Electrochemical sensor based on novel two-dimensional nanohybrids: MoS₂ nanosheets conjugated with organic copper nanowires for simultaneous detection of hydrogen peroxide and ascorbic acid

Abstract

There has been increasing interest in the use of molybdenum disulfide (MoS₂) for the synthesis and application of functional hybrid nanomaterials. Here, for the first time, we prepared MoS₂ nanosheets that were modified with organic copper (OCu) nanowires by a facile two-step hydrothermal synthesis. Multi-characterization techniques such as scanning electron microscopy, transmission electron microscopy, and atomic force microscopy were used to observe the conjugation of OCu with MoS₂ and the morphology of the fabricated MoS₂–OCu nanohybrids. In addition, other methods such as X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction were utilized to understand the structure and properties of the prepared MoS₂–OCu nanohybrids. Finally, the synthesized MoS₂–OCu nanohybrids were further utilized to fabricate multi-functional electrochemical sensors for highly sensitive detection of hydrogen peroxide (H₂O₂) and ascorbic acid (AA). The obtained results indicate that the synthesized novel MoS₂–OCu nanohybrids exhibit excellent electrocatalytic activity towards both analytes, and present potential applications for detecting H₂O₂ and AA with high stability, sensitivity, and selectivity.

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1. Introduction

Electrochemical sensors are novel devices that have been widely applied in various fields such as biomedicine, environmental science, and food analysis due to their quick current response, wide detection range, high stability, and universal detection of different analytes.^1–4 Nowadays, many small molecules and macromolecules, including glucose, ascorbic acid (AA), hydrogen peroxide (H₂O₂), dopamine (DA), DNA, and protein, have been detected by various electrochemical sensors or biosensors.^5–8 Among them, H₂O₂, as one of the most important substances of many bio-related reactions in environmental and biological fields, is an important target for biological analysis. Therefore, it is essential to analyze and detect H₂O₂ in a precise and convenient way. AA is another familiar compound that is present in almost all organisms. It assists with maintaining normal metabolism and is necessary for life and health. Hence, it is also necessary to determine the content of AA in a biological system.

Previously, various methods, such as spectrophotometry,^9,10 fluorescence spectroscopy,^11,12 and electrochemistry,⁶ have been utilized to detect H₂O₂ and AA. Among these methods, the electrochemical method has been thought of as one of the most effective methods to detect H₂O₂ and AA due to its high sensitivity, good selectivity, and high stability. The electrochemical sensors can be divided into enzymatic biosensors and non-enzymatic sensors, which have been fabricated and used for various high-performance detection applications.^3,13 Previous studies indicated that non-enzymatic electrochemical sensors can be employed in a wider range of applications in various fields as compared to enzymatic biosensors due to their higher stability, increased sensitivity, and easier fabrication process.^14,15

In order to design high performance non-enzymatic electrochemical sensors, the modification of electrodes with highly active material is a basic but important step. In recent years, two-dimensional (2D) nanomaterials, such as graphene, that possess unique chemical and physical properties, have shown promising prospects in the fields of nanotechnology, materials science, analytical science, and biomedicine.^16,17 With the development of graphene-like materials, transition-metal dichalcogenides (TMDs), another type of 2D material that includes WS₂, TiS₂, and others, have aroused intensive interest due to their unique structure and properties.^18–20 As a typical 2D layered material, molybdenum disulfide (MoS₂) possesses excellent thermal stability and high electrocatalytic activity, which are desirable properties for its use in various applications such as sensors,²¹ electrocatalysts,²² supercapacitors,²³ and energy storage devices.²⁴ For instance, recently Wang et al. fabricated an electrochemical H₂O₂ sensor with high sensitivity and selectivity based on ultra-small MoS₂ nanoparticles.²⁵

To improve the sensing performance of MoS₂-based non-enzymatic electrochemical sensors, the combination of MoS₂ with metal oxides, noble metals, or carbon materials has been proven to be an ideal strategy.²⁶ In our previous studies, MoS₂-based electrochemical sensors were fabricated that showed enhanced electrocatalytic activity and sensing performances towards a few analytes.^21,27 For example, we have decorated MoS₂ nanosheets with platinum nanoparticles (PtNPs) and utilized the synthesized MoS₂–PtNP nanohybrids as electrode materials to fabricate an electrochemical H₂O₂ sensor.²⁷ The experimental results indicated that the created MoS₂–PtNP-based H₂O₂ sensor exhibits high sensitivity, good reproducibility, and long-term stability. In another work, we successfully fabricated 3D coral-like structural MoS₂–Cu₂O porous nanohybrids, which were utilized as a bifunctional material for electrochemical sensor and oxygen reduction reaction (ORR) applications.²¹ Due to the combination of p-type Cu₂O nanoparticles and n-type MoS₂ nanosheets, the created MoS₂–Cu₂O nanohybrids exhibit great potential in both electrochemical non-enzymatic H₂O₂ detection and ORR catalysis.²¹ In other studies, MoS₂ has been also decorated with prussian blue nanocubes,²⁸ gold nanoparticles,²⁹ and others³⁰ to fabricate electrochemical sensors. All these experiments proved that MoS₂ is an excellent candidate nanomaterial for creating electrochemical sensors for chemical and biological molecule determinations.

For organic–inorganic nanocomposites, the polymer chains insert into the layered inorganic materials and the mobility of starch chains is reduced, which can improve the dimensional stability of materials. A combination of inorganic materials with organic materials also has unique properties in the field of electrochemistry.^31,32 In this work, novel organic copper (OCu) nanowires were synthesized and decorated onto the surface of MoS₂ nanosheets through a two-step hydrothermal reaction. Furthermore, the as-prepared MoS₂–OCu nanohybrids were used to modify glassy carbon electrodes (GCEs) for fabricating both electrochemical H₂O₂ and AA sensors. It is expected that the MoS₂–OCu nanohybrid-based non-enzymatic electrochemical sensors will show enhanced performances for detecting both H₂O₂ and AA, and there will be more potential applications in materials science and analytical science for the synthesized OCu nanowires. In addition, it is likely that this work will supply new ideas to readers so that they can design and apply new types of electrochemical sensors and biosensors in several related areas.

2. Experimental section

2.1 Reagents and materials

Sulphur molybdenum powder, N-methyl pyrrolidone (NMP, ≥99.9% purity), AA, o-phenetidine, uric acid (UA), and dopamine (DA) were purchased from J&K Scientific Ltd, China. Nafion solution was purchased from Sigma-Aldrich Company, China. NaOH, ethanol, H₂O₂ (30% aqueous solution), disodium hydrogen phosphate (Na₂HPO₄), acetic acid (AC), and sodium dihydrogen phosphate (NaH₂PO₄) were purchased from the Beijing Chemical Co., Ltd, China. The water used in this study was purified by a Millipore reverse osmosis system (18.2 MΩ cm).

2.2 Synthesis of MoS₂ nanosheets and MoS₂–OCu nanohybrids

MoS₂ nanosheets were prepared using the liquid-phase exfoliation method. In brief, bulk MoS₂ powder was dispersed in NMP (7.5 mg mL⁻¹) for 4 h using a horn-probe tip sonicator (Sonics Vibra-cell VCX-650W ultrasonic processor) operating at 285 W with an on (6 s) and off (2 s) pulse. The obtained raw dispersion was centrifuged for 60 min at 1500 rpm to remove the unexfoliated MoS₂. Then, the supernatant was decanted and centrifuged for another 90 min at 4500 rpm. The sediment was then collected for subsequent synthesis.

MoS₂–OCu nanohybrids were synthesized via a two-step hydrothermal process. In brief, 1.6 mmol Cu(AC)₂ was dissolved in 64 mL distilled water, and then 80 mL of 0.04 M o-phenetidine solution was added to the reaction system. Subsequently, 16 mL 0.05 M AC was quickly added. The mixture was maintained at 90 °C for 10 h under stirring. The sediment was collected by filtration. In the second step, 80 mL NMP and MoS₂ was mixed with the sediment at 50 °C for 24 h under stirring. The obtained MoS₂–OCu products were collected by centrifugation, washed with ethanol several times, and dried at air and room temperature for future use.

2.3 Fabrication of MoS₂-, OCu, and MoS₂–OCu-modified electrodes and electrochemical experiments

Firstly, a GCE with a diameter of 3.0 mm was polished with alumina slurry. After that, the polished GCE was washed several times with distilled water and ethanol in an ultrasonic bath, and then dried at room temperature. In order to prepare the MoS₂- (OCu and MoS₂–OCu)-modified GCE, 10 mg MoS₂ (OCu and MoS₂–OCu), 100 μL Nafion (0.1%), and 1 mL ethanol were blended in a glass test tube and then transferred to an ultrasonic bath for 10 min. Afterwards, 10 μL of the mixture solution was dropped onto the surface of the GCE and dried at room temperature. These modified GCEs were then used for both electrochemical amperometric response (I–T) and cyclic voltammogram (CV) measurements with an electrochemical workstation (Chenhua CHI760D, Shanghai, China) at room temperature.

A three-electrode system was used with these modified GCEs as working electrodes, with a potassium chloride (KCl)-saturated calomel electrode as the reference electrode, and a Pt electrode as the auxiliary electrode. For the H₂O₂ sensing application, 0.1 M phosphate buffer solution (PBS, pH 7.4) was selected as the test solution. The PBS solution was prepared by mixing 0.1 M NaH₂PO₄ and 0.1 M Na₂HPO₄. The PBS solution was deoxygenated with pure N₂ for 10 min before every electrochemical experiment. The test solution for the AA sensor application was a 0.1 M NaOH solution. The amperometric response tests were performed with constant stirring.

2.4 Experimental techniques

The scanning electron microscopy (SEM) images were recorded with a JSM-6700F scanning electron microscope (JEOL). Atomic force microscopy (AFM) images were obtained using a MultiMode 8 (Bruker Nano) atomic force microscope. A Tecnai G220 transmission electron microscope was used at an accelerating voltage of 200 kV to obtain transmission electron microscopy (TEM) images. Magnified TEM images were collected on a JEM-2100F field emission transmission electron microscope operated at 200 kV. Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, ThermoFisher Scientific Inc., Waltham, MA), X-ray diffraction (XRD, Rigaku D/max-2500 VB+/PC), and X-ray photoelectron spectroscopy (XPS, ThermoVG, ESCALAB 250) were utilized to measure the structures and characteristics of both MoS₂ nanosheets and MoS₂–OCu nanohybrids.

3. Results and discussion

3.1 Synthetic mechanism of MoS₂–OCu nanohybrids

Fig. 1 shows the synthetic process of MoS₂–OCu nanohybrids through a two-step hydrothermal method. Bulk MoS₂ was initially exfoliated to MoS₂ nanosheets by the ultrasonic exfoliation method in NMP solution. It is well known that liquid-phase exfoliation is a facile method to obtain a range of layered 2D materials such as graphene and TMDs from a monolayer to few-layer nanosheets.³³ The layered materials can be effectively exfoliated in solvents with a surface tension closely matching the surface energy of these materials, and NMP has previously been proven to be a good solvent for the liquid-phase exfoliation of MoS₂.³⁴

Fig. 1 Schematic depiction of the two-step hydrothermal synthesis of MoS₂–OCu nanohybrids and electrochemical sensing mechanisms.

In the hydrothermal reaction, AC, o-phenetidine, and Cu(AC)₂ were added to promote the chemical synthesis of OCu nanowires. O-Phenetidine underwent a polymerization reaction and generated poly(o-phenetidine), AC readily protonates poly(o-phenetidine), and carboxylic groups strongly coordinate Cu²⁺ ions, resulting in the formation of polymer chain-associated Cu²⁺ ions.³⁵

We suggest that the association of polymers with Cu²⁺ is also ascribed to the chemical interaction between Cu²⁺ and the amine ligand group (–NH–) (Fig. 2).³⁶ The obtained metal organic complex was then transferred to another flask with NMP and MoS₂ nanosheets. Once OCu nanowires take part in the system, they spontaneously assemble on the surfaces of 2D MoS₂ nanosheets through van der Waals interaction to form MoS₂–OCu nanohybrids, as shown in Fig. 1.

Fig. 2 Molecular structures and synthesis mechanism for OCu nanowires. The complexes of Cu²⁺ with carboxylic acids are associated with protonated polymer chains.

3.2 Morphological characterizations of MoS₂ nanosheets and MoS₂–OCu nanohybrids

The morphological structure of the as-prepared MoS₂ nanosheets is shown in Fig. 3a–d. Fig. 3a displays a typical SEM image that clearly reveals that the formed MoS₂ nanosheets are stacked together. In order to examine the detailed structure of the MoS₂ nanosheets, the prepared MoS₂ nanosheets were further characterized with TEM and AFM. It can be seen in Fig. 3b that the as-prepared MoS₂ sheets have a lamellar structure with multi-layers. The typical AFM image of the synthesized MoS₂ nanosheets is displayed in Fig. 3c, which further confirms that MoS₂ has a lamellar structure and the height of the MoS₂ nanosheets is approximately 2–5 nm. A high-resolution TEM (HRTEM) edge image of MoS₂ was also obtained, in which two levels of lamellar structure can be seen (Fig. 3d).

Fig. 3 Morphological characterizations of (a–d) MoS₂ and (e, f) MoS₂–OCu nanohybrids: (a) SEM, (b) TEM, (c) AFM, and (d) magnified TEM picture of MoS₂ nanosheets; and (e) SEM and (f) magnified TEM image of MoS₂–OCu nanohybrids. The OCu nanowires are labelled with black arrows in (f).

Fig. 3e presents a typical SEM image of the hydrothermally synthesized MoS₂–OCu nanohybrids. A comparison of the images without (Fig. 3a) and with (Fig. 3e) OCu nanowires reveals that the conjugation of OCu nanowires onto the MoS₂ nanosheets reduced the stacking and increased the dispersity of the nanosheets. The modified OCu nanowires can also be clearly found in the magnified TEM image of MoS₂–OCu nanohybrids (Fig. 3f), which illustrates that MoS₂ nanosheets are covered with many OCu nanowires, as indicated by the arrows.

To understand the structure of OCu nanowires, the pure OCu nanowires that did not undergo conjugation with MoS₂ nanosheets were measured with SEM. Typical images are provided in Fig. 4a and b, which show that the synthesized OCu nanowires are twisted with a length from a few to tens μm and a diameter of 20–40 nm. The elemental composition of the OCu nanowires was characterized by elemental mapping and is presented in Fig. 4c–e. The signals of three elements (Cu, O, and C) are clearly detected. Fig. 4f displays the energy dispersive X-ray spectroscopy (EDX) results for OCu nanowires, proving the presence of Cu, O, and C.

Fig. 4 (a, b) SEM images of OCu nanowires at different magnifications. (c–e) The carbon, copper, and oxygen elemental mapping of a selected area of an individual OCu nanowire. (f) EDX of OCu nanowires, and the insert is a table of element weight.

3.3 Characterizations of MoS₂–OCu nanohybrids

The structure and properties of the prepared MoS₂ nanosheets and MoS₂–OCu nanohybrids were further studied with XRD and XPS techniques. Fig. 5a and b present the XPS spectra of MoS₂ nanosheets and MoS₂–OCu nanohybrids, respectively. It can be seen that both samples show bands at 162, 229, 285, 395, and 533 eV, which could be assigned to the characteristic peaks of S 2p, Mo 3d, C 1s, N 1s, and O 1s, respectively. We suggest that the binding energy between Mo and S is very strong after the addition of OCu, and therefore, OCu cannot change the crystallinity of MoS₂. Additionally, in Fig. 3b, two new peaks appeared at 953 and 933 eV, which are ascribed to the characteristic bands of Cu 2p_1/2 and Cu 2p_3/2, respectively. The obtained XPS data indicate that MoS₂ nanosheets were successfully modified by OCu.

Fig. 5 Structural characterizations of MoS₂ and MoS₂-OCu nanohybrids: (a, b) XPS spectra, (c) XRD patterns, and (d) FT-IR spectra.

Fig. 5c displays the XRD patterns of MoS₂ nanosheets and MoS₂–OCu nanohybrids. It can be found that both samples contain the lattice planes of (002), (100), (103), (105), and (110), representing the broad diffraction peaks of MoS₂ nanosheets.³⁷ However, in the XRD pattern of MoS₂–OCu nanohybrids, an obviously broader peak (22°) occurred, illustrating that an additional amorphous structure was attached to the MoS₂ nanosheets. In order to determine the structure of MoS₂–OCu, the synthesized MoS₂ nanosheets and MoS₂–OCu nanohybrids were measured with FT-IR, and the corresponding spectra are shown in Fig. 5d. Compared with MoS₂, the spectrum of MoS₂–OCu reveals three obvious absorption peaks at 1479 (–CH₂–), 1237 (C–O–C), and 739 cm⁻¹ (two substituted benzene rings). The above XRD and FT-IR results demonstrate that the synthesized MoS₂–OCu nanohybrids are OCu-containing compounds.

3.4 Non-enzymatic electrochemical H₂O₂ sensor

The created MoS₂–OCu nanohybrids can be applied as an electrode material to modify GCEs and then to fabricate an electrochemical non-enzymatic sensor. Therefore, firstly we used the MoS₂–OCu-modified GCE (MoS₂–OCu/GCE) to fabricate an electrochemical sensor for detecting H₂O₂. A control experiment with the MoS₂/GCE was also carried out in order to prove that this type of material can improve the performance of the sensors. In this work, it is expected that the fabricated electrochemical H₂O₂ sensor should show enhanced electrochemical sensing performance compared to some previously reported sensors.

Fig. 6 shows the electrochemical detection of H₂O₂ with the fabricated MoS₂–OCu/GCE-based sensor. Fig. 6a presents the CV data for both the MoS₂/GCE and MoS₂–OCu/GCE with the addition of 10 mM H₂O₂ under a scan of 100 mV s⁻¹. It can be seen that the fabricated MoS₂/GCE did not show redox processes, while for the fabricated MoS₂–OCu/GCE, there is a strong current response with a clear reduction peak. The broad reduction peak begins at approximately −0.2 V and reached a maximum at −0.3 V due to the reduction of H₂O₂. The current responses of this electrode were examined at different H₂O₂ concentrations from 0 to 40 mM in order to further investigate the electrochemical behaviour of the fabricated MoS₂–OCu/GCE, as shown in Fig. 6b. The obtained result proved that the redox peak current clearly increases with increasing H₂O₂ concentration.

Fig. 6 Electrochemical detection of H₂O₂: (a) CV images of GCEs modified with MoS₂, and MoS₂–OCu nanohybrids; (b) CV images of the MoS₂–OCu/GCE under different H₂O₂ concentrations; (c) I–T response images of the MoS₂–OCu/GCE in PBS with successive addition of H₂O₂ at −0.3 V; (d) calibrated line; (e) amperometric responses upon successive addition of 2 mM H₂O₂, 3 mM UA, 3 mM AA, 3 mM DA, and 2 mM H₂O₂; (f) storage stability in response to 2 mM H₂O₂.

Hence, an applied potential at −0.3 V was selected for the current–time (I–T) measurement in this work. Fig. 5c displays a typical I–T curve of the fabricated MoS₂–OCu/GCE by successive addition of H₂O₂ at different concentrations in PBS. It can be seen that the fabricated MoS₂–OCu/GCE displays a steady response over a long-term test after the addition of H₂O₂ at different concentrations. Fig. 6d shows the corresponding calibration curve, and a regular response to H₂O₂ can be observed. The linear range (LR) of the fabricated H₂O₂ sensor is from 0.085 to 38.0 mM (R = 0.9897, S/N = 3), and the limit of detection (LOD) was calculated to be 0.0767 μM. The LOD can be defined using the equation LOD = 3σ/S, where σ is the standard deviation of the response, and S is the slope of the curve.

In order to investigate the electrochemical reaction effect of MoS₂–OCu in sensing applications, both MoS₂ and OCu were used for the individual fabrication of electrochemical sensors. Fig. 7 shows the electrochemical detection of H₂O₂ with the fabricated MoS₂/GCE- and OCu/GCE-based sensors. It can be seen that the MoS₂/GCE and OCu/GCE also display a steady response over a long-term test after addition of H₂O₂ at different concentrations. According to Fig. 7a and c, the MoS₂-based sensor is not stable at low analyte concentrations, but the OCu-based sensor exhibits better stability. Fig. 7b and d give the corresponding calibration curves. It is clear that the LR of the fabricated MoS₂-based H₂O₂ sensor is from 0.95 to 43.9 mM (R = 0.9897), and the LOD was calculated to be 0.85 μM. For the OCu-based H₂O₂ sensor, the LR is only from 0.015 to 3.58 mM (R = 0.9952), and the LOD is 0.038 μM. We suggest that combining MoS₂ with OCu nanowires can reduce its LOD and expand its LR. Synergistic effects between MoS₂ and OCu can improve the performance of the electrochemical sensor.

Fig. 7 Electrochemical detection of H₂O₂: (a) I–T response images of the MoS₂/GCE in PBS with successive addition of H₂O₂ at −0.3 V; (b) calibrated line (MoS₂); (c) I–T response images of the OCu/GCE in PBS with successive addition of H₂O₂ at −0.3 V; (d) calibrated line (OCu).

Compared with previous reports describing electrochemical H₂O₂ sensors, our fabricated MoS₂–OCu-based H₂O₂ sensor exhibits a lower LOD and wider LR, as shown in Table 1.

Table 1 Comparison of various materials used in H₂O₂ biosensors

Materials	LR (mM)	LOD (μM)	Ref.
MWCNT–PtNP	0.1–75	0.61	1
Graphene–AuNP	0.25–22.5	6.2	2
MWCNT–AgNP	0.5–30	18.6	5
MoS2–Cu2O	0.1–17.6	0.275	21
Graphene–AgNP	0.005–47	0.56	38
NP–PtNi	0.2–11	10	39
MoS2–PtNP	0.02–4.72	0.345	27
RGO–PANI–PtNP	0.02–8	1.1	40
MoS2–OCu	0.085–38	0.077	This work

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The selectivity of the MoS₂–OCu/GCE was measured under the potential of −0.30 V. Fig. 6e shows that the experiment tested the amperometric response when adding three other relevant electroactive species, UA, DA, and AA. It was found that there was no obvious interference, and a response occurred after successive addition of 3 mM AA, UA, and DA, but a stable electrical response could be seen after adding 2 mM of H₂O₂, indicating that our electrochemical sensor possesses high selectivity when detecting H₂O₂. Finally, Fig. 6f shows the long-term stability of the MoS₂–OCu/GCE over 14 days. The fabricated GCE was then stored in air at room temperature and tested every 2 days. It can be seen from the image that the current response still maintained more than 95.9% of its initial value in response to 2 mM H₂O₂ after 14 days, indicating that the stability of our electrochemical H₂O₂ sensor is acceptable. The decrease in the value of the current response for the H₂O₂ sensor may have possibly occurred because of the removal of material from the GCE and electrochemical corrosion of the working electrode during multiple tests.

3.5 Non-enzymatic electrochemical ascorbic acid sensor

The synthesized MoS₂–OCu nanohybrids were further used to modify GCEs and fabricate a non-enzymatic electrochemical AA sensor. Fig. 8a presents the CV data for both the MoS₂/GCE and MoS₂–OCu/GCE in the absence of 1 mM AA in 0.1 M NaOH, and it was found that both oxidation processes can be seen in the MoS₂/GCE and MoS₂–OCu/GCE. For the MoS₂–OCu/GCE, there is an obvious current response with a broad oxidation peak, with the oxidation process occurring from +0.2 to −0.3 V due to the oxidation of AA. The current responses of the MoS₂–OCu/GCE were tested at different AA concentrations from 0 to 4 mM to investigate the electrochemical behavior of the fabricated MoS₂–OCu/GCE, as shown in Fig. 8b. The above result indicates that the oxidation peak current obviously increases with the increasing AA concentration, and therefore, the fabricated MoS₂–OCu/GCE could be potentially used as an electrochemical sensor.

Fig. 8 Electrochemical detection of AA: (a) CV images of the MoS₂/GCE and MoS₂–OCu/GCE; (b) CV images of the MoS₂–OCu/GCE under different AA concentrations; (c) I–T response images of the MoS₂–OCu/GCE in 0.1 M NaOH with successive addition of AA at −0.25 V; (d) calibrated line; (e) amperometric responses upon successive addition of 1 mM AA, 3 mM UA, 3 mM DA, 3 mM H₂O₂, 3 mM glucose, and 1 mM AA; (f) storage stability in response to 2 mM AA.

Therefore, we chose an applied potential at −0.25 V for the I–T experiment in this test. Fig. 8c displays the obtained I–T curve of the fabricated sensor under successive addition of AA at different concentrations in the NaOH system. The fabricated MoS₂–OCu/GCE displays a stable response over the long-term test upon the addition of AA solution at different concentrations. Fig. 8d shows the calibration curve, and a regular response to AA can be seen. The LR of the fabricated AA sensor is from 0.015 to 11.75 mM (R = 0.9991, S/N = 3), and the LOD is calculated to be 0.0222 μM. MoS₂ and OCu were also individually used for fabricating AA sensors. Both sensors displayed a steady response over a long-term test after addition of AA at different concentrations (Fig. 9). For the MoS₂-based AA sensor, the LR is from 0.585 to 13.6 mM with a LOD of 0.21 mM (R = 0.9990). The LR of the OCu-based AA sensor is from 0.07 to 9.1 mM with a LOD of 0.031 mM (R = 0.9954). Comparing these three sensors, we can draw a conclusion that the introduction of OCu can reduce the LOD of the sensor and increase its sensitivity when the analyte concentration is low. All the results proved that there are synergistic effects of the sensors that can be used to improve the sensing performance in the field of electrochemical sensors.

Fig. 9 Electrochemical detection of AA: (a) I–T response images of the MoS₂/GCE in 0.1 M NaOH with successive addition of AA at −0.25 V; (b) calibrated line (MoS₂); (c) I–T response images of the OCu/GCE in 0.1 M NaOH with successive addition of AA at −0.25 V; (d) calibrated line (OCu).

Compared with previous studies on electrochemical AA sensors, our fabricated MoS₂–OCu sensor also shows a wider LR and lower LOD in the detection of AA, as is shown in Table 2.

Table 2 Comparison of various materials used in AA biosensors

Materials	LR (mM)	LOD (μM)	Ref.
N-Doped graphene	0.005–1.3	2.2	7
Graphene	0.4–6	0.12	41
o-Aminophenol	0.002–0.2	0.86	42
Protein–AuNC	0.015–0.1	0.2	43
Cu4(OH)6SO4	0.017–6.4	6.4	44
Fe2O3/Au	0.025–10	1	45
Copper vanadate	0.001–2	0.9	46
C-MWCNT/PANI	0.002–0.206	1.1	47
MoS2–OCu	0.015–11.75	0.022	This work

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The anti-interference test of the fabricated MoS₂–OCu/GCE was observed at the potential of −0.25 V. There was no obvious interference, and a response can be seen after successive addition of 3 mM UA, DA, H₂O₂, and glucose (Fig. 8e). However, a stable electrical response occurs after adding 1 mM AA, indicating that our AA sensor possesses high selectivity when detecting AA. Finally, we tested the long-term stability of the MoS₂–OCu/GCE over 14 days (Fig. 8f). The obtained result indicates that 98.1% of the initial value of the catalytic current response still has been maintained when 2 mM AA was added after 14 days, indicating that our electrochemical AA sensor possesses good stability.

4. Conclusions

In summary, we successfully decorated MoS₂ nanosheets with OCu nanowires to fabricate the novel MoS₂–OCu nanohybrid. We found that the MoS₂ nanosheets act as active materials and provide improved electrochemical activities for the fabricated non-enzymatic electrochemical H₂O₂ and AA sensors. First, the large surface area of the MoS₂ nanosheets ensures sufficient reactive active sites, making the nanosheet an ideal active material as an electrochemical substrate. Second, the synthesized OCu nanowires act as spacers to prevent MoS₂ nanosheets from restacking and increase the surface area and active sites of the MoS₂ nanosheets. Because of these two attributes, the synthesized MoS₂–OCu nanohybrids are ideal potential electrode materials that can be used in the fabrication of both H₂O₂ and AA sensors with a wider LR and lower LOD. A very low LOD for H₂O₂ (0.0767 μM) and AA (0.0222 μM) was found in this study. It is expected that further applications of OCu nanowires in materials science, analytical science, and nanotechnology will be explored, and the two-step hydrothermal synthesis strategy shown in this work will be valuable for guiding the design and synthesis of other functional hybrid nanomaterials with 1D, 2D, and 3D structures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC, grant no. 51573013) and Deutsche Forschungsgemeinschaft (DFG) under grant WE 5837/1-1.

Notes and references

1. P. Zhang, X. Zhao, X. Zhang, Y. Lai, X. Wang, J. Li, G. Wei and Z. Su, ACS Appl. Mater. Interfaces, 2014, 6, 7563–7571.
2. P. Zhang, X. Zhang, S. Zhang, X. Lu, Q. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2013, 1, 6525.
3. Y. Ma, M. G. Zhao, B. Cai, W. Wang, Z. Z. Ye and J. Y. Huang, Biosens. Bioelectron., 2014, 59, 384–388.
4. J. H. James, O. David, E. L. Paul, M. W. George and S. W. Mark, Science, 1991, 254, 501–502.
5. Z. Ouyang, J. Li, J. Wang, Q. Li, T. Ni, X. Zhang, H. Wang, Q. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2013, 1, 2415.
6. J. Ding, K. Zhang, G. Wei and Z. Su, RSC Adv., 2015, 5, 69745–69752.
7. Z. H. Sheng, X. Q. Zheng, J. Y. Xu, W. J. Bao, F. B. Wang and X. H. Xia, Biosens. Bioelectron., 2012, 34, 125–131.
8. H. Peng, C. Soeller, N. Vigar, P. A. Kilmartin, M. B. Cannell, G. A. Bowmaker, R. P. Cooney and J. Travas-Sejdic, Biosens. Bioelectron., 2005, 20, 1821–1828.
9. J. Guan, J. Peng and X. Jin, Anal. Methods, 2015, 7, 5454–5461.
10. X. Wang, Q. Han, S. Cai, T. Wang, C. Qi, R. Yang and C. Wang, Analyst, 2017, 142, 2500–2506.
11. M. Xu, J. M. Han, Y. Zhang, X. Yang and L. Zang, Chem. Commun., 2013, 49, 11779–11781.
12. S. Huang, F. Zhu, Q. Xiao, W. Su, J. Sheng, C. Huang and B. Hu, RSC Adv., 2014, 4, 46751–46761.
13. D. Li, W. Zhang, X. Yu, Z. Wang, Z. Su and G. Wei, Nanoscale, 2016, 8, 19491–19509.
14. P. Si, Y. Huang, T. Wang and J. Ma, RSC Adv., 2013, 3, 3487.
15. H. Zhu, L. Li, W. Zhou, Z. Shao and X. Chen, J. Mater. Chem. B, 2016, 4, 7333–7349.
16. M. Zhang, Y. Li, Z. Su and G. Wei, Polym. Chem., 2015, 6, 6107–6124.
17. E. B. Bahadır and M. K. Sezgintürk, Trends Anal. Chem., 2016, 76, 1–14.
18. W. Zhang, P. Zhang, Z. Su and G. Wei, Nanoscale, 2015, 7, 18364–18378.
19. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147–150.
20. Y. M. Chhowalla, H. S. Shin, G. Eda, L. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275.
21. X. Zhao, Y. Li, Y. Guo, Y. Chen, Z. Su and P. Zhang, Adv. Mater. Interfaces, 2016, 3, 1600658.
22. Y. Cheng, S. Lu, F. Liao, L. Liu, Y. Li and M. Shao, Adv. Funct. Mater., 2017, 27, 1700359.
23. Y. X. Wang, S. L. Chou, D. Wexler, H. K. Liu and S. X. Dou, Chemistry, 2014, 20, 9607–9612.
24. P. Zhang, X. Lu, Y. Huang, J. Deng, L. Zhang, F. Ding, Z. Su, G. Wei and O. G. Schmidt, J. Mater. Chem. A, 2015, 3, 14562–14566.
25. T. Wang, H. Zhu, J. Zhuo, Z. Zhu, P. Papakonstantinou, G. Lubarsky, J. Lin and M. Li, Anal. Chem., 2013, 85, 10289–10295.
26. J. Ping, Z. Fan, M. Sindoro, Y. Ying and H. Zhang, Adv. Funct. Mater., 2017, 27, 1605817.
27. D. Lin, Y. Li, P. Zhang, W. Zhang, J. Ding, J. Li, G. Wei and Z. Su, RSC Adv., 2016, 6, 52739–52745.
28. S. Su, X. Y. Han, Z. W. Lu, W. Liu, D. Zhu, J. Chao, C. H. Fan, L. H. Wang, S. P. Song, L. X. Weng and L. H. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 12773–12781.
29. J. Chao, M. Zou, C. Zhang, H. Sun, D. Pan, H. Pei, S. Su, L. Yuwen, C. Fan and L. Wang, Nanotechnology, 2015, 26, 274005.
30. H. Sun, J. Chao, X. Zuob, S. Su, X. Liu, L. Yuwen, C. Fan and L. Wang, RSC Adv., 2014, 4, 27625–27629.
31. C. Zhu, G. Yang, H. Li, D. Du and Y. Lin, Anal. Chem., 2015, 87, 230–249.
32. S. Fu, C. Zhu, J. Song, M. Engelhard, H. Xia, D. Du and Y. Lin, ACS Appl. Mater. Interfaces, 2016, 8, 22196–22200.
33. A. O'Neill, U. Khan and J. N. Coleman, Chem. Mater., 2012, 24, 2414–2421.
34. J. N. Coleman, M. Lotya, A. O'Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science, 2011, 331, 568–571.
35. J. Shi, J. Li, X. Huang and Y. Tan, Nano Res., 2011, 4, 448–459.
36. Y. W. Tan, X. Xue, Q. Peng, H. Zhao, T. Wang and Y. Li, Nano Lett., 2007, 7, 3723–3728.
37. Y. Liu, X. He, D. Hanlon, A. Harvey, J. N. Coleman and Y. Li, ACS Nano, 2016, 10, 8821–8828.
38. Y. Li, P. Zhang, Z. Ouyang, M. Zhang, Z. Lin, J. Li, Z. Su and G. Wei, Adv. Funct. Mater., 2016, 26, 2122–2134.
39. C. Xu, J. Wang and J. Zhou, Sens. Actuators, B, 2013, 182, 408–415.
40. X. Du, Y. Chen, W. Dong, B. Han, M. Liu, Q. Chen and J. Zhou, Oncotarget, 2016, 8, 13039–13047.
41. G. P. Keeley, A. O'Neill, N. McEvoy, N. Peltekis, J. N. Coleman and G. S. Duesberg, J. Mater. Chem., 2010, 20, 7864.
42. H. M. Nassef, L. Civit, A. Fragoso and C. K. O'Sullivan, Analyst, 2008, 133, 1736–1741.
43. X. Wang, P. Wu, X. Hou and Y. Lv, Analyst, 2013, 138, 229–233.
44. C. Xia and W. Ning, Analyst, 2011, 136, 288–292.
45. Y. Yin, J. Zhao, L. Qin, Y. Yang and L. He, RSC Adv., 2016, 6, 63358–63364.
46. L. Pei, N. Lin, T. Wei, H. Liu and H. Yu, J. Mater. Chem. A, 2015, 3, 2690–2700.
47. N. Chauhan, J. Narang and C. S. Pundir, Analyst, 2011, 136, 1938–1945.

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