Published: February 04, 2011 r2011 American Chemical Society 1658 dx.doi.org/10.1021/ac102716s | Anal. Chem. 2011, 83, 1658–1664 ARTICLE pubs.acs.org/ac On-Chip Drop-to-Drop Liquid Microextraction Coupled with Real-Time Concentration Monitoring Technique Pavithra A. L. Wijethunga, † Yasith S. Nanayakkara, ‡ Praveen Kunchala, † Daniel W. Armstrong, ‡ and Hyejin Moon* ,† † Department of Mechanical and Aerospace Engineering, and ‡ Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States b S Supporting Information ABSTRACT: 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 electro- wetting 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 concentra- tion 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. L iquid-liquid extraction is one of the most widely used sample pretreatment techniques in basic sciences. Traditional liquid- liquid extraction methods can consume large quantities of organic solvents, which lead to environmentally hazardous waste products. In addition, typically liquid-liquid extraction can be time-consuming and require trained personnel, which elevates cost for unitary or multiplexed industrial procedures. 1 To minimize these drawbacks, miniaturization methods, such as liquid-phase microextraction and single-drop microextraction, were proposed. 2-4 Despite recent deve- lopments, pros and cons of di fferent versions of microextraction platforms reveal the need for continuous innovation toward inte- grated analytical systems. 5 For instance, an extraction device that can facilitate multiplexing or high-throughput operation has never been considered. Thus, we introduce a new drop-to-drop liquid-liquid microextraction (DTD-LLME) approach, a lab-on-chip method, which can easily handle parallel multiple extraction processes. The device is based on electrowetting on dielectric (EWOD) digital microfluidics, where both sample and extractant are handled in micro/nano droplet formats. The proposed device provided en- hanced performance because it combines the use of ionic liquids as solvents and the utility of an EWOD digital microfluidic, which can be operated via remote computer programming. EWOD digital microfluidic is a new type of microfluidic lab-on- a-chip system in which liquid drops are created and actuated by appli- cation of digitized voltages. 6 The interest in EWOD digital micro- fluidic devices has increased tremendously given their versatility over traditional microfluidic devices. They have been used to perform sample preparations for matrix-assisted laser desorption ionization mass spectrometry for protein analysis as well as to provide an efficient platform for chemical reactions. 7-12 Hence, their potential to be integrated into micro total analysis systems is apparent. Previous efforts on EWOD digital microfluidic demonstrated not only the advantage of handling liquids in small volumes with high accuracy but also its capability to perform various multiplexing processes which empowers an EWOD device to be an advantageous platform for high-throughput screening processes. Hence, identifying, developing, and analyzing microextractions on EWOD digital microfluidic plat- forms would be beneficial in achieving such total analysis systems. Recently, Abdelgawad et al. reported a liquid-liquid micro extraction process on a speci fic nonplanar EWOD platform. 13 Although they showed the puri fication of a contaminated DNA droplet by driving it back and forth into a pool of water-immiscible extractant, the device needs to be partially immersed into the extractant pool. Clearly, a microextraction device where both donor and extractant are handled in terms of micro/nanoscale droplets would be preferred. Such a device would provide a better control over the donor-to-extractant ratio, comparatively lower extractant and donor consumption, and a much simpler device configuration and operation. As will be shown, Received: October 19, 2010 Accepted: January 5, 2011