Anal. Chem. 1993, 65,3430-3434
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Design and Characterization of a Spectroelectrochemistry Cell for Absorption and Luminescence Measurements Yoke Foo Lee and Jon R. Kirchhoff Department of Chemistry, University of Toledo, Toledo, Ohio 43606
A long optical path spectroelectrochemical cell, which can be used for both luminescence and absorption measurements in aqueous and nonaqueous solutions, is described. The cell is designed to replace a standard spectrophotometer cuvette holder and utilize the three-dimensional structure of reticulated vitreous carbon (RVC)as the working electrode. Optical channels (2 mm) of a tee configuration are used to provide efficient excitation and detection of the emitted light. The electrochemical and spectral characteristics of the cell were evaluated by cyclic voltammetry, chronoluminescence,and chronoabsorptometryof standard aqueous solutions of o-tolidine and potassium ferricyanide. Spectropotentiostatic experiments of a 5 pM o-tolidine solution resulted in E"' = +0.638 V vs Ag/AgCl and n = 1.91 for the luminescence measurement and E"' = +0.647 V vs Ag/AgCl and n = 1.83 for the absorption measurement. Solutions of [Re(dmpe)2Clz]+,where dmpe is If-bis(dimethylphosphino)ethane,in N,N-dimethylformamide were used toevaluate the cell performance in a nonaqueous solvent.
INTRODUCTION The combination of electrochemistry and spectroscopy provides a powerful tool for the study of chemical systems.' Electrochemistry allows selective control of the redox state of a species, while characterization of the in situ generated species can be performed simultaneously with spectroscopy. Spectroelectrochemistry has therefore permitted a significant expansion in the types of chemical investigations that are possible. Some examples include studies and characterizations of synthetically inaccessible oxidation states, surfaceconfined species, mechanistic electrochemistry, and reactivity of electrogenerated intermediates.24 Spectroelectrochemical methods have been developed utilizing numerous spectroscopic techniques. By far the most widely used spectroelectrochemical methods couple transmission spectroscopy with electrochemistry. One reason for the popularity of UV-visible spectroelectrochemistry has been the development of the optically transparent thin-layer electrode (OTTLEIe5The OTTLE is easy to construct and use, and the thin-layer feature allows rapid and exhaustive electrolysis on small amounts of material.6 Numerous other (1) Heineman, W. R. J . Chem. Educ. 1983, 60, 305-308. (2) Kuwana, T.;Winograd, N. In Electroanalytical Chemistry;Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 1-78. (3) Heineman, W. R. Anal. Chem. 1978,50, 390A-402A. (4) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 1-113. (5) Murray, R. W.; Heineman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 39, 1666-1668.
UV-visible spectroelectrochemical cell designs have been tailored to specific experimental application^.^^^ The coupling of luminescence spectroscopy to electrochemistry in a spectroelectrochemical cell has received relatively little attention. Early experiments by Yildiz et al.' with a vapor-deposited platinum film working electrode in an OTTLE cell design demonstrated the feasibility of electroluminescence measurements, as well as the advantage of the inherent sensitivity of fluorescence over transmission. Subsequent spectroelectroluminescence experiments have tried to exploit the electrochemical features of the gold micromesh OTTLE.aI2 To accommodate detection of the emitted light and the short path length of the OTTLE, cells were placed at a 45' angle relative to the excitation and emission slits. In general, the advantage of a short electrolysis time in the OTTLE cell is offset by interferences from scattered radiation off the face of the cell and nonreproducible cell positioning. Alternatively, detection from the back of a modified OTTLE, i.e., 180' relative to the reflected light, has been used to reduce the effect of scattered light.1°J2 In order to minimize both of these difficulties, a long optical path luminescence spectroelectrochemical cell has been described that permits detection of emitted light a t 90° from a cuvettebased configuration with agold resinate film electrode.13More recent applications of luminescence and electrochemistry include the monitoring of emission from electropolperization reactions at transparent tin oxide electrodes14 and the development of fiber optic luminescence spectroelectrochemistry probes as biosensors.15 This report describes the design and characterization of a new long optical path length luminescence spectroelectrochemistry cell that utilizes reticulated vitreous carbon (RVC) as the working electrode material. Narrow optical channels are drilled through the electrode to provide efficient 90' excitation and detection with minimum interference from scattered light and a reasonable electrolysis time. The optical channels also allow compatibility with both absorption and luminescence measurements. Additional advantages that are realized from this cell design are the cell (i) easily replaces a standard spectrophotometer cell holder, (ii) is easily constructed and assembled, (iii) is resistant to nonaqueous solvents, and (iv) requires only small volumes of solution. The use of RVC as the working electrode provides anelectrode (6) DeAngelis, T. P.; Heineman, W. R. J . Chem. Educ. 1976,53,594597. (7) Yildiz, A.; Kissinger, P. T.; Reilley, C. N. Anal. Chem. 1968, 40, 1018-1024. (8) Simone, M. J.; Heineman, W. R.; Kreishman, G. P. J. Colloid Interface Sci. 1982, 86, 295-298. (9) McLeod, C. W.; West, T. S. Analyst 1982, 107, 1-11. (10)Cousins, B. L.; Fausnaugh, J. L.; Miller, T. L. Analyst 1984,109, 723-726. (11) Turner-Jones, E. T.; Faulkner, L. R. J . Electroanal. Chem. 1984, 179, 53-64. (12) Compton, R. G.; Fisher, A. C.; Wellington, R. G . Electroanalysis 1991. - - - - , 3-,. 27-29. -.
(13) Simone, M. J.; Heineman, W. R.; Kreishman, G. P. Anal. Chem. 1982, 54, 2382-2384. (14) Kamat, P. V. Anal. Chem. 1987,59, 1636-1638. (15)VanDyke, D. A,; Cheng, E.-Y. Anal. Chem. 1989, 61, 633-636.
0003-2700/93/0365-3430$04.00/00 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23,DECEMBER 1, 1993 8431
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Figure 1. (a) Long optical path length cell. dimensions In mllllmeters; (b) lid wlth holes for electrodes and Teflon tubing; (c)cell support for luminescence spectrophotometer;and (d)cell support for transmission
spectrophotometer.
that (i) is removable and easily replaced if required, (ii) is easily machined, and (iii) has a wide potential window for electrochemistry, in both aqueous and nonaqueous solvents.
EXPERIMENTAL SECTION Reagents. Solutions of potassium ferricyanide (Fisher Scientific) in 1 M KN03 and o-tolidine (Aldrich) in 0.5 M acetic acid/l M perchloric acid were freshly prepared before use with distilled deionized water purified by a Barnstead Organicpure fiitration system. [Re(dmpe)&12]PFe, where dmpe is l,a-bis(dimethy1phosphino)ethane (Strem),was prepared by a previous literature procedure.lBN,N-Dimethylformamide (DMF;Burdick and Jackson) and tetrabutylammonium hexafluorophosphate (TBAHFP; Aldrich) were used as the solvent and supporting electrolyte for the nonaqueous experiments. Prior to use, the TBAHF'P was recrystallized from ethanol and dried at 70 O C in vacuo. All other reagents were ACS grade and were used without further purification. RVC with a porosity of 100 poredin. was obtained from Electrosynthesis Co., Inc. Apparatus. Electrochemical experiments were performed with a Bioanalytical Systems, Inc. (BAS) Model CV-27 potentiostat coupled to an Allen Datagraph Model 725M XYT recorder. The potentials were monitored in the spectroelectrochemical experiments by a Keithley Model 179A digital multimeter. Luminescence measurements were obtained with an AmincoBowman Series 2 (SLM Instruments, Inc.) spectrophotometer. A Varian, Model Cary 5E UV-Vis-NIR spectrophotometer was used for all absorption measurements. All potentials were measured with respect to a Ag/AgCl (BAS, Model RE11 reference electrode. Design and Construction of the Spectroelectrochemical Cell. Schematic drawings of the spectroelectrochemical cell, lid, and base supporta are shown in Figure 1. The cell was machined from a block of polyethylene (35 X 35 X 58 mm) in three stages to resemble a standard Spectrophotometer cell holder (Figure la). It consists of two compartments with windows on four sides of the cell body. The larger top compartment was first milled to accommodate the reference and platinum auxiliary electrodes. The smaller rectangular hole (13 X 13 X 12 mm) was then milled (16)Vanderheyden,J.-L.; Heeg, M.J.; Deutach, E.Znorg. Chem. 1985, 24,1666-1673.
(C)
Schematlc drawings of RVC electrode wlth optical channels: (a)front vlew of electrode, (b) lower sectlon of RVC with cross configuration, and (c) tee conflguratlon. Flgure 2.
at the center for the working electrode. Finally, four slots were machined in line with the working electrode chamber. A liner for the working electrode compartment was made from the bottom portion of a standard quartz luminescence cell. The cuvette was cut to fit the dimensions of the working electrode compartment and epoxied in place. The liner provided optical windows for spectroscopic measurements with dimensions of 9 x 9 mm. Various size holes were drilled in the cover (Figure lb) to provide stable electrode connections and purging capability. Two different base supports (Figure lc,d), which replaced the commercial cell holder and support, were constructed for the two spectrophotometers. The designs allow flexibility in positioning and stabilizing the cell so that the light beam passes through the center of the optical openings. Adjustments are easily made with the four screws found on either base plate. The support (150 X 100 X 5 mm) used in the luminescence spectrophotometer (Figure IC)has two vertically mounted rectangular Plexiglas blocks (57 X 12 x 20 mm) that are glued approximately 46 mm apart. Holes (3 mm) were drilled at each corner for the mounting screws. In the case of the transmission spectrophotometer, a rectangular slot (42 X 41 mm) was cut in the Plexiglas plate (107 X 71 X 9 mm) to accommodate the cell body. The cell was secured by four screws and rests on a nylon block so the working cavity was in line with the light beam; additional holes (6mm) and a slot (7 X 17 mm) were also drilled in the plate for the mounting screws and pins. Both the cell and support were painted black to minimize reflection of stray light. The working electrode was prepared by pressing a slightly oversized 100 pores/in. RVC block into the center cavity and cutting to length with about 8 mm extending into the upper compartment. Perpendicular optical channels of either tee or cross configuration (Figure 2) were drilled in the RVC electrode to provide efficient excitation and collectionof the emitted light. The holes varied in size from 1,2,3,and 4 mm in diameter and were positioned at the center of the optical windows. Electrical contact was achieved by inserting a platinum wire into the RVC electrode. A platinum wire with a diameter of 0.5 mm, which surrounded the RVC, was used as the auxiliary electrode. Approximately 2-3 mL of solution is required to f i i the cell for operation. The solution volume occupied by the RVC working electrode was determined by coulometry of a standard ferricyanide solution to be approximately 0.4 mL! The path length is defined by the quartz liner and is 10 mm.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1 1993
Methods. All solutions and the electrochemical cell were purged for 20 min with argon prior to the electrochemical experiments. Degassed solutions were transferred into the electrochemical cell in a glovebag by positive pressure of argon. The cell was filled to a level sufficient to provide electrical communication between all three electrodes. The RVC electrode was then inserted into the working electrode chamber. The reference and auxiliary electrodes were placed in the upper solution chamber and the cell was covered. The cell was blanketed with argon by a Teflon tube inserted into the cell through the top cover. For spectroscopic measurements, the spectrophotometer compartment was continuously purged with argon prior to the transfer of the cell from the glovebag and during spectroelectrochemical measurements. In a typical chronoabsorptometry experiment with [Re(dmpe)zC12]+, [Fe(CN)e]", or o-tolidine, or a chronoluminescence experiment with o-tolidine, the spectral response was monitored at the maximum absorption or emission wavelength as a function of time after a potential step. Absorbance measurements took advantage of the absorption of the oxidized form of o-tolidine (438nm), [Fe(CN)6]3-(418 nm), and [Re(dmpe)zClzl+(408nm), while the luminescencemeasurements used the change in emission intensity of o-tolidine at 405 nm with excitation at 270 nm. The potential step required for exhaustive electrolysiswas determined from the cyclic voltammograms for each species. Electrolysis times were determined from plots of emission intensity and absorbance as a function of time at the point when the response plateaus. Spectropotentiostatic experiments were conducted as previously described.6 Each spectrum was recorded under equilibrium conditions. Equilibrium times were determined by the observation of a steady-state spectroscopic response after a potential step.
RESULTS AND DISCUSSION Evaluation of the Cell Design. The two main considerations for spectroelectroluminescence measurements are to (i) have efficient excitation and detection of the emitted light relative to the scattered radiation and (ii) have as short an electrolysis or equilibration time as possible within the optical channels. Since conventional detection for luminescence spectroscopy requires 90° detection relative to the excitation beam, a working electrode with a three-dimensional structure such as RVC" or a drilled solid electrodel8 is advantageous for the development of an electrode for luminescence spectroelectrochemistry. This cell design utilizes RVC as the working electrode material for several reasons. RVC is easily machined to obtain electrodes with perpendicular optical channels that can be reproducibly aligned with the spectrophotometer optics. An electrode can be cut, fitted, and drilled in a matter of minutes. Furthermore, if an electrode becomes fouled, a new electrode can easily be inserted into the cell. From an electrochemical point of view, RVC is a versatile electrode material since it is inert and exhibits a wide potential window for electrochemical analysis." The wide potential window is a significant advantage over metal electrodes for electrochemical measurements in acidic or basic aqueous solutions. The porous electrode also provides good communication between electrodes without the need to drill ports specific for the insertion of the reference and auxiliary electrodes. In addition, the pores are small enough that diffusional mixing in the optical channels from the upper compartment does not occur, vide infra. Two design configurations, a tee and a cross, with various size optical channels were evaluated. A diagram of each configuration is shown in Figure 2. Both the tee and cross configurations permit absorption and luminescence spectroelectrochemical studies. Furthermore, the long optical path of this cell allows spectroelectrochemical analysis of lower (17) Wang, J. Electrochim. Acta 1981,26, 1721-1726. (18)A reviewer has pointed out that drilledsolid electrodesmay achieve similar advantages.
Flgure 3. Cyclic voltammogram of 1 mM &tolidine in 0.5 M CH3COOH/1 M HCIO, in the spectroelectrochemical cell. Electrode is RVC with 2-mm tee configuration. Initial potential, +0.4 V vs Ag/ ASCI. Scan rate, 2 mV/s.
comentrations than those typically required for OTTLE experiments. Chronoluminescence and chronoabsorptometry with aqueous solutions of 5 pM o-tolidine in 0.5 M CHsCOOH, 1 M HC104 and 0.5 mM K3Fe(CN)6 in 1 M KN03, respectively, were used to evaluate the electrode design and optical channel size. A 100 poreslin. RVC was used for the fabrication of the working electrode in order to provide the shortest electrolysis time within the bulk of the working electrode compartment. Thus, the limiting factor for the electrolysis time is the size of the optical channels drilled into the RVC. I t is observed for a given hole design that the electrolysis time increases with hole size. Also, for a given hole size, the tee design is more efficient than the cross configuration in electrolyzing the solution. These results are not surprising in view of the facts that the diffusion distances and solution volume are increased with larger hole sizes or the cross configuration. Although the 1-mm-diameter tee channel electrode yields the shortest electrolysis time for 5 pM o-tolidine (15 min), it is difficult to align the cell to yield a satisfactory spectral response. This is especially true for luminescence experiments. An additional disadvantage of the small channel diameter is the possibility of bubbles forming and clogging the optical passage. This is minimized with larger holes. The 3- and 4-mm holes exhibit good signalto-noise ratios, but the electrolysis times for similar 5 1 M o-tolidine solutions are quite long (>65 min). Therefore, the 2-mm tee channel design provides the best tradeoff between spectral and electrolysis characteristics for subsequent experiments. The comparable electrolysis time for the 2-mm tee configuration is 38 min. Exhaustive electrolysis times were found to be dependent on the optical channel size, solvent, electroactive species, analyte concentration, and electrolyte. Typical times for the 2-mm tee electrode design were as follows: 0.5 mM [Fe(CN)6]>, 23 min; 0.14 mM [Re(dmpe)zClz]+?22 min; 5 pM o-tolidine, 38 min. Electrochemical Characterization. Cyclic voltammograms of aqueous solutions of 1mM o-tolidine in 0.5 M CH3COOH/l M HCIOl and 4 mM K$e(CN),j in 1M KN03in the spectroelectroluminescence cell are shown in Figures 3 and 4, respectively. The electrochemical parameters determined for o-tolidine were Eo' = +0.635 V, AE, = 90 mV, and &JiW = 1.0, and for K$e(CN)e were E"' = +0.255 V, AE, = 210 mV, and i,,/i,, = 1.0. The E"' values calculated for both
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
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I
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E , V vs. Ag/AgCI Flgwe 4. Cyclic voltammogram of 4 mM K3Fe(CN)( in 1 M KN03 In the spectroelectrochemlcal cell. Electrode Is RVC with 2-mm tee configuration. Initial potential, +0.8 V vs Ag/AgCi. Scan rate, 2 mV/s.
0.0
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360
400 WAVELENGTH,nrn
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500
Flgue6. Spectra recordeddwing a IWinescence speclropotentbstatk experiment of 5 pM o-toikllne in 0.5 M CH&OOH/l M HCIO, at the RVC electrode with 2-mm tee configuration. Applied potentials In V vs Ag/AgCI are as follows: (A) +0.405, (B) +0.608, (C) 4-0.828, (D) +0.637, (E) +0.648, (F) +0.658, (G) +0.670, and (H) +0.806. hx Is 270 nm.
-0.8
E, V vs. Ag/AgCL Figure 5. Cycllc voltammogram of 1 mM [Re(dmpe)&iP]+ in 0.5 M TBAHFP/DMF in the spectroelectrochemlcalcell. Electrode is RVC with 2 m m tee conflguratbn. I n M l potential, +O.O V vs Ag/AgCi. Scan rate, 2 mV/s.
systems are in good agreement with reported values.6J3JgThe peak separation observed in Figure 4 is larger than the Nernstian value of 0 mV for a thin-layer cell, but is consistent with the value measured for KsFe(CN)ein OTTLE cells made with RVC as the working electrode (AE,= 200 mV1.19 The cyclic voltammogram of 1 mM [Re(dmpe)zClzl+ in 0.5 M TBAHFP/DMF is shown in Figure 5. Eo’ is determined to be -0.395 V with AE, = 230 mV and &,i = 0.97. The nonaqueous electrochemicaldata are comparable to previous measurements of [Re(dmpe)&!lz]+ in DMF a t a gold OTTLE.20 Any deviation from thin-layer behavior in either aqueous or nonaqueous solution is most probably due to a combination of the uncompensated resistance in the cell, the electrolysis of sample solution in contact with the working electrode in the upper compartment, and the increased diffusion distances in the optical channels. SpectroelectrochemicalCharacterization. Spectra recorded during luminescence and absorption spectropotentioetatic experimentsof a solution of 5MMo-tolidine are shown in Figures 6 and 7. Due to the long optical path of the cell, a solution concentration compatible with both luminescence and absorption measurements is possible. A 25-30-min equilibration time was determined to be sufficient for the (19)Sorrela, J. W.;Dewald, H. D. A d . Chem. 1990,62,1640-1643. (20)Kirchhoff,J.R.;Heineman, W. R.;Deutach,E.Znorg. Chem. 1987, 26,3108-3113.
I
360
400
WAVELENGTH.nrn
450
1
0
Flgure 7. Spectra recorded duringan absorptionspectropotentiastatlc experiment of 5 pM etolkllne in 0.5 M CH&OOH/l M HCIO, at a RVC electrode with 2-mm tee conflguratbn. Applied potentlals In V vs Ag/AgCI are as follows: (A) +0.822, (B) +0.669, (C) i-0.659, (D) +0.649, (E) +0.628, (F) +0.607, and (0)+0.403.
spectropotentiostatic experiments. This equilibration time is relatively long compared to the few minutes obtained with an OTTLE,BJg but is improved relative to the 30-40 min reported for the long optical path luminescence spectroelectrochemistry cell with a gold resinate working electrode.13 Chronoabsorptometry experiments indicate that about 80 5% of the electrolysis was achieved within 8-11 min after the change in potential. The two-electron-redox reaction of o-tolidine provides a good test system for both spectroscopic methods.6 The versatility of o-tolidine is demonstrated by the spectral changes that occur upon oxidation. Oxidation results in an increase in the absorbance at 438nm, while the oxidized form exhibits a reduction in emission intensity when excited a t 270 nm. Eo’ and n were calculated from the intercept and slope of Nernst plots of log [Oxl/[Redl vs the applied potentials from the absorption and luminescence data. The method of calculatingthe ratio of oxidized to reduced species is in accordance with the methods of Faulkner” and Heineman.6J3 In the case of the luminescence experiment of Figure 6, the Nernst plot (405nm) is linear and gives Eo’ = +0.638 V and n = 1.91. For the absorption data in Figure 7 (438nm), Eo’ is found to be +0.647 V with n = 1.83. These
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
data agree very well with Eo’ values determined by cyclic voltammetry measurements in the spectroelectroluminescence cell and with literature values determined fromprevious spectroelectrochemical measurements of o-tolidine.6J3Jg The nonaqueous spectropotentiostatic experiment for 0.14 mM [Re(dmpe)zClz]+ in 0.5 M TBAHFP/DMF is also in agreement with reported electrochemical and spectral data for the Re(III/II) redox couple.20 The Re(II1) complex exhibits absorptioxt maxima at 408 and 439 nm, while maxima are observed at 347 and 384 nm for the Re(I1) complex. An isosbestic point is maintained at 396 nm throughout the experiment. The Nernstian analysis at 408 nm for the absorption spectropotentiostatic experiment of [Re(dmpe)&l*l+ yields Eo’ = -0.381 V and n = 1.07. The small deviation from n = 2.0for o-tolidine and n = 1.0 for [Re(dmpe)zClz]+in the spectropotentiostatic experiments is indicative of a well-established equilibrium in the optical channels of the spectroelectroluminescence cell. This observation is further underscored by chronoabsorptometry or chronoluminescence measurements that obtain steady-state behavior. Therefore, any electrolysis that may occur at the RVC electrode in the upper compartment does not effect the establishment of equilibrium conditions in the optical channels or the subsequent spectroscopic measurements.
CONCLUSIONS A new and versatile long optical path length spectroelectrochemistry cell has been developed. The cell body is designed to replace the cuvette holder of a standard spectrophotometer, while the electrochemical cell utilizes the three-dimensional structure and favorable electrochemical characteristics of RVC. The cell is easy to construct, is useful for aqueous and nonaqueous solutions, is compatible with both luminescence and absorption measurements, provides good signal-to-noise ratios with reasonable electrolysis times, and allows for a wide variety of luminescence, absorption, and electrochemical investigations to be performed.
ACKNOWLEDGMENT Financial support from the donors to The Petroleum Research Fund, administered by the American Chemical Society (Grant 23968-G3), and The University of Toledo is gratefully acknowledged. RECEIVED for review April 19, 1993. Accepted August 31, 1993.* 0
Abstract published in Advance ACS Abstracts, October 1, 1993.
