Improved method for liquid-liquid extraction on low-cost polymer chips

The efficiency of on-chip microfluidic liquid-liquid extraction was improved by two orders of magnitude by spraying one liquid in another compared to the state-of-the-H-filters. We are developing a portable, integrated optical microsystem in the mid-infrared range to detect cocaine in human saliva. To ensure a good signal-to-noise ratio, the analyte needs to be transferred to an IR-transparent organic solvent by liquid-liquid extraction for subsequent on-chip analysis by IR-spectroscopy. In microfluidics, liquid-liquid extractions are performed by diffusion in a microfluidic H-filter, where two fluids flow in parallel. Recently, microfluidic droplet based liquid-liquid extraction was presented using droplet diameters as large as the channel size. Transfer times between a fraction and a few seconds were achieved with this approach, which is much faster than the conventional H-filter. We improved this system by spraying an organic solvent into an aqueous, external phase (containing the analyte of interest) using a microfluidic flow focusing device. The smaller droplets (higher surface-to-volume ratio) generated with this approach provided a larger interface between the two fluids, what enhanced the extraction process. Furthermore a smaller ratio between the droplet phase and the external phase used for spraying made more analytes available to be extracted by the droplet phase. The experiments were performed on microfluidic chips fabricated by rapid prototyping using a low-cost, commercially available UV glue (NOA81, Norland Optical Adhesive). For the extraction of cocaine from human saliva by tetrachloroethylene, mass spectrometry measurements indicate an improved efficiency of two orders of magnitude compared with values obtained by H-filters. We are now working towards the integration of the microfluidic part with the IR laser, the optical waveguide and detector.