Ru(bpy)32+ trapped in silica glass thin films to build solid electrochemiluminescent (ECL) sensors

Silica glass thin films containing Ru(II) tris-bipyridine complex were deposited via a sol-gel dipping procedure on K-glass substrates to build electrochemiluminescent (ECL) sensors. Films were prepared starting from an ethanolic solution of prehydrolised Si(OC2H5)4 and Ru(bpy)3Cl2. Transparent, crack free and homogeneous reddish Ru(II) tris-bipyridine complex doped silica layers, 200±20 nm thick 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 characterised. 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), atomic force microscopy (AFM) and UV-Vis spectroscopy. Experimental data showed that the complex maintains its integrity either in the as-prepared and in the annealed films and that it is homogeneously distributed inside the thin layer. Electrochemical and ECL characterisation 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 behaviour 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 hydrating period. The 300°C treated sensor was the best performing sample both in terms of low complex leakage and sensitivity.