Anal. Chem. 1999, 71, 1196-1203
Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 3. Effect of Signal Averaging on Limit of Detection Andrew F. Slaterbeck, Thomas H. Ridgway,* Carl J. Seliskar,* and William R. Heineman*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172
A new sensor that combines electrochemistry and spectroscopy with selective partitioning through an applied film has been demonstrated.1-6 This sensor consists of a light-guiding medium coated with an optically transparent electrode (OTE), which is then coated with a selective thin film. For an analyte to be detected with this device, it must first partition into the sensing film, second be electrochemically active at the potentials applied to the working electrode, and third have a change in its absorbance, concomitant with the electrochemistry, at the wavelength propagated within
the light guide. The analytical signal is optical; the amplitude of the absorbance change that occurs when the analyte is electrolyzed corresponds to the concentration of analyte that partitioned into the film. The significant difference between a traditional optical sensor and the new spectroelectrochemical sensor is that the attenuation of the light modulates with time as the potential at the working electrode is modulated. There are two distinct advantages to be gained by modulation of the optical signal: (1) the attenuation that results from the analyte, which changes with time, can be distinguished from that of interferences that also partition into the sensing film yet do not undergo electrochemistry at the applied potentials and (2) by repetitive application of the potential waveform, a steady-state situation should be quickly approached in which the response can be ensemble averaged, thereby reducing random noise contributions to the analytical signal and lowering the limit of detection. This second aspect is the focus of the work presented here. Ensemble averaging, also known as signal averaging or coaddition, is a technique common to spectrometric methods.7-9 By performing repeated measurements of a signal, and then subsequently averaging over the number of samples collected, random noise contributions to the measured signal can be reduced. Two main requirements of this technique must be met: only the contributions to the signal from noise change with each measurement, and the signal must be reproducible over the time of the measurement. This technique has seen limited application to electrochemical methods for two reasons. Frequently in electrochemistry, the background double-layer charging current is more significant than the noise contributions. Electrochemistry is relatively slow to arrive at the steady-state situation required for signal averaging. Several notable exceptions exist, however.10-22
(1) Slaterbeck, A. F.; Shi, Y.; Seliskar, C. J.; Ridgway, T. H.; Heineman, W. R. In Proceedings of the Symposium on Chemical and Biological Sensors and Analytical Electrochemical Methods, Vol. 97-19; Ricco, A. J., Butler, M. A., Vanysek, P., Horvai, G., Silva, A. F., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1997; pp 50-60. (2) Shi, Y.; Slaterbeck, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679-86. (3) Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4819-27. (4) Seliskar, C. J.; Heineman, W. R.; Shi, Y.; Slaterbeck, A. F.; Aryl, S.; Ridgway, T. H.; Nevin, J. H. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3258, 56-65. (5) Seliskar, C. J.; Shi, Y.; Gao, L.; Clager, M. R.; Slaterbeck, A. F.; Heineman, W. R. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3258, 66-74. (6) Shi, Y. Doctoral Dissertation, University of Cincinnati, Cincinnati, OH, 1998.
(7) Hieftje, G. M. Anal. Chem. 1972, 44, 81A-88A. (8) Binkley, D.; Dessy, R. J. Chem. Educ. 1979, 56, 148-53. (9) Tardy, D. C. J. Chem. Educ. 1986, 63, 648-50. (10) Wipf, D. O.; Wightman, R. M. Anal. Chem. 1988, 60, 2460-64. (11) Kuwana, T.; Winograd, N. Spectroelectrochemistry at Optically Transparent Electrodes I. Electrodes Under Semi-Infinite Diffusion Conditions. In Electroanalytical Chemistry A Series of Advances, Vol. 7; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; pp 1-78. (12) Winograd, N.; Kuwana, T. J. Am. Chem. Soc. 1970, 92, 224-26. (13) Winograd, N.; Kuwana, T. Anal. Chem. 1971, 43, 252-59. (14) Winograd, N,; Kuwana, T. J. Am. Chem. Soc. 1971, 93, 4343-50. (15) Heineman, W. R.; Kuwana, T. Anal. Chem. 1972, 44, 1972-78. (16) Jan, C.; Lavine, B. K.; McCreery, R. L. Anal. Chem. 1985, 57, 752-58.
The effect of ensemble averaging on the response of a new spectroelectrochemical sensor has been investigated. The sensor consists of a selective film coated over an optically transparent electrode (OTE). The mode of detection is attenuated total reflection (ATR). For an analyte to be detected, it must first partition into the sensing film, second be electroactive at the applied potential, and third have a change in its absorbance at the wavelength of light monitored by ATR. Four different excitation potential waveforms were investigated: pulsed, step, triangular, and sinusoidal. Under the condition of continuous cycling of the excitation potential, the optical response approaches a steady-state condition within as few as five cycles. Once the response has reached steady state, ensemble averaging over successive cycles is shown to improve the signal-to-noise ratio, allowing an order of magnitude improvement in the limit of detection. A model sensor consisting of a cationically selective sol-gelderived Nafion composite film coated on an indium tin oxide OTE is employed to demonstrate the signal acquisition techniques under investigation. Tris(2,2′-bipyridyl)ruthenium(II) chloride was used as a model analyte.
1196 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
10.1021/ac9807464 CCC: $18.00
© 1999 American Chemical Society Published on Web 02/10/1999
Wipf and Wightman applied ensemble averaging to the current response from fast-scan cyclic voltammetry.10 Double-layer charging was minimized by use of very small, disk-shaped electrodes, while scanning at very fast rates minimized the time required to approach steady state. More frequently, ensemble averaging has been applied to techniques whose mode of detection was not electrochemical. Kuwana and Winograd have used ensemble averaging to measure very small changes in absorbance in the UV-visible internal reflectance spectroelectrochemical study of chemical reactions coupled to heterogeneous electron exchange at the interface.11-14 A good example of the effectiveness of this technique in spectroelectrochemistry is shown in Figure 5 of ref 13. Ensemble averaging has also been employed when other optical modes of detection were coupled to electrochemistry: UV-visible spectroelectrochemistry based on normal incidence transmission,11,15 parallel geometry,16 and reflectance,17 and also infrared spectroelectrochemistry.18,19 The ability imparted by this technique to measure very small changes in absorbance has made tenable the study of solution species very near the electrode,11-14,16,18 species adsorbed on the electrode surface,15,17-20 and also optical changes in the electrode itself.19,21 More recently, Collinson and Wightman employed signal averaging in their observations of electrochemically generated chemiluminescense.22 The commonality of these applications is the analytical signal; though the changes being monitored are induced by electrochemical excitation, the measured signals are optical. Since only the solution very near to the electrode surface (
