On-chip drop-to-drop liquid microextraction coupled with real-time concentration monitoring technique

This paper demonstrates a novel drop-to-drop liquid-liquid micro-extraction (DTD-LLME) device, which is based on an electrowetting on dielectric (EWOD) digital microfluidic chip. Droplets of two immiscible liquids, one of which is an ionic liquid, are formed in nanoliter volumes, driven along electrodes, merged and mixed for extraction, and finally separated upon the completion of the extraction process. All the steps are carried out on a microfluidic chip using combined electrowetting and dielectrophoretic forces, which act on the droplet upon the application of electric potential. Specially, the phase separation of two immiscible nanoliter-scale liquid drops was achieved for the first time on an EWOD digital microfluidic chip. To study the on-chip extraction kinetics, an image-based concentration measurement technique with suitable color parameters was studied and compared with the typical UV absorption based technique. Finally, the effect of applied ac voltage frequency on the extraction kinetics was studied. The observations on DTD-LLME, particularly phase separation, are discussed. The image-based method was found to be applicable for precise concentration measurements with the right choice of the color parameter. Results from experiments on finding the frequency dependence on extraction kinetics demonstrate that the application of higher frequencies can be a factor in accelerating the extraction on the proposed microextraction device.