis(aminomethyl)-benzene and triethylamine were purchased from Merck (Germany) and Fluka. All other chemicals were of analytical-reagent grade and used without further purification. All solutions were prepared with doubly distilled and deionized water. All experiments were carried out at the ambient temperature of 25 ± 1 °C. Phosphate-buffer solutions were prepared by mixing suitable amounts of salts 0.1 M NaH2PO4, Na2HPO4, Na3PO4, 0.1 M NaOH and 0.1 M HCl.

2.2. Apparatus Electrochemical experiments were performed via using a μAutolab III (Eco Chemie B.V.) potentiostat/galvanostat by NOVA 1.8 software. A conventional three-electrode cell was used with a saturated Ag/AgCl as reference electrode, a Pt wire as counter electrode and a modified Au (3 mm diameter) as working electrode. The cell was a one-compartment cell with an internal volume of 10 ml. JENWAY pH-meter (model 3345) was also applied for pH measurements. The cyclic voltammograms were recorded in the potentials ranging from − 0.1 to + 0.4 V using different scan rates. The electrochemical impedance experiment was carried out in 0.1 M KCl solution containing 5 mM Fe(CN)46 −/3 − at frequencies ranging from 100 kHz to 0.1 Hz with an amplitude of 5 mV using an Autolab instrument in combination with the FRA2 module. In order to perform electrochemical impedance spectroscopy measurements the FRA module must be installed and the hardware setup in NOVA must be set up accordingly. The morphology and particle sizes of the samples were characterized by scanning electron microscope (SEM) (X-30 Philips) and transmission electron microscope (TEM) (Zeiss-EM10C − 80 kV).

2.3. Synthesis of CdS-quantum dots by seeds assistant technique L-cysteine capped CdS quantum dots were synthesized according to the previously published seeds-assistant modified technique [38]. Briefly, to a three-necked flask, 1 mmol L-cysteine was dissolved in 100 mL of deionized water and vigorously stirred under nitrogen gas at room temperature for at least 60 min. The pH of the solution was adjusted using 0.5 M Tris buffer solution (pH 9). Subsequently, a 0.50 mmol of Cd (NO3)2 solution was dropped slowly into the solution and reacted for 30 min. Then 10 mL of 0.25 mmol of Na2S was injected slowly into the solution. The seeds solution was prepared by mixing 2 mL of 10−4 M of Cd2+ solution and the same amount of S2− solution and was added into the mixture solution. All steps were performed under magnetic stirring. The solution was aged at 47 °C in bath water for 2 h to form Lcysteine capped CdS-QDs followed by 30 min nitrogen flushing and 0.5 h stirring [39].

2.4. Synthesis of 1, 4-bis(N-methyl)-benzene-bis(N-phenyl, N-benzoylphosphoramidate) (C 6 H5 –C(O)–NH)(C 6 H5 –NH)P(O)Cl was prepared by the reaction of aniline with (C6H5–C(O)–NH)P(O)Cl2 in 2:1 M ratio. The aniline was added drop wise to a CH3CN solution (20 ml) of (C6 H5– C(O)–NH)P(O)Cl 2 at 0 °C. After 4 h stirring, the solvent was removed in vacuum and the resulting was washed with distilled water and dried. In the next step, a solution of 1, 4-bis(aminomethyl)benzene (1 mmol) and triethylamine (2 mmol) in CH 2Cl2 was added at 0 °C to a solution of (C 6H5 –C(O)–NH)(C 6 H5–NH)P(O)Cl (2 mmol) in CH2Cl2. After 4 h stirring, the solvent was removed in vacuum and the resulting white powder was washed with distilled water. Powder sample; 1H NMR (500.13 MHz, d6–DMSO, 25 °C, TMS); δ = 6.42 (d, 3JHH = 8.5 Hz, 2H–Ph), 6.85 (m, 4H–Ph), 7.16 (m, 8H–C6H4, Ph), 7.44 (t, 3JHH = 7.4 Hz, 4H–Ph), 7.55 (t, 3JHH = 7.7 Hz, 4H–Ph), 7.71 (d, 3 JHH = 9.05 Hz, 2H–NHAn), 7.90 (d, 3JHH = 8.3 Hz, 2H–Ph), 9.77 (br, 2H–NHamide). 13C NMR (125.75 MHz, d6–DMSO, 25 °C, TMS); δ = 114.48 (phenyl), 117.64–117.70 (d, phenyl), 118.76–118.81 (d, phenyl), 120.37 (phenyl), 128.10 (phenyl), 128.29 (phenyl), 128.70 (phenyl), 132.26 (phenyl), 134.6 (phenyl), 141.39 (phenyl). 31P{1H} and 31 P NMR (202.45 MHz, d6–DMSO, 25 °C, H3PO4 external); δ = − 4.61 ~ = 3217 (N–H), 1660 (C_O), 1220 and − 3.94 (m). IR (KBr): υ (P_O), 935 (P–N).

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2.5. Preparation of the modified electrode The procedure of the fabrication of the sensor is illustrated in Scheme 1. Firstly, the gold electrode was carefully polished with alumina powder (0.05 μm) on polishing cloth. Then it washed with distilled water and ultrasonically cleaned with ethanol and double distilled water to remove adsorbed alumina particles. Functional MWCNTs suspension was prepared by dispersing 1 mg of MWCNTs in 1 mL of DMF and then immersed in an ultrasound bath during 30 min. Afterward, 5 μL of MWCNTs suspension was cast on the surface of Au electrode and dried in air to form a MWCNTs film at the electrode surface. The MWCNTs/Au electrode was immersed into phosphate buffer solution (PBS) containing 1 mM of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride EDC and N-hydroxysuccinimide NHS (pH 7.0) as a coupling agent for about 2 h to activate the carboxylterminated surface of the MWCNTs, followed by rinsing with buffer solution and drying in air. Then the resulted electrode was immersed into solution containing CdS-QDs (pH 7.0) in the presence of 0.1 mM EDC and NHS for 12 h to form Cds-QDs/MWCNTs/Au electrode. During this process the cysteine capped, CdS-QDs can be covalently bonded to the carboxylic group of the functional MWCNTs. The electrode was removed from the solution and washed with double distilled water, and then it was drying in air again. The assembling of BMBPBP on the surface of CdS-QDs/MWCNTs/Au electrode was carried out by dipping the electrode into DMSO solution containing 1 mM of BMBPBP for 30 min. Finally, the modified electrode (labeled as BMBPBP/CdS-QDs/MWCNTs/Au electrode) was soaked in PBS for 10 min to remove unbound BMBPBP and then after washing it dried in air and kept at room temperature for further use. As the amounts of MWCNTs, CdS-QDs and bisphosphoramidate (BMBPBP) have important role in the analytical response of the proposed modified electrode, their amounts on the performance of the modified electrode were tested. In each case the amounts of two variables were fixed while the other was varied to achieve the maximum response. The amounts of MWCNTs (1 mg/mL), CdS-QDs (1:10 V% in buffer solution) and bisphosphoramidate (BMBPBP) were varied in the range of 2–10 μL (MWCNTs), 50–150 μL (CdS-QDs in solution containing of EDC and NHS for 12 h) and 0.1–5 mM (BMPBP in solution). The best response was obtained by 5 μL of MWCNTs, 100 μL of CdS-

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QDs and 1 mM of BMBPBP in solution which were used for construction of the modified electrode. 2.6. Preparation of real samples The average weight of ten tablets (Sobhan Pharmaceutical Co.), with labeled amounts of 5 mg/tablet of OLZ was determined; then, they were finely powdered and homogenized in a mortar. An aliquot amount of the homogenized powdered sample was dissolved in distilled water and after 30 min sonication, the mixture was filtered and diluted to volume (100 mL) with distilled water to achieve a final concentration of 10 μM. Blood serum samples were collected from healthy volunteers. An aliquot volume of the sample was fortified with known amounts of the OLZ, and then it was treated with 0.5 mL methanol as serum protein denaturation and precipitating agent, and diluted up to 10 mL with doubly distilled water to achieve the final concentration of 50 μM. After vortexing for 60 s, the mixture was centrifuged for 10 min at 10,000 rpm. The clear supernatant layer was filtrated through a 0.45 μm Milli-pore filter to produce a protein free human serum. 3. Results and discussion 3.1. TEM and SEM studies The morphology of the synthesized L-cysteine capped CdS-QDs was studied by TEM. TEM results showed that the L-cysteine capped CdSQDs are close to spherical, with the average particle semidiameter of 30–40 nm (Fig. 1a). The surface morphology of the bare Au, MWCNTs/Au and CdS-QDs/ MWCNTs/Au electrodes was inspected by scanning electron microscopy (SEM) (Fig.1b–d). The morphology of the bare Au electrode (Fig. 1b) reveals a smooth, homogeneous surface and featureless morphology, characterized by the presence of polishing streaks. The MWCNTs/Au electrode (Fig. 1c) exhibited a net structure. This Fig. 1c confirms that the surface of Au electrode was well covered by MWCNTs. It is obvious that MWCNTs were distributed uniformly on the surface of Au electrode. After immobilization of the CdS-QDs, the morphology of the electrode surface was obviously changed indicating that the CdS-QDs are

Scheme 1. Stepwise fabrication process of BMBPBP/CdS-QDs/MWCNTs/Au electrode.

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Fig. 1. (a) TEM image of CdS-QDs, SEM images of bare Au electrode (b), MWCNTs/Au electrode (c) and CdS-QDs/MWCNTs/Au electrode (d).

immobilized onto MWCNTs/Au electrode (Fig. 1d). It can be seen that CdS-QDs/MWCNTs formed a film with high surface area. The results confirmed the successful preparation of CdS-QDs/MWCNTs/Au electrode, which could be applied to further uses. 3.2. Electrochemical impedance measurements Electrochemical impedance spectroscopy (EIS) provides more details about the interfacial properties of a modified electrode during the fabrication process. The impedance spectrum includes a semicircle portion at high frequencies, corresponding to the electron transfer limiting process, and a linear part at low frequencies, resulted from the diffusion limiting step of the electrochemical process. Furthermore, the diameter of the semicircle represents the electron transfer resistance (Rct) [40]. Fig. 2 shows the typical Nyquist plots recorded at frequencies ranging from 0.01 Hz to 10 kHz and a formal potential of 0.17 V in 0.1 M KCl solution containing 1 mM [Fe(CN)6]3−/4− as an electrochemical redox probe for bare Au, MWCNTs/Au, CdS-QDs/MWCNTs/Au and BMBPBP/CdS-QDs/MWCNTs/Au electrodes. The EIS at a bare Au electrode displays a small semicircle, with a charge transfer resistance of 610 Ω (Fig. 2a). After modification of the Au electrode with the MWCNTs, the Rct was decreased (96 Ω), which confirms the attachment

Fig. 2. Nyquist plots of 0.1 M KCl containing 1 mM K3[Fe(CN)6] at: bare Au electrode (a), MWCNTs/Au electrode (b), CdS-QDs/MWCNTs/Au electrode (c) and BMBPBP/CdS-QDs/ MWCNTs/Au electrode (d). EIS conditions: initial potential, 0.17 V; amplitude voltage, 10 mV; frequency range, 100 kHz to 0.1 Hz. Insets: the equivalent circuit model used to fit the impedance data.

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Fig. 3. Effect of pH on the response of the BMBPBP/CdS-QDs/MWCNTs/Au electrode (A) and its anodic peak potential plot vs. pH (B). Scan rate: 50 mV s−1.

of MWCNTs at the electrode surface and makes the electron transfer easier (Fig. 2b). After immobilization of CdS-QDs, on the surface of MWCNTs/Au electrode, the Rct was increased to 260 Ω, suggesting that the semiconductor film was successfully immobilized on the mentioned electrode surface (Fig. 2c). The observed increase in Rct can be attributed to the electrostatic repulsion between [Fe(CN)6]3−/4− redox probe and carboxyl moiety of cysteine capped CdS-QDs (SCH2(NH2) CHCOO− complexes) which accompanied with difficulty in electron transfer kinetics on the electrode surface. Further increasing in Rct (470 Ω) after modification of the CdS-QDs/MWCNTs/Au electrode with BMBPBP, illustrated that the BMBPBP has been successfully immobilized on the CdS-QDs/MWCNTs/Au electrode (Fig. 2d). On the other hand, the enhancement in the Rct may be triggered by adsorption of neutral BMBPBP molecule at the surface of electrode which creates a thick and extended pathway for electron charge transfer between electrode and redox probe. Detailed analysis of the data obtained during an electrochemical impedance measurement is usually performed by fitting the experimental data in an equivalent circuit, based on the Boukamp model [41]. Many circuits will be checked for selection of the best circuit for fitting the experimental data. The electrochemical circle fit tool is available from the Analytical tools. 3.3. Electrochemistry of modified electrode To the best of our knowledge, there is no prior report on the electrochemical properties and, in particular, the electrocatalytic activity of

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bisphosphoramidate derivatives in aqueous media. It should be noted that one of the advantages of bisphosphoramidate derivatives as an electrode modifier is its insolubility in aqueous media and thus increases the stability of the modified electrode (BMBPBP/CdS-QDs/ MWCNTs/Au). The electrochemical behavior of BMBPBP immobilized onto the CdS-QDs/MWCNTs/Au electrode was studied in phosphate buffer solutions at pH values ranging from 2.0 to 12.0. The cyclic voltammetry technique was used with a scan rate of 50 mV s−1. A welldefined redox peak was observed in all studied pHs (Fig. 3A). The pH dependence of the peak potentials are shown in Fig. 3B. The potentials of the redox peaks of BMBPBP were shifted linearly toward fewer positive potential values with increasing the pH between 2.0 and 12.0 by slopes of − 55.1 and − 54.5 mV per pH unit for anodic and cathodic peaks, respectively (Fig. 3B). The obtained slopes were close to the theoretical value (− 59 mV per pH unit), revealed that, the equal numbers of electron and proton are involved in the redox reaction of BMBPBP immobilized onto the CdS-QDs/MWCNTs/Au electrode. Based on this information the following mechanism was suggested for redox reaction of the BMBPBP. According to the suggested mechanism, the group that undergoes reduction or oxidation is the amine group, which is marked with the letter a. Similar mechanism for amine group has been reported previously [42]. (see Scheme 2) Fig. 4 shows the cyclic voltammograms of the BMBPBP/CdS-QDs/ MWCNTs/Au electrode in 0.1 M phosphate buffer solution (pH 7) at different scan rates (10–350 mV s−1). Fig. 4A shows that the peak currents increased by raising the scan rate. As shown in Fig. 4B, for sweep rates below 100 mV s−1 the anodic and cathodic peak currents are linearly proportional to the scan rate (ʋ) as would be expected for redox surface-controlled process. In addition, the peak-to-peak potential separation is about 40 mV for sweep rates below 100 mV s−1, suggesting the quasi-reversible behavior of redox reaction of BMBPBP in aqueous solution over this range of sweep rate. At higher sweep rates, the plot of peak currents vs. scan rate plot deviates from linearity, and the peak current becomes proportional to the square root of the scan rate (Fig. 4C), indicating a diffusion controlled process, which is the reflection of the relatively slow diffusion of counter ions into the electrode surfaces. At higher sweep rates, peaks potential separations, begin to increase, that may be due to the limitation in charge transfer kinetics. The apparent charge transfer rate constant (ks) and the charge transfer coefficient (α) of a surface-confined redox couple can be evaluated from CV experiments by using the variation of anodic and cathodic peak potentials with the logarithm of scan rate according to the Laviron theory [43]. Fig. 4D shows the plots of the Ep vs. log v at scan rates higher than

Scheme 2. Mechanism oxidation of BMBPBP.

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Fig. 4. (A) Effect of scan rate on the CVs of BMBPBP/CdS-QDs/MWCNTs/Au electrode in 0.1 M phosphate buffer (pH 7.0), in the range of 10 to 350 mV s−1. (B) Plot of Ip vs. ʋ; (C) Plot of Ip vs. ʋ1/2 and (D) Plot of Ep vs. log ʋ.

0.09 Vs−1 which was used to extract the transfer coefficient (α). The slopes of the linear segments are equal to − 2.303RT/αnF and 2.303RT/(1 − α)nF for the cathodic and anodic peaks, respectively. The evaluated value for the α was found to be 0.5. Additionally, the electron transfer rate constant was obtained from the following equation:

Logks ¼ α logð1–α Þ þ ð1–α Þlogα– logðRT=nFvÞ–α ð1–α Þ  ðnFΔE=2:3RTÞ:

ð1Þ

The apparent surface electron transfer rate constant, ks = 24.7 s−1, was estimated by introducing the α value in Eq. (1). The surface coverage (Γ) can be estimated from the equation Γ = Q/nFA, where Q is the charge which is obtained by integrating the anodic peak under the background correction, at a low scan rate of 10 mV s−1, and other symbols have their usual meanings. The degree of the surface coverage directly or indirectly effects on the electron transfer kinetic of the electrode toward the adsorbed compounds, its stability and the nature of electrodic process (diffusion or kinetic limited) [44]. Therefore, the surface coverage was calculated at low scan rate [45]. The surface coverage value for the BMBPBP/CdS-QDs/MWCNTs/Au electrode was 9.7 × 10−9 mol cm−2, which corresponds to the presence of multilayer of surface species [46].

3.4. Electrocatalytic oxidation of olanzapine Fig. 5A shows the recorded cyclic voltammograms of 60 μM OLZ at the bare Au (curve b), MWCNTs/Au (curve e) and CdS-QDs/MWCNTs/ Au electrodes (curve f) in 0.1 M phosphate buffer solution with pH (7.0) at the scan rate of 50 mV s−1. As can be seen that the OLZ oxidation peak at the bare Au electrode was broad and weak due to slow electron transfer, while its current increased and its potential shifted toward less positive values when the surface of the bare electrode were modified by MWCNTs and CdS-QDs/MWCNTs (curves e and f). Although, these modifiers catalyze the oxidation of the OLZ but their sensitivities are not sufficient for low level monitoring of the drug. Thus, the CdS-QDs/ MWCNTs/Au electrode was further modified by immobilization of BMBPBP on its surface and then, the electro-oxidation of OLZ was investigated at the resulted electrode (BMBPBP/CdS-QDs/MWCNTs/Au). Curves a, c and d in Fig. 5A, represent the CVs of 0.1 M phosphate buffer solution (pH 7.0) in the absence of the drug as background current at the bare Au, MWCNTs/Au and CdS-QDs/MWCNTs/Au electrodes respectively. Comparison of these results with those in curves e and f indicate the role of MWCNTs and CdS-QDs/MWCNTs nanocomposite in improvement of the electrochemical response, which is due to the excellent characteristics of modifiers such as good electrical conductivity, high surface area, high porosity and more electroactive interaction

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Fig. 5. The CVs of phosphate buffer (pH = 7 in the absence and presence of 60 μM OLZ) at (A) bare Au (a, b), MWCNTs/Au (c, e) and CdS-QDs/MWCNTs/Au (d, f) electrodes and (B) at the BMBPBP/CdS-QDs/MWCNTs/Au electrode (a, c) and CdS-QDs/MWCNTs/Au electrode (b). Scan rate 50 mV s−1.

sites that improve the mass transport and provide the easier accessibility to the active sites. Thus, oxidation of OLZ became facile on these modified electrodes. On the other hand, porous network structure induced by the MWCNTs in CdS-QDs/MWCNTs nanocomposite acted as a pathway for mass transfer of the chemical reactants and products and as an electrical bridge [47]. Additionally, the background current at the CdS-QDs/MWCNTs/Au electrode (curve d, Fig. 5A) is larger than that of the MWCNTs/Au electrode (curve c) due to larger charging current and higher surface area. Curves a and c of Fig. 5B represent the CVs of BMBPBP/CdS-QDs/ MWCNTs/Au electrode immersed in 0.1 M phosphate buffer solution (pH 7.0) at a scan rate of 50 mV s− 1 in the absence and presence of 60 μM OLZ. The electrocatalytic activity of the modified electrode was evidenced by increasing the anodic peak of the modifier (BMBPBP) associated with decreasing in its cathodic peak. Curve b depicts the CV of electrochemical oxidation of 60 μM OLZ at the CdS-QDs/MWCNTs/ Au electrode. Comparison of the results obtained by BMBPBP/CdSQDs/MWCNTs/Au electrode with those of CdS-QDs/MWCNTs/Au electrode (curves b) show the role of BMBPBP in performance of the proposed sensor. In other words, the data clearly show that the combination of CdS-QDs/MWCNTs and mediator (BMBPBP) definitely improve the performance of the electrode for OLZ oxidation. The enhancement in peak current and shift in peak potential reveals the presence BMBPBP in the matrix of the CdS-QDs/MWCNTs/Au electrode which can act as an efficient electron mediator for the electrocatalytic oxidation of OLZ. On the other hand, along with the anodic sweep of potential, OLZ diffuses toward the electrode surface and reduces the BMBPBP (ox) to BMBPBP (red) while, the simultaneous oxidation of regenerated BMBPBP (red) causes an increase in the anodic current. For the same reason, the cathodic current of the BMBPBP/CdS-QDs/MWCNTs/Au electrode disappeared in the presence of OLZ, indicating that BMBPBP (ox) is consumed during a chemical step. In order to optimize the electrocatalytic response of modified electrode (BMBPBP/CdS-QDs/MWCNTs/Au) toward OLZ oxidation, the effect of pH on the catalytic behavior was investigated. The cyclic voltammograms of OLZ at the modified electrode at different pH values 2–12 were recorded (not shown). At pH range of 2–12, the modified electrode shows the electrocatalytic activity, but higher peak currents observed at pH 7 and this value was chosen as optimized. The effect of scan rate on the electrocatalytic oxidation of OLZ at the BMBPBP modified electrode was also investigated by cyclic voltammetry (not shown). Results showed that the oxidation peak potential gradually shifts toward more positive potentials with increasing the scan rate, suggesting a kinetic limitation in the electrochemical process. The anodic peak currents obtained were linear with respect to the square root of the potential scan rate, which indicates the transfer of OLZ is a diffusion controlled process. Also a plot of the scan rate-normalized

current versus scan rate (Ip/v1/2), exhibited the characteristic of a typical EC΄ process. The Tafel plot (Ep versus log v) was used for estimation of the kinetic parameter such as transfer coefficient (α). The Tafel slope (b) was obtained using the following Eq. (2) [48]: Ep ¼ ðb=2Þ logv þ constants:

ð2Þ

On the basis of Eq. (2), the slope of Ep versus log v plot is b/2, where b indicates the Tafel slope (b = 2.3RT/nαF(1 − α)). The plot of Ep versus log v indicates a linear variation for scan rates ranging 2–100 mV s−1. Assuming that the number of electrons transferred in the rate-limiting step is equal to 2, a transfer coefficient of α was estimated as 0.48. 3.5. Chronoamperometric studies Double-step potential chronoamperometry was employed to investigate the electrochemical processes of BMBPBP/CdS-QDs/MWCNTs/Au electrode. Chronoamperometric measurements of OLZ at BMBPBP modified electrode were carried out by setting the working electrode potential at 0.13 V (at the first potential step) and at 0 V (at second potential step) vs. Ag/AgCl for the various concentration of OLZ in phosphate buffer solution (pH 7.0) (Fig. 6). For an electroactive material with a diffusion coefficient of D, the current observed for the electrochemical reaction at the mass transport limited condition is described by the Cottrell equation [45]. Experimental plots of I vs. t−1/2 were employed, with the best fits for different concentrations of OLZ (Fig. 6A). The slopes of the resulting straight lines were then plotted vs. OLZ concentration. From the resulting slope and Cottrell equation the mean value of the D was found to be 2.36 × 10−5 cm2/s. Chronoamperometry can also be employed to evaluate the catalytic rate constant, k, for the reaction between OLZ and the BMBPBP modified electrode according to the method described in the literature [49]: h
 i IC =IL ¼ γ1=2 π1=2 erf γ1=2 þ expð−γ Þ=γ 1=2

ð3Þ

where IC is the catalytic current of OLZ at the BMBPBP modified electrode, IL is the limited current in the absence of OLZ and γ = kC0t is the argument of the error function (C0 is the bulk concentration of OLZ). In cases where γ exceeds the value of 2, the error function (erf (γ1/2)) is almost equal to unity and therefore, the above equation can be reduced to:
 γ1=2 π1=2 ¼ π 1=2 kCt 1=2 :

ð4Þ

The above equation can be used to calculate the value of catalytic rate constant (k) from the slope of IC/IL vs. t1/2 at a given OLZ

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Fig. 6. Chronoamperograms of 0.1 M PBS (pH 7.0) at applied potential of 0.13 V, for 0.0–500 μM of OLZ at the BMBPBP/CdS-QDs/MWCNTs/Au electrode. Inset indicates the plots of I vs. t−1/2 (A) and Ic/IL vs. t1/2 (B).

concentration (Fig. 6B). From the values of the slopes, the average value of k was found to be 2.8 × 104 M−1 s−1. 3.6. Amperometric detection of OLZ at the modified electrode In order to increase the sensitivity of the proposed method the amperometry technique was employed. The amperometric experiments were performed at a constant potential of − 0.13 V and under continuous stirring. Fig. 7 shows the typical steady-state electrocatalytic current time response of the rotated different modified electrodes for the successive additions of OLZ in phosphate buffer solution (pH 7.0).

Fig. 7. Amperometric response of 0.1 M PBS (pH 7.0 and applied potential of 0.13 V) after successive addition of 5 μM OLZ at: the bare Au electrode (a), MWCNTs/Au electrode (b), CdS-QDs/MWCNTs/Au electrode (c), BMBPBP/MWCNTs/Au electrode (d) and BMBPBP/ CdS-QDs/MWCNTs/Au electrode (e).

After successive injection of OLZ (5 μM), no detectable amperometric response was observed at the bare Au electrode (curve a) while it was increased after modification of the bare electrode by MWCNTs (curve b), CdS/MWCNTs (curve c) and BMBPBP/MWCNTs (curve d). As shown the best results with highest sensitivity were obtained when the bare Au electrode was modified by BMBPBP/CdS-QDS/ MWCNTs modifier (curve e). On the other hand, during the successive addition of OLZ, a well-defined response was observed when BMBPBP/ CdS-QDS/MWCNTs/Au was used as working electrode, demonstrating the efficient electrocatalytic ability of the BMBPBP/CdS/MWCNTs/Au electrode. The comparison of this results with that of BMBPBP/ MWCNTs/Au electrode (curve d), indicating the role of CdS quantum dot. The CdS-QD attached to MWCNTs, not only increases the surface area, it also increases the amount of assembled BMBPBP on the electrode surface that provides a large electrocatalytic surface area and hence increases its sensitivity. Therefore, the proposed electrode was utilized for low level detection of OLZ. The amperometric responses and calibration curves for the sensor under the optimized experimental conditions are shown in Fig. 8. The sensor responded rapidly when OLZ was added and reached a steady state (95% of the maximum value) within 10 s, indicating a fast diffusion of the substrate in the modified film on the electrode and the high sensitivity of the sensor. The proposed electrode exhibited two linear ranges for OLZ determination ranging from 20 nM to 10 μM (R2 = 0.9959, n = 21) and 10–100 μM (R2 = 0.9953, n = 21) with sensitivities of 0.16 μA μM−1 and 0.25 μA μM−1, respectively. The limit of detection (LOD) was found to be 6 nM OLZ which is calculated by LOD = 3Sb/m, where Sb is the standard deviation of the blank response, and m is the slope of the first calibration curve. Detection limit, linear calibration range and working pH of the proposed sensor were compared with those obtained in the other

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Fig. 8. Amperometric response of PBS (pH 7.0) containing 0.02 to 100 μM of OLZ at the BMBPBP/CdS-QDs/MWCNTs/Au electrode (A) and its calibration curve (B) at the applied potential of 0.13 V.

The selectivity of the proposed sensor was evaluated by determination of OLZ in the presence of various inorganic and organic foreign species, which may be found with OLZ in pharmaceutical formulations and biological fluids such as ascorbic acid, uric acid, glucose, cystine, glutathione, oxalic acid, dopamine, tryptophan, and histidine. Mg2+, Cu2+, 2−, K+, Cl−, SO2− NO− 4 , CO3 3 and antidepressant and antipsychotic drugs such as fluoxetine, risperidone and thioridazine by amperometric technique (Fig. S1 as an example, represented the effect of above drugs on OLZ determination). The tolerance limit was taken as the maximum concentration of foreign substances causing an approximately ±5% relative error in the determination. The results showed that all tested compounds had no significant effect on the response of the sensor when their concentrations are 100 fold greater than that of OLZ. The obtained results show the satisfactory selectivity of the proposed modified electrode toward electrooxidation of OLZ.

potential of OLZ, after recording of 10 successive cyclic voltammograms of 60 μM OLZ in 0.1 M PBS (pH 7) at the modified electrode, indicating the antifouling property of the nanocomposite modified electrode toward OLZ and its oxidation products. The relative standard deviation for the peak currents in CVs based on ten replicates was 3.84%, indicating excellent repeatability of the response of the modified electrode. The long-term stability of the BMBPBP/CdS-QDs/MWCNTs/Au electrode was tested after being stored in dry conditions at room temperature for 25 days (Fig. S2). The current responses decreased less than 7% of its original response detected for the same OLZ solution, indicating the good stability of this modified electrode, which can be attributed to the strong interaction between CdS-QDs and the immobilization substrate. To estimate the fabrication reproducibility of the proposed sensor, four BMBPBP/CdS-QDS/ MWCNTs/Au electrodes were fabricated in the same conditions. The electrode reproducibility was examined for 60 μM OLZ and there was no significant change in current responses (RSD b 3.8%, n = 6). This good reproducibility can be attributed to the fact that the BMBPBP molecule immobilized firmly on the surface of CdS-QDs/MWCNTs/Au electrodes. These experiments demonstrate that the BMBPBP/CdS-QDs/ MWCNTs/Au electrode has good stability, repeatability and reproducibility for the determination of OLZ.

3.8. Repeatability and reproducibility of the modified electrode

3.9. Real sample analysis

The great difficulties in the electrochemical methods for analysis of some of the biological compounds are their high oxidation overpotentials at the conventional electrodes. The fouling of the electrode surface with the oxidation products may be the other major problem for determination of these compounds. In the present study, stability and antifouling properties of the modified electrode toward OLZ oxidation were tested by measuring the decrease in the electrocatalytic oxidation current of OLZ during successive potential cycling of the modified electrode. The recognizable changes were not observed in the peak current or peak

To investigate the applicability of the proposed sensor for monitoring of the OLZ in real samples, the blood serum and OLZ tablet were analyzed for their OLZ contents. The standard addition method was used for measuring OLZ concentrations in the samples. The obtained results were summarized in Table 2. As it is illustrated, the recovery of OLZ is obtained between 98.53% and 103% using the BMBPBP/CdSQDs/MWCNTs/Au electrode. The results demonstrated the capability of the proposed sensor for determination of OLZ in real samples with good accuracy.

reports and the results are summarized in Table 1. As it is obvious, the BMBPBP/CdS-QDS/MWCNTs/Au electrode shows the lowest LOD in comparison with other reported sensors. 3.7. Selectivity of OLZ detection

Table 1 Analytical parameters for OLZ detection by several methods. Methods

Linear range (μM)

LOD (μM)

Conditions

Real sample

References

ZnS NPsa/CPEb SWCNTsc/Au electrode NH2–TiO2d/MWCNTse/GCEf BMBPBP/CdS-QDs/MWCNTs

20–65 0.64–32 0.12–33 0.02–100

– 0.3 0.09 0.006

PBS buffer (pH = 7.0) NH3/NH4Cl (pH = 8.5) PBS buffer (pH = 5.0) PBS buffer (pH = 7.0)

Tablet, urine Tablet, urine Tablet, human blood serum Tablet, human blood serum

[19] [20] [21] This work

a b c d e

ZnS Nanoparticles. Carbon Paste Electrode. Single-walled carbon nanotubes. Amine-functionalized TiO2. Multi-walled carbon nanotubes.

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Table 2 Determination of OLZ in real samples. Sample

Added amount (μM)

Found amounta (μM)

Recovery (%)

RSD (%)

Serum

1.0 2.0 – 1.0 2.0 3.0

1.03 1.98 1.78 2.81 3.85 4.71

103 99.0 – 101.1 101.8 98.53

2.21 2.44 1.93 2.54 2.6 2.52

Tablet

a

Average of six replicate measurements (rounded).

4. Conclusions A novel sensor was successfully fabricated by immobilizing bisphosphoramidate derivative (BMBPBP) on the surface of Au electrode modified with CdS-QDs/MWCNTs for the determination of low level of OLZ using the amperometric method. The modified electrode shows stable and reproducible electrochemical behavior at wide pH range of 2–12. The experimental results indicate that BMBPBP immobilized onto CdS-QDs/MWCNTs has an excellent electrocatalytic activity toward oxidation of OLZ. The detection limit, sensitivity and linear concentration range of the BMBPBP modified electrode for OLZ detection are better than those reported in the literature [19–21]. The fast response time, good stability and selectivity were the other advantages of the proposed modified electrode. Moreover, the BMBPBP/CdS-QDs/MWCNTs/Au electrode can be used for voltammetric determination of OLZ in real samples with different matrices. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2015.10.068. Acknowledgments The authors gratefully acknowledge the support of this work by the Razi University Research (57021) and Tarbiat Modares University's for their financial support. References [1] N. Bergemann, A. Frick, P. Parzer, J. Kopitz, Olanzapine plasma concentration, Average daily dose, and interaction with co-medication in schizophrenic patients, Pharmacopsychiatry 37 (2004) 63–68. [2] G. Zhang, A.V. Terry, M.G. Bartlett, Bioanalytical methods for the determination of antipsychotic drugs, Biomed. Chromatogr. 22 (2008) 671–687. [3] M. Aravagiri, Y. Teper, S.R. Marder, Pharmacokinetics and tissue distribution of olanzapine in rats, biopharm, Drug Dispos. 20 (1999) 369–377. [4] M. Josefssona, M. Roman, E. Skogh, M.-L. Dahl, Liquid chromatography/tandem mass spectrometry method for determination of olanzapine and N-desmethylolanzapine in human serum and cerebrospinal fluid, J. Pharm. Biomed. Anal. 53 (2010) 576–582. [5] M.A. Saracino, O. Gandolfi, R. Dall'Olio, L. Albers, E. Kenndler, M.A. Raggi, Determination of olanzapine in rat brain using liquid chromatography with coulometric detection and a rapid solid-phase extraction procedure, J. Chromatogr. A 1122 (2006) 21–27. [6] S.C. Kasper, E.L. Mattiuz, S.P. Swanson, J.A. Chiu, J.T. Johnson, C.O. Garner, Determination of olanzapine in human breast milk by high-performance liquid chromatography with electrochemical detection, J. Chromatogr. B 726 (1999) 203–209. [7] J. Bao, B.D. Potts, Quantitative determination of olanzapine in rat brain tissue by highperformance liquid chromatography with electrochemical detection, J. Chromatogr. B 752 (2001) 61–67. [8] M.A. Raggi, G. Casamenti, R. Mandrioli, V. Volterra, A sensitive high-performance liquid chromatographic method using electrochemical detection for the analysis of olanzapine and desmethylolanzapine in plasma of schizophrenic patients using a new solid-phase extraction procedure, J. Chromatogr. B 750 (2001) 137–146. [9] D.W. Boulton, J.S. Markowitz, C.L. DeVane, A high-performance liquid chromatography assay with ultraviolet detection for olanzapine in human plasma and urine, J. Chromatogr. B 759 (2001) 319–323. [10] O.V. Olesen, K. Linnet, Determination of olanzapine in serum by high-performance liquid chromatography using ultraviolet detection considering the easy oxidability of the compound and the presence of other psychotropic drugs, J. Chromatogr. B 714 (1998) 309–315. [11] K. Basavaiah, A.K.U. Rangachar, K. Tharpa, Quantitative determination of olanzapine in pharmaceutical preparations by HPLC, J. Mex. Chem. Soc. 52 (2008) 120–124.

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