Poly (DL-valine) electro-polymerized carbon nanotube paste sensor for determination of antihistamine drug cetirizine

Poly (DL-valine) modified multiwalled carbon nanotube paste sensor (PVLMCNTPS) was prepared by electro-polymerization route. PVLMCNTPS and bare multiwalled carbon nanotube paste sensor (BMCNTPS) morphologies and sensing properties for cetirizine (CTZ) were confirmed through a field emission scanning electron microscope (FE-SEM) and electrochemical studies, respectively. In contrast to BMCNTPS, PVLMCNTPS surface composite creates an electrocatalytic impact on the oxidation of CTZ. PVLMCNTPS properties were optimized using parameters such as accumulation time, number of polymerization cycles, solution pH, and scan rate. The optimized PVLMCNTPS was applied for the determination of cetirizine in 0.1 M phosphate buffer solution (PBS) of pH 7.0, using cyclic voltammetry (CV). It is shown that PVLMCNTPS provided analytical linearity from 2.0 to 80 μM, with a detection limit of 0.11 μM for CTZ determination. PVLMCNTPS is found highly selective for CTZ in presence of some interfering organic molecules. The stable and selective PVLMCNTPS was applied for CTZ determination in pharmaceutical pills with satisfactory results.

polished with a tissue to provide uniform surface and to avoid disturbance in electrochemical measurements.

Preparation of PVLMCNTPS through electro-polymerization
Electrochemical polymerization on BMCNTPS was performed in PBS (pH 5.5, 0.1 M) containing 1.0 mM DL-valine in the potential window -0.2 to 1.7 V with a scan rate of 0.1 V s -1 , and shown in Figure  1a. As electrochemical scanning time passes the CV curves sloped down at positive potentials. This shows the formation of a polymer layer on electrode surface. The number of polymerization scans can affect the electrode properties, and so it was optimized by varying cycles from 5-20 as shown in Figure  1b. Five CV cycles were fine-tuned for CTZ sensing, while after 5 cycles the CV shape and peak current declined. The structure of electro-generated poly (DL-valine) is shown in Figure 1c

Electrochemical characterization and calculation of effective surface area
Electrochemical characterizations of BMCNTPS and PVLMCNTPS were performed in 0.1 M KCl containing 2.0 mM [Fe(CN) 6 ] 3-/4redox system using CV at 0.1 Vs -1 scan rate, as depicted in Figure 3. BMCNTPS surface interaction with [Fe(CN) 6 ] 3-/4gives poor current signals with higher separation of peak potentials (ΔE p ). At the other side, PVLMCNTPS gives higher current response with lower ΔE p upon interaction with Fe(CN) 6 ] 3-/4-. The surface area of BMCNTPS and PVLMCNTPS was determined using Randles-Ševčik formula [36]: Here, K is the constant equal to 2.69×10 5 (C mol -1 V -1/2 ), I p is peak current (A), n is the number of

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PVLMCNTPS gives superior signals at 0.765 V and 0.888 V. At PVLMCNTPS, the current signal is 5 times higher and over-potential is reduced by 15.1 % compared to BMCNTPS. Since CV of CTZ does not show a peak in the reverse scan, the CTZ process is considered irreversible.

Effect of accumulation time
Accumulation of analyte on electrode surface influences the electrode performance. The effect of accumulation time (0-60 s) on CTZ (0.1 mM) detection at PVLMCNTPS was analyzed in 0.1 M PBS, pH 7.0, as shown in Figure 5. The maximum current signal is achieved at 10 s, whereas for further increases of accumulation time, peak currents were decreased. It is probably due saturation in accumulation of analyte on sensor surface. Hence, 10 s of accumulation was optimized for further measurements. Influence of the potential scan rate was carried out in order to elucidate the kinetics of the process at the electrode-analyte interface. CVs as functions of sweep rate (0.100-0.250 V/s) were recorded for CTZ oxidation in 0.1 M PBS, pH 7.0, as shown in Figure 6a. The peak current values of CTZ oxidation are increased with the change in the scan rate, while peak potential values are shifted towards more positive side. The plots of peak current vs. square root of the scan rate (Figure 6b), and log I pa against log scan rate (Figure 6c) give clear evidence for diffusion process as per following equations:

Time, s
log I p =0.56 log  + 2.12

R=0.99
(3) The number of electrons involved in the electrochemical oxidation of CTZ is calculated using the following equation [37]: where n is number of electrons involved, R is gas constant (8.314 J K −1 mol −1 ), T is temperature, F is Faraday's constant (96,485 C mol -1 ) and  is coefficient of charge transfer, assumed to be 0.5 for irreversible behaviour [38]. The number of electrons transferred for CTZ electro-oxidation was found to be 1.027 ≈ 1. The electron transfer mechanism of CTZ oxidation is sketched in Scheme 1.

Selection of pH
According to the given mechanism of CTZ oxidation, the electrochemical sensing is strongly affected by pH of the supporting electrolyte. The effect of pH changes from 5.5 to 7.5 on the sensing of CTZ (0.1 mM) in 0.1 M PBS at PVLMCNTPS is represented in Figure 7a. Obviously, the maximum anodic peak current (I pa ) of CTZ oxidation was obtained at pH 7.0. Hence, PBS with pH 7.0 was chosen for further experimentation. As shown in Figure 7b, the plot of peak potential (E pa ) against buffer pH is found linear according to the following equation: From the linear plot in Figure 7b, the slope was estimated to be 0.051, which is near the theoretical slope of 0.059. This strongly suggests that the electrochemical oxidation of CTZ at PVLMCNTPS involves equal number of electrons and protons.

Construction of calibration plot
In order to determine the analytical performance of sensor, the effect of different concentrations of CTZ were studied by CV and shown in Figure 8a. As presented in Figure 8b, peak currents increased linearly with increase of the concentration of CTZ in the range 2.0-80 µM. The linearity of curve is described as I p / A = 9.68×10 -6 + 0.223 M with R = 0.99. The detection limit (LOD) and limit of quantification (LOQ) values were found to be 0.11 µM (3 S/N) and 0.372 µM (10 S/N) [39], with sensitivity of 0.223 A/M.  Table 1 presents a comparison of detection limit (LOD) values for CTZ determination of here proposed PVLMCNTPS with already existing sensors described in the literature [40][41][42][43][44]. It is clear that the obtained result for PVLMCNTPS is comparable with previous literature results.

Stability and reproducibility
Reproducibility was tested at three newly formed PVLMCNTPS, where the results of CTZ determination were obtained with relative standard deviation (RSD) of 1.24 %. This RSD result shows good reproducibility of the prepared sensor. The stability of the proposed sensor was examined by keeping the sensor in a dry place for 24 hours prior CTZ detection. Even after 1 day of standing, the sensor provided 93.15 % current retention for CTZ determination. This clarifies good storage stability of the proposed PVLMCNTPS.

Selectivity
To test reliability of the proposed PVLMCNTPS, the binding selectivity for 0.1 mM CTZ detection in presence of various organic molecules such as ascorbic acid (AA), glucose (GU), riboflavin (RF), paracetamol (PC), ciprofloxacin (CF), oxalic acid (OA), anthrone (AN), rhodamine (RH) (0.1 mM) was tested in 0.1 M PBS using CV technique. Figure 10 shows the change of peak potential (E p ) value of CTZ oxidation with addition of 0.1 mM of each organic molecule. The results in Figure 10 suggest no significant interference of these organic molecules for CTZ detection, confirming thus the specificity of PVLMCNTPS. To examine the practicability of the proposed strategy, PVLMCNTPS was utilized to detect CTZ concentration in CTZ tablets. CTZ tablets were firstly powdered uniformly using mortar and then 0.1 mM CTZ solution was prepared using tablet powder. The analysis is performed in 0.1 M PBS, pH 7.0 using the standard addition method. The results are given in Table 2, showing that CTZ recovery in the pharmaceutical product is obtained between 90-102.4 %.

Conclusions
A simple and effective sensor for CTZ detection was prepared by electrochemical polymerization of DL-valine on the surface of multiwalled carbon nanotube paste sensor. The sensor modified by poly (DL-valine) has strong affinity towards CTZ molecules as compared to the unmodified. The proposed CTZ sensor provides low detection limit of 0.11 µM with good linear range of concentrations within 2-80 µM. These results are either similar or even better than those obtained by already existing methods. Moreover, the proposed sensor is found stable, reusable, and specific for CTZ detection in presence of other interfering organic molecules, and also, very efficient for routine pharmaceutical analysis.