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Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid
Accepted Manuscript Title: Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid Author: Mingji Li Wenlong Guo Hongji Li Wei Dai Baohe Yang PII: DOI: Reference:
S0925-4005(14)00990-3 http://dx.doi.org/doi:10.1016/j.snb.2014.08.022 SNB 17305
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-7-2014 4-8-2014 6-8-2014
Please cite this article as: M. Li, W. Guo, H. Li, W. Dai, B. Yang, Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid
Tianjin Key Laboratory of Film Electronic and Communication Devices, School of
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a
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Mingji Li a, Wenlong Guo a, Hongji Li b,*, Wei Dai c, Baohe Yang a
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Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,
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b
School of Chemistry & Chemical Engineering, Tianjin University of Technology,
School of Precision Instrument and Optoelectronics Engineering, Tianjin University,
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Tianjin 300072, P. R. China
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c
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Tianjin 300384, P. R. China.
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*Corresponding author. Hongji Li; Mingji Li E-mall: [email protected]; [email protected] Tel.: +86 022 60214259.
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Highlights 1. MgO tadpoles, nanobelts, and nanorods have been synthesized by DC arc plasma
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jet CVD.
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2. MgO nanobelts show high electrocatalytic activity toward small biomolecules.
3. The electrocatalytic activity of the MgO nanobelts depends on their surface
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morphology.
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4. A MgO nanobelt-based electrochemical biosensor detects ascorbic acid, uric acid,
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and dopamine with high sensitivity and selectivity.
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ABSTRACT
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One-dimensional (1D) MgO nanostructures of various morphologies including tadpole-like nanobelts (tadpoles), nanobelts, and nanorods were synthesized via direct
current (DC) arc plasma jet chemical vapor deposition (CVD). The effect of morphology on the biosensing properties of the nanostructures was investigated by comparing their electrochemical properties. Compared with tadpoles and nanorods, the MgO nanobelts had excellent electrocatalytic activity toward ascorbic acid (AA), dopamine (DA) and uric acid (DA). The response of the MgO nanobelts to the analytes was twice that of the tadpoles. A MgO nanobelt-modified electrode was thus fabricated for the simultaneous determination of AA, DA, and DA. The peak separations between AA and DA, DA and UA, and AA and UA for this electrode were 2
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111, 161, and 272 mV, respectively. The linear response ranges of the electrodes were 2.5–15 and 25–150 μM for AA, 0.125–7.5 μM for DA, and 0.5–3 and 5–30 μM for UA. The calculated detection limits were 0.2, 0.05, and 0.04 μM (S/N=3), respectively.
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The excellent electrocatalytic activity of the MgO nanobelts can be attributed to
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various surface defects such as low-coordination anions (O5C2− and O4C2− at the
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terrace and edge sites, and O3C2− at the corner and kink sites). Additionally, electron tunneling between these surface defects is possible. These defects have a strong
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adsorption capacity toward AA, DA, and UA. This affinity improves sensitivity and decreases the detection limits of the MgO nanobelt electrodes.
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Keywords: One-dimensional MgO nanostructures; Ascorbic acid; Dopamine; Uric
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acid; Simultaneous detection.
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1. Introduction Electrochemically active compounds such as ascorbic acid (AA), dopamine (DA), and uric acid (UA) usually coexist in physiological fluids (serum and urine). They play an
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important role in human metabolic processes [1-3]. The detection and quantification
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of AA, DA, and UA in biological fluids such as blood and urine is very important.
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Improper levels of these compounds in the body can lead to various fatal diseases. Therefore, the development of a sensitive and selective method for their simultaneous
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determination is highly desirable for analytical applications and in diagnostic research. Because DA, AA, and UA have high electrochemical activity the use of an
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electrochemical method for their simultaneous determination would be convenient, rapid, and highly sensitive [4-7]. However, the oxidation peaks of these three species
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overlap when using traditional electrodes and this makes their simultaneous determination difficult. Various chemically modified electrodes have been developed
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to overcome this problem but most research into AA, DA, and UA detection has only addressed selective detection. Typically, two of the three molecules are interferents [8-17]. For the simultaneous detection of all three analytes, the correct selection and preparation of a sensitive layer is important. Previous work has only focused on complex composites or carbon based nanomaterials as modifiers for the sensitive layer. These materials include DpAu/PTCA-Cys [18], Pd3Pt1/PDDA-RGO [19],
nano-Cu/PSA III [20], CTAB-GO/MWCNT/GCE [21], Fe3O4@Au-S-Fc/GSchitosan/GCE [22], and graphene-based nanomaterials [6]. They have been reported to be effective for the simultaneous detection of AA, DA, and UA. However, the 4
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sensitive membranes that are composed of these materials are too complex and several processing steps are required for production. Magnesium oxide (MgO), an alkaline earth metal oxide, is well known because of
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its industrial applications as adsorbents, catalysts, and catalyst supports [23-25].
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Because of their large electrochemically active surface area, unique biocompatibility,
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chemical stability, and catalytic properties, MgO nano-/microstructures can be used to fabricate highly sensitive amperometric biosensors [26, 27]. In most of these
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biosensors, the MgO nanostructures serve as highly absorbent materials for enzyme immobilization. This allows for highly sensitive MgO-based biosensors. In a previous
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study into MgO nanobelts we obtained excellent electrochemical performance. A modified MgO nanobelt electrode gave excellent electrocatalytic response during the
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selective determination of hydroquinone and catechol [28]. One-dimensional (1D) MgO nanomaterials such as belts and rods have good surface conductivity [29, 30],
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large electrochemically active surface areas [31], good chemical stabilities and good catalytic activities [32].
Herein, we report the production of 1D MgO nanostructures by direct current (DC)
arc plasma jet chemical vapor deposition (CVD). Tadpole, nanobelt, and nanorod morphologies were selectively obtained when using different reaction times under specific reaction conditions. We also investigated the influence of morphology on the electrochemical properties of the MgO 1D nanostructures. A MgO nanobelt-modified electrode was fabricated for the simultaneous determination of AA, DA, and UA.
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2. Experimental 2.1 Synthesis of 1D MgO nanostructures 1D MgO nanostructures were prepared using DC arc plasma jet CVD with
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Mg(NO3)2·6H2O as the precursor and Mo nanoparticles on the Mo substrate surface
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as the catalyst. Tadpoles, nanobelts, and nanorods were prepared after 1, 5, and 12 min, respectively. The main deposition parameters were as follows: Arc power of 18
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kW, argon flow rate of 1.5 L min−1, hydrogen flow rate of 10 L min−1, substrate
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temperature maintained at 950 °C, and pressure in the reaction chamber maintained at
2.2 Material characterization
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4.0 kPa.
Products were characterized by powder X-ray diffraction (XRD; Rigaku
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D/max-2500/PC, CuKα, λ=0.15406 nm, Japan), scanning electron microscopy (SEM; JEOL, JSM-6700F, 10.0 kV, Japan), transmission electron microscopy (TEM; JEOL,
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JEM-2100, 200 kV, Japan), Fourier transform infrared spectrometry (FT-IR; Bruker VERTEX 70, Germany), and fluorescence spectrophotometry (PL; Hitachi F-4500, 150 W xenon lamp, Japan).
2.3 Electrochemical measurements To prepare the 1D MgO nanostructure-modified electrode, a glassy carbon electrode (GCE, 3.0 mm diameter) was polished successively with 0.3 and 0.05 μm alumina
powder, rinsed ultrasonically with dilute nitric acid, ethanol, and deionized water, each for 5 min. It was then dried under N2 flow at room temperature. 8 mg of the 1D MgO nanostructures and 20 mg chitosan were dispersed in 2.5 mL of 1 % acetic acid. 6
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The dispersion was ultrasonicated to form a homogeneous 3.2 mg mL −1 suspension. The obtained 1D MgO nanostructure suspension was then ultrasonicated in a water bath for 15 min before use. In a typical preparation, the GCE was placed in the
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suspension and electrodeposition of the 1D MgO nanostructures was performed at −5
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V for 20 min. The obtained MgO nanostructure/GCE electrode was then allowed to
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dry slowly.
Electrochemical measurements were performed using a CHI 660D series
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electrochemical system. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using a three electrode system comprising the 1D
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MgO nanostructure-modified GCE as the working electrode, a Ag/AgCl reference
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electrode, and a platinum (Pt) wire counter electrode.
3. Results and discussion
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Fig. 1 shows SEM images and electrochemical property curves of the as-prepared MgO tadpoles, the MgO nanobelts, and the MgO nanorods. The tadpoles consist of many MgO nanobelts with each containing a spherical Mo nanoparticle at their tip. The nanobelts are 10–30 nm wide and ~2 μm in length, and the diameters of the
spherical tips are about 40–100 nm. The MgO nanobelts formed after the disappearance of the spherical tips when the reaction time was extended. The width
and length of the nanobelts increased to 30–100 nm and ∼3 μm, respectively. The MgO nanorods are 0.5–1 μm in length and 100–200 nm in width. 7
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Fig. 1 X The interfacial characteristics of the MgO tadpole-, the MgO nanobelt-, and the
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MgO nanorod-modified electrodes were studied by EIS in a 0.1 M KCl solution. This solution contained a redox couple of 5 mM K4Fe(CN)6 and 5 mM K3Fe(CN)6. Fig. 1D
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shows the EIS that was recorded for the different electrodes. The equivalent
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impedance analysis circuit is shown in the insert of Fig. 1D and lists the parameters used. These parameters include the resistance of the electrolyte solution (Rs), the
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charge transfer resistance at the MgO nanostructure layer/electrolyte interface (Rct), the charge–transfer resistance in the MgO nanostructure layer interior (R1), the value
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of the constant phase element (CPE), the charge–transfer resistance at the MgO
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nanostructure layer/GCE interface (R2), the bulk Faradic pseudo-capacitance (C1 and
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C2), the Warburg impedance (Zw), the double-layer capacitance of the counter
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electrode/electrolyte solution interface (C3), and the charge transfer resistance at the counter electrode/electrolyte interface (Rint). The Rct values were found to be 145.4 Ω cm2 for the GCE, 28.2 Ω cm2 for the MgO tadpoles/GCE, 24.2 Ω cm2 for the MgO
nanobelts/GCE, and 32.3 Ω cm2 for the MgO nanorods/GCE. The Rct of the MgO
nanobelts/GCE was smaller than that of the bare GCE, the MgO tadpoles/GCE, and the MgO nanorods/GCE. The Rct represents the electrode resistance and is closely related to the surface area and conductivity of the working electrode. The MgO nanobelts contribute to a decrease in the resistance at the interface. CV curves between −600 and 1000 mV for the four different electrodes are shown in Fig. 1E. For the bare GCE, the MgO tadpoles/GCE, the MgO belts/GCE and the MgO rods/GCE 8
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the [Fe(CN)6]3-/4- redox couple was reversible with peak to peak separations (△Ep) of
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500, 471, 260, and 444 mV, respectively. After modification with MgO nanobelts, the
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electrode’s △Ep decreased significantly and the redox current surpassed that of the other electrodes. This suggests that the nanobelts provide the necessary conduction
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pathways to accelerate electron transfer between the redox couples and the electrode surface. These results are consistent with the EIS analysis.
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Fig. 1F shows the CVs obtained for a mixture of AA, DA, and UA when using the bare GCE, the MgO tadpoles/GCE, the MgO nanobelts/GCE, and the MgO
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nanorods/GCE. For the bare GCE, the AA and DA oxidation peaks completely overlap and the peak potentials of DA and UA are indistinguishable. This makes the
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simultaneous determination of these compounds on bare GCE impossible. In contrast, the MgO 1D nanostructure-modified GCEs oxidized AA, DA, and UA at 176–246, 315–357, and 486–518 mV, respectively. Additionally, the current response was also enhanced. For the MgO nanobelts/GCE, the peak separations between AA and DA, DA and UA, and AA and UA were 111, 161, and 272 mV, respectively. The onset potential of the mixture for the MgO tadpole-modified electrode was 100 mV while that of the MgO nanobelt electrode shifted to around −100 mV. This was accompanied
by a concomitant increase in the peak current density. The response of the MgO nanobelts to AA, DA, and UA was twice that of the tadpoles. The above results 9
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indicate that the MgO nanobelts had significantly higher electrocatalytic activity than the nanorods and the tadpoles. Fig. 2A, C, and E shows typical TEM images of the as-prepared MgO nanobelts.
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The spacing between the two neighboring parallel fringes was 0.24 nm, which is in
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good agreement with the interplanar spacing of the {111} lattice plane of cubic MgO
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(Fig. 2 B). From regions II and III, the interplanar spacing was calculated to be 0.21 nm, which corresponds to the {100} planes of cubic MgO (JCPDS 45-0946). The
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fringe pattern in the high-resolution TEM images (Fig. 2D and F) clearly indicates that the MgO nanobelts were monocrystalline. Their high crystal quality is evident.
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The surface details were observed under high magnification and growth steps are present (Fig. 2D and F). This nanobelt is also composed of terraces and corners and
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O3C2− and O4C2− [33].
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these sites contribute to the formation of low-coordinated surface ions, including
Fig. 2
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The electrochemical behavior of the MgO nanobelts/GCE was studied at different
pH values using CV (Fig. 3A). The oxidation peak potentials and the magnitude of the current responses were found to be pH dependent. In the pH range from 4.5 to 7.0, the oxidation peak potentials shift negatively with an increase in pH. This indicates that protons participate in the electrode reaction process. When AA, DA, and UA coexist in solution, a stable and well-defined peak is observed at pH 5.0. This pH may, therefore, be optimal for the adsorption and electrochemical oxidation of AA, DA, and UA on the MgO nanobelts/GCE. Additionally, a maximum separation of the peak 10
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potentials for AA–DA and DA–UA is also observed at pH 5.0. We thus selected 0.1 M PBS at pH 5.0 for subsequent experiments. To investigate the reaction kinetics, the influence of scan rate on the peak currents and the peak potentials of AA, DA, and UA
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in the mixture was also investigated. As shown in Fig. 3B, the oxidation peak currents
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(Ipa) for AA, DA, and UA increase consistently with scan rate from 10 to 800 mV s −1.
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The oxidation peak currents of all three compounds increased in a linear manner with scan rate over this range (insert in Fig. 3B). This suggests that electron transfer at the
Fig. 3
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MgO nanobelts/GCE is surface adsorption controlled for all the analytes. X
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The CV results show that a simultaneous determination of AA, DA, and UA is
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possible when using the MgO nanobelts/GCE (Fig. 4A). Three well-defined oxidation
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peaks are present at potentials of approximately 200, 357, and 518 mV. Figure 4 B–D
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shows the relationship between the peak current and the concentration of AA, DA, and UA, respectively. The oxidation peak currents increase with analyte concentration. The peak potentials are steady and the calibration curves are linear in the concentration ranges of 2.5–15 and 25–150 μM for AA, 0.125–7.5 μM for DA, and
0.5–3 and 5–30 μM for UA. The linear equations for AA, DA, and UA are Ipa (μA) =
2.391+0.014 [AA] (μM) (R=0.991), Ipa (μA) = 2.603+0.002 [AA] (μM) (R=0.993); Ipa (μA) = 0.793+0.559 [DA] (μM) (R=0.995); Ipa (μA) = 0.07+0.2 [UA] (μM) (R=0.993),
and Ipa (μA) = 0.361+0.068 [UA] (μM) (R=0.995), respectively. The simultaneous detection limits calculated for AA, DA, and UA are 0.2, 0.05 and 0.04 μM (S/N=3), respectively. These values are comparable to those obtained for other materials. As 11
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shown in Table 1 [6, 18-22, 34-38], the MgO nanobelt-modified electrode has a satisfactory detection limit and linear range for the simultaneous detection of AA, DA,
Fig. 4
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and UA. X
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Table 1 X
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The sensing mechanism of the MgO nanobelts is shown in Fig. 5. The anodic peak response (Fig. 1F) corresponds to the oxidation of the furan ring hydroxyl groups
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(−OH) to carbonyl groups (C=O) in AA [39], the oxidation of catechol to o-quinone in DA, and the oxidation of the bridging double bond to −OH followed by
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dehydration in UA. According to the electrocatalytic oxidation mechanism, the electrode was clearly electrocatalytic toward AA, DA, and UA as the analytes lost
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two electrons. The AA and DA oxidation processes are related to the interactions between the MgO nanobelts and the analytes. This comes from intermolecular effects
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(hydrogen bonds) between the oxo-surface groups or the O atoms in the lattice of the MgO nanobelt surface and the −OH groups of the analytes. A close interaction between the low-coordination defect sites and the −OH groups at the surface of the
MgO nanobelts is evident in the FTIR spectrum (see Fig. S1). The UA oxidation process is related to the interactions between the Mg atoms at the MgO nanobelt surface, the adsorbed H2O molecules, and the bridging double bond in UA. The O
atom in H2O is close to the Mg site [40], and the H atom in H2O is close to the C atom in the bridging double bond of UA [41]. This may be the mechanism for the catalytic oxidation of the three analytes by the MgO nanobelts. 12
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Fig. 5
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We also monitored the PL emission of the MgO nanobelts at an excitation wavelength of 256 nm (Fig. 5). The broad emission spectrum was centered at 355 nm
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and was de-convoluted to eight distinct peaks at 320 (3.9), 355 (3.5), 378 (3.3), 398
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(3.1), 432 (2.9), 466 (2.7), 505 (2.5), and 543 (2.3 eV) nm. These are attributed to
low-coordination oxide ions such as O3C2−, O4C2−, and O5C2−, and other defects (F and
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F+ centers) in the MgO nanobelts [33, 42, 43]. These surface defects increase the
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specific surface area of the nanobelts thus enhancing their adsorption capacity toward the three analytes. The low-coordination oxide ions and the other defects have band
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gaps of 2.3–3.9 eV in the energy band structure of the nanobelts. The bandgap of bulk MgO is 7.8 eV resulting in MgO being a prototypical ionic insulator. However, the
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surface of MgO (including the nanobelts) exhibits semiconducting properties [44, 45]. The conductivity of 1D nanostructures can vary from a fully non-conductive state
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to a conductive state and this is determined by their surface chemistry [46]. MgO nanobelt surfaces have many defects and the surface electronic properties depend strongly on low-coordination sites such as kinks, corners, and steps [44]. Additionally, the (111) lattice plane of MgO nanobelt surfaces has a quasi-1D graphene ribbon electronic structure [30]. Electron transfer is a
fundamental process
in
electrochemistry, and structural defects play a major role in electrocatalytic activity [47]. The decrease in MgO nanobelt resistance is caused by electron tunneling between surface defects [29]. Electron transfer between MgO nanobelts and analytes depend on these surface defects. Therefore, the electron transfer rate accelerates 13
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quickly as electrons are captured and transported to nanobelt surfaces. This may be the main reason for the improved sensitivity and the lower detection limit of MgO nanobelt-modified electrodes compared with those of other electrodes. Therefore, the
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sensitive membranes listed in Table 1 cannot be compared with MgO nanobelts.
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The influence of various substances on the determination of 500 μM AA, 25 μM
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DA, and 100 μM UA was studied by CV. We found that 100-fold (vs. DA) CH3COONa, CaCl2, KCl, K2HPO4, FeCl3, Mg(NO3)2, and Ni(NO3)2, 10-fold (vs. DA)
L-cysteine, L-glycine, L-lysine, L-tyrosine,
L-alanine,
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aniline, catechol, hydroquinone, resorcinol, and catechol, and 4-fold
glucose, L-tryptophan and L-serine do not
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interfere with the determination (change in signal was less than 5%). 100-fold CaCl2 and K2HPO4 were found to interfere significantly with the determination owing to
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their co-adsorption onto the sensor surface (see Fig. S2). The repeatability of MgO nanobelts/GCE electrode sensing was also evaluated by
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CV (re-modified). The relative standard deviations (RSDs %) in the peak currents of AA, DA, and UA, were 7.3, 4.5 and 6.8%, respectively. The stability of the MgO nanobelts/GCE was also examined by subjecting the electrode to 100 successive measurements. The RSDs of the peak currents were found to be 7.6, 4.4 and 5.5%, respectively. The reproducibility of the MgO nanobelts/GCE was examined by CV using a mixture of 500 μM AA, 25 μM DA and 100 μM UA. Five MgO
nanobelts/GCEs were prepared independently under the same conditions and we found acceptable reproducibility with RSDs of 1.12, 0.34, and 0.62% for AA, DA and UA, respectively (see Fig. S3). 14
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4. Conclusion In this study, we propose the use of a novel biosensor based on MgO nanobelts for
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the simultaneous determination of AA, DA, and UA. The production of the electrode
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and its use in analyte detection was based on the electrocatalytic oxidation of the
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electrode and the MgO nanobelts. The mechanism was mainly surface electron transfer at the MgO nanobelts. Structural defects were found to play a major role in
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the electrocatalysis and in electron transfer. The defects resulted in improved sensitivity and lower detection limits for the MgO nanobelt-modified electrodes
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compared with other electrodes. The surfaces of the MgO nanobelts were covered by O3C2−, O4C2−, O5C2−, F, and F+ defects that increased the specific surface area of the
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nanobelts. Their adsorption capacity toward the analytes was thus enhanced. Additionally, the formation of electron transfer tunnels between these surface defects
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caused the surface to exhibit semiconducting properties. This led to accelerated electron transfer in the sensing system. Acknowledgments
This work was supported by the Hi-tech Research and Development Program of China (863 Program, 2013AA030801), the National Nature Science Foundation of China
(no. 61301045), the Natural Science Foundation of Tianjin
(no.
13JCZDJC36000), and the Excellent Young Teachers Program of Tianjin.
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enhanced porous PTCA–Cys layer for simultaneous detection of ascorbic acid, dopamine and uric acid, Sens. Actuators B-Chem 183(2013) 157-162. [19] J. Yan, S. Liu, Z. Zhang, G. He, P. Zhou, H. Liang, et al., Simultaneous electrochemical detection of ascorbic acid, dopamine and uric acid based on graphene anchored with Pd–Pt nanoparticles, Colloids Surf., B 111(2013) 392-397. [20] L. Zhang, W.J. Yuan, B.Q. Hou, Nano-Cu/PSA III modified glassy carbon electrode for simultaneous determination of ascorbic acid, dopamine and uric acid, J. Electroanal. Chem. 689(2013) 135-141. [21] Y.J. Yang, W. Li, CTAB functionalized graphene oxide/multiwalled carbon 18
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nanotube composite modified electrode for the simultaneous determination of ascorbic acid, dopamine, uric acid and nitrite, Biosens. Bioelectron. 56(2014) 300-306.
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[22] M. Liu, Q. Chen, C. Lai, Y. Zhang, J. Deng, H. Li, et al., A double signal
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amplification platform for ultrasensitive and simultaneous detection of ascorbic acid,
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48(2013) 75-81.
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[24] G. Martra, T. Cacciatori, L. Marchese, J.S.J. Hargreaves, I.M. Mellor, R.W. Joyner, et al., Surface morphology and reactivity of microcrystalline MgO - Single
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[26] L. Lu, L. Zhang, X. Zhang, Z. Wu, S. Huan, G. Shen, et al., A MgO Nanoparticles Composite Matrix-Based Electrochemical Biosensor for Hydrogen Peroxide with High Sensitivity, Electroanalysis 22(2010) 471-477. [27] A. Umar, M.M. Rahman, Y.B. Hahn, MgO polyhedral nanocages and 19
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nanocrystals based glucose biosensor, Electrochem. Commun. 11(2009) 1353-1357. [28] H. Li, M. Li, W. Guo, X. Wang, C. Ge, B. Yang, The effect of microstructure and crystal defect on electrochemical performances of MgO nanobelts, Electrochim. Acta
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tunneling between defects in MgO, Phys. Rev. B 86(2012) 245110.
[30] Y.G. Zhang, H.Y. He, B.C. Pan, Structural Features and Electronic Properties of
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MgO Nanosheets and Nanobelts, J. Phys. Chem. C 116(2012) 23130-23135. [31] M.J. Li, X.F. Wang, H.J. Li, W. Dai, G.J. Qiu, F.D. Liu, et al., Electrochemical
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properties of tadpole-like MgO nanobelts, Mater. Lett. 106(2013) 45-48. [32] N. Sutradhar, A. Sinhamahapatra, B. Roy, H.C. Bajaj, I. Mukhopadhyay, A.B.
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[33] M.L. Bailly, G. Costentin, H. Lauron Pernot, J.M. Krafft, M. Che, Physicochemical and in situ photoluminescence study of the reversible transformation of oxide ions of low coordination into hydroxyl groups upon interaction of water and methanol with MgO, J. Phys. Chem. B 109(2005) 2404-2413. [34] F. Wantz, C.E. Banks, R.G. Compton, Direct oxidation of ascorbic acid at an edge plane pyrolytic graphite electrode: A comparison of the electroanalytical response with other carbon electrodes, Electroanalysis 17(2005) 1529-1533. [35] R.T. Kachoosangi, C.E. Banks, R.G. Compton, Simultaneous determination of uric acid and ascorbic acid using edge plane pyrolytic graphite electrodes, 20
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Electroanalysis 18(2006) 741-747. [36] D.A.C. Brownson, C.W. Foster, C.E. Banks, The electrochemical performance of graphene modified electrodes: An analytical perspective, Analyst 137(2012)
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1815-1823.
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peroxide, ascorbic acid and nitrite, Anal. Sci. 23(2007) 165-170. [39] J. Juan, Antonio Aldaz, Manuel Dominguez, Mechanism of L-ascorbica cid
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[40] J. Beheshtian, A.A. Peyghan, Z. Bagheri, Ab initio study of NH3 and H2O adsorption on pristine and Na-doped MgO nanotubes, Struct. Chem. 24(2013) 165-170.
[41] M.U.A. Prathap, R. Srivastava, Tailoring properties of polyaniline for simultaneous determination of a quaternary mixture of ascorbic acid, dopamine, uric acid, and tryptophan, Sens. Actuators B-Chem 177(2013) 239-250. [42] R. Hacquart, J.M. Krafft, G. Costentin, J. Jupille, Evidence for emission and transfer of energy from excited edge sites of MgO smokes by photoluminescence experiments, Surf. Sci. 595(2005) 172-182. 21
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[43] J. Zhang, L.D. Zhang, Intensive green light emission from MgO nanobelts, Chem. Phys. Lett. 363(2002) 293-297. [44] M. Henyk, K.M. Beck, M.H. Engelhard, A.G. Joly, W.P. Hess, J.T. Dickinson,
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structured nanoporous MgO thin films, Surf. Sci. 593(2005) 242-247.
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Surface electronic properties and site-specific laser desorption processes of highly
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[45] S. Stankic, M. Muller, O. Diwald, M. Sterrer, E. Knozinger, J. Bernardi, Size-dependent optical properties of MgO nanocubes, Angew. Chem.-Int. Ed. 44(2005)
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4917-4920.
[46] H. Huang, C.Y. Ong, J. Guo, T. White, M.S. Tse, O.K. Tan, Pt surface
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modification of SnO2 nanorod arrays for CO and H2 sensors, Nanoscale 2(2010) 1203-1207.
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[47] O.V. Cherstiouk, A.N. Gavrilov, L.M. Plyasova, I.Y. Molina, G.A. Tsirlina, E.R. Savinova, Influence of structural defects on the electrocatalytic activity of platinum, J.
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Solid State Electrochem. 12(2008) 497-509.
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Figure Captions Fig. 1. SEM images of MgO (A) tadpoles, (B) nanobelts, and (C) nanorods. (D) Nyquist plots of EIS and (E) CVs of MgO tadpoles/GCE, MgO nanobelts/GCE, and
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MgO nanorods/GCE in a 0.1 M KCl solution containing 5.0 mM K3Fe(CN)6/K4
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Fe(CN)6 (1:1). Inset in D: Randles equivalent circuit. (F) CVs of 500 μM AA, 25 μM DA, and 100 μM UA when using the MgO tadpoles/GCE, the MgO nanobelts/GCE,
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and the MgO nanorods/GCE in a pH 5.0 PBS solution. CV scan rate: 100 mV s−1.
Fig. 2. HRTEM characterization of the MgO nanobelts: (A, C, and E) HRTEM
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images of a single MgO nanobelt; (B) IFFT and FFT patterns of area “I” in (A); (D)
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higher magnification of area “II” in (C); (F) higher magnification of area “III” in (E).
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Fig. 3. (A) CVs obtained for the MgO nanobelts/GCE in PBS solutions with different pH values (4.5, 5.0, 6.0, and 7.0) containing 500 μM AA, 25 μM DA, and 100 μM UA at scan rate of 100 mV s−1. (B) Effect of scan rate on the redox behavior of 500 μM AA, 25 μM DA, and 100 μM UA in pH 5.0 PBS for the MgO nanobelts/GCE.
Scan rates: 10, 20, 30, 40, 50, 60, 80, 100, 120, 150, 180, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, and 800 mV s−1. Insert: Plots of peak current
versus scan rate.
Fig. 4. (A) CVs of the MgO nanobelts/GCE in 0.1 M PBS (pH 5.0) containing AA, 23
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DA, and UA mixtures with concentrations of 2.5 + 0.125 + 0.5, 5 + 0.25 + 1, 10 + 0.5 + 2, 15 + 0.75 + 3, 25 + 1.25 + 5, 50 + 2.5 + 10, 100 + 5 + 20, and 150 + 7.5 + 30 µM, respectively. Peak currents plotted as functions of (B) AA concentration; (C) DA
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concentration; (D) UA concentration.
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Fig. 5. Schematic representation of the biosensing mechanism of the MgO nanobelts.
24
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Table Captions Table 1. Comparison of the analytical performance of the MgO nanobelt-modified
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M
an
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cr
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GCE with those of other modified electrodes reported in the literature.
25
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te
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M
an
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cr
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Fig. 1.
26
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te
d
M
an
us
cr
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Fig. 2.
27
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te
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M
an
us
cr
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Fig. 3.
28
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M
an
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cr
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Fig. 4.
29
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M
an
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cr
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Fig.5.
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Table 1.
DpAu/PTCA–Cys/GCE
Pd3Pt1/PDDA-RGO/GC E
Nano-Cu/PSA III/GCE
5–1300
2.2
DA
0.5–170
0.25
UA
0.1–20
0.045
AA
2–82
DA
1–59
UA
0.4–24
AA
40–1200
0.61
DA
4–200
0.04
UA
4–400
0.1
AA
0.30–730
0.15
DA
0.02–65
0.01
0.25–107
0.1
AA
5.0–300
1.0
DA
5.0–500
1.5
UA
3.0–60
1.0
AA
6–350
5
DA
0.5–50
0.08
1–90
0.1
200 – 2200
71
UA
d
CTAB-GO/MWCNT/GC
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E Fe3O4@Au-S-Fc/GS-chit
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osan/GCE Edge
-
plane
graphite
UA
References
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AA
d
DPV
[6]
cr
graphene/GCE
(μM)
Metho
DPV
us
doped
range (μM)
n limit
an
Nitrogen
Analyte
Detectio
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Modified electrode
Linear
[18]
DPV
[19]
DPV
[20]
DPV
[21]
DPV
[22]
CV
[34]
LSV
[35]
CV
[36]
pyrolytic
electrode AA
(EPPG)
AA
EPPG
UA
Graphene /EPPG CVD-graphene
0.1 – 1000
0.1 0.05
DA
5 – 50
3.78
UA
20– 160
11.25
UA
10 – 100
8.84
CV
[37]
–
0.4
CV
[38]
2.5–15,
0.2
CV
This work
MnO2 modified carbon
powder
epoxy AA
composite electrode MgO nanobelts/GCE
AA
31
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DA
25–150
0.05
UA
0.125–7.5
0.04
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te
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M
an
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cr
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0.5–3, 5–30
32
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Author Biographies
Mingji Li received his B.Sc. degree in physics from the Department of Physics of Jilin
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University in 2001, and received a Ph.D. degree in the National Laboratory of
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Super-hard Materials from the Jilin University in 2006. Currently, he works as a
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professor at Tianjin University of Technology. His research interests include
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nanomaterials, carbon-based thin films, and electrochemical sensors and biosensors.
Wenlong Guo was born in 1988 in Shijiazhuang (China). He received his B.Sc. degree
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from the School of science at Hebei University of Technology in 2012. He is currently pursuing an MS degree in the School of Electronics Information Engineering at
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Tianjin University of Technology. His research interests include nano-materials and
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their applications for electrochemical sensors.
Hongji Li received her B. Eng. degree in material science from the College of Material Science and Engineering of Jilin University in 2001. She received her doctoral degree from the College of Material Science and Engineering of Jilin University in 2006. Currently, she is a lecturer at Tianjin University of Technology, China. Her research interests include electrochemical sensors and related nanomaterials.
Wei Dai was born in 1982 in Tianjin (China). He received his B.S. degree from the 33
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Bio-medical Engineering School in Tianjin Medical University in 2006. Later, he received his M.S. degree from the School of Electronics Information Engineering in Tianjin University of Technology. He is currently pursuing his Ph.D. in the School of
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Precision Instrument and Opto-electronics Engineering in Tianjin University. His
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research interests include Diamond, Ni nano-materials and their applications for
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electrochemical sensors.
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Baohe Yang received his Ph.D. in the Department of Microelectronic and Solid Electronic from Hebei University of Technology in 2003 and is currently professor in
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the School of Electronics Information Engineering, Tianjin University of Technology. His research interests focus on microwave communication devices, high-frequency
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surface acoustic wave devices, and thin film electronic devices. He is the Director of
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the Tianjin Key Laboratory of Film Electronic and Communicate Devices.
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Graphical Abstract
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S0925-4005(14)00990-3 http://dx.doi.org/doi:10.1016/j.snb.2014.08.022 SNB 17305
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-7-2014 4-8-2014 6-8-2014
Please cite this article as: M. Li, W. Guo, H. Li, W. Dai, B. Yang, Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical biosensor based on one-dimensional MgO nanostructures for the simultaneous determination of ascorbic acid, dopamine, and uric acid
Tianjin Key Laboratory of Film Electronic and Communication Devices, School of
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a
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Mingji Li a, Wenlong Guo a, Hongji Li b,*, Wei Dai c, Baohe Yang a
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Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,
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b
School of Chemistry & Chemical Engineering, Tianjin University of Technology,
School of Precision Instrument and Optoelectronics Engineering, Tianjin University,
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Tianjin 300072, P. R. China
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c
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Tianjin 300384, P. R. China.
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*Corresponding author. Hongji Li; Mingji Li E-mall: [email protected]; [email protected] Tel.: +86 022 60214259.
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Highlights 1. MgO tadpoles, nanobelts, and nanorods have been synthesized by DC arc plasma
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jet CVD.
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2. MgO nanobelts show high electrocatalytic activity toward small biomolecules.
3. The electrocatalytic activity of the MgO nanobelts depends on their surface
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morphology.
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4. A MgO nanobelt-based electrochemical biosensor detects ascorbic acid, uric acid,
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and dopamine with high sensitivity and selectivity.
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ABSTRACT
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One-dimensional (1D) MgO nanostructures of various morphologies including tadpole-like nanobelts (tadpoles), nanobelts, and nanorods were synthesized via direct
current (DC) arc plasma jet chemical vapor deposition (CVD). The effect of morphology on the biosensing properties of the nanostructures was investigated by comparing their electrochemical properties. Compared with tadpoles and nanorods, the MgO nanobelts had excellent electrocatalytic activity toward ascorbic acid (AA), dopamine (DA) and uric acid (DA). The response of the MgO nanobelts to the analytes was twice that of the tadpoles. A MgO nanobelt-modified electrode was thus fabricated for the simultaneous determination of AA, DA, and DA. The peak separations between AA and DA, DA and UA, and AA and UA for this electrode were 2
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111, 161, and 272 mV, respectively. The linear response ranges of the electrodes were 2.5–15 and 25–150 μM for AA, 0.125–7.5 μM for DA, and 0.5–3 and 5–30 μM for UA. The calculated detection limits were 0.2, 0.05, and 0.04 μM (S/N=3), respectively.
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The excellent electrocatalytic activity of the MgO nanobelts can be attributed to
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various surface defects such as low-coordination anions (O5C2− and O4C2− at the
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terrace and edge sites, and O3C2− at the corner and kink sites). Additionally, electron tunneling between these surface defects is possible. These defects have a strong
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adsorption capacity toward AA, DA, and UA. This affinity improves sensitivity and decreases the detection limits of the MgO nanobelt electrodes.
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Keywords: One-dimensional MgO nanostructures; Ascorbic acid; Dopamine; Uric
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acid; Simultaneous detection.
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1. Introduction Electrochemically active compounds such as ascorbic acid (AA), dopamine (DA), and uric acid (UA) usually coexist in physiological fluids (serum and urine). They play an
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important role in human metabolic processes [1-3]. The detection and quantification
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of AA, DA, and UA in biological fluids such as blood and urine is very important.
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Improper levels of these compounds in the body can lead to various fatal diseases. Therefore, the development of a sensitive and selective method for their simultaneous
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determination is highly desirable for analytical applications and in diagnostic research. Because DA, AA, and UA have high electrochemical activity the use of an
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electrochemical method for their simultaneous determination would be convenient, rapid, and highly sensitive [4-7]. However, the oxidation peaks of these three species
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overlap when using traditional electrodes and this makes their simultaneous determination difficult. Various chemically modified electrodes have been developed
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to overcome this problem but most research into AA, DA, and UA detection has only addressed selective detection. Typically, two of the three molecules are interferents [8-17]. For the simultaneous detection of all three analytes, the correct selection and preparation of a sensitive layer is important. Previous work has only focused on complex composites or carbon based nanomaterials as modifiers for the sensitive layer. These materials include DpAu/PTCA-Cys [18], Pd3Pt1/PDDA-RGO [19],
nano-Cu/PSA III [20], CTAB-GO/MWCNT/GCE [21], Fe3O4@Au-S-Fc/GSchitosan/GCE [22], and graphene-based nanomaterials [6]. They have been reported to be effective for the simultaneous detection of AA, DA, and UA. However, the 4
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sensitive membranes that are composed of these materials are too complex and several processing steps are required for production. Magnesium oxide (MgO), an alkaline earth metal oxide, is well known because of
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its industrial applications as adsorbents, catalysts, and catalyst supports [23-25].
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Because of their large electrochemically active surface area, unique biocompatibility,
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chemical stability, and catalytic properties, MgO nano-/microstructures can be used to fabricate highly sensitive amperometric biosensors [26, 27]. In most of these
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biosensors, the MgO nanostructures serve as highly absorbent materials for enzyme immobilization. This allows for highly sensitive MgO-based biosensors. In a previous
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study into MgO nanobelts we obtained excellent electrochemical performance. A modified MgO nanobelt electrode gave excellent electrocatalytic response during the
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selective determination of hydroquinone and catechol [28]. One-dimensional (1D) MgO nanomaterials such as belts and rods have good surface conductivity [29, 30],
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large electrochemically active surface areas [31], good chemical stabilities and good catalytic activities [32].
Herein, we report the production of 1D MgO nanostructures by direct current (DC)
arc plasma jet chemical vapor deposition (CVD). Tadpole, nanobelt, and nanorod morphologies were selectively obtained when using different reaction times under specific reaction conditions. We also investigated the influence of morphology on the electrochemical properties of the MgO 1D nanostructures. A MgO nanobelt-modified electrode was fabricated for the simultaneous determination of AA, DA, and UA.
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2. Experimental 2.1 Synthesis of 1D MgO nanostructures 1D MgO nanostructures were prepared using DC arc plasma jet CVD with
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Mg(NO3)2·6H2O as the precursor and Mo nanoparticles on the Mo substrate surface
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as the catalyst. Tadpoles, nanobelts, and nanorods were prepared after 1, 5, and 12 min, respectively. The main deposition parameters were as follows: Arc power of 18
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kW, argon flow rate of 1.5 L min−1, hydrogen flow rate of 10 L min−1, substrate
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temperature maintained at 950 °C, and pressure in the reaction chamber maintained at
2.2 Material characterization
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4.0 kPa.
Products were characterized by powder X-ray diffraction (XRD; Rigaku
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D/max-2500/PC, CuKα, λ=0.15406 nm, Japan), scanning electron microscopy (SEM; JEOL, JSM-6700F, 10.0 kV, Japan), transmission electron microscopy (TEM; JEOL,
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JEM-2100, 200 kV, Japan), Fourier transform infrared spectrometry (FT-IR; Bruker VERTEX 70, Germany), and fluorescence spectrophotometry (PL; Hitachi F-4500, 150 W xenon lamp, Japan).
2.3 Electrochemical measurements To prepare the 1D MgO nanostructure-modified electrode, a glassy carbon electrode (GCE, 3.0 mm diameter) was polished successively with 0.3 and 0.05 μm alumina
powder, rinsed ultrasonically with dilute nitric acid, ethanol, and deionized water, each for 5 min. It was then dried under N2 flow at room temperature. 8 mg of the 1D MgO nanostructures and 20 mg chitosan were dispersed in 2.5 mL of 1 % acetic acid. 6
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The dispersion was ultrasonicated to form a homogeneous 3.2 mg mL −1 suspension. The obtained 1D MgO nanostructure suspension was then ultrasonicated in a water bath for 15 min before use. In a typical preparation, the GCE was placed in the
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suspension and electrodeposition of the 1D MgO nanostructures was performed at −5
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V for 20 min. The obtained MgO nanostructure/GCE electrode was then allowed to
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dry slowly.
Electrochemical measurements were performed using a CHI 660D series
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electrochemical system. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using a three electrode system comprising the 1D
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MgO nanostructure-modified GCE as the working electrode, a Ag/AgCl reference
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electrode, and a platinum (Pt) wire counter electrode.
3. Results and discussion
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Fig. 1 shows SEM images and electrochemical property curves of the as-prepared MgO tadpoles, the MgO nanobelts, and the MgO nanorods. The tadpoles consist of many MgO nanobelts with each containing a spherical Mo nanoparticle at their tip. The nanobelts are 10–30 nm wide and ~2 μm in length, and the diameters of the
spherical tips are about 40–100 nm. The MgO nanobelts formed after the disappearance of the spherical tips when the reaction time was extended. The width
and length of the nanobelts increased to 30–100 nm and ∼3 μm, respectively. The MgO nanorods are 0.5–1 μm in length and 100–200 nm in width. 7
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Fig. 1 X The interfacial characteristics of the MgO tadpole-, the MgO nanobelt-, and the
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MgO nanorod-modified electrodes were studied by EIS in a 0.1 M KCl solution. This solution contained a redox couple of 5 mM K4Fe(CN)6 and 5 mM K3Fe(CN)6. Fig. 1D
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shows the EIS that was recorded for the different electrodes. The equivalent
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impedance analysis circuit is shown in the insert of Fig. 1D and lists the parameters used. These parameters include the resistance of the electrolyte solution (Rs), the
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charge transfer resistance at the MgO nanostructure layer/electrolyte interface (Rct), the charge–transfer resistance in the MgO nanostructure layer interior (R1), the value
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of the constant phase element (CPE), the charge–transfer resistance at the MgO
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nanostructure layer/GCE interface (R2), the bulk Faradic pseudo-capacitance (C1 and
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C2), the Warburg impedance (Zw), the double-layer capacitance of the counter
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electrode/electrolyte solution interface (C3), and the charge transfer resistance at the counter electrode/electrolyte interface (Rint). The Rct values were found to be 145.4 Ω cm2 for the GCE, 28.2 Ω cm2 for the MgO tadpoles/GCE, 24.2 Ω cm2 for the MgO
nanobelts/GCE, and 32.3 Ω cm2 for the MgO nanorods/GCE. The Rct of the MgO
nanobelts/GCE was smaller than that of the bare GCE, the MgO tadpoles/GCE, and the MgO nanorods/GCE. The Rct represents the electrode resistance and is closely related to the surface area and conductivity of the working electrode. The MgO nanobelts contribute to a decrease in the resistance at the interface. CV curves between −600 and 1000 mV for the four different electrodes are shown in Fig. 1E. For the bare GCE, the MgO tadpoles/GCE, the MgO belts/GCE and the MgO rods/GCE 8
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the [Fe(CN)6]3-/4- redox couple was reversible with peak to peak separations (△Ep) of
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500, 471, 260, and 444 mV, respectively. After modification with MgO nanobelts, the
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electrode’s △Ep decreased significantly and the redox current surpassed that of the other electrodes. This suggests that the nanobelts provide the necessary conduction
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pathways to accelerate electron transfer between the redox couples and the electrode surface. These results are consistent with the EIS analysis.
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Fig. 1F shows the CVs obtained for a mixture of AA, DA, and UA when using the bare GCE, the MgO tadpoles/GCE, the MgO nanobelts/GCE, and the MgO
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nanorods/GCE. For the bare GCE, the AA and DA oxidation peaks completely overlap and the peak potentials of DA and UA are indistinguishable. This makes the
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simultaneous determination of these compounds on bare GCE impossible. In contrast, the MgO 1D nanostructure-modified GCEs oxidized AA, DA, and UA at 176–246, 315–357, and 486–518 mV, respectively. Additionally, the current response was also enhanced. For the MgO nanobelts/GCE, the peak separations between AA and DA, DA and UA, and AA and UA were 111, 161, and 272 mV, respectively. The onset potential of the mixture for the MgO tadpole-modified electrode was 100 mV while that of the MgO nanobelt electrode shifted to around −100 mV. This was accompanied
by a concomitant increase in the peak current density. The response of the MgO nanobelts to AA, DA, and UA was twice that of the tadpoles. The above results 9
Page 9 of 35
indicate that the MgO nanobelts had significantly higher electrocatalytic activity than the nanorods and the tadpoles. Fig. 2A, C, and E shows typical TEM images of the as-prepared MgO nanobelts.
ip t
The spacing between the two neighboring parallel fringes was 0.24 nm, which is in
cr
good agreement with the interplanar spacing of the {111} lattice plane of cubic MgO
us
(Fig. 2 B). From regions II and III, the interplanar spacing was calculated to be 0.21 nm, which corresponds to the {100} planes of cubic MgO (JCPDS 45-0946). The
an
fringe pattern in the high-resolution TEM images (Fig. 2D and F) clearly indicates that the MgO nanobelts were monocrystalline. Their high crystal quality is evident.
M
The surface details were observed under high magnification and growth steps are present (Fig. 2D and F). This nanobelt is also composed of terraces and corners and
te
Ac ce p
O3C2− and O4C2− [33].
d
these sites contribute to the formation of low-coordinated surface ions, including
Fig. 2
X
The electrochemical behavior of the MgO nanobelts/GCE was studied at different
pH values using CV (Fig. 3A). The oxidation peak potentials and the magnitude of the current responses were found to be pH dependent. In the pH range from 4.5 to 7.0, the oxidation peak potentials shift negatively with an increase in pH. This indicates that protons participate in the electrode reaction process. When AA, DA, and UA coexist in solution, a stable and well-defined peak is observed at pH 5.0. This pH may, therefore, be optimal for the adsorption and electrochemical oxidation of AA, DA, and UA on the MgO nanobelts/GCE. Additionally, a maximum separation of the peak 10
Page 10 of 35
potentials for AA–DA and DA–UA is also observed at pH 5.0. We thus selected 0.1 M PBS at pH 5.0 for subsequent experiments. To investigate the reaction kinetics, the influence of scan rate on the peak currents and the peak potentials of AA, DA, and UA
ip t
in the mixture was also investigated. As shown in Fig. 3B, the oxidation peak currents
cr
(Ipa) for AA, DA, and UA increase consistently with scan rate from 10 to 800 mV s −1.
us
The oxidation peak currents of all three compounds increased in a linear manner with scan rate over this range (insert in Fig. 3B). This suggests that electron transfer at the
Fig. 3
an
MgO nanobelts/GCE is surface adsorption controlled for all the analytes. X
M
The CV results show that a simultaneous determination of AA, DA, and UA is
d
possible when using the MgO nanobelts/GCE (Fig. 4A). Three well-defined oxidation
te
peaks are present at potentials of approximately 200, 357, and 518 mV. Figure 4 B–D
Ac ce p
shows the relationship between the peak current and the concentration of AA, DA, and UA, respectively. The oxidation peak currents increase with analyte concentration. The peak potentials are steady and the calibration curves are linear in the concentration ranges of 2.5–15 and 25–150 μM for AA, 0.125–7.5 μM for DA, and
0.5–3 and 5–30 μM for UA. The linear equations for AA, DA, and UA are Ipa (μA) =
2.391+0.014 [AA] (μM) (R=0.991), Ipa (μA) = 2.603+0.002 [AA] (μM) (R=0.993); Ipa (μA) = 0.793+0.559 [DA] (μM) (R=0.995); Ipa (μA) = 0.07+0.2 [UA] (μM) (R=0.993),
and Ipa (μA) = 0.361+0.068 [UA] (μM) (R=0.995), respectively. The simultaneous detection limits calculated for AA, DA, and UA are 0.2, 0.05 and 0.04 μM (S/N=3), respectively. These values are comparable to those obtained for other materials. As 11
Page 11 of 35
shown in Table 1 [6, 18-22, 34-38], the MgO nanobelt-modified electrode has a satisfactory detection limit and linear range for the simultaneous detection of AA, DA,
Fig. 4
ip t
and UA. X
cr
Table 1 X
us
The sensing mechanism of the MgO nanobelts is shown in Fig. 5. The anodic peak response (Fig. 1F) corresponds to the oxidation of the furan ring hydroxyl groups
an
(−OH) to carbonyl groups (C=O) in AA [39], the oxidation of catechol to o-quinone in DA, and the oxidation of the bridging double bond to −OH followed by
M
dehydration in UA. According to the electrocatalytic oxidation mechanism, the electrode was clearly electrocatalytic toward AA, DA, and UA as the analytes lost
te
d
two electrons. The AA and DA oxidation processes are related to the interactions between the MgO nanobelts and the analytes. This comes from intermolecular effects
Ac ce p
(hydrogen bonds) between the oxo-surface groups or the O atoms in the lattice of the MgO nanobelt surface and the −OH groups of the analytes. A close interaction between the low-coordination defect sites and the −OH groups at the surface of the
MgO nanobelts is evident in the FTIR spectrum (see Fig. S1). The UA oxidation process is related to the interactions between the Mg atoms at the MgO nanobelt surface, the adsorbed H2O molecules, and the bridging double bond in UA. The O
atom in H2O is close to the Mg site [40], and the H atom in H2O is close to the C atom in the bridging double bond of UA [41]. This may be the mechanism for the catalytic oxidation of the three analytes by the MgO nanobelts. 12
Page 12 of 35
Fig. 5
X
We also monitored the PL emission of the MgO nanobelts at an excitation wavelength of 256 nm (Fig. 5). The broad emission spectrum was centered at 355 nm
ip t
and was de-convoluted to eight distinct peaks at 320 (3.9), 355 (3.5), 378 (3.3), 398
cr
(3.1), 432 (2.9), 466 (2.7), 505 (2.5), and 543 (2.3 eV) nm. These are attributed to
low-coordination oxide ions such as O3C2−, O4C2−, and O5C2−, and other defects (F and
us
F+ centers) in the MgO nanobelts [33, 42, 43]. These surface defects increase the
an
specific surface area of the nanobelts thus enhancing their adsorption capacity toward the three analytes. The low-coordination oxide ions and the other defects have band
M
gaps of 2.3–3.9 eV in the energy band structure of the nanobelts. The bandgap of bulk MgO is 7.8 eV resulting in MgO being a prototypical ionic insulator. However, the
te
d
surface of MgO (including the nanobelts) exhibits semiconducting properties [44, 45]. The conductivity of 1D nanostructures can vary from a fully non-conductive state
Ac ce p
to a conductive state and this is determined by their surface chemistry [46]. MgO nanobelt surfaces have many defects and the surface electronic properties depend strongly on low-coordination sites such as kinks, corners, and steps [44]. Additionally, the (111) lattice plane of MgO nanobelt surfaces has a quasi-1D graphene ribbon electronic structure [30]. Electron transfer is a
fundamental process
in
electrochemistry, and structural defects play a major role in electrocatalytic activity [47]. The decrease in MgO nanobelt resistance is caused by electron tunneling between surface defects [29]. Electron transfer between MgO nanobelts and analytes depend on these surface defects. Therefore, the electron transfer rate accelerates 13
Page 13 of 35
quickly as electrons are captured and transported to nanobelt surfaces. This may be the main reason for the improved sensitivity and the lower detection limit of MgO nanobelt-modified electrodes compared with those of other electrodes. Therefore, the
ip t
sensitive membranes listed in Table 1 cannot be compared with MgO nanobelts.
cr
The influence of various substances on the determination of 500 μM AA, 25 μM
us
DA, and 100 μM UA was studied by CV. We found that 100-fold (vs. DA) CH3COONa, CaCl2, KCl, K2HPO4, FeCl3, Mg(NO3)2, and Ni(NO3)2, 10-fold (vs. DA)
L-cysteine, L-glycine, L-lysine, L-tyrosine,
L-alanine,
an
aniline, catechol, hydroquinone, resorcinol, and catechol, and 4-fold
glucose, L-tryptophan and L-serine do not
M
interfere with the determination (change in signal was less than 5%). 100-fold CaCl2 and K2HPO4 were found to interfere significantly with the determination owing to
te
d
their co-adsorption onto the sensor surface (see Fig. S2). The repeatability of MgO nanobelts/GCE electrode sensing was also evaluated by
Ac ce p
CV (re-modified). The relative standard deviations (RSDs %) in the peak currents of AA, DA, and UA, were 7.3, 4.5 and 6.8%, respectively. The stability of the MgO nanobelts/GCE was also examined by subjecting the electrode to 100 successive measurements. The RSDs of the peak currents were found to be 7.6, 4.4 and 5.5%, respectively. The reproducibility of the MgO nanobelts/GCE was examined by CV using a mixture of 500 μM AA, 25 μM DA and 100 μM UA. Five MgO
nanobelts/GCEs were prepared independently under the same conditions and we found acceptable reproducibility with RSDs of 1.12, 0.34, and 0.62% for AA, DA and UA, respectively (see Fig. S3). 14
Page 14 of 35
4. Conclusion In this study, we propose the use of a novel biosensor based on MgO nanobelts for
ip t
the simultaneous determination of AA, DA, and UA. The production of the electrode
cr
and its use in analyte detection was based on the electrocatalytic oxidation of the
us
electrode and the MgO nanobelts. The mechanism was mainly surface electron transfer at the MgO nanobelts. Structural defects were found to play a major role in
an
the electrocatalysis and in electron transfer. The defects resulted in improved sensitivity and lower detection limits for the MgO nanobelt-modified electrodes
M
compared with other electrodes. The surfaces of the MgO nanobelts were covered by O3C2−, O4C2−, O5C2−, F, and F+ defects that increased the specific surface area of the
te
d
nanobelts. Their adsorption capacity toward the analytes was thus enhanced. Additionally, the formation of electron transfer tunnels between these surface defects
Ac ce p
caused the surface to exhibit semiconducting properties. This led to accelerated electron transfer in the sensing system. Acknowledgments
This work was supported by the Hi-tech Research and Development Program of China (863 Program, 2013AA030801), the National Nature Science Foundation of China
(no. 61301045), the Natural Science Foundation of Tianjin
(no.
13JCZDJC36000), and the Excellent Young Teachers Program of Tianjin.
15
Page 15 of 35
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ip t
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Ac ce p
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22
Page 22 of 35
Figure Captions Fig. 1. SEM images of MgO (A) tadpoles, (B) nanobelts, and (C) nanorods. (D) Nyquist plots of EIS and (E) CVs of MgO tadpoles/GCE, MgO nanobelts/GCE, and
ip t
MgO nanorods/GCE in a 0.1 M KCl solution containing 5.0 mM K3Fe(CN)6/K4
cr
Fe(CN)6 (1:1). Inset in D: Randles equivalent circuit. (F) CVs of 500 μM AA, 25 μM DA, and 100 μM UA when using the MgO tadpoles/GCE, the MgO nanobelts/GCE,
an
us
and the MgO nanorods/GCE in a pH 5.0 PBS solution. CV scan rate: 100 mV s−1.
Fig. 2. HRTEM characterization of the MgO nanobelts: (A, C, and E) HRTEM
M
images of a single MgO nanobelt; (B) IFFT and FFT patterns of area “I” in (A); (D)
te
d
higher magnification of area “II” in (C); (F) higher magnification of area “III” in (E).
Ac ce p
Fig. 3. (A) CVs obtained for the MgO nanobelts/GCE in PBS solutions with different pH values (4.5, 5.0, 6.0, and 7.0) containing 500 μM AA, 25 μM DA, and 100 μM UA at scan rate of 100 mV s−1. (B) Effect of scan rate on the redox behavior of 500 μM AA, 25 μM DA, and 100 μM UA in pH 5.0 PBS for the MgO nanobelts/GCE.
Scan rates: 10, 20, 30, 40, 50, 60, 80, 100, 120, 150, 180, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, and 800 mV s−1. Insert: Plots of peak current
versus scan rate.
Fig. 4. (A) CVs of the MgO nanobelts/GCE in 0.1 M PBS (pH 5.0) containing AA, 23
Page 23 of 35
DA, and UA mixtures with concentrations of 2.5 + 0.125 + 0.5, 5 + 0.25 + 1, 10 + 0.5 + 2, 15 + 0.75 + 3, 25 + 1.25 + 5, 50 + 2.5 + 10, 100 + 5 + 20, and 150 + 7.5 + 30 µM, respectively. Peak currents plotted as functions of (B) AA concentration; (C) DA
cr
ip t
concentration; (D) UA concentration.
Ac ce p
te
d
M
an
us
Fig. 5. Schematic representation of the biosensing mechanism of the MgO nanobelts.
24
Page 24 of 35
Table Captions Table 1. Comparison of the analytical performance of the MgO nanobelt-modified
Ac ce p
te
d
M
an
us
cr
ip t
GCE with those of other modified electrodes reported in the literature.
25
Page 25 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 1.
26
Page 26 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 2.
27
Page 27 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 3.
28
Page 28 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 4.
29
Page 29 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig.5.
30
Page 30 of 35
Table 1.
DpAu/PTCA–Cys/GCE
Pd3Pt1/PDDA-RGO/GC E
Nano-Cu/PSA III/GCE
5–1300
2.2
DA
0.5–170
0.25
UA
0.1–20
0.045
AA
2–82
DA
1–59
UA
0.4–24
AA
40–1200
0.61
DA
4–200
0.04
UA
4–400
0.1
AA
0.30–730
0.15
DA
0.02–65
0.01
0.25–107
0.1
AA
5.0–300
1.0
DA
5.0–500
1.5
UA
3.0–60
1.0
AA
6–350
5
DA
0.5–50
0.08
1–90
0.1
200 – 2200
71
UA
d
CTAB-GO/MWCNT/GC
te
E Fe3O4@Au-S-Fc/GS-chit
Ac ce p
osan/GCE Edge
-
plane
graphite
UA
References
ip t
AA
d
DPV
[6]
cr
graphene/GCE
(μM)
Metho
DPV
us
doped
range (μM)
n limit
an
Nitrogen
Analyte
Detectio
M
Modified electrode
Linear
[18]
DPV
[19]
DPV
[20]
DPV
[21]
DPV
[22]
CV
[34]
LSV
[35]
CV
[36]
pyrolytic
electrode AA
(EPPG)
AA
EPPG
UA
Graphene /EPPG CVD-graphene
0.1 – 1000
0.1 0.05
DA
5 – 50
3.78
UA
20– 160
11.25
UA
10 – 100
8.84
CV
[37]
–
0.4
CV
[38]
2.5–15,
0.2
CV
This work
MnO2 modified carbon
powder
epoxy AA
composite electrode MgO nanobelts/GCE
AA
31
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DA
25–150
0.05
UA
0.125–7.5
0.04
Ac ce p
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d
M
an
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cr
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0.5–3, 5–30
32
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Author Biographies
Mingji Li received his B.Sc. degree in physics from the Department of Physics of Jilin
ip t
University in 2001, and received a Ph.D. degree in the National Laboratory of
cr
Super-hard Materials from the Jilin University in 2006. Currently, he works as a
us
professor at Tianjin University of Technology. His research interests include
an
nanomaterials, carbon-based thin films, and electrochemical sensors and biosensors.
Wenlong Guo was born in 1988 in Shijiazhuang (China). He received his B.Sc. degree
M
from the School of science at Hebei University of Technology in 2012. He is currently pursuing an MS degree in the School of Electronics Information Engineering at
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Tianjin University of Technology. His research interests include nano-materials and
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their applications for electrochemical sensors.
Hongji Li received her B. Eng. degree in material science from the College of Material Science and Engineering of Jilin University in 2001. She received her doctoral degree from the College of Material Science and Engineering of Jilin University in 2006. Currently, she is a lecturer at Tianjin University of Technology, China. Her research interests include electrochemical sensors and related nanomaterials.
Wei Dai was born in 1982 in Tianjin (China). He received his B.S. degree from the 33
Page 33 of 35
Bio-medical Engineering School in Tianjin Medical University in 2006. Later, he received his M.S. degree from the School of Electronics Information Engineering in Tianjin University of Technology. He is currently pursuing his Ph.D. in the School of
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Precision Instrument and Opto-electronics Engineering in Tianjin University. His
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research interests include Diamond, Ni nano-materials and their applications for
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electrochemical sensors.
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Baohe Yang received his Ph.D. in the Department of Microelectronic and Solid Electronic from Hebei University of Technology in 2003 and is currently professor in
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the School of Electronics Information Engineering, Tianjin University of Technology. His research interests focus on microwave communication devices, high-frequency
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d
surface acoustic wave devices, and thin film electronic devices. He is the Director of
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the Tianjin Key Laboratory of Film Electronic and Communicate Devices.
34
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Ac ce p
te
d
M
an
us
cr
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Graphical Abstract
35
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