Detection of Selenium and Nickel Metal Ion in Water Using Mn3O4-Cn-Modified Electrode

+e present study reports the design of the Mn3O4-Cn electrode and its use for simultaneous detection of selenium and nickel in water. +e designed electrode can be used as a convenient electrochemical device for on-site testing of Se (IV) and Ni (II) levels in affected regions. +e best responses are obtained with 0.1M phosphate buffer saline (PBS) and 5mM Fe (CN)6 as supporting electrolyte. +e scan rate and the number of cyclic repetitions have a great effect on peak shape and intensity. It is seen from our study that peak intensity is directly proportional to Se (IV) and Ni (II) concentrations in the range of 5 to 250 μg/L (correlation coefficients 0.952 and 0.984) when the optimized parameters are used. +e detection limit of 0.533 μg/L Se (IV) and 0.718 μg/L for Ni (VI) with a response time of 18 s for 5–250 μg/L concentration is obtained, respectively. Enhanced analytical results for different water samples establish that the proposedmethod is appropriate for Se (IV) andNi (II) detection. FESEM images confirm the Mn3O4-Cn nanocomposite formation on the electrode.


Introduction
Heavy metals in the environment pose a great hazard to organisms owing to their nonbiodegradability [1]. Numerous research laboratories are using systematic procedures to analyze metal ions in various mediums.
is consists of atomic absorption spectrometry, atomic fluorescence spectrometry, high-pressure liquid chromatography, gas chromatography, and flow injection analysis [2]. But, these techniques have their own drawbacks. Electrochemical detection of metal ions has several advantages such as low cost, on-site availability, stability, repeatability, accuracy, and detection of multiple ions. Cyclic voltammetry (CV) is a method to recognize and differentiate redox behavior, stability, and surface area of the working electrode used for electrochemical sensing [3,4].
is technique using modified electrodes is to provide selectivity toward few metal ions. However, the selection of working electrodes is vital for the achievement of CV analysis. ere are many reports based on different metal oxide nanoparticles [5][6][7][8][9][10][11][12][13]. Manganese oxide nanoparticles are among the most suitable contenders as electrode material owing to their catalyst nature, low cost, stability, nontoxic nature, large surface area, low detection limit, natural abundance, and high energy density [14][15][16][17][18][19][20]. John reported the excellent gas sensing properties of Mn 3 O 4 nanorods [20]. Nowadays, due to the progress of nanoscience and nanotechnology, there are numerous ways of fabricating chemically modified electrodes using nanocomposites. A clever candidate to form nanocomposite with Mn 3 O 4 is Chitosan. Chitosan (Cn) is a polysaccharide polymer with attractive properties like thin film-forming capability, good adhesion, and outstanding sensitivity [13]. Chitosan consists of volatile amino and hydroxyl functional groups which enable strong enzyme affinity and chemical modifications [18]. ere are only a few works reported for metal ion detection based on these materials [2][3][4]13]. Mn 3 O 4 is known for its high electrocatalytic nature which makes it an excellent electrode for gas sensors and biosensors. However, a mixture of Cn and Mn 3 O 4 is yet to be tried for trace metal ion detection. In the present work, an electrochemical sensor is developed for selenium and nickel detection in various water sources by modifying a platinum (Pt) electrode with manganese oxide (Mn 3 O 4 ) and chitosan. Selenium and nickel are toxic to the environment if their levels are above the required limit. Freshwaters are being contaminated by the dumping of toxic metal ions as a result of industrialization. So, it is very vital to determine these metal ion traces in freshwaters before usage. To the best of our knowledge, this is the first report of electrochemical selenium and nickel sensor based on Mn 3 O 4 -Cn nanocomposite, that too for selenium and nickel level detection in lake water. , and Fe(CN) 6 are brought from Merck and stored in refrigeration. Water for selenium and nickel detection is collected from a lake in South India, which is known for its toxicity due to the immense dumping of the waste from industrial and fertilizer factories. In commercial fertilizers, selenium is added in trace amounts. But, an ample amount is added up in the waste which polluted the surrounding water resources. Selenium and nickel act as a pollutant due to factory waste dumped into the water resources and lakes especially. Samples are collected from different sites of lakes and rivers during monsoon and summer. Anodic aluminum oxide (AAO) template with a length of 60 µm and having a pore diameter of 350 nm (Smart membranes, Germany) are purchased for the synthesis. We took 1 M manganese acetate (5.13 g) is dissolved in 100 ml of deionized (DI) water and stirred it for 20 minutes [20]. While stirring, we added 1 M ammonium hydroxide (100 ml) slowly.

Experimental
is solution is magnetically stirred for around one hour, resulting in a brown solution. Later, it is subjected to hydrothermal reaction, setting the temperature to 170°C for 7 hr. e resultant mixture is then washed with distilled water and dried at 60°C for 9 hr. e dried powder is annealed at 300°C for 2 hr, which produced pearl-shaped nanostructures, which are sonicated for 60 s in ethanol. is mixture is then filled into the pores of the AAO template by vacuum infiltration technique [20]. e AAO template is then air-dried at room temperature.
is process leads to the development of aligned nanorods. ese nanorods from AAO are separated to fabricate the biosensor, Mn 3 O 4 nanorods are sonicated in ethanol for one hour, and the residue is washed again in ethanol. e residue obtained is then dried at 60°C for 5 hours, which resulted in an ensemble of Mn 3 O 4 nanorods without any support.

Fabrication of Pt/Mn 3 O 4 /Cn Working Electrode.
e working electrode is prepared where 20 mg of Mn 3 O 4 nanorods separated from AAO is added to 5 ml of chitosan solution prepared when 1 gm chitosan is dispersed in 100 ml of 0.1 m acetic acid [13]. When the solution is stirred, Mn 3 O 4 -Cn suspension is formed. e Mn 3 O 4 -Cn electrode is formed when 2 µL of this aliquot is uniformly coated on the Pt working electrode by drop cast method. Performance of Pt/Mn 3 O 4 -Cn electrode for selenium and nickel detection in lake water is investigated as a real-time application. At first, the electrode is exposed to different trials to gain recovery when spiking different concentrations of selenium and nickel in supporting electrolytes [10,11].

Field Emission Spectroscopy and Fourier Transform
Analyses.
e morphology of Mn 3 O 4 nanorods separated from AAO is characterized using FESEM which is illustrated in Figure 1(a). e surface morphology of Mn 3 O 4 -Cn is observed using FESEM confirming nanocomposite formation as shown in Figure 1 3.2. Electron Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) is an efficient method to understand interfacial electron transfer due to changes at the electrode surface while being modified [10]. Fe (CN) 6 is used as the redox probe in impedance measurements conducted in PBS media at Mn 3 O 4 and Mn 3 O 4 -Cn electrodes. EIS measurements are carried out using 1 Hz to 100 kHz with an alternating voltage of 5 mV and a bias potential of 0.5 V ( Figure 2). e semicircle at higher frequencies denotes the electron transfer resistance (R ct ), and the linear part at lower frequencies connotes the diffusion process. A definite semicircle is exhibited by Mn 3 O 4 at a higher frequency. It is noted that the semicircle is reduced, which implies the charge relocation resistance once Mn 3 O 4 is modified with Cn. is shows that Mn 3 O 4 -Cn is an excellent conducting material and it can accelerate electron transfer.

Electrochemical Characterization.
When a potential is applied to the working electrode, it sweeps back and forth for a definite number of cycles according to the given range of voltage and speed of voltage sweep, and subsequent current at the working potential is measured [5]. is current is then plotted against the potential to draw a CV graph that provides a deep perception about the material regarding the anodic peak current (I pa ) from the oxidation process and about cathodic peak current (I pc ) from the reduction process that occurred on working potential. Potentials, where peak currents happen, are called peak potentials (Ep). We can examine the reversibility of reactions happening at the electrode surface by increasing the scan rates [6].
Electrochemical reversible reactions arise from fast electron transfer between ions and electrodes, which will be slower in irreversible reaction [6][7][8][9][10]. e applicability of modified Mn 3 O 4 -Cn as a potential electrochemical sensing platform for a metal solution such as Se (IV) and Ni (II) is investigated using cyclic voltammetry at a deposition potential of 400 mV. All the experiments are conducted at room temperature. e electrochemical analyses are carried out in 10 mL electrochemical cell with a known amount of Se (IV) and Ni (II) in 0.1 M phosphate buffer solution (pH 2) and 5 mM Fe (CN) 6 as supporting electrolyte, respectively.
In CV, the electrolyte has a vital role in the dispersal of analyte types onto the electrode [18][19][20]. Phosphate buffer solution (PBS) gives rise to a sharp and intense oxidation signal and is used as a supporting electrolyte in order to avoid all interferences of H + and complex compound formation that happens while using HCl, HNO 3 , and H 2 SO 4 [21]. PBS (0.1 M) with 5 mM Fe (CN) 6 acting as a redox couple is used as the supporting electrolyte and provides a good response to selenium and nickel detection [21]. PBS is examined for pH from 1-6 in order to detect Se (IV) and Ni (II) using Mn 3 O 4 -Cn sensor [21]. We observe the optimized condition for metal ion detection takes place at pH � 2 for 100 µg/L Se (IV) and Ni (II). Voltammetric response of Pt/ Mn 3 O 4 and Pt/Mn 3 O 4 -chitosan electrodes in PBS (0.1 M) and 5 mM Fe (CN) 6 is carried out at a scan rate of 100 mVs −1 (Figure 3). We tested for different scan rates too, but without any vast variation obtained for 100 mVs −1 .
We noticed that the bare Pt electrode in PBS and Fe (CN) 6 is not showing any peak current. As we used the Pt/ Mn 3 O 4 and Pt/Mn 3 O 4 -Cn electrodes, there is a current response. Peak current with Pt/Mn 3 O 4 -Cn electrode is more intense than that with other electrodes. is confirms that peak intensity is due to Pt/Mn 3 O 4 -Cn electrode only. Based on the profile of the voltammogram, peak current and peak potential of Se (IV) and Ni (II) in 0.1 M PBS and 5 mM Fe (CN) 6 where R is the correlation coefficient and ʋ is the scan rate [21,22]. Evidently, peak currents of Mn 3 O 4 -Cn electrode are International Journal of Electrochemistry 3 higher than Mn 3 O 4 , due to more active surface area and, hence, increased conductivity after adding Cn [21]. e linear dependence between scan rate and oxidation current indicates that selenium and nickel are adsorbed on the surface of the Pt/Mn 3 O 4 -Cn electrode which is due to a surface-confined electron transfer. Moreover, the current passing through the electrode is restricted by the diffusion of selenium and nickel to the electrode surface. is diffusion change is caused by the concentration gradient near the electrode [19,20,22,23]. is gradient is affected by the concentration of selenium and nickel at the electrode and the rate at which it can diffuse through the solution [8,21]. So, the diffusion coefficient can be obtained using Randles-Sevcik formula: erefore, the obtained diffusion coefficient is 8.2 × 10 −6 cm 2 s −1 , where I pa is anodic peak current, A is the active electrode surface area, n is the number of mobile electrons in PBS, ʋ is the scan rate, and C is the concentration of PBS [21]. Electroactive surface area is 0.055 cm 2 and 0.043 cm 2 for Mn 3 O 4 -Cn and Mn 3 O 4 , respectively. Afterward, cyclic voltammograms of metal ion solution of Se (IV) and Ni (II) concentrations between 5 µg/L and 250 µg/L are observed using the optimized conditions (Figure 4). e enhanced current response is due to the large specific surface area of the Mn 3 O 4 -Cn. e improved electrochemical response of Mn 3 O 4 -Cn arose due to the enhanced electroactive surface area and higher catalyst activity of Mn 3 O 4 [4,16].
is enhancement of electrochemical current response due to the uniform and well-aligned grid of Pt/Mn 3 O 4 -Cn electrode delivers a good pathway for the passage of electrons created during oxidation of selenium and nickel which are transported to the electrode surface through Mn ion [4,16]. Also, the existence of nanorod-shaped morphology of Mn 3 O 4 -Cn electrode gives forth better electronic and ionic transport because of uniformly distributed Mn 3 O 4 -Cn rather than bulk matrix [4,16]. Usually, the redox kinds get absorbed on the electrode surface, but due to the electrochemical property of Mn 3 O 4 , it would decrease the absorption of redox species which will enable the electron to be electrochemically reversible. is suggests that this sensor can increase the electron transfer rate at the electrodes which enhance its performance. More conductive pathways are created for electron transfer on the surface of Mn 3 O 4 -Cn in PBS [4,16,21]. e peak current of Se (IV) and Ni (II) increased from 400 mV to 1000 mV because of strong absorption by the Mn 3 O 4 -Cn sensor. Se (IV) and Ni (II) are adsorbed due to electrostatic interaction between the electrode and metal ions resulting in a high peak signal due to the intermetallic bond between Mn and ions [16,19,20,23]. e high peak signal can also arise due to higher electron transfer capability, increase in electroactive surfaces, and conductivity resulting in higher sensitivity toward Se (IV) and Ni (II) ions. e potential is set at 400 mV and reversed at 1000 mV with a scan rate of 100 mV/s. During the analysis, oxidation peak and reduction peak are detected at 730 mV and 850 mV for Se (IV) with 500 mV and 600 mV for Ni (II), respectively ( Figure 4) [11][12][13][14][15][16][17][18][19][20]23].
ose peaks are apparently associated with the redox reaction of Se (IV) and Ni (II) at the surface of Mn 3 O 4 -Cn. As the reduction of metal ions can occur by different mechanisms, the overall process that occurs on the surface of the electrode is considered to include reduction of Se (IV) to Se (0) and Ni (II) to Ni (0). is fact is confirmed by the alteration of the peak intensity during variation of Se (IV) and Ni (II) concentrations [11]. e analytes concentration played a vital role in the shifting of both cathodic and anodic peaks toward lower potential, but the potential difference of reduction and oxidation peaks is relatively consistent as shown in Figure 4. Based on the profile of the voltammogram, the peak current and the peak potential of Se (IV) and Ni (II) in 0.1 M PBS and 5 mM Fe (CN) 6 are determined.

Variation of the Scan Rate.
e effects of scan rate from 25 mV/s to 200 mV/s on the electrochemical response of Mn 3 O 4 /chitosan electrode in PBS (0.1 M) and 5 mM Fe (CN) 6 toward 250 μg/L Se (IV) and Ni (II) are investigated. From Figure 5, one can observe that some anodic peaks appeared at potential around 900 mV to 1000 mV, while the cathodic peak is observed at a potential of around 350 mV to 400 mV. Anodic peak (1000 mV and 1100 mV) and cathodic peak (360 mV and 370 mV) are observed for Se (IV), respectively, while, for Ni (II), the anodic peak is from 410 mV to 650 mV with a cathodic peak between 800 mV and 1000 mV.
Majorly, electron transfer on the surface of Mn 3 O 4 -Cn is controlled by diffusion. We conclude that the working curves of Se (IV) and Ni (II) investigations are least limited by concentration. We observed that the reduction of Se (0) and Ni (0) is obtained toward the lower potential. e experiment is accomplished using Se (IV) and Ni (II) of 20 µg/ L in 0.1 M PBS and 5 mM Fe(CN) 6 to 7 times of repetition at a scan rate of 25 mV/s, which resulted in only a slight variation in current.

Calibration Curves and Detection Limit.
e fabricated Mn 3 O 4 -Cn electrodes exhibit excellent reusability, repeatability, and stability. e electrodes are recyclable up to 35 successive runs with relative standard deviation <3%, while a substantial reduction is observed after it, which may be due to consecutive deposition of Se (0) and Ni (0) during reduction [22,23]. Reproducibility of Mn 3 O 4 -Cn electrode is confirmed using five different sets, which shows outstanding reproducibility with a standard deviation of ∼0.8%. To estimate the stability of this electrode, CV response is recorded for 14 days. We noticed that the current response is diminished by ∼4.2%, which still could be enhanced by storage situations.
us, the Mn 3 O 4 -Cn electrode provides improved stability, reusability, and reproducibility characteristics toward selenium (IV) and Ni (II). It is observed that a linear calibration equation exists from 5 to 250 μg/L. Linear regression model for Se (IV) and Ni (II) for 5 to 250 μg/L is fitted using the following expression: e calibration curve is plotted using peak current versus the concentration of Se (IV) and Ni (II) under the recommended conditions from Figures 6 and 7. ere is a linear relationship held over the concentration ranges of 5-250 μg/L (correlation coefficient R 2 � 0.952) for Se (IV) and Ni (II) (R 2 � 0.984) as depicted in Figure 7. e limit of detection (LOD) is calculated as where C L , S D , and m are the limit of detection, the standard deviation of the blank, and slope of the calibration graph, respectively [22]. LOD obtained is 0.533 μgL −1 Se (IV), and for Ni (II), it is 0.718 μgL −1 , respectively, with a detection time of 18 s. e fabricated sensor displays a satisfactory detection limit and a varied detection range essential for practical utility. Table 1 reveals the superior characteristics of Mn 3 O 4 -Cn electrode for detection of selenium and nickel comparable to reported values.
e obtained results after reacting with each ion confirm the negligible interference on Se (IV) and Ni (II) for the current at 50 µg/L, 100 µg/L, and 100 µg/L concentrations of interfering ions (Figure 8). e species evaluated could not generate any current signal within the range of potentials used in voltammograms. However, a decrease in current is observed in the presence of 1000 µg/L of Li (II). But, the probabilities of existence of this much concentration of interfering ions along with Se (IV) and Ni (II) in real water are negligible. So, the developed sensor can be used for the detection of Se (IV) and Ni (II) in real water. It is important to access inference from organic compounds too since natural water contains them [24,25]. We did the extraction of each organic compound from the sample and dried it.  After the drying process, the weight of each compound is taken [11-20, 23, 24]. e tolerance levels of organic compounds like L-cysteine, sodium dodecylbenzene sulfonate, polyethylene glycol mono-p-nonyl phenyl ether, and EDTA-2Na are 1.2 mg L −1 , 0.9 mg L −1 , 1.0 mg L −1 , and 0.09 mg L −1 , respectively [6].

Real-Time Analysis.
e analysis is done for the purpose of the practical application of Pt/Mn 3 O 4 -Cn electrochemical sensor in environmental detection of Se (IV) and Ni (II) in the real sample, tap water (St. Berchmans College Lab) and lake water (South Kerala, India). e pH of the samples is at the optimum condition as mentioned above. e standard addition method is used as a validation method to evaluate the analytical applicability in the determination of selenium and nickel using the proposed technique. e acquired water samples are first filtered through a 0.45-micron filter to get rid of suspended particles and then added with 10 µg/L of selenium and nickel. With the standard addition method, 10 mL electrochemical cells comprising 2 mL of both tap water and lake water sample alternatively, 1 mL 0.1 M PBS, and known Se (IV) and Ni (II) concentrations are analyzed consecutively. Even without spiking, the Mn 3 O 4 -Cn electrode is sensitive enough to sense Se (IV) and Ni (II) in tap water ensuring its applicability and sensitivity. It is necessary to validate the accuracy of this technique; therefore, the electrodes are tested via a certified reference material (TM-15) with a concentration of 15 µg/L Se (IV) and Ni (II). However, another approach using ICP-MS for water samples is done to specifically rule out any interference from foreign ions. We observed reasonable recovery, which showed the capability of Mn 3 O 4 -Cn for detecting Se (IV) and Ni (II) in real water samples. e analysis is carried out in quadruple where a relative error of 5.98% is obtained for Pt/Mn 3 O 4 -Cn. However, no major differences between the theoretical and experimental values are observed. Table 2 summarizes the analytical results of Se (IV) and Ni (II) concentrations in tap water and lake water on four replicate analyses. e lake water sample has higher concentrations of selenium with reference to the World Health Organization (WHO) guideline of 10 μg/L for Se (IV) and 20 μg/L for Ni (II). But, a higher concentration is noted during the summers while, in the monsoon season, lower concentrations are recorded. is might have occurred due to dilution from   higher rainwater runoff and flows. e concentration of Se (IV) and Ni (II) in tap water are within the limit of WHO guidelines. Our projected technique could be used for the speciation investigation of natural water samples with adequate results. We also collected the sample from the lake during summer when the factory is closed due to a strike for above one month. e results are noteworthy as there is a decrease in the traces of Se (IV) and Ni (II) in the collected samples and detailed out in Table 3.

Conclusions
e detection of Se (IV) and Ni (II) using Mn 3 O 4 -Cn is successfully demonstrated. In low concentrations, CV is a more appropriate method as Se (IV) and Ni (II) exhibit characteristic voltammetric waves of peak heights relevant to selenium concentration. e working electrode of Mn 3 O 4 -Cn is fabricated and used to evaluate the voltammetric behavior of selenium and nickel using the cyclic voltammetry method. CV is a relevant technique to understand electron transfer at the electrode-solution interface, especially for low concentrations. Supporting electrolyte concentration, scan rate, and the number of cyclic repetition is observed for their influence on selenium and nickel peak shape and intensity e results from CV indicate that the presence of Mn 3 O 4 helps to enable the electrochemically reversible electron transfer. Optimization of the parameters of this technique has been carried out for consistent and reproducible results by parallel assemblies of Mn 3 O 4 , which amplifies the current response. Mn 3 O 4 -Cn electrode offers advantages of low cost, low detection limit, low response time, and high sensitivity.
e Se and Ni redox reaction appears to be kinetically faster and more reversible. Selenium and nickel concentrations are detected in tap water and lake water. Pt/Mn 3 O 4 -Cn is successfully working on the analysis of real water samples, and the results allied with the ICP-MS analysis with recoveries greater than 96%. Due to the practical protection of fabricated electrodes toward frequently found interfering ions during quantification, it is effectively verified for real water sample analysis. We had limitations in obtaining results for concentrations below 5 µg/L and for higher concentrations of the detected species. Further work is underway to demonstrate the use of these sensors for testing sediment samples providing rapid on-site analysis for selenium and nickel using differential pulse voltammetry.

Data Availability
Data obtained have been plotted in graphs.

Conflicts of Interest
e authors declare that there are no conflicts of interest. conduct electrochemical analysis. e authors also