Fluorescence Monitored Voltammetry of Single Attoliter Droplets.

The lipid soluble fluorophore Nile Red (9-diethylamino-5-benzo[α]phenoxazinone) is used to fluorescently and electrochemically label an organic-in-water emulsion, where the organic phase is an ionic liquid [P6,6,6,14][FAP]/toluene mixture. The optical detection of the individual droplets is enabled facilitating the in situ tracking and sizing of the suspended particles (average diameter = 530 nm, interquartile range = 180 nm). Through the use of a combined thin-layer optical/electrochemical cell, the irreversible accumulation of the droplets at an optically opaque carbon fiber electrode (diameter ∼7.5 μm) can be monitored. Potentiostatic control of the system enables the fluorescence of the surface bound particles to be electrochemically switched via control of the redox state of the dye. Subsequent measurements of the individual particle fluorescence intensities as a function of the applied electrode potential enables construction of an effective, dynamically recorded cyclic voltammogram of an individual particle. The confined volume voltammetry (∼tens of attoliters) yields insight into the asymmetry of the kinetics of the redox switching process, where it is proposed that the reformation of the fluorescent Nile Red becomes chemically "gated" in the organic phase.