Silver-doped titania modified carbon electrode for electrochemical studies of furantril

An electrochemical method for the determination of an antrallinic acid derivative based on nanoparticles modified electrode was studied through cyclic and differential pulse voltammetry. Modification of electrode with silver-doped titania nanoparticles enhanced the peak current for the electro-oxidation of Furantril. The silver-doped titania nanoparticles were prepared by simple wet chemical methods and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD). Silver-doped TiO2 voltamogramms suggested that pH 5.0 was suitable for electrochemical investigation of furantril. Rate constant, diffusion coefficient, electrode process and number of electrons involved were calculated. Based on these investigations a feasible mechanism for electrode reaction was presented. Limit of detection and quantification were found to be 1.98 nM and 6.6 nM respectively.

Wide bind gap and unique properties of oxide semiconductors such as like ZnO, 1 In 2 O 3 , 2 SnO 2 3 and TiO 2 4 have attracted researchers more and enforced more usage of these materials for different applications. TiO 2 nanoparticles due to their extraordinary properties such as stability, low toxicity, high surface area, and enhanced catalytic activity as attracted a lot. 5 TiO 2 nanoparticles possess exceptional photo-catalytic property and are used as coating material in optical thin film due to its chemical stability. 6,7 However, the short life of electron hole-pairs with poor adsorption capacity has restricted the applicability of TiO 2 in many fields. 8 Doping of TiO 2 nanoparticles with different metals would allow overcoming with these deficiencies and increasing the catalytic activity of TiO 2 nanoparticles. The dopants added work by trapping electrons on the surface of semiconductor and reducing the electron hole pair combination. By altering the chemical and physical properties with a metal ion dopant the applicability of TiO 2 nanoparticles for new devices can be increased. In past reports, it has been observed that metals like Au, Pt, Rh, Ag, Ru, and Pd 9,10 are employed for TiO 2 doping. In addition, photo induced separation of electrons and hole can be enhanced by incorporating low cost transition metals like Ni, Fe, and Cr. 11 The recombination of electron-hole pair remains very high when TiO 2 doped with p-block elements. This restricts the redox capacity of materials surface. 12 To hinder the recombination process and increase the catalytic effect it's suitable to use transition metal such as Ag, Pd, or Pt. Further, with their unique properties Au, Pt and Rh doped TiO 2 have also gained a lot of interest. 13,14 Since use of Pt, Pd, Rh and Au is commercially viable for large scale production, Agdoped TiO 2 nanoparticles (STNP) has gained a large practical value. 15 In addition, Ag shows inimitable sensing properties in chemical and biological field as compared to that of other noble metals. 16 Doping TiO 2 with Ag enables higher electron-hole pair separation by forming Schotthy barrier, which enhances photocatalytic activity of TiO 2 . 17 STNP has also shown greater water splitting activity than TiO 2 . 18 It is been also reported that through density function theory (DFT) calculations Ag can modify TiO 2 anatase surface to rutile. 19 Introduction of Ag on the lattice creates oxygen vacancies closer to TiO 2 , which provides a separate channel for electron transport. 20 Silver is a well-known noble metal that can trap the electrons leading to z E-mail: npshetti@kleit.ac.in easy electron hole pair separation for enhanced catalytic activity of TiO 2 nanoparticles. 21 Silver can also assist in the electronic excitation by producing a localized electric field. 22 Further, silver doping can increase the anatase to rutile conversion, which enhances the surface area and catalytic activity. It is observed that noble metal such as silver and gold in the nano-size exhibits useful application, such as catalytic, electrical, and optical properties. In addition, TiO 2 nanoparticles undergo equilibrium in Fermi level enhancing the catalytic behavior. Thus, STNP nanoparticles studies have gained a large practical value as sensor. Preparation technique greatly affects the physical and chemical properties.
Furantril is a commonly used diuretic. It is administered either orally or by intravenous to treat edema linked to renal impairment, 23 nephrotic syndrome, hypertension, heart failure 24 hepatic cirrhosis.
In the present work, we report the usage of STNP to construct an electrode which has greater sensitivity compared to that of TiO 2 nanoparticles. The synthesized nano-particles were used to develop a sensor for the electrochemical determination of furantril. The synthesized nanoparticles were characterized by SEM, TEM, XRD and EDX. Further, the developed method was applied for the analysis of furantril in pharmaceuticals and spiked human urine samples. The electrode prepared was checked for the electrochemical activity toward furantril using cyclic voltammetric method. In addition, the analytical application of electrode was studied using differential pulse voltammetric method and applied for human urine sample and pharmaceutical dose form. The recovery from these samples obtained, economically low cost as to the other methods, easiness in the fabrication and higher sensitivity allows an alternate method for furantril determination.

Experimental
Synthesis of STNP nano particles.-In 500 ml Pyrex beaker, 500 mg of TiO 2 and 200 ml of double-distilled water was added. (molar ratio) silver nitrate solution was added to the TiO 2 and water mixture to carry out doping process. The resulting suspensions was settled at room temperature for a night after thoroughly mixing by vigorous stirring. Moisture contents remained was removed by drying the mixture at 100 • C for 12 hours in an oven. Further, calcined at below 500 • C for 3 hours in muffle furnace to get STNP nanoparticles. Preparation of STNP modified carbon paste electrode.-Prior to the modification, CPE was prepared by mixing graphite powder (1.0 gm) with paraffin oil (0.5 ml) using a mortar to obtain homogeneous paste. The paste was filled in the cavity of polytetrafluoro ethylene tube (PTFE) to get a smooth surface. The surface activation was carried at pH 5.0, by cycling it in the potential range 0.6-1.4 V and keeping scan rate at 50 mVs −1 . 25,26 For the modification of the electrode, STNP were finely homogenized with carbon paste and filled in the cavity of PTFE tube. Amount of STNP was optimized by using different weight % of STNP nanoparticles. The voltamogramms acquired using cyclic voltammetric technique had two anodic peaks and it was observed for both the peaks that, maximum peak current for oxidation of 0.1 mM furantril was for 1% STNP. Hence, the ratio was opted for further investigations.

Material characterization and electrochemical investigations.-
Identification of phase and crystal structure JEOL, JSM-6360 was used to obtain surface morphology. TEM images were obtained from JEOL, JEM-2010. X-ray diffractometer AXS D5005 was used for the characterization of nano-particles. To perform the electrochemical experiments, D630 analyzer form CHI Company was connected to three-electrode system. Carbon paste electrode (CPE) modified with STNP nanoparticles (working electrode), Ag/AgCl (reference electrode) and platinum electrode (counter electrode). pH meter (Elico Ltd., India) was used to optimize the pH of buffer solutions.
Reagents and chemicals.-Furantril stock solution was prepared by transferring apposite weight into 25 ml volumetric flask and then dissolving it with ethanol to the mark. Supporting electrolyte, phosphate buffer solution was used to investigate the effect of pH in a range 3.0-11.2 pH (I = 0.2 M). KH 2 PO 4 , Na 2 HPO 4 , H 3 PO 4 , and Na 3 PO 4 purchased from Sigma Aldrich were used to prepare the buffer solutions according to the literature. 27 For all the studies, double distilled water and analytical grade reagents were utilized.
Sample preparation.-Furantril tablets (Lasix) were finely powdered. In 100 ml calibration flask, weight equaling to 1.0 mM was transferred and dissolved in ethanol. Good dissolution was accomplished by keeping in ultrasonic bath for 10 min. The peak current of the solution for different addition was recorded using differential pulse voltammetric method. Recovery experiments were performed to investigate accuracy of the proposed method.

Results and Discussion
Characterization of STNP.-Characterization of STPN was performed using XRD, EDX, SEM and TEM. In Figure 1A the XRD pattern of STPN obtained is shown. The presence of highly crystalline films is indicated by sharp and intense peak in the spectra. In addition, the crystal structure was found to be anatase as main diffraction peak obtained at 25 28 Due to small amount of Ag in the TiO 2 matrix the XRD peaks do not show and peaks corresponding to Ag or its oxides. 29 From the peak with strongest intensity at 2θ = 25.36 the crystallite size was determined by Scherrer equation 30 as D = kλ/βcosθ, where k = 0.94, θ = Diffracting angle, β = Full Width Half Maximum (FWHM) and λ = 1.5407. Crystallite size of STNP was 14.2 nM, which is in good agreement with early reported method. The past reports have suggested that doping with Ag the crystallization of TiO 2 is enhanced in some case, hindered or not affected in other cases. 31 Surface morphology of STNP was studied using SEM ( Figure 1B) and TEM, ( Figure 1D) and energy dispersive X-ray spectroscopy (EDX) ( Figure 1C). The SEM image obtained for STNP confirmed the presence of porous and spongy like structure with high roughness suggesting high surface area. SEM images also showed the uneven aggregates of cylindrical shape crystalline structure of STNP. Due to aggregation of tiny crystals, the SEM image showed the varied distribution of Ag on surface of TiO 2 and STNP contained irregular shaped particles. Information regarding shape size and state of agglomeration was obtained by transmission electron microscopy (TEM). Silver doping on TiO 2 nanoparticles were presumed to appear as black spots in TEM images. Elemental composition of nanoparticles was obtained from EDX analysis. The elemental composition of prepared nanoparticles confirmed the presence of Ti and O with small amount of silver.
The Specific Surface area of STNP obtained from BET surface area measurement was 82.62 m 2 /g. The Tauc plot of Kulbeka-Munk function obtained from UV-Vis diffused reflectance spectra was used to determine the bandgap. The bandgap of STNP was 3.1 eV (Figure 2).

Cyclic voltammetric behavior of furantril.-Voltammetric be-
havior studies were carried out using cyclic voltammetric method (Figure 3). The voltammogram recorded shows the presence of two anodic peaks at 0.950 V and 1.12 V with anodic current of 12.43 μA and 23.36 μA respectively for STNP modified carbon paste electrode. Reversing the scan rate, peaks were absent suggesting the irreversible electrode process. The oxidation current for CPE and Ag-TiO 2 /CPE was compared and was observed that the oxidation current for Ag-TiO 2 /CPE was enhanced about 4-5 times. Enhancement of the peak current may be attributed to better catalytic effect of silver doping and larger interaction of the analyte with the electrode surface. The bare electrode and TiO 2 nanoparticles modified electrode shows sluggish electron transfer and hence, the peak current corresponding to them was less intense and broader. Therefore, presumed that these electrodes are not suitable for the electrochemical studies of furantril. Further, electron hole pair separation is also affecting the peak current. Since, TiO 2 nanoparticles have a drawback of wide energy band and short life of electron hole pairs with poor adsorption capacity its catalytic activity is reduced. In Figure 4, a working setup is shown.   The voltammetric behavior was compared with past reported methods and was observed that the furantril oxidation in our investigations was recorded at lower potential as compared to other electrode material. 32,33 Effect of accumulation time.-Accumulation time studies can improve the analyte concentration at the electrode surface. High amount of analyte at electrode surface can improve the sensitivity of the electrode prepared and low detection limit. Varying the accumulation time in the range 0-180 s the voltamogramms were recorded using cyclic voltammetric method. It was observed that the oxidation current for furantril was highest at 120 s. Therefore, all the further experiments were performed keeping accumulation time 120 s as optimum.

Effect of variation of pH.-Alonso et al. 32 used glassy carbon electrode for the determination of furantril in BR buffer solution.
Medeiros et al. 33 proposed a electrochemical method for determination of furantril on Basal-Plane Pyrolytic Graphite, Boron-Doped Diamond, and Amorphous Carbon Nitride Electrodes in 0.10 mol L −1 sulfuric acid, 0.040 mol L −1 BR buffer of pH 4.5, and 0.10 mol L −1 potassium nitrate. It was observed that the investigations were lagging in analysis in PBS. Voltamogramms were recorded in PBS in the range 3.0-11.2 pH ( Figure 5) and compared with past reported methods. Voltamogramms shows two peaks for all the pH in the range 3.0-9.0 pH. As the pH increased above 9.0, the intensity of the peak current decreased. The above obtained results were as similar to the past reported method were the oxidation behavior of furantril was frail above pH 5.0. As the pH was increased above 9.0 pH the second peak diminished.
Plot for pH versus peak current shows that the peak current was maximum at pH 5.0. Therefore, pH 5.0 was opted as optimum for the investigations. In addition, as the pH increased the peak potential shifted to a value that is more negative suggested that there is involvement of protons in the electrode reaction. The regression equation for the plot of peak potential versus pH in the range 3. Effect of variation of scan rate.-Effect of variation in scan rate on the peak current for oxidation of furantril was investigated using cyclic voltammetric method ( Figure 6A). As the scan rate increases, Ip also increased with Ep shifting toward a higher positive value. The shift indicates that the electrode reactions are irreversible. The process was found to be diffusion controlled from the observed linear plot for Ip versus υ; with equation; Ip1 = 42.68 υ -3.87; R 2 = 0.970 and Ip2 = 94.18 υ -5.81; R 2 = 0.991 ( Figure 6B). Diffusion controlled process was affirmed from the plot log Ip versus log υ with a slope closer to  Figure 6C). Further, a plot for peak potential and log scan rate with regression equation: Ep 1 = 0.05 log υ + 1.00; R 2 = 0.992 and Ep 2 = 0.08 log υ + 1.23; R 2 = 0.998; was obtained ( Figure 6D). For an irreversible process, Laviron's equation is as: 34 Ep = E 0 + [2.303RT/αnF] log RTk 0 /αnF From slope, Ep versus log υ, 'αn' was calculated to be 0.909 and 0.732 for peak 1 and 2 respectively. Bard and Faulkner 35 equation can be used for calculation of α: We got α as 0.65 from the above equation and electrons transferred as 2. Further, from equation, E0 can be calculated from intercept of Ep versus υ curve by extrapolating to the vertical axis at υ = 0. 36-38 k • value was calculated to be 5.2 × 10 3 s −1 and 4.98 × 10 3 s −1 respectively.
Tablet analysis and recovery test.-The proposed method was applied for pharmaceutical sample analysis using available tablet, Lasix (100 mg per tablet). Concentration of furantril was prepared to in the range of calibration plot. 45,46 Further, effect of some excipients  was studied. 47,48 Except other than oxalic acid, none interfered with the oxidation signals of furantril.
Detection of furantril in urine samples.-Spiked human urine samples, from healthy volunteers was used to investigate the applicability of proposed method for real sample analysis. Differential pulse voltammetric method was used to investigate the recovery from the samples. The recoveries for different sample were between 99-102% with R.S.D 1.2%. Good recoveries suggested that these methods are well applicable for analysis in biological fluid (Table II).

Conclusions
In this work, an electrochemical investigation of furantril was demonstrated based on silver-doped TiO 2 nanoparticles. The electrochemical method adopted had advantages such as speed of detection, easy sample preparation and a low detection limit as compared to past reported methods. The practical applicability of the proposed method was successfully demonstrated in analysis of real sample such pharmaceutical dosage form and human urine samples. In addition, it was observed that the presence of excipients except oxalic acid had no influence on electrochemical behavior of furantril. In conclusion, based on the facts the proposed electrochemical method offers a selective, sensitive, low cost and simple approach for determination of furantril without any complex pre-treatment of samples in real samples.