Magnetic Fe₃O₄@MOFs decorated graphene nanocomposites as novel electrochemical sensor for ultrasensitive detection of dopamine

Abstract

A novel hybrid nanocomposite of magnetic Fe₃O₄@ZIF-8 (zeolitic imidazolate framework-8 coated Fe₃O₄ nanocomposites denoted as Fe₃O₄@ZIF-8) decorated RGO (reduced graphite oxide) was prepared by a simple method for the first time and denoted as Fe₃O₄@ZIF-8/RGO. After the Fe₃O₄/RGO was formed by solvothermal approach, the MOFs (ZIF-8) was coated on the surface of Fe₃O₄ to get the Fe₃O₄@ZIF-8/RGO nanocomposite. The resulted Fe₃O₄@ZIF-8/RGO nanocomposite was characterized by means of the transmission electron microscope (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), X-ray diffraction spectrometry (XRD), X-ray photoelectron (XPS), and vibrating sample magnetometer (VSM). This nanocomposite was modified on the glassy carbon electrode to fabricate biosensor which used to electrochemical determination for dopamine (DA) in phosphate buffer solution. The results demonstrated the fabricated biosensor showed great potential applications in the detection of DA with remarkable enhanced effect on voltammetric response of DA. The linear relationship between the response peak currents and DA concentration was in range from 2.0 × 10⁻⁹ to 1.0 × 10⁻⁵ M, the limits of detection is 6.67 × 10⁻¹⁰ M. Moreover, the prepared biosensor also showed good selectivity for DA detection in the presence of ascorbic acid and uric acid and satisfactory result in real samples detection.

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1. Introduction

Metal–organic frameworks (MOFs), as new structured hybrid materials that consist of inorganic connectors and organic linker molecules, have attracted enormous interest because of their large accessible surface areas, high porosity, tunable pore sizes, ordered crystalline structures and excellent mechanical stability.^1–3 These remarkable characteristics helped MOFs showed great potential application in gas adsorption, catalysis, separation and purification.⁴ Because many metal ions used in MOFs are electrochemically active, MOFs are also received growing concerns in electrochemical biosensors field.^5,6 To enhance the conductivity, stability in aqueous solution and electro-catalytic activities of target analytes, the introduction of the other highly conductive and mechanically durable materials into MOFs has been proposed.^7,8

Graphene, composed of a single-atom-thick two-dimensional sheet of covalently bonded carbon atoms, has extraordinary electronic conductivity, high specific surface area, exceptional electron transfer rate, optical, structural and mechanical properties.^9–11 In recent years, the graphene has been pay close attention to be the outstanding candidate for potential electrode modifying material because its bio-electrocatalytic properties and physical stability.^12,13 As one kind of chemically derived graphene, reduced graphite oxide (RGO) with similar characteristics to graphene in many aspects has shown great utilization potentiality for application as highly sensitive biosensors.¹⁴ To overcome the poor dispersibility and being prone to aggregation owing to the strong stacking tendency in the synthesis process of bulk-quantity RGO, decorating the RGO nano-sheets with inorganic nanoparticles during the preparation is the reasonable way which not only decreases restacking for RGO nano-sheets but also enhances the physical and chemical properties.¹⁵

Dopamine (DA) is one of the most significant and representative catecholamine neurotransmitter mediating the transmission of messages within the central nervous system of mammals and humans. An abnormal dopaminergic neuron process may lead to neurological illnesses, such as Parkinson's, Alzheimer's and Schizophrenia diseases.¹⁶ As the trace level concentration of DA change has been related to various diseases, the rapid, sensitive and accurate measurement to detect the trace amount of DA is of extreme importance. Several methods have been established for DA detection, such as liquid chromatography, chemiluminescence, capillary electrophoresis, fluorescence, and absorbance and colorimetric methods. Because DA can be easily electrochemically oxidized, the electrochemical biosensors have been considered to be superior to other techniques for the determination of DA in view of its high accuracy, fast response, bulk modification with simple instruments, and low operation and instrumental expenses.¹⁷

Zeolitic imidazolate frameworks (ZIFs) is an attractive subfamily of MOFs due to their exceptional chemical and thermal stabilities and the ease of synthesis. In this work, we have developed a simple method to prepare a novel hybrid nanocomposite of magnetic Fe₃O₄@ZIF-8 decorated graphene with excellent dispersibility and stability in aqueous solution.¹⁸ In this process, Fe₃O₄ decorated RGO is formed firstly by the solvothermal approach, and then ZIF-8 is synthesized on the surface of Fe₃O₄ to get the final products, Fe₃O₄@ZIF-8/RGO nanocomposite. The designed novel structure of anchoring the Fe₃O₄@MOFs nanospheres on graphene nano-sheets could not only restrained restacking of the graphene nano-sheets, but also incorporated of a rapid response towards an assistant magnetic field and fascinating electro-catalytic properties. To explore the potential electrochemical application of the Fe₃O₄@ZIF-8/RGO, the nanocomposite was immobilized on a glassy carbon electrode (GCE) to form a sensing platform for the detection of DA. The results demonstrated that Fe₃O₄@ZIF-8/RGO modified electrode had a high sensitivity, favorable performance and reproducibility, and excellent selectivity for determination of DA.

2. Experiment

2.1. Materials and reagents

Ferric chloride hexahydrate (FeCl₃·6H₂O), sodium acetate, ethylene glycol (EG), and Zn(NO₃)₂·6H₂O were obtained from Sinopharm Chemical Reagent, Co., Ltd (Shanghai, China). Stock solution of dopamine (0.01 mol L⁻¹) was prepared by dissolving dopamine (0.1531 g) in 100 mL deionized water. The phosphate buffer solution (PBS, 0.1 mol L⁻¹), which was prepared by mixing Na₂HPO₄ and NaH₂PO₄ stock solution and adjusted to the pH value of 5.0–7.5 with 0.1 mol L⁻¹ H₃PO₄ or NaOH solution, was used as the supporting electrolyte during all measurements.

2.2. Apparatus

The FT-IR was recorded with a Nicolet Magna-IR spectrophotometer between 4000 and 450 cm⁻¹ using the KBr pellet technique. Transmission electron microscopy (TEM, FEI Tecnai G20) was obtained to elucidate the dimensions of the nanoparticle. The crystalline structures of samples were characterized by X-ray diffraction (XRD) (RigakuD/max-2400). The chemical analysis for the composites were conducted by X-ray photoelectron spectroscopy (XPS, ESCALAB210, VG, UK). Magnetization measurements were performed on a vibrating sample magnetometry (VSM, LAKESHORE-7304, USA). Amperometric measurements were performed on a CHI 660E electrochemical workstation.

2.3. Preparation of Fe₃O₄/RGO nanocomposites

Graphite oxide (GO) was prepared according to the modified Hummers method.¹⁹ Then, Fe₃O₄/RGO nanocomposites were synthesized via the solvothermal approach. Typically, 40 mg of GO was introduced into 30 mL of ethylene glycol (EG) and sonicated for 30 min to get brown dispersion solution. Then 0.987 g of FeCl₃·6H₂O and 1.946 g of sodium acetate were dissolved in the EG solution of GO by stirring for 30 min. After that, the mixture was transferred to a Teflon-lined autoclave and treated at 200 °C for 8 h. The obtained Fe₃O₄/RGO nanocomposites were collected by magnetic decantation and washed with ethanol and distilled water thoroughly. Finally, the nanocomposites were dried at 60 °C of 24 h under vacuum.²⁰

2.4. Preparation of Fe₃O₄@ZIF-8/RGO nanocomposites

0.12 g of Zn(NO₃)₂ was dissolved in 15 mL of 50% ethanol solution containing 2 mmol HCl. 0.35 g of obtained Fe₃O₄/RGO nanocomposites was dispersed in above solution by sonication for 20 min. After that, 30 mL of 50% ethanol solution containing 0.34 g 2-methylimidazole was added to the suspension and the mixture was stirred with ultrasound at room temperature for 10 min. The product, Fe₃O₄@ZIF-8/RGO nanocomposites, were collected by a magnet and washed with distilled water and ethanol thoroughly, then dried at 60 °C under the vacuum for 24 h.²¹

2.5. Preparation of Fe₃O₄@ZIF-8/RGO modified electrode

1 mg of Fe₃O₄@ZIF-8/RGO was dispersed in 1 mL of dimethyl formamide (DMF) and the mixture was sonicated for 30 min to achieve a well-dispersed suspension. Prior to modification, the bare GCE was polished to a mirror-like surface sequentially with 1.0, 0.3 mm and 0.05 μm of α-Al₂O₃, and then rinsed ultrasonically with water and ethanol and de-ionized water. After the solvent was evaporated, 5 μL of Fe₃O₄@ZIF-8/RGO suspension was cast onto the electrode surface. Thus the Fe₃O₄@ZIF-8/RGO nanocomposites modified electrode (Fe₃O₄@ZIF-8/RGO/GCE) was obtained after it was dried in air for approximately 3 h. For comparison, the Fe₃O₄@ZIF-8 modified GCE (Fe₃O₄@ZIF-8/GCE) and GO modified GCE (GO/GCE) were prepared only by replacing the Fe₃O₄@ZIF-8/RGO suspension with Fe₃O₄@ZIF-8 or GO suspension. Fig. 1 shows the preparation of Fe₃O₄@ZIF-8/RGO/GCE and its application for sensing analysis of DA.

Fig. 1 Scheme for the preparation of Fe₃O₄@ZIF-8/RGO, and its application for the determination of DA.

2.6. Electrochemical measurement

Electrochemical characterizations of the modified electrodes were performed in 0.1 M phosphate buffer solution (PBS) (pH 5.0) through cyclic scan in the potential range from −1.0 to 0.8 V. The electrochemical sensing test of the modified electrode was carried out with a CHI 660E electrochemistry workstation by a conventional three-electrode system, comprising a platinum wire as the auxiliary electrode, a saturated calomel electrode as the reference electrode and the modified GCE as the working electrode in the following procedure: a 15 mL solution containing an appropriate amount of dopamine and 0.1 M PBS was transferred into a voltammetric cell, and then cyclic voltammetry (CV) and differential-pulse voltammetry (DPV) measurements were recorded. Real samples determination was experimented by the standard addition method. The urine and serum samples were diluted with PBS (0.1 M, pH 5.5) and spiked with different amounts of known concentrations of DA, and measured under the optimal conditions.

3. Results and discussion

3.1. Characterization of Fe₃O₄@ZIF-8/RGO nanocomposites

The morphologies of the Fe₃O₄/RGO, Fe₃O₄@ZIF-8 and Fe₃O₄@ZIF-8/RGO are characterized by SEM and TEM, and the TEM and SEM images of each synthesized composites are shown in Fig. 2. As shown in Fig. 2a and d, Fe₃O₄ nanoparticles have been decorated on the surface of graphene nano-sheets with a diameter of about 200 nm. The monodisperse core–shell structure of Fe₃O₄@ZIF-8 is just like the reported.²¹ The nearly spherical shape of Fe₃O₄@ZIF-8 and the formation of the ZIF-8 shell could be observed by the SEM image (Fig. 2b) and TEM image (Fig. 2e). Compared with Fe₃O₄/RGO and Fe₃O₄@ZIF-8, the Fe₃O₄@ZIF-8/RGO nanocomposites (Fig. 2c and f) combined two features of each composites, the folds of graphene sheets and core–shell structure of Fe₃O₄@ZIF-8 are exhibited obviously in the images. Therefore, demonstrated by TEM and SEM results, the novel Fe₃O₄@ZIF-8/RGO nanocomposites have been successfully prepared.

Fig. 2 TEM images of the Fe₃O₄/RGO (a), Fe₃O₄@ZIF-8 (b) and Fe₃O₄@ZIF-8/RGO (c) and SEM images of the Fe₃O₄/RGO (d), Fe₃O₄@ZIF-8 (e) and Fe₃O₄@ZIF-8/RGO (f).

The chemical structures of the Fe₃O₄, Fe₃O₄/RGO, Fe₃O₄@ZIF-8 and Fe₃O₄@ZIF-8/RGO are characterized by FT-IR spectra, as presented in Fig. 3. The spectrum of Fe₃O₄ is just same to the report, and the peak at 580 cm⁻¹ is related to the vibration of Fe–O functional groups.²² While in the FT-IR spectrum of Fe₃O₄/RGO, the bands at 3490 cm⁻¹ and 1200 cm⁻¹ appear, which correspond to the stretching vibration of C–H and C–N (in the –C–NH–C– group), respectively.^23,24 Compared to the spectrum of Fe₃O₄, the spectrum of Fe₃O₄@ZIF-8 displays different peaks contributed by the ZIF-8 shell. The bands in region of 900–1330 cm⁻¹ and band at 1440 cm⁻¹ are attributed to the imidazole ring, and the band at 422 cm⁻¹ could be assigned to the Zn–N stretch mode.^21,25 The spectrum of Fe₃O₄@ZIF-8/RGO nanocomposites also combines the features of above composites, and shows the characteristic absorption bands of each constituent part. Overall, the FT-IR spectra confirmed the formation of Fe₃O₄@ZIF-8/RGO structure.

Fig. 3 FT-IR spectrum of Fe₃O₄, Fe₃O₄@ZIF-8, Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO nanocomposite.

The crystal structure of the as prepared samples have been identified by X-ray power diffraction techniques and the XRD spectrum are show in Fig. 4. The intense and sharp peak at 10.6° in the spectrum of GO is attributed to the crystalline plane of graphite oxide, and the diffraction peaks of Fe₃O₄ can be assigned to a superposition of standard XRD pattern of face-centered cubic Fe₃O₄.²⁶ However, the peak at 10.6° has entirely disappeared after the Fe₃O₄ decorated on the graphene through hydrothermal reaction because EG could reduce GO and Fe³⁺ to graphene and Fe²⁺, respectively.^20,27 Simultaneously, standard Fe₃O₄ XRD pattern in the spectrum of Fe₃O₄/RGO demonstrating the Fe₃O₄ nanoparticles are successfully decorated on graphene sheets. The XRD pattern of Fe₃O₄@ZIF-8 is consistent with the characteristic peaks of Fe₃O₄ and the simulated pattern of published ZIF-8 structure data.^21,28 The diffraction peaks of Fe₃O₄@ZIF-8/RGO are very similar to Fe₃O₄@ZIF-8, indicating the coexistence of Fe₃O₄@ZIF-8 and graphene in the resulted nanocomposite.

Fig. 4 XRD spectrum of Fe₃O₄, Fe₃O₄@ZIF-8, Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO nanocomposite.

To investigate the chemical elements on the surface of the Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO, the XPS analysis was performed. The wide-scan XPS spectra for Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO are shown in Fig. 5a. The characteristic peaks of Fe 2p, Fe 3p, O 1s and C 1s appeared in the spectrum of Fe₃O₄/RGO. Compared with the Fe₃O₄/RGO, new peaks assigned to Zn 2p1, Zn 2p3 and N 1s are clearly observed in the spectrum of Fe₃O₄@ZIF-8/RGO, demonstrated the MOFs (ZIF-8) have been successfully reacted on the surface of Fe₃O₄. Moreover, as the ZIF-8 was covered on the Fe₃O₄, the peaks of Fe 2p and Fe 3p are disappeared and the peak of O 1s decreased sharply. Fig. 5b shows the C 1s spectrums of Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO, the only peak at 527.95 eV could be assigned to the C–C in the spectrum of Fe₃O₄/RGO, and two peaks in Fe₃O₄@ZIF-8/RGO corresponding to C–C (183.51 eV) and C=C (186.16 eV) demonstrated the 2-methylimidazole has covered on the Fe₃O₄. The N 1s spectrum of Fe₃O₄@ZIF-8/RGO (Fig. 5c) shows two species of N 1s peak corresponding to –NH– (397.53 eV) and –N= (400.59 eV) which also characterized the 2-methylimidazole on the nanocomposite.^20,29

Fig. 5 XPS spectra of (a) wide scan of Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO; (b) N 1s of Fe₃O₄@ZIF-8/RGO; (c) C 1s of Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO.

The magnetic property of Fe₃O₄@ZIF-8/RGO was studied using a superconducting quantum interference device (SQUID) magnetometer at room temperature, as shown in Fig. 6. The hysteresis loops of the Fe₃O₄ nanoparticle, Fe₃O₄/RGO, Fe₃O₄@ZIF-8 and Fe₃O₄@ZIF-8/RGO nanocomposite are characterized the magnetic measurements of each product. The saturation magnetization of Fe₃O₄, Fe₃O₄/RGO, Fe₃O₄@ZIF-8 and Fe₃O₄@ZIF-8/RGO is 79.08, 60.87, 50.41 and 41.65 emu g⁻¹, respectively. Although the magnetic intensity of Fe₃O₄@ZIF-8/RGO decreases obviously after the ZIF-8 was reacted on the Fe₃O₄, the Fe₃O₄@ZIF-8/RGO nanocomposite can be easily separated conveniently by using magnetic field, and facilitated collection and operation.³⁰

Fig. 6 VSM curves (B) of Fe₃O₄, Fe₃O₄@ZIF, Fe₃O₄/RGO and Fe₃O₄@ZIF-8/RGO nanocomposite.

3.2. Voltammetric behavior of DA at Fe₃O₄@ZIF-8/RGO/GCE

To exploit the potential application of the Fe₃O₄@ZIF-8/RGO nanocomposite, the electrochemical behaviors of DA were investigated on the Fe₃O₄@ZIF-8/RGO/GCE. Fig. 7 shows typical CVs of 1 mM DA on the bare GCE, the Fe₃O₄@ZIF-8/GCE, the Fe₃O₄/RGO/GCE and the Fe₃O₄@ZIF-8/RGO/GCE. On the bare GCE, DA exhibits a reversible electrochemical behavior and the small current indicates it is inefficient to detect of DA on the bare GCE. At the Fe₃O₄@ZIF-8, a couple of small redox peaks appeared and the oxidation peak current of DA enhanced notable, it is because that the relative large surface area of electrode increased significantly after immobilization of Fe₃O₄@ZIF-8. Due to the excellent electric conductivity of reduced graphite oxide, the anodic peak current at the Fe₃O₄/RGO/GCE is increased strikingly. However, when the Fe₃O₄@ZIF-8/RGO/GCE was applied for determination, the redox peaks of DA increased dramatically with a well-defined peak shape, and the redox process became more reversible as judged from the more symmetric peak profiles. Therefore, the component of MOFs (ZIF-8) material in the graphene nano-sheets have positive effect on improving the electrochemical response, which is likely caused by the outstanding porous structure and favorable electron transfer mediating function of the electroactive MOFs.^7,31

Fig. 7 Cyclic voltammograms at the bare GCE, the Fe₃O₄@ZIF-8/GCE, the Fe₃O₄/RGO/GCE and the Fe₃O₄@ZIF-8/RGO/GCE in the presence of DA in PBS (0.1 M, pH 5.5). DA concentration: 1 mM, and scan rate: 20 mV s⁻¹.

3.3. Electrochemical parameters of DA at Fe₃O₄@ZIF-8/RGO/GCE

The influence of the scan rate (ν) on the oxidation current of 0.1 mM DA was examined using Fe₃O₄@ZIF-8/RGO/GCE in 0.1 M pH 5.0 PBS by varying the scan rates from 10 to 50 mV s⁻¹. As shown in Fig. 8, the anodic and cathodic peak currents are both increased gradually when increasing the scan rate. Farther more, with the increase of scan rate, the anodic peak potential and cathodic peak potential slightly shifted to more positive and negative potentials, respectively, indicating that the electron transfer rate decreased and the electrochemical reaction of DA tended to be less reversible. A good linear relationship among the scan rate, the values of the anodic peak current (I_pa) and the cathodic peak current (I_pc) is also obtained. The linear regression equation for the anodic peak current and cathodic current is I_pa (μA) = 5.6328 + 0.21468ν (mV s⁻¹), and I_pc (μA) = −3.1756 − 0.1023ν (mV s⁻¹), respectively, and the correlation coefficient is R_pa = 0.9965, R_pc = 0.9903, respectively. This result suggests that the electrochemical oxidation is an absorption-controlled process for DA at the Fe₃O₄@ZIF-8/RGO/GCE surface.³²

Fig. 8 Cyclic voltammograms of 0.1 mM DA in 0.1 mM PBA at the Fe₃O₄@ZIF-8/RGO/GCE at scan rate of 10–50 mV s⁻¹. The inset shows the plots of anodic and cathodic peak currents vs. scan rates.

Since the redox system was affected by the change of pH due to the involvement of protons in the electrode reaction and the electro-catalytic reaction at the Fe₃O₄@ZIF-8/RGO/GCE is a two electron, two proton process, the influence of pH on the biosensor performance was investigated by measuring the electrode response in 0.1 mM DA with pH values ranging from 5.0 to 7.5.³³ As shown in Fig. 9, the oxidation peak potential (E_pa) of DA shifted negatively with the increased of pH. A good linear relationship between E_pa and pH was constructed and described with a linear equation: E (V) = −0.0704 pH + 0.7773 (R = 0.9935). Farther more, the oxidation peak current (I_pa) also changed with the pH of DA solution ranged from 5.0 to 7.5, and the maximum response current was obtained at pH of 5.5. Thus, a phosphate buffer solution of pH 5.5 was chosen as the optimum pH and used in all experiments below.

Fig. 9 Effects of pH on E_pa and I_pa of 0.1 mM in 0.1 M PBS on the Fe₃O₄@ZIF-8/RGO/GCE.

3.4. Analytical performance and applications

Voltammetric current responses of successive additions of DA were recorded by differential pulse voltammetry (DPV) to check the sensitivity of the sensor under the optimal experimental conditions. As shown in Fig. 10, with the increasing concentration of DA, the oxidation peak current (I_pa) increased relatively. Linearly proportional of peak current and DA concentration is observed in the range from 2.0 × 10⁻⁹ to 1.0 × 10⁻⁵ M (inset of Fig. 10). The linear regression equations for DA is I_pa (μA) = 1.144 + 0.0056C (10⁻⁹ M) and the correlation coefficient is 0.9966. According to signal to noise (S/N) = 3, the detection limits of DA was estimated to be 6.67 × 10⁻¹⁰ M. Table 1 summarized the comparison of analytical results of the analytical performance Fe₃O₄@ZIF-8/RGO/GCE with other dopamine sensor reported recently. Through the comparison, the obtained biosensor in this work has comparable and even better performance than the others, demonstrating that Fe₃O₄@ZIF-8/RGO/GCE has extraordinary application potential in determination of DA.

Fig. 10 DPV for different concentration of DA at the Fe₃O₄@ZIF-8/RGO/GCE. The inset shows the relationship between the peak current and DA concentration.

Table 1 Comparison of analytical performance of DA at different modified electrodes

Modified materials	Methods	Linear ranges (M)	Detection limits (M)	Ref.
GO	DPV	1 × 10^−6 to 1.5 × 10^−7	0.27 × 10^−6	35
Fe3O4/RGO	DPV	4 × 10^−7 to 1.6 × 10^−4	8 × 10^−8	36
Pd/graphene/chitosan	DPV	5 × 10^−7 to 2 × 10^−4	1 × 10^−7	37
Polypyrrole@RGO	DPV	6 × 10^−8 to 8 × 10^−6	6 × 10^−9	38
Cu(tpa)-EGR	DPV	1 × 10^−6 to 5 × 10^−5	2.1 × 10^−7	7
Nafion/C/Al-MIL-53-(OH)2	DPV	3 × 10^−8 to 1 × 10^−5	8 × 10^−9	8
Porphyrin–RGO	DPV	1 × 10^−6 to 7 × 10^−5	9.5 × 10^−9	39
RGO–MWNTs–PTA	DPV	5 × 10^−7 to 2 × 10^−5	1.14 × 10^−6	40
Fe3O4@ZIF-8/RGO	DPV	2 × 10^−9 to 1 × 10^−5	6.67 × 10^−10	This work

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As an important parameter for a biosensor, discriminating the target between the interfering species in similar physiological environments is necessary to be investigated. For the determination of DA, the electrochemical behaviors of ascorbic acid (AA), and uric acid (UA) (usually coexist with DA) on Fe₃O₄@ZIF-8/RGO/GCE has be studied. As shown in Fig. 11, a couple of shape redox peaks is exhibited in the CV of 1 mM of DA, on the contrary, no obvious peak appeared in the CV of 1 × 10⁻³ M of AA and UA, which suggested that DA can be detected in the presence of AA and UA.³⁴ To demonstrate the selectivity of Fe₃O₄@ZIF-8/RGO/GCE for DA more comprehensively, various potential organic compounds and inorganic ions such as NaCl, K₂SO₄, CaCl₂ solutions, lysine, cysteine and glucose solutions were added to the 1 mM of DA solution. The peak currents of DA showed almost no interference in the presence of these influences since their peak currents changes were below 5%. A possible reaction mechanism was discussed here, the negatively charged Fe₃O₄@ZIF-8/RGO acted as electro-catalysts and selective reagents simultaneously, thus, the positively charged DA was attracted and electro-catalyzed on the sensor, otherwise, the negatively charged analytes such as AA and UA were repelled. Therefore, with a wide linear range, low detection potential, and high sensitivity, the Fe₃O₄@ZIF-8/RGO could be used as promising candidates to prepare the dopamine biosensors.

Fig. 11 Cyclic voltammograms of the Fe₃O₄@ZIF-8/RGO/GCE in 0.1 PBS with 1 mM DA, AA and UA, respectively.

3.5. Reproducibility of biosensor and real sample determination

The reproducibility and stability of the Fe₃O₄@ZIF-8/RGO/GCE were further examined. When the electrode was successively scanned in 1 mM DA in 0.1 M pH 5.0 PBS for 50 cycles, there were no obvious changes in the peak current in the CV curves. Repeated DPV experiment was also conducted in the same condition, and the relative standard deviation (RSD) was 2.13% after 15 successive measurements. The long-term stability of the electrode was investigated by measuring its CV response over a 10 day period. The fabricated electrodes were stored under normal conditions at room temperature, and no obvious changes were found when measured it periodically. Therefore, the results indicated the excellent reproducibility and stability of the prepared Fe₃O₄@ZIF-8/RGO/GCE biosensor. Real samples were analyzed by the standard addition method to evaluate the potential application of Fe₃O₄@ZIF-8/RGO/GCE biosensor. Urine and serum samples were diluted with PBS (0.1 M, pH 5.5) in order to avoid the interferences of the complicated matrix in the real samples and fit into the linear ranges of DA. Then the diluted samples were spiked with different amounts of known concentrations of DA, and measured under the optimal conditions. The analytical results are shown in Table 2. As can be observed, the recovery of the spiked samples was in the range of 98.20–102.73% (n = 5) with the relative standard derivation (RSD) values were calculated to be less than 2.69%, indicated that the method is reliable and sensitive for determination of DA in real samples.

Table 2 Determination of DA in real samples using Fe₃O₄@ZIF-8/RGO/GCE

Samples	Added (μM)	Found (μM)	RSD (%)	Recovery (%)
Serum 1	—	0.093	1.91	—
	0.200	0.301	2.23	102.73
	0.500	0.597	2.54	100.81
Serum 2	—	0.223	2.36	—
	0.200	0.429	1.87	101.42
	0.500	0.719	1.61	99.44
Urine 1	—	0.134	2.62	—
	0.200	0.328	2.69	98.20
	0.500	0.639	2.28	100.88
Urine 2	—	1.102	1.77	—
	0.200	1.309	1.89	100.73
	0.500	1.597	2.01	99.61

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4. Conclusions

In this paper, a novel hybrid nanocomposite of magnetic Fe₃O₄@ZIF-8 decorated graphene was prepared by a simple method for the first time. After the Fe₃O₄/RGO was formed by solvothermal approach, the MOFs (ZIF-8) was fabricated on the surface of Fe₃O₄ to get the Fe₃O₄@ZIF-8/RGO nanocomposite. By combing the unique properties of large specific surface area and high conductivity derived from both of MOFs and graphene, the Fe₃O₄@ZIF-8/RGO nanocomposites modified glassy carbon electrode was successfully constructed and used for electrochemical detection of DA. The fabricated biosensor showed great potential applications in the detection of DA with extraordinary advantages such as wide linear range (2.0 × 10⁻⁹ to 1.0 × 10⁻⁵ M), low detection limit (6.67 × 10⁻¹⁰) and good selectivity for DA detection in the presence of ascorbic acid and uric acid. Moreover, the prepared sensor also showed satisfactory result in real samples detection. Our present study demonstrated the combination of MOFs and graphene composites could fabricate a new kind of high sensitive biosensor for electrochemical detection.

Acknowledgements

The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (no. 21304040), Natural Science Foundation of Gansu Province (1308RJYA027) and Chinese Postdoctoral Funds (2013M532090). This paper is dedicated to memory of pro. Yanfeng Li, who passed away recently.

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