Synthesis and Characterization of Porous MnCo2O4 for Electrochemical Determination of Cadmium ions in Water Samples

To utilize the maximum activity of nanomaterials, it was specifically synthesized by appropriate physicochemical properties. In that aspect, we have described the synthesis of porous MnCo2O4 by simple chemical route and applied for the selective detection of cadmium (Cd (II)). The as-prepared porous MnCo2O4 was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) adsorption isotherm, X-ray diffraction pattern analysis (XRD), Fourier transform infra-red spectroscopy (FT-IR), energy dispersive X-ray (EDX) and electrochemical techniques. The porous MnCo2O4 exhibited an excellent electrochemical behaviour and good analytical response towards the determination of Cd (II). Those analytical factors such as pH, deposition potential and deposition time are optimized by using differential pulse anodic stripping voltammetry (DPASV). A wide linear concentration range from 2.3 to 120 µg L−1, limit of detection (LOD) of 0.72 µg L−1 and the limit of quantification (LOQ) of 0.91 µg L−1 were achieved for determination of Cd (II). The selectivity of the developed sensor was explored in the presence of co-interfering ions. Also our sensor exhibits a good stability, reproducibility and repeatability. In addition, the practicability of proposed sensor was evaluated for the detection of Cd (II) in real water samples.

and environmentally friendly [27][28][29] . Hence, MnCo 2 O 4 has been accepted as an alternative electrode material for the Cd sensor due to its excellent conductivity and tunable structural features 30,31 . MnCo 2 O 4 lies on the normal spinel family that consists of Mn 2+ ions in tetrahedral sites, Co 3+ ions in the octahedral sites and O −2 ions tend to coordinate both positions to frame the face centred cubic structure 32 . Besides sensors, MnCo 2 O 4 has been widely utilized in alkaline fuel cells 33 , solid oxide fuel cells 34 , water treatment 35 and glucose sensors 36 . In this work, we have demonstrated the synthesis of porous MnCo 2 O 4 by simple chemical route at the low processing temperature. To the best of our knowledge, this is the first time we have reported the determination of Cd by porous MnCo 2 O 4. Moreover, MnCo 2 O 4 exhibited high sensitivity, excellent selectivity, wide linear concentration range and acceptable storage stability for the determination of Cd (II). The overall preparation and electrochemical pathway for the determination of Cd (II) was illustrated in Fig. 1.

Results and Discussion
Material characterizations. The crystallinity of as-prepared MnCo 2 O 4 was confirmed by X-ray diffraction (XRD) analysis. Figure 2A depicts the XRD patterns of MnCo 2 O 4 exhibiting the noisiest diffraction peaks which can be assigned as spinel compound. Those noises are due to the low crystallite size of the as-prepared compound. MnCo 2 O 4 has a very noisy reflection peaks which indexed as a face centred cubic structure of MnCo 2 O 4 with the space group fd3m (JCPDS No. 23-1237). The manganese and cobalt ions are well dispersive over octahedral and tetrahedral interstices to form the mixed valence ternary oxides as spinel MnCo 2 O 4 crystal. The Debye-Scherrer formula (1) was used to calculate the average crystallite size of the as-prepared compound which exhibited the average crystallite size as 10 nm 37 .
= . λ β θ D 0 9 /( cos ) (1) λ is the X-Ray wavelength, β is the full width at the half maximum and θ is the diffraction angle. The FT-IR spectrum is a very essential tool to investigate the functional group analysis. Figure 2B displays the FT-IR spectrum of MnCo 2 O 4 , the stretching frequency at 3405 cm −1 reveals the broad band adsorption peaks for adsorbed water (H-O-H) 38 . It can be clearly seen that the angular deformation of the adsorbed water molecules band appeared at 1628 cm −1 . The bands at 650 and 561 cm −1 are corresponding to the metal oxide characteristic   (Fig. 3). Such type of the surface morphology was having a good adsorption capacity towards toxic metal ions.
The particle size and surface morphology of the porous MnCo 2 O 4 were investigated by TEM. Figure 4A shows the TEM image of MnCo 2 O 4 which exhibited the particles like porous morphology. The high magnification TEM image in Fig. 4B, displayed a distinct lattice fringes with an interplanar distance indexed to the crystal lattice (311) and (220) planes of spinel MnCo 2 O 4 . The specific surface area (SSA) and pore size of MnCo 2 O 4 were examined by the N 2 adsorption-desorption isotherms. MnCo 2 O 4 adsorption isotherms and pore size distribution (PSD) curves were shown in Fig. 4(C,D). The specific surface area (58.82 m 2 g −1 ) was calculated by Brunauer-Emmett-Teller (BET) method for the as-prepared MnCo 2 O 4 compound. The wide pore size distribution range (2-33 nm) was observed and also the total pore volume was calculated as 0.2454 cm 3 g −1 by using the BJH method.   Figure 6 shows the electrochemical response of bare GCE (a) which exhibited the weak redox peak for 50 µg L −1 of Cd (II). It can be obviously seen that there were no peaks appeared for MnCo 2 O 4 /GCE (b) in the absence of Cd (II). However, a sharp and well-defined redox peak was observed for the porous MnCo 2 O 4 /GCE (c) with higher current response when compared with bare GCE. The obtained oxidation peak current from the porous MnCo 2 O 4 was higher than that of the bare GCE. In addition, the porous MnCo 2 O 4 provides a highly rough surface of the electrode which has a larger surface area with more active sites for Cd (II) accumulation.
Optimization of parameters. The effect of pH (acetate buffer solution) on the electrochemical performance of our proposed electrode was examined in the range of pH 2.0-6.0. It can be seen from the Fig. 7A, the maximum peak current was appeared at pH 5.0. Therefore, the pH 5.0 acetate buffer solution was used as electrolyte for the further detection of the Cd (II). The deposition potential is an important parameter in the detection of Cd (II). Therefore, the effect of deposition potential was investigated in presence of Cd (II) at pH 5.0 solution after 200 s of accumulation in the potential range from −0.6 to −1.2 V. Figure 7B    peak current was increased with increasing the concentration of Cd (II). The gradual positive shift in the stripping potentials of Cd (II) might be due to the increase in the interfacial thickness of Cd (II)-MnCo 2 O 4 . A linear concentration range was obtained from 2.3 to 120 µg L −1 with the correlation coefficient of R 2 = 0.9954 (Fig. 8B). The limit of detection was calculated as 0.72 µg L −1 , limit of quantification was obtained as 0.91 µg L −1 . The sensitivity of the proposed sensor was calculated by dividing the slope of the calibration plot by electroactive area, the calculated sensitivity is 7.7355 µA µg −1 L cm −2 . The analytical performances of the proposed sensor have been compared with previously reported other sensors (Table 1).
Interference study. In order to evaluate the selectivity of the fabricated sensor, it was investigated in the presence of various potentially interfering ions. Figure 9 shows the current responses for (50 µg L −1 ) Cd (II) in the presence of a 3-fold excess concentration of metal ions such as Cu 2+ , Pb 2+ and Hg 2+ . The results showed that the interference signal was less than 1% for interfering ions. Therefore, the proposed sensor material selectively    Real sample analysis. In order to evaluate the reliability of proposed sensor, the determination of Cd (II) was examined in water samples. An optimized experimental condition was applied for the detection of Cd (II) in tap water. The standard addition method was used to detect Cd (II) and the calculated recovery results were presented in Table 2. It can be seen that the average recoveries of Cd (II) were 95.33-100.3% in tap water samples. These results evinced that the practicability of the proposed sensor towards the determination of Cd (II) in water samples.
Stability, repeatability and reproducibility. The five different sensing electrodes were prepared under the same experimental conditions followed from section (Electrochemical behavior of Cd (II)) and observed the efficiency of the Cd (II) detection. The fabricated sensors showed almost same response for all the electrodes and revealed reproducibility with the RSD of 3.80% for the determination of Cd (II). These sensor electrodes were stored at room temperature over 30 days. After that it was retained about 90% of its initial response which confirmed that MnCo 2 O 4 modified electrode has good storage stability. In addition, the sensor also has a good repeatability with RSD value of 3.2%, for the five repeated successive measurements of single modified electrode. These results disclose the proposed sensor material has excellent stability repeatability and reproducibility which endorsed that MnCo 2 O 4 /GCE is suitable for the practical applications.

Conclusions
In summary, the honeycomb-like porous MnCo 2 O 4 spinel was prepared from the simple facile method and subsequent calcination.

Experimental
Chemicals and apparatus. Manganese chloride, cobalt chloride, cadmium nitrate, sodium hydroxide and ascorbic acid were obtained from Sigma-Aldrich. The supporting electrolyte of acetate buffer (pH 5) solution was prepared by using 0.05 M sodium acetate and glacial acetic acid. All the chemicals used were of analytical grade and used as received without purification. Cyclic voltammetry (CV) and differential pulse anodic stripping voltammetry (DPASV) measurements were performed by the CHI 900 electrochemical workstations. Scanning  electron microscopy (SEM) was performed using Hitachi S-3000 H electron microscope, Fourier transform infrared spectroscopy (FT-IR) was carried out by using JASCO FT/IR-6600 instrument. The conventional three-electrode system was used for the electrochemical experiments, the modified glassy carbon electrode (GCE) was used as a working electrode (electrode area: 0.07 cm 2 ), saturated Ag/AgCl used as a reference electrode and platinum electrode used as the auxiliary electrode. Fabrication of electrode and operating condition. Glassy carbon electrode was precleaned with alumina powder and sonicated about 2 mins in ethanol and double distilled water. The adsorbed alumina slurry on the surface of GCE was removed by washing with double distilled water and dried in hot air oven. As-prepared composite was re-dispersed in water and sonicated to get well uniform suspension. A mirror cleaned GCE was fabricated with the suspension of MnCo 2 O 4 by drop cast (6 µL) method and dried in a hot air oven at 35° C for 10 min. The fabricated MnCo 2 O 4 /GCE was further used for the electrochemical measurements. All the experimental conditions were optimized by voltammograms. The electrochemical cell contains 10 mL of desired pH of acetate buffer (0.05 M) solution. The differential pulse Anodic stripping voltammograms were recorded from −0.3 to 1.1 V, applied potential of preconcentration of Cd (II) was −1.0 V and 200 s of preconcentration time. The calibration curve was obtained by plotting the peak current against deposition of cadmium potential range and deposition of cadmium accumulation time. The electrode was polished after each measurement with a clean paper.