Electrochemical formation of PEDOT/Schiff
The poly(EDOT-Schiff base) film was prepared potentiodynamically with a voltage ranging between ̶ 0.50 V and 1.40 V over ten scans from a 0.1 M solution of monomer in CH2Cl2 containing Bu4NPF6 onto a Pt electrode at a scan rate of 10 mV s-1 at 25 ± 2 ºC. Fig. 1 illustrates the cyclic voltammograms that were recorded during the synthesis of poly(EDOT-Schiff base). As shown in Fig. 1, the onset of the oxidation potential for the PEDOT-Schiff base film was 1.22 V. During polymerization, a black-coloured layer of polymer film formed on the Pt electrode. The current peak of the polymer film increased with increasing number of cycles, which corresponds to the systematic growth of the polymer film on the Pt electrode [31]. The cyclic voltammogram in Fig. 1 shows a nucleation loop in the first cycle where this anodic current related to the oxidation of the monomer and nucleation of poly(EDOT/Schiff) [32]. The nucleation loop vanished in the following cycles because of the persistent growth of the film preventing further nucleation in the subsequent scans. The redox processes of poly(EDOT/Schiff) has led to the emergence of anodic and cathodic peaks at 0.53V and -0.07 V, respectively, in the cyclic voltammogram. Moreover, the increase in redox peak current values in the voltammogram implies that the amount of electrodeposited PEDOT polymer has increased on the Pt electrode, which led to an increased thickness of the deposited polymer [33]. In addition, two peaks probably indicate the potential peaks of the monomers, which depended on the type and of size the anion present in the reaction medium. In other words, the emergence of redox peaks in the voltammogram can be significantly influenced by the nature and size of the ionic species present in the electrolyte.
Fig. 2 shows the cyclic voltammogram (scan 10) of the electropolymerisation of the (EDOT/ Schiff) monomer at various scan rates ranging from 5 - 100 mV s-1 under the same polymerisation conditions used for the voltammogram in Figure 1. The molar coverage of polymer films per unit area can be calculated from the amount of charge of the last scan of the deposited polymer using Faraday’s law and Eq. 1 [34].
See formula 1 in the supplementary files.
Here, F represents the Faraday constant (C mol-1), Γ is the molar coverage (mol cm-2), A is the modified electrode area (cm2), Q represents the cathodic charge and n is the number of electrons involved in the electropolymerisation. Herein, n is equal to 2.3 [35] and the modified electrode has a surface area of 0.00785 cm2. The molar coverage is applied to estimate the thickness of the polymer surface (h/μm) using Eq. 2 and 3 [36].
See formula 2 in the supplementary files.
See formula 3 in the supplementary files.
Here, c and ρ are the concentration and density of the monomer, respectively, and Mr is the molecular mass of the monomer (273.3 g/mol). h is the thickness of polymer film. The relative standard deviation (RSD%) from three consecutive experiments was calculated, as shown in Table 1, which also reports the cathodic charge, surface coverage and thickness of prepared polymer films.
Table 1 Charge of reduction peak of poly(EDOT/Schiff) at different scan rates (5-100 mV s‑1). Electrode area was 0.00785 cm-2.
|
Polymer films
|
Scan rate
(mV s-1)
|
Charge of reduction peak Q (C)
|
RSD%
n=3
|
Coverage Г
(mol cm-2)
|
Thickness h (µm)
|
|
|
|
|
|
|
poly(EDOT/Schiff)
|
10
|
1.30 × 10-2
|
1.32
|
7.46 × 10-6
|
14
|
20
|
1.12 × 10-2
|
2.45
|
6.42 × 10-6
|
12
|
30
|
9.15 × 10-3
|
1.85
|
5.25 × 10-6
|
10.5
|
50
|
8.02 × 10-3
|
2.06
|
4.60 × 10-6
|
9.22
|
100
|
6.11 × 10-3
|
1.47
|
3.50 × 10-6
|
7.01
|
Electrochemical characterisation of polymer films
In order to obtain a greater insight into the electrochemical behaviour and stability of the polymer films, the electrochemical features were analysed carefully using cyclic voltammetry in a background electrolyte (monomer-free) of dichloromethane and aqueous solution-Bu4NPF6 (0.1 M) for polymer film which was prepared using a scan rate of 10 mV s-1 (Fig. 1), as depicted in Fig. 3A and 3C, respectively. Voltammetric study findings have shown broad redox current peaks which can probably be attributed to the counterions diffusing into the chain polymer film for both electrolytes [20]. The voltammogram of poly(EDOT/Schiff) films exhibit a broad positive peak at 0.61 V vs. Ag/AgCl and a negative peak at 0.08 V vs. Ag/AgCl in DCM electrolyte. On the other hand, a broad positive peak appears at 0.52 V vs. Ag/AgCl and a negative peak at 0.16 V vs. Ag/AgCl in aqueous electrolyte, which are representative of the oxidation and reduction of the film produced, respectively. Herein, we observed a decrease in the peak currents as a function of increasing scan number; thus, there is decay in the peak potentials and associated CV shapes were changed. This could be attributed to degradation of the film when switching to the overoxidation potential, which led to poor stability during redox cycling.
In this work, the effect of scan rate on electrochemical response has been studied in a background electrolyte (monomer-free) of dichloromethane and aqueous solution-Bu4NPF6 (0.1 M), as shown in Fig. 3B and Fig. 3D. From these two curves, we can observe that the current of the peak was proportional to the scan rate [37]; this property supports the assumption of good electro-activity and stability of the polymer film. This finding indicates that redox current peaks are proportional to the scan rates for the same polymer electrode [38]. Further, both oxidation and reduction peaks have a linear relationship with scan rate, which is indicative of surface-confined control, as revealed in Figs. 4A and 4B. Tables 2 and 3 show charges for the electrochemical response of polymer growth in Fig. 1 as cycled in the background electrolyte (monomer-free) at different scan rates, as presented in Fig. 3B and 3D. The charge was calculated from the CV curves in Fig. 3B and 3D by integration of the current of the redox peaks with respect to time. The charges found for cycles 1 and 10 are shown in Tables 2 and 3.
Table.2 Cathodic charge values of PEDOT/Schiff panels in Figure 1 exposed to 0.1 M Bu4NPF6 in DCM electrolyte (monomer-free) at various scan rates.
|
Scan rate mV s-1
|
Q red, 1st cycle C
|
|
Q red, 10th cycle C
|
|
% Q red 10th / Q red 1st retention
|
|
|
|
|
|
|
10
|
8.45 × 10-3
|
|
6.12 × 10-3
|
|
72
|
20
|
5.89 × 10-3
|
|
4.78 × 10-3
|
|
81
|
30
|
4.78 × 10-3
|
|
3.96 × 10-3
|
|
82
|
50
|
4.08 × 10-3
|
|
3.08 × 10-3
|
|
75
|
100
|
2.64 × 10-3
|
|
2.25 × 10-3
|
|
85
|
Table.3 Cathodic charges of PEDOT/Schiff panels in Figure 1 exposed to 0.1 M Bu4NPF6 in aqueous electrolyte (monomer-free) at various scan rates.
|
Scan rate mV s-1
|
Q red, 1st cycle C
|
|
Q red, 10th cycle C
|
|
% Q red 10th / Q red 1st retention
|
10
|
8.19 × 10-3
|
|
6.12 × 10-3
|
|
77
|
20
|
3.71 × 10-3
|
|
3.08 × 10-3
|
|
83
|
30
|
2.97 × 10-3
|
|
2.68 × 10-3
|
|
90
|
50
|
2.13 × 10-3
|
|
2.45 × 10-3
|
|
87
|
100
|
1.86 × 10-3
|
|
1.70 × 10-3
|
|
91
|
FTIR characterisation of EDOT/NH2 and PEDOT/Schiff base structures
Fig. 5 illustrates the FTIR spectra of EDOT/NH2 and electrochemically prepared poly(EDOT/Schiff base) structures. An FTIR spectrum of PEDOT/NH2 has the distinctive features of PEDOT and the amino group. The bands of the NH2 group vibrations were observed in the range 3380 - 3240 cm-1. The peaks at 3090 cm-1 and 2975 -1 cm were assigned to C–H aromatic and C–H aliphatic stretches, respectively. The vibrations for the C-O-C and C-S groups in EDOT were observed at 1100 cm-1, 940 cm-1 and 615cm-1, respectively. The bands at 1641, 1535 and 1362 cm-1 were attributed to the C=C and C-C stretches in the thiophene cycle. The FTIR spectrum of the prepared polymer Schiff base had characteristics peaks at 1645 cm-1 and 3365 cm-1 that could be attributed to the imine group (C=N) and OH group, respectively. The band at 2939 cm-1 was assigned to the C–H aromatic and dioxyethylene bridge stretching modes of the EDOT molecule. Further, the bands at 1128 cm-1 and 975 cm-1 were noted for EDOT/Schiff [37, 38].
Morphological characterisation of PEDOT/Schiff base
Properties of electroactive polymers such as activity and stability are closely related to surface morphology. The surface morphology of the PEDOT-Schiff film electrodeposited on the Pt electrode was examined by scanning electron microscopy (SEM), as shown in Fig. 6. The polymer film was preformed potentiodynamically by applying the potential range −0.5 to 1.5 V vs. Ag/AgCl using a scan rate of 10 mV s−1 and 100 mV s−1 for 10 cycles. The surface film clearly exhibits growth processes on surface whereby the existence of a number of small globules and clusters which are linked together like chains can be observed. These electrode surfaces have uniform nanostructures and compactness. In general, the morphology of the polymer surface is highly affected by the conditions of the experiment such as the pH, temperature, solvent type, ions present, and scan rates [39]. SEM micrographs of PEDOT/Schiff electrodes at different scan rates are shown in Fig. 6. SEM images indicated that different scan rates result in a change of primary particle size. It was observed that the film prepared at 100 mV s-1 (Fig. 6B) has smoother structure with fused particles (grain sizes on the order of a few micrometres) compared to the film prepared at 10 mV s-1, which has a much more globular morphology, having grain sizes in the order of nanometres ranges (Fig. 6A).
These variations in the surface morphology can be attributed to the fast and slow nucleation and growth rate of the polymer which is affected by timescale. At high scan rate, the nucleation and growth rate of the polymer are fast (instantaneous nucleation reaction). As a result, the redox system does not maintain an equilibrium state during the potential scan, producing larger and different crystal sizes of morphology. In contrast, at slow a scan rate, equilibrium processes and progressive nucleation is predominant. Therefore, the movement of neutral species is slower than the movement of charged species, forming highly homogeneous crystal sizes of morphology.
The Influences of pH
It is unquestionable that pH is one of the parameters which directly influence the voltammograms’ shapes, and then it is significant to study the effect of pH on electrochemical processes. This process aims to reach to the highest peak current during experiments [40, 41]. Analyte solution containing 5 µg L-1 of Cd (II) and Pb (II) ions in media with different pH values were used for the voltammetric investigation. The square wave voltammograms exhibited clear peaks for metal ions in media with diverse pH values. It was found that the peak current shapes and heights in solution of pH 5 were well-defined and higher compared to other pH values. Thus, pH 5 was considered suitable for determination of ion concentrations in solution. The findings obtained are graphically depicted in Fig. 7 where, as shown, the modified electrodes have low current peaks in highly acidic solution (pH 2) and after which the current peak increases with increasing pH until it reaches 5, at which we have tested for cadmium and lead cations.
Measurement of Cd (II) and Pb (II) using SWV
The procedure for the voltammetric measurements for the electro-analytical determination of cadmium and lead concentrations in aqueous solution was divided into two steps. Firstly, the modified electrode was immersed in sample solution containing the analyte (Cd (II) or Pb (II)) at a known pH=5 and a selected concentration (5-100 μg L-1), where metal ions were binding chemically to the ligands at the surface of the electrode; and secondly, the polymer electrode was removed from the metal ion solution and rinsed with deionized water, and then transferred to a voltammetric cell containing only a supporting electrolyte (acetate buffer solution). The square wave voltammograms were performed using different cadmium and lead concentrations. Fig. 8 shows the proposed interaction between the polymer ligand and metal ions.
The determination of Cd (II) and Pb (II) ions
Optimal practical conditions for the determination of Cd (II) and Pb (II) ions by the PEDOT/Schiff electrode using the SWV technique, were assessed separately for each electrode. Initially, the current responses were recorded for the polymer electrodes using a blank solution without metal ions. The blank solution responses do not have any current signals in the voltage range from −1.2 to 0 V, as illustrated in Fig. 9 (black line). Accumulation of the Cd (II) and Pb (II) ions occurred by immersion of the modified electrode in buffer solution at pH 5 containing Cd (II) and Pb (II) ions. This process led to complex formation between the metal ions and the PEDOT/Schiff base layer. The chemical accumulation process which occurred for Cd (II) and Pb (II) ions probably affected the accumulation of other reducible species at the voltage used during the preconcentration processes. After the immersion of metal ions in the PEDOT/Schiff electrode, they were washed with pure water. After that, the modified electrode was moved to the electrochemical cell which contained buffer solution. The SWV response was registered for each metal ion, i.e. Cd (II) and Pb (II) [42, 43].
The square wave voltammograms of modified electrodes, after 15 min of immersion in buffer solution, at pH 5 with 5 µg L-1 of Cd (II) and Pb (II) ions, are shown in Fig. 9. The determination of Cd (II) and Pb (II) ion concentrations was investigated between −1.2 to 0.0 V (vs. Ag/AgCl). As can be noted in Fig. 9, the interaction of the Cd (II) and Pb (II) ions with the modified electrode surface leads to the alteration of the electrochemical properties of the electrode. Clearly, the anodic peak has increased due to Cd (II) and Pb (II) ions, which formed a complex on the modified electrode (Fig. 9), compared with the peak current recorded after immersion of the same electrode in solution without metal ions (Fig. 9, A and B). The calibration equations and correlation coefficients (R2) were calculated for the Cd (II) and Pb (II) ions as y = 1.639 + 0.434x (x: µg L-1, y: µA), R2 = 0.9989 for Cd (II) and y = 0.492 + 0.159x, R2 = 0.9961 for Pb (II), respectively, as shown in Fig. 10. The limits of detection (LOD) were measured as 0.95 μg L-1 and 1.84 μg L-1for the Cd (II) and Pb (II) ions, respectively, which demonstrates the high sensitivity of the modified polymer electrode towards heavy metal ion detection. The separation in peaks locations for metal ions offer an accurate strategy to detect Cd (II) and Pb (II) ions, significantly reducing interfering effects from other heavy metal ions [44, 45].
Calibration was achieved for the determination of metal ions at pH 5 in buffer solution. Fig. 10 shows square wave voltammograms recorded using consecutive additions of ion metals over the 5-100 μg L-1 concentration range at the EDOT/Schiff modified electrode. Peak currents appeared at -0.77 V and -0.50 V for the various concentrations of Cd (II) and Pb (II), respectively (Fig. 9). A linear relationship between the concentration ion metals and current peaks was evident from the experimental findings.
Simultaneous electrochemical determination of Cd (II) and Pb (II) in a binary mixture
The analytical signal of various concentrations of Cd (II) and Pb (II) ions is illustrated in Figs. 9A and 9B, respectively. Subsequently, the simultaneous determination of Cd (II) and Pb (II) ions with the PEDOT/Schiff electrode was carried out to detect Cd (II) and Pb (II) ions in the same solution[46]. SWV voltammograms of the PEDOT/Schiff electrode after sequential additions of different concentrations of Cd (II) and Pb (II) are shown in Fig. 11. The characteristic peaks of Cd (II) and Pb (II) were seen at −0.77 and −0.50 V, respectively. These findings were in agreement with the individual species’ characteristics (Fig. 9). The effects of ion concentrations were examined under optimum conditions. Determination of metal ion concentrations was examined between −1.2 to 0.0 V (vs. Ag/AgCl) at different concentrations such as 5 μg L-1 and 100 μg L-1 for Cd (II) and Pb (II), respectively. Fig. 11 represents the square voltammograms recorded at the PEDOT/Schiff electrode with scan rate of 5 mV s−1. From this figure, it can be seen that the individual peak currents increased linearly with increasing concentrations of the individual metal ions in the binary solutions [38].
The findings of the study confirmed that the modified electrode shows the appropriate reliability and efficiency to be used for detecting Cd (II) and Pb (II) ions. Furthermore, the analytical performances of the PEDOT/Schiff electrode in this project was compared with previous work in the literature for Cd (II)) and Pb (II) detection, as illustrated in Table 4.
Table 4 Comparison of the analytical performance of PEDOT/Schiff electrode with other modified electrodes.
|
Electrodes
|
Methods
|
Analytes
|
Detection limit
|
References
|
5-Br-PADAP
|
ASV
|
Pb (II)
|
0.1 μg L−1
|
[12]
|
|
|
|
|
|
P1,2‐DAAQ
|
SWASV
|
Cd (II)
|
0.3 μg ⋅ L−1
|
[40]
|
Pb (II)
|
0.58 μg ⋅ L−1
|
|
|
GC/ p-1,8-DAN
|
SWV
|
Cd (II)
|
19 ng.L−1
|
[46]
|
Pb (II)
|
30 ng.L−1
|
|
|
|
|
|
CNFs
|
ASV
|
Cd (II)
|
0.38 μg·L−1
|
[47]
|
Pb (II)
|
0.33 μg·L−1
|
|
|
|
|
|
polyamide 6/ Chitosan
|
SWV
|
Cd (II)
|
0.88 μg ⋅ L−1
|
[48]
|
|
|
|
|
|
PMTB
|
DPV
|
Cd (II)
|
0.35 μg L−1
|
[49]
|
Pb (II)
|
0.18 μg L−1
|
|
|
|
|
|
polyaniline
|
SWASV
|
Cd (II)
|
4.43 μg L−1
|
[50]
|
Pb (II)
|
3.30 μg L−1
|
|
|
|
|
|
Poly(1,5-DAN)/MWCNTs
|
SWASV
|
Cd (II)
|
3.2 μg L−1
|
[51]
|
Pb (II)
|
2.1 μg L−1
|
Reactivation of PEDOT/Schiff electrode
Repeated usage of the modified electrode to determination of metal ions necessitates the regeneration of the PEDOT/Schiff electrode. Reactivation of PEDOT/Schiff electrode was achieved by immersing the electrode in 0.1 M EDTA solution for 15 min and then washing with ultra-pure water. A voltammogram recorded for PEDOT/Schiff electrode after reactivation was nearly congruous with the voltammogram curve of the PEDOT/Schiff electrode when reacted with metal ions, as shown in Fig. 12 (red curve). This result indicates that the incorporated metal ions had been totally removed from the PEDOT/Schiff electrode. Therefore, the reactivated PEDOT/Schiff electrode could be applied for the detection of metal ions without any appreciable effect on the electro-activity.
Repeatability and reproducibility study
The repeatability of the PEDOT/Schiff electrode was determined under the optimized conditions using 20 µg L-1 Cd (II)) and Pb (II), respectively. Five consecutive measurements were taken using the same polymer electrode; the estimated relative standard deviations (RSD) were 3.4% and 2.8% for Cd (II)) and Pb (II), respectively. Moreover, the reproducibility of the PEDOT/Schiff electrode was examined. This process required the preparation of five modified electrodes which were then used in the detection of 20 µg L-1 Cd (II) and Pb (II), respectively. The RSD of the PEDOT/Schiff electrode was 3.8% and 3.1% for Cd (II)) and Pb (II), respectively, which indicated that the PEDOT/Schiff electrode prepared has good repeatability and reproducibility.
Interference study
To assess the selectivity of the modified electrode for the detection of metal ions, the effect of other ions on the response of Cd (II) and Pb (II) was investigated. In this study, various ions were chosen to act as interfering ions to investigate the selectivity of the PEDOT/Schiff electrode. Different ions (Na+, K+, Ca2+, Mg2+, Ba2+, Cu2+, Hg2+, Al3+, Fe3+, NO3- and Cl-) were added to a solution containing 20 µg L-1 Cd (II) and Pb (II). Addition of interfering ions did not lead to any perceptible difference in measurements, and these findings demonstrated that the electrochemical responses for Cd (II) and Pb (II) were not influenced by the interfering ions in any apparent way.
The interference experiments proved that additive ions have no perceptible interference effect to detection of target ions even when their concentrations exceeds those ion of interest in the solution, at 20 μg L−1 Cd (II) and Pb (II), by 50-fold. However, a 30-fold concentration of Fe3+, Cu2+ and Hg2+ were found to have a slight influence on the determination of Cd (II) and Pb (II) concentrations. The intermetallic compounds which can form between metal ions is a general problem in voltammetric methods, though this small change could be due to competition between iron and the target metal ions for active sites on the surface modified electrode.
Table 5 Three interference analyses for various metal ions on the current response of Cd (II) and Pb (II)
|
Interfering ions
|
Relative current change %
|
Cd (II)
|
Pb (II)
|
Na
|
0.19
|
0.25
|
K
|
0.36
|
0.42
|
Ca
|
-0.92
|
-0.85
|
Mg
|
1.17
|
1.09
|
Ba
|
1.67
|
1.35
|
Cu
|
6.34
|
5.92
|
Hg
|
-4.93
|
-5.62
|
Al
|
-0.56
|
-0.54
|
Fe
|
5.21
|
4.12
|
NO3
|
0.47
|
0.85
|
Cl
|
0.78
|
0.23
|