Anal. Chem. 1997, 69, 2866-2869
Long Optical Path Length Cell for Thin-Layer Spectroelectrochemistry Neal J. Simmons and Marc D. Porter*
Ames LaboratorysU.S. DOE, Microanalytical Instrumentation Center, and Department of Chemistry, Iowa State University, Ames, Iowa 50011
This paper describes the design and construction of a thinlayer spectroelectrochemical cell with a long optical path length. This cuvette-based cell, which can be reproducibly filled by capillary action, facilitates the use of metal thinfilm electrodes such as those constructed on glass, silicon, and mica substrates. Electrochemical and spectroscopic data are presented that demonstrate the thin-layer behavior of the cell and other important performance characteristics (e.g., optical sensitivity, electrolysis time, and oxygen exclusion capability). Recent studies have exploited the advantages of thin-layer spectroelectrochemistry in explorations of a variety of heterogeneous and homogeneous electrochemical processes.1-34 In many (1) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9, 241-248. (2) Heineman, W. R.; Anderson, C. W.; Halsall, H. B.; Hurst, M. M.; Johnson, J. M.; Kreishman, G. P.; Norris, B. J.; Simone, M. J.; Su, C.-H. In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K. M., Ed.; American Chemical Society: Washington, DC, 1982; Vol. 201, pp 1-21. (3) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1984; Vol. 13, pp 1-113. (4) Niu, J.; Dong, S. In Reviews in Analytical Chemistry; Zangen, M., Ed.; Freund Publishing House: London, 1996; Vol. 15, pp 1-171. (5) DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-597. (6) Kusu, F.; Kuwana, T. Chem. Lett. 1988, 3, 531-534. (7) Murray, R. W.; Heineman, W. R.; O’Dom, G. W. Anal. Chem. 1967, 39, 1666-1668. (8) Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983, 55, 2219-2222. (9) Gui, Y.-P.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985, 57, 1474-1476. (10) Gui, Y.-P.; Kuwana, T. Langmuir 1986, 2, 471-476. (11) Gui, Y.; Kuwana, T. J. Electroanal. Chem. 1987, 222, 321-330. (12) Gui, Y.; Soper, S. A.; Kuwana, T. Anal. Chem. 1988, 60, 1645-1648. (13) Gui, J. Y.; Hance, G. W.; Kuwana, T. J. Electroanal. Chem. 1991, 309, 7389. (14) Porter, M. D.; Kuwana, T. Anal. Chem. 1984, 56, 529-534. (15) McCreery, R. L. Anal. Chem. 1977, 49, 206-209. (16) Finklea, H. O.; Boggess, R. K.; Trogdon, J. W.; Schultz, F. A. Anal. Chem. 1983, 55, 1177-1179. (17) Blubaugh, E. A.; Doane, L. M. Anal. Chem. 1982, 54, 329-331. (18) Flowers, P. A.; Nealy, G. Anal. Chem. 1990, 62, 2740-2742. (19) Taniguchi, I.; Fujiwara, T.; Tominaga, M. Chem. Lett. 1992, 1992, 12171220. (20) Arciero, D. M.; Hooper, A. B. J. Electroanal. Chem. 1994, 371, 277-281. (21) Nagy, T. R.; Anderson, J. L. Anal. Chem. 1991, 63, 2668-2672. (22) Simone, M. J.; Heineman, W. R.; Kreishman, G. P. Anal. Chem. 1982, 54, 2382-2384. (23) Brewster, J. D.; Anderson, J. L. Anal. Chem. 1982, 54, 2560-2566. (24) Mosier-Boss, P. A.; Newbery, R.; Szpak, S.; Lieberman, S. H.; Rovang, J. W. Anal. Chem. 1996, 68, 3277-3282. (25) Niu, J.; Dong, S. Electroanalysis 1995, 7, 1059-1062. (26) Paulson, S. C.; Elliott, C. M. Anal. Chem. 1996, 68, 1711-1716. (27) Dong, S.; Zhu, Y. Langmuir 1991, 7, 394-397. (28) Dong, S.; Zhu, Y.; Cheng, G. Langmuir 1991, 7, 389-393. (29) Hartl, F. Inorg. Chim. Acta 1995, 232, 99-108. (30) Atwood, C. G.; Geiger, W. E.; Bitterwolf, T. E. J. Electroanal. Chem. 1995, 397, 279-285.
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instances, optical monitoring is accomplished by irradiating the solution confined in the thin-layer cavity with the propagation axis of the optical beam parallel to the electrode/solution interface.8-12,21-23,25,33,34 The configuration of such long optical path length thin-layer spectroelectrochemical cells (LOPTLCs) yields path lengths of ∼1 cm. These path lengths result in an enhancement in sensitivity of ∼100-fold over that generally achieved using conventional thin-layer cells and optically transparent electrodes that have path lengths of ∼100 µm. A key feature of the LOPTLC is that the long optical path length and large electrode surface area to solution volume ratio provide ample optical sensitivity for the quantitative monitoring of interfacial processes such as adsorption, desorption, and electrocatalysis.8-11,14,35 The work described herein stems from interest in applying the LOPTLC to our ongoing investigations of the electrode reactions of monolayers formed by the chemisorption of organosulfur compounds (i.e., alkanethiols, dialkyl disulfides, and dialkyl sulfides) at gold.35,36 These monolayers are usually adsorbed at thin (200-300 nm) gold films coated onto fragile glass, silicon, and mica substrates by vapor deposition techniques. Many existing LOPTLC designs, however, employ pressure seals between the electrode and the cell for use with vacuum degasfilling procedures,8-12,14,17,37 requiring mechanically strong electrodes constructed to critical size specifications. The purpose of this paper is to present the design and performance attributes of a LOPTLC that facilitates investigations of electrodes constructed by vapor deposition techniques and can be readily filled without using vacuum procedures. EXPERIMENTAL SECTION Reagents. Potassium ferrocyanide (Fisher), sodium perchlorate (Aldrich), sodium hydroxide (Aldrich), methanol (Fisher, HPLC grade), and ethanol (Quantum, punctilious grade) were used as received. Solutions were prepared using deionized water (Millipore). Electrode Preparation. Glass microscope slides were initially cut into substrates that were 0.99 cm wide and ∼3.5 cm long. (31) Yao, C.-L.; Capdevielle, F. J.; Kadish, K. M.; Bear, J. L. Anal. Chem. 1989, 61, 2805-2809. (32) Flowers, P. A.; Mamantov, G. Anal. Chem. 1989, 61, 190-192. (33) Rubinson, K. A.; Harry B. Mark, J. Anal. Chem. 1982, 54, 1204-1206. (34) Anderson, J. L. Anal. Chem. 1979, 51, 2312-2315. (35) Simmons, N. J.; Zak, J.; Zhong, C.-J.; Porter, M. D. Manuscript in preparation. (36) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114, and references therein. (37) Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973, 45, 1021-1027. S0003-2700(97)00165-0 CCC: $14.00
© 1997 American Chemical Society
Figure 1. Schematic diagram (not to scale) of the thin-layer cell. (a) An exploded side view of all components down the optical axis of the cell (x) including the plate assembly, thickness spacers (ts), back plate (bp), electrode positioning foot (f), positional spacer (ps), thinfilm electrode (tfe), and a compression spring. (b) Top view showing all components, including the reference (RE) and auxiliary (AE) electrodes, assembled inside the cuvette.
These substrates were cleaned by sonication in a dilute Micro (Cole-Parmer) solution, rinsed with deionized water and methanol, dried under a stream of ultrahigh-purity argon (Air Products), and loaded into the vacuum deposition chamber. The electrodes were prepared by vacuum depositing a 15 nm adhesive layer of chromium at 0.1 nm/s, followed by a 300 nm layer of gold (99.9%) at 0.3 nm/s, using an Edwards E306A coating system. During the coating process, the pressure in the deposition chamber was ∼8 × 10-6 Torr. Instrumentation. Electrochemical data were collected with a CV-27 potentiostat (Bioanalytical Systems) and X-Y recorder (Houston Instruments). A Pt coil served as the auxiliary electrode, and all potentials are reported against a Ag/AgCl/saturated NaCl electrode. Due to space limitations within the cuvette-based cell, the diameter of each electrode was constructed to be e4 mm. The optical data were collected with a Hewlett-Packard 8452A diode array spectrophotometer operating in the kinetics software mode for the cyclic voltammetry and chronoamperometry experiments. For cyclic voltammetric experiments, the absorbance at 420 nm was collected at 5 s intervals with an integration time of 1 s. For chronoamperometric experiments, data were collected at 1 s intervals with an integration time of 1 s. In all cases, the absorbance at 600 nm was used as a baseline absorbance. Cell Fabrication. Figure 1 presents a schematic diagram of our LOPTLC, which fits inside a standard 1.00 cm quartz cuvette (Starna). Figure 1a is a side view along the axis of the optical beam (x) showing the plate assembly, a positional spacer (ps), a thin-film working electrode (tfe), and a compression spring. Figure 1b is a top view of the components inside the cuvette, depicting the locations of all three electrodes, the thin-layer cavity, and the optical path. The plate assembly, along with the thin-film working electrode, defines the dimensions of the thin-layer cavity. The plate assembly consists of three thickness spacers (ts), a back plate (bp), and a foot (f), which are all cut from a 1.6 mm thick quartz plate. Two of the thickness spacers are mounted on the top of the back plate
and one is mounted on the bottom. The back plate defines the area of the rectangular-shaped thin-layer cavity, the foot serves as a positioning element for the vertical location of the electrode, and the thickness spacers define the thickness of the thin-layer cavity. The back plate is 0.99 cm wide and 2.47 cm long, resulting in a geometric area for the electrode/solution contact of 2.45 cm2. The thickness spacers (4 mm by 3 mm by 1.6 mm) are attached to the back plate by covering first the front face (i.e., the face in contact with the liquid in the thin-layer cavity) of the back plate with a piece of Teflon tape (Fluorocarbon, Dielectric Division), ∼100 µm thick, that is trimmed to match the dimensions of the back plate. The back plate is then clamped with the Teflon-taped face down onto a flat, nonstick work surface (i.e. covered by Teflon tape). Next, a small amount of EP-21 ARLV two-component epoxy resin (Master Bond, Inc.) is applied to the ends of the back plate. This epoxy is stable in acidic, basic, and nonaqueous (i.e., ethanol) environments. The thickness spacers, the thickness of which is equal to that of the back plate (1.6 mm), are subsequently pressed against the ends of the back plate and the flat working surface while the epoxy cures. After curing, the Teflon tape is removed from the back plate; this leaves the ends of the spacers extending ∼100 µm past the front face of the back plate. The foot is attached with epoxy resin to the single thickness spacer on the bottom of the back plate. In Figure 1a, the optical axis of the probe beam (x) is located near the vertical center of the thin-layer cavity. This arrangement is achieved by epoxying the back plate to a fabricated quartz positional spacer (0.99 cm wide by ∼3 cm long) whose thickness (∼3.2 mm) also places the cavity close to the horizontal center of the cuvette. To optimize the cell position for maximum light throughput, a cuvette holder was attached to a small translation stage and mounted in the sample compartment. Black ink was applied with a fine-tip marker to the outside of the cuvette to ensure that only light passing through the thin-layer cavity reaches the detector. A top view of the assembled cell components is shown in Figure 1b. The spacer assembly is inserted into the cuvette against an interior wall. The working electrode is then placed against the top of the foot and the thickness spacers, as indicated in Figure 1a. A spring is inserted into the cuvette to compress the working electrode firmly against the thickness spacers. The spring was constructed from a 4 mm wide copper strip bent as shown in Figure 1a. The left side of the spring presses against the inside wall of the cuvette that is opposite the wall in contact with the back of the positional spacer. A Pt coil auxiliary electrode (AE) and Ag/AgCl/saturated NaCl reference electrode (RE) are placed behind the working electrode and extend well below the bottom of the spring for solution contact. The cell is filled using a glass pipet to introduce ∼1.5 mL of solution into the bottom of the cuvette. When the liquid level reaches the bottom of the thin-layer cavity, capillary action fills the cavity. For working with degassed solutions, the top of the cell is covered by a Teflon cuvette cap with four access holes to accommodate a reference electrode, an auxiliary electrode, a working electrode contact, and a purge gas line. In such cases, the cell and Teflon tubing (5 cm) are first flooded with argon and then filled by switching a valve to an external, argon-purged solution reservoir. A continuous flow of argon is maintained in the headspace of the cuvette for the duration of the experiment. Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
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Figure 2. (a) Cyclic voltammetric current-potential curve and (b) cyclic voltabsorptometric absorbance-potential curve for 0.506 mM K4Fe(CN)6 and 0.5 M NaClO4 in the LOPTLC. Scan rate 2 mV/s.
RESULTS AND DISCUSSION Cyclic Voltammetry and Voltabsorptometry. As a starting point, the performance of the LOPTLC was evaluated voltammetrically. Parts a and b of Figure 2a,b present the respective cyclic voltammetric current-potential (i-E) curve and cyclic voltabsorptometric absorbance-potential (A-E) curve, respectively, for an electrolytic solution composed of 0.506 mM ferrocyanide and 0.5 M sodium perchlorate. The scan was initiated at -0.10 V at a rate of 2 mV/s and reversed at +0.60 V. From the cyclic voltammetric data, the formal reduction potential (E°′) for ferri-/ferrocyanide was found to be +0.239 V. The shapes and small peak separation (∼50 mV) of the i-E curves are consistent with the expectations for the exhaustive transformation of ferrocyanide to ferricyanide by the completion of the anodic scan and the exhaustive reversal of the process by the completion of the cathodic scan.8,12,14,25,38 That is, the integrated charge, as averaged from eight experiments, passed for the anodic scan [Qa ) 1245 ((39) µC] equals that passed for the cathodic scan [Qc ) 1213 ((48) µC]. This agreement confirms the effective retention of the redox couple within the thin-layer cavity throughout the duration of the voltammetric experiment. From the integrated charges, the computed cell volume is 25.0 ((1.0) µL. This value, coupled with the geometric area of the electrode exposed to solution (i.e., 2.45 cm2), translates to a cavity thickness of 102 ((4) µm. The A-E responses in Figure 2b are also diagnostic of the exhaustive electrolysis of a small volume of solution confined within the thin-layer cavity of the LOPTLC.8,12,14,25 The wavelength for monitoring optically the electrochemically induced changes is 420 nm, which is the absorbance maximum for ferricyanide.39,40 Upon initiation of the anodic scan, the absorbance remains at a baseline level until ∼+0.2 V. At ∼+0.2 V, the absorbance increases rapidly as the applied potential (Eappl) becomes more positive because of the generation of ferricyanide. The continuation of the scan to more positive values of Eappl results in a constant absorbance. This constancy infers both the completion of the oxidation of ferrocyanide to ferricyanide and the retention of the redox species within the restricted volume of the thin-layer (38) Hubbard, A. T.; Anson, F. C. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1970; Vol. 4, pp 129-214. (39) Strojek, J. W.; Kuwana, T. J. Electroanal. Chem. 1968, 16, 471-483. (40) Winograd, N.; Blount, H. N.; Kuwana, T. J. Phys. Chem. 1969, 73, 34563462.
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Figure 3. (a) Double potential step chronoamperometric curve and (b) chronoabsorptometric absorbance-time curve for 0.519 mM K4Fe(CN)6 and 0.5 M NaClO4 in the LOPTLC. The applied potential was first stepped from -0.10 V to +0.60 V for ∼70 s and then stepped back to its initial value.
cavity. The shape of the A-E curve for the reverse sweep mirrors that for the forward sweep, with the absorbance returning to its initial value by the end of the scan. We note that the difference in Eappl at the inflection points of the anodic and cathodic scans in Figure 2b is the same as the peak separation between the two i-E curves in Figure 2a. Chronoamperometry and Chronoabsorptometry. Chronoamperometric (i-t) and chronoabsorptometric (A-t) data were collected simultaneously in a double potential step format to assess the time required to electrolyze exhaustively a redox species confined within the thin-layer cavity of the LOPTLC. The i-t data are presented in Figure 3a and the A-t data in Figure 3b. This experiment was conducted by stepping Eappl from an initial value of -0.10 V to a value of +0.60 V for ∼70 s and then back to the initial value. The A-t curves were obtained at a monitoring wavelength of 420 nm. Upon initiation of the anodic potential step, a large transient oxidative current flows that decays to background levels in ∼32 s. The reversal of the potential step results in a cathodic i-t response that mirrors the profile of the initial potential step. An analysis of the integrated charges under the two i-t profiles, after accounting for double-layer charging found from a double potential step experiment using only supporting electrolyte, gave an average value of 1286 ((64) µC, a value consistent with those determined from the i-E curves. The shapes of the A-t curves in Figure 3b confirm the interpretations of the i-t curves in Figure 3a. That is, the application of the anodic potential step results in a rapid increase in the absorbance at 420 nm, as expected for the conversion of ferrocyanide to ferricyanide. Furthermore, the response reaches a limiting value in ∼38 s, indicating that the exhaustive transformation of the redox species in the thin-layer cavity is achieved. An analysis of the profile obtained upon the reversal of the potential step yields a comparable electrolysis time. The optical data in Figure 3b, when coupled with the molar absorptivity for ferricyanide (1010 M-1 cm-1 39,40) yields an optical path length of 0.99 ((0.01) cm, which is expected from the cell dimensions. Solution Injection Reproducibility. The reproducibility of successive sample injections into the LOPTLC was investigated by the repetitive injection of a 0.506 mM ferrocyanide and 0.5 M NaClO4 solution. After each injection, the cell was disassembled,
rinsed, dried, and reassembled. Integration of the cyclic voltammetric charge required for the exhaustive transformation of the redox species for six separate experiments equaled 1233 ( 48 µC. Equilibrium Redox Studies. One of the most used applications of thin-layer spectroelectrochemical cells is the determination of E°′ and the stoichiometry, n, of redox reactions.2,4,38,41 These determinations take advantage of the rapid establishment of the redox equilibrium for a solution species confined in the thin-layer cavity with Eappl. Using the absorbance of ferricyanide to calculate the relative amounts of the oxidized and reduced forms of the redox couple as a function of Eappl,3-5 the values of the slope and y-intercept of a plot of Eappl vs log({Fe(CN)64-}/{Fe(CN)63-}) can be used to determine the respective values of n and E°’. The analysis of the data from such an experiment gave a value of +0.241 V for E°’ and a value of 0.97 for n. Both values agree well with those in previous reports.5,8,14,22,42 As noted earlier, the cyclic voltammetric data yielded a value of E°′ of +0.239 V. Oxygen Exclusion Capability. For many applications, the ability of thin-layer spectroelectrochemical cells to minimize oxygen is of value.10,13,15,20,33,37 This ability is also important for our investigations of the electrode reactions of chromophoric thiolderived monolayers, since the one-electron reductive desorption of the gold-bound thiolate monolayers in alkaline solutions occurs at large negative potentials.35,36 To test the performance of this cell for these desorption experiments, the cell was filled with an alkaline electrolytic solution (0.5 M NaClO4 and 0.015 M NaOH) that had been purged vigorously with ultrahigh-purity argon. The resulting voltammetric curve was featureless, whereas a scan of (41) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980. (42) Kolthoff, I. M.; Tomsicek, W. J. J. Phys. Chem. 1935, 39, 945-954.
an air-saturated solution exhibited a broad, poorly defined wave at ∼-0.25 V. Based on the smallest detectable voltammetric peak area and assuming a four-electron reduction process, the estimated minimum concentration of oxygen detectable in this cell is ∼0.5 µM. This result demonstrates the potential utility of this cell for our future studies of the reductive desorption of chromophoric thiol-based monolayers at thin-film electrodes. CONCLUSIONS This paper describes the design and performance attributes of a thin-layer spectroelectrochemical cell with a long optical path length. In contrast to earlier designs, this cell can be easily constructed and used for investigations with electrodes consisting of thin metal films on supporting substrates such as glass, silicon, and mica. The assembly and disassembly of the cell are facile, which simplifies the cleaning of components between experiments. The ability to minimize the presence of dissolved oxygen opens the way to studies of the cathodic reactions of monolayers formed at gold from chromophoric organosulfur compounds. These experiments are underway. ACKNOWLEDGMENT Discussions with Jerzy Zak and Chuan-Jian Zhong are gratefully acknowledged. This work was supported by the Office of Basic Energy Research of the Chemical Sciences Division of the U.S. Department of Energy. The Ames laboratory is operated by Iowa State University for the U.S. Department of Energy under Contract W-7405-eng-82. Received for review February 10, 1997. Accepted April 24, 1997.X AC970165Z X
Abstract published in Advance ACS Abstracts, June 1, 1997.
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