Amplifying the electrochemical footprint of <1000 molecules in a dissolving microdroplet

The ability of analytical strategies to detect and positively identify molecules under extremely dilute conditions is important for the growth and expansion of analytical techniques and instrumentation. At present, few measurement science techniques can robustly approach the measurement of just a few thousand molecules. Here, we present an electrochemical platform for the detection and positive identification of fewer than 1000 molecules of decamethylferrocene ((Cp*)₂Fe^II). We achieve this remarkable detection threshold by trapping (Cp*)₂Fe^II in a 1,2-dichloroethane microdroplet, which is allowed to dissolve into an aqueous continuous phase while on a gold microelectrode (radius ∼6.25 μm). Because electrochemistry is not sensitive enough to observe the charge of less than 1000 molecules, we dissolved μM amounts hexacyanoferrate(III) in the aqueous continuous phase. The biphasic reaction between hexacyanoferrate(III) and Cp₂*(Fe)^II allows for a feedback loop when the microelectrode is biased sufficiently negative to reduce Cp₂*(Fe)^III. This feedback loop, a typical EC' catalytic mechanism, amplifies the electrochemical signal of Cp₂*(Fe)^II when the droplet is of small enough dimensions for feedback to occur. Our results demonstrate that clever biphasic reactions can be coupled with dissolving microdroplets to access extremely low limits of quantitation in electroanalysis.