Facile, low-temperature synthesis of tungsten carbide (WC) flakes for the sensitive and selective electrocatalytic detection of dopamine in biological samples

Abstract

Transition metal carbides have shown a promising potential for use in electrochemical applications due to their excellent electronic conductivity, stability and electrocatalysis. Herein, we described a facile, low-temperature method for synthesizing tungsten carbide (WC) used as an electrode modifier to fabricate a highly active electrochemical sensor for the determination of dopamine (DA). The as-synthesized WC was characterized by transmission electron microscopy, elemental mapping, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. TEM results reveal that WC appeared as a flake-like structure with an average particle size of ≈250 nM. Furthermore, WC/GCE shows much lower impedance and higher heterogeneous electron transfer rate than bare GCE, indicating the immobilization of WC that accelerates ionic conductivity of the GCE. Owing to its higher electron transfer rate and excellent ionic conductivity, WC/GCE exhibits a tremendous electrochemical sensing performance for the detection of DA. Under optimized conditions, the working range and limit of detection of DA at WC/GCE were estimated to be 0.05 to 700 μM and 14 nM, respectively. The proposed WC/GCE showed good selectivity, stability and reproducibility, and satisfactory recoveries were found in real sample analysis.

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Introduction

Neurotransmitters are the chemical messengers, regulating human brain function and human activities; variations in the metabolic transformation and biosynthesis lead to neurodegenerative and psychiatric disorders (depression and Parkinson's disease).¹ Dopamine (DA) is one of the excitatory catecholamine neurotransmitters widely distributed in mammalian central nervous system, serving as an antecedent of adrenaline and noradrenaline and helps to maintain hormonal balance; moreover, it plays a significant role in functional activities, such as attention span, emotions and cognition as well as other movements, of the body. Abnormal concentrations of DA in the human body may be associated with many disorders such as Huntington's disease, Alzheimer's disease, schizophrenia, senile dementia, euphoria and HIV infection.² Hence, the determination of DA in vivo/vitro is quite necessary for the diagnostic and biomedical research, and there is an urgent need for reliable, sensitive and selective efforts to determine DA. Till date, numerous strategies such as high-performance liquid chromatography,³ fluorescence studies,⁴ capillary electrophoresis,⁵ spectrophotometry,⁶ and electrochemical methods have been proposed for the detection of DA.⁷ Even though these techniques can offer low detection limits and good selectivity, they are often expensive, require sophisticated instrumentation, and are time consuming. Compared with other described methods, the electrochemical techniques are rapid, low-cost, easy to operate and sensitive.^8–10 Owing to the redox property and high electroactive behavior of DA, the electrochemical methods appear to be a suitable technique for its quantitative determination.¹¹ The major problem in the electrochemical detection of DA is the presence of interferences, such as AA and UA, which are present in high concentrations in biological fluids and possess close oxidation potential to DA.^10,12 In this context, due to the poor electrocatalytic property of conventional bare electrodes unable to selectively detect DA and the formation of oxidation products at the electrode surface, thus can affect the reproducibility of the electrode. To resolve these difficulties, numerous modified electrodes, such as carbon-based nano materials,¹³ conducting polymers,¹⁴ noble metals and metal oxides, have been utilized as an electrocatalyst for the quantitative detection of DA.^15–17 However, all these materials suffer some limitations with respect to poor controllability, sensitivity, complicated material preparation process and expensiveness. Thus, it is highly necessary to develop a profitable electrocatalyst that is not only selective, sensitive and reliable, but also easy to operate and economical to achieve convincing results in pharmacological and biological fields.

Very recently, non-noble metal material-based carbides,^18,19 nitrides,²⁰ sulfides,²¹ and phosphides²² have been explored as better metal catalysts for various electrochemical applications. There are few studies reported on the use of metal sulfides and nitrides as electrode modifiers for the electrochemical sensing applications. For example, Liu et al. proposed a novel electrochemical sensor-based FeS/RGO nanocomposite for the simultaneous determination of dopamine and acetaminophen,²³ Yin et al. reported Ni₃N as an efficient catalyst electrode for sensing non-enzymatic glucose and hydrogen peroxide,²⁴ Haldorai et al. proposed TiN/RGO nanocomposites for the electrochemical determination of dopamine,²⁵ and Xie et al. synthesized metallic NiN nanosheets for sensitive and selective non-enzymatic glucose sensing application.²⁶

Among them, transition metal carbides (TMCs), due to their excellent physical properties, electronic conductivity and chemical stability, have emerged as a new type of electrocatalyst for electrochemical and photochemical processes. In addition, TMCs show remarkable catalytic performances, due to their electronic and catalytic behaviour similar to Pt-group metals (by inducing carbon to form metal lattices) and unique d-band structures,²⁷ making them the most promising candidates as reducing agents or substitutes to Pt in catalytic reactions. Normally, TMCs (Ti, V, Mo, Ta, and W) are more abundant and less expensive than Pt-group metals (Ru, Rh, Pd, Ir, and Pt), and their electrochemical properties can vary depending on the current density, electrochemical potential as well as pH environment and used in various electrochemical applications such as capacitors,²⁸ hydrogen and oxygen evolution reaction (OER & HER) and photochemical or electrochemical methods.^18,29–31 To conclude, TMCs are evolving as auspicious electrocatalysts than metal oxides, alloys and carbon materials. However, there is no report yet available on the use of metal carbides for electrochemical sensing applications.

Furthermore, the group VI transition metal carbides, such as molybdenum (Mo) and tungsten (W), are of specific interest in electrocatalysts.³² Typically, tungsten carbide (WC) is one of the non-toxic and low-cost electrocatalysts, and this electronic state density closely resembles that of Pt.³³ There are very few reports on the use of WC as an electrode material based on electrochemical applications. For example, Morishita et al. developed carbon-coated tungsten and molybdenum carbides for use as electrochemical capacitors.²⁸ Meyer et al. synthesized transition metal carbides, such as WC, Mo₂C, TaC and NbC, which were used as electrocatalysts in HER,³⁰ Xu et al. synthesized tungsten carbide nanoparticles for efficient electrocatalytic hydrogen evolution,³⁴ and Liu et al. proposed WC/carbon composites for hydrogen evolution reactions.²⁹ So far, no attempt has been made to use WC for the electrochemical detection of DA.

In the view of the above-mentioned facts, we herein report a facile method to synthesise WC nanoparticles at low temperatures, and their application as a novel electrode modifier for the electrochemical determination of DA. The electrocatalytic performances of the as-prepared WC/GCE were determined. As expected, it was found that the portable sensor exhibited superior catalytic activity toward the oxidation of DA with a wide linear range and low detection limit. The excellent sensitivity and selectivity were attributed to the unique physicochemical properties, such as high heterogeneous electron transfer rate and desirable ionic conductivity by the electrocatalyst. In addition, the sensor was successfully applied to determine the DA concentration in human serum and urine samples with acceptable recoveries. The synthesis of WC nanoparticles and the electrochemical detection of DA are shown in Scheme 1.

Scheme 1 Pictorial illustration of the present study.

Experimental section

Materials and methods

Disodium hydrogen phosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), tungsten hexacarbonyl (W(CO)₆), ethylene glycol (EG) and oleylamine (70%) were purchased from Sigma-Aldrich. All commercial chemicals and reagents were of analytical grade and used without further purification. For the purpose of electrochemical experiments, 0.05 M phosphate buffer solution (PBS) was used as the supporting electrolyte, which was prepared by mixing two stock solutions of Na₂HPO₄ and NaH₂PO₄. Deionized water (DIW) was used to prepare all the required solutions. All the electro-chemical experiments were performed using cyclic voltammetry (CV) and amperometry techniques on CHI 1205C workstation (CH Instruments Company, made in the USA). The conventional three-electrode system was used including a glassy carbon electrode (GCE area: 0.071 cm²) and a rotating disk electrode (RDE; area: 0.2 cm² (rpm = 1400)) serving as the working electrode, and saturated Ag/AgCl and a platinum wire used as the reference and auxiliary electrodes, respectively.

The surface morphology and the formation of WC nanoparticles were determined using transmission electron microscopy (TEM, JEOL 2100F) at an accelerating voltage of 200 kV. The crystal structure of WC NPs was scrutinized using an X-ray diffractometer (XRD, XPERT-3 diffractometer with Cu-Kα; K = 1.54 Å). The electronic states of W and C were established by X-ray photoelectron spectroscopy (XPS) using a Thermo scientific Multilab 2000. The interfacial electrocatalytic performance of the modified electrode was examined using electrochemical impedance spectroscopy (EIS, IM6ex ZAHNER impedance measurement system) within a frequency range from 0.01 to 100 kHz at a potential of 0.2 V (AC potential: 5 mV).

Synthesis of WC and preparation of modified electrode

To synthesise the WC, a mixture of EG (10 mL) and oleylamine (5 mL) was degassed with N₂ gas for 5 min at room temperature in a three-necked flask. Then, 250 mg W(CO)₆ was added to the mixture of EG and oleylamine, and the reaction mixture was vigorously stirred at 100 °C in a N₂ atmosphere for 2 h. Then, the solution was allowed to cool at room temperature. Subsequently, the solution was centrifuged and washed several times with water and ethanol and dried in a vacuum oven at 80 °C for 12 h. To receive WC, the crude product was carbonized at 800 °C for 2 h under N₂ flow. Finally, the desired WC was prepared and it was used for further applications. Prior to fabrication, the WC dispersion was prepared by adding 2 mg WC in 1 mL DI water, and then sonicated for 30 min. Before that, the glassy carbon electrode (GCE) was polished with 0.05 mm alumina powder and washed with DI water and ethanol to get a mirror-like surface, 6 μL of the WC dispersion was drop-cast on the pre-cleaned GCE surface and dried in a hot air oven, and then the modified electrode was used for further electrochemical studies.

Results and discussion

Surface characterization

The surface morphology of the WC catalyst was envisioned using transmission electron microscopy (TEM). Fig. 1A & B exhibits the low-magnification TEM images of WC, revealing that the particles are generally flake-like structures with an average particle size of around 250 nm. Further, the high-magnification TEM image (Fig. 1C) depicts the ultrafine nature of nanoparticles with leathery surface. The HRTEM (Fig. 1D) image of WC shows well-arranged lattice fringes with ‘d’ spacing of 0.283, 0.251 and 0.188 nm, which are consistent to the (001), (100) and (101) planes of hexagonal WC phases, respectively. Then, the selected-area electron diffraction (SAED) (Fig. 1E) patterns were also validated, which are also consistent with the above-mentioned results. To further confirm the chemical composition of WC nanoparticles, the EDX analysis was performed (Fig. 1F), which clearly indicates the presence of W and C in WC nanoparticles without any other elemental peaks. Furthermore, the corresponding elemental mapping images are shown in Fig. 2B–D, which confirms the presence and homogeneous distribution of W (green) (Fig. 2B) and C (red) (Fig. 2C) in WC Mix (Fig. 2D).

Fig. 1 TEM images (A–C), HRTEM (D), SAED pattern (E), and EDX (F) of WC.

Fig. 2 STEM image of WC (A), and elemental mapping of W (B), C (C), and mixed (D).

The crystallographic attributes of WC were investigated by the X-ray diffraction (XRD) studies (Fig. 3A). The XRD spectrum of WC exhibits the diffraction peaks at 31.5° (001), 35.6° (100), 48.3° (101), 64° (110), 65.7° (002), 73.1° (111), 75.5° (200), 77.12° (102), and 84° (201) consistent with the crystal planes of hexagonal WC (diamond; JCPDS 073-0471). The spectrum validates that there are no more diffraction peaks observed for the possible impurities, which confirms the high-phase crystallographic purity of the as-prepared WC nanoflakes. Subsequent to the elemental analysis, the as-synthesized WC nanoparticles were subjected to X-ray photo electron microscopy (XPS) to explore the electronic properties of the corresponding elements. The XPS spectra of WC are displayed in Fig. 3B, confirming that the WC nanoparticles comprises W and C. Employing a Gaussian fitting, the high-resolution spectrum of W (Fig. 3C) was fitted with two major peaks, W 4f_7/2 and W 4f_5/2 at 33.8 and 36.0 eV, respectively, which confirms the formation of WC. In addition, the two small intense signals are obtained at 37.6 (W 4f_7/2) and 39.8 eV (W 4f_5/2), which are assigned to the W 4f orbits of surface WOx passivation layer.³⁵

Fig. 3 (A) XRD of WC, (B) XPS survey and (C) W 4f spectrum of WC.

Electrochemical behavior of different modified electrodes

In order to evaluate the electrical and interfacial properties of modified electrodes, the electrochemical impedance spectroscopy (EIS) was performed. EIS is an effective non-destructive tool to examine the electrochemical composition of modified electrodes. The impedance measurements were recorded for bare GCE and WC/GCE in 0.1 M KCl solution, containing 5 mM Fe(CN)₆^3−/4− (1 : 1). A typical Randles circuit (inset of Fig. 4A) was used to fit the impedance spectrum, where R_ct, R_s, W, and C_dl are the charge transfer resistance, resistance of solution, Warburg constant, and double-layer capacitance, respectively. The Nyquist plot of the EIS spectrum, a linear part at lower frequencies and semicircle portion at higher frequencies, represents the diffusion limited process and electron transfer-limiting step, revealed in Fig. 4A. The semicircle diameter corresponds to the charge transfer resistance of the relevant electrodes. As can be observed from Fig. 4A, the bare GCE (curve a) exhibits the higher charge transfer resistance with an R_ct value of 337 Ω. However, the very low impedance observed for the WC/GCE (curve b) and the resultant charge transfer resistance (R_ct = 175 Ω) imply fast electron transfer kinetics over the WC-modified electrode surface, which favors an ease diffusion of Fe(CN)₆^3−/4−. These results indicated that WC/GCE exhibits excellent ionic conductivity and electron transfer ability than the bare GCE, which is an excellent pathway to an electrochemical sensing application.

Fig. 4 (A) EIS of bare GCE (a), and WC/GCE (b). (B) CVs of bare GCE (a), WC/GCE (b) in 0.1 KCl solution containing 5 mM ferricyanide redox at a scan rate of 50 mV s⁻¹. (C) CVs of WC/GCE in 0.1 KCl solution containing 5 mM ferricyanide redox at different scan rates from 20 to 200 mV s⁻¹. (D) Plot between the anodic (I_pa) and cathodic (I_pc) current response and square root of the scan rate (ν^1/2).

Electrochemical behavior of 5 mM [Fe(CN)₆]^3−/4− in 0.1 M KCl was examined by CV at GCE (a), and WC/GCE (b) (Fig. 4B). As can be observed in Fig. 4B, the WC/GCE (I_pa = 123.8 μA, ΔE_p = 115 mV) displays higher redox current and lower peak separation values than the bare GCE (I_pa = 80.11 μA, ΔE_p = 135 mV), confirming that WC facilitates electron transfer in the [Fe(CN)₆]^3−/4− redox process. Further, the electroactive surface area of the modified electrodes was analyzed by CV. The CVs of WC-modified electrodes were performed in 0.1 M KCl, containing 5 mM Fe(CN)₆^3−/4− at different scan rates (20 to 200 mV s⁻¹) (Fig. 4C). The response current (I_pa and I_pc) has a linear relationship with the square root of the scan rates (ν^1/2) (Fig. 4D), demonstrating that the overall redox process of Fe(CN)₆^3−/4− is a characteristic diffusion-regulated practice. The Randles–Sevick equation (1) was used to calculate the electroactive surface area:³⁶

I_p = 2.69 × 10⁵AD^1/2n^3/2ν^1/2C (1)

where I_p, A, D, n, ν, and C represent the peak current, active surface area, diffusion coefficient of [Fe(CN)₆]^3−/4−, number of electron transfer (n = 1), scan rate (mV s⁻¹), and the concentration of [Fe(CN)₆]^3−/4−, respectively. By applying these values in eqn (1), the calculated active surface areas (A) of bare GCE and WC/GCE are about 0.078, and 0.113 cm², correspondingly. The large electroactive surface area of WC/GCE can provide massive reactive sites for the electrochemical sensing process.

In addition, the heterogeneous electron transfer rate constant (k_s) for different modified electrodes was investigated using eqn (2):³⁷

R_ct = RT/n²F²ACk_s (2)

where R (8.314 J mol⁻¹ K⁻¹) represents the gas constant, T (298 K) denotes the temperature, n is the number of electrons involved in the reaction, F (96 485 C mol⁻¹) is signified as the Faraday constant, A (cm²) is the electrode surface area and C is the concentration of required [Fe(CN)₆]^3−/4− electrolyte. The k_s value was calculated for bare GCE (1.58 × 10⁻⁷ cm s⁻¹) and WC/GCE (3.89 × 10⁻⁷ cm s⁻¹). The higher k_s value proves that the WC-modified electrode owns a quick electron transfer capability.

Under optimum conditions, the electrochemical behaviour of the as-prepared WC toward the oxidation of DA was studied via cyclic voltammetry (CV). Fig. 5A depicts the CVs obtained using GCE and WC/GCE with 200 μM in 0.05 M PBS (pH 7) at a scan rate of 50 mV s⁻¹. As can be observed from Fig. 5A, there is no observable redox peaks in the blank PBS (pH 7) for the bare GCE (curve a) and WC/GCE (curve b). Moreover, a pair of broad and asymmetric redox peaks were observed for bare GCE (curve a′), while adding DA into the PBS with a weak peak current. The observed ΔE_p = 0.14 V (E_pc = 0.13 and E_pa = 0.29 V) and the attained cathodic (I_pc) and anodic peak currents (I_pa) were 3.6 and 7 μA, demonstrating that the bare GCE does not contain any electro-active material on the surface. However, a pair of well-defined redox peaks with the ΔE_p = 0.03 V (E_pc = 0.19 and E_pa = 0.22 V) was observed for the WC/GCE (curve b′), producing a significant current response (I_pc = 16.06 μA and I_pa = 28.82 μA). The enhanced peak currents and lower anodic peak potentials of DA on WC/GCE may be ascribed to the high ionic conductivity, large surface area and excellent electron transfer kinetics. These results suggest that the WC/GCE has superior electrocatalytic activity toward the oxidation of DA. The mechanism for the electrochemical detection of DA can be expressed as follows:

Dopamine (DA) ⇌ o-Dopaminoquinone (DOQ) + 2H⁺ + 2e⁻

Fig. 5 (A) CVs for bare GCE and WC/GCE in absence (a′ and b′) and presence (a and b) of 200 μM DA in 0.05 M PBS (pH 7) at a sweep rate 50 mV s⁻¹. (B) Current response of WC/GCE for 200 μM DA at different pH (0.05 M PBS) solutions (3, 5, 7, 9, and 11). (C) CVs for WC/GCE in the presence of 200 μM DA in 0.05 M PBS (pH 7) at different sweep rates in the range of 20 to 200 mV s⁻¹. (D) Calibration plot between the scan rate and anodic (I_pa) and cathodic peak current (I_pc).

To study the effect of pH for the electrochemical detection of DA at WC/GCE, CV measurements were conducted for 200 μM DA in solutions with various pH values from 3 to 11 (0.05 M PBS) at a scan rate of 50 mV s⁻¹. As shown in Fig. 5B, the anodic current response (I_pa) of DA was increased upon increasing the pH value of the solution from 3 to 7. The I_pa of DA decreased when the pH of the solution was more than 7, which indicates that the DA has lost its proton. Hence, pH 7 was selected as optimum for the electrolyte solution for DA determination.

The electrocatalytic activity and the kinetic mechanism of DA on WC/GCE were studied via CV in 0.05 M PBS containing 200 μM DA at various sweep rates (20–200 mV s⁻¹). It is apparent from Fig. 5C that the redox peak currents of DA linearly improved with increasing scan rates and the anodic peak potentials slightly shift, suggesting the manifestation of a quasi-reversible reaction. Moreover, the calibration plot (Fig. 5D) between the redox peak currents (I_pa and I_pc) and the scan rate (ν) showed a linear relationship with the corresponding correlation coefficient of R² = 0.9932 (I_pa) and 0.9971 (I_pc), respectively. This implied that the electroactive redox process of DA at the WC electrode surface is a adsorption-controlled process.^10,38

Amperometric i–t study for DA at WC-modified electrode

The amperometric current–time curve observed at WC/GCE with different mass fractions of DA in 0.05 M PBS (pH 7) with constant stirring by a rotating disk electrode (RDE) were recorded to investigate the sensing behaviour of WC/GCE. The oxidation current responses of DA were examined at a constant working potential (0.2 V), and the respective results are displayed in Fig. 6A. The WC/GCE sensor signals rapidly in response to change in DA addition, and the maximum steady-state current is achieved within 2 s, owing to the rapid absorption and activation of DA on the WC electrode surface. As can be observed from Fig. 6A, the anodic peak current increased progressively with the consecutive addition of DA in the concentration range from 0.05 μM to 700 μM. The obtained anodic peak current was linearly correlated with the wide range of concentration (0.05 μM to 700 μM) of DA and manifested in the calibration plot of DA concentrations, and anodic currents were observed (Fig. 6B). The corresponding linear regression coefficient is about R² = 0.9989 and the lowest detection limit (LOD) was 14 nM as calculated from the signal-to-noise ratio of 3 (S/N = 3).³⁹ From these results, our linear range is not only applicable for common level of DA in both the human brain (between 0.01 and 1 μM) and the cerebrospinal fluid (between 0.5 and 25 nM), but also valid for severe diseases caused by abnormally high levels of DA. For example, 20–50 μM of DA reduces the neuronal NMB cell growth and causes cell damage. Even very high DA levels (100–300 μM) are neurotoxic and produce apoptosis.⁴⁰ Consequently, our proposed sensor was very useful to detect some nerve cell damage relating to the abnormality of DA. Further, to illuminate the advantage of the sensor, the analytical performances of WC/GCE were compared with other reported DA sensors (Table 1), which clearly explains that our proposed sensor shows good sensing performances than other modified electrodes with respect to not only the linear range but also the limit of detection, which could be ascribed to that the WC nanoparticle provides superior electrocatalytic activity and rapid electron transfer rate, resulting from the unique flake-like structure. In addition, the modified electrocatalyst in this study can be prepared rapidly with ease at low temperatures. Therefore, WC/GCE could serve as a potential pathway toward DA at trace levels in several bio-samples.

Fig. 6 (A) Amperometric current response of a WC-modified electrode at different concentrations of DA in a constantly stirred 0.05 M PBS (pH 7) solution at an applied potential of 0.2 V. (B) Calibration plot among the concentration of DA and amperometric current response. (C) Amperometric current response of WC-modified electrode in a constantly stirred 0.05 M PBS (pH 7) solution for consecutive addition of 200 μM DA and 10-fold concentrations of CA, UA, AA and PA. (D) Operational stability of the WC-modified electrode.

Table 1 Comparison of the analytical parameters of WC-modified electrodes with several reported DA sensors

Electrode	Technique	Linear range (μM)	LOD (μM)	Ref.
mMWCNT/SPE – magnetic multiwalled carbon nanotube/screen-printed electrode, PANI-polyaniline, CTAB – cetyl trimethylammonium bromide, rGO – reduced graphene oxide, pCNFs – electrospun nanoporous carbon nanofibers, PABSA – rMoS2/CPE-poly(m-aminobenzenesulfonic acid)-reduced MoS2/carbon paste electrode.
rGO/Co3O4/GCE	Amperometry	0–30	0.277	10
Ag–Pt/pCNFs/GCE	DPV	10–500	0.11	13
PABSA-rMoS2/CPE	DPV	1–1000	0.22	14
CeO2/Au-GCE	Amperometry	10–500	0.056	16
mMWCNT/SPE	SWV	5–180	0.43	40
ERGO/GCE	DPV	0.5–60	0.5	41
PANI/Au/nanoelectrode	DPV	0.3–200	0.1	42
CTAB/rGO/ZnS/GCE	DPV	1–500	0.5	43
N-Doped graphene	DPV	0.5–170	0.25	44
Au/polyaniline/GCE	Amperometry	3–115	0.8	45
WC/GCE	Amperometry	0.05–700	0.014	This work

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Anti-interference study

Selectivity and stability are the crucial parameters in electrochemical sensing applications. In order to study the anti-interference effect of the sensors, the WC-modified electrode was evaluated by injecting some possible potentially active species in the evenly stirred PBS (pH 7) with DA via amperometric i–t studies and the change in response current was noted. As it is well known that the anodic peak potentials of caffeic acid (CA), uric acid (UA), ascorbic acid (AA) and paracetamol (PA) are similar to that of DA; therefore, the selective determination of DA is quite difficult due to their overlapping signals. The amperometric i–t responses of WC-modified electrode in the presence of 200 μM DA and 10-fold concentrations of CA, UA, AA and PA in PBS (pH 7) can be observed in Fig. 6C. The response current obviously increased linearly after each addition of 200 μM DA. At the same time, the WC-modified GCE exhibited a negligible current response for the successive injection of aforesaid interfering compounds, and it does not interfere with the current response of DA. As shown in Fig. 6C, the i–t curves of the aforesaid coexisting compounds at the WC/GCE reveal a very weak signal. These results imply that our proposed WC/GCE sensor possesses good anti-interferents ability for the quantitative detection of DA and good tolerance to common biological interferents.

Stability, reproducibility and repeatability of the sensor

Further to investigate the long-term stability of the established sensor, amperometric technique was recorded at WC-modified electrode with 200 μM DA in 0.05 M PBS (pH 7) as far as 1700 s, shown in Fig. 6D. From this result, the current response of DA at WC-modified electrode retains 94.4% of its original current value over 1700 s, confirming that the good reliable performance of WC-modified electrode-based DA sensor. Then, to evaluate the reproducibility of the DA sensor on WC-modified electrode, it was scrutinized by measuring the oxidation current toward 200 μM DA in 0.05 M PBS (pH 7) with five different WC-modified electrodes prepared using the same protocol. No significant decrease was observed in the anodic peak current intensity of these five different electrodes with the relative standard deviation (RSD) of 3.4% for DA determination. To examine the repeatability of the sensor, seven parallel measurements were performed in 0.05 M PBS (pH 7) with 200 μM DA at the same electrode. The DA anodic peak current maintained around 96.9% of their initial current values with an RSD of about 1.12% for WC-modified electrodes. These results suggest that the proposed DA sensor on WC-modified electrode possessed good stability, reproducibility and repeatability, which is a suitable parameter for DA in real-time monitoring.

Real sample analysis

In order to verify the practicality of the sensor, the WC-modified electrode was used for the detection of DA in biological samples such as human serum and urine samples collected from healthy persons by the standard addition method. Before determination, the collected samples were diluted with 0.05 M PBS (pH 7), and then various amounts of known DA concentrations were injected into this appropriate solution. The amperometric i–t technique was performed, and the obtained recoveries of DA from human serum and urine samples are shown in Table 2. Therefore, the WC-modified electrode exhibited excellent applicability with suitable recovery (99.35–99.6% and 99–99.4) toward the determination of DA in biological samples.

Table 2 Detection of dopamine in biological samples like human blood serum and urine samples at WC-modified electrode

Sample	Spiked (μM)	Found (μM)	Recovery (%)	RSD (%) (n = 3)
Blood serum	10.0	9.95	99.5	0.1
	20.0	19.87	99.35	0.08
	30.0	29.9	99.6	0.11
Urine	10.0	9.94	99.4	0.05
	20.0	19.85	99.25	0.07
	30.0	29.7	99	0.1

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Conclusion

In conclusion, we have proposed a new type of electrochemical sensor based on tungsten carbide (WC), which was synthesized via a simple and facile method and applied as a successful electrocatalyst for the determination of dopamine (DA). The most significant advantages of this assay are that the as-synthesized WC modified electrode possessed good ionic conductivity, superior electrocatalytic activity and high heterogeneous rate constant. Such a novel proposed sensor exhibited rapid current response (2 s), lowest detection limit (14 nM) with a wide linear range (0.05 μM to 700 μM), and good working stability. Furthermore, it possessed good anti-interferents ability and good tolerance to common interfering compounds. The performances of the DA sensor were applied in biological samples (human serum and urine) and obtained good recoveries.

Conflicts of interest

Authors do not have a conflict of interest.

Acknowledgements

The authors thankfully acknowledge the financial support of the Ministry of Science and Technology, Taiwan through contract no. MOST 107-2113-M-027-005-MY3.

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