Voltammetric determination of itopride using carbon paste electrode modified with Gd doped TiO2 nanotubes

In the present work TiO2 nanotubes (TNT) have been synthesized by alkaline hydrothermal transformation. Then they have been doped with Gd element. Characterizations of doped and undoped TNT have been done with TEM and SEM. The chemical composition was analyzed by EDX, Raman and FTIR spectroscopy. The crystal structure was characterized by XRD. Carbon paste electrode has been fabricated and mixed with Gd doped and undoped TNT to form a nanocomposite working electrode. Comparison of bare carbon paste electrode and Gd doped and undoped TNT carbon paste electrode for 1.0 ×10−3 M K4 [Fe(CN)6] voltammetric analysis; it was observed that Gd doped TNT modified electrode has advantage of high sensitivity. Gd doped TNT modified electrode has been used as working electrode for itopride assay in a pharmaceutical formulation. Cyclic voltammetry analysis showed high correlation coefficient of 0.9973 for itopride (0.04–0.2 mg/mL) with a limit of detection (LOD) and limit of quantitation values (LOQ) of 2.9 and 23.0 μg.mL−1 respectively.

Electrochemical analysis methods have grown in recent years as alternative to other analytical methods [18][19][20]. Electroanalytical methods have some advantages making them better alternative such as high sensitivity, selectivity, low instrumentation and running cost, easy to handle and short analysis time. Several parameters have played a role in the performance of the electroanalytical methods; one of them is working electrode. Several studies have been done to enhance the performance of working electrodes. In this field carbon paste working electrode achieved a special importance; this importance comes from its simple fabrication steps, in addition to low cost and wide potential window. Various materials such as nanoparticles could be added to carbon paste mixture during preparation to enhance the sensitivity and selectivity of the electrodes [21,22].
Titanium dioxide (TiO 2 ) is belonging to metal oxide semiconductor that considered as the perfect materials in widespread environmental and medical applications [23][24][25][26]. TiO 2 -based nanomaterials as nanotubes have been intensively studied and widely used due to their excellent electrolytic and electrolysis performance, high chemical stability and efficiency, nontoxicity and low cost [27][28][29][30]. The high cation exchange capacity on titanium dioxide nanotubes (TNT) provides the possibility of achieving a high loading of active compound on it, which makes it one on the best sensor. Otherwise, the high specific surface area and absence of micropores in TNT which facilitate transport of reagent the active sites. The bandwidth between the valence and conduction bands limits its activity [31]. By addressing this issue, doping with elements as rare-earth that have a large atomic number have been devoted [32][33][34][35]. The electronic energy levels in rare-earth elements are rich and improve its photocatalytic and electrocatalytic activity.
In this study, TiO 2 nanotubes have been prepared then doped with Gd, after the characterization Gd doped TiO 2 nanotubes. Carbon paste electrode has been mixed with different doses of TiO 2 nanotubes and Gd doped TiO 2 nanotubes, cyclic voltammetry has been established to study the performance of each fabricated electrode, then the electrode exhibited the best performance that has been used in voltammetric analysis of itopride in a pharmaceutical formulation.

Materials and reagents
The standard pharmaceutical formulation of itopride hydrochloride was obtained from Trium pharma (Jordan), sodium sulphate anhydrous Na 2 SO 4 from Janssen Chemica. The supporting electrolyte 1.0 M Na 2 SO 4 was prepared using Milli-Q water. 1.0 M Na 2 SO 4 supporting electrolyte was used for the preparation of stock solutions and standard working solutions. K 4 Fe(CN) 6 .3H 2 O was obtained from (Sigma Aldrich), Graphite powder from (BDH), and Paraffin liquid light BP from (Pacegrove).

Synthesis of TNTs
The method of preparation of TNT and doped Gd-TNT was based on alkaline hydrothermal transformation.
A weighted amount of TiO 2 powder [P25, (99.5%, 21 nm), Sigma-Aldrich, USA] was added to 30 mL of 10 mol dm −3 potassium hydroxide [KOH,Sigma-Aldrich, USA] solution. After stirring for 30 min and the mixture was transferred into a Teflon-lined stainless-steel autoclave and was heated for 24 h at 150°C. The white Powderly precipitate was thoroughly washed with deionized water then with dilute HCl until the pH of washing solution reached 6.5 then with deionized water again, followed by drying for 10 h at 90°C, and calcinating at 400°C for 2 h. Gd-TNT was synthesized by adding Gd(NO 3 ) 3 to the TiO 2 in the KOH solution followed by the hydrothermal and postsynthetic treatments as described above for the undoped TNTs [35].

Characterizations
The morphology of undoped TNTs and doped Gd-TNTs was examined by Transmission Electron Microscope (TEM, JEOL JEM 1400, Japan) and scanning electron microscopy equipped (SEM, Superscan SS-550, Shimadzu, Japan). The chemical composition was analyzed by energy dispersive X-ray analysis equipped (EDX, Superscan SS-550, Shimadzu, Japan). The crystal structure of the as-prepared sensors were characterized by a X-ray diffraction (XRD, Shimadzu, XRD-7000, Japan) at 40 kV and 30 mA, using CuK α incident beam ( λ = 0.154nm). Raman spectroscopy was performed on a Raman microscope (Raman, Sentrarra, Bruker, USA) from 50 cm −1 to 1200 cm −1 . Infrared (FT-IR) absorption spectra of the KCl disks containing powder samples were recorded on a Thermo IS-10 instrument FT-IR spectrometer (Thermo Fisher Scientific Inc., Madison, WI, USA) at a resolution of 4 cm −1 in the range of 400-4000 cm −1 .

Modified carbon paste electrode fabrication
For fabrication of carbon paste modified electrodes, graphite powder, TNT, and Gd-TNT have been mixed as in the Table 1.  After that mixture powder was dispersed in 1.0 mL dimethyl formamide (DMF) then homogenized for 20 min in ultrasonic bath. After that DMF was vaporized from the mixture using oven at 80°C overnight. Dry mixture was mixed with 100 µL paraffin oil using spatula. Micropipette tip of 2 mm end was filled with mixture paste. For electrical connection copper wire connection was made passed through the edge of the tip.

Voltammetric analysis apparatus
Potentiostat (Metrohm Autolab) PGSTAT 204 was used for voltammetric measurements. All measurements were carried out using a 3 electrodes system; where Ag/AgCl (3 M KCl) was used as reference electrode, platinum (Pt) sheet as counter electrode, and fabricated carbon paste as working electrode.   were attributed to anatase TiO 2 which agree with previous studies [35,36] and reveal the characteristics of the anatase phase of the sensors. This result is also an evidence to verify that Gd introduced into the lattice or interstitial site of TiO 2 .

FTIR analysis
IR spectra of TNT (a) and Gd-TNT (b) are depicted in Figure 4. Both spectra display the broad band at around at 3432.53 cm −1 , corresponding to the surface adsorbed water and hydroxyl groups in tubular structure sensors. The large amount of hydroxyl groups on sensors wall enhance their performance for the photo excited electrons capture and profiled the holes to produce the reactive oxygen species in the photocatalytic [40]. The

Electrochemical performance of fabricated electrodes
Cyclic voltammetry was carried out to study the electrochemical performance of fabricated electrodes. Figure 5 shows voltammograms of 1.0 ×10 −3 M K 4 [Fe(CN) 6 ] with fabricated working electrodes. It could be concluded that doping TiO 2 with Gd enhances both anodic and cathodic peak currents, where G2 electrode anodic peak current reaches 56 µA compared to 31 µA for bare C paste electrode. Furthermore, G2 cathodic peak current reaches 52 µA which is the highest compared to other studied electrodes. According to the voltammograms of G1 and G2 of Figure 5, it can be concluded that increasing Gd-TNT portion in the fabricated electrodes has a positive impact on electrode sensitivity.
A comparison has been established between fabricated working electrodes for itopride pharmaceutical formulation assay. The voltammograms in Figure 6 show significant difference in performance between studied working electrodes. Figure 6 shows drastic increase in the anodic peak current of G2 working electrode for itopride compared to G1, bare C paste, and (F) electrodes.
When G2 was used as working electrode it showed ∆E p of 1.1 V; where ( ∆E p = E pa -E pc ) , which is greater than the value of 59/n mV expected for a reversible system [42] suggesting that itopride with G2 working electrode has irreversible behavior in aqueous medium.

Influence of scan rate ( υ ):
Itopride oxidation mechanism was investigated by study the effect of scan rate on the electrode response.
Applied scan rate is ranging from 40 to 180 mV/s. Results are summarized in Figure 7, which indicated that as scan rate increases, anodic peak current increases (Figures 7a and 7b), furthermore it shifts the anodic peak potential (Epa) positively (Figure 7a), which interprets the irreversibility of the electrode process. Figure 7c shows log (ip) vs. log ( υ ) plot which verified linear relationship with slope 0.693, slope value is closer to 0.5, the theoretical value indicated redox process controlled typically by diffusion mass transport only, rather than 1.0 value which typically indicated redox processes controlled by adsorption [42,43].

Analytical performance
To evaluate the performance of fabricated sensor (G2), a calibration curve was established in the acquired optimum conditions for itopride assay in a pharmaceutical formulation. Figure 8 shows cyclic voltammograms of itopride in a pharmaceutical formulation (0.04-0.2 mg/mL). Standard calibration curve illustrates high correlation (R² = 0.9973) in addition to high sensitivity. Each concentration has been done triplicate with relative standard deviation (RSD) of all concentrations less than 1%. Limit of detection (LOD) and limit of quantitation values (LOQ) found to be 2.9 and 23.0 µg.mL −1 , respectively. Where LOD and LOQ of itopride were determined based on signal-to-noise ratio of 3 and 10, respectively. Table 2 shows a comparison between present work and other methods used for itopride determination. This comparison includes precession and LOD. Data in Table 2 indicate that CV analysis of itopride using Gd-TNT electrode has a comparable LOD and precession with chromatographic and spectroscopic methods.

Conclusions
In the present work, TiO 2 nanotubes have been synthesized and doped with Gd element, and then it has been fully characterized. A composite of carbon paste modified with Gd doped TiO 2 nanotubes electrode have shown higher sensitivity compared to bare and undoped TiO 2 nanotubes carbon paste electrode. When Gd doped TiO 2 nanotubes electrode has been applied for cyclic voltammetry of itopride in a pharmaceutical formulation, it has shown high performance compared to commercially available electrode.