Fabrication of Screen-Printed Electrodes Modified by Hydrothermal MnO 2 Microflowers and Carbon for Electrochemical Sensors in Copper Ions Detection

Porous MnO 2 microfowers with a hexagonal crystalline structure were facilely prepared at a low hydrothermal temperature of 90 ° C, without using any template or capping agent. Te as-prepared MnO 2 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 Nafon as binder. In the present work, to evaluate the detection ability of copper (II) in the MnO 2 microfowers, chips of screen-printed electrodes (SPEs) having a polyurethane substrate, a silver working electrode, a carbon counter electrode, and a silver pseu-doelectrode, were designed. Ten, the SPEs chips were modifed with MnO 2 microfowers 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 15nM ( R 2 � 0.9737), and a low detection limit (0.5nM), high sensitivity (214.05 μ A/cm 2 nM), and rapid response (180s) demonstrated high application potential for electrochemical sensors in the detection of copper in water resources.


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][3][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 sufer some disadvantages, viz.requirement of bulky and complex measurement device systems and professionally trained operators, or the difculty in developing portable devices for in-feld 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][9][10].Compared with the above analysis methods, the electrochemical analysis method is more reliable, more economic, and more suitable for in-feld analysis applications.Te 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].
Besides, for the sensor applications, the MnO 2 material reveals a disadvantage of poor electrical conductivity.To overcome this drawback, coupling MnO 2 with another conducting agent to produce composites has been proposed, for instance, the coupling of MnO 2 with graphene oxide (GO) or reduced graphene oxide (RGO) [28,29].Introduction of graphene or GO into the MnO 2 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 MnO 2 /GO composites is very difcult.In addition, the obtained composites normally sufer the nonuniform distribution of MnO 2 on the GO supporting material.Tus, the reliability in reproductivity and repeatability of the electrochemical sensors based on such MnO 2 /GO composites are insufciently high.To fabricate a stable and durable electrochemical sensor, Hao and coworker synthesized MnO 2 nanowires on a nickel foam substrate using the hydrothermal method for copper detection in water [20].Te sensor based on the MnO 2 nanowires/nickel foam electrode posed a limit of copper detection at 0.17 μM.Tis 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 MnO 2 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-feld devices, the nickel foam substrate with low anticorrosion sufers 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 specifc 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.Te 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, efective, 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.Tis material is expensive and has difculty 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.Tey are also suitable for in-feld electrochemical devices.Hence, in the present work, we propose the utilization of the MnO 2 /SP-modifed SPEs for electrochemical sensors to detect copper in water sources.Wherein, MnO 2 was synthesized by the hydrothermal method.SP carbon was a commercial product, and the modifed SPEs were fabricated via a facile dropping method.

Fabrication of Modifed 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.Te 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)).Ten, 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.Te masking and printing processes were performed so that the exposed areas of the working and counter electrodes were 2.25 mm 2 and 7.25 mm 2 , respectively.Finally, the bare SPEs chips were fabricated successfully.
To fabricate the modifed SPEs chip, MnO 2 microfowers and super P ® (SP) carbon black were mixed well with diferent mass ratios and then dispersed in distilled water.A homogeneous suspension of MnO 2 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 MnO 2 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.% Nafon solution was drop-cast on the top layer of the chip and followed by a natural drying process.Herein, Nafon is a cation exchange polymer.It shows chemical stability and selective absorption [33,34].Tus, 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 Nafon flm on the sample surface in the fnal step enables the structural integrity of the modifed SPEs chip after being covered by a layer of MnO 2 / SP.Te preparation process for the modifed SPEs can be illustrated in Scheme 1(b).

Electrochemical Detection of Copper.
Investigation of the copper detection property of the modifed SPEs was performed in a three-electrode cell, in which the SPEs were immersed in the interest solutions as electrolytes.Te electrochemical behavior of Cu(II) ions was recorded using a square wave anodic stripping voltammetry (SWASV) technique.During the process of the preconcentration, the modifed SPEs were dipped in 0.1 M NaAc-HAc bufer solution (pH 5.0) containing CuSO 4 with various concentrations.Copper was frst electrodeposited on the fabricated SPEs at a certain potential for a given period of time, followed by an anodic stripping process.Te 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 2.1.4software.

Physicochemical Properties of the MnO 2 Microfowers.
Figure 1 shows the morphology and microstructure of the synthesized MnO 2 material.As seen from the low-resolution SEM image in Figure 1(a), the prepared sample is composed of uniform microfowers with an average diameter of about 2 μm.
It is recognized that the microfowers 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 microfowers, it can be expected that the obtained MnO 2 material possesses high porosity.Meanwhile, the XRD pattern in Figure 1(c) verifes the crystalline phase structure of the synthesized material.Herein, the difraction peaks of the sample located at 2 θ � 21.4 °, 37.1 °, 42.4 °, 56 °, and 66.8 °totally match with the standard difraction lines of ε-MnO 2 (JCPDS card 00-030-0820) with a hexagonal structure.On the other hand, the background in the XRD pattern of the synthesized MnO 2 sample seems relatively high.In addition, the difraction peaks have insufcient sharpness.Tis indicates the low crystallinity of the synthesized MnO 2 material.
To further investigate the porous structure of the MnO 2 nanofowers, nitrogen adsorption, and desorption measurements were conducted at 77 K.As depicted in Figure 1(d), the isotherm plot of the synthesized MnO 2 sample shows a hysteresis loop at the relative pressure of 0.6−1.0.Tis is the typical shape of type V according to the IUPAC classifcation.Tis is indicative of the mesoporous structure of the synthesized MnO 2 material.According to the measured result, the specifc surface area and porosity of the MnO 2 nanofowers were 20.94 m 2 g −1 and 0.14 cm³ g −1 , respectively.From the pore size distribution curve of the MnO 2 sample (the inset in Figure 1(d)), it is observable that the pore size of the MnO 2 microfowers varied in a wide range of 2−100 nm.However, the main pore size is mainly distributed over a range of over 20 nm.Tus, it can be stated that the MnO 2 microfowers were composed of two levels of hierarchically porous organization with mesopores (2-50 nm) and macropores (>50 nm).Te average pore size was determined to be 17.93 nm.Te 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.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.
As for the SWASV response of the SP-modifed SPEs chip, a broadening anode stripping peaks is observed.Tis implies that the modifed SPEs hardly have ability to detect Cu(II) ions.Nevertheless, compared with the MnO 2 -modifed SPEs, the SPmodifed SPEs show the high current density signal.Te increase in the anodic current of the SP-modifed SPEs results from the higher electrical conductivity of the SP carbon material compared with the MnO 2 material.Tus, 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 MnO 2 /SP-modifed SPEs showed a considerably high anode stripping current density (Figure 2).Te shape of the SWASV plot of the MnO 2 /SPmodifed SPEs appeared relatively similar to that of the MnO 2modifed SPEs.However, the anode stripping current density was signifcantly improved because of the presence of the SP carbon ingredient on the surface of the SPEs after modifcation.Te current density at the anodic peak of the MnO 2 /SPmodifed SPEs achieved 4495 μA cm −2 .Eventually, among the investigated SPEs, the MnO 2 /SP-modifed SPEs are likely suitable to detect the presence of Cu(II) ions in water sources.

Efect of the Loading Mass.
As discussed in Figure 2, the SP carbon and MnO 2 materials show their own advantages and disadvantages.In addition, the loading mass of MnO 2 on the SPEs is also an important parameter needed for investigation.Figure 3(a) presents the change in the SWASV response of the MnO 2 /SP-modifed SPEs chip when the amount of the MnO 2 /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 MnO 2 microfowers to SP carbon was fxed to be 90 : 10.It is recognized that, when the volume of the MnO 2 /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 MnO 2 /SP from 0.415 to 2.076 mg cm −2 , the signal of the anode stripping current was found to increase signifcantly.To be specifc, 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.Ten, 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.Tis can be explained by the fact that the increase in the loading mass of MnO 2 on the SPEs 'surface induced an increase in the real surface area of the working electrode.Tis leads to a larger amount of deposited copper on the surface of the modifed SPEs chip, followed by a higher anodic stripping current.Nevertheless, as the loading mass of MnO 2 increased over 2.076 mg cm −2 corresponding to the sample 10 μL, the thickness of the MnO 2 layer prevented the difusion of Cu (II) ions into the bulk working electrode.On the other hand, because of the low conductivity of MnO 2 the high loading mass of MnO 2 caused the high initial resistance of the working electrode.As a result, the anodic stripping current density of the modifed SPEs decreased.Hence, the optimum loading mass of MnO 2 was determined to be 2.076 mg cm −2 corresponding to 10 μL of the used MnO 2 /SP suspension solution.Tis optimum value was used for further experiments.

Efect of the Mass Ratio of SP Carbon and MnO 2
Microfowers.Due to the low conductivity of MnO 2 the introduction of SP carbon as a conducting agent into the SPEs is crucial.In the present work, to evaluate the efect of the mass ratio of SP carbon to MnO 2 microfowers on the copper detection performance of the modifed SPEs, the mass ratio of SP carbon to MnO 2 microfowers used in the MnO 2 /SP-modifed SPEs was changed from 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.%. Figure 3(b) illustrates the SWASV response of the MnO 2 /SP-modifed SPEs against the change in the mass ratio of SP carbon to MnO 2 microfowers.Noticeably, as for the SPEs modifed by MnO 2 /SP with 10 wt.% SP, the current density signal of copper stripping was the highest.Te 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 modifed SPEs having the high mass ratios of SP to MnO 2 such as 20 wt.% and 30 wt.%, correspondingly.Tis 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 MnO 2 microfowers of 10 wt.% was regarded as the most reasonable ratio to fabricate the modifed SPEs for efective copper detection.Tus, this mixing ratio of SP carbon to MnO 2 microfowers was fxed for the subsequent experiments.

Efect of the Loading Mass of Nafon.
In the present work, Nafon was used as a polymer binder for the modifed 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, Nafon can also decrease the electrical conductivity of the working electrode in the modifed SPEs chip.Te loading mass of Nafon on the working electrode surface, in other words, the loading mass of Nafon on the modifed SPEs frmly impacts the copper detection behavior of the modifed SPEs chip.To evaluate the efect of the loading mass of Nafon, the MnO 2 / SP-modifed SPEs chip was fabricated by dropping 10 μL of a suspension solution containing MnO 2 and SP carbon with a concentration of 2 mg mL −1 , in which the mass ratio of SP carbon to MnO 2 was 10 wt.%, on the bare SPEs chip.After that, the chip was dried at 60 °C and followed by dropping x 6 Journal of Chemistry μL of 0.5 wt.% Nafon 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 Nafon solution from 0 μL to 2 μL.Tis demonstrates that, apart from the function of a binder, Nafon served as an ion exchange membrane, which allows Cu (II) ions to difuse and electrodeposit on the working electrode of the modifed SPEs chip as well as restrict the efect of the other impurities present in the water sources.In addition, a reasonable amount of Nafon binder could ensure the intimate electrical contact between MnO 2 /SP and the screen-printed silver substrate.Accordingly, the MnO 2 /SP material revealed the highest utilization efciency.As the amount of the used Nafon solution reached over 2 μL, the anodic stripping current signal was found to reduce.To be specifc, the current density at the anodic peak only achieved 2744 μA cm −2 for the modifed SPEs chip which was prepared from 4 μL of the Nafon solution.Tis can be explained by the increasing thickness of the Nafon membrane, accompanied by the high resistance of the SPEs.Tus, the reasonable amount of 0.5 wt.% Nafon solution for the fabrication of the modifed SPEs chip was 2 μL.

Efect 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.Te deposition time has a signifcant infuence 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 CuSO 4 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 MnO 2 /SP-modifed SPEs chip corresponding to the diferent 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.Te 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.Tis 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.Tus, 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 sufciently good, the period of 180 s is the recommended proper time for the preconcentration step in the copper detection process.

Efect 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 bufer solution of 0.1 M HAc-NaAc containing 10 nM Cu 2+ , the MnO 2 /SP-modifed 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.Te 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 modifed 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.Terefore, 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.

Te Stability and Reproducibility of the Screen-Printed
Electrodes.To evaluate the stability and reproducibility of the electrodes for electrochemical sensors application, a series of fve MnO 2 /SP-modifed SPE chips were fabricated at the optimum conditions and measured with SWASV in 0.1 M NaAc-HAc (pH 5.0) bufer solution containing 10 nM CuSO 4 with the optimum experimental parameters.From Figure 4(a) it is observable that the stripping current signals of the fve chip samples are stable.Te 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)).Tis suggests the excellent reliability of the SWASV measurements and the high reproducibility of the SPEs chip for the electrochemical detection of Cu (II) ions.
In addition, for electrochemical sensor applications, the reutilization demand of the detection probe is indispensable.Terefore, to examine the repeatability of the MnO 2 /SPmodifed SPEs chip in the copper detection process, the optimum MnO 2 /SP-modifed 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.Te 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.Tis demonstrates the good repeatability of the MnO 2 /SPmodifed SPEs chip for consecutively seven repeated analyses.

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: 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/cm 3 ), A is the area of the (planar) electrode (cm 2 ), D o is the difusion coefcient for species (cm 2 /s), and t is the time (s) In case the parameters such as D o and t are fxed, 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 MnO 2 /SP-modifed SPE chips were measured with SWASV in 0.1 M NaAc-HAc (pH 5.0) solution containing Cu (II) ions with two diferent concentration ranging from 5 μM to 100 μM 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.Te 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 μM to 100 μM was found (Figure 5(b)).Tis relationship can be expressed as follows: where i pa is the anodic stripping current density (µA cm −2 ) and C M is the concentration of Cu (II) ions (μM).Tis equation possesses a correlation coefcient of R 2 � 0.9786 and sensitivity was 34.62 μA cm −2 μM −1 .Likewise, at the concentration range of Cu (II) ions from 0.625 nM to 15 nM, the linear regression equation was found to be Journal of Chemistry i pa � 2280.9 + 214.05 C M , (3) where i pa is the anodic stripping current density (μA cm −2 ) and C M is the concentration of Cu(II) ions (nM).Te correlation coefcient of this equation was R 2 � 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 MnO 2 /SP-modifed 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 MnO 2 /SPmodifed SPE chip as an electrochemical sensor, namely, MnO 2 /SP/SPEs, in the electrochemical detection of Cu (II) ions, the criteria such as sensitivity and LOD of the MnO 2 / SP/SPEs in the present work were compared with the other electrodes as reported previously [35][36][37], and the comparison results are listed in Table    recognized that the electrochemical sensors based on the MnO 2 /SP-modifed 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.

Conclusion
Te 3D porous MnO 2 microfowers were successfully synthesized using the hydrothermal method.After synthesis, the MnO 2 microfowers were used as electrode active materials for the SPEs.Because of the low electrical conductivity of MnO 2 , the MnO 2 -modifed SPEs showed inferior electrocatalytic ability towards copper detection.Along with the inclusion of SP carbon as a conducting agent, the MnO 2 /SPmodifed SPEs demonstrated excellent electrocatalytic ability in the amperometric detection of Cu(II) ions.Te sensor using the optimized MnO 2 /SP-modifed 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 coefcient of R 2 � 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 coefcient of R 2 � 0.9786 and detection sensitivity of 34.644 μA cm −2 μM −1 .Te high stability and reliable reproducibility of the fabricated SPEs were identifed through almost repeated current signals for a series of the fve different SPE chips fabricated at the same time and after ten cycling tests for each chip.Tese fndings showed the promising applicability of the MnO 2 /SP-modifed SPE chips as reliable electrochemical sensors for copper detection in water sources.

Scheme 1 :
Scheme 1: Preparation process of (a) the bare and (b) modifed SPEs chip.

Figure 3 :
Figure 3: SWASV responses of the MnO 2 /SP-modifed SPEs (a) with the diferent loading mass of MnO 2 /SP, (b) with the diferent contents of SP carbon, and (c) with the diferent loading mass of Nafon.SWASV responses of the MnO 2 /SP-modifed SPEs (d) at the diferent deposition potentials for 180 s and (e) for the diferent deposition times at the deposition potential of −1 V in 0.1 M NaAc-HAc (pH 5.0) solution containing 10 nM CuSO 4 .
. From Table 1, it is

Table 1 :
Comparison of the MnO 2 /SP-modifed SPEs and other previously reported electrodes for electrochemical detection of Cu (II).