Fabrication of Screen-Printed Electrodes Modified by Hydrothermal MnO2 Microflowers and Carbon for Electrochemical Sensors in Copper Ions Detection

Abstract

Porous MnO₂ microflowers with a hexagonal crystalline structure were facilely prepared at a low hydrothermal temperature of 90°C, without using any template or capping agent. The as-prepared MnO₂ only presented an excellent detection ability for copper (II) by a square wave anodic stripping voltammetry in the presence of super P carbon black as conducting agent, and Nafion as binder. In the present work, to evaluate the detection ability of copper (II) in the MnO₂ microflowers, chips of screen-printed electrodes (SPEs) having a polyurethane substrate, a silver working electrode, a carbon counter electrode, and a silver pseudoelectrode, were designed. Then, the SPEs chips were modified with MnO₂ microflowers and/or super P carbon and used as electrochemical sensors for the detection of copper (II) present in water sources. From the measured results, the fabricated sensors with excellent copper detection in a linear range from 0.625 nM to 15 nM (R² = 0.9737), and a low detection limit (0.5 nM), high sensitivity (214.05 μA/cm² nM), and rapid response (180 s) demonstrated high application potential for electrochemical sensors in the detection of copper in water resources.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

According to a report of the World Health Organization (WHO), when a human being drinks water sources containing copper (Cu) ions with a concentration above 1.3 mg L⁻¹, the symptoms of digestive disorders such as nausea and vomiting appear [1]. Copper accumulated in the liver and other organs causes liver failure, nervous breakdown, loss of vision, muscle atrophy, and kidney failure. [2]. Copper is a natural element present in domestic water sources. Currently, in many rural areas of Vietnam, people still have the habit of using untreated borehole water sources. Very possibly, these water sources contain an abundant amount of copper and other heavy metals arising from wastewater pollution problems from manufacturing factories, companies, or farming activities. Long-term use of water sources contaminated with heavy metals exceeding the allowable limit can cause many health risks. Also, according to Vietnam standards issued by the Ministry of Health, the copper content in domestic water must not exceed a threshold of 1 mg L⁻¹, similar to some other countries in the world [2–4].

Currently, there are some used methods to detect and analyze the copper content in water, for example, high performance liquid chromatography (HPLC) [5], inductively coupled plasma mass spectrometry (ICP-MS) [6] or ultraviolet-visible spectroscopy (UV-VIS) [7]. Each of the above methods has its own advantages, but basically, they can detect and determine the trace amount of copper in water. However, these methods also suffer some disadvantages, viz. requirement of bulky and complex measurement device systems and professionally trained operators, or the difficulty in developing portable devices for in-field analyses. In very recent years, the development of electrochemical sensors to detect heavy metals and toxic organic compounds in water has been among the research study directions of great interest [8–10]. Compared with the above analysis methods, the electrochemical analysis method is more reliable, more economic, and more suitable for in-field analysis applications. The electrochemical analysis method demonstrates a simple analysis and sampling procedure. For this analysis method, small electrical circuit boards can be easily produced and integrated into small portable devices, which are favorable for in-situ monitoring of contaminated water samples. Furthermore, the electrochemical analysis techniques enable shorter analysis times as well as online water monitoring [9].

To date, various electrode active materials have been investigated for electrochemical sensors to detect copper (II). The materials include novel metals (nanosilver and nanogold) [11, 12], silica-based materials [13], carbon-based materials (graphite, carbon nanotube, graphene) [13–17], and metal oxides (Sb₂O₃, MnO₂, SnO₂, and Fe₃O₄) [18–21]. Among them, manganese dioxide (MnO₂) has gained great attraction due to its merits such as cheap cost, simple synthesis, low toxicity, and environmental friendliness. Accordingly, the MnO₂ materials with various morphologies such as zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanotubes, nanorods, nanowires and two-dimemsional (2D) nanosheets [22], and 3D complex architectures, for example, nanoflowers [23], sea-urchins [24, 25], hollow microspheres [26], and nanobowls [22, 27, 28] have synthesized and proved excellent heavy metal detection abilities.

Besides, for the sensor applications, the MnO₂ material reveals a disadvantage of poor electrical conductivity. To overcome this drawback, coupling MnO₂ with another conducting agent to produce composites has been proposed, for instance, the coupling of MnO₂ with graphene oxide (GO) or reduced graphene oxide (RGO) [28, 29]. Introduction of graphene or GO into the MnO₂ material not only enhances the electrical conductivity property but also increases surface area and improves chemical stability for the synthesized composite materials. Unfortunately, the practical synthesis of MnO₂/GO composites is very difficult. In addition, the obtained composites normally suffer the nonuniform distribution of MnO₂ on the GO supporting material. Thus, the reliability in reproductivity and repeatability of the electrochemical sensors based on such MnO₂/GO composites are insufficiently high. To fabricate a stable and durable electrochemical sensor, Hao and coworker synthesized MnO₂ nanowires on a nickel foam substrate using the hydrothermal method for copper detection in water [20]. The sensor based on the MnO₂ nanowires/nickel foam electrode posed a limit of copper detection at 0.17 μM. This value is even lower than the minimum value of copper concentration, 0.23 μM, present in drinking water as recommended by the WHO. Despite the initiation of a new investigation on MnO₂ sensors that were fabricated directly on a nickel substrate, the copper detection limit of the mentioned sensors was reported to be much lower than that of the sensors fabricated by the conventional coating methods, for example, the slurry casting method, the paint-coating method, or the dropping method, which often employ a glassy carbon substrate or a carbon paste substrate as a current collector. In addition to these substrates, a nickel foam substrate with a larger surface area and high porosity was also employed as a current collector for an electrode used for the copper detection sensors. However, if used for the in-field devices, the nickel foam substrate with low anticorrosion suffers from unstable detection abilities and low reliability regarding reproductivity and repeatability of measurements.

In this context, we recognize that super P (SP) carbon black possesses good adsorption and a high specific surface area, leading to its popular utilization in lithium ion batteries as a conducting agent [30, 31]. If so, inclusion of SP carbon into the working electrodes of electrochemical sensors likely enhances the detection signal due to enhancement of electrical conductivity as well as improvement of the capturing ability of analytes for the electrodes. The inclusion of SP carbon enables the application of the dropping or slurry casting method to fabricate the electrodes on a large scale. In future, this is an easy, effective, and economic approach to deploy industrial production for the electrochemical sensors. In addition, it is found that most recent investigations on the electrochemical sensors are involved glassy carbon as an electrode substrate. This material is expensive and has difficulty in manufacturing as well as mechanical processing, showing poor feasibility in mass production. Meanwhile, screen-printed electrodes (SPEs) show plenty of advantages, such as quick in-situ analysis and high reproducibility, sensitivity, and accuracy. They are also suitable for in-field electrochemical devices. Hence, in the present work, we propose the utilization of the MnO₂/SP-modified SPEs for electrochemical sensors to detect copper in water sources. Wherein, MnO₂ was synthesized by the hydrothermal method. SP carbon was a commercial product, and the modified SPEs were fabricated via a facile dropping method.

2. Experimental Section

2.1. Reagents and Equipment

Manganese sulfate (MnSO₄.H₂O), ammonium persulfate ((NH₄)₂S₂O₈), and 5 wt.% Nafion solution were purchased from Sigma Aldrich Co., Germany. Sulfuric acid (H₂SO₄), acetic acid (CH₃COOH), sodium acetate (CH₃COONa), copper sulfate (CuSO₄.5H₂O), and isopropanol (IPA) were supplied by Xilong Scientific Co., China. Conductive silver and carbon inks were purchased from Sandoz Building Materials Ltd., China. Carbon black super P® (C, 99+%) was purchased from Alfa Aesar. An acetate buffer solution of a concentration of 0.1 M with a pH of 5 was prepared by dissolving sodium acetate (NaAc) and acetic acid (HAc) in distilled water.

2.2. Synthesis of MnO₂ Microflowers

MnO₂ microflowers were prepared using the hydrothermal method. In detail, 0.2114 g of MnSO₄.H₂O and 0.285 g of (NH₄)₂S₂O₈ were dissolved, respectively, in 12.5 mL of distilled water to obtain two individual solutions. After that, the solution containing (NH₄)₂S₂O₈ was poured and mixed well with the MnSO₄ solution under magnetic stirring conditions for 15 min. Next, the solution mixture was poured into a 50 mL Teflon-lined autoclave, accompanied by a hydrothermal synthesis at 90°C for 24 h. After cooling down, the obtained sample was centrifuged and rinsed with distillation water. The centrifugation and rinsing processes were repeated several times until the obtained supernatant after centrifugation reached pH 7. The solid product was dried at 60°C in a vacuum oven for 24 h.

2.3. Characterizations of MnO₂ Nanoflowers

The phase structure of the synthesized MnO₂ material was determined using an X-ray diffractometer (XRD, EQUINOX 5000, France) with Cu K $\alpha$ radiation. The microstructure of the material was examined with field emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan). The N₂ adsorption and desorption measurements were performed on the Micromeritics Gemini VII 2390 Surface Analyzer to evaluate the specific surface area and the porosity of the material.

2.4. Fabrication of Modified Screen-Printed Electrodes

An integrated three-electrode chip, the so-calledscreen-printed electrodes (SPEs) chip, was fabricated by integrating a counter electrode, a working electrode, and a pseudoreference electrode on a polyurethane pad wherein, the working electrode was made of silver, the counter electrode was made of carbon, the pseudoreference electrode was made of silver, and the electric contacts were made of silver. Herein, the following micromachining processes are not described in detail. The electric contacts were deposited on the polyurethane pad with dimensions of approximately 4.5 mm wide, 18 mm long, and 0.9 mm thick using the 3D printing technique as the early designed pattern (Scheme 1(a)). Then, the pad was masked and printed by a carbon layer. In the next step, the pad was covered by a suitable mask and printed by a silver layer. The masking and printing processes were performed so that the exposed areas of the working and counter electrodes were 2.25 mm² and 7.25 mm², respectively. Finally, the bare SPEs chips were fabricated successfully.

[figure(s) omitted; refer to PDF]

To fabricate the modified SPEs chip, MnO₂ microflowers and super P® (SP) carbon black were mixed well with different mass ratios and then dispersed in distilled water. A homogeneous suspension of MnO₂ and SP with a concentration of 2 mg mL⁻¹ was only achieved after the mixture was ultrasonically treated for 30 min and mixed using a Vortex mixer. After that, 2 μL of the prepared suspension of MnO₂ and SP was sucked and dropped on the surface of the bare SPEs chip with a micropipette [32]. After each drop, the chip was dried at 60°C. Finally, a given volume of 0.5 wt.% Nafion solution was drop-cast on the top layer of the chip and followed by a natural drying process. Herein, Nafion is a cation exchange polymer. It shows chemical stability and selective absorption [33, 34]. Thus, it is considered a suitable binder for the fabrication of the electrode to detect metallic cations in natural water resources as well as wastewater. In addition, covering a thin Nafion film on the sample surface in the final step enables the structural integrity of the modified SPEs chip after being covered by a layer of MnO₂/SP. The preparation process for the modified SPEs can be illustrated in Scheme 1(b).

2.5. Electrochemical Detection of Copper

Investigation of the copper detection property of the modified SPEs was performed in a three-electrode cell, in which the SPEs were immersed in the interest solutions as electrolytes. The electrochemical behavior of Cu(II) ions was recorded using a square wave anodic stripping voltammetry (SWASV) technique. During the process of the preconcentration, the modified SPEs were dipped in 0.1 M NaAc-HAc buffer solution (pH 5.0) containing CuSO₄ with various concentrations. Copper was first electrodeposited on the fabricated SPEs at a certain potential for a given period of time, followed by an anodic stripping process. The SWASV response was recorded during the anodic stripping process within a potential window between −0.2 V and 0.3 V with a potential step of 0.5 mV, a modulation amplitude of 20 mV, and a frequency of 25 Hz. All the electrochemical measurements were conducted by connecting with an AUTOLAB workstation using Nova 2.1.4 software.

3. Results and Discussion

3.1. Physicochemical Properties of the MnO₂ Microflowers

Figure 1 shows the morphology and microstructure of the synthesized MnO₂ material. As seen from the low-resolution SEM image in Figure 1(a), the prepared sample is composed of uniform microflowers with an average diameter of about 2 μm. It is recognized that the microflowers are assembled by a bunch of interconnected nanowires with a diameter of approximately 40–50 nm and a length of 300–400 nm (Figure 1(b)). With the three-dimensional (3D) structural feature of the microflowers, it can be expected that the obtained MnO₂ material possesses high porosity. Meanwhile, the XRD pattern in Figure 1(c) verifies the crystalline phase structure of the synthesized material. Herein, the diffraction peaks of the sample located at 2 $\theta$ = 21.4$°$, 37.1$°$, 42.4$°$, 56$°$, and 66.8$°$ totally match with the standard diffraction lines of $\epsilon$-MnO₂ (JCPDS card 00-030-0820) with a hexagonal structure. On the other hand, the background in the XRD pattern of the synthesized MnO₂ sample seems relatively high. In addition, the diffraction peaks have insufficient sharpness. This indicates the low crystallinity of the synthesized MnO₂ material.

[figure(s) omitted; refer to PDF]

To further investigate the porous structure of the MnO₂ nanoflowers, nitrogen adsorption, and desorption measurements were conducted at 77 K. As depicted in Figure 1(d), the isotherm plot of the synthesized MnO₂ sample shows a hysteresis loop at the relative pressure of 0.6$-$1.0. This is the typical shape of type V according to the IUPAC classification. This is indicative of the mesoporous structure of the synthesized MnO₂ material. According to the measured result, the specific surface area and porosity of the MnO₂ nanoflowers were 20.94 m² g⁻¹ and 0.14 cm³ g⁻¹, respectively. From the pore size distribution curve of the MnO₂ sample (the inset in Figure 1(d)), it is observable that the pore size of the MnO₂ microflowers varied in a wide range of 2$-$100 nm. However, the main pore size is mainly distributed over a range of over 20 nm. Thus, it can be stated that the MnO₂ microflowers were composed of two levels of hierarchically porous organization with mesopores (2–50 nm) and macropores (>50 nm). The average pore size was determined to be 17.93 nm. The novel 3D structure, along with the presence of the hierarchical pores is expected to promote the favorable penetration of waste source to the electrode surface in the copper detection.

3.2. Electrochemical Behavior of the MnO₂ Microflowers

Figure 2 shows the typical SWASV plots of the bare, SP, MnO₂, and MnO₂/SP-modified SPEs in the 0.1 M NaAc-HAc solution containing 10 nM CuSO₄. These SWASV responses were measured at the electrodeposition potential of $-$1 V for the electrodeposition time of 180 s. As seen, the electrochemical response signal of the bare SPEs, which is indicated by the anodic current density, was very small, and the anodic peak was blunt. When the SPEs chip was modified with MnO₂, the anodic peak of the working electrode became sharper. In addition, the stripping current of copper increased by two fold in comparison with that of the bare SPEs chip. The current density of the anodic peak of the MnO₂-modified SPEs chip, 2850 μA cm⁻², was recorded at the potential value of 0.103 V. This indicates the enhanced copper detection ability of the SPEs after being modified by the MnO₂ microflowers. This is also a positive signal for the sensor application. The increase in the stripping current of copper probably originates from the significant increase in the electrode surface area after the SPEs chip was modified by the MnO₂ microflowers. In addition, the MnO₂ material with a tunnel structure and high surface area could promote the adsorption of Cu (II). Accordingly, a large amount of Cu (II) ions was accumulated easily on the electrode surface, deposited to form metallic copper, and then stripped from the electrode in the subsequent step of SWASV. As a result, the anode stripping signal was higher, and the shape of the anodic peak was sharper.

[figure(s) omitted; refer to PDF]

As for the SWASV response of the SP-modified SPEs chip, a broadening anode stripping peaks is observed. This implies that the modified SPEs hardly have ability to detect Cu(II) ions. Nevertheless, compared with the MnO₂-modified SPEs, the SP-modified SPEs show the high current density signal. The increase in the anodic current of the SP-modified SPEs results from the higher electrical conductivity of the SP carbon material compared with the MnO₂ material. Thus, SP carbon as a conductive agent should be used as a component of the working electrode for the fabrication of electrochemical sensors. To take the advantages of these two materials, their combination is necessary. Indeed, the MnO₂/SP-modified SPEs showed a considerably high anode stripping current density (Figure 2). The shape of the SWASV plot of the MnO₂/SP-modified SPEs appeared relatively similar to that of the MnO₂-modified SPEs. However, the anode stripping current density was significantly improved because of the presence of the SP carbon ingredient on the surface of the SPEs after modification. The current density at the anodic peak of the MnO₂/SP-modified SPEs achieved 4495 μA cm⁻². Eventually, among the investigated SPEs, the MnO₂/SP-modified SPEs are likely suitable to detect the presence of Cu(II) ions in water sources.

3.3. Optimization of Experimental Conditions

3.3.1. Effect of the Loading Mass

As discussed in Figure 2, the SP carbon and MnO₂ materials show their own advantages and disadvantages. In addition, the loading mass of MnO₂ on the SPEs is also an important parameter needed for investigation. Figure 3(a) presents the change in the SWASV response of the MnO₂/SP-modified SPEs chip when the amount of the MnO₂/SP suspension covering on the surface of the SPEs chip increased from 6 μL to 14 μL, corresponding to the increase in the loading mass of the SPEs from 0.415 to 2.906 mg cm⁻². Herein, the mass ratio of MnO₂ microflowers to SP carbon was fixed to be 90 : 10. It is recognized that, when the volume of the MnO₂/SP suspension dropping on the surface of the SPEs increased from 6 μL to 10 μL, corresponding to the increase in the loading mass of MnO₂/SP from 0.415 to 2.076 mg cm⁻², the signal of the anode stripping current was found to increase significantly. To be specific, the current density of the anodic peak increased from 2938 μA cm⁻² as for the sample 6 μL to 3525 μA cm⁻² as for the sample 8 μL and reached the maximum of 4495 μA cm⁻² as for the sample of 10 μL. Then, the current density of the anodic peak decreased to 2599 μA cm⁻² and 2049 μA cm⁻² as for the samples 12 μL and 14 μL, respectively. This can be explained by the fact that the increase in the loading mass of MnO₂ on the SPEs ‘surface induced an increase in the real surface area of the working electrode. This leads to a larger amount of deposited copper on the surface of the modified SPEs chip, followed by a higher anodic stripping current. Nevertheless, as the loading mass of MnO₂ increased over 2.076 mg cm⁻² corresponding to the sample 10 μL, the thickness of the MnO₂ layer prevented the diffusion of Cu (II) ions into the bulk working electrode. On the other hand, because of the low conductivity of MnO₂ the high loading mass of MnO₂ caused the high initial resistance of the working electrode. As a result, the anodic stripping current density of the modified SPEs decreased. Hence, the optimum loading mass of MnO₂ was determined to be 2.076 mg cm⁻² corresponding to 10 μL of the used MnO₂/SP suspension solution. This optimum value was used for further experiments.

[figure(s) omitted; refer to PDF]

3.3.2. Effect of the Mass Ratio of SP Carbon and MnO₂ Microflowers

Due to the low conductivity of MnO₂ the introduction of SP carbon as a conducting agent into the SPEs is crucial. In the present work, to evaluate the effect of the mass ratio of SP carbon to MnO₂ microflowers on the copper detection performance of the modified SPEs, the mass ratio of SP carbon to MnO₂ microflowers used in the MnO₂/SP-modified SPEs was changed from 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.%. Figure 3(b) illustrates the SWASV response of the MnO₂/SP-modified SPEs against the change in the mass ratio of SP carbon to MnO₂ microflowers. Noticeably, as for the SPEs modified by MnO₂/SP with 10 wt.% SP, the current density signal of copper stripping was the highest. The current density at the anodic peak reached 4495 μA cm⁻². As the percentage of SP carbon increased over 10 wt.%, the stripping current signal showed a declining tendency. Especially, the measured current density at the copper stripping peak was 2531 μA cm⁻² and 1811 μA cm⁻² for the modified SPEs having the high mass ratios of SP to MnO₂ such as 20 wt.% and 30 wt.%, correspondingly. This likely results from the high electrical conductivity and poor Cu (II) detection of SP carbon, as shown in Figure 2. Accordingly, the mass ratio of SP carbon to MnO₂ microflowers of 10 wt.% was regarded as the most reasonable ratio to fabricate the modified SPEs for effective copper detection. Thus, this mixing ratio of SP carbon to MnO₂ microflowers was fixed for the subsequent experiments.

3.3.3. Effect of the Loading Mass of Nafion

In the present work, Nafion was used as a polymer binder for the modified SPEs with the aim of making an intimate contact between the electrode active material and the screen-printed silver which was used as the current collector. However, Nafion can also decrease the electrical conductivity of the working electrode in the modified SPEs chip. The loading mass of Nafion on the working electrode surface, in other words, the loading mass of Nafion on the modified SPEs firmly impacts the copper detection behavior of the modified SPEs chip. To evaluate the effect of the loading mass of Nafion, the MnO₂/SP-modified SPEs chip was fabricated by dropping 10 μL of a suspension solution containing MnO₂ and SP carbon with a concentration of 2 mg mL⁻¹, in which the mass ratio of SP carbon to MnO₂ was 10 wt.%, on the bare SPEs chip. After that, the chip was dried at 60°C and followed by dropping x μL of 0.5 wt.% Nafion solution (x = 0, 2, 4). Finally, the sample was dried naturally.

According to Figure 3(c), the stripping current density at the anodic peak elevated from 2098 μA cm⁻² to 4494 μA cm⁻² with the increase in the used volume of Nafion solution from 0 μL to 2 μL. This demonstrates that, apart from the function of a binder, Nafion served as an ion exchange membrane, which allows Cu (II) ions to diffuse and electrodeposit on the working electrode of the modified SPEs chip as well as restrict the effect of the other impurities present in the water sources. In addition, a reasonable amount of Nafion binder could ensure the intimate electrical contact between MnO₂/SP and the screen-printed silver substrate. Accordingly, the MnO₂/SP material revealed the highest utilization efficiency. As the amount of the used Nafion solution reached over 2 μL, the anodic stripping current signal was found to reduce. To be specific, the current density at the anodic peak only achieved 2744 μA cm⁻² for the modified SPEs chip which was prepared from 4 μL of the Nafion solution. This can be explained by the increasing thickness of the Nafion membrane, accompanied by the high resistance of the SPEs. Thus, the reasonable amount of 0.5 wt.% Nafion solution for the fabrication of the modified SPEs chip was 2 μL.

3.3.4. Effect of the Deposition Time of SWASV

Similarly, the electrodeposition time is among the important parameters, like the electrodeposition potential of the preconcentration step in the SWASW technique. The deposition time has a significant influence on the response signal of the analytes of interest. When the electrodeposition time is prolonged, the amount of copper is accumulated increasingly in the preconcentration step, accompanied by the higher anodic stripping signal and the requirement of a longer period of analysis time. So, in the present work, evaluation and selection of the proper electrodeposition time, meeting the requirement of the high response signal, and reasonable analysis time were carried out. Herein, the preconcentration step prior to copper detection was performed in the solution of 0.1 M NaAc/HAc (pH 5.0) and 10 nM CuSO₄ at the electrodeposition potential of $-$1 V for various electrodeposition times from 120 s to 240 s.

Figure 3(d) presents the measured SWASW responses of the MnO₂/SP-modified SPEs chip corresponding to the different deposition times at the deposition potential of $-$1 V. It was found that, for the sample electrodeposited in the time periods of 120, 150, 180, 210, and 240 s, the copper stripping current density measured at the anodic peak was 2792, 3676, 4495, 4565, and 4731 μA cm⁻², respectively. The considerably enhanced signal of the anodic stripping current density is accounted for a large amount of accumulated copper for the elongated electrodeposition time in the previous preconcentration step. Besides, as for the samples with the electrodeposition times of 180 s, 210 s, and 240 s, the increase in the current density of the anodic stripping peak was negligible. This can be explained in the following way: when the electrodeposition time increased from 180 s to 240 s, the amount of copper metal deposited on the surface of the electrode also increased, but increased negligibly and almost achieved a limiting value. Thus, in the subsequent anodic stripping step, the amount of the deposited copper would dissolve. Correspondingly, the obtained current density of the anodic peak from the SWASV response increased negligibly. In brief, to reduce the analysis time in the copper detection process providing the anodic stripping signal is sufficiently good, the period of 180 s is the recommended proper time for the preconcentration step in the copper detection process.

3.3.5. Effect of the Deposition Potential of SWASV

In general, for the stripping analysis technique, the selection of a proper deposition potential is very important to achieve the best detection signals. So, in this study, in the buffer solution of 0.1 M HAc-NaAc containing 10 nM Cu²⁺, the MnO₂/SP-modified SPEs chip performed the preconcentration step by electrodeposition of copper at a constant potential ranging from $-$0.8 V to $-$1.1 V for 180 s. The anodic stripping responses of the SPEs chip were then recorded.

According to the measured results in Figure 3(d), the current density of the anodic stripping peak increased from 2952 μA cm⁻² and reached the maximum value of 4495 μA/cm^-2 when the electrodeposition potential declined from $-$0.8 V to $-$1.0 V. However, when the electrodeposition potential decreased to $-$1.1 V, the anodic peak current density deteriorated to 2143 μA cm⁻². Obviously, at the more negative electrodeposition potential like $-$1.1 V, the total cathodic current increased, but the electrodeposition current of copper reduced due to the discharge competition of water to form hydrogen gas, namely, the hydrogen evolution phenomenon. At that time, the working electrode surface of the modified SPEs chip was partially covered by hydrogen bubbles preventing the approach of Cu (II) ions to the working electrode surface to deposit [28]. As a result, following the diminished amount of electrodeposited copper on the working electrode surface, the anodic stripping current signal of copper in the correspondingly subsequent step degraded as well. Therefore, the electrodeposition potential of $-$1.0 V was regarded as the best deposition potential for the preconcentration step in the SWASV technique used for the copper detection.

3.4. The Stability and Reproducibility of the Screen-Printed Electrodes

To evaluate the stability and reproducibility of the electrodes for electrochemical sensors application, a series of five MnO₂/SP-modified SPE chips were fabricated at the optimum conditions and measured with SWASV in 0.1 M NaAc-HAc (pH 5.0) buffer solution containing 10 nM CuSO₄ with the optimum experimental parameters. From Figure 4(a) it is observable that the stripping current signals of the five chip samples are stable. The current densities of the anodic stripping peaks were measured at around 4391 μA cm⁻² with the highest deviation of 2.4% (Figure 4(b)). This suggests the excellent reliability of the SWASV measurements and the high reproducibility of the SPEs chip for the electrochemical detection of Cu (II) ions.

[figure(s) omitted; refer to PDF]

In addition, for electrochemical sensor applications, the reutilization demand of the detection probe is indispensable. Therefore, to examine the repeatability of the MnO₂/SP-modified SPEs chip in the copper detection process, the optimum MnO₂/SP-modified SPEs chip was measured with SWASV repeatedly in 0.1 M NaAc-HAc (pH 5.0) solution containing 10 nM Cu (II) under the optimum experimental conditions. Figure 4(c) displays the SWASV responses of the SPEs chip for 10 consecutive measurements. As shown, the resultant anodic peaks appear clear and sharp. Remarkably, for seven initial SWASV measurements, the positions of these anodic stripping peaks almost coincide after each measurement. The recorded current density of the anodic peaks after seven measurements was 4171 μA cm⁻² (Figure 4(d)). However, from the 8^th to 10^th SWASV measurements, the obtained anodic peak signal degraded with 84% retention of the initial anodic peak response. This demonstrates the good repeatability of the MnO₂/SP-modified SPEs chip for consecutively seven repeated analyses.

3.5. Determination of the Copper Content in Water

According to Cottrell equation [35], the dependence of the electric current on the analyte concentration can be written as follows:$$\begin{array}{}\text{(1)}& i=\frac{nFA{C_{o}D}_o^{1/2}}{{\pi }^{1/2}{t}^{1/2}}\text{,}\end{array}$$where i is the current (A), n is the number of electron, F is the Faraday constant, 96485 C/mol, C_o is the initial concentration of the reducible analyte (mol/cm³), A is the area of the (planar) electrode (cm²), D_o is the diffusion coefficient for species (cm²/s), and t is the time (s)

In case the parameters such as D_o and t are fixed, the current, i, is referred to a linear function of the analyte concentration. Based on the above Cottrell equation, to construct the linear calibration equation showing the relationship between the anodic stripping peak current density and the concentration of Cu (II) ions present in the water sources, the optimum MnO₂/SP-modified SPE chips were measured with SWASV in 0.1 M NaAc-HAc (pH 5.0) solution containing Cu (II) ions with two different concentration ranging from 5 $\mu$M to 100 $\mu$M and from 0.625 nM to 15 nM. Prior to the SWASV measurements, the preconcentration step was conducted at the electrodeposition potential of $-$1.0 V for 180 s. The obtained results are displayed in Figure 5. It is easy to recognize the signal intensity of the copper stripping process, which is manifested by the current density of the anodic peak, increased linearly against the concentration of Cu (II) ions present in the analysis solution (Figures 5(a) and 5(c)). Based on the ordinary least square method, the linear regression equation within the high concentration ranging from 5 $\mu$M to 100 $\mu$M was found (Figure 5(b)). This relationship can be expressed as follows:$$\begin{array}{}\text{(2)}& i_{pa}=7418.2+34.62C_M\text{,}\end{array}$$where i_pa is the anodic stripping current density (µA cm⁻²) and C_M is the concentration of Cu (II) ions (μM). This equation possesses a correlation coefficient of R² = 0.9786 and sensitivity was 34.62 μA cm⁻²μM⁻¹.

[figure(s) omitted; refer to PDF]

Likewise, at the concentration range of Cu (II) ions from 0.625 nM to 15 nM, the linear regression equation was found to be$$\begin{array}{}\text{(3)}& i_{pa}=2280.9+214.05C_M\text{,}\end{array}$$where i_pa is the anodic stripping current density (μA cm⁻²) and C_M is the concentration of Cu(II) ions (nM). The correlation coefficient of this equation was R² = 0.9737, and the sensitivity of the electrochemical sensor was 214.05 μA cm⁻² nM⁻¹. From the calculation result based on the standard deviation of the response and the slope approach [36], the limit of copper detection (LOD) of the MnO₂/SP-modified SPE chips was 0.5 nM, which is much smaller than the lower standard as recommended by WHO (0.23 μM).

To clarify the outstanding performance of the MnO₂/SP-modified SPE chip as an electrochemical sensor, namely, MnO₂/SP/SPEs, in the electrochemical detection of Cu (II) ions, the criteria such as sensitivity and LOD of the MnO₂/SP/SPEs in the present work were compared with the other electrodes as reported previously [35–37], and the comparison results are listed in Table 1. From Table 1, it is recognized that the electrochemical sensors based on the MnO₂/SP-modified SPE chips had good sensitivity and a low detection limit for copper in the water sources, which can be superior or comparable to those based on other electrodes.

Table 1

Comparison of the MnO₂/SP-modified SPEs and other previously reported electrodes for electrochemical detection of Cu (II).

Electrodes	Method	Sensitivity	LOD (μM)	Reference
Fe-OSA/GCE	SWASV	0.909 μA/μM	0.0878	[37]
NH2-CMS/GCE	SWASV	5.791 μA/μM	0.011	[38]
Alk-Ti3C2/GCE	SWASV	20.59 μA/μM	0.032	[39]
GO/MnO2/GCE	SWASV	63.93 μA/μM	0.0017	[28]
MnO2@RGO/GCE	SWASV	—	0.0002	[29]
MnO2/Ni foam	SWASV	59.5 μA/μM	0.17	[20]
MnO2/SP/SPEs	SWASV	214.05 μA/cm2 nM	0.0005	This study

4. Conclusion

The 3D porous MnO₂ microflowers were successfully synthesized using the hydrothermal method. After synthesis, the MnO₂ microflowers were used as electrode active materials for the SPEs. Because of the low electrical conductivity of MnO₂, the MnO₂-modified SPEs showed inferior electrocatalytic ability towards copper detection. Along with the inclusion of SP carbon as a conducting agent, the MnO₂/SP-modified SPEs demonstrated excellent electrocatalytic ability in the amperometric detection of Cu(II) ions. The sensor using the optimized MnO₂/SP-modified SPEs displayed the high sensitivity of 214.05 μA cm⁻² nM⁻¹ in copper detection in the low linear concentration range of 0.625 nM to 15 nM, with a correlation coefficient of R² = 0.9737 and a very low limit of detection index of 0.5 nM. Besides, the sensor also illustrated the excellent detection ability in the concentration range of 5 μM to 100 μM, with the correlation coefficient of R² = 0.9786 and detection sensitivity of 34.644 μA cm⁻²μM⁻¹. The high stability and reliable reproducibility of the fabricated SPEs were identified through almost repeated current signals for a series of the five different SPE chips fabricated at the same time and after ten cycling tests for each chip. These findings showed the promising applicability of the MnO₂/SP-modified SPE chips as reliable electrochemical sensors for copper detection in water sources.

Acknowledgments

This research was funded by the Hanoi University of Science and Technology (HUST) under the project number T2020-SAHEP-028.

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