Microdroplet-Based Potentiometric Redox Measurements on Gold Nanoporous Electrodes

Potentiometric redox measurements were made in subnanoliter droplets of solutions using an optically transparent nanoporous gold electrode strategically mounted on the stage of an inverted microscope. Nanoporous gold was prepared via dealloying gold leaf with concentrated nitric acid and was chemisorbed to a standard microscope coverslip with (3-mercaptopropyl)trimethoxysilane. The gold surface was further modified with 1-hexanethiol to optimize hydrophobicity of the surface to allow for redox measurements to be made in nanoscopic volumes. Time traces of the open-circuit potential (OCP) were used to construct Nernst plots to evaluate the applicability of the droplet-based potentiometric redox measurement system. Two poised one-electron transfer systems (potassium ferricyanide/ferrocyanide and ferrous/ferric ammonium sulfate) yielded Nernstian slopes of −58.5 and −60.3 mV, respectively, with regression coefficients greater than 0.99. The y-intercepts of the two agreed well to the formal potential of the tw...