Electrochemical Detection in Nanoliter Droplets

The growing interest for in situ sensing in nanoliter droplets warrants a deeper study of the sensing capabilities and wetting principal of very small electrochemical sensing electrodes in contact with microfluidic droplets. For this study, a device consisting of a droplet generator followed by a 100 mu m wide channel with a three-electrode set-up with a 10 mu m wide working electrode is used. For measurements 5 mM ferri-/ferrocyanide was used as standard redox couple. It was experimentally found that a thin layer of the droplet phase was left behind on the electrodes, suggesting that the droplets do not need to wet the electrodes but rather merge with the water layer present on the electrodes. With this device, it was possible to detect the redox couple inside the microfluidic droplets and the currents reached a well-defined plateau almost immediately. However, the currents measured inside the droplets deviated from the expected trends for currents measured in flow. Also, a linear concentration curve could be obtained indicating that electrochemical quantitative sensing remains possible.