Introduction

Dopamine (DA) is a well-known neurotransmitter, which plays an extremely important role in the central nervous system, kidney and cardiovascular system [1, 2]. Some neuropsychiatric diseases are related with it [3, 4]. Therefore, it is vitally important to determine DA for the diagnosis and treatment of these diseases. Many methods have been reported for the detection of DA, such as electrochemical method, chemiluminescence, capillary electrophoresis, spectrophotometry, and flow injection [5,6,7,8,9,10,11]. Among them, the electrochemical methods have the advantages of simple operation, high selectivity and sensitivity, fast response and low cost [12], Some new nanomaterials modified electrodes have been used for electrochemical detection of DA [13,14,15,16,17]. However, these modified electrodes have some shortcomings, such as low sensitivity and high detection limit. How to improve the sensitivity of the modified electrodes is the focus of current researchers.

Carbon nanofibers (CNFs), an interesting one-dimensional carbon material, are widely used in solar cells, supercapacitors and nanoprobes [18,19,20]. CNFs have unique nanofiber network structure, good conductivity, chemical stability and biocompatibility, and they are ideal candidate materials for high-performance nanoprobes [21]. However, CNFs have a small specific surface area and a small number of active sites, resulting in lower sensitivity in actual detection [22]. The nanofiber network structure of CNFs is an ideal carrier for combining with other materials, which will further enhance the electrochemical properties.

Molybdenum disulfide (MoS2) is a layered compound with graphene-like structure, which has attracted a wide attention and has been widely used in various fields [23,24,25]. In order to further enhance its physical and chemical properties, considerable efforts have been made to prepare MoS2 nanomaterials with different morphologies. Among them, MoS2 nanosheet balls (MoS2 NSBs) composed of numerous nanosheets have high surface ratio, catalytic efficiency, porous surface, biocompatibility and chemical stability, which are considered as excellent materials for nanoprobes [26,27,28].

Here, a novel golf ball-like MoS2 NSBs were successfully grown on the CNFs. The hybrid MoS2 NSB/CNFs was used for electrochemical determination of DA under the interference of uric acid (UA). Due to the synergistic effect of MoS2 NSBs with large specific surface area and CNFs with high conductivity, the hybrid shows a high sensitivity, low LOD, good selectivity, reproducibility and stability for the determination of DA. It is expected to be used in practical DA detection.

Experimental

Preparation of the molybdenum(IV) disulfide nanosheet balls (MoS2 NSBs) on carbon nanofibers (CNFs)

CNFs were synthesized by electrospinning, which is same as previously reported [29]. The prepared CNFs were cut into a size of 1 cm × 2 cm and put in concentrated nitric acid (Sinopharm Chemical Reagent Co., Ltd. China http://www.reagent.com.cn/) for 18 h to prepare the activated CNFs. Then, sodium molybdate (Na2MoO4·2H2O, 0.5 mmol) and thiourea (CH4N2S, 2.5 mmol) (Sinopharm Chemical Reagent Co., Ltd. China http://www.reagent.com.cn/) were added to 0.075 L deionized water (Sichuan Wortel Water Treatment Equipment Co., Ltd. China http://www.sc-woter.com/) and stirred for 20 mins to prepare hydrothermal solution. The activated CNFs and hydrothermal solution were transferred to a 0.1 L Teflon-lined stainless-steel autoclave and heated at 220 °C for 22 h, and then cooled to ambient temperature. Finally, the samples were washed with deionized water and dried in a vacuum freeze dryer for 1 h to obtain MoS2 NSB/CNFs. All the chemical reagents in this experiment are analytical reagents.

Characterizations

The morphologies of CNFs and MoS2 NSB/CNFs were characterized by JSM7000F scanning electron microscope (SEM). X-ray diffraction (XRD) experiments were recorded by Rigaku Ratoflex D/MAX diffractometer.

Electrochemical measurements

All electrochemical measurements were conducted on a VMP3 electrochemical workstation (Biologic Science Instrument, France). The electrochemical measurements include cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS) and amperometric responses. MoS2 NSB/CNFs electrode was used as the working electrode, Ag/AgCl as the reference electrode and platinum wire as the counter electrode. The effective plane area of the MoS2 NSB/CNFs electrode was 0.7 cm2. The specific parameters of electrochemical measurements are the same as those reported previously [30].

Results and discussion

Preparation and characterizations of the MoS2 NSB/CNFs

Figure 1 shows the preparation process schematic of MoS2 NSB/CNFs and electrochemical redox reactions of DA and UA at the MoS2 NSB/CNFs electrode. First, the polyacrylonitrile nanofibers (PAN NFs) were prepared by electrospinning (Fig. 1a-b). Then, PAN NFs were stabilized at 300 °C and carbonized at 900 °C to obtain CNFs (Fig. 1c). After that, CNFs were placed in hydrothermal solution and heated at 220 °C for 22 h to prepare MoS2 NSB/CNFs (Fig. 1d). The electrochemical redox reactions of DA and UA occurs on the surface of MoS2 NSB/CNFs (Fig. 1e-f).

Fig. 1
figure 1

│ Schematic of preparation process of the MoS2 NSB/CNFs and electrochemical redox reactions of DA and UA at the electrode

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Figure 2 shows the SEM morphologies of CNFs and MoS2 NSB/CNFs. The CNFs are interwoven into a network structure, whose average diameter is ~ 200 nm (Fig. 2a). After hydrothermal reaction, discontinuous MoS2 NSBs were grown on the CNFs (Fig. 2b). By observing SEM images with high-magnification, MoS2 NSBs with the diameter of ~ 2 μm are made up of nemerous bent nanosheets to form a porous structure and CNFs connect them through the center of the ball (Fig. 2c-d), which further increases the adsorption sites of biological molecules. XRD image of MoS2 NSB/CNFs clearly shows the diffraction peaks of CNFs and MoS2 NSB (Fig. S1). All above results indicate that golf ball-liked MoS2 NSB/CNFs is synthesized successfully.

Fig. 2
figure 2

│ SEM images of CNFs and MoS2 NSB/CNFs. a, CNFs. b-d, MoS2 NSB/CNFs with different magnifications

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Electrochemical properties

Figure 3 shows the CV curves of MoS2 NSB/CNFs electrode in 0.1 mM DA and UA at a scan rate of 100 mV s−1. The results show that the DA and UA oxidation peaks of MoS2 NSB/CNFs electrode are significantly higher than those of bare CNFs electrode. This may be the synergistic effect of MoS2 NSBs with large specific surface area and abundant active sites, and CNFs with high conductivity. The calculated ECSA of the MoS2 NSB/CNFs electrode is 3.9 cm2 (Fig. S2a). The charge transfer resistance of CNFs and MoS2 NSB/CNFs is 380 and ~ 10 Ω (Fig. S2b). The results show that the conductivity of CNFs can be significantly improved by combination of MoS2 NSBs. Moreover, this indicates that DA and UA are both adsorption control processes of MoS2 NSB/CNFs at the scanning range of 10–100 mV·s−1 (Fig. S3).

Fig. 3
figure 3

│ CV curves of CNFs and MoS2 NSB/CNFs in 0.1 mM DA and UA at a scan rate of 100 mV s−1. a, DA. b, UA. The oxidation peak potential for LD and UA appears at ~ 0.25 and 0.45 V, respectively

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DPV has a higher current sensitivity than CV, so DPV technology is used for the determination of DA [31]. Figure 4 depicts the DPV curves of MoS2 NSB/CNFs electrodes in DA solution with different concentrations. The linear fitting equation is IDA = (28.22 ± 7.51) + (4.37 ± 0.25)CDA (R2 = 0.9846) indicates the linear relationship between the oxidation peak current of DA and its concentration. Hence, the sensitivity of the MoS2 NSB/CNFs electrode for detecting DA is 6.24 μA·μM−1·cm−2. The LOD of the MoS2 NSB/CNFs electrode is obtained using the formula: LOD = 3Sb/m, where m is the slope of the fitting curve (4.37 μA·μM−1), Sb is the standard deviation of the blank signal (Fig. S4), so the LOD is 0.036 μM. The sensitivity and LOD are superior than those of the previous reported (Table 1) [32,33,34,35,36,37,38,39].

Fig. 4
figure 4

Differential pulse voltammetric (DPV) curves of MoS2 NSB/CNFs electrode in DA with different concentrations. a, The DA concentration from bottom to top is 0, 1, 5, 10, 20, 40 and 60 μM. b, Relationship of the DA oxidation peak current vs. concentration. DPV conditions: pulse height is 50 mV, pulse width is 0.2 s, step height is 4 mV and step time is 0.5 s

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Table 1 Summary of the previously published results for individual detection of DA by DPV using different electrodes
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Since UA and DA coexist in human body fluid, it is necessary to investigate the interference of UA on the detection of DA. Figure 5 depicts the DPV curves of MoS2 NSB/CNFs electrode in mixtures of DA and UA. The concentration of DA (CDA) and its oxidation current (ILD) is linearly correlated, and the linear equation is IDA = (13.56 ± 4.91) + (4.33 ± 0.18)CDA (R2 = 0.9916). The result shows that the presence of UA has no significant influence on the detection of DA.

Fig. 5
figure 5

DPV curves of MoS2 NSB/CNFs electrode in DA with different concentrations in the presence of 40 μM UA. a, The DA concentrations from bottom to top is 0, 1, 5, 10, 20, 40 and 60 μM. b, Relationship of the DA oxidation peak current vs. concentration. The DPV conditions are the same as in Fig. 4

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The selectivity of the MoS2 NSB/CNFs electrode for DA detection was investigated by continuous injection of potentially interfering ions, such as uric acid, ascorbic acid, folic acid, KCl, Na2SO4, NaCl, NaNO3 and NaOH, as shown in Fig. S5. There is no current response when adding interfering substances, which confirms that the MoS2 NSB/CNFs electrode has a good selectivity performance.

However, DA cannot be detected precisely when it coexists with neurotransmitters, such as, levodopa (LD), norepinephrine (NE) and epinephrine (E) due to the same oxidation potential.

Reproducibility and stability

To analyze the repeatability and stability of MoS2 NSB/CNFs electrode, 10 μM DA was tested multiple times with DPV. After repeating 9 measurements every 10 min at room temperature, the relative standard deviations (RSD) was 1.9% (Fig. S6a), indicating that the MoS2 NSB/CNFs electrode had a good reproducibility. The MoS2 NSB/CNFs electrode was placed in a 0.01 M phosphate buffered saline and the DPV test was performed in 10 μM DA every 2 days. After 14 days, the oxidation peak current of DA decreased by 7.1% (Fig. S6b), indicating that the MoS2 NSB/CNFs electrode had an excellent stability.

Real sample analysis

The feasibility of the MoS2 NSB/CNFs electrode was investigated by the detection of human urine samples. The experimental results obtained are shown in ESM. The recoveries of the detection of 10, 20 and 40 μM DA are 101.6, 99.8 and 107.8%. The RSD is 1.9, 2.2 and 1.7%, respectively. The results show that MoS2 NSB/CNFs electrodes can be used for real urine test and have a great potential for clinical applications.

Conclusions

A novel MoS2 NSB/CNFs hybrid was prepared by combining electrospinning preparation of CNFs and in-situ growth of MoS2 NSBs on the surface of CNFs. The golf ball-like MoS2 NSBs are made up of numerous bent nanosheets and CNFs connect them through the center of the ball. The MoS2 NSB/CNFs electrode exhibits excellent electrochemical properties for the detection of DA due to the synergistic effect of the two nanomaterials. It is also expected to be used for clinical determination of DA.