Electrochemiluminescence of Ru(II) complexes immobilized on a magnetic microbead surface: Distribution of magnetic microbeads on the electrode surface and effect of azide ion

The electrochemiluminescence (ECL) of magnetic microbeads modified with tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)(3)](2+)) was studied in the presence of tri-n-propylamine (TPA) to develop highly sensitive ECL detection system, where the employed microbead has a diameter of 4.5 mu m. The ECL signal of the [Ru(bpy)(3)](2+) derivative-modified magnetic microbeads was found to be affected by the geometrical distribution of the magnetic microbeads on the electrode surface. The ECL peak intensity increased with increasing the number of the beads on the electrode surfaces up to 1.6 x 10(6) beads cm(-2), although above 1.6 x 10(6) beads cm(-2), it decreased. The ECL decrease arises from the physical prevention of the ECL from reaching the photomultiplier tube by the excessive beads. The observed peak ECL signal of the [Ru(bpy)(3)](2+) derivative-modified magnetic microbeads in the presence of NaN3, which serves as a preservative substance, mainly appeared at a potential of +0.90 V vs Ag/AgCl where [Ru(bpy)(3)](2+) is hardly oxidized, whereas the ECL signal in the absence of NaN3 appeared at a potential of +1.15 V. The presence of NaN3 on the electrode surface retards formation of an oxide layer on the electrode surfaces and promotes TPA oxidation. The ECL response at +0.90 V was mainly attributed to ECL reaction of excited-state [Ru(bpy)(3)](2+*) formed by oxidation of [Ru(bpy)(3)](+) with TPA radical cation, where the [Ru(bpy)(3)](+) was generated by reduction of [Ru(bpy)(3)](2+) with TPA radical.