Single Liquid Aerosol Nano-Impact Electrochemistry: Accessing the Droplet|air Interface

Over the past decade, many groups have shown that chemistry changes drastically in confined volumes compared to large volumes. As a robust analytical chemistry tool, electrochemistry has augmented the discussion because of its ability to study reactivity of single atoms, molecules, and nanoparticles, one at a time. Unfortunately, this field of science, known as nano-impact electrochemistry, has been limited to studying sub-femtoliter liquid droplets suspended in oil because measurements require at least two electrodes. Here, we develop a miniaturizable platform for the nano-impact electrochemical study of single liquid aerosol droplets. We achieve detection by the miniaturization of laser-pulled, dual-barrel ultramicroelectrodes, where two Pt ultramicroelectrodes are separated by a distance of a few micrometers. When aerosols loaded with a 1 : 1 solution of 300 mM hexacyanoferrate(ii/iii) are nebulized, discrete transients, indicative of single aerosol droplet collisions with the nanoprobe, can be observed in the amperometric i-t trace. Using finite element modeling, we demonstrate the amperometric signal depends on the aerosol droplet geometry at the nanoprobe. Our results push the limit of what is measurable by taking electroanalysis to the single aerosol level. These experiments also allow access to the study of chemical reactions at the droplet|air interface.