Development of Modified Pencil Graphite Electrode Sensors by Using Polyaniline and Polypyrrole Based Poly(Acrylamide-Co-Amps) Hydrogels for the Detection of Methylene Blue

Introduction

Hydrogels are cross linked three dimensional polymer network systems synthesized via polymerization of monomers  which are hydrophilic in nature with the cross linker. The applications of the superabsorbent polymers are employed  in  super capacitors, conducting materials and in sensors. The hydrogels synthesized by using conducting polymers have shown significant advantages in various aspects. The conducting polymer based hydrogels have an  extensive applications in chemical separations,  drug delivery and controlled release in agriculture. Only a few studies have been reported on the synthesis of super absorbent polymers with the conducting polymers and inorganic   materials. The effect of monomer concentration, amount of cross linker and initiator  and conducting materials like aniline, pyrrole, graphite, copper, etc., on the conductivity of hydrogels synthesized are also investigated. The versatility of the hydrogels has various applications as conducting materials, sensors, drug release materials and biomaterials¹.

Sensors in electrochemical analysis have been  developed  and successfully applied in different areas in the last few decades. Substantial efforts have been made in the development of portable electrochemical sensors to investigate organic and inorganic environmental pollutants^2-5. The electrodes were often prepared with suitable modifier materials to achieve improvements in selectivity and sensitivity^6-9. The development of the newer electrode modifier material is a progressive effort to fulfill the demands of modern electrochemical technology. Application of surface modified carbon electrode with nanomaterials in environmental pollutant monitoring is a promising electrochemical sensing technique. Because of their higher sensitivity, good conducting property with smaller residual current and durable nature, the electrochemical sensors exhibit their performance in different fields. Carbon paste electrodes demonstrate their extensive applications since the introduction by Adams in 1958^10-13.

The Glassy Carbon, Carbon paste, Carbon filler, Screen printed carbon strips, etc.,  are the most frequently used electrodes as electrochemical sensors.  Apart from the carbon based electrodes, some commonly known materials are also used for the preparation of the novel electrodes  for various electrochemical sensing^14,15. Now a days, a huge variety of nanomaterials have been synthesized  and their applications in electrochemical sensor development show immense progress. The utilization of  unique and distinct  characteristics of nanomaterials by   modern electroanalytical methods  is an emerging field on the characterization of electroactive species.

Nevertheless, the main issue of  conventional carbon based electrode is fouling of their surface during the electrochemical determinations. The  intermediate products formed by  the electrochemical reactions on the electrode surface essentially cleaned prior to each measurement for good results.  In order to resolve these issues of the conventional glassy carbon based sensors, various surface modifications have been developed like metallization, derivatization and doping^16-18. Some cost effective disposable electrodes such as screen printed electrodes and the pencil graphite electrodes have been used to overcome such difficulties. The pencil graphite electrodes have more advantages over the screen printed electrodes due to their good mechanical strength, easy availability and low cost. The development of pencil graphite electrode based electrochemical sensors is a significant one to overcome the drawbacks of the conventional GC based sensors. Now a days demand of low cost, easy to fabricate onsite voltammetric sensors are progressively increasing.

The sensitivity and selectivity of pencil graphite electrodes can be enhanced by the surface modification with nanomaterials, conducting polymers or by a combination of both. Different methods are available to modify the surface characteristics of the electrodes. This is achieved with one or more modifier compounds over the electrode surface by coating, casting or dispersion to combine the chemical, electrochemical and catalytic properties of the modifier.

In the present study a novel hydrogels based on Poly(acrylamide/AMPS), Poly(acrylamide/AMPS)/polyaniline and Poly(acrylamide/AMPS)/polypyrrole synthesized via free radical copolymerization using the cross linker methylene bisacrylamide and the initiator potassium per sulfate. The synthesized hydrogels were coated on the Pencil Graphite Electrodes which were used as sensors for the determination of methylene blue dye. Detailed characterization study was performed by Cyclic Voltammetry (CV), Square Wave Voltammetry (SWV), Differential pulse voltammetry (DPV), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD) and Scanning Electron Microscopic (SEM) techniques.

Experimental

Materials

Acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS)  (Merck) was used as purchased. Potassium persulfate (KPS) as an initiator and N,N’-methylene bisacrylamide (MBA) as a cross linker, aniline, pyrrole and methanol were purchased from Sigma Aldrich. Aniline and pyrrole monomer were distilled prior to use. All the reagents were analytical grade and used directly without further purification. Pencil leads (0.7mm, Kokuyo Camlin Ltd, India) were purchased from a stationary shop. Deionised water was used in the preparation of hydrogels.

Synthesis of Poly(acrylamide/AMPS) polymer

Polymerization reaction was carried out in 1:2 methanol/water  solvent mixture at  60^°C in nitrogen atmosphere and  in the presence of KPS as initiator and MBA as a cross linker. Aqueous solution containing AM (0.5g), AMPS (0.5g,0.3g,0.1g), MBA (0.05g)and KPS (0.05g) were prepared using methanol water mixture. The contents were placed in a polymerization tube and  nitrogen gas was purged  for 15 minutes. Polymerization reaction was controlled in a thermostatic water bath at  60^°C along with constant stirring by a magnetic stirrer for 3 hours. The  gelatinous polymer mass thus obtained was  used for further studies.

Preparation of Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR hydrogels

The freshly prepared gelatinous Poly(acrylamide/AMPS)  hydrogels were taken in two sets of  polymerization tubes. Aniline and pyrrole monomer concentration was varied as 0.1ml, 0.3ml and 0.5ml and was added to the hydrogels. The mixtures were allowed overnight to disperse aniline  and pyrrole monomers uniformly in to the hydrogel polymer matrix at room temperature. A dark green colored Poly(acrylamide/AMPS)/polyaniline and black colored poly(acrylamide/AMPS)/polypyrrole  hydrogel polymer resulted was used for electrode preparations.

Preparation of Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR hydrogels coated Pencil Graphite Electrode Sensor

The Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR hydrogel was taken in the separate polymerization tubes. Pencil leads were dipped into 1cm height in the hydrogel matrices for 24 hours. The polymer coated pencil leads were taken outside and dried at room temperature. Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR  coated Pencil Graphite Electrodes  were used to study the electrochemical response of MB dye in aqueous medium.

Methylene blue absorption

A stock solution of 400ppm MB dye was prepared with deionized water. 40, 120 and 200 ppm of MB working solutions was prepared by diluting the stock solution.  Then hydrogel coated electrodes (PGEs) were dipped in 5ml of working solutions and allowed to swell for 10min, 20min and 30min in each concentration of MB solution. The electrodes are washed with water and electrochemical investigations were carried out to study their sensing behavior.

Measurements and Characterization Techniques

Poly(acrylamide/AMPS), Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR  hydrogels were characterized with  PerkinElmer, Spectrum Two FTIR spectrometer  using KBr pellets between the wave  region of 4000cm^-1 to 400cm^-1. Morphological study of samples was carried out using the  CarelZeiss, Model EVO-18 scanning electron microscope. The X- ray diffraction studies of the hydrogels were carried out using a Bruker D8 Advance X-ray powder diffractometer using Cu Ka radiation. The electrochemical studies were carried  out using a CH Instruments model 620 electrochemical workstation in a single compartment three electrode cell assembly.

Results and Discussion

Cyclic voltammetry

Cyclic voltammetry characterization of hydrogel coated PGEs performed using CH620 software. Voltammetric experiments were performed using a single compartment three electrode system using hydrogel coated PGEs as working electrodes. A Pt wire was an auxillary electrode and KCl saturated calomel electrode as reference electrode in a potential window ranging from 1.4V to -1.0V at a scan rate of 0.05V/s.The bare PGE, plain hydrogel coated PGE and polyaniline, polypyrrole based hydrogel coated PGE were subjected to voltammetric investigations. The electrochemical response was studied in the pH ranging from 1.0 to 13.0 with methylene blue dye as an analyte. MB dye showed a well defined cyclic voltammogram of the fabricated electrodes with a better current response in pH 1.0 (Fig. 1). Effect of scan rate studies between 0.025V/s and 0.25V/s showed a better response at 0.050V/s was used for further studies. The influence of monomer ratio was studied for plain, polyaniline and polypyrrole based hydrogels. The hydrogels prepared with 0.5g AM, 0.5g AMPS, 0.05g MBA plain and same with 0.1ml of aniline and with 0.1ml of pyrrole showed better characteristic response to MB dye. The role of MB dye concentration was also studied from 40ppm to 400ppm. Maximum peak current observed with 200ppm MB solution (Fig. 1, 2). The influence of swelling time was studied  by allowing the hydrogel coated electrode into a 200ppm MB solution for 10, 20, 30, 40 and 50 minutes. MB absorption reached maximum at 30 minutes and then there was no significant increase in the peak current.  The effect of  sensing behavior with different concentration of pyrrole is shown in figure (2) and the polymer gel with 0.1ml pyrrole has more  influence than the other concentrations. The figure (3) shows the sensing behavior of polymer gel with 0.1ml of aniline and different concentrations of pyrrole monomer in which polymer gel with aniline monomer has more sensing behavior. Overall CV analysis of the PGEs coated with Poly
(acrylamide-co-AMPS)/polyaniline hydrogel showed higher current response than the PGEs coated with Poly(acrylamide-co-AMPS), Poly(acrylamide-co-AMPS)/PPyr and bare electrode.

Polymer gel with 0.1ml of aniline in 200 ppm of MB and (b) Polymer gel in  200 ppm of MB, Polymer gel in 120 ppm of MB, (d) Polymer gel in 40 ppm of MB at 30min swelling time.

Figure 1: Cyclic voltammogram of PGEs coated with Poly(Acrylamide/AMPS)/PANI and Poly(Acrylamide/AMPS) hydrogels at the scan rate of 50mV/s. Click here to View Figure

Figure 2: Cyclic voltammogram of PGEs coated with Poly(acrylamide/AMPS)/PPyr and  Poly(acrylamideAMPS) hydrogels at the scan rate of 50 mV/s. Click here to View Figure

Figure 3: Cyclic voltammogram of PGEs coated with Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPyr hydrogels at the scan rate of 50 mV/s. Click here to View Figure

 DPV and SWV studies

Figure 4: DPV of PGEs coated with Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPyr hydrogels at the scan rate of 50 mV/s Click here to View Figure

Figure 5: SWV of PGEs coated with Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPyr hydrogels at the scan rate of 50 mV/s. Click here to View Figure

The optimum parameters arrived with CV studies of hydrogel coated PGEs and MB dye concentration was taken for DPV and SWV studies. DPV and SWV experimental parameters were optimized for further studies. The DPV studies reveals that the PGEs hydrogel containing polyaniline has good sensing behavior with a maximum current response of 726.9 µA than the electrodes coated with polypyrrole  (Fig. 4). The peak current is shown in table 1. In the same way PGEs subjected to optimized square wave voltammetric studies. As in the DPV studies, similar type of response was noticed but with a lower peak currents. The 0.1 ml aniline based hydrogel showed a maximum peak current of 609.2 µA with the electrode dipped with 120ppm of MB dye for a swelling period of 30 min. Typical square wave voltammogram are illustrated in figure 5.

Table 1: Voltammetric behavior of PGE coated with Poly(acrylamide/AMPS)/PANI and P(acrylamide/AMPS)/PPyr in 120ppm MB dye: swelling time 30 min.

Plain gel composition	Poly(acrylamide/AMPS)/PANI		Poly(acrylamide/AMPS)/PPyr
	Anilne (ml)	Peak current (µA)	Pyrrole (ml)	Peak current (µA)
0.5g AM+0.5g AMPS+ 0.05g	0.1	726.9	0.1	311.50
	0.3	300.1	0.3	129.60
	0.5	214.7	0.5	074.67

FTIR analysis of the Hydrogels

Figure 6: FTIR Spectra of Poly(acrylamide/AMPS) Hydrogel Click here to View Figure

Figure 7: FTIR Spectra of Poly(acrylamide/AMPS)/PANI  Hydrogel Click here to View Figure

Figure 8: FTIR Spectra of Poly(acrylamide/AMPS)/PPYR  Hydrogel Click here to View Figure

The FT spectrum of Poly(acrylamide/AMPS) and Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR hydrogels were characterized with  PerkinElmer, Spectrum Two FTIR spectrometer  using KBr pellets between the wave  region of 4000cm^-1 to 400cm^-1 and are shown in figures (6-8). A broad peak corresponding to NH stretching is formed at 3434.08cm^-1. The peak observed at 1645.75cm^-1corresponds to C=O stretching of AM. The peaks around 1110cm^-1 ,1045cm^-1and 620cm^-1 corresponds to asymmetric stretching of SO₂ , symmetric stretching of the SO₂ and C-S stretching of AMPS respectively. The peak at 2923.53cm^-1 is due to CH stretching of polymer backbone. The  stretching of C-N band is seen at around 1456cm^-1.

XRD Analysis

The XRD patterns of the copolymer hydrogels were carried out using a Bruker D8 Advance X-ray powder diffractometer using Cu Ka radiation and are shown in figures (9-11). 2  values range from 10 to 80. The peak pattern of Poly(acrylamide/AMPS), Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR is shown in the figures. The XRD analysis showed that the co-polymeric hydrogel materials are more amorphous in nature.

Figure 9: X-ray Diffraction pattern of Poly(acrylamide/AMPS) hydrogel Click here to View Figure

Figure 10: X-ray Diffraction pattern of Poly(acrylamide/AMPS)/PANI) hydrogel Click here to View Figure

Figure 11: X-ray Diffraction pattern of Poly(acrylamide/AMPS)/PPYR hydrogel Click here to View Figure

SEM Analysis

The Surface Morphology of copolymer hydrogels was studied by CarelZeiss, Model EVO-18 scanning electron microscope. The SEM images of the hydrogels were shown in figures (12-14). The image indicates the presence of copolymers which are uniformly distributed throughout the polymer matrix.

Figure 12: SEM analysis  of Poly(acrylamide/AMPS) hydrogel Click here to View Figure

Figure 13: SEM analysis of Poly(acrylamide/AMPS)/PANI hydrogel Click here to View Figure

Figure 14: SEM analysis of Poly(acrylamide/AMPS)/PPYR hydrogel Click here to View Figure

It can be seen from the images that the Poly(acrylamide/AMPS) hydrogel shows a smooth, less dense and less compact structure. But the Poly(acrylamide/AMPS)/PANI shows a coarse, more dense and highly compact network surface suggesting the connection and formation of polyaniline chain within the Poly(acrylamide/AMPS) polymer matrix. The image of Poly(acrylamide/AMPS)/PPYR shows an irregular, coarse with more porous structure.

Conclusions

In the present work Poly(acrylamide/AMPS), Poly(acrylamide/AMPS)/PANI and Poly(acrylamide/AMPS)/PPYR hydrogels were synthesized and  used for the coating of the pencil leads which are considered as Pencil Graphite Electrodes (PGEs). The polymer hydrogel electrodes were characterized by Cyclic Voltammetry, DPV, SWV, FTIR, XRD and SEM analyses. The electrodes coated with hydrogels prepared with aniline showed higher sensitivity over hydrogel with pyrrole and bare electrodes in the electrochemical response of MB dye under optimum concentration and swelling time. The CV studies revealed that the hydrogels, prepared with polyaniline coated pencil graphite electrodes showed good sensing efficiency. Due to their excellent sensing behavior, the coated PGEs could thus potentially substitute to the expensive electrodes. Because of the cost effective easy to fabrication, they may be a potential sensor in sensing environmental pollutants.

Conflict of Interests

The authors declare that there is no conflict of interest including any financial, personal or other relationships with other people or organizations that can influence their work.

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