Introduction

Ultra-long nanofibers with large anisotropy can be used in many fields such as composites, microelectronics, separation, and biosensing [1, 2]. Among them, ZnO nanofibers are widely used in sensors due to their high sensitivity, low detection limit, and fast electron transfer kinetics [3,4,5,6]. At present, electrospinning and sol–gel, combined with magnetron sputtering methods, are widely used for preparing ultra-long nanofibers, and hydrothermal growth methods using carbon fibers as a template are also included [7, 8]. These preparation methods result in ordered ultra-long nanofibers to further achieve the desired function [9, 10]. The carbon fiber here is chemically resistant and electrically conductive and is often used as the electrode material [11] in addition to being used as a template. Therefore, the carbon fiber and the nanomaterial grown thereon can be directly served as an unsupported electrode which does not require a commercial electrode as support [12,13,14,15]. Yang [3] used this type of electrode to achieve the detection of dopamine, and Liu [16] achieved simultaneous detection of uric acid and ascorbic acid.

The detection of dopamine (DA) is of great significance for the diagnosis of neurological diseases such as schizophrenia and Parkinson’s disease [17]. However, the current electrochemical detection of DA is disturbed because uric acid (UA) and DA have similar redox potentials in vivo [18]. The solution to this problem is usually to use a modified electrode so that the redox potentials of the two do not overlap. At present, complex of cyclodextrin and graphene [19, 20], precious metal composite [21,22,23,24], inorganic compound boron nitride [25], semiconductor composite indium tin oxides [26], graphene composite [27, 28], and electrochemically pretreated pencil graphite electrodes [29], these material modification electrodes have achieved a distinction for the oxidation–reduction potential of UA and DA.

Inspired by the above work, we sputtered ZnO seed layer on carbon fiber and prepared ZnO nanofibers/carbon fibers (ZnO NF/CF) free-standing electrodes by hydrothermal method. The vision is to use this electrode to distinguish the oxidative–reduction potentials of UA and DA while achieving their simultaneous detection.

Experiment

Reagent

DA and UA were purchased from Sigma-Aldrich. Different pH phosphate, buffer solutions (PBS) were prepared by mixing NaH2PO4 and Na2HPO4 solutions and then adjusting the pH with 0.1 M NaOH and H3PO4. All other chemicals are of analytical grade and are used as received. All solutions were prepared using ultra-pure water (≥ 18 MΩ, Milli-Q, Millipore). The DA hydrochloride and UA solutions were prepared on the same day and stored in the refrigerator.

Instrument

Scanning electron microscopy (SEM, JSM-6330F) and energy-dispersive spectroscopy (EDS) were used to measure the surface morphology, and elemental analysis was performed on the sample. The X-ray diffraction pattern was obtained using XRD-6000 diffracted radiation (Cu K, λ = 0.15406 nm).

Electrochemical measurements of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using a CHI 660E workstation (Shanghai Chenhua Instrument Corporation, China). CV experiments were performed at a scan rate of 50 mV s−1 unless otherwise stated. The DPV experiment was performed with the following parameters: amplitude, 0.05 V; pulse width, 0.2 s; sample width, 0.0167 s; pulse period, 0.5 s; and quiet time, 2 s.

Preparation of ultra-long ZnO NF/CF electrode

The ultra-long ZnO NF/CF free-standing electrode was obtained in two steps, as shown in Scheme 1; the first step is to sputter a ZnO seed layer on the surface of the carbon fiber by RF magnetron sputtering (MSP-30 °C). Briefly, a ZnO (5 N, 7.5 cm diameter) target was used to deposit a ZnO layer (length and diameter of 4 cm and 2 mm, respectively) on a carbon fiber substrate. A gas mixture of Ar (50%) and N2 (10%) having a total pressure of 1.0 Pa was used as a sputtering gas. All samples were sprayed for 5 min under conditions of 3.3 Pa, 50 sccm Ar, and 10 sccm N2. Next, a mixed solution of zinc acetate dihydrate (0.05 M) and hexamethylenetetramine (HMTA, 0.05 M) was prepared, and after stirring for 5 min, it was transferred to a Teflon lined stainless steel autoclave having a volume of 50 mL. Carbon fibers sputtered with a ZnO seed layer were immersed in the above-mixed solution. The mixture was hydrothermally treated at 90 °C for 3 h and then to room temperature. Finally, ultra-long ZnO NF/CF was rinsed with deionized water and dried under vacuum at 60 °C for 6 h and used to detect UA and DA. The optimization of the morphologies and cross-sections of ZnO NF/CF is shown in Support Information Figs. S1–S4.

Scheme 1
scheme 1

Schematic diagram of ZnO NF/CF preparation and sensor assembly

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Results and discussion

Characterization of ultra-long ZnO NF/CF electrode

Here, ZnO is in situ hydrothermally grown on the carbon nanofiber to form a stand-by electrode. Figure 1a, b shows the typical structure of ZnO NF/CF. It can be seen that the material prepared by this method has an ordered orientation, and the ZnO on the surface of the carbon fiber is needle-like (Fig. 1b). This needle-like ZnO is uniform and covers the entire surface of each carbon fiber and is not aggregated (Fig. 1c). Further, the single ZnO has a diameter of about 18 nm and a length of 4.5 μm (Fig. 1d). The components of these prepared materials are mainly O, C, and Zn (Fig. 1e). Further component analysis is shown in the XRD diagram (Fig. 1f), where the 2θ = 26.5 and the 44° peak is assigned to (002) and (100) of the carbon fiber. Other results show a hexagonal wurtzite ZnO structure with no characteristic peaks of other impurities, indicating that the composition of the above nanofibers is ZnO and carbon fiber. Therefore, these results indicate that the nanocomposites are ZnO NF/CF. It is worth noting that the topography of the resulting sample of Fig. 1 shows that the needle-like ZnO nanolayer is uniform and orderly sputtered on the carbon nanofiber, which increases the specific surface area of the electrode and enhances the electron transport rate [3,4,5]. So, it can be expected that the as-prepared materials are competitive for the electrochemical applied.

Figure 1
figure 1

a–c Morphology with different magnifications, d length, e EDS, and f XRD of ultra-long ZnO NF/CF

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Redox of UA or DA on ZnO NF/CF electrode

Figure S5 shows the cyclic voltammogram of the different electrodes. On the bare carbon fiber electrode, a weak oxidation peak of 0.4 V was observed and there was almost no reduction peak, which may be the hydrophobicity and chemical inertness of carbon fibers [25]. In contrast, a large oxidation current on the ZnO NF/CF electrode may be caused by several factors: (1) The central carbon fiber provides a channel for rapid electron transport; (2) the ZnO is a graphite-like structure [26], which forms a synergistic effect between ZnO and carbon fiber, and (3) the ZnO microstructure on the prepared ZnO/CF material is well dispersed, making UA easy to access conductive carbon fiber. In addition, ZnO/carbon fiber heterojunctions result in higher electrochemical activity [24]. For DA redox, there are similarities to UA, except for their oxidation potential [27, 28] (Fig. 2a). In addition, the chemical processes of UA and DA on the surface of the electrode are all oxidation processes controlled by surface diffusion (Fig. S6).

Figure 2
figure 2

DPV electrochemical response (1) and linear range (2) of a DA and b UA on the ZnO NF/CF electrode

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The electrical signal detection on the ZnO NF/CF electrode for UA or DA could be affected by electron transfer from the ZnO surface, as well as the electrolyte acidity, scan rate, and analyte concentration. As shown in Scheme 2, in the outermost layer of the ZnO crystal, there are many exposed zinc atoms, each of which has one or two free unoccupied empty orbitals [30]. Meanwhile, the valence electron orbitals of the oxygen atoms in the phenolic hydroxyl groups of DA and UA are sp2 hybrid, and the lone pair electrons can be bonded to the upper orbital of Zn. In the electrochemical oxidation process, first, DA and UA molecules are adsorbed on the surface of the electrode. The unshared pair of oxygen atoms in the phenolic hydroxyl group is close to the unoccupied orbital of the zinc atom, and then, the compound is formed by orbital overlap. Since electrons are shared with zinc atoms, the electron density of oxygen atoms is lowered. The bond polarity of “OH” is increased, and H+ can make DA and UA molecules as free positive ions. At the electrode voltage, the electrons in the coordinate-like will shift and transfer to the electrode. Then, a coordinate-like bond is broken, and the oxygen atom of the organic compound that loses hydrogen and two electrons will be positively charged, and thus, the electron cloud of the oxygen atom will deviate. For DA, the other hydrogens on the phenolic hydroxyl group will leave as H+ and the large π bond of the benzene ring will be destroyed. Finally, a new π bond is formed between oxygen and carbon to produce o-benzoquinone. For UA, the hydrogen on the imidazole ring will leave as H+ and produce a new conjugated molecule called ninhydrin.

Scheme 2
scheme 2

Proposed mechanism of the oxidation process of DA and UA on the surface of ZnO NF/CF

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DPV behavior of DA in the presence of UA

DPV is used here to detect DA and UA because it has higher sensitivity and better resolution than cyclic voltammetry [29]. The results of different concentrations of UA and DA are shown in Fig. 2a, b. Three important characteristics that must be considered are as follows: (1) Different oxidation potentials (0.35 V UA and 0.2 V DA) were observed, and (2) as the concentration increases, the oxidation current increases linearly, indicating a linear trajectory of UA and DA; (3) the oxidation current is affected by the adsorption force. UA and DA have aromatic rings in their molecular structure (Fig. S7), resulting in high oxidation currents (Fig. 2a, b). In addition, the specificity of such electrodes also shows better acceptability. The presence of glucose, potassium chloride, and sodium chloride did not affect the detection of UA or DA (Fig. S8).

For the electrochemical detection of the mixture of UA and DA, we changed the concentration of DA therein, while the concentration of UA was constant. As shown in Fig. 3, the DA concentration varied between 6 and 20 μM and the UA was fixed at a relatively large concentration of 0.1 mM. When the DA concentration increases, the oxidation peak current of DA also increases, but the oxidation peak current of UA is always constant. The peak potential of DA is 0.19, and the peak potential of UA is 0.38, which separates DA and UA well. The DA detection here has a linear range of 6–20 μM and a detection limit of ~ 0.402 μM, which is even better than some complex sensor results (see Table 1 for more information). These results also indicate that the ZnO NF/CF electrode can be used for the quantitative detection of DA in the coexistence of UA. Therefore, the ZnO NF/CF electrode is also a promising candidate for the selective determination of DA in the presence of UA.

Figure 3
figure 3

a DPV of DA with different concentrations containing 0.1 mM UA. b The corresponding plots of peak current versus the concentration of DA

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Table 1 UA and DA by DPV using different electrodes
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Conclusion

In summary, the ZnO seed layer was successfully sputtered on carbon fiber by magnetron sputtering to prepare ultra-long ZnO NF/CF and directly used as a free-standing electrode. The electrode was characterized, and the electrochemical processes of UA and DA on the electrode surface were discussed. The DPV method can be used to detect sensitive DA or UA, and the detection of DA can be realized under the high concentration of UA (0.1 mM). The DA detection here has a linear range of 6–20 μM and a detection limit of ~ 0.402 μM. These results indicate that this electrode can perform effective electrochemical oxidation of UA and DA and the resolution of oxidation peaks and can be used as an alternative for the simultaneous detection of UA and DA.