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A scanning electron microscope provides details surface information about nanoparticle including morphological characterization, homogeneity and the size of the particles. Fig. 2 indicates the SEM micrograph of NiTiO3 nanoparticles. As can be observed the NiTiO3 nanoparticles present homogeneous morphology with spherical form and the average diameter of the nanoparticles is about 38.0 nm.
X-ray diffraction patterns have been widely used in nanoparticles research as a primary characterization tool for getting critical features such as strain, crystallite size and structure. So, XRD analysis was accomplished for investigation of the crystal phases of NiTiO3 nanopowders. Fig. 3 shows the XRD spectra of the NiTiO3 nanoparticles after heat-treatment to 750 °C in air for 2 h that is the lowest temperature has been reported so far for the formation of NiTiO3 nanopowders by the sol-gel method. Sharp and intense peaks of nanoparticles at this temperature represent fine crystalline rhombohedral NiTiO3 phase and all peaks were well-coordinated associated with the database in JCPDS (file number: 83-0199). The particle size computed with Scherrer’s formula  (Eq. 1):
D = (0.9λ)/(βcosθ) (1)
where λ (0.15418 nm) is the X-ray wavelength, β is the full-width at half maximum (FWHM) in radians of the X-ray diffraction peak, θ is the diffraction angle and D is the average size of the particles. According to Eq. 1, the average particle size of the NiTiO3 nanopowders was estimated about 33.0 nm with compatible value with SEM results.
Characterization of the modified electrode by EIS
In this study, EIS technique was used to indicate the additional effect of nickel titanate nanoparticles on the modified electrode. EIS graphs of the modified and unmodified electrodes in [Fe(CN)6]3-/4- (Fe2+/Fe3+) as negatively charged redox probe is shown in Fig. 4. The value of the electron transfer resistance (Rct, semicircle diameter) depends on the dielectric and insulating features at the electrode/electrolyte interface . The results were approximated by an equivalent circuit. As can be observed the presence of NiTiO3 nanoparticles on the surface of the carbon paste electrode increases the electron transfer at the surface of the modified electrode. By the other hand, the modified electrode compared to a bare electrode, had lower charge transfer resistance.
The surface morphology of the bare and modified electrodes was characterized by SEM technique (Figs. 5A and B). It can be seen in Fig. 5A, the pure graphite electrode surface is nonuniform and free of any coating. Fig. 5B shows that a homogeneous distribution of NiTiO3 nanoparticles on the surface of a modified electrode.
Kinetic parameters of the NiTiO3/CPE were investigated by cyclic voltammetry (CV) method. Cyclic voltammograms of the modified electrode on Fe2+/Fe3+ probe solution in the range of scan rates from 10.0 to 70.0 mV s-1 are shown in Fig. 6. It can be seen in Fig. 6(c) for scan rate values higher than 300.0 mV s-1 the anodic potential is directly proportional to the logarithm of scan rate. Afterward, electron transfer rate constant (ks, s−1) and charge transfer coefficient (α) can be calculated by the Laviron’s equation (Eq. 2) .
Log ks = α log (1-α) + (1-α) logα – log (RT/nFv) –α (1-α) nαF∆Ep/2.3RT (2)
Where v is different scan rate values and n is the number of electrons involved in the redox reaction. From these expressions, α can be determined by measuring the variation of the peak potential with respect to the scan rate, and ks can be determined by measuring the ΔEp values. According to these results, the value of α and ks were obtained to be 0.32 and 0.14 s-1, respectively.
Application of the nano structured modified sensor in electrochemical studies of monohydroxy benzoic acid isomers
Oxidation of OHB and PHB on the unmodified and modified electrodes
The electrochemical behaviors of OHB and PHB have been investigated by DPV. Fig. 7 shows differential pulse voltammograms of the NiTiO3/CPE and CPE electrodes in B-R buffer solution (pH 5.0) containing 50.0 µM PHB and 50.0 µM OHB. As shown in Fig. 7b and c, a significant enhancement in voltammetric responses of NiTiO3/CPE compared with bare CPE prove the effect of nickel titanate nanoparticles on the modified electrode. So, the modified electrode was used for simultaneous determination of OHB and PHB with high sensitivity and suitable detection limit.
The electrochemical behavior of OHB and PHB at NiTiO3/CPE were studied in the presence of B-R buffer with different pHs (2.0 to 9.0) using differential pulse voltammetry (DPV). Differential pulse voltammograms of the modified electrode toward OHB and PHB were recorded and were shown in Figs. 8A and 9A, respectively. As can be seen, the anodic peak potentials of OHB and PHB shift to negative values with increasing of pH. So, protons participate in the OHB and PHB oxidation reaction and acidity of the electrolyte has a significant effect on the oxidation. Moreover, it indicates that the optimum pH 2.0 can be used for the determination of OHB and PHB individually (Figs. 8B and 9B). But when OHB and PHB determine simultaneously at pH 2.0, both of them have only one peak in DPV (Fig. 10). Therefore, we used another pH value to resolve the two isomers peaks from each other. As is shown in Fig. 11, in pH 5.0 there are two separate peaks with good sensitivity for two isomers. Therefore pH= 5.0 buffer solution was selected for simultaneous determination of these isomers.
The ability of the proposed nanostructured sensor for determination of OHB and PHB in the presence of common interfering substances was investigated by DPV technique. The experiments were carried out by analyzing a standard solution containing 50.0 µM OHB and PHB using an increasing amount of interfering species. The tolerance limit was defined as the concentrations which give an error less than ±5.0% in the oxidation peak current of OHB and PHB . Some common cations and anions such as Na+, K+, NH4+, Ca2+, Mg2+, Cl-, CO32-, NO3- and I- were studied for its interference with detection of OHB and PHB. The results demonstrate that these ions virtually have no obvious interference to the DPV signals of the targets at the NiTiO3/CPE. Some organic compounds such as gallic acid, uric acid and dopamine have been considered to have no influence on the signals of OHB and PHB with deviations below 5%. These results have been reported in Table 1.
The ability to generate a modified electrode with a stable surface was investigated in optimized experimental conditions, using continuous DPV determination of OHB and PHB. Successive measurements of oxidation currents of 50.0 µM OHB and PHB at the same NiTiO3/CPE for fifteen days period were performed. The relative standard deviation (RSD) for the current signal was obtained at 4.2%. The results demonstrate NiTiO3/CPE has an excellent long-time stability for the determination of OHB and PHB without any fouling.
Moreover, for reproducibility investigation, five parallel modified electrodes were fabricated in the same way, and their electrochemical responses toward 50.0 µM OHB and PHB were recorded. The RSD of the measurements was estimated to be 4.5% and 3.4% for OHB and PHB, respectively, confirming that the modified electrode is highly reproducible. According to these results, NiTiO3/CPE is a repeatable and stable electrode for determination of OHB and PHB.
Since DPV has much higher current sensitivity and insignificant charging current contribution to the background current than CV, it was chosen for simultaneous determination and evaluation of the detection limit of OHB and PHB. The differential pulse voltammograms of OHB and PHB with a different concentration in 0.2 M B-R buffer (pH 5.0) at the surface of NiTiO3/CPE are shown in Fig. 12. Under the optimized conditions, two linear dynamic ranges were observed for OHB and PHB. Fig. 12B indicates the oxidation peak current of OHB has two linear calibration ranges of 10.0-90.0 µM and 90.0-1000.0 µM. The specific calibration equations for these concentration ranges are:
Ipa= 0.012 C + 0.023 R2= 0.995
Ipa= 0.0066 C + 0.1095 R2= 0.990
The linear regression equations for PHB (ranges of 10.0-90.0 µM and 90.0-1000.0 µM) were also obtained to be (Fig. 12C):
Ipa= 0.039 C + 0.076 R2= 0.998
Ipa= 0.0109 C + 2.8241 R2= 0.997
Detection limit is defined as the lowest concentration that can be distinguished from the noise level . According to the slope of the above equations, the detection limits were calculated, which were 0.38 μM and 0.1 μ M for OHB and PHB, respectively.
In order to estimate the validity of the proposed method in a cosmetic product, the NiTiO3/CPE was applied to measure the OHB and PHB concentration simultaneously in a commercial peeling skin lotion using standard addition method. Table 2 indicates the obtained analytical results. As can be seen from the results, the novel working electrode could be applied for simultaneous determination of OHB and PHB in real samples.
Also, the nanostructured modified electrode was used for the analysis in the resulting solution of the Kolbe-Schmitt reaction. The standard addition method was applied for testing recovery. The results of the spiked samples are acceptable and are shown in Table 3. This result reveals that simultaneous or independent measurements of the two products are possible without much interference and also demonstrates the applicability of CPEs in organic chemistry. Therefore, the NiTiO3 nanostructured modified electrode could be applied for the determination of OHB and PHB in real samples with satisfactory results.
In the present study, NiTiO3 nanoparticles were introduced as a novel CPE modifier for simultaneously electrochemical detection of OHB and PHB without any interference. Because of similar physicochemical properties, the two isomers interfere with each other during their identification. Moreover, these isomers are formed in Kolbe-Schmitt reaction at the same time and simultaneous determination of them has particular importance. The proposed method enables sensing of OHB without interference by PHB. Under optimal conditions, the sensor represents excellent performance for detecting OHB and PHB with a detection limit of 0.38 µm and 0.1 µm, respectively. The desirable selectivity and sensitivity of the modified electrode are ascribed to the excellent conductivity and electrocatalytic ability of NiTiO3 nanoparticles. This modified electrode is a promising approach to the simultaneous voltammetric determination of the isomers in real samples without any separation with satisfactory results.
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