Electrochemical Sensing of Thiocyanate Using Gold Electrodes Modified with an Underpotentially Deposited Silver Monolayer

Abstract

Gold electrodes modified by underpotential deposition to expose a layer of silver atoms on their surfaces were used to measure thiocyanate concentrations in aqueous solutions. When exposed to thiocyanate, the ion adsorbs onto the modified electrode and causes changes in the electrochemical properties of the silver adlayer. Coulometric measurement of the fraction of the silver adlayer that remains in its original state provides a means for determining thiocyanate concentrations. The adsorption of thiocyanate onto the electrode follows a first-order process with a rate constant of ca. 440 L/mol s that defines its concentration/time response.Keywords: Electrochemical sensors, Thiocyanate, Gold electrode, Underpotential deposition, Sensors, Monolayers DOI: 10.1002/elan.200900288 Thiocyanate ( À SCN) is an important ion in metabolism, often being affected by the consumption of specific foods or the smoking of tobacco [1]. For the latter, thiocyanate measurements can be used to distinguish between smokers and nonsmokers, as thiocyanate is a metabolic degradation product of tobacco smoke [2]. Present methods for measuring thiocyanate levels include a variety of spectroscopic and chromatographic methods as well as number of electrochemical approaches [1,3]. Thiocyanate-selective electrodes have been reported employing metal porphyrins [4 -8], selected metal complexes [9 -12], and other complexing agents with specificity for thiocyanate [1,3,13] that have been incorporated into membranes cast onto an electrode surface. Many of these agents require chemical synthesis [3, 5 -8, 10 -13], limiting their availability. Wang et al. [14] reported an amperometric sensor for thiocyanate that employed a glassy carbon electrode modified to expose silver nanoparticles on its surface. The silver nanoparticles had redox properties that were different from those of bulk silver and underwent a shift in potential upon the adsorption of thiocyanate onto the silver surface.Underpotentially deposited (upd) metal films have electrochemical properties that are different from their bulk counterparts [15]. For example, silver adlayers on gold, present as a one-atom thick layer on a gold surface at coverages of up to a monolayer, are less prone to oxidation than their bulk counterparts and can have redox properties > 500 mV positive of bulk silver. This shift reflects differences in interaction for the silver adatoms with underlying silver atoms in one case, and with underlying gold atoms in the other. The more noble character of the underlying gold and its ability to stabilize the silver adatoms better than a bulk silver substrate are responsible for the difference in electrochemical properties of a silver layer on the two materials. The adsorption of species onto an upd metal adlayer can further alter the electrochemical properties of the metal adatoms. Previously, we have shown that chloride and other halides adsorb onto upd films of silver on gold and produce readily identified electrochemical changes that relate directly to the ha...

Photochemically mediated synthesis of a gold colloid by dithizone and its application in the amperometric sensing of thiocyanate

“…NH 2 -NH 2 (hydrazine), NaBH 4 (sodium borohydride) etc.) [29][30][31][32][33] However, most of them require a colorometric probe or redox dye indicator etc. However, photochemical schemes also do not utilise any external reducing agent, so they have an advantage over the common existing methods in which an excess of the reducing agent (e.g.…”

“…chromatography, potentiometry and uorometry etc., for the determination of environmentally hazardous thiocyanate. [29][30][31][32][33] However, most of them require a colorometric probe or redox dye indicator etc. and they are also time consuming, costly or require a sophisticated instrumentation work-up or have a high detection limit which prevents them from being used for on-site measurement and eld tests.…”