A novel electrochemical sensor for glucose detection based on a Ti₃C₂T_x/ZIF-67 nanocomposite

Abstract

A Ti₃C₂T_x/ZIF-67 nanocomposite with outstanding conductivity has been prepared by loading ZIF-67 onto a two-dimensional Ti₃C₂T_x nanosheet. Ti₃C₂T_x sheets were synthesized by etching Ti₃AlC₂, and then ZIF-67 was grown in situ on the Ti₃C₂T_x nanosheet. The Ti₃C₂T_x/ZIF-67 nanocomposite exhibits excellent detection performance for glucose, with a low LOD of 3.81 μM and wide linear detection range of 5–7500 μM. This terrific result is contributed by the synergistic effect of the high electrically conductive ability of Ti₃C₂T_x and active catalytic performance of ZIF-67. Moreover, the electrochemical sensor prepared using the Ti₃C₂T_x/ZIF-67 nanocomposite also shows excellent selectivity, stability and repeatability for glucose detection. The Ti₃C₂T_x/ZIF-67 nanocomposite with outstanding performance has potential applications for electrochemical sensors.

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1. Introduction

Glucose is the direct energy source for human life activities. Many chronic diseases, such as diabetes, cardiovascular disease and other complications are closely related to blood glucose level.^1–5 Therefore, sensitive detection of glucose content in blood is of great value for monitoring human health. The advantage of an electrochemical method for detecting glucose compared to other methods is low cost, portability and high sensitivity.^6–8 The glucose sensors currently studied are mainly divided into enzyme-based glucose sensors and enzyme-free glucose sensors.^9–11 Although the traditional enzyme-based glucose sensor has good sensitivity and linearity, it needs to maintain the activity of the enzyme on the electrode surface, so its use and storage conditions are more stringent, which limits the development and application of glucose sensors to a certain extent.^12–14 In contrast, the enzyme-free sensor has the advantages of easy preparation, high sensitivity, and good stability, which has attracted the wide attention of researchers.^15–18 The development of highly active catalysts to promote electrocatalytic glucose oxidation is the key to non-enzymatic glucose sensing. So far, many precious metals, metal alloys and metal derivatives have been used to prepare non-enzymatic glucose sensors.^19–21

Among them, metal–organic frameworks (MOFs) have attracted much attention due to their rich metal active sites, high porosity and electrochemical activity.^22–24 Based on these unique characteristics, MOFs have great potential in the fields of separation, energy storage, catalysis, etc.^25–27 It is worth noting that in recent years, MOFs have been used more as electrochemical sensing platform to detect certain substances, such as hydrazine,^6,28 dopamine,^29,30 hydrogen peroxide^31,32 and ascorbic acid. Generally, glucose electrochemical sensors related to MOF mainly focus on using MOF as an immobilized substrate or precursor of electrocatalyst.^33,35 For example, Ma et al. constructed an integrated dehydrogenase-based electrochemical sensor using zeolite imidazole frameworks (ZIFs) as substrate for glucose detection.³⁵ Shi et al. encapsulated Cu nanoparticles (NPs) on the ZIF-8 matrix for glucose sensing.³⁴ However, the poor electrical conductive ability of MOFs would greatly decrease the sensitivity of MOFs, which severely limits its application as an electrocatalyst for glucose detection in the electrocatalytic process.³⁶ Therefore, MOFs are usually composited with other conductive materials to overcome its shortcoming of poor conductivity.^37–40 MXene, a new type of two-dimensional nanomaterial, has attracted tremendous attention in recent years due to its large specific surface area, high hydrophilicity and intrinsic electrical conductivity.^41–43 These inherent conductivity of MXene makes it very suitable for improving the poor conductivity of MOF materials.

Based on the above discussion, the porous Co-based MOF (ZIF-67) was selected as the electrocatalyst for glucose detection and loaded on the two-dimensional MXene material (Ti₃C₂T_x) to prepare the Ti₃C₂T_x/ZIF-67 composite material. Then the obtained Ti₃C₂T_x/ZIF-67 composite was modified on glass–carbon electrode (GCE) to detect the electrochemical performance of glucose. Ti₃C₂T_x/ZIF-67 composite shows good electrochemical sensing performance for glucose, since Ti₃C₂T_x makes up for the lack of conductivity of ZIF-67. This work provides an avenue to manufacture electrochemical sensors using MXene/MOFs composite materials.

2. Experimental section

2.1 Materials

Lithium fluoride (LiF, 98.5%), hydrochloric acid (HCl, 37%) were obtained from Sigma Aldrich. Ti₃AlC₂ powder (400 mesh) was purchased from 11 Technology Co., Ltd (JiLin, China). Co(NO₃)₂·6H₂O, 2-methylimidazole, glucose and NaOH were bought from Fuchen Chemical Reagent Co., Ltd,. (Tianjin, China). All chemical reagents were analytical grade and used as received without further purification.

2.2 Preparation of MXene (Ti₃C₂T_x)

In the MXene (Ti₃C₂T_x) preparation process, LiF (1 g) was added into 20 mL HCl solation (9 mol L⁻¹) in a Teflon flask. After the LiF completely dissolved, Ti₃AlC₂ (1 g) powder was dispersed in the above mixture solution under magnetically stirring for 48 h at 35 °C. The resulting dispersion was washed with deionized water for a few times until pH ≈ 6, and the precipitate was collected and dried. Then multilayer Ti₃C₂T_x (200 mg) was added into 100 mL deionized water under ultrasonic dispersion for 30 min. Ti₃C₂T_x nanosheets were obtained after being centrifuged at 3500 rpm for 1 h.

2.3 Preparation of Ti₃C₂T_x/ZIF-67 composite

To prepare Ti₃C₂T_x/ZIF-67 composite, Co(NO₃)₂·6(H₂O) (1.455 g) was added in the mixed solution of methanol (40 mL)/ethanol (40 mL) under ultrasonication, and then 10 mL Ti₃C₂T_x dispersion (5 mg mL⁻¹) was added under stirring for 0.5 h. And then 2-methylimidazole (1.642 g) was dispersed in another mixture of methanol (40 mL)/ethanol (40 mL) and directly added to the mixture solution, stirring magnetic force. After incubated for 24 h, the Ti₃C₂T_x/ZIF-67 was obtained by vacuum suction filtration, methanol washing and freeze-drying. For comparison, pure ZIF-67 without Ti₃C₂T_x was also prepared by the same process.

2.4. Fabrication of the modified electrode

The glassy carbon electrode (GCE) was polish with alumina powder (0.3 μm and 0.05 μm). To prepare the modified GCE, Ti₃C₂T_x/ZIF-67 (10 mg) was added into ethanol (5 mL) under sonication for 30 min to obtain uniform dispersion. Ti₃C₂T_x/ZIF-67 suspension (10 μl) was pipetted and dropped onto GCE surface and drying at 25 °C. The other modified electrodes were prepared by the similar procedure.

2.5 Characterization

X-ray powder diffraction (XRD) spectra analyses the crystal structure of samples were carried out by the Bruker D8 with a Cu Kα radiation at a scan speed of 5°/min and a step size of 0.02°. The morphologies of the samples were observed by scanning electron microscopy (SEM, SU8020). X-ray photoelectron spectroscopy (XPS) analyses of the Ti₃C₂T_x/Cu-BTC nanocomposite were carried out by the Thermo ESCLAB 250 Xi (ThermoFisher Scientific, USA). All electrochemical properties were recorded by the IviumStat Electrochemical Workstation with a three electrodes system. In this system, glassy carbon electrode (GCE) was used as working electrode, Ag/AgCl and platinum flat were used as reference electrode and counter electrode respectively.

3. Results and discussion

3.1 Formation of Ti₃C₂T_x/ZIF-67 nanocomposite

According to the schematic diagram of the fabrication of the Ti₃C₂T_x/ZIF-67 nanocomposite (Scheme 1), the Ti₃C₂T_x/ZIF-67 was prepared by the Ti₃C₂T_x-templated growth of ZIF-67. The Co²⁺ ions were adhered to the surface of Ti₃C₂T_x due to the electrostatic attraction of the oxygenic groups of Ti₃C₂T_x. The highly crystalline ZIF-67 was in situ grown on Ti₃C₂T_x by the coordination reaction of Co²⁺ and N atom of 2-methylimidazole to form Ti₃C₂T_x/ZIF-67 nanocomposite.

Scheme 1 Schematic diagram of the preparation of the Ti₃C₂T_x/ZIF-67 nanocomposite.

3.2 Morphology of Ti₃C₂T_x/ZIF-67 nanocomposite

The morphology and microstructure of Ti₃C₂T_x/ZIF-67 nanocomposite were characterized by scanning electron microscope. The SEM image of the Ti₃AlC₂ particles was shown in Fig. 1(a), and it can be seen that the raw material Ti₃AlC₂ has a clear and compact layered three-dimensional structure. As shown in Fig. 1(b), Ti₃AlC₂ transformed into a well-resolved loosely stacked accordion-like structure after etching reaction. Ti₃C₂T_x nanosheets (Fig. 1(c)) exhibited the single/few layered structure (f-Ti₃C₂T_x) by sonication treatment. The Fig. 1(d) was shown that ZIF-67 clearly presented the typical dodecahedron morphology with a uniform size ∼250 nm. In order to prove the homogeneous particles on the surface of Ti₃C₂T_x were ZIF-67 in Fig. 1(e) and (f), the EDS mapping was employed. The result confirmed that the elements (Ti, Co, N, C and O) were existed on the surface of Ti₃C₂T_x/ZIF-67 nanocomposite (Fig. 2). Among them, Ti was belonged to Ti₃C₂T_x, while N and Co was attributed to ZIF-67. The EDS mapping further verified the presence of Ti₃C₂T_x and ZIF-67 in the composite, demonstrating that ZIF-67 particles have been successfully distributed on the Ti₃C₂T_x nanosheets.

Fig. 1 SEM image of (a) Ti₃AlC₂, (b) Ti₃C₂T_x, (d) ZIF-67, (e) and (f) Ti₃C₂T_x/ZIF-67 with different magnifications. TEM image of (c) f-Ti₃C₂T_x flake.

Fig. 2 The EDX mapping analysis of Ti₃C₂T_x/ZIF-67.

3.3 Crystal structure of Ti₃C₂T_x/ZIF-67 nanocomposite

The crystal structure of Ti₃AlC₂, Ti₃C₂T_x, ZIF-67 and Ti₃C₂T_x/ZIF-67 nanocomposite was analyzed by the XRD spectra. The diffraction peaks at 9.58°, 19.2° and 39.04° in the XRD spectrum of Ti₃AlC₂ was corresponded to the (002), (004) and (104) planes of crystalline Ti₃AlC₂, respectively. Compared with Ti₃AlC₂, the characteristic peaks at 19.2° and 39.04° were not appeared and the diffraction peak (002) shifted to a smaller angle in the XRD pattern of Ti₃C₂T_x, which proved the effectiveness of the synthesis procedure. The characteristic diffraction peaks of the ZIF-67 were entirely consistent with the well-known structure of ZIF-67 crystal, which demonstrate that the synthesized ZIF-67 purity is high. The diffraction peaks of ZIF-67 could be well recognized in the XRD spectrum of Ti₃C₂T_x/ZIF-67 indicating that the presence of Ti₃C₂T_x does not affect the growth of ZIF-67 crystals. All the rustles illustrated that ZIF-67 particles have grown on the Ti₃C₂T_x nanosheets. The surface characteristic of Ti₃C₂T_x/ZIF-67 nanocomposite was analyzed by XPS (Fig. 3(b)). It can be seen that the six peaks of 285 eV, 399 eV, 455 eV, 531 eV, 685 eV and 811 eV of Ti₃C₂T_x/ZIF-67 correspond to the elements C 1s, N 1s, Ti 2p, O 1s, F 1s and Co 2p. Among them, Co 2p and N 1s are characteristic elements of ZIF-67, while Ti 2p and F 1s are derived from Ti₃C₂T_x, which further indicates the successful preparation of Ti₃C₂T_x/ZIF-67 nanocomposites.

Fig. 3 (a) XRD patterns of Ti₃AlC₂, Ti₃C₂T_x, ZIF-67 and Ti₃C₂T_x/ZIF-67. (b) XPS survey scan of Ti₃C₂T_x/ZIF-67.

3.4 Surface area and porosity of Ti₃C₂T_x/ZIF-67 nanocomposites

The surface area and porosity of ZIF-67 and Ti₃C₂T_x/ZIF-67 nanocomposites were measured by nitrogen adsorption/desorption experiments. Fig. 4(a) and (c) are the N₂ adsorption/desorption isotherms of ZIF-67 and Ti₃C₂T_x/ZIF-67, respectively. It can be seen that ZIF-67 and Ti₃C₂T_x/ZIF-67 both exhibit typical type I curve, suggesting that ZIF-67 and Ti₃C₂T_x/ZIF-67 are both microporous structures. Fig. 4(b) and (d) are the pore size distribution curves of ZIF-67 and Ti₃C₂T_x/ZIF-67, respectively. It can be seen that the pore size distribution is mainly concentrated at about 1.9 nm, mainly micropores, which is consistent with the corresponding adsorption/desorption isotherm. In addition, the specific surface area of ZIF-67 and Ti₃C₂T_x/ZIF-67 are 1546 m² g⁻¹ and 678 m² g⁻¹, respectively. It means that the Ti₃C₂T_x/ZIF-67 specific surface area is smaller than that of ZIF-67. This is due to the Ti₃C₂T_x nanosheets with dense film structure do not contribute much to the specific surface area.

Fig. 4 N₂ adsorption/desorption isotherms of ZIF-67 (a) and Ti₃C₂T_x/ZIF-67 (c); pore size distribution curves of ZIF-67 (b) and Ti₃C₂T_x/ZIF-67 (d).

3.5 Impedance analysis

The electrochemical property of Ti₃C₂T_x/ZIF-67 was studies by electrochemical impedance spectroscopy (EIS). In this research, EIS experiments was performed in 5 mM [Fe(CN)₆]^3−/4− with 0.1 M KCl at 10 mV AC voltage in the frequency range of 0.1 Hz to 10 kHz. The EIS of these nanocomposites and bare GCE were shown in Fig. 5. The bare GCE EIS had a semicircular portion at higher frequencies. The semicircle diameter of EIS of ZIF-67/GCE has increased due to the inherent insulation of ZIF-67. However, the curve of Ti₃C₂T_x/GCE had no obvious semicircular part, even close to linear, which was related to the outstanding metal conductivity of Ti₃C₂T_x. The semicircle diameter of Ti₃C₂T_x/ZIF-67/GCE was between Ti₃C₂T_x/GCE and bare GCE, which suggested that its electron transfer rate was greatly improved compared to ZIF-67/GCE. The EIS result proved that The Ti₃C₂T_x nanosheets were an important component to enhance the electrochemical conductivity of Ti₃C₂T_x/ZIF-67.

Fig. 5 Nyquist plots of EIS for GCE, Ti₃C₂T_x/GCE, ZIF-67/GCE and Ti₃C₂T_x/ZIF-67/GCE.

3.6 Electrocatalytic behaviors of the sensor

The electrocatalytic ability of Ti₃C₂T_x/ZIF-67 composite for glucose was studied by CV curve characterization analysis. The typical CV curves of GCE, ZIF-67/GCE and Ti₃C₂T_x/ZIF-67/GCE with or without 1 mM glucose in 0.1 M NaOH electrolyte were shown in Fig. 6(a). It can be seen that no obvious redox peaks were observed for bare GCE with or without glucose, which indicated that the bare GCE cannot catalyze glucose. Compared with the bare GCE, the redox peaks value of ZIF-67/GCE were nearby 0.30 V because of the conversion of Co(II) and Co(III). It was worth noting that the anodic peak current increased significantly after adding glucose. This provided that ZIF-67 has prominent catalytic activity for the oxidation of glucose. The reaction mechanism can be explained by eqn (1) and (2):

[Co(II)(min)₂]_n + nOH⁻ → [Co(III)(min)₂(OH)]_n + ne⁻ (1)

Co(III) + glucose → Co(II) + gluconolactone (2)

Fig. 6 (a) CVs of GCE, ZIF-67/GCE and Ti₃C₂T_x/GCE in the presence and absence of 1 mM glucose in 0.1 M NaOH solution at a scanning rate of 50 mV s⁻¹. (b) CVs of Ti₃C₂T_x/ZIF-67/GCE with different scan rate (10–500 mV s⁻¹).

The oxidation process of glucose by Ti₃C₂T_x/ZIF-67 was displayed in Fig. 7. Co(II) was oxidized to Co(III) initially, and then to oxidize glucose molecules to gluconolactone through Co(III), and finally Co(III) was reduced to Co(II). For the Ti₃C₂T_x/ZIF-67/GCE, an obvious current response appeared with glucose added, which was owing to the presence of Ti₃C₂T_x with high conductivity that provides a greater electron transfer rate. Meantime, the cyclic voltammograms of Ti₃C₂T_x/ZIF-67 at 10–500 mV s⁻¹ were also tested. As shown in Fig. 6(b), the peak current increased accordingly with the scan rate increasing from 10 to 500 mV s⁻¹, and exhibited good linear, which indicated a diffusion-controlled electrochemical process.

Fig. 7 The mechanism of Ti₃C₂T_x/ZIF-67/GCE to detect glucose in NaOH solution.

3.7 Amperometric detection of glucose on Ti₃C₂T_x/ZIF-67/GCE

Amperometric determination test result (Fig. 8(a)) exhibited the ampere response of Ti₃C₂T_x/ZIF-67/GCE towards glucose by continuously adding different amount of glucose to a stirred PBS at the fixed potential of 0.35 V. It can be seen that a quick and sensitive current response for each addition of glucose, and as the glucose concentration increasing, the response current was significantly enhanced. Fig. 8(b) presented the corresponding I–C calibration curve, it can be seen that the current response increased linearly over the glucose concentration ranging from 5 μmol L⁻¹ to 7.5 mmol L⁻¹. The standard method (LOD = 3σ/q) was used to calculate the detection limit (LOD), where σ is the standard deviation and q is the slope of the calibration graph. The LOD and linear range of Ti₃C₂T_x/ZIF-67/GCE was 3.81 μM and 5 μM −7.5 mM, respectively. As showed in Table 1, the Ti₃C₂T_x/ZIF-67 based glucose sensor had a great advantage compared with previously reported sensors, which provided an up-and-coming prospect for MOF-based glucose sensors. The excellent detection performance can be attributed to the synergetic effect of superior conductivity of Ti₃C₂T_x-MXene and the catalytic activity of ZIF-67. All in all, Ti₃C₂T_x/ZIF-67 was an effective electrode material with great potential in the trace detection of glucose.

Fig. 8 (a) Typical amperometric i–t response on successive addition of glucose for Ti₃C₂T_x in 0.1 M NaOH; (b) calibration plot of current versus concentration of glucose. Potential: 0.35 V.

Table 1 Comparison of performance of Ti₃C₂T_x/ZIF-67/GCE with those of reported sensors for glucose detection

Electrode material	Linear range (μM)	LOD (μM)	Ref.
AuNP/NG/ITO	40–16 100	12	19
Ni@Cu-MOF	5–2000	1.67	43
NiMoO4	10–8000	4.6	44
ZIF-67/GCE	5–1000	1.6	2
CoP	500–5000	9	45
Co3O4/NCNTs	5–2650, 4650–13 650	5	46
Ti3C2Tx/ZIF-67	5–7500	3.81	This work

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3.8 Selectivity and stability of the sensor

Considering the real-time applications of the sensor, anti-interference performance has been evaluated. Fig. 9(a) showed the amperometric current response of Ti₃C₂T_x/ZIF-67/GCE for glucose and interference including UA (2,6,8,-trihydroxypurine), AP (Ascorbic Acid) and AA (Acetaminophen) which was often co-exist with glucose in the biological systems. It can be seen that 1 mM glucose was added at the beginning, and then a significant increase in the current response was observed. Subsequently, the same amount of AA, UA and AP were added successively, and the current response remains unchanged. To prove the selectivity of Ti₃C₂T_x/ZIF-67 nanocomposite, glucose was added again, and the current response was shown immediately. Actually, the Ti₃C₂T_x/ZIF-67 nanocomposite showed excellent selectivity and anti-interference performance for glucose sensing.

Fig. 9 (a) Amperometric I–t curve of Ti₃C₂T_x/ZIF-67/GCE with successive addition of 1 mM glucose and 0.5 mM AA, UA and AP in 0.1 M NaOH solution. Potential: 0.35 V; (b) change of catalytic activity of Ti₃C₂T_x/ZIF-67/GCE after 10 time of recycling detection in 0.1 M NaOH; (c) the amperometric I–t curve of Ti₃C₂T_x/ZIF-67/GCE toward 1 mM glucose over 2500 consecutive seconds; (d) reproducibility for glucose sensing with Ti₃C₂T_x/ZIF-67/GCE.

The stability of the prepared sensor was further studied by the amperometric method. As shown in Fig. 9(b), Ti₃C₂T_x/ZIF-67 still maintained more than 90% of the activity after 10 times of detection. The result indicated that the electrocatalytic activity of the sensor can be maintained well during the recycling measurement. As shown in Fig. 9(c), the stability of the sensor was further evaluated by recording the current–time curve continuously. The current signal value kept steady within 2500 s, which showed a good stability of the sensor. Finally, the repeatability was studied by continuously testing five different Ti₃C₂T_x/ZIF67/GCE samples under the same conditions. As shown in the Fig. 9(d), all electrodes had similar current response and its relative standard deviation (RSD) was only 1.18%, which indicated that the sensors has an excellent repeatability.

Conclusions

In conclusion, this paper reported the preparation of ZIF-67 loaded on 2D MXene (Ti₃C₂T_x) nanosheets using a facile method for the first time. With the in situ growing ZIF-67 on 2D Ti₃C₂T_x, the prepared Ti₃C₂T_x/ZIF-67 nanocomposite combined the advantages of high electric conductivity and unique electrocatalytic activity, resulting in excellent electrocatalytic glucose oxidation performance with a wide dynamic range (5–7500 μM) and a low detection limit (3.81 μM). This work suggested that the effective combination of MOF with excellent electrocatalytic activity and 2D Ti₃C₂T_x with ultra-high conductivity was a promising method for the development of high-performance electrochemical sensors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was funded by Beijing Nova Program (Z201100006820113) and National Natural Science Foundation of China (11805016).

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Footnote

† These authors contributed equally to this work and should be considered co-first authors.

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