Skip to main content
Chemistry LibreTexts

Results and Discussion

The cyclic voltammograms in Figures 2(a), illustrate the selectivity principle in the electrochemical detection of catechol in the presence of ascorbic acid. Ascorbic acid illustrates irreversible behavior and catechol illustrates reversible behavior. As mentioned previously, the detection of catechol can be effected by the presence of ascorbic acid. The tailor-designed porous structure facilitated the electrochemical response to catechol in the presence of ascorbic acid. Figure 2(a), (Blue Line) shows clearly that the SGC/TiO2 electrode was able to electrocatalyze the oxidation of catechol. The oxidation peaks of catechol and ascorbic acid are being resolved at different potentials and the reduction peak of the catechol oxidized in the forward sweep is detected. However, the cyclic voltammogram at the bare sonogel-carbon electrode with no titanium oxide as designated in Figure 2(a), (Red Line) illustrates that the oxidation peaks of catechol and ascorbic acid are not resolved, and the reduction and oxidation peaks for catechol are not detectable.

Figure 2 (a): Cyclic voltammogram of 5mM catechol +5mM ascorbic acid in 10 mM H2SO4 (100 mV/s) vs Ag/AgCl /3 M NaCl, on bare sonogel-carbon electrode (Bare SGC, Red Line) and compare to (SGC-TiO2, Blue Line) modified electrode [current (mA) vs. potential (mV)].

A similar study by coworkers has been conducted in the past for the detection of catechol in the presence of ascorbic acid with a modified poly (3-methylthiophene) electrode (P3MT) [4]. Like the SGC-TiO2 electrode, the P3MT electrode has indicated the reversibility of catechol oxidation is significantly improved compared with the bare sonogel-carbon (SGC) electrode. Studies have shown that catechol and ascorbic acid can be difficult to distinguish in a cyclic voltammogram (refer to lab appendix for example of student data) with a P3MT electrode and there is deterioration of the P3MT electrode over time [4, 26]. The problem found with the P3MT modified electrode is poor stability and electrode fouling. The SGC-TiO2 electrode was found to be more stable than the P3MT modified electrode after repeated sweeping cycles. In Figure 2(b), the stability of the SGC/TiO2 electrode is compared to the P3MT modified electrode in the detection of catechol over 20 cycles. As shown in Figure 2(b), the SGC/TiO2 electrode did not show any change when it was swept repetitively, indicating that the TiO2 sol-gel has a good adherence to the sonogel-carbon electrode surface and this electrode has a good stability over 20 cycles. Figure 2(b), the P3MT modified electrode (Red Line) shows instability over 20 cycles. The cyclic voltammogram for the P3MT modified electrode shows that as the number of cycles increase, the anodic and cathodic peak currents vary. This indicates that the P3MT adherence to the sonogel-carbon electrode surface weakens as the number cycles increases, exhibiting instability. Another sign that the P3MT lacked stability was the result of black P3MT particles falling off the electrode into the sulfuric acid solution during the 20 cycles. There was a problem with adherence of the P3MT (polymer) over several scans.

Figure 2(b): Comparison of the stability of SGC/TiO2 electrode (Blue Line) and P3MT modified electrode (Red Line) in the detection of 5 mM catechol in 10 mM H2SO4 (100 mV/S) over 20 scans [current (mA) vs. potential (mV)].

The modified SGC/TiO2 electrode shows improved electrocatalysis and improved selectivity towards catechol compared to the bare SGC electrode and the modified P3MT electrode. Profitable features that students learn about the advantages of the SGC/TiO2 synthesized electrode include relative chemical inertness, good mechanical properties, physical rigidity, stability, and enhanced catalytic properties for the nanostructured modified titanium oxide electrode. {Note the results shown in Figure 2(a), and Figure 2(b), are examples of actual student data and illustrate the inconsistent peak splitting for catechol can be due to not obtaining reproducible sol-gel synthesis results of modified electrodes}

Although the syntheses of the modified electrode surfaces are given to the students in recipe format, the investigation of the oxidation and reduction reactions of catechol in the presence of ascorbic acid is left open. The student must determine from the electroanalytical results which sensor would be the best sensor to detect catecholamines in the presence of interferents. A justification of this choice is an expectation in the lab report. The emphasis on grading of this lab can be placed on the student's analysis of the oxidation and reduction reactions for catechol (reversible behavior, oxidation and reduction peaks) and ascorbic acid (irreversible behavior, oxidation peak).

The cyclic voltammogram data presented are student results. Our experience has shown that a degree of student independence is welcomed at the level that this lab is presented. The experiment can be expanded to require students to create an optimized sensor that is applicable for real world analysis at pH of 7.4. However, due to the extra time needed to prepare phosphate buffer and the pH adjustments that are needed to prepare each solution to keep a physiological pH of 7.4, sulfuric acid was employed in this experiment due to the limited lab time at our university. Students typically have difficulty making solutions at the undergraduate level and adjusting pH can be tough in a limited time period. Additional interferents that may be studied to expand the lab could be acetaminophen, ascorbic acid and uric acid in the presence of catechol and other catecholamines (L-Dopa, dopamine, serotonin, etc.) by cyclic voltammetry. Salimi et al. discuss the need for simultaneous determination of ascorbic acid, acetaminophen, uric acid and neurotransmitters with a carbon electrode prepared by the sol-gel technique [28]. Simultaneous detection of neurotransmitters, ascorbic acid, and uric acid are oxidized at nearly the same potential with poor sensitivity at solid electrodes, resulting in overlapped voltammetric responses. Simultaneous detection is a problem of critical importance not only in the field of biomedical chemistry and neurochemistry but also diagnostic and pathological research. For the instructor, the possibilities for modification are abundant, and our hope is that ideas presented here are the seeds for many more. See the Lab Appendix for pre- or post-test questions and suggested readings to assist students with the experiment.