(Invited) Structure and Reactivity of Ruthenium-Substituted Prussian-Blue-Type Oxo-Cyanide Bridged Networks: Prospects for Electrocatalysis and Analytical Determinations

Electrochemical deposition of crosslinked oxo-cyanoruthenate, Ru-O/CN-O, from a mixture of RuCl3 and K4Ru(CN)6 is known to yield a film on glassy carbon that promotes oxidations by a combination of the electron and oxygen transfers. The initial suggestion that the film could be a simple ruthenium analogue of Prussian Blue, in which the Ru ions reach higher then III oxidation state, was later clarified using Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was postulated that the electrochemical deposition process yielded the -Ru-O-Ru- network with cyano-groups crosslinks, Ru-O/CN-O, where the ruthenium could exist in mixed oxidation states. Such mixed-valent conditions, facilitated the overall charge distribution and conductivity of the catalytic material. In addition, contrary to iron ions in Prussian Blue, ruthenium can be readily oxidized to higher oxidation states to form catalytically active ruthenium(IV,III) oxo species that include Ru3O2 6+ or Ru2O5 + groups immobilized within the cyanide-bridged network. Furthermore, at pH = 2, the hexacyanoruthenate anions tended to transformation to dinuclear anionic species, [(CN)5-RuIICN-RuIII-(CN)5]6 −. Strong electrostatic attractive interactions as well as the polynuclear character of deposits led to the overall stabilization of the system. Films of Ru-O/CN-O were demonstratedto have low ohmic resistances as a result of effective charge transport involving hydrogen and/or potassium ions. Numerous applications of Ru-O/CN-O, which have been reported or recently demonstrated, will be considered during presentation. The oxidation of alcohols, inorganic and organic sulfur compounds, glucose, hydrazine, aldehydes, nitrosamines, insulin, and tetracycline are representative examples. From the electrochemical analysis point of view, as it is typical with electrode reactions involving mediated electron transfers, the signal-to-background ratios with amperometry under potentiostatic conditions are preferred over potentiodynamic methodology for analytical applications. By the former approach, detection limits of nmol dm− 3 in an injection are achieved. For example, a linear calibration curve has been obtained over the range 8–200 ng insulin in a 7.5 µL injection into a flow system with measurement of current under potentiostatic conditions . Analogous to mixed metal oxides as modifiers, the efficacy of this catalyst should be related to the combination of fast electron displacements and specific oxygen transfers. The system’s resistance to interfacial passivation should be mentioned here. Finally, the carefully designed multicomponent systems provide a route to improving electrocatalytic efficiency over that at single-component materials because of electronic, geometric or stability factors. Combination of Ru-O/CN-O with silica sol-gels as solid electrolytes expands the scope of electrochemical applications and adapts electrocatalysis to chemical analysis of complex samples.