Introduction

The process of modifying polymers with good conductive materials stirs the attention of the scientists in recent times. One of these methods is irradiation of grafting polymers and modified with conductive materials to be used for multiple purposes, especially in the field of conductors and semiconductors, as well as in electrochemistry [1–4].

A composite polymer membrane working electrode has been studied at cyclic voltammetric for famotidin and terthiophene determination of drug in pharmaceutical formulation. This composite film of polymer electrode showed good catalytic behavior, which includes a good current response [5, 6].

The modified electrode functionalized with polyacrylic acid (PAA) tethered to indium tin oxide (ITO) and covered with gold nanoparticle (ITO/PAA/Au) demonstrated switchable interfacial properties discriminating different pH. The switchable electrochemical and plasmonic process was characterized by cyclic voltammetry (CV), electrochemistry impedance spectroscopy (EIS), and localized surface plasmon resonance (LSPR) [7].

Grafted molecularly imprinted (MIP) polymer thin films onto Au electrodes was studied as good sensor in the presence of N,N′-methylenebis(acrylamide) to detection of hydroquinone [8].

Another study for using a molecularly imprinted polymer (MIP) as a recognition element in the development of a heparin sensor for real-time monitoring. An indium tin oxide (ITO) electrode grafted with a heparin-specific MIP was used as a working electrode to perform cyclic voltammetry of ferrocyanide. The anodic current was found to be dependent on heparin concentration. Therefore, the MIP-grafted electrode is suitable for real-time monitoring of heparin in blood. The advantage of these method is that a very small volume of blood is needed, which is very important, especially when regular measurements are required [9].

The electrical connectivity of grafted polymer with pair of electrons located on the nitrogen atom within the structure of grafted polymer, and in addition to that the presence of high conductivity in a series of a activated carbon is one of the reasons used in the manufacture of electrodes operating polymeric high connectivity characteristic, which is mainly used in the analysis electrochemistry, especially in a cyclic voltammetry [4].

One of the syntheses of grafting polymers is electropolymerization of statistically exposed dithiobenzoate moieties served as a chain transfer agent for the polymerization of styrene on a matrix of an electrodeposited conjugated polymer. The polystyrene modified substrate was then used as the macro chain transfer agent for the synthesis of the second block of poly-tert-butyl acrylate on the surface. The polymerization from the surface was characterized by surface analytical methods including AFM, XPS, and contact angle measurements [10–14].

In this work, grafted polymer was modified with activated carbon to fabrication grafted polymer electrode self modified with activated carbon by gamma ray. The new grafted polymer electrode was electrochemically characterized in K3Fe(CN)6 with KCl aqueous electrolyte by CV technique.

Experimental

Synthesis of grafted polymer modified with activated carbon (GP/AC)

Polystyrene-acrylonitrile was grafted with activated carbon (AC) as an improvement material, using gamma-irradiation. The new grafted polymer modified with activated carbon indicated that the best grafting percentage ratio could be revealed when the concentration of the catalyst ferrous ammonium sulphate (FAS) is about 2 % and the monomer concentration is 90 %, at gamma ray dose 1.25 M rad [4].

Instrument and electroanalytical methods

Electrochemical workstations of NuVant Systems Inc., USA (EZ stat series with potentiostat/glvanostat driven by electroanalytical measuring software) were connected to a PC computer in order to perform cyclic voltammetry (CV), chronoamperometry (CC), and chronoamperometry (CA). An Ag/AgCl (3 M NaCl) and platinum wire (1 mm diameter) were used as the reference and counter electrodes, respectively.

Grafted polymer electrode self modified with AC was used as a working electrode in this study. The voltammetric experiments were carried out with 0.1 M KCl as supporting electrolyte and using K3Fe(CN)6 solution for calibration. The solution was degassed with nitrogen gas for 10–15 min prior to recording the voltammogram.

Reagents

All reagents were analytical reagents or electrochemical grade purity. All solutions were prepared using double distilled water. Unless otherwise specified, the supporting electrolyte was used 0.1 M KCl in aqueous media at room temperature.

Fabrication of grafted polymer electrode self modified with activated carbon (GPESMAC)

GPESMAC has been fabricated from grafted polymer modified with activated carbon. The diameter of electrode was 3 cm. A (1 mm) hole was made in the grafted polymer to allow 1 cm length of platinum wire out from the other side of electrode. A copper wire was then joined with the platinum wire. After that, all parts of fabricated electrode were covered with glassy tube and then fixed with epoxy resin.

Results and discussion

Electrochemical properties of grafted polymer electrode self modified with activated carbon (GPESMAC)

Potential window of GPESMAC

0.1 M of potassium chloride solution is commonly used as a reference standard for the purpose of calibrating a voltammetric system in aqueous solutions. During the calibration process of the electroanalytical workstation using GPESMAC as a working electrode, comparison with the working potential window of GCE/AC (using attachment method for modification GCE with AC) was found to be in the range (−1.8 to +1.8 V) with large value of capacity as show in Fig. 1a with small significant peaks appearing at +1.4 V. A considerable extension of the working window with the absence of each, especially at the anodic region, is observed when GPESMAC is used. Although the working window at the anodic and cathodic regions deteriorates, this is due to the ‘‘electrocatalyzed’’ oxidation–reduction of the aqueous electrolyte or the reactivity of GPESMAC giving rise to a limiting current appearing at about −1.5 to +2.0 V with low capacity as show in Fig. 1b. This limiting current, however, diminishes in subsequent cycles allowing the potential working window to be widened at the anodic region to near 2.0 V.

Fig. 1
figure 1

Cyclic voltammetry of different electrodes a glassy carbon electrode modified with activated carbon by mechanical attachmemt method, b grafted polymer electrode self modified with activated carbon in 0.1 M KCl as supporting electrolyte versus Ag/AgCl and 100 mV s−1

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Effect of GPESMAC on the redox reaction of K3[Fe(CN)6] during CV

It was found that the redox potential of GPESMAC in 1 mM K3Fe(CN)6 with 0.1 M KCl as supporting electrolyte with anodic and cathodic potential peaks as shown in Fig. 2:

$${\text{Epa }} = \, + 200{\text{ mV}} \left( {\text{oxidation potential of anodic peak}} \right)$$
$${\text{Epc }} = \, + 100{\text{ mV}} \left( {\text{reduction potential of cathodic peak}} \right)$$

with peak separation of 100 mV (for n = 1) in 0.1 M KCl electrolyte indicates the reversible reaction of the Fe(III)/Fe(II) couples in KCl solution, and agrees well with the accepted values.

Fig. 2
figure 2

Cyclic voltammetry of GPESMAC in 1 mM K3Fe(CN)6 with 0.1 M KCl as supporting electrolyte versus Ag/AgCl and 100 mV s−1

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Effect of varying scan rate

The effect of varying scan rates on the cyclic voltammograms using GPESMAC as working electrode in 0.1 M KCl supporting electrolyte was studied with 1 mM K3Fe(CN)6 over a scan rate range of 5–500 mV s−1. Oxidation and reduction currents of the Fe(III)/Fe(II) couple was observed to increase with scan rate due to heterogeneous kinetics and IR effect. Based on a plot of log(Ipa) versus log (scan rate) for the oxidation current of the first cycle, a straight line was obtained in Fig. 3 fulfilling the equation: y = 0.4388 X + 1.7912 with R 2 = 0.9988. A slope of 0.44 is quite comparable with the theoretical slope of 0.5 for the diffusion controlled process.

Fig. 3
figure 3

Plot log Ipa (anodic current) versus log (scan rate) of GPESMAC in 0.1 M KCl and 1 mM K3Fe(CN)6 versus Ag/AgCl and scan rate 5–500 mVs−1

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Effect of varying temperature

Effect of temperature on the redox process of K3Fe(CN)6 was studied on GPESMAC. The current increases gradually at a temperature of 10–90 °C. Plots of log (Ia) oxidation current of K3Fe(CN)6 versus the reciprocal of temperature, which is found to be fairly linear in agreement with the thermodynamic expectation of Arrhenius equations [15]:

$$\sigma = \sigma {^\circ }{\text{Exp }}( - {\text{ Ea}}/ \, RT \, )$$
(1)
$$D \, = \, D{^\circ }{\text{Exp }}( - {\text{Ea}}/ \, RT)$$
(2)

where: σ/D are conductivity/diffusibility and σ°/D° are standard conductivity/the initial diffusibility.

From the relationship between log(Ia) and 1/T of the oxidation K3Fe(CN)6 as Fe(II)/Fe(III) the activation energy was found 6.32 kJ/mol in GPESMAC comparison with other studies [16].

Effect of varying K3Fe(CN)6 concentration

Figure 4 shows the linear current dependent on K3Fe(CN)6 concentration; observed at concentration range (5–10 mM) which is described by the equation of y = 28.372X + 2.2268 with R 2 = 0.9904. The slope of the linear line for K3Fe(CN)6 showed that a considerably high sensitivity response of 28 μA/mM is readily obtained at GPESMAC during cyclic voltammetry.

Fig. 4
figure 4

Plot Ipc(cathodic current) versus different concentration K3[Fe(CN)6] (0.3–1 mM) in 0.1 M KCl at scan rate 100 mV s−1 using GPESMAC versus Ag/AgCl

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Reproducibility

The potential cycling of the redox of GPESMAC in 1 mM K4Fe(CN)6 and 1 M KCl aqueous solution as a supporting electrolyte was carried out during cyclic voltammetry. Continuous potential cycling did not seem to affect the redox current of GPESMAC as the faradic activity appears reproducible even after 15 cycles, reflecting the stability and reproducibility at the surface of GPESMAC.

Scanning electron microscopy (SEM) of GPE/AC

Before modification grafted polymer surface appears compact and nonporous. The uniformity of the grafted polymer surface slightly increases since occurrence of protrusion observed phase as shown in Fig. 5a. After modification with AC, although many of the species deposits with homogenous distribution of AC still remain at about <1 μm as show in Fig. 5b.

Fig. 5
figure 5

SEM of a grafted polymer, b grafted polymer modified with CNT

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Atomic force microscopy (AFM)

The surface image of AFM in an area of 20 μm × 20 μm of the grafted polymer (polystyrene acrylonitrile) before and after modified with AC as shown in Fig. 6a, b respectively. The surface of the electrode appeared to be compact and rough. According to AFM images, the average species size and thickness of the film were estimated to be 11.23 and 28.69 μm, respectively.

Fig. 6
figure 6

AFM of grafted acrylonitrile on polystyrene a before modified with activated carbon, b after modification

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Conclusion

A grafted polymer electrode self modified with activated carbon GPESMAC has been fabricated in laboratory for using in cyclic voltammetric technique as a working electrode. The new electrode was used as alternative of working electrodes modified with deposit materials such as activated carbon using classical modification methods (attachment or solvent evaporation methods). The CV analysis results of these electrodes have an extended potential working region as compared to the routine modification method of GCE with activated carbon and have high stability and reproducibility. Redox peaks of Fe(II)/Fe(III) showed high current as compared with other modified electrodes.

Electro-catalytic activity of GPESMAC is therefore evident in this study, it was studied by redox process of K3Fe(CN)6 during cyclic voltammetry. The redox peaks potential shifts slightly to less negative value by about 100 mV for oxidative peak and 50 mV for reductive peak with current enhancement of about 3-fivefolds. The sensitivity under conditions of cyclic voltammetry is significantly dependent on the concentration, scan rate and temperature. Excellent reproducibility and stability of the current is observed, provided a fabricated electrode is used for each experiment without any polishing.