Highly reproducible chronoamperometric analysis in microdroplets

Abstract

Here we report a method for highly reproducible chronoamperometric analysis of the contents of microdroplets. Aqueous microdroplets having volumes on the order of 1 nL and separated by a fluorocarbon solvent are generated within a microfluidic device using a T-shaped junction. The key finding is that stable and reproducible quasi-steady-state currents are observed if the electrochemical measurements are made in a narrowed segment of a microchannel. Under these conditions, the microdroplets are stretched, here by a factor of 10, leading to desirable intradroplet mass transfer characteristics. Microdroplet frequencies up to 0.67 s⁻¹ are accessible using this method. The quasi-steady-state currents resulting from chronoamperometric analysis of microdroplets containing 1.0 mM Ru(NH₃)₆³⁺ have relative standard deviations of just 1.8% and 2.8% at flow rates of 30 nL min⁻¹ and 60 nL min⁻¹, respectively. Importantly, the design of the microelectrochemical device ensures direct contact between intradroplet redox molecules and the electrode surface. That is, the fluorocarbon between microdroplets does interfere with inner-sphere electrocatalytic processes such as the oxygen reduction reaction. Finite-element simulations are presented that are in accord with the experimental findings.