Results and Discussion

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The anodic peak potentials obtained for catechol and ascorbic acid on a PBTP electrode were found to be dependant on the electropolymerization potentials. In this study the polymer film was grown electrochemically on a platinum substrate at four different potentials; +1.5V, +1.6V, +1.7V, +1.8V. The results obtained from the cyclic voltammograms carried out utilizing these PBTP electrodes showed that the ∆Ep values for the catechol, decreased from +1.5V to +1.8V and the peak separation became smaller. Table 1 and 2 lists the Ep_a values obtained for catechol and ascorbic acid, which displays that the bare platinum electrode the anodic peaks to be closer together for catechol at 625 mV and ascorbic acid at 699 mV. The modified PBTP polymer film there is a greater separation between the catechol and ascorbic acid anodic peaks. These values were obtained from the positive scan of the first voltammetric cycle at a scan rate of 50mVs^-1. Therefore, the electrocatalytic activity of the PBTP polymer film can be seen from the fact that these Ep_a values were consistently more negative than those obtained from the bare platinum. These results suggest that there is less problem of overlap between the anodic peaks for the modified PBTP platinum electrode between catechol and ascorbic acid.

Table 1. Comparison of oxidation potentials of bare platinum and modified platinum PBTP in the detection of catechol

Catechol	Ep_a/ mV	Ep_c/mV	∆E mV
Bare Platinum	625	404	221
Platinum PBTP (1.8V)	573	450	123

Table 2. Comparison of oxidation potentials of bare platinum and modified platinum PBTP in the detection of ascorbic acid

Ascorbic Acid	Ep_a/mV
Bare Platinum	699
Platinum PBTP (1.8V)	460

Table 3. Electrochemistry of catechol and ascorbic acid at platinum electrode modified by 2,2-bithiophene synthesized at different potentials, AA=Ascorbic Acid

Catechol (Pt)	Ep_a /mV	Ep_c/mV	?E mV
1.5V	601	414	187
1.6V	590	427	163
1.7V	579	448	131
1.8V	573	450	123
AA
1.5V	470	--	--
1.6V	437	--	--
1.7V	407	--	--
1.8V	402	--	--

Table 4. Electrochemistry of catechol in the presence of ascorbic acid (AA) at a platinum electrode by 2,2-bithiophene synthesized at different potentials

Catechol	Ep_a/mV (AA)	Ep_a/mV	Ep_c/mV	?E mV
1.5V	450	582	458	124
1.6V	421	577	458	116
1.7V	402	575	464	108
1.8V	402	575	464	108

Table 3, and 4 presents the data illustrating the consistent decrease in the peak separation as the electropolymerization potentials increased. The data strongly suggests electrodes modified with polymer films electrochemically grown at +1.8V exhibits a greater sensitivity, selectivity and effectiveness in detecting catechol and ascorbic acid separately and catechol in the presence of AA with data from Figures 2 and 4.

Figure 2. Cyclic voltammogram for 5mM catechol diluted with 0.1 M H₂SO₄ at PBTP modified Pt electrode

Figure 3. Cyclic voltammogram of 5mM ascorbic acid diluted with 0.1 M H₂SO₄ at a PBTP modified Pt electrode; Black= 1.5V, Red= 1.6V, Green= 1.7V and Blue= 1.8V

Figure 4. Cyclic voltammogram of an equimolar mixture of 5mM ascorbic acid and catechol diluted with 0.1 M H₂SO₄; Black= 1.5V, Red= 1.6V, Green= 1.7V and Blue = 1.8V

Figure 3 displays that the response of the PBTP electrode to ascorbic acid is irreversible and Figure 4 shows a mixture that catechol can be detected in the presence of common interferent ascorbic acid. Figure 4 demonstrates the selectivity principle in the electrochemical detection of catechol in the presence of ascorbic acid. We can conclude that PBTP modified electrodes have demonstrated that the reversibility of catechol is significantly improved compared to that of the bare platinum electrode. The PBTP modified electrodes are effective in detecting catechol and ascorbic acid simultaneously.

Polymers prepared at +1.8V has an improved catalytic activity compared to lower potentials. The SEM data in Figure 5 confirms that the PBTP polymerized at +1.8V has a more cross-linked/uniform film surface compared to +1.5 V. Thus the higher potential produces a more conductive matrix exhibiting a smaller and more reversible peak separation with higher peak current intensities as can be seen by cyclic voltammetry.