On-site drug testing using easily accessible body fluids, such as saliva, has gained a lot of importance both for road safety and forensic applications. However, saliva is a complex, inhomogeneous amalgam with many different compounds, whose concentrations change between people and over time. This leads to a wide range of fluid properties, e.g., viscosity, surface tension, making saliva pretreatment in microfluidic systems extremely challenging. We present the development, fabrication and characterization of a low-cost, portable microsystem to detect cocaine in human saliva. The chip combines multiphase liquid-liquid extraction to transfer the cocaine from the infrared (IR) light absorbing saliva to the IR-transparent solvent, perchloroethylene (PCE), together with on-chip cocaine detection by IR-spectroscopy (IR-laser, waveguide, detector). This miniaturized sensing platform has been developed as part of the Nano-Tera project IrSens. Saliva is a suitable matrix for the analysis of small molecules such as cocaine, but it is found to be a challenge to process this complex fluid on a microfluidic chip due to the risk of clogging. The characterization of the rheological behavior of saliva is described and then a suitable sample pretreatment method is presented. The low-cost UV-curable polymer NOA81 was chosen as microfluidic chip material and characterized for microfluidic applications. For research applications a new, highly flexible, and low-cost NOA81 rapid prototyping method is presented based on scotch-tape masters. Furthermore, this thesis introduces a microfluidic chip design combining a simple and robust droplet generation method together with a geometry for droplet merging in order to perform continuous droplet-based liquid-liquid extraction. The extraction efficiency of the system was evaluated by state-of-the-art mass spectrometry measurements. The microfluidic chip has been integrated in an optofluidic microsystem transferring cocaine from human saliva to the IR-transparent organic solvent PCE and thereby enabling cocaine detection by IR-spectroscopy. In this thesis we achieved to develop a simple and robust droplet generation method re- sulting in precise and controlled droplet sizes, and which allowed to process complex fluids such as human saliva without clogging. Furthermore, a droplet-based liquid-liquid extraction system which transfers the cocaine continuously from human saliva to the organic solvent PCE was established. For these liquids the developed system yielded a two to three times higher extraction efficiency than state-of-the-art devices. In addition, we demonstrated that our system has the potential for cocaine preconcentration in PCE, which is crucial for IR spectroscopy, our final application.
Carlotta Guiducci, Saurabh Tomar, Thomas Ernst