Controlled growth of single-crystalline nanostructured dendrites of α-Fe₂O₃ blended with MWCNT: a systematic investigation of highly selective determination of L-dopa

Abstract

α-Fe₂O₃ dendritic nanostructures were prepared by simple hydrothermal method and then blended with MWCNT (multiwall carbon nanotubes) to construct a novel biosensor for the determination of L-dopa. The structure of the new material was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and the electrochemical behavior of L-dopa was also studied by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometry in phosphate buffer solution (PBS) at pH 7.2. The experimental results suggested that α-Fe₂O₃ blended MWCNT composite showed 3-fold increase of the oxidation peak current compared to that of the bare electrode. The DPV current responses of L-dopa were increased linearly in the range from 5.0 × 10⁻⁸ to 3.8 × 10⁻⁶ M with a lower detection limit of 30 nM (3σ). Finally, the proposed sensor was demonstrated for the sensitive determination of L-dopa in pharmaceutical samples and the obtained results were quite promising.

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1. Introduction

Nanostructured materials have gained great interest to design new materials and devices as they exhibit unique electronic, optoelectronic and catalytic properties.^1–4 A variety of 3D structures like nanocages,⁵ cantaloupe-like superstructures,⁶ hollow nanospheres,^7,8 multimode,⁹ flower-like microspheres,¹⁰ airplane-like nanostructures,¹¹ shuttle-like nanocrystals,¹² nanoflowers¹³ and dendrites^14–16 have been investigated.

Recently, a variety of dendritic structures of metals, metal oxides and chalcogenides^17–19 have been extensively investigated for various applications. Among them, α-Fe₂O₃ has been identified as a suitable material for a variety of applications in magnetic devices, catalysis, biosensors, photo electrodes, lithium ion batteries and pigments.^20,21 Hence much attention has been focused to explore new strategies in order to tailor the morphology of α-Fe₂O₃.

In order to tune the size and the shape of the required nanostructures, hydrothermal precipitation technique has been identified as a well established wet chemistry approach using surfactants that can form different morphology by the self assembly process. Nevertheless, during charge/discharge process, Fe₂O₃ suffers from crumbling, pulverization, consequent fast capacity fading^22–24 and severe agglomeration of active materials, which cause the reduction of active surface area resulting low conductivity.²⁴

So, huge amount of efforts have been made to circumvent the above issues. One of the ways to overcome these shortcomings is to fabricate hybrid nanostructure by linking transition metals to CNT (carbon nanotubes) and the superior property of this functional nanomaterial could be used with enhanced conductivity for various applications.^25,26 The iron oxide (γ-Fe₂O₃) nanoparticle doped with CNT has been successfully investigated to sense the dopamine with a detection limit of 3.3 × 10⁻⁷ M.²⁷ As reported in the literature, α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (magnetite) have different physical, chemical and electronic properties. Thus α-Fe₂O₃ and γ-Fe₂O₃ behave as two different semiconducting materials demonstrating dissimilar electrochemical behaviors.

L-Dopa, the precursor of dopamine, is an important neurotransmitter which is commonly used for the treatment of neural disorders such as parkinson's syndrome. Traditional methods for the determination of biomolecules include spectrophotometry, chemiluminescence, chromatography and electrochemical approaches. Among these techniques, the electrochemical method has received a lot of attention because of their high sensitivity, selectivity and convenience.^28–30 To the best of our knowledge, the simple and novel surfactant assisted α-Fe₂O₃ blended with MWCNT (multiwall carbon nanotubes) for L-dopa biosensing application has not been explored in the literature.

In this investigation we have described the use of an electrochemical procedure, to fabricate a α-Fe₂O₃/MWCNT composite onto the surface of glassy carbon electrode (GCE) in order to achieve the challenge of determination of L-dopa by differential pulse voltammetry (DPV) technique (Scheme 1). The composite has been used to determinate the electrochemical redox properties of L-dopa. Thus, the present work exhibited good sensitivity, selectivity and stability toward the determination of neurotransmitter with a low detection limit of 30 nM.

Scheme 1 Schematic diagram for L-dopa sensing of α-Fe₂O₃/MWCNT composite.

2. Experimental section

2.1 Materials and instrumentation

α-Fe₂O₃ (99.99%), MWCNT (99%) potassium ferricyanide K₃[Fe(CN)₆] 98.5% and hexadecyl trimethyl ammonium bromide were purchased from Sigma Aldrich, India. L-Dopa was procured from Sisco Research Laboratories, India. All other chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared using double-distilled (DD) water.

The surface morphology of the samples was investigated by transmission electron microscopy (TEM) JEOL 2010 EX microscope and scanning electron microscopy (SEM), performed with an acceleration voltage of 200 kV by placing the powder on a copper grid and X-ray diffraction (XRD) (Bruker Germany D8 Advance) with Cu Kα radiation (λ = 1.5418 Å). Fourier transformed infrared (FTIR) spectroscopy was performed on a Thermo Nicolet 200 FTIR spectrometer using the KBr wafer technique. Electrochemical signals were measured with a CHI 6005D electrochemical workstation (Austin, USA) using a GC working electrode (0.07 cm²), an Ag/AgCl (3 M KCl) reference electrode and a platinum wire auxiliary electrode. The prepared phosphate buffer solution (PBS) (pH 7.2) was purged with nitrogen gas for 10 minutes prior to all the electrochemical experiments. All the measurements were carried out in PBS under nitrogen atmosphere at room temperature.

2.2 Fe₂O₃/MWCNT composite preparation

α-Fe₂O₃ nanodendrites decorated MWCNT with desired concentration were synthesized by a facile and environmental friendly hydrothermal method. In a typical synthesis, 0.02 M concentration of potassium ferricyanide K₃[Fe(CN)₆] 98.5% and 5 mg hexadecyl trimethyl ammonium bromide were dissolved in 80 mL of DD water. After magnetic stirring for until completely dissolved, the solution was transferred to a Teflon lined stainless steel autoclave (100 mL capacity) and then kept at 180 °C for 24 h, after cooling naturally, the collected precipitate was harvested by centrifugation and washed thoroughly with water and ethanol before being dried at 100 °C for 12 h.

As the oxygen functionalities on the surface of MWCNT improve the electrochemical behavior, acid wash of MWCNT was done. The acid wash of MWCNT was refluxed in a 1 : 1 mixture of 98 wt% H₂SO₄ and 78 wt% HNO₃ for 24 h at 60 °C. The functionalization process allows the oxidation of the graphitic sp² carbon into –COOH and –OH groups on the side walls of the nanotubes, thus introducing negative charges on their surface, which in turn will be easily ready to interact with doping material for composite preparation. The acid-treated MWCNT washed thoroughly with DD water and sonicated for 30 minutes. The diameter of the MWCNT is given from the manufacturer of the company as 40 nm. Different ratio of α-Fe₂O₃ and MWCNT were taken to study the optimum mechanism for sensor applications. From Fig. S1 and S2,† It was observed that the composite of α-Fe₂O₃ and MWCNT in the ratio 4 : 1 showing maximum current in the [Fe(CN)₆]^3−/4− solution and also in the 1 mM L-dopa solution in PBS (pH 7.2). Hence 1.25 mg of MWCNT in water was then mixed with 5 mg of red-hued nanostructured α-Fe₂O₃ powder synthesized by the hydrothermal process and allowed magnetic stirring for 30 minutes at ambient temperature to ensure the binding of MWCNT with α-Fe₂O_3.

2.3 Fabrication of GCE modified with α-Fe₂O₃/MWCNT composite

Prior to surface modification, the bare GCE was polished with gamma alumina suspensions 1.0, 0.3 and 0.5 μM respectively. After that, the electrode was successively washed in ethanol and DD water for 15 minutes by ultrasonic method. The as prepared α-Fe₂O₃ and MWCNT composite was dispersed in DD water (1 mL) and drop casted (10 μL) onto the GCE and air dried at room temperature for 1 h. The merely blend of α-Fe₂O₃ and functionalized MWCNT are attached due to the negative charges of MWCNT and positive charges of α-Fe₂O₃. This composite transfers the maximum electrons in the [Fe(CN)₆]^3−/4− solution confirming attached platform's strong affinity and exhibits maximum current. In electrochemical behavior of L-dopa study, the composite has drastically decreased the oxidation potential to 0.19 V from 0.40 V (oxidation potential of α-Fe₂O₃) owing to chemical bond between negative charges of MWCNT and positive charges of α-Fe₂O₃. These evidences strongly confirm the chemical bonding between MWCNT and α-Fe₂O₃.

3. Results and discussion

3.1 Materials characterization

The morphology of the nanostructured α-Fe₂O₃, MWCNT, α-Fe₂O₃/MWCNT was characterized by SEM as shown in Fig. 1. The surfactant assisted α-Fe₂O₃ exhibits uniform dendritic shaped nanostructures with hierarchical arrangement of a well-defined main trunk, branches and sub-branches as indicated in Fig. 1(A). In fact, the branch and sub-branch structures itself is yet another dendritic structure but of a smaller size. The sub-branch is of a spindle shape and is the fundamental building block of the entire special shape. Fig. 1(B) shows the individual MWCNT has smooth surface, typical tube morphology and randomly align to form a network structure, which is beneficial for accommodating the volume expansion and facilitates the electron transfer mechanism. As can be observed in Fig. 1(C), broken branches are seen in the dendritic structures after blending with MWCNT and MWCNT found hidden in the dendrites.

Fig. 1 SEM images of (A) Fe₂O₃ nanodendrites (B) MWCNT (C) Fe₂O₃/MWCNT nanocomposite and (D) XRD patterns of (a) pristine α-Fe₂O₃ (b) MWCNT, (c) α-Fe₂O₃/MWCNT nanocomposite.

The morphologies and structural properties of α-Fe₂O₃ and α-Fe₂O₃/MWCNT composite were also characterized under TEM (Fig. 2A and B), showing the α-Fe₂O₃ dendritic structure in strict contact with the carbon nanotube mats and creating a favorable electrical contact between the composite layer and the electrodes. This favors the mutual dispersion of the inorganic and organic components eliminating the formation of aggregates. The configuration of this hybrid composite is particularly advantageous for sensing applications.

Fig. 2 TEM images of (a) Fe₂O₃ nanodendrites (b) Fe₂O₃/MWCNT nanocomposite.

Fig. 1D(a) shows powder XRD pattern of the micro pin dendrites of pure α-Fe₂O₃ which has the rhombohedral structure with the characteristic peaks at 2θ of 33.3, 35.6, 49.5, 54.1, 62.4 and 64.0 are indexed as (104), (110), (024), (116), (214) and (300) depicting high purity and crystallinity. It is noted from the JCPDS cards no: 33-0664 that well matched for iron oxide and no other hydroxide, maghemite, magnetite peaks are observed exhibiting the hematite phase.

A weak broadening diffraction peak at 26.3 is well indexed as the (002) reflection of graphite (JCPDF 75-2078) as shown in Fig. 1D (curve b) depicting the MWCNT presence. From the diffraction pattern observed for the α-Fe₂O₃/MWCNT nanocomposite, the blend formation is confirmed (Fig. 1D (curve c)). This is also supported by FTIR studies (Fig. S3†).

From Debye–Scherrer equation

d = Kλ/cos θ

the size of α-Fe₂O₃ nanodendrites and was estimated as 57 nm.

3.2 Cyclic voltammetric (CV) studies of the modified electrode

Fig. S4† shows the effect of scan rate on peak currents (10–100 mV s⁻¹) for α-Fe₂O₃ nanodendrites, MWCNT, α-Fe₂O₃/MWCNT nanocomposite modified electrodes in 0.1 M KCl solution containing [Fe(CN)₆]^3−/4− (1 mM). The oxidation peak current varied linearly with the scan rate as shown inset Fig. S2† and the oxidation peak potential shift to more positive potential with increasing scan rate, confirm the kinetic limitation in the electrochemical reaction. Also, a plot of peak current versus the square root of scan rate was found to be linear in the range of 10–100 mV s⁻¹, suggesting that, at sufficient over potential, the process is diffusion rather than surface controlled.

The CV of the bare, α-Fe₂O₃, MWCNT and α-Fe₂O₃/MWCNT nanocomposite modified electrodes recorded in the presence of 1 mM [Fe(CN)₆]^3−/4− in 0.1 M KCl at a scan rate 50 mV s⁻¹ are shown in Fig. 3. As shown in figure at the bare GCE, a pair of well defined redox peak is observed with a peak to peak separation of 65 mV (ΔE_p = (E_pa − E_pc)). Deposition of the α-Fe₂O₃ onto the electrode surface decreases the reversibility of [Fe(CN)₆]^3−/4− (i_pa: 1.50 × 10⁻⁵ A and ΔE_p: 140 mV) due to the barrier properties of Fe₂O₃ film, whereas MWCNT deposited electrode (curve c) increases the reversibility to produce i_pa: 2.73 × 10⁻⁵ A. Interestingly, the decoration of MWCNT onto the α-Fe₂O₃ remarkably enhances the reversible nature of [Fe(CN)₆]^3−/4− (i_pa: 4.89 × 10⁻⁵ A and ΔE_p: 35 mV), due to the electron transfer kinetics towards [Fe(CN)₆]^3−/4−]. This is due to the fact that –OH and –COOH groups on the side walls of the MWCNT introducing negative charges on their surface, which in turn interacts with Fe³⁺ ions at the oxidation potential 0.22 V.⁴⁰ The α-Fe₂O₃ modified GCE showed larger ΔE_p value compared to that of the other modified electrodes, the larger peak separation was assumed resulting lower electrical conductivity while in the α-Fe₂O₃/MWCNT modified GCE depicted smaller value exhibiting maximum conductivity.

Fig. 3 (A) CV and (B) EIS behavior of (a) bare (b) α-Fe₂O₃ (c) MWCNT and (d) α-Fe₂O₃/MWCNT nanocomposite modified electrodes recorded in the presence of 1 mM [Fe(CN)₆]^−3/4 in 0.1 M KCl.

3.3 Impedimetric measurements of the modified electrodes

Although impedance spectroscopy of electrodes is not a new method, it exhibits very useful information about the electrochemical process. EIS (electrochemical impedance spectroscopy) behavior of the modified GC electrode measured by impedance in the frequency region from 100 kHz to 1 Hz and the DC potential 250 mV and AC potential ±5 mV in the presence of (1 mM) [Fe(CN)₆]^3−/4− in 0.1 M KCl. The value of charge transfer resistances (R_CT) was determined from the Randle's equivalent circuit. As it can be seen in Fig. 3(B), the MWCNT and α-Fe₂O₃/MWCNT modified electrodes showed nearly straight lines (curves-b and d) implying that a diffusion-limited electrochemical behavior of the redox couple at the electrode surface, while the diameter of the high frequency semicircle obviously for α-Fe₂O₃ modified electrode was resulted due to the increase in electron transfer resistance suggesting that a significant blocking of [Fe(CN)₆]^3−/4− in the redox reaction (curve c). The R_CT value for bare GC, MWCNT, α-Fe₂O₃, and α-Fe₂O₃/MWCNT nanocomposite modified electrodes have been estimated as 118, 60, 2663, and 1923 Ω respectively. These results are consistent with the reported literature.⁴¹ Impedance plots obtained were in good agreement with the peak current values obtained from CV measurements (Fig. 3(A)). Hence, we have used α-Fe₂O₃/MWCNT modified GC electrode for further investigations.

3.4 Electrochemical oxidation of L-dopa

The electrochemical behavior of 1 mM L-dopa is investigated by cyclic voltammetry (Fig. 4A) at bare GC (curve a), α-Fe₂O₃ (curve b), MWCNT (curve c) and α-Fe₂O₃/MWCNT (curve d) electrode, respectively. As can be seen in Fig. 4(A), a well-fined oxidation peak for 1 mM L-dopa with a large peak current 8.06 × 10⁻⁵ A response of α-Fe₂O₃/MWCNT electrode appears at +0.19 V (curve d), and the oxidation of L-dopa at the MWCNT electrode exhibits an enhanced current response of 6.55 × 10⁻⁵ A at the same potential (curve c). However, the oxidation of L-dopa at the bare GC and α-Fe₂O₃ coated GC electrode require a relatively large over potential at 0.30 V and 0.40 V, respectively (curve a and b), and the current response of L-dopa on bare GC electrode is higher than that of α-Fe₂O₃ modified electrode. In addition, at α-Fe₂O₃/MWCNT GC electrode, the oxidation peak current response is almost 3-fold as large as that at the bare GC electrode. Thus MWCNT-modified α-Fe₂O₃ shows a good redox behavior towards L-dopa (as seen in Fig. 4(A)), indicating that the MWCNT-modified GCE have effectively decreased the oxidation potential of L-dopa to 0.19 V. The variation in peak responses may be ascribed to the variation in the morphology of the MWCNT on the hematite surface. For further investigations and use as a sensor to detect L-dopa, we chose α-Fe₂O₃/MWCNT as the electrode material and, in order to conduct control experiments, we used the rest of the electrode materials. The promotion of charge transfer at L-dopa the α-Fe₂O₃/MWCNT modified electrode was much smoother possibly on account of a lower resistance. As evident from Fig. 4(A), the peak currents increased upon modification. L-dopa showed maximum current sensitivity at α-Fe₂O₃/MWCNT modified electrode. Enhancement in peak current is attributed to combined effect of MWCNTs and α-Fe₂O₃ film. The presence of MWCNTs promotes electron transfer at α-Fe₂O₃/MWCNT modified electrodes. Since L-dopa which is being positively charged at pH 7.2 gets attracted towards negatively charged composite film and results in enhancement of peak current.

Fig. 4 (A) CV behavior of (a) bare electrode (b) Iron oxide (c) MWCNT (d) α-Fe₂O₃/MWCNT in PBS solution containing 1 mm of L-dopa (B) CV obtained for 1 mm of L-dopa in (pH 7.2) buffer solution with different scan rate of α-Fe₂O₃/MWCNT modified electrode (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, (f) 60, (g) 70, (h) 80, (i) 90 and (j) 100 mV s⁻¹. (b) Inset: oxidation current vs. square root of scan rate.

The effect of scan rate on the oxidation of L-dopa is shown in Fig. 4(B). It can be noted that the L-dopa peak current increases with increase in scan rate. A good linearity between the anodic peak current and the square root of the scan rate with a correlation coefficient of 0.984 was obtained within the range of 10–100 mV s⁻¹, suggesting that the electrochemical oxidation of L-dopa a diffusion controlled process.

3.5 Linear sweep voltammogram

Fig. 5 shows the linear sweep voltammogram (LSV) obtained for L-dopa sensing in the concentration range of 50–500 μM at the nanodendrites α-Fe₂O₃/MWCNT modified electrode. The oxidation current linearly increases with increasing concentration of L-dopa towards a slight negative potential shift upon each increment of 50 μM with a correlation coefficient of 0.990.

Fig. 5 LSV obtained for L-dopa in the concentration ranging from 50 to 500 μm L-dopa was added in steps of 50 μm each at the nanodendrites α-Fe₂O₃/MWCNT modified electrode in PB (pH 7.2) solution.

3.6 DPV measurements

DPV is a pulse technique having much higher sensitivity than cyclic voltammetry while detecting lower concentration of analyte. Hence, α-Fe₂O₃/MWCNT modified electrode has been chosen to examine the sensitivity in PBS (pH 7.2) for different concentration of L-dopa. In Fig. 6 curves show the well-defined and stable anodic oxidation peak current curves for L-dopa. Clearly, the DPV anodic peak current increases linearly with L-dopa concentration from 5.0 × 10⁻⁸ to 3.8 × 10⁻⁶ M with correlation coefficient 0.998 and the lower detection limit found to be 30 nM (3σ). The present α-Fe₂O₃/MWCNT modified electrode has been compared with other modified electrodes to examine its superiority and the results are shown in Table 1.

Fig. 6 DPV of the α-Fe₂O₃/MWCNT modified GC electrode in PB (pH 7.2) solution with various concentration of L-dopa (a–n) 0, 5.00 × 10⁻⁸, 1.00 × 10⁻⁷, 2.00 × 10⁻⁷, 4.00 × 10⁻⁷, 7.00 × 10⁻⁷, 9.00 × 10 ⁻⁷, 1.20 × 10⁻⁶, 2.00 × 10⁻⁶, 2.40 × 10⁻⁶, 2.80 × 10⁻⁶, 3.00 × 10⁻⁶, 3.30 × 10⁻⁶ and 3.80 × 10⁻⁶ M; scan rate V s⁻¹; amplitude V and step potential V; Inset shows the resulting calibration plot.

Table 1 Performance of the α-Fe₂O₃/MWCNT based L-dopa sensor in comparison with reported sensors

Electrode	pH	Linear range (M)	Detection limit (M)	Ref.
AuNP–CNT–PGE	7.0	1.5 × 10^−8–0.1 × 10^−6	50 × 10^−9	31
SWCNT/GC	5.0	5.0 × 10^−7–2.0 × 10^−5	3.0 × 10^−7	32
p-NiTAPc–GCE	4.0	1.0 × 10^−7–7.0 × 10^−7	10 × 10^−8	33
Ru-red/NaY/CPE	4.8	1.2 × 10^−4–1.0 × 10^−2	8.5 × 10^−5	34
GNP–MEA–NIHCF	7.0	8.2 × 10^−7–2.5 × 10^−3	53 × 10^−8	35
MCPE	4.0	2.6 × 10^−4–1.2 × 10^−3	2.5 × 10^−5	36
Oxavandium salen	6.0	1.0 × 10^−6–1.0 × 10^−4	80 × 10^−8	37
p-NiIITAPcc	4.0	1.0 × 10^−7–7.0 × 10^−7	1.0 × 10^−7	38
PEDOT–SWCNT	7.0	2.0 × 10^−5–1.0 × 10^−1	10 × 10^−8	39
PEDOT/SWNT/Pt	7.4	1.0 × 10^−7–2.0 × 10^−5	1.0 × 10^−8	29
α-Fe2O3/MWCNT	7.2	5.0 × 10^−8–3.8 × 10^−6	30 × 10^−9	This work

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3.7 Selective determination of L-dopa in the presence of interfering compounds using α-Fe₂O₃/MWCNT modified electrode

We have prepared the possible interfering foreign compounds for the accurate determination of L-dopa (Fig. 7). The oxidation peak current did not change, even in the presence of 10-fold excess of physiological interfering compounds such as urea, glucose and ascorbic acid (1 mM). These physiological interfering compounds got oxidized above the applied potential of 0.25 V at the α-Fe₂O₃/MWCNT modified electrode. Thus, α-Fe₂O₃/MWCNT modified film was highly selective towards L-dopa from the aforementioned interfering compounds.

Fig. 7 Amperometric i–t curve for the addition of 0.15 mM of L-dopa curves (a and b) and 1 mM of urea, glucose and ascorbic acid (c–e) and final addition of 0.15 mM of L-dopa (f) at α-Fe₂O₃ decorated MWCNT modified electrode in PBS (pH 7.2). Its applied potential = +0.50 V.

3.8 The stability and reproducibility of the α-Fe₂O₃/MWCNT modified electrode

Stability and reproducibility are the two important factors which influence the analytical performance of modified electrodes. In order to investigate the reproducibility of the present results, six different GC electrodes were modified with α-Fe₂O₃/MWCNT and their response towards the oxidation of 5.0 × 10⁻⁶ M L-dopa was tested. The peak currents obtained in the measurements of five independent electrodes showed a relative standard deviation (RSD) of 1.5%, indicating that the modified electrode's remarkable reproducibility. After DPV experiments, the electrode was kept in a PBS at room temperature and the current responses decreased about 3% in one month. These results clearly confirm that the chemical attachment between the negative charges of MWCNT and positive charges of α-Fe₂O₃ is so strong, stable and reproducible.

3.9 Real sample analysis

In order to test the practical applicability of the proposed electrode, the analysis of tablet of L-dopa containing carbidopa (Syndopa) using the standard addition method was carried out. The obtained results are presented in ESI Table 1.† The tablets include both L-dopa and carbidopa. The electro-oxidation of L-dopa is nearly 0.19 V but the electro-oxidation of carbidopa around 0.5 V. Therefore the carbidopa has not any interference to L-dopa. The RSD of each sample for three time's parallel detections replica is less than 1.08% confirming that the proposed sensor is reliable for L-dopa determination in pharmaceutical preparations.

4. Conclusion

The electrochemically fabricated α-Fe₂O₃/MWCNT nanocomposite demonstrated a novel electron transfer with a significant enlargement in peak current and a great decrease in peak potential. The proposed method was applied for the detection of L-dopa in pharmaceutical preparations. The α-Fe₂O₃/MWCNT platform gives a fast response, good linearity range, low detection limit with satisfactory stability, repeatability, reproducibility and good potential applications toward the determination of L-dopa. Further, selective determination of L-dopa was also achieved by this proposed sensor in the presence of common physiological interferents such as urea, glucose and ascorbic acid.

Acknowledgements

Wilson would like to acknowledge Alagappa University for unstinted support to execute the work and Ahmad Umar would like to acknowledge the support of the Ministry of Higher Education, Kingdom of Saudi Arabia through a grant (PCSED-13-22) under the Promising Centre for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01451k

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