Chemiluminescence (CL) of luminol with hydrogen peroxide is useful for the highly sensitive determination of biochemicals by conjugation with oxidase. However, the method has stability problems because reactant-mixing conditions influence the results. However, electrochemiluminescence (ECL) of luminol can be performed on well-mixed solutions of luminol and hydrogen peroxide. ECL has not yet been applied to the quantitative analysis of biochemicals due to low quantum yields at physiological pH. In this work, we evaluated the effect of modifying the electrode with cationic polymer on the ECL intensity of luminol. Transparent indium-tin oxide (ITO) electrode was treated with 1 wt% aqueous solution of polyethyleneimine (MW: 70,000) buffered by 0.1 M borate at pH 8.0. ECL intensity at the ITO electrode was measured in a mixed solution of 1.0 mM luminol and 10-30 μM hydrogen peroxide buffered by phosphate or Tris-HCl (pH 7.4). The electrode potential was applied in alternate pulses of 0.00 V versus Ag/AgCl for 3 s and 1.00 V for 3 s. The sensitivity of ECL to hydrogen peroxide concentration at the treated ITO increased remarkably. Results indicate that the cationic polymer adsorbed on the electrode enhances ECL by increasing pH in the vicinity of the electrode surface. However, the cationic polymer also enhanced inhibition of ECL by ascorbic acid, an anionic reducing reagent.
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