Selective electrochemical detection of dopamine using nitrogen-doped graphene/manganese monoxide composites

Abstract

Nitrogen doping plays a critical role in regulating the electronic properties of graphene, which has shown fascinating applications in bioelectronics and biosensors. Besides, the surface properties of graphene could be adjusted via chemical modification, which facilitates its use in composite materials. Furthermore, a covalent assembly of graphene into an ordered hierarchical structure provides an interconnected conductive network beneficial to electrolyte transfer on the electrode surface. In this paper, we developed a novel nitrogen doped reduced graphene oxide/manganese monoxide composite (N-RGO/MnO) by incorporating a covalent assembly and nitrogen doping. The as prepared N-RGO/MnO was further applied for highly selective and sensitive detection of dopamine (DA) in the presence of uric acid (UA) and ascorbic acid (AA) by differential pulse voltammetry. The separation of the oxidation peak potentials for DA-UA was about 131 mV. This excellent electrochemical performance can be attributed to the unique structure of N-RGO/MnO. The response of the electrochemical sensor varies linearly with the DA concentrations ranging from 10 μM to 180 μM with a detection limit of 3 μM (S/N = 3). This work is promising to open new possibilities in the study of novel graphene nanostructures and promote its potential electrochemical applications.

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1. Introduction

Dopamine (DA), as a catecholamine neurotransmitter, plays a critical role in the function of the central nervous and cardiovascular systems. Abnormal DA concentration in the brain has been related with various diseases, such as Schizophrenia and Parkinson's disease.^1,2 Thus, trace level measurement of DA has been a long-standing goal for investigating its physiological functions and diagnosing nervous diseases. Over the past several decades, tremendous effort has been made and various techniques have been developed for this goal. Electrochemical technique has received considerable interest in the detection of DA for its simplicity, cost-effectiveness, reproducibility and potential for miniaturization. However, the presence of many electro-active substances in the extracellular fluid of the brain has interfered this process. For example, ascorbic acid (AA) and uric acid (UA) are both electro-active compounds and have the similar electrochemical oxidation peak potentials with DA.^3–6 Thus, many attempts have been made to circumvent this problem for selective determination of DA in the presence of UA and/or AA. The fundamental approach is to improve the selectively and electro-catalytic performance on the electrode surface. Following this strategy, various chemically modified electrodes have been fabricated using metal oxides, metal nanoparticles, carbon nanomaterials, and so on.

Graphene, a carbon-based two-dimensional nanomaterial with sp²-hybridized carbon networks, has received tremendous attention for its unique physical and chemical properties, such as high surface-to-volume ratio, excellent conductivity, and rich surface chemistry.^7,8 Due to these fascinating properties, graphene provides an ideal base for electric devices and biosensors.^9,10 Hence, tailoring and developing the electronic characteristics of graphene to achieve unique properties possess promising technological significances in practical utility.

Chemical doping with foreign atoms is an effective method to modify materials intrinsically, tailor electronic properties, manipulate surface chemistry, and produce local changes to the elemental composition of host materials.^11,12 As a promising material, much research effort has been devoted for the doping of graphene to enhance its electrochemical behaviors. Among the numerous potential dopants, nitrogen is considered to be an excellent element for the chemical doping of carbon materials because it is of comparable atomic size and contains five valence electrons available to form strong valence bonds with carbon atoms.¹³ Consequently, nitrogen doping has great potential to be used for intrinsic graphene modification.

Development of hierarchical architectures from interconnected nanoscale building blocks is intriguing for the successful practical use of nanomaterials. For example, interconnected networks can not only offer a large number of accessible open pores to electrolytes in electrochemistry, but also allow for the growth and anchor of functional inorganic nanomaterials with high loading amount.^14,15 They possess promising technological significances in catalysts, energy conversion systems and sensors etc.^16–18 Recently, some attempts have been made to develop interconnected graphitic solid structures such as hierarchical architectures, which exhibits high conductivity and porosity, and excellent mechanical strength.¹⁹ However, keeping the properties of the individual graphene while constructing macroscopic structures from them still remains a challenge due to irreversible aggregation. Even though the widespread utilization of these non-covalent assembly methods, the unstable structure of the assembly hinders the development of graphene based electrochemical platform. Graphene oxide²⁰ possesses plenty oxygen-containing groups, which can act as sufficient reaction sites for covalent conjunctions. Therefore, the covalent assembly is highly desirable for making and engineering stable graphene superstructures, and bringing superior properties to such structure.²¹

Meanwhile, numerous hybrids such as graphene/Au,²² graphene/metal oxide,²³ graphene/polymer²⁴ composites have been reported for producing graphene hybrid with synergy or multiple functionalities. Manganese oxides, as a class of transition metal oxides, are promising candidates for active electrode materials, due to their low cost, high specific capacitance, and environmentally friendly nature. The electron transfer of manganese oxides has offered substantial potentials in many fields, including catalysis, energy storage, and biological sensing.^25–27 Therefore, the synergetic effect arising from heteroatom doping, covalent assembly, and hybrid will improve the electronic properties of graphene to a great extent, which is a promising electrode material for the determination of DA.

In this work, we present a strategy to synthesize nitrogen doped reduced graphene oxide/manganese monoxide composite (N-RGO/MnO) through covalent assembly and urea adulteration. The graphene oxide (GO) network was fabricated by covalent assembly through glutaraldehyde and resorcinol aided cross linking. After pyrolysis reaction with urea, which serves as reducing-doping agents, the structure of N-RGO was further achieved without destroying the macroscopic interconnected morphology. Before GO network being constructed, graphene oxide sheets absorb most Mn²⁺ ions through electrostatic interactions or coordination with abundant surface oxygen-containing groups, giving rise to the nucleation sites of MnO nanoparticles (MnO NPs). Then the urea can promote pyrolysis of Mn(NO₃)₂ precursors, resulting in the formation of MnO NPs in the process of thermal treatment. The graphene hybrids exhibit an interconnected macroporous framework of graphene sheets with uniform dispersion of MnO nanoparticles. The structure of the as prepared nanomaterial was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectra. We also investigated the basic electrochemical properties of the nanostructure with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Furthermore, the as prepared N-RGO/MnO was applied for DA detection by using differential pulse voltammetry (DPV) techniques. Even in the presence of UA, the response of the electrochemical sensor varies linearly with the DA concentrations. Therefore, this novel material allows for the selectively detection of DA and has great potential to be developed for effective electrochemical sensing.

2. Experimental section

2.1 Chemicals and materials

For N-RGO/MnO synthesis, graphite powder (99.99%, 325 mesh) was obtained from Alfa Aesar. K₂S₂O₈, H₂SO₄, KNO₃, H₂O₂ (30%), P₂O₅, KMnO₄, HCl, urea, resorcinol, glutaraldehyde, Na₂B₄O₇·10H₂O, K₃[Fe(CN)₆], K₄[Fe(CN)₆], Na₂HPO₄, NaH₂PO₄, NaCl, KCl, Mn(NO₃)₂ (50%), catechol and hydroquinone were purchased from Beijing Chemical Company. All reagents were analytically pure and used without further purification.

2.2 Synthesis of nitrogen doping graphene/manganese monoxide composite (N-RGO/MnO)

GO was fabricated according to the previously reported Hummers' method.²⁸ The GO network structure was constructed through covalent conjugation between hydroxyl groups on GO surface employing glutaraldehyde as the link reagent.²⁹ The addition of resorcinol initiated the polycondensation reaction to form aerosol-type amorphous structure with enhanced mechanical stability. Briefly, 5 mg mL⁻¹ GO dispersion was treated with 22 mM glutaraldehyde, 0.06 mM borax, 11 mM resorcinol and 5 mg mL⁻¹ Mn(NO₃)₂ under rigorous string. Afterward, the mixture turned viscous and the reaction was completed assisted by ultrasonication for 2 h. The GO/Mn(NO₃)₂ was obtained after freeze drying in lyophilizer for 48 h. In this process, GO absorbed most Mn²⁺ ions through electrostatic interactions or coordination with abundant surface oxygen-containing groups,²⁹ giving rise to the nucleation sites of MnO NPs. As for nitrogen doping and the synthesis of MnO NPs, a mixture of 100 mg GO/Mn(NO₃)₂ and 500 mg urea was ground with an agate mortar. The yielding homogeneous mixture was transferred to a furnace and then was pyrolyzed at 900 °C for 3 h at a rate of 3 °C min⁻¹ under nitrogen atmosphere followed by lyophilization to get nitrogen-doped graphene/manganese monoxide composite (N-RGO/MnO).

2.3 Characterization

The surface morphology of as prepared samples was characterized by transmission electron microscopy (TEM) operated on a JEOL JSM 7401 and H-7650B, respectively. The crystallographic structures of the N-RGO and N-RGO/MnO were investigated by powder X-ray diffraction (XRD) on a Bruker D8-Advance X-ray powder diffractometer. The scattering angles used in the measurements varied from 10° to 80°. Raman spectra was employed to characterize the reduction degree and the nitrogen doping state of the samples.

2.4 Electrochemical detection

Electrochemical measurements were carried out using a conventional three-electrode system composed of a glassy carbon working electrode (GCE), a KCl saturated Ag/AgCl reference electrode, and a Pt wire counter electrode. Before usage, the GCE was polished with 0.3 and 0.05 μm α-Al₂O₃ slurry on an abrasive cloth successively. After rinsing with water, ethanol, and water respectively under ultrasonication, the electrode was dried with high-purity nitrogen. The samples were prepared by drop-casting 5 μL of 1 mg mL⁻¹ suspension over the glassy carbon electrode and evaporating the remaining water at room temperature. Cyclic voltammetry and differential pulse voltammetry (DPV) measurements were performed on a CHI 1030 electrochemical workstation with corresponding electrochemical software. Electrochemical impedance spectroscopy (EIS) was measured on a PARSTAT 2273. The EIS was tested in 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1) redox probe solution with frequency from 100 mHz to 100 kHz by applying an ac voltage of 10 mV amplitude. All measurements for dopamine detection were conducted in 0.1 M phosphate buffer saline (PBS, pH 7.40) at room temperature. Dopamine, ascorbic acid and urea solutions were freshly prepared in PBS prior to use.

3. Results and discussion

3.1 Characteration of N-RGO/MnO

The crystallographic structures of N-RGO and N-RGO/MnO were characterized by X-ray diffraction (Fig. 1A). The strong diffraction peak observed at about 25° was in accordance with the previously reported value of RGO.^30,31 XRD pattern of N-RGO/MnO show that the attached MnO NPs are manganosite phase (JCPDS no. 75-1090). The fact that no other characteristic peak appeared reveals that there was no impurity in the resulting materials. As shown in Fig. 1B, a large amount of MnO NPs with diameters of ∼50 nm are uniformly distributed on the graphene sheets. The wrinkles in the TEM images indicate the successful cross-linking between single building blocks under the glutaraldehyde-aided reaction. What's more, the N-RGO/MnO structure was highly interconnected and well dispersed without aggregations. The dispersed nanostructure provides sufficient pathways to facilitate the electron transfer on the electrode surface and the diffusion of electrolyte.

Fig. 1 (A) XRD patterns of N-RGO and N-RGO/MnO; (B) TEM image of N-RGO/MnO.

Raman spectroscopy is a powerful nondestructive and on-site technique for the structural characterization of carbonaceous materials, which offers clear evidence of the reduction and N-doping in the carbon bones of GO after reaction. As shown in Fig. 2A, the 2D band shows the most prominent character of graphene in Raman spectrum. Lower I_D/I_G value (1.06) of N-RGO and N-RGO/MnO (1.12) compared to RGO (1.20) was attributed to the electron-donating capability of N heteroatom induced graphitic degree decrease.^32,33 The specific Raman vibrational modes located at 650.3 cm⁻¹ and 366.3 cm⁻¹ confirm that the attached MnO NPs are present. XPS spectrum was employed to characterize the heteroatom doping state of 3D-N-RGO/MnO. As shown in Fig. 2B, an obvious N1s peak was clearly observed in the XPS spectrum, illustrating that the nitrogen heteroatom had been dopped into the graphene backbones successfully. We also get the concentrations of nitrogen and MnO NPs quantitatively from the XPS spectra, which were calculated to be 5.70% and 24.4% (wt%) in the 3D-N-RGO/MnO respectively.

Fig. 2 (A) Raman spectra of RGO, N-RGO and N-RGO/MnO; (B) XPS survey spectra of 3D-N-RGO/MnO.

3.2 Electrochemical performance of N-RGO/MnO modified electrode

The electrochemical properties of the as prepared material were tested in the Fe(CN)₆^3−/4− redox system, which is almost an ideal quasi-reversible system on carbon-based electrode. The cyclic voltammetric performances of Fe(CN)₆^3−/4− on the bare glassy electrode and different modified glassy electrodes are shown in Fig. 3A. N-RGO/MnO modified electrode exhibits a redox peak with the peak current higher than that of bare glassy electrode. The obvious increase of the peak current density from GCE, RGO modified GCE, N-RGO modified GCE, N-RGO/MnO modified GCE in sequence was contributed to the multidimensional electron transfer pathways benefited from the interconnected structure and the electroactive sites resulting from nitrogen doping and MnO composite. The electron transfer ability of N-RGO was further investigated by electrochemical impedance spectroscopy. As shown in Fig. 3B, the diameter of the semicircle of N-RGO/MnO modified GCE in the characteristic impedance curves (Nyquist plots) decreased compared to GCE, which represents improved electron transfer capability. Additionally, by fitting the results with an appropriate equivalent circuit, the charge transfer resistance values are calculated to be 135.10 Ω at MnO modified GCE, 32.91 Ω at RGO modified GCE, 14.48 Ω at N-RGO modified GCE and 2.74 Ω at N-RGO/MnO modified GCE respectively. These data represent the enhancement of electron transfer ability, which is in consistent with the result obtained from the CV measurements. These results verify that the nitrogen doped reduced graphene oxide/manganese monoxide composite greatly improved the electrochemical performance on the glassy carbon interface with fast electron transfer rate.

Fig. 3 (A) CV and (B) EIS of (a) N-RGO/MnO, (b) N-RGO, (c) RGO, (d) MnO modified GCE, and (e) bare GCE in 5 mM Fe(CN)₆^3−/4− containing 0.5 M KCl. Scan rate, 100 mV s⁻¹. The frequency range is from 100 Hz to 100 KHz with signal amplitude of 10 mV.

3.3 Dopamine detection on the electrochemical sensor

Selectivity is a very important aspect of sensor performance. Thus, commonly existing species such as UA and AA are chosen to evaluate the selectivity of the as-fabricated DA sensor. To investigate the electrocatalytic activity of the DA sensor towards the electrochemical oxidation of DA, UA, and AA, the N-RGO/MnO modified GCE was characterized by cyclic voltammetry ranging from −0.1 to 0.6 V (vs. Ag/AgCl) in 0.1 M phosphate buffer solution. The cyclic voltammograms of N-RGO/MnO, N-RGO, RGO, and MnO in 0.1 M phosphate buffer solution (pH 7.4) containing 1 mM DA are shown in Fig. 4. The obvious increase of the oxide peak current from MnO, bare GCE, RGO, N-RGO, and N-RGO/MnO in sequence contributed to the multidimensional electron transfer pathways benefited from the interconnected structure and the electroactive sites resulting from nitrogen doping and MnO composite.

Fig. 4 CV curves of (a) N-RGO/MnO, (b) N-RGO, (c) RGO, (d) bare GCE and (e) MnO in 0.1 M phosphate buffer solution (pH 7.4) containing 1 mM DA.

The cyclic voltammograms of 1 mM DA, 0.5 mM UA, and 1 mM AA in 0.1 M phosphate buffer solution (pH 7.4) at the bare GCE and N-RGO/MnO modified GCE are shown in Fig. 5A–C, respectively. It can be seen that well-defined and resolved voltammetric responses were obtained for the direct oxidation of DA, UA, and AA at the N-RGO/MnO modified GCE in comparison to bare GCE. Moreover, the N-RGO/MnO modified electrode exhibits negative shift of the anodic potentials and increased current responses. The peak-to-peak separations (ΔE_p) for redox of DA are 49 and 98 mV for the N-RGO/MnO modified electrode and bare GCE, respectively. The ΔE_p for the redox of DA at the N-RGO/MnO modified electrode is much less than at bare GCE. This represents the fast electron transfer kinetics of DA on the N-RGO/MnO modified electrode. For the electrochemical reaction of UA, an oxidation peak at 479 mV and no obvious cathodic peak potential are observed at the bare GCE. In contrast, a pair of redox peaks was observed at the N-RGO/MnO modified electrode. The oxidation and reduction peak potentials are located at 344 and 276 mV (Fig. 5B), respectively. For the electrochemical reaction of AA (Fig. 5C), the absence of obvious anodic peak suggests the reactionlessness of the electrochemical process, which means that the existence of AA hasn't a great influence on the detection of DA or UA. For the redox reaction of DA, well-defined oxidation and reduction peaks are observed at 198 and 149 mV at the N-RGO/MnO modified electrode. The negative shift and increased peak current of the oxidation peaks of DA, and UA indicate that the N-RGO/MnO modified electrode exhibits an excellent catalytic effect on DA and UA oxidation. This can be attributed to the modification of the surface of GCE with the N-RGO/MnO composite. In order to demonstrate the selective behavior at N-RGO/MnO modified electrode, the electrochemical behavior of a mixture of 1 mM DA, 0.5 mM UA, and 1 mM AA in a phosphate buffer solution was studied (Fig. 5D). For the ternary mixture of DA, UA, and AA, an overlapped oxidation peak is seen at the bare GCE. Their oxidation peaks can't be separated well. In contrast, well-defined anodic peaks of DA, UA, and AA are observed at the N-RGO/MnO modified electrode. A strong cathodic peak of DA can also be seen. The anodic peak potential separations are 131 mV for DA/UA and the existence of AA hasn't a great influence on the detection of DA or UA. This result shows that the N-RGO/MnO modified electrode exhibits excellent selective electrocatalytic behavior for the electrooxidation of DA, UA, and AA, which allows the simultaneous determination of DA, UA, and AA. Because the existence of AA did not have a great influence on the detection of DA or UA, the next experiments were performed in PBS containing DA and UA.

Fig. 5 CV responses at the bare GCE (dash line) and N-RGO/MnO modified GCE (solid line) in 0.1 M phosphate buffer solution (pH 7.4) containing (A) 1 mM DA, (B) 0.5 mM UA, (C) 1 mM AA, and (D) mixture of 1 mM DA, 0.5 mM UA and 1 mM AA. Scan rate, 50 mV s⁻¹.

To further investigate the electrocatalytic mechanism of N-RGO/MnO modified electrode toward DA and UA detection, we studied the influence of scan rate on peak current (Fig. 6A). When the scan rate changed from 5 to 90 mV s⁻¹, the oxidative and reduction current peaks of the N-RGO/MnO modified electrode increased with the square root of scan rate linearly (Fig. 6B). The linear regression equation is I_pc(DA) (μA) = 3.21 − 2.20 × ν^1/2 (mV s⁻¹)^1/2 with the correlation coefficient of r = 0.989, I_pa(DA) (μA) = 1.38 + 3.38 × ν^1/2 (mV s⁻¹)^1/2 with correlation coefficient of r = 0.998 and I_pa(UA) (μA) = −1.20 + 4.35 × ν^1/2 (mV s⁻¹)^1/2 with correlation coefficient of r = 0.990, where I_pc and I_pa is the reduction and oxidative current peaks and ν is the scan rate. These data are in consistence with previous reports,³⁴ manifesting that the redox processes were typically diffusion controlled.

Fig. 6 (A) CVs of the N-RGO/MnO modified GCE at different scan rates (5, 10, 20, 30, 40, 50, 60, 70, 90 mV s⁻¹). (B) Plots of the I_pa and I_pc on the N-RGO/MnO modified GCE as functions of the square root of the scan rates. Measured in 0.1 M phosphate buffer solution (pH 7.4) containing 1 mM DA and 0.5 mM UA.

Compared to CV technique, the square-wave voltammetry is a more sensitive and selective electrochemical technique that has been widely used for quantitative determination. In order to improve the sensitivity and lower the detection limit, DPV of selective determination of DA in the presence of UA in a phosphate buffer solution was performed (Fig. 7A). Even in the presence of 20 μM UA, the detection ability of the sensor wasn't interfered. Peak currents of the sensor increased linearly with increasing concentration of DA from 10 μM to 180 μM with detection limit of 3 μM (S/N = 3), which is comparable to the results obtained by other electrochemical methods.³⁵ The linear regression equation was described as: I (μA) = −0.23 + 0.09 × C_DA (μM) with the correlation coffecient of r = 0.997, where I is the peak current and C_DA is the concentration of dopamine (Fig. 7B). The relative small standard deviation reveals good reproducibility and stability of as proposed sensor. These results were obtained from three parallel experiments. Compared with other methods for DA detection, this sensor is easy to opera, response fast and excluded the signal interference from UA and AA, which is contributed to the good electrochemical performances of N-RGO/MnO resulting from hierarchical structure, nitrogen doping and composite.

Fig. 7 (A) DPVs of N-RGO/MnO modified GCE in 0.1 M phosphate buffer solution (pH 7.4) containing different concentrations of DA (from top to bottom: 0, 4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180 μM) with the presence of 20 μM UA. (B) Linear relationship between the peak current and DA concentration. Pulse amplitude: 50 mV. Pulse period: 0.4 s.

4. Conclusion

In summary, electrochemical behaviors of DA, UA and AA at the N-RGO/MnO modified electrode and the selective determination of DA in the presence of UA and AA were demonstrated. The N-RGO/MnO modified electrode presents high sensitivity and low detection limit for DA detection, which is a good candidate for practical applications. The fast electron transfer kinetic results from the hierarchical structure material, heteroatom doping and material composite endows graphene with excellent electrocatalytic activity for sensitive and selective DA detection. This study proposes a simple and versatile protocol to promote the potential applications of graphene for the development of novel sensors and catalysis material.

Acknowledgements

This work was financially supported by National Basic Research Program of China (No. 2013CB934004), the National Natural Science Foundation of China (No. 21235004, No. 51572139).

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