Fabrication of Ni–Fe₂O₃ magnetic nanorods and application to the detection of uric acid

Abstract

Doping of Ni into Fe₂O₃ lattices has been achieved by co-precipitation followed by thermal decomposition method. The structural, morphological, and magnetic properties of the fabricated samples were investigated by X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared (FT-IR) spectroscopy, UV-visible absorption (UV-vis) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The results reveal that the Ni is well doped within the lattices of Fe₂O₃. The Ni dopant suppresses the formation of more stable α-Fe₂O₃ at higher calcination temperature. Further, the Ni-doped Fe₂O₃ nanoparticles were used to fabricate an electrochemical sensor (Ni–Fe₂O₃/GCE) for the detection of uric acid (UA) in biological conditions by cyclic voltammetry (CV) and chronoamperometry (CA). It was found that 5%Ni–Fe₂O₃/GCE exhibits best response towards UA with less positive potential and larger current response. Furthermore, the sensor gives good linear current response in the concentration range of 6.6 to 112.4 μM with the higher sensitivity of 0.849 μA (μM cm²)⁻¹. Such fabricated sensors are appropriate for newly emerging non-enzymatic electrochemical nanobiosensors.

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1. Introduction

Uric acid (2,6,8-trihydroxypurine) is an important biomolecule which is produced from purine degradation metabolism in many mammals and plays vital roles especially in human health. It can penetrate cell membranes and accumulate in extracellular fluids. The alterations of UA concentration in body fluids such as human serum and urine may lead to several pathological disorders such as arthritis, gout, neurological diseases, and kidney disease. Hence, accurate determination of UA in body fluids is important in clinical diagnosis at early stages of related diseases. Therefore, various electrode materials, such as polymers^1,2 metal nanoparticles³ metal complexes,^4,5 nanohybrids,^6,7 and enzyme based biosensors^8,9 are reported. However, the major problem associated with the determination of UA is the interference from the coexistence of large amount of ascorbic acid (AA). Hence, fabrication of reproducible, more stable and selective electrode is more favorable in practical applications.

Metal oxide nanomaterials have received extensive interest in the field of electrochemical sensor. Among them, Fe₂O₃ nanoparticles have considerable attention in different fields because of its low toxicity, biocompatibility and structural stability. They have been extensively studied for applications in the field of photocatalysis, solar cells, pigments, lithium ion-battery, field emission devices and gas sensors.^10,11 Doping of different metal ions or metal oxides into Fe₂O₃ will find new application or improve the performance of existing applications. For instance, Niu et al.¹² found that the material mixed with rare earth oxides through a sol–gel method in citric acid system presented high gas sensitivity to gasoline. Jing et al.¹³ reported that the Ni-doped γ-Fe₂O₃ exhibits higher gas sensitivity and better selectivity to ethanol than the pure γ-Fe₂O₃. Recently, we have reported that the Co doped α-Fe₂O₃ exhibits higher electrochemical sensing property than the pure α-Fe₂O₃.¹⁴ It is noted that until now most of the researchers focused mainly on doping of metal ion into γ-Fe₂O₃, for their gas sensing properties. Herein we applied Ni-doped Fe₂O₃ nanorods to fabricate the electrochemical sensor for the detection of UA. On the contrary to the enzymes-based biosensor which cannot provide the biosensors with long-term stability, Ni-doped Fe₂O₃ nanorods could be an outstanding substitute for enzymes. Ni-doped Fe₂O₃ nanorods are considerably more stable over a long time, and wide range of temperature compared with enzyme based sensors.

In this respect, an approach to fabricate Ni-doped Fe₂O₃ nanorods by co-precipitation followed by thermal decomposition method has been developed. XRD, XPS, FT-IR, UV-visible, VSM, SEM and TEM have been used to characterize the nanoparticles. This method resulted in the doping of Ni into Fe₂O₃ via the simultaneous precipitation of metal oxalates. The liberation of gaseous products enhances the surface area which is a key need for any catalysts. The application of the fabricated Ni–Fe₂O₃ nanorods as an electrochemical sensor for UA was compared to that of Fe₂O₃/GCE and bare GCE. The UA can easily bind to the Ni–Fe₂O₃ nanorods through surface OH group. Further, the presence of Ni enhances the activity of the proposed sensor.

2. Materials and methods

2.1. Materials

Ferrous sulphate (FeSO₄·7H₂O), nickel chloride, oxalic acid, sodium dodecyl sulphate (SDS), sodium dihydrogen phosphate and disodium hydrogen phosphate were purchased from Qualigens and used without further purification. Uric acid was purchased from Sigma-Aldrich and used as received. Doubly distilled water was used as the solvent for all the experiments.

2.2. Fabrication of Ni-doped Fe₂O₃ nanorods

In order to synthesis Ni-doped Fe₂O₃ nanorods, the mixture of 2.78 g of anhydrous ferrous sulphate and required amount (2, 5, and 10 mol% with respect to Fe²⁺ ion) of nickel chloride were dissolved in 125 mL of 0.01 M SDS solution and heated at 70 °C for 5 h under constant stirring condition. To the above solution, 125 mL of 0.1 M oxalic acid solution was slowly added. The obtained precipitate was filtered, washed, dried and calcined at 450 °C for 2 h. Pure Fe₂O₃ nanorods have been prepared in the same way except using nickel chloride.¹⁵

2.3. Characterization of Ni-doped Fe₂O₃ nanorods

The diffraction analysis was obtained from Rich Siefert 3000 diffractometer with Cu-Kα₁ radiation (λ = 1.5406 Å). XPS spectrum of the sample was measured on Omicron Nanotechnology, GmBH, Germany XM1000 and FT-IR spectra were recorded by using Schimadzu FT-IR 8300 series instrument. The UV-vis absorption spectrum was obtained on a CARY 5E UV-vis-NIR spectrophotometer. The morphology and size of the sample was analyzed by FE-SEM and TEM using a HITACHI SU6600 field emission-scanning electron microscopy and PHILIPS CM200 transmission electron microscopy respectively. The magnetic studies were carried at room temperature using VSM: EG & G Princeton Applied Research, Model 4500 instrument.

2.4. Fabrication of working electrodes and electrochemical experiments

In order to fabricate (x)Ni–Fe₂O₃/GCE, under the optimized condition (Fig. S1†), 10 μL of the freshly prepared dispersion containing 5 mg of nanorods in 5 mL of ethanol is drop casted on the highly polished GCE. Thereafter, the prepared modified electrodes were extensively rinsed with PBS to remove loosely bound nanorods. The electrochemical experiments were performed on a CHI 1103A electrochemical instrument using the as-modified electrode and bare GCE (surface area, 0.0707 cm²) as working electrode, a platinum wire was the counter electrode, and saturated calomel electrode (SCE) was the reference electrode.

3. Results and discussion

3.1. XRD, XPS, FTIR and UV-visible analysis

Fig. 1 shows the XRD patterns of (x)Ni–Fe₂O₃ (x = 2%, 5%, and 10%) nanorods along with pure Fe₂O₃. The diffraction patterns indicate that the Ni-doped samples are composed of hematite (JCPDS no. 089-0599) and maghemite (JCPDS no. 089-5892) phases. The diffraction peaks responsible for hematite phase are found at 24.7°, 33.5°, 35.4°, 41.9°, 49.1°, 54.0°, 61.3° and 64.2° which are corresponding to (012), (104), (110), (113), (024), (116), (214) and (300) planes respectively. The diffraction peaks corresponding to maghemite phase are found at 30.4° and 57.6° which corresponds to (220) and (511) planes respectively. It can be found that the nickel dopant did not exist as nickel oxide in the XRD pattern. It implies that Ni ions are well doped in Fe₂O₃ lattices. It is noticed that the diffraction peaks corresponding to hematite phase decreases with increasing amount of Ni. i.e., the γ-Fe₂O₃ phase content increases with increasing Ni concentration and becomes the dominant phase at higher Ni concentration. Similar observations was reported by Deka et al. in Zn doped Fe₂O₃.¹⁶ Hence, the XRD studies show that doping of Ni suppresses the formation of α-Fe₂O₃ and enhances the formation of γ-Fe₂O₃. Further, all the peaks in the XRD pattern of doped samples are slightly shifted to lower angle (2θ) with increasing percentage of Ni doping (Fig. S2†) than the pure Fe₂O₃, which probably results from the incorporation of Ni²⁺ into Fe₂O₃ lattice. It indicated that the doping of Ni into Fe₂O₃ lattice inhibit the growth of Fe₂O₃ nanorods. From the above results, it can be concluded that the obtained Fe₂O₃ crystal lattice distortion is caused by the doping of Fe sites by Ni ions, because the ionic radius of Ni²⁺ (0.69) is similar to that of Fe²⁺ (0.64) in the six coordination environment.

Fig. 1 XRD patterns of (a) Fe₂O₃, (b) 2%Ni–Fe₂O₃, (c) 5%Ni–Fe₂O₃ and (d) 10%Ni–Fe₂O₃ nanorods.

In order to confirm the doping of Ni²⁺ into Fe₂O₃, XPS analysis was performed and the obtained result is shown in Fig. 2. The XPS survey spectrum (Fig. 2a) reveals that the nanoparticles are composed of Fe, Ni and O. The high-resolution Fe 2p spectrum (Fig. 2b) shows two distinct peaks with binding energies of about ∼710.9 eV for Fe 2p_3/2 and ∼724.0 eV for Fe 2p_1/2. The shakeup satellite peak present at 719–722 eV, is characteristic for Fe³⁺ ions in Fe₂O₃.¹⁷ It also shows that the synthesized Fe₂O₃ nanoparticles contain both Fe³⁺ and Fe²⁺ ions. It is obvious that the amount of Fe³⁺ is higher than the amount of the Fe²⁺ ions. Generally, Fe₂O₃ has an inverse spinel structure which has both Fe³⁺ and Fe²⁺ in its lattice. Hence, the XPS results showed the synthesized Fe₂O₃ contain both α- and γ-phase Fe₂O₃. High-resolution Ni 2p core-level spectrum (Fig. 2c) exhibits two peaks present at 855.5 and 873.9 eV, corresponding to Ni 2p_3/2 and Ni 2p_1/2, respectively. The binding energy of Ni 2p_3/2 is the characteristic value for NiO (854–855 eV), which reveals that the Ni dopant exist in the form of divalent.¹⁸ The XPS result confirms that the Fe exists as Fe²⁺/Fe³⁺and Ni as Ni²⁺ in the prepared samples. The O 1s core level spectrum (Fig. 2d) exhibits peak at about 530.9 eV, which can be assigned to O²⁻. The peak with higher binding energy at 533.0 eV is usually attributed to surface hydroxyl group.

Fig. 2 (a) Survey (b) Fe 2p, (c) Ni 2p, and (d) O 1s XPS core level spectra of 10%Ni–Fe₂O₃ nanorods. The peak is calibrated at 284.6 eV with adventitious carbon.

The FTIR spectra of (x)Ni–Fe₂O₃ are shown in Fig. 3. (x)Ni–Fe₂O₃ showed the bands at 3424, 1637, 1394, 638, 548, and 462 cm⁻¹. The broad band centered at 3424 cm⁻¹ was assigned to the hydroxyl stretching vibration of surface water molecules. Further, the band at 1637 cm⁻¹ is attributed to the bending mode of water molecule. In Fig. 3b, the band at 638 and 462 cm⁻¹ are attributed to Fe–O bond in tetrahedral and octahedral site in γ-Fe₂O₃ whereas the bands at 548 is due to the Fe–O stretching vibrational mode of α-Fe₂O₃.¹⁹ When the concentration of Ni increases, the band position corresponding to α-Fe₂O₃ disappears and that of γ-Fe₂O₃ increases abruptly. Particularly, 10%Ni–Fe₂O₃ shows the bands at 583 and 468 cm⁻¹. In general, the inverse spinel compounds exhibit two high frequency infrared lattice vibrations which are sensitive to changes in interaction between cations and oxygen in the octahedral (O_h) and tetrahedral (T_d) positions. The highest band observed in the range of 600–500 cm⁻¹ corresponding to intrinsic stretching vibrations of the metal at the T_d site, M_tetra ↔ O [A site], whereas the lowest band observed in the range of 450–385 cm⁻¹ is assigned to O_h site, M_octa ↔ O [B site].²⁰ Hence, the bands observed for the high concentration Ni doped samples are all due to the γ-Fe₂O₃ i.e., doping of Ni stabilizes the γ-Fe₂O₃. This result is well matched with XRD analysis. Further, it was observed that the absorption band at 456 cm⁻¹ shifts slightly to a higher wavenumber side and 643 cm⁻¹ shifts to a lower wavenumber side with an increase in Ni²⁺ concentration (up to 10%). The shifts in the bands are due to the perturbation occurring in the Fe³⁺–O²⁻ bond by substituting Ni²⁺ ions, and also, it is attributed to an increase in bond length on the B site.

Fig. 3 FT-IR spectra of (a) Fe₂O₃, (b) 2%Ni–Fe₂O₃, (c) 5%Ni–Fe₂O₃ and (d) 10%Ni–Fe₂O₃ nanorods.

Fig. 4 shows the UV-vis absorption spectra of (x)Ni-doped samples. All the (x)Ni-doped samples exhibit two bands at 350 nm and 550 nm. The absorption band at about 350 nm is according to the direct charge transfer transitions from O²⁻ 2p to Fe³⁺ 3d.²¹ The absorption in the visible region (550 nm) is due to the Fe³⁺ 3d–3d spin forbidden transition (indirect transition). It is found that the Ni-doped samples show an obvious red-shift in the bandgap transition which may result from the following: (i) (x)Ni–Fe₂O₃ is characterized by an inverse spinel structure in which the tetrahedral and octahedral sites within the lattice are occupied by Ni²⁺ and Fe³⁺ cations.²² (ii) The energy band structures of (x)Ni–Fe₂O₃ are generally defined by considering the O 2p orbital as the valence band and the Fe 3d orbital as the conduction band absorption of (x)Ni–Fe₂O₃ in the visible region can be ascribed to the photoexcited electron transition from the O-2p level into the Fe-3d level.²³ (iii) According to literature,²⁴ it is attributable to the lattice expansion which causes the energy decrease of the Ni d-like conduction band. Furthermore, the additional band at 450 and 750 nm are assigned to Ni²⁺ species incorporated into the Fe₂O₃ lattice. As previously reported in the literature,²⁵ the absorption band at about 450 and 750 nm can be attributed to the ³A_2g(F) → ³T_2g(F) and ³A_2g(F) → ³T_1g(F) transition of Ni²⁺ in distorted octahedral environment. Hence, the appearance of a new peak confirms that Ni is doped into the Fe₂O₃ lattice. To determine the bandgap of the Ni-doped samples, we have used Tauc's plot which is shown in Fig. S3.† The obtained direct bandgap is in the range of 2–2.15 eV, which is consistent with Hashimoto et al.²⁶ As the amount of Ni increases, the bandgap slightly reduces. These phenomena suggest that the conduction band edge of Fe₂O₃ is slightly affected by doping of Ni²⁺.²⁷ Similar phenomenon has been observed by Liu et al. in Ni doped Fe₂O₃ thinfilms.²⁸

Fig. 4 DRS UV visible absorption spectra of (a) Fe₂O₃, (b) 2%Ni–Fe₂O₃, (c) 5%Ni–Fe₂O₃ and (d) 10%Ni–Fe₂O₃ nanorods.

3.2. Morphological analysis

The SEM image of (x)Ni–Fe₂O₃ nanorods, prepared by calcination of the mixture of ferrous oxalate and nickel oxalate at 450 °C for 2 h is shown in Fig. 5A. Fig. 5B and 6 show the lower and higher magnified FE-SEM and TEM images of individual nanorod. As can be seen, the calcined sample was composed of numerous worm-like particles of about ∼40 nm size distributed homogeneously within the whole particle. This rough surface is similar to that of ‘rudraksha’ (Fig. 5B, image c). It shows that the synthesized product is mostly in rod-like form, with cracks on the surfaces of the sample. These cracks were mainly formed by the release of gases from the interior of the prepared oxalates particles during calcination process. Considering the large volume of gases released out from the metal oxalates with limited dimensional shrinkage during calcination, there must be a significant porosity left inside the particles.²⁹ Further, the morphology of the Ni-doped samples didn't undergo any change with respect to pure Fe₂O₃. Based on the above results, solution–solid–solid growth mechanism has been proposed. When the mixture of ferrous and nickel precursors are dissolved in 0.1 M SDS solution, M²⁺–SDS complex will be formed. After the addition of oxalic acid in the above aliquot, metal oxalates nuclei have formed which subsequently grow as metal oxalates nanorods. The as-prepared metal oxalates are calcined at 450 °C to get Ni–Fe₂O₃ nanorods. The pictorial representation of the above possible mechanism has been shown in Scheme 1. Further EDS analysis (Fig. 5A, image d) shows that the sample is composed of Fe, Ni and O which is consistent with XPS results. The SAED (Fig. 6d) pattern reveals the polycrystalline nature of the sample.

Fig. 5 (A) SEM images of (a) 2%Ni–Fe₂O₃, (b) 5%Ni–Fe₂O₃, and (c) 10%Ni–Fe₂O₃ nanorods. (d) EDS spectrum of 5%Ni–Fe₂O₃ nanorods. (B) (a) Lower and (b) higher magnified FESEM images of 5%Ni–Fe₂O₃ nanoparticles. (c) Image of ‘rudraksha’.

Fig. 6 TEM images of 5%Ni–Fe₂O₃ nanorods in different magnifications (a–c). (d) SAED pattern 5%Ni–Fe₂O₃ nanorods.

Scheme 1 Possible formation mechanism of Ni–Fe₂O₃ nanorods.

3.3. Magnetic property

Fig. 7A shows the M(H) curve of the (x)Ni–Fe₂O₃ nanoparticles at room temperature. It can be seen that the (x)Ni–Fe₂O₃ nanoparticles show typical ferromagnetic behaviour with higher saturation magnetization (M_s) than the pure Fe₂O₃. It is clear that the saturation magnetization increases with Ni concentration (upto 5%Ni) and decreases for 10%Ni–Fe₂O₃. Hence, we have inferred that this as being due to the presence of γ-Fe₂O₃ which would be stabilized by doping of Ni. Because of the very large M_s value of γ-Fe₂O₃ as compared to the α-Fe₂O₃, the presence of very small amount of γ-Fe₂O₃ is sufficient enough to provide a significant increase of magnetic M_s as compared to pure Fe₂O₃. We have estimated the percentage of γ-Fe₂O₃ for all the samples, using the following equation

M_s = (x)(M_α-Fe2O3) + (1 − x)(M_γ-Fe2O3)

where, (x) and (1 − x) are the fractions of α-Fe₂O₃ and γ-Fe₂O₃. M_α-Fe2O3 and M_γ-Fe2O3 are standard values of the saturation magnetization of the two phases in the bulk i.e. 0.2 emu g⁻¹ for the α-Fe₂O₃ and 85 emu g⁻¹ for the γ-Fe₂O₃. The results suggest that the 5%Ni–Fe₂O₃ has higher M_s value because of the presence of higher γ-Fe₂O₃ content. The coercivity (H_C) of the (x)Ni–Fe₂O₃ increases with increasing Ni²⁺ content, and is significantly higher than that of the Fe₂O₃. The increasing coercivity is due to the effect of doping of Ni²⁺ in the γ-Fe₂O₃ lattice. As Ni²⁺ prefers T_d sites in the spinel ferrite lattice, as in NiFe₂O₄, it is possible that the doped Ni²⁺ ions are occupied in the T_d sites and corresponding amounts of Fe³⁺ ions are pushed from the T_d sites to the vacant O_h sites. This increases the magnetocrystalline anisotropy contributions from Fe³⁺ in the O_h sites and therefore increases the coercivity. The increase in H_C and M_s with increasing Ni content is in agreement with previously published results.³⁰

Fig. 7 Hysteresis loops of (a) Fe₂O₃, (b) 2%Ni–Fe₂O₃, (c) 5%Ni–Fe₂O₃ and (d) 10%Ni–Fe₂O₃ nanorods.

3.4. Electrochemical sensing property

3.4.1. Electrochemical behaviour of the (x)Ni–Fe₂O₃ nanoparticles. Typical cyclic voltammograms (CV's) of bare, Fe₂O₃/GCE, 2%Ni–Fe₂O₃/GCE, 5%Ni–Fe₂O₃/GCE and 10%Ni–Fe₂O₃/GCE in 0.1 M PBS at the scan rate of 50 mV s⁻¹ is shown in Fig. S4.† As expected, voltammetric profile of the electrode built with Ni–Fe₂O₃ yields higher current response with a pair of well-defined redox peak at 0.71 V/0.37 V corresponding to Fe(II)/Fe(III) couple. The improved activity is mainly attributed to presence of Fe²⁺ which would be stabilized by Ni²⁺ doping process. From the cyclic voltammogram presented in Fig. S4,† the concentration of Ni–Fe₂O₃ coated on the electrode surface was calculated from,

Γ = Q/nFA

where n represents the number of electrons involved in the redox process (n = 2), A is the surface area of the electrode (0.0707 cm²) and Γ represents the surface coverage concentration (mol cm⁻²). From the above, the calculated surface coverage of Ni–Fe₂O₃ layer was 3.42 × 10⁻⁹ mol cm⁻².

The analysis of scan rate on the voltammogram profile of the 5%Ni–Fe₂O₃/GCE in the range of 30–500 mV s⁻¹ was presented as inset in Fig. S4.† A linear relationship between the scan rate and the peak current was observed. It indicates that the electrode process is diffusion controlled process.

Electrochemical impedance spectroscopy (EIS) was used to study the modified electrode. The electron-transfer resistance (R_et) at the electrode surface is equal to the semicircle diameter of the Nyquist plots and can be used to describe the interface properties of the modified electrodes. Fig. S5† shows the EIS results of bare GCE, Fe₂O₃/GCE, 2%Ni–Fe₂O₃/GCE, 5%Ni–Fe₂O₃/GCE and 10%Ni–Fe₂O₃/GCE in 5 mM [Fe(CN)₆]^3−/4−. After modifying with 5%Ni–Fe₂O₃, the R_et (2945.7 Ω) increased due to the blocking effect of 5%Ni–Fe₂O₃ in the charge transfer process and demonstrated that the 5%Ni–Fe₂O₃ film was successfully immobilized on to the GCE surface. This is probably due to the negative charges of surface hydroxyl groups on the 5%Ni–Fe₂O₃ surface limit the access of [Fe(CN)₆]^3−/4− to the electrode surface. For the 10%Ni–Fe₂O₃/GCE, the R_et decreased to 498.8 Ω, indicated that the negatively charged surface hydroxyl groups on 10%Ni–Fe₂O₃ were effectively decreased.

3.4.2. Influence of Ni doping on electrochemical sensing of UA. Fig. 8 depicts the CV's of bare, Fe₂O₃/GCE and (x)Ni–Fe₂O₃/GCE in presence 0.1 mM UA in 0.1 M PBS (pH = 7.4) at the scan rate of 50 mV s⁻¹. The modified electrodes show shift in anodic peak potential with enhanced anodic peak current than the bare GCE, indicates that the modified electrode has better catalytic activity. As shown in Fig. 8d, the oxidation peak potential and anodic peak current of UA at 5%Ni–Fe₂O₃/GCE is about 0.56 V and 17.4 μA respectively which showed the electrochemical response of UA at 5%Ni–Fe₂O₃/GCE is higher than the other modified electrode under the same experimental condition. Hence, the experimental results suggest that 5%Ni–Fe₂O₃ nanoparticles can enhance the electron transfer rate and lower the overpotential of UA oxidation. This enhanced electrochemical sensing property was mainly attributed to (i) the larger electroactive sites of the modifying layer due to the large pores on the sample as evident by the FE-SEM and TEM analysis. (ii) The (x)Ni–Fe₂O₃ layer contains free surface –OH group as seen in FTIR and XPS studies which could form hydrogen bonding with UA. It is known that hydrogen bond acceptor strength of amide group is stronger than the ester group. This hydrogen bond facilitates easier oxidation of UA at the modified electrode surface. Hence, the carbonyl group at C-8 of UA forms hydrogen bonding with surface –OH group of (x)Ni–Fe₂O₃ nanorods. (iii) As evident from the Fig. S4,† only (Fe²⁺/Fe³⁺) redox couple is electroactive and hence Fe site plays the vital role in the electrocatalytic oxidation of UA.³¹ The electrocatalytic oxidation mechanism of UA at 5%Ni–Fe₂O₃/GCE was similar to that of UA at RTIL-NiHCF-NP-gel modified PIGE.³² Based on the above discussion and the previous reports, the electrochemical redox mechanism of UA at (x)Ni–Fe₂O₃/GCE can be explained by the following way: (a) the first step is the mass transport of bulk UA to the electrode surface by diffusion. (b) The second step is the adsorption of UA on to the (x)Ni–Fe₂O₃/GCE surface active sites. (c) In the third step, Ni–Fe^(II)₂O₃ was electrochemically oxidized to form Ni–Fe^(III)₂O₃ and then the resulting Ni–Fe^(III)₂O₃ reacted with adsorbed UA which caused an electrochemical oxidation of UA. (d) The reduced Ni–Fe^(II)₂O₃ donates the electron to the electrode and regenerate Ni–Fe^(III)₂O₃. The graphical representation of the above discussed mechanism is given in the Scheme 2.

Fig. 8 Cyclic voltammograms of (a) bare, (b) Fe₂O₃/GCE, (c) 2%Ni–Fe₂O₃/GCE, (d) 5%Ni–Fe₂O₃/GCE, (e) 6%Ni–Fe₂O₃/GCE, (f) 7%Ni–Fe₂O₃/GCE, (g) 8%Ni–Fe₂O₃/GCE and (h) 10%Ni–Fe₂O₃/GCE in presence of 0.1 mM UA at a scan rate of 50 mV s⁻¹.

Scheme 2 Possible electrochemical oxidation mechanism of UA at Ni–Fe₂O₃/GCE.

3.4.3. Effect of magnetic property on the electrochemical sensing property. In order to find the influence of magnetic property of the samples on the electrochemical sensing of UA, oxidation peak current was plotted against the concentration of Ni and M_s values. From Fig. 9, it can be seen that the increase of Ni content, both anodic peak current and M_s of the samples increases. 5%Ni–Fe₂O₃ exhibits higher current with maximum magnetization. It indicates that the peak current is directly related to the magnetization of the sample. Similar result has been observed by Ahmad et al. who reported that the current response of magnetic nanoparticles is directly proportional to the induced magnetization of the sample and applying potential.³³

Fig. 9 Plot of concentration of Ni against magnetization and UA peak current.

3.4.4. Effect of scan rate. Electrochemical behaviors of 0.1 mM UA at 5%Ni–Fe₂O₃/GCE with different scan rates are investigated. From Fig. 10A, it can be seen that the oxidation peak current moved linearly with increasing scan rate. Further, Fig. 10B shows the oxidation peak current was linearly related to the scan rate in the range from 30 to 500 mV s⁻¹. The regression equation can be expressed as I_pa (μA) = 0.0729ν (mV s⁻¹) + 14.902 (R² = 0.993), indicating the oxidation of UA at 5%Ni–Fe₂O₃/GCE is an adsorption-controlled electrode process.³⁴ It also suggests that the 5%Ni–Fe₂O₃ layer has good electrochemical activity and fast electron transfer at pH 7.4. Moreover, a linear correlation of log(ν) versus log(I_pa) (Fig. 10C) is observed and it follows the linear regression equation of log(I_pa) = 0.43 log(ν) + 0.524 (R² = 0.9883). The number of electrons involved in the overall reaction can be obtained by equation.
where, n is the number of electrons involved in the oxidation reaction, I_Pa is the anodic peak current (μA), R is the gas constant, T is the temperature (K), F is the Faraday constant, Q is the charge (C) and v corresponds to the scan rate (mV s⁻¹). The total number of electrons involved in the redox process of UA is 2. The findings are consistent with that mentioned earlier, on the redox process of UA at 5%Ni–Fe₂O₃/GCE.

Fig. 10 (A) Cyclic voltammograms of 5%Ni–Fe₂O₃/GCE in 0.1 mM UA (pH = 7.4) at the scan rate of (a) 30, (b) 50, (c) 70, (d) 100, (e) 150, (f) 200, (g) 250, (h) 300, (i) 350, (j) 400, (k) 450 and (l) 500 mV s⁻¹. (B) The linear relationship between anodic current (I_pa) versus scan rate (ν). (C) Plot of log(I_pa) versus log(ν).

3.4.5. Determination of UA. The calibration curve for the UA oxidation at the 5%Ni–Fe₂O₃/GCE was obtained under the optimal experimental conditions using CA method. The chronoamperometric response of the 5%Ni–Fe₂O₃/GCE is illustrated in Fig. 11. In order to conduct this experiment, 0.2 mL of 0.1 mM UA was added to 30 mL of 0.1 M PBS solution and the UA oxidation current was measured at 0.54 V in a stirred condition. The successive addition of UA to continuously stirred 0.1 M PBS produces a significant increase in the current. The nearly equal current steps for each addition of UA demonstrate that the 5%Ni–Fe₂O₃/GCE is stable and has an efficient electrocatalytic activity towards the UA. The linear relationship between current and UA concentration was obtained for the UA concentration in the range of 6.6 μM to 112.4 μM and it was shown as inset in Fig. 11. The obtained linear regression equation is I_pa (μA) = 0.06C_UA (μM) + 0.937 with the correlation coefficient of 0.9900. The obtained sensitivity of 5%Ni–Fe₂O₃/GCE was 0.849 μA (μM cm²)⁻¹. According to the calculation method for the detection limit mentioned in the reference,³⁵ the detection limit of 5%Ni–Fe₂O₃/GCE was found to be 3.1 μM (S/N = 3). The obtained results reveal that the proposed sensor had high sensitivity, wide linear range, lower detection limit and good repeatability. The comparison of this sensor with other electrochemical sensors for the determination of UA was listed Table. S1.†,^36–44

Fig. 11 Chronoamperometric responses of 5%Ni–Fe₂O₃/GCE for the successive additions of 0.2 mL of 0.1 mM UA to 30 mL of 0.1 M PBS. Applied potential is 0.54 V. Inset is the plot of current versus concentration of UA.

The reproducibility of 5%Ni–Fe₂O₃/GCE was examined by measurement of the CV response to 0.1 mM UA. The relative standard deviation (RSD) for 5 successive determinations was about 2.8%, indicating an excellent reproducibility. Additionally, eight 5%Ni–Fe₂O₃/GCE based on the same fabrication procedure were prepared and used for the determination of 0.1 mM UA, and the RSD was 1.18%, revealing an excellent repeatability of the electrode preparation procedure. The storage stability of the modified electrode was also determined. The CV response current decreased to 96.6% of the initial value after 10 days and 87.9% remained after thirty days storage, demonstrating an excellent long-term stability. Moreover, the long-term stability of 5Ni–Fe₂O₃/GCE was also examined by scanning 75 times consecutively in 0.12 mM UA at the scan rate of 50 mV s⁻¹ using PBS buffer. The result indicates that the anodic current response decreased by 9.8% only and the peak potential has shifted by 2.5%. When the 5Ni–Fe₂O₃/GCE was scanned 130 times consecutively in 0.12 mM at the scan rate of 50 mV s⁻¹, the anodic peak current decreased by 50% and the peak potential have shifted by 15%. Hence the 5Ni–Fe₂O₃/GCE has reasonable stability upto 75 scans consecutively. Hence, the proposed modified electrode resulted in an acceptable sensitivity, detection limit, repeatability and stability of UA, suggests a promising feature for the applicability of the modified electrode for the direct determination of UA in real samples.

Further, we investigate the chronoamperometric response of 5%Ni–Fe₂O₃/GCE for the interfering substances like dopamine (DA), which normally coexists with UA in the human blood. The chronoamperometric response of the 5%Ni–Fe₂O₃/GCE toward 1 mM DA in 0.1 M PBS (pH = 7.4) is shown in Fig. S6.† The results suggest that there is no appreciable interference from the DA. Interference studies were also carried out with other possible compounds. The experimental results shows that 150-fold concentration of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, nitrate, 120 fold concentration of urea, 100-fold concentration of ascorbic acid and glucose, did not have any significant interference on the determination of 1.0 × 10⁻⁵ M UA.

The practical application of 5%Ni–Fe₂O₃/GCE was examined by estimating UA in human urine samples. In order to avoid the interferences of the real samples matrix and fit into the linear range of UA, only diluted urine samples were added into the electrochemical cell. To establish the correctness of the results, the diluted samples mentioned above were spiked with certain amounts of UA and then detected. The recovery of the spiked samples ranged between 98.5% and 101.6% (Table. S2†).

4. Conclusions

In this paper, Ni doped Fe₂O₃ nanorods were successfully synthesized by coprecipitation followed by thermal decomposition method. The characterization techniques confirm the doping of Ni into Fe₂O₃ lattices. Further, (x)Ni–Fe₂O₃ nanorods modified electrodes were successfully fabricated. The as-prepared (x)Ni–Fe₂O₃/GCE exhibited an excellent electrocatalytic activity towards the oxidation of UA. The anodic peak currents were dramatically changed as a function of Ni concentration. The relatively smaller crystallite size, higher surface area and the presence of surface hydroxyl group are responsible for the observed enhanced electrocatalytic activity of the modified electrode. Many outstanding advantages, like excellent sensitivity, wide linear range, low detection limit, reproducibility, and stability of the modified electrode confirmed that the 5%Ni–Fe₂O₃/GCE had excellent analytical performance. Moreover, the fabricated electrode has been utilized for the determination of UA in human urine with reasonable results. Therefore, we believe that the 5%Ni–Fe₂O₃/GCE can be a convenient tool for the assay of UA in research assays and in clinical diagnosis.

Acknowledgements

One of the authors (RS) acknowledges the CSIR, New Delhi, India, for the financial assistance in the form of Senior Research Fellowship. We acknowledge the FE-SEM, and XPS facility provided by the National Centre for Nanoscience and Nanotechnology, University of Madras.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00725e

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