Introduction

Discharge of toxic heavy metal ions to the environment causes most harmful issues to the human and ecosystem. For occasion, the industry of ceramic, semiconductors, pharmaceutical, metallurgical, agricultural and petrochemicals contaminates the surrounding water and soil1,2,3. The Cd is one of the toxic heavy metal, which causes nephrotoxicity, flu-like symptoms, renal tubular dysfunction, bone demineralization and cancers even for the intake of trace level through contaminated food and water4,5,6,7,8. Therefore, the international agency of research of cancer (IARC) classified the cadmium as a carcinogenic substance9. Moreover, the world health organization (WHO) and United States Environmental Production Agency (EPA) have defined the maximum level of Cd in drinking water as 0.003–0.005 µg L−110, 11. Therefore, the detection of Cd is essential in water treatment and soil analysis. There are several methods such as inductively coupled plasma mass spectrometry12, 13, inductively coupled plasma atomic emission spectrometry14, atomic absorption spectrometry15, atomic fluorescence spectrometry16 and UV–Visible spectroscopy17. All the aforementioned methods offer good precision and high resolution, however, they are more expensive and needs trained technicians to handle the instruments. Moreover, the determination of Cd by those methods is time-consuming process. In contrast, the electrochemical methods have been accepted for the detection of heavy metal ions because of their excellent sensitivity, rapid analysis and low cost18, 19. Hence, the electrochemical method has emerged as a powerful technique for the detection of heavy metal ions when compared to other methods.

In recent years, variety of porous metal oxides such as MgO20, Co3O4 21 and NiO22 were reported for the electrochemical sensors. Because, these porous metal oxides have high surface area and open porous structure which helps in the detection of heavy metal ions. The transition metal oxides with a spinel structure have attracted more interest in the wide area of research field due to their unique properties such as magnetic, electrical and optical properties23,24,25. The common chemical formula of the spinel is AB2O4, where A and B are the divalent and trivalent metal ions, coordinated in tetrahedral and octahedral sites, respectively26. Among the materials developed for the Cd detection, the spinel metal oxides are the promising materials due to high earth abundance, low cost and environmentally friendly27,28,29. Hence, MnCo2O4 has been accepted as an alternative electrode material for the Cd sensor due to its excellent conductivity and tunable structural features30, 31. MnCo2O4 lies on the normal spinel family that consists of Mn2+ ions in tetrahedral sites, Co3+ ions in the octahedral sites and O−2 ions tend to coordinate both positions to frame the face centred cubic structure32. Besides sensors, MnCo2O4 has been widely utilized in alkaline fuel cells33, solid oxide fuel cells34, water treatment35 and glucose sensors36. In this work, we have demonstrated the synthesis of porous MnCo2O4 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 MnCo2O4. Moreover, MnCo2O4 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.

Figure 1
figure 1

Preparation of porous MnCo2O4 and electrochemical detection of Cd2+.

Results and Discussion

Material characterizations

The crystallinity of as-prepared MnCo2O4 was confirmed by X-ray diffraction (XRD) analysis. Figure 2A depicts the XRD patterns of MnCo2O4 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. MnCo2O4 has a very noisy reflection peaks which indexed as a face centred cubic structure of MnCo2O4 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 MnCo2O4 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 nm37.

$${\rm{D}}=0.9{\rm{\lambda }}/({\rm{\beta }}\,\cos \,{\rm{\theta }})$$
(1)

λ is the X-Ray wavelength, β is the full width at the half maximum and θ is the diffraction angle.

Figure 2
figure 2

(A) XRD pattern (B) FTIR spectrum of porous MnCo2O4 spinel compound.

The FT-IR spectrum is a very essential tool to investigate the functional group analysis. Figure 2B displays the FT-IR spectrum of MnCo2O4, 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 peaks which revealed the formation of MnCo2O4 compound39. The surface morphology of MnCo2O4 was investigated by SEM. The SEM images of MnCo2O4 show the flake like morphology that consists of uniform macropores (Fig. 3). Such type of the surface morphology was having a good adsorption capacity towards toxic metal ions.

Figure 3
figure 3

SEM image of porous MnCo2O4 spinel compound.

The particle size and surface morphology of the porous MnCo2O4 were investigated by TEM. Figure 4A shows the TEM image of MnCo2O4 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 MnCo2O4. The specific surface area (SSA) and pore size of MnCo2O4 were examined by the N2 adsorption-desorption isotherms. MnCo2O4 adsorption isotherms and pore size distribution (PSD) curves were shown in Fig. 4(C,D). The specific surface area (58.82 m2 g−1) was calculated by Brunauer-Emmett-Teller (BET) method for the as-prepared MnCo2O4 compound. The wide pore size distribution range (2–33 nm) was observed and also the total pore volume was calculated as 0.2454 cm3 g−1 by using the BJH method.

Figure 4
figure 4

TEM image (A) and HR-TEM (B) of porous MnCo2O4 spinel compound (C) N2 adsorption–desorption isotherms and (D) the pore size distribution of the MnCo2O4 spinel compound.

Moreover, the presence of elements in MnCo2O4 compound was confirmed by EDX analysis and shown in Fig. 5. The EDX spectrum of MnCo2O4 exhibits the signal for Mn, Co and O with the weight percentage of 11.47%, 26.12% and 65.41%, respectively. These results confirmed that the as-prepared compound was porous MnCo2O4 spinel.

Figure 5
figure 5

EDX spectrum and element weight % bar diagram of MnCo2O4.

Electrochemical behavior of Cd (II)

CV response of MnCo2O4/GCE was tested in the presence of 50 µg L−1 Cd (II) at a scan rate of 50 mVs−1 in acetate buffer solution. 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 MnCo2O4/GCE (b) in the absence of Cd (II). However, a sharp and well-defined redox peak was observed for the porous MnCo2O4/GCE (c) with higher current response when compared with bare GCE. The obtained oxidation peak current from the porous MnCo2O4 was higher than that of the bare GCE. In addition, the porous MnCo2O4 provides a highly rough surface of the electrode which has a larger surface area with more active sites for Cd (II) accumulation.

Figure 6
figure 6

CV response of bare GCE in the presence of Cd (II) (a) MnCo2O4/GCE (c) and in the absence of MnCo2O4/GCE (b) in the acetate buffer (pH 5) solution containing 50 µg L−1 of Cd (II).

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 depicts electrochemical activity for deposition potential showing the maximum current reached in the deposition potential of −1.0 V. Therefore, −1.0 V was fixed as the optimum deposition potential. The effect of the deposition time on MnCo2O4/GCE current response was also investigated from 50 s to 300 s. As shown in Fig. 7C, the current response was higher for the accumulation time of 200 s. Therefore, the 200 s of deposition time was chosen for the further experiment.

Figure 7
figure 7

Effect of (A) pH of acetate buffer (B) Deposition potential of cadmium (C) Deposition time of cadmium on the DPSAV current in acetate buffer solution contained 50 µg L−1 of Cd (II) ion.

The determination of MnCo2O4 was tested by the successive additions of different concentrations of Cd (II) ions (Fig. 8A). Response of MnCo2O4/GCE was recorded towards various concentrations of Cd (II) from 2.3 to 120 µg L−1. The noticeable current response was appeared for the addition of 2.3 µg L−1 of Cd (II), further the 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)-MnCo2O4. A linear concentration range was obtained from 2.3 to 120 µg L−1 with the correlation coefficient of R2 = 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).

Figure 8
figure 8

(A) Responses of MnCo2O4 modified GCE for the addition of different concentrations of Cd (II) from 2.3 to 120 µg L−1. (B) The corresponding linear plot for I pa vs. concentration of Cd (II).

Table 1 Comparison for the analytical performance of other reported methods.

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 Cu2+, Pb2+ and Hg2+. The results showed that the interference signal was less than 1% for interfering ions. Therefore, the proposed sensor material selectively detected Cd (II) in the presence of interferents. These studies resulted that MnCo2O4 exhibited good selectivity for the detection of Cd (II) in the presence of other interference. Therefore, the proposed sensor material is suitable for the practical applications.

Figure 9
figure 9

Effect of interference species on the detection of cadmium at MnCo2O4. Peak current response of 50 μg L−1 Cd (II) in the presence of 3-fold of Cu2+, Pb2+ and Hg2+.

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.

Table 2 Determination of Cd (II) in water samples by MnCo2O4 modified GCE.

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 MnCo2O4 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 MnCo2O4/GCE is suitable for the practical applications.

Conclusions

In summary, the honeycomb-like porous MnCo2O4 spinel was prepared from the simple facile method and subsequent calcination. The simple electrochemical technique was applied to detect the Cd ion by DPASV based on porous MnCo2O4 modified electrode. The developed sensor shows a wide linear concentration range from 2.3 to 120 µg L−1, lowest detection limit of 0.72 µg L−1 and excellent sensitivity of 7.7355 µA µg−1 L cm−2. The analytical performances of the developed sensor were comparable with the previous results. Moreover, MnCo2O4/GCE exhibits good storage stability, acceptable selectivity, excellent repeatability and reproducibility. In addition, the developed MnCo2O4/GCE validates the practicability towards the determination of Cd (II) in water samples.

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 cm2), saturated Ag/AgCl used as a reference electrode and platinum electrode used as the auxiliary electrode.

Synthesis of porous MnCo2O4 spinel oxide

For the preparation of porous MnCo2O4 composite, the molar ratio (1:2) of MnCl2 and CoCl2.6H2O were dissolved in 50 ml distilled water and stirred for 30 minutes. Then, 0.2 M NaOH was added drop-wise with constant stirring and after that 0.1 M ascorbic acid was slowly added to the suspension. Suspension temperature was maintained at 60–70° C for 1 hr. Finally, the precipitate was filtered and washed with ethanol and distilled water. The precipitate was dried at room temperature. After that the dried samples were calcined in a hot air oven at 450° C for 2 hr. Finally, the black color porous MnCo2O4 was obtained.

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 MnCo2O4 by drop cast (6 µL) method and dried in a hot air oven at 35° C for 10 min. The fabricated MnCo2O4/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.