Electrochemical Detection of Dopamine at Fe3O4/SPEEK Modified Electrode

Reported here is the design of an electrochemical sensor for dopamine (DA) based on a screen print carbon electrode modified with a sulphonated polyether ether ketone-iron (III) oxide composite (SPCE-Fe3O4/SPEEK). L. serica leaf extract was used in the synthesis of iron (III) oxide nanoparticles (Fe3O4NPs). Successful synthesis of Fe3O4NP was confirmed through characterization using Fourier transform infrared (FTIR), ultraviolet–visible light (UV–VIS), X-ray diffractometer (XRD), and scanning electron microscopy (SEM). Cyclic voltammetry (CV) was used to investigate the electrochemical behaviour of Fe3O4/SPEEK in 0.1 M of phosphate buffer solution (PBS) containing 5 mM of potassium ferricyanide (III) solution (K3[Fe(CN)6]). An increase in peak current was observed at the nanocomposite modified electrode SPCE-Fe3O4/SPEEK) but not SPCE and SPCE-Fe3O4, which could be ascribed to the presence of SPEEK. CV and square wave voltammetry (SWV) were employed in the electroxidation of dopamine (0.1 mM DA). The detection limit (LoD) of 7.1 μM and 0.005 μA/μM sensitivity was obtained for DA at the SPCE-Fe3O4/SPEEK electrode with concentrations ranging from 5–50 μM. LOD competes well with other electrodes reported in the literature. The developed sensor demonstrated good practical applicability for DA in a DA injection with good resultant recovery percentages and RSDs values.


Introduction
Dopamine (3,4-dihydroxyphenethylamine), an organic chemical of the catecholamine neurotransmitters, is one of the most researched neurotransmitters (NTs) because of its major role in the human body such as in the hormonal, renal, central, and cardiovascular systems, and the human metabolism [1][2][3][4][5][6][7]. DA plays an important role within and outside the brain's rewards system by reinforcing certain behaviour that results in rewards. In addition, DA controls movements, emotional response functions as a vasodilator, and expands the urine output in the pancreas and kidneys by reducing the formation of insulin. However, elevated dopamine concentration in the brain could cause many neurological disorders such as Tourette's syndrome, restless leg syndrome (RLS), and several illnesses such as drug dependence, schizophrenia, Parkinson's disease, depression, degenerative diseases, and attention deficit hyperactivity disorder (ADHD) [8][9][10][11][12][13][14]. As a result, maintaining and controlling the high levels of DA in the human body is important. The health import of DA calls for the need to develop a cost-effective, simple, sensitive, and selective assay with a fast response for DA investigation in pharmaceutical samples.

UV-Visible Study
The formation of Fe 3 O 4 NPs is ascribed to ferrous, ferric salts (iron (II) chloride tetrahydrate, iron (III) chloride tetrahydrate, and the leaf extract of L. serica. The reduction that occurred on the Fe 2+ ions is explained by the visible colour change in the reaction mixture which physically confirms the Fe-O nanoparticles by using a UV-Visible spectrophotometer. Figure 1 shows the UV-visible spectra of green mediated Fe 3 O 4 NPs with an absorbed peak of approximately 296 nm, which is close to absorption peaks (296, 259, and 282) reported in previous studies [35]. However, the obtained results show a great biomolecule capping surface of the Fe 3 O 4 NPs without the presence of a Plasmon resonance surface. The energy band gap was calculated to be 4.19 eV according to Equation (1) using the obtained maximum absorption peak (296 nm):

FTIR Study
FTIR spectra of SPEEK, Fe3O4NPs, and Fe3O4/SPEEK nanocomposites recorded on the wavenumber from 400-4000 cm −1 presented in Figure 2 gives the information on different functional groups of the compounds present. The absorption peak at 3485, 2929, and 2858 cm −1 correspond to the -OH stretching of the phenol group and -C-H stretch, which agrees with the literature [36]. The absorption at the 1656, 1596, and 1460 cm −1 band corresponds to -C=C stretch which indicates the nitriles group [37]. The 1639 and 1591 cm −1 peaks reflect the -C=C stretch aromatic vibrations [38]. The intense peaks at 1090, 1242, and 1215 cm −1 are attributed to the -C-O stretch, phenol or alcohol group, and deformation bands in the lignin [37]. The absorption peaks at 469 and 654 cm −1 corresponding to Fe-O stretch, confirms the successful synthesis of Fe3O4NPs, and Fe3O4/SPEEK nanocomposites, respectively. It is possible that the presence of the phenol -OH group and the amide -N-H group played a role in the reduction of the precursor compound into iron oxide nanoparticles. The polymer SPEEK and iron oxide nanoparticles showed a significant interaction as shown in the composite peaks absorbed ( Figure 2).    Figure 2 gives the information on different functional groups of the compounds present. The absorption peak at 3485, 2929, and 2858 cm −1 correspond to the -OH stretching of the phenol group and -C-H stretch, which agrees with the literature [36]. The absorption at the 1656, 1596, and 1460 cm −1 band corresponds to -C=C stretch which indicates the nitriles group [37]. The 1639 and 1591 cm −1 peaks reflect the -C=C stretch aromatic vibrations [38]. The intense peaks at 1090, 1242, and 1215 cm −1 are attributed to the -C-O stretch, phenol or alcohol group, and deformation bands in the lignin [37]. The absorption peaks at 469 and 654 cm −1 corresponding to Fe-O stretch, confirms the successful synthesis of Fe 3 O 4 NPs, and Fe 3 O 4 /SPEEK nanocomposites, respectively. It is possible that the presence of the phenol -OH group and the amide -N-H group played a role in the reduction of the precursor compound into iron oxide nanoparticles. The polymer SPEEK and iron oxide nanoparticles showed a significant interaction as shown in the composite peaks absorbed ( Figure 2).

FTIR Study
FTIR spectra of SPEEK, Fe3O4NPs, and Fe3O4/SPEEK nanocomposites recorded on the wavenumber from 400-4000 cm −1 presented in Figure 2 gives the information on different functional groups of the compounds present. The absorption peak at 3485, 2929, and 2858 cm −1 correspond to the -OH stretching of the phenol group and -C-H stretch, which agrees with the literature [36]. The absorption at the 1656, 1596, and 1460 cm −1 band corresponds to -C=C stretch which indicates the nitriles group [37]. The 1639 and 1591 cm −1 peaks reflect the -C=C stretch aromatic vibrations [38]. The intense peaks at 1090, 1242, and 1215 cm −1 are attributed to the -C-O stretch, phenol or alcohol group, and deformation bands in the lignin [37]. The absorption peaks at 469 and 654 cm −1 corresponding to Fe-O stretch, confirms the successful synthesis of Fe3O4NPs, and Fe3O4/SPEEK nanocomposites, respectively. It is possible that the presence of the phenol -OH group and the amide -N-H group played a role in the reduction of the precursor compound into iron oxide nanoparticles. The polymer SPEEK and iron oxide nanoparticles showed a significant interaction as shown in the composite peaks absorbed ( Figure 2).

XRD Study
The crystallographic structure of the samples can be determined using XRD. Figure 3 shows the XRD pattern of Fe 3 [39]. The diffraction peaks observed confirm the crystalline nature of Fe 3 O 4 nanoparticles, and the planes of the magnetite Fe 3 O 4 NPs also confirm the rhombohedral hematite phase. The diffraction peak obtained in the XRD patterns indicates that there was no trace of additional planes observed, which indicates the mediated Fe 3 O 4 NPs were obtained in high purity at room temperature [40,41].

XRD Study
The crystallographic structure of the samples can be determined using XRD. Figure  3 shows the XRD pattern of Fe3O4NPs with diffraction peaks and their corresponding planes at 2thetha (θ) values of 26.75° (120), 35.16° (200), 39.22° (123), 52.01° (115), and 55.99° (122), which are similar to a reported study [39]. The diffraction peaks observed confirm the crystalline nature of Fe3O4 nanoparticles, and the planes of the magnetite Fe3O4NPs also confirm the rhombohedral hematite phase. The diffraction peak obtained in the XRD patterns indicates that there was no trace of additional planes observed, which indicates the mediated Fe3O4NPs were obtained in high purity at room temperature [40,41].

SEM Study
The surface morphology of the prepared Fe3O4NPs from the green synthesis of L. serica leaf extract was analyzed by scanning electron microscopy (SEM). Figure 4a,b represent the SEM images of Fe3O4NPs and Fe3O4/SPEEK nanocomposites, respectively. Figure 4a depicts the morphology of Fe3O4NPs that appears to be roughly agglomerated spherical particles in shape, which could be due to the steric effect associated with the magnetic Fe3O4NPs surface interaction by the active sites [42]. Figure 4b shows the morphology of the Fe3O4/SPEEK nanocomposites that appeared to have some crystal-like structure which indicates the presence of the SPEEK polymer in the nanocomposite, indicating the occurrence of interaction between Fe3O4 and SPEEK. Hence, particles appeared to be clustered together, thus, maintaining the agglomerated spherical shape [43].

SEM Study
The surface morphology of the prepared Fe 3 O 4 NPs from the green synthesis of L. serica leaf extract was analyzed by scanning electron microscopy (SEM). Figure 4a,b represent the SEM images of Fe 3 O 4 NPs and Fe 3 O 4 /SPEEK nanocomposites, respectively. Figure 4a depicts the morphology of Fe 3 O 4 NPs that appears to be roughly agglomerated spherical particles in shape, which could be due to the steric effect associated with the magnetic Fe 3 O 4 NPs surface interaction by the active sites [42]. Figure 4b shows the morphology of the Fe 3 O 4 /SPEEK nanocomposites that appeared to have some crystal-like structure which indicates the presence of the SPEEK polymer in the nanocomposite, indicating the occurrence of interaction between Fe 3 O 4 and SPEEK. Hence, particles appeared to be clustered together, thus, maintaining the agglomerated spherical shape [43].

Electrochemical Characterization of Electrodes
The electrochemical efficiency and electron transport properties of electrodes (bare-SPCE, SPCE-Fe3O4NPs, SPCE-SPEEK, and SPCE-Fe3O4/SPEEK nanocomposites) were investigated using cyclic voltammetry (CV) at a scan rate of 25 mV/s within −0.2-1.0 V potential window in 0.1 M PBS of pH 7.4 containing 5 mM K3[Fe(CN)6]. A comparative cyclic voltammogram of the electrodes is presented in Figure 5. The current response was enhanced at the SPCE-SPEEK and SPCE-Fe3O4/SPEEK electrodes as opposed to the bare and nanoparticle-modified electrodes, which could be due to the presence of SPEEK which has excellent electrocatalytic properties. Table 1 shows the parameters measured at the electrodes.    Figure 5. The current response was enhanced at the SPCE-SPEEK and SPCE-Fe 3 O 4 /SPEEK electrodes as opposed to the bare and nanoparticle-modified electrodes, which could be due to the presence of SPEEK which has excellent electrocatalytic properties. Table 1 shows the parameters measured at the electrodes.

Electrochemical Characterization of Electrodes
The electrochemical efficiency and electron transport properties of electrodes (bare-SPCE, SPCE-Fe3O4NPs, SPCE-SPEEK, and SPCE-Fe3O4/SPEEK nanocomposites) were investigated using cyclic voltammetry (CV) at a scan rate of 25 mV/s within −0.2-1.0 V potential window in 0.1 M PBS of pH 7.4 containing 5 mM K3[Fe(CN)6]. A comparative cyclic voltammogram of the electrodes is presented in Figure 5. The current response was enhanced at the SPCE-SPEEK and SPCE-Fe3O4/SPEEK electrodes as opposed to the bare and nanoparticle-modified electrodes, which could be due to the presence of SPEEK which has excellent electrocatalytic properties. Table 1 shows the parameters measured at the electrodes.   Where I pa = Anodic peak current, I pc = Cathodic peak current, E pa = Anodic peak potential, E pc = Cathodic peak potential, , and ∆E p = Peak potential separation.

Scan Rate Study at SPCE-Fe 3 O 4 /SPEEK Electrode
The effect of scan rate variation on peak currents of modified SPCE-Fe 3 O 4 /SPEEK in 5 mM prepared in 0.1 M PBS, pH 7.4 solution was studied using CV in the range from 25-500 mV/s scan rate as shown in Figure 6a. As the scan rate increases, the oxidation peak potentials shifted to the more positive. In consequence, a linear plot of peak currents versus square root of scan rate (v 1/2 ) was deduced (Figure 6b). The graph clearly shows an increase of the peak currents with an increase in the square root of the scan rate, indicating a diffusion-controlled electrochemical process, which was also confirmed by the correlation coefficient (R 2 ) value of 0.99 [44]. The surface area of the modified nanocomposite electrode (SPCE-Fe 3 O 4 /SPEEK) was found to be 2.799 cm 2 , using a Randle-Sevcik Equation (2) which is higher than the geometry of the bare SPCE (0.125 cm 2 ): where I p represents peak current (A), n is the number of electron transfer, A represents surface area (cm 2 ), D represents diffusion coefficient (cm 2 /s), C represents concentration (mol/cm 3 ), and v represents scan rate (V/s).  Where Ipa = Anodic peak current, Ipc = Cathodic peak current, Epa = Anodic peak potential, Epc = Cathodic peak potential, E / = , and ∆Ep = Peak potential separation.

Scan Rate Study at SPCE-Fe3O4/SPEEK Electrode
The effect of scan rate variation on peak currents of modified SPCE-Fe3O4/SPEEK in 5 mM prepared in 0.1 M PBS, pH 7.4 solution was studied using CV in the range from 25-500 mV/s scan rate as shown in Figure 6a. As the scan rate increases, the oxidation peak potentials shifted to the more positive. In consequence, a linear plot of peak currents versus square root of scan rate (v 1/2 ) was deduced (Figure 6b). The graph clearly shows an increase of the peak currents with an increase in the square root of the scan rate, indicating a diffusion-controlled electrochemical process, which was also confirmed by the correlation coefficient (R 2 ) value of 0.99 [44]. The surface area of the modified nanocomposite electrode (SPCE-Fe3O4/SPEEK) was found to be 2.799 cm 2 , using a Randle-Sevcik Equation (2) which is higher than the geometry of the bare SPCE (0.125 cm 2 ): where Ip represents peak current (A), n is the number of electron transfer, A represents surface area (cm 2 ), D represents diffusion coefficient (cm 2 /s), C represents concentration (mol/cm 3 ), and v represents scan rate (V/s). From the cyclic voltammetric measurement in Figure 6a, a linear plot of potential peaks (Epa/Epc) versus the log of scan rate (Figure 7) gave two straight lines with slopes of equal Equations (3) and (4): From the cyclic voltammetric measurement in Figure 6a, a linear plot of potential peaks (E pa /E pc ) versus the log of scan rate (Figure 7) gave two straight lines with slopes of equal Equations (3) and (4): charge transfers coefficient (α) were calculated to be 1 and 0.53, respectively. Additionally, the Tafel value (b) was found to be 0.375 Vdec −1 using Equation (5) which is higher than the theoretical value (0.118 Vdec −1 ), suggesting adsorption on the electrode surface by reactants:  The behaviour of DA on bare and modified screen-printed electrodes (bare-SPCE, SPCE-Fe3O4NPs, SPCE-SPEEK, and SPCE-Fe3O4/SPEEK) was studied using cyclic voltammetry at a 25 mV/s scan rate as shown in Figure 9. Redox peaks were observed in all the According to Laviron's Equations (3) and (4), the number of electron transfer (n) and charge transfers coefficient (α) were calculated to be 1 and 0.53, respectively. Additionally, the Tafel value (b) was found to be 0.375 Vdec −1 using Equation (5) which is higher than the theoretical value (0.118 Vdec −1 ), suggesting adsorption on the electrode surface by reactants: According to Laviron's Equations (3) and (4), the number of electron transfer (n) and charge transfers coefficient (α) were calculated to be 1 and 0.53, respectively. Additionally, the Tafel value (b) was found to be 0.375 Vdec −1 using Equation (5) which is higher than the theoretical value (0.118 Vdec −1 ), suggesting adsorption on the electrode surface by reactants:  The behaviour of DA on bare and modified screen-printed electrodes (bare-SPCE, SPCE-Fe3O4NPs, SPCE-SPEEK, and SPCE-Fe3O4/SPEEK) was studied using cyclic voltammetry at a 25 mV/s scan rate as shown in Figure 9. Redox peaks were observed in all the The behaviour of DA on bare and modified screen-printed electrodes (bare-SPCE, SPCE-Fe 3 O 4 NPs, SPCE-SPEEK, and SPCE-Fe 3 O 4 /SPEEK) was studied using cyclic voltammetry at a 25 mV/s scan rate as shown in Figure 9. Redox peaks were observed in all the electrodes. The oxidation peak current of the SPCE-Fe 3 O 4 /SPEEK electrode was slightly higher than the bare, which could be due to the presence of SPEEK ascribed to its good electrical conductivity, but smaller than the SPCE-SPEEK electrode. However, obtained oxidation potential for DA at the SPCE-Fe 3 O 4 /SPEEK electrode was nearer to 0.25 V expected for DA. The high peak current observed on the SPCE/SPEEK electrode, compared with SPCE-Fe 3 O 4 /SPEEK, could be due to the excellent electronic conductivity property of SPEEK, which enhanced the reactivity of Fe 3 O 4 . The Fe 3 O 4 nanoparticles-modified electrode showed a lower redox peak current than the bare-SPCE on the DA probe, due to the quick assembling of nanoparticles that conduct to larger particles of Fe 3 O 4 , which may crucially reduce the electrochemical properties of the electrode and, thus, be electroinactive [45,46]. Parameters determined in cyclic voltammetric detection of DA on bare and modified electrodes are shown in Table 2. electrodes. The oxidation peak current of the SPCE-Fe3O4/SPEEK electrode was slightly higher than the bare, which could be due to the presence of SPEEK ascribed to its good electrical conductivity, but smaller than the SPCE-SPEEK electrode. However, obtained oxidation potential for DA at the SPCE-Fe3O4/SPEEK electrode was nearer to 0.25 V expected for DA. The high peak current observed on the SPCE/SPEEK electrode, compared with SPCE-Fe3O4/SPEEK, could be due to the excellent electronic conductivity property of SPEEK, which enhanced the reactivity of Fe3O4. The Fe3O4 nanoparticles-modified electrode showed a lower redox peak current than the bare-SPCE on the DA probe, due to the quick assembling of nanoparticles that conduct to larger particles of Fe3O4, which may crucially reduce the electrochemical properties of the electrode and, thus, be electro-inactive [45,46]. Parameters determined in cyclic voltammetric detection of DA on bare and modified electrodes are shown in Table 2.

Scan Rate Study on Dopamine
In Figure 10a, the electrochemical impact of varying scan rates in the range from 25-400 mV/s on the anodic peak currents of the nanocomposite-modified SPCE-Fe 3 O 4 / SPEEK toward 0.1 mM DA oxidation was investigated using cyclic voltammetry. An increase in the scan rate resulted in shifts of peak potentials to the more positive, and an increase of peak currents, suggesting a diffusion-controlled process. Figure 10b shows the linear plot of peak currents against the square root of scan rate (v 1/2 ) with 0.98 regression values for both anodic and cathodic lines (I pa and I pc ), confirming a diffusion-controlled process for DA oxidation. In Figure 10a, the electrochemical impact of varying scan rates in the range from 25-400 mV/s on the anodic peak currents of the nanocomposite-modified SPCE-Fe3O4/SPEEK toward 0.1 mM DA oxidation was investigated using cyclic voltammetry. An increase in the scan rate resulted in shifts of peak potentials to the more positive, and an increase of peak currents, suggesting a diffusion-controlled process. Figure 10b shows the linear plot of peak currents against the square root of scan rate (v 1/2 ) with 0.98 regression values for both anodic and cathodic lines (Ipa and Ipc), confirming a diffusion-controlled process for DA oxidation.  Figure 11 represents the linear plot of peak potentials (Epa/Epc) against the logarithm of scan rate (v). The Tafel slope value was found to be 0.693 V/dec for DA from the slope value of Figure 11 by applying Equation (5). Obtained values were higher than the expected theoretical value of 0.118 V/dec, suggesting adsorption of the reactant on the electrode surface.

Concentration Study of DA
The impact of different concentrations on the DA current response was studied using square wave voltammetry as shown in Figure 12a under the optimal parameters of 0.01 V potential step, 0.001 V amplitude, deposition time of 10 s, and frequency of 25 Hz. The result obtained shows the dependence of reduction peak currents of dopamine on increasing DA concentrations (5 to 50 μM). The poorly defined reduction peak current could be due to the nature of the electrode modifiers. The linear relationship between peak currents and DA concentrations (Figure 12b Figure 11 represents the linear plot of peak potentials (E pa /E pc ) against the logarithm of scan rate (v). The Tafel slope value was found to be 0.693 V/dec for DA from the slope value of Figure 11 by applying Equation (5). Obtained values were higher than the expected theoretical value of 0.118 V/dec, suggesting adsorption of the reactant on the electrode surface.
Molecules 2021, 26, x 10 of 16 μM by applying Equation (6). The LoD competes well with previous works investigated in the literature (Table 3): SD stands for the standard deviation of the peak current, over the slope of the calibrated plot.

Electro-Analysis of DA Concentration Study of DA
The impact of different concentrations on the DA current response was studied using square wave voltammetry as shown in Figure 12a under the optimal parameters of 0.01 V potential step, 0.001 V amplitude, deposition time of 10 s, and frequency of 25 Hz. The result obtained shows the dependence of reduction peak currents of dopamine on increasing DA concentrations (5 to 50 µM). The poorly defined reduction peak current could be due to the nature of the electrode modifiers. The linear relationship between peak currents and DA concentrations (Figure 12b) yielded a linear regression equation of I pc = 0.005087 [DA] + 4.320144, and regression value of 0.98. The detection limit was calculated to be 7.2 µM by applying Equation (6). The LoD competes well with previous works investigated in the literature (Table 3): SD stands for the standard deviation of the peak current, over the slope of the calibrated plot. brated plot.    Abbreviations: Ppy = Polypyrrole; Au = Gold; Ag = Silver; E-RGO = Electrochemically-reduced graphene oxide; GCE = Glass carbon electrode, PoPD = poly(o-phenylenediamine). Figure 13a,b show the SW voltammogram of DA and UA accordingly with peak potential observed at 0.23 and 0.36 V for the respective analyte. Figure 13c represents the simultaneous detection of DA and UA of the same concentration with potentials noticed at 0.16 V (DA) and 0.31 V (UA). The shifts in the peak potentials and the peak separation (0.15 V) between DA and UA indicate non-interference of the UA signal with that of DA, successful detection of DA in the presence of UA at the designed electrode, and selectivity of the electrode.
Abbreviations: Ppy = Polypyrrole; Au = Gold; Ag = Silver; E-RGO = Electrochemically-reduced graphene oxide; GCE = Glass carbon electrode, PoPD = poly(o-phenylenediamine). Figure 13a,b show the SW voltammogram of DA and UA accordingly with peak potential observed at 0.23 and 0.36 V for the respective analyte. Figure 13c represents the simultaneous detection of DA and UA of the same concentration with potentials noticed at 0.16 V (DA) and 0.31 V (UA). The shifts in the peak potentials and the peak separation (0.15 V) between DA and UA indicate non-interference of the UA signal with that of DA, successful detection of DA in the presence of UA at the designed electrode, and selectivity of the electrode.

Analytical Application of the Proposed Sensor for Determination of DA in Pharmaceutical Sample
The practical applicability of the designed sensor for DA determination was investigated using a diluted DA hydrochloride injection (dopamine HCl-Fresenius 200 mg/5 mL) sample, spiked with different concentrations of DA standards in accordance with well-

Analytical Application of the Proposed Sensor for Determination of DA in Pharmaceutical Sample
The practical applicability of the designed sensor for DA determination was investigated using a diluted DA hydrochloride injection (dopamine HCl-Fresenius 200 mg/5 mL) sample, spiked with different concentrations of DA standards in accordance with wellestablished standard addition procedure. The results recorded from the SWV measurements under optimum conditions (0.01 potential step, 0.001 amplitude, deposition time 10 s, and frequency of 25 Hz) are summarized in Table 4. Satisfactory recoveries in the range from 99.9% to 100% were obtained with good relative standard deviations (RSDs), illustrating the promising application of the SPCE-Fe 3 O 4 /SPEEK electrode for the determination of DA in real samples.

Repeatability and Stability Study for Fe 3 O 4 /SPEEK
The repeatability study of the SPCE-Fe 3 O 4 /SPEEK modified electrode was conducted using cyclic voltammetry at a 25 mVs −1 scan rate, for 10 repetitive scans in 0.1 mM DA ( Figure 14). Relative standard deviations of 0.75 and 2.45% were obtained for oxidation peak potential and peak current, accordingly, suggesting acceptable repeatability, stability, and reproducibility of the electrochemical sensor. Peak current was monitored for 28 days at an interval of 5 days, and the electrode was stored in the refrigerator when not in use. A 45% increase of the initial peak current was observed which could be attributed to increased assimilation of the nanocomposite (SPCE-Fe 3 O 4 /SPEEK) onto the electrode surface over time.

Repeatability and Stability Study for Fe3O4/SPEEK
The repeatability study of the SPCE-Fe3O4/SPEEK modified electrode was conducted using cyclic voltammetry at a 25 mVs −1 scan rate, for 10 repetitive scans in 0.1 mM DA ( Figure 14). Relative standard deviations of 0.75 and 2.45% were obtained for oxidation peak potential and peak current, accordingly, suggesting acceptable repeatability, stability, and reproducibility of the electrochemical sensor. Peak current was monitored for 28 days at an interval of 5 days, and the electrode was stored in the refrigerator when not in use. A 45% increase of the initial peak current was observed which could be attributed to increased assimilation of the nanocomposite (SPCE-Fe3O4/SPEEK) onto the electrode surface over time.

Materials and Methods
The L. serica plant was collected from Kwa-Zulu Natal province. Iron (II) chloride tetrahydrate (FeSO4·4H2O) and iron (III) chloride hexahydrate (FeCl3·6H2O) are products from BDH and LABCHEM, South Africa. Sodium phosphate salts (Na2HPO4 and NaH2PO4) products of LABCHEM and GlassWorld, South Africa, were used in the preparation of 0.1 M phosphate buffer solution (PBS) of pH 7.4. Potassium ferricyanide (III) (K3[Fe(CN)6]) and dopamine hydrochloride were purchased from Sigma-Aldrich (St. Louis, MO, USA). Dimethyl formamide (DMF), sodium hydroxide (NaOH), and distilled water was produced by Emplura ® Merck (The Chemical Center from Maharashtra, India). All chemicals used were of analytical grade.

Preparation of Plant Leaf Extract
Approximately 10 g of ground fine powdered leaves of the L. serica plant was weighed and transferred into a conical flask followed by the addition of 200 mL of distilled water and heated for several minutes at 60 • C until a change in colour (dark green-brown solution) was observed. Leaf extract was filtered using Whatman No. 1 filter paper and a Buchner flask [51].

Synthesis of Iron Oxide Nanoparticles
The green meditation of Fe 3 O 4 NPs derived from L. serica leaf extract following the prescribed method with a few minor changes [52,53]. 2:1 M volume ratio of iron (II) chloride tetrahydrate and iron (III) chloride tetrahydrate solution was added to the L. serica extract with a resultant black-coloured precipitate, indicating the formation of precipitates (iron oxide nanoparticle). The pH of the mixture was adjusted to 11 by the addition (drop-wise) of 1.0 M of NaOH solution under continuous stirring. The solution was thereafter stirred for 1 h to complete the reaction homogeneity, filtered using vacuum filtered precipitates (Fe 3 O 4 NPs) washed several times with distilled water, and air-dried in the fume wood overnight. The dried sample was stored in an airtight container for further characterization.

Preparation of SPEEK Polymer
Details on SPEEK preparation (sulphonation of polyether ether ketone) have been reported in our previous work [54].
3.4. Synthesis of Iron Oxide/SPEEK Nanocomposites 20 mg of SPEEK and 40 mg of iron oxide nanoparticles were dissolved in N, Ndimethylformamide solution, sonicated for 48 h at room temperature, and stored for further characterization.

Preparation of Real Sample for Analysis
A dopamine hydrochloride injection (the mL taken) was diluted with distilled water in a 100 mL flask and 2 mL each of the diluted solution was transferred into six 50 mL volumetric flasks. Five of the flasks were spiked with different concentrations of DA stock solution while the sixth flask was held as a control. The flasks were made to the mark using 0.1 M PBS of pH 7.4, and analyzed using SWV in triplicate.

Conclusions
In this study, the synthesis of Fe 3 O 4 NPs from the L. serica plant and the fabrication of nanocomposite-modified SPCE (SPCE/SPEEK/Fe 3 O 4 ) for DA detection is reported. The amplified SPCE-Fe 3 O 4 /SPEEK peak current, in contrast to Fe 3 O 4 NPs, could be attributed to the presence of SPEEK. The plot of peak currents versus the square root of scan rate gave a 0.98 regression value, suggesting the occurrence of a diffusion-controlled electrochemical process. The calculated detection limit competes well with previous studies investigated. In addition, the proposed sensor was selective to DA in the presence of uric acid (UA) and yielded good recovery with excellent RSDs in real sample sensing of DA. The results suggest the potential application of the designed sensor for DA monitoring in the pharmaceutical sample.