Ultrasensitive Electrochemistry by Radical Annihilation Amplification in a Solid-Liquid Microgap

We report a technique to amplify the electrochemical signal within micro- and nanodroplets via radical annihilation amplification. Toluene droplets filled with decamethylferrocene (DmFc) are suspended in an aqueous solution containing 10 mM NaClO₄ and 10 μM Na₂C₂O₄. When a toluene droplet irreversibly collides with an ultramicroelectrode biased sufficiently positive for concurrent oxidation of DmFc and oxalate (C₂O₄^2-), blip-type responses are observed in the amperometric i-t trace even when the concentration of DmFc is 50 nM. The toluene droplet wetting the ultramicroelectrode effectively creates a microgap, where DmFc molecules are oxidized to DmFc⁺. In the continuous phase, the oxidation of oxalate (C₂O₄^2-) produces a strong reducing agent, CO₂^•-. Regeneration of DmFc via radical annihilation amplifies the current, similar to conventional nanogap experiments. This experiment allows one to observe the electrochemistry of hundreds to thousands of molecules trapped in a femtoliter droplet, enhancing the sensitivity of droplet-based electrochemistry by 5 orders of magnitude. Finite element simulations validate our experimental results and indicate the importance of the droplet geometry to amplification.