Introduction

Dopamine (DA) is a crucial catecholamine neurotransmitter of the central nervous system (CNS) [1, 2]. Abnormal dopamine levels can lead to neurological disorders and other diseases, such as schizophrenia, Parkinson’s, and Alzheimer’s [3,4,5]. Therefore, the accurate and rapid determination of dopamine is of great significance in developing biomedical science and human health. Due to the simple operation, high sensitivity and selectivity, and immediate response of the electrochemical method, it has attracted considerable attention to detecting dopamine. However, the sensitivity and selectivity of conventional electrodes are not satisfied with the detection of dopamine due to the overlapping in the electrochemical potential window of dopamine with other substances such as uric acid (UA) and ascorbic acid (AA) [6]. Hence, to avoid this disadvantage, novel sensing nanomaterials need to be developed to improve the sensitivity and selectivity of electrochemical sensors.

MXene is a large category of two-dimensional (2D) nanomaterials. It is composed of transition metal carbides and nitrides [7,8,9,10,11]. Recently, MXene-based sensors have been increasingly reported. The functionalized MXene surfaces or combined MXene with other 2D nanomaterials can give the secondary component beneficial properties, resulting in the fabricated MXene-based sensors having more excellent performance than original MXene-based sensors [12,13,14]. Nevertheless, the conventional challenges for MXene-based sensors are the barrier to commercialization, including realizing significant performance, high stability, versatility, and achieving homogeneous and repeatable amplification processing of MXene-based sensors [12]. Thus, it is vital to improving the properties of composites that the excellent features of MXene construct the required properties of other nanomaterials through self-assembly and additive manufacturing [15].

Metal–organic framework materials (MOF) have become promising nanomaterials in sensing fields due to their high porosity, excellent adsorption performance, film-forming ability, controllable modification synthesis, etc. Therefore, they are considered sensing nanomaterials for fabricating electrochemical sensors [16,17,18]. However, most MOFs have relatively poor conductivity in aqueous solutions due to the property of coordination bonds. Therefore, because of the excellent features and particular structure of both MXene and MOF, the hybrid materials of MXene combined with MOF can further extend their widespread application in electrochemical sensors.

Herein, a new Ti3C2 membrane was prepared from Ti3C2 and UIO-66-NH2 using a hydrogen-bonded self-assembly approach in an aqueous solution with optimal reaction conditions at 60 °C. Ti3C2 membrane was realized by the hydrogen bonds between the –OH groups in Ti3C2 and the oxygen atoms of O=C groups in UIO-66-NH2. After the Ti3C2 film was freeze-drying, the hierarchical cave-like morphology was found via the scanning electron microscope (SEM). Moreover, the fabricated Ti3C2 membrane/GCE was used to detect dopamine via differential pulse voltammetry (DPV). The DPV response for sensing dopamine was obtained through selective oxidation of dopamine forming dopamine quinone in the electrochemical potential window of dopamine, proposed in Fig. 1.

Fig. 1
figure 1

The synthetic strategy of Ti3C2 membrane and the constructed Ti3C2 membrane/GCE sensor for sensing dopamine

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Experimental section

Preparation of Ti3C2 membrane

Typically, Ti3C2 (0.0343 g) and UIO-66-NH2 (0.0057 g) were dispersed in ultrapure water (10 mL), and the mixture was magnetically stirred at 60 °C for 6 h. Finally, the obtained sample was freeze-dried.

Fabricating Ti3C2 membrane/GCE sensor

Firstly, the bare GCE was polished on the chamois leather with Al2O3 (0.05 μm) and repeatedly washed with ultrapure water. Finally, the electrode was dried at room temperature. Then, the solution of the Ti3C2 membrane was cast onto the electrode surface. Again, the fabricated electrode was dried at room temperature. Meanwhile, the surface of GCE has formed a uniform film. This film is not fall off or collapse during the test.

Results and discussion

Structural characterization

The morphological features of the Ti3C2 and Ti3C2 membranes were studied via SEM and TEM. As shown in Fig. 2A, Ti3C2 exhibits an accordion-like structure with multilayers less than 50 nm. For hierarchical cave-like Ti3C2 membrane composite (Fig. 2B), it indicates that UIO-66-NH2 particles with typical octahedral morphology grow uniformly on the surface of Ti3C2. As shown in Fig. 2C, a compact composite Ti3C2 film is formed on the electrode surface. Furthermore, as illustrated in Fig. 2D, the diffraction peaks (002) in Ti3C2 shift to a smaller angle, agreeing with the reported literature [19]. XRD spectrogram of Ti3C2 membrane shows that the typical peak of UIO-66-NH2 at 6.26° matched well with the peak of Ti3C2, revealing that UIO-66-NH2 particles are successfully grown on Ti3C2 nanosheets [20]. Ti3C2 nanosheets and UIO-66-NH2 nanoparticles form a hybrid scaffold as displayed in TEM images of composites at different magnifications (Fig. 2E,F) and energy dispersive X-ray spectrum (EDS) elemental maps (Fig. S2A).

Fig. 2
figure 2

SEM for Ti3C2 (A), Ti3C2 membrane (B), Ti3C2 film on electrode (C). XRD graph (D). TEM for Ti3C2 membrane (E–F). Inset: The magnified SEM images of Ti3C2 (A), Ti3C2 membrane (B)

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Besides, the Ti3C2 membrane on the electrode was characterized using AFM. As shown in Fig. S2B, the Ti3C2 film has a flexible nanosheet structure with wrinkles on the surface and offers outstanding transparency. Furthermore, the 3D AFM image of the Ti3C2 film provides a more apparent characterization where no cracks, pinholes, or other visible defects, but rather rough surface, demonstrating that the fabricated Ti3C2 membrane has outstanding mechanical properties and particular morphology.

In order to further research the mechanism of Ti3C2 membrane formation, XPS spectra of the O1s of Ti3C2, UIO-66-NH2, and Ti3C2 membrane were recorded. Figure 3A exhibits that the Al element disappears in Ti3C2 under the etching action of HF, which makes for its excellent hydrophilicity and dispersion. Moreover, excessive -OH are introduced into Ti3C2 during the etching process of Ti3AlC2, which will significantly increase the hydrophilicity of Ti3C2 and thus form hydrogen bonds. Furthermore, the O1s XPS spectra Ti3C2 in Fig. 3B show three peaks at 528.8, 530.0, and 531.6 eV vested in the oxidized TiO2 phase and the functionalized C−Ti−Ox and C−Ti−O–H phases, respectively [21]. The fourth peak at 532.7 eV is referred to as adsorbed H2O. For the O1s spectrum of UIO-66-NH2, the prominent peak, related to the C=O bonds, lies at 531.7 eV, and the second one put down to Zr-O-C locates at lower energy (530.2 eV) [22]. Nevertheless, for Ti3C2@UIO-66-NH2, the O1s spectra exhibit that the main peak appears at lower energy (529.3 eV), indicating the formation of oxygen-containing hydrogen bonds (C-Ti–O∙∙∙H∙∙∙O=C). Such conversion from Ti3C2 to Ti3C2@UIO-66-NH2 can also be reflected in the O1s XPS spectra, where the binding energy of C-Ti–O-H (531.6 eV) is decreased to 529.6 eV because of the transformation from C-Ti–O-H bonds to C-Ti–O∙∙∙H∙∙∙O=C bonds.

Fig. 3
figure 3

A XPS survey spectra of Ti3C2, UIO-66-NH2, and Ti3C2 membrane. B The XPS image of O1s regions of Ti3C2 membrane

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Electrochemical performance

The CVs for different fabricated electrodes were carried out in Fe(CN)63−/4− solution (1 mM) containing KCl (0.1 M) at the potential ranging from − 0.1 V to 0.9 V with a scan rate of 100 mV/s. As can be seen from Fig. 4A, bare GCE and UIO-66-NH2/GCE exhibit minor symmetrical redox peaks. Nevertheless, the reduction peak current responses of Ti3C2 membrane/GCE were significantly increased, indicating that Ti3C2 membrane/GCE has sensitive electronic characteristics and strong adsorption capacity. Therefore, it can be suggested that Ti3C2 membrane has a better effect on promoting electron transfer, which may root in unique morphology and property of that.

Fig. 4
figure 4

A CVs of bare GCE (a), UIO-66-NH2/GCE (b), and Ti3C2 membrane/GCE (c) in Fe(CN)63−/4− solution of 1 mM including KCl of 0.1 M. Scan rate: 100 mV s−1. Inset: Amplified CV plot of bare GCE (a) and UIO-66-NH2/GCE (b). B Nyquist diagrams of bare GCE (a), UIO-66-NH2/GCE (b), and Ti3C2 membrane/GCE (c) in Fe(CN)63−/4− solution of 1 mM including 0.1 M KCl

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Figure 4B shows the EIS of different modified GCE with 0.19 V open-circuit voltage, 5 mV voltage amplitude, and a frequency ranging from 0.1 to 100 kHz. As can be seen from Fig. 4B, the semicircle diameter of UIO-66-NH2/GCE (Rct = 133.32 Ω) is much smaller than the case of bare GCE (Rct = 216.00 Ω), indicating that the UIO-66-NH2 accelerates the electron transport. Whereas Ti3C2 membrane/GCE (Rct = 62.53 Ω) has a significantly smaller semicircle diameter than UIO-66-NH2/GCE (Rct = 133.32 Ω), indicating that the electron transfer on the surface of the electrode was accelerated through Ti3C2 membrane, that is to say, Ti3C2 outstanding enhances the electrical conductivity. The results are consistent with CV results.

Sensor capability of the Ti3C2 membrane/GCE

The Ti3C2 membrane/GCE sensor capability for detecting DA was studied via DPV method under the optimum experimental conditions (See Supporting Information), at the potential ranging from − 0.1 to 0.6 V with 100 mV s−1 scan rate. As can be seen from Fig. 5A,B, the DPV response of the fabricated sensor was significantly enhanced as the concentration of DA increased from 0 to 100 pM. However, when DA concentration reached 100 pM and above, the DPV response of the fabricated sensor was gradually stabilized, indicating that the conductance of the fabricated sensor was gradually saturated [23]. Furthermore, as shown in Fig. 5C, the DPV responses heightened with increasing DA concentration, demonstrating an excellent linear dependence between the current (I) and negative logarithm of DA concentration (− lgC) ranging from 1 to 250 fM with 14.72 µA fM−1 cm−2 sensitivity. The linear regression equation was I (μA) =  − 10.396 (− lgC) + 15.885 (R2 = 0.9965). The limit of detection (LOD) was 0.81 fM. The sensing performances of the sensor and the reported electrochemical sensors for identifying DA are shown in Table 1. Moreover, the reported other methods for detecting DA are listed in Table S1. As can be seen from Table 1 and S1, the performance of the Ti3C2 membrane/GCE sensor was superior to the reported electrochemical sensors and even suitable for placement among the reported other methods for the sensing DA with a long linear range and low LOD. The remarkable capability of the sensor was ascribed to the synergistic outcomes of the unique morphology and good conductivity of the Ti3C2 membrane.

Fig. 5
figure 5

A DPV of Ti3C2 membrane/GCE sensor with different concentrations of DA (a ~ o: 0, 1, 1.5, 2, 3, 5, 7, 10, 15, 20, 30, 50, 100, 200, 250 fM, respectively) in PBS (1 mM, pH 7.0). B Analyzed the DA detection range of the fabricated sensor. C Calibration chart for DA concentrations from 1 to 250 fM. D The interference of various species for DA detection

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Table 1 The reported electrochemical sensors for detecting DA
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Interference, reproducibility, and stability studies

Because the oxidative potential of DA overlaps with the signals of AA and UA, it is easy to affect the detection of DA on the electrode. Therefore, the selectivity for the constructed sensor was determined with potential interfering substances containing K+, Na+, Glu, LA, BSA, AA, and UA. As shown in Fig. 5D, tested substances do not interfere obviously with 200 fM DA. Thus, this sensor has an outstanding anti-interference capability for detecting DA.

Besides, the Ti3C2 membrane/GCE sensor’s long-term stability was investigated by examining its DPV response after storage at 4 °C. As shown in Fig. S8, after 15 days, the sensor maintains more than 92.1% of its initial response to DA, resulting in excellent standing stability, and the reproducibility of this sensor was further evaluated via detecting 200 fM DA under the same conditions (Fig. S9). Five independent sensors yielded 3.16% relative standard deviation (RSD) under the same method, demonstrating excellent reproducibility.

Real sample detection

To appraise the application of the Ti3C2 membrane/GCE sensor in practice, which was used to detect spiked DA from human serum samples [37,38,39]. Drug-free human blood samples were collected from healthy volunteers at the Guilin University of Technology Hospital. All the blood samples were obtained through venipuncture, and the coagulant was added rapidly. Due to the high protein of DA bonding in the blood plasma, these serum samples were pretreated using a high-speed refrigerated centrifuge to eliminate the interferences and improve the recovery [40,41,42,43]. Then, 100 μL of blood serum was diluted with 5 mL of 0.1 M PBS (pH = 7.0) buffer to prevent the matrix effect of actual samples [44]. Different concentrations of DA standard solution were selected according to the linear range and added to the diluted serum samples to prepare the spiked samples. Then, the determination of dopamine was carried out through DPV with the applied potential (+ 0.264 V vs. Ag/AgCl). Finally, the recovery rates were calculated according to reference [45]. As shown in Table 2, the results indicated that the recoveries were 101.2% to 103.5%, with the RSD in the range of 1.2–2.4%. The obtained result shows that the Ti3C2 membrane/GCE sensor has remarkable recoveries of DA on biological samples.

Table 2 Results for the detection of dopamine in human serum samples (n = 5)
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Conclusion

A novel Ti3C2 membrane has been synthesized from hydrogen-bonded self-assembly in a water solution with a large specific area, distinguishing electronic conductivity network, and excellent dispersion in the aqueous phase. These outstanding performances played a significant role in electron transfer and fabricating sensors. Astonishingly, the fabricated sensor of the Ti3C2 membrane exhibited a substantial performance for DA with high selectivity and sensitivity. Moreover, the fabricated sensor was further applied to fast-sensing DA in the real samples with excellent recoveries. This study indicates that the applications of some novel prepared MXene membranes in sensor fields are still in the initial stage, meaning that the fabricated hybrid materials of MXene combined with properties of other materials will have promising application prospects. Nowadays, the COVID-19 pandemic spreads around the world; more advanced sensors are urgently needed with high sensitivity and selectivity, rapid determination, and commercialization. This work opens a direction to building advanced sensors using MXene-based nanomaterials.