3.1. Electropolymerization of Dyes (Gentian Violet, Brilliant Green, Aniline, and Methyl Violet)
Electrochemical polymerization of gentian violet at the glassy carbon electrode was done by using a cyclic voltammetric method in an aqueous solution that contains 2 mM of gentian violet in 0.2 M PBS (pH 5.8) and 0.1 M Na2SO4. Electropolymerisation was achieved by the formation of film that grew between the potential ranges of -1,500 to 2,500 mV at a scan rate of 100 mV/s for 15 cycles using cyclic voltammetry (Fig. (a)).
The voltammogram of GV on GCE showed one peak on anodic scanning with no corresponding peak on the reverse scan. The appearance of a single anodic peak in the first cycle only, with no cathodic peak on the reverse scan, is indicative of the irreversible nature of the oxidation process. On progressive cycle, the anodic peak at 1,500 mV and the cathodic peak at around − 400 mV both increased with the number of cycles. This shows polymer growth on the electrode [8].
Polymerizing BG on GCE gives a continuous increase of the corresponding polymer peaks at around + 150, +800, -300 and − 1,400 mV, indicating that BG is electropolymerised up to the last cycle since GCE provides a more open structure for the passage of the monomer, thicker films of PBG being achieved as observed in Fig. 1. (b). At high positive potentials, ~+800 mV, irreversible monomer oxidation occurs. The redox couple at the potentials + 150 mV (oxidation peak) is due to the oxidation of brilliant green monomer and − 300 mV (reduction peak) is because of the reduction of the poly(brilliant green) (PBG) film formed at the surface of the GCE and in both cases, the height of the peaks increase with potential cycle, showing PBG growth [11, 12].
The polymerization of methyl violet takes place when the potential reaches about 1700 mV vs. Ag/AgCl. The growth of the polymer on the electrode can be seen in the increase of all the peaks, and it is more evident at the reduction one. As can be seen from Fig. 1 (c), an irreversible anodic peak with a peak potential of 1,750 mV vs. Ag/AgCl is found for MV oxidation on GCE. This result confirms that MV can be directly oxidized on GCE.
A couple of reversible (oxidation/reduction) peak is observed between the potential range of -400 mV and 250 mV. The height of this reversible peak increased successively with increasing potential cycles, indicating the formation of the polymer. The cathodic peak current of the polymer increases rapidly and its peak potentials shift a little towards a more negative potential with increasing the number of potential cycles while its anodic peaks remain nearly constant. This reason is that the film thickness of the poly(methyl violet) on the glassy carbon electrode was gradually growing during the electrolysis and caused a potential drop [16, 17].
In the electrodeposition of polyaniline in Fig. s1, the GC electrode proceeds via a radical cation mechanism. From the first cycle, there is only one anodic and one cathodic peak and up on cycling (cycles 2–7) three anodic and three cathodic peaks appeared at the potentials of 260 mV, 520 mV, and 770 mV (anodic peaks) and 16 mV, 370 mV, and 570 mV (cathodic peaks). As the number of cycles increases, the peak currents of these peaks increase. The peak at 260 mV indicates the transformation of the reduced leucoemeraldine salt form into the emeraldine salt form i.e., the electron transfer from the PANI film; the peak at 770 mV is assigned to the further oxidation into the pernigraniline form. The middle peak was assigned to the presence of cross-linking of PANI caused by the reaction of nitrenium species being present as intermediates or to over-oxidation products. On the reverse scan, the cathodic peak at 16 mV represents the electron transfer to the electrodeposited polyaniline film. The one at 570 mV probably corresponds to side reactions such as decomposition of the polymer.
The large currents observed at the positive end of the CV are due to the superposition of two distinct processes: one is the electron transfer from the PANI film corresponding to the oxidation of the PANI film and the other is the electron transfer from the aniline monomer to the electrode corresponding to the oxidation of the aniline monomer to produce a precursor for the PANI film [13–15].
The cyclic voltammetry curve of a deposited PANI film in monomer-free solution is shown in Fig. s1. The CV was scanned between the potential range − 200 and 1,200 V at a scan rate of 100 mV/s in a monomer-free 0.3 M H2SO4 solution. The first redox couple corresponds to the redox reaction between leucoemeraldine and emeraldine, and the third redox couple to the redox reaction between emeraldine and pernigraniline, respectively. The second redox process belongs most probably to some quinone or quinoneimine-type degradation products, formed during the electrochemical preparation of the polymers. In order to compensate for the charge of the PANI film, anion doping/dedoping of the polyaniline film occurs [2].
Besides the proton/cation exchange, the anion is also expelled from the polyaniline film during deprotonation [16–18].
3.1.1. Voltammetric characterization of GC/(PGV, PBG, and PMV) modified electrode
3.1.1.1. Scan rate effect
To study the electrode behavior, the scan rate for PGV was varied between 10 and 300 mV/s and the change in current response of 0.1 M H2SO4 was observed. The peak intensities (both the cathodic and the anodic) depend linearly on the scan rate, i.e., as the scan rate increases, the peak intensities also increase linearly.
As one can easily observe from Fig. 2, there is no shift of peak potential with the increment of the scan rate. This indicates that the redox reversal process is adsorption-controlled.
Figure s2 shows the relationship (the scan rate for PGV) between Ipa/Ipc and v1/2(mV/s) 1/2. Both anodic and cathodic peak currents depend linearly on the square root of the whole scan rate range examined, which suggested a diffusion-controlled process in solution [9, 10].
The slope of the curve Ipa versus υ1/2 is slightly greater than the corresponding reduction process, meaning that the oxidation process occurs with a higher electron transfer rate [11, 12].
Electrochemical activity (redox behavior) of the PBG film on the GCE was investigated in 0.1 M H2SO4 at various scan rates as observed in Fig. 2 (b). The peak intensities (both the cathodic and the anodic) depend linearly on the scan rate. Up on the increment of sweep rate, there is a little shift of peak potentials in both cathodic and anodic peak potentials, i.e. as the sweep rate increases, the anodic peak potentials shift to positive and that of the cathode shift toward more negative. This shifting is the characteristic of multi-layer modified electrodes, where mass transfer is done through the film, and also of the processes with slow electron transfer or influenced by an ohmic drop effect. They all show a linear dependence of the peak current on the square root of the potential scan rate, so that the electrochemical process at the electrode is controlled by diffusion of the counterion.
Electrochemical activity (redox behavior) of the PMV film on the GCE was investigated in 0.1 M H2SO4 at various scan rates and the peak currents are observed in Fig. 2 (c). enhanced proportionally with scan rates imply a diffusion-controlled process and the peak potentials shifted toward positive and negative potentials for the anodic and cathodic peak potentials, respectively which is the characteristics of multi-layer modified electrodes, where a mass transfer is done through the film, and also of the processes with slow electron transfer or influenced by an ohmic drop effect.
3.1.1.2. Effect of solution pH
It has been found that when organic dye polymers are in solution or chemically adsorbed at electrode surfaces, they possess a midpoint potential which is highly pH-dependent. It was therefore necessary to investigate the variation of midpoint potential with pH for the electropolymerised dyes since this influences the applied potential [11].
The pH indicates the number of hydrogen ions in a solution. Since the solution pH has a significant effect on chemical reactions, the measurement and control of pH are very important. The effect of solution pH has a significant contribution to electrochemical oxidation and was carefully investigated by the CV technique. Its effect on the response of PGV-modified GCE was investigated in the range of pH 1.56–5.30 on the CV.
From Fig. 3 (a) both the anodic and cathodic peak potentials shifted toward negative and the current peaks decreased as pH values increased. Shift in the negative direction with a rising value of pH, suggesting that protons have participated in electrode reactions. The voltammogram also becomes more irreversible with the increment of the pH.
Figure s3 shows the peak potential dependence of the redox couple of PGV on the pH. The difference between both peaks is that the slope of the cathodic peak potential dependence (3.09 mV/pH) is roughly twice larger than the slope of the anodic one (1.45 mV/pH). This result points out the participation of two hydrogen ions in the electrochemical reaction associated with the cathodic peak and only one proton in the anodic one.
For the GCE coated with PBG film (Fig. 3 (b)), the peak potential moves to more negative potentials as pH increases, suggesting that a protonation, prior to the electron transfer, must occur in the reduction, as well as deprotonation in the oxidation. The peak width increases with pH. This means as pH increases the peak becomes more irreversible. The intensity of the peaks decreases as pH values increase and this is associated with a decrease of the hydrogen ion concentration.
From Fig. 3 (c) (the pH dependence voltammogram of PMV film coated GCE) as the pH value increases, there is a shift toward the negative potential from the potential of 250 mV for the smallest pH value (1.56) to 50 mV for the highest value of pH (5.30). Shift in the negative direction with a rising value of pH, suggesting that protons have participated in electrode reactions.
3.1.2. Electropolymerisation of Copolymers
Before electropolymerisation of the copolymers, the optimum mole ratio of the two dyes was determined from different mole ratios after scanning the mixtures of two dyes in the supporting electrolyte, as shown in Fig. 4.
The inset graphs show the comparison where the maximum peak current is produced for the mixtures of two dyes in each ratio. By this method the optimum ratio selected for the mixtures of (a) poly(gentian violet-brilliant green) is 1:3 (GV: BG), (b) poly(aniline-methyl violet) is 1:3 (PANI : MV), (c) poly(brilliant green-methyl violet) is 1:3 (BG : MV) and that of (d) poly(gentian violet-methyl violet) is 1:5 (GV : MV).
3.2. Electropolymerization of BG with GV, aniline with MV, GV with MV, BG with MV
The cyclic voltammograms of copolymerization of brilliant green and gentian violet (Fig. 5).
From Fig. 5, for the copolymerization of BG and GV the peak found at -550 mV is closer to that of the GV monomer than that of BG (the peaks of the copolymer and the GV are found roughly at about the same potential). Thus, the copolymer property is more related to the GV. But this cathodic peak of the copolymer is more intense by one and a half (1.5) times than the peak observed during the electropolymerization of the GV in the absence of BG. However, the anodic peak currents at around a peak potential of 1,500 mV and the high currents observed at far positive potential are almost equal for the electropolymerization of both gentian violet alone and gentian violet in the presence of brilliant green. This shows the presence of brilliant green enhances the rate of polymerization of gentian violet at the surface of glassy GCE. This shows BG has a catalytic effect on the electropolymerization of GV as it increases the rate of polymerization of GV.
During the electropolymerization of the aniline and MV, the change in current is similar to that of the voltammograms of electropolymerization of methyl violet. But, the cathodic peaks observed at about the same potential (-400 mV) are different in their intensity, i.e., the peak current of the copolymer (-3.80 mA) is greater than that of poly(MV) in the absence of aniline (-2.90 mA). This shows that the presence of aniline produces a high current.
For the copolymerization of the selected mole ratio of MV and GV, both the oxidation and reduction peaks were increased upon cycles and this is very visible in the first six cycles. The voltammogram has a shape similar to that of MV polymerization; therefore, the copolymer behaves as the poly(MV). The presence of GV has made a very small change in electropolymerization. Thus, the presence of GV enhances a little the rate of polymerization of MV at the surface of GCE. The cathodic peak current of the copolymer poly(GV-MV) observed at about − 500 mV is about ten times when compared to that of the peak current of poly(GV) in the absence of MV. This shows MV has a catalytic effect on the electropolymerization of GV as it increases the rate of polymerization of GV.
On copolymerization of brilliant green and methyl violet, both the oxidation and reduction peaks were increased with cycles. The intensity of the oxidation and reduction peak currents of the copolymer is much greater than that of poly(brilliant green) which shows the catalytic effect of methyl violet.
3.3. Determination of Ascorbic Acid and Uric Acid by GC/Polydyes Modified Electrodes and Bare GC Electrode
The oxidation potentials of UA and AA are too close to be separately determined by using bare electrodes.
3.3.1. Determination of Ascorbic Acid and Uric Acid by Bare GC Electrode
The electrode was modified in that the 1:3 mole ratio of the two dyes (optimum mole ratio of the two dyes) was selected and scanned for fifteen cycles.
Figure 6 (A) shows the CV and (B) shows the DPV of the mixtures of ascorbic acid and different concentrations of uric acid on bare GCE. The two solution mixture voltammograms show only one irreversible potential peak in CV i.e., peaks: (a) and (b) at 566 mV and also one potential peak in DPV i.e., peaks: (a) and (b) at 380 mV on bare GCE.
3.3.2. Determination of AA and UA mixture with copolymer poly(brilliant green-gentian violet) modified GC Electrode
Figure 7 depicts the CV and DPV of a mixture of the solution AA and UA on GCE modified by the copolymer of BG and GV (poly(BG-GV)). The electrode was modified in the mixtures of the two dyes by a 1:3 mole ratio (optimum mole ratio of the two dyes) and scanned for fifteen cycles in the solution.
Figure 7 shows the existence of two separate potential peaks for the concentrations of AA and UA in both CV and DPV. The first potential peaks around 200 mV on CV and 100 mV on DPV are the peaks of UA and those at potentials 600 mV on CV and 450 mV on DPV are the peak potentials of AA. The peak currents of AA are equal and that of UA are not. This is because the concentration of AA is the same (2 mM) and the concentrations of UA are 2.5 mM and 5 mM for the smaller and higher peak currents, respectively.
Therefore, one can easily conclude from this that the modified GCE resolved the overlapped voltammetric responses of AA and UA to well-separated voltammetric peaks. The results showed that the electrocatalytic activity of the poly(BG-GV) copolymer-modified GCE were excellent when compared to bare GCE.
3.3.3. Determination of AA and UA mixture with copolymer poly(brilliant green-methyl violet) modified GC Electrode
Two separated peaks at around 300 mV and 600 mV on CV and 150 mV (for the small potential peaks) and 450 mV and 500 mV on DPV are observed in Fig. 8.
In both cases, the peaks at small potentials are the potential peaks of UA which were in different concentrations and those at high potentials are the potential peaks of AA. The peak current of the voltammogram (a) which is for 2.5 mM UA and 2 mM AA is lower than that of (b) (5 mM UA and 2 mM AA) on the peak of UA due to their concentration difference and that of AA is the same. From this, again the separation of potential peaks by Brilliant Green-Methyl Violet copolymer-modified GCE shows the ability of this copolymer film to enhance the electrode to separate the potential peaks of samples that show almost one peak on bare GCE.
3.4. Determination of the Area of the Modified Electrode
The determination of the area of the modified electrode was done with all polymers investigated, and their copolymers. The CV in bare GCE was scanned at different scan rates of 4 mM K3Fe(CN)6 in 1 M KNO3 solution.
The square root of scan rate dependence of peak current was plotted and the Randles-Sevcik equation was applied to determine the Diffusion coefficient of Fe(CN)63− ion. This was used to determine the area after running the CV of 4 mM K3Fe(CN)6 in 1 M KNO3 at different scan rates. From the plot of peak current vs. square root of scan rate shown in Fig. 9 (a), the slope was determined and this was used to determine the area from the Randles-Sevcik equation and the result is summarized in Table 1.
Table 1
Surface area of the different modified electrodes
Modified Electrode Area/cm2 Ip of DPV for 2 mM AA
|
Poly(BG-GV) 8.75 ˣ 10-2 2.87 ˣ 10-4
BG 1.20 ˣ 10-1 3.02 ˣ 10-4
poly(ANI- MV) 1.27 ˣ 10-1 3.66 ˣ 10-4
poly(BG-MV) 7.88 ˣ 10-2 4.12 ˣ 10-4
GV 1.10 ˣ 10-1 4.89 ˣ 10-4
MV 1.44 ˣ 10-1 5.39 ˣ 10-4
poly(GV-MV) 1.15 ˣ 10-1 7.12 ˣ 10-4
PANI 7.86 ˣ 10-1 8.50 ˣ 10-4
Bare GCE 7.10 ˣ 10-2 5.40 ˣ 10-5
|
The area of modified electrodes is decreased from PANI to poly(brilliant green-gentian violet) in the order of:
PANI > MV > MV-PANI > BG > GV-MV > GV > BG-GV > BG-MV
And in the decrement order of the peak current it can be put as:
PANI > GV-MV > MV > GV > BG-MV > MV-PANI > BG > BG-GV
After being modified with CV each one with fifteen cycles, the modified electrodes were scanned by DPV in 2 mM AA. From Table 1, the peak currents of the modified electrodes are greater than that of the peak currents of bare GCE raised from the increment of the areas of the electrodes.
The peak current is increased with the increase of the area of the modified electrodes and it is highest for the electrode with the largest area which is PANI with an area of 11 times and peak current 16 times greater than that of bare GCE and smaller for the smallest one, i.e., poly(BG-MV) copolymer with area 1.1 times greater than bare GCE and the peak current is 71/2 times greater than that of bare GCE, respectively. This increment is because as the surface area of the electrode increases the microporous hole of the films produced over the modified electrode surface increases and this allows more and more electron transfer [19–21]. Though this case does for the two extremes and for some others, it is not consistent in all cases as can be seen, for instance, on BG and poly(BG-MV). In this case, the area of the electrode modified by poly(BG) is about 1.7 times and the peak current is 5.6 times greater than that of bare GCE whereas the area of the electrode modified by poly(BG-MV) is 1.1 times and the peak current 71/2 times greater than that of bare GCE. Even though the area of the first (1.7 times) is greater than the latter (1.1 times), the peak current of the electrode modified by poly(BG) is less than that of poly(BG-MV). This inconsistency is raised from another factor which is the kinetics of the nature of the film which slags the electron transfer to be increased by the area of the electrode. This inconsistency is also observed between poly(GV) and poly(MV-PANI), poly(MV) and poly(GV-MV) and some others due to the same reason [19–21].
Table 2
Electrochemical parameters of the studied modified electrodes (BG/GC, GV/GC, MV/GC, PANI/GC, BG-GV/GC, GV-MV/GC, BG-MV/GC, and PANI-MV/GC) at scan rate of 100 mV/s.
Modified Electrode
|
Epa/mV
|
Epc/mV
|
Ipa/mA
|
Ipc/mA
|
Ipa/Ipc
|
ΔE/mV
|
GV
BG
MV
PANI
Poly(BG-GV)
Poly(BG-MV)
poly(MV-GV)
poly(ANI-MV)
|
348
203
396
306
500
483
438
464
|
204
67
215
17
437
271
233
216
|
0.058
0.05
0.33
1.01
0.13
0.11
0.23
0.33
|
-0.056
-0.05
-0.33
-0.78
-0.12
-0.07
-0.21
-0.33
|
1.03
1
1
1.3
1.08
1.57
1.09
1
|
144
136
181
289
63
212
205
248
|
From Table 2, the ratio of anodic peak current to cathodic peak current (Ipa/Ipc) for poly(BG), poly(MV), and poly(ANI-MV) is equal to one. The (Ipa/Ipc) of the others are also close to one. This shows the electron transfer is high and therefore the processes are reversible.