Concentration Enrichment in a Dissolving Microdroplet: Accessing Sub-nanomolar Electroanalysis

Droplet evaporation has previously been used as a concentration enrichment strategy; however, the measurement technique of choice requires quantification in rather large volumes. Electrochemistry has recently emerged as a method to robustly probe volumes even down to the attoliter (10^-18 L) level. We present a concentration enrichment strategy based on the dissolution of a microdroplet placed on the surface of a Au ultramicroelectrode (radius ∼ 6.25 μm). By precisely positioning a 1,2-dichloroethane microdroplet onto the ultramicroelectrode with a microinjector, we are able to track the droplet's behavior optically and electrochemically. Because the droplet spontaneously dissolves over time, given the relative solubility of 1,2-dichloroethane in the water continuous phase, the change in volume with time enriches the concentration of the redox probe (Cp₂*(Fe)^II) in the droplet. We demonstrate robust electrochemical detection down to sub-nM (800 pM) concentrations of Cp₂*(Fe)^II. For this droplet, 800 pM constitutes only about 10⁶ molecules. We extend the strategy in a single-blind study to determine unknown concentrations, emphasizing the promise of the new methodology. These results take voltammetric quantification easily to the sub-μM regime.