Anal. Chem. 2006, 78, 6833-6839
Evanescent Wave Cavity Ring-Down Spectroscopy in a Thin-Layer Electrochemical Cell Mikhail Mazurenka,† Lucas Wilkins, Julie V. Macpherson, Patrick R. Unwin,* and Stuart R. Mackenzie*,†
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK
The application of evanescent wave cavity ring-down spectroscopy (EW-CRDS) in monitoring electrogenerated species within a thin-layer electrochemical cell is demonstrated. In the proof-of-concept experiments described, ferricyanide, Fe(CN)63-, was produced by the transportlimited oxidation of ferrocyanide, Fe(CN)64-, in a thinlayer solution cell (25-250 µm) formed between an electrode and the hypotenuse of a fused-silica prism. The prism constituted one element of a high-finesse optical cavity arranged in a triangular ring geometry with light being totally internally reflected at the silica/solution interface. The cavity was pumped with the output (∼417 nm) of a single-mode external cavity diode laser, which was continuously scanned across the cavity modes. The presence of electrogenerated ferricyanide within the resulting evanescent field, beyond the optical interface, was detected by the enhanced loss of light trapped within the cavity, as measured by the characteristic cavity ring down. In this way, the EW-CRDS technique is sensitive to absorption in only the first few hundred nanometers of solution above the silica surface. The cavity ring-down response accompanying both cyclic voltammetric and step potential chronoamperometry experiments at a variety of electrode-surface distances is presented, and the results are shown to be well reproduced in modeling by finite element methods. The studies herein thus provide a foundation for further applications of EW-CRDS combined with electrochemistry. The application of spectroscopy to electrochemical systems1,2 has provided a wealth of information on electrode reactions, such as the identification of intermediates and pathways, that would be difficult to obtain by voltammetric methods alone. Among the wide range of spectroscopic techniques that have been applied to electrochemical processes,3 UV-visible absorption spectroscopy is among the oldest and most widely used.4 Traditionally, * To whom correspondence should be addressed. e-mail: P.R.Unwin@ warwick.ac.uk;
[email protected]. † Current address: Department of Chemistry, University of Cambridge, Lensfield Rd., Cambridge, CB2 1EW. (1) Gale, R. J. Spectroelectrochemistry; Plenum: New York, 1988. (2) Varma, R.; Selman, J. R. Techniques for Characterization of Electrodes and Electrochemical Processes; Wiley: New York, 1991. (3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; Wiley: New York, 2001. (4) Crayston, J. A. Encyclopedia of Electrochemistry; Wiley-VCH: New York, 2003. 10.1021/ac060678i CCC: $33.50 Published on Web 08/26/2006
© 2006 American Chemical Society
this spectroscopy technique has operated in transmission mode, demanding the use of semitransparent electrodes, as in the optically transparent thin-layer electrode cell.5 However, development of alternative cell designs such as the long optical path length cell,6,7 reflection cells,8 and attenuated total reflection (ATR) cells9 has expanded the range of electrode materials and processes that are compatible with in situ UV-visible spectroscopy. In this paper, we present the application of a recently developed spectroscopic technique that is ideally suited to the investigation of dynamic processes occurring at interfaces. The technique represents an extension of cavity ring-down spectroscopy (CRDS) into the condensed phase. The CRDS method, originally developed as a test for optical reflectivity,10-13 has matured in various guises over the last 10 years into an ultrahigh-sensitivity gas-phase absorption technique. As a result of its simplicity, high sensitivity, and ability to measure absolute absorptions, CRDS has found applications in trace detection, weak absorption spectroscopy, low vapor pressure optical activity, and gas-phase dynamical studies. Its importance among the multitude of existing spectroscopic techniques is reflected in a number of recent reviews.14-17 In its simplest form, the CRDS technique involves the injection of light into a high-finesse cavity comprising two highly reflective mirrors. The light level within the cavity is then monitored via the small fraction of light that leaks through the back mirror every round trip. The decay in the light intensity, the “ring down”, is characterized by an exponential decay constant and represents a measure of both the inherent cavity losses and any intracavity (5) Murray, R. W.; Heineman, W. R.; O’Dom, W. O. Anal. Chem. 1967, 39, 1666. (6) Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983, 55, 2219-2222. (7) Gui, Y. P.; Kuwana, T. Langmuir 1986, 2, 471-476. (8) Tolmachev, Y. V.; Wang, Z. H.; Hu, Y. N.; Bae, I. T.; Scherson, D. A. Anal. Chem. 1998, 70, 1149-1155. (9) Slaterbeck, A. F.; Stegemiller, M. L.; Seliskar, C. J.; Ridgway, T. H.; Heineman, W. R. Anal. Chem. 2000, 72, 5567-5575. (10) Kornienko, L. S.; Skubin, B. G. Opt. Spectrosc. 1976, 40, 323-324. (11) Herbelin, J. M.; McKay, J. A.; Kwok, M. A.; Ueunten, R. H.; Urevig, D. S.; Spencer, D. J.; Benard, D. J. Appl. Opt. 1980, 19, 144-147. (12) Herbelin, J. M.; McKay, J. A. Appl. Opt. 1981, 20, 3341-3344. (13) Kwok, M. A.; Herbelin, J. M.; Ueunten, R. H. Opt. Eng. 1982, 21, 979982. (14) Scherer, J. J.; Paul, J. B.; Okeefe, A.; Saykally, R. J. Chem. Rev. 1997, 97, 25-51. (15) Wheeler, M. D.; Newman, S. M.; Orr-Ewing, A. J.; Ashfold, M. N. R. J. Chem. Soc., Faraday Trans. 1998, 94, 337-351. (16) Berden, G.; Peeters, R.; Meijer, G. Int. Rev. Phys. Chem. 2000, 19, 565607. (17) Mazurenka, M.; Orr-Ewing, A. J.; Peverall, R.; Ritchie, G. A. D. Annu. Rep. C 2005, 101, 100-142.
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absorptions. The high quality of optical coatings available, for which reflectivities routinely exceed 99.995% across narrow spectral regions, means that in gas-phase studies, especially those in which the cavity is evacuated except for the sample of interest, ring-down times as long as several tens of microseconds are achievable. This corresponds to effective path lengths in a typical 1-m cavity of more than 104 m. The presence of even a very weak intracavity absorber (or indeed scatterer) significantly increases the cavity losses per round trip, reducing the ring-down time. The characterization of intracavity absorption by a simple decay constant has the further advantage of rendering this technique relatively immune to the shot-to-shot intensity fluctuations typical of pulsed laser systems, which can limit the sensitivity of comparative absorption methods. By comparison with the gas phase, the application of CRDS to problems in the condensed phase has been slow. Bulk solution absorption CRDS has been demonstrated by simply filling the cavity with the solution of interest, but the ring-down times are much shorter (typically ∼100 ns) than for gas-phase cavities due to absorption by the solvent.18 To overcome this restriction, smallvolume cells or coated slides placed at Brewster’s angle within the cavity have been used to minimize reflective losses.19-21 This particular application has been demonstrated to be considerably more sensitive than UV-visible spectroscopy in the detection of components separated by HPLC.21 One of the most promising condensed-phase variants of the CRDS method is the recent development by several groups of a method based on evanescent waves.22-25 Evanescent wave CRDS (EW-CRDS) utilizes the evanescent waves that exist whenever radiation undergoes total internal reflection (TIR) at a boundary with an optically less dense medium. Upon TIR, an evanescent field is established that extends beyond the boundary of the two media with amplitude decaying exponentially with distance from the boundary. Species within the evanescent field can absorb (or scatter) radiation, resulting in a net loss of light intensity from the cavity, which manifests itself as a more rapid ring-down decay. The evanescent field extends only a short distance (∼200-500 nm) beyond the surface, which makes this method exclusively sensitive to the interfacial region. As such the technique has thus far found applications in the study of gas-surface interactions,23,24,26-29 polymer surface-solvent interactions,30 and fundamental solid-solution interactions.31 The potential of EW-CRDS as a medical diagnostic tool has also been explored.32 (18) Hallock, A. J.; Berman, E. S. F.; Zare, R. N. Anal. Chem. 2002, 74, 17411743. (19) Xu, S.; Sha, G.; Xie, J. Rev. Sci. Instrum. 2002, 73, 255-258. (20) Muir, R. N.; Alexander, A. J. Phys. Chem. Chem. Phys. 2003, 5, 1279-1283. (21) Snyder, K. L.; Zare, R. N. Anal. Chem. 2003, 75, 3086-3091. (22) Shaw, A. M.; Hannon, T. E.; Li, F. P.; Zare, R. N. J. Phys. Chem. B 2003, 107, 7070-7075. (23) Pipino, A. C. R. Phys. Rev. Lett. 1999, 83, 3093-3096. (24) Pipino, A. C. R.; Hudgens, J. W.; Huie, R. E. Chem. Phys. Lett. 1997, 280, 104-112. (25) Tarsa, P. B.; Rabinowitz, P.; Lehmann, K. K. Chem. Phys. Lett. 2004, 383, 297-303. (26) Pipino, A. C. R.; Hudgens, J. W.; Huie, R. E. Rev. Sci. Instrum. 1997, 68, 2978-2989. (27) Pipino, A. C. R. Appl. Opt. 2000, 39, 1449-1453. (28) Pipino, A. C. R.; Hoefnagels, J. P. M.; Watanabe, N. J. Chem. Phys. 2004, 120, 2879-2888. (29) Aarts, I. M. P.; Pipino, A. C. R.; Hoefnagels, J. P. M.; Kessels, W. M. M.; van de Sanden, M. C. M. Phys. Rev. Lett. 2005, 95.
6834 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006
Figure 1. Schematic of the thin-layer electrochemical cell cavity ring-down spectrometer. A ring cavity is formed by two highly reflective spherical mirrors and the total internal reflection from the back surface of a fused-silica prism. Absorption within the evanescent field is monitored by the ring-down time. ECDL, external cavity diode laser; AOM, accoustooptic modulator; PMT, photomultiplier tube.
In this paper, we describe the first application of EW-CRDS coupled to dynamic electrochemistry, through studies of the mass transport-limited oxidation of ferrocyanide (Fe(CN)64-) to ferricyanide (Fe(CN)63-) in aqueous solution within a thin-layer cell formed between an electrode and a prism surface at which TIR takes place. This represents the first implementation of a hybrid spectroelectrochemical technique that we are developing for the study of a variety of interfacial phenomena. The electrogenerated product in all experiments, Fe(CN)63-, was detected via its absorbance at 417 nmsconveniently close to the minimum in the liquid water absorption spectrum, which with our present sensitivity has negligible effect on the ring-down time.33 In other regions of the electromagnetic spectrum, it is possible that bulk water absorption may provide a limit to the sensitivity of the method to other absorbers. In this experiment, the electrode served to alter the concentration distribution of Fe(CN)64- and Fe(CN)63- in a well-defined manner, and the change in ring-down time was measured. Studies with both cyclic voltammetry and potential step chronoamperometry over a range of time scales and at different electrode-prism separations and bulk concentrations were performed to examine the characteristics of the technique. EXPERIMENTAL SECTION Apparatus. The CRD spectrometer was built as a ring cavity as shown schematically in Figure 1. The cavity consisted of two high-reflectance concave mirrors (R ) 99.997% at 415 nm, Los Gatos Research). The mirrors with radii of curvature of 1 m, were mounted in adjustable gimbal mounts separated by 35 cm. The third element of the cavity was a standard commercial right-angle fused-silica prism (CVI) antireflection coated (AR, R
