Introduction

Dopamine (DA) is an important catecholamine neurotransmitter in the mammalian central nervous system and plays a key role in the function of central nervous, renal, hormonal and cardiovascular systems [1–3]. The DA concentration in biological systems is usually in the range of 10−8 M to 10−6 M [4]. Abnormal levels of DA have been associated with several neurological disorders such as Parkinson’s disease, schizophrenia, Alzheimer’s disease, Huntington’s disease, attention deficit hyperactivity disorder and drug addiction [3, 5, 6]. Therefore, the rapid and sensitive detection of DA in biological system is of great clinical importance. At present, various analytical techniques have been developed for the determination of DA that includes mass spectrometry [7], spectrophotometry [8], high performance liquid chromatography (HPLC) [9, 10], chemiluminescence [11] and electrochemical methods [3, 5, 12–15]. Among them, the electrochemical method is considered as a useful approach owing to its high sensitivity, fast response, easy operation, cost-effectiveness and capability of in situ detection [2, 16]. Moreover, the electrochemical detection of DA can be easily realized since it has good electrochemical activity [5].

Transition metal oxides, such as MnO2 [16], Fe3O4 [17], Co3O4 [18] and TiO2 [19] have been used as electrocatalysts for the detection of DA. At the same time, copper oxides including CuO and Cu2O have attracted intensive interest as promising candidates for electrochemical sensors due to their low cost, environmental friendship and significant catalytic activities [20–24]. For example, Song et al. [21] reported a non-enzymatic H2O2 sensor based on CuO nanoflowers whose linear range was from 4.25 × 10−5 to 4 × 10−2 mol L−1, with a detection limit of 0.167 μmol L−1. Liu et al. [24] constructed an electrochemical sensor with the hybrid nanomaterial of Cu2O nanocubes wrapped by graphene nanosheets, which exhibited the excellent performance toward the detection of glucose and H2O2. In particular, several research groups have also explored CuxO nanocomposites modified electrodes for the detection of DA [25–27]. Reddy et al. [25] synthesized the flake-shaped CuO nanoparticles with enhanced current response for DA. Zhang et al. [27] reported an electrochemical DA sensor based on the Cu2O/graphene nanocomposite. It is noted that the detection of DA at these CuxO-based modified electrodes is almost limited to the voltammetry method. On the other hand, the size and structural morphology of nanoparticles are known to have a significant effect on enhancing the electrochemical response of sensors, and well-controlled nanostructures are thereby essential for achieving efficient electrocatalysts [28]. From this prospect, the shape-controlled synthesis of CuxO nanoparticles has attracted enormous attention, and CuxO nanoparticles with different shapes were obtained, such as cubes [20, 24], rods [29], nanowires [30, 31], nanoflowers [21, 32] and polyhedra [33]. It is found that the detection performance of sensing electrodes can be improved by changing the morphology of the CuxO nanostructures. However, among the various morphologies of CuxO nanoparticles, hollow structures have received considerable attention due to their high specific surface area, low density and potential applications in the area of sensing [34–36]. To the best of our knowledge, the studies on CuxO hollow microspheres for the amperometric determination of DA have not yet been reported.

Deep eutectic solvents (DESs) are the promising solvents to be used in the shape-controlled synthesis of functional materials due to their unique physicochemical properties, such as high conductivity, viscosity, surface tensions, polarity, thermal stability and negligible vapor pressure [37, 38]. Herein we report the synthesis of Cu2O hollow microsphere (HMS)/carbon black (CB) hybrid material (Cu2O HMS/CB) in the DESs/H2O mixed solvent and its application in the electrochemical determination of DA. The materials were characterized by XRD, SEM and TEM. The Cu2O HMS/CB composites were used for the fabrication of the modified glassy carbon electrode (GCE) and thus for the electrochemical investigation of DA in PBS solution (pH = 5.7). By taking advantages of high electronic conductivity of carbon black and high electrocatalytic activity of Cu2O hollow microsphere, the Cu2O HMS/CB composites exhibit strong and sensitive current responses to DA.

Experimental

Reagents

Vulcan XC-72 carbon black (CB) was supplied from Cabot Corporation (http://shminxing027203.11467.com). Dopamine (DA), uric acid (UA) and 5 wt.% Nafion solution were purchased from Sigma-Aldrich (http://www.sigmaaldrich.com/china-mainland/chemistry-product.html). (−)–Epinephrine (+)–bitartrate salt was purchased from J&K Scientific Ltd (Beijing, China) (http://www.jkchemical.com). Choline chloride, urea, absolute ethanol, polyvinylpyrrolidone (PVP), ascorbic acid (AA), CuSO4, NaOH, H2O2 and D–glucose were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) (http://www.sinoreagent.com). Phosphate buffer saline (PBS, 0.1 mol L−1, pH 5.7) was used as the supporting electrolyte. All the chemicals are of analytical grade and used as received without further purification. All aqueous solutions were prepared using tridistilled water.

Apparatus and measurements

The size and morphology of Cu2O samples were analyzed by scanning electron microscopy (SEM, LEO-1530) and transmission electron microscopy (TEM, FEI Tecnai-F30). Energy dispersive X-ray (EDX) spectroscopy characterization was conducted on the same apparatus (SEM, LEO-1530), and the Cu2O content of the Cu2O HMS/CB composites could be obtained according to the EDX results. X-ray diffraction (XRD) measurements were carried out on an X-ray diffractometer (Rigaku D/MAX 2500 v/pc, Japan) with a Cu Kα radiation source (λ = 1.5406 Å). Electrochemical experiments were performed on a CHI 660D electrochemical workstation with a standard three-electrode system comprising a piece of Pt foil (1 cm2) as auxiliary electrode, a saturated calomel electrode (SCE) as reference electrode, against which all potentials were quoted, and the prepared modified electrode as working electrode. All experiments were carried out at room temperature around 25 °C.

Synthesis of Cu2O hollow microsphere/carbon black composite

The choline chloride/urea DESs were synthesized according to the previously reported method [38]. In a typical procedure for the synthesis of Cu2O HMS/CB composite, a DESs/H2O mixed solvent was first prepared by ultrasonically mixing the DESs and tridistilled water (1:2 in volume). 16 mg ascorbic acid was completely dissolved into 10 mL DESs/H2O mixed solvent, and the pH value of system was adjusted by the NaOH/DESs solution to 11 (denoted as solution A). Then, 22.3 mg CuSO4, 0.9 g PVP and 25 mg Vulcan XC-72 were added in 20 mL DESs/H2O mixed solvent under ultrasonic treatment for 30 min, and the pH value of system was adjusted by the NaOH/DESs solution to 11 (denoted as solution B). Subsequently, the solution B was stirred for 15 min at 40 °C, and then the solution A was added dropwise into the solution B. After reaction for 3 h under constant stirring, the as-obtained suspension was allowed to stand overnight. The resultant Cu2O HMS/CB products were collected by centrifugation, washed with tridistilled water and absolute ethanol for several times, and dried in vacuum at 60 °C for 24 h. As a comparison, the Cu2O HMS was prepared with the similar procedure as described above except for the addition of Vulcan XC-72.

Fabrication of modified electrodes

The modified electrode substrate was a glassy carbon electrode (GCE, 5 mm diameter), which was polished sequentially with 5.0 μm, 1.0 μm, 0.3 μm Al2O3 powder and then washed ultrasonically in tridistilled water before each experiment. Then 1 mg of the prepared Cu2O HMS/CB was dispersed ultrasonically in 400 μL Nafion solution (2 wt.%), and 5 μL of the suspension was pipetted and air-dried on the pretreated GCE at room temperature. The resulting modified electrode was denoted as Cu2O HMS/CB/GCE. In addition, the Cu2O HMS/GCE and CB/GCE electrodes were also prepared with the similar procedure.

Results and discussion

Physical characterization

In order to analyze the crystal structure and phase purity of the Cu2O products, the X-ray diffraction measurements were carried out. As shown in Fig. 1, in the case of Cu2O HMS (curve a), the diffraction peaks at ca. 29.7°, 36.4°, 42.5°, 61.6° and 73.8° originate from the crystal planes of (110), (111), (200), (220) and (311) of the cubic symmetry Cu2O, respectively [33, 39]. No impurity is detected in this curve, which demonstrates that the high-purity Cu2O product is successfully synthesized. The average crystallite size was estimated to be 9.7 nm by the Scherrer’s equation based on the peak assigned to the (220) plane [40], suggesting that the Cu2O microspheres are constructed by smaller nanoparticles. For the Cu2O HMS/CB composite (curve b), the strong peak located at the 2θ value of ca. 24.6° is evidently attributed to the (002) phase of Vulcan XC-72 carbon black [41, 42], and the other three peaks at ca. 36.4°, 42.5° and 63.7° can be ascribed to the diffractions of Cu2O(111), Cu2O(200) and Cu2O(220) planes, respectively. It is noted that, due to the coating of CB, the diffraction signals of Cu2O become weaker or even disappear.

Fig. 1
figure 1

XRD patterns of Cu2O HMS (a) and Cu2O HMS/CB (b)

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The morphology of Cu2O products were further investigated by SEM and TEM methods, and their size distribution was evaluated statistically by measuring the diameter of 200 Cu2O microspheres in the magnified TEM images. As can be seen from the SEM image of Cu2O HMS (Fig. 2a), the obtained Cu2O HMS particles are spherical. The corresponding high magnification SEM image (inset of Fig. 2a) indicates that the Cu2O microsphere is in fact an agglomerate of abundant Cu2O nanoparticles. The broken sphere suggests that the spheres are hollow. However, in the SEM image of Cu2O HMS/CB composite (Fig. 2b), the Cu2O microspheres are difficult to be observed, which may be due to the embedding of Cu2O microspheres in CB nanoparticles. Additionally, the TEM image of Cu2O HMS is shown in Fig. 2c and its particle size distribution is shown in Fig. 2e. The microspheres exhibits paler contrast in the middle region compared to the dark edges, further confirming their hollow structure. The average diameter of the hollow spheres is about 371 nm. The TEM image of Cu2O HMS/CB composite is shown in Fig. 2d and its particle size distribution is shown in Fig. 2f. It is found that the Cu2O microspheres are embedded in the CB aggregation, which is in agreement with the XRD results. Their average diameter is about 198 nm, much smaller than that of the Cu2O HMS product. These results demonstrate that the coating of CB in the Cu2O HMS/CB composite can not only reduce the size of Cu2O HMS but also improve the electronic conductivity of the hybrid material effectively. It will be responsible for the enhanced electrocatalytic properties of Cu2O HMS/CB composite discussed below.

Fig. 2
figure 2

SEM (a), TEM (c) images and the size distribution histogram (e) of Cu2O HMS, the inset is the corresponding high-magnification SEM image. SEM (b), TEM (d) images and the size distribution histogram (f) of Cu2O HMS/CB composite

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Electrochemical behavior of modified electrodes

The electrochemical behaviors of different modified electrodes were studied by cyclic voltammetry (CV) measurements. Figure 3 shows the CV curves of CB/GCE, Cu2O HMS/GCE and Cu2O HMS/CB/GCE electrodes in 0.1 mol L−1 PBS (pH = 5.7) solution containing 50 μmol L−1 DA at a scan rate of 50 mV s−1. As can be seen from the inset of Fig. 3, the Cu2O HMS/CB/GCE has no redox peaks in blank PBS solution, implying that the Cu2O HMS/CB/GCE is non-electroactive in the selected potential region. Upon the addition of 50 μmol L−1 DA, it can be observed that the Cu2O HMS/CB/GCE shows a pair of well-defined peaks with anodic peak potential (E pa) at 0.248 V and cathodic peak potential (E pc) at 0.193 V (Fig. 3a). The corresponding peak potential separation (ΔE p) is 55 mV, much smaller than that of the Cu2O HMS/GCE (Fig. 3b, 108 mV) and CB/GCE (Fig. 3c, 78 mV) electrodes, suggesting a fast electron transfer kinetics on the Cu2O HMS/CB/GCE electrode [27, 43, 44]. Moreover, the anodic and cathodic peak currents of DA on the Cu2O HMS/CB/GCE are 12.7 and −13.3 μA, respectively, much larger than those on the CB/GCE (3.3 and −3.3 μA) and Cu2O HMS/GCE (5.8 and −7.6 μA) electrodes. These results indicate that the coating of CB significantly improve the electron transfer rate of Cu2O HMS and thus enhance the electrocatalytic activity towards DA.

Fig. 3
figure 3

Cyclic voltammograms of Cu2O HMS/CB/GCE (a), Cu2O HMS/GCE (b) and CB/GCE (c) electrodes in 0.1 mol L−1 PBS solution (pH = 5.7) with 50 μmol L−1 DA. Scan rate: 50 mV · s−1. Inset: the cyclic voltammogram of Cu2O HMS/CB/GCE in 0.1 mol L−1 PBS solution (pH = 5.7)

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The influence of scan rate on the CV response of Cu2O HMS/CB/GCE in 0.1 mol L−1 PBS solution (pH = 5.7) with 50 μmol L−1 DA is shown in Fig. 4a. It can be observed that the scan rate affects the positions of the redox peaks and the values of the redox peak currents. With increasing scan rates from 10 to 200 mV s−1, the peak potentials shift to more positive and more negative values for the anodic and cathodic peaks, respectively, and the redox peak currents also increase gradually. The peak potential separation (ΔE p) become higher with increasing scan rate owing to the increased irreversibility of the electrode process [27, 45]. Furthermore, both the oxidation and reduction peak currents (I pa and I pc) increase linearly with the square root of scan rates (Fig. 4b, linear regression equations: I pa (μA) = 3.516 ν 1/2–9.995, R 2 = 0.9929; I pc (μA) = −3.9166 ν 1/2 + 12.5316, R 2 = 0.9913). These characteristics indicate that the redox behavior of DA at the Cu2O HMS/CB/GCE is a typical diffusion-controlled electrochemical process [44, 45].

Fig. 4
figure 4

a Cyclic voltammograms of Cu2O HMS/CB/GCE in 0.1 mol L−1 PBS solution (pH = 5.7) with 50 μmol L−1 DA at different scan rates from 10 mV · s−1 to 200 mV · s−1. b The plot of the redox peak current versus the square root of scan rate

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Effect of solution pH

The effect of pH on the determination of DA in 0.1 mol L−1 PBS solution at the Cu2O HMS/CB/GCE was carefully investigated in the pH range of 4.0–9.5. As shown in Fig. 5a, both anodic and cathodic peak potentials are shifted negatively with the increasing pH values, and the peak current of DA reaches a maximum at about pH = 5.7. The results indicate that the electrocatalysis of DA at the composite electrode is a pH dependent reaction and pH 5.7 should be selected as the optimum condition for the electrochemical determination of DA. Figure 5b shows the calibration curve of anodic peak potential versus different pH value. It is found that, in the pH range of 4.0–9.5 the anodic peak potential (E pa) decreases linearly with the increase of pH. The linear regression equation is E pa (V) = 0.6079–0.0639pH with a correlation coefficient of R 2 = 0.9951. In addition, the slope value can be calculated to be −63.9 mV/pH using the regression equation, which is close to the theoretical value of −59 mV/pH at 25 °C, indicating a two-proton reaction coupled with a two-electron transfer process [14, 26, 27]. Therefore, the electrochemical reaction of DA at the Cu2O HMS/CB/GCE electrode can be expressed as the following:

Fig. 5
figure 5

a Cyclic voltammograms of Cu2O HMS/CB/GCE in 0.1 mol L−1 PBS solution with 50 μmol L−1 DA at different pH values from 4.0 to 9.5. Scan rate: 50 mV · s−1. b The plot of anodic peak potential (E pa) of DA in CV curves versus pH values

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figure b

Effect of Cu2O content in the Cu2O hollow microsphere/carbon black composite

The Cu2O content in the Cu2O HMS/CB composite should play a significant role in the oxidation of DA at the composite modified electrode, so we studied the effect of different Cu2O mass contents on the current response of Cu2O HMS/CB/GCE to 50 μmol L−1 DA in 0.1 mol L−1 PBS solution (pH = 5.7) at a scan rate of 50 mV s−1. We synthesized five Cu2O HMS/CB samples with different Cu2O mass content by changing the amount of CuSO4 precursor, and from the corresponding EDX results their Cu2O mass contents were determined to be 4.84, 20.46, 27.73, 30.79 and 42.83 %, respectively. As depicted in Fig. 6, the current density was expressed by the normalized current per milligram of Cu2O loading. It can be seen that, the anodic peak current density increases evidently with the increased Cu2O mass content, and the maximum response is approached at the Cu2O content of 20.46 %. Moreover, as described above, smaller peak potential separation (ΔE p) between the anodic and cathodic peaks responds to faster electron transfer kinetics. When the mass content of Cu2O in the composite is 20.46 %, the ΔE p value reaches the minimum, suggesting the fastest electron transfer rate. Thus, 20.46 % Cu2O was chosen as the optimal mass content for the sensor fabrication.

Fig. 6
figure 6

Effect of Cu2O mass content in the composite on the ΔE p value and response current of Cu2O HMS/CB/GCE electrode in 0.1 mol L−1 PBS solution (pH = 5.7) with 50 μmol L−1 DA. Scan rate: 50 mV · s−1

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DA determination of the Cu2O hollow microsphere/carbon black/GCE sensor

The experiment was performed under the optimized conditions in a stirred system. Figure 7a displays the current-time plot for the Cu2O HMS/CB/GCE with successive addition of DA into a stirring PBS solution (pH 5.7). As can be seen, the response time is very fast and the steady-state current reaches another steady-state value within 3 s. The present sensor exhibits good linear amperometric response to DA concentration ranging from 9.9 × 10−8 to 7.08 × 10−4 mol L−1 (R 2 = 0.9979), with the sensitivity of 0.0492 μA μM−1 and detection limit of 3.96 × 10−8 mol L−1 at signal to noise ratio of 3 (Fig. 7b). The performance of the Cu2O HMS/CB/GCE sensor is compared with those of other published DA electrochemical sensor in Table 1. It can be observed that our proposed sensor exhibits better performance in terms of wide linear range, low detection limit, high sensitivity and fast response time.

Fig. 7
figure 7

a Amperometric responses of Cu2O HMS/CB/GCE to the successive addition of DA at the operating potential of 0.25 V in 0.1 mol L−1 PBS solution (pH = 5.7). b The calibration curve of Cu2O HMS/CB/GCE to different concentration of DA

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Table 1 Comparison of present work with other published electrodes for DA determination
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Reproducibility and stability of the Cu2O hollow microsphere/carbon black/GCE sensor

The Cu2O HMS/CB/GCE sensor had a good reproducibility. For eleven electrodes modified identically, the relative standard deviation (RSD) of the current response to 50 μmol L−1 DA was 4.67 %. In addition, the storage stability of the sensor was also investigated. When the sensor was stored at the ambient environment and measured intermittently the current response to 50 μmol L−1 DA, it still retained 80.3 % of its initial activity after 22 days, indicating that the sensor had a satisfactory stability.

Selectivity of the Cu2O hollow microsphere/carbon black/GCE sensor

The selectivity of the Cu2O HMS/CB/GCE sensor was also investigated by using the chronoamperometry technique at the operating potential of 0.25 V. In order to check the effect of substances that might interfere with the sensor performance, five kinds of possible interfering substances, H2O2, uric acid, ascorbic acid, glucose and epinephrine were used for measurement in our experiments and the results are shown in Fig. 8. It can be observed that the five tested interferents could not cause obvious interference to the determination of DA, demonstrating the high selectivity of the Cu2O HMS/CB/GCE sensor.

Fig. 8
figure 8

Chronoamperometry curve of Cu2O HMS/CB/GCE in 0.1 mol L−1 PBS solution (pH = 5.7) with successive addition of 1.0 mmol L−1 DA, 1.0 mmol L−1 H2O2, 1.0 mmol L−1 UA, 1.0 mmol L−1 glucose, 0.1 mmol L−1 AA , 0.05 mmol L−1 epinephrine and 1.0 mmol L−1 DA at applied potential of 0.25 V

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Conclusion

The synthesis of Cu2O HMS/CB hybrid material in the DESs/H2O mixed solvent and its use in the preparation of amperometric sensor for the detection of DA are reported. SEM, TEM and XRD results reveal that the Cu2O microspheres are embedded in the CB aggregation and the presence of CB in the Cu2O HMS/CB composite obviously reduces the size of Cu2O HMS. The electrocatalytic properties of modified electrodes were studied by using cyclic voltammetry and chronoamperometry methods. Due to high electronic conductivity of CB and high electrocatalytic activity of Cu2O HMS, the Cu2O HMS/CB/GCE electrode exhibits higher electrocatalytic activity to DA oxidation compared to other modified electrodes. The Cu2O HMS/CB/GCE has been employed as an electrochemical sensor for determination of DA in the wide range from 9.9 × 10−8 to 7.08 × 10−4 mol L−1 with a low detection limit of 3.96 × 10−8 mol L−1 (S/N = 3). In addition, the sensor also displays advantages including high sensitivity, good reproducibility, long-term stability and selectivity. The work reported here provides a new platform for preparing an amperometric DA sensor with high performance and low cost.