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

Abstract

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 NaClO 4 and 10 μM Na 2 C 2 O 4 . When a toluene droplet irreversibly collides with an ultramicroelectrode biased sufficiently positive for concurrent oxidation of DmFc and oxalate (C 2 O 4 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 (C 2 O 4 2− ) produces a strong reducing agent, CO 2 •− . 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 dropletbased electrochemistry by 5 orders of magnitude. Finite element simulations validate our experimental results and indicate the importance of the droplet geometry to amplification.

Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets

“…The blip occurs because of a rapid rise in current when the droplet irreversibly adsorbs to the electrode followed by an exponential decay back to baseline due to the electrolysis of droplet contents. Studying single entities, one at a time, opens the door to studying physicochemical properties of single nanoparticles. − ,− Such stochastic electrochemical experiments have elucidated physicochemical properties of single atoms, molecules, and nanoparticles …”

Electrochemistry under confinement

“…In the presented work, the polymerization was induced by a chemical compound, but this process might be replaced by electropolymerization 221 for a more controlled electrode decoration. Recently, Dick and coworkers 216 presented a promising technique to greatly amplify electrochemical sensitivity in single-droplet impact measurements by radical amplification, usually known from nano gap experiments. 222 Toluene nanocarriers, encapsulating bis-(pentamethylcyclopentadienyl) iron (DmFc) were dispersed in an aqueous solution supported by sodium perchlorate.…”

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

“…À at the emulsion/solution. 63 The strong reducing reagent, CO 2 À , was generated from oxalate at the solution/ electrode interface. Liquid/liquid interfaces can be used instead of UMEs to electrochemically detect single NEs.…”