A WSe 2 @poly(3,4-ethylenedioxythiophene) nanocomposite-based electrochemical sensor for simultaneous detection of dopamine and uric acid

In the present work, a nanocomposite of two-dimensional WSe 2 nanosheets with poly-(3,4-ethylenedioxythiophene (WSe 2 @PEDOT) was prepared by facile hydrothermal method and characterized in terms of structural and morphological analyses. This nanocomposite was used to modify glassy carbon electrode for the construction of an electrochemical sensing platform for simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). It was found that the incorporation of PEDOT into WSe 2 nanosheets exhibited enhanced electrochemical behaviors and electro¬catalytic activity against DA and UA. Using differential pulse voltammetry (DPV) measurements, the WSe 2 @PEDOT modified electrode displayed wide linear detection ranges of 16 to 466 µM for DA and 20 to 582.5 µM for UA. The electrode also exhibited high selectivity against DA and UA in the presence of major interference of ascorbic acid and other interferent substances.


Introduction
Dopamine (DA) and uric acid (UA) are two important biological compounds that play important roles in human physiological functions [1].Abnormal concentrations of these molecules in biological fluids lead to many diseases, including schizophrenia, Alzheimer, Parkinson's disease, drug addiction, hyperuricemia, chronic renal sickness, etc. [2][3][4].Therefore, early analyses of these molecules are highly important to deal with the concerned conditions.Recently, electrochemical methods have become attractive for the determination of DA and UA [5,6].However, these two electroactive molecules have close oxidation potentials, which limits their selective detection in biological samples on traditional electrodes.Therefore, tremendous research efforts have been directed toward the development of novel electrodes based on various materials.Among these, two-dimensional (2D) nanomaterials such as graphene and its inorganic analogues have attracted significant interest in electrochemical applications because of their unique chemical and physical properties [7].For example, transition metal dichalcogenides (TMDs) have been widely investigated as electrode materials for electrochemical sensing platforms due to their high electrocatalytic properties, chemical stability, low cost and facile synthesis [8][9][10].WSe 2 , an important member of TMDs, has received special attention in electrochemical applications [11,12].However, WSe 2 tends to stack easily due to high interactions between its layers, which decrease the number of electroactive sites and electrochemical performance.Therefore, various strategies have been applied to improve the electrochemical performance to decrease the number of layers and enhance the number of active sites.Among the alternative strategies, the production of WSe 2 in the presence of conductive supports is an effective way to reduce the number of layers by preventing layer stacking.Conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) can be incorporated with TMD materials to prepare composite electrode materials [13,14].PEDOT possesses high conductivity and electrochemical stability and shows high potential to be applied to an electrode material [15].PEDOT-like supporting materials not only prevent the stacking of WSe 2 layers but also increase the active sites of WSe 2 .
In this work, WSe 2 @PEDOT composite was prepared via a hydrothermal method to enhance the electrochemical properties of WSe 2 .The as-prepared composite was coated on a glassy carbon electrode (GCE) for electrochemical detection of DA and UA.

Synthesis of WSe 2 @PEDOT
The preparation of WSe 2 @PEDOT was carried out as follows: first, 0.1 g of as-prepared PEDOT was ultrasonically dispersed in 30 mL of DMF for 60 min.Then, 0.32 g Se powder and 0.1 g NaBH 4 as a reducing agent were added and stirred for 60 min to obtain good dispersion.Later, 0.49 g H 2 WO 4 was slowly added to this dispersion and stirred for an additional 30 min.The final dispersion was transferred to a 45 mL Teflon-lined stainless steel autoclave reactor and heated at 200 °C for 12 h.After being cooled to room temperature, the product was filtered and washed with DW at least three times and finally dried at 60 °C.The obtained powder was annealed at 500 °C for 3 h under N 2 flow to enhance the crystallinity.
The synthesis of WSe 2 as control was the same as that of WSe 2 @PEDOT, except PEDOT was not added to the reaction mixture.

Electrode modification
Before coating, the cleaning process was applied to polish the GCE surface by using 0.3 and 0.05 µm alumina powder.GCE was then sonicated in DW and ethanol to remove any adsorbed particles on its surface.On the other hand, well-dispersed electrode ink was prepared by sonication of WSe 2 @PEDOT in DMF for 1 h to obtain a concentration of 5 mg mL -1 WSe 2 @PEDOT.Later, 10 μL from this ink was drop-coated on GCE and left to dry in an open atmosphere.Finally, WSe 2 @PEDOT coated-GCE was carefully rinsed with 0.1 M phosphate buffer solution (PBS) before experiments.
For the control, GCE was also modified with pristine WSe 2 in the same way described above for GCE/WSe 2 @PEDOT.

Electrochemical measurements
Electrochemical experiments were conducted via Ivium CompactStat model potentiostat.Threeelectrode configuration was used for electrochemical works, and in addition to GCE as a working electrode, Pt wire was used as a counter electrode and Ag/AgCl as a reference electrode.

Characterization
The crystal structure of the as-prepared WSe 2 @PEDOT was determined through X-ray diffraction (XRD) using Bruker/D8 Advance diffractometer.Morphological characterization was conducted using scanning electron microscopy (FEI Model: Quanta 400F) and transmission electron microscopy (FEI Tecnai G2 Spirit Biotwin model).

Materials characterization
WSe 2 @PEDOT composite was successfully obtained via hydrothermal reaction, as schematically represented in Figure 1.In this synthesis procedure, metallic Se particles were initially reduced by a reducing agent of NaBH 4 to produce Se 2-ions.The Se 2-ions were then reacted with W cations, forming WSe 2 nanosheets embedded in PEDOT polymeric structure.
The structure of the WSe 2 @PEDOT composite was evaluated by X-ray diffraction (XRD).Figure 2 shows the XRD patterns of the WSe 2 @PEDOT composite as well as pristine WSe 2 nanosheets.XRD pattern of WSe 2 displays major diffraction peaks (planes) at 13.3 o (002), 31.9 o (100), 37.The surface morphology of the WSe 2 @PEDOT composite was investigated using SEM and TEM.The SEM images (Figure 3a-b) of pristine WSe 2 and WSe 2 @PEDOT indicate that nanosheets incorporated flower-like structures were obtained by hydrothermal treatment.However, the WSe 2 @PEDOT possesses looser, dispersed and thinner nanosheets due to the incorporation of PEDOT.This difference is more obvious in the TEM images, as shown in Figure 3c-d.Thus, the resulting composite can have more active sites for analyte adsorption, leading to an improved electrochemical signal.
In order to determine the electrocatalytic activities of bare, WSe 2 and WSe 2 @PEDOT modified electrodes toward a binary mixture of DA and UA, the CV and DPV measurements were performed in 0.1 M PBS solution containing 0.19 mM DA and 0.28 mM UA. Figure 4c shows CVs of different electrodes performed in the potential range between -0.2 to +0.6 V, at 50 mV/s of scan rate.It is clearly seen that the current response of bare GCE is very low, which does not provide a sensitive sensing system for the determination of DA and UA.On the other hand, as it is expected by modifying the GCE electrode, the current response was improved and the highest activity was obtained with the electrode modified with WSe 2 @PEDOT composite.
Electrocatalytic activities of electrodes were further investigated using the DPV technique, which is known as a more sensitive electrochemical method.Figure 4d shows that oxidation peak positions and their current responses are more obvious than was observed in CV measurements.Therefore, the DPV technique was chosen for the simultaneous determination of DA and UA.Moreover, from the CV and DPV curves, it is obviously seen that the current response of UA was greatly increased.This enhancement of the WSe 2 @PEDOT modified electrode toward UA is due to the presence of PEDOT conducting polymer.At the working pH, DA has a cationic character and UA exists as an anionic form, which leads to enhanced electrostatic interaction between UA and cationic PEDOT [18].Therefore, different interactions of DA and UA with the WSe 2 @PEDOT composite result in peak separation of DA and UA.
Cyclic voltammetry was also used to evaluate the effect of various scan rates on the electrocatalytic behaviors of DA and UA on the WSe 2 @PEDOT electrode and to understand the reaction kinetics on its surface.Figure 5(a) displays CVs of DA (0.28 mM) and UA (0.37 mM) in 0.1 M PBS at varied scan rates from 25 to 250 mV/s.Figure 5b shows the corresponding oxidation peak currents, which are proportional to the scan rate, indicating an adsorption-controlled process on WSe 2 @PEDOT coated electrode [19].The simultaneous determination of DA and UA on GCE-WSe 2 @PEDOT was performed using DPV. Figure 6a displays the DPVs for increasing DA and UA concentrations in 0.1 M PBS.It is obvious that oxidation peak positions of DA and UA maintain the peak-to-peak potential separation by gradually increasing their concentrations.Figure 6(b-c) displays calibration curves of DA and UA and linear detection ranges of 16 to 466 µM for DA and 20 to 582.5 µM for UA, were determined.The linear regression equations with corresponding regression coefficients were also placed in Figure 6b, c.Furthermore, considering the slopes of the regression equations, LODs were evaluated to be 8 µM for DA and 14 µM UA, respectively.The selectivity of the GCE-WSe 2 @PEDOT electrode was investigated through DPV technique.The major interference molecule is ascorbic acid, found in a much higher concentration than DA and UA in biological fluids.Figure 7a shows DPVs of GCE-WSe 2 @PEDOT in 0.1 M PBS solution containing 0.38 mM DA, 0.38 mM UA and 1.0 mM AA and other interference substances (e.g., NaCl, KCl, NaNO 3 , MgSO 4 , glucose and citric acid).It is clearly seen that adding interfering substances in much higher concentrations to the mixed DA and UA solution does not have an obvious change in their peak currents, indicating high selectivity of the sensor.It is also worth noting that the electrode modified with WSe 2 @PEDOT inhibits the oxidation of AA, which has a close oxidation potential to DA and UA.The stability test of the GCE-WSe 2 @PEDOT electrode was also investigated using the DPV technique.The stability data was collected by obtaining the peak current values of DA and UA for fifteen days and given in Figure 7b, indicating an acceptable change compared with the initial currents.

Conclusions
WSe 2 @PEDOT composite was successfully prepared via a simple hydrothermal method and applied as an electrochemical sensor for the detection of DA and UA.It was found that the incorporation of conducting PEDOT polymer into the WSe 2 nanostructure improved electrochemical behavior and electrocatalytic activity of composite material.Furthermore, WSe 2 @PEDOT modified electrode gave separated oxidation peaks for DA and UA with better current responses.WSe 2 @PEDOT-based sensor exhibited a linear detection range of 16 to 466 µM for DA and 20 to 582.5 µM for UA and detection limits of 8 µM for DA and 14 µM UA, respectively.The selectivity and stability of the modified electrode were also studied through DPV measurements, exhibiting high selectivity in the presence of various interferents and high stability.

Figure 1 .
Figure 1.Schematic representation of synthetic procedure and electrochemical detection

Figure 6 .
Figure 6.(a) DPVs of GCE-WSe 2 @PEDOT in 0.1 M PBS (pH 7.0) containing various concentrations of DA and UA; calibration plots of (b) DA and (c) UA Selectivity, reproducibility and stability of the sensor