- Research article
- Open Access
Tailoring of nanostructured material as an electrochemical sensor and sorbent for toxic Cd(II) ions from various real samples
© The Author(s). 2018
- Received: 16 June 2018
- Accepted: 13 September 2018
- Published: 10 October 2018
The aim of the present study is the fabrication of electrochemical sensor and sorbent for toxic Cd(II) ion using ion-imprinting technique on vinyl-functionalized multiwalled carbon nanotube. Multiwalled carbon nanotube-based ion-imprinted polymer (MWCNT-IIP) were synthesized using meth acrylic acid as the functional monomer, N,N′methylene-bis-acrylamide as the cross linking agent, and potassium peroxo disulphate as an initiator. The template and porogen used were cadmium chloride and water. To know the importance of MWCNT, ion-imprinted polymer without MWCNT was also prepared. For the purpose of comparison, non-imprinted polymers were also synthesized. The synthesized products were analyzed by FT-IR, XRD, TEM, EDAX, and TGA. An electrochemical sensor was made up by modifying platinum electrode with MWCNT-IIP. Experimental factors that control the routine of the sensor were investigated and optimized. Under optimal conditions, a calibration curve was obtained with a detection limit of 0.03 μM by using differential pulse voltammetric technique. Selectivity studies show irrelevant significance with Zn (II), Cu (II), and Ni (II) ions. The feasibility of modified platinum electrode shows a prospective application in real water sample collected from a lake, pigments, cosmetics, and fertilizers. The synthesized nanostructured material is also used for the extraction of Cd(II) ion from real water samples. The maximum adsorption of Cd(II) by various imprinted and non-imprinted sorbents was calculated, and it was found that maximum adsorption takes place at pH 6. The kinetic studies show that the adsorption of Cd(II) increases with time and reaches equilibrium at 70 min and the kinetic data follow pseudo-second-order kinetics. The adsorption data fitted to the Langmuir adsorption model which confirms the monolayer formation of an IIP layer on MWCNT surface. The selectivity co-efficient of the imprinted sorbent shows high selectivity and specificity towards Cd(II) ion than other metal ions.
- Cadmium (II)
- Molecular imprinting
- Cyclic voltammetry
A new method for Cd(II) ion sensing with the advantage of ion-imprinted and electrochemical sensor.
The sensor displays lower detection limit compared with the existed methods.
MWCNT-IIP sensor displays good selectivity towards Cd(II) ion over other metal ions.
Sensor with high sensitivity can be used for sensing and extraction of Cd(II) ion in real samples.
Metal pollution in the environment is generally due to the industrial development (Das et al. 2016). Heavy metal ions such as cadmium, lead, chromium, and arsenic are highly toxic to the human health (Das et al. 1997). Cadmium grades ninth among the toxic substance by US disease and poison registry. Cadmium is a dispensable heavy metal which inhibits the enzyme, and it has been considered as the most hazardous pollutant due to its solubility and toxicity in the surroundings. Cd(II) is classified as a type I carcinogen. Exposure to cadmium leads to a variety of undesirable effects such as cancer, growth in the liver and kidney, and softening of bones (Waalkes 2003; Parameswaran and Mathew 2014). There are lots of methods which reported the detection of cadmium ions such as atomic absorption spectrometry (Parham et al. 2009), inductively coupled plasma mass spectrometry (D’Ilio et al. 2005), and inductively coupled plasma atomic emission spectroscopy (Zougagh 2002). Most of these techniques are even though perceptive and precise but are costly and laborious and cause difficulties for in situ sensing. Thus, it is vital to build up an uncomplicated, fast, and cost-effective method for Cd(II) ion detection.
Currently, molecular imprinting technique has been widely used for the development of artificial receptors with high selectivity and affinity towards the target molecule. Due to its advantage such as stable, inexpensive, versatile, and easy to prepare, its application spread to many fields in sensors, catalysis, separation technique, etc. (Kan et al. 2008; Sooraj and Mathew 2014; Mazzotta and Malitesta 2010). MWCNT has large surface area and pore size, so it has been used for electrochemical sensing as the supporting materials for the IIPs. The electrochemical detection is usually performed with a three electrode system containing a working electrode, a reference electrode, and a counter electrode. The working electrode can be modified with different materials for specific recognition of metal ions (Gumpu et al. 2015; Gong et al. 2004; Martínez-Huitle et al. 2010). The presence of metal ions causes the alteration of current, potential which can be used for the sensing of metal ions. Voltammetric method is used for this sensing because of their intrinsic specificity, fast response, low cost, and soaring sensitivity and due to the short analysis time (Wong et al. 2015; Lee et al. 2016a).
In the present work, cadmium ion-imprinted polymers based on multiwalled carbon nanotubes (MWCNT-IIP) for the sensing of cadmium ions have been developed using cadmium ion as a template, methacrylic acid (MAA) as a functional monomer, and N,N′methylene-bis-acrylamide (NNMBA) as a cross linking agent. For smooth interaction and bonding between carbon nanotubes and polymer, MWCNTs are modified with carboxyl functional group which is then converted into acid chloride group and finally to vinyl-functionalized MWCNT. Then, NNMBA and MAA would copolymerize with vinyl groups on the surface of MWCNTs, forming uniform IIP layer. Cyclic voltammetry (CV) technique and differential pulse voltammetry (DPV) technique are used for the sensing of Cd(II) ions using modified platinum electrode (Lee et al. 2016b; Roy et al. 2014). The potential for the sensing of MWCNT-IIP-Pt towards Cd(II) ion was tried against real samples. Subsequent to the flourishing sensing of Cd(II) ion, MWCNT-IIP was also utilized for the extraction of Cd(II) ions from the same sample itself.
Materials and methods
MWCNT of 10–15 nm was obtained from Reinsto Nano ventures private limited India; N,N′methylene-bis-acrylamide (NNMBA), potassium peroxo disulphate (K2S2O8), and nafion were purchased from Sigma Aldrich (Germany). Thionylchloride (SOCl2), dimethyl formamide (DMF), and tetrahydrofuran (THF) were obtained from Merck (India). Methacrylic acid (MAA) and metal chlorides were purchased from SRL (India).
The FT-IR spectra were recorded on a Perkin-Elmer 400 FT-IR spectrophotometer. X-ray diffractogram was recorded by PAN analytic XPERT-PRO. TEM-EDAX was investigated by a JEOL-2100 model tunneling electron microscope. Surface area measurements were carried out using Thermofisher Scientific Surfer Analyzer SRFA13/0004. Absorption spectra of Cd(II) ion were recorded by Perkin-Elmer Atomic Absorption Analyser 300. The wavelength selected for the determination of Cd(II) ion is 228.80. Thermo gravimetric analysis was obtained from NETZCHSTA449C instrument. Electrochemical studies were carried out with an electrochemical work station (Biologic SP-200), and cyclic voltammetry and differential pulse voltammetry technique were conceded with platinum wire auxiliary electrode, saturated calomel reference electrode, and a platinum electrode modified with imprinted MWCNT-IIP which was used as the working electrode.
Purification and functionalization of MWCNT
The purchased MWCNT was purified as reported in literature (Shen et al. 2008). The pristine MWCNT was heated in an air oven at 650 °C for 2 h and then allowed to cool followed by the treatment with hydrochloric acid for 2 h. The above mixture was then centrifuged to get the solid. It was then washed with distilled water until the pH becomes neutral. The solid thus obtained was filtered and dried under vacuum.
Purified MWCNT (0.5 g) was treated with 60 ml of con.HNO3 and sonicated for 10 min. This sonicated mixture was refluxed at 60 °C for 24 h. The mixture was filtered and washed with distilled water until the pH of the final wash came down to neutral. The filtered solid was dried under vacuum to obtain carboxyl group incorporated MWCNT (MWCNT-COOH). Carboxylic acid capacity of MWCNT-COOH is calculated by acid–alkali titration and found to be 1.8 mmol/g.
MWCNTs-COOH, 0.4 g, was suspended in a mixture of 10 ml thionyl chloride and 30 ml chloroform in a round bottom flask, refluxed for 24 h at 60 °C. The solid was washed by anhydrous tetrahydrofuran to remove the excess thionyl chloride present on it. It is then dried under vacuum to obtain MWCNT-COCl.
For introducing vinyl group on MWCNT, MWCNTs-COCl was dispersed in 30 ml THF. Twenty microliters of allyl amine dissolved in 10 ml of DMF was added drop wise to the mixture. The mixture was stirred at 60 °C for 24 h and washed with anhydrous THF to remove unreacted reagent. The solid collected was washed with THF and dried under vacuum to get vinyl-functionalized MWCNT (MWCNT-CH=CH2).
Synthesis of IIP on the surface of vinyl-functionalized MWCNTs
Vinyl group incorporated MWCNT (MWCNT-CH=CH2, 0.6 g) was added to the porogen in a 250-ml round bottom flask. Cadmium chloride (0.6 g) and methacrylic acid (0.025 mmol) were added to it and stirred well. Finally, NNMBA (8 g) and potassium peroxo disulphate (100 mg) were also added to it. Then, the mixture was stirred at 70 °C. The obtained polymer was washed with distilled water until no cadmium chloride was detected by AAS in the eluent. MWCNT-NIP was also prepared by the same procedure without using the template molecule. For the purpose of comparison, imprinted polymer was synthesized without using vinyl-functionalized MWCNT and also non-imprinted polymer was prepared without using template molecule.
Modification of platinum electrode and electrochemical sensing
Application of sensor using real samples
The modified platinum electrode was analyzed for Cd(II) ion in real samples collected from a lake (Ashtamudi Lake), pigments, cosmetics, and fertilizers. Real samples were filtered through the 0.22-μm polar size filter paper and used for the analysis. The abovementioned procedure was followed with the sample using MWCNT-IIP and MWCNT-NIP. Electrochemical studies were carried out at a potential range of − 300 to 1000 mV with a scan rate of 100 mV/s.
Optimization of adsorption study
where Q (μmol/g) is the adsorption capacity, V (ml) is the volume of the solution, M (g) is the mass of the polymer, and Co and Ce (mmol L−1) are the concentration of template before and after extraction process.
Application of the extraction method
The water samples collected from a lake, pigments, cosmetics, and fertilizers were filtered using Whatman no. 1 filter paper. The Cd(II) ion extraction using various ion-imprinted and non-imprinted polymers were carried out using the above explained extraction procedure.
The surface area analysis of various tailored polymers was investigated using BET method. The specific surface area of the MWCNT-IIP and IIP calculated was 296.8871 and 176.2139 m2/g. Compared with IIP, MWCNT-IIP has a high specific surface area, due to the presence of MWCNT which could effectively enhance the adsorption of Cd(II) on the surface of the adsorbent.
EWC% of Cd(II) ion binding on MWCNT-IIP, IIP, MWCNT-NIP and NIP
Effect of concentration
Effect of pH
Effect of scan rate
Differential pulse voltammetry
Application of modified electrode
Sensing of Cd(II) ion in real samples by the modified electrode
Cd (II) ion
1.216 ± 0.0120
99.30 ± 0.0020
3.319 ± 0.0030
99.42 ± 0.0010
0.997 ± 0.0010
99.28 ± 0.0030
4.236 ± 0.0020
99.38 ± 0.0010
0.829 ± 0.0010
68.61 ± 0.0030
2.210 ± 0.0012
66.96 ± 0.0010
0.6500 ± 0.0030
65.70 ± 0.0100
2.659 ± 0.0020
63.31 ± 0.002
Comparison with other reported electrodes
Comparative studies of modified platinum electrode with other reported sensors
Template–monomer interaction study
Effect of solvent
Effect of concentration
Effect of pH
Effect of time
Effect of mass of the polymer
selectivity factor various adsorbent verses Cd(II) ion
α MWCNT-IIP / α MWCNT-NIP
α IIP / α NIP
Extraction of Cd(II) ion from real samples
Analysis of real sample
Cd (II) ion
1.221 ± 0.0010
99.19 ± 0.0120
3.905 ± 0.0020
99.64 ± 0.0030
1.110 ± 0.0010
99.19 ± 0.0010
4.027 ± 0.0120
99.52 ± 0.0010
0.819 ± 0.0010
67.68 ± 0.0030
2.565 ± 0.0010
65.92 ± 0.0020
0.742 ± 0.0020
67.45 ± 0.0030
2.629 ± 0.0010
65.68 ± 0.0010
This study reveals a simple and well-designed approach for the sensing of Cd(II) ions from the real samples through adsorption on mutiwalled carbon nanotube-based ion-imprinting polymer. The modified platinum electrode exhibited a low detection limit of 0.03 μM and a good selectivity, practical applicability for the sensing of Cd(II) ions. At pH 5, maximum sensing was obtained. The plot of current versus concentration of Cd(II) ion gives a straight line passing through the origin, portraying that it is acquiescent for electro analysis of Cd(II) ion. Extraction studies were carried out by batch wise method. The effect of concentration, pH, contact time, and reusability was studied in details. At pH 6, maximum adsorption was obtained. The Langmuir model of cadmium sorption was fitted with Langmuir isotherm which assumed that the sorption was monolayer adsorption with no interaction between adsorbed ions. The adsorption isotherm verified that a homogeneous distribution of binding site with monolayer coverage within MWCNT-IIP takes place, and the sorption process is chemisorption. A pseudo-second-order rate equation can be used to determine the adsorption of Cd(II) onto MWCNT-IIP. Investigations showed that the selectivity of MWCNT-IIP was significantly higher than that of bulk IIP and it can be used as perceptive substances for the sensing of Cd(II) from real samples. This low cost, superficial synthetic route and strength of the surface-imprinted nanostructure IIP provide a useful intermediate for Cd(II) ion extraction from other metal ions which was also confirmed by its high selectivity coefficient.
The analytical facilities from Cashew Export Promotion Council of India, Kollam, Kerala, is gratefully acknowledged.
There is no funding for this study.
Availability of data and materials
The authors have no data to share since all data are revealed in the submitted manuscript.
MWCNT-based ion-imprinted polymer used as an electrochemical sensor and sorbent for toxic Cd(II) ions from real samples was discussed in this paper. Both authors read and approved the final manuscript.
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