Voltammetric examination of carbon and platinum nanoelectrodes under mixed diffusion – migration mass transport conditions

Abstract

The majority of electroanalytical experiments are performed with supporting electrolytes to minimize the migration of electroactive ions and other unwanted effects of the low conductivity of the system. Microelectrodes and nanoelectrodes allow for voltammetric measurements in solutions of substantial resistivity, however, their application in non-supported electrolytes is vaguely studied in detail. This work demonstrates the employment of carbon and platinum nanoelectrodes in solutions of ionic and neutral ferrocene derivatives in the presence and absence of supporting electrolyte. The ferrocene derivatives of various charges served as redox probes for the studies of diffusion-migration transport to nanoelectrodes. The obtained results are confronted with the theoretical predictions derived for microelectrodes. An addition of a supporting electrolyte stabilizes the voltammetric signal. It makes the voltammetric waves at nanoelectrodes well developed irrespective of the type of the redox system examined. The results revealed that the migration of electroactive species to nanoelectrodes in the absence of a supporting electrolyte does not introduce significant changes to the voltammetric signal recorded with a supporting electrolyte as much as for bigger microelectrodes. In many cases, the complete elimination of supporting electrolyte led to the poor development of the voltammetric waves at nanoelectrodes, possibly due to the breakdown of the electroneutrality in the region adjacent to the nanoelectrode, since the electrical double layer for nanoelectrodes may occupy a significant fraction of the transport depletion layer. The uncertainty in the concentrations of ionic impurities (that determine the actual ionic strength) in the layer adjacent to the electrode is another factor that may produce unpredictable and, to some extent, random voltammetric behavior of charged redox probes at nanoelectrodes in the absence of supporting electrolyte.