Morphologically Tunable MnO₂ Nanoparticles Fabrication, Modelling and Their Influences on Electrochemical Sensing Performance toward Dopamine

Abstract

The morphology or shape of nanomaterials plays an important role in functional applications, especially in the electrochemical sensing performance of nanocomposites modified electrodes. Herein, the morphology-dependent electrochemical sensing properties of MnO₂-reduced graphene oxide/glass carbon electrode (MnO₂-RGO/GCE) toward dopamine detection were investigated. Firstly, various morphologies of nanoscale MnO₂, including MnO₂ nanowires (MnO₂ NWs), MnO₂ nanorods (MnO₂ NRs), and MnO₂ nanotubes (MnO₂ NTs), were synthesized under different hydrothermal conditions. Then the corresponding MnO₂-RGO/GCEs were fabricated via drop-casting and the subsequent electrochemical reduction method. The oxidation peak currents increase with the electrochemical activity area following the order of MnO₂ NWs-RGO/GCE, MnO₂ NTs-RGO/GCE, and MnO₂ NRs-RGO/GCE. The spatial models for MnO₂ NWs, MnO₂ NTs, and MnO₂ NRs are established and accordingly compared by their specific surface area, explaining well the evident difference in electrochemical responses. Therefore, the MnO₂ NWs-RGO/GCE is selected for dopamine detection due to its better electrochemical sensing performance. The response peak current is found to be linear with dopamine concentration in the range of 8.0 × 10⁻⁸ mol/L–1.0 × 10⁻⁶ mol/L and 1.0 × 10⁻⁶ mol/L–8.0 × 10⁻⁵ mol/L with a lower detection limit of 1 × 10⁻⁹ mol/L (S/N = 3). Finally, MnO₂ NWs-RGO/GCE is successfully used for the determination of dopamine injection samples, with a recovery of 99.6–103%. These findings are of great significance for understanding the relationship between unlimited nanoparticle structure manipulation and performance improvement.

1. Introduction

Dopamine is an important neurotransmitter that plays a vital role in the regulation of cognitive and neuroendocrine functions, as well as emotions and sleep [1]. An abnormal level of dopamine may cause several central nervous system diseases such as depression, anxiety, schizophrenia, and Parkinson’s disease [2,3,4,5]. Therefore, it is of great significance to detect dopamine accurately at a physiological level in the earlier prevention and clinical diagnosis of these neurological diseases. To date, various techniques have been developed to detect dopamine, including but not limited to high performance liquid chromatography [6], mass spectrometry, fluorescent spectrometry [7], capillary electrophoresis [8,9], and electrochemiluminescence [10,11]. Despite being reliable and precise, these methods often suffer from drawbacks such as expensive instruments, complex and time-consuming analytical procedures, and the requirement of experienced technical staff. Recently, electrochemical methods have been widely applied to detect biomolecules, contaminants, and food additives, due to overwhelming advantages including cost- and time- effectiveness, rapidness and simplicity, as well as good selectivity and sensitivity. The key issue to electroanalytical chemistry lies in the development of ultrasensitive modified electrodes. Precious metal nanoparticles and nanoalloys modified electrodes have exhibited superior sensing performances (i.e., wide linear ranges, low detection limit, good stability and selectivity) [12,13,14], but their scarcity and cost has limited practical applications. To solve this problem, various transition metals or metal oxides modified electrodes have become ideal alternative modification materials [15,16,17,18].

MnO₂, an important transition metal oxide, has been extensively used in rechargeable batteries [19], supercapacitors [20,21], electrocatalysis [22,23], and sensors [24,25,26,27,28], owing to its prominent advantages such as cheapness, low toxicity, and excellent electrocatalytic performances. Moreover, the electrochemical performance can be tailored by tuning the morphologies of nano-MnO₂ [25]. However, the poor electrical conductivity has hindered its broad applications in electrochemical sensors, due to the semiconductor property of itself [26]. Hence, much effort has been devoted to composite or hybrid nano-MnO₂ with conductive components, aiming to enhance electrical conductivity, decrease charge transfer resistance, and eventually improve sensing performances [20,21,29]. Graphene, an emerging 2D carbon nanomaterial, has become one of the most preferred electrode modification materials due to its large surface area, high electrical conductivity, and rapid heterogeneous electron transfer rate [27,30,31]. In recent years, MnO₂-graphene nanocomposite modified electrodes have been widespread in electrochemical sensors. For example, Wu and co-workers developed a non-enzymatically catalyzed H₂O₂ sensor based on a MnO₂/reduced graphene oxide nanoribbons composite modified electrode, which exhibits excellent electrochemical performance and high precision, as well as good selectivity, reproducibility, and stability [26]. Mahmoudian et al. constructed a H₂O₂ sensor based on MnO₂ nanotubes/reduced graphene oxide nanocomposite modified glassy carbon electrode, and the charge transfer resistance reduced significantly [24]. α-MnO₂ nanorods/reduced graphene oxide modified glassy carbon electrodes have been used to detect uric acid with satisfactory results, even in the presence of large amounts of ascorbic acid [32]. In addition, MnO₂ nanowire/chitosan modified gold electrodes [28] and Pt nanodendrites/graphene/MnO₂ nanoflower modified electrodes [33] have also been successfully used for the detection of dopamine, but precious metal (i.e., Au, Pt) components are very expensive. To our best knowledge, MnO₂-graphene binary composite modified electrodes for dopamine detection is rarely reported.

The morphology or shape of nanomaterials play a vital role in the electrochemical sensing performance of modified electrodes. For example, the surface adsorption is tuned via controlled preparation of various shapes of hematite α-Fe₂O₃ (shuttle-like, pseudo-shuttle-like, polyhedron-like, and drum-like α-Fe₂O₃), and shuttle-like Fe₂O₃ exhibits better electrochemical detection ability than other shapes of Fe₂O₃ nanoparticles [34]. Various morphologies of manganese dioxide (MnO₂) electrocatalysts, including nanoflowers, nanorods, nanotubes, nanoplates, nanowires, and microspheres were prepared via facile hydrothermal synthesis and precipitation methods, and their electrochemical properties were found to be strongly dependent on the morphology [35]. Among these various morphologies, the nanoflowers-like MnO₂, coupled with GO, exhibited relatively high sensitivity toward the simultaneous determination of guaiacol and vanillin [35]. However, the effect of different morphologies of nano-MnO₂ on the dopamine detection is not yet clear. Thus, it is well worth further investigation.

Herein, three different morphologies of nano-MnO₂, including nanowires (MnO₂ NWs), MnO₂ nanorods (MnO₂ NRs), and MnO₂ nanotubes (MnO₂ NTs), were prepared by hydrothermal method firstly, then composited with graphene oxide (GO) to obtain nano-MnO₂ counterparts. The MnO₂-reduced graphene oxide modified glass carbon electrode (MnO₂-RGO/GCE) was prepared by drop-casting MnO₂-GO dispersion on the surface of GCE and subsequently an electrochemical reduction process [36,37,38]. The response peak currents were measured by second-order derivative linear sweep voltammetry in dopamine solution. The corresponding peak current densities were estimated by dividing by their electrochemical active area, which was obtained from the cyclic voltammograms using [Fe(CN)₆]^3−/4− as redox probe. The response peak currents and peak current densities among three various morphologies of MnO₂-RGO/GCE have been compared to find out the main factors on the enhancement of electrochemical response. The response peak current is supposed to be proportional to the surface area of the modified electrode, because of the presence of an adsorption-controlled process during electrochemical oxidation of dopamine [34,37,38,39,40,41]. Based on the hypothesis, spatial models for three various morphologies of nano-MnO₂ were established to compare their specific surface area, and find the main factors on the specific surface area. Finally, the MnO₂-RGO/GCE with the largest response peak current was chosen to detect dopamine in real samples.

2. Results

2.1. Materials Characterization

The surface morphologies of MnO₂ NRs, MnO₂ NTs, MnO₂ NWs and their corresponding MnO₂-RGO nanocomposites were characterized by scanning electron microscopy (SEM, Hitachi S-3000N, Tokyo, Japan). The SEM images of MnO₂ NRs, MnO₂ NTs, and MnO₂ NWs are shown in Figure 1A–C, respectively. MnO₂ NTs show distinct hollow tubular structures with uniform diameters (Figure 1B). MnO₂ NWs show obvious line-like structures, and the diameter is uniform (Figure 1C). It can been seen clearly that thin sheets are attached to the surface of MnO₂ NRs, MnO₂ NTs, and MnO₂ NWs (Figure 1D–F) when they were composited with RGO.

The MnO₂ NRs, MnO₂ NTs, and MnO₂ NWs were further characterized by X-ray diffractometer (XRD, PANalytical, Almelo, Holland), operating at 40 kV and 40 mA with Cu Kα radiation (λ = 0.1542 nm). The XRD patterns of MnO₂ NRs, MnO₂ NTs, and MnO₂ NWs are presented in Figure 2. All of the nano-MnO₂ appears with obvious diffraction peaks. These diffraction peaks are located at 2θ of 12.78°, 17.68°, 28.28°, 37.48°, 42.36°, 49.94°, 56.34°, 60.16°, and 69.08°, indexing into (110), (200), (310), (211), (301), (411), (600), (521), and (541) facets (JSPDS44-0141), indicating that tetrahedral crystalline α-MnO₂ was synthesized.

2.2. Voltammetric Responses of Dopamine on the MnO₂-RGO/GCEs

The second-order derivative linear sweep voltammetric (SDLSV) response of 1 × 10⁻⁵ mol/L dopamine on the bare GCE, RGO/GCE, MnO₂ NRs-RGO/GCE, MnO₂ NTs-RGO/GCE, and MnO₂ NWs were summarized in

Table 1. The second-order derivative linear sweep voltammetric (SDLSV) response of dopamine on the different electrodes ^a.

Electrodes	Epa/mV b	Ipa/μA c	Jpac/(μA/cm2) d
Bare GCE	392	1.396	——
RGO/GCE	444	22.56	——
MnO2 NRs-RGO/GCE	452	25.74	272.13
MnO2 NTs-RGO/GCE	452	27.86	265.33
MnO2 NWs-RGO/GCE	440	30.26	232.77

^a The second-order derivative linear sweep voltammetry for the bare GCE, RGO/GCE, MnO₂ NRs-RGO/GCE, MnO₂ NTs-RGO/GCE and MnO₂ NWs were recorded in the 0.1 mol/L PBS containing 1 × 10⁻⁵ mol/L dopamine at 100 mV/s. ^b E_pa denotes oxidation peak current. ^c I_pa denotes oxidation peak current. ^d J_pa denotes oxidation peak current density.

, respectively. The oxidation peak of dopamine on the bare GCE is very weak, and the response current is 1.396 μA. The response peak of dopamine on the RGO/GCE is obvious and the oxidation peak current increases to 22.56 μA, which is an order of magnitude greater than that of the bare GCE. The significant increase in the oxidation peak current is highly related to excellent electrical conductivity and large specific surface area of RGO. The electrical conductivity of RGO is enhanced greatly due to the restoration of the conductive carbon-conjugate networks, which will decrease the electron transfer resistance and accelerate the electron transfer rate. The adsorption capacity also increase due to the large specific surface area of RGO. Besides, the presence of residual oxygen containing functional groups will also promote the dopamine adsorption with the help of π–π interaction. The response peak current of MnO₂-RGO/GCE further increases, which is mainly due to the synergistic enhancement effect between nano-MnO₂ and RGO. The extra increase mainly arises from the excellent electrocatalytic performance of nano-MnO₂. Among three different morphologies of nano-MnO₂ composited with RGO, the largest oxidation peak current is obtained at MnO₂ NWs-RGO/GCE, the second one on the MnO₂-NTs-RGO, and the smallest one on the MnO₂ NRs-RGO/GCE. The variation among different morphologies depends on electrochemical active area and electrocatalytic activity per unit area.

2.3. Electrochemical Active Area of MnO₂-RGO/GCEs

In order to compare the electrochemical active area of different electrodes, the cyclic voltammograms of the MnO₂ NRs-RGO/GCE, MnO₂ NTs-RGO/GCE, and MnO₂ NWs-RGO/GCE were investigated in the 0.1 mol/L PBS solution containing 1 × 10⁻³ mol/L [Fe(CN)₆]^3−/4− (Figure 3). Their reduction peak currents (i_pc) are 2.158 × 10⁻⁵ A, 1.738 × 10⁻⁵ A, and 1.375 × 10⁻⁵ A, respectively. According to Randles-Sevcik Equation [42]

$$i_{\mathrm{pc}}=(2.69\times {10}^5)n^{3/2}{D}^{1/2}{v}^{1/2}AC$$

(1)

where i_pc is the reduction peak current of K₃[Fe(CN)₆] (A); n is the electron transfer number during the redox process; A is the electrochemical active area (cm²); D is diffusion coefficient of K₃[Fe(CN)₆] (D = 7.6 × 10⁻⁶ cm²·s⁻¹ [43]); C is the concentration of K₃[Fe(CN)₆] (mol·cm⁻³); v is the scanning rate (V·s⁻¹). Therefore, the electrochemical active areas of MnO₂ NWs-RGO/GCE, MnO₂ NTs-RGO/GCE and MnO₂ NRs-RGO/GCE were 0.130 cm², 0.105 cm² and 0.0829 cm², respectively, which are much larger than those of bare GCE (Φ3.0 mm, 0.0710 cm²). It means that the MnO₂-RGO composite film increases the specific surface area. The electrochemical active area follows the order of MnO₂ NWs-RGO/GCE > MnO₂ NTs-RGO/GCE > MnO₂ NRs-RGO/GCE. The order of electrochemical active area is consistent with that of their corresponding peak current i_pa, confirming that the response peak current (i_pa) is closely related to the electrochemical active area. The large electrochemical active area not only facilitates the accumulation of dopamine on the electrode surface, but also increases the catalytic sites on the surface of the modified electrodes.

In order to explore the effect of electrocatalytic activity on the response peak current, the peak current densities are estimated by dividing by their electrochemical area, aiming to exclude the influence of the electrochemical active area. The peak current densities (J_a) of MnO₂ NWs-RGO/GCE, MnO₂ NRs-RGO/GCE, and MnO₂ NTs-RGO/GCE are listed in the fourth column in Table 1. Interestingly, the order of peak current densities is the reverse of that of the electrochemical active area, namely MnO₂ NRs-RGO/GCE > MnO₂ NTs-RGO/GCE > MnO₂ NWs-RGO/GCE. It implies that the response peak current is also highly related to the electrocatalytic activity. The peak current density on the MnO₂ NWs-RGO/GCE is smallest while the peak current is largest, suggesting the electrochemical active area plays a major role in enhancing the electrochemical response towards dopamine.

Why does there exist such an evident disparity between different morphological MnO₂ nanomaterials? What is the driving force to differentiate electrochemical performances between these three types of morphology? We are trying to simulate their spatial models to deeply reveal the secrets behind them.

2.4. Spatial Models for Various Morphologies of Nano-MnO₂

It has been reported that MnO₂ is the reaction active site for the oxidation of electrochemically active species [44]. As is well known, electrocatalytic oxidation of dopamine on the surface of modified electrodes is mainly controlled by the adsorption process [34,37,38,39,40,41]. Hence, the response peak current can be calculated according to the Laviron Equation [45]:

$$i_{\mathrm{p}}=\frac{n^2{F}^2\Gamma }{4RT}Av$$

(2)

where Г denotes surface coverage, n denotes the electron transfer number, A denotes the surface area of the electrode, v denotes the scan rate, and T denotes the Kelvin temperature. The surface coverage (Г) can be considered as a constant due to the same bulk dopamine concentration for three morphologies of MnO₂.

Nanorods and nanowires are generally distinguished by their length-to-diameter (l/d) ratio [46,47]. Nanomaterials with l/d ratio of 1–10, diameter (d) less than 100 nm, and length (l) less than 1000 nm are often defined as nanorods. Nanomaterials with l/d ratio greater than 10, d less than 100 nm, and l greater than 1000 nm are often referred as nanowires. Nanorods with a hollow structure and l/d ratio of about 1–10 are often called nanotubes. In order to investigate the specific surface area among different morphologies of nano-MnO₂, cylinder models were proposed to simulate the MnO₂ NWs, MnO₂ NTs, and MnO₂ NRs (Figure 4).

The surface area (S), volume (V) and specific surface area of cylinder (S/V) are calculated according to Equations (3)–(5):

$$S=\pi dl+\frac{\pi d^2}{2}$$

(3)

$$V=\frac{\pi d^{2}l}{4}$$

(4)

$$\frac{S}{V}=\frac{\pi dl+\frac{\pi d^2}{2}}{\frac{\pi d^{2}l}{4}}=\frac{4l+2d}{dl}$$

(5)

Assuming that the MnO₂ NWs, MnO₂ NRs, and MnO₂ NTs are of equal volume, only their surface areas need to be compared. d₁ and d₂ denotes the diameter of the MnO₂ NWs and MnO₂ NRs. d₃ and d₄ denote the outer and inner diameter of MnO₂ NTs. l₁, l₂, and l₃ represents the length of MnO₂ NWs, MnO₂ NRs, and MnO₂ NTs, respectively.

The surface areas of MnO₂ NWs (S₁), MnO₂ NRs (S₂), and MnO₂ NTs (S₃) are calculated according to Equations (6)–(8):

$$S_1=\pi d_1{l}_1+\frac{\pi d_1{}^2}{2}$$

(6)

$$S_2=\pi d_2{l}_2+\frac{\pi d_2{}^2}{2}$$

(7)

$$S_3=\pi d_3{l}_3+\frac{\pi (d_3{}^2-d_4{}^2)}{2}+\pi d_4{l}_4$$

(8)

The volume (V) are assumed to be the same for MnO₂ NWs, MnO₂ NRs, and MnO₂ NTs, and the volumes (V) are calculated as follows:

$$V=\frac{\pi d_1{}^2{l}_1}{4}=\frac{\pi d_2{}^2{l}_2}{4}=\frac{\pi d_3{}^2{l}_3}{4}$$

(9)

The difference on surface areas between MnO₂ NWs and MnO₂ NRs:

$$S_1-S_2=\pi d_1{l}_1+\frac{\pi d_1{}^2}{2}-\pi d_2{l}_2-\frac{\pi d_2{}^2}{2}$$

(10)

According to Equation (9), the d₂ can be expressed as Equation (11):

$$d_2=\sqrt{\frac{d_1{}^2{l}_1}{l^2}}$$

(11)

d₂ in the Equation (10) is replaced with Equation (11) and Equation (10) was simplified to Equation (12).

$$S_1-S_2=\pi d_1(l_1-\sqrt{l_1{l}_2})+\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})$$

(12)

Obviously, l₂ for nanorods is less than l₁ for nanowires, we can conclude that

$$\pi d_1(l_1-\sqrt{l_1{l}_2})>0,\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})<0$$

(13)

To further compare the first part and second part of Equation (12), the first part was divided by the negative of the second part,

$$\frac{\pi d_1(l_1-\sqrt{l_1{l}_2})}{-\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})}=\frac{2l_2(l_1-\sqrt{l_1{l}_2)}}{d_1(l_1-l_2)}=\frac{2l_2}{d_1}\cdot \frac{l_1-\sqrt{l_1{l}_2}}{l_1-l_2}$$

(14)

From the definitions of nanorods, nanowires and nanotubes, the boundary conditions are as follows:

$$\begin{array}{l}1\mathrm{nm}<d_{\mathrm{i}=1,2,3}<100\mathrm{nm}\\ 0\mathrm{nm}<d_4<d_3\\ l_1>1000\mathrm{nm};l_{i=2,3}<1000\mathrm{nm}\\ \frac{l_1}{d_1}>10>\frac{l_2}{d_2}>1\end{array}$$

According to Equation (9), we can infer that d₁ < d₂. Combining with the boundary conditions, we can conclude that

$$\frac{2l_2}{d_1}>\frac{2l_2}{d_2}>2(d_2>d_1,1<\frac{l_2}{d_2}<10),\frac{l_1-\sqrt{l_1{l}_2}}{l_1-l_2}>\frac{1}{2}(when\frac{l_1}{l_2}>1)$$

(15)

Hence, $\frac{\pi d_1(l_1-\sqrt{l_1{l}_2})}{-\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})}>1$, namely

$$S_1-S_2=\pi d_1(l_1-\sqrt{l_1{l}_2})+\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})=\pi d_1(l_1-\sqrt{l_1{l}_2})-[-\frac{\pi d_1{}^2}{2}(1-\frac{l_1}{l_2})]>0$$

(16)

Hence, S₁ − S₂ > 0, meaning that the surface area of MnO₂ NWs (S₁) is larger than that of MnO₂ NRs (S₂) when they have the same volume. Considering that l₁ and l₂ is in the microscale and nanoscale, d₁ and d₂ are in the same order of magnitude, the influence of the length on the surface area is more significant. In other words, the difference in surface area between the MnO₂ NWs and the MnO₂ NRs mainly arise from the lateral area rather than basal areas. The surface area of MnO₂ NTs (S₃) is relatively larger than that of MnO₂ NRs (S₂) when they have the same volumes, due to the presence of inner surfaces in MnO₂ NTs. Similarly, the surface area of the MnO₂ NWs (S₁) can be roughly regarded to be larger than that of the MnO₂ NTs (S₃), due to l₁ (microscale) being much longer than l₃ (nanoscale). In summary, the specific surface area follows the sequence of MnO₂ NWs > MnO₂ NTs > MnO₂ NRs. Correspondingly, the order of surface areas of MnO₂-RGO nanocomposites increases in the following order: MnO₂ NWs-RGO, MnO₂ NTs-RGO and MnO₂ NRs-RGO, which is consistent with the result of CV in the [Fe(CN)₆]^3−/4− system. According to the Laviron equation, the response peak current (i_pa) is proportional to the surface area. As a result, the largest response current (i_pa) is obtained on the MnO₂ NWs-RGO/GCE, and the smallest one on the MnO₂ NRs-RGO/GCE.

The diameter and length of nano-MnO₂ with different morphologies can be acquired from the SEM images, and their average specific surface areas were also estimated according to Equation (5) (Figure 4). The specific surface areas of MnO₂ NWs, MnO₂ NTs, and MnO₂ NRs are 0.133 nm⁻¹, 0.116 nm⁻¹, and 0.0940 nm⁻¹, respectively. It can also be inferred that the specific surface area of nano-MnO₂ follows the order of MnO₂ NWs > MnO₂ NTs > MnO₂ NRs, which is consistent with the result from the proposed spatial models. To conclude, the variation on the peak current among MnO₂-RGO nanocomposites with different morphologies mainly comes from the specific surface area, dominating by lateral area. Therefore, MnO₂ NWs-RGO/GCE was employed to detect dopamine in subsequent experiments.

2.5. Electrochemical Kinetics of Dopamine on MnO₂ NWs-RGO/GCE

Cyclic voltammograms of 1 × 10⁻⁵ mol/L dopamine on the MnO₂ NWs-RGO/GCE were recorded at various scanning rates, and the results are shown in Figure 5A. It is observed that a pair of redox peaks appear on the MnO₂ NWs-RGO/GCE. With the increasing of scanning rates, the oxide peak currents shift toward a positive direction while the reduction peak currents shift negatively. In other words, the peak separation increases with the scanning speed, suggesting the electrochemical oxidation of dopamine on the MnO₂ NWs-RGO/GCE is a quasi-reversible process. As presented in Figure 5B, the oxidation peak currents (i_pa) increase linearly with scanning rate (v), suggesting the electrochemical oxidation of dopamine is an adsorption-controlled process. This result is consistent with previous reports [34,37,38,39,40,41], and also favors our assumption for model simulation.

2.6. Electrochemical Sensing Perfomances of MnO₂ NWs-RGO/GCE

The pH of supporting electrolytes plays an important role in the electrochemical oxidation of dopamine, so the pH dependence of the dopamine response was also investigated. As shown in Figure 6, the maximum response peak current was obtained at pH 3.0. Hence, pH 3.0 is selected for quantitative analysis. Since dopamine oxidation is an adsorption-controlled process, accumulation was often used to improve the sensitivity. The electrochemical sensing performances of MnO₂ NWs-RGO/GCE were investigated using second-order derivative linear sweep voltammetry, and the results have been published in Chinese Journal of Analytic Chemistry [48]. The MnO₂ NWs-RGO/GCE exhibits two linear response ranges (6 × 10⁻⁸ mol/L–1 × 10⁻⁶ mol/L and 1 × 10⁻⁶ mol/L–8 × 10⁻⁵ mol/L) and a low detection limit (S/N = 3, 1.0 × 10⁻⁹ mol/L) towards dopamine detection. A comparison of the electrochemical sensing performance between the MnO₂ NWs-RGO/GCE and previous reports is summarized in

Table 2. Comparison of the electrochemical sensing performances between the MnO₂ NWs-RGO/GCE and previous reports.

Electrodes	Method	Linear Range (μM)	Detection Limit (μM)	Ref.
MnO2 nanowires/chitosan-modifed gold electrode	CA a	0.10–12.0	0.04	[28]
ZnO-modified carbon paste electrode	DPV b	0.1–20	0.03	[51]
Cu2O/graphene-modified glassy carbon electrode	CV c	0.3–1.4; 2–20	0.055	[16]
CuO-modified carbon paste electrode	DPV b	0.1–10	0.01	[49]
Mn3O4-modified graphite electrode	DPV b	10–70	0.1	[52]
SWCNT/Fe2O3-modified graphite electrode	SWV d	3.2–31.8	0.36	[53]
rGO-Mn3O4/Nafion film supporting Au nanoparticles modified gold electrode	CA a	1.0–1450	0.25	[50]
Pt nanodendrites/reduce graphene oxide/MnO2 nanoflowers modified glassy carbon electrode	DPV b	1.5–215.56	0.1	[33]
MnO2 NWs-ErGO/GCE	SDLSV e	0.06–1.0 1.0–80	0.001	This work

^a CA: chronoamperometry. ^b DPV: differential pulse voltammetry. ^c CV: cyclic voltammetry. ^d SWV: square wave voltammetry. ^e SDLSV: second-order derivative linear sweep voltammetry.

. Remarkably, the detection performance of the MnO₂ NWs-RGO/GCE is comparably even better than previous reports [16,28,33,49,50,51,52,53]. Last but not least, the linear response ranges basically overlap with the physiological level of dopamine (generally 10⁻⁶ mol/L to 10⁻⁸ mol/L [54]), indicating that MnO₂ NWs-RGO/GCE shows great prospects for dopamine detection in various real samples such as brain fluids, blood serum, and urine. The MnO₂ NWs-RGO/GCEs were successfully applied to detect dopamine in dopamine hydrochloride injection samples with a recovery rate of 99.6%~103%. Together with low cost, rapidness, and simplicity, the MnO₂ NWs-RGO/GCEs are expected to detect dopamine in various real samples.

3. Materials and Methods

3.1. Materials and Chemicals

Graphite powder, potassium permanganate (KMnO₄), manganese sulfate monohydrate (MnSO₄·H₂O), ammonium persulfate ((NH₄)₂S₂O₈), concentrated sulfuric acid (H₂SO₄), concentrated hydrochloric acid (HCl), hydrogen peroxide (H₂O₂), potassium ferricyanide (K₃[Fe(CN)₆]), potassium ferrocyanide (K₄[Fe(CN)₆]), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate (Na₂HPO₄), sodium hydroxide (NaOH), and ethyl alcohol were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Dopamine was purchased from Sigma-Aldrich Co (St. Louis, CA, USA). All these reagents were used as received without further purification, and deionized water was used throughout the experiments.

3.2. Preparation of Nano-MnO₂ with Various Morphologies

3.2.1. Preparation of MnO₂ NRs

To begin, 0.1 mol MnSO₄·H₂O and 0.1 mol KMnO₄ were dissolved in 30 mL of deionized water. Then, 1 mL of 60% H₂SO₄ was added into the above mixture solution and stirred for 30 min, afterwards the mixture solution was transferred to a Teflon-lined stainless-steel autoclave and reacted at 150 °C for 30 min. The resulting product was centrifuged at 8000 rpm for 30 min, washed alternately by deionized water and ethanol three times, and vacuum-dried at 60 °C to obtain MnO₂ NRs. Finally, MnO₂ NRs were dispersed into deionized water to form 1 mg/mL dispersion solution.

3.2.2. Preparation of MnO₂ NTs

Firstly, 0.45 g KMnO₄ and 1 mL of concentrated hydrochloric acid was added into 40 mL of deionized water, then the mixture solution was stirred for 20 min. Afterwards the reaction solution was transferred into a Teflon-lined stainless-steel autoclave and autoclaved at 80 °C for 10 h. The resulting product was centrifuged at 8000 rpm for 30 min, washed repeatedly three times with deionized water and ethanol, and dried under vacuum at 60 °C to obtain MnO₂ NTs. Finally, MnO₂ NTs were dispersed into deionized water to form 1 mg/mL dispersion solution.

3.2.3. Preparation of MnO₂ NWs

Briefly, 0.008 mol MnSO₄·H₂O and 0.015 mol (NH₄)₂S₂O₈ were dissolved into 35 mL of deionized water, and the mixture was sealed into to a Teflon-lined stainless-steel autoclave and reacted at 120 °C for 10 h. The as-obtained product was centrifuged at 8000 rpm for 30 min, washed repeatedly three times with deionized water and ethanol, and vacuum-dried at 60 °C to obtain MnO₂ NWs. Finally, MnO₂ NWs were dispersed into deionized water to obtain 1 mg/mL dispersion solution.

3.3. Preparation of MnO₂-GO Nanocomposite Dispersion

GO was prepared from cheap graphite powder by modified Hummers method referred to our previous reports [36,37,38]. The resulting GO was dispersed in 100 mL of deionized water under ultrasonication for 2 h, centrifuged twice at 6000 rpm to remove trace precipitates, and the supernatant was taken out to obtain a gold GO solution with approximately 1 mg/mL. Then, 1 mL of MnO₂ NRs, MnO₂ NTs, and MnO₂ NWs dispersion (1 mg/mL) was added into 20 mL GO dispersion (1 mg/mL) and ultrasonicated for 2 h to obtain the corresponding nano-MnO₂-GO dispersion.

3.4. Fabrication of MnO₂-RGO Modified Electrodes

Prior to electrode modification, the bare glassy carbon electrodes (GCEs) were polished to form mirror-like surfaces with 0.05 μm Al₂O₃ fine particles. Then the polished GCEs were subjected to ultrasonication in deionized water and absolute ethanol (each for 1 min), and dried by pure N₂ gas. 5 μL of nano-MnO₂-GO dispersions were dropped on the surface of the GCEs, and dried with an infrared lamp to obtain nano MnO₂-GO modified glass carbon electrodes (i.e., MnO₂ NRs-GO/GCEs, MnO₂ NRs-GO/GCEs, and MnO₂ NRs-GO/GCEs). Afterwards the MnO₂-GO/GCEs were electrochemically reduced into MnO₂-RGO/GCEs by the potentiostatic method. Specifically, the nano-GO/GCEs was immersed into 0.1mol/L PBS solution (pH 7.0), and then electrochemically reduced at a fixed potential of −1.5 V for 120 s. For comparison, RGO modified glass carbon electrodes (RGO/GCE) were fabricated by similar method.

3.5. Electrochemical Measurements

All the electrochemical experiments were carried out with standard three-electrode setup, consisting of bare GCEs, RGO/GCEs, or MnO₂-RGO/GCEs as working electrode, a saturated calomel electrode (SCE) as reference electrode, and platinum wire electrode acted as auxiliary electrode. The electrochemical responses of 10 μM dopamine on the different electrodes were evaluated by second-order derivative linear sweep voltammetry. The electrochemical active areas of MnO₂ NWs-RGO/GCEs, MnO₂ NTs-RGO/GCEs, and MnO₂ NRs-RGO/GCEs were estimated by recording cyclic voltammograms in 1 mM [Fe(CN)₆]^3−/4− probe solution.

4. Conclusions

In this study, three different morphologies of MnO₂ nanomaterials (MnO₂ NWs, MnO₂ NTs, and MnO₂ NRs) were prepared and then composited with GO to obtain MnO₂-RGO counterparts. MnO₂-RGO/GCE were fabricated by drop-casting MnO₂-RGO dispersion on the surface of polished GCE and subsequent electrochemical reduction method. Both the response peak currents and electrochemical active areas increase in the following order of MnO₂ NWs-RGO/GCE, MnO₂ NTs-RGO/GCE, and MnO₂ NRs-RGO/ GCE, while the response peak current densities increase in the reverse order. Morphology-dependent electrochemical sensing properties toward dopamine were well explained by establishing spatial models for MnO₂ NWs, MnO₂ NTs, and MnO₂ NRs. By comparing the specific surface areas of these three morphologies of MnO₂ nanomaterials, the enhancement of response peak currents mainly arises from the specific surface area, dominating by length to diameter ratio. The higher length to diameter ratio favors the electrochemical sensing toward dopamine, which provides valuable technical guidance for the development of novel electrode modification materials. The MnO₂ NWs/GCE was chosen for detection of dopamine due to having the largest response peak current. The MnO₂ NWs/GCEs exhibit wide linear dynamic ranges (6.0 × 10⁻⁸ mol/L ~ 1.0 × 10⁻⁶ mol/L and 1.0 × 10⁻⁶ mol/L ~ 8.0 × 10⁻⁵ mol/L) and a low detection limit (1 × 10⁻⁹ mol/L). Finally, MnO₂ NWs-RGO/GCEs were successfully used for the determination of dopamine injection samples with satisfactory results.