In the Laboratory
W
Variable Effects during Polymerization S. K. Lunsford Department of Chemistry, Wright State University, Dayton, OH 45435;
[email protected] An instrumental analysis course is offered during the junior or senior year for the undergraduate degree in chemistry. The goal for this course has been to develop more advanced labs that integrate and incorporate many science components together, such as electrochemistry, analytical techniques, and biological and neurological compounds. For example, catechol is of interest to neuroscientists as it is secreted in the brain, and altered levels have been associated with mental and behavioral disorders such as schizophrenia, attention deficient disorder, Alzheimer’s disease, Parkinson’s disease, eating disorders, epilepsy, and amphetamine and cocaine addiction (1– 7). Since catechol undergoes oxidation within the usable potential range for aqueous electrochemistry, it can be detected by electrochemical methods using a modified electrode. An optimized poly(3-methylthiophene) (P-3MT) platinum modified electrode that can be used to detect catechol by in vivo cyclic voltammetry has been developed. The modified electrode has displayed electrochemical reversibility as well as enhanced sensitivity and selectivity to catechol compared to the conventional bare platinum electrode (8). With this in mind, we developed a lab experiment in which students polymerize 3-methylthiophene onto a polished platinum electrode surface and optimize the conditions (deposition temperature, monomer concentration, deposition time, applied voltage, and electrolyte concentration) for the selective electrode to determine catechol by utilizing cyclic voltammetry (9–12). The students learn to use a potentiostat–galvanostat and a cyclic voltammetry instrument while determining the best parameters to detect catechol. The use of only an electrochemical analyzer can be sufficient to polymerize the electrode and study the variable effects during polymerization if both instruments are not available. Experimental Procedure Chemicals Acetonitrile (HPLC grade), tetrabutylammonium tetrafluoroborate (TBATFB), sulfuric acid (ACS Reagent), methanol (HPLC grade), and catechol (certified) were obtained from Fisher Scientific. The 3-methylthiophene monomer was obtained from Aldrich and stored at 4 ⬚C. Monomer solutions were freshly prepared for each polymerization by dissolution in a previously prepared 0.05 or 0.075 M TBATFB in acetonitrile solution.
Electropolymerization Preparation Before electropolymerization the platinum electrode was first polished with a nylon cloth using 2-µm diamond paste then polished with a microcloth using 0.05-µm alumina. The electropolymerization of the 3-methylthiophene on a polished platinum electrode was carried out in a three-electrode singlecompartment cell by applying a constant potential of 1.6 or 1.7 V using an EG&G model 173 potentiostat–galvanostat (Princeton Applied Research). The reference electrode was AgAgCl3 M NaCl (MF-2074, BAS). A platinum auxiliary electrode was a 120-mm long, 0.3-mm diameter platinum wire 1830
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(Alfa Aesar) sealed in the tip of a disposable glass pipette. The working electrode was inserted in the center of the platinum auxiliary electrode during the electropolymerization process. The electrolyte solutions of TBATFB and acetonitrile were deaerated with nitrogen for 10 minutes. An ice–water bath was used for polymerization at 0.0 ⬚C and an acetone–isopropanol bath cooled by dry ice was used for polymerization at ᎑20 ⬚C. After the electropolymerization, the P-3MT electrode was rinsed with methanol and deionized water. A BAS-100 electrochemical analyzer (BAS) was employed for the cyclic voltammetry study. A scan rate of 100 mV s᎑1 was used throughout the different variables studied in the experiment. Hazards Sulfuric acid is corrosive and causes severe burns, and is a strong dehydrating agent, do not breathe vapors, do not get in eyes, or on skin or clothing. 3-Methylthiophene is highly flammable and harmful by inhalation and if swallowed. Acetonitrile is flammable, toxic, harmful by inhalation and harmful if in contact with skin and if swallowed. This material can produce a cyanide-like effect and targets areas of the body such as the central nervous system and liver. Tetrabutylammonium tetrafluoroborate (TBATFB) is harmful by inhalation, contact with skin, and if swallowed. Also TBATFB is irritating to eyes and respiratory system. Catechol is harmful if in contact with skin or swallowed and it reacts readily with light. Target organs for catechol are the liver and the central nervous system. Methanol is flammable and toxic by inhalation, in contact with skin, and if swallowed. Target organs for methanol are eyes and kidneys. Acetone is flammable and irritating to eyes, with target organs liver, kidneys, and nerves. Dry ice can cause severe frostbite to skin. Isopropanol is flammable and irritating to eyes and can damage liver, kidneys, or nerves if it comes in contact with skin. Results and Conclusions The following variables were tested in the lab: deposition temperature, monomer concentration, deposition time, applied voltage, and electrolyte concentration. Each variable was tested at two levels (Table 1). The results from combinations of the variables on the peak separation in the cyclic voltammograms are shown in Table 2 (9–12). The cyclic voltammograms from runs 2 and 15 (Table 2) are shown in Figure 1. The peak separation decreases in the cyclic Table 1. Variables for Polymerization Quantity
Level (−)
Deposition Temperature (⬚C) Monomer Concentration (M) Deposition Time (s)
᎑20 0.05 10
Level (+) 0.0 0.075 20
Applied Voltage (V)
1.6
1.7
Electrolyte Concentration (M)
0.05
0.1
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In the Laboratory Table 2. Variable Effects and Peak Separation during Polymerization Run
Deposition Temperature/ ⬚C
Monomer Concentration/ (mol L᎑1)
Deposition Time/ s
Applied Voltage/ V
1
---
---
---
2
+
---
---
3
---
+
4
+
5
Electrolyte Concentration/ (mol L᎑1)
Peak Separation/ mV
---
+
446
---
---
504
---
---
---
229
+
---
---
+
206
---
---
+
---
---
232
6
+
---
+
---
+
222
7
---
+
+
---
+
171
8
+
+
_
---
---
255
9
---
---
---
+
---
173 183
10
+
---
---
+
+
11
---
+
---
+
+
226
12
+
+
---
+
---
184
13
---
---
+
+
+
246
14
+
---
+
+
---
169
15
---
+
+
+
---
168
16
+
+
+
+
+
180
NOTE: Cyclic voltammograms: 0.005 M catechol in 0.01 M sulfuric acid tested by P-3MT/platinum electrodes. The + and ---- refer to the levels listed in Table 1.
Figure 1. Cyclic voltammograms of P-3MT/Pt disc electrodes comparing run 2 and run 15 in Table 2. Run 2: (A) Electrolyte solution in the absence of catechol and (C) with catechol. Run 15: (B) Electrolyte solution in the absence of catechol and (D) with catechol. Experimental conditions: scan rate 100 mV s᎑1; electrolyte 0.01 M sulfuric acid in aqueous solution, pH = 1.6; 0.005 M catechol; solution temperature 22 ⬚C.
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In the Laboratory
voltammogram of run 15 compared to run 2 Figure 1D versus 1C). The decrease in peak separation is associated with an increase in the monomer concentration, deposition time, and applied voltage. The P-3MT formed under optimized conditions had improved electrochemical reversibility, selectivity, and reproducibility for the detection of catechol. The effects of the monomer concentration, deposition time, and voltage during electropolymerization on catechol peak separation were demonstrated in this experiment. The peak separation decreases with the increase in the monomer concentration, supporting electrolyte, deposition time, and voltage as Table 2 illustrates. The change in temperature and monomer concentration together had no significant effect on the peak separation as run 14 and 15 show (Table 2). This laboratory allowed the students to learn electrochemical techniques, lab skills, and instrumentation while optimizing polymerization parameters to best detect catechol. This experiment takes three lab periods to complete successfully. The undergraduate students work in groups of two to three. A lecture is needed to explain principles of cyclic voltammetry and electrochemical methods before the lab. In addition the students are required to read supplemental materials from books and literature to assist in the understanding of the electrochemical methods and studies performed in this experiment. A pretest and posttest were administered to assess the students’ knowledge before and after the instrumental analysis lab. The results showed normalized gains of more than 0.8.1 The pre- and posttest questions can be obtained in the Supplemental Material.W Conclusion The students gained experience with instrumental and analytical skills used in today’s modern bioanalytical chemistry laboratories. These skills allowed the students to see practicality and applicability in developing electrochemical sensors for neurological diseases and disorders. Such skills were the establishment of a correct base line that is essential for accurate peak current in cyclic voltammetry.
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Acknowledgment The technical assistance of Dr. Taylor at the Faraday Electrochemical Company is gratefully acknowledged. W
Supplemental Material
Student handout, including the pre- and postlab questions, is available in this issue of JCE Online. Note 1. Improvements were assessed by calculating a normalized gain, n: n = (% average gain)(possible average % gain); n > 0.7 = high gain, 0.69–0.3 = medium gain, and low gain is less than 0.3.
Literature Cited 1. Hassan, S. S.; Elnemma, E. M. Anal. Chem. 1989, 61, 2189. 2. Ma, Y. L.; Galal, A.; Zimmer, H.; Huang, Z. F.; Bishop, P. L.; Mark, H. B., Jr. Anal. Chem. Acta 1994, 21, 289. 3. Kissinger, P. T.; Heineman, W. R. In Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Marcel Dekker: New York, 1976. 4. Walczak, M. W.; Dryer, D. A.; Jacobson, D. D.; Foss, M. G.; Flynn, N. T. J. Chem. Educ. 1997, 74, 1195–1197. 5. Mark, H. B., Jr.; Atta, N.; Ma, Y. L.; Petticrew, K. L.; Zimmer, H.; Shi, Y.; Lunsford, S. K.; Rubinson, J. F.; Galal, A.; Bioelectrochem. Bioenerg.1995, 38, 229. 6. Hubbard, A. T. J. Electroanal. Chem. 1969, 22, 165. 7. Evans, D. H.; O’Connell, K. M.; Peterson, R. A.; Kelly, M. J. J. Chem. Educ. 1983, 60, 290–292. 8. Adams, R. N. Anal. Chem. 1976, 48, 1126A. 9. Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W. Y. J. Electroanal. Chem. 1981, 129, 115. 10. Genis, E. M.; Bidan, G.; Diaz, A. F. J. Electrochem. Soc. 1983, 149, 101. 11. Wang, S.; Tanaka, K.; Yamabe, T. Synth. Met. 1989, 32, 141. 12. Lunsford, S. K.; Galal, A.; Akmal, N.; Ma, Y. L.; Zimmer, H.; Mark, H. B., Jr. Anal. Lett. 1994, 27, 2141.
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