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Effect of Electrode Modification by Polyion Adsorption on Electrochemiluminescence of Luminol

Yasuo Yoshimi, Akitoshi Hatamochi, Kamada Masayuki, and Shin-ichi Sekine. Applied Chemistry, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, 135-8548, Japan

The chemiluminescence of luminol is useful for the high sensitive quantitative analysis of biochemicals that can be converted into hydrogen peroxide by an oxidase. Conveniently, luminol is inexpensive and possesses low toxicity. However, the method has stability problems because reactant-mixing conditions influence the results. Electrochemiluminescence (ECL) is generated by electrolytic reaction instead of catalysis. ECL of luminol can be performed on completely mixed solutions of luminol and hydrogen peroxide. Especially, the characteristic of ECL is applicable for analysis of biochemicals in small amount of body fluid sample for clinical use. The ECL enables microanalysis with very simple system, then it is applicable for micro total analytical system (μTAS). However, the optimal pH for the ECL reaction of luminol is far from intensity at physiological pH. The ECL is inhibited seriously in anionic reducers, ascorbic acid and uric acid, which is included in body fluids. We previously reported that electrode modification by double adsorption of cationic polymers firstly and anionic polymer secondly increases the sensitivity of ECL to hydrogen peroxide concentration [1]. However, the ECL at the modified electrode is inhibited by the anionic reducers more seriously that that at the unmodified electrode. Cyclic voltammetry indicated that the anionic layer blocks the reducers from the modified electrode, effectively. We estimated the ECL in the presence of dye (beet red or eosin yellowish) without affinity with the electrode surface. Neither of the dye affected on the electrochemical reaction of luminol and hydrogen peroxide. Either of the dye decreased the ECL at the modified electrode more remarkably than the unmodified electrode. The dye in the test solution absorbs photons emitted in bulk. Then, the result shows that ECL at the electrode modified with polycation and polyanion generates at the bulk of the solution rather than the electrode surface. It would be the reason why the electrostatic repulsion of anionic reducers by the polyanion cannot prevent the inhibition of ECL from the reducers. We must design new strategy to overcome the problem of the inhibition by the reducers.

Reference [1] Y. Yoshimi, M. Kamada, Y. Ohkawara, K. Hattori and K. Sakai: Improvement of Hydrogen Peroxide Sensitivity of Electrochemiliminescent Method with Luminol by Polyion Adsorption onto the Electrode, Electrochemistry, 361, 747-750, 2004.