Introduction

Dopamine (DA) as an important neurotransmitter, plays a significant role in the function of the central nervous, renal and hormonal systems [1]. Deficiency of DA is believed to be related to several diseases such as Schizophrenia, Huntington and Parkinson [2]. Electrochemical techniques for detecting dopamine (DA) have received considerable interest due to its electrochemical activity [3–6]. However, the main challenge to electrochemical detection of DA is its very low concentration and the strong interference resulting from much higher concentration of ascorbic acid (AA) [7]. To resolve this problem, various materials such as polymer film [8–11], nanoparticles [12–18] and nanostructures [19–23], have been used to modify working electrode for developing highly sensitive and selective methods. Among these materials, carbon nanotubes, due to the remarkable electrocatalytic properties and large surface area, have attracted great attentions [24–26]. The use of carbon nanotubes for modifying electrode can improve the sensitivity of electrochemical detection [7, 27]. Moreover, carbon nanotubes have been reported to modify electrodes for detecting DA with obviously improved sensitivity [28–30].

Recently, semiconductor nanostructured materials have gained increasing interests in the electrochemical sensing fields due to excellent properties [31, 32]. In2S3 are important semiconductors with the band-gap values of 2.8 eV [33]. Due to its unique catalytic, optical, electronic and gas-sensing properties, In2S3 can be used as catalysts, luminophors, electrodes, solar cells and gas sensors [34]. Additionally, the nanostructured In2S3 often exhibits shape- and size-dependent physical and chemical properties, which are of importance and interest to scientific research [33, 34].

In this work, urchin-like In2S3 microspheres were synthesized chemically with hydrothermal method. Then multi-walled carbon nanotubes (MWCNTs) functionalized urchin-like In2S3 was prepared to develop a novel electrochemical senor for detection of DA in the presence of ascorbic acid (AA). The electrochemical properties of MWCNTs-In2S3 were investigated by cyclic voltammetry and differential pulse voltammetry. The developed MWCNTs-In2S3 composite exhibited superior electrocatalytic activity and significantly enhanced the sensitivity for DA determination. Meanwhile, MWCNTs-In2S3 composite modified glassy carbon electrode (GCE) increased peak separation between DA and AA and showed obviously improved selectivity. The designed excellent MWCNTs-In2S3 nanostructure offers a promising application in electroanalysis and biosensing.

Experimental

Chemicals and apparatus

Indium trichloride (InCl3. 4H2O), thioacetamide (TAA), DA and AA were supplied by Sinopharm Chemical Reagent Co., Ltd (China, http://sinoreagent.cn.alibaba.com). MWCNTs (≥98% purity, 60–100 nm diameter and 1–2 μm length) were purchased from Shenzhen Nanoport Co. Ltd (China, http://www.nanotubes.com.cn). Phosphate buffer solution was 0.1 M Na2HPO4 and NaH2PO4 and its pH was adjusted with H3PO4 or NaOH solutions. All other chemicals and reagents are of analytical grade and were prepared using distilled water.

All electrochemical measurements were performed on a CHI 852C electrochemical workstation (Co., CHI, USA) at room temperature using a conventional three-electrode system with a modified GCE (Φ = 3 mm) as working electrode, a platinum wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. Scanning electron micrographs (SEM) and energy dispersive X-ray spectroscopy (EDX) were recorded on a Hitachi S-4800 scanning electron microscope (Japan) with an EDX attachment at an acceleration voltage of 15 kV.

Synthesis of urchin-like In2S3 nanostructure

Urchin-like In2S3 microspheres were simply synthesized according to previous method [34]. Briefly, InCl3. 4H2O (2.0 mM) and TAA (3.0 mM) powders were firstly dissolved in 30 mL water and then transferred in Teflon-lined stainless-steel autoclaves. After 2.0 mL of acetic acid (>99.5%) was added to the mixed solution, the autoclaves were sealed and maintained at 80 °C for 12 h, then cooled to room temperature. The as-formed precipitates were filtered, washed carefully with distilled water and ethanol and finally dried in air at 60 °C. Afterward, the orange red In2S3 powders were obtained.

Preparation of MWCNTs-In2S3 modified GCE

The GCE was firstly polished successively with 0.3 and 0.05 μm alumina slurry (Buhler) followed by rinsing thoroughly with distilled water, successive ultrasonication in 1:1 nitric acid, acetone and distilled water and dried in air. MWCNTs were firstly dispersed in 30% nitric acid and then refluxed for 24 h at 140 °C. The resulting suspension was centrifuged, and the sediment was washed with water until the pH reached about 7.0. Then 0.75 mL of carboxylated MWCNTs (1.0 mg L−1) were mixed by 0.25 mL of urchin-like In2S3 (1.0 mg L−1) with ultrasonication for 0.5 h. After 5.0 μL of the mixed solution was dropped on the surface of the pretreated GCE and dried in silica gel desiccators, the MWCNTs-In2S3 modified electrode was obtained.

Results and discussion

Characterizations of MWCNTs-In2S3 modified GCE

Figure 1 displays the EDX spectrum of the resulted In2S3 nanostructure. No EDX peaks resulting from the possible contaminations of Cl, N, C and O are visible, suggesting the formation of pure phase In2S3. The atomic ratio of In to S was estimated to be 40.67:50.33, which nearly accords with stoichiometric ratio of In2S3.

Fig. 1
figure 1

EDX spectrum of the obtained In2S3

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SEM image in Fig. 2A shows that the synthesized In2S3 microspheres. These urchin-like microspheres are constructed with about 15–30 nm arrayed nanoflakes with the diameter of about 2 μm. As seen from SEM image of MWCNTs-In2S3 in Fig. 2B, urchin-like morphology of In2S3 disappears and MWCNTs have been uniformly wrapped on the surface of urchin-like nanostructure. The MWCNTs-In2S3 composite can not only greatly increase the effective electrode surface area but also remarkable improve the electrochemical activity of MWCNTs. These features thus are beneficial to significantly enhance the sensitivity for detecting analytes.

Fig. 2
figure 2

SEM images of In2S3 (A) and MWCNTs-In2S3 (B)

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The cyclic voltammograms (CVs) of 50 μM DA in pH 5.0 phosphate buffer solution at different modified GCEs were shown in Fig. 3. A pair of redox peaks of DA were observed at bare GCE (curve a), while In2S3 modified GCE (curve b) shows no obvious peaks, indicating that single In2S3 is not favorable to the electrocatalytic oxidation of DA. However, MWCNTs-In2S3 modified GCE exhibits a pair of remarkable and well-defined redox peaks (curve d). The anodic peak current of DA at MWCNTs-In2S3 modified GCE is 2 times larger than that at MWCNTs modified GCE (curve c) and 3 times larger than that at bare GEC, which is ascribed to the excellent electrocatalytic capability and larger surface area of MWCNTs-In2S3.

Fig. 3
figure 3

CVs of 50 μM DA in pH 5.0 phosphate buffer solution on bare (a), In2S3 (b), MWCNTs (c) and MWCNTs-In2S3 (d) modified GCEs at a scan rate of 100 mV s−1

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Electrochemical behavior of DA at MWCNTs-In2S3 modified GCE

Figure 4 shows the CVs of 50 μM DA in pH 5.0 phosphate buffer solution at MWCNTs-In2S3 modified electrode at different scan rates. The reduction and oxidation peak currents of DA at MWCNTs-In2S3 GCE increased linearly with the increasing scan rate from 10 to 300 mV s−1 (The linear regression equations are I pa (μA) = 0.099 v + 2.18 with a correlation coefficient of R 2 = 0.9940 and I pc (μA) = −0.082 v−0.45 with a correlation coefficient of R 2 = 0.9981). In addition, the ratio of anodic peak current to the cathodic peak current is approximately equal (I pa/I pc ≈ 1), indicating a quasi-reversible surface-controlled process. The surface coverage (Г *) of the electroactive species (DA) on surface of the modified electrode can be calculated from the charge integration of anodic peak in CVs at 50 mV s−1, according to Г * = Q/nFA, where Q is the charge consumed in the reaction, n is the number of electrons transferred, F is the Farady constant, and A is the electrode area. The separation of the peak potential (ΔE P) is 34 mV, the number of electron transferred in the electrochemical process is 2 (n ≈ 1.7 based on the formula of 59/n mV at 25 °C). The surface coverage was calculated to be 3.7 × 10−10 mol cm−2, which is close to the value of 4.18 × 10−10 mol cm−2 reported previously [21].

Fig. 4
figure 4

CVs of 50 μM in pH 5.0 phosphate buffer solution at 10, 30, 50, 80, 100, 150, 200, 250 and 300 mV s−1 (from a to i), Inset: plots of anodic and cathodic peak currents vs. scan rates

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The influence of the pH value on the electrochemical behavior of DA on MWCNTs- In2S3 modified electrode was investigated and shown in Fig. 5. With the increase of solution pH from 2.0 to 7.0, the redox potentials of DA shifted linearly as a function of solution pH with a slope of −50.3 mV pH−1 (Inset A of Fig. 5, The linear regression equations is E p = 0.478–0.0503pH with a correlation coefficient of R 2 = 0.9978). This slope is close to the theoretical value of −59.0 mV pH−1 [20], indicating two protons and two electrons participating in the electron transfer process (As the oxidization of DA is a two-electron process). Otherwise, when increasing pH value from 2.0 to 7.0, the anodic peak current of DA reaches a maximum value at pH 5.0 (Inset B of Fig. 5). Therefore, pH 5.0 phosphate buffer solution was employed due to the more sensitive electrochemical response of DA.

Fig. 5
figure 5

CVs of 50 μM DA in 0.1 M phosphate buffer solution with 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0 pH values on MWCNTs-In2S3 modified GCEs at a scan rate of 100 mV s−1 (from a to f), Insets: plot of E o′of DA vs. pH (A) and plot of anodic peak currents vs. pH (B)

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Interference study from AA

For electrochemical detection of DA, the strong interference from coexisted AA must be considered because their oxidation peaks are usually overlapped. As shown in Fig. 6A, a much smaller redox peaks of DA (ΔE P = 101 mV) was observed at bare GCE (curve a), indicating the sluggish electron transfer of DA. However, DA at MWCNTs- In2S3 modified electrode (curve b) shows a pair of quasi-reversible redox peaks at 0.23 V (ΔE P = 33 mV) with remarkable enhanced oxidization current, suggesting the obviously improved electron transfer rate. In Fig. 6B, only one broad oxidization peak of AA at 0.09 V appeared at bare electrode, while a sharp oxidization peak of AA at 0.01 V was obtained at MWCNTs-In2S3 modified electrode. The MWCNTs-In2S3 modified electrode also exhibits intriguing electrocatalytic activity to AA by decreasing the over-potential and increasing the oxidization current. As can be seen form Fig. 6C, a broad and irreversible oxidization peak of the mixture of DA and AA was observed at bare electrode. However, for the MWCNTs-In2S3 modified electrode, two well separated oxidization peaks of DA and AA were obtained with the peak separation of 176 mV.

Fig. 6
figure 6

CVs of 0.1 mM DA (A), 0.1 mM AA (B) and 0.1 mM DA + 0.1 mM AA (C) in pH 5.0 phosphate buffer solution on bare GCE (a) and MWCNTs-In2S3 GCE (b) at a scan rate of 50 mV s−1

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Selective detection of DA at MWCNTs-In2S3 modified GCE

Differential pulse voltammogram (DPV) of the mixture (DA + AA) (Fig. 7A) at MWCNTs-In2S3 modified electrode shows two well separated oxidization peaks with greatly increased oxidization peak currents. The peak separation of DA and DA at MWCNTs-In2S3 modified electrode is 225 mV, which is larger than that of 148 mV at bare electrode. These results indicate that MWCNTs-In2S3 modified electrode not only obviously improves the selectivity for DA toward AA but also remarkably enhances the sensitivity of DA detection.

Fig. 7
figure 7

DPV of 0.1 mM DA + 0.1 mM AA in pH 5.0 phosphate buffer solution on bare GCE (a) and MWCNTs-In2S3 GCE (b) at a scan rate of 50 mV s−1 (A) and DPVs of 0.5, 1, 5, 10, 50, 100, 200, 300, 400, 500 and 600 μM DA (from a to k) on MWCNTs-In2S3 modified GCE in pH 5.0 phosphate buffer solution, inset: calibration curve for DA (B)

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The potential separation of DA and AA at MWCNTs-In2S3 modified GCE is 225 mV, which is sufficient enough for the selective determination of DA. Figure 7B shows the DPVs of different concentrations of DA. The peak currents increased linearly with the increase of DA concentration ranging from 0.5 μM to 300 μM with a low detection limit of 0.1 μM at signal-to-noise of 3 (Inset of Fig. 7B). The comparisons for the analytical performance by use of the developed and several other modified electrodes were listed in Table 1. It can be seen from Table 1, the obtained detection limit for DA is much lower than those of other electrochemical biosensors. Meanwhile, the linear range of this work was wider than that of previous methods listed in Table 1. Furthermore, the sensitivity for MWCNTs-In2S3 modified GCE was calculated to be 594.9 μA mM−1 cm−2, which is higher than that of 121.6 μA mM−1 cm−2 at carbon nanotubes modified GCE for DA [40].

Table 1 Comparison between MWCNTs-In2S3 modified electrode and other modified electrodes for detection of DA
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Analytical application

To evaluate the potential application of MWCNTs-In2S3 modified electrode, DA in pharmaceutical product, dopamine hydrochloride injection, was determined with differential pulse voltammetry. The DA samples (65.28 μM) were obtained by diluting DA injection (10 mg mL−1) at a ratio of 1:1,000 with pH 5.0 phosphate buffer solution. The DA level was detected to be 65.51 μM close to 65.28 μM standard value with five measurements. The recovery was tested by detecting the DA standard solution spiked in the DA samples and shown in Table 2. The obtained recovery values were acceptable, indicating good accuracy for determining of DA in real samples.

Table 2 Determination of DA in DA injection
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Conclusions

A novel MWCNTs-In2S3 modified electrode is developed for highly efficient electrocatalytic detection of DA. Compared to the bare and MWCNTs modified GCEs, This MWCNTs-In2S3 composite has larger specific surface area and exhibits intriguing electrocatalytic activity with remarkably enhanced sensitivity for DA. The resulting electrode can separate the peak potentials of DA and AA by about 225 mV in the presence of AA and shows perfect selectivity for DA detection. The constructed DA electrochemical sensor has high sensitivity, lower detection limit and wide linear range. The MWCNTs-In2S3 composite opens a new avenue for developing promising electrochemical biosensor.