Construction and characterization of Ru(II)tris(bipyridine)- based silica thin film electrochemiluminescent sensors

Ru(II) tris-bipyridine based ECL sensors were produced by embeddingthe complex inside silica glass thin films deposited via a sol-gel dipping procedure on K-glass conducing substrates. Films were prepared starting from a prehydrolyzed ethanolic solution of Si(OC2H5)4 and Ru(bpy)3Cl2. Transparent, crack-free and homogeneous reddish silica layers, havinga thickness of 20020 nm, were obtained. The films, either deposited at room temperature or thermally annealed at 100, 200 and 300 C for 30 h, were structurally and chemically characterized. Ru(bpy)3Cl2 thermal stability was previously checked by thermogravimetric analysis (TGA). The films were investigated by X-Ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and UV-vis spectroscopy. XPS in-depth profiles revealed a homogeneous distribution of the ruthenium complex inside the silica thin layers. SIMS data suggested that the embedded Ru(bpy)3Cl2 did not react with oxygen inside the oxygen-rich silica matrix to give Ru-O bonds. Electrochemical and ECL characterization of the thin film electrodes were made by means of cyclic voltammetry (CV) and controlled potential step experiments. The ECL sensor showed a diffusive redox behavior of the Ru(bpy)3 2+/Ru(bpy)3 3+ system. Light emission produced from the reaction between oxalic acid and the electrogenerated Ru(bpy)3 3+ was larger and stable when thermally treated electrodes were used after a suitable hydration period. The 300°C treated sample was the best performingsensor both in terms of low complex leakage and sensitivity. Calibration plots relative to oxalic acid were obtained both in stationary and in flowingsolutions in the concentration range 2uM - 300 uM. A linear behavior appeared in the former case, while in the latter a slight curvature was evident as a consequence of a finite diffusion time of the analyte inside the thin film. The signal repeatability, obtained by multiple 100 uL of 10 uM oxalic acid injections in flowingsolutions , was better than 4%. The obtained detection limit (computed as three times the standard deviation of the base-line noise) was 10-6M as oxalic acid.