Electroactivity of redox probes encapsulated within sol-gel-derived silicate films

The electroactivity of redox probes encapsulated within a silicate film prepared by the sol-gel process has been investigated to provide specific information about dopant stability, entrapment, and activity. Redox probes of different charges and sizes were physically doped into a sol prepared by the acid-catalyzed hydrolysis and condensation of tetramethoxysilane. The doped sols were then spin cast on pretreated glassy carbon electrodes, dried, and placed in a 0.1 M KNO3 electrolyte solution. The voltammetry of the gel-doped anionic probes (i.e., potassium ferricyanide (Fe(CN)6(3-/4-)), iridium(IV) chloride (IrCl62-/3-), potassium octacyanomolybdate(IV) (Mo(CN)(8)(4-/3-))) was Stable as the film remained in solution and/or the electrode potential continuously cycled. For the gel-doped cationic or neutral probes (i.e., ruthenium(III) hexaammine (RU(NH3)(6)(3+/2+)), ruthenium(II) tris(bipyridine) (RU(bpy)(3)(2+/3+)), ferrocenemethanol (FcCH(2)OH(0/+))), a significant reduction in the Faradaic current was observed. UV-vis spectroscopy of the electrolyte solution after electrochemical cycling of gel-encapsulated Ru(bpy)(3)(2+/3+) confirmed that the dopant leached Out of the film. The percentage of electroactive reagent was determined for Fe(CN)(6)(3-/4-) and RU(bpy)(3)(2+) and found to be 3-9% and 5-10%, respectively, consistent with entrapment in a compact, dense matrix. With the exception of the much larger RU(bpy)(3)(2+/3+), which exhibited peak-shaped voltammetry, the electrochemical behavior of the electroactive fraction of the entrapped redox probes corresponded to a surface-confined process.