In the Laboratory
Gold Electrodes Modified with Self-Assembled Monolayers for Measuring l-Ascorbic Acid An Undergraduate Analytical Chemistry Laboratory Experiment Takashi Ito,* D. M. Neluni T. Perera, and Shinobu Nagasaka Department of Chemistry, Kansas State University, Manhattan, KS 66506; *
[email protected] We describe an electrochemistry laboratory experiment for an undergraduate laboratory course that allows students to measure the l-ascorbic acid (AA) content of a real sample using gold electrodes modified with self-assembled monolayers (SAMs). The experiment has the following three objectives: (i) to teach the basics of voltammetry; (ii) to expose students to recent electroanalytical chemistry research, that is, chemically modified electrodes based on SAMs; and (iii) to demonstrate an application of SAM-modified electrodes for measuring AA in a real sample. This experiment can be completed within three hours and is suitable as an experiment for an undergraduate laboratory course. A number of electrochemistry laboratory experiments have been reported in this Journal. These articles can be classified into four categories on the basis of their objectives: (a) to teach students various electrochemical techniques (1–3); (b) to determine the thermodynamic parameters of compounds (4–7); (c) to measure the concentration of chemical species of environmental or medical interest in real samples (8–13); and (d) to introduce recent achievements in electrochemistry research to undergraduate laboratories (9, 10, 14–19). In undergraduate laboratory courses, it is important to teach the basic concepts of electrochemistry with experiments in categories (a) and (b), but, in our experience, these experiments are not attractive to many students because they cannot link such experiments to real-life problems. In contrast, laboratory experiments in category (c) often stimulate students’ interest in electroanalytical methods. Many of the previous reports in this category (8, 11, 12), however, require students to deal with toxic mercury as
an electrode material because they are designed to measure the concentration of heavy metal ions via stripping voltammetry. Laboratory experiments regarding real-sample analysis using enzyme-modified electrodes have also been reported (9, 10), but the limited stability of the enzymes sometimes gives large variation in data. The development of simple analysis-oriented experiments that provide the students with basic training in electrochemical methods while simultaneously exposing them to recent research achievements will enhance student interest and learning. The experiment reported in this article aims to measure AA using electrodes modified with SAMs. AA in a real sample is directly measured from the peak current in cyclic voltammograms (CVs) using SAM-modified electrodes. This method is different from those used in previously reported laboratory experiments designed to determine AA concentrations in real samples (20, 21). The fundamental properties and applications of thiolate SAMs on gold have been widely studied (22) and have also been introduced in undergraduate laboratory experiments (7, 23–25). However, the long preparation time required to obtain densely-packed SAMs limits the general utility of these experiments in the undergraduate laboratory. Here, thioctic acid and cysteamine hydrochloride (Figure 1), which have short alkyl chains, are used in place of more common alkanethiols having long alkyl chains. These molecules do not require long preparation time to obtain SAMs giving reproducible results in electrochemistry experiments (26, 27), thus making it possible to design laboratory experiments that can be completed during limited lab hours. Materials and Methods
COOH S
S
thioctic acid
cysteamine hydrochloride
COOH S
S
á
á
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ź
NH3 Cl cysteamine hydrochloride
Figure 1. The compounds used to modify the gold electrode.
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ź
NH3 Cl
HS
Gold disk electrodes (2.0 mm in diameter; CH Instruments) are polished with wet alumina slurry (50 nm) on a clean microcloth (Buehler). For SAM modification, polished electrodes are immersed in an aqueous solution of cysteamine hydrochloride (10 mM) or in an ethanol solution of thioctic acid (10 mM) for at least one hour (26, 27). Electrochemical measurements were carried out in a standard three-electrode cell containing a Ag/AgCl reference electrode (CH Instruments) and a Pt wire counter electrode. A CH Instruments 600B electrochemical analyzer was used to record the data. Aqueous solutions of AA containing 0.1 M NaCl and 50 mM NaH2PO4–Na2HPO4 buffer (pH 7.4) are purged with argon during electrochemical measurements to minimize AA oxidation. All aqueous solutions are prepared with water having a resistivity of 18 MΩ cm or higher (Barnstead Nanopure Systems).
Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
Results This experiment consists of (i) comparison of CVs of AA on electrodes modified with different SAMs and (ii) determination of the AA content of a vitamin C tablet using a SAMmodified electrode. Comparison of CVs of AA on Different Electrodes The students record CVs of AA obtained on an unmodified gold electrode, a gold electrode modified with cysteamine hydrochloride (a cysteamine-modified electrode), and a gold electrode modified with thioctic acid. The purpose of these experiments is to study the effects of SAM modification on the electrode reaction. Figure 2 shows typical CVs of AA (1.0 mM) observed on these three electrodes at pH 7.4. As reported previously (26–29), the electrode reaction of AA is irreversible. A cysteamine-modified electrode gives a relatively sharp AA oxidation peak around 0 V vs Ag/AgCl. In contrast, a bare electrode exhibits a broad oxidation peak at more positive potential. Likewise, AA is oxidized at an even more positive potential on a gold electrode modified with thioctic acid. At pH 7.4, protonation of the amino group of cysteamine and deprotonation of the carboxyl group of thioctic acid offer positively- and negatively-charged surfaces, as estimated from the pKa values of compounds, 10.81 (30) and ~5 (31), respectively. AA (pKa1 = 4.17) (32) is negatively charged at this pH. Hence, the difference in CVs as shown in Figure 2 can be explained on the basis of electrostatic interactions between AA and the SAM surface: the oxidation of AA is facilitated on the positively-charged SAM owing to electrostatic attraction and is inhibited on the negatively-charged SAM owing to electrostatic repulsion (26–29, 33). In addition, the larger peak current in curve c of Figure 2 offers higher detection sensitivity, indicating that a cysteamine-modified electrode is best suited for determining the concentration of AA in solution. Subsequently, CVs of AA (1.0 mM) are measured at different scan rates on a cysteamine-modified electrode, as shown Figure 3A. The results provide a valuable opportunity to discuss the origin of the oxidation current: diffusion-controlled systems or adsorbed systems (3, 33). In the former, the redox current originates from the electrode reaction of species diffusing to the electrode surface, whereas in the latter, it is ascribed to electron transfer reactions between the electrode and redox species adsorbed onto the electrode. Regardless of the reversibility of the electrode reaction, the peak current (ip) is proportional to the square root of the scan rate (v) for diffusion-controlled systems, whereas ip is proportional to v for adsorbed systems. As shown in Figure 3B, ip is proportional to v 1∙2 rather than v in this measurement, indicating that AA diffusing from the solution to the electrode gives the oxidation current.
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(E vs Ag/AgCl) / V Figure 2. Cyclic voltammograms of 1.0 mM l-ascorbic acid in 0.1 M NaCl + 50 mM NaH2PO4–Na2HPO4 buffer (pH 7.4) on an (a) unmodified gold electrode, (b) a gold electrode modified with thioctic acid, and (c) a gold electrode modified with cysteamine hydrochloride. Scan rate, 0.1 V/s.
A 0
Current / NA
Ethanol is flammable. Cysteamine hydrochloride and thioctic acid cause irritation of the eyes, skin, digestive tract, and respiratory tract, and thus exposure should be minimized by wearing gloves and goggles and by working in a fume hood. Razor blades should be handled with care and properly stored and disposed of to avoid injury.
scan rate: 0.02 V/s 0.05 V/s 0.1 V/s 0.5 V/s 1 V/s
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In the Laboratory
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v1/2 / (V1/2 sź1/2)
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Figure 3. (A) Cyclic voltammograms of 1.0 mM l-ascorbic acid in 0.1 M NaCl + 50 mM NaH2PO4–Na2HPO4 buffer (pH 7.4) on a cysteamine-modified electrode (2.0 mm diameter) at different scan rates. (B) Relationship between peak current (ip) and the square root of scan rate (v 1/2), obtained from the data shown in Figure 3A.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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In the Laboratory A
Current / NA
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[AA]: 0.2 mM 0.4 mM 0.6 mM 0.8 mM 1.0 mM
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Summary and Conclusions
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This experiment provides a means for teaching these important issues in an undergraduate analytical laboratory course: (i) the basics of voltammetry, including information obtained from the scan rate dependence of the redox current; (ii) the effect of SAM modification on redox reaction by comparing the three electrodes; and (iii) determination of analyte concentration using electrochemical sensors. In particular, the analysis of a vitamin C tablet will draw students’ interest to the experiment. This experiment only requires a potentiostat, electrodes, and common chemicals and can be completed within a three-hour lab period. Thus, this experiment can be readily incorporated into an undergraduate analytical chemistry laboratory.
ip / NA
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Acknowledgments
C
Current / NA
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The authors thank Daniel A. Higgins (Department of Chemistry, Kansas State University) and Maryanne M. Collinson (Department of Chemistry, Virginia Commonwealth University) for contributing an earlier version of the Lab Documentation and their suggestions. The authors gratefully acknowledge financial support from Kansas State University.
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Literature Cited
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AA Concentration / mM Figure 4. (A) Cyclic voltammograms of l-ascorbic acid at different concentrations on a cysteamine-modified electrode (2.0 mm diameter). Measured in 0.1 M NaCl + 50 mM NaH 2 PO 4 – Na2HPO4 buffer (pH 7.4) at 0.1 V/s. (B) Relationship between peak current (ip) and l-ascorbic acid concentration obtained from the data shown in Figure 3A. The line fit to the data was obtained using the least-squares method. (C) Cyclic voltammogram measured on the cysteamine-modified electrode in a sample solution prepared from a vitamin C tablet. Measured in 0.1 M NaCl + 50 mM NaH2PO4–Na2HPO4 buffer (pH 7.4) as a supporting electrolyte at 0.1 V/s just after CVs in Figure 3A were measured.
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Determination of the AA Content of a Vitamin C Tablet As discussed above, cysteamine-modified electrodes are suitable for detecting AA in an aqueous solution. Figure 4A shows CVs observed on a cysteamine-modified electrode at different AA concentrations. The shape of the CVs is similar, although the peak position changes slightly, probably due to gradual fouling of the electrode surface (27). However, ip increases linearly with increasing AA concentration (Figure 4B), indicating that the cysteamine-modified electrode can be used to determine AA concentration in the range of 0.2–1 mM. Figure 4C shows a CV of a sample solution prepared from a vitamin C tablet, as a real sample, obtained using the cysteamine-modified electrode. Since the shape of the CV is very similar to those in Figure 4A, the concentration of AA in the solution, and thus the AA content of the original tablet, can be determined from ip.
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Supporting JCE Online Material
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