Hematite Nanoparticles-Modified Electrode Based Electrochemical Sensing Platform for Dopamine

Hematite (α-Fe2O3) nanoparticles were synthesized by the solid transformation of ferrous hydroxide and ferrihydrite in hydrothermal condition. The as-prepared α-Fe2O3 nanoparticles were characterized by UV-vis, PL, XRD, Raman, TEM, AFM, FESEM, and EDX analysis. The experimental results indicated the formation of uniform hematite nanoparticles with an average size of 45 nm and perfect crystallinity. The electrochemical behavior of a GC/α-Fe2O3 electrode was studied using CV and EIS techniques with an electrochemical probe, [Fe(CN)6]3−/4− redox couple. The electrocatalytic activity was investigated toward DA oxidation in a phosphate buffer solution (pH 6.8) by varying different experimental parameters. The chronoamperometric study showed a linear response in the range of 0–2 μM with LoD of 1.6 μM for DA. Square wave voltammetry showed a linear response in the range of 0–35 μM with LoD of 236 nM for DA.


Introduction
Hematite ( -Fe 2 O 3 ) is thermodynamically the most stable iron oxide with n-type semiconductor properties [1]. Its stability, favorable band-gap (2.1-2.2 eV), and low cost make -Fe 2 O 3 one of the most promising materials, and it is used in catalysis, electrocatalysis, photocatalysis, electrochemical sensors, and gas sensors [2][3][4][5]. The lower band-gap of hematite compared to titanium dioxide enables this material to absorb light in the visible range [6]. The properties of -Fe 2 O 3 are highly affected by the morphology, crystallite size, and crystallinity of the material. Many synthetic methods for preparing -Fe 2 O 3 have been investigated. These processes involve the use of a surfactant, very high temperature, corrosive, toxic, or expensive raw materials, or a long synthesizing period [7][8][9]. Discovering a low-cost and facile method to synthesize -Fe 2 O 3 with excellent quality and desirable properties has always been a challenge for scientists. In this work, we describe a low-cost, facile method of synthesizing nanosized -Fe 2 O 3 particles. Dopamine (DA) is a neurotransmitter that plays an important role in the brain and nervous system. DA is measured in the body using electrochemical sensors for monitoring HIV infection and the diagnosis of Parkinson's disease. The importance of DA in the nervous system has resulted in the fabrication of electrochemical sensors for its detection [6,7]. Some work has been done to investigate the performance of -Fe 2 O 3 -modified electrode in supercapacitors, but few studies have been done on the electrochemical properties of -Fe 2 O 3 for electrochemical sensors [4].
The overall objective of the present study was the improvement of the sensitivity toward the electrochemical detection of DA by using an - were prepared using a simple hydrothermal method and then used to fabricate the sensor electrode. A ferricyanide redox couple was used as a marker to probe the -Fe 2 O 3 nanoparticle-modified electrode interface with a tunable kinetic barrier. The GC/ -Fe 2 O 3 electrode showed a better electrocatalytic response toward DA oxidation compared to bare GC electrode. The square wave voltammetry and chronoamperometric techniques were also employed for the detection of DA and showed excellent selectivity and sensitivity. a 25% ammonia solution was dropwise added to the above reaction mixture until the pH of the attained solution was 10. The reaction mixture was vigorously stirred for 1 h at room temperature. The sample was gathered by repeated centrifugation (6000 rpm for 2 min) and washed with 100 mL of deionized water; this procedure was repeated three times, and it was redispersed into the deionized water. Further, the solution was poured into a 50 mL Teflon-lined autoclave and subjected to a hydrothermal process at 180 ∘ C for 12 h. Finally, the resulting brownish-orange color precipitate was centrifuged, washed several times with water, and then dried in a hot air oven at 100 ∘ C to completely remove the moisture.

Characterization Techniques.
The UV-visible absorption spectrum of the -Fe 2 O 3 nanoparticles in the 190-900 nm spectral range was recorded using a Thermo Scientific Evolution-300 UV-vis absorption spectrophotometer. The PL spectrum was acquired from a Renishaw inVia 2000 system with a laser emitting at 325 nm. The phase identification was recorded by using a Siemens-D5000 X-ray diffractometer with copper K radiation ( = 1.5418Å) at a scan rate of 0.02 ∘ /s. The surface morphology and elemental composition of the -Fe 2 O 3 nanoparticles were examined using a JEOL field emission scanning electron microscope operated at 10 kV. The surface topography and roughness of the thin film were studied using Agilent's atomic force microscope (AFM) in AC mode. The size and shape of the obtained -Fe 2 O 3 nanoparticles were studied using a JEOL JEM-2100F high resolution transmission electron microscope.

Electrochemical
Studies. An -Fe 2 O 3 nanoparticle-modified glassy carbon electrode (GC/ -Fe 2 O 3 ) was fabricated by dispersing 1 mg of the synthesized -Fe 2 O 3 nanoparticles in 1 mL of deionized water and then sonicating them for 30 min to ensure a homogeneous dispersion. A 5 L volume of the colloidal -Fe 2 O 3 solution was cast on a glassy carbon electrode and then dried in a hot air oven at 65 ∘ C for 1 h. This GC/ -Fe 2 O 3 NP-modified electrode was used for the electrocatalytic oxidation of dopamine. All the electrochemical measurements were carried out using a VersaSTAT-3 electrochemical analyzer (Princeton Applied Research, USA) with a conventional three-electrode system under a nitrogen atmosphere at room temperature (27 ∘ C). The -Fe 2 O 3 nanoparticle-modified GC electrode, platinum wire, and saturated calomel electrode were used as the working, counter, and reference electrodes, respectively. A 0.1 M phosphate buffer solution (PBS) (pH = 6.8) was used as the supporting electrolyte. All the potentials are quoted against the saturated calomel electrode unless otherwise mentioned.  200-400 nm in the UV region and weak absorption at 400-800 nm of the visible region. The two types of optical absorptions in the UV and visible regions are mainly attributed to the two kinds of electronic transition mechanisms. The former is due to the contribution of the direct charge transition of O 2 − 2p → Fe 3+ 3d (UV absorption), and the latter originates from the indirect charge transition of Fe 3+ 3d → 3d (visible absorption) [8][9][10]. The band-gap energy ( bg ) value of the prepared -Fe 2 O 3 nanoparticles was derived from the relationship between the absorption coefficient and the incident photon energy of the -Fe 2 O 3 nanoparticles using Tauc's plot method [11,12]. Consider

Results and Discussion
where is the absorption coefficient, bg is the band-gap energy, and is the absorption constant. The absorption coefficient ( ) was determined from the equation = (2.303 × 10 3 ) ( )/ by using the measured absorbance ( ) and optical path length ( ) (1 cm). The is equal to 1/2 and 2 for allowed direct and allowed indirect transitions, respectively. By extrapolating the linear region in the plots of ( ℎ]) versus ℎ], the band-gap energy values were obtained for the -Fe 2 O 3 nanoparticles (Figures 1(b) and 1(c)). The calculated bandgap energy values of 2.58 eV and 2.13 eV are due to the allowed direct transition and allowed indirect transition, respectively, for the -Fe 2 O 3 nanoparticles. This observed bg value is slightly higher and blue shifted compared to the results of previous reports because of the quantum size confinement in the nanoparticles. The photoluminescence spectrum of the -Fe 2 O 3 nanoparticles was recorded using excitation by 325 nm laser irradiation and is shown in Figure 1(d). The broad and intense PL peak at ∼710 nm and shoulder peak at 590 nm are due to the band-edge emission of the -Fe 2 O 3 nanoparticles, which correspond to the band-gap energies of 1.75 and 2.1 eV for indirect and direct transitions.   [2,9,13]. The absence of diffraction peaks due to Fe(OH) 2 and Fe(OH) 3 indicates the complete transformation of these hydroxides to hematite during the hydrothermal process. The crystallite size was calculated to be ∼40 nm for hematite particles using Scherrer's equation [13]. The EDX spectra and elemental mapping for the prepared -Fe 2 O 3 nanoparticles were recorded and are shown in  The electroactive surface area ( ) of an electrode can be easily calculated from the slope obtained by the Randles-Sevcik plot described by the following:

Morphological Studies of -Fe
where , ], , , and are the number of electrons, scan rate, diffusion coefficient (cm 2 s −1 ), surface area of the electrode, and concentration (in mol⋅L −1 ), respectively [14]. The diffusion coefficient ( ) can be estimated using the Cottrell equation as follows: where , , , and are the bulk concentration (mol cm −3 ), area of the electrode (cm 2 ), Faraday constant 6 The Scientific World Journal An increase in the peak current resulted from increasing the electroactive surface area, as observed in the hematitemodified electrode [15]. The peak-to-peak separation (Δ ) of the CV results can be used to investigate the reversibility of the electrochemical process in the electrodes. The peak separation for a reversible process can be calculated as follows: The Scientific World Journal where Δ is the minimum theoretical peak-to-peak separation, pa is the anodic peak potential, and pc is the cathodic peak potential. In a reversible reaction process, Δ is greater than Δ , whereas in an irreversible reaction process, only one peak will be observed in one of the potential scans [14].  The observed diameter of the semicircle is equal to the charge transfer kinetics of the redox probe at the electrode interface, where re and im are real and imaginary parts of the impedance spectra ( ). The bare GC electrode shows a semicircle with a larger diameter. Interestingly, the GC electrode modified with -Fe 2 O 3 nanoparticles showed a significant decrease in the semicircle diameter of the Nyquist plot. This indicated that the charge transfer resistance decreased upon the modification of the -Fe 2 O 3 nanoparticles. The charge transfer resistance ( ct ) values for both the bare GC and the -Fe 2 O 3 nanoparticle-modified electrode were calculated by applying the following equation [16]: where ct is the charge transfer resistance due to the transferring of electrons at the electrode/electrolyte solution interface, is a gas constant, is the experimental temperature, is the electron transfer number of the redox process, which could be considered to be equal to one for [Fe(CN) 6 ] 4− /[Fe(CN) 6 ] 3− couple, is the Faraday constant, and 0 is the exchange current of the redox probe [16].
The bare GC and -Fe 2 O 3 nanoparticle-modified electrode had ct values of 180 and 25 kΩ, respectively. It should be noted that the ct value of the -Fe 2 O 3 nanoparticle-modified electrode is lower than that of the bare GC electrode, which is mainly attributed to the high electron conductivity of the -Fe 2 O 3 nanoparticle-modified electrode and the sluggish electron transfer behavior of the bare GC. This clearly shows that the -Fe 2 O 3 nanoparticle-modified electrode is a promising candidate for electrocatalysis and sensor applications.  Figure 9. It can be observed that the bare GCE has quasireversible, poorly defined redox peaks and a low electrochemical response toward DA. However, a significant increase in the redox peak current was detected for the GC/ -Fe 2 O 3 -modified electrode, contributing to its improved conductivity performance. Hence, the GC/ -Fe 2 O 3 -modified electrode exhibited reversible behavior and possessed faster electron transfer kinetics, as reflected in its low Δ value. The effect of the scan rate on the DA oxidation was studied for the bare GC and GC/ -Fe 2 O 3 -modified electrode by varying the scan rate from 5 to 200 mVs −1 with 50 M of DA in a 0.1 M PBS (pH = 6.8) (Figure 10). At the 200 mVs −1 scan rate, we observed a redox peak with an anodic-to-cathodic ratio of 1.37, which was slightly higher than the unit value expected for the ideal voltammetric behavior of a GC/ -Fe 2 O 3 -modified electrode for reversibility. The peak separations (Δ ) of the bare electrode and GC/ -Fe 2 O 3 -modified electrode for all the scan rates in the range of 5-200 mVs −1 were greater than 295 mV (due to the two electron transfers), which indicated a reversible redox process. Increasing the scan rate increased the peak separation (Δ ) because of the chemical interaction between the DA and the modified electrode. Simultaneous increases in the anodic and cathodic current intensities were also observed when increasing the scan rate. A plot of the anodic and cathodic current ( pa ) peaks versus the square root of the scan rate is shown in Figure 10. It shows the linear relation for the GC/ -Fe 2 O 3 -modified electrode ( 2 = 0.9943 and 0.9931), which indicates a diffusion controlled redox process [3,[17][18][19][20]. various pH values ranging from 3 to 11, and the observed cyclic voltammograms are shown in Figure 11(a). It can be observed that increasing the pH of the electrolyte solution leads to negative shifts in both the anodic and the cathodic peak potentials, which suggests that the redox reaction of DA includes some protons transfers at the modified electrode.

Influence of pH on Electrocatalytic Activity of -Fe
In addition, the influence of the pH on the anodic peak potential of DA was also studied, and the obtained results are shown in Figure 11(b). From Figure 11(c), it can be noted that the applied potential ( pa ) versus the pH shows a linear relationship in the pH range of 3 to 11, with a slope value of −75.05 mV/pH and this observed slope value is higher than the theoretical value of −59 mV/pH.

Chronoamperometric Detection of Dopamine.
The chronoamperometric technique was also used to determine the applicability of the GC/ -Fe 2 O 3 -modified electrode for DA sensing. Figure 12 shows the chronoamperometric response of the GCE/ -Fe 2 O 3 -modified electrode upon the stepwise addition of 500 nM into the 0.1 M PBS (pH = 6.8) at an applied potential of 200 mV. Figure 12 shows the increase in the current with increasing DA concentration in the range to be ∼3 s, which indicates a fast electron transfer with this modified electrode.  Figure 13. It is observed that when the DA concentration increases, the peak current also increases up to 90 M of DA. When the DA concentration was greater than 90 M, the modified electrode did not show any further increase in the peak current. This phenomenon is mainly attributed to the saturation of the catalytic sites of the modified electrode. The calibration plot obtained for the modified electrode current response against the dopamine concentration is shown in Figure 13

Selectivity Study of -Fe 2 O 3 Nanoparticle-Modified Electrode.
In order to study the selectivity of the -Fe 2 O 3 nanoparticle-modified electrode toward DA, chronoamperometric curves were recorded with common interference sources such as ascorbic acid, glucose, sulphate, and nitrate and are shown in Figure 14. It can be seen that upon the addition of DA, the current is increased, whereas when 100fold higher concentration of an interference source is added, such as AA, Glucose (Glu), SO 4 − , or NO 2 − , there is no obvious increase in the current. Further, when DA is added to the same solution, the current increases. This clearly suggests that the fabricated -Fe 2 O 3 nanoparticle-modified electrode is more selective toward DA.

Conclusion
Hematite ( -Fe 2 O 3 ) nanoparticles were synthesized using a hydrothermal method and characterized by UV-vis, PL, XRD, TEM, AFM, FESEM, and EDX analyses. The experimental results indicated the formation of uniform hematite nanoparticles with an average size of 45 nm. The so-formed -Fe 2 O 3 nanoparticles were used to fabricate a sensor electrode for the detection of DA. The electrochemical behavior of this GC/ -Fe 2 O 3 electrode was studied by using CV and EIS techniques with an electrochemical probe, an [Fe(CN) 6 ] 3−/4− redox couple. The electrocatalytic activity was investigated toward DA oxidation in a phosphate buffer solution (pH 6.8) by varying different experimental parameters. The chronoamperometric study showed a linear response in the range of 0-2 M with LoD of 1.6 M for DA. The square wave voltammetry showed a linear response in the range of 0-35 M with LoD of 236 nM for DA. In a biological system containing a micromolar concentration of DA [23], submicromolar detection would be beneficial using an SWV and CA method for a modified electrode. The results of this study suggested that -Fe 2 O 3 nanoparticles could be a promising candidate for the fabrication of ultramicroelectrodes for in vivo DA detection applications.