titanium dioxide nanocomposite sensor

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Jan 2, 2018 - dispersing 2 mg of the synthesized Ppyl/TiO2 nanocomposite in 2 mL of N ...... 8 Square wave voltammograms of SLM at the Ppyl/TiO2/GCE sensor at ... square root of the scan rate (ν1/2) and can be expressed by Eq. (6):. I μA.
Ionics (2018) 24:2473–2488 https://doi.org/10.1007/s11581-017-2365-6

ORIGINAL PAPER

Polypyrrole/titanium dioxide nanocomposite sensor for the electrocatalytic quantification of sulfamoxole Ab Lateef Khan 1 & Rajeev Jain 1 Received: 20 April 2017 / Revised: 3 October 2017 / Accepted: 21 November 2017 / Published online: 2 January 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract A highly sensitive electrocatalytic polymer nanocomposite material of polypyrrole (Ppyl) and titanium dioxide (TiO2) was synthesized by interfacial polymerization of pyrrole and titanium dioxide. The synthetic nanocomposite material was used as a voltammetric sensor by modifying glassy carbon electrode (GCE) for the quantification of sulfamoxole (SLM). Prior to the modification, synthetic nanocomposite material was studied for its various spectral characteristics by FTIR, XRD, and scanning electron microscopy (SEM). Furthermore, the developed sensor (Ppyl/TiO2/GCE) was also characterized by electrochemical impedance spectroscopy and various voltammetric techniques. The developed Ppyl/TiO2/GCE sensor exhibited an exceptional electroanalytical performance as compared to Ppyl/GCE, TiO2/GCE, and bare GCE. Under optimized experimental conditions, the fabricated sensor exhibits a linear response for the oxidation of SLM over a concentration range from 1.25 to 12.50 μg mL−1 with the correlation coefficient of 0.9980 (r2), the detection limit of 1.24 ng mL−1 and quantification limit of 4.15 ng mL−1. Keywords Nanocomposite . Sulfamoxole . Polypyrrole . Glassy carbon electrode . Voltammetric sensor

Introduction Polymer nanocomposites (PNCs) of metallic nanoparticles show synergy in their chemical and physical properties resulting from the constituent polymer and introduced metal. This remarkable behavior of the nanocomposite material in their physicochemical properties can be ascribed to high surface area and quantum size effect [1–4]. According to the latest analyses, it was found that the electroactive polymer composites of noble metal or metal oxide nanoparticles are superb substitutes for the detection of various analytes [5–8]. There is a large extent of literature on synthesis, characterization, and application of nanocomposites with a range of combinations of conducting polymers, e.g., polypyrrole ( P p y l ) , p o l y a n i l i n e ( PA N I ) , p o l y ( 3 , 4 ethylenedioxythiophene), and metallic nanoparticles [1, Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11581-017-2365-6) contains supplementary material, which is available to authorized users. * Rajeev Jain [email protected] 1

School of Studies in Chemistry, Jiwaji University, Gwalior 474011, India

9–15]. The mounting importance of polymer nanocomposites is because of their potential applications in a number of fields such as sensors [16–20], batteries [21–24], electrochromic displays [25–29], electrocatalysis [30–32], and so on. Hence, all the mentioned conducting polymers have earned unique acknowledgment in electrochemistry because of their interesting redox behavior. But among them, polypyrrole is quite ahead to make its mark in electrochemistry due to the various promising aspects like the ease of synthesis, control of the properties with much easiness, appropriate redox behavior, high electrical conductivity, flexibility, and superior environmental stability, for being widely explored in a range of commercial applications [31, 33–40]. On the contrary, TiO2-embedded nanocomposite materials have made their mark in electrochemistry as one of the potential and fascinating electrode-modifying materials to develop electrochemical sensors and biosensors by amplifying its electrical conductivity due to the shared effect of the amalgamating materials, thereby lending it greater sensitivity. While TiO2 nanoparticles are found to contain diverse characteristics like superb catalytic activity, immense regularity, enormous surface area, smart biocompatibility, and reasonably sensible electric conduction and most of all, it is an inexpensive transition metal chemical compound and procurable in plenty [41–48].

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Hybrid composite materials are known to possess the novel properties and thus enhanced performance that cannot be achieved from the individual components. The nanocomposites of conducting polymer/inorganic nanoparticles present a new useful hybrid between inorganic and organic materials. In the majority of the well-known conducting polymers, Ppyl has been found to possess numerous advantages in the hybrid nanocomposites as compared to others. Hence, there has been an extreme focus on the research of Ppyl/inorganic nanocomposite. With the progress of material science, it has been shown that a nanocomposite of Ppyl and TiO2 combines the merits of nano-TiO2 and Ppyl to develop the potential applications in many fields [49–56]. These Ppyl/TiO2 nanocomposites exhibit characteristic properties as compared to their individual components, for example, thermal or mechanical stability and controlled conductivity, and these properties have offered them potential applications in photocatalytic materials [53, 56, 57] or anode electrodes for dye solar cells [58] and as anode materials for lithium-ion batteries [59]. However, as per the literature available and to the best of our knowledge, there is no report on the application of PPy/TiO2 nanocomposite as an electrochemical sensor for the electrocatalytic study of molecules of pharmaceutical interest. Sulfamoxole [4-amino-N-(4,5-dimethyl-1,3-oxazol-2yl)benzenesulfonamide] is a sulfonamide used as an antibiotic (Supplementary data, Scheme 1). The sulfonamides are synthetic antibiotics and are bacteriostatic in nature either by killing or inhibiting a wide range of gram-positive and gram-negative bacteria and are thereby also called as broad-spectrum antibiotics. Sulfamoxole competitively inhibits the bacterial enzyme dihydropteroate synthetase required for the proper processing of para-aminobenzoic acid (PABA) which is very important for the synthesis of folic acid. The folic acid synthesis reaction inhibited by sulfamoxole is very important for bacterial growth [60–63]. There is very little amount of literature in particular reference to the quantification of sulfamoxole by any of the analytical methods, and so far to the best of our knowledge, no systemic voltammetric study of the SLM is available. In the present work, a nanocomposite sensor PpylTiO2/glassy carbon electrode (GCE) has been developed for the electrocatalytic determination of SLM by the voltammetric method. The constructed Ppyl/TiO2/ GCE sensor demonstrated an outstanding electroanalytical performance as compared to Ppyl/GCE, TiO2/GCE, and bare GCE. The prepared nanocomposite sensor confirmed some significant improvements, for example, high conductivity, low detection limit, excellent reproducibility, and comparatively extensive linear dynamic range.

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Materials and methods Materials All the chemicals were used as received without any further treatment. Furthermore, the TiO2 (rutile) nanomaterial powder and reference standard of SLM was procured from SigmaAldrich and the tablets of Sulfuno containing SLM equivalent to 500 mg manufactured by German Remedies, India, was obtained from the commercial sources. All the solution preparations were carried out in double-distilled water.

Instrumentation The spectral characterization studies were carried out at Fourier transform infrared (A2 technologies ExoScan FT-IR serial No. D0909001258). The phase formation of powder samples was verified by X-ray diffraction (XRD) technique using an X-ray powder diffractometer (Rigaku Corporation Japan, SmartLab 3kW) with CuK α radiation (λ = 1.5405 Å) in the slow scan in the 2θ range of 10–80°. The morphological characteristics were studied by a scanning electron microscope (Quanta 400 ESEM, the Netherlands) to obtain the scanning electron microscopic images. Electrochemical studies were carried out using BAUTOLAB PGSTAT204^ (Metrohm Autolab B.V., S.No. AUT50302, made in the Netherlands) potentiostat/galvanostat fitted with NOVA 1.10 software. The electrochemical impedance spectroscopic analysis was done by μ-Autolab type PGSTAT204 (Eco-Chemie B.V., Utrecht, the Netherlands) potentiostat-galvanostat equipped with NOVA 1.10 software using FRA module. All the voltammetric measurements were performed using a three-electrode system where the synthetic Ppyl/TiO2/GCE was used as the working electrode, Ag/AgCl (3.0 M KCl) as the reference electrode, and platinum wire as an auxiliary electrode. The pH measurements were carried out using a digital pH meter (Decibel DB-1011).

Preparation of standard and sample solutions of SLM A stock solution of SLM reference standard of 1000 μg mL−1 concentration was prepared by dissolving the desired mass in 100 mL of methanol and kept in a cold storage place. Furthermore, the powdered sample of Sulfuno tablets containing 10 mg SLM was dissolved in a requisite amount of methanol to obtain the same concentration as that of the standard solution. Before the sample preparation, the average weight of 20 tablets was calculated and later on the tablets were crushed finely with a mortar pestle. The sample solution was sonicated for about 20 min to dissolve the sample substance properly in order to get the homogenous solution. After proper intermixing, the prepared solution was centrifugated at

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4000

3000

2500

2000

cm

1500

-1

Fig. 1 FTIR spectra of a Ppyl, b TiO2, and c Ppyl/TiO2

418.65 516.8

414.6

1000

615.11

1208.6 1102.2 1049 926.18 795.2

1699.8

3111.8

3500

1650.6

(a) Ppyl 3877.2 3787.2 3701.2

The synthesized Ppyl/TiO2 nanocomposite along with its different constituents, i.e., Ppyl and TiO2, were characterized by various spectroscopic (IR, XRD) and voltammetric techniques including electrochemical impedance spectroscopy as well. Further, the morphological characteristics of the synthesized materials were carried out by scanning electron microscopy (SEM). Besides it, the lone bare GCE was also studied for its

676.5

Results and discussion

1356 1319.1

(b)TiO2

926.18 926.18

(c) Ppyl/TiO2

%T

The bare GCE before modification was thoroughly rinsed with ethanol followed by ultrapure water in an ultrasonicator. The electrode was then cleaned mechanically on micro cloth pad by polishing with an alumina powder of 0.05 μm to 0.1 μm particle size followed by regular washing with ultrapure water in order to obtain a clean and smooth surface and subsequently dried in the vacuum desiccator for about 30 min. Later on, the GCE surface was modified by Ppyl/TiO2 nanocomposite suspension by casting the known volume (6 μL) and dried at room temperature. The fabricated Ppyl/TiO2 nanocomposite after drying was employed for the voltammetric analysis.

1560.6 1401 1302.7

Fabrication of Ppyl/TiO2 nanocomposite-modified GCE voltammetric sensor

The synthetic Ppyl/TiO2 nanocomposite and its constituents, viz., Ppyl and TiO2, were analyzed by FTIR spectroscopy, and the results obtained for the former were compared with the results of the latter to ascertain the formation of the Ppyl/TiO2 nanocomposite. Figure 1a–c demonstrates the FTIR spectra of Ppyl, TiO2, and Ppyl/TiO2. The FTIR spectrum of PPy (Fig. 1a) exhibits the typical band at 3112 cm−1 related to the N–H stretching whereas the band at 1561 cm−1 can be ascribed to the pyrrole ring due to the C=C stretching, 1401 cm−1 due to C=N vibrations, and 1303 cm−1 for C–H or C–N in-plane deformation [8, 33, 55, 65, 66]. However, the band appearing at 1200 cm−1 may be attributed to the C–C stretching band whereas the bands at 1102 and 1049 cm−1 could be related to the in-plane deformation vibration of the pyrrole ring. Furthermore, the characteristic band at 926 cm−1 indicates the C–H out-of-plane deformation of the pyrrole unit while the bands exhibited at 795 and 615 cm−1 may be due to C–H and C–C out-of-plane ring deformations, respectively [8, 33, 55, 65, 66]. However, the absorption band of 1700 cm−1 in the spectrum of Ppyl characterizes the existence of the carbonyl group formed by the nucleophilic attack of water during the synthesis of polypyrrole. A broad, downhill baseline can be seen from 1700 to 3100 cm−1 of the spectrum, which could be ascribed to the conduction of free electron in the conducting polymers [8, 33, 55, 65, 66]. On the other hand, the FT-IR

1703.8

The suspension for the modification of GCE was prepared by dispersing 2 mg of the synthesized Ppyl/TiO2 nanocomposite in 2 mL of N,N-dimethylformamide (DMF) to give a suspension of 1 mg mL−1 with ultrasonication for 1 h so as to obtain the homogenous suspension.

FTIR characterization

1654.7 1556.5 1482.8 1405 1315 1200 1098 1044.9

Preparation of Ppyl/TiO2 nanocomposite suspension

Spectral studies of Ppyl, TiO2, and Ppyl/TiO2 nanocomposite materials

1650.6 1544.2

A Ppyl/TiO2 nanocomposite was synthesized by liquid-liquid interfacial polymerization using pyrrole and TiO2 nanoparticles [64]. Initially, 300 mg of FeCl3·6H2O and 30 mg of TiO2 were dissolved in 15 mL of H2O. Afterwards, 0.8 mL of pyrrole was dispersed in 15 mL of chloroform in a beaker and mixed properly to obtain an organic phase. The aqueous phase was then poured dropwise into the organic phase, and the beaker was left uninterrupted for 24 h. After the completion of the polymerization, it was found that a black film was formed on the interface of the two phases. The product was filtered and rinsed several times with water and alcohol and air dried. Similarly, a pure Ppyl was synthesized without TiO2 by the same method for comparison.

3111.8

Preparation of Ppyl/TiO2 nanocomposite

various characteristics like surface area, charge transfer resistance, and heterogeneous electron transfer (HET) rate constant (k0eff) for comparison with its modifying components, namely, Ppyl and TiO2, to find out their effect on the conductivity of the GCE.

3746.2

3000 rpm for about 15 min. An aliquot of the sample solution was then analyzed.

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500

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spectrum (Fig. 1b) of TiO2 shows the characteristic band at 1654 cm−1 resulting from the bending vibration of Ti–OH as well as from coordinated H2O. The bands existing at 516 and 414 cm−1 could be possible because of the vibration of the Ti–O bond in TiO2. The peak appearing at 676 cm−1 could be assigned to the vibration of the Ti–O–O bond [54, 67]. However, the spectrum of Ppyl/TiO2 nanocomposites (Fig. 1c) depicts the stretching frequencies of N–H, C=C, C=N, and C–N slightly reallocated and were detected at 3118, 1557, 1405, and 1315 cm−1, respectively. Moreover, the band at 1200 cm−1 relates to C–N+ stretching and the band at ~ 926 cm−1 characterizes the C=N+–C stretching which are ascribed to the formation of bipolarons, which clearly supports their doping by FeCl3. It further implies that polypyrrole was oxidized by FeCl3, having positively charged bodies. The small shifting in the stretching frequencies of N–H, C=C, C=N, and C–N may possibly be accredited to the interaction of TiO2 nanoparticles with Ppyl [33, 55, 65, 66]. XRD analysis The XRD patterns of the synthesized Ppyl and Ppyl/TiO2 nanocomposite are shown in Fig. 2a–c. The TiO2 (rutile) nanopowder was used as such in the synthesis of the Ppyl/TiO2 nanocomposite.The XRD pattern of Ppyl (Fig. 2a) can be seen exhibiting a characteristic wider peak near about 26.44, which possibly could be attributed to the replicate units of the pyrrole ring indicating the amorphous nature of Ppyl [8, 33, 65, 66]. The XRD pattern of TiO2 (Fig. 2b) reveals some sharp peaks which are allocated to (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), and (112) planes. The observed Bd^ values are parallel to the standard Bd^ values (JCPDS card no. 21-1276) and thus confirms the rutile form of TiO2 [68]. However, the XRD analysis of the Ppyl/TiO2 nanocomposite (Fig. 2c) shows the typical peaks at 2θ equal to 27.34°, 28.64°, 36.1°, 41.12°, 54.3°, 56.58°,

(301) (112)

(002) (310)

(b) TiO2

(220)

(200) (111) (210)

(211)

(101)

(110)

Intensity (arb. units)

(c) Ppyl/TiO2

(a) Ppyl 10

20

30

40

50

60

2 (degree)

Fig. 2 XRD pattern of a Ppyl, b TiO2, and c Ppyl/TiO2

70

80

62.86°, 62.98°, and 69.62° corresponding to (905), (880), (212), (310), (435), (152), (197), (162), and (135) planes, respectively. Further, the Ppyl/TiO2 nanocomposite (Fig. 2c) shows comparatively the weaker peaks of TiO2 in conjunction with a wide peak of Ppyl and hence implies the fabrication of the Ppyl/TiO2 nanocomposite. Also, the XRD pattern of the Ppyl/TiO2 nanocomposite (Fig. 2) reveals the well-defined and sharper peaks than that in Ppyl (Fig. 2a) demonstrating the greater extent of crystallinity of the Ppyl/TiO2 nanocomposite. This could be possibly due to the TiO2 nanoparticles filling the voids of the polypyrrole matrix, thereby giving rise to a well-defined growth of Ppyl [8, 33, 55, 65, 66]. Scanning electron microscopy The surface and morphological studies of Ppyl, TiO2, and Ppyl/TiO2 nanocomposite materials were investigated by scanning electron microscopy as shown in Fig. 3. The SEM images with high magnification (Fig. 3a) evidently shows that the synthetic Ppyl exists as a bunch of spherical-shaped particles. Similarly, the SEM images of TiO2 nanoparticles as revealed in Fig. 3b demonstrates that there is a huge mass of extremely small and granular particles. Furthermore, Fig. 3c reveals the blending of Ppyl and TiO2 nanoparticles to form the nanocomposite with the lump-like appearance. TiO2 nanoparticles in the synthetic Ppyl/TiO2 nanocomposite can be seen to stuck with the surface of large Ppyl polymer granules and well scattered in the nanocomposite material. The SEM study evidently indicates the morphological transformation of Ppyl and TiO2 to Ppyl/TiO2 nanocomposite which is in accordance with the previous literature report [33].

Electrochemical characterization of GCE, Ppyl, TiO2, and Ppyl/TiO2 nanocomposite Surface area study of bare GCE, Ppyl/GCE, TiO2/GCE, and Ppyl/TiO2/GCE One of the objectives of the present work was to observe the effect of modifying components, i.e., Ppyl, TiO2, and Ppyl/TiO2 nanocomposite, on the surface area of GCE because the surface area is considered as one of the concrete indications for the successful modification of the electrode. The cyclic voltammetry (CV) of potassium ferricyanide K3[Fe(CN)6] is frequently an appreciated and acceptable mode to analyze the surface of the modified electrode. Hence, it was preferred as a benchmark to investigate the effect of the modification on the surface area of GCE by Ppyl, TiO2, and Ppyl/TiO2 nanocomposite material. The effective surface area analyses were executed by using 1.0 mM solution of K3[Fe(CN)6] in 0.1 M KCl as the electrochemical probe at various scan rates (10– 100 mV s −1 ) to study the behavior of the fabricated

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Fig. 3 Scanning electron micrograph (SEM) of a Ppyl, b TiO2/GCE, and c Ppyl/TiO2/GCE

Ppyl/TiO2/GCE, Ppyl/GCE, TiO2/GCE, and bare GC electrodes. Figure 4 represents the voltammograms obtained in 1.0 mM solution of K3 [Fe(CN)6] containing 0.1 M KCl at the scan rate of 100 mV s−1 for bare GCE (curve a), TiO 2 /GCE (curve b), Ppyl/GCE (curve c), and Ppyl/TiO2/GCE (curve d), respectively. The distinct pair of redox peaks can be seen on the bare GCE having a peak-to-peak separation (ΔEp) of 106 mV. The GCE after modifying singly with polypyrrole and TiO2 shows an increase in the redox peak currents with the decrease in ΔEp in both cases. This behavior of the modified electrodes could be ascribed to the electroactive nature of the modifying agents which after successful modification led to an increase in the conductivity of the electrodes. Similarly, when the electrode (GCE) was coated with synthetic Ppyl/TiO2, the redox peaks improved drastically having a peak-to-peak separation 94.13 mV in contrast to the bare GCE, TiO2/GCE, and Ppyl/GCE, demonstrating that Ppyl/TiO2 can efficiently enhance the electron transfer rate of Fe(CN)63− owing to its large surface area and high electrical conductivity. The peak separation for TiO2/GCE and Ppyl/GCE was found to be 102.22 and 98.37 mV, respectively. A higher value of the peak current also demonstrated the effective modification of the electrode and the synergistic effect of the synthetic Ppyl/TiO2 nanocomposite material. The surface area of the different

electrodes was determined from the slope of the plot I vs. ν1/2 using Randle’s Sevcik equation (Eq. (1)) [69, 70]  ð1Þ I ¼ 2:69  105 ACD1=2 n3=2 ν 1=2

Fig. 4 Surface area studies of bare GCE (curve a), at TiO2/GCE (curve b), Ppyl/GCE (curve c), and Ppyl/TiO2/GCE (curve d) with 1.0 mM K3[Fe(CN)6] in 0.1 M KCl by cyclic voltammetry

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where A corresponds to the effective surface area of the electrodes in square centimeters, n is the number of electrons involved in the charge transfer process, D is the diffusion coefficient of the analyte in the solution, and C is the concentration of K3[Fe(CN)6] solution. The value of n and D for K3[Fe(CN)6] are 1 and 7.6 × 10−6 cm2 s−1, respectively [46, 70, 71]. The surface area of the Ppyl/TiO2/GCE nanocomposite sensor was determined to be equal to 0.0965 cm2 which is much greater than Ppyl/GCE (0.0784 cm 2), TiO 2/GCE (0.0558 cm2), and bare GCE (0.0244 cm2) indicating an enhanced voltammetric performance towards oxidation of SLM (Fig. 4).

ð3Þ

HET rate constant, k 0 eff , by inner sphere redox probe ([K3Fe(CN)6]) Inner-sphere reactions are the reactions in which transfer of electron takes place in an activated complex where a ligand is shared between the donor and acceptor molecules (and where the bridging ligand may or may not be transferred during the reaction). Further, in the case of heterogeneous electrode reactions, there is a strong interaction of reactant or product with the surface of the electrode in an inner-sphere reaction. Hence, in heterogeneous inner-sphere reactions, the reactants, intermediates, or products are often specifically adsorbed on the surface of electrode. To determine the HET rate constant, k 0 eff , of GCE, Ppyl/GCE, TiO2/GCE, and Ppyl/TiO2/GCE for studying their electrochemical characteristics, 1 mM potassium ferricyanide (III) ([Fe(CN)6]3−) in 0.1 M KCl was chosen as inner-sphere redox species because of its sensitivity for the surface oxides [79, 80]. The cyclic voltammograms (CVs) obtained for GCE, Ppyl/GCE, TiO2/GCE, and Ppyl/TiO2/GCE (Supplementary data, Fig. S1) reveals the characteristic peaks for the oxidation and reduction processes, with peak-to-peak separation (ΔEP) of 96.51 mVat 100 mV s,−1 for Ppyl/TiO2/GCE indicating the process to be quasi-reversible [81, 82]. Also, the peak-to-peak separation of Ppyl/GCE, TiO2/GCE, and bare GCE was found to be 100.55, 105.34, and 110.00 mV, respectively. It is thus obvious from the CVs shown in the plot that Ppyl/TiO2/GCE shows the phenomenal redox peak height with larger reversibility, having a peak-to-peak separation (ΔEP) of 96.51 mV (at 100 mV s−1), therefore suggesting the rapid kinetics as compared to Ppyl/GCE, TiO2/GCE, and bare GCE. It is to be noted that ΔEP is one of the required aspects to ascertain the sensitivity of the electrode and is applied to determine the HET rate constant. The smaller the value of ΔEP for a given electrode, the greater its reversibility and the faster its HET kinetics [46]. For more investigation and characterization of the developed electrodes together with bare GCE, k0 was found out from the slope by plotting the kinetic parameter ψ against k0eff [πDn F/(RT)]−1/2 [70, 73] via Nicholson’s method as expressed by Eqs. (3) and (4). The k0eff of the electrodes was observed to be equal to 0.0143, 0.0187, 0.0276, and 0.0715 cm − 1 for GCE, TiO 2 /GCE, Ppyl/GCE, and Ppyl/TiO2/GCE, respectively, after being deduced from the slope of the plot (Supplementary data, Fig. S2).

where X is equal to ΔEP and facilitates the calculation of ψ as a function of ΔEP from the voltammograms obtained experimentally. Therefore, Eq. (2) can be broadly used to determine k0eff by plotting the graph between ψ and [πDn F/ (RT)]−1/2 [73, 78]. It is to mention here that all k0eff values have been calculated at the scan rate of 10–100 mV s−1. The heterogeneous rate constant (k 0 eff ) of synthetic Ppyl/TiO2/GCE was compared with other electrodes, i.e., bare GCE, modified Ppyl/GCE, and TiO2/GCE, to ascertain its voltammetric performance.

HET rate constant, k 0 eff , by outer sphere redox probe ([Ru(NH3)6]Cl3) Outer-sphere reactions are the reactions in which transfer of electron takes place between two species having no bond between them, with the electron tunneling from one to the other, most likely across the layer of solvation. Furthermore, in the case of heterogeneous electrode reactions, the reactants, products, and intermediates do not interact strongly with the electrode material and electron transfer occurs by tunneling across at least a monolayer of solvent in an outer-sphere reaction.

Effective heterogeneous electron transfer rate constant, k0eff After surface area studies, the attention was next turned towards the determination of effective heterogeneous electron transfer (HET) rate constant, k0eff, of the fabricated Ppyl/TiO2/ GCE, Ppyl/GCE, and TiO2/GCE along with bare GCE by cyclic voltammetry using potassium ferricyanide(III) as inner-sphere and hexaammineruthenium(III) chloride as outer-sphere redox probes in 0.1 M KCl to study the effect of modication on electrode kinetics. The HET rate constant, k0eff, was determined by Nicholson’s method [72, 73] for quasi-reversible systems by the equation given below: ψ ¼ k 0 eff ½πDnυF=ðRT Þ−1=2

ð2Þ

where ψ is called the kinetic parameter, D corresponds to the diffusion coefficient (D = 7.6 × 10 −6 cm 2 s −1 [71] for [K 3 Fe(CN) 6 ] and 9.1 × 10 − 6 cm 2 s − 1 [73–75] for Ru(NH3)62+/3+ in 0.1 M KCl as supporting electrolyte), n represents the number of electrons taking part in the process, F stands for Faraday’s constant, R denotes the gas constant, and T is the temperature. In one step and one electron process at a constant temperature (298 K), ψ is computed as a function of peak-to-peak separation (ΔEP) [72, 73, 76, 77]. The function of ψ (ΔEP), which fits Nicholson’s data, for practical usage (rather than producing a working curve) [73, 77, 78] is given by the following equation: ψ ¼ ð−0:6288 þ 0:0021XÞ=ð1−0:017XÞ

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Likewise, the HET rate constant, k0eff, studies were also carried out for the electrodes under investigation, i.e., GCE, Ppyl/GCE, TiO2/GCE, and Ppyl/TiO2/GCE, with 1 mM hexaammin-eruthenium(III) chloride in 0.1 M KCl chosen as redox probe to characterize the electrode surfaces electrochemically. The reason to choose [Ru(NH3)6]Cl3 is because it acts as an excellent outer-sphere redox probe and is primarily dependent on the electronic structure of carbon-based electrode materials while investigating the carbon surfaces [74, 77]. The voltammograms (Supplementary data, Fig. S3) obtained (at 100 mV s−1) for GCE, Ppyl/GCE, TiO2/GCE, and Ppyl/TiO2/ GCE against the Ru(NH3)62+/3+ redox probe testified that Ppyl/TiO2/GCE exhibits ΔEp of 68.21 mV, Ppyl/GCE of 71.11 mV, and TiO2/GCE of 77.70 mV and the bare GCE possess ΔEp of 82.73 mV. Hence, it is evident that Ppyl/TiO2/ GCE possesses greater reversibility because of its smaller peakto-peak separation (ΔEp) of 64.84 mV indicating thus more rapid kinetics and a larger HET rate constant. The value of ΔEp was further explored to determine the k0eff of all the mentioned electrodes from the plot drawn between kinetic factor ψ and [πDn F/(RT)]−1/2 [73, 78] via Eqs. (2) and (3) using Nicholson’s method. The k0eff for the given electrodes evaluated from the slope of the plot (Supplementary data, Fig. S4) was detected to be 0.0414, 0.0532, 0.1527, and 0.3681 cm−1 for GCE, TiO2/GCE, Ppyl/GCE, and Ppyl/TiO2/GCE, respectively, using hexaammineruthenium(III) chloride in 0.1 M KCl as redox probe. Ppyl/TiO2/GCE therefore strongly elucidates the fast HET kinetics and accordingly testifies the excellent electrochemical properties, thereby giving rise to the exceptional voltammetry in comparison to other given electrodes towards Ru(NH3)62+/ 3+ as shown in Supplementary data, Fig. S4. Electrochemical impedance spectroscopy While continuing the electrochemical characterization of the electrodes under observation, it was intended to study the charge transfer resistance and other kinetic parameters of these electrodes to authenticate the above results further by electrochemical impedance spectroscopy (EIS). EIS is the most commonly, highly precise, and well-recommended technique used to investigate the electrode processes and the interfacial properties of the modified electrodes in order to study their charge transfer resistance and other kinetic parameters. Therefore, in the present work EIS studies were conducted at an open circuit potential (OCP) in the frequency range of 0.1–100,000 Hz by applying a sinusoidal signal of 10 mV amplitude and using 5.0 mM of K3[Fe(CN)6] in 0.1 M phosphate buffer solution (pH 7.0) as the redox probe. The characteristic Nyquist plots obtained for the GCE, TiO2/GCE, Ppyl/GCE, and Ppyl/TiO2/GCE nanocomposite are depicted in Fig. 5, where each of the plots can be seen composed of a semicircular part in the high-frequency region

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Fig. 5 Nyquist plot of GCE (curve a), TiO2/GCE (curve b), Ppyl/GCE (curve c), and Ppyl/TiO2/GCE (curve d) in 5.0 mM K3[Fe(CN)6]. Inset: Randle’s circuit used to fit the impedance data for Ppyl/TiO2/GCE

pertaining to the electron transfer-limited process and a slightly linear part in the low-frequency region signifying a diffusion process. The electrode resistance due to the charge transfer resistance (Rct) is approximately equivalent to the diameter of the semicircle whereas the capacitance of the electrode is attributed to the vertical linear portion in the Nyquist plot. EIS data is usually interpreted by applying the equivalent electrical circuit fitting model system. The majority of the circuit elements incorporated in a typical model are common electrical elements like resistors, capacitors, inductors, and some specialized electrochemical elements (e.g., Warburg diffusion elements), and each element in the model has a known impedance behavior. Therefore, the EIS experimental data was gathered by fitting an equivalent circuit model based on Randle’s model and used to determine the simulated values of kinetic parameters. This equivalent circuit is fitted with the charge transfer resistance (Rct), solution resistance (Rs), and a constant phase element (CPE). Constant phase element (CPE) is required in the circuit for simulating the impedance data and is described as Z CPE ¼ T CPE ðjwÞ−n

ð4Þ

where TCPE and n refers to frequency-independent constants; w is the angular frequency. The exponent n as the correction factor is related to the roughness of the electrode and used to calculate the amount of the variation from Randle’s model. The CPE depicts the inhomogeneous nature of the surface, and the changeover of the double-layer capacitance by CPE enhanced the agreement of fitting. When n = 1, 0, and 0.5, the

2480 Table 1 Kinetic parameters obtained by simulation of the best fitted values of the Randle’s equivalent circuit elements from EIS data for different electrodes given below

Ionics (2018) 24:2473–2488

Electrode

Rs (Ω cm2)

Rct (kΩ cm2)

CPE (nM ho)

W (μM ho)

N

χ2

GCE

350 350

23.10 12.30

811 116

61.8 613

0.94 0.92

0.84 0.12

350 350

8.91 5.65

870 947

245 462

0.90 0.88

0.80 0.87

TiO2/GCE Ppyl/GCE Ppyl/TiO2/GCE

CPE is designated as ideal capacitor, pure resistor, and Warburg element, respectively [83]. The values of the fitted parameters are given in Table 1. The smaller values of χ2 indicate that the experimental data is best fit to Randle’s model [84]. It is also evident from the data (Table 1) that there is a significant decrease in the Rct value from the bare GCE to Ppyl/TiO2/GCE which can be further confirmed from the Nyquist plot as the shortening in the diameter of the semicircular portion from GCE, TiO2/GCE, and Ppyl/GCE to Ppyl/TiO2/GCE. The higher values of the CPE and the lower values of n (Table 1) for Ppyl/TiO2/GCE reveal their greater surface area availability as compared to Ppyl/GCE, TiO2/ GCE, and GCE. The results thus clearly testify that the interfaces between TiO2 and Ppyl could markedly promote the charge transfer reactions of the composite film and so is its super conductivity. Further, the Nyquist plot also elucidates the diffusion-controlled process (Warburg impedance) because of the frequency-dependent ion diffusion/transport from the solution to the interface for all the given electrodes as evident from linear fraction tilting at an angle of about 45° to the Zre axis in the low-frequency zone of the impedance spectrum. While evaluating the EIS plot obtained for the investigated electrodes, it can be seen that Ppyl/TiO2/GCE reveals a more vertical fraction with a smaller semicircle than that of Ppyl/GCE, TiO2/GCE, and bare GCE in the lowfrequency part with rapid amplification in the imaginary region of impedance, thereby confirming the superior conducting behavior [25, 85]. Therefore, the results procured from EIS measurements supports the results obtained from CV and hence authenticates again the successful casting and fabrication of synthetic Ppyl/TiO2 on bare GCE for the sensory application.

Electroactive studies of SLM at TiO2, Ppyl, and Ppyl/TiO2 sensors The electroactive behavior of SLM was studied at synthetic Ppyl/TiO2 together with TiO2 and Ppyl. These sensors were used to study the redox nature of SLM by voltammetric techniques like square wave (SWV), differential pulse voltammetry (DPV), and cyclic voltammetry (CV) by monitoring the current response of each electrode towards the SLM. Under optimal conditions, each of the electrodes under investigation shows the oxidation of SLM in pH 5.56 Britton-Robinson buffer (BRB), and

the current responses for SLM at bare GCE, TiO2/GCE, Ppyl/GCE, and Ppyl/TiO2/GCE nanocomposite sensors were compared. Figure 6 reveals the SWV (Fig. 6a), DPV (Fig. 6b) , and CV ( Fig. 6 c) resp ons es of 5 μg mL−1 SLM in methanol at the bare GCE (curve b), TiO2/GCE (curve c), Ppyl/GCE (curve d), and Ppyl/TiO2/ GCE (curve e) in pH 5.56 Britton-Robinson buffer. The curve a in SWV (Fig. 6a), DPV (Fig. 6b), and CV (Fig. 6c) represents the blank voltammograms of Ppyl/TiO2/ GCE. Moreover, it is also evident from each figure that the bare GCE exhibits a very weak anodic peak for SLM whereas TiO2/GCE shows a dramatic increase in the peak current. The much improved voltammetric response of SLM on TiO2/GCE could be evidently attributed to the large surface area and electrocatalytic activity of TiO2. Similarly, in the case of Ppyl/GCE, it could also be observed that the oxidation peak current of SLM is greater than that on bare GCE or TiO2/GCE, which could be due to greater conductivity and surface area of polypyrrole. However, Fig. 6a (SWV), Fig. 6b (DPV), and Fig. 6c (CV)elucidate that SLM exhibits a well-defined and excellent peak height at the Ppyl/TiO 2/GCE sensor. The heightening of the current at the proposed electrode (Ppyl/TiO2/GCE) could reasonably be the synergetic effect of polypyrrole and TiO2. Thus, both polypyrrole and TiO2 exhibiting a large surface area increased the electron transfer between the electrode and analyte. The synergy of both enormously enhanced the electrode response for SLM. Also, Ppyl/TiO2/GCE shows no electrochemical response in the blank solution. Hence, the oxidation of SLM at all the corresponding electrodes clearly elucidates the electroactive nature of Ppyl, TiO2, and Ppyl/TiO2 and further their peak heights in the order of electroactivity.

Polypyrrole/TiO2 nanocomposite as voltammetric sensor for SLM The studies were further extended to the voltammetric quantification of SLM after the satisfactory characterization studies of the interested electrode material and its allies (viz., Ppyl and TiO2). Thus, the excellent electroactive characteristics of the Ppyl/TiO2 nanocomposite made it best fit to the electrodemodifying material for the bare GCE surface in order to develop a modified voltammetric electrode (Ppyl/TiO2/GCE) for the electroanalysis of SLM so as to determine the sensitivity of

Ionics (2018) 24:2473–2488

2481

Fig. 6 Voltammetric performance of Ppyl/TiO2/GCE sensor for oxidation of SLM. (a) Square wave voltammograms of 5 μg mL−1 SLM: blank (curve a), at GCE (curve b), at TiO2/GCE (curve c), at Ppyl/GCE (curve d), and at Ppyl/TiO2/GCE (curve e). (b) Differential pulse voltammograms of 5 μg mL−1 SLM: blank (curve a), at GCE (curve b), at TiO2/

GCE (curve c), at Ppyl/GCE (curve d), and at Ppyl/TiO2/GCE (curve e). (c) Cyclic voltammograms of 5 μg mL−1 SLM: blank (curve a), at GCE (curve b), at TiO2/GCE (curve c), at Ppyl/GCE (curve d), and at Ppyl/TiO2/GCE (curve e)

the synthetic Ppyl/TiO2/GCE voltammetric sensor after quantifying the proposed drug. The efficiency of the proposed electrode was further improved by optimizing the various experimental parameters in order to increase its (Ppyl/TiO2/ GCE) electroanalytical performance in terms of the sensitivity and limit of detection (LOD) towards the target analyte (SLM). Various experimental parameters like supporting electrolyte, pH of the medium, selection of the solvent, and casting

volume of the electrode modifier (Ppyl/TiO2) have also been observed.

Fig. 7 Effect of pH (pH 2.5–12.0) (inset: plot of peak potential, Ep vs. pH)

Optimization of pH The effect of the pH on the oxidation of SLM in BrittonRobinson buffer over a pH range of 2–12 was studied to select the optimum pH in order to carry out the voltammetric

2482 Scheme 1 Proposed electrochemical reaction (oxidation) of SLM at Ppyl/TiO2/ GCE

Ionics (2018) 24:2473–2488

H3C

N

NH O

H3C

analysis of the proposed drug at Ppyl/TiO2/GCE. Figure 7 represents the square wave voltammograms obtained for electro-oxidation of SLM (1000 μg mL−1 in methanol) in BR buffer at different pHs (2–12). It is apparent from Fig. 7 that the oxidation current increases with the rise in pH and reaches the maximum at pH 5.56 with a single and welldefined peak height. Further, the current height starts diminishing while increasing the pH after 5.56 pH and completely disappeared at pH 12. It was also observed that the anodic peak potential shifts towards a less positive value with the increase in pH and therefore indicates the involvement of protons in the oxidation process of SLM. The dependence of peak potential (Ep) on pH of the medium shows a linear relationship which may be expressed by the linear regression equation as SWV; Ep =V ðAg=AgClÞ ¼ −0:0279 pH þ 1:2027; r2 ¼ 0:9810 ð5Þ

Therefore, pH 5.56 was selected as the most optimum pH to carry out further voltammetric studies (Fig. 7).

O

NH

N

S

-2e-, -1H+

O NH2

H3C

+H2O

O H3C

O S

O NH OH

Optimization of solvent Ten milligrams of SLM was dissolved into 10 mL of different solvents, and the prepared solutions were then subjected to analysis by square wave voltammetry. The solvents studied included water, methanol, ethanol, DMF, and DMSO, and the surfactants like sodium lauryl sulfate (SLS), cetyltrimethylammonium bromide (CTAB), Tween-20, Tween-40, and Triton X-100 were selected on the basis of the solubility of the drug under investigation (SLM). The effect of different solvents on the oxidation of SLM at the synthetic electrode (Ppyl/TiO2/GCE) has been shown in Supplementary data, Fig. S5. The bar graph (Supplementary data, Fig. S5) obtained for the oxidation of SLM in different solvents clearly shows that SLM exhibits a well-defined peak with sharp and greater height in methanol as compared to other solvents which suggest that methanol is a highly optimizing solvent for the electroanalysis of SLM.

Optimization of modifying film The different volumes of Ppyl/TiO2 ranging from 2 to 15 μL was drop casted on the GCE surface and dried to develop a very thin film of the modifier (Ppyl/TiO2). The modified electrode (Ppyl/TiO2/GCE) was used to analyze the electro-oxidation of SLM. The influence of different casting volumes of Ppyl/TiO2 on the anodic peak current can be observed from Supplementary data Fig. S6 in which the curve obtained clearly describes higher current at the GCE surface with 6 μL of Ppyl/TiO2. It is thus apparent that 6 μL of Ppyl/TiO2 is the most effective for carrying out the electroanalytical studies of the SLM. Table 2 Method validation parameters for standard linearity of SLM using SWV

Fig. 8 Square wave voltammograms of SLM at the Ppyl/TiO2/GCE sensor at different concentrations from 1.25 to 12.50 μg mL−1. Inset: plot of peak current, I vs. SLM concentration

Linearity parameters

Results

Slope Standard deviation Correlation coefficient Limit of detection (ng mL−1) Limit of quantification (ng mL−1)

0.3867 0.1548 0.9980 1.24 4.15

Ionics (2018) 24:2473–2488 Table 3 Accuracy and precision for determination of SLM by SWV (n = 3)

2483

Added (μg mL−1)

Precisiona (μg mL−1)

Found (μg mL−1)

5.0

5.04

10.0 12.0

10.08 12.09

Coefficient of variation (%)

5.06 ± 0.12

2.34

0.80

10. 08 ± 0.11 12. 09 ± 0.14

1.09 1.16

0.80 0.75

a

Mean ± S.D.

b

Accuracy = [found − added/added] × 100, coefficient of variation = S.D./mean × 100

 logI ðμAÞ ¼ 0:3691 log ν mV s−1 −0:1178; r2 ¼ 0:9950 ð9Þ

Scan rate studies Cyclic voltammetric studies were carried out at various scan rates ranging from 10 to 100 mV/s to ascertain the influence of scan rate on the peak current. It can be observed that the oxidation current (I) of SLM varies linearly (Supplementary data, Fig. S7) with the square root of the scan rate (ν1/2 mV/s) which indicates that current (I) is directly proportional to the square root of the scan rate (ν1/2) and can be expressed by Eq. (6):  IðμAÞ ¼ 0:3503υ1=2 mV s−1 −0:7155; r2 ¼ 0:9938

ð6Þ

It was further observed that increasing scan rate ν, (10– 100 mV/s) results in the shifting of peak potential (Epa) towards a more positive value and exhibits a linear relationship with the logarithm of the scan rate ln ν, (Supplementary data, Fig. S7) as shown by Eq. (7), Epa ðV vs:Ag=AgCl Þ ¼ 0:0314ðln ν Þ þ 0:9937; r2 ¼ 0:9850

ð7Þ

It clearly suggests that the electrode process for the oxidation of SLM is an irreversible [86–88] and diffusion-controlled process [89–91]. Furthermore, the completely diffusion-controlled and irreversible process should comply with the Laviron equation (Eq. (8)) [86] in which Epa can be defined as  E p ¼ E° þ ðRT =αnF Þ ln RT k° =αnF þ ðRT =αnF Þ ln ν

Accuracyb (%)

ð8Þ

where E° indicates the formal redox potential, R is the universal gas constant (8.314 J/K mol), T is the absolute temperature (298 K), F is known as Faraday constant (96,485 C mol−1), k0 is the standard rate constant of the reaction, n is the number of electrons involved in the ratedetermining step, is the scan rate, and α depicts the transfer coefficient and is supposed to be 0.5 for a completely irreversible electrode process [71]. Hence, in order to confirm the irreversible and diffusion-controlled behavior further, the transfer coefficient (α) was determined from the slope by plotting log I vs. log ν (Supplementary data, Fig. S7) and was found to be 0.3691 which is very close to the theoretical value (0.5) and therefore testifying the mentioned behavior of the electrode given by Eq. (9).

Moreover, the linear relationship between Epa and ln as stated previously (Eq. 7) gives a slope of RT/αnF (0.0314) from which the number of electrons (n) involved in the reaction can be calculated by substituting the values of α (0.5), R (8.314 J/K mol), T (298 K), and F (96,485 C mol−1) and was found to be equal to 1.64 which is nearest to 2. Thus, results obtained suggest that the electrode reaction of SLM at Ppyl/TiO2/GCE is a two-electron and two-proton process. While taking the above facts into consideration, the following feasible electron transfer mechanism may be suggested for the oxidation of SLM (Scheme 1).

Electrode sensitivity After establishing the optimizing conditions, the sensitivity of fabricated Ppyl/TiO2/GCE was monitored by square wave voltammetry. Limit of detection (LOD) and limit of quantification (LOQ) of the analyte (SLM) were determined by measuring the peak current against the concentration of SLM. Results were accomplished by plotting the calibration curve of the peak current against the concentration of SLM. The square wave voltammograms represented in Fig. 8 illustrates the effect of different concentrations of SLM on the sensitivity of electrode in which the oxidation peak current can be seen increasing with SLM concentrations from 1.25 to 12.50 μg mL−1 in BR Table 4 Reproducibility and repeatability data for 5 μg mL−1 SLM at Ppyl/TiO2/GCE sensor Sensor reproducibility Sensor

Single sensor repeatability

Mean current (I/A) RSD (%) Mean current (I/A) RSD (%)

Sensor 1 2.62a a

Sensor 2 2.72

Sensor 3 2.55a Average 2.63b

0.67 0.27 0.36 3.19

a

Mean of seven replicate readings

b

Mean of three sensors

2.62a

0.67

2484 Table 5 Recovery study of SLM in pharmaceutical tablets

Ionics (2018) 24:2473–2488

Original conc. (μg mL−1)

Added (μg mL−1)

5.00

2.00

7.00

6.83

5.00 5.00

2.00 2.00

7.00 7.00

6.91 7.18

5.00

3.00

8.00

7.82

5.00 5.00

3.00 3.00

8.00 8.00

7.94 8.12

5.00

5.00

10.00

10.22

5.00 5.00

5.00 5.00

10.00 10.00

9.87 10.13

buffer (pH 5.56) at Ppyl/TiO2/GCE as expressed by Eq.. (10):  I ðμAÞ ¼ ð0:3867Þ SLM μg mL−1 þ 0:2708; r2 ¼ 0:9980

ð10Þ

Thus, the regression graph (inset of Fig. 8) clearly depicts the direct relationship of the current intensity with the concentration of SLM. The calibration curve was further explored to estimate the limit of detection (LOD) as 3 S/m and was observed to be 1.24 ng mL−1, and the limit of quantification (LOQ) was 10 S/m and was calculated as 4.15 ng mL−1, with S corresponding to the standard deviation of the peak currents (n = 9) and m representing the slope of the calibration curve. The different statistical parameters obtained from the linear regression equation are reported in Table 2. The values of correlation coefficient (r2) and standard deviation clearly indicate the good linearity of the calibration curve and a very narrow scattering of the experimental points. Therefore, the sensitive determination of SLM was carried out effectively with a broader linear range.

Accuracy and precision of the proposed method The accuracy of the electrode sensitivity was testified by determining exactly the weighed amounts of the SLM standard from the three different known concentrations by square wave voltammetry. The accuracy of the developed electrode was established by evaluating the percentage of relative error between the added amount and the found amount from the Table 6 Comparison of detection limit of proposed method with other electrochemical methods reported for SLM

Net conc. (μg mL−1)

Total found (μg mL−1)

Mean (μg mL−1)

St. dev

RSD %

Recovery %

6.97

0.18

2.63

99.62

7.96

0.15

1.90

99.50

10.07

0.18

1.80

100.73

standard solution of SLM. Table 3 illustrates the excellent accuracy acquired by using the proposed Ppyl/TiO2/GCE for the quantification of the drug under observation. However, the precision of the synthetic electrode for the SLM was established in terms of the relative standard deviation (% RSD) and coefficient of variation (CV). The mean variation coefficient for electrode was found to be 1.53% (Table 3) which is less than 2.0% indicating superb precision. Thus, the results summarized in Table 3 validates both accuracy and the precision of the proposed electrode for the quantification of the SLM.

Repeatability, reproducibility, and stability of synthetic voltammetric sensor In order to look at the reproducibility, repeatability, and stability of synthetic Ppyl/TiO2/GCE, the SLM solution of known concentration was analyzed by SWV. The repeatability of the electrode was carried out by determining the relative standard deviation (RSD) of the current response for seven replicate measurements obtained from the oxidation of SLM with known concentration (5 μg/mL). The RSD of the peak currents for the proposed electrode was found to be 0.67% (Table 4), thereby depicting superb repeatability. However, the reproducibility parameter was determined from the RSD of three similar constructed sensors (Ppyl/TiO2/GCE) in the SLM solution of the same concentration (5 μg/mL). The reproducibility was determined by evaluating the oxidation peak current of SLM at the three prepared electrodes using SWV. Meanwhile, the calculated RSD for the electrodes was 3.19% (Table 4) and indicates fine reproducibility

Reference method

Detection parameter (LOD)

Reference

HPLC

7.97 μg kg−1

SPELC-MS/MS) Spectrophotometric (alkaline and acidic sols) Coulometric Voltammetry (Ppyl/TiO2/GCE)

1–5 ng L−1 0.07–0.79 μg mL−1 0.02–0.16 mg mL−1 1.24 ng mL−1

[92] [93] [94] [95] Present work

Ionics (2018) 24:2473–2488

as well. Further, the one more important parameter of the developed sensor checked was stability. The stability of the synthetic voltammetric sensor was verified by determining the decrease in sensitivity of the modified electrode from the drop in the peak current after storing the electrode for over 15 days at room temperature. It was observed that the sensor shows a negligible decrease in its sensivity (decreased current) over the first week and sustained about 94% of its sensivity after storing for 15 days in an open air at room temperature (25–30 °C), thus indicating the long-term stability of the synthetic voltammetric Ppyl/TiO2/ GCE sensor.

Application of the synthetic Ppyl/TiO2/GCE sensor to pharmaceutical formulations The sensing property of the fabricated voltammetric Ppyl/TiO2/GCE was further extended for the determination of SLM in pharmaceutical tablet Sulfuno (500 mg/tablet). The recovery test was used to look at the accuracy of the method and hence the analytical significance of the sensor. The recovery test was carried out by following the standard addition method in which a standard solution of SLM with known concentration (2–5 μg/mL) was added to the previously analyzed sample solution and analyzed by SWV at Ppyl/TiO2/GCE. All the test solutions were prepared as stated in the BPreparation of standard and sample solutions of SLM^ section. The recovery percentage of SLM obtained from the average of five replicate measurements was estimated to be in between 99.50 and 100.73% with the relative standard deviation of 1.80 to 2.63% (Table 5). The data summed up in Table 5 satisfactorily describes the accuracy of the method with the recovery range indicating that the synthetic sensor could be efficiently applied to the quantification of SLM in commercial pharmaceutical products.

Comparison of the proposed method with previous reported methods The analytical performance of the present voltammetric method for the quantification of SLM at the Ppyl/TiO2/GCE sensor has been compared with other methods reported previously in literature in terms of the limit of detection (Table 6) [92–95]. The limit of detection obtained for SLM by the proposed method is the lowest in comparison to other reported methods which indicates that the developed Ppyl/TiO2/GCE sensor is very sensitive for the quantification of SLM.

Conclusion A novel sensor was fabricated by modifiying the GCE with a highly conductive Ppyl/TiO2 nanocomposite material to carry out electrocatalytic studies of the SLM by SWV technique in

2485

order to develop a simple and sensitive method of quantification for SLM. It was observed that the developed Ppyl/TiO2/ GCE nanocomposite sensor exhibits remarkable sensitivity, good stability, repeatability, reproducibility, wide linear response range, and low detection limit. The results obtained clearly imply that Ppyl/TiO2 can be used as a voltammetric sensor for the quantification of SLM. Acknowledgements Authors are highly thankful to the school of studies in Chemistry, Jiwaji University Gwalior, for providing us all the essential services.

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