Anal. Chem. 1995, 67,2393-2400
Impedance Analysis of NMP-TCNQ at Glucose Oxidase Interfaces C. C. Jung and E. A. H. Hall* University of Cambridge, Institute of Biotechnology, Tennis Court Road, Cambridge CB2 lQT, U.K.
Organic conducting salts are known as electrode materials that can accept electrons from biological molecules including a variety of different enzymes. This unusual property is not fully understood yet. This study investigates the biocatalytic propertiesof the organic conducting salt N-methylphenaziniumtetracyano-p-quinodethane (NMP-TCNQ) as a prototype system in connection with the enzyme (glucose oxidase using impedance spectroscopy. A mathematical model is applied to provide an understanding of the observed effects in the impedance spectrum, and a model involvingTCNQO as the mediating species is proposed to account for the observed experimental data. Since the first observation of bioelectrocatalytic currents in the glucose oxidase (GOx) /glucose system on organic conducting salt (OCS) electrodes,1,2several different enzymes have been shown to work with different OCS materials, including FAD- and NAD+-dependent system^.^,^ Nevertheless, the mechanism of current generation is still debated,5 although there seems to be agreement about the fact that direct electron transfer from the enzyme to a conducting salt electrode is very unlikely. Kulys6v7proposed an oxidation pathway via dissolved components of the salt. He determined the solubility of N-methylphenazinium tetracyano-pquinodimethane (NMP-TCNQ) in buffer solution to be 9-18 pM,7 which is in the region of the enzyme concentrations employed, whereas Albery and co-workers8were not able to detect concentrations of > 1pM employing rotating ring disk electrode studies. They proposed an alternative mechanism for the ?TF-TCNQ (TTF, tetrathiafulvalene) electrode involving heterogeneous catalysis following dissociation of the salt into its neutral species l"'and TCNQO at the surface of the electrode and the reaction of the reduced enzyme with TCNQ0.899 Hill et al.1° found that addition of LiTCNQ to a solution of reduced enzyme caused the current measured on the TI"-TCNQ electrode to decrease, whereas addition of lTF-C104 resulted in an increase in current. This was interpreted to be due to two (1) Kulys, J. J.; Samalius, A. S.: Svirmickas, G. J. S. FEBS Lett. 1980,114, 7. (2) Cenas. N. IC; Kulys, J. J. Bioelectrochem. Bioenerg. 1981,8, 103. (3) Ward, M. D. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16, pp 181-312. (4) Bartlett, P. N. In Biosensors; Cass, A. E. G., Ed.; IRL Press: Oxford, 1990; pp 47-95. (5) Hall, E. A. H. Biosenson; Open University Press: Oxford, 1990. (6) Kulys, J. J.; Cenas, N. K. Biochim. Biophys. Acta 1983,10, 385. (7) Kulys, J. J. Biosenson 1986,2,3. (8) Albery, W. J.; Bartlett, P. N.; Craston, D. H.]. Electroanal. Chem. Intetfacial Electrochem. 1985,194,223. (9) Albery, W. J.; Bartlett, P. N.; Cass, A E. G . Phil. Trans. R. SOC.London B 1987,316, 107. (10) Hill, B. S.; Scolari, C. A,; Wilson, G . S. Phil. Trans. R. SOC.London A 1990, 333,63.
0003-2700/95/0367-2393$9.00/0 0 1 995 American Chemical Society
Table 1. Expected Redox Potentials for the Electroactive Mediator Couple Candidates
TCNQ-12pyocyanin
crCNQ-)z/ZTCNQzNMP+/O ZTCNQo/(TCNQ-)2 TCNQo/-
Ei/z
ref
-0.320 -0.245 -0.133 -0.050 +0.115 t-0.211
13 14,15 13 16 13 13
competing mechanisms, the homogeneous mediation by ?TF+ and the heterogeneous reaction of the enzyme with TCNQO. Zhao et al.11 more recently suggested mediation by TCNQ- at an HMlTeF-TCNQ (HMlTeF, hexamethylenetetratellurofulvalene) electrode at potentials of
