Fabrication of novel polymer-modified graphene-based electrochemical sensor for the determination of mercury and lead ions in water and biological samples
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This paper presents the application of polyglycine-modified graphene paste electrode (PGMGPE) for the electrochemical detection of Hg (II) and Pb (II) ions in the water and biological samples.
The developed electrode was characterized by field emission scanning electron microscopy. Electrochemical techniques such as cyclic voltammetry and differential pulse voltammetry were used to study the behavior of metal ions.
The modification process improves the electrochemical behavior of heavy metal ions. The peak current varied linearly with the increase of the concentration leading to a detection limit of 6.6 μM (Hg (II)) and 0.8 μM (Pb (II)), respectively.
The developed electrode exhibits good sensitivity, selectivity, stability, and lower detection limit, and was successfully applied to the determination of heavy metal ions in water and biological samples with a good recovery range.
KeywordsHgCl2 PbCl2 Graphene Glycine Cyclic voltammetry Blood serum Water sample
Differential pulse voltammetry
Phosphate buffer solution
Polyglycine-modified graphene paste electrode
Bare graphene paste electrode
Field emission scanning electron microscopy
Linear sweep voltammetry
Differential pulse anodic stripping voltammetry
Square wave voltammetry
Carbon paste electrode
Chitosan/glassy carbon electrode
Glassy carbon electrode/polyaniline
Heavy metal ions are one of the most hazardous pollutants in the environment and are highly toxic. Mainly, environmental pollution by heavy metals is attributed to lead, cadmium, mercury, and copper. Trace heavy metal elements are fundamental to living organisms for a normal and healthy life. Excessive level of heavy metal pollution in the environment could cause harm (Li et al. 2013; Guascito et al. 2008). The accumulation of heavy metal ions in the human body can cause diseases in the central nervous system, liver, kidney, and skin (Bodo et al. 2010). So, it has become an important task to develop a cost-effective and sensitive method for the detection of heavy metal ions. Currently, various techniques have been reported for the detection of heavy metals such as atomic absorption spectroscopy (Liu et al. 2005) and fluorescence spectrometry (Liu et al. 2005). However, these methods are expensive and time-consuming. In contrast, electrochemical methods (Huang and He 2013; Gherasim et al. 2014; Huakun et al. 2016; Sathish Reddy et al. 2012a, b, 2018; Gururaj et al., 2018; Ongera Gilbert et al. 2009; Mahanthesha et al. 2010; Liu et al, 2012) have advantages over the above techniques such as being inexpensive, highly sensitive, and easy to use. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are sensitive techniques for the detection of heavy metals in water.
Many approaches are used for the modification of the working electrode surface by chemicals such as thiolamine (Fu et al. 2013), polymers (Ahuja et al. 2007; McQuade et al., 2000; Mohadesi and Taher, 2007), and monomolecular ligands (Canpolat et al. 2007; Zheng et al. 2006; Zhang et al., 2002; Anandhakumar and Mathiyarasu 2013). Graphene shows a wide application as an electrode material, because it shows very high electrical conductivity, good mechanical properties, fast electron transfer rate, and a high specific surface area (Zhao et al. 2012). Graphene has a thin layer of sp2-bonded carbon atoms, stacked in a two-dimensional (2D) honeycomb lattice and belongs to the class of carbon nanomaterials (Konios et al. 2014). It is widely used in various fields including electronics, biological, medicine, energy storage, and catalysis (Gupta et al. 2016, Gupta et al. 2012; Khani et al. 2010; Jain et al. 2006, Jain et al. 1997; Wei et al. 2019; Beitollahi et al. 2014, 2017; Tajik et al. 2014; Movlaee et al. 2017).
In this paper we report the development of an electrochemical sensor based on polyglycine-modified graphene for the determination of Hg2+ and Pb2+ ions using the cyclic voltammetry (CV) technique. The electrochemical behavior of the heavy metal ions was studied at the bare and modified electrode and it was shown that the redox signal of these two heavy metal ions was promoted by the modified electrode. The developed sensor is less expensive and shows a wide linear response range with low detection limit, good stability, and reproducibility. Short or limited shelf life is the main disadvantage of the developed sensor. To our best knowledge, no research has been conducted to study the heavy metal ion detection at polyglycine-modified graphene paste electrode.
Chemicals and reagents
Graphene was obtained from Tokyo Chemical Industries. Glycine and silicone oil were obtained from Nice Chemical Pvt Ltd., Kerala. Disodium hydrogen phosphate and sodium dihydrogen phosphate were obtained from Himedia. Phosphate buffer solution (PBS) 0.1 M was prepared by mixing 0.1 M disodium and sodium dihydrogen phosphate, and diluted with water and used as a supporting electrolyte. The pH was adjusted by adding an acid solution. Other chemicals such as HgCl2 and PbCl2 were of analytical grade and used as received from the manufacturers. Distilled water was used for the preparation of all the solutions.
CV and DPV were performed using the CHI6038E (CH Instrument, USA). The electrochemical cell comprises three electrodes; platinum wire and calomel electrode were used as a counter electrode and a reference electrode, respectively. The working electrode used in this study was a 3 mm-diameter unmodified and modified graphene paste electrode. All the experiments were performed at room temperature. pH measurements were carried out using EQ-610 pH meter, which was calibrated before use. Field emission scanning electron microscopy (FESEM) was obtained using the instrument operating at 5.00 kV (DST-PURSE Laboratory, Mangalore University). All the peak currents were taken with background current.
Preparation of bare paste electrode
Bare graphene paste electrode was prepared by thoroughly mixing (15 min) 60:40 w/w% of graphene and silicone oil in a small agate mortar. A portion of the paste was then packed into the tip of the electrode of 3 mm diameter. Before each experiment, the surface was smoothed with a tissue paper until a shiny and clean electrode surface was obtained.
Results and discussion
Preparation of modified paste electrode
Structural features of BGPE and PGMGPE
Electrochemical behavior of K4[Fe(CN)6]
where Ip is the peak current in A, C0 is the concentration of the electroactive species (mol/cm3), n is the number of electrons involved in the reaction, D0 is the diffusion coefficient in cm2/s, v is the scan rate (V/s), and A is the electroactive surface area (cm2). For the modified electrode, the surface area is maximum (0.03 cm2) as compared to the bare electrode (0.02 cm2).
Effect of varying pH
Effect of scan rate
Figure 7c shows the cyclic voltammograms for PbCl2 for different scan rates. It was seen that as the scan rate increased, the peak current also increased and peak potential was shifted toward less negative from 0.1 to 0.250 V/s in 0.1 M PBS having pH 4.5. There is a linear relationship between the peak current and scan rate as observed in the linear regression equation of Ipa (μA) =60.17 + 1334.4 v (V/s), with the correlation coefficient of 0.994 (Fig. 7d), indicating that the electrode process is adsorption controlled.
Effect of concentration
Comparison between the detection limit of the proposed method with other previously reported methods
Application of the modified electrode to the real sample
Result for the determination of Hg (II) and Pb (II) ions in groundwater
Determination of metal ions in the blood sample
Sensor stability and reproducibility
To investigate the stability of the sensor, the current behavior was studied before and after 25 cycles in 0.1 M PBS, at 4.5 pH and scan rate of 0.1 V/s. It was observed that even after 25 cycles, 85% of the initial current was retained, showing that the developed sensor had good stability. Reproducibility was studied with three successive removals of the electrode, keeping the solution same. It was observed that the relative standard deviation of 2.85% for the analyte indicated that the electrode had acceptable reproducibility.
In this work, a novel electroanalytical method which showed good sensitivity and comparable with previous reports was presented. The method is based on the subduing of the peak current of the modifier due to the analyte in a modified carbon paste electrode.
In this study, an electrochemical sensor based on polyglycine-modified graphene paste electrode (PGMGPE) was developed for the quantification of trace Pb (II) and Hg (II). The developed electrode exhibits good sensitivity, selectivity, good stability, and lower detection limit. It was shown that the developed electrode provides an alternative strategy for the electrochemical determination of heavy metal ions. The modified electrode has been used to determine the Pb (II) and Hg (II) in water and blood samples. The result obtained in the analysis of Hg (II) and Pb (II) in water and blood samples had a good recovery, demonstrating the applicability of the method for real sample analysis.
We gratefully acknowledge the financial support from the VGST, Bangalore, under Research project. No. KSTePS/VGST-KFIST (L1) 2016-2017/GRD-559/2017-18/126/333, 21/11/2017.
This research work was planned and executed by CR and JGM. The experimental work and analysis were carried out by CR and JGM, and the manuscript was written by CR. All authors read and approved the final manuscript.
This research did not receive any specific grant from funding agencies.
The authors declare that they have no competing interests.
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