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 (IrCl6 2-/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 (FcCH2OH0/+)), 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.