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multiwalled carbon nanotube nanocomposite modified glassy carbon electrode
Journal of Alloys and Compounds 802 (2019) 326e334
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A ratiometric electrochemical sensor for dopamine detection based on hierarchical manganese dioxide nanoflower/multiwalled carbon nanotube nanocomposite modified glassy carbon electrode Yong Wang*, Luyao Wang, Qianfen Zhuang College of Chemistry, Nanchang University, Nanchang, 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2019 Received in revised form 6 June 2019 Accepted 10 June 2019 Available online 13 June 2019
A ratiometric electrochemical sensor is developed for dopamine detection based on hierarchical manganese dioxide (MnO2) nanoflower/multiwalled carbon nanotube (MWCNT) nanocomposite modified glassy carbon electrode (GCE). The hierarchical MnO2 nanoflower was electrodeposited onto the electrochemically pretreated GCE modified with MWCNT (MWCNT/EPGCE) to prepare an inner reference electrochemical probe. The fabrication process of the MnO2/MWCNT/EPGCE electrode was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The introduction of MWCNT can not only improve the electrochemical signals of dopamine, but also increase MnO2 signals. The origin of the dual signal enhancement for both dopamine and MnO2 is reasonably explained by using electrochemical techniques. The results show that the high sensitivity for dopamine detection at MnO2/ MWCNT/EPGCE predominately originates from the high surface coverage of dopamine, and the signal enhancement of MnO2 on MWCNT/EPGCE is ascribed to the electrocatalytic activity of MWCNT towards the electrodeposited MnO2 and the increase of MnO2 surface coverage. This sensor based on MnO2/ MWCNT/EPGCE has a low detection limit of 0.17 mM and a wide linear range from 0.5 to 30.0 mM. In addition, the sensor displays high selectivity, good reproducibility, and good stability, and can be applied for dopamine detection in human serum samples with satisfactory results. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ratiometric electrochemical sensor Composite materials Manganese dioxide nanoflower Multiwalled carbon nanotube Electrodeposition Dopamine
1. Introduction Dopamine (DA) is a kind of important catecholamine neurotransmitter molecules [1e3]. It is widely distributed in mammalian brain tissue and body fluids, and plays a vital role in the central nervous system [1e3]. Changes of DA's concentration usually lead to HIV infection and various neurological diseases such as Alzheimer's and Parkinson's [1e3]. Therefore, the development of a simple, fast and effective assay method for DA determination is of great significance for clinical diagnosis. Up to date, many methods have been proposed for DA detection, including high-performance liquid chromatography [4], capillary electrophoresis [5], and spectroscopic methods [6e8]. However, most of these methods require expensive equipment, and involve time-consuming pretreatment steps. Compared with these
* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jallcom.2019.06.124 0925-8388/© 2019 Elsevier B.V. All rights reserved.
methods, electrochemical sensor becomes an attractive alternative due to their unique advantages like simplicity, fast response, miniaturization, low cost and high sensitivity [9e11]. Currently, most of the electrochemical sensors focus on the development of single-signal sensing strategy which only uses a single electrochemical signal to realize analyte's detection [9e11]. Generally speaking, the kind of sensing strategy is easily influenced by many factors such as environmental changes, the sensor or reagent concentrations, and instrumental efficiency, which give false positive or negative errors for analyte's detection [12,13]. To overcome the drawbacks, ratiometric electrochemical sensor represents an acceptable alternative owning to its superior built-in self-calibration ability [12,13]. The kind of sensing strategy usually determines the ratios of the peak current at two different redox potentials for analyte detection. Both of the peak current signals usually originate from the analyte itself and the inner reference probe, respectively. Importantly, the strong electrochemical signal of the inner reference probe can increase the accuracy of signal ratio value, which causes the improvement of the accuracy of
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analyte detection. Therefore, the development of an electrochemical probe with good inner reference ability is always attractive. Manganese dioxide (MnO2) is a kind of transition metal oxide, and has been recently used for the construction of electrochemical sensors due to its high specific surface area and good electrocatalytic activity [14e17]. However, the use of MnO2 as the inner reference probe in the construction of ratiometric electrochemical sensors is still limited due to its poor electrical conductivity. Therefore, rational choice of support materials with excellent electrical conductivity is vitally important for the construction of MnO2-modified electrochemical sensors. Multiwalled carbon nanotube (MWCNT) has many specific properties such as high electrical conductivity, large surface area, good electrocatalytic activity and great chemical stability, and thus it is an ideal support material in electrochemical modified sensors [18e21]. Recently, many researchers have reported that the MnO2/MWCNT nanocomposite could remarkably improve the performance of electrochemical modified sensors [22e25]. Xu et al. reported that the MnO2/MWCNT nanocomposite could be used for sensitive electrochemical detection of hydrogen peroxide because the combination of MnO2 and MWCNT increased the electrocatalytic active area and promoted electron transfer [22]. Tang et al. found that the MnO2/MWCNT nanocomposite could enhance the sensitivity of the detection of hydroquinone and catechol due to its excellent catalytic activity, high surface area and nanosized structure [23]. VesaliNaseh et al. found that the MnO2/MWCNT nanocomposite enhanced charge transfer efficiency and reversibility of the glucose oxidase mediated redox processes, resulting in the increase of sensitivity for glucose detection [24]. Jung et al. found that the MnO2/MWCNT nanocomposite could be used to construct a sensitive sensor for the detection of hydrogen gas due to their large surface area and fast electron-transport capability [25]. In this study, we prepared the hierarchical manganese dioxide (MnO2) nanoflower/multiwalled carbon nanotube (MWCNT) nanocomposite modified electrode for the construction of ratiometric electrochemical dopamine sensor. The hierarchical MnO2 nanoflower was electrodeposited onto the electrochemically pretreated glassy carbon electrode modified with multiwalled carbon nanotube (MWCNT/EPGCE) to prepare an inner reference electrochemical probe. The presence of multiwalled carbon nanotube enhanced the electrochemical signals of the analyte, DA, and the inner reference, the inorganic MnO2 nanoflower, and thus facilitated the fabrication of sensitive ratiometric electrochemical sensor for DA detection. In addition, the possible origin of the signal enhancement of MnO2 on MWCNT/EPGCE and the high sensitivity for DA detection was reasonably explained. The brief protocol for the fabrication of the ratiometric electrochemical sensor for DA detection based on MnO2/MWCNT nanocomposite is displayed in Scheme 1. 2. Experimental 2.1. Reagents and chemicals Multiwalled carbon nanotube (MWCNT) was purchased from Shenzhen Nanotech. Port Co., Ltd. Dopamine (DA), sulfuric acid (H2SO4), nitric acid (HNO3), manganese (II) acetate tetrahydrate (MnAc2$4H2O), sodium sulfate (Na2SO4), disodium hydrogen phosphate dodecahydrate (Na2HPO4$12H2O), sodium dihydrogen phosphate dehydrate (NaH2PO4$2H2O), and other inorganic salts were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Phosphate buffer solutions (0.1 M PBS, pH 7.0) were employed as the supporting electrolyte. All chemicals were used as received without any further refinement. Ultrapure water
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Scheme 1. Schematic illustration of the construction processes of the developed ratiometric electrochemical sensor and its application for DA detection.
(18.25 MU cm) was used throughout the experiments. 2.2. Apparatus Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) were conducted using a JSM-6701F microscope system (JEOL Co., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were done on an ESCALAB 250Xi Microprobe (Thermo Scientific, USA) with a monoenergetic Al Ka radiation (200 W, hn ¼ 1486.6 eV). Electrochemical experiments were operated on a CHI-660A electrochemical workstation (Chenhua Apparatus Co., Shanghai, China) in conjunction with a typical three-electrode system. The Ag/AgCl (3 M NaCl) electrode and platinum wire were respectively used as the reference electrode and the counter electrode. The unmodified glassy carbon electrode (GCE, 3 mm in diameter) or modified GCE served as working electrodes. 2.3. Preparation of MnO2/MWCNT/EPGCE 100 mg of MWCNT was suspended into 100 mL of concentrated HNO3 and H2SO4 (1:3 vol ratio) mixture, followed by ultrasonication in a water bath for 3 h at 40 C. After that, the asprepared MWCNT was washed with water several times [26,27], and dried in a vacuum-drying oven overnight at 80 C. Finally, the resultant MWCNT was dissolved in water, and ultrasonicated at room temperature for 1 h to produce a homogeneous dispersion. The aforementioned pretreatment procedure can introduce oxygen-containing moieties mainly on the open ends and defects on the sidewall of the nanotubes to yield MWCNT functionalized with carboxylic groups. Before modification, GCE was successively polished with 1.0, 0.3 and 0.05 mm alumina slurries. Then, the GCE was ultrasonicated for 5 min in dilute nitric acid, ethanol, and water, respectively. After that, the polished GCE was subjected to electrochemical pretreatment in 0.5 M H2SO4, at þ2.0 V for 30 s, 1.0 V for 10 s, and then from 0 to 1.0 V at a scanning rate of 0.1 V/s until a stable voltammogram was achieved [28]. The electrochemically pretreated glassy carbon electrode (EPGCE) was firstly modified by dropping 10 mL of 0.2 mg/mL MWCNT functionalized with carboxylic groups, and dried in air. Next, the electrodeposition of MnO2 nanoflower on the surface of the MWCNT/EPGCE was performed by cyclic potential scanning from 0.0 to 0.9 V at 200 mV/s in the 10 mM of manganese (II)
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acetate solution containing 10 mM Na2SO4 for 10 cycles [29]. Finally, the as-prepared MnO2/MWCNT/EPGCE was washed carefully with water. 2.4. Electrochemical measurements Various concentrations (0e80.0 mM; total 15 samples) of DA were added to 10 mL of PBS (0.1 M, pH 7.0), and then mixed well. After that, the differential pulse voltammetric (DPV) measurements for DA detection were performed. Parameters in the DPV measurements were as follows: the increment potential was 4 mV, the pulse amplitude was 50 mV, the pulse width was 0.06 s, the sample width was 0.0167 s, the pulse period was 0.2 s, the quiet time was 2 s, and the potential window was form 0.1e0.8 V. Electrochemical impedance spectroscopy (EIS) measurements were conducted in 5 mM [Fe(CN)6]3/4- (1:1) solution containing 0.1 M KCl with a voltage frequency range of 0.1 Hze100 kHz at amplitude of 5 mV. 3. Results and discussion 3.1. Preparation and characterization of MnO2/MWCNT nanocomposite Fig. S1 displays cyclic voltammograms of the electrodeposition of MnO2 on the surface of MWCNT/GCE. An oxidation peak over the potential ranges from 0.5 to 0.8 V and a reduction peak over the potential ranges from 0.2 to 0.6 V can be clearly observed, which can be ascribed, as reported in literature, to the redox reaction between Mn2þ and MnO2 [30]. SEM is used to characterize the surface morphologies of the MWCNT and MnO2/MWCNT nanocomposite, which are distributed on the surface of glassy carbon electrode. As shown in Fig. 1A, a typical crossing tubular structure can be seen on the surface of GCE, suggesting the successful immobilization of MWCNT onto GCE. Fig. 1B shows the surface morphology of MnO2 electrodeposited onto the surface of MWCNT/ GCE. It can be clearly that the hierarchical MnO2 nanoflowers are uniformly distributed on the surface of MWCNT/GCE with size distribution ranging from 130 nm to 200 nm, and the surface of the MnO2 nanoflowers is composed of many randomly oriented
nanorod-like structures. The results from both the EDX spectrum and the XPS survey spectrum indicate that the elements C, O, and Mn are present at the MWCNT/MnO2 nanocomposite (Fig. 1C and D). In addition, as shown in the Mn 2p XPS spectrum (Fig. 1E), two main peaks appear at 653.6 and 641.9 eV, which are respectively assigned to Mn 2p1/2 and Mn 2p3/2 due to the presence of MnO2 in the nanocomposite [31,32]. Simultaneously, the spin-energy separation is ca. 11.7 eV, further confirming the presence of MnO2 in the nanocomposite [31,32]. All the results confirm the successful preparation of MWCNT/MnO2 nanocomposite onto GCE. The possible growth process of MnO2 can be explained as following [29,30,33]: Mn2þ / Mn3þ þ e
(1)
Mn3þ þ 2H2O / MnOOH þ 3Hþ
(2)
MnOOH / MnO2 þ Hþ þ e
(3)
Overall: Mn2þ 2H2O / MnO2 þ 4Hþ þ 2e
(4)
The growth of MnO2 depends on the active support material [29,30,33,34]. In this work, MWCNT was employed. Growth began as the thin film on the surface of MWCNT, and soluble Mn3þ was produced initially. After that, Mn3þ underwent hydrolysis and precipitated as MnOOH on the MWCNT surface, followed by the formation of MnO2 nanoparticles via a further solid state oxidation process. The MnO2 nanoparticles were nucleated to act as the reactive sites for the subsequent growth of complex MnO2 nanorods. The formed MnO2 nanorods rapidly accumulated on the MWCNT surface to form hierarchical MnO2 nanoflowers. 3.2. Electrochemical behaviors of different electrodes CV is a simple, rapid and powerful technique for studying redox reactions at the electrode solution interfaces. The CV behaviors of 5 mM [Fe(CN)6]3/4- (1:1) solution containing 0.1 M KCl on the bare and modified GCE were studied. As shown in Fig. 2A, the CV of the
Fig. 1. (A) SEM images of MWCNT on the surface of GCE. (B) SEM images of MnO2/MWCNT on the surface of GCE. (C) EDX spectrum of MnO2/MWCNT on the surface of GCE. XPS survey spectrum (D) and Mn 2p XPS spectrum (E) of MnO2/MWCNT on the surface of GCE.
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Fig. 2. CV (A) and EIS (B) curves of (a) GCE, (b) EPGCE, (c) MnO2/EPGCE, (d) MWCNT/EPGCE, (e) MnO2/MWCNT/EPGCE in 5 mM [Fe(CN)6]3-/4- (1:1) containing 0.1 M KCl.
GCE shows a pair of weak redox peaks with the peak-to-peak potential separation (DEp) of 0.268 V. After electrochemical pretreatment, the GCE displays a pair of well-defined redox peak with DEp of 0.124 V, and the corresponding redox peak currents raise due to the fast electron transfer rate of the electrode. When modified with MnO2, the MnO2/EPGCE exhibits the DEp of 0.142 V, which is a little larger than that of the EPGCE. Simultaneously, the redox peak currents of the MnO2/EPGCE are less than those of the EPGCE. All these suggest the poor electrical conductivity of the electrodeposited MnO2. When modified with MWCNT, the MWCNT/ EPGCE exhibits the DEp of 0.124 V, and has the redox peak currents greater than the GCE, which indicate that MWCNT can accelerate the electron transfer rate of the electrode. Moreover, the EPGCE modified with MWCNT and MnO2 gives a DEp value similar to MWCNT/EPGCE, but its peak current slightly increases, suggesting that the MnO2/MWCNT nanocomposite can further improve the electrical conductivity of the electrode due to the synergetic effect between MWCNT and MnO2. It is worth noting that the all the electrodes possess a pair of redox peaks, implying that they undergo quasi-reversible processes. The electrochemical active areas of different electrodes can be estimated and compared according to the Randles-Sevcik equation [35]: ipc ¼ (2.69 105) n3/2 D1/2 v1/2 A c
MWCNT/EPGCE, and MnO2/MWCNT/EPGCE can be estimated to be 444.9 U, 0.0033U, 25.08 U, 0.0069 U, and 0.0147 U, respectively, denoting that the electrical conductivity of the MnO2/MWCNT/ EPGCE is far higher than those of the GCE and MnO2/EPGCE, but somewhat lower than those of the MWCNT/EPGCE and EPGCE. On the basis of the Rct values from EIS plots, we can estimate the standard electron transfer rate constants (k0) for the [Fe(CN)6]3/4system at different electrodes according to the following equation [35]:
k0 ¼
RT ðnFÞ2 AcRct
(6)
where n the electron transfer number, F the Faraday constant, A the electroactive surface areas of the bare or modified electrode, c the concentration of Fe(CN)3/4, R the ideal gas con6 stant, and T thermodynamic temperature. The electron transfer rate constants of the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE were estimated to be 1.7 103, 134.4, 0.018, 58.0, and 27.8 cm/s, respectively. The results suggest that the electron transfer rate of the MnO2/MWCNT/EPGCE is far larger than those of the GCE and MnO2/EPGCE, but slightly less than those of the MWCNT/EPGCE and EPGCE.
(5)
Where ipc the reduction peak current, n the electron transfer number, D the diffusion coefficient of Fe(CN)3/4in the solution 6 (7.6 106 cm2/s [36]), v the scanning rate, A the electroactive surface areas of the bare or modified electrode, and c the concentration of Fe(CN)3/4. The electrochemical active areas of the 6 GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/ EPGCE were estimated to be 0.070 cm2, 0.120 cm2, 0.117 cm2, 0.133 cm2, and 0.131 cm2, respectively. The results suggest that the MnO2/MWCNT nanocomposite can increase the electroactive surface area of the electrode, which causes the increase of peak current. EIS is further used to assess the interfacial behavior of different electrodes. The Nyquist plots of the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE are shown in Fig. 2B. The semicircular at the higher frequency region and the linear part at the lower frequency region respectively stand for the electron transfer and diffusion limited process of the electrochemical reaction. A typical Randle's equivalent electric circuit model is given in the inset of Fig. 2B. The model consists of an active electrolyte resistance (Rs), a constant phase element (CPE, it is a double-layer capacitance Cdl in the work), a charge transfer resistance (Rct), and a Warburgh impedance (Zw). By fitting the impedance data from the Nyquist plots on the basis of the Randle's equivalent circuit model, the Rct values of the GCE, EPGCE, MnO2/EPGCE,
3.3. Electrochemical responses of dopamine and MnO2 at different electrodes The DPV responses of the GCE, EPGCE, MnO2/EPGCE, MWCNT/ EPGCE, and MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with or without 50.0 mM DA are presented in Fig. 3A and B. It is clearly from Fig. 3A that the five different electrodes provide an oxidation peak between 0 and 0.3 V due to the DA redox reaction. Looking at Fig. 3C, the magnitude of the oxidation peak current for DA at the five different electrodes is MnO2/MWCNT/EPGCE z MWCNT/ EPGCE z MnO2/EPGCE > EPGCE [ GCE, suggesting that the MnO2/ MWCNT/EPGCE has a high sensitivity for DA detection. As shown in Fig. 3B, the MnO2 electrodeposited at the EPGCE has a DPV peak at 0.59 V, which is assigned to the redox reaction between Mn2þ and MnO2. When the MnO2 is electrodeposited at the MWCNT/EPGCE, the DPV peak of MnO2 negatively shifts to 0.54 V with a remarkably enhanced peak current of MnO2. The large peak current implies that compared with the MnO2/EPGCE, the MnO2/MWCNT/EPGCE is more suitable for the construction of ratiometric sensor. Moreover, Fig. 3D displays that with the increasing concentration of DA, the DPV peak current of DA at the MnO2/MWCNT/EPGCE gradually increases, but the peak current of MnO2 at the electrode almost remains unchangeable, denoting the validity of the construction of the ratiometric electrochemical sensor based on the MnO2/ MWCNT/EPGCE. Therefore, in the following work, the peak
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Fig. 3. DPV graphs of the (a) GCE, (b) EPGCE, (c) MnO2/EPGCE, (d) MWCNT/EPGCE, and (e) MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with (A) and without (B) 50.0 mM DA. (C) The corresponding bar graph of the oxidation peak current for DA at the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE. (D) DPV graphs of the MnO2/MWCNT/ EPGCE with different concentrations of DA ranging from 0 to 30.0 mM in 0.1 M PBS (pH 7.0).
potential of 0.54 V at the MnO2/MWCNT/EPGCE was chosen as the measuring potential of the MnO2 inner reference probe, while the oxidation peak potential of 0.15 V of DA at the MnO2/MWCNT/ EPGCE was selected as the measuring potential of the analyte DA. Additionally, the ratio of the net peak current [DI (DA)/DI(MnO2)] without and with DA was employed for subsequent quantitative detection of DA. To understand the origin of high sensitivity for DA detection at MnO2/MWCNT/EPGCE, we used CV technique for investigating the effect of scanning rate on the peak currents and potentials of DA to obtain the electrochemical kinetics information (see Fig. 4A). Fig. 4B and C depict the effect of scanning rate on the peak currents and potentials of DA at MnO2/MWCNT/EPGCE. As shown in Fig. 4B, the oxidation (Ipa) and reduction (Ipc) peak currents enhance gradually as the scanning rate (20e500 mV/s) increases. In the selected potential scanning rate range, the linear regression equations between the oxidation or reduction peak currents and the scanning rate can be respectively expressed as Ipa (mA) ¼ 0.226 v (mV/s) e 1.099 (r ¼ 0.999), and Ipc (mA) ¼ 0.197 v (mV/s) þ 0.174 (r ¼ 0.999). Clearly, the oxidation or reduction peak currents of DA is proportional to the scanning rate, implying that the electrode process is controlled by the adsorption of DA on the MnO2/MWCNT/EPGCE surface [35]. In addition, it is clear from Fig. 4C that with the increase of the scanning rate, the oxidation and reduction peak shifts to more positive potentials and more negative potentials, respectively. When the scanning rate is above 200 mV/s, the values of the oxidation (Epa) and reduction (Epc) peak potentials are proportional to the logarithm of scanning rate (lgv), and the linear relationship between Epa or Epc and lgv can be respectively expressed as Epa (V) ¼ 0.073 lgv (mV/s) þ 0.018 (r ¼ 0.994) and Epc (V) ¼ 0.067 lgv (mV/s) þ 0.304 (r ¼ 0.999). As for an adsorption-controlled process, the electron transfer coefficient (a), the electron transfer number (n), and the apparent rate constant (ks) can be estimated on the basis of the following Laviron's equation [37]:
Epa ¼ Eq þ 0
2:3RT RTks 2:3RT lg þ lg v ð1 aÞnF ð1 aÞnF ð1 aÞnF
Epc ¼ Eq þ 0
2:3RT RTks 2:3RT lg lg v anF anF anF
lgks ¼ a lgð1 aÞ þ ð1 aÞ lga lg
(7)
(8)
að1 aÞnF DEp RT nFv 2:3RT (9)
DEp ¼ Epa Epc
(10)
Where Eq' the formal redox potential, R the ideal gas constant, F the Faraday constant, and T thermodynamic temperature. The a, n and ks values are estimated to be 0.52, 1.69, and 3.52 s1 respectively. The result suggest that two electrons are involved in the electrochemical oxidation process of DA, which agrees well with that reported previously [38]. In addition, the surface coverage (G) of DA on the MnO2/MWCNT/EPGCE surface can be obtained from the following Laviron's equation [35]:
Ip ¼
n2 F2 AGv 4RT
(11)
where n the electron transfer number, R the ideal gas constant, F the Faraday constant, T thermodynamic temperature, A the electroactive surface areas, and v the scanning rate. On the basis of the slope of the Ipev curve and the aforementioned Laviron's equation, the surface coverage of DA adsorbed on the electroactive surface area of MWCNT/MnO2/EPGCE is estimated to be 4.60 107 mol/cm2. We further studied the effect of scanning rate on the peak
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Fig. 4. (A) CV curves of 50.0 mM DA at the MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) at various scanning rates (from inner to outer: 20e500 mV/s). (B) Plot of the oxidation and reduction peak currents of DA versus the scanning rate. (C) Plot of the oxidation and reduction peak currents of DA versus the logarithm of scanning rate. (D) Plot of the oxidation peak potentials of DA versus the pH. (E) Plot of the oxidation peak currents of MnO2 at the MnO2/MWCNT/EPGCE versus the scanning rate.
currents and potentials of DA at the other four electrodes (the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE). The results show that the electrochemical redox behavior of DA on the other four electrode surface is a surface adsorption-controlled electrochemical process (Figs. S2AeS2D). Therefore, we estimated the apparent rate constants of the DA electrochemical reaction and the surface coverages of DA at the four different electrode according to the aforementioned Laviron's equations. The apparent rate constants at the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE are 1.90, 3.63, 4.81 and 4.08 s1, and the related surface coverages of DA are 3.78 108, 2.46 107, 1.59 107, and 1.60 107 mol/cm2. By analyzing the electroactive surface areas, electrical conductivities, the apparent rate constants of the DA electrochemical reaction, and the surface coverages of DA at the five different electrodes, we can find that compared with the other four electrodes (the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE), the large oxidation peak current and high sensitivity for DA detection at MnO2/MWCNT/EPGCE predominately originates from the high surface coverage of DA at the MnO2/MWCNT/EPGCE, which is attributed to the strong synergistic effect between MnO2 and MWCNT. At the same time, without any doubt, the large electroactive surface area, good electrical conductivity, and fast electrochemical rate at the MnO2/ MWCNT/EPGCE also make contributions to the oxidation peak current and detection sensitivity of DA. In addition, the effect of solution pH (3.0e8.0) on the oxidation peak potential of DA at the MnO2/MWCNT/EPGCE was studied using CV technique. As displayed in Fig. 4D, the Epa has a linear relationship with pH values. The linear regression equation can be obtained as Epa (V) ¼ 0.051 pH þ 0.558 (r ¼ 0.982). The slope (0.051 V per pH) approaches that expected theoretically Nernstian value (0.059 V per pH) for an electrochemical process with protons/electrons ratio of 1:1. In view of the above experimental results, the oxidation process of DA at the MnO2/MWCNT/EPGCE is a two-electron and two-proton process, which is consistent with the previous report [39]. On the other hand, to understand the origin of the signal
enhancement of the electrodeposited MnO2 on MWCNT/EPGCE, the effect of the scanning rate on the peak currents and potentials of MnO2 at two different electrodes (MnO2/MWCNT/EPGCE and MnO2/EPGCE) was also investigated by CV technique (Fig. 4A and Fig. S2C). As shown in Fig. 4E and Fig. S3, the oxidation peak currents of MnO2 is proportional to the scanning rate (20e500 mV/s), suggesting that the electrode process of the MnO2/MWCNT/EPGCE and MnO2/EPGCE is controlled by the adsorption of MnO2. On the basis of the Laviron's equations [35], the electron transfer number (n) of the MnO2 electrochemical reaction and the surface coverage of MnO2 at the MnO2/MWCNT/EPGCE or MnO2/EPGCE can be calculated. The electron transfer number of MnO2 at the MnO2/ MWCNT/EPGCE or MnO2/EPGCE is respectively estimated to be 2.05 and 2.03, suggesting that the overall redox process of MnO2 is a two-electron transfer process. Simultaneously, the corresponding surface coverage of MnO2 at the MnO2/MWCNT/EPGCE or MnO2/ EPGCE is 1.78 107 and 6.01 107 mol/cm2, respectively. In addition, on the basis of the Laviron's equation, the ratio value of apparent rate constant at the MnO2/MWCNT/EPGCE and MnO2/ EPGCE can be estimated to be 1.8:1. These results indicate that MWCNT has electrocatalytic activity towards the electrodeposited MnO2, and increases the surface coverage of the electrodeposited MnO2, which lead to large signal enhancement of the electrodeposited MnO2 on MWCNT/EPGCE. 3.4. Optimization of the experimental conditions The numbers of cycle for MnO2 deposition and the pH value of PBS were optimized. Fig. S4 shows the effect of the number of electrodeposition cycles (from 10 to 20) of MnO2 on the ratio of the net peak current. It can be found that almost no remarkable change for the ratio of the net peak current appears when the number of cycles is 10. Ten cycles for MnO2 deposition were selected for the following experiments. Moreover, on the basis of high sensitivity, high stability and environmental friendliness, pH 7.0 was chosen as
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the optimal pH value of PBS buffer. 3.5. Ratiometric electrochemical sensing of DA Fig. 5A shows typical DPV response curves of DA at the MnO2/ MWCNT/EPGCE upon the addition of different concentrations (0e80.0 mM) of DA. It can be observed that the concentration increase of DA leads to the enhancement of DA oxidation peak current, but the oxidation peak current of MnO2 almost does not change. The ratio of the net peak current, DI (DA)/DI(MnO2), is proportional to the concentration of DA over the range of 0.5e30.0 mM with a correlation coefficient (r) of 0.998 and a detection limit of 0.17 mM at a signal/noise ratio of 3 (Fig. 5B). This linear fitting equation between DI (DA)/DI(MnO2) and DA concentration can be represented as DI (DA)/DI(MnO2) ¼ 0.35 c (mM) 0.010. The LOD value for the DA electrochemical sensor is comparable with or superior to those values earlier literature reported (Table 1). 3.6. Selectivity, reproducibility, and stability To test the selectivity of the proposed sensor, some common organic and inorganic substances like Naþ, Kþ, Ca2þ, Mg2þ, Cl, 3 NO 3 , PO4 , ascorbic acid (AA), uric acid (UA), citric acid, L-cysteine and glucose, were examined at the MnO2/MWCNT/EPGCE as the potential competitors of DA, as shown in Fig. 6. Clearly, almost no remarkable change appears for the DI (DA)/DI(MnO2) value upon the addition of each interfering substance at the concentration of 50.0 mM. When the mixture of all interfering substances was added, the DI (DA)/DI(MnO2) value approached that value in the presence of each interfering substance alone. In addition, it can be found that the addition of DA alone gave a noticeable increase in the DI (DA)/ DI(MnO2) value, and the DI (DA)/DI(MnO2) value in the presence of DA and the mixture of all interfering substances was close to that of DA alone. All these results suggest that the developed ratiometric electrochemical sensor has high selectivity for DA detection. The high selectivity for DA detection is ascribed to two possible reasons. One reason is that the large electroactive surface area, good electrical conductivity, and fast electrochemical rate at the MnO2/ MWCNT/EPGCE significantly enhance the peak response current of DA, and the other is that the MnO2/MWCNT nanocomposite at the electrode improves the electrocatalytic property of the sensor, and facilitates the signal separation of analyte and other interfering substances. The intra-reproducibility and inter-reproducibility test of the MnO2/MWCNT/EPGCE for DA detection were also tested. As can be seen from Fig. S5A, one MWCNT/MnO2/EPGCE electrode is repeatedly used for five times, and the relative standard deviation (RSD) value is calculated as 4.3%. Five identically modified
Table 1 Comparison of the performance of the constructed MnO2/MWCNT/EPGCE-based ratiometric electrochemical sensor with other reported sensor for dopamine detection. Electrode b
PS/MCPE Co3O4/MWCNT/GCEc RGO/MWCNT/PTA/GCEd (SWCNT/CPB)3/GCEe CTAB-GO/MWCNT/GCEf BPVCM-e/MWCNT/GCEg poly-VA/MWCNT/GCEh IMWCNT/CPEi MnO2/MWCNT/EPGCE
Linear range (mM)
LODa (mM)
Reference
10e90 1e20 0.5e20 4e120 5e500 5e300 and 300e1000 5e120 1.9e79.4 and 79.4e714.3 1.0e50.0
9.3 0.176 1.14 0.6 1.5 2.28 4.5 0.52 0.8
[11] [38] [39] [40] [41] [42] [43] [44] This work
a
Limit of detection. Poly (sudan III) modified carbon paste electrode. c Cobalt oxide/multilwall carbon nanotube modified glassy carbon electrode. d Reduced graphene oxide/mutilwall carbon nanotube/phospotungstic acid modified glassy carbon electrode. e Multilayer films of negatively charged single-wall carbon nanotube and positively charged cetylpyridinium bromide modified glassy carbon electrode. f Hexadecyl trimethyl ammonium bromide functionalized graphene oxide/multiwalled carbon nanotube modified glassy carbon electrode. g Branched amphiphilic photosensitive and electroactive polymer/multilwall carbon nanotube modified glassy carbon electrode. h Electropolymerized vanillic acid/multilwall carbon nanotube modified glassy carbon electrode. i Indenedione derivative and multilwall carbon nanotube modified carbon paste electrode. b
Fig. 6. Selectivity of the ratiometric sensor based on MnO2/MWCNT/EPGCE. The concentration of DA and each interfering substance is 5.0 mM and 50.0 mM, respectively. The mixture contains all interfering substances.
Fig. 5. (A) DPV curves of the MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with different concentrations (0e80.0 mM) of DA. (B) Plot of the ratio of the net peak current, DI (DA)/ DI(MnO2), versus the DA concentration. Inset: the enlarged calibration plot with DA concentration of 0.5e30.0 mM.
Y. Wang et al. / Journal of Alloys and Compounds 802 (2019) 326e334
electrodes were used for DA detection (Fig. S5B), and the RSD value was 8.4%. All these confirm that the constructed ratiometric sensor has high reproducibility. Moreover, the MWCNT/MnO2/EPGCE electrode was stored for a week, and the related DI (DA)/DI(MnO2) kept above 90% current response. In addition, the MWCNT/MnO2/ EPGCE electrode was tested by 15 consecutive scans, and the corresponding current had no obvious change in the DI (DA)/DI(MnO2) ratio. The results show that the sensor has an acceptable stability. 3.7. Detection of DA in real samples To test the usefulness of the developed sensor, it was applied for DA determination in human serum samples. The human serum samples were collected from the local hospital. The samples were first filtered using a 0.22 mm membrane, and then centrifuged at 12,000 rpm for 30 min. Prior to the measurement, the samples were 10-fold diluted with PBS buffer solution, and then transferred to the electrochemical cell without any pretreatment process. It was found that no DA in such samples was detected by the sensor. Hence, a recovery test was done on the samples spiked with DA at different concentration levels to evaluate the feasibility of the sensor. The results are listed in Table 2. Clearly, the recoveries of the spiked samples are in the range of 94e110%, and the RSD value ranges from 1.0% to 6.0%, suggesting that the ratiometric electrochemical sensor based on MWCNT/MnO2/EPGCE has high reproducibility and precision. On the basis of the aforementioned results, it is very clear that this proposed sensor has great potential for DA detection in real samples. 4. Conclusion In this study, a ratiometric electrochemical sensor based on MnO2/MWCNT nanocomposite-modified electrode was developed for DA detection. The hierarchical MnO2 nanoflower was electrodeposited onto the MWCNT/EPGCE to prepare an inner reference electrochemical probe. The introduction of multiwalled carbon nanotube can not only improve the electrochemical signals of DA, but also increase MnO2 signals. The dual signal enhancement facilitates the construction of ratiometric electrochemical sensor for sensitive and accurate detection of DA. The possible origin of the high sensitivity for DA detection and the signal enhancement of MnO2 on MWCNT/EPGCE is reasonably explained. The results show that the high sensitivity for DA detection at MnO2/MWCNT/EPGCE predominately originates from the high surface coverage of DA at the MnO2/MWCNT/EPGCE, and the signal enhancement of MnO2 on MWCNT/EPGCE is ascribed to the electrocatalytic activity of MWCNT towards the electrodeposited MnO2 and the increase of MnO2 surface coverage. This sensor based on MnO2/MWCNT/ EPGCE displays a wide linear range from 0.5 to 30.0 mM with a LOD value of 0.17 mM. In addition, the sensor displays high selectivity, good reproducibility, and good stability, and can be successfully applied for DA detection in human serum samples. It is believed that the nanocomposite consisting of the hierarchical MnO2 nanoflower and multiwalled carbon nanotube can be further applied to construct ratiometric electrochemical sensors for the detection of other analytes of interest.
Table 2 Detection of DA in human serum samples by the proposed ratiometric sensor. Spiked (mM)
Detected (mM)a
Mean recovery (%)
RSD (%)
5.0 10.0 20.0
4.7 11.0 21.5
94 110 108
1.9 5.6 3.2
a
Mean value of three determinations.
333
Acknowledgements The authors acknowledge support from the National Natural Science Foundation of China (NSFC21864017 and NSFC21305061), the Natural Science Foundation of Jiangxi Province (20181BAB213008 and 20171BAB203018), the Education Department of Jiangxi Province (GJJ160006 and GJJ160204), the Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC-201802), the Open Project Program of State Key Laboratory of Food Science and Technology of Nanchang University (SKLF-KF-201810), and the State Key Laboratory of Chemo/Biosensing and Chemometrics of Hunan University (SKLCBC-2018007). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.124. References [1] R.A. Mitchell, N. Herrmann, K.L. Lanctot, The role of dopamine in symptoms and treatment of apathy in Alzheimer's disease, CNS Neurosci. Ther. 17 (2011) 411e427. [2] P.J. Gaskill, D.R. Miller, J. Gamble-George, H. Yano, H. Khoshbouei, HIV, Tat and dopamine transmission, Neurobiol. Dis. 105 (2017) 51e73. [3] D.J. Surmeier, Determinants of dopaminergic neuron loss in Parkinson's disease, FEBS J. 285 (2018) 3657e3668. [4] D.W. Qi, Q. Zhang, W.H. Zhou, J.J. Zhao, B. Zhang, Y.F. Sha, Z.Q. Pang, Quantification of dopamine in brain microdialysates with high-performance liquid chromatography-tandem mass spectrometry, Anal. Sci. 32 (2016) 419e424. [5] L.Y. Zhang, S.F. Qv, Z.G. Wang, J.K. Cheng, Determination of dopamine in single rat pheochromocytoma cell by capillary electrophoresis with amperometric detection, J. Chromatogr. B 792 (2003) 381e385. [6] Y.X. Guo, J. Lu, Q. Kang, M. Fang, L. Yu, Fabrication of biocompatible, luminescent supramolecular structures and their applications in the detection of dopamine, Langmuir 34 (2018) 9195e9202. [7] Y.Y. Wang, L. Yang, Y.Q. Liu, Q.B. Zhao, F. Ding, P. Zhou, H.B. Rao, X.X. Wang, Colorimetric determination of dopamine by exploiting the enhanced oxidase mimicking activity of hierarchical NiCo2S4-rGO composites, Microchim. Acta 185 (2018) 496. [8] L. Zhang, Z.R. Tang, Y.P. Dong, Silicon quantum dot involved luminol chemiluminescence and its sensitive detection of dopamine, Anal. Methods 10 (2018) 4129e4135. [9] M. Sajid, M.K. Nazal, M. Mansha, A. Alsharaa, S.M.S. Jillani, C. Basheer, Chemically modified electrodes for electrochemical detection of dopamine in the presence of uric acid and ascorbic acid: a review, Trends Anal. Chem. 76 (2016) 15e29. [10] M.Z.H. Khan, Graphene oxide modified electrodes for dopamine sensing, J. Nanomater. 2017 (2017) 8178314. [11] T.S.S.K. Naik, Fabrication of poly (Sudan III) modified carbon paste electrode sensor for dopamine: a voltammetric study, J. Electroanal. Chem. 834 (2019) 71e78. [12] H. Jin, R.J. Gui, J.B. Yu, W. Lv, Z.H. Wang, Fabrication strategies, sensing modes and analytical applications of ratiometric electrochemical biosensors, Biosens. Bioelectron. 91 (2017) 523e537. [13] T. Yang, R.Z. Yu, Y.H. Yan, H. Zeng, S.Z. Luo, N.Z. Liu, A. Morrin, X.L. Luo, W.H. Li, A review of ratiometric electrochemical sensors: from design schemes to future prospects, Sens. Actuators, B 274 (2018) 501e516. [14] E. Turkusic, J. Kalcher, E. Kahrovic, N.W. Beyene, H. Moderegger, E. Sofic, D. Begic, K. Kalcher, Amperometric determination of bonded glucose with an MnO2 and glucose oxidase bulk-modified screen-printed electrode using flow-injection analysis, Talanta 65 (2005) 559e564. [15] Y.H. Bai, H. Zhang, J.J. Xu, H.Y. Chen, Relationship between nanostructure and electrochemical/biosensing properties of MnO2 nanomaterials for H2O2/ choline, J. Phys. Chem. C 112 (2008) 18984e18990. [16] H. Liu, X.L. Yu, H.F. Chen, Y.H. Liu, Preparation of porous carbon-manganese dioxide nanocomposite for sensitive determination of cadmium ion, Int. J. Electrochem. Sci. 12 (2017) 9736e9746. [17] W. Cao, Y. Wang, Q.F. Zhuang, L.Y. Wang, Y.N. Ni, Developing an electrochemical sensor for the detection of tertbutylhydroquinone, Sens. Actuators, B 293 (2019) 321e328. [18] J. Yang, H. Lee, M. Cho, J. Nam, Y.K. Lee, Nonenzymatic cholesterol sensor based on spontaneous deposition of platinum nanoparticles on layer-by-layer assembled CNT thin film, Sens. Actuators, B 171e172 (2012) 374e379. [19] K.Q. Ding, Y.H. Wang, H.W. Yang, C.B. Zheng, Y.L. Cao, H.G. Wei, Y.R. Wang, Z.H. Guo, Electrocatalytic activity of multi-walled carbon nanotubessupported PtxPdy catalysts prepared by a pyrolysis process toward ethanol oxidation reaction, Electrochim. Acta 100 (2013) 147e156. [20] X. Dong, X.C. Lu, K.Y. Zhang, Y.Z. Zhang, Chronocoulometric DNA biosensor
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A ratiometric electrochemical sensor for dopamine detection based on hierarchical manganese dioxide nanoflower/multiwalled carbon nanotube nanocomposite modified glassy carbon electrode Yong Wang*, Luyao Wang, Qianfen Zhuang College of Chemistry, Nanchang University, Nanchang, 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2019 Received in revised form 6 June 2019 Accepted 10 June 2019 Available online 13 June 2019
A ratiometric electrochemical sensor is developed for dopamine detection based on hierarchical manganese dioxide (MnO2) nanoflower/multiwalled carbon nanotube (MWCNT) nanocomposite modified glassy carbon electrode (GCE). The hierarchical MnO2 nanoflower was electrodeposited onto the electrochemically pretreated GCE modified with MWCNT (MWCNT/EPGCE) to prepare an inner reference electrochemical probe. The fabrication process of the MnO2/MWCNT/EPGCE electrode was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The introduction of MWCNT can not only improve the electrochemical signals of dopamine, but also increase MnO2 signals. The origin of the dual signal enhancement for both dopamine and MnO2 is reasonably explained by using electrochemical techniques. The results show that the high sensitivity for dopamine detection at MnO2/ MWCNT/EPGCE predominately originates from the high surface coverage of dopamine, and the signal enhancement of MnO2 on MWCNT/EPGCE is ascribed to the electrocatalytic activity of MWCNT towards the electrodeposited MnO2 and the increase of MnO2 surface coverage. This sensor based on MnO2/ MWCNT/EPGCE has a low detection limit of 0.17 mM and a wide linear range from 0.5 to 30.0 mM. In addition, the sensor displays high selectivity, good reproducibility, and good stability, and can be applied for dopamine detection in human serum samples with satisfactory results. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ratiometric electrochemical sensor Composite materials Manganese dioxide nanoflower Multiwalled carbon nanotube Electrodeposition Dopamine
1. Introduction Dopamine (DA) is a kind of important catecholamine neurotransmitter molecules [1e3]. It is widely distributed in mammalian brain tissue and body fluids, and plays a vital role in the central nervous system [1e3]. Changes of DA's concentration usually lead to HIV infection and various neurological diseases such as Alzheimer's and Parkinson's [1e3]. Therefore, the development of a simple, fast and effective assay method for DA determination is of great significance for clinical diagnosis. Up to date, many methods have been proposed for DA detection, including high-performance liquid chromatography [4], capillary electrophoresis [5], and spectroscopic methods [6e8]. However, most of these methods require expensive equipment, and involve time-consuming pretreatment steps. Compared with these
* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jallcom.2019.06.124 0925-8388/© 2019 Elsevier B.V. All rights reserved.
methods, electrochemical sensor becomes an attractive alternative due to their unique advantages like simplicity, fast response, miniaturization, low cost and high sensitivity [9e11]. Currently, most of the electrochemical sensors focus on the development of single-signal sensing strategy which only uses a single electrochemical signal to realize analyte's detection [9e11]. Generally speaking, the kind of sensing strategy is easily influenced by many factors such as environmental changes, the sensor or reagent concentrations, and instrumental efficiency, which give false positive or negative errors for analyte's detection [12,13]. To overcome the drawbacks, ratiometric electrochemical sensor represents an acceptable alternative owning to its superior built-in self-calibration ability [12,13]. The kind of sensing strategy usually determines the ratios of the peak current at two different redox potentials for analyte detection. Both of the peak current signals usually originate from the analyte itself and the inner reference probe, respectively. Importantly, the strong electrochemical signal of the inner reference probe can increase the accuracy of signal ratio value, which causes the improvement of the accuracy of
Y. Wang et al. / Journal of Alloys and Compounds 802 (2019) 326e334
analyte detection. Therefore, the development of an electrochemical probe with good inner reference ability is always attractive. Manganese dioxide (MnO2) is a kind of transition metal oxide, and has been recently used for the construction of electrochemical sensors due to its high specific surface area and good electrocatalytic activity [14e17]. However, the use of MnO2 as the inner reference probe in the construction of ratiometric electrochemical sensors is still limited due to its poor electrical conductivity. Therefore, rational choice of support materials with excellent electrical conductivity is vitally important for the construction of MnO2-modified electrochemical sensors. Multiwalled carbon nanotube (MWCNT) has many specific properties such as high electrical conductivity, large surface area, good electrocatalytic activity and great chemical stability, and thus it is an ideal support material in electrochemical modified sensors [18e21]. Recently, many researchers have reported that the MnO2/MWCNT nanocomposite could remarkably improve the performance of electrochemical modified sensors [22e25]. Xu et al. reported that the MnO2/MWCNT nanocomposite could be used for sensitive electrochemical detection of hydrogen peroxide because the combination of MnO2 and MWCNT increased the electrocatalytic active area and promoted electron transfer [22]. Tang et al. found that the MnO2/MWCNT nanocomposite could enhance the sensitivity of the detection of hydroquinone and catechol due to its excellent catalytic activity, high surface area and nanosized structure [23]. VesaliNaseh et al. found that the MnO2/MWCNT nanocomposite enhanced charge transfer efficiency and reversibility of the glucose oxidase mediated redox processes, resulting in the increase of sensitivity for glucose detection [24]. Jung et al. found that the MnO2/MWCNT nanocomposite could be used to construct a sensitive sensor for the detection of hydrogen gas due to their large surface area and fast electron-transport capability [25]. In this study, we prepared the hierarchical manganese dioxide (MnO2) nanoflower/multiwalled carbon nanotube (MWCNT) nanocomposite modified electrode for the construction of ratiometric electrochemical dopamine sensor. The hierarchical MnO2 nanoflower was electrodeposited onto the electrochemically pretreated glassy carbon electrode modified with multiwalled carbon nanotube (MWCNT/EPGCE) to prepare an inner reference electrochemical probe. The presence of multiwalled carbon nanotube enhanced the electrochemical signals of the analyte, DA, and the inner reference, the inorganic MnO2 nanoflower, and thus facilitated the fabrication of sensitive ratiometric electrochemical sensor for DA detection. In addition, the possible origin of the signal enhancement of MnO2 on MWCNT/EPGCE and the high sensitivity for DA detection was reasonably explained. The brief protocol for the fabrication of the ratiometric electrochemical sensor for DA detection based on MnO2/MWCNT nanocomposite is displayed in Scheme 1. 2. Experimental 2.1. Reagents and chemicals Multiwalled carbon nanotube (MWCNT) was purchased from Shenzhen Nanotech. Port Co., Ltd. Dopamine (DA), sulfuric acid (H2SO4), nitric acid (HNO3), manganese (II) acetate tetrahydrate (MnAc2$4H2O), sodium sulfate (Na2SO4), disodium hydrogen phosphate dodecahydrate (Na2HPO4$12H2O), sodium dihydrogen phosphate dehydrate (NaH2PO4$2H2O), and other inorganic salts were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Phosphate buffer solutions (0.1 M PBS, pH 7.0) were employed as the supporting electrolyte. All chemicals were used as received without any further refinement. Ultrapure water
327
Scheme 1. Schematic illustration of the construction processes of the developed ratiometric electrochemical sensor and its application for DA detection.
(18.25 MU cm) was used throughout the experiments. 2.2. Apparatus Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) were conducted using a JSM-6701F microscope system (JEOL Co., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were done on an ESCALAB 250Xi Microprobe (Thermo Scientific, USA) with a monoenergetic Al Ka radiation (200 W, hn ¼ 1486.6 eV). Electrochemical experiments were operated on a CHI-660A electrochemical workstation (Chenhua Apparatus Co., Shanghai, China) in conjunction with a typical three-electrode system. The Ag/AgCl (3 M NaCl) electrode and platinum wire were respectively used as the reference electrode and the counter electrode. The unmodified glassy carbon electrode (GCE, 3 mm in diameter) or modified GCE served as working electrodes. 2.3. Preparation of MnO2/MWCNT/EPGCE 100 mg of MWCNT was suspended into 100 mL of concentrated HNO3 and H2SO4 (1:3 vol ratio) mixture, followed by ultrasonication in a water bath for 3 h at 40 C. After that, the asprepared MWCNT was washed with water several times [26,27], and dried in a vacuum-drying oven overnight at 80 C. Finally, the resultant MWCNT was dissolved in water, and ultrasonicated at room temperature for 1 h to produce a homogeneous dispersion. The aforementioned pretreatment procedure can introduce oxygen-containing moieties mainly on the open ends and defects on the sidewall of the nanotubes to yield MWCNT functionalized with carboxylic groups. Before modification, GCE was successively polished with 1.0, 0.3 and 0.05 mm alumina slurries. Then, the GCE was ultrasonicated for 5 min in dilute nitric acid, ethanol, and water, respectively. After that, the polished GCE was subjected to electrochemical pretreatment in 0.5 M H2SO4, at þ2.0 V for 30 s, 1.0 V for 10 s, and then from 0 to 1.0 V at a scanning rate of 0.1 V/s until a stable voltammogram was achieved [28]. The electrochemically pretreated glassy carbon electrode (EPGCE) was firstly modified by dropping 10 mL of 0.2 mg/mL MWCNT functionalized with carboxylic groups, and dried in air. Next, the electrodeposition of MnO2 nanoflower on the surface of the MWCNT/EPGCE was performed by cyclic potential scanning from 0.0 to 0.9 V at 200 mV/s in the 10 mM of manganese (II)
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acetate solution containing 10 mM Na2SO4 for 10 cycles [29]. Finally, the as-prepared MnO2/MWCNT/EPGCE was washed carefully with water. 2.4. Electrochemical measurements Various concentrations (0e80.0 mM; total 15 samples) of DA were added to 10 mL of PBS (0.1 M, pH 7.0), and then mixed well. After that, the differential pulse voltammetric (DPV) measurements for DA detection were performed. Parameters in the DPV measurements were as follows: the increment potential was 4 mV, the pulse amplitude was 50 mV, the pulse width was 0.06 s, the sample width was 0.0167 s, the pulse period was 0.2 s, the quiet time was 2 s, and the potential window was form 0.1e0.8 V. Electrochemical impedance spectroscopy (EIS) measurements were conducted in 5 mM [Fe(CN)6]3/4- (1:1) solution containing 0.1 M KCl with a voltage frequency range of 0.1 Hze100 kHz at amplitude of 5 mV. 3. Results and discussion 3.1. Preparation and characterization of MnO2/MWCNT nanocomposite Fig. S1 displays cyclic voltammograms of the electrodeposition of MnO2 on the surface of MWCNT/GCE. An oxidation peak over the potential ranges from 0.5 to 0.8 V and a reduction peak over the potential ranges from 0.2 to 0.6 V can be clearly observed, which can be ascribed, as reported in literature, to the redox reaction between Mn2þ and MnO2 [30]. SEM is used to characterize the surface morphologies of the MWCNT and MnO2/MWCNT nanocomposite, which are distributed on the surface of glassy carbon electrode. As shown in Fig. 1A, a typical crossing tubular structure can be seen on the surface of GCE, suggesting the successful immobilization of MWCNT onto GCE. Fig. 1B shows the surface morphology of MnO2 electrodeposited onto the surface of MWCNT/ GCE. It can be clearly that the hierarchical MnO2 nanoflowers are uniformly distributed on the surface of MWCNT/GCE with size distribution ranging from 130 nm to 200 nm, and the surface of the MnO2 nanoflowers is composed of many randomly oriented
nanorod-like structures. The results from both the EDX spectrum and the XPS survey spectrum indicate that the elements C, O, and Mn are present at the MWCNT/MnO2 nanocomposite (Fig. 1C and D). In addition, as shown in the Mn 2p XPS spectrum (Fig. 1E), two main peaks appear at 653.6 and 641.9 eV, which are respectively assigned to Mn 2p1/2 and Mn 2p3/2 due to the presence of MnO2 in the nanocomposite [31,32]. Simultaneously, the spin-energy separation is ca. 11.7 eV, further confirming the presence of MnO2 in the nanocomposite [31,32]. All the results confirm the successful preparation of MWCNT/MnO2 nanocomposite onto GCE. The possible growth process of MnO2 can be explained as following [29,30,33]: Mn2þ / Mn3þ þ e
(1)
Mn3þ þ 2H2O / MnOOH þ 3Hþ
(2)
MnOOH / MnO2 þ Hþ þ e
(3)
Overall: Mn2þ 2H2O / MnO2 þ 4Hþ þ 2e
(4)
The growth of MnO2 depends on the active support material [29,30,33,34]. In this work, MWCNT was employed. Growth began as the thin film on the surface of MWCNT, and soluble Mn3þ was produced initially. After that, Mn3þ underwent hydrolysis and precipitated as MnOOH on the MWCNT surface, followed by the formation of MnO2 nanoparticles via a further solid state oxidation process. The MnO2 nanoparticles were nucleated to act as the reactive sites for the subsequent growth of complex MnO2 nanorods. The formed MnO2 nanorods rapidly accumulated on the MWCNT surface to form hierarchical MnO2 nanoflowers. 3.2. Electrochemical behaviors of different electrodes CV is a simple, rapid and powerful technique for studying redox reactions at the electrode solution interfaces. The CV behaviors of 5 mM [Fe(CN)6]3/4- (1:1) solution containing 0.1 M KCl on the bare and modified GCE were studied. As shown in Fig. 2A, the CV of the
Fig. 1. (A) SEM images of MWCNT on the surface of GCE. (B) SEM images of MnO2/MWCNT on the surface of GCE. (C) EDX spectrum of MnO2/MWCNT on the surface of GCE. XPS survey spectrum (D) and Mn 2p XPS spectrum (E) of MnO2/MWCNT on the surface of GCE.
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Fig. 2. CV (A) and EIS (B) curves of (a) GCE, (b) EPGCE, (c) MnO2/EPGCE, (d) MWCNT/EPGCE, (e) MnO2/MWCNT/EPGCE in 5 mM [Fe(CN)6]3-/4- (1:1) containing 0.1 M KCl.
GCE shows a pair of weak redox peaks with the peak-to-peak potential separation (DEp) of 0.268 V. After electrochemical pretreatment, the GCE displays a pair of well-defined redox peak with DEp of 0.124 V, and the corresponding redox peak currents raise due to the fast electron transfer rate of the electrode. When modified with MnO2, the MnO2/EPGCE exhibits the DEp of 0.142 V, which is a little larger than that of the EPGCE. Simultaneously, the redox peak currents of the MnO2/EPGCE are less than those of the EPGCE. All these suggest the poor electrical conductivity of the electrodeposited MnO2. When modified with MWCNT, the MWCNT/ EPGCE exhibits the DEp of 0.124 V, and has the redox peak currents greater than the GCE, which indicate that MWCNT can accelerate the electron transfer rate of the electrode. Moreover, the EPGCE modified with MWCNT and MnO2 gives a DEp value similar to MWCNT/EPGCE, but its peak current slightly increases, suggesting that the MnO2/MWCNT nanocomposite can further improve the electrical conductivity of the electrode due to the synergetic effect between MWCNT and MnO2. It is worth noting that the all the electrodes possess a pair of redox peaks, implying that they undergo quasi-reversible processes. The electrochemical active areas of different electrodes can be estimated and compared according to the Randles-Sevcik equation [35]: ipc ¼ (2.69 105) n3/2 D1/2 v1/2 A c
MWCNT/EPGCE, and MnO2/MWCNT/EPGCE can be estimated to be 444.9 U, 0.0033U, 25.08 U, 0.0069 U, and 0.0147 U, respectively, denoting that the electrical conductivity of the MnO2/MWCNT/ EPGCE is far higher than those of the GCE and MnO2/EPGCE, but somewhat lower than those of the MWCNT/EPGCE and EPGCE. On the basis of the Rct values from EIS plots, we can estimate the standard electron transfer rate constants (k0) for the [Fe(CN)6]3/4system at different electrodes according to the following equation [35]:
k0 ¼
RT ðnFÞ2 AcRct
(6)
where n the electron transfer number, F the Faraday constant, A the electroactive surface areas of the bare or modified electrode, c the concentration of Fe(CN)3/4, R the ideal gas con6 stant, and T thermodynamic temperature. The electron transfer rate constants of the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE were estimated to be 1.7 103, 134.4, 0.018, 58.0, and 27.8 cm/s, respectively. The results suggest that the electron transfer rate of the MnO2/MWCNT/EPGCE is far larger than those of the GCE and MnO2/EPGCE, but slightly less than those of the MWCNT/EPGCE and EPGCE.
(5)
Where ipc the reduction peak current, n the electron transfer number, D the diffusion coefficient of Fe(CN)3/4in the solution 6 (7.6 106 cm2/s [36]), v the scanning rate, A the electroactive surface areas of the bare or modified electrode, and c the concentration of Fe(CN)3/4. The electrochemical active areas of the 6 GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/ EPGCE were estimated to be 0.070 cm2, 0.120 cm2, 0.117 cm2, 0.133 cm2, and 0.131 cm2, respectively. The results suggest that the MnO2/MWCNT nanocomposite can increase the electroactive surface area of the electrode, which causes the increase of peak current. EIS is further used to assess the interfacial behavior of different electrodes. The Nyquist plots of the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE are shown in Fig. 2B. The semicircular at the higher frequency region and the linear part at the lower frequency region respectively stand for the electron transfer and diffusion limited process of the electrochemical reaction. A typical Randle's equivalent electric circuit model is given in the inset of Fig. 2B. The model consists of an active electrolyte resistance (Rs), a constant phase element (CPE, it is a double-layer capacitance Cdl in the work), a charge transfer resistance (Rct), and a Warburgh impedance (Zw). By fitting the impedance data from the Nyquist plots on the basis of the Randle's equivalent circuit model, the Rct values of the GCE, EPGCE, MnO2/EPGCE,
3.3. Electrochemical responses of dopamine and MnO2 at different electrodes The DPV responses of the GCE, EPGCE, MnO2/EPGCE, MWCNT/ EPGCE, and MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with or without 50.0 mM DA are presented in Fig. 3A and B. It is clearly from Fig. 3A that the five different electrodes provide an oxidation peak between 0 and 0.3 V due to the DA redox reaction. Looking at Fig. 3C, the magnitude of the oxidation peak current for DA at the five different electrodes is MnO2/MWCNT/EPGCE z MWCNT/ EPGCE z MnO2/EPGCE > EPGCE [ GCE, suggesting that the MnO2/ MWCNT/EPGCE has a high sensitivity for DA detection. As shown in Fig. 3B, the MnO2 electrodeposited at the EPGCE has a DPV peak at 0.59 V, which is assigned to the redox reaction between Mn2þ and MnO2. When the MnO2 is electrodeposited at the MWCNT/EPGCE, the DPV peak of MnO2 negatively shifts to 0.54 V with a remarkably enhanced peak current of MnO2. The large peak current implies that compared with the MnO2/EPGCE, the MnO2/MWCNT/EPGCE is more suitable for the construction of ratiometric sensor. Moreover, Fig. 3D displays that with the increasing concentration of DA, the DPV peak current of DA at the MnO2/MWCNT/EPGCE gradually increases, but the peak current of MnO2 at the electrode almost remains unchangeable, denoting the validity of the construction of the ratiometric electrochemical sensor based on the MnO2/ MWCNT/EPGCE. Therefore, in the following work, the peak
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Fig. 3. DPV graphs of the (a) GCE, (b) EPGCE, (c) MnO2/EPGCE, (d) MWCNT/EPGCE, and (e) MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with (A) and without (B) 50.0 mM DA. (C) The corresponding bar graph of the oxidation peak current for DA at the GCE, EPGCE, MnO2/EPGCE, MWCNT/EPGCE, and MnO2/MWCNT/EPGCE. (D) DPV graphs of the MnO2/MWCNT/ EPGCE with different concentrations of DA ranging from 0 to 30.0 mM in 0.1 M PBS (pH 7.0).
potential of 0.54 V at the MnO2/MWCNT/EPGCE was chosen as the measuring potential of the MnO2 inner reference probe, while the oxidation peak potential of 0.15 V of DA at the MnO2/MWCNT/ EPGCE was selected as the measuring potential of the analyte DA. Additionally, the ratio of the net peak current [DI (DA)/DI(MnO2)] without and with DA was employed for subsequent quantitative detection of DA. To understand the origin of high sensitivity for DA detection at MnO2/MWCNT/EPGCE, we used CV technique for investigating the effect of scanning rate on the peak currents and potentials of DA to obtain the electrochemical kinetics information (see Fig. 4A). Fig. 4B and C depict the effect of scanning rate on the peak currents and potentials of DA at MnO2/MWCNT/EPGCE. As shown in Fig. 4B, the oxidation (Ipa) and reduction (Ipc) peak currents enhance gradually as the scanning rate (20e500 mV/s) increases. In the selected potential scanning rate range, the linear regression equations between the oxidation or reduction peak currents and the scanning rate can be respectively expressed as Ipa (mA) ¼ 0.226 v (mV/s) e 1.099 (r ¼ 0.999), and Ipc (mA) ¼ 0.197 v (mV/s) þ 0.174 (r ¼ 0.999). Clearly, the oxidation or reduction peak currents of DA is proportional to the scanning rate, implying that the electrode process is controlled by the adsorption of DA on the MnO2/MWCNT/EPGCE surface [35]. In addition, it is clear from Fig. 4C that with the increase of the scanning rate, the oxidation and reduction peak shifts to more positive potentials and more negative potentials, respectively. When the scanning rate is above 200 mV/s, the values of the oxidation (Epa) and reduction (Epc) peak potentials are proportional to the logarithm of scanning rate (lgv), and the linear relationship between Epa or Epc and lgv can be respectively expressed as Epa (V) ¼ 0.073 lgv (mV/s) þ 0.018 (r ¼ 0.994) and Epc (V) ¼ 0.067 lgv (mV/s) þ 0.304 (r ¼ 0.999). As for an adsorption-controlled process, the electron transfer coefficient (a), the electron transfer number (n), and the apparent rate constant (ks) can be estimated on the basis of the following Laviron's equation [37]:
Epa ¼ Eq þ 0
2:3RT RTks 2:3RT lg þ lg v ð1 aÞnF ð1 aÞnF ð1 aÞnF
Epc ¼ Eq þ 0
2:3RT RTks 2:3RT lg lg v anF anF anF
lgks ¼ a lgð1 aÞ þ ð1 aÞ lga lg
(7)
(8)
að1 aÞnF DEp RT nFv 2:3RT (9)
DEp ¼ Epa Epc
(10)
Where Eq' the formal redox potential, R the ideal gas constant, F the Faraday constant, and T thermodynamic temperature. The a, n and ks values are estimated to be 0.52, 1.69, and 3.52 s1 respectively. The result suggest that two electrons are involved in the electrochemical oxidation process of DA, which agrees well with that reported previously [38]. In addition, the surface coverage (G) of DA on the MnO2/MWCNT/EPGCE surface can be obtained from the following Laviron's equation [35]:
Ip ¼
n2 F2 AGv 4RT
(11)
where n the electron transfer number, R the ideal gas constant, F the Faraday constant, T thermodynamic temperature, A the electroactive surface areas, and v the scanning rate. On the basis of the slope of the Ipev curve and the aforementioned Laviron's equation, the surface coverage of DA adsorbed on the electroactive surface area of MWCNT/MnO2/EPGCE is estimated to be 4.60 107 mol/cm2. We further studied the effect of scanning rate on the peak
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Fig. 4. (A) CV curves of 50.0 mM DA at the MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) at various scanning rates (from inner to outer: 20e500 mV/s). (B) Plot of the oxidation and reduction peak currents of DA versus the scanning rate. (C) Plot of the oxidation and reduction peak currents of DA versus the logarithm of scanning rate. (D) Plot of the oxidation peak potentials of DA versus the pH. (E) Plot of the oxidation peak currents of MnO2 at the MnO2/MWCNT/EPGCE versus the scanning rate.
currents and potentials of DA at the other four electrodes (the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE). The results show that the electrochemical redox behavior of DA on the other four electrode surface is a surface adsorption-controlled electrochemical process (Figs. S2AeS2D). Therefore, we estimated the apparent rate constants of the DA electrochemical reaction and the surface coverages of DA at the four different electrode according to the aforementioned Laviron's equations. The apparent rate constants at the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE are 1.90, 3.63, 4.81 and 4.08 s1, and the related surface coverages of DA are 3.78 108, 2.46 107, 1.59 107, and 1.60 107 mol/cm2. By analyzing the electroactive surface areas, electrical conductivities, the apparent rate constants of the DA electrochemical reaction, and the surface coverages of DA at the five different electrodes, we can find that compared with the other four electrodes (the GCE, EPGCE, MnO2/EPGCE, and MWCNT/EPGCE), the large oxidation peak current and high sensitivity for DA detection at MnO2/MWCNT/EPGCE predominately originates from the high surface coverage of DA at the MnO2/MWCNT/EPGCE, which is attributed to the strong synergistic effect between MnO2 and MWCNT. At the same time, without any doubt, the large electroactive surface area, good electrical conductivity, and fast electrochemical rate at the MnO2/ MWCNT/EPGCE also make contributions to the oxidation peak current and detection sensitivity of DA. In addition, the effect of solution pH (3.0e8.0) on the oxidation peak potential of DA at the MnO2/MWCNT/EPGCE was studied using CV technique. As displayed in Fig. 4D, the Epa has a linear relationship with pH values. The linear regression equation can be obtained as Epa (V) ¼ 0.051 pH þ 0.558 (r ¼ 0.982). The slope (0.051 V per pH) approaches that expected theoretically Nernstian value (0.059 V per pH) for an electrochemical process with protons/electrons ratio of 1:1. In view of the above experimental results, the oxidation process of DA at the MnO2/MWCNT/EPGCE is a two-electron and two-proton process, which is consistent with the previous report [39]. On the other hand, to understand the origin of the signal
enhancement of the electrodeposited MnO2 on MWCNT/EPGCE, the effect of the scanning rate on the peak currents and potentials of MnO2 at two different electrodes (MnO2/MWCNT/EPGCE and MnO2/EPGCE) was also investigated by CV technique (Fig. 4A and Fig. S2C). As shown in Fig. 4E and Fig. S3, the oxidation peak currents of MnO2 is proportional to the scanning rate (20e500 mV/s), suggesting that the electrode process of the MnO2/MWCNT/EPGCE and MnO2/EPGCE is controlled by the adsorption of MnO2. On the basis of the Laviron's equations [35], the electron transfer number (n) of the MnO2 electrochemical reaction and the surface coverage of MnO2 at the MnO2/MWCNT/EPGCE or MnO2/EPGCE can be calculated. The electron transfer number of MnO2 at the MnO2/ MWCNT/EPGCE or MnO2/EPGCE is respectively estimated to be 2.05 and 2.03, suggesting that the overall redox process of MnO2 is a two-electron transfer process. Simultaneously, the corresponding surface coverage of MnO2 at the MnO2/MWCNT/EPGCE or MnO2/ EPGCE is 1.78 107 and 6.01 107 mol/cm2, respectively. In addition, on the basis of the Laviron's equation, the ratio value of apparent rate constant at the MnO2/MWCNT/EPGCE and MnO2/ EPGCE can be estimated to be 1.8:1. These results indicate that MWCNT has electrocatalytic activity towards the electrodeposited MnO2, and increases the surface coverage of the electrodeposited MnO2, which lead to large signal enhancement of the electrodeposited MnO2 on MWCNT/EPGCE. 3.4. Optimization of the experimental conditions The numbers of cycle for MnO2 deposition and the pH value of PBS were optimized. Fig. S4 shows the effect of the number of electrodeposition cycles (from 10 to 20) of MnO2 on the ratio of the net peak current. It can be found that almost no remarkable change for the ratio of the net peak current appears when the number of cycles is 10. Ten cycles for MnO2 deposition were selected for the following experiments. Moreover, on the basis of high sensitivity, high stability and environmental friendliness, pH 7.0 was chosen as
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the optimal pH value of PBS buffer. 3.5. Ratiometric electrochemical sensing of DA Fig. 5A shows typical DPV response curves of DA at the MnO2/ MWCNT/EPGCE upon the addition of different concentrations (0e80.0 mM) of DA. It can be observed that the concentration increase of DA leads to the enhancement of DA oxidation peak current, but the oxidation peak current of MnO2 almost does not change. The ratio of the net peak current, DI (DA)/DI(MnO2), is proportional to the concentration of DA over the range of 0.5e30.0 mM with a correlation coefficient (r) of 0.998 and a detection limit of 0.17 mM at a signal/noise ratio of 3 (Fig. 5B). This linear fitting equation between DI (DA)/DI(MnO2) and DA concentration can be represented as DI (DA)/DI(MnO2) ¼ 0.35 c (mM) 0.010. The LOD value for the DA electrochemical sensor is comparable with or superior to those values earlier literature reported (Table 1). 3.6. Selectivity, reproducibility, and stability To test the selectivity of the proposed sensor, some common organic and inorganic substances like Naþ, Kþ, Ca2þ, Mg2þ, Cl, 3 NO 3 , PO4 , ascorbic acid (AA), uric acid (UA), citric acid, L-cysteine and glucose, were examined at the MnO2/MWCNT/EPGCE as the potential competitors of DA, as shown in Fig. 6. Clearly, almost no remarkable change appears for the DI (DA)/DI(MnO2) value upon the addition of each interfering substance at the concentration of 50.0 mM. When the mixture of all interfering substances was added, the DI (DA)/DI(MnO2) value approached that value in the presence of each interfering substance alone. In addition, it can be found that the addition of DA alone gave a noticeable increase in the DI (DA)/ DI(MnO2) value, and the DI (DA)/DI(MnO2) value in the presence of DA and the mixture of all interfering substances was close to that of DA alone. All these results suggest that the developed ratiometric electrochemical sensor has high selectivity for DA detection. The high selectivity for DA detection is ascribed to two possible reasons. One reason is that the large electroactive surface area, good electrical conductivity, and fast electrochemical rate at the MnO2/ MWCNT/EPGCE significantly enhance the peak response current of DA, and the other is that the MnO2/MWCNT nanocomposite at the electrode improves the electrocatalytic property of the sensor, and facilitates the signal separation of analyte and other interfering substances. The intra-reproducibility and inter-reproducibility test of the MnO2/MWCNT/EPGCE for DA detection were also tested. As can be seen from Fig. S5A, one MWCNT/MnO2/EPGCE electrode is repeatedly used for five times, and the relative standard deviation (RSD) value is calculated as 4.3%. Five identically modified
Table 1 Comparison of the performance of the constructed MnO2/MWCNT/EPGCE-based ratiometric electrochemical sensor with other reported sensor for dopamine detection. Electrode b
PS/MCPE Co3O4/MWCNT/GCEc RGO/MWCNT/PTA/GCEd (SWCNT/CPB)3/GCEe CTAB-GO/MWCNT/GCEf BPVCM-e/MWCNT/GCEg poly-VA/MWCNT/GCEh IMWCNT/CPEi MnO2/MWCNT/EPGCE
Linear range (mM)
LODa (mM)
Reference
10e90 1e20 0.5e20 4e120 5e500 5e300 and 300e1000 5e120 1.9e79.4 and 79.4e714.3 1.0e50.0
9.3 0.176 1.14 0.6 1.5 2.28 4.5 0.52 0.8
[11] [38] [39] [40] [41] [42] [43] [44] This work
a
Limit of detection. Poly (sudan III) modified carbon paste electrode. c Cobalt oxide/multilwall carbon nanotube modified glassy carbon electrode. d Reduced graphene oxide/mutilwall carbon nanotube/phospotungstic acid modified glassy carbon electrode. e Multilayer films of negatively charged single-wall carbon nanotube and positively charged cetylpyridinium bromide modified glassy carbon electrode. f Hexadecyl trimethyl ammonium bromide functionalized graphene oxide/multiwalled carbon nanotube modified glassy carbon electrode. g Branched amphiphilic photosensitive and electroactive polymer/multilwall carbon nanotube modified glassy carbon electrode. h Electropolymerized vanillic acid/multilwall carbon nanotube modified glassy carbon electrode. i Indenedione derivative and multilwall carbon nanotube modified carbon paste electrode. b
Fig. 6. Selectivity of the ratiometric sensor based on MnO2/MWCNT/EPGCE. The concentration of DA and each interfering substance is 5.0 mM and 50.0 mM, respectively. The mixture contains all interfering substances.
Fig. 5. (A) DPV curves of the MnO2/MWCNT/EPGCE in 0.1 M PBS (pH 7.0) with different concentrations (0e80.0 mM) of DA. (B) Plot of the ratio of the net peak current, DI (DA)/ DI(MnO2), versus the DA concentration. Inset: the enlarged calibration plot with DA concentration of 0.5e30.0 mM.
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electrodes were used for DA detection (Fig. S5B), and the RSD value was 8.4%. All these confirm that the constructed ratiometric sensor has high reproducibility. Moreover, the MWCNT/MnO2/EPGCE electrode was stored for a week, and the related DI (DA)/DI(MnO2) kept above 90% current response. In addition, the MWCNT/MnO2/ EPGCE electrode was tested by 15 consecutive scans, and the corresponding current had no obvious change in the DI (DA)/DI(MnO2) ratio. The results show that the sensor has an acceptable stability. 3.7. Detection of DA in real samples To test the usefulness of the developed sensor, it was applied for DA determination in human serum samples. The human serum samples were collected from the local hospital. The samples were first filtered using a 0.22 mm membrane, and then centrifuged at 12,000 rpm for 30 min. Prior to the measurement, the samples were 10-fold diluted with PBS buffer solution, and then transferred to the electrochemical cell without any pretreatment process. It was found that no DA in such samples was detected by the sensor. Hence, a recovery test was done on the samples spiked with DA at different concentration levels to evaluate the feasibility of the sensor. The results are listed in Table 2. Clearly, the recoveries of the spiked samples are in the range of 94e110%, and the RSD value ranges from 1.0% to 6.0%, suggesting that the ratiometric electrochemical sensor based on MWCNT/MnO2/EPGCE has high reproducibility and precision. On the basis of the aforementioned results, it is very clear that this proposed sensor has great potential for DA detection in real samples. 4. Conclusion In this study, a ratiometric electrochemical sensor based on MnO2/MWCNT nanocomposite-modified electrode was developed for DA detection. The hierarchical MnO2 nanoflower was electrodeposited onto the MWCNT/EPGCE to prepare an inner reference electrochemical probe. The introduction of multiwalled carbon nanotube can not only improve the electrochemical signals of DA, but also increase MnO2 signals. The dual signal enhancement facilitates the construction of ratiometric electrochemical sensor for sensitive and accurate detection of DA. The possible origin of the high sensitivity for DA detection and the signal enhancement of MnO2 on MWCNT/EPGCE is reasonably explained. The results show that the high sensitivity for DA detection at MnO2/MWCNT/EPGCE predominately originates from the high surface coverage of DA at the MnO2/MWCNT/EPGCE, and the signal enhancement of MnO2 on MWCNT/EPGCE is ascribed to the electrocatalytic activity of MWCNT towards the electrodeposited MnO2 and the increase of MnO2 surface coverage. This sensor based on MnO2/MWCNT/ EPGCE displays a wide linear range from 0.5 to 30.0 mM with a LOD value of 0.17 mM. In addition, the sensor displays high selectivity, good reproducibility, and good stability, and can be successfully applied for DA detection in human serum samples. It is believed that the nanocomposite consisting of the hierarchical MnO2 nanoflower and multiwalled carbon nanotube can be further applied to construct ratiometric electrochemical sensors for the detection of other analytes of interest.
Table 2 Detection of DA in human serum samples by the proposed ratiometric sensor. Spiked (mM)
Detected (mM)a
Mean recovery (%)
RSD (%)
5.0 10.0 20.0
4.7 11.0 21.5
94 110 108
1.9 5.6 3.2
a
Mean value of three determinations.
333
Acknowledgements The authors acknowledge support from the National Natural Science Foundation of China (NSFC21864017 and NSFC21305061), the Natural Science Foundation of Jiangxi Province (20181BAB213008 and 20171BAB203018), the Education Department of Jiangxi Province (GJJ160006 and GJJ160204), the Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC-201802), the Open Project Program of State Key Laboratory of Food Science and Technology of Nanchang University (SKLF-KF-201810), and the State Key Laboratory of Chemo/Biosensing and Chemometrics of Hunan University (SKLCBC-2018007). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.124. References [1] R.A. Mitchell, N. Herrmann, K.L. Lanctot, The role of dopamine in symptoms and treatment of apathy in Alzheimer's disease, CNS Neurosci. Ther. 17 (2011) 411e427. [2] P.J. Gaskill, D.R. Miller, J. Gamble-George, H. Yano, H. Khoshbouei, HIV, Tat and dopamine transmission, Neurobiol. Dis. 105 (2017) 51e73. [3] D.J. Surmeier, Determinants of dopaminergic neuron loss in Parkinson's disease, FEBS J. 285 (2018) 3657e3668. [4] D.W. Qi, Q. Zhang, W.H. Zhou, J.J. Zhao, B. Zhang, Y.F. Sha, Z.Q. Pang, Quantification of dopamine in brain microdialysates with high-performance liquid chromatography-tandem mass spectrometry, Anal. Sci. 32 (2016) 419e424. [5] L.Y. Zhang, S.F. Qv, Z.G. Wang, J.K. Cheng, Determination of dopamine in single rat pheochromocytoma cell by capillary electrophoresis with amperometric detection, J. Chromatogr. B 792 (2003) 381e385. [6] Y.X. Guo, J. Lu, Q. Kang, M. Fang, L. Yu, Fabrication of biocompatible, luminescent supramolecular structures and their applications in the detection of dopamine, Langmuir 34 (2018) 9195e9202. [7] Y.Y. Wang, L. Yang, Y.Q. Liu, Q.B. Zhao, F. Ding, P. Zhou, H.B. Rao, X.X. Wang, Colorimetric determination of dopamine by exploiting the enhanced oxidase mimicking activity of hierarchical NiCo2S4-rGO composites, Microchim. Acta 185 (2018) 496. [8] L. Zhang, Z.R. Tang, Y.P. Dong, Silicon quantum dot involved luminol chemiluminescence and its sensitive detection of dopamine, Anal. Methods 10 (2018) 4129e4135. [9] M. Sajid, M.K. Nazal, M. Mansha, A. Alsharaa, S.M.S. Jillani, C. Basheer, Chemically modified electrodes for electrochemical detection of dopamine in the presence of uric acid and ascorbic acid: a review, Trends Anal. Chem. 76 (2016) 15e29. [10] M.Z.H. Khan, Graphene oxide modified electrodes for dopamine sensing, J. Nanomater. 2017 (2017) 8178314. [11] T.S.S.K. Naik, Fabrication of poly (Sudan III) modified carbon paste electrode sensor for dopamine: a voltammetric study, J. Electroanal. Chem. 834 (2019) 71e78. [12] H. Jin, R.J. Gui, J.B. Yu, W. Lv, Z.H. Wang, Fabrication strategies, sensing modes and analytical applications of ratiometric electrochemical biosensors, Biosens. Bioelectron. 91 (2017) 523e537. [13] T. Yang, R.Z. Yu, Y.H. Yan, H. Zeng, S.Z. Luo, N.Z. Liu, A. Morrin, X.L. Luo, W.H. Li, A review of ratiometric electrochemical sensors: from design schemes to future prospects, Sens. Actuators, B 274 (2018) 501e516. [14] E. Turkusic, J. Kalcher, E. Kahrovic, N.W. Beyene, H. Moderegger, E. Sofic, D. Begic, K. Kalcher, Amperometric determination of bonded glucose with an MnO2 and glucose oxidase bulk-modified screen-printed electrode using flow-injection analysis, Talanta 65 (2005) 559e564. [15] Y.H. Bai, H. Zhang, J.J. Xu, H.Y. Chen, Relationship between nanostructure and electrochemical/biosensing properties of MnO2 nanomaterials for H2O2/ choline, J. Phys. Chem. C 112 (2008) 18984e18990. [16] H. Liu, X.L. Yu, H.F. Chen, Y.H. Liu, Preparation of porous carbon-manganese dioxide nanocomposite for sensitive determination of cadmium ion, Int. J. Electrochem. Sci. 12 (2017) 9736e9746. [17] W. Cao, Y. Wang, Q.F. Zhuang, L.Y. Wang, Y.N. Ni, Developing an electrochemical sensor for the detection of tertbutylhydroquinone, Sens. Actuators, B 293 (2019) 321e328. [18] J. Yang, H. Lee, M. Cho, J. Nam, Y.K. Lee, Nonenzymatic cholesterol sensor based on spontaneous deposition of platinum nanoparticles on layer-by-layer assembled CNT thin film, Sens. Actuators, B 171e172 (2012) 374e379. [19] K.Q. Ding, Y.H. Wang, H.W. Yang, C.B. Zheng, Y.L. Cao, H.G. Wei, Y.R. Wang, Z.H. Guo, Electrocatalytic activity of multi-walled carbon nanotubessupported PtxPdy catalysts prepared by a pyrolysis process toward ethanol oxidation reaction, Electrochim. Acta 100 (2013) 147e156. [20] X. Dong, X.C. Lu, K.Y. Zhang, Y.Z. Zhang, Chronocoulometric DNA biosensor
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